SENSITIVITY OF HIV-1 TO NEUTRALIZATION BY ANTIBODIES AGAINST O-LINKEDCARBOHYDRATE EPITOPES DESPITE DELETION OF O-GLYCOSYLATION SIGNALS IN THE V3 LOOP

It has been suggested that threonine or serine residues in the V3 loop of HIV-1 gp120 are glycosylated with the short-chain O-linked oligosa ccharides Tn or sialosyl-Tn that function as epitopes for broadly neut ralizing carbohydrate specific antibodies. In this study we examined w hether mutation of such threonine or serine residues could decrease th e sensitivity to infectivity inhibition by Tn or sialosyl-Tn specific antibodies. All potentially O-glycosylated threonine and serine residu es in the V3 loop of cloned HIV-1(BRU) were mutagenized to alanine thu s abrogating any O-glycosylation at these sites. Additionally, one of these T-A mutants (T308A) also abrogated the signal for N-glycosylatio n at N306 inside the V3-loop. The mutant clones were compared with the wild type virus as to sensitivity to neutralization with monoclonal a nd polyclonal antibodies specific for the tip of the V3 loop of BRU or for the O-linked oligosaccharides Tn or sialosyl-Tn. Deletion of the N-linked oligosaccharide at N306 increased the neutralization sensitiv ity to antibodies specific for the tip of the loop, which indicates th at N-linked glycosylation modulates the accessibility to this immunodo minant epitope. However, none of the mutants with deletions of O-glyco sylation signals in the V3 loop displayed any decrease in sensitivity to anti-Tn or anti-sialosyl-Tn antibody. This indicates that these bro adly specific neutralization epitopes are located outside the V3 loop of gp120.