Myeloid DAP12-associating lectin (MDL)-1 is a cell surface receptor involved in the activation of myeloid cells

PSGL-1 engagement by E-selectin signals through Src kinase Fgr and ITAM adapters DAP12 and FcR gamma to induce slow leukocyte rolling

E-selectin binding to P-selectin glycoprotein ligand-1 (PSGL-1) can activate the β₂ integrin lymphocyte function-associated antigen-1 by signaling through spleen tyrosine kinase (Syk). This signaling is independent of Gα_i-protein–coupled receptors, results in slow rolling, and promotes neutrophil recruitment to sites of inflammation. However, the signaling pathways linking E-selectin engagement of PSGL-1 to Syk activation are unknown. To test the role of Src family kinases and immunoreceptor tyrosine-based activating motif (ITAM)–containing adaptor proteins, we used different gene-deficient mice in flow chamber, intravital microscopy, and peritonitis studies. E-selectin–mediated phosphorylation of Syk and slow rolling was abolished in neutrophils from fgr^−/− or hck^−/− lyn^−/− fgr^−/− mice. Neutrophils from Tyrobp^−/− Fcrg^−/− mice lacking both DAP12 and FcRγ were incapable of sustaining slow neutrophil rolling on E-selectin and intercellular adhesion molecule-1 and were unable to phosphorylate Syk and p38 MAPK. This defect was confirmed in vivo by using mixed chimeric mice. Gα_i-independent neutrophil recruitment into the inflamed peritoneal cavity was sharply suppressed in Tyrobp^−/− Fcrg^−/− mice. Our data demonstrate that an ITAM-dependent pathway involving the Src-family kinase Fgr and the ITAM-containing adaptor proteins DAP12 and FcRγ is involved in the initial signaling events downstream of PSGL-1 that are required to initiate neutrophil slow rolling.

Mincle is an ITAM-coupled activating receptor that senses damaged cells

Macrophage-inducible C-type lectin (Mincle) is expressed mainly in macrophages and is induced after exposure to various stimuli and stresses. Here we show that Mincle selectively associated with the Fc receptor common gamma-chain and activated macrophages to produce inflammatory cytokines and chemokines. Mincle-expressing cells were activated in the presence of dead cells, and we identified SAP130, a component of small nuclear ribonucloprotein, as a Mincle ligand that is released from dead cells. To investigate whether Mincle is required for normal responses to cell death in vivo, we induced thymocyte death by irradiating mice and found that transient infiltration of neutrophils into the thymus could be blocked by injection of Mincle-specific antibody. Our results suggest that Mincle is a receptor that senses nonhomeostatic cell death and thereby induces the production of inflammatory cytokines to drive the infiltration of neutrophils into damaged tissue.

CLEC5A is critical for dengue-virus-induced lethal disease

Dengue haemorrhagic fever and dengue shock syndrome, the most severe responses to dengue virus (DV) infection, are characterized by plasma leakage (due to increased vascular permeability) and low platelet counts. CLEC5A (C-type lectin domain family 5, member A; also known as myeloid DAP12-associating lectin (MDL-1)) contains a C-type lectin-like fold similar to the natural-killer T-cell C-type lectin domains and associates with a 12-kDa DNAX-activating protein (DAP12) on myeloid cells. Here we show that CLEC5A interacts with the dengue virion directly and thereby brings about DAP12 phosphorylation. The CLEC5A-DV interaction does not result in viral entry but stimulates the release of proinflammatory cytokines. Blockade of CLEC5A-DV interaction suppresses the secretion of proinflammatory cytokines without affecting the release of interferon-alpha, supporting the notion that CLEC5A acts as a signalling receptor for proinflammatory cytokine release. Moreover, anti-CLEC5A monoclonal antibodies inhibit DV-induced plasma leakage, as well as subcutaneous and vital-organ haemorrhaging, and reduce the mortality of DV infection by about 50% in STAT1-deficient mice. Our observation that blockade of CLEC5A-mediated signalling attenuates the production of proinflammatory cytokines by macrophages infected with DV (either alone or complexed with an enhancing antibody) offers a promising strategy for alleviating tissue damage and increasing the survival of patients suffering from dengue haemorrhagic fever and dengue shock syndrome, and possibly even other virus-induced inflammatory diseases.

