Oral Diseases (2014) 20, 633–636 doi:10.1111/odi.12248 © 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd All rights reserved www.wiley.com
ANNIVERSARY REVIEW
Immunomodulation regulates mesenchymal stem cell-based bone regeneration Y Su1, S Shi2, Y Liu1 1
Laboratory of Tissue Regeneration and Immunology and Department of Periodontics, Beijing Key Laboratory for Tooth Regeneration and Function Reconstruction, Capital Medical University School of Stomatology, Beijing, China; 2Center for Craniofacial Molecular Biology, University of Southern California, Los Angeles, CA, USA
Mesenchymal stem cell (MSC)-based regenerative medicine represents a promising frontier for bone reconstruction. Significant efforts have been devoted to clarifying the capacities of MSCs to repair or reconstruct bone tissue. This review provides a concise summary of current knowledge pertaining to the possible mechanisms of MSC action in the regeneration of bone, with particular focus on the interplay between donor MSCs and host immune response in the process of new bone regeneration. Oral Diseases (2014) 20, 633–636 Keywords: mesenchymal stem cell; regeneration; immune response
Introduction Mesenchymal stem cells (MSCs) belong to a primitive cell type originating from the mesodermal germ layer and have been isolated from a wide range of adult tissues, including but not limited to bone marrow, peripheral blood, umbilical cord blood, dental tissues, adipose tissue, spleen, liver, and heart (Liu et al, 2012). Mesenchymal stem cells are characterized by their capacities for self-renewal and differentiation into multiple cell lineages, such as osteoblasts, chondrocytes, adipocytes, tenocytes, and myocytes (Rahaman and Mao, 2005; Krampera et al, 2006). The multilineage differential potential of MSCs facilitates them to be an ideal tool for regenerative medicine. Multiple preclinical and clinical studies have shown that MSC-seeded constructs can be used for tissue regeneration, such as Correspondence: Yi Liu, Laboratory of Tissue Regeneration and Immunology and Department of Periodontics, Beijing Key Laboratory for Tooth Regeneration and Function Reconstruction, Capital Medical University School of Stomatology, Tian Tan Xi Li No. 4, Beijing 100050, China. Tel: +86 10 57099450, Fax: +86 10 57099450, E-mail: liuyi@ ccmu.edu.cn Received 11 February 2014; revised 23 March 2014; accepted 30 March 2014
accelerating bone repair in critical-sized defects in long bones, craniofacial deformities, and alveolar bone defects (Mauney et al, 2005; Ding et al, 2010). In addition, MSCs have the capacity to modulate immune response through the interactions between the immune cells associated with both the innate and adaptive immune systems (Uccelli et al, 2008; Han et al, 2012), which make them capable of reducing hyperactivated immune responses (English, 2013). However, the interactions between MSCs and immune cells do not belong to mesenchyme lineage only. The stem cells from non-mesenchymal origin such as neural stem cells and amnion epithelial cells also have immunomodulatory properties (Ben-Hur, 2008; Castillo-Melendez et al, 2013). Within the bone marrow microenvironment, MSCs maintain the hematopoietic stem cell (HSC) niche, from which all immune cells derive (Frenette et al, 2013). Recently, MSCs have shown immunomodulatory properties including the ability to regulate the proliferation/apoptosis and cytokine secretion of immune cells. They are able to suppress proliferation of several subsets of lymphocytes, including CD4+ helper T lymphocytes (Ths), CD8+ cytotoxic T lymphocytes (CTLs), and natural killer (NK) cells, as well as induce CD3+ T-cell apoptosis (Uccelli et al, 2008; Akiyama et al, 2012; Spaggiari and Moretta, 2013). Also, MSCs can modulate the intensity of an immune response by directly or indirectly promoting the generation of regulatory T cells (Tregs) (Burr et al, 2013). The therapeutic effect of MSCs in graft-vs-host disease has been shown to depend on MSCs initiating cytokine and nitric oxide (NO) signaling to block the activity of migrating T cells (Ren et al, 2008). Allogenic MSC transplantation (MSCT) ameliorates disease phenotypes in a mouse model of systemic lupus erythematosus (SLE) and in human SLE patients via reconstructing the bone marrow osteoblastic niche to enhance regulatory T cells and re-establish immune homeostasis (Sun et al, 2009). Additionally, systemic infusion of MSCs has been demonstrated to dramatically restore alveolar bone healing and regeneration via reestablishment of host immune homeostasis in mouse and minipig models of bisphosphonate-associated osteonecrosis of the jaw (BRONJ) (Kikuiri et al, 2010; Li et al, 2013).
