Association of DAP12 with activating CD94/NKG2C NK cell receptors

Immune responses to Pneumocystis murina are robust in healthy mice but largely absent in CD40 ligand-deficient mice

Pneumocystis is a pathogen of immunocompromised hosts but can also infect healthy hosts, in whom infection is rapidly controlled and cleared. Microarray methods were used to examine differential gene expression in the lungs of C57BL/6 and CD40 ligand knockout (CD40L-KO) mice over time following exposure to Pneumocystis murina. Immunocompetent C57BL/6 mice, which control and clear infection efficiently, showed a robust response to infection characterized by the up-regulation of 349 primarily immune response-associated genes. Temporal changes in the expression of these genes identified an early (Week 2), primarily innate response, which waned before the infection was controlled; this was followed by primarily adaptive immune responses that peaked at Week 5, which coincided with clearance of the infection. In conjunction with the latter, there was an increased expression of B cell-associated (Ig) genes at Week 6 that persisted through 11 weeks. In contrast, CD40L-KO mice, which are highly susceptible to developing severe Pneumocystis pneumonia, showed essentially no up-regulation of immune response-associated genes at Days 35-75. Immunohistochemical staining supported these observations by demonstrating an increase in CD4+, CD68+, and CD19+ cells in C57BL/6 but not CD40L-KO mice. Thus, the healthy host demonstrates a robust, biphasic response to infection by Pneumocystis; CD40L is an essential upstream regulator of the adaptive immune responses that efficiently control infection and prevent development of progressive pneumonia.

Signal transduction during natural killer cell activation

Natural killer (NK) cells are a subpopulation of lymphocytes that can mediate cytotoxicity of certain tumor cells, virus-infected cells, and normal cells. In addition to their cytotoxic potential, NK cells secrete a variety of cytokines and chemokines that can modulate the function, growth, and differentiation of other immune cells. These different responses are initiated by the interaction of specific NK surface receptors with defined soluble or cell-associated ligands. There are several different types of receptors on the NK cell surface including "triggering" receptors, adhesion molecules, cytokine receptors, and MHC-recognizing killer-cell inhibitory receptors. The functional response of an NK cell is the result of the integration of signals transduced by these different types of receptors. Some of these signaling pathways are similar to other lymphoid cells, but there are also unique features employed by NK cells. This overview focuses on receptor-initiated signaling pathways that modulate NK functions.

Influence of Hsp70 and HLA-E on the killing of leukemic blasts by cytokine/Hsp70 peptide-activated human natural killer (NK) cells

This study compared the effects of the human 70-kDa stress protein (Hsp70) peptide, TKDNNLLGRFELSG (TKD), proinflammatory cytokines, or a combination of both on the repertoire of receptors expressed by human natural killer (NK) cells and their capacity to kill human CX colon carcinoma cells, K562 erythroleukemic cells, and leukemic blasts from two patients with acute myelogenous leukemia. Low-dose interleukin (IL) 2/IL-15 and TKD increase the expression density of activatory (NKG2D, NKp30, NKp44, NKp46, CD94/NKG2C) and inhibitory (CD94/NKG2A) receptors on NK cells. Concomitantly, IL-2/TKD treatment enhances the cytotoxicity of NK cells (as reflected by their secretion of granzyme B) against Hsp70 membrane-positive and human leukocyte antigen (HLA)-E membrane-negative (Hsp70(+)/HLA-E(-)) CX(+) and K562 cells. However, it had no effect on the responsiveness to Hsp70(-)/HLA-E(-) CX(-) cells over that induced by IL-2 alone. The cytotoxicity of IL-2/TKD-activated, purified NK cells and peripheral blood mononuclear cells against Hsp70(+)/HLA-E(+) leukemic blasts was weaker than that against Hsp70(+)/HLA-E(-) K562 cells. Hsp70-blocking and HLA-E transfection experiments confirmed membrane-bound Hsp70 as being a recognition/activatory ligand for NK cells, as cytotoxicity was reduced by the presence of the anti-Hsp70 monoclonal antibody cmHsp70.2 and by inhibiting Hsp70 synthesis using short interference ribonucleic acid. HLA-E was confirmed as an inhibitory ligand, as the extent of NK cell-mediated lysis of K562 cell populations that had been transfected with HLA-E(R) or HLA-E(G) alleles was dependent on the proportion of HLA-E-expressing cells. These findings indicate that Hsp70 (as an activatory molecule) and HLA-E (as an inhibitory ligand) expression influence the susceptibility of leukemic cells to the cytolytic activities of cytokine/TKD-activated NK cells.

Restricted expression of C-type lectin-like natural killer receptors by CD8 T cells in the murine small intestine

The intestinal mucosa represents a challenging environment for CD8+ T cells, which must tolerate nutrient antigens and commensal microorganisms while responding efficiently to pathogens. Consequently, specific regulatory mechanisms apply for CD8+ T cells in the intestinal environment, which should also be reflected in a tissue-specific gene expression profile of these cells. This study investigates whether such tissue-specific gene expression can be observed in CD8+ T cells primed during bacterial infection. To identify intestine-specific gene expression in conventional CD8alphabeta+ T cells, mice were infected with Listeria monocytogenes expressing ovalbumin (LmOVA). Using OVA257-264 tetramers, specific CD8+ T cells were sorted from spleen, liver and the small intestinal mucosa, and RNA samples from these cells were compared using microarrays. This approach allowed the identification of differences in gene expression in a highly defined CD8+ T-cell population with identical antigen specificity generated during infection. One group of genes with reduced expression in the intestinal mucosa comprised members of the C-type lectin-like natural killer receptor (NKR) family. Fluorescence-activated cell sorting analysis was used to assess protein expression of NKR. NKR expression on CD8+ T cells from the intestinal mucosa was dependent on the route of listeria application and consequently on the site of T-cell priming. Retinoic acid influenced NKR expression consistent with an imprinting of the NKR expression profile in intestine-associated lymphoid tissues. In contrast, NKR expression was largely independent from intestinal flora. Our results demonstrate that in the intestinal mucosa, conventional CD8alphabeta+ T cells lack NKR expression and thereby lose responsiveness to NKR ligands, which otherwise could possibly cause adverse activation or inhibition of T cells in this environment.

Heat shock protein 70 (Hsp70): membrane location, export and immunological relevance

Stress or heat shock proteins (HSPs) are remarkably conserved in all living organisms. Their expression is induced in response to a variety of physiological and environmental insults. In the cytosol these proteins play an essential role as molecular chaperones by assisting the correct folding of nascent and stress-accumulated misfolded proteins, preventing protein aggregation, transport of proteins, and supporting antigen processing and presentation. Following stress, intracellularly located HSPs fulfill protective functions and thus prevent lethal damage. In contrast, membrane-bound or extracellularly located HSPs act as danger signals and elicit immune responses mediated either by the adaptive or innate immune system. Here, HSPs act as carriers for immunogenic peptides, induce cytokine release or provide recognition sites for natural killer (NK) cells. This article will discuss methods for the detection of membrane-bound and extracellular HSPs and methods for determining their immunological functions.

