Patients who undergo pelvic radiotherapy may develop severe and chronic complications resulting from gastrointestinal alterations. The lack of curative treatment highlights the importance of novel and effective therapeutic strategies. We thus tested the therapeutic benefit of mesenchymal stem cells (MSC) treatment and proposed molecular mechanisms of action. MSC efficacy was tested in an experimental model of radiation-induced severe colonic ulceration histologically similar to that observed in patients. In this model, MSC from bone marrow were administered intravenously, immediately or three weeks (established lesions) after irradiation. MSC therapy reduces radiation-induced colonic ulceration and increases animal survival. MSC treatment induces therapeutic efficacy whatever the time of cell infusion. Infused-MSC engraft in the colon but also increase endogenous MSC mobilization in blood that have lasting benefits over time. In vitro analysis demonstrates that the MSC effect is mediated by paracrine mechanisms through the non-canonical WNT (Wingless integration site) pathway. In irradiated rat colons, MSC treatment increases the expression of the non-canonical WNT4 ligand by epithelial cells. The epithelial regenerative process is improved after MSC injection by stimulation of colonic epithelial cells positive for SOX9 (SRY-box containing gene 9) progenitor/stem cell markers. This study demonstrates that MSC treatment induces stimulation of endogenous host progenitor cells to improve the regenerative process and constitutes an initial approach to arguing in favor of the use of MSC to limit/reduce colorectal damage induced by radiation.

Non-neoplastic tissues around an abdomino-pelvic tumor can be damaged by the radiotherapy protocol, leading to chronic gastrointestinal complications that affect the quality of life with substantial mortality. Stem cell-based approaches using immunosuppressive bone marrow mesenchymal stem cells (MSCs) are promising cell therapy tools. In a rat model of radiation proctitis, we evidenced that a single MSC injection reduces colonic mucosa damages induced by ionizing radiation with improvement of the re-epithelization process for up to 21 days. Immune cell infiltrate and inflammatory molecule expressions in the colonic mucosa were investigated. We report that MSC therapy specifically reduces T-cell infiltration and proliferation, and increases apoptosis of radiation-activated T cells. We assessed the underlying molecular mechanisms and found that interleukin-10 and regulatory T lymphocytes are not involved in the immunosuppressive process in this model. However, an increased level of corticosterone secretion and HSD11b1 (11β-hydroxysteroid dehydrogenase type 1)-steroidogenic enzyme expression was detected in colonic mucosa 21 days after MSC treatment. Moreover, blocking the glucocorticoid (GC) receptor using the RU486 molecule statistically enhances the allogenic lymphocyte proliferation inhibited by MSCs in vitro and abrogates the mucosal protection induced by MSC treatment in vivo. Using the irradiation model, we found evidence for a new MSC immunosuppressive mechanism involving GCs.

Radiation proctitis is an insidious disease associated with substantial morbidity and mortality. It may develop following the treatment of several cancers by radiotherapy when normal colorectal tissues are present in the irradiation field. There is no unified approach for the assessment and treatment of this disease, partly due to insufficient knowledge about the mechanism involved in the development of radiation proctitis. However, unresolved inflammation is hypothesized to have an important role in late side effects. This study aimed to analyse the involvement of specific immunity in colorectal damage developing after localized irradiation, and evaluate the benefit of immunomodulatory mesenchymal stromal cells isolated from adipose tissue (Ad-MSCs) for reduction of late side effects. Our experimental model of colorectal irradiation induced severe colonic mucosal damage and fibrosis that was associated with T-cell infiltration. Immune cell activation was investigated; adoptive transfer of T cells in nude rats showed stronger colonization by T cells isolated from irradiated rats. The predominant role of T cells in late radiation-induced damage and regeneration processes was highlighted by in vivo depletion experiments. Treatments using Ad-MSCs reduced T-cell infiltration in the colon and reduced established colonic damage as measured by histological score, functional circular muscle contractibility, and collagen deposition. Here, we have demonstrated for the first time the predominance of the TH17 population compared to TH1 and TH2 in radiation-induced bowel disease, and that this is reduced after Ad-MSC treatment. Additionally, we demonstrated in vitro that IL17 acts directly on colonic smooth muscle cells to induce expression of pro-inflammatory genes that could participate in the development of radiation-induced injury. Our data demonstrate that the TH17 population is specifically induced during development of radiation-induced side effects in the colon. Moreover, Ad-MSC treatment modulates the TH17 population and reduces the extracellular matrix remodelling process induced following irradiation.

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