HIV-1 gp120-CD4i Antibody Complex Elicits CD4 Binding Site Specific Antibody Response in Mice

Elicitation of broadly neutralizing antibody (bNAb) responses toward the conserved HIV-1 Envelope (Env) CD4 binding site (CD4bs) by vaccination is an important goal for vaccine development and yet to be achieved. The outcome of previous immunogenicity studies suggests that the limited accessibility of the CD4bs and the presence of predominant non-neutralizing determinants (nND) on Env may impede the elicitation of bNAbs and their precursors by vaccination. Here we designed a panel of novel immunogens that 1) preferentially expose the CD4bs by selective elimination of glycosylation sites flanking the CD4bs, and 2) minimize the nND immune response by engineering fusion proteins consisting of gp120 core and one or two CD4-induced (CD4i) monoclonal antibodies (mAbs) for masking nND epitopes, referred to as gp120-CD4i fusion proteins. As expected, the fusion proteins possess improved antigenicity with retained affinity for VRC01-class CD4bs-directed bNAbs and dampened affinity for non-neutralizing antibodies. We immunized C57BL/6 mice with these fusion proteins and found that overall the fusion proteins elicit more focused CD4bs antibody response than prototypical gp120 core by serological analysis. Consistently, we found that mice immunized with selected gp120-CD4i fusion proteins have higher frequencies of germinal center activated B cells and CD4bs-directed memory B cells than those inoculated with parental immunogens. We isolated three mAbs from mice immunized with selected gp120-CD4i fusion proteins and found that their footprints on Env are similar to VRC01-class bNAbs. Thus, utilizing gp120-CD4i fusion proteins with selective glycan deletion as immunogens could focus antibody response toward CD4bs epitope.