INSIGHTS INTO THE ROLE OF COMPLEMENT DURING INTESTINAL INFLAMMATION
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Complement is arguably the most ancient and effective innate defensesystemthat can also promote pathogenesis in multiple disorders. The focus of this study was to understand the impact of complement in diseases of the intestinal tract. To this end, murine models of colitis (dextran sulfate sodium, Citrobacter rodentium, interleukin-10deficient) and small intestinal mucositis were used.We measured local and systemic complement activationand subsequentlyexplored the impact of defective complement activation either by using mice lacking properdin, the only known positive regulator of complement, orby treating mice with a C5a (complement activation product) receptor antagonist.Complement activation correlated with the pathology in all models and its ablation either protected or exacerbated the injury. In this regard, C5a receptor antagonist ameliorated the symptoms of dextran sulfate sodium induced colitisbut complement activation was found to be protective in C. rodentium and IL-10-/- models of colitis. In the enteric infection model, properdin deficient mice (PKO) had increased diarrhea and exacerbated inflammation combined with defective epithelial cell derived IL-6 and greater numbers of colonizing bacteria. Importantly, exogenous properdin was sufficient to rescue PKO mice. Then using in vitro and in vivo approaches, we show that the mechanism behind the exacerbated inflammation of PKO mice is due to a failure to increase local C5a levels. We show that C5a directly stimulates IL-6 production from colonic epithelial cells and that inhibiting C5a in infected WT mice resulted in defective epithelial IL-6 production and exacerbated inflammation. Overall, our findings indicate that systemic properdin in the colonic environment promotes C5a generation that protects, possibly by inducing IL-6 production. In accordance with the infection model, loss of properdin resulted in defective terminal complement activation and exacerbated colitis in IL-10-/- mice. Notably, neutrophil recruitment to the colon, but not IL-6 production, was defective in double knockout mice and was accompanied by markedly higher local and systemic bacterial numbers compared to IL-10-/- mice. To the best of our knowledge, these are the first reports implicating properdin mediated complement activation in modulating mucosal environment during colonic injury.We next used properdin deficient mice to investigate the impact of complement activation during chemotherapy induced small intestinal mucositis.Loss of properdin did not impair complement activation, but protected mice from mucositis that was associated with elevated IL-10. Importantly, protective effects were lost in PKO mice that lacked IL-10, suggesting the protection was dependent on IL-10 production. Collectively, these results suggest that complement proteins, especially properdin and C5a, modulate intestinal pathology in a context dependent manner.