Platelet-Mediated Protection of Cancer Cells From Immune Surveillance - Possible Implications for Cancer Immunotherapy

Abstract

The growing insights in the complex interactions between metastatic cancer-cells and platelets have revealed that platelet tumor cell interactions in the blood stream are an important factor supporting tumor metastasis. An increased coagulability of platelets facilitates the vascular evasion and establishment of solid tumor metastasis. Furthermore, platelets can support an immunosuppressive tumor microenvironment or shield tumor cells directly from engagement of cytotoxic lymphocytes as e.g., natural killer (NK) cells. Platelets are both in the tumor microenvironment and systemically the quantitatively most important source of TGF-β, which is a key cytokine for immunosuppression in the tumor microenvironment. If similar platelet-tumor interactions are of physiological relevance in hematological malignancies remains less well-studied. This might be important, as T- and NK cell mediated graft vs. leukemia effects (GvL) are well-documented and malignant hematological cells have a high exposure to platelets compared to solid tumors. As NK cell-based immunotherapies gain increasing attention as a therapeutic option for patients suffering from hematological and other malignancies, we review the known interactions between platelets and NK cells in the solid tumor setting and discuss how these could also apply to hematological cancers. We furthermore explore the possible implications for NK cell therapy in patients with solid tumors and patients who depend on frequent platelet transfusions. As platelets have a protective and supportive effect on cancer cells, the impact of platelet transfusion on immunotherapy and the combination of immunotherapy with platelet inhibitors needs to be evaluated.

Keywords: NK cells; antitumor immunity; cytotoxicity; immunosuppressive; immunotherapy; metastasis; platelets; tumor microenvironment.

Figure 1

Metastasizing tumor cells exploit platelets as physical and immunological shielding from NK cells in peripheral blood. Platelets are activated upon encountering tumor cells, which release adenosine diphosphate (ADP) and tissue factor (TF), depicted as blue dots. Activated platelets adhere to the tumor cells surface, providing a physical and an immunological shield by presenting ligands to inhibitory NK cell receptors. Moreover, activated platelets exchange surface receptors with tumor cells. Several ligands to inhibitory NK cell receptors have been shown to be passed over from platelets to tumor cells, including HLA class I, glucocorticoid-induced TNF-related protein (GITR) ligand, receptor activator of NFκB (RANK) ligand and PD-L1. The figure furthermore delineates the shedding of MICA and MICB, which are ligands to the activating NK cell receptor NKG2D.

Figure 2

Depicted is a growing metastasis, highlighting the interactions between platelets and different lymphocytes. Tumor cells can activate platelets through release of ADP and TF. ADP directly activates platelets through engagement of the ADP receptor, while TF activates platelets together with other coagulation factors. Platelet activation results in the release of the dense granules and α-granules that contain amongst others more ADP, different clotting factors, growth factors, and TGF-β, which can impact different tumor infiltrating lymphocytes. The impact of TGF-β on T cells and NK cells is indicated by solid line arrows and the suspected impact on transfused cellular therapy products is shown by dashed line arrows.