Immune checkpoint inhibitors (ICIs) have revolutionized cancer treatment in the last decade. Among various checkpoints identified so far, interaction between programmed death-1 (PD-1) with programmed death ligand 1 (PD-L1) and their targeting using human monoclonal antibodies has attracted the most attention and is considered as the most prominent treatment with the best clinical outcomes. Accumulating evidence is the witness for the impact of gut microbiota on clinical responses and ICI efficacy. Specific bacterial species are identified in fecal specimens of cancer patients responding to the anti-PD-(L)1 immunotherapy, while non-responders demonstrate high abundance of other bacterial sources. Thus, the composition of gut microbiota may suggest potential biomarker in identification of patients with the best responses to immunotherapy. Notably, fecal microbial transplantation (FMT) from responders to non-responders has shown hopeful results in improving clinical outcomes and overcoming resistance to ICIs. Additionally, some bacterial components, such as the use of antibiotics and probiotic supplements have been shown to affect the efficacy of ICIs treatment. However, employment of these findings requires further investigations and precise understanding of the impact of gut microbiota on the host's immune responses. In the current review, we aim to discuss the roles of PD-1/PD-L1 checkpoint pathway, their therapeutic significance, and the impact of gut microbiota/products on the PD-1/PD-L1 immunotherapy outcomes.
