4. Discussion
Although Vit D deficiency has been shown to correlate with poor prognosis in cancer therapy-induced mucositis, no animal model study experimentally tested whether Vit D supplementation could have therapeutic effects on mucositis course. IL-22 is a critical cytokine for gastrointestinal homeostasis and restoration of damaged intestinal/mucosal tissues. Absence of IL-22 exacerbates mucositis outcome in the murine models and Vit D-mediated signaling was shown to regulates IL-22 production in both mice and humans. However, whether recombinant IL-22 or its genetic overexpression would have therapeutic benefit has not yet been put to test neither in animal models, nor in humans. In this study, our data revealed that in a preclinical model of methotrexate-induced mucositis, Vit D supplementation could improve mucositis, and the protection may be IL-22 mediated.
Our data revealed that Vit D injections ameliorated mucositis pathology. Injections were initiated 4 days prior to methotrexate injection in order to give Vit D time to have its effects on mucosal cells, thus, it was considered for prophylaxis. These results are consistent with previous findings that Vit D deficiency is associated with more severe mucositis both in adults and pediatrics.9,21Nejatinamini et al. reported that lower intake of vitamins and plasma 25-hydroxy Vit D (25-OHD) was associated with mucositis in head and neck cancers treatments.21 The patients included both radiation and chemotherapy or combined therapy groups (22% radiotherapy, 71% chemotherapy, 7% both). Similar observations were made in acute lymphoblastic leukemia in children.9Safety of Vit D supplementation in pediatric cases prior to hematopoietic cell transplantation have also been reported.22 Data from a study by Anand et al.23 with adult oral cancer patients revealed that Vit D supplementation significantly improved treatment associated mucositis. Another study with smaller patient cohort size (n=14) found no significant benefit.24 Therefore collectively, these data argue for a beneficial role for Vit D in improving treatment-associated mucositis in cancers and perhaps hematopoietic stem cell transplantation.25
IL-22 is an important cytokine in gut homeostasis. Intestinal epithelial cells express IL-22 receptors and IL-22R signaling in these cells promotes regeneration of epithelial tissue. Indeed, its absence renders mice susceptible to dextran sulfate sodium (DSS)-induced models of colitis.26 This also is valid for CD4+ naïve T cell induced models of colitis. 19, 26, 27 In addition to intestinal tissue remodeling IL-22 signaling stimulates production of mucus, anti-microbial peptides which are crucial for epithelial barrier immunity and homeostasis.28 In the intestinal lamina propria or other mucosal tissues, IL-22 is produced by Th17 cells, ILC3s, mucosal-associated invariant T (MAIT) and gamma delta T cells. Vit D and VDR signaling was shown to be critical for development and function of ILC3s and forkhead box P3 (FOXP3)+ Treg cells in gastrointestinal system. 11, 12 Thus, diminished number of ILC3s and subsequently reduced levels of IL-22 and IL-10 was reported in the deficiency of Vit D.11, 12Our cytokine data of IL-22 levels are in line with these reports. Vit D treatment augmented intestinal IL-22 production. IL-22 hydrodynamic delivery also increased IL-22 levels. The effect of Vit D IL-22 was more profound than hydrodynamic delivery group. This was also reflected in the histopathology scores. In our model, Vit D did not appear have an impact on total ILC levels. This might be due to relatively short treatment time.
In summary, our results show that Vit D treatment may provide partial protection from chemically-induced mucositis in mice.