On the one hand, given that multiple suppressive immune checkpoint molecules [e.g., PD-1, cytotoxic T-lymphocyte associated protein 4 (CTLA-4) and TIM-3] are highly expressed on the surface of CD103+CD8+ TRMs (59, 60), several studies have identified CD103+CD8+ TRMs as the population serving major antitumor efficacy in patients with different tumor types treated with anti-PD-1/PD-L1 immune checkpoint therapy. Here, PDCD1 is linked to neoplasm.