Microbial biomarkers for immune checkpoint blockade therapy against cancer

Three major standard treatments, i.e., surgery, chemotherapy, and radiotherapy, were traditionally applied to the treatment of cancer and saved many patients. Meanwhile, clinical studies as well as basic research of immunotherapy are being actively conducted for intractable or advanced malignancies that cannot be cured by the conventional standard treatments. Remarkable therapeutic efficacies have been recently reported in clinical trials on some cancer types, and immunotherapy is now being recognized as the "fourth" standard therapy against cancer. In particular, immune checkpoint inhibitor therapy (ICI) has demonstrated the effectiveness of immunotherapy through large-scale randomized clinical trials, leading to the paradigm-shift in cancer treatment. Immune checkpoint molecules transduce co-inhibitory signals to immunocompetent cells including T cells, and crucially contribute to the formation of an immunosuppressive microenvironment in tumor tissues, which intrinsically confers the treatment resistance. Programmed death-1 (PD-1, CD279) is one of the typical immune checkpoint molecules. Anti-tumor therapies targeting PD-1 and its ligands had been developed and approved in many countries, and various studies utilizing clinical specimens are currently progressing. In this review, we provide an overview of the biomarkers based on the analysis of enteric microbiota that correlate with the clinical efficacy/inefficacy of PD-1-based therapy.