LILRA2 activation inhibits dendritic cell differentiation and antigen presentation to T cells

The differentiation of monocytes into dendritic cells (DC) is a key mechanism by which the innate immune system instructs the adaptive T cell response. In this study, we investigated whether leukocyte Ig-like receptor A2 (LILRA2) regulates DC differentiation by using leprosy as a model. LILRA2 protein expression was increased in the lesions of the progressive, lepromatous form vs the self-limited, tuberculoid form of leprosy. Double immunolabeling revealed LILRA2 expression on CD14+, CD68+ monocytes/macrophages. Activation of LILRA2 on peripheral blood monocytes impaired GM-CSF induced differentiation into immature DC, as evidenced by reduced expression of DC markers (MHC class II, CD1b, CD40, and CD206), but not macrophage markers (CD209 and CD14). Furthermore, LILRA2 activation abrogated Ag presentation to both CD1b- and MHC class II-restricted, Mycobacterium leprae-reactive T cells derived from leprosy patients, while cytokine profiles of LILRA2-activated monocytes demonstrated an increase in TNF-alpha, IL-6, IL-8, IL-12, and IL-10, but little effect on TGF-beta. Therefore, LILRA2 activation, by altering GM-CSF-induced monocyte differentiation into immature DC, provides a mechanism for down-regulating the ability of the innate immune system to activate the adaptive T cell response while promoting an inflammatory response.

The TREM-1/DAP12 pathway

DNAX activation protein of 12kDa (DAP12) is an immunoreceptor tyrosine-based activation motif (ITAM)-bearing adapter, which couples to multiple receptors expressed on natural killer (NK) cells, monocytes, and neutrophils. Initially, DAP12-mediated signaling was mainly investigated downstream of receptors expressed on NK cells. In myeloid cells, one of the receptors associating with DAP12 is the triggering receptor expressed on myeloid cells (TREM)-1. Since the real nature of TREM-1L(s) is still illusive, TREM-1 biology was so far only studied using agonistic monoclonal antibodies for receptor ligation. Triggering via TREM-1 results in the production of pro-inflammatory cytokines, chemokines, reactive oxygen species (ROS), and leads to rapid degranulation of neutrophilic granules, and phagocytosis. Furthermore, application of a TREM-1/Ig fusion protein in an animal model of experimentally induced sepsis increases survival. It is obvious that targeting components of the TREM-1/DAP12 pathway could be a promising therapeutic strategy for the treatment of inflammatory diseases. Therefore, it is of great importance to get further insight into the signaling cascade downstream of TREM-1. This review summarizes the current understanding of the TREM-1/DAP12 pathway in monocytes and neutrophils.

The adaptor protein CARD9 is essential for the activation of myeloid cells through ITAM-associated and Toll-like receptors

Immunoreceptor tyrosine-based activation motifs (ITAMs) are crucial in antigen receptor signaling in acquired immunity. Although receptors associated with the ITAM-bearing adaptors FcRgamma and DAP12 on myeloid cells have been suggested to activate innate immune responses, the mechanism coupling those receptors to 'downstream' signaling events is unclear. The CARMA1-Bcl-10-MALT1 complex is critical for the activation of transcription factor NF-kappaB in lymphocytes but has an unclear function in myeloid cells. Here we report that deletion of the gene encoding the Bcl-10 adaptor-binding partner CARD9 resulted in impaired myeloid cell activation of NF-kappaB signaling by several ITAM-associated receptors. Moreover, CARD9 was required for Toll-like receptor-induced activation of dendritic cells through the activation of mitogen-activated protein kinases. Although Bcl10-/- and Card9-/- mice had similar signaling impairment in myeloid cells, Card11-/- (CARMA1-deficient) myeloid cell responses were normal, and although Card11-/- lymphocytes were defective in antigen receptor-mediated activation, Card9-/- lymphocytes were not. Thus, the activation of lymphoid and myeloid cells through ITAM-associated receptors or Toll-like receptors is regulated by CARMA1-Bcl-10 and CARD9-Bcl-10, respectively.

