Platelets upregulate tumor cell programmed death ligand 1 in an epidermal growth factor receptor-dependent manner in vitro

Programmed death ligand 1 (PD-L1) is an immune checkpoint protein that suppresses cytotoxic T lymphocytes and is often overexpressed in cancers. Due to favorable clinical trial results, immune checkpoint inhibition (ICI) is part of Food and Drug Administration approved immuno-oncology therapies; however, not all patients benefit from ICI therapy. High blood platelet-to-lymphocyte ratio has been associated with failure of ICI treatment, but whether platelets have a role in hindering ICI response is unclear. Here, we report that coculturing platelets with cancer cell lines increased protein and gene expression of tumor cell PD-L1, which was reduced by antiplatelet agents, such as aspirin and ticagrelor. Platelet cytokine arrays revealed that the well-established cytokines, including interferon-γ, were not the main regulators of platelet-mediated PD-L1 upregulation. Instead, the high molecular weight epidermal growth factor (EGF) is abundant in platelets, which caused an upregulation of tumor cell PD-L1. Both an EGF-neutralizing antibody and cetuximab (EGF receptor [EGFR] monoclonal antibody) inhibited platelet-induced increases in tumor cell PD-L1, suggesting that platelets induce tumor cell PD-L1 in an EGFR-dependent manner. Our data reveal a novel mechanism for platelets in tumor immune escape and warrant further investigation to determine if targeting platelets improves ICI therapeutic responses.