STAT1 is dispensable for IL-12–induced expression of IL-21 in human CD4⁺ T cells. (A-E) The frequency of naive (CD45RA⁺CCR7⁺), memory (CD45RA⁺CCR7^−/+), and CXCR5⁺CD45RA⁻ CD4⁺ T cells in PBMCs was determined for healthy donors and STAT1-deficient patients. (A-B) Representative dot plots from 1 donor and 1 STAT1-deficient patient. (C-E) The frequency of (C) naive (CD45RA⁺CCR7⁺), (D) memory (CD45RA⁺CCR7^−/+), and (E) CXCR5⁺CD45RA⁻ CD4⁺ T cells from all healthy donors (total CD4⁺ T cells, n = 54; naive CD4⁺ T cells, n = 70; memory CD4⁺ T cells, n = 70; CXCR5⁺CD45RA⁻ CD4⁺ T cells, n = 61) and STAT1-deficient patients (n = 6) was examined. (F-I) Total CD4⁺ T cells isolated from healthy donors and STAT1-deficient patients (n = 3) were cultured for 5 days under neutral (nil) or Th1-polarizing (ie, IL-12) conditions, and expression of intracellular IL-21 (F-G) and IFNγ (H-I) was then determined. The graphs in panels F and H show the frequency of cytokine-positive cells; those in panels G and I depict cytokine expression after Th1 polarization as fold-increase relative to the nil culture in each experiment. The values represent the mean ± SEM (n = 3).