Platelets Impair NK Cell Immunosurveillance Of Metastasizing Tumor Cells By Altering Surface Expression and Shedding Of Ligands For The Activating Immunoreceptor NKG2D

NK cells play an important role in the immunosurveillance of tumor cells. 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 like e.g. NKG2D (NKG2DL) are preferentially recognized and eliminated by NK cells. Besides the direct interaction with their target cells, NK activity is further influenced by various other hematopoietic cells. In mouse models, thrombocytopenia impairs metastasis, and this is reversed by additional depletion of NK cells. However, the knowledge regarding the molecular mechanisms by which platelets influence NK cells is still fragmentary. We recently reported that release of TGF-β by platelets upon their interaction with (metastasizing) tumor cells downmodulates NKG2D on NK cells (Kopp et al., Cancer Res. 2009; Placke et al., 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 NKG2DL surface expression 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 decreased NKG2D-dependent cytotoxicity of NK cells as revealed by blocking experiments using NKG2D antibody and NKG2DL-specific F(ab)₂ fragments targeting the specific 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.