Maintenance of hair follicle immune privilege is linked to prevention of NK cell attack

Hair follicles (HFs) enjoy a relative immune privilege (IP) that is characterized by downregulation of major histocompatibility complex (MHC) class I and local expression of potent immunosuppressants. Normally, natural killer (NK) cells attack cells with absent/low MHC class I expression. However, because few perifollicular NK cells are found around healthy human anagen HFs, we asked how HFs escape from NK cell attack. This study suggests that this happens via an active NK cell suppression. Alopecia areata (AA), an organ-specific autoimmune disease thought to result from a collapse of HF-IP, in contrast, shows striking defects in NK cell inhibition/containment. We show that the NK cell inhibitor macrophage migration inhibitory factor is strongly expressed by the HF epithelium, and very few CD56(+)/NKG2D(+) NK cells are observed in and around normal anagen HFs compared to AA with prominent aggregations of CD56(+)/NKG2D(+) NK around AA-HFs. By flow cytometry, many fewer NK function-activating receptors (NKG2D, NKG2C) and significantly more killer cell Ig-like receptors-2D2/2D3 were found to be expressed on peripheral blood CD56(+) NK cells of healthy controls than on those of AA patients. In addition, only weak immunoreactivity for MHC class I chain-related A gene was observed in normal anagen HFs compared to AA. To our knowledge, this defect is previously unreported and must be taken into account in AA pathogenesis and its management.

The heterodimeric assembly of the CD94-NKG2 receptor family and implications for human leukocyte antigen-E recognition

The CD94-NKG2 receptor family that regulates NK and T cells is unique among the lectin-like receptors encoded within the natural killer cell complex. The function of the CD94-NKG2 receptors is dictated by the pairing of the invariant CD94 polypeptide with specific NKG2 isoforms to form a family of functionally distinct heterodimeric receptors. However, the structural basis for this selective pairing and how they interact with their ligand, HLA-E, is unknown. We describe the 2.5 A resolution crystal structure of CD94-NKG2A in which the mode of dimerization contrasts with that of other homodimeric NK receptors. Despite structural homology between the CD94 and NKG2A subunits, the dimer interface is asymmetric, thereby providing a structural basis for the preferred heterodimeric assembly. Structure-based sequence comparisons of other CD94-NKG2 family members, combined with extensive mutagenesis studies on HLA-E and CD94-NKG2A, allows a model of the interaction between CD94-NKG2A and HLA-E to be established, in which the invariant CD94 chain plays a more dominant role in interacting with HLA-E in comparison to the variable NKG2 chain.

Immune-refractory cancers and their little helpers--an extended role for immunetolerogenic MHC molecules HLA-G and HLA-E?

There is strong evidence to support a role for non-classical MHC class I (class Ib) molecules, most notably HLA-E and HLA-G in tumour immune escape. In this article, we summarize the current knowledge on their expression, regulation and functional relevance in various malignancies, particularly brain tumours. Special emphasis is devoted to the phenomenon that these tolerogenic molecules are expressed by non-transformed cells that are found in close neighborhood to tumour cells representing either parenchymal cells or immune cells attracted to the tumour microenvironment. Here they may act as "natural" or "inducible" suppressors of anti-tumoural immune responses. We thus speculate about the role of HLA-G expressing T cells, a novel population of natural regulatory cells that was identified recently. It is suggested that various cell types within a tumour cooperate in order to inhibit anti-tumour immunity-and that immunetolerogenic HLA-G may play a major role in this context.

Common themes in the assembly and architecture of activating immune receptors

Each of the many different cell types of the immune system expresses one or several activating receptors which serve a central role in the cell's surveillance function. Many of these cell-surface receptors share a distinctive modular design that consists of a ligand-binding module with no intrinsic signalling capability that is non-covalently associated with one or more dimeric signalling modules. Receptor assembly is directed by unique polar contacts within the transmembrane domains, whereas extracellular contacts can contribute to stability and specificity. This Review discusses the structural basis of receptor assembly and the implications of these findings for the mechanisms of receptor triggering.

Qa-1(b)-dependent modulation of dendritic cell and NK cell cross-talk in vivo

Dendritic cells (DC) trigger activation and IFN-gamma release by NK cells in lymphoid tissues, a process important for the polarization of Th1 responses. Little is known about the molecular signals that regulate DC-induced NK cell IFN-gamma synthesis. In this study, we analyzed whether the interaction between Qa-1(b) expressed on DC and its CD94/NKG2A receptor on NK cells affects this process. Activation of DC using CpG-oligodeoxynucleotides in Qa-1(b)-deficient mice, or transfer of CpG-oligodeoxynucleotide-activated Qa-1(b)-deficient DC into wild-type mice, resulted in dramatically increased IFN-gamma production by NK cells, as compared with that induced by Qa-1(b)-expressing DC. Masking the CD94/NKG2A inhibitory receptor on NK cells in wild-type mice similarly enhanced the IFN-gamma response of these cells to Qa-1(b)-expressing DC. Furthermore, NK cells from CD94/NKG2A-deficient mice displayed higher IFN-gamma production upon DC stimulation. These results demonstrate that Qa-1(b) is critically involved in regulating IFN-gamma synthesis by NK cells in vivo through its interaction with CD94/NKG2A inhibitory receptors. This receptor-ligand interaction may be essential to prevent unabated cytokine production by NK cells during an inflammatory response.

Downregulation of CD94/NKG2A inhibitory receptors on CD8+ T cells in HIV infection is more pronounced in subjects with detected viral load than in their aviraemic counterparts

The CD94/NKG2A heterodimer is a natural killer receptor (NKR), which inhibits cell-mediated cytotoxicity upon interaction with MHC class I gene products. It is expressed by NK cells and by a small fraction of activated CD8+ T lymphocytes. Abnormal upregulation of the CD94/NKG2A inhibitory NKR on cytotoxic T cells (CTLs) could be responsible for a failure of immunosurveillance in cancer or HIV infection. In this study, CD94/NKG2A receptor expression on CD8+ T lymphocytes and NK cells was assessed in 46 HIV-1-infected patients (24 viraemic, 22 aviraemic) and 10 healthy volunteers. The percentage of CD8+ T lymphocytes expressing the CD94/NKG2A inhibitory heterodimer was very significantly decreased in HIV-1-infected patients in comparison with non-infected controls. Within the HIV infected patients, the proportion of CD8+ T lymphocytes and NK cells expressing CD94/NKG2A was higher in subjects with undetectable viral loads in comparison with their viraemic counterparts. No significant difference was detected in the proportion of CD8+ T lymphocytes expressing the activatory CD94/NKG2C heterodimer between the HIV-1 infected patients and the healthy donors, nor between the vireamic and avireamic HIV-1 infected patients. In conclusion, chronic stimulation with HIV antigens in viraemic patients leads to a decreased rather than increased CD94/NKG2A expression on CD8+ T lymphocytes and NK cells.

Activating and inhibitory receptors on synovial fluid natural killer cells of arthritis patients: role of CD94/NKG2A in control of cytokine secretion

Natural killer (NK) cells are activated early during inflammatory events and contribute to the shaping of the ensuing adaptive immune response. To further understand the role for NK cells in inflammation, we investigated the phenotype and function of synovial fluid (SF) NK cells from patients with chronic joint inflammation, as well as from patients with transient inflammation of the knee following trauma. We confirm that synovial NK cells are similar to the well-characterized CD56(bright) peripheral blood (PB) NK-cell subset present in healthy individuals. However, compared to this PB subset the synovial NK cells express a higher degree of activation markers including CD69 and NKp44, the latter being up-regulated also on CD56(bright) NK cells in the PB of patients. Activated synovial NK cells produced interferon-gamma and tumour necrosis factor, and the production was further up-regulated by antibody masking of CD94/NKG2A, and down-regulated by target cells expressing human leucocyte antigen-E in complex with peptides known to engage CD94/NKG2A. We conclude that synovial NK cells have an activated phenotype and that CD94/NKG2A is a key regulator of synovial NK-cell cytokine synthesis.

