Maternal neutralizing antibodies against a CRF01_AE primary isolate are associated with a low rate of intrapartum HIV-1 transmission

Abstract

Mother-to-child transmission (MTCT) of HIV-1 provides a model for studying the role of passively acquired antibodies in preventing HIV infection. We determined the titers of neutralizing antibodies (NAbs) against six primary isolates of clades B and CRF01_AE in sera from 45 transmitting and 45 nontransmitting mothers matched for the main independent factors associated with MTCT in Thailand. A lower risk of MTCT, particularly for intrapartum transmission, was associated only with higher NAb titers against the CRF01_AE strain, MBA. The envelope glycoprotein of this strain showed an unusually long V2 domain of 63 amino acids, encoding six potential N-linked glycosylation sites. We provided experimental data indicating that the extended V2 domain contributed to the higher level of resistance to neutralization by mothers' sera in this strain. Taken together the data suggest that some primary isolates with specific properties may be useful indicators for identifying protective antibodies.

Figure 1. Comparison of neutralizing antibody titers in transmitting (T) and nontransmitting (NT) mothers

(A) Comparison of NAb titers against the six strains in transmitting and nontransmitting mothers. Box plots show the distribution of maternal antibody titers; for each distribution, the horizontal lines represent the lower adjacent 25^th, median, 75^th, and upper adjacent percentiles. (B) Comparison of NAb titers against the MBA strain in nontransmitting mothers and transmitting mothers according to time of transmission (in utero or intrapartum). Nab titers were compared using the Wilcoxon matched-pairs signed-ranks test. NS: not significant.

Figure 2. Sequence characteristics, and their location, in the MBA envelope glycoprotein

The Env amino-acid sequences of C1712, LEA and MBA were aligned using the BioEdit package version 5.0.9. Only regions showing characteristics specific to MBA are indicated. Amino-acid numbering is based on MBA amino-acid Env sequence. Identical amino acids and insertions are indicated by dots or dashes, respectively. PNGS (NXT or NXS) are highlighted in grey.

Figure 3. Effect of the MBA V2 domain on LEA neutralization

Neutralization activity in sera from 10 mothers was determined against viruses pseudotyped with the parental Env proteins from LEA (crosses) and MBA (open squares), and with the chimeric envelope protein V2MBAEnvΔV2LEA (closed triangles). Percentage of neutralization is plotted against serum dilution. The sera used are indicated at the top of each panel.

Figure 4. Construction of the V2 chimeric Envs

(A) Env sequences encoding donor V2 domains (MBA and LEA) were amplified by PCR using specific primers that anneal at both ends of the V2 domain. (B) Env sequences encoding recipient env backbones within the pCR2.1 vector (pCR2.1–envΔV2MBA and pCR2.1–envΔV2LEA) were amplified by PCR using specific primers that anneal to regions adjacent to the V2 primers. A 5’ phosphate group was added to these primers for ligation to the V2 amplicon. (C) Sequences encoding the V2 domains and recipient backbones were blunt-end ligated to generate chimeric env genes in pCR2.1 (pCR2.1–V2MBA-envΔV2LEA and pCR2.1–V2LEA-envΔV2MBA).