Immunoreceptor DAP12 bearing a tyrosine-based activation motif is involved in activating NK cells

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.

DAP12: an adapter protein with dual functionality

Expressed predominantly on myeloid and natural killer (NK) cells, DAP12 is an adapter protein that can associate with a variety of receptors. To date, DAP12 has predominantly been characterized as an adapter protein that activates various myeloid and NK cell effector functions; however, recent findings have demonstrated that DAP12 can also inhibit myeloid functions. Here we review the dual functionality of DAP12 and present evidence that DAP12 can suppress as well as activate NK cells.

The structure of the zetazeta transmembrane dimer reveals features essential for its assembly with the T cell receptor

The T cell receptor (TCR) alphabeta heterodimer communicates ligand binding to the cell interior via noncovalently associated CD3gammaepsilon, CD3deltaepsilon, and zetazeta dimers. While structures of extracellular components of the TCR-CD3 complex are known, the transmembrane (TM) domains that mediate assembly have eluded structural characterization. Incorporation of the zetazeta signaling module is known to require one basic TCRalpha and two zetazeta aspartic acid TM residues. We report the NMR structure of the zetazeta(TM) dimer, a left-handed coiled coil with substantial polar contacts. Mutagenesis experiments demonstrate that three polar positions are critical for zetazeta dimerization and assembly with TCR. The two aspartic acids create a single structural unit at the zetazeta interface stabilized by extensive hydrogen bonding, and there is evidence for a structural water molecule (or molecules) within close proximity. This structural unit, representing only the second transmembrane dimer interface solved to date, serves as a paradigm for the assembly of all modules involved in TCR signaling.

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.

The role of the NKG2D receptor for tumor immunity

Natural killer (NK) cells have originally been identified based on their capacity to kill transformed cells in a seemingly non-specific fashion. Over the last 15 years, knowledge on receptor ligand systems used by NK cells to specifically detect transformed cells has been accumulating rapidly. One of these receptor ligand systems, the NKG2D pathway, has received particular attention, and now serves as a paradigm for how the immune system is able to gather information about the health status of autologous host cells. In addition to its significance on NK cells, NKG2D, as well as other NK cell receptors, play significant roles on T cells. This review aims at summarizing recent insights into the regulation of NKG2D function, the control over NKG2D ligand expression and the role of NKG2D in tumor immunity. Finally, we will discuss first attempts to exploit NKG2D function to improve immunity to tumors.

The dynamics of natural killer cell tolerance

Natural killer (NK) cells are important mediators of resistance against tumor growth and metastasis. NK cell reactivity is regulated by a balance of signals from activating and inhibitory receptors. While reactivity against tumor cells is beneficial, it is essential that NK cells do not attack normal tissue. The distinction between tumor cells and normal cells is partly made at the level of activating receptors: transformation often results in induction of ligands for such receptors. In addition, NK cells discriminate self from non-self using MHC class I-binding inhibitory receptors. Host MHC class I molecules regulate development of NK cell reactivity and tolerance, a process that is not well understood. Recent data suggest that functional maturation may not be a binary phenomenon: quantitative aspects, with regards to avidity and frequency in interactions between developing NK cells and normal cells, may be important for the generation of NK cells that are 'tuned' to optimally sensing the absence of self-MHC class I. In this article, we discuss models for development of NK cell reactivity and tolerance. Our understanding of this process may have significant implications for the use of NK cells in cancer therapy.

Binding of sialyl Lewis X antigen to lectin-like receptors on NK cells induces cytotoxicity and tyrosine phosphorylation of a 17-kDa protein

BACKGROUND

Natural killer (NK) cells mediate cytotoxicity through cell-surface receptors including lectin-like receptors. We have investigated whether sialyl Lewis X (sLe(X)) antigen, Neu5Acalpha2,3Galbeta1,4(Fucalpha1,3) GlcNAc-R, can bind to the lectin-like receptors on human NK-derived KHYG cells, using transferrin secreted by human hepatoma-derived HepG2 cells (Hep-TF), whose N-glycans are rich in alpha1,3-fucosylated bi-, tri-, and tetra-antennary type complexes, and commercially available human transferrin (Nor-TF), which is comprised of bi-antennary N-glycans without alpha1,3-fucosylation.

RESULTS

High sLeX-expressing erythroleukemia-derived K562 cells isolated from fucosyltransferase-3-transfected cells were 2.5-fold more susceptible than wild-type K562 cells to KHYG cells. Fluorescein isothiocyanate (FITC)-labeled Hep-TF bound 1.8-fold more strongly to KHYG cells than did FITC-labeled Nor-TF; the binding was suppressed by treatment with anti-NKG2D, anti-NKG2C, anti-CD94 and anti-CD161 antibodies. FITC-labeled Hep-TF bound more strongly to human monocyte-derived U937 cells transfected with NKG2D and CD94 than to wild-type U937 cells. Moreover, tyrosine phosphorylation of a 17-kDa protein in the KHYG cells was enhanced by incubation on a Hep-TF coated plate and treatment with an anti-NKG2D antibody, but not by a Nor-TF coated plate and an anti-CD94 antibody.

CONCLUSION

The interaction of sLe(X) antigen with lectin-like receptors on NK cells induces cytotoxicity that is mediated through a tyrosine-phosphorylated 17-kDa protein.