Association of CD94/NKG2A, CD94/NKG2C, and its ligand HLA-E polymorphisms with Behcet's disease

Inhibitory CD94/NKG2A and activating CD94/NKG2C receptors are expressed on natural killer, CD4, and CD8 T cells and recognize human leukocyte antigen (HLA)-E, resulting in the modulation of cytotoxic activity and cytokine production. An imbalance in cytotoxic activity and cytokine production has been implicated in Behcet's disease (BD). The results of this study showed that the NKG2A c.-4258*C, c.338-90*G, and CD94 c.-134*T alleles (P= 0.015, OR = 0.8; P < 0.0001, OR = 0.5; and P= 0.034, OR = 0.8, respectively) were associated with decreased risk and that NKG2A c.284-67_-62del, c.1077*C, and the activating receptor, NKG2C c.305*T were not associated with 345 patients with BD. But a significant difference in NKG2C c.305*T was detected among BD patients with ocular lesions and arthritis (P < 0.0001, OR = 2.1 and P= 0.0001, OR = 1.8, respectively). We already showed in our previous research that HLA-E*0101 also appears to contribute to a reduction in risk through the inhibitory CD94/NKG2A-mediated immune response. This result led us to the analyses of the combined risk of the HLA-E and the NKG2A for BD. Individuals harboring HLA-E*0101, NKG2A c.-4258*C, and c.338-90*G evidenced a reduced risk of BD compared with healthy controls (21.1% vs 40.1%, P < 0.0001, OR = 0.4). By way of contrast, individuals without the HLA-E*0101, NKG2A c.-4258*C, and c.338-90*G alleles evidenced a twofold increased risk of BD (P= 0.014, OR = 2.0). Individuals without HLA-E*0101, NKG2A c.-4258*G/*G, and c.338-90*G evidenced a 4.8-fold increase in BD risk (P= 0.0002, OR = 4.8). Although the effects of these single nucleotide polymorphisms (SNPs) remain unclear, our results indicate that the SNPs of the inhibitory receptor CD94/NKG2A and its haplotypes, as well as its ligand HLA-E, are associated with BD immune systems.

Variation in the ligand binding domains of the CD94/NKG2 family of receptors in the squirrel monkey

Natural killer cells are regulated, in part, by cell surface expression of the inhibitory CD94/NKG2A heterodimer and the activating CD94/NKG2C heterodimer. In the present study, we characterize the CD94/NKG2 family in the squirrel monkey, a New World monkey species. Full-length CD94, NKG2A, and NKG2CE complementary deoxyribonucleic acid molecules were identified in three unrelated squirrel monkeys. Three alternatively spliced forms of CD94 were detected in which part of intron 4 was included in the mature transcript, suggesting evolutionary pressure for changes in the corresponding loop 3 region of the lectin domain in squirrel monkeys. Squirrel monkey NKG2A contains a three-nucleotide indel that results in an additional amino acid in the predicted NKG2A protein compared to NKG2A in other species. This NKG2A insertion tracks to loop five of the lectin domain, as is seen with the recently described marmoset NKG2CE indel. Transmembrane-deleted forms of CD94 and NKG2CE were also expressed in the squirrel monkey. Analysis of full-length squirrel monkey and additional primate CD94/NKG2 sequences demonstrated statistically significant increases in the Ka/Ks ratio in the putative major histocompatibility complex E (MHC-E) binding domain compared to the non-binding domain. Furthermore, positive selection was detected in the MHC-E binding domain of primate NKG2 family members, and purifying selection was detected in the primate CD94 binding domain. Purifying selection was also detected in the nonbinding domains of primate CD94 and NKG2 molecules.

Phenotypic and functional changes of cytotoxic CD56pos natural T cells determine outcome of acute hepatitis C virus infection

Innate CD56(pos) natural killer (NK) and natural T (NT) cells comprise important hepatic antiviral effector lymphocytes whose activity is fine-tuned through surface NK receptors (NKRs). Dysregulation of NKRs in patients with long-standing hepatitis C virus (HCV) infection has been shown, but little is known regarding NKRs in acute infection. Treatment-naïve patients with acute HCV (n = 22), including 10 with spontaneous recovery, were prospectively studied. CD56(pos) NT levels were reduced early in acute HCV infection and did not fluctuate over time. In resolving HCV infection, NT cells with a more activated phenotype (lower CD158A and higher natural cytotoxicity receptor expression) at baseline predated spontaneous recovery. Moreover, NKG2A expression on CD56(+) NT cells correlated directly with circulating HCV RNA levels. Deficient interleukin-13 (IL-13) production by NT cells and reduced IL-2-activated killing (LAK) at baseline were associated with the ultimate development of persistence. These results indicate a previously unappreciated role for NT cells in acute HCV infection and identify a potential target for pharmacologic manipulation.

Complexity in the cattle CD94/NKG2 gene families

Natural killer cell responses are controlled to a large extent by the interaction of an array of inhibitory and activating receptors with their ligands. The mostly nonpolymorphic CD94/NKG2 receptors in both humans and mice were shown to recognize a single nonclassical MHC class I molecule in each case. In this paper, we describe the CD94/NKG2 gene family in cattle. NKG2 and CD94 sequences were amplified from cDNA derived from four animals. Four CD94 sequences, ten NKG2A, and three NKG2C sequences were identified in total. In contrast to human, we show that cattle have multiple distinct NKG2A genes, some of which show minor allelic variation. All of the sequences designated NKG2A have two tyrosine-based inhibitory motifs in the cytoplasmic domain and one putative gene has, in addition, a charged residue in the transmembrane domain. NKG2C appears to be essentially monomorphic in cattle. All of the NKG2A sequences are similar apart from NKG2A-01, which, in contrast, shares the majority of its carbohydrate recognition domain with NKG2-C. Most of the genes appear to generate multiple alternatively spliced forms. These findings suggest that the CD94/NKG2A heterodimers in cattle, in contrast to other species, are binding several different ligands. Because NKG2C is not polymorphic, this raises questions as to the combined functional capacity of the CD94/NKG2 gene families in cattle.

Non-T Cell Activation Linker (NTAL) Negatively Regulates TREM-1/DAP12-Induced Inflammatory Cytokine Production in Myeloid Cells

The engagement of triggering receptor expressed on myeloid cells 1 (TREM-1) on macrophages and neutrophils leads to TNF-alpha and IL-8 production and enhances inflammatory responses to microbial products. For signal transduction, TREM-1 couples to the ITAM-containing adapter DNAX activation protein of 12 kDa (DAP12). In general, ITAM-mediated signals lead to cell activation, although DAP12 was recently implicated in inhibitory signaling in mouse macrophages and dendritic cells. To date, signals downstream of the TREM-1 and DAP12 complex in myeloid cells are poorly defined. By analyzing receptor-induced tyrosine phosphorylation patterns, we discovered that the ligation of TREM-1 leads to tyrosine phosphorylation of the non-T cell activation linker (NTAL; also called linker of activation in B cells or LAB) in a myelomonocytic cell line and primary human granulocytes. Using RNA interference to decrease the expression levels of NTAL, we demonstrate that in NTAL knockdown cell lines the phosphorylation of ERK1/2 is enhanced. In addition, low levels of NTAL are correlated with decreased and delayed mobilization of Ca(2+) after TREM-1 triggering. Most importantly, we demonstrate that NTAL acts as a negative regulator of TNF-alpha and IL-8 production after stimulation via TREM-1. Our results show that activation signals delivered via DAP12 can be counterbalanced by the adaptor NTAL, identifying NTAL as gatekeeper of TREM-1/DAP12-induced signaling in myeloid cells.

