STAT3-deficient cells show impaired Tfh cell function in vitro. Naive CD4⁺ T cells isolated from healthy donors (HD) and STAT3_MUT patients (Pt) were cultured under neutral (nil) or Th1 polarizing conditions. After 5 days, the cells were harvested and treated with mitomycin C before being cocultured with allogeneic naive B cells, in the absence or presence of exogenous IL-21, for an additional 7 days. After this time secretion of IgM, IgG, and IgA was determined. (A-B) The data were derived from experiments that used naive CD4⁺ T cells isolated from different STAT3_MUT patients; (C) the data show the effect of exogenous IL-21 on the ability of STAT3_MUT CD4⁺ T cells to provide B-cell help. Each graph represents the mean ± SEM of triplicate cultures; similar results were obtained in 5 (A-B) and 2 (C) experiments.