The present disclosure describes compositions and methods for increasing the abundance of commensal bacteria belonging to the order Clostridiales, including Blautia, Ruminococcus, Clostridium, Eubacterium, Holdemania and Dorea species, that are as-sociated with reduced lethal GVHD and improved overall survival following bone mar-row or hematopoietic stem cell transplant. The present disclosure, therefore, provides methods for reducing the likelihood, incidence or severity of GVHD by (1) avoiding the loss of endogenous beneficial species through antibiotic selection； (2) by administering a therapeutically effective amount of a composition comprising one or more Clostridiales associated with reduced GVHD to individuals who may lack or have lost those strains from their intestinal microbiota. Additionally, support for endogenous or reestablished Clostridiales related to reduced GVHD as a treatment option for reducing GVHD can also be provided in the form of nutritional supplementation, for example, sugars fermented by some species of Clostridiales with GVHD reducing activity.
