SOCS1 Regulates the IFN but Not NFκB Pathway in TLR-Stimulated Human Monocytes and Macrophages

Abstract SOCS1 can regulate TLR-mediated signal transduction, yet mechanistic studies in murine macrophages have been confusing and contradictory. This study has used an adenoviral transfection system to determine the role of SOCS1 in the regulation of TNF-α production by activated human monocytes. Monocytes were infected with AdV-SOCS1 or with an empty vector control, AdV-GFP, for 24 h before activation with the TLR4 ligand, LPS. SOCS1 did not regulate TNF-α mRNA or protein production within the first two hours of TLR4 activation. However, SOCS1 suppressed the sustained production of TNF-α by primary human monocytes and synovial fluid macrophages ex vivo. In addition, SOCS1 regulated the production of IL-6, but not IL-10, by monocytes. Analysis of the early signaling pathway downstream of TLR4 demonstrated that SOCS1 had no regulatory effect on the activation or on the DNA binding capacity of NFκB. The late effects of LPS are mediated in part through the MyD88-independent pathway activating IRF3 and initiating the production of IFN-β. In response to adenoviral infection and before LPS exposure, monocytes expressed enhanced levels of IFN-β and Myxovirus A mRNA, an anti-viral molecule characterizing IFN-β activity. These two genes were reduced in AdV-SOCS1-infected cells. Further, SOCS1 regulated IFN-dependent pathways in LPS-activated cells as evidenced by reduced IFN-β production and STAT1 phosphorylation. Using AdV-infection to dissect SOCS1 control of IFN-dependent pathways, this study suggests that SOCS1-regulation of the IFN-dependent component of the LPS-induced TLR4 signaling pathway may contribute to the down-regulation of inflammatory cytokine production by AdV-SOCS1-infected human monocytes.