IL-6 promotes humoral responses. (A) CD62L⁺CD4⁺ T cells purified from WT or KO mice were stimulated for 48 hours with plastic-coated anti-CD3 and anti-CD28 mAbs. Where indicated, IL-6 was added in the primary culture medium. Cells were washed, rested 1 day in fresh media, and restimulated as in Figure 1. (B) CD62L⁺CD4⁺ T cells were stimulated in the presence or absence of recombinant IL-4 or IL-6 (20 ng/mL). Recovered cells were rested 1 day in fresh medium and tested for B-cell help capacity. (C) Flow cytometric analyses of IL-4 and IFN-γ production for each T-cell population obtained in B. (D) CD62L⁺CD4⁺ DO11.10 T cells were stimulated in the presence or absence of IL-6, rested 1 day in fresh medium, and restimulated in the presence of CD19⁺ B cells purified from TNP-KLH–immunized mice and OVA-TNP (500 ng/mL). Culture supernatants were tested for TNP-specific IgG₁ and IgG₂a antibody contents on day 7. (E) Balb/C inoculated with NP-KLH (50 μg in CFA, intraperitoneally) were further inoculated 1 day later with IL-6–encoding or control (eGFP) plasmid DNA by hydrodynamic tail vein injection. Sera were collected on day 14 and tested for NP-specific isotypes. *P < .05; **P < .01. Results in panels A, B, and D are mean plus or minus SD of triplicate T-B cocultures. Similar results were obtained in 2 (D), 3 (A,C,E), and up to 30 (B) independent experiments.