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.

Semimature stage: a checkpoint in a dendritic cell maturation program that allows for functional reversion after signal-regulatory protein-alpha ligation and maturation signals

CD47 on live cells actively engages signal-regulatory protein-alpha (SIRP-alpha) on phagocytes and delivers a negative signal that prevents their elimination. We evaluated the biological consequences of SIRP-alpha ligation on the dendritic cell (DC) response to maturation signals and the potential interplay with the IL-10/IL-10R inhibitory pathway. At first, CD47/SIRP-alpha allowed the generation of mature migratory DCs not producing IL-12, IFN-gamma-inducible protein-10, and CCL19. Rather, they secreted neutrophils attracting chemokine CXCL5 and IL-1beta, reflecting a partial block in functional DC maturation. Afterward, semimature DCs functionally regressed in an IL-10-independent fashion toward cells that retrieved the cardinal features of immature DCs: re-expression of CCR5, loss of DC-lysosome-associated membrane protein, high endocytosis, and impaired allostimulatory functions. The global gene expression profile of IL-10 and SIRP-alpha-ligated DC demonstrated two distinct molecular pathways. IL-10R and SIRP-alpha expression were reciprocally down-regulated by CD47 and IL-10, respectively. These results emphasize that the SIRP-alpha pathway might be part of the molecular machinery used by the DC to dampen or resolve an inflammatory response in an IL-10-independent manner.

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.

Critical Residues at the Ly49 Natural Killer Receptor’s Homodimer Interface Determine Functional Recognition of m157, a Mouse Cytomegalovirus MHC Class I-Like Protein

NK cell function is regulated by Ly49 receptors in mice and killer cell Ig-like receptors in humans. Although inhibitory Ly49 and killer cell Ig-like receptors predominantly ligate classical MHC class I molecules, recent studies suggest that their activating counterparts recognize infection. The quintessential example is resistance to the mouse CMV in C57BL/6 mice, which depends on the functional recognition of m157, a mouse CMV-encoded MHC class I-like molecule, by Ly49H, an activating NK cell receptor. We have taken advantage of the natural variation in closely related members of the Ly49C-like receptors and the availability of Ly49 crystal structures to understand the molecular determinants of the Ly49H-m157 interaction and to identify amino acid residues discriminating between m157 binding and nonbinding receptors. Using a site-directed mutagenesis approach, we have targeted residues conserved in receptors binding to m157 (Ly49H and Ly49I(129)) but different from receptors lacking m157 recognition (Ly49C, Ly49I(B6), and Ly49U). Wild-type and mutant receptors were transfected into reporter cells, and physical binding as well as functional activation by m157 was studied. Our findings suggested that the Ly49 MHC class I contact "site 2," I226, may not be involved in m157 binding. In contrast, residue Y146 and G151, mapping at the receptor homodimer interface, are likely critical for functional recognition of the m157 glycoprotein. Our combined functional and three-dimensional modeling approach suggested that the architecture of the Ly49H dimer is crucial to accessing m157, but not MHC class I. These results link Ly49 homodimerization variability to the direct recognition of pathogen products.

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.

Myeloid C-type lectins in innate immunity

C-type lectins expressed on myeloid cells comprise a family of proteins that share a common structural motif, and some act as receptors in pathogen recognition. But just as the presence of leucine-rich repeats alone is not sufficient to define a Toll-like receptor, the characterization of C-type lectin receptors in innate immunity requires the identification of accompanying signaling motifs. Here we focus on the known signaling pathways of myeloid C-type lectins and on their possible functions as autonomous activating or inhibitory receptors involved in innate responses to pathogens or self.

