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SMAD4 is frequently mutated and inactivated in human gastric cancer (GC). Although the epithelial cell-autonomous functions of Smad4 have been extensively studied, its contribution to tumor immunity is largely undetermined. Here, we report that the loss of Smad4 expression in GC cells endows them with the ability to evade tumor immunity. Unlike their Smad4-proficient counterparts, Smad4-deficient stomach organoids can evade host immunity to form tumors in immunocompetent mice. Smad4-deficient GC cells show expanded CD133

ST2 functions as a receptor for the cytokine IL-33. It has been implicated in carcinogenesis. In this study, we sought to mechanistically determine how ST2 and IL-33 function to support cancer stem cell (CSC) activity and drive gastric cancer (GC) pathogenesis. ST2+ subpopulation spontaneously arose during gastric tumorigenesis. A thorough evaluation of ST2 and IL-33 expression in gastric tumors revealed that they show an overlapping expression pattern, notably in poor differentiated GC and metastasis foci. Moreover, their expression levels are clinically correlated to cancer progression. Using a genetic model of CSC-driven gastric carcinogenesis, ST2+ subpopulation displays increased tumorigenicity, chemoresistance and metastatic potentials through increased survival fitness endowed by an elevated MAPK-regulated Bcl-xL. The IL-33/ST2 axis enhances the self-renewal and survival of GC stem cells and organoids. Importantly, we observed a synergistic cooperation between IL-33/ST2 and the canonical Wnt pathway in transactivating Wnt-dependent transcription and supporting CSC activity, a partnership that was abrogated by inhibiting Bcl-xL. Concordant with this, ST2+ subpopulation was targeted by MEK1/2 and Bcl-xL-specific inhibitors. These findings establish ST2 as a functional CSC marker that fortifies the Wnt signal while availing a novel therapeutic strategy to suppress GC progression by targeting the IL-33/ST2/Bcl-xL signaling axis.