| Peripheral B cell development occurs in a stepwise fashion from transitional 1 (T1) to transitional 2 (T2) to mature B cell stages. This results in the generation of three mature B cell types: follicular mature (FM), marginal zone (MZ), and B1 cells. Though these B cell populations have been identified, the specific signals driving their development have not been well characterized. To begin to address such questions, we first tested the BCR responsiveness of the T1, T2, MZ, and FM populations. In response to BCR engagement, T2, but not T1, cells generated proliferative and pro-survival signals, at both the cellular and molecular levels. The distinct differences in BCR responsiveness between T1 and T2 cells suggest a model for peripheral B lymphocyte selection, whereby T1 cells function in B cell negative selection while T2 cells serve as targets for BCR driven positive selection. In addition, while T2 cells proliferate robustly to both BCR or toll like receptor (TLR) stimulation, MZ and FM cells are selectively responsive to either TLR or BCR stimulation, respectively. Furthermore, TLR activation drives T2 cells to differentiate into MZ cells, while BCR engagement drives T2 cells into the FM pool. These results suggest that the T2 population may act as a pivotal checkpoint to generate either an adaptive vs. "innate" B cell immune response. Finally, we investigated the molecular requirements for BCR signaling in T2 cells. We demonstrate that both Bruton's tyrosine kinase (Btk) and protein kinase C-beta (PKCbeta) are required for BCR mediated NFkappaB survival signaling. In contrast, Btk, but not PKCbeta, is required for BCR dependent T2 cell differentiation. These results distinguish the pathways regulating BCR mediated maintenance vs. maturation. Furthermore, the specific role for PKCbeta in B cell survival suggest PKCbeta as an attractive drug target for B cell malignancies characterized by dysregulated NFkappaB signaling. Testing this idea, we demonstrate the efficacy of small molecule PKC inhibitors in blocking the survival of such lymphomas. Together, these results make major advances in our current understanding of peripheral B lymphocyte development and provide the molecular basis for a potential new treatment against certain B cell cancers. |