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MSCs in cell-based bone regeneration Mesenchymal stem cells express low levels of class II major histocompatibility complex (MHC-II) and costimulatory molecules, including CD40, CD80, and, CD86, which renders these cells immunoprivileged, allowing them to be used for tissue repair (Javazon et al, 2004). With respect to repairing bone defects, site-directed delivery of MSCs using a bioscaffold is a practical approach for encouraging the growth of missing tissue. Numerous studies have shown the efficacy of autologous MSC implantation for repairing critical-sized defects in long bones in animal models (Bruder et al, 1998; Schantz et al, 2003; Kraus and Kirker-Head, 2006; Yuan et al, 2007). Based on these findings, human autologous MSCs have been successfully used in the clinical treatment of long bone defects and osteonecrosis of the femoral head (Garg et al, 1993; Sim et al, 1993; Chapman et al, 1997; Connolly, 1998; Kawate et al, 2006). In addition, allogeneic MSCs derived from adipose tissue and dental tissue have the capacity to regenerate bone without the need for immunosuppressive therapy and show similar regenerative effects to those observed in autologous MSC implantation (Ding et al, 2010; Liu et al, 2013a,b). Nevertheless, although a variety of literature reports have shown the efficacy and safety of using MSCs for bone regeneration, no such use has yet been approved by the US Food and Drug Administration (FDA). It is complicated to develop procedures and protocols for isolating and expanding MSCs that reach the high quality required for clinical use. As such, fundamental insights into the therapeutic mechanisms of MSCs, especially those that regulate autologous and allogeneic MSC survival and terminal differentiation locally in wounded tissues, are required before clinical use of MSCs in regenerative therapies can be successful on a larger stage.
Host immune response and MSC-based bone regeneration Factors produced by lymphocytes and other mononuclear cells can dramatically influence bone turnover by altering the delicate balance between bone formation and bone resorption, processes mediated by osteoblasts and osteoclasts, respectively (Raggatt and Partridge, 2010). Moreover, complex crosstalk between implanted MSCs and recipient immune cells can also affect the fate and function of MSCs, thereby playing a critical role in MSC-mediated tissue regeneration. It has been reported that when bone marrow MSCs (BMMSCs) from normal mice were cocultured with lymph node (LN) T cells from C3H or immunocompromised mice, BMMSCs could be induced to apoptosis by activated LN cells from normal mice but not from T-cell-deficient immunocompromised mice in vitro (Yamaza et al, 2008). When implanted into immunocompromised mice subcutaneously using hydroxyapatite– tricalcium phosphate (HA-TCP) particles as a carrier, MSCs are capable of forming bone and hematopoietic marrow elements. However, when implanted into immune competent C57BL/6 mice, MSCs fail to regenerate bone marrow structure (Liu et al, 2011). Oral Diseases
These data collectively suggest that T cells can impair MSCs’ function and inhibit MSC-mediated bone formation. Further studies indicated the distinct roles of T-cell subpopulations in mediating the interaction between immune cells and MSCs in MSC-based osteogenesis. Infusion with Pan T, CD4+, or CD4+CD25 T cells prior to MSCs’ subcutaneous implantation was found to block bone formation in immunocompromised mice. Mechanically, a lack of bone formation was correlated with high levels of TNF-a and IFN-c secreted by host immune cells, which inhibited MSC differentiation and induced cell apoptosis. In contrast, infusion with CD4+CD25+ Treg cells showed no inhibitive effect on bone formation, suggesting that modulating host immune response might facilitate MSC-based tissue regeneration. Indeed, systemic infusion of CD4+CD25+ Treg cells markedly reduced TNF-a and IFN-c production and abolished TNF-a/IFN-c-induced cell apoptosis, thereby dramatically improving MSC-mediated bone regeneration of critical-sized calvarial bone defects. Importantly, site-specific treatment with aspirin also markedly improved bone regeneration in a calvarial injury model (Liu et al, 2011). In addition to T cells, macrophages are described as another critical cell population for the regulation of MSCbased osteogenesis. Macrophages are recognized as a highly heterogeneous cell population, and the function of individual populations may vary significantly in vivo (Gordon and Taylor, 2005; Mosser and Edwards, 2008). It has been reported that the conditioned medium from differentiated