IN ALLOIMMUNITY AND TRANSPLANTATION Myeloid-Derived Suppressor Cells: Mechanisms of Action and recent advances in their role in transplant tolerance Nahzli Dilek, Romain Vuillefroy de Silly, Gilles Blancho and Bernard Vanhove Journal Name: Frontiers in Immunology ISSN: 1664-3224 Article type: Review Article Received on: 27 Mar 2012 Accepted on: 30 Jun 2012 Provisional PDF published on: 30 Jun 2012 Frontiers website link: www.frontiersin.org Citation: Dilek N, Vuillefroy de silly R, Blancho G and Vanhove B(2012) Myeloid-Derived Suppressor Cells: Mechanisms of Action and recent advances in their role in transplant tolerance. 3:208. doi:10.3389/fimmu.2012.00208 Article URL: http://www.frontiersin.org/Journal/Abstract.aspx?s=1251& name=alloimmunity%20and%20transplantation&ART_DOI=10.3389 /fimmu.2012.00208 (If clicking on the link doesn't work, try copying and pasting it into your browser.) Copyright statement: © 2012 Dilek, Vuillefroy de silly, Blancho and Vanhove. This is an open-access article distributed under the terms of the Creative Commons Attribution Non Commercial License, which permits non-commercial use, distribution, and reproduction in other forums, provided the original authors and source are credited. This Provisional PDF corresponds to the article as it appeared upon acceptance, after rigorous peer-review. Fully formatted PDF and full text (HTML) versions will be made available soon. Myeloid-Derived Suppressor Cells: Mechanisms of Action and recent advances in their role in transplant tolerance Nahzli DILEK *, §, Romain VUILLEFROY DE SILLY *, ‡, Gilles BLANCHO *, †, ‡ , and Bernard VANHOVE *, †, ‡ * INSERM, UMR-S 1064, Nantes, F-44093 France ; † CHU de Nantes, ITUN, Nantes, F-44093 France ; ‡ Université de Nantes, Faculté de Médecine, Nantes, F-44093 France. § Effimune S.A.S, Nantes, France. Abstract word count: 66 Text word count: 30102 2 figures 1 table 92 references 1 Abstract Myeloid derived suppressor cells (MDSC) are a heterogeneous population of immature hematopoietic precursors known to suppress immune responses in infection, chronic inflammation, cancer and autoimmunity. In this paper we review recent findings detailing their mode of action and discuss recent reports that suggest that MDSC are also expanded during transplantation and that modulation of MDSC can participate in preventing graft rejection as well as graft-versus-host disease. 2 Introduction In the 80’s, a new cell population known as natural suppressor cells, distinct from T and NK cells, was described in tumor-bearing mice (1, 2). Generated in bone marrow under the influence of soluble factors produced by tumors, these cells derive from a mixed and heterogeneous population of myeloid cells found at different differentiation stages. They have been defined as myeloid suppressive cells (MSC) because of their ability to suppress immune responses (3-5). To minimize the confusion with existing mesenchymal stem cells (MSC), Gabrilovich proposed to name these cells “Myeloid-Derived Suppressor Cells” (MDSC) (6). In mice, MDSC accumulate in the lymphatic organs (7) after the development of various diseases such as infections (8-10), chronic inflammation, tumor growth, graft versus host disease (GVHD) (11) and immune stress due to superantigen stimulation (staphylococcal endotoxin A, SEA) (12). In mice, MDSC are characterized by the expression of myeloid cell markers, such as GR-1 (Ly6G and Ly6C) and CD11b (3), as well as immature cell markers, such as CD31 (5). Two subsets of MDSC were also described: monocytic MDSC, which have CD11b +Ly6G -Ly6C High phenotype, and granulocytic MDSC, which have CD11b +Ly6G +Ly6C +/- phenotype (13, 14). Other markers correlated to their suppressive function have been identified as CD80 (9), CD115 (15) or CD16 (10). They also express MHC class I molecules, but not MHC class II molecules (16). In humans, MDSC accumulate in cancer patients (17, 18) and are defined by the expression of immature markers such as CD34, CD33, CD15 and CD16. Moreover, CD14+HLA-DR -/low MDSC have been recently characterized in cancer patients (19), suggesting that as is the case with mice, various human tumors induce different MDSC subsets. In the presence of appropriate growth factors (IL-4 + GM-CSF or TNF-α + GM-CSF), MDSC can differentiate into efficient antigen-presenting 3 cells (APC), either DC or macrophages by increasing the expression of costimulatory molecules and MHC class II molecules (5, 20). Control of MDSC by cytokines (Figure 2) Many studies have shown that inflammatory environments induce the production and the accumulation of MDSC able to block CD4 and CD8 mediated immune responses and lead to cancer development. Indeed, tumor cells secrete a large variety of cytokines that allow the recruitment of MDSC in lymphoid organs or peripheral blood and direct their differentiation into suppressor cells (21). That global inflammation controls MDSC recruitment is best illustrated by observations showing that the reduction of inflammatory potential in IL-1R-/- mice allows delaying MDSC accumulation and then reducing tumor and metastatic growth (22). One key factor controlling