Fig. (4) The effect of anti-Env antibodies on syncytium formation between Env⁺ cells and SupT1 cells. The Env+ cells were incubated with the antibodies for 30 min before the addition of the SupT1 cells. The cells were then incubated for 3 h at 37°C, washed twice with PBS, and then observed by fluorescence microscopy. (A) 4 µg/ml 4E10 antibody; (B) 4 µg/ml 2F5 antibody; (C) 4 µg/ml IgG1 b12 antibody; (D) 4 µg/ml IgG1 m14 antibody. (E) 10 µg/ml 2G12 antibody; (F) Untreated control cells. IgG1 b12 and IgG1 m14 had some fusion-inhibitory activity, whereas 4E10 and 2F5 antibodies were ineffective. The anti-gp41 4E10 and 2F5 antibodies did not inhibit the binding of the redorange SupT1 cells to the green Env⁺ cells.