Bugs in the system: bringing the human microbiome to bear in cancer immunotherapy

The influence of the composition of the human microbiome on the efficacy of cancer directed immunotherapies, such as antibodies directed against the programmed cell death 1 protein (PD-1) or its ligand (PD-L1), has garnered increasing attention as the role of immunotherapies in the care of cancer has grown. Dysbiosis (altered microbiota) has recently been reported to adversely affect the efficacy of cancer directed immunotherapies, and correction of this dysbiosis has the potential to improve the efficacy of these treatments. However, the exact mechanisms underlying this relationship remains unknown. Current methods for characterizing the microbiome likely capture only a small portion of the highly complex interaction between the microbiome and the immune system. Here we discuss the recent reports of the influence of dysbiosis on cancer immunotherapy, methods to more fully characterize the interaction between the microbiome and the immune system, and methods of modulating the immune system to improve the efficacy of cancer immunotherapy.