macrophages suppressed the osteogenic differentiation of MSCs and osteoprogenitor cells (Chen et al, 2012; Lee et al, 2012). TNF-a and IL-1b secreted by macrophages were indicated to contribute to these inhibitive effects. In MSC-mediated bone regeneration, the unique roles of different macrophage populations remain unclear. Following tissue injury or infection, macrophages usually exhibit an inflammatory phenotype (M1) and secrete mediators to kill invading organisms or influence the polarization of TH1 and TH17 cells. In contrast to M1 macrophages, M2 macrophages exhibit potent anti-inflammatory effects and play critical roles in tissue repair by releasing active molecules and interacting with stem and progenitor cells (Murray and Wynn, 2011). However, the pro-osteogenic effect of activated pro-inflammatory macrophages (M1) on MSCs has been observed in a coculture system (Guihard et al, 2012). This leads to the identification of macrophages as key players in the resolution of inflammation and the repair/regeneration of damaged tissue and hints at the question of whether they can change their phenotype (M1/M2 paradigm) in response to the local microenvironment. Moreover, macrophages commonly serve as antigen-presenting cells (APCs), and as such their function in propagating T-cell response in bone regeneration warrants exploration. In addition, as the most professional APCs, DCs were suggested to be involved in inflammation-induced osteogenesis by acting as osteoclast precursors (Alnaeeli et al, 2007). However, there have been no reports about the function of DC cells on MSC osteogenesis or MSC-mediated bone regeneration so far, which indicates that investigations on DCs are required. Taken together, these pieces of evidence provide insight into the interplay between the immune system and implanted MSCs in a graft–host environment and point to
MSC-based regeneration and immune response
Y Su et al
strategies that may improve therapeutic outcomes for MSCbased tissue engineering. The distinctive roles that various components of the host immune system play in mediating communication between immune cells and stem cells could also be occurring during homeostatic bone metabolism as well as during osteogenesis. The next set of challenges includes deciphering the molecular mechanisms of how various immune cells regulate MSCs and their precise roles in bone regeneration in vivo. This will be important for the eventual manipulation of the immune compartments to improve MSC-based bone regeneration.
Summary and future directions All cell therapies face challenges regarding how to best achieve integration of the transplanted cells into the host environment, which is a complex process involving recruitment, homing, and integration into local niches, connection to the host blood supply, immune compatibility, and graft durability (Daley, 2012). The prospects for more widespread stem cell-based bone engineering depend on understanding the biology of transplanted MSCs in vivo, as well as their relationship with the native microenvironment and host immune system. First, the mechanisms underlying migration of MSCs remain to be clarified, although evidence suggests that both chemokines and their receptors are involved. Studying the mechanisms of MSC migration may allow MSCs delivered systemically to be recruited to the site of damaged bone tissue. Next, the use of MSCs as immunomodulators has been tested in inflammatory and autoimmune diseases. However, the possible roles of MSCs in regulating immune and inflammatory responses when delivered locally to regenerate bone tissue are still unclear and deserve more attention. Moreover, it will be interesting to reveal the individual adaptive immunity profile in pathological bone diseases, which may help to identify markers of conditions with poor bone healing, thus allowing early supportive interventions or targeted therapies. Lastly, so far, investigations of the influence of immune cells on the skeletal system have mainly focused on interactions between immune cells and osteoclasts under physiological or pathological conditions. However, understanding of how the host immune system may interact with implanted MSCs in bone tissue engineering remains primitive. Additionally, beyond T cells, many components of the immune system, such as macrophages, DCs, and NK T cells, remain uncharacterized regarding their effects on MSC-based bone regeneration. Also, there is limited knowledge of how implanted MSCs may in turn influence the immune system. Answers to these and similar questions await further studies and will surely benefit both basic and clinical research in the field of tissue engineering.
