Antibody 17b binding at the coreceptor site weakens the kinetics of the interaction of envelope glycoprotein gp120 with CD4

HIV-1 utilizes CD4 and the chemokine coreceptor for viral entry. The corece ptor CCR5 binding site on gp120 partially overlaps with the binding epitope of 17b, a neutralizing antibody of HIV-1. We designed a multicomponent bio sensor assay to investigate the kinetic mechanism of interaction between gp 120 and its receptors and the cooperative effect of the CCR5 binding site o n the CD4 binding site, using 17b as a surrogate of CCR5. The Env gp120 pro teins from four viral strains (JRFL, YU2, 89.6, and HXB2) and their corresp onding C1-, V1/V2-, C5-deleted mutants (Delta JRFL, Delta YU2, Delta 89.6, and Delta HXB2) were tested in this study. We found that, across the primar y and lab-adapted virus strains, 17b reduced the affinity of all four full- length Env gp120s for sCD4 by decreasing the on-rate and increasing the off -rate. This effect of 17b on full-length gp120 binding to sCD4 contrasts wi th the enhancing effect of sCD4 on gp120-17b interaction. For the correspon ding loop-deleted mutants of Env gp120, the off-rates of the gp120-sCD4 int eraction were greatly reduced in the presence of 17b, resulting in higher a ffinities (except for that of Delta HXB2). The results suggest that, when 1 7b is prebound to full-length gp120, the V 1/V2 loops may be relocated to a position that partially blocks the CD4-binding site, leading to weakening of the CD4 interaction. Given the fact that the 17b binding epitope partial ly overlaps with the binding site of CCR5, the kinetic results suggest that coreceptor CCR5 binding could have a similar "release" effect on the gp120 -CD4 interaction by increasing the off-rate of the latter. The results also suggest that the neutralizing effect of 17b may arise not only from partia lly blocking the CCR5 binding site but also from reducing the CD4 binding a ffinity of gp120. This negative cooperative effect of 17b may provide insig ht into approaches to designing antagonists for viral entry.