Complement activation does not play a direct role in model chemotherapeutic - induced mucositis

Abstract

Chemotherapeutic agents are highly efficient in the treatment of various cancers; however, many of the drugs also lead to unwanted toxicities. “Mucositis” describes the injury caused by these cytotoxic agents to the healthy mucosa of the digestive tract. Depending on the dose regimen and drug, mucositis afflicts a significant fraction of patients, from 40-100%. Currently, the underlying molecular mechanism(s) responsible for mucositis are not fully understood and there are no efficient treatments. Research into the mechanism underlying mucositis has identified superoxide radicals early followed by changes in mediators of inflammation and subsequent tissue injury. It was previously reported that complement becomes activated in the jejunum of mice injected with 5-fluorouracil to elicit mucositis and that properdin deficient mice, which should have compromised alternative pathway activation, were protected though in a complement-activation-independent mechanism. While that discovery implicated properdin in the inflammation, the experiment did not rule-out other pathways of complement activation possibly contributing to the inflammation. Therefore, the objective of this study was to determine whether activation of complement by other routes contributes to mucositis, and the hypothesis; that preventing complement activation would protect mice from mucositis. Mice were injected daily with 5-fluorouracil or methotrexate for 5 days then euthanized one day later. Each mouse’s jejunum, colon, tongue and cheek mucosae were harvested for histopathological analysis. To determine whether the lectin pathway was involved, mice lacking mannose binding lectins 1 and 2 were used. Deficient mice responded similar to wildtype mice to 5-fluorouracil and methotrexate, showing weight loss at the same rate and similar pathological features in both the jejunum and colon. Neither strain of mouse manifested with oral mucositis. Having ruled-out the mannose-binding lectin pathway, it was decided to use mice deficient in the molecule central to complement, C3. C3 deficient mice were bred with wild type mice, then the heterozygous offspring bred to a second generation which were used in experiments. The stool bacteria showed a pattern consistent with the F2 generation mice becoming more similar though different from their parent’s. C3 deficient and C3 wildtype drug-treated mice lost weight to a similar extent, while C3 heterozygotes lost weight beginning on the third day of the experiment. All the mice had similar histopathological features in their jejunums and colons after the treatments. No mice developed oral mucositis. One experiment extending the period of time the mice were treated was conducted but none of the mice developed oral mucositis. I conclude that complement activation does not contribute mechanistically to mucositis.