The role of the immune microenvironment in bone, cartilage, and soft tissue regeneration: from mechanism to therapeutic opportunity
Military Medical Research volume 9, Article number: 65 (2022)
Bone, cartilage, and soft tissue regeneration is a complex spatiotemporal process recruiting a variety of cell types, whose activity and interplay must be precisely mediated for effective healing post-injury. Although extensive strides have been made in the understanding of the immune microenvironment processes governing bone, cartilage, and soft tissue regeneration, effective clinical translation of these mechanisms remains a challenge. Regulation of the immune microenvironment is increasingly becoming a favorable target for bone, cartilage, and soft tissue regeneration; therefore, an in-depth understanding of the communication between immune cells and functional tissue cells would be valuable. Herein, we review the regulatory role of the immune microenvironment in the promotion and maintenance of stem cell states in the context of bone, cartilage, and soft tissue repair and regeneration. We discuss the roles of various immune cell subsets in bone, cartilage, and soft tissue repair and regeneration processes and introduce novel strategies, for example, biomaterial-targeting of immune cell activity, aimed at regulating healing. Understanding the mechanisms of the crosstalk between the immune microenvironment and regeneration pathways may shed light on new therapeutic opportunities for enhancing bone, cartilage, and soft tissue regeneration through regulation of the immune microenvironment.
Bone, cartilage, and soft tissue injury are common clinical conditions that can severely impair function and limit the quality of life [1,2,3]. Although the field of tissue engineering has rapidly developed, thus providing promising means of adequately repairing tissue damage, soft tissue defects, particularly large-area tissue injuries or complex injuries involving multiple tissues, remain major challenges for clinicians [4, 5]. Regenerative medicine is an important branch of translational medicine in bioengineering and cellular biology that involves the replacement, reconstruction, or regeneration of cells, tissues, or organs. The purpose is to stimulate the body’s inherent repair mechanisms to effectively cure injured tissue . Bone, cartilage, and soft tissue regeneration is a dynamically balanced process involving the metabolism, differentiation, and migration of tissue cells, including complex interactions between the immune and musculoskeletal systems [7, 8].
Immune cells are involved in the regulation of tissue homeostasis; in contrast, tissue cells influence the survival and function of immune cells; as such, immunomodulation plays a critical role in tissue repair and regeneration [9, 10]. The inflammatory response is important in maintaining tissue homeostasis and has dual roles in this regulation process . It serves as a protective response in the promotion of tissue regeneration and is often a major cause of tissue damage in infectious diseases, immunologic alteration, and trauma [12,13,14]. When the inflammatory response is activated by injury, abundant cells derived from peripheral blood monocytes are present in tissues, and the specific immune microenvironment drives these cells’ sequence of repair .
The immune microenvironment plays an important role in the healing, repair, and regeneration of tissues, and can be reshaped by intrinsic and exoteric factors, such as stem cells . Currently, the major roles of stem cells in the regulation of the immune microenvironment and the connection between stem cells and tissue regeneration has been well studied . The reparative functions of mesenchymal stem cells (MSCs) in a wide array of inflammatory diseases rely on their immunomodulation and the release of various bioactive cytokines . Specifically, tissue regeneration is closely associated with the immune microenvironment surrounding the injured tissue. The immunoregulatory function of MSCs affects the clinical application and translation of MSC-based regenerative therapy .
Stem cells exert their immunomodulatory roles by producing various regulatory cytokines, such as interleukin (IL)-4, IL-7, IL-10, interferon-γ (IFN-γ), and prostaglandin E2 (PGE2) . Interestingly, emerging evidence indicates that small extracellular vesicles (sEVs) secreted in a paracrine manner are an important means through which stem cells regulate the immune microenvironment . For example, MSC-derived sEVs significantly ameliorate the development of autoimmune and neurodegenerative disorders by re-programming the immune environment under pathological conditions . sEVs are versatile membrane vesicles with a regulatory function through delivery of various bioactive molecules throughout the intercellular microenvironment. sEVs mediate the crosstalk between cells and modulate the immune microenvironment in a paracrine manner . Evidence has indicated a correlation between sEVs and stem cells, thus suggesting their high potential for promoting cellular proliferation and migration to the injured tissue [10, 13]. Moreover, stem cell-derived sEVs facilitate tissue regeneration through modulation of the immune microenvironment . However, stem cells and sEVs also have substantial disadvantages in clinical application, such as complex components, unstable biological activity, low targeting, and difficulty in preservation [23, 24]. Therefore, interest has increased in developing new strategies to maximize the therapeutic effects of stem cells and EVs.
Currently, various tissue-specific biomaterials with cytokines and immunomodulatory effects promoting tissue regeneration have been developed and implanted into sites of damaged tissue to enhance the therapeutic efficacy of tissue regeneration. The biomaterial-based strategy can provide physical support to transplanted cells, and stem cells can rapidly proliferate and differentiate to compensate for the lost tissue cells . Furthermore, the therapeutic effects can be enhanced if biomaterials exert immunomodulatory effects and inhibit the local overactivated immune responses . The properties of the biomaterial for tissue regeneration vary depending on the target damaged tissue, and the immunosuppression provided by biomaterials has prominent effects on tissue repair and regeneration .
The role of the immune microenvironment in regulating tissue regeneration has attracted attention [28,29,30,31,32]. For example, Yang et al.  have summarized the roles of multiple immune cells and immune cytokines in bone regeneration. Similarly, a recent study has systematically introduced the new developments in cellular crosstalk between immune cells and stem cells, and provided advanced insights into the application of biomaterial-based strategies in the promotion of tissue regeneration . However, few studies have comprehensively summarized the regulatory roles of stem cells and the immune microenvironment, and how to balance these roles through immunomodulatory biomaterials in bone, cartilage, and soft tissue regeneration. Therefore, we will discuss the role of the immune microenvironment in tissue regeneration, focusing on stem cells and immune cells, to discuss the immune mechanisms in the tissue repair and regeneration processes, and shed light on promoting the curative effects of treatments for severe tissue injury.
Stem cell signals in the regulation of tissue regeneration
Stem cells are a critical primitive cell type with differentiation and regeneration potential. Tissue regeneration is coordinated by stem cells, which not only compensate for lost functional cells but also exert self-renewal functions [18, 21]. Stem cells reside in damaged tissues and are the cellular source of the regeneration process . In response to tissue damage, stem cells accelerate the production of specific types of differentiated cells, thus promoting tissue regeneration. Classic stem cell signals are often activated within tissue cells, and consequently enable damaged tissues to self-renew and proliferate . Thus, stem cell signals play a major role in tissue regeneration, and stem cell-based therapies are a prominent trend in regenerative medicine.
Stem cells and bone regeneration
Bone regeneration is an intricate and highly orchestrated biological regulatory process involving different cell types and their activated signaling pathways [32, 33]. Stem cells, particularly skeletal stem cells or MSCs, engage in bone regeneration, owing to their self-renewal and differentiation ability, secretion of active cytokines, and modulation of other cells in host tissues . The involvement of MSCs in bone regeneration processes is mediated by active molecules, such as hormones and growth factors, and their stimulated cellular networks  (Table 1).
Bone morphogenetic protein-2 (BMP-2) is a signaling molecule with critical roles in bone regeneration. Recombinant human BMP-2 (rhBMP-2) has been widely used in clinical settings to enhance bone regeneration. Our previous research has indicated that the osteoporotic phenotype is reversed in mice with systematic injections of rhBMP-2. rhBMP-2 injection enhances the osteogenic activity of MSCs, thus suggesting that rhBMP-2 and other active anabolic compounds are effective in targeting MSCs . Through a skeletal gene therapy approach, we have found that MSCs infected with a recombinant adenoviral vector encoding human BMP-2 can repair bone defects in ectopic sites through engrafting and forming bone and cartilage in mice [36, 37]. Freeman et al.  have reported that 3D bioprinted implants containing a vascular endothelial growth factor (VEGF) gradient, paired with spatially defined BMP-2 localization and release kinetics, expedites the healing of defects in large bone with minimal formation of heterotopic bone. A major advancement in the field of bone regeneration has been the development of mRNA-based BMP-2 therapy. For example, to avoid the high costs and adverse effects of rhBMP-2, such as inflammatory complications, ectopic bone formation, and tumor formation , De La Vega et al.  have reported that using a modified mRNA encoding BMP-2 is another approach to bone regeneration. This novel approach, compared with the recombinant protein approach, results in significantly better healing of large, critically sized, segmental osseous defects of long bones and has no adverse effects, possibly because of its transient and anatomically restricted expression of BMP-2 
In recent years, several high-impact studies have reported the critical intracellular signals in bone regeneration. De Simone et al.  reported that rhythmic traveling waves of extracellular signal-regulated kinase activity modulate the growth of bone temporally and spatially in regenerating zebrafish. After injury, inflammatory signals cause bone regeneration to commence simultaneously with infiltration by sensory nerve fibers. Xu et al.  demonstrated that cranial bone injuries stimulate nerve growth factor expression and signaling via p75 in resident osteogenic precursors that affect their migration into the damaged tissue, thus suggesting that nerve growth factor-p75 signaling has potential roles in bone regeneration. Ambrosi et al.  reported that intrinsic skeletal stem cell aging in mice alters signaling in the bone marrow niche to a degenerative inflammatory niche, thus leading to poorly regenerated bones because of fragility. BMP-2 has been used to activate skeletal stem cells together with a CSF1 antagonist to inhibit bone resorption, thus eliciting youthful bone regeneration in older bone . Using chromatin and transcriptional profiling, Ransom et al.  demonstrated that mechanotransduction via the focal adhesion kinase signaling pathway in skeletal stem cells promotes stem cell-mediated regeneration of adult skeletal tissue. The dedifferentiation of mature cells is a cellular process strongly associated with tissue regeneration. Osteoblasts dedifferentiate into osteogenic progenitors during zebrafish fin regeneration, thus providing source cells for bone restoration . Through in vivo chemical identification of mediators of osteoblast dedifferentiation and fin regeneration, Mishra et al.  have found that the NF-κB pathway is active in mature osteoblasts and is downregulated before dedifferentiation. In contrast, inhibition of NF-κB signaling has been found to enhance dedifferentiation, thus clarifying the molecular regulation of regenerative cellular plasticity 
In summary, although debates remain regarding the origins, functions, developmental potential, and possible therapeutic uses of MSCs —mainly because MSCs are commonly defined by their in vitro functions, whereas their functions in vivo are insufficiently defined —these stem cells and the signaling molecules associated with their proliferation, migration, and differentiation are critical in bone homeostasis and regeneration [48, 49]
Stem cell signals in the regulation of cartilage regeneration
Cartilage is mainly composed of collagen and proteoglycans. As an avascular and aneural tissue, cartilage lacks self-healing ability after damage, which can be triggered by trauma, aging, obesity, immune diseases, tumor resection, and osteoarthritis (OA). After injury is initiated in cartilage, the two opposing bones rub against each other, and joint replacement is eventually required in the absence of early intervention. Since the 1930s, clinical interventions for cartilage lesions, including surgical and non-surgical approaches, have evolved from palliative to reparative and most recently to regenerative strategies . However, the complex bi-phasic structure of the osteochondral unit and the relatively low metabolic activity of chondrocytes in articular cartilage substantially hinder repair. Cartilage is remodeled dynamically by signaling pathways that are controlled by cells and the extracellular matrix (ECM) . Using cell signals has therefore provided a longer tether for the regenerative management of cartilage injury. Chondrocytes are one of the primary choices in cartilage regeneration, because they are the prominent resident cell type in articular cartilage. However, their application is largely limited by their inferior isolation efficiency, low proliferation rate, and high possibility of dedifferentiation into fibroblasts during expansion [52, 53]. Hence, the use of stem cells has gained momentum in the field, owing to their ability to proliferate and directionally differentiate into chondrocytes. Preclinical and clinical studies involving stem cells have demonstrated significantly better outcomes with cartilage regeneration than with traditional cell-free strategies [54, 55].
Stem cells used for the restoration of cartilage defects can be classified into three main categories: adult stem cells (ASCs), embryonic stem cells (ESCs), and induced pluripotent stem cells (iPSCs) . The most widely used form of stem cells is ASCs, which are found in adult body tissues; these cells include skeletal stem cells and MSCs, adipose tissue MSCs, joint synovium MSCs, and peripheral blood MSCs. These cells have benefits of the relative ease of isolation and greater availability than ESCs and iPSCs, which are found in mammalian embryos and genetically reprogrammed somatic cells, respectively [57, 58]. The age-associated decline in proliferation, and the association with hypertrophic cartilage or fibrocartilage formation limit the application of ASCs in cartilage regeneration. ESCs are considered the most suitable type for articular cartilage regeneration, because they can indefinitely self-renew and can be directed to differentiate into both lineages of bone and cartilage, owing to their pluripotency [55, 59]. The disadvantages of using ESCs in clinical practice include the difficulty in obtaining functional chondrocytes from human ESCs and ethical issues . The discovery of iPSCs in 2006 opened a gateway for cartilage regeneration, because these cells possess pluripotency and the potential for self-renewal, similarly to ESCs but without ethical concerns [60, 61]. However, standard and simple protocols to guide iPSCs toward chondrogenic differentiation are lacking, and these cells’ effectiveness in hyaline cartilage production is highly reliant on environmental cues [62,63,64,65,66]. For example, Lee et al.  have found that chondrocytes derived from iPSCs through mesodermal and ecto-mesodermal differentiation have distinct activities and functions. Similar to the search for better scaffold design and effective biological stimulation, appropriate stem cell selection remains a challenge for functioning cartilage regeneration.
In terms of the therapeutic mechanisms involving stem cells in cartilage regeneration, cartilage tissue restoration was previously believed to be achieved via the directional chondrogenic differentiation of implanted stem cells triggered by the surrounding microenvironment (differentiation theory) [67,68,69]. However, recent evidence suggests that exogenous (donor) stem cells do not directly contribute to the formation of regenerated cartilage tissue by turning into chondrocytes . Instead, they regulate the microenvironment of the defect area by producing various derivatives, including growth factors, extracellular vesicles (EVs), and ECM, which can alter the fate of host cells such as endogenous (host) stem cells, chondrocytes, and macrophages (paracrine effect) [71, 72]. For example, these paracrine signaling pathways induce the homing and proliferation of resident chondrocytes, promote the chondrogenic differentiation of endogenous stem cells, and positively modulate the anti-inflammatory process by influencing host macrophages to facilitate cartilage regeneration [70, 71, 73,74,75].
Although the detailed mechanisms underlying stem cell-regulated cartilage regeneration remain unclear, the patterning, growth, maturation, and homeostasis of cartilage tissue are known to be exquisitely tuned by a series of signaling pathways, which govern the fate of stem cells [76, 77]. Consequently, a better understanding of these signaling pathways should offer therapeutic opportunities for cartilage regeneration. In Table 2, we summarize the crucial signaling pathways that are responsive to cartilage functional behavior, along with recent examples of use of these signals in stem cell-mediated cartilage regeneration; the mechanisms underlying each of these signals have been investigated in different microenvironment cues. Healthy cartilage function is finely tuned through synergistic functions of multiple signaling pathways, and extensive crosstalk exists among these pathways to maintain a dynamic balance between synthetic and catabolic activities of cartilage tissue.
The correlation between stem cells and soft tissue regeneration
Soft tissue defects, such as those following trauma, tumor resection, and infection, are common clinical encounters, and adequate tissue regeneration is a substantial challenge. Stem cell-based therapy has gained momentum in regenerative medicine [113, 114]. Stem cells modulate tissue metabolism and regeneration mainly via two unique abilities: 1) the ability to self-renew with symmetric division, and 2) the ability to multi-directionally differentiate with asymmetric division  (Table 3).
c-Jun N-terminal kinase (JNK) signaling is among the most important regulatory signals in soft tissue regeneration; JNK regulates the activity of stem cells involved in soft tissue repair and regeneration . JNKs are important molecules mediating the intracellular responses of stem cells to many different types of stimuli in the external cellular microenvironment . JNK function is essential for achieving a delicate balance between cell death and stem cell survival to promote soft tissue repair, remodeling, and regeneration . Dhoke et al.  have reported that transplantation of preconditioned stem cells enhances soft tissue regeneration with a robust antioxidant defensive mechanism through activation of JNK signaling. Similarly, Jiang et al.  have found that JNK signaling plays a critical role in regulating the differentiation of MSCs into keratinocytes and promotes tissue regeneration. An in-depth understanding of the mechanism underlying how JNK signaling mediates soft tissue regeneration would aid in the development of new effective therapies.
