Altered CD4 Interactions of HIV Type 1 LAI Variants Selected for the Capacity to Induce Membrane Fusion in the Presence of a Monoclonal Antibody to Domain 2 of CD4

We selected HIV-1-LAI variants with the ability to induce syncytium formation of C8166 cells in the presence of a monoclonal antibody (MAb), 5A8, to domain 2 of CD4. Five biologically cloned variants with at least 60-fold greater resistance than wild type to 5A8-mediated inhibition of syncytium formation were obtained. The variants exhibited reduced relative sensitivity to inhibition of syncytium formation and virus infection, not only by the selecting anti-domain 2 MAb, but also by MAbs to domains 1 and 3 of CD4. By contrast, the sensitivity of these variants to neutralization by soluble CD4 and bivalent CD4–IgG was greater than for the parental clone. The affinities of soluble CD4 for Env protein, in either solubilized or membrane-anchored form, did not differ significantly between the variants and LAI. Analyses of sCD4-induced exposure of the transmembrane protein at 4 and 37°C suggested, however, that the variants had acquired an increased susceptibility to the triggering of conformational changes in their Env oligomers at 37°C. This may represent a mechanism of both the increased resistance to the CD4 MAbs and the enhanced sensitivity to soluble CD4.