Background:  Neutralizing antibodies that inhibit infection with a broad array of primary isolates are rarely detected in sera from HIV-1-infected humans, and only a few broadly neutralizing monoclonal antibodies have been isolated so far. Two major relatively conserved epitopes, corresponding to the neutralizing monoclonal antibodies 2G12 and b12, and a cluster of 2 adjacent epitopes, corresponding to the neutralizing monoclonal antibodies 2F5 and 4E10, have been defined on the envelope glycoproteins gp120 and gp41, respectively. Previous studies showed that broadly neutralizing antibodies are more frequent in long-term non-progressors (LTNP) than they are in other infected persons, but nothing is known about the envelope regions targeted by these antibodies. Here, we addressed the question of a putative association between the neutralizing capacity of sera and the detection of specific antibodies directed to the known conserved broadly neutralizing epitopes.

Methods:  The capacity of 67 sera from the French ANRS cohort of LNTP patients to neutralize 4 heterologous, primary isolates of various clades was analyzed by means of an assay using CD4⁺CXCR4⁺CCR5⁺ HeLa cells. Competitive and non-competitive enzyme-linked immunosorbent assays were developed to compare 2F5 (and/or 4E10), b12 or 2G12-like antibody levels in neutralizing and non-neutralizing LTNP sera. As controls, antibodies directed to 2 non-broadly neutralizing major epitopes, the V3 loop and the gp41 immunodominant epitope, were also targeted.

Results:  Of the 67 sera, 11 samples (16%) showed broadly neutralizing activity to the 4 strains. The levels of 2G12-like antibodies in these broadly neutralizing sera were higher than in all other sera (median of 11.3 µg/mL vs 4.6 µg/mL with interquartile range from 5.4 µg/mL to 18.7 µg/mL vs 2.3 µg/mL to 7.8 µg/mL). This difference was statistically significant (non-parametric Mann-Whitney test, p = 0.03). The levels of antibody to the other envelope target epitopes were not statistically different among broadly and non-broadly neutralizing sera.

Conclusions:  Higher 2G12-like antibody levels are associated with broad neutralizing activity in LTNP sera, suggesting that the antigenicity of the “silent face” of gp120 should continue to be analyzed in depth to help find ways to induce broadly neutralizing antibodies.