IFN-I can actively promote quiescence in virus-specific CD8⁺ T cells. Cell-cycle progression in activated CD8⁺ EBV tetramer⁺ T cells and in noncycling resting PBMC control cells was determined by tritiated thymidine incorporation (A). Active EBV virus-specific CD8⁺ EBV-tetramer⁺ T cells were subsequently cultured in fresh medium alone or fresh medium supplemented with exogenous IL-15 or IFN-I. Proliferation and cell survival were determined 4 days later by tritiated thymidine incorporation (B) and trypan blue exclusion (C), respectively (dotted line represents 100% survival). These experiments are representative of 5 separate experiments. cpm indicates counts per minute. Error bars represent standard error from the mean.