Figure 3.

Binding of X5, Fab b12, and 17b to cell-surface-associated oligomeric gp120-gp41. The T-cell line H9 was infected with the TCLA X4 HIV-1_MN isolate for 10 days. At this time postinfection there was no detectable CD4 remaining at the cell surface, no syncytium formation in the culture, but strong Env expression detected using gp120-specific mAbs and flow cytometry. These H9 cells were preincubated with sCD4 (20 μg/ml) or buffer alone for 1 h at 20°C, then incubated with Fab X5, 17b, or Fab b12 at the indicated concentrations. The amount of bound antibodies was measured by flow cytometry and represented in arbitrary units. The continuous lines represent fitting of the binding data by using the Langmuir adsorption isotherm as described in the legend of Fig. 1.