Cyclosporin A, antagonist of C-18, interferes with its CCR5 fusion-blocking activity. (A) Cyclosporin A (CsA) reduces C-18 blocking of HIV-1 R5-env cell fusion. Serial 3-fold dilutions of C-18 (from 10 to 0.36 μg/mL) were preincubated with CsA (50 μg/mL) for one hour at 37°C and were then added to PM1 cells for an additional one hour at 37°C. The untreated or treated PM1 cells were mixed with effector cells expressing either JR-FL Env (shown) or Ba-L Env (not shown). Syncytia were scored between 3 and 4 hours. Calculated ID₅₀ is as follows: C-18, 4.2 μg/mL; C-18 + CsA, 130 μg/mL. The experiment shown is representative of 3 experiments performed. (B) Preincubation of C-18 with Cyclosporin A (CsA) significantly reduces its ability to inhibit HIV-1 infection. C-18 (serial dilutions) was incubated with CsA (50 μg/mL) for one hour at 37°C before adding to PM1 cells for one hour at 37°C. Virus was then added at 100 TCID₅₀/well (5 replicates per group) and incubated with cells for 2 days before extensive washings. The ID₅₀ values were calculated according to Reed and Muench. Data shown are for day 11 (Ba-L) or day 21 (JR-FL). CsA alone at the same concentration had no inhibitory or stimulatory effects on HIV infection in any of the assays. Error bars represent SD of the means of 3 replicates per data point.