Gut microbiome, metabolites, and tissue metabolism in graft-versus-host disease

Immune reactions promote not only graft-versus-tumor events but also graft-versus-host disease (GVHD) characterized by the activation of T-cells and antigen-presenting cells. Recent studies have unveiled the effects of microbial flora on GVHD and brought the mechanism other than immune cells. The role of tissue-intrinsic factors contributes to target-tissue resilience, repair, and regeneration and mitigates the severity of GVHD without altering alloreactive immune cells. Antibiotics, specific bacteria, and low microbiota diversity are identified as risk factors for GVHD and mortality. The current focus has shifted to target-tissue homeostasis in GVHD. Most of the gut microbiota symbiotically colonize the gut, and this homeostasis is finely tuned in the gastrointestinal tract, which has a relatively hypoxic environment dominated by anaerobic organisms. Therefore, intestinal epithelial cells (IECs) are uniquely adapted to this hypoxic environment, and cells programmed by "physiological hypoxia" tonally regulate barrier function with commensal bacteria. Metabolic changes in IECs post allo-HCT can affect the gut environment, leading to dysbiosis. This review focuses on the role of metabolism in IECs with microbiota and microbial metabolites in the GVHD setting to promote tissue tolerance.