SCF and IL-33 regulate intestinal mouse mast cell plasticity in colon cancer

Background: Mast cells (MC) are innate immune cells that derive from bone marrow pluripotent cells, circulate in the peripheral blood, mature and differentiate in the peripheral tissues under the influence of the stem cell factor (SCF). Otherwise, SCF together with other soluble mediators such as IL33, released upon danger signals by tissue resident cells, support MC activation and mediator release. MCs are mainly known for their involvement in IgE-mediated allergic disorders but are also frequently observed in tumor microenvironment (TME). Moreover, MC-derived mediators can either exert pro-tumorigenic functions, causing progression and spread of the tumor, or anti-tumorigenic functions, limiting tumor growth. MCs can be divided into at least two subtypes: mucosal MCs that are mainly associated with the lung and gastrointestinal tract epithelia and connective MCs found in intestinal submucosa, peritoneal cavity and skin. However, since MCs are characterized by plasticity nature, we hypothesize that in pathological condition such as colorectal cancer MCs display different phenotypes and functions. Thus, the exact effect of unique MC subset(s) during tumor progression is far from being understood. Purpose: The aim of this study is to evaluate the effect of unique MC subset(s) during tumor progression and the molecular players able to modulate MC functions during progression from chronic intestinal inflammation to colorectal cancer (CRC). Results: We used a conventional colitis-induced CRC mouse model (AOM/DSS) and observed by multiparametric flow cytometry that the frequency of c-kit/FceRIa double positive MCs increases in the tumor of AOM/DSS mice compared to unaffected tissue. By confocal microscopy we demonstrated that tumor-associated MCs harbor a main connective phenotype. We have then evaluated MC functionality by assessing the presence of cytokines by flow cytometry and RNA-SCOPE and found that connective MC subset showed a much higher production of both IL-6 and TNF-a. Moreover, we observed that SCF and IL-33 are expressed at high levels on tumor lesions. In primary cultures of peritoneum-derived MCs and bone marrow derived MCs we demonstrated by real time PCR that SCF and IL-33 can promote a connective phenotype. Moreover by flow cytometry and Western blot analysis we observed that both molecules act synergistically to induce TNFa and IL-6 release through the phosphorylation of key signaling molecules and transcription factors. Conclusion: Overall, our data suggest that during CRC development, connective MCs are activated by the combined action of SCF and IL-33 and contribute to tumor transformation through the secretion of IL-6 and TNFa.