IL-6–derived DC promotes humoral responses in vivo. (A) WT and IL-6–KO mice were inoculated with NP-KLH (50 μg in CFA, intraperitoneally). Sera were collected on day 14 and tested for NP-specific isotypes. (B) KLH-pulsed DCs purified from WT or IL-6^−/− mice (3 × 10⁵ cells) were inoculated intravenously into C57BL6 mice. Sera were collected on day 14 and tested for KLH-specific isotypes. (C,D) KLH-pulsed DCs obtained in panel B were inoculated in f.p., and draining lymph nodes were recovered on day 7. Expression of CXCR5⁺ T_FH cells was analyzed by FACS (C), and lymph nodes cells were tested for IL-4 secretion in the presence of graded doses of KLH (D). (E) CD4⁺ T cells purified from lymph nodes of immunized mice were tested for B-cell help delivery in the presence of CD19⁺ B from WT naive C57BL6 mice and KLH (10 μg/mL). Culture supernatants were tested on day 7 for IgG₁ contents (mean ± SD of triplicate cultures). Numbers in dot-plot quadrants in panel C represent the percentages. Dashed lines in panel A represent normal serum antibody concentrations. **P < .01; ***P < .001. Similar results were obtained in 2 (C,E) or 3 (A,B,D) independent experiments.