Effect of sialylation of lipopolysaccharide of Neisseria gonorrhoeae on recognition and complement-mediated killing by monoclonal antibodies directed against different outer-membrane antigens.

Growth of gonococci in the presence of CMP-N-acetylneuraminic acid (CMP-NANA) has previously been shown to induce resistance to the bactericidal effect of normal human serum and is accompanied by sialylation of the gonococcal lipopolysaccharide (LPS). We have used monoclonal antibodies (mAbs) to compare the effect of LPS sialylation on recognition of gonococci and complement-mediated killing by antibodies directed either against LPS or against defined epitopes on outer-membrane protein PI. Despite differences in binding to sialylated LPS on Western blots, all three mAbs directed against LPS showed considerably reduced binding to gonococci grown in the presence of CMP-NANA and a concomitant reduction in ability to promote complement-mediated killing. In contrast, mAbs directed against previously defined epitopes on a surface exposed loop of PI showed little difference in binding between sialylated and non-sialylated gonococci and promoted killing of the sialylated gonococci. Similarly a mAb directed against an epitope on a loop of the outer-membrane Rmp protein, which had previously been shown to block killing by antibodies directed against other surface antigens, also exerted a blocking effect with sialylated gonococci. Thus in the present study the continued biological effect of mAbs was correlated with the ability of the antibody to recognize surface-exposed epitopes on sialylated gonococci. Despite the presence of the sialylation which is likely to occur in vivo, it should be possible to induce complement-mediated killing by focusing the immune response to those surface-exposed epitopes which are least susceptible to the potential inhibitory effect of LPS sialylation.