MDSC expansion and the development of cancer is peroxisome proliferator-activated receptor-gamma (PPAR γ) (23). Also VEGF (Vascular Endothelial Growth Factor) (24), M-CSF (Macrophage Colony-Stimulating Factor) (21) or IL-6 (22) are required for MDSC expansion (25). Indeed, they prevent MDSC differentiation into mature DC through a mechanism involving the activation of STAT3 signaling pathway (26, 27). By contrast, in a mouse cancer model, the use of siRNA blocking expression of SCF (Stem Cell Factor) or blockade of SCF/c-kit receptor interaction allowed to reduce MDSC expansion and restore T lymphocyte proliferation, thus resulting in tumor rejection (28). GM- CSF (Granulocyte Macrophage Colony-Stimulating Factor) also induces MDSC expansion which suppresses tumor-specific CD8 + T cell response. However, in combination with IL-4, GM-CSF induces MDSC differentiation into mature DC capable to activate immune responses (4, 29). PGE2 also, as well as other COX2 activators as lipopolysaccharide, IL- 1beta, and IFN-γ, by inducing expression of COX2 in monocytes, blocks their differentiation 4 into mature DCs and induces a typical MDSC phenotype (30, 31). In addition IFN-γ produced by T cells in tumor-bearing mice was shown to make MDSC responsive to IL-13 and suppressive (32). Another important factor is Hsp72 that was shown essential for expansion, activation and suppressive function of murine and human MDSC, also through STAT3 signaling pathway (33). Another study demonstrated that injection of Flt3L (fms-like tyrosine kinase 3 ligand) encoding adenoviruses in tumor-bearing mice resulted in the increase of spleen DC, T, B lymphocytes and NK cells but also of MDSC which dominated and blocked anti-tumor activity of effector cells (34). Finally, it was recently shown that the complement anaphylatoxin C5a increases tumor infiltrating MDCS and gives them a suppressive activity through reactive oxygen species (ROS) and reactive nitrogen species (RNS) regulation (35). Several tumor-derived factors such as TGF-β, IL-3, IL-6, IL-10, platelet-derived growth factors and GM-CSF could also induce ROS production by MDSC (36). Beside soluble factors, MDSC are controlled by their expression of Fas which leads to cell apoptosis after contact with Fas-L positive activated T cells (37). Mechanisms of suppression Several regulatory mechanisms have been associated to MDSC and new ones are being uncovered (Summarized in Figure 1), a phenomenon probably due to their heterogeneity. Following an immune stress due to GM-CSF production by tumor cells, MDSC accumulate in lymphoid organs where they suppress proliferation of and cytokine production by T and B cells activated by alloantigens (38) or by CD3 stimulation (39). Indeed, MDSC block the cell cycle at the G0/G1 phases in a contact dependent manner (27, 40). The suppressive activity of MDSC also depends on the release of IFN-γ by target T cells (41) . MDSC can also inhibit NK cell activity through membrane-bound TGF-β1, resulting in inhibition of IFN-γ and 5 NKG2D expression (42). The effect shows a high efficacy since addition in vitro of only 3% of MDSC was able to completely block T cell proliferation (41). To control T cell response and in response to signals provided by activated T cells, activated MDSC use 2 enzymes involved in L-arginine metabolism: iNOS which allows NO generation (43) and Arg1 (arginase 1) which depletes arginine from the environment (32, 44, 45). These two mechanisms of action appear to be used by monocytic and granulocytic subtypes of MDSC, respectively (13). In vitro, iNOS inhibitors (L-NMMA) combined or not with Arg1 inhibitors (43, 46) block inhibition of T cells by MDSC. Similarly, phosphodiesterase-5 inhibitors delay tumor progression by decreasing Arg1 and iNOS expression and by regulating the suppressive machinery of MDSC. The activation of either of these enzymes inhibits T cell proliferation by interfering with the transduction of intracellular signals and by inducing T cell apoptosis (47, 48). In fact, the loss of L-arginine inhibits T cell proliferation through several mechanisms such as the decrease of CD3 ζ chain expression and the inhibition of Cyclin D3 and Cyclin - dependent Kinase (cdk)-4 upregulation (49-52). Interestingly, arginine deprivation of T cells can reproduce the activity of MDSC by blocking the cell cycle at the G0/G1 stage (49). Regulation of L-arginine concentration in the microenvironment is therefore an important mechanism to modulate CD3 ζ chain expression of T-cell Receptor (TCR) and T cell function. Another important consequence of Arg1 activity is the induction of expansion of natural T regulatory cells (nTreg) (53). The second mechanism of action involving iNOS and NO production suppresses T cell function through other mechanisms involving the inhibition of JAK3 and STAT5, a mechanism shared with suppressive macrophages (54), the inhibition of MHC class II expression (55) and the induction of T cell apoptosis (56, 57). De Wilde and colleagues showed for the first time that another enzyme, heme oxygenase 1 (HO-1), is also associated with suppressive function of MDSC (58).