Cytopathic variants of an attenuated isolate of human immunodeficiency virus type 2 exhibit increased affinity for CD4.

Naturally occurring isolates of human immunodeficiency virus (HIV) have been described which are deficient in their ability to fuse with and kill CD4+ target cells. Although the molecular basis for their attenuation has not yet been defined, several lines of evidence point toward the viral envelope gene as a key determinant of viral pathogenicity. In the present article, we report the biological characterization of two highly cytopathic variants derived by repeated cell-free passage of an attenuated isolate of HIV type 2 (HIV-2), termed HIV-2/ST. Unlike the parental virus, the cytopathic variants were found to infect Sup-T1 cells with great efficiency and to induce both cell fusion and profound killing in these cultures. To determine whether changes in the viral envelope gene were responsible for the observed phenotypic differences, we examined the CD4 binding affinity of these viruses using a novel assay designed to quantitate the binding of fluoresceinated CD4 to viral envelope in its native configuration on the cell surface. The results demonstrated that the affinity of parental HIV-2/ST envelope for CD4 was 2 orders of magnitude reduced, while the cytopathic variants exhibited a high CD4 binding affinity, comparable to that of cytopathic HIV-1 and HIV-2 isolates. From these data, we conclude that the cytopathic potential of HIV depends, at least in part, on its receptor-binding affinity. In addition, our study documents strong selection pressures for viruses with increased CD4 affinity during propagation in immortalized T-cell lines, thus emphasizing the need to study HIV envelope biology in natural target cells.