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When stem cells meet immunoregulation
International Immunopharmacology 9 (2009) 596–598
Contents lists available at ScienceDirect
International Immunopharmacology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i n t i m p
When stem cells meet immunoregulation Roberta Tasso a, Giuseppina Pennesi b,⁎ a b
Department of Oncology, Biology, and Genetics, University of Genova, Largo Rosanna Benzi 10, 16132, Genova, Italy Stem Cells Laboratory, Advanced Biotechnology Center, Largo Rosanna Benzi 10, 16132, Genova, Italy
a r t i c l e
i n f o
Article history: Received 21 January 2009 Accepted 22 January 2009 Keywords: Stem cells Adult stem cells Immunoregulation Mesenchymal stem cells
a b s t r a c t The clinical use of stem cells to prevent tissue injury or reconstruct damaged organs is constrained by different ethical and biological issues. Whereas the use of adult stem cells isolated from differentiated tissues is advantageous from the ethical point of view, the immune response of a host to implants of either embryonic or adult stem cells remains a critical problem. Embryonic stem cells can be rejected by an immunocompetent recipient as well as some types of adult stem cells. There is, however, a population of adult stem cells able to differentiate into the three mesenchymal lineages, osteocytes, chondrocytes, adipocytes that have the additional capacity of modulating the immune response by the activation of disparate mechanisms, among which the generation of antigen-specific CD4+CD25+FoxP3+ regulatory T lymphocytes. This short review will focus on the immunological properties of embryonic and adult stem cells are, with particular emphasis on the immunomodulatory function of mesenchymal stem cells and their interactions with regulatory T lymphocytes. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The use of stem cells appears to be one of the most promising approaches for the therapy of severely debilitating diseases. Due to their ability to self renew and to differentiate into many cell types, they can be applied to repair or replace damaged cells or tissues. Stem cells can be identified as embryonic stem cells (ES) that can be isolated from the inner cell mass of the embryo and possess the capacity to differentiate into cells of all histological lineages (totipotency), or as adult stem cells that can be isolated from different tissues of an adult individual and that are able to differentiate into a limited range of cell types (pluripotency). If ES could be considered the best choice to regenerate new tissues, there are important ethical and safety issues that need to be addressed before they can be used in the clinic. One of the most crucial point is that they have the potential to be rejected due to the fact that they express low levels of major histocompatibility complex (MHC) class I antigen which can increase after differentiation both in vitro and in vivo [1,2]. They do not express MHC class II molecules [1,2]. Moreover, the absence of MHC class II molecules and low-level expression of MHC class I may also lead to rejection mediated by natural killer (NK) cells; however, several studies have reported that this is not always the case [1]. In addition, the host's immune system can recognize and respond to fetal antigens or other antigens expressed by ES cells [2] as well as ABO blood groups [1].
⁎ Corresponding author. Stem Cells Laboratory, Advanced Biotechnology Center, Largo Rosanna Benzi 10, 16132, Genova, Italy. Tel.: +39 010 5737511; fax: +39 010 5737505. E-mail address: [email protected] (G. Pennesi). 1567-5769/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.intimp.2009.01.014
The immunogenicity of adult stem cells is not uniformly shared by cells derived from different tissues, because of the diversity of their source and the heterogeneity of cell types in each source. Moreover, the differentiation process can modify their immunological properties [3]. In general allogeneic adult stem cells are recognized by the recipient's immune system, but there are some populations of tissue-specific stem cells that acquire an immunoprivileged status as a result of endogenous immunosuppressive properties, i.e. some populations of neural progenitor cells [4] and mesenchymal stem cells (MSC) [5], although only the use of MSC is currently explored in clinical trials for the therapy of graft versus host's disease [6] or other conditions in which the immune system is abnormally activated (see www.clinicatrials.gov and www.controlled-trials.com).
2. Mesenchymal stem cells Mesenchymal stem cells primary reside in the bone marrow but can be found in other tissues, i.e. fat, and are capable of self-renewal and multilineage differentiation into the three mesenchymal lineages, osteocytes, adipocytes, and chondrocytes [7,8]. Heterogeneity of MSC population is reflected by the absence of a unique, specific molecular marker [9]. The landmark of MSC immunophenotype is considered the absence of CD45 [10]. MSC derived from different tissues express developmental markers of mesenchymal, endothelial, and hematopoietic tissues [10–13], but they also produce molecules involved in the regulation of the immune system, such as MHC class I antigens, molecules of the stimulatory or inhibitory pathways of immune response, and an array of different cytokines [14–16]. Through these molecules, MSC can regulate the immune response.
