IL-2 has multiple functions in T cell-mediated adaptive immunity. The stringent control of its expression is important for T cell activation, proliferation and the subsequent T cell clone contraction. Our lab has recently shown that the transcriptional repressor Blimp-1 is part of a negative feedback loop which controls IL-2 gene expression in mice. Understanding the molecular mechanisms of this signaling loop in T cells might help us to better understand the regulation as well as the role of IL-2 in T cell immunity. The human ortholog to murine Blimp-1 is termed PRDI-BF1 (each encoded by the respective Prdm1 gene). Both genes contain five zinc finger regions, whereby the first two zinc fingers are dispensable for DNA binding. In case of the human protein they are instead required to recruit the G9?Ñ methyltransferase to the gene promotor. We found that the human wild-type PRDI-BF1 protein suppressed IL-2 production in murine T cells, while deletion of the first two zinc fingers abolished this ability. Thus, a similar Blimp-1-mediated methylation mechanism might exist in IL-2 gene silencing. IL-2/IL-2R signaling is indispensable for Blimp-1 induction. PI-3Kinase and Stat5 are downstream of the IL-2 receptor complex and are known to contribute to IL-2 inhibition in T cells from C57BL/6 mice. However, activating only these two pathways are still not sufficient to induce Blimp-1 or suppress IL-2 expression in in IL-2R beta-/- mice. The Blimp-1-dependent IL-2 self regulatory loop is not functional in IL-2R beta-/-mice. In order to conveniently study this dysregulation we crossed these mice with a GFP transgenic strain in which the GFP transgene is under the control of IL-2 promoter sequence. In IL-2R beta-/-IL-2p-GFP mice about five times as many spleenic CD4+ T cells transcribe IL-2pGFP, compared to the littermate IL-2R beta+/-IL-2p-GFP control animals. And most of the GFP cells demonstrate activated phenotype (CD44HighCD62Llow). Blimp-1 is known as a master regulator of B cell terminal differentiation. Since a recent report indicated that IL-2 signaling via STAT5 constrains Th17 Cell differentiation, we speculated that Blimp-1 might play a similar role in effector T cell differentiation. In order to evaluate this possibility, activated CD4+ T cells from C57BL/6 mice were transduced with Blimp-1 and cultured under Th17 polarizing conditions. Blimp-1 overexpression in did not change the profile of IL-17 production.