The immune recognition receptor complex NKG2D-DAP10 on natural killer cells is stimulated by specific ligands carried on virus-infected and malignant cells. Because DAP10 does not have an immunoreceptor tyrosine-based activation motif (ITAM) in its cytoplasmic tail, its ability to trigger killing has been debated. Here we show that a crucial Tyr-Ile-Asn-Met amino acid motif in the cytoplasmic tail of DAP10 couples receptor stimulation to the downstream activation of phosphatidylinositol 3-kinase, Vav1, Rho family GTPases and phospholipase C. Unlike that of ITAM-containing receptors, the activation of NKG2D-DAP10 proceeds independently of Syk family protein tyrosine kinases. Yet the signals initiated by NKG2D-DAP10 are fully capable of inducing killing. Our findings identify a previously unknown mechanism by which receptor complexes that lack ITAM motifs can trigger lymphocyte activation.

T cell receptor (TCR)-mediated cytoskeletal reorganization is considered to be actin-related protein (Arp) 2/3 complex dependent. We therefore examined the requirement for Arp2/3- and formin-dependent F-actin nucleation during T cell activation. We demonstrated that without Arp2/3-mediated actin nucleation, stimulated T cells could not form an F-actin-rich lamellipod, but instead produced polarized filopodia-like structures. Moreover, the microtubule-organizing center (MTOC, or centrosome), which rapidly reorients to the immunological synapse through an unknown mechanism, polarized in the absence of Arp2/3. Conversely, the actin-nucleating formins, Diaphanous-1 (DIA1) and Formin-like-1 (FMNL1), did not affect TCR-stimulated F-actin-rich structures, but instead displayed unique patterns of centrosome colocalization and controlled TCR-mediated centrosome polarization. Depletion of FMNL1 or DIA1 in cytotoxic lymphocytes abrogated cell-mediated killing. Altogether, our results have identified Arp2/3 complex-independent cytoskeletal reorganization events in T lymphocytes and indicate that formins are essential cytoskeletal regulators of centrosome polarity in T cells.

T-cell antigen receptor (TCR) engagement activates multiple protein tyrosine kinases (PTKs),

NKG2D is an important immunosurveillance receptor that responds to stress-induced ligand expression on tumors and virus-infected cells. Human natural killer cells express NKG2D and require the transmembrane adaptor DAP10 to initiate their full cytotoxic activation. However, DAP10 has no immunoreceptor tyrosine-based activation motif and thus the mechanism of recruiting 'downstream' effector proteins is unclear. We show here that binding of the p85 subunit of phosphatidylinositol-3- kinase to DAP10 could not by itself trigger cell-mediated cytotoxicity and that binding of an intermediate consisting of the DAP10 binding partner Grb2 and the effector molecule Vav1 (Grb2-Vav1) to DAP10 was sufficient to initiate tyrosine-phosphorylation events. For full calcium release and cytotoxicity to occur, both Grb2-Vav1 and p85 had to bind to DAP10. These findings identify a previously unknown mechanism by which NKG2D-DAP10 mediates cytotoxicity and provides a framework for evaluating activation by other receptor complexes that lack immunoreceptor tyrosine-based activation motifs.

Here, we present data suggesting a novel mechanism for regulation of natural killer (NK) cell cytotoxicity through inhibitory receptors. Interaction of activation receptors with their ligands on target cells induces cytotoxicity by NK cells. This activation is under negative control by inhibitory receptors that recruit tyrosine phosphatase SHP-1 upon binding major histocompatibility class I on target cells. How SHP-1 blocks the activation pathway is not known. To identify SHP-1 substrates, an HLA-C-specific inhibitory receptor fused to a substrate-trapping mutant of SHP-1 was expressed in NK cells. Phosphorylated Vav1, a regulator of actin cytoskeleton, was the only protein detectably associated with the catalytic site of SHP-1 during NK cell contact with target cells expressing HLA-C. Vav1 trapping was independent of actin polymerization, suggesting that inhibition of cellular cytotoxicity occurs through an early dephosphorylation of Vav1 by SHP-1, which blocks actin-dependent activation signals. Such a mechanism explains how inhibitory receptors can block activating signals induced by different receptors.In many cell types, activation induced by cell contact is controlled by inhibitory receptors that bind ligands on target cells (41, 48). The importance of this type of negative regulation is illustrated by the protection of normal cells from lysis by NK cells, of red blood cells from ingestion by macrophages (44), and of cardiomyocytes from immunoglobulin G (IgG)-mediated autoimmunity (43). A common feature of the receptors involved in these protective functions (i.e., CD158, SIRP-␣, and PD-1, respectively) and of many other receptors that inhibit cellular responses is the presence of immunoreceptor tyrosine-based inhibition motifs (ITIMs) in the cytoplasmic tail (12,48). Upon Tyr phosphorylation, ITIMs bind to the SH2 domains of protein tyrosine phosphatases (PTPases) SHP-1 and SHP-2, thereby releasing the catalytic site from autoinhibition (2, 12). Activation of human NK cell and T cell cytotoxicity is controlled by several inhibitory receptors, including the CD158 killer cell Ig-like receptors (KIR) and the lectin-like CD94/NKG2A, which bind to major histocompatibility complex (MHC) class I molecules expressed on target cells (5).Whereas the functional consequence of SHP-1 recruitment by ITIM-containing receptors in NK cells is well established, the specific point at which SHP-1 blocks activation signals has not been defined. Engagement of inhibitory receptors by ligands on target cells blocks conjugate formation (13), Ca 2ϩ flux (38, 52), polarization of lipid rafts in NK cells (42), and actin cytoskeleton rearrangement in T cells (26). As NK cell cytotoxicity depends on the activity of Tyr kinases (40), a number of Tyr-phosphorylated proteins are potential substrates for dephosphorylation by SHP-1. Two nonexclusive models, a "promiscuous" and a "selective" model, can be proposed for SHP-1-mediated inhibition. First, recruitment and activation of SHP-1 by inhibitory receptors at the NK-target cell interfac...