FIG. 1.

Enhanced kinetics of syncytium formation at 37°C after preincubation of E/T cells at 31.5°C. Effectors were 12E1 cells infected for 18 h with recombinant vaccinia virus expressing 89.6 (dual tropic) Env (A) or JR-FL (R5) Env (B). Effector and target (PM1) cells were mixed at a 1:1 ratio and either incubated at 37°C or preincubated at 31.5°C (A and B) or at 23°C (B, broken line) for 60 min before transfer to 37°C. (C) TF228 (IIIB Env) effector cells were mixed with target (PM1) cells and preincubated at 31.5°C for the indicated times before transfer to 37°C. No syncytia were scored at the time of transfer. The data were fitted according to the mathematical model described in Materials and Methods. Statistically significant differences in lag time and fusion rate constant were seen after 30 and 60 min of preincubation at 31.5°C (see Table 1).