Stat4 regulates multiple components of IFN-γ-inducing signaling pathways

Stat4 is activated in response to IL-12. Most functions of IL-12, including the induction of IFN-γ, are compromised in the absence of Stat4. Since the precise role of Stat4 in IFN-γ induction has not been established, experiments were conducted to examine Stat4 activation of IFN-γ and other genes required for cytokine-induced expression of IFN-γ. We first examined IL-12 signaling components. Basal expression of IL-12Rβ1 and IL-12Rβ2 is decreased in Stat4-deficient cells compared with that in control cells. However, IL-12 was still capable of inducing equivalent phosphorylation of Jak2 and Tyk2 in wild-type and Stat4-deficient activated T cells. We have further determined flint other cytokine signaling pathways that induce IFN-γ production are defective in the absence of Stat4. IL-18 induces minimal IFN-γ production from Stat4-deficient activated T cells compared with control cells. This is due to defective IL-18 signaling, which results from the lack of IL-12-induced, and Stat4-dependent, expression of the IL-18R. Following IL-12 pretreatment to induce IL-18R, wild-type, but not Stat4-deficient, activated T cells demonstrated IL-18-induced NF-κB DNA-binding activity. In addition, IL-12-pretreated Stat4-deficient activated T cells have minimal IFN-γ production followed by stimulation with IL-18 alone or in combination with IL-12 compared with control cells. Thus, Stat4 activation by IL-12 is required for the function of multiple cytokine pathways that result in induction of IFN-γ.