FIG. 2.

Chemokine receptor and gp120 requirements for membrane exchanges and syncytium formation. Quantification of syncytia formed after overnight coculture (A) and of double-fluorescent cells formed after 2-h coculture of HeLa-P4 cells or U373-CCR5 target cells with an equivalent number of Env-negative HeLa cells (lanes 1 and 4), X4 Env⁺ cells (HeLa-Env/LAI, lanes 2 and 5) or R5 Env⁺ cells (HeLa-Env/ADA, lanes 3 and 6) (B). Cocultures were initiated by adding ∼2 × 10⁵ target cells to an equivalent number of adherent cells. Target cells were stably transfected with a Tat-inducible lacZ transgene, and their coculture partners expressed HIV-1 Tat, which allowed us to detect syncytia as blue foci after staining with X-Gal. In panel B, target cells were labeled with DiI, and coculture partners were labeled with DiO. Approximately 5,000 cells were analyzed by flow cytometry, and the percentage of double-fluorescent cells was determined as described in the legend of Fig. 1. Results represent the mean values from three independent experiments ± standard errors of the means.