Nature of allelic sequence polymorphism at the KIR3DL3 locus

KIR3DL3 is a framework gene of the Leukocyte Receptor Complex, present in all individuals and haplotypes analysed to date. We describe 17 novel KIR3DL3 alleles, including seven single nucleotide polymorphic (SNP) positions within the coding region. Sequence variation within introns included a VNTR within intron 1. As KIR3DL3 mRNA is known to be expressed in decidual NK cells, we investigated the impact of KIR3DL3 allelic variation on pre-eclampsia. No statistical difference in allele frequency or polymorphism was observed between pre-eclampsia patient and control cohorts. Linkage disequilibrium (LD) analysis of exonic SNPs suggested that recombination may be a mechanism of generating sequence diversity within KIR3DL3. A potential recombination hotspot was located within intron 5. A strong LD was detected between polymorphism in exon 6 of KIR3DL3 and the KIR gene -2DL3 or -2DS2 loci, which define the centromeric end of two main haplotypes (A and B) of the KIR cluster. Comparison of primate KIR sequences indicated that the Ig domains of KIR3DL3 are highly conserved between chimpanzee, gorilla and humans. Investigation of KIR3DL3 dN/dS ratios indicated a greater level of synonymous mutations consistent with purifying selection, although positive selection was detected acting on two sites within the stem region.

The killer immunoglobulin-like receptor gene cluster: tuning the genome for defense

Killer immunoglobulin-like receptors (KIRs) are molecules expressed on the surface of natural killer (NK) cells, which play an important role in innate immunity. KIR recognition of major histocompatability complex (MHC) class I allotypes represents one component of the complex interactions between NK cells and their targets in determining NK cell reactivity. KIRs are encoded by a gene cluster at human chromosome 19q13.4. Despite their high degree of sequence identity, KIR genes encode proteins that have diverse recognition patterns (specific HLA class I allotypes) and confer opposing signals (activating or inhibitory) to the NK cell. The KIR gene cluster is highly polymorphic, with individual genes exhibiting allelic variability and individual haplotypes differing in gene content. The polymorphism of the KIR locus parallels that of the MHC, facilitating the adaptation of the immune system to a dynamic, challenging environment. This variation is associated with a growing number of human diseases, which is likely to extend to levels observed for the HLA loci. Here we review current progress in understanding KIR biology and genetics.

NKG2D and cytotoxic effector function in tumor immune surveillance

NKG2D is a type II transmembrane-anchored glycoprotein expressed as a disulfide-linked homodimer on the surface of all mouse and human natural killer cells (NK cells). Stimulation of NK cells through NKG2D triggers cell-mediated cytotoxicity and in some cases induces the production of cytokines. NKG2D binds to family of ligands with structural homology to MHC class I, however, unlike conventional MHC class I molecules, NKG2D ligands often display up-regulated surface expression on stressed cells and are frequently over expressed by tumors. Recent evidence clearly implicates that NKG2D recognition plays an important role in tumor immune surveillance and that NKG2D primarily acts to trigger perforin-mediated apoptosis. The data begin to place the NKG2D pathway into the context of other recognition-effector systems used by NK cells.

Contemplating the murine test tube: lessons from natural killer cells andCryptococcus neoformans

Murine experimentation has provided many useful tools, including the ability to knockout or over-express genes and to perform experiments that are limited by ethical considerations. Over the past century, mice have imparted valuable insights into the biology of many systems, including human immunity. However, although there are many similarities between the immune response of humans and mice, there are also many differences; none is more prominent than when examining natural killer cell biology. These differences include tissue distribution, effector molecules, receptor repertoire, and cytokine responses, all of which have important implications when extrapolating the studies to the human immune responses to Cryptococcus neoformans.

The balancing act: inhibitory Ly49 regulate NKG2D-mediated NK cell functions

NK cells use NKG2D receptor to recognize 'induced-self'. In apparent violation of the 'missing-self' hypothesis, NK cells stimulated through NKG2D can lyse target cells despite normal expression levels of MHC class I molecules. Although, 'overriding' of the inhibitory by the activating signals had been postulated the precise role of inhibitory Ly49 receptors on NKG2D-mediated activation has only started emerging. We propose that NKG2D-mediated activation is a function of 'altering the balance' in the signaling strength between the activating NKG2D and inhibiting Ly49 receptors. Balance in the signaling strength depends on the expression levels of activating ligands on the target cells. Qualitative and quantitative variations of MHC class I molecules expressed on the target cells also plays a major role in determining this 'altered-balance'. Consequently, the nature of Ly49 receptors expressed on specific NK subsets determines the level of NKG2D-mediated NK cell activation. These observations provide a firm basis of 'altered-balance' in NK signaling and describe an active interplay between inhibitory Ly49 and activating NKG2D receptors.

