CD21 AUGMENTS ANTIGEN PRESENTATION IN IMMUNE INDIVIDUALS

CD21 (complement receptor 2, CR2) binds the C3 degradation products, i C3b and C3d, which are covalently linked to antigen or immune complexe s in the process of complement activation. The ability of antigen-nons pecific B cells to present immune complexes containing high titers of acquired antibodies was tested. Influenza virus was incubated with ser um from immune donors to create complement-containing complexes. These bound specifically to CD21 on transfected fibroblasts and B cell line s, as measured by microcytofluorimetry. Binding of immune complexes wa s ablated by inactivation of serum complement. In addition, the immuno globulin in immune human serum blocked influenza binding to cells in t he absence of complement, implying a minimal role for immunoglobulin-F c receptor interactions in this system. Significant immune complex bin ding required a threshold level of CD21 expression, suggesting that on ly those cells with the highest levels of CD21 are likely to participa te in the processing of macromolecular antigens. B cells pulsed with c omplement-influenza complexes elicited an augmented response from a pa nel of influenza-specific, class II-restricted T cell clones, as compa red with those which had bound immunoglobulin-influenza complexes lack ing complement. This enhanced response did not require CD35. In additi on, B cell lines expressing higher levels of CD21 were more efficient in processing antigen than those with lower levels. These findings sug gest that presentation of antigen by B cells in immune individuals is dependent on the binding of complement-antigen immune complexes to CD2 1.