IL15 Stimulation with TIGIT Blockade Reverses CD155-mediated NK-Cell Dysfunction in Melanoma

Purpose: Natural Killer (NK) cells play a critical role in tumor immunosurveillance. Multiple activating and inhibitory receptors regulate NK cell-mediated tumor control. The inhibitory receptor TIGIT and its counter-receptor CD226 exert opposite effects on NK cell-mediated tumor reactivity. Experimental design: We evaluated the frequency, phenotype and functions of NK cells freshly isolated from healthy donors and melanoma patients with multiparameter flow cytometry. We assessed TIGIT and CD226 cell surface expression and internalization upon binding to CD155. We evaluated the role of Interleukin (IL)-15 and TIGIT blockade in increasing NK cell-mediated cytotoxicity in vitro and in two mouse models. Results: NK cells are present at low frequencies in metastatic melanoma, are dysfunctional and downregulate both TIGIT and CD226 expression. As compared with TIGIT- NK cells, TIGIT+ NK cells exhibit higher cytotoxic capacity and maturation but paradoxically lower cytotoxicity against CD155+ MHC class I-deficient melanoma cells. Membrane-bound CD155 triggers CD226 internalization and degradation, resulting in decreased NK cell-mediated tumor reactivity. IL-15 increases TIGIT and CD226 gene expression by tumor-infiltrating NK cells (TiNKs) and, together with TIGIT blockade, increases NK cell-mediated melanoma cytotoxicity in vitro and decreases tumor metastasis in two mouse melanoma models. Specific deletion of TIGIT on transferred NK cells enhances the anti-metastatic activity of IL-15, while CD226 blockade decreases the effects of IL-15 and TIGIT blockade. Conclusion: Our findings support the development of novel combinatorial immunotherapy with IL-15 and TIGIT blockade to promote NK cell-mediated destruction of MHC class I-deficient melanoma, which are refractory to CD8+ T cell-mediated immunity.