Gene-expression profiling and array-based CGH classify CD4+CD56+ hematodermic neoplasm and cutaneous myelomonocytic leukemia as distinct disease entities

CD4+CD56+ hematodermic neoplasm (CD4+CD56+HN) is an aggressive hematopoietic malignancy with distinct clinicopathologic and immunophenotypic features that commonly involve the skin, bone marrow, and blood. Differentiation from cutaneous myelomonocytic leukemia (c-AML) may be exceedingly difficult and requires extensive phenotyping. The molecular mechanisms involved in the development of CD4+CD56+HN are largely unresolved. Moreover, recurrent chromosomal alterations have not yet been precisely defined in CD4+CD56+HN and c-AML. In the present study an integrated genomic analysis using expression profiling and array-based comparative genomic hybridization (CGH) was performed on lesional skin biopsy samples of patients with CD4+CD56+HN and c-AML. Our results demonstrate that CD4+CD56+HN and c-AML show distinct gene-expression profiles and distinct patterns of chromosomal aberrations. CD4+CD56+HN is characterized by recurrent deletion of regions on chromosome 4 (4q34), chromosome 9 (9p13-p11 and 9q12-q34), and chromosome 13 (13q12-q31) that contain several tumor suppressor genes with diminished expression (Rb1, LATS2). Elevated expression of the oncogenes HES6, RUNX2, and FLT3 was found but was not associated with genomic amplification. We noted high expression of various plasmacytoid dendritic-cell (pDC)–related genes, pointing to the cell of origin of this malignancy.

Innate immune response gene expression profiles of N9 microglia are pathogen-type specific

Glial cells, particularly microglia, are thought to play a pivotal role in initiating and guiding innate immune responses to CNS infections and in perpetuating inflammation and pathology in CNS diseases such as multiple sclerosis and Alzheimer's disease. We describe here the development and use of a new microarray designed to specifically profile transcript expression of innate immunity genes. Microarray analysis validated by quantitative PCR demonstrated an extensive range of pattern recognition receptor gene expression in resting N9 microglia, including Toll-like receptors, scavenger receptors and lectins. Stimulation with LPS or infection with virus modulated pattern recognition receptor, cytokine, chemokine and other innate immune transcripts in a distinct and stimulus-specific manner. This study demonstrates that a single glial cell phenotype has an innate capability to detect infection, determine its form and generate specific responses.

The C-type lectin-like domain superfamily

The superfamily of proteins containing C-type lectin-like domains (CTLDs) is a large group of extracellular Metazoan proteins with diverse functions. The CTLD structure has a characteristic double-loop ('loop-in-a-loop') stabilized by two highly conserved disulfide bridges located at the bases of the loops, as well as a set of conserved hydrophobic and polar interactions. The second loop, called the long loop region, is structurally and evolutionarily flexible, and is involved in Ca2+-dependent carbohydrate binding and interaction with other ligands. This loop is completely absent in a subset of CTLDs, which we refer to as compact CTLDs; these include the Link/PTR domain and bacterial CTLDs. CTLD-containing proteins (CTLDcps) were originally classified into seven groups based on their overall domain structure. Analyses of the superfamily representation in several completely sequenced genomes have added 10 new groups to the classification, and shown that it is applicable only to vertebrate CTLDcps; despite the abundance of CTLDcps in the invertebrate genomes studied, the domain architectures of these proteins do not match those of the vertebrate groups. Ca2+-dependent carbohydrate binding is the most common CTLD function in vertebrates, and apparently the ancestral one, as suggested by the many humoral defense CTLDcps characterized in insects and other invertebrates. However, many CTLDs have evolved to specifically recognize protein, lipid and inorganic ligands, including the vertebrate clade-specific snake venoms, and fish antifreeze and bird egg-shell proteins. Recent studies highlight the functional versatility of this protein superfamily and the CTLD scaffold, and suggest further interesting discoveries have yet to be made.

