Abstract 3652: Surface expression and shedding of NKG2D ligands by (metastasizing) tumor cells is altered by platelets leading to impaired NK cell immunosurveillance

Abstract NK cells play an important role in anti-tumor immunity. The mechanisms leading to NK cell activation are described by the ‘missing-self’ and ‘induced-self' hypotheses, implying that cells with low or absent expression of MHC class I and stress-induced expression of ligands for activating receptors such as NKG2D (NKG2DL), are targeted by NK cells. Besides the direct interaction with their target cells, NK activity is also influenced by various other hematopoietic cells. In mouse models, thrombocytopenia impairs metastasis, and this is reversed by additional depletion of NK cells. Hence, the knowledge of the underlying molecular mechanisms by which platelets influence NK cells is still fragmentary. We already showed that release of TGF-β by platelets upon their interaction with (metastasizing) tumor cells downmodulates the activating receptor NKG2D on NK cells (Kopp et al, Cancer Res. 2009; Placke, J Innate Immun. 2011). Moreover, platelets transfer ´healthy´ MHC class I to the tumor cell surface. Thus, platelets may facilitate metastasis by interfering with both, ‘induced and missing self' NK cell recognition. Here we provide evidence for a yet unknown mechanism by which platelets further impair NKG2D-mediated immunosurveillance. Tumor cells were incubated with platelets from healthy donors resulting in coating of tumor cells and activation of the platelets, or treated with platelet-derived soluble factors (releasate) obtained either by tumor cell-induced platelet activation (TCIPA) or the platelet agonist thrombin. Presence of platelet derived factors derived either from coating of tumor cells or contained in platelet releasate substantially reduced surface expression of NKG2DL on tumor cells. This was paralleled by enhanced levels of soluble NKG2DL in culture supernatants, indicating that platelet-derived factors mediate NKG2DL shedding from the tumor cell surface. Diminished NKG2DL surface expression resulted in diminished NKG2D-dependent natural cytotoxicity of NK cells, as revealed by blocking experiments using NKG2D antibody and NKG2DL-specific F(ab)2 fragments targeting the specifically modulated NKG2DL. Our data thus identify induction of NKG2DL shedding as novel mechanism by which interaction of platelets with metastasizing tumor cells impairs NK cell immunosurveillance. Citation Format: Stefanie Raab, Korbinian Nepomuk Kropp, Alexander Steinle, Gerd Klein, Hans-Georg Kopp, Helmut R. Salih. Surface expression and shedding of NKG2D ligands by (metastasizing) tumor cells is altered by platelets leading to impaired NK cell immunosurveillance. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3652. doi:10.1158/1538-7445.AM2014-3652