Author contributions YY. Su wrote the review. ST. Shi and Y. Liu made substantial contributions to the conception of the paper, revised the article critically for important intellectual content, and were involved in the final approval of the version to be published.
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References Akiyama K, Chen C, Wang D et al (2012). Mesenchymal-stemcell-induced immunoregulation involves FAS-ligand-/FASmediated T cell apoptosis. Cell Stem Cell 10: 544–555. Alnaeeli M, Park J, Mahamed D, Penninger JM, Teng YT (2007). Dendritic cells at the osteo-immune interface: implications for inflammation-induced bone loss. J Bone Miner Res 22: 775–780. Ben-Hur T (2008). Immunomodulation by neural stem cells. J Neurol Sci 265: 102–104. Bruder SP, Kraus KH, Goldberg VM, Kadiyala S (1998). The effect of implants loaded with autologous mesenchymal stem cells on the healing of canine segmental bone defects. J Bone Joint Surg Am 80: 985–996. Burr SP, Dazzi F, Garden OA (2013). Mesenchymal stromal cells and regulatory T cells: the Yin and Yang of peripheral tolerance? Immunol Cell Biol 91: 12–18. Castillo-Melendez M, Yawno T, Jenkin G, Miller SL (2013). Stem cell therapy to protect and repair the developing brain: a review of mechanisms of action of cord blood and amnion epithelial derived cells. Front Neurosci 7: 194. Chapman MW, Bucholz R, Cornell C (1997). Treatment of acute fractures with a collagen-calcium phosphate graft material. A randomized clinical trial. J Bone Joint Surg Am 79: 495–502. Chen C, Uluda g H, Wang Z, Rezansoff A, Jiang H (2012). Macrophages inhibit migration, metabolic activity and osteogenic differentiation of human mesenchymal stem cells in vitro. Cells Tissues Organs 195: 473–483. Connolly JF (1998). Clinical use of marrow osteoprogenitor cells to stimulate osteogenesis. Clin Orthop Relat Res 355: S257– S266. Daley GQ (2012). The promise and perils of stem cell therapeutics. Cell Stem Cell 10: 740–749. Ding G, Liu Y, Wang W et al (2010). Allogeneic periodontal ligament stem cell therapy for periodontitis in swine. Stem Cells 28: 1829–1838. English K (2013). Mechanisms of mesenchymal stromal cell immunomodulation. Immunol Cell Biol 91: 19–26. Frenette PS, Pinho S, Lucas D, Scheiermann C (2013). Mesenchymal stem cell: keystone of the hematopoietic stem cell niche and a stepping-stone for regenerative medicine. Annu Rev Immunol 31: 285–316. Garg NK, Gaur S, Sharma S (1993). Percutaneous autogenous bone marrow grafting in 20 cases of ununited fracture. Acta Orthop Scand 64: 671–672. Gordon S, Taylor PR (2005). Monocyte and macrophage heterogeneity. Nat Rev Immunol 5: 953–964. Guihard P, Danger Y, Brounais B et al (2012). Induction of osteogenesis in mesenchymal stem cells by activated monocytes/macrophages depends on oncostatin M signaling. Stem Cells 30: 762–772. Han Z, Jing Y, Zhang S, Liu Y, Shi Y, Wei L (2012). The role of immunosuppression of mesenchymal stem cells in tissue repair and tumor growth. Cell Biosci 2: 8. Javazon EH, Beggs KJ, Flake AW (2004). Mesenchymal stem cells: paradoxes of passaging. Exp Hematol 32: 414–425. Kawate K, Yajima H, Ohgushi H et al (2006). Tissue-engineered approach for the treatment of steroid-induced osteonecrosis of the femoral head: transplantation of autologous mesenchymal stem cell cultured with beta-tricalcium phosphate ceramics and free vascularized fibula. Artif Organs 30: 960–962. Kikuiri T, Kim I, Yamaza T et al (2010). Cell-based immunotherapy with mesenchymal stem cells cures bisphosphonaterelated osteonecrosis of the jaw-like disease in mice. J Bone Miner Res 25: 1668–1679.
