STAT3 is required for the acquisition of a B-cell help function by T cells. (A) Naive T cells from WT, STAT4^−/−, STAT6^−/−, and T cell–specific STAT3^−/− mice were stimulated as in Figure 1, in the presence or absence of IL-6, and tested for B-cell help activity in the presence of B cells (5 × 10⁵ cells/well) and anti-CD3 mAbs (500 ng/mL). Culture supernatants were tested on day 7 for IgG₁ and IgG₂a/c contents. (B) Naive T cells were activated 48 hours in the presence of medium, IL-6 (20 ng/mL) or IL-6 plus JSI-124 STAT3 inhibitor (100 mM) and tested for ThB function as in panel A. (C) Control and T cell–specific STAT3^−/− mice were immunized with NP-KLH (50 μg in CFA, intraperitoneally). Sera were collected on day 14 and tested for NP-specific antibodies. (D) RAG-deficient mice were transferred with CD4⁺ T cells from control and T cell–specific STAT3^−/− mice together with C57BL6 purified B cells and immunized on day 1 as in panel C. Results in panels A and B are expressed as mean plus or minus SD of triplicate T-B coculture. SDs are not visible on the line graph. Similar results were obtained in 3 independent experiments. *P < .05; **P < .01.