R. Tasso, G. Pennesi / International Immunopharmacology 9 (2009) 596–598
3. MSC-mediated immunoregulation When injected in an immunocompetent recipient, human or murine allogeneic MSC are not actively rejected and do not elicit a proliferative response when used as stimulators in an in vitro mixed lymphocyte reaction (MLR) [13,14,17–22]. This is not a passive phenomenon, but is the result of the combined effect of soluble factors and of mechanisms mediated by cell-to-cell contact that act on any cell components of the immune system [23]. In 2002 papers were published clearly showing that human MSC could inhibit proliferation of T cells that had either been cultured in a MLR or stimulated by polyclonal activators [24], and that upon in vivo infusion, they could prolong skin engraftment in nonhuman primates [25]. It was further demonstrated that the anti-proliferative activity of murine MSC was also exerted on antigen-specific responses [26] and could have been mediated by allogeneic MSC (absence of MHC restriction) [19,26]. It appears that MSC-mediated immunosuppression is not due to the induction of apoptosis in proliferating T lymphocytes [24,27,28], but that MSC-targeted cells are blocked in the G0 phase of the cell cycle [29]. Inhibition of T cell proliferation resulted in the decreased production of Th1 cytokines by effector T cells [22,26,27], that are rendered also unresponsive to cytokine stimulation by downregulation of the intracellular signaling [14,30]. Inhibition of T cell proliferation by MSC appears to be subsequent both to cell-to-cell interaction and to the release of soluble factors, and it is conceivable that discrepant findings reported in the literature reflect differences in the experimental conditions used [14,18,24,27,31]. The PD-1 pathway was described to be involved in MSC-mediated immunomodulation occurring by cell-to-cell contact [14], while TGF-β1 and hepatocyte growth factor (HGF) [24], indoleamine 2,3-dioxygenase (IDO) [32], and prostaglandin E2 (PGE-2) [33] are molecules responsible for the immunomodulation by humoral factors. MSC inhibit also B lymphocyte proliferation; a clue to the underlying molecular mechanism(s) was the observation that B cell inhibition was in part due to the physical contact between MSC and B cells, partially mediated by PD-1/PD-L1,-L2 [14], and in part to soluble factors released by MSC in culture supernatants [14,29]. However, culture conditions might drastically change the immunomodulatory effect of MSC on activation of B lymphocytes [28,34]. A few studies concordantly demonstrate that interactions between human MSC inhibit IL-2- or IL-15-driven NK proliferation [28,35,36], but the effect of MSC on NK cell-mediated cytotoxicity is more controversial, likely due to differences in the experimental approaches [28,35]. Similarly to what was observed for T cells, soluble factors such as TGF-β1 and PGE-2 play a role in the MSC-mediated suppression of NK cell proliferation [35]. On the other hand, MSC were highly susceptible to lysis by autologous or allogeneic IL-2-activated NK cells [35–37]. The effect of MSC alters the function of dendritic cells inhibiting the maturation of monocyte-derived myeloid dendritic cells (DC) by downregulation of MHC and costimulatory molecules, and the production of IL-12 upon Toll-like receptor (TLR)-mediated DC activation [38–41]. Following interaction with MSC, DC of myeloid origin produce a decreased amount of TNF-α, while plasmacytoid DC increase the production of IL-10 [33]. This effect, in turn, lead to decreased IFN-γ production by Th1 cells, increased IL-4 secretion by Th2 cells, and an increased number of regulatory T cells [33,39]. The mechanism of MSC-induced inhibition of DC differentiation and function appears to be mediated by soluble factors, such as PGE-2, released upon cell-to-cell contact [33,41]. Similarly to what was observed in T cells, the cell cycle in DC is arrested in the G0/G1 phase, upon interaction with MSC [42]. It is important to notice that the differentiation process modify the ability of MSC to suppress the immune response. Although in vitro experiments show that MSC retain the immunomodulatory property when induced to differentiate into any of the mesenchymal lineages
