A large number of reports continue to refine the differentiation stages of murine lymphopoiesis. However very little is known about the earliest stages of human lymphoid differentiation, those few reports that exist have mostly used umbilical cord blood. The studies in this research proposal will address a critical gap in our knowledge of adult human lymphopoiesis, through the identification and characterization of the most primitive lymphoid progenitors in the human bone marrow able to generate T cells. The overall goal of these studies is to identify the lymphoid progenitors in adult human bone marrow with maximum T cell production capacity. We have identified a progenitor population in human bone marrow that expresses high levels of the homing molecule L-selectin (CD62L) and possesses robust lymphoid potential in vitro. We hypothesize that this CD34+ CD10- CD62L++ (CD10-) lymphoid-restricted progenitor population marks the first stage of commitment by human hematopoietic stem cells to the lymphoid differentiation pathway. Further, we propose that the CD10- population is a more primitive precursor with superior T lymphoid potential to the previously described CD34+ CD10+ (CD10+) LP population in human bone marrow. The studies in Aim 1 address the biology of human lymphoid commitment by defining the molecular characteristics of the CD10- and CD10+ LPs. These studies define the lineage relationship between the two LP populations based on their gene expression profiles, both globally and specifically using genes that are known to be critical to lymphoid development. The studies in Aim 2 address clinical relevance by determining the ability of the CD10- and CD10+ LPs to home and reconstitute the thymus and by quantifying the relative functional output of T cells from each population using in vitro assays and in vivo using a xenogeneic transplant model. This knowledge is essential for the development of diagnostic and therapeutic advances toward the process of lymphoid differentiation and thymic reconstitution.