Epitope exposure on functional, oligomeric HIV-1 gp41 molecules

We have used cells infected with the HIV-1 molecular clone HX10 to study the binding of monoclonal antibodies (mAbs)to different epitopes within the extracellular domain of the HIV-1 transmembrane glycoprotein gp41. Gp41 mAb binding to the infected cells at 4° was variable but weaker than the binding of an anti-gp120/V3 loop mAb and increased substantially for three of the gp41 antibodies at 37°. Treatment of the cells with soluble CD4 (sCD4) at 37° increased gp41 mAb binding to epitopes spanning residues 521-663, implying that these regions had probably been masked by gp120, which following interaction with sCD4 had subsequently dissociated from gp41. By contrast, the binding of a mAb to residues 662-667 which form a neutralization epitope was reduced by sCD4 binding. Another region which has been described as containing a neutralization epitope spans residues 725-750. MAbs to this region bound equally well to noninfected and HIV-infected cells, and binding was not increased in the presence of sCD4. These data strongly imply that this epitope is not exposed on the external surface of the membrane, a finding in accord with the proposed cytoplasmic localization of this region.