Epithelial regeneration is a crucial component of soft tissue regeneration, and an in-depth understanding of the regulatory roles of ESCs and their effects on tissue homeostasis might elucidate soft tissue regeneration . Among the critical signaling pathways associated with epithelial stem cell function, phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) signaling has attracted extensive attention in soft tissue repair and regeneration [121, 122]. Akt activation occurs after Thr308 and Ser473 phosphorylation, and active Akt controls multiple cellular regulatory processes, including cell survival and cell metabolism [121, 123]. Akt activation is negatively regulated by molecules that antagonize PI3K signaling, whereas in vivo results have indicated that double-knockout of Akt1 and Akt2 leads to deficient activation of mTOR . A prior study has indicated that ESCs provide protective mechanisms for inducing stem cell differentiation under the aberrant activation of mTOR . Thus, strategies aimed at indirectly activating mTOR may be a feasible approach to increase epithelial migration into injured sites and wound beds, thereby promoting soft tissue regeneration.
Wnt/β-catenin signaling is a well-documented Wnt signaling pathway, and its roles in soft tissue homeostasis and regeneration have received ample interest in recent decades. A previous study has revealed that Wnt/β-catenin signaling is critically involved in the regulation of stem cell function and tissue repair, as well as in the progression of chronic inflammatory diseases [125, 126]. Within the nucleus, β-catenin binds T-cell factor transcription enhancers, thus promoting the transcription of specific genes and leading to specific Wnt/β-catenin transduction . Prior studies have demonstrated that activation of this β-catenin-dependent pathway enhances the proliferation and function of stem cells, such as ESCs and MSCs, thus markedly promoting soft tissue regeneration [128,129,130]. Therefore, selective enhancement of Wnt/β-catenin signaling may be an effective strategy to induce soft tissue regeneration.
In addition, the role of nuclear factor erythroid 2-associated factor 2 (Nrf2) during soft tissue regeneration is an important research topic from the therapeutic perspective. Nrf2 is the primary mediator of active redox homeostasis. Several biofactors have been found to ameliorate cellular oxidative stress and enhance stem cell function, thus accelerating tissue repair by promoting Nrf2 activation . The important role of Nrf2 in regeneration involves the prevention of reactive oxygen species (ROS) accumulation in damaged tissues and activation of the antioxidant defense system . In a previous study, Nrf2 signaling has been demonstrated to have a protective role against cellular ROS via activation of the antioxidative system during tissue regeneration . Excessive ROS suppresses the proliferation of stem cells, stimulates cell apoptosis, and impairs tissue regeneration . Nrf2 is expressed in a wide array of cell types, including stem cells, endothelial cells, and fibroblasts. A recent study has indicated that Nrf2 deficiency impedes corneal epithelial wound healing in an Nrf2 knockout murine model . Similarly, an in vivo study has indicated that Nrf2 deficiency inhibits the activation of the antioxidative system in keratinocytes . Although these findings have indicated an important role of Nrf2 signaling during soft tissue repair and regeneration, more in-depth studies are needed to gain a better understanding of Nrf2 function in soft tissue regeneration. In Table 3, we summarize the crucial signaling pathways involved in soft tissue regeneration, each of which has been investigated to determine the underlying mechanisms in response to different microenvironment cues.
Immune microenvironment in tissue regeneration
The immune microenvironment plays a crucial role in tissue regeneration. Tissue regeneration generally begins with early immune-inflammatory responses, thus triggering the boosting of immune cells and secretion of inflammatory cytokines and chemokines, which subsequently mobilize and recruit immune cells to injured sites [140, 141]. Simultaneously, stem cells can cope with an immune microenvironment and regulate immune-inflammatory responses during tissue regeneration . Therefore, we will discuss the immunomodulatory effects of various immune cells and their roles in tissue regeneration.
The role of macrophages in tissue regeneration
The process of tissue regeneration has been described as four continuous and overlapping stages: hemostasis, inflammation, repair, and remodeling . To achieve an ideal outcome, these stages should be tightly controlled, because aberrations can lead to damage that increases the likelihood of regeneration failure. The development of these stages is dependent on the regulatory roles of immune cells, particularly at the inflammatory stage, thus determining the effectiveness of the subsequent repair and remodeling stages (Fig. 1) [140, 141].
For bone, cartilage, and soft tissue regeneration, inflammation initiates an influx of neutrophils, followed by monocytes, which then differentiate into macrophages . Signals from innate immune cells further recruit lymphocytes into the wound bed, where they participate in intercellular communication and affect inflammatory responses. The next repair stage is characterized by neo-angiogenesis, secretion of ECM, and collagen synthesis. The ultimate remodeling stage involves maturation of the newly formed blood vessels and tissue remodeling . Immune cells are crucial for the removal of cell debris and for modulating regeneration through the regulation of tissue-specific stem cells. This function is exemplified in bone repair and regeneration, in which the interaction between immune cells and osteogenic cells is critical for the completion of the inflammation stage and the progression to repair and remodeling . In previous research, we have found that excessive inflammation hinders bone remodeling, whereas effective control of the inflammatory response induces bone regeneration . Increasing evidence indicates that abnormal cellular activity during inflammation impairs tissue regeneration [144,145,146]. Given the complex roles of the immune microenvironment in tissue regeneration, in-depth knowledge of the underlying mechanisms that regulate tissue regeneration is essential.
Macrophages, an important immune cell type with multiple functions, have prominent roles in both innate and adaptive immunity. Macrophages extensively infiltrate damaged tissues and are key players in tissue regeneration . In addition to their ability to eliminate cell debris, neutrophils, invading organisms, and other apoptotic cells through phagocytosis, macrophages actively mediate tissue repair and exhibit different phenotypes during tissue regeneration [147, 148]. M1 macrophages produce pro-inflammatory cytokines, are highly phagocytic, and can engulf apoptotic neutrophils and remove pathogens and debris from local tissues. M2 macrophages have anti-inflammatory effects, and regulate angiogenesis, fibroblast regeneration, myofibroblast differentiation, and collagen production . Macrophages exert a crucial role in tissue regeneration through phenotypic polarization, and they participate in almost all stages of tissue regeneration. Several major cytokines are involved in regulating tissue regeneration via the mediation of macrophage phenotype polarization. Generally, IFN-γ, IL-2, IL-3, IL-12, tumor necrosis factor-α (TNF-α), lipopolysaccharide, and Toll-like receptor agonists induce macrophage M1 polarization [150,151,152]. M1 macrophages secrete pro-inflammatory cytokines and chemokines, such as IL-1β, IL-6, IL-12, IL-23, chemokine (C-X-C motif) ligand (CXCL)-9, and CXCL-10, and participate in inflammatory responses [153,154,155]. Accordingly, several cytokines, including IL-4, IL-10, IL-13, transforming growth factor-β (TGF-β), and granulocyte-macrophage colony-stimulating factor, induce macrophage M2 polarization and secretion of various anti-inflammatory molecules, thus enhancing anti-inflammatory activity and promoting tissue regeneration [156,157,158].
In early stages, macrophages infiltrate the wound area and are activated to the M1 phenotype, which participates in the phagocytosis of pathogens and cell fragments, and the secretion of inflammatory factors to recruit circulating monocytes. In the repair stage, macrophages produce active cytokines, thus promoting more apoptosis of neutrophils; elicit a switch from the pro-inflammatory phenotype (M1) to the anti-inflammatory phenotype (M2); and phagocytose apoptotic neutrophils, thereby alleviating local inflammation of the damaged tissue . The pro-inflammatory ability of macrophages is important in the early stages of tissue regeneration, but proper tissue regeneration requires a timely transformation of macrophages to the anti-inflammatory phenotype.
The ability of macrophages to induce inflammatory deactivation is seen with the formation and maintenance of regulatory T cells (Tregs). Tregs create an anti-inflammatory microenvironment conducive to tissue regeneration and maintain the anti-inflammatory phenotype of macrophages . Enhanced switching from a pro-inflammatory to an anti-inflammatory macrophage phenotype facilitates tissue regeneration. Scavenger receptor class B1 has been found to promote M1 macrophages switching to M2 macrophages for tissue regeneration . Similarly, Kim et al.  have introduced an exosome-guided macrophage reprogramming technique, in which M2 macrophage-derived exosomes induce a complete switch from M1 to M2 macrophages, thereby markedly promoting cutaneous wound healing via enhancement of angiogenesis, re-epithelialization, and collagen deposition.
Different immune cells and immunomodulators are involved in the multiple stages of tissue regeneration. Tissue regeneration can be promoted by regulation of the immune system, particularly critical immune cell subsets . However, the mechanism through which the immune system regulates the regeneration of various organs and tissues requires further study . Macrophages are essential in most stages of tissue regeneration, but the mechanisms through which they switch their phenotypes and promote tissue regeneration remain elusive and require further exploration.
The correlation between natural killer (NK) cells and tissue regeneration
NK cells are another important innate immune cell type recruited to sites of injury . NK cells secrete active factors, which effectively mediate the host’s immune response. The key function of NK cells is to identify foreign, virally infected, and metabolically altered cells, and to induce their apoptosis or cell lysis . The exact role of NK cells in the regulation of tissue regeneration remains unknown. NK cells have been well documented to remove injured cells at the site of damage, and the cytotoxicity of NK cells is regulated by an array of receptors and the distribution of ligands on the membranes of target cells . Activated cytotoxic NK cells exert killing by delivering lytic granules or secreting death-inducing cytokines . Dastagir et al.  have found that NK cells are recruited to regenerating digit tips, and have observed NK cytotoxicity against osteoclast and osteoblast progenitors. The authors have concluded that stem cell proliferation and differentiation are mediated through multiple routes by distinct NK cell subsets. In-depth knowledge of NK cell-stem cell crosstalk may provide novel strategies for regenerative medicine.
The interaction of NK cells and MSCs in regeneration has recently become an important research area. MSCs are trophoblasts that are likely to exist in all tissues to support the survival and growth of many cell types, including hematopoietic cells, ESCs, tumor cells, nerve cells, liver cells, and endothelial cells [170, 171]. The “cell empowerment” and “cell replacement” concepts describe how MSCs modulate the immune system and provide a source of undifferentiated cells for tissue regeneration . Previous work has demonstrated that undifferentiated MSCs suppress NK cell proliferation, cytokine release, and cytotoxicity . Interestingly, more recent evidence has suggested that in appropriate contexts, MSCs can also support NK cell function . A prior study has further examined the crosstalk between MSCs and NK cells, and indicated that MSC activation enhances the regenerative functions of NK cells; moreover, NK cell-modulated neo-angiogenesis and tissue proliferation during trophoblast invasion are also used by MSCs in peripheral tissues to induce regeneration after inflammation . However, MSCs have been demonstrated to impair the cytotoxic capabilities of NK cells . Thus, more in-depth studies on the crosstalk between NK cells and MSCs are needed. The crucial issue in determining the outcome of using MSCs in clinical trials may be the interaction between implanted donor MSCs and recipient immune cells.
Dendritic cells (DCs) in the regulation of tissue regeneration
DCs are antigen-presenting cells that are critical in orchestrating adaptive immune responses and tissue homeostasis [177,178,179]. DCs initiate T cell responses and link innate and adaptive immunity by directing T cell differentiation into effector lineages [177, 180]. Except for immediate antigen processing and presentation, DCs are involved in homeostasis and disease regulation through cytokine secretion and the shaping of peripheral tolerance through local immunity . Although the precise role of DCs during tissue healing and regeneration remains under investigation, many studies have shown that DCs are fundamental in the tissue repair process. DCs recognize foreign substances at injury sites and immediately contribute to the healing of damaged tissue, acting as an immunoregulator of tissue regeneration through the modulation of macrophage homeostasis . In a burn wound murine model, DC-deficient mice show significantly delayed wound healing associated with the inhibition of early cellular proliferation, wound levels of Transforming growth factor-β1 (TGF-β1), and neo-angiogenesis in the wound beds. These findings suggest that DCs may have an essential role in the acceleration of events that promote early wound healing, and this acceleration is likely to be caused by the secretion of factors that activate cell proliferation and enhance cell functions .
DCs interact with skeletal cells, which have critical roles in tissue repair and regeneration. MSCs inhibit DC maturation in vitro and impair the ability of DCs to prime T cells in vivo . MSCs diminish major histocompatibility complex class II, CD40, and CD86 costimulatory molecules’ expression on mature DCs, and IL-6 is involved in the MSC-mediated immunoregulatory mechanism through partial inhibition of the differentiation of DCs . MSC migration is promoted through EVs from DCs, which can be manipulated to locally recruit endogenous or transplanted cells to injury sites . DCs have crucial roles in bone metabolism , including: 1) contribution to inflammation-mediated osteoclastogenesis and participation in inflammatory bone disease; 2) activation of T cells that produce bone remodeling cytokines and soluble factors; 3) pathogenesis of postmenopausal osteoporosis; 4) and transdifferentiation into osteoclasts in the presence of RANKL and macrophage colony-stimulating factor (M-CSF) or IL-17, wherein RANKL/RANK regulates the immune crosstalk between CD4 T cells and DCs. An in vitro study has indicated that DCs inhibit the differentiation and mineralization of osteoblasts . The field of tissue engineering in regenerative medicine often relies on strategies to appropriately modulate the immune response. DCs directly interact with biomaterials and which are critical for exerting biomaterial function . The crosstalk and mechanisms of regulation between bone cells and DCs must be further investigated before DCs can become a focus of new clinical therapies involving tissue regeneration and repair.
T cells in tissue regeneration
Tregs are an important group of T cells that maintain the body’s immune tolerance. Tregs are produced by the thymus and exported to the periphery. They actively inhibit the activity of potentially autoreactive T cells, thus modulating the body’s immunity and preventing the occurrence of autoimmune diseases . Tregs are not only critical for immune homeostasis but also exert a variety of non-immune functions, including mediation of stem and progenitor cell activity. Consequently, Tregs have become a crucial cell type for tissue repair and regeneration . In a prior study, we have demonstrated cross-communication between Tregs and bone-forming cells, with the potential for osteogenic differentiation and angiogenesis promoting bone remodeling and regeneration. Mechanistically, Treg-induced TGFBR1/SMAD2 signaling inhibition has been shown to be involved in the Tregs’ beneficial effects on bone healing . Similarly, Tregs modulate the activity of many other types of stem and progenitor cells involved in regeneration. For example, enrichment in IL-33 derived from Tregs has been found to have a prominent role in the regulation of fibro/adipogenic progenitor cells, and diminished IL-33 is the main reason for failed tissue regeneration in aging mice . Tregs systemically maintain the balance between immune homeostasis and inflammation, and are particularly abundant in soft tissue . Tregs in soft tissue have also been found to enhance the regenerative process, mainly by enforcing immune tolerance and suppressing excessive inflammation . For instance, Moreau et al.  have reported that Treg-derived amphiregulin induces tissue-resident T cell proliferation upon injury, thus leading to an immune-suppressive microenvironment and tissue regeneration (Fig. 2a).
Gamma delta (γδ) T cells are an important T cell type distributed in various tissues [196, 197]. Although the definitive effects of these cells on tissue regeneration are unknown, reports have identified the role of γδ T cells in promoting tissue regeneration, possibly through communication with tissue stem cells . The involvement of γδ T cells in wound repair and regeneration has been determined on the basis of the dynamics of γδ T cells and γδ T cell-derived activators during tissue regeneration . Similarly, the important role of γδ T cells has clearly been demonstrated in diabetic mice, which show delayed formation of granulation tissue and wound closure after γδ T cell ablation . In bone regeneration, IL-17 A-producing γδ T cells promote bone regeneration by inducing osteogenesis in a fracture mouse model. Mechanistically, γδ T cells enhance the proliferation and osteogenic differentiation of injured MSCs, and consequently stimulate bone regeneration after injury  (Fig. 2b).
