PP2-156 STAT3 negatively regulates antiviral responses through suppression of TLR and type I IFN-mediated responses

Type I interferons (IFNs) are crucial cytokines for innate immunity to combat viral or bacterial infections. Upon ligand binding to IFN receptors, signal transducer and activator of transcription (STAT) proteins, including STAT1, STAT2, and STAT3 are activated. While an essential role of STAT1 and STAT2 for type I IFN-mediated antiviral response has been demonstrated in gene-targeted mice, the role of STAT3 remains to be determined. Here, we report that STAT3KO mouse embryonic fibroblasts (MEFs) and bone marrow-derived macrophages (BMMs) display enhanced IFNα responses, including increased activation of STAT1 and expression of IFN downstream genes compared to their wild-type (WT) counterparts. Moreover, STAT3KO cells also display increased IFN-mediated antiviral response to EMCV or VSV infections, with increased survival rates and decreased viral titers. Restoration of STAT3 back to STAT3KO MEFs suppresses the otherwise enhanced IFN responses, suggesting that the altered phenotypes in STAT3KO cells are intrinsic to the loss of STAT3. Interestingly, enhanced expression of type I IFN and activation of both STAT1 and STAT2 are also observed in STAT3KO MEFs following infection of EMCV. In addition, expression of MDA5 and RIG-I, two cytosolic sensors for viral RNAs is also increased in STAT3KO cells as opposed to WT cells. Knockdown of MDA5 in STAT3KO MEFs significantly reduces IFN-mediated antiviral response, suggesting that increased levels of MDA5 contribute to enhanced antiviral response in STAT3KO cells. In reporter assays, overexpression of STAT3 down-regulates IFNα-driven ISRE but not GAS promoter activities. Similarly, STAT3 also suppresses poly I:C-driven IFNβ and ISRE promoter activities in a dose-dependent manner in 293-TLR3 cell line. Lastly, enhanced production of proinflammatory cytokines such as TNFα, IL-6, and IL-12 and type I IFNs is observed in STAT3KO mice in vivo in response to stimulation of poly I:C or R848, two TLR ligands mimicking viral infections. Taken together, these results suggest that STAT3 negatively regulates antiviral responses through, at least, two mechanisms, namely suppression of TLR-mediated responses and type I IFN signaling.