Binding of the soluble 2DLT and D1D2 proteins to gp120 protein or to gp120 expressed on the cell surface. A) Binding of sCD4, D1D2, 2DLT, T1144 to rgp120 as measured by ELISA. The data are representative of results from three similar experiments performed in triplicate (means ± SD). B) The binding affinity of sCD4 (a), D1D2 (b), 2DLT (c), and T1144 (d) to gp120 as determined by SPR assay. The recombinant gp120 (rgp120) was immobilized onto the CM3 sensor chip. The proteins and peptide at various concentrations were injected onto the surface. The affinity constant of each sample was calculated in one-site binding modes by BIAcore evaluation software. C) Binding of sCD4, D1D2, 2DLT, and T1144 to the HIV-1 Env (gp120/gp41) expressed on CHO-WT cell surfaces as determined by flow cytometry. CHO-EE cells expressing no HIV-1 Env molecule were used as control cells.