Rabbit
immunoglobulin G (
IgG)
antibodies raised against the major outer
membrane protein of the Chlamydia trachomatis
lymphogranuloma venereum strain 434 neutralized the infectivity of the parasite for HeLa 229 cells. The mechanism by which anti-major outer
membrane protein IgG prevented C. trachomatis from establishing
infection was studied by using intrinsically 14C-radiolabeled elementary bodies. Neutralized elementary bodies were filterable through a
polycarbonate filter (pore diameter, 600 nm), demonstrating that reduction in infectivity was not due to the aggregation of elementary bodies by cross-linking
IgG. Antibody-neutralized elementary bodies attached to and penetrated HeLa cells at rats nearly identical to those for infectious organisms exposed to nonneutralizing control
IgG. These results suggest that antibody interferes with the infectious process of the parasite after its internalization. Anti-major outer
membrane protein Fab fragments could not be substituted for neutralizing
IgG antibodies. The requirement for intact
IgG implies that cross-linking of
antibodies to the major outer
membrane protein on the surfaces of the organisms may be instrumental in neutralization.