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.

Platelet (PLT) transfusions are potentially life saving for individuals with low PLT numbers; however, previous work revealed that PLT transfusions are associated with increased infection risk. During storage, PLT intended for transfusion continuously shed ectosomes (Ecto) from their surface, which express immunomodulatory molecules like phosphatidylserine or TGF-β1. Recently, PLT-Ecto were shown to reduce proinflammatory cytokine release by macrophages and to favor the differentiation of naive T cells toward regulatory T cells. Whether PLT-Ecto modify NK cells remains unclear. We exposed purified NK cells and full PBMCs from healthy donors to PLT-Ecto. We found a reduced expression of several activating surface receptors (NKG2D, NKp30, and DNAM-1) and decreased NK cell function, as measured by CD107a expression and IFN-γ production. Pretreatment of PLT-Ecto with anti–TGF-β1 neutralizing Ab restored surface receptor expression and NK cell function. We further observed a TGF-β1–mediated upregulation of miR-183, which, in turn, reduced DAP12, an important protein for stabilization and downstream signaling of several activating NK cell receptors. Again, these effects could antagonized, in part, when PLT-Ecto were preincubated with anti–TGF-β1 Ab. Erythrocyte Ecto did not affect NK cells. Polymorphonuclear cell Ecto expressed MHC class I and inhibited NK cell function. In addition, they induced the secretion of TGF-β1 by NK cells, which participated in an auto/paracrine manner in the suppressive activity of polymorphonuclear cell–derived Ecto. In sum, our study showed that PLT-Ecto could inhibit NK cell effector function in a TGF-β1–dependent manner, suggesting that recipients of PLT transfusions may experience reduced NK cell function.

Analysis of killer cell immunoglobulin-like receptor (KIR) expression has been notoriously difficult because of the cross-reactivity of available antibodies, in particular between activating and inhibitory isoforms. We undertook a comprehensive study of available anti-KIR antibodies binding to activating KIRs (a-KIRs). Using cell lines stably transfected with a-KIRs (KIR2DS1-S5 and KIR3DS1), we confirmed documented binding specificities. In addition, we show that clones HPMA4 and 143211-previously assumed to be specific for KIR2DS1/L1 and KIR2DL1, respectively-bind KIR2DS5 and KIR2DS3 (HPMA4), and KIR2DS5 (143211). Other antibodies with previously undocumented binding were JJC11.6 (recognizing KIR2DS3) and 5.133 (recognizing all a-KIRs except KIR2DS1 and KIR2DS3). The novel KIR2DS5 reactivities were confirmed by blocking with soluble KIR-Fc fusion proteins, and by reverse transcriptase-PCR analysis of sorted primary natural killer cells. In conclusion, we show formerly undocumented binding properties of anti-KIR antibodies. These cross-reactivities should be taken into account when analyzing KIR expression.

Natural killer (NK) cells require interaction of inhibitory surface receptors with human leukocyte antigen (HLA) ligands during development to acquire functional competence in a process termed "licensing." The quantity of HLA required for this process is unknown. Two polymorphisms affecting HLA-C surface expression (rs9264942 and rs67384697) have recently been identified, and shown to influence progression of HIV infection. We typed a cohort of healthy donors for the two HLA-C-related polymorphisms, KIR2DL1 and KIR2DL3, and their respective HLA-C ligands and analyzed how HLA ligands influenced licensing status of killer cell immunoglobulin-like receptor (KIR)+ NK cells in terms of degranulation and cytokine production in response to HLA-deficient target cells. The presence of respective HLA class I ligands increased the function of KIR2DL1+ and KIR2DL3+ NK cells in a dose-dependent manner. In contrast, neither of the HLA-C-related polymorphisms nor the quantity of cell surface HLA-C had any significant effect on NK cell function. Interestingly, HLA-Cw7-an HLA-C allele with low surface expression-licensed KIR2DL3+ NK cells more strongly than any other KIR2DL3 ligand. The quantity of cell surface HLA-C does not appear to influence licensing of NK cells, and the HLA-C-related polymorphisms presumably influence HIV progression through factors unrelated to NK cell education.