Epitopes corresponding to the envelope genetic subtype are present on the surface of free virions of HIV-1 group M primary isolates and can be detected in neutralization assays with extended incubation phases.

The hypothesis is that there are neutralizing epitopes on the surface of free virions of human immunodeficiency virus type 1 (HIV-1) that correspond to the genetic subtype of the envelope glycoprotein. Assays with extended incubation and reduced absorption phases are required to demonstrate neutralization with antibodies to these epitopes. These assays quantify virus infectivity, rather than reductions in release of antigen into culture supernatants. Neutralizing antibodies reduce virus infectivity by at least 80%, as scored by the presence/absence of antigen released after 14 days in culture of mitogen-transformed peripheral blood mononuclear cells (PBMCs). The epitopes are shared within different subtypes of group M, but not group O, isolates. Individual plasma, selected from three, independent panels of seropositive individuals, cross-neutralize within each subtype as well as the combinations of A with C, B with D or G, and C with CRF01_AE. Isolates within subtype B show the greatest variation in their resistance to neutralization, ranging from highly sensitive to highly resistant. No highly sensitive subtype D isolates were identified. Isolates from subtypes A, C, and CRF01_AE were all resistant. The strategic implication for vaccine design is that antibodies to a limited number of epitopes can neutralize more than 90% of the HIV-1 isolates that are circulating currently in the world. Also, since only antibodies that produce an all-or-nothing loss in virus infectivity can reasonably be expected to prevent the viremic phase after in vivo infection, assays with extended incubation, and culture phases should be used to monitor current efficacy trials.