Other important T cell types, such as cytotoxic T cells (CD8 T cells) and T helper cells (CD4 T cells), have been well documented to be essential activators in tissue regeneration. Adoptive T-cell therapy is a promising therapeutic approach against diseases . CD4 T cells play an important role in the immune system and guide the body to fight against microorganisms . CD8 T cells, another subgroup of T cells that can be further activated into effective T cells, known as cytotoxic T lymphocytes, exist in the tonsils, spleen, and other organs . A prior animal experiment has demonstrated that CD8 T cells impair cancellous bone repair . Similar results have also been found in humans, thus suggesting that impaired bone regeneration is closely linked to the elevation of CD8 cells . Interestingly, in skin regeneration, whereas CD4 and CD8 T cells are enriched in wounds, showing peak levels at days 5–10 and 7–10 post-injury, neither CD4 nor CD8 T cells are believed to impair skin regeneration . Thus, CD8 and CD4 T cells may have different regulatory roles in the regenerative process, depending on the target tissue. In a rat model, in vivo, CD8 and CD4 T cells have opposing roles in mediating wound regeneration: CD4 T cells are associated with enhanced repair, whereas CD8 T cells are associated with impaired healing . Mechanistically, T cells release a wide array of cytokines that affect both macrophages and fibroblasts, both of which play prominent roles in the regulation of tissue regeneration  (Fig. 2c).
Roles of sEVs-derived immunomodulation in tissue regeneration
sEVs are nano-sized extracellular vesicles involved in the regulation of cell-to-cell communication, which have attracted attraction as a promising cell-free therapeutic strategy in clinical applications . Generally, sEVs contain molecules including cytokines, lipids, and nucleic acids, which are important mediators of the biological behaviors of target cells . sEVs derived from stem cells can achieve enhanced cell proliferation and function with little immune response by creating a beneficial immune microenvironment . sEVs from MSCs have been demonstrated to enhance tissue regeneration and immune regulation, similarly to MSCs . Thus, the regulatory roles of sEVs in tissue regeneration are discussed and summarized in the following section.
sEVs-mediated immunomodulation and bone regeneration
In various models of diseases and cell-free regenerative medicine, sEVs appear to be beneficial in improving recovery [210, 211]. sEVs have been identified to play a major role in intercellular communication, particularly between MSCs and immune cells . Tissue regeneration after injury requires two major conditions: 1) a pro-inflammatory microenvironment to neutralize injury and eradicate dead or injured tissue, and 2) a subsequent anti-inflammatory microenvironment to regenerate new tissue through the migration, differentiation, and proliferation of reparative cell types, thus increasing vascularization and nutrient supply .
MSC-mediated therapeutic activities are an important aspect of immune modulation, including MSC-derived sEVs [213, 214]. By comparing the immunomodulatory functions of human gingival mesenchymal stem cells (GMSCs) and GMSC-sEVs in a murine in vitro T cell co-culture model of collagen-induced arthritis, Tian et al.  have found that GMSC-sEVs have similar or sometimes greater effects than GMSCs in inhibiting IL-17 A and promoting IL-10, thus decreasing the frequency and intensity of bone erosion in arthritis; these findings suggest that GMSC-sEVs may be a promising new cell-free therapy strategy for treating rheumatoid arthritis. The sEVs collected from human Bone marrow-derived mesenchymal stem/stromal cells (hBMSCs) significantly decrease the expression of proinflammatory genes, such as IL-1β, TNF‐α, IL‐6, and inducible nitric oxide synthase (iNOS), in macrophages and greatly promote the expression of early osteogenic markers in hBMSCs, thus suggesting that sEVs derived from differentiating mesenchymal stem/stromal cells have a unique function in the regulation of bone dynamics through their osteoimmunomodulatory role . An in vitro study has reported that T cell proliferation is suppressed by MSC-derived EVs, thereby supporting the application of a cell-based in vitro potency assay for determining the immunomodulatory potential of EVs . Another in vitro experiment has characterized the immunomodulatory function of human adipose MSC-derived exosomes on in vitro stimulated T cells. The investigation confirmed that these exosomes repress the differentiation and activation of T cells, and decrease T cell proliferation and IFN-γ release in in vitro stimulated cells . Yang et al.  have discovered that human umbilical cord MSC-derived exosomes released from hydrogels aid in bone regeneration in animal studies. Although this work had limitations regarding the overall mechanisms and efficacy, the strategy provides tremendous promise for prospective treatments in tissue and organ repair through sEV-based therapy. A recent porcine model study has identified MSC-derived sEVs paired with hyaluronic acid (HA) to aid in osteochondral repair by increasing trabecular bone thickness and improving the biomechanical properties of bone .
Immune cells affect bone regeneration through cell signaling regulation and osteoblastogenesis in MSCs. In an in vitro experiment, Li et al.  have identified that sEVs derived from M2 macrophages might have promise as a therapeutic tool in bone diseases, owing to their ability to inhibit adipogenesis and promote osteogenesis of BMSCs through the miR-690/IRS-1/TAZ axis. Our previous study has shown that hematopoietic cells stimulate proliferation and osteoblastogenesis, and inhibit cellular senescence of MSCs . Hematopoietic cells express TNF-α, platelet-derived growth factor beta (PDGF-β), Wnt1, 4, 6, 7a and 10a, secreted frizzled-related protein 3 (sFRP-3), and sFRP-5. The increase of TNF-α expression in hematopoietic cells in older people is associated with activation of NF-κB signaling and/or Wnt/β-catenin signaling, which negatively affects the interactions of hematopoietic cells on MSCs via TNF-α receptors, through inducing cellular senescence while also inhibiting osteoblast differentiation in MSCs. Our data have established paracrine interactions of hematopoietic cells on human MSCs; these findings, together with those from other reports, suggest that declining skeletal stem cell function may involve the extrinsic mechanisms of immunosenescence [221,222,223]. sEVs play crucial roles in the cellular regulation of MSCs and immune cells, thus positively influencing bone regeneration. However, more studies are needed to precisely identify the mechanisms underlying exosomal immunomodulation and bilateral interactions between skeletal cells and immune cells in bone regeneration.
Important roles of sEVs-mediated immunomodulation in regulation of cartilage regeneration
A variety of studies have examined the use of sEVs for cartilage regeneration. Notably, in these studies, the sEVs have been from an array of sources and applied at different concentrations, among which MSC-derived sEVs have been the most widely applied (Table 4). Previous studies have demonstrated that MSCs exert critical immunomodulatory roles in cartilage regeneration, mainly through paracrine secretion of trophic factors [224,225,226]. However, the immunomodulatory function of MSCs cannot be sufficiently imitated by any cytokine alone, e.g., IL-6, IL-8, IL-10, IL-33, monocyte chemoattractant protein-1 (MCP-1), or TGF-β, thus indicating that the immunomodulatory function of MSCs necessitates synergism among multiple cytokines [227, 228]. MSC-derived sEVs loaded with more than 100 immunomodulatory proteins are considered a perfect vehicle for this synergism . A prior study has demonstrated that MSC-derived sEVs are immunomodulatory and not immunosuppressive in mice, and these sEVs induce Tregs with active immune reactivity triggered by the grafting of allogenic skin . Furthermore, an in vivo study in an immunocompetent rat osteochondral defect model has indicated that MSC-derived sEVs alleviate OA by promoting M2 macrophage polarization and enhancing cartilage regeneration [230, 231]. MSC-derived sEVs are now widely accepted as a feasible therapeutic agent to regulate cartilage regeneration.
Considerable developments have been achieved in the application of sEVs for cartilage regeneration, and the mechanisms, therapeutic strategies, and production have been widely studied . In recent decades, clinical trials for sEVs have been undertaken, and standard guidelines for sEV extraction have been established . Prior study has detected various bioactive molecules, including non-coding RNA, proteins, lipids, and cholesterin, in the content of sEVs . However, full use of all characteristics of sEVs has not yet been accomplished, owing to the complexity of the bioactive cargo; most studies have focused on single bioactive factors within sEVs. An in-depth understanding of sEV biogenesis with the emergence of multiple bioreactors for elevating production would improve exosomal production in the laboratory setting, because the current low production scale greatly limits their clinical potential.
sEVs regulate soft tissue regeneration via immunomodulation
Excessive and persistent inflammation after injury impairs soft tissue regeneration and leads to the formation of chronic tissue defects. Effective and rapid soft tissue regeneration can be achieved by suppression of the overactivity of immune cells at injury sites . Normally, in the first few days after injury, immune cells, including macrophages, NK cells, and T cells, are recruited to the defect site by chemoattractants, such as complement, clotting components, and cytokines, to clear cell debris and bacteria from the wound. Thus, strategies that effectively modulate the overactive immune microenvironment have the potential to enhance and accelerate tissue regeneration .
Recent investigations have focused on identifying the mechanisms underlying the inhibitory effects of sEVs on the overactive immune response, including their suppressive activity on NK cells and CD8 T cells, their inhibitory effects on the differentiation and maturation of DCs, and their promotive effects on the function of Tregs [1, 242, 243]. Hence, sEVs are considered to have high potential as therapeutic vesicles for immunomodulation and for promoting soft tissue regeneration. For instance, Su et al.  have successfully engineered PD-L1-overexpressing sEVs, and demonstrated their promotive effects on wound healing through immunosuppressant activity.
Immunomodulation to ameliorate damage-induced inflammation and construct an appropriate immune microenvironment conducive to tissue regeneration may potentially be mediated through innate or adaptive immune responses . Prior proteomic profiling research has revealed enrichment of exosomal proteins during inflammation or complement activation . For example, MSC-derived sEVs have been reported to induce M2 phenotype polarization and decrease pro-inflammatory cytokines, thus enhancing tissue regeneration . A crucial immunomodulatory advantage of sEVs allowing them to promote soft tissue regeneration involves the promotion of anti-inflammatory and pro-regenerative macrophages (M2) over pro-inflammatory macrophages (M1). Although the underlying mechanism has yet to be uncovered, the macrophage phenotype polarization observed in sEV-mediated tissue regeneration is attributable mainly to the ability of sEVs to communicate directly with monocytes, and modulate active molecule production and release. In addition to macrophages, Tregs have attracted attention for their link with sEVs in attenuating the activated immune system . Interestingly, MSC-derived sEVs polarize CD4 T cells to Tregs when CD4 T cells are activated by allogenic CD11C+ antigen-presenting cells instead of CD3/CD28 co-stimulation . This finding suggests that the extent of the immunosuppressive functions of sEVs depends on the immune-reactive microenvironment; consequently, exosomal immunomodulation may ameliorate the immune system without leading to adverse effects.
Biomaterials involved in the regulation of the immune microenvironment for promoting regeneration
Biomaterials with different formulations have shown great promise in tissue regeneration . The potential of immune microenvironment modulation by implanted biomaterials in vivo has attracted considerable attention, and the development of biomaterials for the regulation of immune responses is expected to promote tissue regeneration . Microenvironment-regulating biomaterials may perform multiple functions in facilitating tissue regeneration. In the following section, advances in, and applications of, biomaterials used for tissue regeneration are discussed in depth.
Biomaterial-based immunomodulation and its critical role in regulation of bone regeneration
A wide array of biomaterials have been identified as stabilizing structures for injured bone or inducers of bone regeneration. These differ in chemical composition, shape, porosity, and mechanical properties . During the past few decades, extensive strides have been attempted to deliver immunomodulatory signals for bone regeneration through the use of a variety of biomaterials . The “soft” biomaterials, represented by hydrogels, have structural and chemical properties that allow for: 1) dynamic changes in their mechanical characteristics; 2) processing into various forms with unique surface topographies; 3) activation of biological responses; and 4) sustained delivery of biofactors . Our recent research has provided an example of how the bone immune microenvironment can be mimicked ; we developed a “cocktail therapy” to simultaneously regulate osteoblast differentiation and macrophage phenotype polarization. The cocktail therapy, comprising an HA-based hydrogel, engineered sEVs, and an inositol-requiring enzyme-1α (IRE-1α) inhibitor, has provided new insights into biomaterial strategies for effective bone regeneration therapy. Similarly, an in situ injectable hydrogel has been constructed via single-step equal volume mixing of a PBS solution of oxidized HA and hydrazide grafted gelatin, and its immunomodulatory function by the release of sEVs overexpressing PD-L1 in bone regeneration has been verified . Additionally, with the rapid development of bio-nanotechnology, various nano-structured materials in soft materials, such as anisotropic nanoscale ligands  and composite nanoparticles , have also been applied to regulate cell behaviors and promote bone regeneration. For example, Wong et al.  have conjugated RGD-bearing magnetic nanoparticles (Fe3O4 coated with silica) to increase RGD tether mobility, which can be decreased through the application of an external magnetic field, thus increasing MSC adhesion, spreading, and osteogenic differentiation. Furthermore, a “self-regeneration” biomaterial concept has recently been proposed, whereby the promotion of vascularization and bone formation can be achieved without the need for introducing cells or other therapeutics . This strategy is based on the presence of layered topographic cues, particularly in biomaterials that arrange the nanomorphologic cues into layered three-dimensional (3D) structures. A recent example of mimicking an extracellular tissue environment has been introduced by Hasani-Sadrabadi et al. , who have reported a novel periodontal membrane for guided tissue regeneration, which can mimic the complex extracellular environment of periodontal tissue and serve as functional tissue constructs for periodontal regeneration.
Among the immune cells in the bone microenvironment, macrophages have emerged as central to the immunomodulation of tissue regeneration. Particular focus has been placed on the role of phenotype switching, which remains controversial . For instance, a recent study has revealed the utilization of a nanostructured polycaprolactone (PCL)/polyvinylpyrrolidone electrospun biomaterial in bone tissue regeneration via polarization of macrophages toward the M2 phenotype . Generally, immunomodulation through biomaterials can be achieved via various strategies, including: 1) mediation of their physicochemical properties, 2) delivery of immunoregulatory activators, and 3) alteration of their mechanical properties . For example, Lin et al.  have introduced a new application of sodium alginate hydrogels with different stiffness to mimic tissue repair in vivo and have found that MSCs in a stiffer matrix show faster migration than those in a softer matrix; this novel platform promotes MSC migration through mimicking the natural tissue repair process. In addition to the physicochemical properties, direct incorporation of immunoregulatory activators has been used to initiate a specific immune response and bone regeneration. In our prior work, we constructed mesoporous silica and FeO composite-targeted nanoparticles loaded with baicalein, which were capable of promoting bone regeneration via the delivery of active baicalein to injured bone sites . Similarly, the ability of ECM proteins to promote M2 macrophage polarization has been used to enhance the immunomodulatory function of implanted biomaterials . The emerging understanding of the role of the biomaterial-regulated immune microenvironment in bone regeneration has led to a paradigm shift in strategies that harness immune cells to promote regeneration.
Immunomodulatory biomaterials for cartilage regeneration
Cartilage is susceptible to damage but cannot easily heal because of its avascular nature. Cartilage regeneration remains a research focus, because most currently used clinical methods cannot achieve satisfactory results . Evidence indicates that synovial inflammation is closely associated with chondrocyte apoptosis and cartilage damage . Macrophages reside in the synovial lining of joints and are involved in the regulation of synovial inflammation . Furthermore, anti-inflammatory M2 macrophages have been proposed to produce pro-chondrogenic factors, such as TGF-β and insulin-like growth factor (IGF), which are considered novel targets for cartilage repair . Therefore, innovative biomaterials scaffolds with immunomodulatory activity have been developed to promote cartilage regeneration in recent decades.
