Background and Methods The link between gut microbiota composition and response to immunotherapy has been clearly identified, as microbiota modulation has been shown to induce immune checkpoint inhibitor (ICI) sensitivity in preclinical models and patients. While fecal microbiota transplantations, probiotic or antibiotic administrations, and diet changes have been shown to impact on melanoma tumor growth and response to ICI, the various mechanisms behind these anti-tumor responses and the causal role of tumor-residing microbes remain incompletely understood. While detection of bacterial signatures within tumors has demonstrated the presence of a diverse, tumor-intrinsic microbiota, the viability and the functional profiles of those tumor-resident microbes remains unknown. Here, we utilize a preclinical melanoma model to investigate the impact of gut microbiota manipulation on i) commensal bacterial translocation to systemic sites and ii) the profile of viable tumor-residing microbes.

Results and Conclusions Using a broad-spectrum culturomics approach, we identified that gut microbiota composition manipulation drives gut-distal organ and melanoma tumor microbiota changes, while failing to impact on gut barrier permeability. We demonstrate that commensal translocation occurs independent of disease state, the host microbiome, or host genetics, and occurs rapidly following oral administration of exogenous commensal microbes. While our investigation demonstrates the critical nature of microbial-host crosstalk within the tumor microenvironment, in regard to inducing spontaneous anti-tumor immunity and ICI efficacy in the context of Lactobacillus probiotic administration, further studies are required to determine how endogenous, translocated, tumor-residing commensals impact on melanoma development and ICI efficacy.

Ethics Approval Study obtained ethics approval by IACUC at the University of Pittsburgh.