Inter‐subtype cross‐neutralizing antibodies recognize epitopes on cell‐associated HIV‐1 virions

Abstract

HIV-1 infected individuals with cross-neutralizing antibodies against primary HIV-1 isolates belonging to Group M (envA-H) and O, are identified. To investigate the neutralization-kinetics of primary isolates with these antibodies, different neutralization assay conditions are compared. Each set is summarized as a/b/c where a is the time in hours for which antibody is incubated with virus, b is the time in hours allowed for virus to absorb to cells, c is the total culture period in days, from the cells' first exposure to virus, before antigen production (peripheral blood mononuclear cells) or number of fluorescent cells (GHOST) are measured. In HIV-infected individuals, neutralizing antibodies can be detected against a wide range of primary isolates (Group M; A-H and Group O) in PBMC-assays with short incubation phases (1/2/7 or 1/24/7). If cultures are extended (1/2/14 or 1/24/14), however, neutralization can be lost. In kinetic experiments, neutralization can even be seen without pre-incubation (a=0 hr). This study shows that neutralization of primary HIV isolates by cross-reactive antibodies can continue after the virus has bound to its target cell. This neutralization, however, is not an all or nothing loss in virus infectivity. Most often it leads only to a reduction in viral replication rates.

Protection in Macaques Immunized with HIV-1 Candidate Vaccines Can Be Predicted Using the Kinetics of Their Neutralizing Antibodies

“…The results from the present and previous studies [10], [17], [18] indicate that multiple steps are involved in HIV-1 neutralization so that the virus-antibody complex remains infectious. Antibody binds to the free virion since neutralization increases as the incubation phase is extended.…”

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

“…This index quantifies, by definition, a loss in virus infectivity: it does not matter how much antigen a single infectious dose of virus releases into the culture supernatant as long as this is enough to be registered as significant in an antigen capture ELISA. The culture phase has been extended to 14 days since previous experience has shown that a single infectious dose of virus can be detected after this interval in PBMC cultures even if its replication has been slowed down after exposure to HIV-1 seropositive plasma [Donners et al, 2003a]. In order to detect antibodies binding to the outside of the virion, the incubation phase of the assay has been extended.…”

“…The format of a traditional neutralization assay (e.g., 1/2/7) is based on the assumption that antibody binds to a virion when both are suspended in fluid. For primary isolates of HIV, this assumption may not be valid [Spenlehauer et al, 2001;Donners et al, 2003a]. Neutralization with primary HIV isolates may continue, and may only start, after the virus has bound to its target cell.…”

“…Neutralization with primary HIV isolates may continue, and may only start, after the virus has bound to its target cell. From previous studies [Donners et al, 2003a], it would have been expected that the effect of inter-subtype cross-neutralizing antibodies would no longer be detectable after 14 days in culture and yet plasma X1-X3 each neutralize 11 or 12 of 13 sensitive isolates. Therefore, this prediction may not be correct.…”

“…In a previous report from the present laboratory, it was suggested that the antibodies able to neutralize HIV-1 isolates from multiple subtypes recognized epitopes that were exposed when the virions bound to their target cells [Donners et al, 2003a]. The principal contribution to the final neutralization index (the titer of the virus divided by its titer after exposure to antibody) was determined by the absorption phase of the assay.…”