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[43], but there are conflicting results on whether this capacity remains during the in vivo differentiation process. In fact allogeneic MSC loaded onto an osteoinductive biomaterial for tissue engineering purposes are acutely rejected by a non-immunosuppressed mouse [3], while this is not the case when a similar experimental approach is performed in a canine model [44]. 4. MSC and regulatory T lymphocytes In the perspective of using MSC in the clinical practice is of extreme importance the finding that human and murine MSC can induce the generation of CD4 +CD25+ T lymphocytes with regulatory functions (Tregs) in both in vitro and in vivo settings [3,22,39,45]. We have demonstrated that the generation of antigen-specific Treg contributes to the beneficial effect of MSC in the therapy of collagen-induced arthritis, a mouse model for human rheumatoid arthritis [22]. In this model the combined action of MSC and MSC-generated Treg suppress specifically the adaptive immune response to the immunizing antigen blocking the process leading to the formation of permanent tissue damage [22]. As any potent therapy has its downside effects, we cannot avoid considering that the immunosuppressant milieu created by MSC might favor tumor growth [46]. In a new mouse model of sarcoma experimentally induced by subcutaneously implanting porous ceramic seeded with murine MSC, we find that host-derived sarcomas developed when we implanted MSC/bioscaffold constructs into syngeneic recipients and that CD4+CD25+ Tregs were present with a significantly higher incidence in the spleen of tumor-bearing mice. These cells significantly blocked the proliferation of host's splenocytes challenged with syngeneic tumor cells, but not the proliferation of the same responder cells to a different stimulus. In this case, the expansion of specific Tregs impairs the activation of T lymphocytes against sarcoma cells contributing to tumor immune-escape [3]. The mechanisms by which MSC can generate adaptive Treg are not yet unraveled. An important step in this direction is the finding that soluble non-classical MHC class I molecules are involved in this process [45]. This has been clearly demonstrated in vitro using human MSC, but not yet confirmed in in vivo settings. It is interesting to notice that MSC and Treg share common mechanisms of immunosuppression, such as the expression of inhibitory molecules [14,47], or the release of inhibitory cytokines [24,48–50]. Thus we can conclude that MSC are part of a broader family of cells derived from different histological lineages but sharing the same immunomodulatory properties, and that in vivo the immunosuppressant effect could be the final outcome of the concerted action of different cells. References [1] Batten P, Rosenthal NA, Yacoub MH. Immune response to stem cells and strategies to induce tolerance. Philos Trans R Soc Lond B Biol Sci 2007;362:1343–56. [2] Chidgey AP, Layton D, Trounson A, Boyd RL. Tolerance strategies for stem-cellbased therapies. Nature 2008;453:330–7. [3] Tasso R, Augello A, Carida M, Postiglione F, Tibiletti MG, Bernasconi B, et al. Development of sarcomas in mice implanted with mesenchymal stem cells seeded onto bioscaffolds. Carcinogenesis 2009;30:150–7, doi:10.1093/carcin/bgn234. [4] Hori J, Ng T, Shatos M, Klassen H, Streilein J, Young MG, et al. Neural progenitor cells lack immunogenicity and resist destruction as allografts. Stem Cells 2003;21:405–16. [5] Uccelli A, Moretta L, Pistoia V. Mesenchymal stem cells in health and disease. Nat Rev Immunol 2008, doi:10.1038/nri2395. [6] Le Blanc K, Rasmusson I, Sundberg B, Gotherstrom C, Hassan M, Uzunel M, et al. Treatment of severe acute graft-versus-host disease with third party haploidentical mesenchymal stem cells. Lancet 2004;363:1439–41. [7] Bianco P, Riminucci M, Gronthos S, Robey P. Bone marrow stromal stem cells: nature, biology, and potential applications. Stem Cells 2001;19:180–92. [8] Grove JE, Bruscia E, Krause DS. Plasticity of bone marrow-derived stem cells. Stem Cells 2004;22:487–500. [9] Bianco P, Robey PG, Simmons PJ. Mesenchymal stem cells: revisiting history, concepts, and assays. Cell Stem Cell 2008;2:313–9.