Regulation of Ly49D/DAP12 Signal Transduction by Src-Family Kinases and CD45

Activating, DAP12-coupled members of the Ly-49 family of NK cell receptors help control viral infections in mice. However, the kinases and/or phosphatases mediating tyrosine phosphorylation of Ly-49D-associated DAP12 have not been elucidated. In this study, we show for the first time that Src family tyrosine kinases are physically and functionally associated with Ly-49D/DAP12 signaling in murine NK cells. Specifically, we demonstrate the following: 1) inhibition of Src family kinases suppresses DAP12 phosphorylation and downstream DAP12 signals; 2) both Fyn and Lck are capable of phosphorylating DAP12; and 3) both kinases coimmunoprecipitate with the Ly-49D/DAP12 complex in NK cells. Although we detect enhanced phosphorylation of Fyn upon Ly-49D cross-linking in NK cells, Ly-49D-mediated events in both Fyn-/- and Fyn/Lck-/- mice appear normal, reinforcing the theme of redundancy in the ability of Src family kinases to initiate activation events. In contrast to disruption of specific Src family enzymes, Ly-49D/DAP12-mediated calcium mobilization and cytokine production by CD45 null NK cells are defective. Although others have ascribed the effects of CD45 mutation solely on the suppression of Src family activity, we demonstrate in this study that DAP12 is hyperphosphorylated in CD45 null NK cells, resulting in uncoordinated tyrosine-mediated signaling upon Ly-49D ligation. Therefore, although our data are consistent with a Src kinase activity proximally within DAP12 signaling, DAP12 also appears to be a substrate of CD45, suggesting a more complex role for this phosphatase than has been reported previously.

The leukocyte receptor complex in chicken is characterized by massive expansion and diversification of immunoglobulin-like Loci

The innate and adaptive immune systems of vertebrates possess complementary, but intertwined functions within immune responses. Receptors of the mammalian innate immune system play an essential role in the detection of infected or transformed cells and are vital for the initiation and regulation of a full adaptive immune response. The genes for several of these receptors are clustered within the leukocyte receptor complex (LRC). The purpose of this study was to carry out a detailed analysis of the chicken (Gallus gallus domesticus) LRC. Bacterial artificial chromosomes containing genes related to mammalian leukocyte immunoglobulin-like receptors were identified in a chicken genomic library and shown to map to a single microchromosome. Sequencing revealed 103 chicken immunoglobulin-like receptor (CHIR) loci (22 inhibitory, 25 activating, 15 bifunctional, and 41 pseudogenes). A very complex splicing pattern was found using transcript analyses and seven hypervariable regions were detected in the external CHIR domains. Phylogenetic and genomic analysis showed that CHIR genes evolved mainly by block duplications from an ancestral inhibitory receptor locus, with transformation into activating receptors occurring more than once. Evolutionary selection pressure has led not only to an exceptional expansion of the CHIR cluster but also to a dramatic diversification of CHIR loci and haplotypes. This indicates that CHIRs have the potential to complement the adaptive immune system in fighting pathogens.

The immune system developed to cope with a diverse array of pathogens, including infectious organisms. The detection of these pathogens by cells of the immune system is mediated by a large set of specific receptor proteins. Here the authors seek to understand how a particular subset of cell surface receptors of the domestic chicken, the chicken Ig-like receptors (CHIR), has evolved. They demonstrate that at least 103 such receptor loci are clustered on a single microchromosome and provide the first detailed analysis of this region. The sequences of the CHIR genes suggest the presence of inhibitory, activating, and bifunctional receptors, as well as numerous incomplete loci (pseudogenes) that appear to have evolved by duplications of an ancestral inhibitory receptor gene. Multiple regions of very high sequence variability were also identified within CHIR loci which, together with considerable expansion of the number of these genes, suggest that CHIR polypeptides are involved in critical functions in the immune system of the chicken.

The assembly of diverse immune receptors is focused on a polar membrane-embedded interaction site

The majority of receptors responsible for activation of distinct cell types within the immune system assemble with dimeric signaling modules through interaction of a basic transmembrane residue with a pair of acidic residues of the signaling dimer. Because assembly of other membrane proteins requires specific interactions along extended stretches of transmembrane helices, we examined how transmembrane sequences flanking the polar interaction site contribute to assembly for three receptors that associate with different signaling modules—the natural killer cell receptors KIR and NKG2D and the Fc receptor for IgA, FcαRI. The KIR and NKG2D receptors assembled with the DAP12 and DAP10 dimers, respectively, even when the entire KIR or NKG2D transmembrane domains were replaced by polyleucine sequences with a properly positioned basic residue. In contrast, a high degree of specificity for the basic side chain could be observed because the KIR–DAP12 and FcαRI–Fcγ interactions favored lysine or arginine, respectively. Steric hindrance among incompatible extra-membranous domains and competition for signaling modules also contributed to specificity of assembly. These results demonstrate that these interactions are focused on the polar site created by three ionizable transmembrane residues, and explain how the DAP12 and Fcγ signaling modules can assemble with large, non-overlapping sets of receptors that have highly divergent transmembrane sequences.

By mutating all residues but one to polyleucine in diverse immune receptors, the authors find that only one polar interaction embedded in the membrane is required for their specificity and assembly.

Signal regulatory proteins in the immune system

Signal regulatory proteins (SIRPs) constitute a family of transmembrane glycoproteins with extracellular Ig-like domains. Several SIRP family members have thus far been identified on myeloid and other cells in man, mouse, rat, and cattle. In the present study, we provide a description of the SIRP multigene family, including a number of previously undescribed SIRP genes, based on the complete genome sequences of various mammalian and bird species. We discuss this information in the context of the known immunological properties of the individual SIRP family members. Our analysis reveals SIRPs as a diverse multigene family of immune receptors, which includes inhibitory SIRPalpha, activating SIRPbeta, nonsignaling SIRPgamma, and soluble SIRPdelta members. For each species, there appears to be a single inhibitory SIRPalpha member that, upon interaction with the "self" ligand CD47, controls "homeostatic" innate immune effector functions, such as host cell phagocytosis. The activating SIRPbeta proteins show considerable variability in structure and number across species and do not bind CD47. Thus the SIRP family is a rapidly evolving gene family with important roles in immune regulation.

