Complement receptor 2 (CR2/CD21) is part of the B-cell coreceptor and expressed by mature B cells and follicular dendritic cells. CD21 is a receptor for C3d-opsonized immune complexes and enhances antigen-specific B-cell responses.Objective
Genetic inactivation of the murineCR2locus results in impaired humoral immune responses. Here we report the first case of a genetic CD21 deficiency in human subjects.METHODS
CD21 protein expression was analyzed by means of flow cytometry and Western blotting.CD21transcripts were quantified by using real-time PCR. TheCD21gene was sequenced. Wild-type and mutantCD21cDNA expression was studied after transfection of 293T cells. Binding of EBV-gp350 or C3d-containing immune complexes and induction of calcium flux in CD21-deficient B cells were analyzed by means of flow cytometry. Antibody responses to protein and polysaccharide vaccines were measured.Results
A 28-year-old man presented with recurrent infections, reduced class-switched memory B cells, and hypogammaglobulinemia. CD21 receptor expression was undetectable. Binding of C3d-containing immune complexes and EBV-gp350 to B cells was severely reduced. Sequence analysis revealed a compound heterozygous deleterious mutation in theCD21gene. Functional studies with anti-immunoglobulin– and C3d-containing immune complexes showed a complete loss of costimulatory activity of C3d in enhancing suboptimal B-cell receptor stimulation. Vaccination responses to protein antigens were normal, but the response to pneumococcal polysaccharide vaccination was moderately impaired.Conclusions
Genetic CD21 deficiency adds to the molecular defects observed in human subjects with hypogammaglobulinemia.