TNFα-induced M-MDSCs promote transplant immune tolerance via nitric oxide

Efficient induction of functional competent myeloid-derived suppressor cells (MDSCs) will be critical for the clinical application of MDSCs to treat autoimmune diseases and to induce transplantation immune tolerance. In the present study, we tried to establish the MDSC induction system with M-CSF and tumor necrosis factor alpha (TNF alpha) and investigated the immunosuppressive function of M-CSF + TNF alpha-induced MDSCs in transplant mouse models. Monocytic MDSCs (M-MDSCs) were induced by culture of the non-adherent mouse bone marrow cells with M-CSF or M-CSF + TNF alpha, respectively, for 7 days. Phenotype analysis revealed that the majority of M-CSF- and M-CSF + TNF alpha-induced MDSCs express F4/80. The addition of TNF alpha in the induction period increased Gr-1, Ly6C, CD80, and CD274 expressions on these cells. M-CSF + TNF alpha-induced M-MDSCs showed poor TNF alpha, IL-12, and IL-6 expressions after lipopolysaccharide (LPS) stimulation and decreased arginase 1 (Arg-1) and Fizz expressions after IL-4 stimulation compared with M-CSF-induced M-MDSCs. M-CSF + TNF alpha-induced M-MDSCs showed enhanced ability to suppress T cell proliferation and cytokine production than M-CSF-induced M-MDSCs. M-CSF + TNF alpha-induced M-MDSCs express high levels of inducing nitric oxide synthase (iNOS) and blocking iNOS activity by a chemical inhibitor or gene deficiency significantly reversed the inhibitory effects of M-CSF + TNF alpha-induced M-MDSCs on T cells. Adoptive transfer of M-CSF + TNF alpha-induced M-MDSCs promoted immune tolerance in a male-to-female skin-grafted mice, but M-CSF + TNF alpha-induced iNOS-deficient M-MDSCs failed to do so. Thus, M-CSF + TNF alpha-induced M-MDSCs have powerful immunosuppressive activity, which is mediated by an iNOS-dependent pathway. M-CSF + TNF alpha-induced M-MDSCs can promote immune tolerance to donor antigens in a transplant mouse model.