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[10] Tyndall A, Walker UA, Cope A, Dazzi F, De Bari C, Fibbe W, et al. Immunomodulatory properties of mesenchymal stem cells: a review based on an interdisciplinary meeting held at the Kennedy Institute of Rheumatology Division, London, UK, 31 October 2005. Arthritis Res Ther 2007;9:301. [11] Kuznetsov S, Mankani M, Gronthos S, Satomura K, Bianco P, Robey PG. Circulating skeletal stem cells. J Cell Biol 2001;153:1133–9. [12] Jiang Y, Jahagirdar B, Reinhardt R, Schwartz R, Keenek C, Ortiz-Gonzalezk XR, et al. Pluripotency of mesenchymal stem cells derived from adult marrow. Nature 2002;418:41–9. [13] Potian JA, Aviv H, Ponzio NM, Harrison JS, Rameshwar P. Veto-like activity of mesenchymal stem cells: functional discrimination between cellular responses to alloantigens and recall antigens. J Immunol 2003;171:3426–34. [14] Augello A, Tasso R, Negrini SM, Amateis A, Indiveri F, Cancedda R, et al. Bone marrow mesenchymal progenitor cells inhibit lymphocyte proliferation by activation of the programmed death 1 pathway. Eur J Immunol 2005;35:1482–90. [15] Liu CH, Hwang SM. Cytokine interactions in mesenchymal stem cells from cord blood. Cytokine 2005;32:270–9. [16] Stagg J. Immune regulation by mesenchymal stem cells: two sides to the coin. Tissue Antigens 2007;69:1–9. [17] Horwitz E, Prockop D, Fitzpatrick L, Koo W, Gordon P, Neel M, et al. Transplantability and therapeutic effects of bone marrow-derived mesenchymal cells in children with osteogenesis imperfecta. Nat Med 1999;5:309–13. [18] Tse WT, Pendleton JD, Beyer WM, Egalka MC, Guinan EC. Suppression of allogeneic T-cell proliferation by human marrow stromal cells: implications in transplantation. Transplantation 2003;75:389–97. [19] Le Blanc K, Tammik L, Sundberg B, Haynesworth SE, Ringden O. Mesenchymal stem cells inhibit and stimulate mixed lymphocyte cultures and mitogenic responses independently of the major histocompatibility complex. Scand J Immunol 2003;57:11–20. [20] Maitra B, Szekely E, Gjini K, Laughlin M, Dennis J, Haynesworth SE, et al. Human mesenchymal stem cells support unrelated donor hematopoietic stem cells and suppress T-cell activation. Bone Marrow Transplant 2004;33:597–604. [21] Klyushnenkova E, Mosca JD, Zernetkina V, Majumdar MK, Beggs KJ, Simonetti DW, et al. T cell responses to allogeneic human mesenchymal stem cells: immunogenicity, tolerance, and suppression. J Biomed Sci 2005;12:47–57. [22] Augello A, Tasso R, Negrini SM, Cancedda R, Pennesi G. Cell therapy using allogeneic bone marrow mesenchymal stem cells prevents tissue damage in collagen-induced arthritis. Arthritis Rheum 2007;56:1175–86. [23] Bianco P, Gehron Robey P, Pennesi G, Cancedda R. Cell source. In: van Blitterswijk C, Thomsen P, Lindahl A, Hubbell J, Williams DF, Cancedda R, de Bruijn JD, Sohier J, editors. Tissue Engineering. Oxford, UK: Elsevier Inc.; 2008. p. 279–306. [24] Di Nicola M, Carlo-Stella C, Magni M, Milanesi M, Longoni P, et al. Human bone marrow stromal cells suppress T-lymphocyte proliferation induced by cellular or nonspecific mitogenic stimuli. Blood 2002;99:3838–43. [25] Bartholomew A, Sturgeon C, Siatskas M, Ferrer K, McIntosh K, Patil S, et al. Mesenchymal stem cells suppress lymphocyte proliferation in vitro and prolong skin graft survival in vivo. Exp Hematol 2002;30:42–8. [26] Krampera M, Glennie S, Dyson J, Scott D, Laylor R, Simpson E, et al. Bone marrow mesenchymal stem cells inhibit the response of naive and memory antigenspecific T cells to their cognate peptide. Blood 2003;101:3722–9. [27] Zappia E, Casazza S, Pedemonte E, Benvenuto F, Bonanni I, Gerdoni E, et al. Mesenchymal stem cells ameliorate experimental autoimmune encephalomyelitis inducing T-cell anergy. Blood 2005;106:1755–61. [28] Krampera M, Cosmi L, Angeli R, Pasini A, Liotta F, Andreini A, et al. Role for interferon-gamma in the immunomodulatory activity of human bone marrow mesenchymal stem cells. Stem Cells 2006;24:386–98. [29] Glennie S, Soeiro I, Dyson PJ, Lam EW, Dazzi F. Bone marrow mesenchymal stem cells induce division arrest energy of activated T cells. Blood 2005;105:2821–7. [30] Sato K, Ozaki K, Oh I, Meguro A, Hatanaka K, Nagai T, et al. Nitric oxide plays a critical role in suppression of T-cell proliferation by mesenchymal stem cells. Blood 2007;109:228–34.
