Several species of intestinal bacteria have been associated with enhanced efficacy of checkpoint blockade immunotherapy, but the underlying mechanisms by which the microbiome enhances anti-tumor immunity is unclear. Here, we isolated three bacterial species, including Bifidobacterium pseudolongum, Lactobacillus johnsonii and Olsenella species, that significantly enhanced efficacy of immune checkpoint inhibitors in four mouse models of cancer. We found that intestinal B. pseudolongum modulated enhanced immunotherapy response through production of the metabolite inosine. Decreased gut barrier function induced by immunotherapy increased systemic translocation of inosine and activated anti-tumor T cells. The effect of inosine was dependent on T cell expression of the adenosine A2A receptor and required co-stimulation. Collectively, our study identifies a novel microbial metabolite-immune pathway that is activated by immunotherapy that may be exploited to develop microbial-based adjuvant therapies.

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The microbiota plays an important role in regulating both the innate and adaptive immune systems. Many studies have focused on the ability of microbes to shape the immune system by stimulating B‐cell and antibody responses and the differentiation of T helper cell function. However, an important feature of the immune system is its ability to generate memory responses, which provide increased survival for the host. This review will highlight the role of the microbiota in the induction of immune memory with a focus on both adaptive and innate memory as well as vaccine efficacy.

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