Human CD8+intraepithelial lymphocytes: a unique model to study the regulation of effector cytotoxic T lymphocytes in tissue

The epithelium of the human small intestine contains a large population of intraepithelial cytolytic alphabeta T-cell receptor (TCR) CD8 alpha beta T lymphocytes (IE-CTLs), whose main role is to sustain epithelial integrity by rapidly eliminating infected and damaged cells. In mouse, the recognition of inducible/modified self-molecules, i.e. non-classical major histocompatibility complex (MHC) class I molecules, is mediated by the TCR and natural killer receptors (NKRs) co-expressed on the cell surface of a non-conventional autoreactive CD8 alpha alpha alpha beta TCR cell subset. In contrast, in humans, the recognition of non-classical MHC class I molecules induced by stress and inflammation on intestinal epithelial cells (IECs) is principally mediated by NKRs expressed on conventional CD8 alpha beta alpha beta TCR cells. By sensing microenvironmental signals of inflammation and stress through NKRs, IE-CTLs fine tune their TCR activation threshold. Furthermore, IE-CTLs under particular conditions, involving interleukin-15 upregulation, acquire the capacity to kill distressed intestinal epithelial cells in an antigen non-specific manner. Adaptive IE-CTLs appear hence to have autoreactive properties and modulate their immune response based on innate signals, reflecting the fitness of the tissue.

Unusual expression of CD94 on CD8+ TCR-alpha beta T cells in infectious mononucleosis

Infectious mononucleosis, caused by primary Epstein-Barr virus (EBV) infection, is usually a benign, self-limited lymphoproliferative disorder. We report a case of a 21-year-old woman who presented with fever, sore throat, severe neutropenia, and absolute lymphocytosis with atypical lymphocytes. In situ hybridization for EBV-encoded small RNA performed on the marrow aspirate clot specimen demonstrated scattered positive cells. EBV serology was compatible with primary infection. Flow cytometry immunophenotypic studies performed on aspirate material revealed a profoundly expanded population of CD8+ T-cell receptor (TCR)-alphabeta T cells with uniform expression of CD94. No evidence of a monoclonal T-cell population was found as assessed by V(beta) use with flow cytometry and by TCR gamma-chain gene rearrangement using a polymerase chain reaction method. Uniform expression of CD94 in an exuberant reactive proliferation of CD8+ TCR-alphabeta T cells in infectious mononucleosis has not been reported previously, and combined with atypical morphology might be misinterpreted as a malignant neoplasm.

Dissecting natural killer cell activation pathways through analysis of genetic mutations in human and mouse

Natural killer (NK) cell cytotoxicity is mediated by multiple germ line-encoded activating receptors that recognize specific ligands expressed by tumor cells and virally infected cells. These activating receptors are opposed by NK inhibitory receptors, which recognize major histocompatibility complex class I molecules on potential targets, raising the threshold for NK cell activation. Once an abnormal cell has been detected, NK cells are the sentinel source of cytolytic mediators, such as granzymes and perforins, as well as interferon-gamma, which can polarize the immune response to a T-helper 1 cell type. Activation signals are transmitted by adhesion-dependent pathways, immunoreceptor tyrosine-based activation motif (ITAM)-dependent pathways, DAP10 ITAM-independent pathways, and by signaling through immunoreceptor tyrosine-based switch motifs. These pathways activate downstream signaling partners to trigger NK cell cytotoxicity. Some of these downstream molecules are unique to the various pathways, and some of these molecules are shared. Because of the complexity of signals involved in NK cell-target cell interaction, the generation of mice with targeted mutations in signaling molecules involved in adhesion, activation, or inhibition is essential for a precise dissection of the mechanisms regulating NK cell effector functions. Here we review recent advances in the genetic analysis of the signaling pathways that mediate NK cell killing.

Activating and inhibitory functions of DAP12

When associated with different receptors, the signalling adaptor DAP12 has been shown to both potentiate and attenuate the activation of leukocytes. But how can a protein with a single signalling motif elicit qualitatively different cellular responses? We describe a model of DAP12 function, whereby the quality of the cellular response (activation or inhibition) is modulated by the avidity of the interaction between the DAP12-associated receptor and its ligand. This model extends from previous studies of inhibitory signalling mediated by other adaptors, such as the Fc-receptor gamma-chain and CD3zeta, and provides a potential mechanism for the conflicting phenotypes observed in studies of DAP12-deficient mice.

The CD94/NKG2 family of receptors: from molecules and cells to clinical relevance

Immune responses must be tightly regulated to avoid hyporesponsiveness on one hand or excessive inflammation and the development of autoimmunity (hyperresponsiveness) on the other hand. This balance is attained through the throttling of activating signals by inhibitory signals that ideally leads to an adequate immune response against an invader without excessive and extended inflammatory signals that promote the development of autoimmunity. The CD94/NKG2 family of receptors is composed of members with activating or inhibitory potential. These receptors are expressed predominantly on NK cells and a subset of CD8+ T cells, and they have been shown to play an important role in regulating responses against infected and tumorigenic cells. In this review, we discuss the current knowledge about this family of receptors, including ligand and receptor interaction, signaling, membrane dynamics, regulation of gene expression and their roles in disease regulation, infections, and cancer, and bone marrow transplantation.

Molecular and functional characterization of CD300b, a new activating immunoglobulin receptor able to transduce signals through two different pathways

In this study, we describe the characterization of human CD300b, a novel member of the CMRF-35/immune receptor expressed by myeloid cell (IREM) multigene family of immune receptors. Immune receptor expressed by myeloid cell-3 cDNA was cloned from a PHA-activated PBMC library and RT-PCR revealed the gene to be expressed preferentially in cells of myeloid origin. The CD300b cDNA open reading frame encodes a 201-aa type I protein composed of a single extracellular Ig V-type domain followed by a transmembrane region containing a positively charged residue (lysine) which is a common feature among receptors that associate with activating adaptor proteins. Indeed, CD300b was able to associate with DNAX-activating protein of 12 kDa (DAP-12) and deliver different activating signals through this ITAM-based adaptor. Unusually for an activating receptor, the 29-aa cytoplasmic tail of CD300b contains a tyrosine-based motif that, upon c-Fyn phosphorylation, became a docking site for the intracellular signaling mediator growth factor receptor-bound protein 2. Moreover, in the absence of DAP-12, CD300b was able to activate NFAT/AP-1-dependent transcriptional activity in RBL-2H3 cells. This activity could be abolished only by mutating both the cytoplasmic tyrosine and the transmembrane lysine. Our data suggest the existence of an unidentified molecule capable of interacting with CD300b through a charged residue of the transmembrane region and allowing receptor signaling independent of DAP-12. Therefore, CD300b defines a nonclassical Ig receptor able to trigger signals by coupling distinct mediators and thus initiating different signaling pathways.

NK cell-based immunotherapies against tumors

Natural killer (NK) cells provide the first line of defence against pathogens and tumors. Their activation status is regulated by pro-inflammatory cytokines and by ligands that either target inhibitory or activating cell surface receptors belonging to the immunoglobulin-like, C-type lectin or natural cytotoxicity receptor families. Apart from non-classical HLA-E, membrane-bound heat shock protein 70 (Hsp70) has been identified as a tumor-specific recognition structure for NK cells expressing high amounts of the C-type lectin receptor CD94, acting as one component of an activating heterodimeric receptor complex. Full-length Hsp70 protein (Hsp70) or the 14-mer Hsp70 peptide T-K-D-N-N-L-L-G-R-F-E-L-S-G (TKD) in combination with pro-inflammatory cytokines enhances the cytolytic activity of NK cells towards Hsp70 membrane-positive tumors. Based on these findings cytokine/TKD-activated NK cells were adoptively transferred in tumor patients. These findings were compared to results of clinical trials using cytokine-activated NK cells.

