Manipulation of the Complement Pathway to Increase Radiation Sensitivity and Immune Response

One of the research directions of my laboratory is identifying targets that act both to protect against radiation induced normal tissue toxicity but also are able to sensitise tumours to radiation. In our screen for such agents, we found that targeting the complement pathway at the level of C5aR1 can improve radiation response in colorectal cancer models while also reducing radiation-induced small bowel toxicity. The complement system is an innate immunity pathway composed of soluble and cell surface proteins. Proteolytic cleavage of complement components leads to their activation and the formation of downstream effectors. Complement cleavage products signal through cell surface receptors such as C3aR1 and C5aR1. C5aR1 is the main receptor for C5a, which is considered the most potent anaphylatoxin of the complement cascade.
To date, very few molecules have been approved that can modify the radiation response of tumors and only one molecule that has been approved to protect against normal tissue damage-amifostine. The innovation in this proposal is based on the identification of a single molecule C5aR1, which can increase therapeutic index by protecting normal tissue from radiation induced damage and at the same time radiosensitising tumours.

Increasing the therapeutic window of radiotherapy may be achieved by using molecularly targeted therapies against cancer-associated pathways. The complement system is an important pathway in immunity with emerging roles in cancer progression. To date, no study has evaluated the effectiveness of complement inhibition on both tumour and normal tissue radiation response. Our data suggests that targeting complement at the level of C5aR1 can improve radiation response in colorectal cancer models while reducing radiation-induced small bowel disease. The improved tumour radiation response occurs through reduced NF-kB signaling following C5aR1 antagonist treatment, an effect mediated in an IL-10-dependent manner. Importantly, we find that C5aR1 loss also results in increased survival of mice following radiotherapy, through decreased crypt cell apoptosis leading to protection of the gastrointestinal tract from radiation-induced small bowel disease. In the small intestine, C5aR1 loss does not result in decreased NF-kB signaling but instead results in increased total AKT and decreased p53 phosphorylation in an IL-10-dependent manner. Our data suggest that genetic or pharmacologically targeting C5aR1 can improve radiation response of colorectal cancer cell lines while reducing radiation-induced small bowel disease. Together, these findings suggest that targeting the complement system could be a promising approach to increasing the therapeutic window of radiotherapy.
Aim 1. Determine the broad vs specific protection of normal epithelium in C5aR1-/- mice. Our hypothesis is that C5aR1 inhibition will protect normal epithelium, especially in the digestive tract from radiation induced damage. Our preliminary studies have focused on the digestive system, but we will expand our studies to other critical sites such as salivary glands, lung epithelium, and liver to see if they also exhibit increased radioprotection in C5aR1-/- mice. In addition, we will determine if there is protection against early effects induced by conventional (fractionated, hypofractionated) and FLASH radiotherapy.
Aim 2. We will investigate the mechanistic basis of radioprotection of normal tissue when C5aR1 is lost or inhibited. Our preliminary data indicates that normal tissue radioprotection, at least in the intestines, is an IL-10 dependent mechanism and involves recruitment of CX3CR1+ macrophages that promote cell survival and angiogenesis. Our hypothesis is that inhibition of C5aR1 decreases normal tissue damage in the gut epithelium through decreasing crypt cell apoptosis. We will investigate the dependence of this protection on IL-10 signalling and determine the critical downstream effectors that promote crypt cell survival.
Aim 3. We will determine how C5aR1 promotes tumour cell killing in combination with radiotherapy. The hypothesis based on our preliminary data is that C5aR1 inhibition increases IL-10 secretion, which attenuates NF-kB signaling, resulting in IL-10 dependent tumour cell killing both in cell culture and in animal models, suggesting a non-canonical stress specific and likely immune-independent role for C5aR1 in regulating apoptosis.
Aim 4. We will determine the effect of clinical grade C5aR1 inhibitors on radioprotecting intestine, liver, salivary glands, and sensitizing colorectal and pancreatic tumours to radiotherapy. This aim will be essential in providing the framework for taking C5aR1 inhibitors into the clinic as dual radioprotectors/radiosensitizers.