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Krampera M, Pizzolo G, Aprili G, Franchini M (2006). Mesenchymal stem cells for bone, cartilage, tendon and skeletal muscle repair. Bone 39: 678–683. Kraus KH, Kirker-Head C (2006). Mesenchymal stem cells and bone regeneration. Vet Surg 35: 232–242. Lee SS, Sharma AR, Choi BS et al (2012). The effect of TNFa secreted from macrophages activated by titanium particles on osteogenic activity regulated by WNT/BMP signaling in osteoprogenitor cells. Biomaterials 33: 4251–4263. Li Y, Xu J, Mao L et al (2013). Allogeneic mesenchymal stem cell therapy for bisphosphonate-related jaw osteonecrosis in Swine. Stem Cells Dev 22: 2047–2056. Liu Y, Wang L, Klkuiri T et al (2011). Mesenchymal stem cellbased tissue regeneration is governed by recipient T lymphocytes via IFN-c and TNF-a. Nat Med 17: 1594–1601. Liu Y, Wang S, Shi S (2012). The role of recipient T cells in mesenchymal stem cell-based tissue regeneration. Int J Biochem Cell Biol 44: 2044–2050. Liu G, Zhang Y, Liu B, Sun J, Li W, Cui L (2013a). Bone regeneration in a canine cranial model using allogeneic adipose derived stem cells and coral scaffold. Biomaterials 34: 2655–2664. Liu O, Xu J, Ding G et al (2013b). Periodontal ligament stem cells regulate B lymphocyte function via programmed cell death protein 1. Stem Cells 31: 1371–1382. Mauney JR, Volloch V, Kaplan DL (2005). Role of adult mesenchymal stem cells in bone tissue engineering applications: current status and future prospects. Tissue Eng 11: 787–802. Mosser DM, Edwards JP (2008). Exploring the full spectrum of macrophage activation. Nat Rev Immunol 8: 958–969. Murray PJ, Wynn TA (2011). Protective and pathogenic functions of macrophage subsets. Nat Rev Immunol 11: 723–737.
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Raggatt LJ, Partridge NC (2010). Cellular and molecular mechanisms of bone remodeling. J Biol Chem 285: 25103–25108. Rahaman MN, Mao JJ (2005). Stem cell-based composite tissue constructs for regenerative medicine. Biotechnol Bioeng 91: 261–284. Ren G, Zhang L, Zhao X et al (2008). Mesenchymal stem cellmediated immunosuppression occurs via concerted action of chemokines and nitric oxide. Cell Stem Cell 2: 141–150. Schantz JT, Teoh SH, Lim TC, Endres M, Lam CX, Hutmacher DW (2003). Repair of calvarial defects with customized tissue-engineered bone grafts I. Evaluation of osteogenesis in a three-dimensional culture system. Tissue Eng 9(Suppl 1): S113–S126. Sim R, Liang TS, Tay BK (1993). Autologous marrow injection in the treatment of delayed and non-union in long bones. Singapore Med J 34: 412–417. Spaggiari GM, Moretta L (2013). Cellular and molecular interactions of mesenchymal stem cells in innate immunity. Immunol Cell Biol 91: 27–31. Sun L, Akiyama K, Zhang H et al (2009). Mesenchymal stem cell transplantation reverses multiorgan dysfunction in systemic lupus erythematosus mice and humans. Stem Cells 27: 1421–1432. Uccelli A, Moretta L, Pistoia V (2008). Mesenchymal stem cells in health and disease. Nat Rev Immunol 8: 726–736. Yamaza T, Miura Y, Bi Y et al (2008). Pharmacologic stem cell based intervention as a new approach to osteoporosis treatment in rodents. PLoS ONE 3: e2615. Yuan J, Cui L, Zhang WJ, Liu W, Cao Y (2007). Repair of canine mandibular bone defects with bone marrow stromal cells and porous beta-tricalcium phosphate. Biomaterials 28: 1005–1013.
