Gld mutation of Fas ligand increases the frequency and up-regulates cell survival genes in CD25+CD4+ T_R cells

The Fas pathway and regulatory T (T R) cells play intertwining roles in controlling T cell tolerance through deletion and suppression of autoreactive T cells. Impairment of either mechanism causes severe T cell lymphoproliferation albeit with opposing outcomes. T cell lymphoproliferation induced by defective Fas pathway does not cause overt lymphocytic infiltration but rather prevents an important set of T cell-mediated autoimmune diseases. In contrast, deficiency in T_R cell causes fulminant autoimmunity in very early life and fatal lymphocytic infiltration. These observations suggest existence of unidirectional fail/safe mechanism that compensate for defects in the Fas pathway but not in regulatory cells. To gain insights into how animals compensate for defects in the Fas system, we analyzed the impact of generalized lymphoproliferative disease (gld) mutation on survival, function and transcription profile of CD25+CD4+ T_R cells. Our results show that all CD4 T cells expanded in gld mice. However, CD25+CD4+ T_R cells are disproportionately increased in the pool of CD4 T cells perhaps due to their unique apoptosis phenotype. Freshly isolated CD25+CD4+ TR cells, unlike CD25-CD4+ T cells, are highly sensitive to FasL-induced apoptosis in the steady state. CD25+CD4+ T_R cells that accumulate in gld mice express similar level of Foxp3, and have suppression potency and T_R gene expression profile as wild-type CD25+CD4+ T_R cells. Furthermore, the transcription profile of gld CD25+CD4+ T_R cells is characterized by differential expression of genes involved in cell survival, metabolism and innate immune responses. These results provide a strong cellular and molecular basis for understanding why impaired Fas pathway prevents an important subset of T cell-mediated autoimmune diseases.