Passively transmitted gp41 antibodies in babies born from HIV-1 subtype C-seropositive women: correlation between fine specificity and protection

Abstract

HIV-exposed, uninfected (EUN) babies born to HIV-infected mothers are examples of natural resistance to HIV infection. In this study, we evaluated the titer and neutralizing potential of gp41-specific maternal antibodies and their correlation with HIV transmission in HIV-infected mother-child pairs. Specific gp41-binding and -neutralizing antibodies were determined in a cohort of 74 first-time mother-child pairs, of whom 40 mothers were infected with HIV subtype C. Within the infected mother cohort, 16 babies were born infected and 24 were PCR negative and uninfected at birth (i.e., exposed but uninfected). Thirty-four HIV-uninfected and HIV-unexposed mother-child pairs were included as controls. All HIV-positive mothers and their newborns showed high IgG titers to linear epitopes within the HR1 region and to the membrane-proximal (MPER) domain of gp41; most sera also recognized the disulfide loop immunodominant epitope (IDE). Antibody titers to the gp41 epitopes were significantly lower in nontransmitting mothers (P < 0.01) and in the EUN babies (P < 0.005) than in HIV-positive mother-child pairs. Three domains of gp41, HR1, IDE, and MPER, elicited antibodies that were effectively transmitted to EUN babies. Moreover, in EUN babies, epitopes overlapping the 2F5 epitope (ELDKWAS), but not the 4E10 epitope, were neutralization targets in two out of four viruses tested. Our findings highlight important epitopes in gp41 that appear to be associated with exposure without infection and would be important to consider for vaccine design.

Fig 1

Quantification of IgG and IgA in serum samples from all mother-baby pairs. The left panel shows serum IgG quantification. The right panel shows serum IgA quantification. Bars represent immunoglobulin concentrations in HIV healthy control unexposed mothers and in their babies (HC-Mo and HC-N, respectively), nontransmitter mothers and their babies (NTr-Mo and EUN-N, respectively), and transmitter mothers and their babies (Tr-Mo and HIV+N).

Fig 2

Binding antibodies to gp41 in HIV-infected women and their children. The graphs show the percentage of maternal and cord blood serum specimens that were found to contain high-titer antibodies to gp41 peptides (A) and the mean values (1/n) plus standard errors of the means found in mothers and in their children (B). All specimens were tested in duplicate. TR, transmitting mothers; NT, nontransmitting mothers.

Fig 3

HIV neutralization assays of either babies or mothers. Neutralization was calculated as the IC₅₀ (range and means) corresponding to the sample dilution leading to 50% neutralization. (A) SF162 (R5); (B) IIIB (X4); (C) DU156 (C-R5); (D) DU172 (C-R5); (E) SVA.MLV#922 (control). (F) IC₅₀ and IC₉₀ (values and ranges) obtained with TriMab. The y axis reports the concentration (μg/ml) of antibodies instead of the neutralizing titer (1/n). All samples were tested in duplicate.

Fig 4

Neutralization specificity of anti-HIV antibodies from single or pooled cord blood serum samples. All neutralization assays were performed on the DU156 HIV isolate (C-R5) with cord blood specimens from EUN babies; statistical significance (two-tailed t test) is indicated in each panel. Both controls and cord blood samples were tested in three replicates. (A) CB100 preadsorbed with scrambled and 12-C peptides; (B) CB77 preadsorbed with scrambled peptide and peptide 3; (C) CB70 preadsorbed with scrambled peptide and peptide 12-C; (D) CB70 preadsorbed with scrambled peptide and peptide 3; (E) pool A (five random, ELISA-negative samples) preadsorbed with scrambled peptides and peptide 27; (F) pool B (five random, ELISA-positive samples) preadsorbed with scrambled peptides and peptide 27; (G) pool C (five random, ELISA-positive samples) preadsorbed with scrambled peptide and peptide 27; (H) pool of CB serum from 5 HIV-uninfected babies preadsorbed with scrambled peptide and peptides 3, 12-C, and 27; (I) TriMab neutralization control. The x axis from panels A to H report serum dilution as 1/n, panel I reports concentrations of TriMab. Peptide sequences are given in Table 1. Scr., scrambled.

Fig 5

Inhibition of HIV transcytosis across HEC-1 epithelial cells, forming a model of human epithelium. Cord blood specimens from EUN newborns were tested in transcytosis inhibition assays using the HIV-1 SF162 strain (two replicas). Results are expressed in percentages compared to results with 2F5 MAb (>85% inhibition; positive control) and a pool of HIV-negative cord blood samples (100% transcytosis; negative control [CTRL]).