Complement processing and immunoglobulin binding to Neisseria gonorrhoeae determined in vitro simulates in vivo effects

Local inflammation elicited by Neisseria gonorrhoeae correlates closely wit h sensitivity to killing by normal human serum. Serum-sensitive (SS) isolat es are rendered resistant in vitro by lipooligosaccharide sialylation. Diff erences in C3b processing on N. gonorrhoeae in vitro were found to match fi ndings at the cervical level in vivo. Nonsialylated SS gonococci bound 5-fo ld more C3b than did stably serum-resistant (SR) gonococci; most was proces sed to iC3b, yet significant C3b persisted. Sialylated SS gonococci bound 4 -fold less total C3 antigen than did SR gonococci, which was promptly conve rted to iC3b. C3b bound later on stably SR gonococci but again was processe d swiftly to iC3b. In vivo, the iC3b/C3 ratio of SS isolates more closely r esembled nonsialylated SS isolates in vitro, implying heterogeneous sialyla tion or desialylation in vivo. In vitro, total IgM bound was unchanged by s ialylation of SS isolates, but total C4 bound decreased by 75%, suggesting that sialylation may indirectly regulate the classical complement pathway.