Identification of a subset of human natural killer cells expressing high levels of programmed death 1: A phenotypic and functional characterization

Background: PD-1 is an immunologic checkpoint that limits immune responses by delivering potent inhibitory signals to T cells on interaction with specific ligands expressed on tumor/virusinfected cells, thus contributing to immune escape mechanisms. Therapeutic PD-1 blockade has been shown to mediate tumor eradication with impressive clinical results. Little is known about the expression/function of PD-1 on human natural killer (NK) cells. Objective: We sought to clarify whether human NK cells can express PD-1 and analyze their phenotypic/functional features. Methods: We performed multiparametric cytofluorimetric analysis of PD-1+ NK cells and their functional characterization using degranulation, cytokine production and proliferation assays. Results: We provide unequivocal evidence that PD-1 is highly expressed (PD-1bright) on a NK cell subset detectable in the peripheral blood of approximately one fourth of healthy subjects. These donors are always serologically positive for human cytomegalovirus. PD-1 is expressed by CD56dim but not by CD56bright NK cells and is confined to fully mature NK cells characterized by the NKG2A-KIR+CD57+ phenotype. Proportions of PD-1bright NK cells were higher in the ascites of a cohort of ovarian-carcinoma patients suggesting their possible induction/expansion in tumor environments. Functional analysis revealed a reduced proliferative capability in response to cytokines, low degranulation and impaired cytokine production upon interaction with tumor targets. Conclusions: We have identified and characterized a novel subpopulation of human NK cells expressing high levels of PD-1. These cells have the phenotypic characteristics of fully mature NK cells and are increased in ovarian-carcinoma patients. They display low proliferative responses and impaired anti-tumor activity that can be partially restored by antibody-mediated disruption of PD- 1/PD-L interaction.