We have characterized sera from healthy volunteers immunized with a monomeric recombinant gp120 (rgp120) derived from a CCR5/CXCR4 (R5X4)-using subtype B isolate of human immunodeficiency virus type (HIV-1), HIV-1W61D, in comparison to sera from long-term HIV-1-infected individuals, using homologous reagents. Sera from vaccinees and HIV-1 positive subjects had similar binding titers to native monomeric rgp120W61D and showed a similar titer of antibodies inhibiting the binding of soluble CD4

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... 4) to rgp120W61D. However, extensive peptide binding studies showed that the overall pattern of recognition of vaccinee and HIV-1-positive sera is different, with vaccinee sera displaying a wider and more potent recognition of linear V1/V2 and V3 domain epitopes. Neutralization of homologous HIV-1W61D or heterologous HIV-1M2424/4 peripheral blood mononuclear cell (PBMC)-derived virus lines by vaccinee sera could be achieved, but only after adaptation of the viruses to T-cell lines and was quickly lost on readaptation to growth in PBMC. Sera from HIV-positive individuals were able to neutralize both PBMC-grown and T-cell line-adapted viruses. Interestingly, rgp120W61D was recognized by monoclonal antibodies previously shown to neutralize primary HIV-1 isolates. The use of very potent adjuvants and R5X4 rgp120 led to an antibody response equivalent in binding activity and inhibition of binding of sCD4 to gp120 to that of HIV-positive individuals but did not lead to the induction of antibodies capable of neutralizing PBMC-grown virus.