Mesenchymal stem/stromal cells protect against autoimmunity via CCL2-dependent recruitment of myeloid-derived suppressor cells.

Exogenously administered mesenchymal stem/stromal cells (MSCs) suppress autoimmunity despite transient engraftment. However, the mechanism is unclear. In this study, we report a novel mechanism by which MSCs modulate the immune system by recruiting myeloid-derived suppressor cells in a mouse model of experimental autoimmune uveitis (EAU). Intravenous infusion of MSCs blocked EAU development and reduced Th1 and Th17 responses. Time course analysis revealed an increase of MHC class II(lo)Ly6G(-)Ly6C(hi)CD11b(+) cells in draining lymph nodes by MSCs. These Ly6C(hi)CD11b(+) cells suppressed CD4(+) cell proliferation and Th1/Th17 differentiation and induced CD4(+) cell apoptosis. Adoptive transfer of Ly6C(hi)CD11b(+) cells ameliorated EAU, whereas depletion of Ly6C(hi)CD11b(+) cells abrogated the effects of MSCs. 1.8% of MSCs were present in draining lymph nodes 1 d after infusion, and MSCs with CCL2 knockdown did not increase MHC class II(lo)Ly6G(-)Ly6C(hi)CD11b(+) cells and failed to attenuate EAU. Therefore, our findings demonstrate that MSCs suppress autoimmunity by recruiting myeloid-derived suppressor cells into sites of inflammation in a CCL2-dependent manner.