Intracellular and extracellular functions of heat shock proteins: repercussions in cancer therapy

Stress or heat shock proteins (HSPs) are the most conserved proteins present in both prokaryotes and eukaryotes. Their expression is induced in response to a wide variety of physiological and environmental insults. These proteins play an essential role as molecular chaperones by assisting the correct folding of nascent and stress-accumulated misfolded proteins, and preventing their aggregation. HSPs have a dual function depending on their intracellular or extracellular location. Intracellular HSPs have a protective function. They allow the cells to survive lethal conditions. Various mechanisms have been proposed to account for the cytoprotective functions of HSPs. Several HSPs have also been demonstrated to directly interact with various components of the tightly regulated programmed cell death machinery, upstream and downstream of the mitochondrial events. On the other hand, extracellular located or membrane-bound HSPs mediate immunological functions. They can elicit an immune response modulated either by the adaptive or innate immune system. This review will focus on HSP27, HSP70, and HSP90. We will discuss the dual role of these HSPs, protective vs. immunogenic properties, making a special emphasis in their utility as targets in cancer therapy.

Cutting edge: inhibition of TLR and FcR responses in macrophages by triggering receptor expressed on myeloid cells (TREM)-2 and DAP12

DAP12 is an ITAM-containing adapter that associates with receptors in myeloid and NK cells. DAP12-associated receptors can give activation signals leading to cytokine production; however, in some situations, DAP12 inhibits cytokine production stimulated through TLRs and FcRs. Here we show that Triggering Receptor Expressed on Myeloid cells (TREM)-2 is responsible for the DAP12-mediated inhibition in mouse macrophages. A chimeric receptor composed of the extracellular domain of TREM-2 and the cytoplasmic domain of DAP12 inhibited the TLR- and FcR-induced TNF production of DAP12-deficient macrophages, whereas a TREM-1 chimera did not. In wild-type macrophages, TREM-2 knockdown increased TLR-induced TNF production. A TREM-2 Fc fusion protein bound to macrophages, indicating that macrophages express a TREM-2 ligand. Thus, the interaction of TREM-2 and its ligand results in an inhibitory signal that can reduce the inflammatory response.

Reprogramming of CTLs into natural killer-like cells in celiac disease

Celiac disease is an intestinal inflammatory disorder induced by dietary gluten in genetically susceptible individuals. The mechanisms underlying the massive expansion of interferon γ–producing intraepithelial cytotoxic T lymphocytes (CTLs) and the destruction of the epithelial cells lining the small intestine of celiac patients have remained elusive. We report massive oligoclonal expansions of intraepithelial CTLs that exhibit a profound genetic reprogramming of natural killer (NK) functions. These CTLs aberrantly expressed cytolytic NK lineage receptors, such as NKG2C, NKp44, and NKp46, which associate with adaptor molecules bearing immunoreceptor tyrosine-based activation motifs and induce ZAP-70 phosphorylation, cytokine secretion, and proliferation independently of T cell receptor signaling. This NK transformation of CTLs may underlie both the self-perpetuating, gluten-independent tissue damage and the uncontrolled CTL expansion leading to malignant lymphomas in severe forms of celiac disease. Because similar changes were detected in a subset of CTLs from cytomegalovirus-seropositive patients, we suggest that a stepwise transformation of CTLs into NK-like cells may underlie immunopathology in various chronic infectious and inflammatory diseases.

Interactions between natural killer cells, cortisol and prolactin in malaria during pregnancy

Natural killer cells derived from pluripotent hematopoietic stem cells are important cells of the immune system that have two main functions: a cytolytic activity and a cytokine-producing capacity. These functions are tightly regulated by numerous activating and inhibitory receptors, including newly discovered receptors that selectively trigger the cytolytic activity in a major histocompatibility complex independent manner. Based on their defining function of spontaneous cytotoxicity without prior immunization, natural killer (NK) cells have been thought to play a critical role in immune surveillance and cancer therapy. New insights into NK cell biology have suggested their major roles in the control of infections, particularly in Plasmodium falciparum infection and in fetal implantation. P. falciparum is the main protozoan parasite responsible for malaria causing 200-300 million clinical cases and killing over 3 million people each year. This review provides an update on NK cell function, ontogeny and biology in order to better understand the role of NK cells in pregnancy in regions where malaria is endemic. Understanding mechanisms of NK cell functions may lead to novel therapeutic strategies for the treatment of human disease, in general, and particularly in the fight against malaria.

Expression patterns of lectin-like natural killer receptors, inhibitory CD94/NKG2A, and activating CD94/NKG2C on decidual CD56bright natural killer cells differ from those on peripheral CD56dim natural killer cells

The balance of inhibitory and activating natural killer (NK) receptors on maternal decidual NK cells, most of which are CD56bright, is thought to be crucial for the proper growth of trophoblasts in placenta. A lectin-like NK receptor, CD94/NKG2, is the receptor for human leukocyte antigen (HLA)-E, which is expressed on trophoblasts. To clarify the mechanism regulating the activity of decidual NK cells during pregnancy, we investigated the expression patterns of inhibitory NK receptor, CD94/NKG2A, and activating receptor, CD94/NKG2C, on decidual NK cells in an early stage of normal pregnancy and compared them with those on peripheral NK cells, most of which are CD56dim. The rate of NKG2A-positive cells was significantly higher for decidual CD56bright NK cells than for peripheral CD56dim NK cells, but the rates of NKG2C-positive cells were comparable between the two cell types. Interestingly, peripheral CD56dim NK cells reciprocally expressed inhibitory NKG2A and activating NKG2C, but decidual CD56bright NK cells that expressed activating NKG2C simultaneously expressed inhibitory NKG2A. The co-expression of inhibitory and activating NKG2 receptors may fine-tune the immunoregulatory functions of the decidual NK cells to control the trophoblast invasion in constructing placenta.

Heat shock proteins in immunity

This chapter focuses on immunological effects of eukaryotic and microbial heat shock proteins (HSPs), with molecular weights of about 60, 70, and 90 kDa. The search for tumor-specific antigens resulted in the identification of HSPs. They have been found to elicit a potent anti-cancer immune response mediated by the adoptive and innate immune system. Following receptor-mediated uptake of HSP (HSP70 and gp96) peptide complexes by antigen-presenting cells and representation of HSP-chaperoned peptides by MHC class I molecules, a CD8-specific T cell response is induced. Apart from chaperoning immunogenic peptides derived from tumors, bacterial and virally infected cells, they by themselves provide activatory signals for antigen-presenting cells and natural killer (NK) cells. After binding of peptide-free HSP70 to Toll-like receptors, the secretion of pro-inflammatory cytokines is initiated by antigen-presenting cells and thus results in a nonspecific stimulation of the immune system. Moreover, soluble as well as cell membrane-bound HSP70 on tumor cells can directly activate the cytolytic and migratory capacity of NK cells. Apart form cancer, HSPs of different origins, with a molecular weight of about 60, 70, and 90 kDa, also play a pivotal role in viral infections, including human and simian immunodeficiency virus (HIV, SIV), measles, and choriomeningitis. Moreover, HSPs have been found to induce tolerance against autoimmune diseases. In summary, depending on their mode of induction, intracellular/extracellular location, cellular origin (eukaryote/prokaryote), peptide loading status, intracellular ADP/ATP content, concentration, and route of application, HSPs either exert immune activation as danger signals in cancer immunity and mediate protection against infectious diseases or exhibit regulatory activities in controlling and preventing autoimmunity.