[31] Rasmusson I, Ringden O, Sundberg B, Le Blanc K. Mesenchymal stem cells inhibit the formation of cytotoxic T lymphocytes, but not activated cytotoxic T lymphocytes or natural killer cells. Transplantation 2003;76:1208–13. [32] Meisel R, Zibert A, Laryea M, Gobel U, Daubener W, Dilloo D. Human bone marrow stromal cells inhibit allogeneic T-cell responses by indoleamine 2,3-dioxygenasemediated tryptophan degradation. Blood 2004;103:4619–21. [33] Aggarwal S, Pittenger MF. Human mesenchymal stem cells modulate allogeneic immune cell responses. Blood 2005;105:1815–22. [34] Corcione A, Benvenuto F, Ferretti E, Giunti D, Cappiello V, Cazzanti F, et al. Human mesenchymal stem cells modulate B-cell functions. Blood 2006;107:367–72. [35] Sotiropoulou PA, Perez SA, Gritzapis AD, Baxevanis CN, Papamichail M. Interactions between human mesenchymal stem cells and natural killer cells. Stem Cells 2006;24:74–85. [36] Spaggiari GM, Capobianco A, Becchetti S, Mingari MC, Moretta L. Mesenchymal stem cell-natural killer cell interactions: evidence that activated NK cells are capable of killing MSCs, whereas MSCs can inhibit IL-2-induced NK-cell proliferation. Blood 2006;107:1484–90. [37] Poggi A, Prevosto C, Massaro AM, Negrini S, Urbani S, Pierri I, et al. Interaction between human NK cells and bone marrow stromal cells induces NK cell triggering: role of NKp30 and NKG2D receptors. J Immunol 2005;175:6352–60. [38] Zhang W, Ge W, Li C, You S, Liao L, Han Q, et al. Effects of mesenchymal stem cells on differentiation, maturation, and function of human monocyte-derived dendritic cells. Stem Cells Dev 2004;13:263–71. [39] Maccario R, Podesta M, Moretta A, Cometa A, Comoli P, Montagna D, et al. Interaction of human mesenchymal stem cells with cells involved in alloantigenspecific immune response favors the differentiation of CD4+ T-cell subsets expressing a regulatory/suppressive phenotype. Haematologica 2005;90:516–25. [40] Beyth S, Borovsky Z, Mevorach D, Liebergall M, Gazit Z, Aslan H, et al. Human mesenchymal stem cells alter antigen-presenting cell maturation and induce Tcell unresponsiveness. Blood 2005;105:2214–9. [41] Jiang XX, Zhang Y, Liu B, Zhang SX, Wu Y, Yu XD, et al. Human mesenchymal stem cells inhibit differentiation and function of monocyte-derived dendritic cells. Blood 2005;105:4120–6. [42] Ramasamy R, Fazekasova H, Lam EW, Soeiro I, Lombardi G, Dazzi F. Mesenchymal stem cells inhibit dendritic cell differentiation and function by preventing entry into the cell cycle. Transplantation 2007;83:71–6. [43] Le Blanc K, Tammik C, Rosendahl K, Zetterberg E, Ringden O. HLA expression and immunologic properties of differentiated and undifferentiated mesenchymal stem cells. Exp Hematol 2003;31:890–6. [44] Arinzeh TL, Peter SJ, Archambault MP, van den Bos C, Gordon S, Kraus K, et al. Allogeneic mesenchymal stem cells regenerate bone in a critical-sized canine segmental defect. J Bone Joint Surg Am 2003;85-A:1927–35. [45] Selmani Z, Naji A, Zidi I, Favier B, Gaiffe E, Obert L, et al. Human leukocyte antigenG5 secretion by human mesenchymal stem cells is required to suppress T lymphocyte and natural killer function and to induce CD4+CD25highFOXP3+ regulatory T cells. Stem Cells 2008;26:212–22. [46] Djouad F, Bony C, Apparailly F, Louis-Plence P, Jorgensen C, Noel D. Earlier onset of syngeneic tumors in the presence of mesenchymal stem cells. Transplantation 2006;82:1060–6. [47] Paust S, Lu L, McCarty N, Cantor H. Engagement of B7 on effector Tcells by regulatory T cells prevents autoimmune disease. Proc Natl Acad Sci U S A 2004;101:10398–403. [48] Nasef A, Chapel A, Mazurier C, Bouchet S, Lopez M, Mathieu N, et al. Identification of IL-10 and TGF-beta transcripts involved in the inhibition of T-lymphocyte proliferation during cell contact with human mesenchymal stem cells. Gene Expr 2007;13:217–26. [49] Powrie F, Carlino J, Leach MW, Mauze S, Coffman RL. A critical role for transforming growth factor-beta but not interleukin 4 in the suppression of T helper type 1mediated colitis by CD45RB(low) CD4+ T cells. J Exp Med 1996;183:2669–74. [50] Asseman C, Mauze S, Leach MW, Coffman RL, Powrie F. An essential role for interleukin 10 in the function of regulatory T cells that inhibit intestinal inflammation. J Exp Med 1999;190:995–1004.
