Properdin deficiency protects from 5‐fluorouracil‐induced small intestinal mucositis in a complement activation‐independent, interleukin‐10‐dependent mechanism

Intestinal mucositis is a serious complication of chemotherapy that leads to significant morbidity that may require dose or drug adjustments. Specific mitigating strategies for mucositis are unavailable, due partly to an incomplete understanding of the pathogenic mechanisms. We have previously shown an effect of properdin, a positive regulator of complement activation, in models of colitis. Here we use properdin‐deficient (PKO) mice to interrogate the role of properdin and complement in small intestinal mucositis. Mucositis was induced by five daily injections of 5‐fluorouracil (5‐FU) in wild‐type (WT), PKO, interleukin (IL)‐10–/– and properdin/IL‐10–/– double knock‐out (DKO) mice. At the time of euthanasia their jejunum was collected for histology, immunohistochemistry and cytokine and complement activation measurements. Complement became activated in mice receiving 5‐FU, indicated by increased intestinal levels of C3a and C5a. Compared to WT, PKO mice experienced significantly less mucositis, despite C3a levels as high as inflamed WT mice and slightly less C5a. Conversely, PKO mice had higher intestinal levels of IL‐10. IL‐10 expression was mainly by epithelial cells in both uninflamed and inflamed PKO mice. IL‐10–/– mice proved to be highly susceptible to mucositis and DKO mice were equally susceptible, demonstrating that a lack of properdin does not protect mice lacking IL‐10. We interpret our findings to indicate that, to a significant extent, the inflammation of mucositis is properdin‐dependent but complement activation‐independent. Additionally, the benefit achieved in the absence of properdin is associated with increased IL‐10 levels, and IL‐10 is important in limiting mucositis.