The goal of this proposal is to directly study molecular events determining beta lymphoid lineage specification of normal human hematopoietic progenitors. Insights into the regulation of beta cell differentiation are especially relevant to lymphoblastic leukemias, in which differentiation block is a primary feature. In using the beta lymphoid differentiation pathway as a model for lineage commitment of normal and malignant cells, we will take advantage of an unequivocal differentiation endpoint, Ig gene rearrangement, and our ability to isolate unique populations containing clonogenic lymphoid progenitors which proliferate for two weeks in culture with minimal differentiation. The specific contribution of beta lineage associated transcription factors and the IL-7 receptor to beta lineage specification will be tested in aim 1 by retroviral-mediated transfer of these genes into normal early progenitors followed by measurement of beta lineage related gene expression and rearrangements in sorted transduced cells after culture. This approach, which uniquely targets normal nontransformed progenitors at the early stages of development where lineage commitment is occurring, will test the differentiative function of these factors, and provide new information relevant to human and murine lymphopoiesis. The rationale is that early CD34 plus lymphoid progenitors expressing the transcription factor PAX-5 and IL-7 receptor (IL-7R) differentiate in vitro into pro-beta cells, while IL-7R/PAX-5-progenitors, although expressing certain early lymphoid associated genes, do not. PAX-5-/IL-7R-cells may be therefore be poised for entry into the beta lymphoid differentiation pathway, but do not spontaneously take this step in the in vitro microenvironment. We hypothesize that they would be responsive to factors which should normally appear at this differentiation stage to induce beta lineage specification. In vivo, these early differentiation steps require contact between lymphoid progenitors and bone marrow stroma. In the second specific aim, we will test the hypothesis that certain integrin adhesion interactions mediate migration of lymphoid progenitors into specialized stromal microenvironments or deliver activation signals, promoting either differentiation or maintenance of a progenitor phenotype. We propose to study the regulation of progenitor migration and beta lineage differentiation by stromal integrin ligands and progenitor integrin associated proteins, using a unique monoclonal antibody which inhibits lymphoid progenitor migration and identifies a potentially novel stromal VLA-4 ligand.