Critical and Opposing Effects of T Cell-Derived TNFα and Interferon-γ on IL-17 Induction Assessed in Vitro and in Vivo Following Allogeneic Bone Marrow Transplantation

IL-17-producing CD4 T cells (Th17) are a recently identified T helper subset that plays a role in mediating host defense to extracellular bacteria infections and is involved in the pathogenesis of many autoimmune diseases. In vitro induction of IL-17 in murine CD4⁺ T cells has been shown to be dependent on the presence of the proinflammatory cytokines TGF-β and IL-6 whereas IFNγ can suppress the development of Th17 cells. In the current study, we examined the roles of TNFα and IFNγ on IL-17 production by purified T cells in vitro and in vivo after allogeneic bone marrow transplantation (BMT). We present findings that expression of TNFα by the T cell itself is necessary for optimal development of Th17 under in vitro polarizing conditions. A novel role for T cell-derived TNFα in Th17 induction was observed when in vitro polarization of Tnf^−/−CD4⁺ T cells resulted in marked reductions in IL-17⁺CD4⁺ T cells compared to Tnf^+/+CD4⁺ T cells. In marked contrast, T cell-derived IFNγ markedly inhibited Th17 development as more IL-17⁺CD4⁺ T cells were found in Ifnγ^−/−CD4⁺ T cells than in Ifnγ^+/+CD4⁺ T cells, and of particular interest was the dramatic increase in IL-17⁺CD8⁺ cells from Ifnγ^−/− mice. To determine if T cell-derived TNFα or IFNγ can regulate Th17 development in vivo we examined the differentiation of alloreactive donor T cells following allogeneic BMT. We have found that donor-derived Th17 cells can be found in lymphoid tissues and GVHD-affected organs after allogeneic BMT. However, transfer of Tnf^−/− CD4⁺ T cells after allogeneic BMT resulted in marked reductions in Th17 cells in the spleen (18×10³ vs 7×10³, P<0.05). In agreement with the in vitro data and in contrast to what was observed with transfer of Tnf^−/− CD4⁺ T cells, transfer of donor Ifnγ^−/− T cells resulted in marked increases in not only IL-17⁺CD4⁺ but also IL-17⁺CD8⁺ T cells infiltrating the liver (7×10³ vs 14×10³, P<0.05; 4×10⁴ vs 12.5×10⁴, P<0.05). These results suggest that the donor T cell-derived TNFα and IFNγ opposingly regulate IL-17 induction of both CD4⁺ and CD8⁺ T cells in vitro and after allogeneic BMT which correlates with GVHD pathology.