Biomaterials derived from ECM have been reported to play important roles in modulating the host macrophage response . Because of their ability to provide microenvironments similar to native cartilage, decellularized cartilage ECM scaffolds have been used to enhance cartilage regeneration [264, 265]. When implanted into cartilage defect areas, cartilage ECM scaffolds induce M2 phenotype polarization of macrophages, which in turn stimulates the migration, proliferation, and chondrogenic differentiation of MSCs  (Fig. 3a). Type II collagen, the main collagenous component in cartilage, is responsible for the induction of M2 macrophage polarization . Another type of collagen, squid type II collagen, has been applied to repair cartilage lesions in degenerative OA . This collagen polarizes the synovial macrophage response toward an M2 phenotype and increases the levels of TGF-β and IGF simultaneously in vivo, thus inducing a pro-chondrogenic environment for cartilage repair. An ECM-mimicking hydrogel scaffold has been fabricated by incorporating polydopamine (PDA)-modified HA into a collagen matrix  (Fig. 3b). The hydrogel scaffold has immunomodulation ability, through increasing the ratio of M2 macrophages and suppressing the inflammatory response, thus resulting in cartilage regeneration and remodeling in rabbits. Other natural biopolymers have also been suggested to have immunomodulatory functions. A hybrid scaffold composed of alginate, chitosan, hydroxyapatite, and fucoidan has an anti-inflammatory function, as evidenced by inhibition of the production of ROS and inflammatory mediators, including TNF-α, IL-6, and IL-1β, in lipopolysaccharide (LPS)-treated RAW 264.7 cells; thus, this scaffold should be a promising candidate for cartilage tissue engineering . The immunomodulatory effects of some synthetic polymers have also been explored: 3D-printed porous scaffolds made of sulfonated polyetheretherketone have been demonstrated to facilitate cartilage repair by promoting M2 macrophage polarization, increasing secretion of the anti-inflammatory cytokines IL-4 and IL-10, and preventing macrophage-induced cartilage degeneration . Additionally, lithium calcium silicate-based bioactive ceramic scaffolds have been reported to promote cartilage maturation by immunomodulating M2 macrophage polarization, as shown by upregulated expression of IL-10, and downregulated expression of the inflammatory factors TNFα, IL-6, and IL-1β .
IL-4, a Th2-type cytokine, is an important promoter of M2 macrophage polarization . Thus, delivery of IL-4 by bioscaffolds is an effective approach to drive macrophage polarization toward the M2 phenotype in vivo. An IL-4-loaded bilayer scaffold made of gelatin methacrylate (GelMA) (upper layer) and PCL-hydroxyapatite (lower layer) has been developed for the repair of osteochondral defects in rabbits  (Fig. 4a). IL-4 released from the scaffold relieves the inflammatory response and protects chondrocytes, thus improving regeneration of both cartilage and subchondral bone. Platelet-rich plasma has also been found to facilitate M2 macrophage transition. The incorporation of 20% platelet-rich plasma into GelMA hydrogel scaffolds enhances osteochondral repair effects in rabbits by promoting the proliferation, migration, and chondrogenic differentiation of BMSCs, and increasing M2 macrophage infiltration . Recently, MSCs and their sEVs have been shown to have unique immunomodulatory properties and anti-inflammatory abilities [275, 276]. Bioscaffolds loaded with MSCs or their sEVs have been applied in the treatment of cartilage damage. BMSC-based engineered cartilage, produced by culturing BMSCs on PGA/PLA scaffolds, has been reported to suppress in vivo inflammation through the promotion of M2 macrophage polarization, thus improving cartilage regeneration . In a rabbit articular cartilage defect model, the intra-articular injection of human umbilical cord Wharton’s jelly MSC-derived sEVs has been found to increase the infiltration of regenerative M2 macrophages and the expression level of IL-10, and to decrease the ratio of M1 macrophages, thus improving cartilage repair  (Fig. 4b). Additionally, a biocomposite scaffold composed of ECM/GelMA/sEVs has been reported to increase cartilage regeneration by promoting chondrocyte migration, restoring chondrocyte mitochondrial dysfunction, and enhancing the polarization of macrophages toward the M2 phenotype .
The physical properties of the scaffolds are also involved in the regulation of macrophage polarization and cartilage regeneration. PCL/Eucommia ulmoides gum composite scaffolds with a different elastic modulus that overlaps with those of human cartilage tissues with OA (1–5 MPa) have been designed by adjusting the ratio of PCL to Eucommia ulmoides gum. In this range, high scaffold stiffness favors M2 macrophage polarization, and the expression of inflammatory cytokines increases as the scaffold stiffness decreases  (Fig. 4c). However, the mechanism through which scaffold stiffness regulates the immune microenvironment and macrophage behaviors remain to be elucidated. Macrophages can sense mechanical stimulation via integrins, which produce signals to focal adhesion kinases and enhance cytoskeleton reorganization. Cha et al.  have revealed that macrophage polarization within 3D biomaterials can be modulated through integrin-mediated interactions. Inhibition of integrin α2β1 significantly decreases the induction of M2 macrophages. Kang et al.  demonstrated that highly anisotropic ligand-coated gold nanorods facilitate the recruitment of integrin β1 on macrophages, thus enhancing cell adhesion and M2 polarization. In addition, the effects of other scaffold properties, such as porosity, pore size, topography, and hydrophilicity, on macrophage behavior and cartilage regeneration deserve further investigation.
Critical role of immunomodulation in biomaterials-mediated soft tissue regeneration
The skin, the largest organ of the human body, is susceptible to multiple insults including accidental injuries and various diseases. The skin is equipped with an intricate network of immune cells, mainly including neutrophils, lymphocytes, monocytes, and macrophages, which is crucial not only for host defense but also for tissue homeostasis and reconstruction . In the event of injury, a cascade of biological interactions between different cell types (immune cells, fibroblasts, endotheliocytes, and keratinocytes) and ECM components is initiated, thus inducing wound healing and tissue regeneration. The development of biomaterials for the regulation of immune responses will facilitate wound healing and tissue restoration. Naturally, derived biopolymer-based hydrogels have been found to have intrinsic anti-inflammatory activity . Chitosan is a widely used biomaterial for wound healing, owing to its favorable biocompatibility, biodegradability, adhesiveness, and hemostatic ability . A chitosan/aloe vera nanohydrogel, developed to enhance wound healing, has been demonstrated to decrease the ratio of M1 macrophages and the expression of iNOS and TNF-α while increasing the ratio of M2 macrophages, thereby promoting skin tissue regeneration . Recently, the immunomodulatory function of silk fibroin has drawn the attention of researchers. Silk fibroin hydrogels applied in burn wound treatment have been found to induce a transition from the inflammation to proliferation stage, and improve tissue regeneration, as evidenced by the deposition of collagen type I and III fibers . Interestingly, synthetic Ti3C2 MXene quantum dots have been reported to modulate the immune microenvironment by selectively inhibiting the activation of proinflammatory CD4+IFN-γ+ T cells while promoting the proliferation of immunosuppressive CD4+CD25+FoxP3+ T cells, thus enhancing wound healing in rats . Although the immunomodulatory activity of hydrogels has been obtained through composition selection, their effectiveness is considered limited. Local delivery of MSCs, microRNAs (miRNAs), and biological molecules, such as anti-inflammatory cytokines (IL-10, IL-4, IL-2, and MCP-1), peptides (L-12 and LL-37 peptides), and antibodies (anti-TNF-α), has gradually become a common strategy for the regulation of the immune microenvironment at wound sites . For example, an adipose tissue-derived MSCs (ADSCs)-seeded chitosan/difunctional polyurethane hydrogel has been prepared for the treatment of chronic diabetic skin wounds . The hydrogel produces synergistic immunomodulatory effects through activation of C3a and C5a, upregulation of the cytokines stromal cell derived factor 1 (SDF-1) and TGF-β, and decreased secretion of the proinflammatory cytokines TNF-α and IL-1β, thus accelerating wound healing  (Fig. 5a). MiR-223 has recently been suggested to promote M2 macrophage polarization. An adhesive hydrogel containing miR-223-loaded HA nanoparticles has been reported to achieve local delivery of miR-223 and drive the polarization of macrophages to the M2 phenotype, thereby promoting wound healing and the formation of uniform vascularized skin  (Fig. 5b).
Muscle regeneration is also significantly affected by the activities of various immune cells, such as macrophages, CD8 T cells, and Tregs . The immune-mediated muscle regeneration driven by scaffolds holds great promise in muscle regeneration. How skeletal- and cardiac muscle-derived ECM scaffolds regulate the immune microenvironment and stimulate tissue recovery in traumatic muscle wounds has been investigated [295, 296]. The ECM scaffolds induce IL-4-dependent M2 macrophage polarization through activation of the mTOR/Rictor-dependent T helper 2 pathway, thus guiding a pro-regenerative response that facilitates muscle regeneration. The effects of local delivery of RK35, a myostatin inhibitor, by HA/muscle-derived ECM scaffolds on muscle regeneration have been investigated  (Fig. 5c). The scaffolds show a prolonged release of RK35, thereby promoting pro-regenerative M2 macrophages and FoxP3+ Tregs, and increasing anti-inflammatory cytokine expression. In a recent study, PLGA/PCL electrospun nanofiber scaffolds loaded with FTY720, an agonist of the sphingosine-1-phosphate signal, have been demonstrated to promote pro-regenerative local injury milieu formation, as shown by increased numbers of M2 macrophages and muscle stem cells, thus improving muscle regeneration after volumetric loss .
Peripheral nerve regeneration remains a challenge, because the currently used autogenous tissue replacement is limited by tissue availability, secondary deformities, and potentially inappropriate size. To overcome these obstacles, various biomaterial scaffolds have been developed to repair nerve injuries in recent years. Hydrogel-based codelivery of MSCs and bioactive factors has also been applied for the treatment of nerve injuries. Fibrin hydrogels loaded with ADSCs and microspheres containing tacrolimus have been reported to enhance peripheral nerve regeneration via immunomodulatory actions . In another study, on the basis of the acidic microenvironment at injury sites, pH-responsive IL-4 plasmid-loaded liposomes have been incorporated into PLA electrospun fibers to treat acute spinal cord injury (SCI)  (Fig. 5d). The immunoregulatory fiber scaffolds significantly suppress the acute inflammatory response and promote neural differentiation of MSCs, thus decreasing scar tissue formation and enhancing motor function recovery. However, a growing number of studies have demonstrated that the delivery of MSCs has only limited benefits for SCI, possibly because of the heterogeneity of MSCs . Furthermore, some scaffolds negatively affect spine cord regeneration because of the proinflammatory milieu induced by biomaterials . Thus, a more comprehensive understanding of the crosstalk among the immune system, implanted scaffolds or MSCs, and the nervous system will be essential for treating SCI. Immunomodulatory effects driven by biomaterial scaffolds are also an innovative regenerative strategy to repair vessels. Recently, an electrospun chain-extended-ureido-pyrimidinone-PCL scaffold functionalized with IL-4 and heparin has been developed to repair vascular damage in rats. The addition of IL-4 ameliorates the intimal hyperplasia caused by heparin and promotes M2 macrophage polarization and mature neotissue formation  (Fig. 5e).
The immune system is closely associated with tissue injury and regeneration. Therefore, effective modulation and manipulation of the immune response to effectively modulate the innate healing process are crucial for successful tissue restoration. Spatiotemporal regulation of immune cells, their functions, and their communication with tissue-specific cells, including progenitor cells and stem cells, is imperative to enhance tissue regeneration. An in-depth understanding of the immunomodulatory and pro-regenerative activators and their multiple functions will critically contribute to their successful application as therapeutics.
Although much knowledge has been gained regarding immunomodulation and applied to the rational design of strategies to modulate the immune response and promote tissue regeneration, multiple underlying mechanisms remain to be explored. For example, M2 phenotype macrophages are widely accepted to be permissive to tissue repair and regeneration. During the inflammation phase, however, excessive infiltration of M2 macrophages is not conducive to tissue resistance against foreign pathogens and may thus impair tissue healing. The mechanisms underlying this dual function are not well described. Furthermore, other immune cells have subpopulations, and different subpopulations may exert different effects on tissue regeneration. For example, whereas CD8 T cells have adverse effects on tissue regeneration, CD4 T cells and Tregs enhance tissue regeneration. With the development of research technologies such as single-cell sequencing technology, further definitive classification of immune cell subpopulations will be beneficial in the study of the regulation of the immune microenvironment in tissue regeneration. In addition, immune regulation is a complex and delicate dynamic regulatory process. With aging, the function of the immune system gradually declines. Therefore, whether diminished tissue regeneration in older people is closely associated with the functional decline in the immune system must be investigated. An in-depth exploration of the underlying mechanisms would enable the immune system to be effectively harnessed to improve tissue repair.
The incorporation of biomaterials into immunomodulatory therapeutics has significant potential to advance the fields of tissue engineering and regenerative medicine. However, although many biomaterials that enhance tissue regeneration and mediate immunomodulation have been developed, most of the research has yet to be translated into the clinic. To enable the clinical utilization of biomaterials, materials with high biosafety profiles should be selected. Notably, precise targeting and controlled release are two issues requiring attention. Optimization of these two aspects should allow biomaterials to achieve precise regulation of the immune microenvironment and facilitate their clinical translation. Therefore, novel strategies that promote precise and targeted immunomodulation for clinical use in tissue are required.
The datasets used or analyzed during the current study are available from the corresponding author on reasonable request.
Adipose tissue-derived MSCs
Adult stem cells
Adipose tissue derived-MSCs
Bone marrow derived-MSCs
Bone morphogenetic protein-2
Chemokine (C-X-C motif) ligand
Embryonic stem cells
Eucommia ulmoides gum
Focal adhesion kinase
Fibroblast growth factor
- γδT cells:
Gamma delta T cells
Gingival mesenchymal stem cells
Human bone marrow-derived mesenchymal stem/stromal cells
Insulin-like growth factor
Induced pluripotent stem cells
Inducible nitric oxide synthase
c-Jun N-terminal kinase
Monocyte chemoattractant protein-1
Macrophage colony stimulating factor
Mammalian target of rapamycin
Mesenchymal stem cells
- NK cells:
Natural killer cells
Nuclear factor erythroid 2-associated factor 2
Platelet-derived growth factor beta
Recombinant human BMP-2
Reactive oxygen species
Spinal cord injury
stromal cell derived factor 1
Small extracellular vesicles
Secreted frizzled-related protein 3
Signal transducer and activator of transcription
Transforming growth factor-β
Transforming growth factor-β1
Regulatory T cells
Tumor necrosis factor-α
Vascular endothelial growth factor
Volumetric muscle loss
Hao Q, Wu Y, Wu Y, Wang P, Vadgama JV. Tumor-derived exosomes in tumor-induced immune suppression. Int J Mol Sci. 2022;23(3):1461.
Murphy MP, Koepke LS, Lopez MT, Tong X, Ambrosi TH, Gulati GS, et al. Articular cartilage regeneration by activated skeletal stem cells. Nat Med. 2020;26(10):1583–92.
Shin CS, Cabrera FJ, Lee R, Kim J, Ammassam Veettil R, Zaheer M, et al. 3D-bioprinted inflammation modulating polymer scaffolds for soft tissue repair. Adv Mater. 2021;33(4):e2003778.
Hao Z, Li H, Wang Y, Hu Y, Chen T, Zhang S, et al. Supramolecular peptide nanofiber hydrogels for bone tissue engineering: from multihierarchical fabrications to comprehensive applications. Adv Sci (Weinh). 2022;9(11):e2103820.
Zhao Y, Song S, Ren X, Zhang J, Lin Q, Zhao Y. Supramolecular adhesive hydrogels for tissue engineering applications. Chem Rev. 2022;122(6):5604–40.
Zhao T, Sun F, Liu J, Ding T, She J, Mao F, et al. Emerging role of mesenchymal stem cell-derived exosomes in regenerative medicine. Curr Stem Cell Res Ther. 2019;14(6):482–94.
Ye J, Xie C, Wang C, Huang J, Yin Z, Heng BC, et al. Promoting musculoskeletal system soft tissue regeneration by biomaterial-mediated modulation of macrophage polarization. Bioact Mater. 2021;6(11):4096–109.
Bucher CH, Schlundt C, Wulsten D, Sass FA, Wendler S, Ellinghaus A, et al. Experience in the adaptive immunity impacts bone homeostasis, remodeling, and healing. Front Immunol. 2019;10:797.
Zarubova J, Hasani-Sadrabadi MM, Ardehali R, Li S. Immunoengineering strategies to enhance vascularization and tissue regeneration. Adv Drug Deliv Rev. 2022;184:114233.
Lu T, Zhang Z, Zhang J, Pan X, Zhu X, Wang X, et al. CD73 in small extracellular vesicles derived from HNSCC defines tumour-associated immunosuppression mediated by macrophages in the microenvironment. J Extracell Vesicles. 2022;11(5):e12218.
Venugopal D, Vishwakarma S, Kaur I, Samavedi S. Electrospun fiber-based strategies for controlling early innate immune cell responses: towards immunomodulatory mesh designs that facilitate robust tissue repair. Acta Biomater. 2022;S1742-7061(22):00341–5.
