Several studies have demonstrated that the efficacy of chemotherapy is driven by a microbiota-induced immune response.23 Cyclophosphamide was able to promote accumulation of Th17 and Th1-cell response through stimulation of gram-positive commensals.24 Activation of splenic effector CD4+ T cells and tumor-infiltrating lymphocytes by Bacteroidales was found to be correlated with the development of checkpoint-blockade-induced colitis and the efficacy of CTLA-4 blockade.25,26 Infiltration of tumor-specific T cells by anti-PD-L1 was also mediated by the enrichment of Bifidobacterium. Here, CTLA4 is linked to colitis.