Epitopes for neutralizing antibodies induced by HIV-1 envelope glycoprotein BG505 SOSIP trimers in rabbits and macaques

Abstract

The native-like, soluble SOSIP.664 trimer based on the BG505 clade A env gene of HIV-1 is immunogenic in various animal species, of which the most studied are rabbits and rhesus macaques. The trimer induces autologous neutralizing antibodies (NAbs) consistently but at a wide range of titers and with incompletely determined specificities. A precise delineation of immunogenic neutralization epitopes on native-like trimers could help strategies to extend the NAb response to heterologous HIV-1 strains. One autologous NAb epitope on the BG505 Env trimer is known to involve residues lining a hole in the glycan shield that is blocked by adding a glycan at either residue 241 or 289. This glycan-hole epitope accounts for the NAb response of most trimer-immunized rabbits but not for that of a substantial subset. Here, we have used a large panel of mutant BG505 Env-pseudotyped viruses to define additional sites. A frequently immunogenic epitope in rabbits is blocked by adding a glycan at residue 465 near the junction of the gp120 V5 loop and β24 strand and is influenced by amino-acid changes in the structurally nearby C3 region. We name this new site the “C3/465 epitope”. Of note is that the C3 region was under selection pressure in the infected infant from whom the BG505 virus was isolated. A third NAb epitope is located in the V1 region of gp120, although it is rarely immunogenic. In macaques, NAb responses induced by BG505 SOSIP trimers are more often directed at the C3/465 epitope than the 241/289-glycan hole.

Mapping polyclonal antibody responses in non-human primates vaccinated with HIV Env trimer subunit vaccines

“…The primary interaction between the V1/V3 Fab and the apex of the trimer is between the CDRH3 of the Fab, which navigates around glycans at positions N133 and N137 to make peptide contacts at residues N137 and I138, T139 of the trimer V1 loop, with the CDRL3 making contacts with R143 within the same region ( Figure 7C and 7D). We previously reported virus neutralization data that implicated an epitope that includes residues N133 and N137, which are part of the V1 loop, as part of these antibodies' binding (Klasse et al, 2018). Further, Pauthner et al reported up to nearly 100-fold decreases in neutralization titer in this animal upon amino acid insertions in the vicinity of these residues (Pauthner et al, 2017).…”

“…In an attempt to map NHP antibody responses in the context of both differing neutralization titers and levels of protection from a SHIV challenge (Pauthner et al, 2017, we applied our EMPEM approach to identify and characterize vaccine-induced antibodies that correlated with serum neutralizing titers, and consequently protection . In contrast to rabbits, where the 241/289 glycan hole and trimer base are the predominant targets of the antibody response, several additional epitopes were identified to be immunogenic in NHPs, some of which are likely responsible for the bulk of the neutralization activity (Cirelli et al, 2018;Klasse et al, 2018;Pauthner et al, 2017). These epitopes include the FP, V1/V3, and a newly identified epitope at the gp120/gp120 interface.…”

“…, suggesting viral evolution away from the infecting virus and concomitant antibody evolution. The remaining three animals exhibited antibody responses in and around the N289/S241 glycan hole region (GH-1/GH-2) that is a highly immunogenic epitope in rabbits (McCoy et al, 2016;Bianchi et al, 2018;Klasse et al, 2018), and the C3/V5 region of gp120 that we have previously described as the "C3/T465 epitope" in NHPs. Thus, considering a sample of only six animals and a time span of just 20 weeks under viral challenge pressure, the infected control macaques exhibited a relatively narrow repertoire of antibody epitope responses to the founding virus, with five different 3D Fab phenotypes detected against two regions of Env ( Figure 1B and 1C).…”

“…Further, these unwanted responses may be competing with elicitation of more desirable bnAb. We currently only have a limited understanding of the diversity in the responses to SOSIP trimers which has been derived primarily from rabbit immunization studies Klasse et al, 2018;Mccoy et al, 2016;Pauthner et al, 2017). To investigate the specificity of polyclonal antibody responses, we recently developed an electron microscopy-based polyclonal epitope mapping technique, whereby total serum IgG is isolated, digested in fragments antigen binding (Fabs) and complexed with Env SOSIP.664 trimers before they are imaged using negative-stain or cryoEM.…”

Neutralizing antibody responses to an HIV envelope glycan hole are not easily broadened

“…Our previous work with BG505 immunogens in rabbits revealed that the dominant autologous nAbs target the glycan hole created by the absence of glycans at positions 230, 241, and 289 (13). Moreover, autologous neutralization specific for glycan holes was also seen following immunization with trimers from different clades (19). To determine whether the B41 neutralizing mAbs also targeted a glycan hole on the B41 immunogen, we tested the neutralization activity of eight isolated mAbs representing the different autologous nAb families using a panel of B41 mutant pseudoviruses with the N230 and N289 glycans knocked-in (FIG 2A&B).…”