IL-12 selectively induces Tfh phenotype and function in human naive CD4⁺ T cells. Peripheral blood naive CD4⁺ T cells were cultured in media only (Nil) or with TAE beads alone (Th0) or under Th1, Th2, or Th17 conditions. (A–C) After 3 d, the cells were harvested and expression of (A) CXCR5, (B) ICOS, and (C) CD40L were determined by flow cytometry. The histogram plots (top) depict expression of the indicated surface receptor by cells cultured in media (solid gray) or under Th0 (blue), Th1 (green), Th2 (orange), or Th17 (red) conditions. The graphs (bottom) represent fold change in expression of the indicated cell surface marker relative to the Th0 culture (mean ± SEM; n = 4). *, P < 0.05; **, P < 0.01; ****, P < 0.0001 compared with Th0 (ANOVA). (D and E) After 5 d, activated CD4⁺ T cells were treated with mitomycin C and co-cultured with sort-purified allogeneic naive B cells in the presence of TAE beads for 7 d. (D) Secretion of IgM, IgG, and IgA (n = 3) and (E) expression of PRDM1 (Blimp1; n = 2) relative to the Th0 cultures were determined by ELISA and qPCR, respectively. Values represent mean ± SEM. **, P < 0.01.