Role of IFN-γ in induction of Foxp3 and conversion of CD4+CD25- T cells to CD4+ Tregs

IFN-gamma is an important Th1 proinflammatory cytokine and has a paradoxical effect on EAE in which disease susceptibility is unexpectedly heightened in IFN-gamma-deficient mice. In this study, we provide what we believe is new evidence indicating that IFN-gamma is critically required for the conversion of CD4(+)CD25(-) T cells to CD4(+) Tregs during EAE. In our study, the added severity of EAE in IFN-gamma knockout mice was directly associated with altered encephalitogenic T cell responses, which correlated with reduced frequency and function of CD4(+)CD25(+)Foxp3(+) Tregs when compared with those of WT mice. It was demonstrated in both human and mouse systems that in vitro IFN-gamma treatment of CD4(+)CD25(-) T cells led to conversion of CD4(+) Tregs as characterized by increased expression of Foxp3 and enhanced regulatory function. Mouse CD4(+)CD25(-) T cells, when treated in vitro with IFN-gamma, acquired marked regulatory properties as evidenced by suppression of EAE by adoptive transfer. These findings have important implications for the understanding of the complex role of IFN-gamma in both induction and self regulation of inflammatory processes.