Contents lists available at ScienceDirect
International Immunopharmacology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i n t i m p
When stem cells meet immunoregulation Roberta Tasso a, Giuseppina Pennesi b,⁎ a b
Department of Oncology, Biology, and Genetics, University of Genova, Largo Rosanna Benzi 10, 16132, Genova, Italy Stem Cells Laboratory, Advanced Biotechnology Center, Largo Rosanna Benzi 10, 16132, Genova, Italy
a r t i c l e
i n f o
Article history: Received 21 January 2009 Accepted 22 January 2009 Keywords: Stem cells Adult stem cells Immunoregulation Mesenchymal stem cells
a b s t r a c t The clinical use of stem cells to prevent tissue injury or reconstruct damaged organs is constrained by different ethical and biological issues. Whereas the use of adult stem cells isolated from differentiated tissues is advantageous from the ethical point of view, the immune response of a host to implants of either embryonic or adult stem cells remains a critical problem. Embryonic stem cells can be rejected by an immunocompetent recipient as well as some types of adult stem cells. There is, however, a population of adult stem cells able to differentiate into the three mesenchymal lineages, osteocytes, chondrocytes, adipocytes that have the additional capacity of modulating the immune response by the activation of disparate mechanisms, among which the generation of antigen-specific CD4+CD25+FoxP3+ regulatory T lymphocytes. This short review will focus on the immunological properties of embryonic and adult stem cells are, with particular emphasis on the immunomodulatory function of mesenchymal stem cells and their interactions with regulatory T lymphocytes. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The use of stem cells appears to be one of the most promising approaches for the therapy of severely debilitating diseases. Due to their ability to self renew and to differentiate into many cell types, they can be applied to repair or replace damaged cells or tissues. Stem cells can be identified as embryonic stem cells (ES) that can be isolated from the inner cell mass of the embryo and possess the capacity to differentiate into cells of all histological lineages (totipotency), or as adult stem cells that can be isolated from different tissues of an adult individual and that are able to differentiate into a limited range of cell types (pluripotency). If ES could be considered the best choice to regenerate new tissues, there are important ethical and safety issues that need to be addressed before they can be used in the clinic. One of the most crucial point is that they have the potential to be rejected due to the fact that they express low levels of major histocompatibility complex (MHC) class I antigen which can increase after differentiation both in vitro and in vivo [1,2]. They do not express MHC class II molecules [1,2]. Moreover, the absence of MHC class II molecules and low-level expression of MHC class I may also lead to rejection mediated by natural killer (NK) cells; however, several studies have reported that this is not always the case [1]. In addition, the host's immune system can recognize and respond to fetal antigens or other antigens expressed by ES cells [2] as well as ABO blood groups [1].
⁎ Corresponding author. Stem Cells Laboratory, Advanced Biotechnology Center, Largo Rosanna Benzi 10, 16132, Genova, Italy. Tel.: +39 010 5737511; fax: +39 010 5737505. E-mail address: [email protected] (G. Pennesi). 1567-5769/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.intimp.2009.01.014
The immunogenicity of adult stem cells is not uniformly shared by cells derived from different tissues, because of the diversity of their source and the heterogeneity of cell types in each source. Moreover, the differentiation process can modify their immunological properties [3]. In general allogeneic adult stem cells are recognized by the recipient's immune system, but there are some populations of tissue-specific stem cells that acquire an immunoprivileged status as a result of endogenous immunosuppressive properties, i.e. some populations of neural progenitor cells [4] and mesenchymal stem cells (MSC) [5], although only the use of MSC is currently explored in clinical trials for the therapy of graft versus host's disease [6] or other conditions in which the immune system is abnormally activated (see www.clinicatrials.gov and www.controlled-trials.com).
