T lymphocytes must be produced throughout life to replenish losses from cellular senescence, trauma, and antigen-induced activation. Stem cells for this process emigrate from the bone marrow and home to the thymus via the blood, after which they undergo extensive proliferation to generate a pool of cells for TCR/MHC-mediated selection. T lineage commitment is induced by signals found within the thymus, because early intrathymic precursors maintain non-T lineage potential. Stromal cells, mainly epithelial cells and fibroblasts, are believed to be the primary source of such signals. However, the molecular nature of interactions between lymphoid progenitors and stromal cells is poorly understood, and very few of the signals required for lineage commitment and proliferation have been identified. The goal of this project is to define the molecular signals by which thymic stromal cells induce T lineage commitment and proliferation of multipotent precursors from bone marrow. The three-phase approach involves 1) isolation of mRNA from thymocytes at defined stages of development, followed by microarray screening for genes encoding receptors that function in lineage commitment or proliferation; 2) in situ localization of the corresponding ligands in the thymus; and 3) in vivo analysis of mutant cells deficient in the interactions predicted by the first two phases. These studies are expected to reveal the molecular nature of signals by which the thymus induces T lineage commitment and proliferation in the steady-state. A better understanding of this process may also help to predict new strategies for treating age-related deficiencies of thymic stroma, including the prolonged inability to produce T cells that frequently follows hematopoietic reconstitution after myeloablative therapy.