ANNIVERSARY REVIEW
Immunomodulation regulates mesenchymal stem cell-based bone regeneration Y Su1, S Shi2, Y Liu1 1
Laboratory of Tissue Regeneration and Immunology and Department of Periodontics, Beijing Key Laboratory for Tooth Regeneration and Function Reconstruction, Capital Medical University School of Stomatology, Beijing, China; 2Center for Craniofacial Molecular Biology, University of Southern California, Los Angeles, CA, USA
Mesenchymal stem cell (MSC)-based regenerative medicine represents a promising frontier for bone reconstruction. Significant efforts have been devoted to clarifying the capacities of MSCs to repair or reconstruct bone tissue. This review provides a concise summary of current knowledge pertaining to the possible mechanisms of MSC action in the regeneration of bone, with particular focus on the interplay between donor MSCs and host immune response in the process of new bone regeneration. Oral Diseases (2014) 20, 633–636 Keywords: mesenchymal stem cell; regeneration; immune response
Introduction Mesenchymal stem cells (MSCs) belong to a primitive cell type originating from the mesodermal germ layer and have been isolated from a wide range of adult tissues, including but not limited to bone marrow, peripheral blood, umbilical cord blood, dental tissues, adipose tissue, spleen, liver, and heart (Liu et al, 2012). Mesenchymal stem cells are characterized by their capacities for self-renewal and differentiation into multiple cell lineages, such as osteoblasts, chondrocytes, adipocytes, tenocytes, and myocytes (Rahaman and Mao, 2005; Krampera et al, 2006). The multilineage differential potential of MSCs facilitates them to be an ideal tool for regenerative medicine. Multiple preclinical and clinical studies have shown that MSC-seeded constructs can be used for tissue regeneration, such as Correspondence: Yi Liu, Laboratory of Tissue Regeneration and Immunology and Department of Periodontics, Beijing Key Laboratory for Tooth Regeneration and Function Reconstruction, Capital Medical University School of Stomatology, Tian Tan Xi Li No. 4, Beijing 100050, China. Tel: +86 10 57099450, Fax: +86 10 57099450, E-mail: liuyi@ ccmu.edu.cn Received 11 February 2014; revised 23 March 2014; accepted 30 March 2014
accelerating bone repair in critical-sized defects in long bones, craniofacial deformities, and alveolar bone defects (Mauney et al, 2005; Ding et al, 2010). In addition, MSCs have the capacity to modulate immune response through the interactions between the immune cells associated with both the innate and adaptive immune systems (Uccelli et al, 2008; Han et al, 2012), which make them capable of reducing hyperactivated immune responses (English, 2013). However, the interactions between MSCs and immune cells do not belong to mesenchyme lineage only. The stem cells from non-mesenchymal origin such as neural stem cells and amnion epithelial cells also have immunomodulatory properties (Ben-Hur, 2008; Castillo-Melendez et al, 2013). Within the bone marrow microenvironment, MSCs maintain the hematopoietic stem cell (HSC) niche, from which all immune cells derive (Frenette et al, 2013). Recently, MSCs have shown immunomodulatory properties including the ability to regulate the proliferation/apoptosis and cytokine secretion of immune cells. They are able to suppress proliferation of several subsets of lymphocytes, including CD4+ helper T lymphocytes (Ths), CD8+ cytotoxic T lymphocytes (CTLs), and natural killer (NK) cells, as well as induce CD3+ T-cell apoptosis (Uccelli et al, 2008; Akiyama et al, 2012; Spaggiari and Moretta, 2013). Also, MSCs can modulate the intensity of an immune response by directly or indirectly promoting the generation of regulatory T cells (Tregs) (Burr et al, 2013). The therapeutic effect of MSCs in graft-vs-host disease has been shown to depend on MSCs initiating cytokine and nitric oxide (NO) signaling to block the activity of migrating T cells (Ren et al, 2008). Allogenic MSC transplantation (MSCT) ameliorates disease phenotypes in a mouse model of systemic lupus erythematosus (SLE) and in human SLE patients via reconstructing the bone marrow osteoblastic niche to enhance regulatory T cells and re-establish immune homeostasis (Sun et al, 2009). Additionally, systemic infusion of MSCs has been demonstrated to dramatically restore alveolar bone healing and regeneration via reestablishment of host immune homeostasis in mouse and minipig models of bisphosphonate-associated osteonecrosis of the jaw (BRONJ) (Kikuiri et al, 2010; Li et al, 2013).
