COMPLEMENT ACTIVATION BY A B-CELL SUPERANTIGEN

Staphylococcal protein A (SpA), acting as a B cell superantigen, binds to the Fab region of human V(H)3(+) Igs. Using SpA abrogated of its I gG Fc binding activity (Mod SpA) as a model B cell superantigen, we de termined whether such an interaction causes complement activation. Add ition of Mod SpA to human serum led to complement consumption and the generation of C3a. To determine whether this complement activation 1) was due to an interaction between V(H)3(+) Igs and the Fab binding sit e of SpA and 2) proceeded via the classical complement pathway, we tes ted a panel of monoclonal IgM proteins for the ability to bind C1q fol lowing interaction with SpA, Clq binding was restricted to SpA-reactiv e, V(H)3(+) IgM proteins. To formally determine whether the binding of SpA to the reactive V(H)3(+) IgM proteins led to complement activatio n, we reconstituted the serum from a hypogammaglobulinemic patient wit h monoclonal IgM proteins and measured complement consumption and C3a generation following the addition of Mod SpA. We observed complement c onsumption and C3a production only in Mod SpA-treated serum reconstitu ted with a V(H)3(+), SpA-binding, IgM protein. Taken together, these r esults provide compelling evidence that the interaction of the Fab bin ding site of SpA and V(H)3(+) Igs can lead to complement activation vi a the classical pathway. This novel interaction may have significant i mplications for the in vivo properties of a B cell superantigen.