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Background  Passive transfer of antibodies can be protective in the simian human immunodeficiency virus (SHIV) – rhesus macaque challenge model. The human monoclonal antibody IgG1 b12 neutralizes human immunodeficiency type 1 (HIV-1)  in vitro  and protects against challenge by SHIV. Our hypothesis is that neutralizing antibodies can only completely inactivate a relatively small number of infectious virus.    Methods And Findings  We have used GHOST cell assays to quantify individual infectious events with HIV-1 SF162  and its SHIV derivatives: the relatively neutralization sensitive SHIV SF162P4  isolate and the more resistant SHIV SF162P3 . A plot of the number of fluorescent GHOST cells with increasing HIV-1 SF162  dose is not linear. It is likely that with high-dose inocula, infection with multiple virus produces additive fluorescence in individual cells. In studies of the neutralization kinetics of IgG1 b12 against these isolates, events during the absorption phase of the assay, as well as the incubation phase, determine the level of neutralization. It is possible that complete inactivation of a virus is limited to the time it is exposed on the cell surface. Assays can be modified so that neutralization of these very low doses of virus can be quantified. A higher concentration of antibody is required to neutralize the same dose of resistant SHIV SF162P3  than the sensitive SHIV SF162P4 . In the absence of selection during passage, the density of the CCR5 co-receptor on the GHOST cell surface is reduced. Changes in the CD4 : CCR5 density ratio influence neutralization.    Conclusions  Low concentrations of IgG1 b12 completely inactivate small doses of the neutralization resistant SHIV  SF162P3 . Assays need to be modified to quantify this effect. Results from modified assays may predict protection following repeated low-dose shiv challenges in rhesus macaques. It should be possible to induce this level of antibody by vaccination so that modified assays could predict the outcome of human trials.

In Vitro Neutralization of Low Dose Inocula at Physiological Concentrations of a Monoclonal Antibody Which Protects Macaques against SHIV Challenge