Programmed cell death-1: from a T-cell immune checkpoint to a regulator of Natural Killer cell biology.
Programmed cell death protein 1 (PD-1, CD279) is a pivotal inhibitory immune checkpoint receptor that plays a central role in maintaining immune homeostasis and peripheral tolerance. Originally characterized as a negative regulator of T-cell activation, PD-1 limits excessive immune responses and prevents autoimmunity, while its sustained expression under conditions of chronic antigen stimulation contributes to T-cell dysfunction and exhaustion. The discovery that blockade of the PD-1 pathway can restore anti-tumor immunity has revolutionized cancer therapy and established immune checkpoint inhibition as a cornerstone of modern oncology. Although PD-1 has traditionally been viewed as a key regulator of adaptive immunity, accumulating evidence indicates that its biological functions extend beyond T cells. In recent years, PD-1 expression has been identified in several innate immune cell populations, particularly Natural Killer (NK) cells, where it has emerged as an important modulator of effector functions, cytokine production, metabolic fitness, and antitumor activity. These findings have challenged the classical view of PD-1 biology and revealed unexpected similarities between NK-cell dysfunction and the exhausted phenotype described in chronically stimulated T cells. In the tumor microenvironment, PD-1 expression on NK cells has been associated with impaired cytotoxicity and reduced immune surveillance, suggesting that NK cells may also represent relevant targets of PD-1-mediated immunosuppression. At the same time, the mechanisms regulating PD-1 expression and signaling in NK cells appear to differ, at least in part, from those operating in T lymphocytes, highlighting the complexity of this pathway across distinct immune cell subsets. In this review, we summarize the current knowledge of PD-1 biology, from its established role in T-cell regulation to its emerging functions in NK cells. We discuss the molecular mechanisms governing PD-1 expression and signaling, its contribution to immune dysfunction in cancer and chronic diseases, and the potential implications of targeting the PD-1 axis to enhance both adaptive and innate antitumor immunity.