BACKGROUND: The increased expression of programmed cell death ligand 1 (PD-L1) on a subset of immune cells in the peripheral blood has been frequently observed in patients with cancer, suggesting a relationship with PD-L1 expression in tumor tissues. In this study, we investigated the mechanisms underlying PD-L1 expression on various types of immune cells in the peripheral blood of patients with cancer. METHODS: PD-L1 expression on various immune cell populations was analyzed in peripheral blood mononuclear cells of 112 patients with non-small cell lung cancer (NSCLC) using flow cytometry. A mouse model of X-ray-induced acute thrombocytopenia was used to investigate the relationship between thrombopoiesis and PD-L1-expressing platelet generation. The clinical significance of PD-L1-expressing platelets was analyzed in a cohort of patients with stage IV NSCLC who received a combination of anti-programmed cell death 1 (PD-1) therapy and chemotherapy. RESULTS: All immune cell populations, including monocytes, T cells, B cells, and NK cells, showed higher PD-L1 expression in patients with cancer than in healthy controls. However, this increased frequency of PD-L1-expressing cells was not attributed to the expression of the cells themselves. Instead, it was entirely dependent on the direct interaction of the cells with PD-L1-expressing platelets. Notably, the platelet-dependent acquisition of PD-L1 on circulating immune cells of patients with lung cancer was observed in various other cancer types and was mechanistically associated with a surge in thrombopoiesis, resulting in the increased production of PD-L1-expressing reticulated platelets. Clinically, patients with enhanced thrombopoiesis and concurrently high PD-L1-expressing platelets exhibited a better response to anti-PD-1 therapy. CONCLUSIONS: These findings highlight the role of tumor-associated thrombopoiesis in generating PD-L1-expressing platelets that may serve as a resource for PD-L1-positive cells in the circulation and act as a predictive biomarker for anti-PD-1/PD-L1 therapy.