Shaw GS, Samavedi S. Potent particle-based vehicles for growth factor delivery from electrospun meshes: fabrication and functionalization strategies for effective tissue regeneration. ACS Biomater Sci Eng. 2022;8(1):1–15.
Ullah M, Qiao Y, Concepcion W, Thakor AS. Stem cell-derived extracellular vesicles: role in oncogenic processes, bioengineering potential, and technical challenges. Stem Cell Res Ther. 2019;10(1):347.
Juban G. Transcriptional control of macrophage inflammatory shift during skeletal muscle regeneration. Semin Cell Dev Biol. 2021;119:82–8.
Tonkin J, Temmerman L, Sampson RD, Gallego-Colon E, Barberi L, Bilbao D, et al. Monocyte/macrophage-derived IGF-1 orchestrates murine skeletal muscle regeneration and modulates autocrine polarization. Mol Ther. 2015;23(7):1189–200.
Qi K, Li N, Zhang Z, Melino G. Tissue regeneration: the crosstalk between mesenchymal stem cells and immune response. Cell Immunol. 2018;326:86–93.
Soteriou D, Fuchs Y. A matter of life and death: stem cell survival in tissue regeneration and tumour formation. Nat Rev Cancer. 2018;18(3):187–201.
Wang Y, Chen X, Cao W, Shi Y. Plasticity of mesenchymal stem cells in immunomodulation: pathological and therapeutic implications. Nat Immunol. 2014;15(11):1009–16.
Pourgholaminejad A, Aghdami N, Baharvand H, Moazzeni SM. The effect of pro-inflammatory cytokines on immunophenotype, differentiation capacity and immunomodulatory functions of human mesenchymal stem cells. Cytokine. 2016;85:51–60.
Liu H, Li R, Liu T, Yang L, Yin G, Xie Q. Immunomodulatory effects of mesenchymal stem cells and mesenchymal stem cell-derived extracellular vesicles in rheumatoid arthritis. Front Immunol. 2020;11:1912.
Wang Z, Yang R, Zhang J, Wang P, Wang Z, Gao J, et al. Role of extracellular vesicles in placental inflammation and local immune balance. Mediat Inflamm. 2021;2021:5558048.
Riazifar M, Mohammadi MR, Pone EJ, Yeri A, Lässer C, Segaliny AI, et al. Stem cell-derived exosomes as nanotherapeutics for autoimmune and neurodegenerative disorders. ACS Nano. 2019;13(6):6670–88.
Hu Y, Tao R, Chen L, Xiong Y, Xue H, Hu L, et al. Exosomes derived from pioglitazone-pretreated MSCs accelerate diabetic wound healing through enhancing angiogenesis. J Nanobiotechnol. 2021;19(1):150.
Xiong Y, Chen L, Yan C, Zhou W, Endo Y, Liu J, et al. Circulating exosomal miR-20b-5p inhibition restores Wnt9b signaling and reverses diabetes-associated impaired wound healing. Small. 2020;16(3):e1904044.
Abdulghani S, Mitchell GR. Biomaterials for in situ tissue regeneration: a review. Biomolecules. 2019;9(11):750.
Shen P, Chen Y, Luo S, Fan Z, Wang J, Chang J, et al. Applications of biomaterials for immunosuppression in tissue repair and regeneration. Acta Biomater. 2021;126:31–44.
Lee J, Byun H, Madhurakkat Perikamana SK, Lee S, Shin H. Current advances in immunomodulatory biomaterials for bone regeneration. Adv Healthc Mater. 2019;8(4):e1801106.
Yang N, Liu Y. The role of the immune microenvironment in bone regeneration. Int J Med Sci. 2021;18(16):3697–707.
Ko GR, Lee JS. Engineering of immune microenvironment for enhanced tissue remodeling. Tissue Eng Regen Med. 2022;19(2):221–36.
Fang J, Feng C, Chen W, Hou P, Liu Z, Zuo M, et al. Redressing the interactions between stem cells and immune system in tissue regeneration. Biol Direct. 2021;16(1):18.
Mata R, Yao Y, Cao W, Ding J, Zhou T, Zhai Z, et al. The dynamic inflammatory tissue microenvironment: signality and disease therapy by biomaterials. Research (Wash D C). 2021;2021:4189516.
Dimitriou R, Jones E, McGonagle D, Giannoudis PV. Bone regeneration: current concepts and future directions. BMC Med. 2011;9:66.
Hayrapetyan A, Jansen JA, van den Beucken JJJP. Signaling pathways involved in osteogenesis and their application for bone regenerative medicine. Tissue Eng Part B Rev. 2015;21(1):75–87.
Iaquinta MR, Mazzoni E, Bononi I, Rotondo JC, Mazziotta C, Montesi M, et al. Adult stem cells for bone regeneration and repair. Front Cell Dev Biol. 2019;7:268.
Turgeman G, Zilberman Y, Zhou S, Kelly P, Moutsatsos IK, Kharode YP, et al. Systemically administered rhBMP-2 promotes MSC activity and reverses bone and cartilage loss in osteopenic mice. J Cell Biochem. 2002;86(3):461–74.
Turgeman G, Pittman DD, Müller R, Kurkalli BG, Zhou S, Pelled G, et al. Engineered human mesenchymal stem cells: a novel platform for skeletal cell mediated gene therapy. J Gene Med. 2001;3(3):240–51.
Moutsatsos IK, Turgeman G, Zhou S, Kurkalli BG, Pelled G, Tzur L, et al. Exogenously regulated stem cell-mediated gene therapy for bone regeneration. Mol Ther. 2001;3(4):449–61.
Freeman FE, Pitacco P, van Dommelen LHA, Nulty J, Browe DC, Shin JY, et al. 3D bioprinting spatiotemporally defined patterns of growth factors to tightly control tissue regeneration. Sci Adv. 2020;6(33):eabb5093.
Gillman CE, Jayasuriya AC. FDA-approved bone grafts and bone graft substitute devices in bone regeneration. Mater Sci Eng C Mater Biol Appl. 2021;130:112466.
De La Vega RE, van Griensven M, Zhang W, Coenen MJ, Nagelli CV, Panos JA, et al. Efficient healing of large osseous segmental defects using optimized chemically modified messenger RNA encoding BMP-2. Sci Adv. 2022;8(7):eabl6242.
De Simone A, Evanitsky MN, Hayden L, Cox BD, Wang J, Tornini VA, et al. Control of osteoblast regeneration by a train of Erk activity waves. Nature. 2021;590(7844):129–33.
Xu J, Li Z, Tower RJ, Negri S, Wang Y, Meyers CA, et al. NGF-p75 signaling coordinates skeletal cell migration during bone repair. Sci Adv. 2022;8(11):eabl5716.
Ambrosi TH, Marecic O, McArdle A, Sinha R, Gulati GS, Tong X, et al. Aged skeletal stem cells generate an inflammatory degenerative niche. Nature. 2021;597(7875):256–62.
Ransom RC, Carter AC, Salhotra A, Leavitt T, Marecic O, Murphy MP, et al. Mechanoresponsive stem cells acquire neural crest fate in jaw regeneration. Nature. 2018;563(7732):514–21.
Mishra R, Sehring I, Cederlund M, Mulaw M, Weidinger G. NF-κB signaling negatively regulates osteoblast dedifferentiation during zebrafish bone regeneration. Dev Cell. 2020;52(2):167-82.e7.
Sipp D, Robey PG, Turner L. Clear up this stem-cell mess. Nature. 2018;561(7724):455–7.
Park D, Spencer JA, Koh BI, Kobayashi T, Fujisaki J, Clemens TL, et al. Endogenous bone marrow MSCs are dynamic, fate-restricted participants in bone maintenance and regeneration. Cell Stem Cell. 2012;10(3):259–72.
Lin TH, Pajarinen J, Lu L, Nabeshima A, Cordova LA, Yao Z, et al. NF-κB as a therapeutic target in inflammatory-associated bone diseases. Adv Protein Chem Struct Biol. 2017;107:117–54.
Chang J, Liu F, Lee M, Wu B, Ting K, Zara JN, et al. NF-κB inhibits osteogenic differentiation of mesenchymal stem cells by promoting β-catenin degradation. Proc Natl Acad Sci USA. 2013;110(23):9469–74.
Wei W, Dai H. Articular cartilage and osteochondral tissue engineering techniques: recent advances and challenges. Bioact Mater. 2021;6(12):4830–55.
Jacob J, More N, Kalia K, Kapusetti G. Piezoelectric smart biomaterials for bone and cartilage tissue engineering. Inflamm Regen. 2018;38:2.
Deng C, Chang J, Wu C. Bioactive scaffolds for osteochondral regeneration. J Orthop Translat. 2019;17:15–25.
Graceffa V, Vinatier C, Guicheux J, Stoddart M, Alini M, Zeugolis DI. Chasing chimeras–the elusive stable chondrogenic phenotype. Biomaterials. 2019;192:199–225.
Koh YG, Kwon OR, Kim YS, Choi YJ, Tak DH. Adipose-derived mesenchymal stem cells with microfracture versus microfracture alone: 2-year follow-up of a prospective randomized trial. Arthroscopy. 2016;32(1):97–109.
Zhang Y, Liu S, Guo W, Wang M, Hao C, Gao S, et al. Human umbilical cord Wharton’s jelly mesenchymal stem cells combined with an acellular cartilage extracellular matrix scaffold improve cartilage repair compared with microfracture in a caprine model. Osteoarthr Cartil. 2018;26(7):954–65.
Qasim M, Chae DS, Lee NY. Bioengineering strategies for bone and cartilage tissue regeneration using growth factors and stem cells. J Biomed Mater Res A. 2020;108(3):394–411.
Reissis D, Tang QO, Cooper NC, Carasco CF, Gamie Z, Mantalaris A, et al. Current clinical evidence for the use of mesenchymal stem cells in articular cartilage repair. Expert Opin Biol Ther. 2016;16(4):535–57.
Chan CKF, Gulati GS, Sinha R, Tompkins JV, Lopez M, Carter AC, et al. Identification of the human skeletal stem cell. Cell. 2018;175(1):43-56.e21.
Shelat R, Bhatt LK, Paunipagar B, Kurian T, Khanna A, Chandra S. Regeneration of hyaline cartilage in osteochondral lesion model using L-lysine magnetic nanoparticles labeled mesenchymal stem cells and their in vivo imaging. J Tissue Eng Regen Med. 2020;14(11):1604–17.
Castro-Viñuelas R, Sanjurjo-Rodríguez C, Piñeiro-Ramil M, Hermida-Gómez T, Fuentes-Boquete IM, de Toro-Santos FJ, et al. Induced pluripotent stem cells for cartilage repair: current status and future perspectives. Eur Cell Mater. 2018;36:96–109.
Tsumaki N, Okada M, Yamashita A. iPS cell technologies and cartilage regeneration. Bone. 2015;70:48–54.
Lee MS, Stebbins MJ, Jiao H, Huang HC, Leiferman EM, Walczak BE, et al. Comparative evaluation of isogenic mesodermal and ectomesodermal chondrocytes from human iPSCs for cartilage regeneration. Sci Adv. 2021;7(21):eabf0907.
Nam Y, Rim YA, Jung SM, Ju JH. Cord blood cell-derived iPSCs as a new candidate for chondrogenic differentiation and cartilage regeneration. Stem Cell Res Ther. 2017;8(1):16.
Murphy C, Mobasheri A, Táncos Z, Kobolák J, Dinnyés A. The potency of induced pluripotent stem cells in cartilage regeneration and osteoarthritis treatment. Adv Exp Med Biol. 2018;1079:55–68.
Zhang M, Shi J, Xie M, Wen J, Niibe K, Zhang X, et al. Recapitulation of cartilage/bone formation using iPSCs via biomimetic 3D rotary culture approach for developmental engineering. Biomaterials. 2020;260:120334.
Lach MS, Rosochowicz MA, Richter M, Jagiełło I, Suchorska WM, Trzeciak T. The induced pluripotent stem cells in articular cartilage regeneration and disease modelling: Are we ready for their clinical use? Cells. 2022;11(3):529.
Yoo JU, Barthel TS, Nishimura K, Solchaga L, Caplan AI, Goldberg VM, et al. The chondrogenic potential of human bone-marrow-derived mesenchymal progenitor cells. J Bone Joint Surg Am. 1998;80(12):1745–57.
Somoza RA, Welter JF, Correa D, Caplan AI. Chondrogenic differentiation of mesenchymal stem cells: challenges and unfulfilled expectations. Tissue Eng Part B Rev. 2014;20(6):596–608.
Huang J, Huang Z, Liang Y, Yuan W, Bian L, Duan L, et al. 3D printed gelatin/hydroxyapatite scaffolds for stem cell chondrogenic differentiation and articular cartilage repair. Biomater Sci. 2021;9(7):2620–30.
de Windt TS, Vonk LA, Slaper-Cortenbach ICM, van den Broek MPH, Nizak R, van Rijen MHP, et al. Allogeneic mesenchymal stem cells stimulate cartilage regeneration and are safe for single-stage cartilage repair in humans upon mixture with recycled autologous chondrons. Stem Cells. 2017;35(1):256–64.
Jiang S, Tian G, Li X, Yang Z, Wang F, Tian Z, et al. Research progress on stem cell therapies for articular cartilage regeneration. Stem Cells Int. 2021;2021:8882505.
Kandoi LPK, Misra S, Verma RSVKR. The mesenchymal stem cell secretome: a new paradigm towards cell-free therapeutic mode in regenerative medicine. Cytokine Growth Factor Rev. 2019;46:1–9.
De Bari C, Roelofs AJ. Stem cell-based therapeutic strategies for cartilage defects and osteoarthritis. Curr Opin Pharmacol. 2018;40:74–80.
Zhang Y, Guo W, Wang M, Hao C, Lu L, Gao S, et al. Co-culture systems-based strategies for articular cartilage tissue engineering. J Cell Physiol. 2018;233(3):1940–51.
Parate D, Kadir ND, Celik C, Lee EH, Hui JHP, Franco-Obregón A, et al. Pulsed electromagnetic fields potentiate the paracrine function of mesenchymal stem cells for cartilage regeneration. Stem Cell Res Ther. 2020;11(1):46.
Su YJ, Wang PW, Weng SW. The role of mitochondria in immune-cell-mediated tissue regeneration and ageing. Int J Mol Sci. 2021;22(5):2668.
Mariani E, Pulsatelli L, Facchini A. Signaling pathways in cartilage repair. Int J Mol Sci. 2014;15(5):8667–98.
Chen S, Tao J, Bae Y, Jiang MM, Bertin T, Chen Y, et al. Notch gain of function inhibits chondrocyte differentiation via Rbpj-dependent suppression of Sox9. J Bone Miner Res. 2013;28(3):649–59.
Zieba JT, Chen YT, Lee BH, Bae Y. Notch signaling in skeletal development, homeostasis and pathogenesis. Biomolecules. 2020;10(2):332.
Liu Z, Chen J, Mirando AJ, Wang C, Zuscik MJ, O’Keefe RJ, et al. A dual role for NOTCH signaling in joint cartilage maintenance and osteoarthritis. Sci Signal. 2015;8(386):ra71.
Liu X, Du M, Wang Y, Liu S, Liu X. BMP9 overexpressing adipose-derived mesenchymal stem cells promote cartilage repair in osteoarthritis-affected knee joint via the Notch1/Jagged1 signaling pathway. Exp Ther Med. 2018;16(6):4623–31.
Yu HT, Gu CZ, Chen JQ. MiR-9 facilitates cartilage regeneration of osteoarthritis in rabbits through regulating Notch signaling pathway. Eur Rev Med Pharmacol Sci. 2019;23(12):5051–8.
Usami Y, Gunawardena AT, Iwamoto M, Enomoto-Iwamoto M. Wnt signaling in cartilage development and diseases: lessons from animal studies. Lab Invest. 2016;96(2):186–96.
Wu CL, Dicks A, Steward N, Tang R, Katz DB, Choi YR, et al. Single cell transcriptomic analysis of human pluripotent stem cell chondrogenesis. Nat Commun. 2021;12(1):362.
