BackgroundAcute graft-vs.-host disease (aGvHD) is a life-threatening complication of allogeneic hematopoietic cell transplantation (allo-HCT), where allogeneic donor T cells recognize recipient tissues as foreign and damage them. Gut microbiota disruption following allo-HCT has been associated with increased aGvHD incidence, severity, and mortality. Growing clinical evidence suggests the efficacy of fecal microbiotherapy to restore a healthy and diverse gut microbiota and to treat aGvHD. MaaT013 (Xervyteg®), a standardized allogeneic fecal microbiotherapy derived from pooled healthy human fecal microbiota, has demonstrated efficacy in mitigating steroid-refractory aGvHD in the Phase 2 HERACLES trial, and is currently being evaluated in the pivotal ARES trial in ruxolitinib-resistant aGvHD with gastrointestinal involvement.MethodsHere, multi-omics analyses across complementary in vitro and in vivo models suggest that MaaT013 modulates biological processes involved in aGvHD pathogenesis.ResultsFirst, MaaT013-derived metabolites prevented inflammation-induced gut barrier alteration in an in vitro leaky gut model. Second, MaaT013 stimulated dendritic cell-mediated Tregs amplification and subsequent release of IL-10 and TGF-b in vitro. Third, beneficial bacteria from MaaT013 engrafted in the gut of germ-free mice, producing anti-inflammatory metabolites and restoring healthy gut physiology through immune rebalancing. Finally, MaaT013 delayed the appearance of aGvHD symptoms in a specific-pathogen free xenogeneic mouse model of aGvHD.DiscussionTogether, these results provide exploratory, multi-model evidence of MaaT013's potential to mitigate aGvHD by restoring microbial diversity, gut integrity, and immune homeostasis.