ANTIBODY-INDEPENDENT BINDING OF COMPLEMENT COMPONENT C1Q BY LEGIONELLA-PNEUMOPHILA

Incubation of Legionella pneumophila Philadelphia 1 in normal human se rum depleted of either classical-pathway component C1q or alternative- pathway component factor B resulted in activation of the complement sy stem. Experiments focused on the role of the classical pathway in comp lement activation revealed that legionellae bound C1q independently of antibody. Purified preparations of L. pneumophila major outer membran e protein but not serogroup 1 lipopolysaccharide bound C1q independent ly of antibody. This suggests that antibody-independent binding of C1q by L. pneumophila can result in activation of the classical pathway i n normal human serum and that major outer membrane protein may be a C1 q acceptor on the L. pneumophila cell surface.