Our broad objective in this proposal is to determine how normal progression through T cell development is controlled by functional interactions between ubiquitous regulators of chromatin and lineage-specific DNA binding proteins. Our focus is on the chromatin remodeler Mi-2beta and its role in regulating key events in T cell differentiation. In the. first specific aim, we study the role of Mi-2beta in CD4 gene expression in a potentially reciprocal regulation of the CD4 enhancer and silencer. This is pursued by interrogating both the physical and genetic interactions between Mi-2beta, lineage-specific factors, and chromatin remodelers on the CD4 locus during T cell development and by assessing the functional consequences of their absence. In the second specific aim, we examine the role of Mi-2beta in the transition from the thymocyte double negative to the double positive stage and downstream of beta-selection. Effects on gene-specific but also general chromatin events are investigated in what appears to be a novel, Mi-2beta-dependent, checkpoint at this stage of T cell development. In the third specific aim, the role of Mi-2beta as a positive regulator of the T cell proliferative response is addressed. Its effect on expression of key cell cycle regulators as well as on the execution of key cell cycle events is examined. The functional but antagonistic interactions between Mi-2beta and the negative regulator Ikaros in T cell proliferation are explored. Particular emphasis is given on the Mi-2beta effects on Ikaros induced leukemogenesis. These studies, which address the function of a ubiquitously-expressed chromatin remodeler in its ability to be targeted by lineage-specific and developmentally-important DNA binding factors will generate new insights into the normal controls of lymphocyte differentiation and proliferation and provide paradigms for other developmental systems. Another important outcome of these studies will be an increase in our ability to intervene in the fate of T cell precursors as well as of mature naive T cells by targeting such remodeler-DNA binding factor interactions. This investigation may also identify molecular targets for intervention in immune disease, characterized by inappropriate lymphocyte proliferation such as autoimmunity, leukemia and lymphoma.