Induction of perforin expression in response to IL-21 is independent of a requirement for cellular proliferation. PBMCs from healthy controls and patients with HIV were cultured in the presence of the cytokines IL-21 (50 ng/mL) or IL-15 (50 ng/mL) for 5 days and analyzed for (A) proliferation by CFSE dye dilution and (B) perforin expression (mean ± SEM). Analysis was performed on total CD8 T cells as well as maturation subsets. Significance was determined by 1-way ANOVA, cytokine-stimulated versus unstimulated on day 5: *P < .05; **P < .01; ***P < .001. (C) A representative dotplot showing perforin expression in response to IL-21 and IL-15 in CD8 T_EM cells from a healthy control and a patient with HIV. (D) Perforin expression in proliferating and nonproliferating CD8 T-cell maturation subsets of a representative patient with HIV. (E) Perforin and granzyme B expression in CD8 T cells of patients (n = 4) and healthy controls (n = 3). The top 2 panels show prercentages of perforin- or granzyme B–expressing cells; bottom panels show ratios of responses in cytokine-stimulated/unstimulated cultures. In panels A, B, and E, bars represent mean values with one standard error above the mean depicted on top of each bar. For dot plots depicted in panels C and D, numbers in each quadrant represent percentages of cells.