Cancer is a leading cause of death globally. Checkpoint blockade therapies offer a promising treatment for many cancers, but have been ineffective for colorectal cancers. Previous studies have shown a dependency of immunotherapies on the microbiota. Consequently, we hypothesized that specific gut bacteria promote immunotherapy for colorectal cancer. We identify three commensal bacteria and a microbial metabolite, inosine, that enhance the efficacy of immune checkpoint blockade therapy in colorectal cancer. We show that inosine interacts with the adenosine A2A receptor on T cells resulting in intestinal Th1 cell differentiation. Decreased gut barrier function induced by immunotherapy increased the translocation of bacterial metabolites and promoted cancer protective Th1 cell activation. This microbial-metabolite-immune circuit provides a mechanism for a new class of bacteria-enhanced checkpoint blockade therapies. The efficacy of this mechanism differs among colorectal cancer subtypes and highlights the strengths as well as potential limitations of this novel bacterial co-therapy for cancer.

Immunotherapy efficacy in colorectal cancer is dependent on activation of a microbial-metabolite-immune circuit