Dectin-1: a signalling non-TLR pattern-recognition receptor

Dectin-1 is a natural killer (NK)-cell-receptor-like C-type lectin that is thought to be involved in innate immune responses to fungal pathogens. This transmembrane signalling receptor mediates various cellular functions, from fungal binding, uptake and killing, to inducing the production of cytokines and chemokines. These activities could influence the resultant immune response and can, in certain circumstances, lead to autoimmunity and disease. As I discuss here, understanding the molecular mechanisms behind these functions has revealed new concepts, including collaborative signalling with the Toll-like receptors (TLRs) and the use of spleen tyrosine kinase (SYK), that have implications for the role of other non-TLR pattern-recognition receptors in immunity.

Signaling through Mutants of the IgA Receptor CD89 and Consequences for Fc Receptor γ-Chain Interaction

The prototypic receptor for IgA (FcalphaRI, CD89) is expressed on myeloid cells and can trigger phagocytosis, tumor cell lysis, and release of inflammatory mediators. The functions of FcalphaRI and activating receptors for IgG (FcgammaRI and FcgammaRIII) are dependent on the FcR gamma-chain dimer. This study increases our understanding of the molecular basis of the FcalphaRI-FcR gamma-chain transmembrane interaction, which is distinct from that of other activatory FcRs. FcalphaRI is unique in its interaction with the common FcR gamma-chain, because it is based on a positively charged residue at position 209, which associates with a negatively charged amino acid of FcR gamma-chain. We explored the importance of the position of this positive charge within human FcalphaRI for FcR gamma-chain association and FcalphaRI functioning with the use of site-directed mutagenesis. In an FcalphaRI R209L/A213H mutant, which represents a vertical relocation of the positive charge, proximal and distal FcR gamma-chain-dependent functions, such as calcium flux, MAPK phosphorylation, and IL-2 release, were similar to those of wild-type FcalphaRI. A lateral transfer of the positive charge, however, completely abrogated FcR gamma-chain-dependent functions in an FcalphaRI R209L/M210R mutant. By coimmunoprecipitation, we have demonstrated the loss of a physical interaction between FcR gamma-chain and FcalphaRI M210R mutant, thus explaining the loss of FcR gamma-chain-dependent functions. In conclusion, not only the presence of a basic residue in the transmembrane region of FcalphaRI, but also the orientation of FcalphaRI toward the FcR gamma-chain dimer is essential for FcR gamma-chain association. This suggests the involvement of additional amino acids in the FcalphaRI-FcR gamma-chain interaction.

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.

Characterization of Siglec-H as a novel endocytic receptor expressed on murine plasmacytoid dendritic cell precursors

We describe the cloning and characterization of Siglec-H, a novel murine CD33-related siglec-like molecule with 2 immunoglobulin domains. Unlike other CD33-related siglecs, Siglec-H lacks tyrosine-based signaling motifs in its cytoplasmic tail. Although Siglec-H has the typical structural features required for sialic acid binding, no evidence for carbohydrate recognition was obtained. Specific monoclonal and polyclonal antibodies (Abs) were raised to Siglec-H and used to define its cellular expression pattern and functional properties. By flow cytometry, Siglec-H was expressed specifically on plasmacytoid dendritic cell (pDC) precursors in bone marrow, spleen, blood, and lymph nodes. Staining of tissue sections showed that Siglec-H was also expressed in a subset of marginal zone macrophages in the spleen and in medullary macrophages in lymph nodes. Using bone marrow-derived pDC precursors that express Siglec-H, addition of Abs did not influence cytokine production, either in the presence or absence of synthetic oligodeoxynucleotides containing unmethylated cytosine-guanine motifs (CpG). In comparison, Siglec-H functioned as an endocytic receptor and mediated efficient internalization of anti-Siglec-H Abs. By immunizing mice with ovalbumin-conjugated anti-Siglec-H Ab in the presence of CpG, we demonstrate generation of antigen-specific CD8 T cells in vivo. Targeting Siglec-H may therefore be a useful way of delivering antigens to pDC precursors for cross-presentation.

NK Cell Receptors as Tools in Cancer Immunotherapy

Natural killer (NK) cells were identified 30 years ago based on their ability to "spontaneously" kill tumor cells. The basis for NK cell recognition and activation is due to a variety of receptors that bind to specific ligands on tumor cells and normal cells. Some of these receptors have the ability to inhibit NK cell function, and other receptors activate NK cell function. Therapeutic strategies for cancer therapy are being developed based on preventing NK cell inhibition or using NK cell receptors to activate NK cells or T cells. There are intriguing clinical data from studies of bone marrow transplantation that support the idea that preventing NK cell inhibition by human leukocyte antigen (HLA) class I molecules can be a means to promote graft-versus-leukemia (GvL) effects and limit graft-versus-host disease (GvHD) in acute myeloid leukemia (AML) and chronic myeloid leukemia (CML) patients. Experimental findings also support the blockade of NK cell inhibitory receptors as a way to protect against leukemia relapse. It may be possible to use our knowledge of NK cell activating receptors and their ligands to immunize patients with modified tumor cells to promote beneficial NK cell responses and development of host antitumor cytotoxic T lymphocytes (CTLs). Finally, new data support the idea of using modified NK cell receptors as a means to target patients' T cells against their own tumor cells and induce long-term immunity against them. Tumors are essentially tissues that have overcome normal regulation mechanisms, and therefore the ability to distinguish normal cells from abnormal cells is a key part of selectively attacking tumor cells. NK cells have various receptor systems designed to recognize infected and abnormal cells. Understanding NK cell receptors and their recognition mechanisms provides new tools for the development of immunotherapies against cancer.

