Mast cells promote small bowel cancer in a tumor stage-specific and cytokine-dependent manner.

Mast cells (MCs) are tissue resident sentinels that mature and orchestrate inflammation in response to infection and allergy. While they are also frequently observed in tumors, the contribution of MCs to carcinogenesis remains unclear. Here, we show that sequential oncogenic events in gut epithelia expand different types of MCs in a temporal-, spatial-, and cytokine-dependent manner. The first wave of MCs expands focally in benign adenomatous polyps, which have elevated levels of IL-10, IL-13, and IL-33, and are rich in type-2 innate lymphoid cells (ILC2s). These vanguard MCs adhere to the transformed epithelial cells and express murine mast cell protease 2 (mMCP2; a typical mucosal MC protease) and, to a lesser extent, the connective tissue mast cell (CTMC) protease mMCP6. Persistence of MCs is strictly dependent on T cell-derived IL-10, and their loss in the absence of IL-10-expressing T cells markedly delays small bowel (SB) polyposis. MCs expand profusely in polyposis-prone mice when T cells overexpress IL-10. The frequency of polyp-associated MCs is unaltered in response to broad-spectrum antibiotics, arguing against a microbial component driving their recruitment. Intriguingly, when polyps become invasive, a second wave of mMCP5+ /mMCP6+ CTMCs expands in the tumor stroma and at invasive tumor borders. Ablation of mMCP6 expression attenuates polyposis, but invasive properties of the remaining lesions remain intact. Our findings argue for a multistep process in SB carcinogenesis in which distinct MC subsets, and their elaborated proteases, guide disease progression.