A Structural basis for the association of DAP12 with mouse, but not human, NKG2D

ITAM-based signaling beyond the adaptive immune response

Classical immunoreceptors like lymphocyte antigen receptors and Fc-receptors (FcR) are central players of the adaptive immune response. These receptors utilize a common signal transduction mechanism, which relies on immunoreceptor tyrosine-based activation motifs (ITAMs) present in the receptor complex. Upon ligand binding to the receptors, tyrosines within the ITAM sequence are phosphorylated by Src-family kinases, leading to an SH2-domain mediated recruitment and activation of the Syk or the related ZAP-70 tyrosine kinase. These kinases then initiate further downstream signaling events. Here we review recent evidence indicating that components of this ITAM-based signaling machinery are also present in a number of non-lymphoid or even non-immune cell types and they participate in diverse biological functions beyond the adaptive immune response, including innate immune mechanisms, platelet activation, bone resorption or tumor development. These results suggest that the ITAM-based signaling paradigm has much wider implications than previously anticipated.

NKG2D in NK and T Cell-Mediated Immunity

One of the best characterized NK cell receptors is NKG2D, a highly conserved C-type lectin-like membrane glycoprotein expressed on essentially all NK cells, as well as on gammadelta-TcR+ T cells and alphabeta-TcR+ CD8+ T cells, in humans and mice. Here we review recent studies implicating NKG2D in T cell and NK cell-mediated immunity to viruses and tumors, and its potential role in autoimmune diseases and allogeneic bone marrow transplantation.

T Cells Gene-engineered with DAP12 Mediate Effector Function in an NKG2D-dependent and Major Histocompatibility Complex-independent Manner

NKG2D is an important activating/co-stimulatory receptor harnessed by NK and T cells in immune surveillance. In contrast to NK cells, T cells fail to express the activation-signaling molecule DAP12 even when activated and, therefore, ligation of NKG2D alone is insufficient to induce T cell cytolytic function. To test whether we could endow T cells with NK cell-like effector function, we have engineered DAP12 into T cells by retroviral transduction (T-DAP12). T-DAP12 cells were demonstrated to specifically secrete interferon-gamma following receptor ligation and to mediate potent and specific lysis of the NKG2D ligand (NKG2D-L) (Rae-1beta) expressing MHC class I-deficient and class I-sufficient tumors. To circumvent the inability of T-DAP12 cells to proliferate following NKG2D ligation by Rae-1beta expressing tumors, DAP12 was engineered into OT-1 cells with an endogenous T cell receptor specific for chicken ovalbumin peptide (amino acids 257-264). Importantly, following a period of proliferation through endogenous T cell receptor ligation, OT-1-DAP12 cells retained specificity against NKG2D-L expressing major histocompatibility complex class I-deficient tumor. In adoptive transfer experiments, T-DAP12 cells enhanced the survival of NK cell-depleted RAG-1-deficient mice inoculated with RMA-S-Rae-1beta but not parental RMA-S tumors. Overall, this study demonstrated the significant potential of suppressing tumors and other cellular targets expressing NKG2D-L by endowing T cells with innate NK cell-like function.

Comparative genomics of natural killer cell receptor gene clusters

Many receptors on natural killer (NK) cells recognize major histocompatibility complex class I molecules in order to monitor unhealthy tissues, such as cells infected with viruses, and some tumors. Genes encoding families of NK receptors and related sequences are organized into two main clusters in humans: the natural killer complex on Chromosome 12p13.1, which encodes C-type lectin molecules, and the leukocyte receptor complex on Chromosome 19q13.4, which encodes immunoglobulin superfamily molecules. The composition of these gene clusters differs markedly between closely related species, providing evidence for rapid, lineage-specific expansions or contractions of sets of loci. The choice of NK receptor genes is polarized in the two species most studied, mouse and human. In mouse, the C-type lectin-related Ly49 gene family predominates. Conversely, the single Ly49 sequence is a pseudogene in humans, and the immunoglobulin superfamily KIR gene family is extensive. These different gene sets encode proteins that are comparable in function and genetic diversity, even though they have undergone species-specific expansions. Understanding the biological significance of this curious situation may be aided by studying which NK receptor genes are used in other vertebrates, especially in relation to species-specific differences in genes for major histocompatibility complex class I molecules.

