Antibody-mediated neutralization of primary human immunodeficiency virus type 1 isolates: Investigation of the mechanism of inhibition

Human immunodeficiency virus type 1 (HIV-1) neutralization occurs when spec ific antibodies, mainly those directed against the envelope glycoproteins, inhibit infection, most frequently by preventing the entry of the virus int o target cells. However, the precise mechanisms of neutralization remain un clear. Previous studies, mostly with cell lines, have produced conflicting results involving either the inhibition of virus attachment or interference with postbinding events. In this study, we investigated the mechanisms of neutralization by immune sera and compared the inhibition of peripheral blo od mononuclear cells (PBMC) infection by HIV-1 primary isolates (PI) with t he inhibition of T-cell line infection by T-cell line-adapted (TCLA) strain s. We followed the kinetics of neutralization to determine at which step of the viral cycle the antibodies act. We found that neutralization of the TC LA strain HIV-1(MN)/MT-4 required an interaction between antibodies and cel l-free virions before the addition of MT-4 cells, whereas PI were neutraliz ed even after adsorption onto PBMC, In addition, the dose-dependent inhibit ion of HIV-1(MN) binding to MT-4 cells was strongly correlated with serum-i nduced neutralization. In contrast, neutralizing sera did not reduce the ad hesion of PI to PBMC, Postbinding inhibition was also detected for HIV-K,, produced by and infecting PBMC, demonstrating that the mechanism of neutral ization depends on the target cell used in the assay. Finally, we considere d whether the different mechanisms of neutralization may reflect the recogn ition of qualitatively different epitopes on the surface of PI and HIV-1(MN ) or whether they reflect differences in virus attachment to PBMC and MT-4 cells.