PD-1 expression by tumour-associated macrophages inhibits phagocytosis and tumour immunity

Mouse and human tumour-associated macrophages express PD-1, which increases with cancer stage and induces decreased phagocytosis by macrophages; by contrast, PD-L1 removal increases phagocytosis in vivo, decreases tumour burden and increases survival of mice. Therapeutic antibodies that inhibit the interaction of programmed cell death protein (PD-1) with its ligand (PD-L1) are known to activate cytotoxic T cells. Here Irv Weissmann and colleagues study the role for PD-1 on tumour-infiltrating macrophages in mice. The study shows that PD-1-expressing macrophages have limited phagocytic capabilities and that blocking PD-1/PD-L1 enhances phagocytosis and correlates with reduced tumour growth. This finding suggests a T-cell-independent mechanism of therapeutic activity of PD-1/PD-L1 inhibitors which may be clinically relevant. Programmed cell death protein 1 (PD-1) is an immune checkpoint receptor that is upregulated on activated T cells for the induction of immune tolerance1,2. Tumour cells frequently overexpress the ligand for PD-1, programmed cell death ligand 1 (PD-L1), facilitating their escape from the immune system3,4. Monoclonal antibodies that block the interaction between PD-1 and PD-L1, by binding to either the ligand or receptor, have shown notable clinical efficacy in patients with a variety of cancers, including melanoma, colorectal cancer, non-small-cell lung cancer and Hodgkin’s lymphoma5,6,7,8,9. Although it is well established that PD-1–PD-L1 blockade activates T cells, little is known about the role that this pathway may have in tumour-associated macrophages (TAMs). Here we show that both mouse and human TAMs express PD-1. TAM PD-1 expression increases over time in mouse models of cancer and with increasing disease stage in primary human cancers. TAM PD-1 expression correlates negatively with phagocytic potency against tumour cells, and blockade of PD-1–PD-L1 in vivo increases macrophage phagocytosis, reduces tumour growth and lengthens the survival of mice in mouse models of cancer in a macrophage-dependent fashion. This suggests that PD-1–PD-L1 therapies may also function through a direct effect on macrophages, with substantial implications for the treatment of cancer with these agents.