Oral Inflammation and Microbiome Dysbiosis Exacerbate Chronic Graft-versus-host Disease

• Oral mucositis severity was associated with chronic GVHD in both conventional HLA-matched HCT and haplo-identical HCT with PTCy.

• Oral dysbiosis promoted migration of oral pathobionts into regional lymph nodes and the intestine, that continuously activated APCs.

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 the oral cavity and gut with Enterococcaceae significantly increasing in both organs. In this model, the migration of Enterococcaceae to cervical lymph nodes (LNs) both pre- and post-transplantation activated antigen-presenting cells (APCs), thereby promoting the expansion of donor-derived inflammatory T cells. Based on these results, we hypothesize that pathogenic bacteria increased in the oral cavity might not only exacerbate local inflammation but also enhance systemic inflammation throughout the HCT course. Additionally, these bacteria translocated to the gut and formed ectopic colonies, further amplifying systemic inflammation. Furthermore, interventions targeting the oral microbiome mitigated murine cGVHD. Collectively, our findings highlight the importance of oral dysbiosis in cGVHD and suggest that modulation of the oral microbiome during transplantation may be an effective approach for preventing or treating cGVHD.