Synergy among receptors on resting NK cells for the activation of natural cytotoxicity and cytokine secretion

Freshly isolated, resting natural killer (NK) cells are generally less lytic against target cells than in vitro interleukin 2 (IL-2)-activated NK cells. To investigate the basis for this difference, the contribution of several receptors to activation of human NK cells was examined. Target-cell lysis by IL-2-activated NK cells in a redirected, antibody-dependent cytotoxicity assay was triggered by a number of receptors. In contrast, cytotoxicity by resting NK cells was induced only by CD16, and not by NKp46, NKG2D, 2B4 (CD244), DNAM-1 (CD226), or CD2. Calcium flux in resting NK cells was induced with antibodies to CD16 and, to a weaker extent, antibodies to NKp46 and 2B4. Although NKp46 did not enhance CD16-mediated calcium flux, it synergized with all other receptors. 2B4 synergized with 3 other receptors, NKG2D and DNAM-1 each synergized with 2 other receptors, and CD2 synergized with NKp46 only. Resting NK cells were induced to secrete tumor necrosis factor alpha (TNF-alpha) and interferon gamma (IFN-gamma), and to kill target cells by engagement of specific, pair-wise combinations of receptors. Therefore, natural cytotoxicity by resting NK cells is induced only by mutual costimulation of nonactivating receptors. These results reveal distinct and specific patterns of synergy among receptors on resting NK cells.

Natural killer cell receptors: regulating innate immune responses to hematologic malignancy

Critical to innate immunity, the natural killer (NK) cell performs its function of immunosurveillance through its recognition of altered or missing self on damaged, infected, or transformed malignant cells. NK cell receptors responsible for detection of human leukocyte antigen (HLA) class I and class I-like proteins on potential target cells transmit inhibitory and activating signals that integrate to determine NK cell function. Advances in the fields of NK cell receptor biology and immunogenetics have enhanced our understanding of NK cell target recognition and may now guide studies to determine NK cell effects in the clinical setting. Analysis of NK cell receptor-ligand relationships, such as the inhibitory killer immunoglobulin-like receptors (KIRs) and their HLA class I ligands, has revealed the potential for NK cell-mediated benefit in allogeneic hematopoietic stem cell transplantation for hematologic malignancies.

KARAP/DAP12/TYROBP: three names and a multiplicity of biological functions

The signaling adaptor protein KARAP/DAP12/TYROBP (killer cell activating receptor-associated protein / DNAX activating protein of 12 kDa / tyrosine kinase binding protein) belongs to the family of transmembrane polypeptides bearing an intracytoplasmic immunoreceptor tyrosine-based activation motif (ITAM). This adaptor, initially characterized in NK cells, is associated with multiple cell-surface activating receptors expressed in both lymphoid and myeloid lineages. We review here the main features of KARAP/DAP12, describing findings from its identification to recently published data, showing its involvement in a broad array of biological functions. KARAP/DAP12 is a wiring component for NK cell anti-viral function (e.g. mouse cytomegalovirus via its association with mouse Ly49H) and NK cell anti-tumoral function (e.g. via its association with mouse NKG2D or human NKp44). KARAP/DAP12 is also involved in inflammatory reactions via its coupling to myeloid receptors, such as the triggering receptors expressed by myeloid cells (TREM) displayed by neutrophils, monocytes/macrophages and dendritic cells. Finally, bone remodeling and brain function are also dependent upon the integrity of KARAP/DAP12 signals, as shown by the analysis of KARAP/DAP12-deficient mice and KARAP/DAP12-deficient Nasu-Hakola patients.

A requirement for CD45 distinguishes Ly49D-mediated cytokine and chemokine production from killing in primary natural killer cells

Engagement of receptors on the surface of natural killer (NK) cells initiates a biochemical cascade ultimately triggering cytokine production and cytotoxicity, although the interrelationship between these two outcomes is currently unclear. In this study we investigate the role of the cell surface phosphatase CD45 in NK cell development and intracellular signaling from activating receptors. Stimulation via the major histocompatibility complex I-binding receptor, Ly49D on CD45(-/-) primary NK cells resulted in the activation of phosphoinositide-3-kinase and normal cytotoxicity but failed to elicit a range of cytokines and chemokines. This blockage is associated with impaired phosphorylation of Syk, Vav1, JNK, and p38, which mimics data obtained using inhibitors of the src-family kinases (SFK). These data, supported by analogous findings after CD16 and NKG2D stimulation of CD45(-/-) primary NK cells, place CD45 upstream of SFK in NK cells after stimulation via immunoreceptor tyrosine-based activation motif-containing receptors. Thus we identify CD45 as a pivotal enzyme in eliciting a precise subset of NK cell responses.

