Analysis of the regulation of murine B lymphopoiesis by interleukin-7 and the pre-B cell receptor

Generation of the B lymphocyte lineage involves the development of early non-committed hematopoietic stem cells to mature, Ig-producing B cells. Cell-intrinsic regulatory signals combine with signals provided by the surrounding microenvironment to mediate selection checkpoints throughout the developmental process. In this thesis I examine the roles played by the pre-B cell receptor and the interleukin-7 (IL-7) receptor during a major developmental checkpoint. The transition from the pro-B to pre-B cell stage requires the formation of the pre-B cell receptor, and the signaling complexes that arise following its membrane insertion. Prior to the acquisition of the pre-B cell receptor, B cell precursors require exposure to IL-7, a growth factor produced by surrounding stromal cells. I demonstrate a connection between these two intrinsic and extrinsic signals: in the presence of the pre-B cell receptor, pre-B cells experience a reduced threshold for proliferation in response to IL-7. I establish that a signal transmitted by the pre-B cell receptor complex is sufficient to allow proliferation in response to low concentrations of IL-7. Further experiments indicate that activation of the ERK/MAP kinase pathway occurs in response to combined stimulation of the pre-B cell receptor and the IL-7 receptor, and that cells expressing the pre-B cell receptor exhibit increased levels of activated ERK even in the absence of receptor cross-linking. I propose a model for the ERK-dependent selection of pre-B cells in the presence of low concentrations of IL-7. Experimental results reveal that ERK phosphorylation is pivotal in the induction of proliferation, but not maturation or survival of cells undergoing the pro-B to pre-B transition, providing support for my hypothesis. A similar requirement for protein kinase A activity is also indicated. In summary, pre-B cells experience a pre-B cell receptor-dependent growth advantage over more immature cells in an environment that provides limited amounts of interleukin-7, a situation that may exist in the murine bone marrow. The data presented in this thesis constitute a model for the specific selection of pre-B cells that have completed a crucial developmental task, and highlight the regulatory role played by the environmental context in which cells mature.