Molecular characterization of KIR3DL3

Killer-cell immunoglobulin-like receptors (KIRs) are a structurally and functionally diverse family of molecules expressed by natural killer (NK) cells and T-cell subsets. The most centromeric gene in the human KIR cluster is KIR3DL3, a framework gene that is present in all haplotypes. KIR3DL3 has only one immunoreceptor tyrosine-based inhibitory motif and lacks the exon encoding the stem between the Immunoglobulin domains and the transmembrane region. We have investigated expression of KIR3DL3 in blood and decidual NK cells by reverse transcriptase polymerase chain reaction (RT-PCR) and protein analysis using a KIR3DL3-specific monoclonal antibody, CH21. KIR3DL3 mRNA was only detected in the CD56(bright) subset in cells from peripheral blood and in CD56(bright) decidual NK cells. The CD56(bright) NK92 cell line was also positive. Quantitative RT-PCR indicated a trend for higher expression of KIR3DL3 in female peripheral blood mononuclear cells compared to that in male. Using a bisulphite conversion method, we found that the promoter of KIR3DL3 was strongly methylated. Surface protein expression was detectable after demethylation. Like other KIRs, KIR3DL3 is highly polymorphic, and we detected 14 variants in 25 unrelated individuals. Nucleotide substitutions were scattered throughout the sequence, with a cluster of alleles at the start of the transmembrane region at the site where the remnant of the linking stem present in other KIR is found. We conclude that the KIR3DL3 gene is not a pseudogene but encodes a protein that is not expressed in healthy individuals. Protein expression might be induced under certain developmental or pathological situations.

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.

Role of ITAM-containing adapter proteins and their receptors in the immune system and bone

The immunoreceptor tyrosine-based activation motif (ITAM) is a highly conserved region in the cytoplasmic domain of signaling chains and receptors and is a critical mediator of intracellular signals. ITAM-mediated signals depend on the Syk or zeta-associated protein of 70 kDa tyrosine kinases, and ITAM signaling is required for the differentiation and function of B and T cells in adaptive immunity. ITAM-dependent receptors also regulate the function of innate immune cells, including natural killer cells, and myeloid-derived cells such as macrophages, neutrophils, dendritic cells, and mast cells. Myeloid lineage cells also include osteoclasts (OCLs), the cells required for bone resorption, and recent studies show a critical role for the ITAM-containing adapter proteins DAP12 and the FcRgamma chain (Fcepsilon receptor I gamma chain) in OCL differentiation. Mice deficient in both the DAP12 and FcRgamma ITAM-bearing adapters are significantly osteopetrotic with a severe defect in OCL differentiation, demonstrating the requirement for ITAM signals in bone and further implicating this pathway in the development of highly specialized cell functions in hematopoietic cells. Regulation of osteoclastogenesis by ITAM-dependent receptors suggests that OCLs, similar to related myeloid cells, are tightly controlled by arrays of receptors that allow them to sense and respond to their local microenvironment like other innate immune cells.

Epigenetic control of highly homologous killer Ig-like receptor gene alleles

Mature human NK lymphocytes express the highly homologous killer Ig-like receptor (KIR) genes in a stochastic fashion, and KIR transcription precisely correlates with allele-specific DNA methylation. In this study, we demonstrate that CpG methylation of a minimal KIR promoter inhibited transcription. In human peripheral blood NK cells and long-term cell lines, expressed KIR genes were associated with a moderate level of acetylated histone H3 and H4 and trimethylated histone H3 lysine 4. Histone modifications were preferentially associated with the transcribed allele in NK cell lines with monoallelic KIR expression. Although reduced, a substantial amount of histone acetylation and H3 lysine 4 trimethylation also was associated with nonexpressed KIR genes. DNA hypomethylation correlated with increased chromatin accessibility, both in vitro and in vivo. Treatment of NK cell lines and developing NK cells with the DNA methyltransferase inhibitor, 5-aza-2'-deoxycytidine, caused a dramatic increase in KIR RNA and protein expression, but little change in histone modification. Our findings suggest that KIR transcription is primarily controlled by DNA methylation.

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.

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.

Insights into seven and single transmembrane-spanning domain receptors and their signaling pathways in human natural killer cells

Human natural killer (NK) cells are important cells of the innate immune system. These cells perform two prominent functions: the first is recognizing and destroying virally infected cells and transformed cells; the second is secreting various cytokines that shape up the innate and adaptive immune re-sponses. For these cells to perform these activities, they express different sets of receptors. The receptors used by NK cells to extravasate into sites of injury belong to the seven transmembrane (7TM) family of receptors, which characteristically bind heterotrimeric G proteins. These receptors allow NK cells to sense the chemotactic gradients and activate second messengers, which aid NK cells in polarizing and migrating toward the sites of injured tissues. In addition, these receptors determine how and why human resting NK cells are mainly found in the bloodstream, whereas activated NK cells extravasate into inflammatory sites. Receptors for chemokines and lysophospholipids belong to the 7TM family. On the other hand, NK cells recognize invading or transformed cells through another set of receptors that belong to the single transmembrane-spanning domain family. These receptors are either inhibitory or activating. Inhibitory receptors contain the immune receptor tyrosine-based inhibitory motif, and activating receptors belong to either those that associate with adaptor molecules containing the immune receptor tyrosine-based activating motif (ITAM) or those that associate with adaptor molecules containing motifs other than ITAM. This article will describe the nature of these receptors and examine the intracellular signaling pathways induced in NK cells after ligating both types of receptors. These pathways are crucial for NK cell biology, development, and functions.

