The goal of this proposal is to gain a better understanding of the factors that influence alternate /? lineage commitment. Identifying the molecular processes involved is important not only because the divergence of T cells into functionally distinct and ? lineages is essential for the development of a normal immune cell repertoire, but also because differentiation of multipotential precursor cells into distinct cell tpes is a fundamental yet poorly understood process common to all multicellular organisms. Despite much effort, relatively little insight has been gained into the developmental cues that determine whether an immature thymocyte will adopt the or ? lineage. We have generated data that provide compelling evidence for a role of TCR ligand in promoting commitment to the ? lineage. In pursuing these studies, we exploited an ideally suited ? TCR transgenic model (KN6), which has a known ligand whose expression can be manipulated to alter the lineage fate of T cell progenitors. Accordingly, we have demonstrated that engagement of ligand by the KN6 ? TCR complex promotes adoption of the ? fate. This finding was highly controversial when first reported, but is now gaining support as other TCR/ligand pairs are examined. However, it remains unclear whether ligand engagement during development serves to confirm a pre-assigned lineage fate of a T cell progenitor (stochastic model) or, conversely, has a role in promoting a precursor to become either an or ? T cell (instructional model). In addition, studies of T cell lineage commitment have been limited by the lack of reliable markers to identify precursors committed to the ? lineage, aside from expression of the ? TCR. Therefore, we also propose to evaluate genes that are modulated with commitment to the ? lineage to establish a gene signature that identifies thymocytes that adopt the ? fate. In the current proposal, the role of ligand during ? T cell lineage commitment will be examined as follows. In Aim 1, we will assess whether ligand dictates the lineage fate of single progenitor cells in an instructive or stochastic manner. To address the requirement for ligand in vivo and its role in shaping the ? T cell repertoire, Aim 2 is focused on investigating ? T cell development in a novel ? TCR ligand-deficient mouse model. Finally, we are interested in identifying new markers to identify thymocytes committed to the ? fate. Therefore, in Aim 3, we will examine changes in gene expression that are consistent with adoption of the ? fate to establish a unique molecular signature of ? lineage commitment. Our identification of molecular effectors controlling T cell lineage commitment is important for our understanding of thymocyte development, as well as for improving our knowledge of developmental processes since control of cell growth and differentiation is a recurring theme in development and transformation.