The oral microbiota, second in abundance to the gut, is implicated in chronic systemic diseases, but its specific role in GVHD pathogenesis has been unclear. Our study finds that mucositis-induced oral dysbiosis in patients post-hematopoietic cell transplantation associated with increased chronic GVHD (cGVHD) even in patients receiving post-transplant cyclophosphamide. In murine HCT models, oral dysbiosis caused by bilateral molar ligatures exacerbated cGVHD and increased bacterial load in …