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.

Expression of MHC class I receptors confers functional intraclonal heterogeneity to a reactive expansion of gammadelta T cells

NK cell receptors for MHC class I molecules (MHC-NKR) can be expressed by T cell subsets. The restricted repertoire and phenotypic characteristics of MHC-NKR(+) T cells indicate that expression of MHC-NKR is acquired upon antigenic challenge and might promote expansion of T cells. Previous studies performed on in vitro generated alphabeta T cell clones concluded that MHC-NKR expression was not a clonal attribute. Here, we examined a massive monoclonal expansion of a non-leukemic gammadelta T cell population found in the peripheral blood of a lung-transplanted patient who suffered from a cytomegalovirus infection. Despite their monoclonality, these T cells displayed a heterogeneous and stable in vivo Ig- and lectin-like MHC-NKR phenotype. Twenty percent of the cells displayed a CD94(+)NKG2A(+) phenotype, and 10% were labeled with an anti-CD158b1/b2/j monoclonal antibody. A CD158b/j(+) gammadelta T cell clone derived in vitro from patient's peripheral blood lymphocytes was shown to express the activating form CD158j (KIR2DS2), which once cross-linked stimulated the clone cytolytic function and costimulated the TCR-induced production of cytokines, independently of the killer-activating receptor-associated protein (KARAP). In conclusion, heterogeneity of MHC-NKR expression confers a functional intraclonal diversity that may participate to induction of specific gammadelta T cell effector functions or proliferation upon pathogen challenge.

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.

Contribution of nuclear factor of activated T cells c1 to the transcriptional control of immunoreceptor osteoclast-associated receptor but not triggering receptor expressed by myeloid cells-2 during osteoclastogenesis

Bone homeostasis depends on the coordination of osteoclastic bone resorption and osteoblastic bone formation. Receptor activator of NF-kappaB ligand (RANKL) induces osteoclast differentiation through activating a transcriptional program mediated by the key transcription factor nuclear factor of activated T cells (NFAT) c1. Immunoreceptors, including osteoclast-associated receptor (OSCAR) and triggering receptor expressed by myeloid cells (TREM)-2, constitute the co-stimulatory signals required for RANKL-mediated activation of calcium signaling, which leads to the activation of NFATc1. However, it remains unknown whether the expression of immunoreceptors are under the control of NFATc1. Here we demonstrate that the expression of OSCAR, but not that of TREM-2, is up-regulated during osteoclastogenesis and markedly suppressed by the calcineurin inhibitor FK506, suggesting that OSCAR is transcriptionally regulated by NFATc1. NFATc1 expression results in the activation of the OSCAR promoter, which was found to be further enhanced by co-expression of PU.1 and microphthalmia-associated transcription factor (MITF). We further provide evidence that NFATc1 specifically regulates OSCAR by chromatin immunoprecipitation assay and quantification of OSCAR and TREM-2 mRNA in NFATc1-/- cells. Thus, OSCAR but not TREM-2 is involved in the positive feedback loop of the immunoreceptor-NFATc1 pathway during osteoclastogenesis. Although several immunoreceptors have been identified as co-stimulatory molecules for RANKL, the expression and function are differentially regulated. These mechanisms, possibly together with the delicate regulation of their ligands on osteoblasts, may provide the exquisite machinery for the modulation of osteoclastogenesis in the maintenance of bone homeostasis.

Impaired synaptic function in the microglial KARAP/DAP12-deficient mouse

Several proteins are expressed in both immune and nervous systems. However, their putative nonimmune functions in the brain remain poorly understood. KARAP/DAP12 is a transmembrane polypeptide associated with cell-surface receptors in hematopoeitic cells. Its mutation in humans induces Nasu-Hakola disease, characterized by presenile dementia and demyelinization. However, alteration of white matter occurs months after the onset of neuropsychiatric symptoms, suggesting that other neuronal alterations occur in the early phases of the disease. We hypothesized that KARAP/DAP12 may impact synaptic function. In mice deficient for KARAP/DAP12 function, long-term potentiation was enhanced and was partly NMDA receptor (NMDAR) independent. This effect was accompanied by changes in synaptic glutamate receptor content, as detected by the increased rectification of AMPA receptor EPSCs and increased sensitivity of NMDAR EPSCs to ifenprodil. Biochemical analysis of synaptic proteins confirmed these electrophysiological data. In mutants, the AMPA receptor GluR2 subunit expression was decreased only in the postsynaptic densities but not in the whole membrane fraction, demonstrating specific impairment of synaptic receptor accumulation. Alteration of the BNDF-tyrosine kinase receptor B (TrkB) signaling in the mutant was demonstrated by the dramatic decrease of synaptic TrkB with no change in other regulatory or scaffolding proteins. Finally, KARAP/DAP12 was detected only in microglia but not in neurons, astrocytes, or oligodendrocytes. KARAP/DAP12 may thus alter microglial physiology and subsequently synaptic function and plasticity through a novel microglia-neuron interaction.

