STAT3 mutations impair IL-12–induced expression of IL-21 in naive CD4⁺ T cells. (A-D) Naive CD4⁺ T cells isolated from healthy donors (n = 8) and STAT3_MUT patients (n = 9) were cultured for 5 days under neutral conditions (nil); polarizing Th1, Th2, or Th17 conditions; or in the presence of IL-6, IL-21, IL-23, or IL-27, after which time expression of intracellular IL-21 and IFNγ was determined. (A-B) Representative dot plots of cytokine expression by activated naive CD4⁺ T cells from 1 donor and 1 patient. (C-D) Percentage of activated normal and STAT3_MUT naive CD4⁺ T cells induced to express (C) IL-21 or (D) IFNγ in response to the indicated culture. The values represent the mean ± SEM. (E-F) Naive CD4⁺ T cells isolated from healthy donors and STAT3_MUT patients were labeled with CFSE and cultured with anti-CD2/CD3/CD28 Abs in the absence (nil) or presence of Th1 polarizing conditions (+IL-12). After 5 days cells were harvested, and (E) proliferation and expression of (F) IL-21 and (G) IFNγ in cells that had undergone different divisions were then determined. (F-G) The values represent the mean ± SEM (n = 3).