Antigen-Driven B Cell Differentiation and Mechanisms of Peripheral Tolerance During the T Cell-Dependent Immune Response

During a T cell-dependent B cell immune response, cognate interactions with helper T cells result in B cells differentiating into short-lived antibody-forming cells (AFCs) or initiating germinal centers (GCs). B cell entry into GCs is followed by rapid clonal expansion that is coupled with somatic hypermutation of antibody variable regions and stringent selection mechanisms. These events lead to the emergence of B cells with high-affinity antigen receptors during the late phases of the primary immune response and are also critical to the development of long-lived memory populations that contribute to robust responses upon re-immunization. In these studies, mechanisms that influence B cell activation, proliferation, and differentiation at various stages of the T cell-dependent response were explored. To perform high-resolution analyses of the in vivo immune response, transgenic mouse models in which B cells express antigen receptors specific for the hapten arsonate were used. Following immunization, the arsonate-reactive B cells are efficiently recruited into a T cell-dependent immune response and can be readily detected for analysis using histological and flow cytometric methods. Accordingly, these studies track antigen-specific B cells at various stages of a foreign antigen-driven immune response, evaluating changes in their phenotype and location in the microenvironment of the spleen. The results include the identification of a phase of B cell expansion prior to entry into GCs that is not appreciated in current models of the T cell-dependent B cell response. In addition, pathways previously shown to influence B cell activity, including those mediated by Toll-like receptors (TLRs) or cellular FLICE-inhibitory protein (cFLIP), were scrutinized to determine their roles in the maturation of the T cell-iv dependent response. While studies by others have implicated endosomal TLRs in the aberrant activation of autoreactive B cells, signaling through these receptors did not influence the participation of an anti-DNA B cell clonotype in GC or AFC responses in vivo. The studies presented here also suggest a more profound role for expression of cFLIP by B cells than is suggested by previous studies. Analyses of cFLIP-deficient B cells demonstrated substantial deficiencies in their initial activation and proliferative responses, defects that precluded their participation in an immune response. Overall, the data presented in this thesis provide insight on factors that regulate antigen-specific B cell participation in a T cell-dependent immune response and the formation of long-term memory