Association of antibodies blocking HIV-1 gp160-sCD4 attachment with virus neutralizing activity in human sera

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Abstract
Sera, from HIV-1 and HIV-2 seropositive individuals, were tested for the presence of antibodies able to inhibit the binding (BI) of HIV-IIIB gp160 (produced in mammalian cells using a vaccinia expression system) to the extracellular portion of the CD4 receptor. A competition enzyme immunoassay (EIA) with scluble CD4 (sCD4) was used. BI antibodies were highly prevalent among HIV-1 seropositives but not in HIV-2 infected individuals. Attempts to localize the binding site for these BI antibodies on the primary sequence of gp120 by using synthetic peptides encompassing the putative CD4 binding site on gp120 (aa 397–439) were not successful. This study did not reveal a significant correlation between the presence of BI antibodies and disease evolution. BI antibody titres correlated less well with anti-gp160 titres (r=0.51, P⩽0.011) than with neutralizing antibody (NA) titres using either the isolates HIV-SF2 (SF2) (r=0.77, P⩽0.000) and HIV-MN (MN) (r=0.61, P⩽0.002) or the isolate HIV-IIIB (HX10) (r=0.89, P⩽0.000) of which the gp160 for the assays was derived. An HIV-IIIB neutralizing serum, elicited in a rabbit by immunization with a 17-mer synthetic peptide derived from the third variable domain (V3) of gp120, did bind gp160 without inhibiting the subsequent attachment of sCD4 to gp160. These results indicate that (conformational) epitopes on gp160 recognized by antibodies that inhibit the attachment to CD4 may be identical or closely linked to the binding sites (not in the V3 domain) for HIV-1 (group-specific) neutralizing antibodies.