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The two-signal model of T cell activation states that antigen recognition by TCR provides a tolerogenic signal (termed Signal 1) unless the T cell receives simultaneous costimulation (Signal 2) that permits antigen recognition to prime activation. Our efforts to characterize genetic and biochemical factors resulting from Signal 1 alone have identified signaling molecules, transcription factors, and an epigenetic regulator that each contribute to the anergic phenotype observed. However, our most striking finding is that the same factors identified using anergy to model T cell activation versus tolerance also participate in determining the outcome of the effector phenotype of fully activated T cells. We summarize our own findings and other recent advances in the genetic and biochemical understanding of T cell activation, tolerance, and plasticity in this review.

Ag-specific T cell cytokine expression is dictated by the context in which TCR engagement occurs. Recently it has become clear that epigenetic changes play a role in this process. DNA methyltransferase 3a (DNMT3a) is a de novo methyltransferase important to the epigenetic control of cell fate. We have determined that DNMT3a expression is increased following TCR engagement and that costimulation mitigates DNMT3a protein expression. T cells lacking DNMT3a simultaneously express IFN-gamma and IL-4 after expansion under nonbiasing conditions. While global methylation of DNA from wild-type and knockout T cells is similar, DNMT3a-null T cells demonstrate selective hypomethylation of both the Il4 and Ifng loci after activation. Such hypomethylated knockout Th2 cells retain a greater capacity to express IFN-gamma protein when they are subsequently exposed to Th1-biasing conditions. Based on these findings we propose that DNMT3a is a key participant in regulating T cell polarization at the molecular level by promoting stable selection of a context-specific cell fate through methylation of selective targets in T cells.