Glucocorticoids and the cytokines IL-12, IL-15 and IL-18 present in the tumor microenvironment induce PD-1 expression on human Natural Killer cells

Background: PD-1 immune checkpoint blockade has provided significant clinical efficacy across various types of cancer by unleashing both T- and NK cell-mediated anti-tumor responses. However, resistance to immunotherapy occurs for many patients, rendering the identification of the mechanisms that control PD-1 expression extremely important to increase the response to the therapy Objective: To identify the stimuli and the molecular mechanisms that induce the de novo PD-1 expression on human NK cells in the tumor setting Methods: NK cells freshly isolated from peripheral blood of healthy donors were stimulated with different combinations of molecules, and PD-1 expression was studied at the mRNA and protein level. Moreover, ex vivo analysis of tumor microenvironment and NK cell phenotype was performed. Results: Glucocorticoids (GCs) are indispensable for PD-1 induction on human NK cells, in cooperation with a combination of cytokines that are abundant at the tumor site. Mechanistically, GCs together with IL-12, IL-15 and IL-18 not only upregulate PDCD1 transcription, but also activate a previously unrecognized transcriptional program leading to enhanced mRNA translation and resulting in an increased PD-1 protein amount in NK cells. Conclusion: Our results provide evidence of a novel immune suppressive mechanism of GCs involving the transcriptional and translational control of an important immune checkpoint CD56bright NK cells were stimulated with cytokines in the presence of Dex or DMSO (ctrl) for 6 days, then RNA was extracted for RNA-seq. Three biological replicates were generated for each condition (Dex and ctrl).