STAT1 mutations do not affect the generation of plasmablasts but impair sustained Ig secretion. (A–F) Naive (A–C) and memory (D–F) B cells were sort-purified from normal donors or STAT1_MUT patients and then cultured (25 × 10³/well/100 µl) with CD40L alone (open symbols in B and E) or together with IL-21 (closed symbols in B and E). The generation of plasmablasts, defined as cells acquiring a CD38^hiCD27^hi phenotype (A, B, D, and E), as well as secretion of IgM, IgG, and IgA (C and F), was determined after 4, 5.5, and 7 d. The contour plots (A and D) are representative of plasmablasts detected after 5 d of culture. The graphs depicting Ig secretion are from cultures of CD40L/IL-21–stimulated B cells. The values represent the mean (±SEM for B, C, E, and F) of experiments using cells from two normal donors and two STAT1_MUT patients. Similar results were obtained in a second independent experiment.