Human Tumor-Derived Exosomes Down-Modulate NKG2D Expression

Abstract

NKG2D is an activating receptor for NK, NKT, CD8+, and γδ+ T cells, whose aberrant loss in cancer is a key mechanism of immune evasion. Soluble NKG2D ligands and growth factors, such as TGFβ1 emanating from tumors, are mechanisms for down-regulating NKG2D expression. Cancers thereby impair the capacity of lymphocytes to recognize and destroy them. In this study, we show that exosomes derived from cancer cells express ligands for NKG2D and express TGFβ1, and we investigate the impact of such exosomes on CD8+ T and NK cell NKG2D expression and on NKG2D-dependent functions. Exosomes produced by various cancer cell lines in vitro, or isolated from pleural effusions of mesothelioma patients triggered down-regulation of surface NKG2D expression by NK cells and CD8+ T cells. This decrease was rapid, sustained, and resulted from direct interactions between exosomes and NK cells or CD8+ T cells. Other markers (CD4, CD8, CD56, CD16, CD94, or CD69) remained unchanged, indicating the selectivity and nonactivatory nature of the response. Exosomal NKG2D ligands were partially responsible for this effect, as down-modulation of NKG2D was slightly attenuated in the presence of MICA-specific Ab. In contrast, TGFβ1-neutralizing Ab strongly abrogated NKG2D down-modulation, suggesting exosomally expressed TGFβ as the principal mechanism. Lymphocyte effector function was impaired by pretreatment with tumor exosomes, as these cells exhibited poor NKG2D-dependent production of IFN-γ and poor NKG2D-dependent killing function. This hyporesponsiveness was evident even in the presence of IL-15, a strong inducer of NKG2D. Our data show that NKG2D is a likely physiological target for exosome-mediated immune evasion in cancer.

Wiskott‐Aldrich syndrome protein (WASp) and N‐WASp are involved in the regulation of NK‐cell migration upon NKG2D activation

“…In addition, NKG2D ligands may be secreted via exosomes maintaining their ability to modulate NKG2D cell surface levels [12]. Cancer-derived exosomes may downregulate receptor expression and reduce NK and CD8 + T cells functional responses, as reported recently [13,14]. Thus, the NKG2D recognition system has been postulated as a relevant mechanism in the immune response to cancer or in the tumor evasion from immunosurveillance [15,16].…”

Cell-Cell Communication Via Extracellular Membrane Vesicles and Its Role in the Immune Response

“…Several studies have shown the immunosuppressive properties of tumor-derived exosomes as well (Clayton et al, 2008;Raulet et al, 2013). The expression of NKG2D, FasL, and membrane-bound TGF-β, a representative immunosuppressive cytokine, in these exosomes appears responsible for the immunosuppression (Bianco et al, 2007;Borges et al, 2013;Fagiolo, 2004;Yoshimura and Muto, 2011).…”

“…In contrast, exosomes from immature DCs show a different protein profile including NFGE8 (a phagocytosis-inducing factor), FasL, and TRAIL (an apoptosis-inducing ligand) (Li et al, 2006). Recent data suggest that exosomes from tumor cells often express immunosuppressive ligands, such as NKG2D, whose expression allows the evasion of tumor cells from T cell and NK cell killing (Clayton et al, 2008).…”

The exosomes in tumor immunity

“…Clinical researches showed reduced surface expression of NKG2D on circulating NK cells in cancer patients compared to healthy individuals. 40 Human prostate cancer cells-derived exosomes were found to express ligands for NKG2D, resulting in downregulation of NKG2D on NK cells and impaired NK cell cytotoxic function, 41 which might be due to TGF-b1 presented by TEX. 42,43 Neutralizing TGF-b1 antibody strongly abrogated NKG2D reduction on NK cells, suggesting TGF-b1 may be an effective mediator.…”