Natural IgM Secreting Cells of the Mouse

B-1 cells are unique, fetal-origin, self-reactive, innate like B lymphocytes that give rise to a heterogeneous population of cells, including IgM secreting cells. The poly-reactive IgM produced by B-1cells is a first line of defense again invading pathogens and is essential for initiation of robust B-2 adaptive immune responses. Additionally, natural IgM aids protection from autoimmune diseases and neoplasia. IgM secretion by B-1 cells occurs both, at steady state, mostly in bone marrow (BM) and spleen in the absence of microbial stimuli, and as an early response to inflammation or infection, usually in the draining lymphoid tissues. In my dissertation, I aimed to further characterize the subpopulations of IgM secreting B-1 derived cells and the mechanisms that drive their differentiation into three types of antibody secreting cells: Blimp-1 dependent B-1 plasma cells (PC), Blimp-1 independent non-terminally differentiated B-1 cells (B-1sec) and inflammation/infection induced B-1 plasma blasts and plasma cells. In Chapter 1, I review the role of B-1 cells and nIgM in the control of and response to infections. This includes outlining our current understanding of B-1 cell development and maintenance, as well as the events that regulate the activation of B-1 cells to become antibody secreting cells. I explore as yet unanswered questions about the role of innate versus antigen-specific signaling in the ability of B-1 cells to engage in tissue homeostasis and immune defense through their secretion of IgM. In Chapter 2, I examine the developmental requirements and nIgM repertoire contributions of B-1PC. In particular, the data demonstrate that BM B-1PC development requires the presence of CD4 T cells, bucking the dogma that nIgM is always T independent. Additionally, I demonstrate that B-1PC generate a distinct, oligoclonal nIgM repertoire that has relatively little overlap with non-secreting B-1 cells. This B-1PC repertoire was dominated by short 7-8 amino acids. It was encoded by various CDR3 variable immunoglobulin heavy chain genes that nonetheless generated a small number of distinct CDR3 motives, highly suggestive of convergent recombination and thus positive selection. Additionally, the data showed that in the absence of BM B-1PC a population of B-1sec emerged that compensated for the loss B-1PC derived serum nIgM. This population was phenotypically less differentiated than B-1PC and was unable to be reconstituted from adult peritoneal B-1 cells.In Chapter 3, I aimed to identify the B-1sec by flow cytometry and further characterize their function and transcriptional profile. B-1sec were identified as CD19+ CD23- CD43var intracytplasmic IgMhi and CD138+. Using a double-reporter mouse to identify the immunoglobulin joining (J)-chain and Blimp-1, the master transcriptional regulator of plasma cell differentiation, B-1sec were further identified as J-chain+ Blimp-1neg. Despite the lack of Blimp-1 expression, I demonstrated that these cells secrete significant amounts of IgM and have a transcriptional profile of an antibody secreting cell. In Chapter 4, I examine the role of toll like receptor (TLR) and B cell receptor (BCR) signaling on both, steady state nIgM production and infection-induced secretion of IgM by B-1 cells. I found that both were reduced, when B-1 cells lack TLR expression. I further demonstrate that B-1 cells do not use the same level of BCR and TLR signaling coordination to enhance their activation and differentiation as we see in B-2 cells. Instead, B-1 cells were exquisitely sensitive to TLR stimulation, but BCR signaling inhibited TLR-stimulated B-1 cell responses in contrast, while synergistic effects were noted in B-2 cells. Furthermore, when BCR signaling was blocked with a Syk inhibitor, the effects on TLR signaling were much more pronounced in B-2 than B-1 cells, suggesting that B-1 cells respond through a TLR pathway that is uncoupled from the MyD88-Syk axis previously shown to be present in B-2 cells.Together my work adds to the understanding of how B-1 cells contribute to IgM production, and thus increase understanding of their integral role in early defense against pathogens and their role in connecting the innate and adaptive branches of the immune system