CD28 costimulation is required not only to induce IL-12 receptor but also to render Janus kinases/STAT4 responsive to IL-12 stimulation in TCR-triggered T cells

The activation of resting T cells for the acquisition of various functions depends on whether CD28 costimulatory signals are provided upon T cell receptor stimulation. Here, we investigated how CD28 costimulation functions to allow TCR-triggered resting T cells to acquire IL-12 responsiveness. When T cells are stimulated with low doses of anti-CD3 mAb, CD28 costimulation was required for the optimal levels of IL-12 receptor (IL-12R) expression. However, stimulation of T cells with high doses of anti-CD3 alone induced comparable levels of IL-12R expression to those induced upon CD28 costimulation. Nevertheless, there was a substantial difference in IL-12 responsiveness between these two groups of T cells: compared to anti-CD28-costimulated T cells, T cells that were not costimulated with anti-CD28 exhibited decreased levels of Janus kinases (JAK) JAK2/TYK2 and STAT4 phosphorylation and IFN-γ production following IL-12 stimulation. Importantly, STAT6 phosphorylation following IL-4 stimulation was not decreased in anti-CD28-uncostimulated T cells. These resutls indicate that CD28 costimulation not only contributes to up-regulating IL-12R expression but is also required to render JAKs/STAT4 responsive to IL-12 stimulation.