NK cell recognition of mouse cytomegalovirus-infected cells

Abstract Natural killer (NK) cells and cytomegalovirus have been locked in an evolutionary arms race for millions of years in an attempt to overwhelm each other. Cytomegaloviruses deploy cunning disguises to avoid detection by NK cells. Studies of the mouse model of infection have shown that NK cells deploy multiple mechanisms to deal with mouse cytomegalovirus (MCMV) infection, which involve receptors of the C-lectin type superfamily. Remarkably, these receptors have two additional common features: They map to the same genetic region, known as the NK cell gene complex; and they recognize MHC class I-related structures. While reviewing these attack-counterattack measures, this chapter points to the central role that recognition of the MCMV-infected cells by NK cells plays in host resistance to infection.

NKG2 receptor-mediated regulation of effector CTL functions in the human tissue microenvironment

NKG2 receptors and their ligands play an essential role in the control of CTL activation in the tissue microenvironment. We discuss the regulation of NKG2 receptor expression by CTL and how uncontrolled activation of NKG2 receptors can lead to organ-specific autoimmune and inflammatory disorders.

Silencing Human NKG2D, DAP10, and DAP12 Reduces Cytotoxicity of Activated CD8+T Cells and NK Cells

Human CD8+ T cells activated and expanded by TCR cross-linking and high-dose IL-2 acquire potent cytolytic ability against tumors and are a promising approach for immunotherapy of malignant diseases. We have recently reported that in vitro killing by these activated cells, which share phenotypic and functional characteristics with NK cells, is mediated principally by NKG2D. NKG2D is a surface receptor that is expressed by all NK cells and transmits an activating signal via the DAP10 adaptor molecule. Using stable RNA interference induced by lentiviral transduction, we show that NKG2D is required for cytolysis of tumor cells, including autologous tumor cells from patients with ovarian cancer. We also demonstrated that NKG2D is required for in vivo antitumor activity. Furthermore, both activated and expanded CD8+ T cells and NK cells use DAP10. In addition, direct killing was partially dependent on the DAP12 signaling pathway. This requirement by activated and expanded CD8+ T cells for DAP12, and hence stimulus from a putative DAP12-partnered activating surface receptor, persisted when assayed by anti-NKG2D Ab-mediated redirected cytolysis. These studies demonstrated the importance of NKG2D, DAP10, and DAP12 in human effector cell function.

Characterization of CD200-Receptor Expression in the Murine Epidermis

CD 200 is a widely expressed transmembrane glycoprotein that transmits an inhibitory signal after ligation of the structurally homologous CD 200-receptor-1 (CD 200 R1). Recently, we showed that CD 200 is expressed on keratinocytes and plays a role in protecting hair follicles from autoimmune attack. Here, we report the characterization of cell surface and mRNA expression of CD 200 R1 by cells of the murine epidermis. In addition, we report mRNA expression for other members of the CD 200 R-family (R2-R4) by quantitative real-time RT-PCR. Variable levels of CD 200 R1, R2, R3, and R4 mRNA were detected in bulk epidermal cell suspensions. Freshly isolated Langerhans cells (LC) preferentially expressed CD 200 R1. Consistent with an inhibitory role for CD 200:CD 200 R1 interaction, LC obtained from mice deficient in CD 200 (CD 200(-/-)) were in a heightened state of activation as compared with wild-type (CD 200(+/+)) cells. Freshly isolated dendritic epidermal T cells (DETC) expressed low levels of CD 200 R1, R2, and R3 mRNA, but they preferentially increased cell surface and mRNA expression of CD 200 R1 upon activation in vitro. In functional assays using sub-optimal CD3 signaling, immobilized CD 200 inhibited DETC proliferation and cytokine secretion. Collectively, these results suggest that CD 200:CD 200 R interactions may play a role in regulating both LC and DETC in cutaneous immune reactions.

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.

Switch from inhibitory to activating NKG2 receptor expression in HIV-1 infection: lack of reversion with highly active antiretroviral therapy

BACKGROUND

HIV-1 infection is characterized by increase in inhibitory receptors and loss of activating receptors on natural killer (NK) cells, resulting in loss of cell activity. Exceptionally, for an inhibitory receptor, the proportion of NK cells bearing CD94-NKG2A decreases during HIV-1 infection. It is not understood whether HIV-1 itself or other concomitant infections drive these changes.

OBJECTIVES

To investigate the relationship between HIV-1 viraemia and changes in C-type lectin-like receptor expression in NK cells and to investigate the effect of highly active antiretroviral therapy (HAART) on these changes.

METHODS

Three cohorts of patients were studied: (1) before, during and after treatment interruption in aviraemic and viraemic patients receiving HAART (n = 15); (2) HIV-1-positive treatment-naive individuals (n = 13); and (3) HIV-1-positive individuals receiving successful HAART for a minimum of 1 year without interruption (n = 11). Flow cytometry was used to study the expression of NKG2A before and after treatment interruption and to define expanded populations of NK cells in untreated and treated HIV-1-positive individuals. Assays were performed in vitro to assess the cytotoxicity of the expanded populations.

RESULTS

Increases in plasma HIV-1 RNA during treatment interruption in aviraemic HAART-treated individuals did not influence the proportion of NK cells carrying the complex CD94-NKG2A. Loss of NKG2A NK cells corresponded to the dramatic expansion of a distinct population of cells expressing a functional activating CD94-NKG2C receptor with skewed expression of killer cell immunoglobulin-like receptor family and natural cytotoxicity receptors.

CONCLUSION

Changes in the NK cell repertoire during HIV-1 infection were not a result of HIV-1 viraemia alone but resembled those associated with concomitant infections.

Enhanced tryptophan catabolism in the absence of the molecular adapter DAP12

DAP12 is an immunoreceptor tyrosine-based activation motif-bearing membrane adapter molecule expressed by different cell types. Although several receptors associate with DAP12 in murine dendritic cells (DC), the function of these receptors is as yet unknown. Here we report that splenic mature DC with DAP12 overexpression are characterized by an impaired tolerogenic potential. In contrast, inhibition of DAP12 function results in enhanced tolerogenesis and constitutive expression of immunosuppressive tryptophan catabolism mediated by indoleamine 2,3-dioxygenase (IDO). Increased resistance to experimental encephalomyelitis is observed in DAP12 knockin mice, which is dependent on IDO expression. Therefore, DAP12-related receptors act as negative regulators of IDO-mediated tolerance in vivo.

Multiple cytokines regulate the NK gene complex-encoded receptor repertoire of mature NK cells and T cells

Mature NK cells comprise a highly diverse population of lymphocytes that express different permutations of receptors to facilitate recognition of diseased cells and perhaps pathogens themselves. Many of these receptors, such as those belonging to the NKRP1, NKG2, and Ly49 families are encoded in the NK gene complex (NKC). It is generally thought that these NKC-encoded receptors are acquired by a poorly understood stochastic mechanism, which operates exclusively during NK cell development, and that following maturation the repertoire is fixed. However, we report a series of observations that demonstrates that the mature NK cell repertoire in mice can in fact be radically remodeled by multiple cytokines. Thus, both IL-2 and IL-15 selectively induce the de novo expression of Ly49E on the majority of mature NK cells. By contrast, IL-4 not only blocks this IL-2-induced acquisition of Ly49E, but reduces the proportion of mature NK cells that expresses pre-existing Ly49 receptors and abrogates the expression of NKG2 receptors while leaving the expression of several NKRP1 receptors unaltered. IL-21 also abrogates NKG2 expression on mature NK cells and selectively down-regulates Ly49F. IL-4 and IL-21 additionally cause dramatic and selective alterations in the NKC-encoded receptor repertoire of IL-2-activated T cells but these are quite different to the changes induced on NK cells. Collectively these findings reveal an unexpected aspect of NKC receptor expression that has important implications for our understanding of the function of these receptors and of the genetic mechanisms that control their expression.