Targeting Syk as a treatment for allergic and autoimmune disorders

Recent advances in our understanding of allergic and autoimmune disorders have begun to translate into novel, effective and safe medicines for these common maladies. Examples include an anti-IgE monoclonal antibody recently approved for severe asthmatics and the TNF-alpha antagonists that have demonstrated their ability to suppress rheumatoid arthritis, Crohn's disease and other chronic inflammatory processes. However, protein therapies are difficult and expensive to develop, manufacture and administer. Clearly, there is also a need for small-molecule inhibitors of novel targets that have safe and effective characteristics. Syk is an intracellular protein tyrosine kinase that was discovered 15 years ago as a key mediator of immunoreceptor signalling in a host of inflammatory cells including B cells, mast cells, macrophages and neutrophils. These immunoreceptors, including Fc receptors and the B-cell receptor, are important for both allergic diseases and antibody-mediated autoimmune diseases and thus pharmacologically interfering with Syk could conceivably treat these disorders. In addition, as Syk is positioned upstream in the cell signalling pathway, therapies targeting Syk may be more advantageous relative to drugs that inhibit a single downstream event. Syk inhibition during an allergic or asthmatic response will block three mast cell functions: the release of preformed mediators such as histamine, the production of lipid mediators such as leukotrienes and prostaglandins and the secretion of cytokines. In contrast, commonly used antihistamines or leukotriene receptor antagonists target only a single mediator of this complex cascade. Despite its expression in platelets and other non-haematopoietic cells, the role of Syk in regulating vascular homeostasis and other housekeeping functions is minimal or masked by redundant Syk-independent pathways. This suggests that targeting Syk would be an optimal approach to effectively treat a multitude of chronic inflammatory diseases without undue toxicity.

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.

The T cell receptor: critical role of the membrane environment in receptor assembly and function

Recent studies have demonstrated that cell membranes provide a unique environment for protein-protein and protein-lipid interactions that are critical for the assembly and function of the T cell receptor (TCR)-CD3 complex. Highly specific polar interactions among transmembrane (TM) domains that are uniquely favorable in the lipid environment organize the association of the three signaling dimers with the TCR. Each of these three assembly steps depends on the formation of a three-helix interface between one basic and two acidic residues in the membrane environment. The same polar TM residues that drive assembly also play a central role in quality control and export by directing the retention and degradation of free subunits and partial complexes, while membrane proximal cytoplasmic signals control recycling and degradation of surface receptors. Recent studies also suggest that interactions between the membrane and the cytoplasmic domains of CD3 proteins may be important for receptor triggering.

Natural selection drives recurrent formation of activating killer cell immunoglobulin-like receptor and Ly49 from inhibitory homologues

Expression of killer cell Ig-like receptors (KIRs) diversifies human natural killer cell populations and T cell subpopulations. Whereas the major histocompatibility complex class I binding functions of inhibitory KIR are known, specificities for the activating receptors have resisted analysis. To understand better activating KIR and their relationship to inhibitory KIR, we took the approach of reconstructing their natural history and that of Ly49, the analogous system in rodents. A general principle is that inhibitory receptors are ancestral, the activating receptors having evolved from them by mutation. This evolutionary process of functional switch occurs independently in different species to yield activating KIR and Ly49 genes with similar signaling domains. Selecting such convergent evolution were the signaling adaptors, which are older and more conserved than any KIR or Ly49. After functional shift, further activating receptors form through recombination and gene duplication. Activating receptors are short lived and evolved recurrently, showing they are subject to conflicting selections, consistent with activating KIR's association with resistance to infection, reproductive success, and susceptibility to autoimmunity. Our analysis suggests a two-stage model in which activating KIR or Ly49 are initially subject to positive selection that rapidly increases their frequency, followed by negative selection that decreases their frequency and leads eventually to loss.

Decreased DAP12 expression in natural killer lymphocytes from patients with systemic lupus erythematosus is associated with increased transcript mutations

Decreased numbers of natural killer (NK) cells and impaired NK function have been reported in patients with systemic lupus erythematosus (SLE). Since DAP12 plays a pivotal role in activation of NK cells, we analyzed the expressions of DAP12 protein and mRNA in peripheral blood NK cells from patients with SLE. Both DAP12 protein and mRNA expressions in NK cells from the SLE patients were decreased compared with those in NK cells from normal subjects. Sequence analysis of DAP12 cDNA showed increased nucleotide mutations, including both nucleotide substitutions and deletions. In spite of the mRNA mutations, we found no mutations in genomic DNA, suggesting that mRNA was modified during or after transcription. Decreased expression of DAP12 in NK cells from the patients was accompanied by increased expression of ADAR1 (adenosine deaminase that acts on RNA transcripts) and by decreased expression of NKp44. These results suggest that abnormal expression of DAP12 molecules in NK cells may account for the impairment of NK cell function in patients with SLE.

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.