2. Mesenchymal stem cells Mesenchymal stem cells primary reside in the bone marrow but can be found in other tissues, i.e. fat, and are capable of self-renewal and multilineage differentiation into the three mesenchymal lineages, osteocytes, adipocytes, and chondrocytes [7,8]. Heterogeneity of MSC population is reflected by the absence of a unique, specific molecular marker [9]. The landmark of MSC immunophenotype is considered the absence of CD45 [10]. MSC derived from different tissues express developmental markers of mesenchymal, endothelial, and hematopoietic tissues [10–13], but they also produce molecules involved in the regulation of the immune system, such as MHC class I antigens, molecules of the stimulatory or inhibitory pathways of immune response, and an array of different cytokines [14–16]. Through these molecules, MSC can regulate the immune response.
R. Tasso, G. Pennesi / International Immunopharmacology 9 (2009) 596–598
3. MSC-mediated immunoregulation When injected in an immunocompetent recipient, human or murine allogeneic MSC are not actively rejected and do not elicit a proliferative response when used as stimulators in an in vitro mixed lymphocyte reaction (MLR) [13,14,17–22]. This is not a passive phenomenon, but is the result of the combined effect of soluble factors and of mechanisms mediated by cell-to-cell contact that act on any cell components of the immune system [23]. In 2002 papers were published clearly showing that human MSC could inhibit proliferation of T cells that had either been cultured in a MLR or stimulated by polyclonal activators [24], and that upon in vivo infusion, they could prolong skin engraftment in nonhuman primates [25]. It was further demonstrated that the anti-proliferative activity of murine MSC was also exerted on antigen-specific responses [26] and could have been mediated by allogeneic MSC (absence of MHC restriction) [19,26]. It appears that MSC-mediated immunosuppression is not due to the induction of apoptosis in proliferating T lymphocytes [24,27,28], but that MSC-targeted cells are blocked in the G0 phase of the cell cycle [29]. Inhibition of T cell proliferation resulted in the decreased production of Th1 cytokines by effector T cells [22,26,27], that are rendered also unresponsive to cytokine stimulation by downregulation of the intracellular signaling [14,30]. Inhibition of T cell proliferation by MSC appears to be subsequent both to cell-to-cell interaction and to the release of soluble factors, and it is conceivable that discrepant findings reported in the literature reflect differences in the experimental conditions used [14,18,24,27,31]. The PD-1 pathway was described to be involved in MSC-mediated immunomodulation occurring by cell-to-cell contact [14], while TGF-β1 and hepatocyte growth factor (HGF) [24], indoleamine 2,3-dioxygenase (IDO) [32], and prostaglandin E2 (PGE-2) [33] are molecules responsible for the immunomodulation by humoral factors. MSC inhibit also B lymphocyte proliferation; a clue to the underlying molecular mechanism(s) was the observation that B cell inhibition was in part due to the physical contact between MSC and B cells, partially mediated by PD-1/PD-L1,-L2 [14], and in part to soluble factors released by MSC in culture supernatants [14,29]. However, culture conditions might drastically change the immunomodulatory effect of MSC on activation of B lymphocytes [28,34]. A few studies concordantly demonstrate that interactions between human MSC inhibit IL-2- or IL-15-driven NK proliferation [28,35,36], but the effect of MSC on NK cell-mediated cytotoxicity is more controversial, likely due to differences in the experimental approaches [28,35]. Similarly to what was observed for T cells, soluble factors such as TGF-β1 and PGE-2 play a role in the MSC-mediated suppression of NK cell proliferation [35]. On the other hand, MSC were highly susceptible to lysis by autologous or allogeneic IL-2-activated NK cells [35–37]. The effect of MSC alters the function of dendritic cells inhibiting the maturation of monocyte-derived myeloid dendritic cells (DC) by downregulation of MHC and costimulatory molecules, and the production of IL-12 upon Toll-like receptor (TLR)-mediated DC activation [38–41]. Following interaction with MSC, DC of myeloid origin produce a decreased amount of TNF-α, while plasmacytoid DC increase the production of IL-10 [33]. This effect, in turn, lead to decreased IFN-γ production by Th1 cells, increased IL-4 secretion by Th2 cells, and an increased number of regulatory T cells [33,39]. The mechanism of MSC-induced inhibition of DC differentiation and function appears to be mediated by soluble factors, such as PGE-2, released upon cell-to-cell contact [33,41]. Similarly to what was observed in T cells, the cell cycle in DC is arrested in the G0/G1 phase, upon interaction with MSC [42]. It is important to notice that the differentiation process modify the ability of MSC to suppress the immune response. Although in vitro experiments show that MSC retain the immunomodulatory property when induced to differentiate into any of the mesenchymal lineages