Prostaglandin E2 induces the expression of functional inhibitory CD94/NKG2A receptors in human CD8+ T lymphocytes by a cAMP-dependent protein kinase A type I pathway

The CD94/NKG2A heterodimer is a natural killer receptor (NKR), which inhibits cell-mediated cytotoxicity upon interaction with MHC class I gene products. It is expressed by NK cells and by a small fraction of activated T cells, predominantly of CD8+ phenotype. Abnormal upregulation of the CD94/NKG2A inhibitory NKR on cytotoxic T cells (CTLs) could be responsible for a failure of immunosurveillance in cancer or HIV infection. In an attempt to identify the mechanisms leading to inhibitory NKR upregulation on T cells, we analyzed the expression of the CD94/NKG2A heterodimer on human CTLs activated with anti-CD3 mAb in the presence of PGE2 or with 8-CPT-cAMP, an analogue of cyclic AMP. As previously described, anti-CD3 mAb-mediated activation induced the expression of CD94/NKG2A on a small fraction of CD8+ T cells. Interestingly, when low concentrations of PGE2 or 8-CPT-cAMP were present during the culture, the proportion of CD8+ T cells expressing CD94/NKG2A was two- to five-fold higher. This upregulation was partially prevented by PKA inhibitors, such as KT5720 and Rp-8-Br-cAMP (type I selective). We also report that cAMP induces upregulation of NKG2A at the mRNA level. We further demonstrated that cross-linking of CD94 on CD8+ T cells expressing the CD94/NKG2A heterodimer inhibits their cytotoxic activity in a bispecific antibody redirected lysis assay. Our findings clearly demonstrate that the PGE2/cAMP/PKA type I axis is involved in the expression of CD94/NKG2A receptor on human CD8+ T lymphocytes.

SIRPbeta1 is expressed as a disulfide-linked homodimer in leukocytes and positively regulates neutrophil transepithelial migration

Signal regulatory proteins (SIRPs) comprise a family of cell surface signaling receptors differentially expressed in leukocytes and the central nervous system. Although the extracellular domains of SIRPs are highly similar, classical motifs in the cytoplasmic or transmembrane domains distinguish them as either activating (beta) or inhibitory (alpha) isoforms. We reported previously that human neutrophils (polymorphonuclear leukocytes (PMN)) express multiple SIRP isoforms and that SIRPalpha binding to its ligand CD47 regulates PMN transmigration. Here we further characterized the expression of PMN SIRPs, and we reported that the major SIRPalpha and SIRPbeta isoforms expressed in PMN include Bit/PTPNS-1 and SIRPbeta1, respectively. Furthermore, although SIRPalpha (Bit/PTPNS-1) is expressed as a monomer, we showed that SIRPbeta1 is expressed on the cell surface as a disulfide-linked homodimer with bond formation mediated by Cys-320 in the membrane-proximal Ig loop. Subcellular fractionation studies revealed a major pool of SIRPbeta1 within the plasma membrane fractions of PMN. In contrast, the majority of SIRPalpha (Bit/PTPNS-1) is present in fractions enriched in secondary granules and is translocated to the cell surface after chemoattractant (formylmethionylleucylphenylalanine) stimulation. Functional studies revealed that antibody-mediated ligation of SIRPbeta1 enhanced formylmethionylleucylphenylalanine-driven PMN transepithelial migration. Co-immunoprecipitation experiments to identify associated adaptor proteins revealed a 10-12-kDa protein associated with SIRPbeta1 that was tyrosine-phosphorylated after PMN stimulation and is not DAP10/12 or Fc receptor gamma chain. These results provide new insights into the structure and function of SIRPs in leukocytes and their potential role(s) in fine-tuning responses to inflammatory stimuli.

Recombinant CD200 Protein Does Not Bind Activating Proteins Closely Related to CD200 Receptor

CD200 (OX2) is a cell surface glycoprotein that interacts with a structurally related receptor (CD200R) expressed mainly on myeloid cells and is involved in regulation of macrophage and mast cell function. In mouse there are up to five genes related to CD200R with conflicting data as to whether they bind CD200. We show that mouse CD200 binds the inhibitory receptor CD200R with a comparable affinity (Kd = 4 microM) to those found for the rat and human CD200 CD200R interactions. CD200 gave negligible binding to the activating receptors, CD200RLa, CD200RLb, and CD200RLc, by direct analysis at the protein level using recombinant monomeric and dimeric fusion proteins or to CD200RLa and CD200RLb when expressed at the cell surface. An additional potential activating gene, CD200RLe, found in only some mouse strains also did not bind CD200. Thus, the CD200 receptor family consists of both activatory and inhibitory members like several other paired ligand receptors, such as signal regulatory protein, killer cell Ig-like receptor/KAR, LY49, dendritic cell immunoreceptor/dendritic cell immunoactivating receptor, and paired Ig-like type 2 receptor. Although the ligand for the inhibitory product is a widely distributed host protein, the ligands of the activating forms remain to be identified, and one possibility is that they are pathogen components.

Amelioration of acute graft-versus-host disease by NKG2A engagement on donor T cells

Acute graft-versus-host disease (aGVHD) remains a major complication of allogeneic bone marrow transplantation, which is caused by donor T cells specific for host alloantigens. In a murine model, we found that donor T cells expressed a natural killer cell inhibitory receptor, CD94/NKG2A, during the course of aGVHD. Administration of an anti-NKG2A mAb markedly inhibited the expansion of donor T cells and ameliorated the aGVHD pathologies. These results suggested that the CD94/NKG2A inhibitory receptor expressed on host-reactive donor T cells can be a novel target for the amelioration of aGVHD.

The CD94/NKG2C killer lectin-like receptor constitutes an alternative activation pathway for a subset of CD8+ T cells

The CD94/NKG2C killer lectin-like receptor (KLR) specific for HLA-E is coupled to the KARAP/DAP12 adapter in a subset of NK cells, triggering their effector functions. We have studied the distribution and function of this KLR in T lymphocytes. Like other NK cell receptors (NKR), CD94/NKG2C was predominantly expressed by a CD8(+) T cell subset, though TCRgammadelta(+) NKG2C(+) and rare CD4(+) NKG2C(+) cells were also detected in some individuals. Coculture with the 721.221 HLA class I-deficient lymphoma cell line transfected with HLA-E (.221-AEH) induced IL-2Ralpha expression in CD94/NKG2C+ NK cells and a minor subset of CD94/NKG2C(+) T cells, promoting their proliferation; moreover, a similar response was triggered upon selective engagement of CD94/NKG2C with a specific mAb. CD8(+) TCRalphabeta CD94/NKG2C(+) T cell clones, that displayed different combinations of KIR and CD85j receptors, expressed KARAP/DAP12 which was co-precipitated by an anti-CD94 mAb. Specific engagement of the KLR triggered cytotoxicity and cytokine production in CD94/NKG2C(+) T cell clones, inducing as well IL-2Ralpha expression and a proliferative response. Altogether these results support that CD94/NKG2C may constitute an alternative T cell activation pathway capable of driving the expansion and triggering the effector functions of a CTL subset.

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.