MSC-based regeneration and immune response
Y Su et al
634
MSCs in cell-based bone regeneration Mesenchymal stem cells express low levels of class II major histocompatibility complex (MHC-II) and costimulatory molecules, including CD40, CD80, and, CD86, which renders these cells immunoprivileged, allowing them to be used for tissue repair (Javazon et al, 2004). With respect to repairing bone defects, site-directed delivery of MSCs using a bioscaffold is a practical approach for encouraging the growth of missing tissue. Numerous studies have shown the efficacy of autologous MSC implantation for repairing critical-sized defects in long bones in animal models (Bruder et al, 1998; Schantz et al, 2003; Kraus and Kirker-Head, 2006; Yuan et al, 2007). Based on these findings, human autologous MSCs have been successfully used in the clinical treatment of long bone defects and osteonecrosis of the femoral head (Garg et al, 1993; Sim et al, 1993; Chapman et al, 1997; Connolly, 1998; Kawate et al, 2006). In addition, allogeneic MSCs derived from adipose tissue and dental tissue have the capacity to regenerate bone without the need for immunosuppressive therapy and show similar regenerative effects to those observed in autologous MSC implantation (Ding et al, 2010; Liu et al, 2013a,b). Nevertheless, although a variety of literature reports have shown the efficacy and safety of using MSCs for bone regeneration, no such use has yet been approved by the US Food and Drug Administration (FDA). It is complicated to develop procedures and protocols for isolating and expanding MSCs that reach the high quality required for clinical use. As such, fundamental insights into the therapeutic mechanisms of MSCs, especially those that regulate autologous and allogeneic MSC survival and terminal differentiation locally in wounded tissues, are required before clinical use of MSCs in regenerative therapies can be successful on a larger stage.
Host immune response and MSC-based bone regeneration Factors produced by lymphocytes and other mononuclear cells can dramatically influence bone turnover by altering the delicate balance between bone formation and bone resorption, processes mediated by osteoblasts and osteoclasts, respectively (Raggatt and Partridge, 2010). Moreover, complex crosstalk between implanted MSCs and recipient immune cells can also affect the fate and function of MSCs, thereby playing a critical role in MSC-mediated tissue regeneration. It has been reported that when bone marrow MSCs (BMMSCs) from normal mice were cocultured with lymph node (LN) T cells from C3H or immunocompromised mice, BMMSCs could be induced to apoptosis by activated LN cells from normal mice but not from T-cell-deficient immunocompromised mice in vitro (Yamaza et al, 2008). When implanted into immunocompromised mice subcutaneously using hydroxyapatite– tricalcium phosphate (HA-TCP) particles as a carrier, MSCs are capable of forming bone and hematopoietic marrow elements. However, when implanted into immune competent C57BL/6 mice, MSCs fail to regenerate bone marrow structure (Liu et al, 2011). Oral Diseases
These data collectively suggest that T cells can impair MSCs’ function and inhibit MSC-mediated bone formation. Further studies indicated the distinct roles of T-cell subpopulations in mediating the interaction between immune cells and MSCs in MSC-based osteogenesis. Infusion with Pan T, CD4+, or CD4+CD25 T cells prior to MSCs’ subcutaneous implantation was found to block bone formation in immunocompromised mice. Mechanically, a lack of bone formation was correlated with high levels of TNF-a and IFN-c secreted by host immune cells, which inhibited MSC differentiation and induced cell apoptosis. In contrast, infusion with CD4+CD25+ Treg cells showed no inhibitive effect on bone formation, suggesting that modulating host immune response might facilitate MSC-based tissue regeneration. Indeed, systemic infusion of CD4+CD25+ Treg cells markedly reduced TNF-a and IFN-c production and abolished TNF-a/IFN-c-induced cell apoptosis, thereby dramatically improving MSC-mediated bone regeneration of critical-sized calvarial bone defects. Importantly, site-specific treatment with aspirin also markedly improved bone regeneration in a calvarial injury model (Liu et al, 2011). In addition to T cells, macrophages are described as another critical cell population for the regulation of MSCbased osteogenesis. Macrophages are recognized as a highly heterogeneous cell population, and the function of individual populations may vary significantly in vivo (Gordon and Taylor, 2005; Mosser and Edwards, 2008). It has been reported that the conditioned medium from differentiated macrophages suppressed the osteogenic differentiation of MSCs and osteoprogenitor cells (Chen et al, 2012; Lee et al, 2012). TNF-a and IL-1b secreted by macrophages