Allogeneic hematopoietic-cell transplantation (allo-HCT) is an important treatment for hematological malignancies. The intestinal microbiota consists of a community of diverse microbes that reside in the intestine and are critical for host development, homeostasis, and immune regulation. In human analyses and animal experiments, we and others have shown that the intestinal microbiota contribute to the pathophysiology of all three major complications of allo-HCT: infections, GVHD, and relapse. Using 16S ribosomal RNA next-generation sequencing, we examined the intestinal microbiota of allo-HCT patients and found a post- transplant ?microbiota injury?. This dysbiosis is likely due to the combined effects of (a) broad- spectrum antibiotics for the treatment of post-transplant febrile neutropenia and (b) the profound nutritional alterations experienced by these patients. We found an inverse relationship between a loss of the genus Blautia after allo-HCT and GVHD mortality. We observed that broad- spectrum antibiotics that target the anaerobic commensal flora are particularly associated with increases in GVHD-related mortality and in fact worsened intestinal GVHD in our animal model. The protective layer of intestinal mucus that normally contributes to barrier function was depleted in animals suffering from GVHD and treated with anaerobe-targeted antibiotics. Finally, we and others have observed an association between Enterococcus and the development of GVHD in mouse and man. Therefore, we hypothesize that the intestinal microbiota can regulate the development of GVHD and can be targeted to prevent or treat GVHD. We propose to study in Aim 1 the mechanisms by which microbiota (in particular Blautia and Enterococcus) and their metabolites modulate GVHD using gnotobiotic mice. In Aim 2 we will study the role of nutrition in the development of GVHD both in humans and mouse models. In allo-HCT patients we will correlate nutritional intake, microbiota composition and GVHD. In mouse models we will study a) the effects of the diet on the mucus layer, b) sialidase inhibitors to prevent mucus layer degradation, c) prebiotics to mitigate damage to the microbiota and mucus layer, and d) effects of short chain fatty acids on GVHD. In addition to elucidating the interactions of the intestinal microbiota and nutrition in the development of GVHD, this work will form the basis of clinical trials to reduce GVHD and transplant-related mortality.