Convergence on a distinctive assembly mechanism by unrelated families of activating immune receptors

Activating receptors in cells of hematopoetic origin include members of two unrelated protein families, the immunoglobulin (Ig) and C type lectins, which differ even in the orientation of the transmembrane (TM) domains. We examined assembly of four receptors with diverse function: the NK receptors KIR2DS and NKG2C/CD94, the Fc receptor for IgA, and the GPVI collagen receptor. For each of the four different receptors studied here, assembly results in the formation of a three-helix interface in the membrane involving two acidic TM residues from the signaling dimer and a basic TM residue from the ligand recognition module, an arrangement remarkably similar to the T cell receptor (TCR)-CD3 complex. The fact that the TM domains of Ig family and C type lectins adopt opposite orientations proves that these receptor families independently evolved toward the same structural arrangement of the interacting TM helices. This assembly mechanism is thus widely utilized by receptors in cells of hematopoetic origin.

Chimeric NK-receptor-bearing T cells mediate antitumor immunotherapy

NKG2D is an activating cell-surface receptor expressed on natural killer (NK) cells and some T-cell subsets. Its ligands are primarily expressed on tumor cells. The aim of this study was to determine whether chimeric NK-receptor-bearing T cells would directly kill tumor cells and lead to induction of host immunity against tumors. Chimeric NK receptors were produced by linking NKG2D or DNAX activating protein of 10 kDa (Dap10) to the cytoplasmic portion of the CD3zeta chain. Our results showed that chimeric (ch) NKG2D-bearing T cells responded to NKG2D-ligand-bearing tumor cells (RMA/Rae-1beta, EG7) but not to wild-type tumor cells (RMA). This response was dependent upon ligand expression on the target cells but not on expression of major histocompatibility complex (MHC) molecules, and the response could be blocked by anti-NKG2D antibodies. These T cells produced large amounts of T-helper 1 (Th1) cytokines and proinflammatory chemokines and killed ligand-expressing tumor cells. Adoptive transfer of chNKG2D-bearing T cells inhibited RMA/Rae-1beta tumor growth in vivo. Moreover, mice that had remained tumor-free were resistant to subsequent challenge with the wild-type RMA tumor cells, suggesting the generation of immunity against other tumor antigens. Taken together, our findings indicate that modification of T cells with chimeric NKG2D receptors represents a promising approach for immunotherapy against cancer.

Pathways participating in activation of mouse uterine natural killer cells during pregnancy

Activated natural killer (NK) cells proliferate in large numbers in murine mesometrial endometrium from Day 6 to Day 12 of gestation (term = 19 gestation days) to become the most abundant uterine lymphocytes. Early human decidua contains analogous CD56+/CD16- cells. Murine uterine (u)NK cells localize to decidua basalis and mesometrial lymphoid aggregate of pregnancy (MLAp). Decidua and MLAp are transient, pregnancy-associated tissues traversed by maternal arteries to the placentas. Uterine NK cells sensitize these arteries, facilitating their structural changes into high-volume conduits by Gestation Day 10 through release of interleukin (IL)-18, interferon (IFN)-gamma, vascular endothelial growth factor (VEGF), and other molecules. Little information exists concerning where, when, or how murine or human uNK cells become activated. In murine lymphoid tissue, three NK cell adaptor-mediated activation pathways are known: FcRgamma/CD3zeta, DNAX-activating protein (DAP) 10, and DAP12 (genes Fcgr3/Cd3z, Hcst, and Tyrobp, respectively). Expression of ligands for these receptors was demonstrated in implantation sites of normal C57BL/6J mice. Then, histological and morphometric analyses of implantation sites in mice with genetic inactivation of each pathway were undertaken. Implantation sites in DAP10-/- (Hcst deleted) mice appeared normal, spiral artery modification occurred, and concentrations of IFN-gamma in MLAp and decidua basalis were similar to those in time-matched C57BL/6J. Implantation sites of FcRgamma-/-/CD3zeta-/- (Fcgr3/Cd3z double knockout), DAP12 (Tyrobp)-loss-of-function-mutant, and FcRgamma-/-/DAP12-/- (Fcgr3/Tyrobp double knockout) mice differentiated abundant but functionally impaired uNK cells that could not modify spiral arteries. These data reveal key importance of FcRgamma-/-/CD3zeta-/- and thus maternal IgG during activation of mouse uNK cells and assign DAP12 but not DAP10 signaling contributions.

Distribution and signaling of TREM2/DAP12, the receptor system mutated in human polycystic lipomembraneous osteodysplasia with sclerosing leukoencephalopathy dementia

Together with its adaptor protein, the adaptor protein of 12 kDa also known as KARAP and TYROBP (DAP12), triggering receptor expressed in myeloid cells 2 (TREM2) is a stimulatory membrane receptor of the immunoglobulin/lectin-like superfamily, well known in myeloid cells. In humans, however, loss-of-function mutations of TREM2/DAP12 leave myeloid cells unaffected but induce an autosomal recessive disease characterized, together with bone cysts, by a spectrum of pathological lesions in the cortex, thalamus and basal ganglia with clinical symptoms of progressive dementia (polycystic lipomembraneous osteodysplasia with sclerosing leukoencephalopathy). Nothing was known about the role of TREM2/DAP12 in brain cell biology and physiology. By confocal immunocytochemistry we demonstrate that, in both human and mouse cerebral cortex, TREM2/DAP12, strongly expressed by microglia, is also present in a fraction of neurons but not in astrocytes and oligodendrocytes. In contrast, in the hippocampal cortex TREM2-expressing neurons are rare. Both in neurons and microglia the receptor appears to be located mostly intracellularly in a discrete compartment(s) partially coinciding with (or adjacent to) the Golgi complex/trans-Golgi network. Four nerve cell lines were identified as expressing the intracellular receptor system. In living human microglia CHME-5 and glioblastoma T98G cells, activation of TREM2 by its specific antibody induced [Ca2+]i responses, documenting its surface expression and functioning. Surface expression of TREM2, low in resting CHME-5 and T98G cells, increases significantly and transiently (60 min) when cells are stimulated by ionomycin, as revealed by both surface biotinylation and surface immunolabeling. Our results provide the first information about the expression, distribution (mostly intracellular) and functioning of TREM2/DAP12 system in nerve cells, a necessary step in the understanding of the cellular mechanisms affected in polycystic lipomembraneous osteodysplasia with sclerosing leukoencephalopathy.

The genes and gene organization of the Ly49 region of the rat natural killer cell gene complex

We here report the cDNA sequences of 11 new rat Ly49 genes with full and three with incomplete open reading frames. Although obtained from different inbred rat strains, these as well as six previously published cDNA represent non-allelic genes matching different loci in the Brown Norway (BN) rat genome, which is predicted to contain 34 Ly49 loci distributed over the distal part of the NK cell gene complex. Some of the cloned genes appear to be mutated to non-function in the BN genome, which harbors additional genes with full open reading frames, suggesting at least 26 non-allelic functional Ly49 genes in the rat. Of the encoded receptors, 13 are predicted to be inhibitory, eight to be activating, whereas five may be both ('bifunctional'). Phylogenetic analysis bears evidence of a highly dynamic genetic region, in which only the most distally localized Ly49 gene has a clear-cut mouse ortholog. In phylograms, the majority of the genes cluster into three subgroups with the genes mapping together, defining three chromosomal regions that seem to have undergone recent expansions. When comparing the lectin-like domains, the receptors form smaller subgroups, most containing at least one inhibitory and one activating or 'bifunctional' receptor, where close sequence similarities suggest recent homogenization events.