Regulation of B cell receptor signaling: B cell activation through complement and the role of protein tyrosine phosphatase Shp2

My thesis is focused upon mechanisms regulating B cell receptor (BCR) activation and regulation. There are two branches of the immune system, the innate and adaptive. Proper cross-talk between the innate and adaptive immune system is crucial in initiating proper and effective immune responses and preventing inappropriate immune reactions such as autoimmunity and lymphoma.; Arguably the most crucial link between B cell function and the innate immune system is through the complement receptor CD21 (Cr2). CD21 is the receptor for the complement breakdown product C3d. Cross-linking of antigen with C3d has been shown to augment antigen specific antibody titers far above normal immune responses. In chapter 1, I have shown that conjugation of protective antigen (PA) of anthrax with C3d produces 3-fold greater antibody titers as compared to PA in alum---the vaccine currently being developed for human administration. These antigen-specific antibodies are produced very rapidly during the primary response, are of all IgG isotypes, and are sustained long after primary immunization. Finally, I see that C3d may augment this response by enhancing the efficiency of germinal center formation and plasma cell differentiation.; In chapter 2, I focus upon the mechanism of CD21 function on B cells and follicular dendritic cells (FDCs). By immunizing WT and CD21-/- mice, I found that CD21 is important in initiating proper B cell function during the primary immune response but had little role in eliciting secondary or memory immune responses. I further examine CD21 function by examining the mechanisms by which CD21 functions on the surface of B cells. By using deconvolution microscopy and immunoprecipitation, I lend support to the sequestration model of CD21 function. This model implicates CD21 is an inhibitory receptor on B cells which actively sequesters CD19, the cardinal member of the BCR co-receptor complex.; Finally, in chapter 3, I look downstream of surface receptor regulation of BCR function and look at the role of the protein tyrosine phosphatase, Shp2, in B cell development, signaling and function. By using Cre/lox-p technology, I create a B cell specific deletion of Shp2 and show that Shp2 may be an important regulator of marginal zone versus follicular B cell development and is a crucial regulator in the GC reaction.