Molecular mechanisms of peripheral B lymphocyte development

The development of mature B cell populations is dependent on the signals the immature lymphocyte receives from its environment. The signals generated through the B cell receptor of antigen (BCR) are of utmost importance, as its deletion results in a complete absence of all peripheral B cell populations. Here, three questions regarding how BCR signaling influences B cell development are addressed: (1) How is the transcription factor NF-kappaB activated following stimulation through the BCR?, (2) How does the NF-kappaB subunit p50 influence B cell development?, and (3) How does the BCR influence lineage choice during B cell development?; To address the first question we show that a novel protein kinase, PKK (Protein Kinase-C associated Kinase; RIP4/DIK) activates NFkappaB in both a kinase-dependent and a kinase-independent manner and functionally lies downstream of activated PKC. Moreover, we show that while the overexpression of a catalytically inactive form of PKK in the B lineage results in a block in development, a null mutation in PKK has no effect, suggesting a redundant role for PKK in B lymphocyte biology.; Second, in NF-kappaB1/p50 mutant mice there is a significant reduction in the number of marginal zone (MZ) B cells. Here, we attempt to identify the p50 target genes that are required for MZ B cell development. Through gene expression and chromatin immunoprecipitation assays we find that p50 positively and directly regulates the Notch target hairy/enhancer of split (Hes) 5, a gene that may play a role in B lymphocyte development. Additionally, we show that the p50 null mutation enhances the MZ B cell phenotype seen in the Notch2 haploinsufficient mouse. Taken together our data are consistent with the model that NF-kappaB and Notch cooperate in the development of peripheral B cell populations.; Finally, our laboratory has suggested, based on the analysis of Aiolos null and xid mice, that BCR signal strength can influence B cell development. An expectation of this model is that B lymphocytes with the same BCR will receive identical signals and will, therefore, differentiate along the same lineage. Using a "quasi-monoclonal" mouse model we show that this is indeed the case. Additionally, we use this model to better define transitional B cell populations and to place Notch2 in a hierarchy with the BCR.