Background: We recently completed a humoral immune correlate analysis of MTCT risk in the U.S. Women and Infant Transmission study (WITS), a historical cohort followed before availability of antiretroviral therapy. We observed that plasma IgG responses against the Envelope (Env) third variable (V3) loop and tier 1 virus neutralization predicted reduced risk of MTCT. To follow up on this study, we investigated the binding specificity of maternal plasma anti-V3 IgG antibodies and their contribution to the tier 1 virus neutralization response.
Methods: Maternal plasma samples (n=248) were tested for IgG binding against a multi-clade panel of Env V3 peptides by a binding antibody multiplex assay to identify amino acid differences associated with IgG binding strength. A peptide competition neutralization assay with SF162 V3, scrambled and no peptide was used to measure the contribution of V3-specific antibodies to tier 1 virus (SF162) neutralization in TZM-bl cells. A regression model was used to investigate the association between neutralization titers and MTCT risk.
Results: The maternal plasma V3-specific IgG response against the clade B peptide was high in magnitude (median MFI 23,177), but, binding was abolished (median MFI < 100) against the CRF1 V3 peptide, which differed from clade B peptide at 3 amino acid residues (K305T, I307T and A317R). Moderate plasma binding was observed against the clade A peptide (median MFI 8,739), which differed from the clade B peptide at positions I307V and H308R. Based on the shared amino acid differences between the V3 peptides, positions 305, 307, 308, and 317 flanking the V3 loop tip appear to be critical for maternal plasma V3 binding. Maternal plasma tier 1 virus neutralization significantly decreased in the presence of SF162 V3 as compared to the scrambled peptide and no peptide (median ID50 671, 1399, 2930, for SF162 V3 peptide, scrambled peptide, and no peptide, respectively; p<0.0001 for both). The median plasma tier 1 virus neutralization inhibition with the V3 peptide was 50% (IQR 9-75%), yet the proportion of the maternal plasma neutralization inhibited by the V3 peptide did not predict MTCT risk.
Conclusions: Our results suggest that in clade B HIV-1-infected women, V3 is an important tier 1 virus neutralization epitope that is dependent on residues that flank the V3-loop tip for IgG binding. Yet, additional epitope specificities may also contribute to the maternal neutralization responses associated with decreased MTCT risk.