CHARACTERIZATION OF KINETICS AND TARGET PROTEINS FOR BINDING OF HUMAN-COMPLEMENT COMPONENT-C3 TO THE SURFACE-EXPOSED OUTER-MEMBRANE OF CHLAMYDIA-TRACHOMATIS SEROVAR-L2

In order to characterize the interaction of human complement with Chla mydia trachomatis, flow cytometry was used to quantitate binding of co mplement component C3 to elementary bodies of C. trachomatis serovar L 2 preincubated in fresh serum in the presence or absence of human poly clonal chlamydial antibody. Isolation of each of the complement activa tion pathways revealed that C3 was activated most effectively by the a lternative pathway. The degree of binding by the classical pathway was proportional to the concentration of antibody, but dual-pathway-media ted binding was not greater than antibody-independent alternative path way binding. Electrophoresis and immunoblotting of detergent-extracted outer membrane protein-C3b complexes indicated that the chlamydial ma jor outer membrane protein was the primary cell surface moiety binding C3b in both the presence and absence of specific antibody. Hydroxylam ine cleavage of outer membrane protein-C3b complexes provided evidence that the majority of C3b is bound to the major outer membrane protein by hydroxyl ester bonds. This result was also unchanged by the presen ce of specific antibody. An unexpected finding was the apparent bindin g of anti-C3 antibody to a 40-kDa protein of the chlamydial outer memb rane complex, perhaps indicating C3 mimicry on the part of the chlamyd ial major outer membrane protein.