Germinal centers - why and why not? : the humoral response in primary immunodeficiency

Abstract: An efficient and regulated immune system is required for protection against pathogens. B cells are required for a humoral response. For a sustained humoral response against pathogens germinal center (GC) reactions are required. In the GC reaction, B cells with antigen-specific B cell receptors are selected and expanded. The GC contains follicular dendritic cells (FDCs) presenting antigens to the B cells, T follicular helper cells promoting proliferation of cognate B cells and highly proliferating B cells expressing activation induced deaminase (AID) in order to produce high-affinity and isotype-switched antibodies. In this thesis I aimed to define the unique and redundant functions of two actin-polymerizing proteins, WASp and N-WASp, the importance of the RhoGTPase Cdc42 and the consequences of mutated AID in the GC reaction. In paper I we discovered that naïve WASp-deficient (WKO) mice and WASp-deficient mice with N-WASp conditionally deleted only in B cells (cDKO) had increased titers of autoreactive antibodies (IgG and IgM, respectively). Both WKO and cDKO B cells got sufficient help from cognate T cells, while the polarization of GCs and the FDC network was altered upon autoantigen challenge. cDKO mice may have increased activation threshold of the B cell receptor in B cells resulting in reduced autoreactive IgG. In paper II we revealed the distinctive and partly redundant function of WASp and N-WASp in B cells. We observed reduced marginal zone cellularity in WKO mice and even more pronounced in cDKO mice when compared with wild type mice. The mild phenotype of reduced GC and humoral response in WKO mice previously reported was markedly defected in cDKO mice after antigen challenge, promoting the notion of overlapping functions of WASp and N-WASp in B cells. In paper III we investigated the role of Cdc42, that activates both WASp and N-WASp, in B cells. Mice with deleted Cdc42 conditionally in B cells had reduced marginal zone and follicular B cell numbers and reduced GC B cell number associated with reduced specific antibody titers in serum after antigen challenge. In paper IV we identified a mouse model for one of the most common mutations in AID leading to the hyper-IgM syndrome. The replacement mutation in amino acid position 112 resulted in a catalytic dead AID protein with spontaneous GC formation, and lack of high affinity or class-switched antibodies. Altogether, the work presented in this thesis enlightens the significance of tightly regulated B cells for a sufficient, and not autoreactive, humoral immune response. Specifically, this thesis has contributed to our understanding of the importance of a functional AID protein in B cells. Moreover, this thesis has helped in defining the precise activation of the actin regulating proteins in B cells to form antigen-induced GCs and for production of classswitched antibodies with high affinity for the antigen.