Antibody-mediated neutralization of primary isolates of human immunodeficiency virus type 1 in peripheral blood mononuclear cells is not affected by the initial activation state of the cells

Antibody-mediated neutralization of human immunodeficiency virus type 1 (HIV-1) was evaluated with primary isolates and sera from infected individuals, using human peripheral blood mononuclear cells (PBMC) activated with phytohemagglutinin 1 day after virus inoculation (resting-cell assay) or 2 days prior to virus inoculation (blast assay). Assays were performed exclusively with syncytium-inducing (SI) isolates since non-SI isolates replicated poorly or not at all in the resting-cell assay. Ninety percent neutralization was difficult to achieve in both assays for most virus-serum combinations tested. Of particular note, virus replication in the absence of antibody was delayed 2 to 3 days in the resting-cell assay. At least part of this delay was due to a decrease in virus infectivity; the 50% tissue culture infectious dose of primary isolates was 25 to 30 times lower in the resting-cell assay than in the PBMC blast assay. When a broadly neutralizing serum and the same dilution of virus were used in both assays, neutralization was greater in the resting-cell assay than in the blast assay on day 7, but neutralization was equal in both assays when measurements were made 3 days sooner in the PBMC blast assay. Both assays had the same level of detection on day 7 when the amount of virus mixed with antibody and added to cells was standardized according to infectivity for the respective target cells. Thus, when the infectious dose was adjusted, the two assays were equally sensitive for detecting antibody-mediated neutralization of primary isolates of HIV-1. These results indicate that primary isolates of HIV-1 are difficult to neutralize in both assays and that the detection of neutralization is not affected by the initial activation state of PBMC.