Deng Y, Zhang X, Li R, Li Z, Yang B, Shi P, et al. Biomaterial-mediated presentation of wnt5a mimetic ligands enhances chondrogenesis and metabolism of stem cells by activating non-canonical Wnt signaling. Biomaterials. 2022;281:121316.
Lee J, Jeon O, Kong M, Abdeen AA, Shin JY, Lee HN, et al. Combinatorial screening of biochemical and physical signals for phenotypic regulation of stem cell-based cartilage tissue engineering. Sci Adv. 2020;6(21):eaaz5913.
Hua B, Qiu J, Ye X, Liu X. Intra-articular injection of a novel Wnt pathway inhibitor, SM04690, upregulates Wnt16 expression and reduces disease progression in temporomandibular joint osteoarthritis. Bone. 2022;158:116372.
Hata A, Chen YG. TGF-β signaling from receptors to Smads. Cold Spring Harb Perspect Biol. 2016;8(9):a022061.
Wang W, Rigueur D, Lyons KM. TGFβ signaling in cartilage development and maintenance. Birth Defects Res C Embryo Today. 2014;102(1):37–51.
Fang D, Jin P, Huang Q, Yang Y, Zhao J, Zheng L. Platelet-rich plasma promotes the regeneration of cartilage engineered by mesenchymal stem cells and collagen hydrogel via the TGF-β/SMAD signaling pathway. J Cell Physiol. 2019. https://doi.org/10.1002/jcp.28211.
Ye C, Chen J, Qu Y, Liu H, Yan J, Lu Y, et al. Naringin and bone marrow mesenchymal stem cells repair articular cartilage defects in rabbit knees through the transforming growth factor-β superfamily signaling pathway. Exp Ther Med. 2020;20(5):59.
Ying J, Wang P, Zhang S, Xu T, Zhang L, Dong R, et al. Transforming growth factor-beta1 promotes articular cartilage repair through canonical Smad and Hippo pathways in bone mesenchymal stem cells. Life Sci. 2018;192:84–90.
Kumar A, Takada Y, Boriek AM, Aggarwal BB. Nuclear factor-kappaB: its role in health and disease. J Mol Med (Berl). 2004;82(7):434–48.
Wang SN, Xie GP, Qin CH, Chen YR, Zhang KR, Li X, et al. Aucubin prevents interleukin-1 beta induced inflammation and cartilage matrix degradation via inhibition of NF-κB signaling pathway in rat articular chondrocytes. Int Immunopharmacol. 2015;24(2):408–15.
Hossain MA, Adithan A, Alam MJ, Kopalli SR, Kim B, Kang C-W, et al. IGF-1 facilitates cartilage reconstruction by regulating PI3K/AKT, MAPK, and NF-kB signaling in rabbit osteoarthritis. J Inflamm Res. 2021;14:3555–68.
Hsiao HY, Cheng CM, Kao SW, Liu JW, Chang CS, Harhaus L, et al. The effect of bone inhibitors on periosteum-guided cartilage regeneration. Sci Rep. 2020;10(1):8372.
Fernández-Torres J, Martínez-Nava GA, Gutiérrez-Ruíz MC, Gómez-Quiroz LE, Gutiérrez M. Role of HIF-1α signaling pathway in osteoarthritis: a systematic review. Rev Bras Reumatol Engl Ed. 2017;57(2):162–73.
Silagi ES, Schipani E, Shapiro IM, Risbud MV. The role of HIF proteins in maintaining the metabolic health of the intervertebral disc. Nat Rev Rheumatol. 2021;17(7):426–39.
Rankin EB, Giaccia AJ, Schipani E. A central role for hypoxic signaling in cartilage, bone, and hematopoiesis. Curr Osteoporos Rep. 2011;9(2):46–52.
Deng C, Zhu H, Li J, Feng C, Yao Q, Wang L, et al. Bioactive scaffolds for regeneration of cartilage and subchondral bone interface. Theranostics. 2018;8(7):1940–55.
Lutzweiler G, Ndreu Halili A, Engin Vrana N. The overview of porous, bioactive scaffolds as instructive biomaterials for tissue regeneration and their clinical translation. Pharmaceutics. 2020;12(7):602.
Lin R, Deng C, Li X, Liu Y, Zhang M, Qin C, et al. Copper-incorporated bioactive glass-ceramics inducing anti-inflammatory phenotype and regeneration of cartilage/bone interface. Theranostics. 2019;9(21):6300–13.
Mi B, Chen L, Xiong Y, Yang Y, Panayi AC, Xue H, et al. Osteoblast/osteoclast and immune cocktail therapy of an exosome/drug delivery multifunctional hydrogel accelerates fracture repair. ACS Nano. 2022;16:771–82.
Minina E, Kreschel C, Naski MC, Ornitz DM, Vortkamp A. Interaction of FGF, Ihh/Pthlh, and BMP signaling integrates chondrocyte proliferation and hypertrophic differentiation. Dev Cell. 2002;3(3):439–49.
Okamura G, Ebina K, Hirao M, Chijimatsu R, Yonetani Y, Etani Y, et al. Promoting effect of basic fibroblast growth factor in synovial mesenchymal stem cell-based cartilage regeneration. Int J Mol Sci. 2020;22(1):300.
Öztürk E, Arlov Ø, Aksel S, Li L, Ornitz DM, Skjåk-Bræk G, et al. Sulfated hydrogel matrices direct mitogenicity and maintenance of chondrocyte phenotype through activation of FGF signaling. Adv Funct Mater. 2016;26(21):3649–62.
Öztürk E, Stauber T, Levinson C, Cavalli E, Arlov Ø, Zenobi-Wong M. Tyrosinase-crosslinked, tissue adhesive and biomimetic alginate sulfate hydrogels for cartilage repair. Biomed Mater. 2020;15(4):045019.
Haupt JL, Frisbie DD, McIlwraith CW, Robbins PD, Ghivizzani S, Evans CH, et al. Dual transduction of insulin-like growth factor-I and interleukin-1 receptor antagonist protein controls cartilage degradation in an osteoarthritic culture model. J Orthop Res. 2005;23(1):118–26.
Weimer A, Madry H, Venkatesan JK, Schmitt G, Frisch J, Wezel A, et al. Benefits of recombinant adeno-associated virus (rAAV)-mediated insulinlike growth factor I (IGF-I) overexpression for the long-term reconstruction of human osteoarthritic cartilage by modulation of the IGF-I axis. Mol Med. 2012;18:346–58.
An C, Cheng Y, Yuan Q, Li J. IGF-1 and BMP-2 induces differentiation of adipose-derived mesenchymal stem cells into chondrocytes-like cells. Ann Biomed Eng. 2010;38(4):1647–54.
Lo WC, Dubey NK, Tsai FC, Lu JH, Peng BY, Chiang PC, et al. Amelioration of nicotine-induced osteoarthritis by platelet-derived biomaterials through modulating IGF-1/AKT/IRS-1 signaling axis. Cell Transpl. 2020;29:963689720947348.
Gugjoo MB, Amarpal, Abdelbaset-Ismail A, Aithal HP, Kinjavdekar P, Pawde AM, et al. Mesenchymal stem cells with IGF-1 and TGF- β1 in laminin gel for osteochondral defects in rabbits. Biomed Pharmacother. 2017;93:1165–74.
Midgley AC, Wei Y, Li Z, Kong D, Zhao Q. Nitric-oxide-releasing biomaterial regulation of the stem cell microenvironment in regenerative medicine. Adv Mater. 2020;32(3):e1805818.
Sakai D, Andersson GBJ. Stem cell therapy for intervertebral disc regeneration: obstacles and solutions. Nat Rev Rheumatol. 2015;11(4):243–56.
Semba T, Sammons R, Wang X, Xie X, Dalby KN, Ueno NT. JNK signaling in stem cell self-renewal and differentiation. Int J Mol Sci. 2020;21(7):2613.
Widmann C, Gibson S, Jarpe MB, Johnson GL. Mitogen-activated protein kinase: conservation of a three-kinase module from yeast to human. Physiol Rev. 1999;79(1):143–80.
Almuedo-Castillo M, Crespo-Yanez X, Crespo X, Seebeck F, Bartscherer K, Salò E, et al. JNK controls the onset of mitosis in planarian stem cells and triggers apoptotic cell death required for regeneration and remodeling. PLoS Genet. 2014;10(6):e1004400.
Dhoke NR, Geesala R, Das A. Low oxidative stress-mediated proliferation JNK-FOXO3a-catalase signaling in transplanted adult stem cells promotes wound tissue regeneration. Antioxid Redox Signal. 2018;28(11):1047–65.
Jiang X, Wu F, Xu Y, Yan JX, Wu YD, Li SH, et al. A novel role of angiotensin II in epidermal cell lineage determination: angiotensin II promotes the differentiation of mesenchymal stem cells into keratinocytes through the p38 MAPK, JNK and JAK2 signalling pathways. Exp Dermatol. 2019;28(1):59–65.
Blanpain C, Fuchs E. Stem cell plasticity. Plasticity of epithelial stem cells in tissue regeneration. Science. 2014;344(6189):1242281.
Jere SW, Houreld NN, Abrahamse H. Role of the PI3K/Akt (mTOR and GSK3β) signalling pathway and photobiomodulation in diabetic wound healing. Cytokine Growth Factor Rev. 2019;50:52–9.
Song MS, Salmena L, Pandolfi PP. The functions and regulation of the PTEN tumour suppressor. Nat Rev Mol Cell Biol. 2012;13(5):283–96.
Hoxhaj G, Manning BD. The PI3K-Akt network at the interface of oncogenic signalling and cancer metabolism. Nat Rev Cancer. 2020;20(2):74–88.
Canaud G, Bienaimé F, Viau A, Treins C, Baron W, Nguyen C, et al. AKT2 is essential to maintain podocyte viability and function during chronic kidney disease. Nat Med. 2013;19(10):1288–96.
Castilho RM, Squarize CH, Chodosh LA, Williams BO, Gutkind JS. mTOR mediates Wnt-induced epidermal stem cell exhaustion and aging. Cell Stem Cell. 2009;5(3):279–89.
Nusse R, Clevers H. Wnt/β-catenin signaling, disease, and emerging therapeutic modalities. Cell. 2017;169(6):985–99.
Jamieson C, Sharma M, Henderson BR. Targeting the β-catenin nuclear transport pathway in cancer. Semin Cancer Biol. 2014;27:20–9.
Deschene ER, Myung P, Rompolas P, Zito G, Sun TY, Taketo MM, et al. β-Catenin activation regulates tissue growth non-cell autonomously in the hair stem cell niche. Science. 2014;343(6177):1353–6.
Zhang B, Han F, Wang Y, Sun Y, Zhang M, Yu X, et al. Cells-micropatterning biomaterials for immune activation and bone regeneration. Adv Sci (Weinh). 2022;9(18):e2200670.
Tan SH, Barker N. Wnt signaling in adult epithelial stem cells and cancer. Prog Mol Biol Transl Sci. 2018;153:21–79.
Süntar I, Çetinkaya S, Panieri E, Saha S, Buttari B, Profumo E, et al. Regulatory role of Nrf2 signaling pathway in wound healing process. Molecules. 2021;26(9):2424.
Beyer TA, Auf dem Keller U, Braun S, Schäfer M, Werner S. Roles and mechanisms of action of the Nrf2 transcription factor in skin morphogenesis, wound repair and skin cancer. Cell Death Differ. 2007;14(7):1250–4.
Guan Y, Gao N, Niu H, Dang Y, Guan J. Oxygen-release microspheres capable of releasing oxygen in response to environmental oxygen level to improve stem cell survival and tissue regeneration in ischemic hindlimbs. J Control Release. 2021;331:376–89.
Long M, Rojo de la Vega M, Wen Q, Bharara M, Jiang T, Zhang R, et al. An essential role of NRF2 in diabetic wound healing. Diabetes. 2016;65(3):780–93.
Sun Y, Liu WZ, Liu T, Feng X, Yang N, Zhou HF. Signaling pathway of MAPK/ERK in cell proliferation, differentiation, migration, senescence and apoptosis. J Recept Signal Transduct Res. 2015;35(6):600–4.
Lee BC, Song J, Lee A, Cho D, Kim TS. Visfatin promotes wound healing through the activation of ERK1/2 and JNK1/2 pathway. Int J Mol Sci. 2018;19(11):3642.
Xin P, Xu X, Deng C, Liu S, Wang Y, Zhou X, et al. The role of JAK/STAT signaling pathway and its inhibitors in diseases. Int Immunopharmacol. 2020;80:106210.
Song Q, Xie Y, Gou Q, Guo X, Yao Q, Gou X. JAK/STAT3 and Smad3 activities are required for the wound healing properties of Periplaneta americana extracts. Int J Mol Med. 2017;40(2):465–73.
Zhu Y, Wang Y, Jia Y, Xu J, Chai Y. Roxadustat promotes angiogenesis through HIF-1α/VEGF/VEGFR2 signaling and accelerates cutaneous wound healing in diabetic rats. Wound Repair Regen. 2019;27(4):324–34.
Rodrigues M, Kosaric N, Bonham CA, Gurtner GC. Wound healing: a cellular perspective. Physiol Rev. 2019;99(1):665–706.
Dekoninck S, Blanpain C. Stem cell dynamics, migration and plasticity during wound healing. Nat Cell Biol. 2019;21(1):18–24.
Claes L, Recknagel S, Ignatius A. Fracture healing under healthy and inflammatory conditions. Nat Rev Rheumatol. 2012;8(3):133–43.
Lin Z, Xiong Y, Meng W, Hu Y, Chen L, Chen L, et al. Exosomal PD-L1 induces osteogenic differentiation and promotes fracture healing by acting as an immunosuppressant. Bioact Mater. 2022;13:300–11.
Eming SA, Wynn TA, Martin P. Inflammation and metabolism in tissue repair and regeneration. Science. 2017;356(6342):1026–30.
Wong SL, Demers M, Martinod K, Gallant M, Wang Y, Goldfine AB, et al. Diabetes primes neutrophils to undergo NETosis, which impairs wound healing. Nat Med. 2015;21(7):815–9.
Knipper JA, Ding X, Eming SA. Diabetes impedes the epigenetic switch of macrophages into repair mode. Immunity. 2019;51(2):199–201.
Wynn TA, Vannella KM. Macrophages in tissue repair, regeneration, and fibrosis. Immunity. 2016;44(3):450–62.
Kharaziha M, Baidya A, Annabi N. Rational design of immunomodulatory hydrogels for chronic wound healing. Adv Mater. 2021;33(39):e2100176.
Mei J, Zhou J, Kong L, Dai Y, Zhang X, Song W, et al. An injectable photo-cross-linking silk hydrogel system augments diabetic wound healing in orthopaedic surgery through spatiotemporal immunomodulation. J Nanobiotechnol. 2022;20(1):232.
de Saint-Vis B, Fugier-Vivier I, Massacrier C, Gaillard C, Vanbervliet B, Aït-Yahia S, et al. The cytokine profile expressed by human dendritic cells is dependent on cell subtype and mode of activation. J Immunol. 1998;160(4):1666–76.
Byles V, Covarrubias AJ, Ben-Sahra I, Lamming DW, Sabatini DM, Manning BD, et al. The TSC-mTOR pathway regulates macrophage polarization. Nat Commun. 2013;4:2834.
Batra R, Suh MK, Carson JS, Dale MA, Meisinger TM, Fitzgerald M, et al. IL-1β (interleukin-1β) and TNF-α (tumor necrosis factor-α) impact abdominal aortic aneurysm formation by differential effects on macrophage polarization. Arterioscler Thromb Vasc Biol. 2018;38(2):457–63.
Ma X. TNF-alpha and IL-12: a balancing act in macrophage functioning. Microbes Infect. 2001;3(2):121–9.
Tong Y, Lear TB, Evankovich J, Chen Y, Londino JD, Myerburg MM, et al. The RNFT2/IL-3Rα axis regulates IL-3 signaling and innate immunity. JCI Insight. 2020;5(3):e133652.
Kohler JB, Cervilha DAdB, Riani Moreira A, Santana FR, Farias TM, Alonso Vale MIC, et al. Microenvironmental stimuli induce different macrophage polarizations in experimental models of emphysema. Biol Open. 2019;8(4):bio040808.