were indicated to contribute to these inhibitive effects. In MSC-mediated bone regeneration, the unique roles of different macrophage populations remain unclear. Following tissue injury or infection, macrophages usually exhibit an inflammatory phenotype (M1) and secrete mediators to kill invading organisms or influence the polarization of TH1 and TH17 cells. In contrast to M1 macrophages, M2 macrophages exhibit potent anti-inflammatory effects and play critical roles in tissue repair by releasing active molecules and interacting with stem and progenitor cells (Murray and Wynn, 2011). However, the pro-osteogenic effect of activated pro-inflammatory macrophages (M1) on MSCs has been observed in a coculture system (Guihard et al, 2012). This leads to the identification of macrophages as key players in the resolution of inflammation and the repair/regeneration of damaged tissue and hints at the question of whether they can change their phenotype (M1/M2 paradigm) in response to the local microenvironment. Moreover, macrophages commonly serve as antigen-presenting cells (APCs), and as such their function in propagating T-cell response in bone regeneration warrants exploration. In addition, as the most professional APCs, DCs were suggested to be involved in inflammation-induced osteogenesis by acting as osteoclast precursors (Alnaeeli et al, 2007). However, there have been no reports about the function of DC cells on MSC osteogenesis or MSC-mediated bone regeneration so far, which indicates that investigations on DCs are required. Taken together, these pieces of evidence provide insight into the interplay between the immune system and implanted MSCs in a graft–host environment and point to
MSC-based regeneration and immune response
Y Su et al
strategies that may improve therapeutic outcomes for MSCbased tissue engineering. The distinctive roles that various components of the host immune system play in mediating communication between immune cells and stem cells could also be occurring during homeostatic bone metabolism as well as during osteogenesis. The next set of challenges includes deciphering the molecular mechanisms of how various immune cells regulate MSCs and their precise roles in bone regeneration in vivo. This will be important for the eventual manipulation of the immune compartments to improve MSC-based bone regeneration.
Summary and future directions All cell therapies face challenges regarding how to best achieve integration of the transplanted cells into the host environment, which is a complex process involving recruitment, homing, and integration into local niches, connection to the host blood supply, immune compatibility, and graft durability (Daley, 2012). The prospects for more widespread stem cell-based bone engineering depend on understanding the biology of transplanted MSCs in vivo, as well as their relationship with the native microenvironment and host immune system. First, the mechanisms underlying migration of MSCs remain to be clarified, although evidence suggests that both chemokines and their receptors are involved. Studying the mechanisms of MSC migration may allow MSCs delivered systemically to be recruited to the site of damaged bone tissue. Next, the use of MSCs as immunomodulators has been tested in inflammatory and autoimmune diseases. However, the possible roles of MSCs in regulating immune and inflammatory responses when delivered locally to regenerate bone tissue are still unclear and deserve more attention. Moreover, it will be interesting to reveal the individual adaptive immunity profile in pathological bone diseases, which may help to identify markers of conditions with poor bone healing, thus allowing early supportive interventions or targeted therapies. Lastly, so far, investigations of the influence of immune cells on the skeletal system have mainly focused on interactions between immune cells and osteoclasts under physiological or pathological conditions. However, understanding of how the host immune system may interact with implanted MSCs in bone tissue engineering remains primitive. Additionally, beyond T cells, many components of the immune system, such as macrophages, DCs, and NK T cells, remain uncharacterized regarding their effects on MSC-based bone regeneration. Also, there is limited knowledge of how implanted MSCs may in turn influence the immune system. Answers to these and similar questions await further studies and will surely benefit both basic and clinical research in the field of tissue engineering.
Author contributions YY. Su wrote the review. ST. Shi and Y. Liu made substantial contributions to the conception of the paper, revised the article critically for important intellectual content, and were involved in the final approval of the version to be published.
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