Project Summary Invariant natural killer T (iNKT) cells are a subset of T lymphocytes that sit at the boarder of adaptive and innate immunity. They have highly restricted T cell receptors (TCRs) that detect glycolipid antigen presented by the non-classical MHC molecule CD1d and they respond rapidly to antigen or cytokine stimulation. NKT cells play an important role in anti-microbial immune responses and because of their rapid production of multiple cytokines they are being considered as therapeutic agents in cancer and other diseases. NKT cells acquire their effector phenotype as a consequence of their unique mechanism of selection and differentiation in the thymus. The range of effector fates that NKT cells can acquire is only beginning to be appreciated but the mechanisms controlling NKT cell effector fate choice are completely unknown. Our research is directed at understanding the molecular mechanisms that control iNKT cell development with a focus on the E protein transcription factors, their inhibitors the ID proteins, and their downstream targets. These proteins are critical regulators of iNKT cell numbers and effector cell fate choice. In this grant application we propose experiments to determine the requirements for E2A in iNKT cell development an the consequences of over- and under-expression of two putative E2A target genes, Lef1 and Bcl6. Our studies will have a major impact on our understanding of how NKT cell number and function is regulated. In addition, our studies will provide insight into mechanisms to manipulate NKT cell effector fate and thereby alter immune responses at their inception.