Natural Killer Cell Receptor Signaling Pathway in Mammals

NK (natural killer) cells are large granular lymphocytes of the innate immune system. They are widespread throughout the body, being present in both lymphoid organs and nonlymphoid peripheral tissues. NK cells are involved in direct innate immune reactions against viruses, bacteria, parasites, and other triggers of pathology, such as malignant transformation, all of which cause stress in affected cells. Importantly, NK cells also link the innate and adaptive immune responses, contributing to the initiation of adaptive immune responses. NK cell responses are mediated through two major effector functions, the direct cytolysis of target cells and the production of cytokines and chemokines. We focus here on how recognition events are integrated to trigger distinct and graded effector functions.

NK cell recognition

The integrated processing of signals transduced by activating and inhibitory cell surface receptors regulates NK cell effector functions. Here, I review the structure, function, and ligand specificity of the receptors responsible for NK cell recognition.

Altered NKG2D function in NK cells induced by chronic exposure to NKG2D ligand-expressing tumor cells

NKG2D is an activation receptor that allows natural killer (NK) cells to detect diseased host cells. The engagement of NKG2D with corresponding ligand results in surface modulation of the receptor and reduced function upon subsequent receptor engagement. However, it is not clear whether in addition to modulation the NKG2D receptor complex and/or its signaling capacity is preserved. We show here that the prolonged encounter with tumor cell-bound, but not soluble, ligand can completely uncouple the NKG2D receptor from the intracellular mobilization of calcium and the exertion of cell-mediated cytolysis. However, cytolytic effector function is intact since NKG2D ligand-exposed NK cells can be activated via the Ly49D receptor. While NKG2D-dependent cytotoxicity is impaired, prolonged ligand exposure results in constitutive interferon gamma (IFNgamma) production, suggesting sustained signaling. The functional changes are associated with a reduced presence of the relevant signal transducing adaptors DNAX-activating protein of 10 kDa (DAP-10) and killer cell activating receptor-associated protein/DNAX-activating protein of 12 kDa (KARAP/DAP-12). That is likely the consequence of constitutive NKG2D engagement and signaling, since NKG2D function and adaptor expression is restored to normal when the stimulating tumor cells are removed. Thus, the chronic exposure to tumor cells expressing NKG2D ligand alters NKG2D signaling and may facilitate the evasion of tumor cells from NK cell reactions.

Anatomy of a murder—signal transduction pathways leading to activation of natural killer cells

Natural killer (NK) cells control the early phases of viral infections, modulate antigen-specific immune responses, and participate in the rejection of tumours and bone marrow grafts. A fine balance between inhibitory and activating receptors tightly regulates NK cell activation. Biochemical studies in human cell lines and primary cells have revealed some of the activating NK cell signalling pathways, however animal models are instrumental to understand the physiological implications of these findings for immune responses in vivo. Gene targeting in mice and biochemical studies in cells are helping to dissect out the various signal transduction pathways that control NK cell activation. A clearer view of these pathways may eventually help designing more effective immune therapies based on the use of NK cells.

NK cells in innate immunity

NK cells have an important role in innate immune responses, particularly in anti-viral immunity. Recent studies have revealed a molecular basis for NK cell recognition of virus-infected cells, implicating the activating KIR and Ly49 receptors and NKG2D in this process. Additionally, mutual cooperation between NK cells and dendritic cells suggests that these innate cells can shape the nature of an adaptive immune response. These findings, as well as advances in understanding NK cell development and homeostasis, indicate that NK cell biology is more sophisticated than previously appreciated.

CD200 receptor family members represent novel DAP12-associated activating receptors on basophils and mast cells

Modulation by balancing activating and inhibitory receptors constitutes an important mechanism for regulating lymphocyte and myeloid cell effector responses. Using a microarray screen during parasitic helminth infection, we identified CD200 receptor-like 3 as a transcript highly expressed in basophils. Novel splice variants were present that generated proteins that differed in surface expression. The second immunoglobulin-like domain, encoded by exon 4, was required for cell surface expression and recruitment of DAP12 to the cell surface. Splice variants also generated unique cytoplasmic domains, which contributed to efficient pairing with DAP12. Despite expression on basophils and mast cells, which are integral components of allergic immunity, the absence of DAP12 did not alter effector cell recruitment or the host response elicited by helminth infection with Nippostrongylus brasiliensis.