Platelet-Derived Proteases ADAM10 and ADAM17 Impair NK Cell Immunosurveillance of Metastasizing Tumor Cells By Diminishing NKG2D Ligand Surface Expression

Introduction: NK cells are cytotoxic lymphocytes the reactivity of which is governed by the principles of ‘missing-self’ and “induced-self’ recognition. This implies that cells with low or absent expression of MHC class I and/or stress-induced expression of ligands for activating receptors like NKG2D (NKG2DL) are preferentially eliminated by NK cells. We and others reported previously that NKG2DL undergo proteolytic cleavage (i.e. shedding) which promotes evasion from NKG2D-mediated tumor immunosurveillance. Notably, the reactivity of NK cells is also influenced by various other components of the hematopoietic system, and we and others provided evidence for the involvement of platelets in tumor immune evasion including impairment of NKG2D-mediated immune surveillance (e.g., Kopp et al., Cancer Res 2009, Raab et al., Blood 2013 122:3488). Here we extend our recent findings and provide further data how platelets affect immunostimulatory NKG2D-NKG2DL interaction. Methods: Tumor cells were incubated with platelets from healthy donors or treated with platelet-derived soluble factors (releasate) obtained by stimulation of platelets with known platelet agonists including ADP and thrombin. NKG2DL and ADAM10 as well as ADAM17 surface expression on tumor cells and platelets, respectively, was measured by FACS, while release of NKG2DL was determined by ELISA. ADAM10 and 17 protein levels were assessed by immunoblotting. NK cell lysis of tumor cells in the presence or absence of coating platelets or platelet releasate was determined by chromium release assays. Results: We found that interaction of platelets with tumor cells resulted in substantially reduced NKG2DL expression on the surface of the malignant cells, which was paralleled by enhanced release of soluble NKG2DL. Similar albeit weaker effects were observed upon treatment of tumor cells with platelet releasate, indicating that platelet-derived factors mediate NKG2DL shedding from the tumor cell surface. Notably, ADAM10 and ADAM17, the known sheddases of NKG2DL, were found to be expressed on the platelet surface, and sheddases could also be detected in platelet releasate, pointing to an involvement in platelet-mediated NKG2DL shedding. Diminished NKG2DL surface expression resulted in reduced NKG2D-mediated NK cell cytotoxicity as revealed by blocking experiments using NKG2D antibody and F(ab)₂ fragments specific for the modulated NKG2DL. Conclusion: We propose that induction of NKG2DL shedding constitutes a novel mechanism by which the interaction of platelets with metastasizing tumor cells impairs NK cell immunosurveillance.