Fecal carbohydrate-degrading bacteria are associated with reduced incidence of lower gastrointestinal GVHD.

Reduction in specific Parabacteroides and Bacteroides species around the time of engraftment contributes to LGI-GVHD risk in allo-HSCT patients.

• Study used a retrospective cohort with metagenomic sequencing of stool from N=90 patients undergoing allogeneic hematopoietic stem cell transplantation.
• The species implicated include Parabacteroides merdae, P. distasonis, and Bacteroides ovatus.
• These findings were identified around the time of engraftment as the critical temporal window.

Parabacteroides merdae, P. distasonis, and Bacteroides ovatus abundances were significantly correlated with carbohydrate-active enzyme (CAZyme) gene abundances in patients who did not develop LGI-GVHD compared with those who did.

• CAZyme gene abundances were investigated given the known diverse carbohydrate-degrading functionality of these bacteria.
• The correlation between bacterial species and CAZyme abundances was specifically observed in the non-LGI-GVHD group compared to the LGI-GVHD group.
• The analysis focused on functional gene abundances rather than taxonomic abundance alone.

Specific gene abundances of xylosidase were significantly associated with a reduced risk of LGI-GVHD.

• Xylosidase genes contribute to the degradation of xylose-containing polysaccharides.
• The association between xylosidase gene abundance and reduced LGI-GVHD risk was statistically significant.
• This finding highlights a specific enzymatic functional pathway—xylose-containing polysaccharide degradation—as relevant to GVHD risk.

The intestinal microbiome makes critical contributions to lower gastrointestinal graft-versus-host disease, which carries morbidity and mortality for allo-HSCT patients.

• LGI-GVHD is an established complication of allogeneic hematopoietic stem cell transplantation.
• The study used metagenomic sequencing to characterize the gut microbiome at a functional gene level in addition to taxonomic composition.
• The retrospective cohort included N=90 patients with stool samples collected around the time of engraftment.

Carbohydrate-degrading functionality of putative beneficial bacteria mediates risk of LGI-GVHD.

• The paper concludes that functional capacity for carbohydrate degradation, not just bacterial presence, is important in LGI-GVHD risk.
• Parabacteroides and Bacteroides species are described as having 'known diverse carbohydrate-degrading functionality.'
• The findings suggest that loss of CAZyme-encoding bacteria around engraftment is a mechanistically relevant feature of LGI-GVHD development.