IL-21 protects cells from apoptosis and induces perforin in antigen-specific CD8 T cells. (A) PBMCs obtained from patients with HIV and healthy individuals were cultured in the presence or absence of IL-21 (50 ng/mL) or IL-15 (50 ng/mL) for 5 days and then stained with annexin V and 7-AAD to detect the percentage of apoptotic cells. Data were analyzed by using FLOWJO software. The results are representative of 2 independent experiments. (B) PBMCs isolated from a patient with HIV were cultured for 5 days with medium, IL-21 (50 ng/mL), and IL-15 (50 ng/mL). CD3⁺CD8⁺ T cells were gated to determine perforin expression in Gag-tetramer⁺ cells.