Fujiwara N, Kobayashi K. Macrophages in inflammation. Curr Drug Targets Inflamm Allergy. 2005;4(3):281–6.
Hamidzadeh K, Christensen SM, Dalby E, Chandrasekaran P, Mosser DM. Macrophages and the recovery from acute and chronic inflammation. Annu Rev Physiol. 2017;79:567–92.
Lu X, Yu C, Zhang C, Zhang H, Li Y, Cheng X, et al. Effects of Salmonella enterica serovar typhimurium sseK1 on macrophage inflammation-related cytokines and glycolysis. Cytokine. 2021;140:155424.
Saini S, Dhiman A, Nanda S. Immunomodulatory properties of chitosan: Impact on wound healing and tissue repair. Endocr Metab Immune Disord Drug Targets. 2020;20(10):1611–23.
Proto JD, Doran AC, Gusarova G, Yurdagul A, Sozen E, Subramanian M, et al. Regulatory T cells promote macrophage efferocytosis during inflammation resolution. Immunity. 2018;49(4):666–77.e6.
Zhang J, Qu C, Li T, Cui W, Wang X, Du J. Phagocytosis mediated by scavenger receptor class BI promotes macrophage transition during skeletal muscle regeneration. J Biol Chem. 2019;294(43):15672–85.
Kim H, Wang SY, Kwak G, Yang Y, Kwon IC, Kim SH. Exosome-guided phenotypic switch of M1 to M2 macrophages for cutaneous wound healing. Adv Sci (Weinh). 2019;6(20):1900513.
Bozorgmehr N, Okoye I, Oyegbami O, Xu L, Fontaine A, Cox-Kennett N, et al. Expanded antigen-experienced CD160CD8 effector T cells exhibit impaired effector functions in chronic lymphocytic leukemia. J Immunother Cancer. 2021;9(4):e002189.
Sadtler K, Wolf MT, Ganguly S, Moad CA, Chung L, Majumdar S, et al. Divergent immune responses to synthetic and biological scaffolds. Biomaterials. 2019;192:405–15.
Sobecki M, Krzywinska E, Nagarajan S, Audigé A, Huỳnh K, Zacharjasz J, et al. NK cells in hypoxic skin mediate a trade-off between wound healing and antibacterial defence. Nat Commun. 2021;12(1):4700.
Backes CS, Friedmann KS, Mang S, Knörck A, Hoth M, Kummerow C. Natural killer cells induce distinct modes of cancer cell death: discrimination, quantification, and modulation of apoptosis, necrosis, and mixed forms. J Biol Chem. 2018;293(42):16348–63.
O’Brien KL, Finlay DK. Immunometabolism and natural killer cell responses. Nat Rev Immunol. 2019;19(5):282–90.
Fauriat C, Long EO, Ljunggren HG, Bryceson YT. Regulation of human NK-cell cytokine and chemokine production by target cell recognition. Blood. 2010;115(11):2167–76.
Dastagir N, Beal Z, Godwin J. Tissue origin of cytotoxic natural killer cells dictates their differential roles in mouse digit tip regeneration and progenitor cell survival. Stem Cell Rep. 2022;17(3):633–48.
Karp JM, Leng Teo GS. Mesenchymal stem cell homing: the devil is in the details. Cell Stem Cell. 2009;4(3):206–16.
Jiang B, Yan L, Wang X, Li E, Murphy K, Vaccaro K, et al. Concise review: mesenchymal stem cells derived from human pluripotent cells, an unlimited and quality-controllable source for therapeutic applications. Stem Cells. 2019;37(5):572–81.
Ha DH, Kim HK, Lee J, Kwon HH, Park GH, Yang SH, et al. Mesenchymal stem/stromal cell-derived exosomes for immunomodulatory therapeutics and skin regeneration. Cell. 2020;9(5):1157–69.
Spaggiari GM, Capobianco A, Abdelrazik H, Becchetti F, Mingari MC, Moretta L. Mesenchymal stem cells inhibit natural killer-cell proliferation, cytotoxicity, and cytokine production: role of indoleamine 2,3-dioxygenase and prostaglandin E2. Blood. 2008;111(3):1327–33.
Thomas H, Jäger M, Mauel K, Brandau S, Lask S, Flohé SB. Interaction with mesenchymal stem cells provokes natural killer cells for enhanced IL-12/IL-18-induced interferon-gamma secretion. Mediat Inflamm. 2014;2014:143463.
Petri RM, Hackel A, Hahnel K, Dumitru CA, Bruderek K, Flohe SB, et al. Activated tissue-resident mesenchymal stromal cells regulate natural killer cell immune and tissue-regenerative function. Stem Cell Rep. 2017;9(3):985–98.
DelaRosa O, Sánchez-Correa B, Morgado S, Ramírez C, del Río B, Menta R, et al. Human adipose-derived stem cells impair natural killer cell function and exhibit low susceptibility to natural killer-mediated lysis. Stem Cells Dev. 2012;21(8):1333–43.
Brown CC, Gudjonson H, Pritykin Y, Deep D, Lavallée V-P, Mendoza A, et al. Transcriptional basis of mouse and human dendritic cell heterogeneity. Cell. 2019;179(4):846–63.e24.
Steinman RM, Banchereau J. Taking dendritic cells into medicine. Nature. 2007;449(7161):419–26.
See P, Dutertre CA, Chen J, Günther P, McGovern N, Irac SE, et al. Mapping the human DC lineage through the integration of high-dimensional techniques. Science. 2017;356(6342):eaag3009.
Zhu FJ, Tong YL, Sheng ZY, Yao YM. Role of dendritic cells in the host response to biomaterials and their signaling pathways. Acta Biomater. 2019;94:132–44.
Lech M, Gröbmayr R, Weidenbusch M, Anders HJ. Tissues use resident dendritic cells and macrophages to maintain homeostasis and to regain homeostasis upon tissue injury: the immunoregulatory role of changing tissue environments. Mediat Inflamm. 2012;2012:951390.
Julier Z, Park AJ, Briquez PS, Martino MM. Promoting tissue regeneration by modulating the immune system. Acta Biomater. 2017;53:13–28.
Vinish M, Cui W, Stafford E, Bae L, Hawkins H, Cox R, et al. Dendritic cells modulate burn wound healing by enhancing early proliferation. Wound Repair Regen. 2016;24(1):6–13.
Chiesa S, Morbelli S, Morando S, Massollo M, Marini C, Bertoni A, et al. Mesenchymal stem cells impair in vivo T-cell priming by dendritic cells. Proc Natl Acad Sci USA. 2011;108(42):17384–9.
Djouad F, Charbonnier LM, Bouffi C, Louis-Plence P, Bony C, Apparailly F, et al. Mesenchymal stem cells inhibit the differentiation of dendritic cells through an interleukin-6-dependent mechanism. Stem Cells. 2007;25(8):2025–32.
Silva AM, Almeida MI, Teixeira JH, Maia AF, Calin GA, Barbosa MA, et al. Dendritic cell-derived extracellular vesicles mediate mesenchymal stem/stromal cell recruitment. Sci Rep. 2017;7(1):1667.
Saxena Y, Routh S, Mukhopadhaya A. Immunoporosis: Role of Innate Immune Cells in Osteoporosis. Front Immunol. 2021;12:687037.
Yang Y, Wang X, Miron RJ, Zhang X. The interactions of dendritic cells with osteoblasts on titanium surfaces: an in vitro investigation. Clin Oral Investig. 2019;23(11):4133–43.
Ferreira LMR, Muller YD, Bluestone JA, Tang Q. Next-generation regulatory T cell therapy. Nat Rev Drug Discov. 2019;18(10):749–69.
Esensten JH, Muller YD, Bluestone JA, Tang Q. Regulatory T-cell therapy for autoimmune and autoinflammatory diseases: The next frontier. J Allergy Clin Immunol. 2018;142(6):1710–8.
Chen L, Xiong Y, Hu Y, Yu C, Panayi AC, Zhou W, et al. Regulatory T cell-exosomal miR-142-3p promotes angiogenesis and osteogenesis via TGFBR1/SMAD2 inhibition to accelerate fracture repair. Chem Eng J. 2022;427:131419.
Kuswanto W, Burzyn D, Panduro M, Wang KK, Jang YC, Wagers AJ, et al. Poor repair of skeletal muscle in aging mice reflects a defect in local, interleukin-33-dependent accumulation of regulatory T cells. Immunity. 2016;44(2):355–67.
Li J, Tan J, Martino MM, Lui KO. Regulatory T-cells: potential regulator of tissue repair and regeneration. Front Immunol. 2018;9:585.
Boothby IC, Cohen JN, Rosenblum MD. Regulatory T cells in skin injury: at the crossroads of tolerance and tissue repair. Sci Immunol. 2020;5(47):eaaz9631.
Moreau JM, Dhariwala MO, Gouirand V, Boda DP, Boothby IC, Lowe MM, et al. Regulatory T cells promote innate inflammation after skin barrier breach via TGF-β activation. Sci Immunol. 2021;6(62):eabg2329.
Silva-Santos B, Mensurado S, Coffelt SB. γδ T cells: pleiotropic immune effectors with therapeutic potential in cancer. Nat Rev Cancer. 2019;19(7):392–404.
Ribot JC, Lopes N, Silva-Santos B. γδ T cells in tissue physiology and surveillance. Nat Rev Immunol. 2021;21(4):221–32.
Hovav AH. Human γδ T cells: rapid, stable and clonally reactive. Cell Mol Immunol. 2017;14(8):646–8.
Xu P, Fu X, Xiao N, Guo Y, Pei Q, Peng Y, et al. Involvements of γδT lymphocytes in acute and chronic skin wound repair. Inflammation. 2017;40(4):1416–27.
Liu Z, Xu Y, Zhang X, Liang G, Chen L, Xie J, et al. Defects in dermal Vγ4 γ δ T cells result in delayed wound healing in diabetic mice. Am J Transl Res. 2016;8(6):2667–80.
Ono T, Okamoto K, Nakashima T, Nitta T, Hori S, Iwakura Y, et al. IL-17-producing γδ T cells enhance bone regeneration. Nat Commun. 2016;7:10928.
Wong WK, Yin B, Rakhmatullina A, Zhou J, Wong SHD. Engineering advanced dynamic biomaterials to optimize adoptive T-cell immunotherapy. Eng Regen. 2021;2:70–81.
Sallusto F. Heterogeneity of human CD4+ T cells against microbes. Annu Rev Immunol. 2016;34:317–34.
Yu Y, Chen Z, Wang Y, Li Y, Lu J, Cui L, et al. Infliximab modifies regulatory T cells and co-inhibitory receptor expression on circulating T cells in psoriasis. Int Immunopharmacol. 2021;96:107722.
Bernhardsson M, Dietrich-Zagonel F, Tätting L, Eliasson P, Aspenberg P. Depletion of cytotoxic (CD8+) T cells impairs implant fixation in rat cancellous bone. J Orthop Res. 2019;37(4):805–11.
Reinke S, Geissler S, Taylor WR, Schmidt-Bleek K, Juelke K, Schwachmeyer V, et al. Terminally differentiated CD8+ T cells negatively affect bone regeneration in humans. Sci Transl Med. 2013;5(177):177ra36.
Chen L, Mehta ND, Zhao Y, DiPietro LA. Absence of CD4 or CD8 lymphocytes changes infiltration of inflammatory cells and profiles of cytokine expression in skin wounds, but does not impair healing. Exp Dermatol. 2014;23(3):189–94.
Davis PA, Corless DJ, Aspinall R, Wastell C. Effect of CD4+ and CD8+ cell depletion on wound healing. Br J Surg. 2001;88(2):298–304.
Vigneswaran Y, Han H, De Loera R, Wen Y, Zhang X, Sun T, et al. This paper is the winner of an SFB Award in the Hospital Intern, Residency category: Peptide biomaterials raising adaptive immune responses in wound healing contexts. J Biomed Mater Res A. 2016;104(8):1853–62.
Elahi FM, Farwell DG, Nolta JA, Anderson JD. Preclinical translation of exosomes derived from mesenchymal stem/stromal cells. Stem Cells. 2020;38(1):15–21.
El Andaloussi S, Mäger I, Breakefield XO, Wood MJA. Extracellular vesicles: biology and emerging therapeutic opportunities. Nat Rev Drug Discov. 2013;12(5):347–57.
Wei F, Li Z, Crawford R, Xiao Y, Zhou Y. Immunoregulatory role of exosomes derived from differentiating mesenchymal stromal cells on inflammation and osteogenesis. J Tissue Eng Regen Med. 2019;13(11):1978–91.
Toh WS, Zhang B, Lai RC, Lim SK. Immune regulatory targets of mesenchymal stromal cell exosomes/small extracellular vesicles in tissue regeneration. Cytotherapy. 2018;20(12):1419–26.
Planat-Benard V, Varin A, Casteilla L. MSCs and inflammatory cells crosstalk in regenerative medicine: concerted actions for optimized resolution driven by energy metabolism. Front Immunol. 2021;12:626755.
Tian X, Wei W, Cao Y, Ao T, Huang F, Javed R, et al. Gingival mesenchymal stem cell-derived exosomes are immunosuppressive in preventing collagen-induced arthritis. J Cell Mol Med. 2022;26(3):693–708.
Pachler K, Ketterl N, Desgeorges A, Dunai ZA, Laner-Plamberger S, Streif D, et al. An in vitro potency assay for monitoring the immunomodulatory potential of stromal cell-derived extracellular vesicles. Int J Mol Sci. 2017;18(7):1413.
Blazquez R, Sanchez-Margallo FM, de la Rosa O, Dalemans W, Alvarez V, Tarazona R, et al. Immunomodulatory potential of human adipose mesenchymal stem cells derived exosomes on in vitro stimulated T cells. Front Immunol. 2014;5:556.
Yang S, Zhu B, Yin P, Zhao L, Wang Y, Fu Z, et al. Integration of human umbilical cord mesenchymal stem cells-derived exosomes with hydroxyapatite-embedded hyaluronic acid-alginate hydrogel for bone regeneration. ACS Biomater Sci Eng. 2020;6(3):1590–602.
Zhang S, Wong KL, Ren X, Teo KYW, Afizah H, Choo ABH, et al. Mesenchymal stem cell exosomes promote functional osteochondral repair in a clinically relevant porcine model. Am J Sports Med. 2022;50(3):788–800.
Li Z, Wang Y, Li S, Li Y. Exosomes derived from M2 macrophages facilitate osteogenesis and reduce adipogenesis of BMSCs. Front Endocrinol (Lausanne). 2021;12:680328.
Zhou S. Paracrine effects of haematopoietic cells on human mesenchymal stem cells. Sci Rep. 2015;5:10573.
Phinney DG, Pittenger MF. Concise review: MSC-derived exosomes for cell-free therapy. Stem Cells. 2017;35(4):851–8.
Tan SHS, Wong JRY, Sim SJY, Tjio CKE, Wong KL, Chew JRJ, et al. Mesenchymal stem cell exosomes in bone regenerative strategies-a systematic review of preclinical studies. Mater Today Bio. 2020;7:100067.
Kim P, Park J, Lee DJ, Mizuno S, Shinohara M, Hong CP, et al. Mast4 determines the cell fate of MSCs for bone and cartilage development. Nat Commun. 2022;13(1):3960.
Wang X, Xu X, Zhang Y, An X, Zhang X, Chen G, et al. Duo cadherin-functionalized microparticles synergistically induce chondrogenesis and cartilage repair of stem cell aggregates. Adv Healthc Mater. 2022;11(13):e2200246.
Djouad F, Bouffi C, Ghannam S, Noël D, Jorgensen C. Mesenchymal stem cells: innovative therapeutic tools for rheumatic diseases. Nat Rev Rheumatol. 2009;5(7):392–9.
Gonzalez-Fernandez P, Rodríguez-Nogales C, Jordan O, Allémann E. Combination of mesenchymal stem cells and bioactive molecules in hydrogels for osteoarthritis treatment. Eur J Pharm Biopharm. 2022;172:41–52.
Ranganath SH, Levy O, Inamdar MS, Karp JM. Harnessing the mesenchymal stem cell secretome for the treatment of cardiovascular disease. Cell Stem Cell. 2012;10(3):244–58.
