Provided herein are methods of treating graft-versus-host disease in a subject by administering pharmaceutical compositions containing bacterial strains of the Clostridia class. Also described herein are exemplary human-derived bacteria belonging to the Clostridia class, combinations of which have been shown to induce accumulation of regulatory T cells (Treg cells) in the colon and suppress immune functions, and are therefore useful for mitigating pathological immune responses. Pharmaceutical compositions containing these and/or related bacteria can be used to prevent and treat immune-mediated diseases such as graft-versus-host disease.