The effect of intestinal flora on immune checkpoint inhibitors in tumor treatment: a narrative review

Tremendous progress has been achieved in understanding of the interaction between tumor microenvironment and intestinal flora in the past decades. Immune checkpoint inhibitors (ICIs) are a promising treatment strategy for advanced tumors, most prominently cytotoxic T-lymphocyte-associated protein (CTLA-4) and programmed cell death protein-1 (PD-1), its major ligand PD-L1, its beneficial to part of the population and obtaining excellent clinical results. However, the majority of patients do not respond or develop early progressive disease. Reached consensus by experts currently believe that the intestinal flora plays an important role in the explanation of the limited therapeutic effect of ICIs, there are differences in the composition of intestinal flora between patients with good response and patients with poor response, cloned mice by fecal microbiota transplantation (FMT) proved that the mice with transplanted feces from patients with good response can reduce tumor volume and obtain a better progress free survival (PFS). Therefore, “beneficial bacteria” seem to be enriched in the intestinal flora of patients who are well-responsive to ICIs and can be potentially used as a marker and cancer immunotherapeutic adjuvant of ICIs. In this review, we aim to summarize some of the studies demonstrating intestinal flora on tumor immunotherapy through anti-PD1, anti-PD-L1, anti-CTLA-4 and discuss possible mechanisms of this effect.