Culture supernatants from mock-, PHA- or MLR-stimulated cells were collected at the indicated time points (1~4 days post-stimulation) and used as conditioned medium after filtered through 0.2-μm nylon filter. Cells from fresh tonsils were treated with medium, PHA (10 μg/ml) or conditioned medium, added at 40% v/v, for 24 hours and washed extensively before virus infection. At 48 hours post-infection, cells were stained and virus infectivity was analyzed on flow by assessing GFP expression in CD4+ (A), CD8+ (B), or CD19+ (C) cells. One representative data of 3 independent experiments is shown.