Ritonavir decreases Fas-L expression, but does not alter Fas-R expression in CD4⁺ cells stimulated with PHA. Samples of normal PBMC were cultured with PHA for 72 hours at 37°C at different concentrations of ritonavir. There was a significant dose-dependent decrease in Fas-L expression with ritonavir, although there was no change in Fas-R expression.