IL-2 and IL-15 predominantly activate STAT5 in CD3 T cells from both healthy individuals and patients with HIV, while IL-21 activates STAT5 and STAT3. PBMCs isolated from healthy individuals (A) and patients with HIV (B) were cultured in medium or stimulated with IL-2 (1000 IU/mL), IL-15 (50 ng/mL), and IL-21 (50 ng/mL) for 15 minutes, and expression of surface CD3 and phosphorylated epitopes for STAT3, STAT4, and STAT5 were determined on fixed and permeabilized cells by flow cytometry in healthy controls (n = 5) and patients with HIV (n = 3). Bars represent mean values of percentages of CD8⁺ T cells positive for phosphorylated STATs. Error bars depict one standard error above mean. (C) Representative histogram plots from 1 healthy control and 1 patient with HIV comparing STAT phosphorylation by medium or the cytokines IL-2, IL-15, and IL-21.