Inhibition of Neisseria gonorrhoeae complement-mediated killing during acute gonorrhoea is dependent upon the IgG2:IgG3 antibody ratio

Excessive binding of antibodies to the bacterial cell surface can paradoxically increase resistance of some Gram-negative pathogens to complement-mediated killing (CMK). We examined CMK of 336 Neisseria gonorrhoeae clinical isolates sampled from participants recruited to a clinical trial. Serum bactericidal assays revealed 3% (9/336) of the autologous participant sera that were tested inhibited CMK. Gonococci isolated from these participants were resistant to the autologous host serum, sensitive to a pool of healthy control sera (HCS) and protected by the host serum in a 1:1 mixture with HCS. Analysis of the clinical metadata showed that there were a significantly higher proportion of inhibitory sera found in participants with urethral infections and from men within the transmission network of men who have sex with women (MSW), when compared to the whole cohort. Following antibody purification from selected participants with inhibitory sera (5/9), IgG and IgM protected the autologous isolates from HCS-mediated killing. Only three of these isolates were protected by purified IgA. A closer examination of IgG subclasses using whole gonococcal cell ELISAs revealed a strong correlation between increased IgG2 binding and decreased IgG3 binding to the bacterial cell surface of isolates that were resistant to CMK. This suggests that IgG2 prevents bactericidal IgG3 from initiating CMK and that the IgG2:IgG3 ratio is important for determining either inhibition or killing of isolates. We therefore reveal a previously unreported mechanism by which inhibitory antibodies prevent CMK of N. gonorrhoeae.