FIG. 1.

Soluble CD4 induction of the 17b epitope on the envelope glycoproteins of primary HIV-1 isolates. (A) Binding of 17b to radiolabeled envelope glycoproteins. COS-1 cells were transfected with plasmids expressing the different HIV-1 envelope glycoproteins and metabolically labeled. 17b antibody (5 μg/ml) was bound in the absence or presence of sCD4 for 90 min at room temperature, the cells were washed, and the amount bound was determined by precipitation of antibody-gp120 complexes with protein A-Sepharose. (B) Binding of radiolabeled 17b to unlabeled envelope glycoproteins. COS-1 cells were transfected as for panel A. The 17b antibody was metabolically labeled and binding was carried out as for panel A. Bound 17b was determined by precipitation with protein A-Sepharose and quantitated by SDS-PAGE. Data were normalized for the cell surface expression of each envelope glycoprotein, as determined by immunoprecipitation with pooled HIV-1-infected patient serum. The experiment shown is representative of at least six independent experiments.