JAK1/STAT3 activation directly inhibits IL-12 production in dendritic cells by preventing CDK9/P-TEFb recruitment to the p35 promoter.

Inhibition of Janus-activated kinase-1 (JAK1) is a promising clinical concept for post-transplant immunosuppression and autoimmunity. However, it also raises concerns regarding possible immunosuppressive side effects. Our study investigates JAK1 signalling in the context of CD40L and bacterially activated human MoDC using siRNA and biological inhibitors. We demonstrate that strong stimuli (e.g. intact Escherichia coli or LPS in addition to IL-1β) induce IL-12p70 via a ROS/RELA/CDK9 pathway that is inhibited by simultaneous JAK1/STAT3 signalling. Transcription is effective if RELA recruits the positive transcription elongation factor b (P-TEFb) component CDK9 to a combined RELA/STAT3 binding site -50 to -20bp upstream of the start site of the IL-12p35 promoter. STAT3 simultaneously attaches to this site and inhibits CDK9 binding. In the presence of IFNγ, JAK1/2 inhibitors block STAT1/IRF1/IRF8-dependent activation and simultaneously enhance CDK9-dependent activation signals. This inverse regulation of IFNγ- vs. E. coli-induced cytokine production by JAK inhibitors including Ruxolitinib was similarly observed for IL-6 and TNF-α production, but not for IL-10 production. Thus, JAK1 inhibition enhances IL-12p70 production in this context by increased DNA binding of CDK9. In contrast, weak RELA-activation signals (CD40L, LPS) depended on IFN-γ induced STAT1/IRF1/IRF8 co-signalling, which was completely blocked by JAK inhibitors as reported before. Our results suggest a novel molecular mechanism of how cytokine responses to invading pathogens are separable from IFNγ-dependent autoimmunity by targeting JAK1/STAT3 activation.