The cell binding site of simian immunodeficiency virus (SIV) is believed to be the CD4 molecule. Several CD4 cell lines are, however, resistant to infection by SIV251 and additional cell membrane molecules have been implicated in SIV251 entry. We investigated the binding, envelope fusion and entry of the viral core proteins (p27) of SIV251 into two human CD4 cell lines (H9 and Sup-T1) which are infectible, and one CD4 (A3.01) and two CD4 cell lines (K562 and Raji) that are resistant to infection. The fusion of the viral and cellular membranes was monitored by a fluorescence assay for lipid mixing. Cell entry of the viral core was evaluated following virus-cell incubation and cell surface trypsinization. We found that SIV251 can bind to and fuse (membrane lipid mixing) in a temperature-dependent but pH-independent fashion with CD4 and CD4 human-derived cell lines. In contrast, lipid mixing with CD4 expressing EL-4 mouse T cells or Mv-1-lu mink lung fibroblasts was absent or limited, suggesting that certain components of human cell membranes in addition to CD4 are involved in SIV envelope-cell fusion. Lipid mixing with the human cells was inhibited partially by soluble CD4. Anti-CD4 antibodies inhibited the lipid inter-mixing with H9, but not with Raji cells, whereas neutralizing anti-SIV sera inhibited fusion with both CD4 and CD4 cells. Out of the five human cell lines tested, efficient entry of p27 and productive infection took place only with H9 and Sup-T1 cells. In these two cases, the amounts of p27 internalized during virus-cell fusion correlated with the extent of infection.


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