CD28 induction by IL-12.

(A) CD4⁺CD28^null clones were stimulated with anti-CD3 mAb alone or in the presence of increasing concentrations of IL-12. Histograms of a representative experiment are shown. IL-12 at 20 ng/mL was optimal to induce CD28 expression. (B) The time course for CD28 expression was analyzed by serial flow cytometric analysis of an anti-CD3 mAb/IL-12–stimulated T-cell clone. Results are expressed as mean fluorescence intensity of CD28 expression (▾) compared with the isotype control (●) at each time point. CD28 expression was maximal after 6 days of culture in IL-12. (C) To examine whether re-expression of CD28 was transient, T cells were stimulated with anti-CD3 mAb and IL-12 for 5 days and then maintained in the absence of IL-12. CD28 expression decreased again and was absent at day 20 after initial stimulation. A representative example of 3 experiments is shown for a clone (upper panel) and a line (lower panel). In all histograms, the shaded areas represent staining with an isotype control.