Triggering of Toll‐like receptors modulates IFN‐γ signaling: involvement of serine 727 STAT1 phosphorylation and suppressors of cytokine signaling

Abstract

Microbial stimuli activate cells of the innate immune system by triggering Toll‐like receptors (TLR). Activation of macrophages and dendritic cells is further enhanced by secondary signals like IFN‐γ. Here we analyzed the interplay of IFN‐γ and TLR signaling in cells of the innate immune system. Using a STAT1‐dependent reporter construct we show that IFN‐γ signaling can be enhanced as well as inhibited by simultaneous stimulation with either defined TLR agonists or whole‐bacterial lysates. Short costimulation resulted in the amplification of IFN‐γ signaling and was attributable to the p38 mitogen‐activated protein kinase (MAPK)‐dependent phosphorylation of signal transducer and activator of transcription (STAT)1 on serine 727. In contrast, prolonged co‐incubation as well as pre‐incubation with TLR agonists led to an inhibition of IFN‐γ signaling. TLR triggering induced expression of suppressor of cytokine signaling (SOCS)‐1, SOCS‐3 and cytokine‐inducible SH2 domain‐containing protein (CIS). Overexpression of SOCS‐1 and, to a lesser extend, of SOCS‐3 and CIS inhibited IFN‐γ signaling as measured by activation of STAT1. Moreover, pre‐incubation with TLR‐dependent stimuli impaired IFN‐γ‐induced MHC class II regulation but enhanced CD40 and CD86 expression. Taken together, the results indicate a tight interplay between TLR and IFN‐γ signaling pathways which involve induction of SOCS proteins and serine phosphorylation of STAT1.