Abstract
Graft-versus-host disease (GVHD) is one of the principal complications seen in the recipients of allogenic hematopoietic stem cell transplantation (allo-HSCT), and persists as a leading cause of post-transplant morbidity and mortality. Increasing evidence highlights the crucial influence of the gut microbiome (GM) on transplant outcomes. Microbial dysbiosis, characterized by reduced bacterial diversity and pathogenic overgrowth, is strongly associated with higher rates of complications and mortality. Patients with lower microbial diversity exhibit poorer overall survival (OS) and an increased incidence of acute GVHD (aGVHD). Conversely, restoration of beneficial commensal communities has been shown to enhance immune homeostasis, mitigate GVHD severity, and decrease infection risk. Emerging therapeutic strategies now focus on modulating the intestinal microbiome through dietary interventions, probiotics, prebiotics, and fecal microbiota transplantation (FMT). It has been demonstrated that bacterial metabolites, such as short-chain fatty acids (SCFAs) from the diet, especially a diet rich in fibers, reduce the occurrence/severity of GVHD by inducing regulatory T cells (Tregs), which release anti-inflammatory cytokines and regulate the host immune system. Hence, the implementation of dietary fibers (DFs) could increase beneficial commensals, Treg induction, and improve outcomes such as GVHD and OS in recipients of allo-HCT. Hereupon, this review addresses how a fiber-rich diet modulates GM composition, reinforces epithelial barrier integrity, and improves the efficacy of Treg-based immunotherapy by stabilizing their regulatory phenotype and increasing their functional persistence, ultimately leading to a reduction in GI complications associated with GVHD. Unlike prior reviews that primarily cover the microbiome–GVHD axis or Treg therapies in isolation, this review emphasizes fermentable dietary fibers as a mechanistically grounded, clinically actionable strategy to support Treg stability and persistence via microbiota-derived metabolites. We integrate mechanistic evidence with emerging clinical feasibility data and ongoing trials of prebiotic supplementation in allogeneic HSCT.