Factors Determining the Breadth and Potency of Neutralization by V3-Specific Human Monoclonal Antibodies Derived from Subjects Infected with Clade A or Clade B Strains of Human Immunodeficiency Virus Type 1

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Abstract
The neutralizing activities of anti-V3 antibodies for HIV-1 isolates is affected both by sequence variation within V3 and by epitope masking by the V1/V2 domain. To analyze the relative contribution of V3 sequence variation, chimeric Env genes that contained consensus V3 sequences from seven HIV-1 subtypes in the neutralization-sensitive SF162 Env backbone were constructed. Resulting viral pseudotypes were tested for neutralization by 15 anti-V3 MAbs isolated from humans infected with viruses of either subtype B (anti-V3 B MAbs) or subtype A (anti-V3 A MAbs). Pseudovirions with the subtype B consensus V3 sequence were potently neutralized (IC 50 < 0.006 μg/ml) by all but one of these MAbs, while pseudovirions with V3 subtypes A, C, F, H, AG, and AE were generally neutralized more effectively by anti-V3 A MAbs than by anti-V3 B MAbs. A V1/V2-masked Env version of SF162 Env with the consensus B V3 sequence was also neutralized by these MAbs, although with considerably lower potency, while similarly masked chimeras with V3 sequences of subtype A, C, or AG were weakly neutralized by anti-V3 A MAbs but not by anti-V3 B MAbs. Mutations in the V1/V2 domain of YU-2 Env increased the sensitivity of this highly resistant Env to a pool of anti-V3 B MAbs several thousand-fold. These results demonstrated (i) the exceptional sensitivity of representative V3 domains of multiple subtypes to neutralization in the absence of epitope masking, (ii) the broader neutralizing activity of anti-V3 A MAbs for viruses containing diverse V3 sequences, and (iii) the generality and dominant effect of V1/V2 masking on restriction of V3-mediated neutralization.

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