HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 MEMBRANE-FUSION MEDIATED BY A LABORATORY-ADAPTED STRAIN AND A PRIMARY ISOLATE ANALYZED BY RESONANCE ENERGY-TRANSFER

Previous studies of human immunodeficiency virus type 1 (HIV-1) envelo pe glycoprotein-mediated membrane fusion have focused on laboratory-ad apted T-lymphotropic strains of the virus. The goal of this study was to characterize membrane fusion mediated by a primary HIV-1 isolate in comparison with a laboratory-adapted strain. To this end, a new fusio n assay was developed on the basis of the principle of resonance energ y transfer, using HeLa cells stably transfected with gp120/gp41 from t he T-lymphotropic isolate HIV-1(LAI) or the macrophage-tropic primary isolate HIV-1(JR-FL). These cells fused with CD4(+) target cell lines with a tropism mirroring that of infection by the two viruses. Of part icular note, HeLa cells expressing HIV-1(JR-FL)gp120/gp41 fused only w ith PM1 cells, a clonal derivative of HUT 78, and not with other T-cel l or macrophage cell lines. These results demonstrate that the envelop e glycoproteins of these strains play a major role in mediating viral tropism. Despite significant differences exhibited by HIVJR-FL and HIV -1(LAI) in terms of tropism and sensitivity to neutralization by CD4-b ased proteins, the present study found that membrane fusion mediated b y the envelope glycoproteins of these viruses had remarkably similar p roperties. In particular, the degree and kinetics of membrane fusion w ere similar, fusion occurred at neutral pH and was dependent on the pr esence of divalent cations. Inhibition of HIV-1(JR-FL) envelope glycop rotein-mediated membrane fusion by soluble CD4 and CD4-IgG2 occurred a t concentrations similar to those required to neutralize this virus. I nterestingly, higher concentrations of these agents were required to i nhibit HIV-1(LAI) envelope glycoprotein-mediated membrane fusion, in c ontrast to the greater sensitivity of HIV-1(LAI) virions to neutraliza tion by soluble CD4 and CD4-IgG2. This finding suggests that the mecha nisms of fusion inhibition and neutralization of HIV-1 are distinct.