EPITOPE EXPOSURE ON FUNCTIONAL, OLIGOMERIC HIV-1 GP41 MOLECULES

We have used cells infected with the HIV-I molecular clone HX10 to stu dy the binding of monoclonal antibodies (mAbs) to different epitopes w ithin the extracellular domain of the HIV-1 transmembrane glycoprotein gp41. Gp41 mAb binding to the infected cells at 4 degrees was variabl e but weaker than the binding of an anti-gp120/V3 loop mAb and increas ed substantially for three of the gp41 antibodies at 37 degrees. Treat ment of the cells with soluble CD4 (sCD4) at 37 degrees increased gp41 mAb binding to epitopes spanning residues 521-663, implying that thes e regions had probably been masked by gp120, which following interacti on with sCD4 had subsequently dissociated from gp41. By contrast, the binding of a mAb to residues 662-667 which form a neutralization epito pe was reduced by sCD4 binding. Another region which has been describe d as containing a neutralization epitope spans residues 725-750. MAbs to this region bound equally well to noninfected and HIV-infected cell s, and binding was not increased in the presence of sCD4. These data s trongly imply that this epitope is not exposed on the external surface of the membrane, a finding in accord with the proposed cytoplasmic lo calization of this region. (C) 1995 Academic Press, Inc.