The ubiquitin ligase, RNF5, regulates the gut microbiota composition and influences the immune checkpoint response to tumors. RNF5 deficient animals exhibit significant inhibition of tumor development as well as an altered gut microbiota composition. Methods of treating cancer by administering to a subject one or more selected bacterial species and/or one or more prebiotics that promote the growth of one or more selected bacterial species are disclosed. Also disclosed are methods of treating cancer by administering to a subject one or more selected bacterial species and/or one or more prebiotics that promote the growth of one or more selected bacterial species in combination with one or more anti-cancer agents.