Nikfarjam S, Rezaie J, Zolbanin NM, Jafari R. Mesenchymal stem cell derived-exosomes: a modern approach in translational medicine. J Transl Med. 2020;18(1):449.
Zhang B, Yin Y, Lai RC, Tan SS, Choo ABH, Lim SK. Mesenchymal stem cells secrete immunologically active exosomes. Stem Cells Dev. 2014;23(11):1233–44.
Shpigelman J, Lao FS, Yao S, Li C, Saito T, Sato-Kaneko F, et al. Generation and application of a reporter cell line for the quantitative screen of extracellular vesicle release. Front Pharmacol. 2021;12:668609.
Zhang S, Teo KYW, Chuah SJ, Lai RC, Lim SK, Toh WS. MSC exosomes alleviate temporomandibular joint osteoarthritis by attenuating inflammation and restoring matrix homeostasis. Biomaterials. 2019;200:35–47.
Liu Y, Lin L, Zou R, Wen C, Wang Z, Lin F. MSC-derived exosomes promote proliferation and inhibit apoptosis of chondrocytes via lncRNA-KLF3-AS1/miR-206/GIT1 axis in osteoarthritis. Cell Cycle. 2018;17(21–22):2411–22.
Mao G, Hu S, Zhang Z, Wu P, Zhao X, Lin R, et al. Exosomal miR-95-5p regulates chondrogenesis and cartilage degradation via histone deacetylase 2/8. J Cell Mol Med. 2018;22(11):5354–66.
Zhang S, Chu WC, Lai RC, Lim SK, Hui JHP, Toh WS. Exosomes derived from human embryonic mesenchymal stem cells promote osteochondral regeneration. Osteoarthr Cartil. 2016;24(12):2135–40.
Zhang S, Chuah SJ, Lai RC, Hui JHP, Lim SK, Toh WS. MSC exosomes mediate cartilage repair by enhancing proliferation, attenuating apoptosis and modulating immune reactivity. Biomaterials. 2018;156:16–27.
Gong M, Yu B, Wang J, Wang Y, Liu M, Paul C, et al. Mesenchymal stem cells release exosomes that transfer miRNAs to endothelial cells and promote angiogenesis. Oncotarget. 2017;8(28):45200–12.
Won Lee G, Thangavelu M, Joung Choi M, Yeong Shin E, Sol Kim H, Seon Baek J, et al. Exosome mediated transfer of miRNA-140 promotes enhanced chondrogenic differentiation of bone marrow stem cells for enhanced cartilage repair and regeneration. J Cell Biochem. 2020;121(7):3642–52.
Zhang M, Chen D, Zhang F, Zhang G, Wang Y, Zhang Q, et al. Serum exosomal hsa-miR-135b-5p serves as a potential diagnostic biomarker in steroid-induced osteonecrosis of femoral head. Am J Transl Res. 2020;12(5):2136–54.
Tkach M, Théry C. Communication by extracellular vesicles: where we are and where we need to go. Cell. 2016;164(6):1226–32.
Su D, Tsai HI, Xu Z, Yan F, Wu Y, Xiao Y, et al. Exosomal PD-L1 functions as an immunosuppressant to promote wound healing. J Extracell Vesicles. 2019;9(1):1709262.
Whiteside TL. Exosomes and tumor-mediated immune suppression. J Clin Invest. 2016;126(4):1216–23.
Chen G, Huang AC, Zhang W, Zhang G, Wu M, Xu W, et al. Exosomal PD-L1 contributes to immunosuppression and is associated with anti-PD-1 response. Nature. 2018;560(7718):382–6.
Syn NL, Wang L, Chow EKH, Lim CT, Goh BC. Exosomes in cancer nanomedicine and immunotherapy: prospects and challenges. Trends Biotechnol. 2017;35(7):665–76.
Collino F, Pomatto M, Bruno S, Lindoso RS, Tapparo M, Sicheng W, et al. Exosome and microvesicle-enriched fractions isolated from mesenchymal stem cells by gradient separation showed different molecular signatures and functions on renal tubular epithelial cells. Stem Cell Rev Rep. 2017;13(2):226–43.
Pina S, Oliveira JM, Reis RL. Natural-based nanocomposites for bone tissue engineering and regenerative medicine: a review. Adv Mater. 2015;27(7):1143–69.
Roohani I, Yeo GC, Mithieux SM, Weiss AS. Emerging concepts in bone repair and the premise of soft materials. Curr Opin Biotechnol. 2022;74:220–9.
Oh J, Xia X, Wong WKR, Wong SHD, Yuan W, Wang H, et al. The effect of the nanoparticle shape on T cell activation. Small. 2022;18(36):e2107373.
Yin B, Yang H, Yang M. Integrating soft hydrogel with nanostructures reinforces stem cell adhesion and differentiation. J Compos Sci. 2022;6(1):19.
Wong DSH, Li J, Yan X, Wang B, Li R, Zhang L, et al. Magnetically tuning tether mobility of integrin ligand regulates adhesion, spreading, and differentiation of stem cells. Nano Lett. 2017;17(3):1685–95.
Christman KL. Biomaterials for tissue repair. Science. 2019;363(6425):340–1.
Hasani-Sadrabadi MM, Sarrion P, Nakatsuka N, Young TD, Taghdiri N, Ansari S, et al. Hierarchically patterned polydopamine-containing membranes for periodontal tissue engineering. ACS Nano. 2019;13(4):3830–38.
Kim SY, Nair MG. Macrophages in wound healing: activation and plasticity. Immunol Cell Biol. 2019;97(3):258–67.
Wu S, Ma J, Liu J, Liu C, Ni S, Dai T, et al. Immunomodulation of telmisartan-loaded PCL/PVP scaffolds on macrophages promotes endogenous bone regeneration. ACS Appl Mater Interfaces. 2022;14(14):15942–55.
Whitaker R, Hernaez-Estrada B, Hernandez RM, Santos-Vizcaino E, Spiller KL. Immunomodulatory biomaterials for tissue repair. Chem Rev. 2021;121(18):11305–35.
Lin C, Tao B, Deng Y, He Y, Shen X, Wang R, et al. Matrix promote mesenchymal stromal cell migration with improved deformation via nuclear stiffness decrease. Biomaterials. 2019;217:119300.
Zhou W, Lin Z, Xiong Y, Xue H, Song W, Yu T, et al. Dual-targeted nanoplatform regulating the bone immune microenvironment enhances fracture healing. ACS Appl Mater Interfaces. 2021;13(48):56944–60.
Kajahn J, Franz S, Rueckert E, Forstreuter I, Hintze V, Moeller S, et al. Artificial extracellular matrices composed of collagen I and high sulfated hyaluronan modulate monocyte to macrophage differentiation under conditions of sterile inflammation. Biomatter. 2012;2(4):226–36.
Kwon H, Brown WE, Lee CA, Wang D, Paschos N, Hu JC, et al. Surgical and tissue engineering strategies for articular cartilage and meniscus repair. Nat Rev Rheumatol. 2019;15(9):550–70.
Steinberg J, Southam L, Fontalis A, Clark MJ, Jayasuriya RL, Swift D, et al. Linking chondrocyte and synovial transcriptional profile to clinical phenotype in osteoarthritis. Ann Rheum Dis. 2021;80(8):1070–4.
Zhang H, Lin C, Zeng C, Wang Z, Wang H, Lu J, et al. Synovial macrophage M1 polarisation exacerbates experimental osteoarthritis partially through R-spondin-2. Ann Rheum Dis. 2018;77(10):1524–34.
Fernandes TL, Gomoll AH, Lattermann C, Hernandez AJ, Bueno DF, Amano MT. Macrophage: A potential target on cartilage regeneration. Front Immunol. 2020;11:111.
Huleihel L, Dziki JL, Bartolacci JG, Rausch T, Scarritt ME, Cramer MC, et al. Macrophage phenotype in response to ECM bioscaffolds. Semin Immunol. 2017;29:2–13.
Kiyotake EA, Beck EC, Detamore MS. Cartilage extracellular matrix as a biomaterial for cartilage regeneration. Ann N Y Acad Sci. 2016;1383(1):139–59.
Changchen W, Hongquan W, Bo Z, Leilei X, Haiyue J, Bo P. The characterization, cytotoxicity, macrophage response and tissue regeneration of decellularized cartilage in costal cartilage defects. Acta Biomater. 2021;136:147–58.
Tian G, Jiang S, Li J, Wei F, Li X, Ding Y, et al. Cell-free decellularized cartilage extracellular matrix scaffolds combined with interleukin 4 promote osteochondral repair through immunomodulatory macrophages: In vitro and in vivo preclinical study. Acta Biomater. 2021;127:131–45.
Dai M, Sui B, Xue Y, Liu X, Sun J. Cartilage repair in degenerative osteoarthritis mediated by squid type II collagen via immunomodulating activation of M2 macrophages, inhibiting apoptosis and hypertrophy of chondrocytes. Biomaterials. 2018;180:91–103.
Gan D, Jiang Y, Hu Y, Wang X, Wang Q, Wang K, et al. Mussel-inspired extracellular matrix-mimicking hydrogel scaffold with high cell affinity and immunomodulation ability for growth factor-free cartilage regeneration. J Orthop Translat. 2022;33:120–31.
Sumayya AS, Muraleedhara Kurup G. Anti-inflammatory potential of marine macromolecules cross-linked bio-composite scaffold on LPS stimulated RAW 264.7 macrophage cells for cartilage tissue engineering applications. J Biomater Sci Polym Ed. 2021;32(8):1040–56.
Yuan Z, Long T, Zhang J, Lyu Z, Zhang W, Meng X, et al. 3D printed porous sulfonated polyetheretherketone scaffold for cartilage repair: Potential and limitation. J Orthop Translat. 2022;33:90–106.
Zhai D, Chen L, Chen Y, Zhu Y, Xiao Y, Wu C. Lithium silicate-based bioceramics promoting chondrocyte maturation by immunomodulating M2 macrophage polarization. Biomater Sci. 2020;8(16):4521–34.
Takenaka M, Yabuta A, Takahashi Y, Takakura Y. Interleukin-4-carrying small extracellular vesicles with a high potential as anti-inflammatory therapeutics based on modulation of macrophage function. Biomaterials. 2021;278:121160.
Gong L, Li J, Zhang J, Pan Z, Liu Y, Zhou F, et al. An interleukin-4-loaded bi-layer 3D printed scaffold promotes osteochondral regeneration. Acta Biomater. 2020;117:246–60.
Jiang G, Li S, Yu K, He B, Hong J, Xu T, et al. A 3D-printed PRP-GelMA hydrogel promotes osteochondral regeneration through M2 macrophage polarization in a rabbit model. Acta Biomater. 2021;128:150–62.
Zhao X, Zhao Y, Sun X, Xing Y, Wang X, Yang Q. Immunomodulation of MSCs and MSC-derived extracellular vesicles in osteoarthritis. Front Bioeng Biotechnol. 2020;8:575057.
Chahal J, Gómez-Aristizábal A, Shestopaloff K, Bhatt S, Chaboureau A, Fazio A, et al. Bone marrow mesenchymal stromal cell treatment in patients with osteoarthritis results in overall improvement in pain and symptoms and reduces synovial inflammation. Stem Cells Transl Med. 2019;8(8):746–57.
Ding J, Chen B, Lv T, Liu X, Fu X, Wang Q, et al. Bone marrow mesenchymal stem cell-based engineered cartilage ameliorates polyglycolic acid/polylactic acid scaffold-induced inflammation through M2 polarization of macrophages in a pig model. Stem Cells Transl Med. 2016;5(8):1079–89.
Jiang S, Tian G, Yang Z, Gao X, Wang F, Li J, et al. Enhancement of acellular cartilage matrix scaffold by Wharton’s jelly mesenchymal stem cell-derived exosomes to promote osteochondral regeneration. Bioact Mater. 2021;6(9):2711–28.
Chen P, Zheng L, Wang Y, Tao M, Xie Z, Xia C, et al. Desktop-stereolithography 3D printing of a radially oriented extracellular matrix/mesenchymal stem cell exosome bioink for osteochondral defect regeneration. Theranostics. 2019;9(9):2439–59.
Xue YZB, Niu YM, Tang B, Wang CM. PCL/EUG scaffolds with tunable stiffness can regulate macrophage secretion behavior. Prog Biophys Mol Biol. 2019;148:4–11.
Cha BH, Shin SR, Leijten J, Li YC, Singh S, Liu JC, et al. Integrin-mediated interactions control macrophage polarization in 3D hydrogels. Adv Healthc Mater. 2017;6(21). https://doi.org/10.1002/adhm.201700289.
Kang H, Wong SHD, Pan Q, Li G, Bian L. Anisotropic ligand nanogeometry modulates the adhesion and polarization state of macrophages. Nano Lett. 2019;19(3):1963–75.
Nguyen AV, Soulika AM. The dynamics of the skin’s immune system. Int J Mol Sci. 2019;20(8):1811.
Parkatzidis K, Chatzinikolaidou M, Kaliva M, Bakopoulou A, Farsari M, Vamvakaki M. Multiphoton 3D printing of biopolymer-based hydrogels. ACS Biomater Sci Eng. 2019;5(11):6161–70.
Shivakumar P, Gupta MS, Jayakumar R, Gowda DV. Prospection of chitosan and its derivatives in wound healing: Proof of patent analysis (2010–2020). Int J Biol Macromol. 2021;184:701–12.
Ashouri F, Beyranvand F, Beigi Boroujeni N, Tavafi M, Sheikhian A, Varzi AM, et al. Macrophage polarization in wound healing: role of aloe vera/chitosan nanohydrogel. Drug Deliv Transl Res. 2019;9(6):1027–42.
Chouhan D, Lohe TU, Samudrala PK, Mandal BB. In situ forming injectable silk fibroin hydrogel promotes skin regeneration in full thickness burn wounds. Adv Healthc Mater. 2018;7(24):e1801092.
Rafieerad A, Yan W, Sequiera GL, Sareen N, Abu-El-Rub E, Moudgil M, et al. Application of Ti C MXene quantum dots for immunomodulation and regenerative medicine. Adv Healthc Mater. 2019;8(16):e1900569.
Chen TY, Wen TK, Dai NT, Hsu SH. Cryogel/hydrogel biomaterials and acupuncture combined to promote diabetic skin wound healing through immunomodulation. Biomaterials. 2021;269:120608.
Saleh B, Dhaliwal HK, Portillo-Lara R, Shirzaei Sani E, Abdi R, Amiji MM, et al. Local immunomodulation using an adhesive hydrogel loaded with miRNA-laden nanoparticles promotes wound healing. Small. 2019;15(36):e1902232.
San Emeterio CL, Hymel LA, Turner TC, Ogle ME, Pendleton EG, York WY, et al. Nanofiber-based delivery of bioactive lipids promotes pro-regenerative inflammation and enhances muscle fiber growth after volumetric muscle loss. Front Bioeng Biotechnol. 2021;9:650289.
Xi K, Gu Y, Tang J, Chen H, Xu Y, Wu L, et al. Microenvironment-responsive immunoregulatory electrospun fibers for promoting nerve function recovery. Nat Commun. 2020;11(1):4504.
Bonito V, Koch SE, Krebber MM, Carvajal-Berrio DA, Marzi J, Duijvelshoff R, et al. Distinct effects of heparin and interleukin-4 functionalization on macrophage polarization and in situ arterial tissue regeneration using resorbable supramolecular vascular grafts in rats. Adv Healthc Mater. 2021;10(21):e2101103.
Tidball JG, Flores I, Welc SS, Wehling-Henricks M, Ochi E. Aging of the immune system and impaired muscle regeneration: A failure of immunomodulation of adult myogenesis. Exp Gerontol. 2021;145:111200.
Sadtler K, Estrellas K, Allen BW, Wolf MT, Fan H, Tam AJ, et al. Developing a pro-regenerative biomaterial scaffold microenvironment requires T helper 2 cells. Science. 2016;352(6283):366–70.
Estrellas KM, Chung L, Cheu LA, Sadtler K, Majumdar S, Mula J, et al. Biological scaffold-mediated delivery of myostatin inhibitor promotes a regenerative immune response in an animal model of Duchenne muscular dystrophy. J Biol Chem. 2018;293(40):15594–605.