Oral and gut microbiome alterations in oral chronic graft-versus-host disease: results from Close Assessment and Testing for Chronic GVHD (CATCH study).

Whether and how the oral microbiome and its changes in allogeneic hematopoietic cell transplantation (alloHCT) recipients may contribute to oral chronic graft-versus-host disease (cGVHD) pathogenesis is unknown. In addition, while the oral and colonic microbiota are distinct in healthy adults, whether oral microbes may ectopically colonize the gut in alloHCT patients is unknown. To address these knowledge gaps, longitudinal oral and fecal samples were collected prospectively in the multicenter CATCH Study (Close Assessment and Testing for Chronic GVHD; NCT04188912). Through shotgun metagenomic sequencing of the samples collected at baseline, oral cGVHD onset, first post-cGVHD onset visit, and 1-year post-HCT timepoints in patients with oral cGVHD (cases; N = 29) or without any cGVHD (controls; N = 51), we examined whether (i) oral and/or gut microbiomes and their longitudinal trajectories differ between cases and controls, and (ii) oral and gut microbiomes overlap in alloHCT recipients, especially those developing cGVHD. A total of 195 samples were analyzed. The onset of oral cGVHD was characterized by an expansion of Streptococcus salivarius and Veillonella parvula in the oral microbiome. High levels of oral/gut microbiota overlap were observed, particularly in patients with oral cGVHD, suggesting ectopic colonization of the gut by oral bacteria. The unusual coalescence of two distant niches in these patients may have short- or long-term consequences for the host, a novel avenue for future research. In addition, this study suggests a contribution of the oral microbiome to oral cGVHD pathogenesis.