Optimal Germinal Center B Cell Activation and T-Dependent Antibody Responses Require Expression of the Mouse Complement Receptor Cr1

Follicular dendritic cells (FDCs) and complement receptor (Cr)1 and complement receptor (Cr) 2 are important for the generation of humoral immunity. Cr1/2 expression on B cells and FDCs was shown to provide a secondary signal for B cell activation, to facilitate transport of Ag in immune follicles, and to enhance retention of immune complexes by FDCs. We show in this study that murine B cells predominantly express the Cr2 product from the Cr2 gene, whereas FDCs almost exclusively express the Cr1 isoform generated from the Cr2 gene. To define the specific role of Cr1, we created an animal that maintains normal cell-restricted expression of Cr2 but does not express Cr1. Cr1-deficient (Cr1KO) mice develop normal B1 and B2 immature and mature B cell subsets and have normal levels of naive serum Abs but altered levels of natural Abs. Immunization of the Cr1KO animal demonstrates deficient Ab responses to T-dependent, but not T-independent, Ags. Germinal centers from the immunized Cr1KO animal possess a deficiency in activated B cells, similar to that seen for animals lacking both Cr1 and Cr2 or C3. Finally, animals lacking only Cr1 respond similarly to wild-type animals to infections with Streptococcus pneumoniae, a pathogen to which animals lacking C3 or both Cr1 and Cr2 are particularly sensitive. Altogether, these data suggest that the production of Cr1, primarily by FDCs, is critical in the generation of appropriately activated B cells of the germinal center and the generation of mature Ab responses.