Fcγ Receptor IIB on Follicular Dendritic Cells Regulates the B Cell Recall Response

Abstract

Generation of the B cell recall response appears to involve interaction of Ag, in the form of an immune complex (IC) trapped on follicular dendritic cells (FDCs), with germinal center (GC) B cells. Thus, the expression of receptors on FDC and B cells that interact with ICs could be critical to the induction of an optimal recall response. FDCs in GCs, but not in primary follicles, express high levels of the IgG Fc receptor FcγRIIB. This regulated expression of FcγRIIB on FDC and its relation to recall Ab responses were examined both in vitro and in vivo. Trapping of IC in spleen and lymph nodes of FcγRII−/− mice was significantly reduced compared with that in wild-type controls. Addition of ICs to cultures of Ag-specific T and B cells elicited pronounced Ab responses only in the presence of FDCs. However, FDCs derived from FcγRIIB−/− mice supported only low level Ab production in this situation. Similarly, when FcγRIIB−/− mice were transplanted with wild-type Ag-specific T and B cells and challenged with specific Ag, the recall responses were significantly depressed compared with those of controls with wild-type FDC. These results substantiate the hypothesis that FcγRIIB expression on FDCs in GCs is important for FDCs to retain ICs and to mediate the conversion of ICs to a highly immunogenic form and for the generation of strong recall responses.