Divergence of natural killer cell receptor and related molecule in the decidua from sporadic miscarriage with normal chromosome karyotype

The aim of this cohort study was to investigate immunophenotypic characteristics of natural killer (NK) cells by assessing specific molecules expressed in the decidua of sporadic miscarriages and induced abortions. The deciduae were obtained from 29 consecutively seen women whose pregnancies ended in first trimester miscarriages (MS), and the fetal chromosome karyotype of these MS was analysed. Additionally, 13 deciduae were obtained from induced abortion (IA) with informed consent. The expression of perforin, CD94, CD161, CD158a, CD158b, CD244 on CD3-CD56+NK cells, and perforin on CD3+CD8+ T cells was analysed by flow cytometry. The CD158a (mean+/-SD, 26.2+/-14.7%) and CD94 (50.2+/-25.7%) expressions in MS with normal chromosome karyotype (MSNK; n=11) were significantly decreased as compared with those (41.5+/-19.5%, 71.4+/-20.4%) in MS with abnormal karyotype (MSAK; n=18) and those (44.3+/-21.9%, 80.8+/-17.5%) in IA (n=13). Conversely, the perforin expression on CD3-CD8-CD56+NK cells (76.3+/-11.0%) and CD3+CD8+T cells (30.6+/-9.2%) in MSNK was significantly increased as compared with those (66.8+/-16.6%, 23.6+/-8.7%) in MSAK and those (62.9+/-11.6%, 19.7+/-8.1%) in IA. A positive correlation between CD94 and CD158a expressions on NK cells, negative correlations between CD94 on NK cells and perforin on NK cells/T cells, and between CD158a on NK cells and perforin on T cells were found in the decidua. A divergence of NK cell repertoire in the decidua might be related to aetiology of sporadic MSNK.

Dap12 and Trem2, molecules involved in innate immunity and neurodegeneration, are co-expressed in the CNS

Polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy (PLOSL) is a recessively inherited disease characterized by early onset dementia associated with bone cysts. Our group has recently established the molecular background of PLOSL by identifying mutations in DAP12 and TREM2 genes. To understand how loss of function of the immune cell activating DAP12/TREM2 signaling complex leads to dementia and loss of myelin, we have analyzed here Dap12 and Trem2 expression in the mouse CNS. We show that Dap12 and Trem2 are expressed from embryonic stage to adulthood, and demonstrate a highly similar expression pattern. In addition, we identify microglial cells and oligodendrocytes as the major Dap12/Trem2-producing cells in the CNS and, consequently, as the predominant cell types involved in PLOSL pathogenesis. These findings provide a good starting point for the study of the molecular mechanisms of this inherited dementia and new evidence for the involvement of the immune system in neuronal degeneration.

The role of TCR stimulation and TGF-beta in controlling the expression of CD94/NKG2A receptors on CD8 T cells

Following antigen recognition, murine CD8 T cells express CD94/NKG2A receptors. Our results show that this up-regulation occurs rapidly in vitro and is accompanied by an approximately 8-fold increase in CD94 and approximately 125-fold increase in NKG2A mRNA. In contrast, only a twofold increase in NKG2C mRNA is noted. The addition of TGF-beta, but not IL-10, IL-12 or IL-15, leads to a further increase in cell membrane expression of these receptors, as well as a approximately 6-fold increase in mRNA for both chains. TGF-beta also increases CD94/NKG2A expression on memory CD8 T cells that are re-exposed to antigen. The effect of TGF-beta on increasing CD94/NKG2A expression on both naive and memory CD8 T cells occurs only when there is a concurrent stimulation through the TCR. In contrast, TGF-beta does not increase expression of CD94/NKG2A on resting or activated NK cells. We also show by using purified CD8 T cells, that TGF-beta acts directly on these cells. These results implicate a role for both antigen and TGF-beta in increasing expression of inhibitory CD94/NKG2A receptors on CD8 T cells.

Innate and adaptive immunity: specificities and signaling hierarchies revisited

The conventional classification of known immune responses by specificity may need re-evaluation. The immune system can be classified into two subsystems: the innate and adaptive immune systems. In general, innate immunity is considered a nonspecific response, whereas the adaptive immune system is thought of as being very specific. In addition, the antigen receptors of the adaptive immune response are commonly viewed as 'master sensors' whose engagement dictates lymphocyte function. Here we propose that these ideas do not genuinely reflect the organization of immune responses and that they bias our view of immunity as well as our teaching of immunology. Indeed, the level of specificity and mode of signaling integration used by the main cellular participants in the adaptive and innate immune systems are more similar than previously appreciated.

Brain and bone damage in KARAP/DAP12 loss-of-function mice correlate with alterations in microglia and osteoclast lineages

Human polycystic lipomembraneous osteodysplasia with sclerosing leukoencephalopathy, also known as Nasu-Hakola disease, has been described to be associated with mutations affecting the immunoreceptor tyrosine-based activation motif-bearing KARAP/DAP12 immunoreceptor gene. Patients present bone fragilities and severe neurological alterations leading to presenile dementia. Here we investigated whether the absence of KARAP/DAP12-mediated signals in loss-of-function (KDelta75) mice also leads to bone and central nervous system pathological features. Histological analysis of adult KDelta75 mice brains revealed a diffuse hypomyelination predominating in anterior brain regions. As this was not accompanied by oligodendrocyte degeneration or microglial cell activation it suggests a developmental defect of myelin formation. Interestingly, in postnatal KDelta75 mice, we observed a dramatic reduction in microglial cell numbers similar to in vitro microglial cell differentiation impairment. Our results raise the intriguing possibility that defective microglial cell differentiation might be responsible for abnormal myelin development. Histomorphometry revealed that bone remodeling is also altered, because of a resorption defect, associated with a severe block of in vitro osteoclast differentiation. In addition, we show that, among monocytic lineages, KARAP/DAP12 specifically controls microglial and osteoclast differentiation. Our results confirm that KARAP/DAP12-mediated signals play an important role in the regulation of both brain and bone homeostasis. Yet, important differences exist between the symptoms observed in Nasu-Hakola patients and KDelta75 mice.