Fc gamma receptor IIB on follicular dendritic cells regulates the B cell recall response

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 c ells (FDCs), with germinal center (GC) B cells. Thus, the expression of rec eptors on FDC and B cells that interact with ICs could be critical to the i nduction of an optimal recall response. FDCs in GCs, but not in primary fol licles, express high levels of the IgG Fc receptor Fc gamma RIIB, This regu lated expression of Fc gamma RIIB on PBC and its relation to recall Ab resp onses were examined both in vitro and in vivo. Trapping of IC in spleen and lymph nodes of Pc gamma RII-/- mice was significantly reduced compared wit h 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 Fc gamma RIIB-/- mice supported only low level A b production in this situation. Similarly, when Fc gamma RIIB-/- mice were transplanted with wild-type Ag-specific T and B cells and challenged with s pecfic Ag, the recall responses were significantly depressed compared with those of controls with wild-type FDC, These results substantiate the hypoth esis that Fc gamma RIIB expression on FDCs in GCs is important for FDCs to retain ICs and to mediate the conversion of ICs to a highly immunogenic for m and for the generation of strong recall responses.