597
[43], but there are conflicting results on whether this capacity remains during the in vivo differentiation process. In fact allogeneic MSC loaded onto an osteoinductive biomaterial for tissue engineering purposes are acutely rejected by a non-immunosuppressed mouse [3], while this is not the case when a similar experimental approach is performed in a canine model [44]. 4. MSC and regulatory T lymphocytes In the perspective of using MSC in the clinical practice is of extreme importance the finding that human and murine MSC can induce the generation of CD4 +CD25+ T lymphocytes with regulatory functions (Tregs) in both in vitro and in vivo settings [3,22,39,45]. We have demonstrated that the generation of antigen-specific Treg contributes to the beneficial effect of MSC in the therapy of collagen-induced arthritis, a mouse model for human rheumatoid arthritis [22]. In this model the combined action of MSC and MSC-generated Treg suppress specifically the adaptive immune response to the immunizing antigen blocking the process leading to the formation of permanent tissue damage [22]. As any potent therapy has its downside effects, we cannot avoid considering that the immunosuppressant milieu created by MSC might favor tumor growth [46]. In a new mouse model of sarcoma experimentally induced by subcutaneously implanting porous ceramic seeded with murine MSC, we find that host-derived sarcomas developed when we implanted MSC/bioscaffold constructs into syngeneic recipients and that CD4+CD25+ Tregs were present with a significantly higher incidence in the spleen of tumor-bearing mice. These cells significantly blocked the proliferation of host's splenocytes challenged with syngeneic tumor cells, but not the proliferation of the same responder cells to a different stimulus. In this case, the expansion of specific Tregs impairs the activation of T lymphocytes against sarcoma cells contributing to tumor immune-escape [3]. The mechanisms by which MSC can generate adaptive Treg are not yet unraveled. An important step in this direction is the finding that soluble non-classical MHC class I molecules are involved in this process [45]. This has been clearly demonstrated in vitro using human MSC, but not yet confirmed in in vivo settings. It is interesting to notice that MSC and Treg share common mechanisms of immunosuppression, such as the expression of inhibitory molecules [14,47], or the release of inhibitory cytokines [24,48–50]. Thus we can conclude that MSC are part of a broader family of cells derived from different histological lineages but sharing the same immunomodulatory properties, and that in vivo the immunosuppressant effect could be the final outcome of the concerted action of different cells. References [1] Batten P, Rosenthal NA, Yacoub MH. Immune response to stem cells and strategies to induce tolerance. Philos Trans R Soc Lond B Biol Sci 2007;362:1343–56. [2] Chidgey AP, Layton D, Trounson A, Boyd RL. Tolerance strategies for stem-cellbased therapies. Nature 2008;453:330–7. [3] Tasso R, Augello A, Carida M, Postiglione F, Tibiletti MG, Bernasconi B, et al. Development of sarcomas in mice implanted with mesenchymal stem cells seeded onto bioscaffolds. Carcinogenesis 2009;30:150–7, doi:10.1093/carcin/bgn234. [4] Hori J, Ng T, Shatos M, Klassen H, Streilein J, Young MG, et al. Neural progenitor cells lack immunogenicity and resist destruction as allografts. Stem Cells 2003;21:405–16. [5] Uccelli A, Moretta L, Pistoia V. Mesenchymal stem cells in health and disease. Nat Rev Immunol 2008, doi:10.1038/nri2395. [6] Le Blanc K, Rasmusson I, Sundberg B, Gotherstrom C, Hassan M, Uzunel M, et al. Treatment of severe acute graft-versus-host disease with third party haploidentical mesenchymal stem cells. Lancet 2004;363:1439–41. [7] Bianco P, Riminucci M, Gronthos S, Robey P. Bone marrow stromal stem cells: nature, biology, and potential applications. Stem Cells 2001;19:180–92. [8] Grove JE, Bruscia E, Krause DS. Plasticity of bone marrow-derived stem cells. Stem Cells 2004;22:487–500. [9] Bianco P, Robey PG, Simmons PJ. Mesenchymal stem cells: revisiting history, concepts, and assays. Cell Stem Cell 2008;2:313–9.
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