In this study, the researchers used a mouse model of HPV-positive oropharyngeal cancer to examine how response to anti-PD-1 immunotherapy relates to both immune cell profiles and the composition of the oral microbiome. They found that in treatment responders, there were higher numbers of activated immune cells — including specific T cell, B cell, dendritic cell and myeloid cell populations — in tumors, tumor-draining lymph nodes, and blood. Moreover, certain bacterial types in the oral cavity (like Allobaculum, Blautia, Faecalibacterium, Dorea, and Roseburia) were more abundant in responders, while non-responders had more Enterococcus. These immune and microbial patterns may serve as biomarkers to predict which HPV-positive oropharyngeal cancer patients will benefit from PD-1 blockade therapies.