SOCS3 Limits Pro‐Inflammatory Signature in Septic Endothelium

During sepsis, the innate immune system releases multiple inflammatory cytokines in a process known as the cytokine storm, which results in a severe and persistent inflammatory response. The cytokine storm affects practically all aspects of endothelial cell function and is thought to be the key factor in the progression from sepsis to organ failure. Interleukin 6 (IL-6), a major mediator of the cytokine storm during shock, activates JAK kinases to phosphorylate the transcription factor STAT3. In turn, STAT3 promotes expression of SOCS3, which leads to a potent negative regulation of this pathway. The aim of this study is to determine the role of SOCS3 in the regulation of the intensity and duration of IL-6 signaling in the endothelium by assessing the response to endotoxin of mice lacking SOCS3 specifically in the endothelial cells (SOCS3iEKO). Two weeks after tamoxifen induced SOCS3 deletion, we induced severe acute inflammation by a single IP injection of LPS. While this treatment was not lethal in control mice, SOCS3iEKO mice died in less than 24 hours after injection, with death starting at ~16 hours post-treatment. Furthermore, a severity score applied at 15 hours showed an increased severity in the endotoxemic mice, which was further aggravated by the SOCS3 deficiency. We also detected a 10-fold increase in whole tissue IL-6 mRNA levels in kidney, lung and liver in these mice, suggesting persistent IL-6 activation in the SOCS3iEKO mice. Collectively, our data suggest that the loss of endothelial SOCS3 exacerbates the LPS-induced pro-thrombotic and type I interferon-like transcriptional response. We detected increased levels tissue factor, as well as MX1, IRF8 and OASL1expression in endotoxin-treated mice that was further aggravated by the loss of endothelial SOCS3. Bioinformatic analysis of gene expression (RNA-Seq) from HUVEC treated with IL-6 showed a similar activation of an IFN-like response. We observed increased NETosis in SOCS3iEKO kidneys upon LPS. Consistently, SOCS3iEKO mice showed severe kidney failure in response to LPS, as determined by direct glomerular filtration rate assays, and confirmed by increased plasma BUN levels. Fluorescent dextran assays showed kidney cortex areas devoid of fluorescence, suggesting tissue hypoperfusion. In summary, levels of endothelial SOCS3 are critical to regulating the extent of the inflammatory response, and overactivation of this pathway in the endothelium of SOCS3iEKO mice leads to endotheliopathy that will promote kidney failure and lethality.