Altogether, our study underpins the importance of functional CD8+ T cells in tumor immunity during PD-1 blockade, eliminates any doubts about effector or off-target effects of antibody treatment for anti-tumor efficacy, and advances the knowledge of specific TIL subpopulations prior to tumor growth inhibition, with the potential to be biomarkers of clinical response. This evidence concerns the gene PDCD1 and neoplasm.