c‐Cbl regulates MICA‐ but not ULBP2‐induced NKG2D down‐modulation in human NK cells

Abstract

The NKG2D activating receptor on human NK cells mediates "altered self" recognition, as its ligands (NKG2DLs) are upregulated on target cells in a variety of stress conditions. Evidence collected in the past years shows that, even though expression of NKG2DLs acts as a danger signal that renders tumor cells susceptible to cytotoxicity, chronic exposure to soluble or membrane-bound NKG2DLs can lead to down-modulation of receptor expression and impairment of NKG2D-mediated cell functions. Here, we evaluated whether different cell-bound NKG2DLs, namely MICA and ULBP2, are equivalently able to induce NKG2D down-modulation on human NK cells. We found that although both ligands reduce NKG2D surface expression, MICA promotes a stronger receptor downmodulation than ULBP2, leading to a severe impairment of NKG2D-dependent NK-cell cytotoxicity. We also provide evidence that the ubiquitin pathway and c-Cbl direct MICAinduced but not ULBP2-induced NKG2D internalization and degradation, thus identifying a molecular mechanism to explain the differential effects of MICA and ULBP2 on NKG2D expression. A better understanding of the molecular mechanisms employed by the different NKG2DLs to control NKG2D surface expression could be useful for the development of anti-tumor strategies to restore a normal level of NKG2D receptors on human NK cells. Keywords:Endocytosis r NK cells r NKG2D ligands r NKG2D receptor Additional supporting information may be found in the online version of this article at the publisher's web-site Introduction NK cells are an important arm of the innate immune system directly involved in the recognition and lysis of virus-infected and tumor cells. NK-cell functional activation mainly depends on the interplay between inhibitory receptors for MHC class I molecules Correspondence: Prof. Rossella Paolini e-mail: rossella.paolini@uniroma1.it and a wide array of activating receptors that act in concert to induce efficient target cell elimination [1][2][3]. One of the best characterized activating receptors is NKG2D (NK receptor group 2, member D), whose expression is not restricted to NK cells, as it is also found on many T-cell subsets [4,5]. NKG2D mediates "altered self" recognition, in that it recognizes ligands absent or poorly expressed on most tissues and upregulated by conditions of stress or infection, in autoimmune diseases and malignant transformation, thus rendering damaged cells susceptible to NKG2D-mediated cellular cytotoxicity [6,7]. Eur. J. Immunol. 2014Immunol. . 44: 2761Immunol. -2770 The early hypothesis claiming a prominent role of NKG2D in tumor immune surveillance [8] has been supported by studies using various animal tumor models and NKG2D-deficient mice [9][10][11][12]. In humans, a remarkable characteristic of NKG2D resides in the interaction of a single invariant receptor with two structurally distinct families of MHC class I-related cell surface glycoproteins that differ in modes of membrane anchor and affinities for their common receptor: the polymorphic MHC class I-related Chain (MIC...

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Modulation of Immune Cell Functions by the E3 Ligase Cbl-b

“…In follow up publications, it was shown that two synergistic activating signals, for example, through NKG2D and CD244 are needed to overcome blockade by c-Cbl (104) and that this synergistic action is mediated by a combined phosphorylation of separate tyrosine residues of the adaptor protein SLP-76 (105). On the other hand, c-Cbl is involved in a kind of negative feedback loop in the down regulation of NKG2D after prolonged exposure to one of its ligands, MICA (106). …”