C-type lectin-like receptors on myeloid cells

Host defence against pathogens requires the recognition of conserved microbial molecules, or 'pathogen-associated molecular patterns' (PAMPs), by their receptors termed 'pattern recognition receptors' (PRRs), represented most notably by toll-like receptors (TLRs) and C-type lectins. The 'non-classical' C-type lectins (these that lack the residues involved in calcium binding, required for carbohydrate binding) are traditionally thought of as being restricted to natural killer (NK) or T cells, playing important roles in immune surveillance. In recent years, however, a growing number of these receptors have been identified on myeloid cells, both of human and mouse origin. In contrast to their NK counterparts that primarily control cellular activation through recognition of major histocompatibility antigen (MHC) class I and related molecules, the myeloid-expressed receptors appear to have a far more diverse range of functions and ligands, including those of exogenous origin. Some of C-type lectin-like molecules possess activating/inhibitory signalling motifs that trigger downstream signalling events, suggesting the role for these receptors as positive/negative regulators of granulocyte and monocyte functions. With the exception of a few myeloid NK-like lectins, the natural ligands for most of these receptors remain unidentified, making it difficult to define their functions in normal physiological, inflammatory or pathological conditions. Importantly, in some cases, these novel C-type lectin-like lectins, encoded by genes from the same gene cluster, can act as receptor/ligand pairs, additionally contributing to the regulation of myeloid cell functions or their interaction with other (like NK) cell types. However, the relevance and importance of such interactions still needs to be assessed. Although few of the myeloid-expressed C-type lectins have been characterized in detail, we review here each of these receptors and highlight their prospective roles in innate and adaptive immunity.

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.

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.

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.

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.

Macrophage receptors and immune recognition

Macrophages express a broad range of plasma membrane receptors that mediate their interactions with natural and altered-self components of the host as well as a range of microorganisms. Recognition is followed by surface changes, uptake, signaling, and altered gene expression, contributing to homeostasis, host defense, innate effector mechanisms, and the induction of acquired immunity. This review covers recent studies of selected families of structurally defined molecules, studies that have improved understanding of ligand discrimination in the absence of opsonins and differential responses by macrophages and related myeloid cells.

Enhanced Toll-like receptor responses in the absence of signaling adaptor DAP12

DAP12 is a signaling adaptor containing an immunoreceptor tyrosine-based activation motif (ITAM) that pairs with receptors on myeloid cells and natural killer cells. We examine here the responses of mice lacking DAP12 to stimulation through Toll-like receptors (TLRs). Unexpectedly, DAP12-deficient macrophages produced higher concentrations of inflammatory cytokines in response to a variety of pathogenic stimuli. Additionally, macrophages deficient in spleen tyrosine kinase (Syk), which signals downstream of DAP12, showed a phenotype identical to that of DAP12-deficient macrophages. DAP12-deficient mice were more susceptible to endotoxic shock and had enhanced resistance to infection by the intracellular bacterium Listeria monocytogenes. These data suggest that one or more DAP12-pairing receptors negatively regulate signaling through TLRs.

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.

C-type lectin-like molecule-1: a novel myeloid cell surface marker associated with acute myeloid leukemia

Acute myeloid leukemia (AML) has a poor prognosis due to treatment-resistant relapses. A humanized anti-CD33 antibody (Mylotarg) showed a limited response rate in relapsed AML. To discover novel AML antibody targets, we selected a panel of single chain Fv fragments using phage display technology combined with flow cytometry on AML tumor samples. One selected single chain Fv fragment broadly reacted with AML samples and with myeloid cell lineages within peripheral blood. Expression cloning identified the antigen recognized as C-type lectin-like molecule-1 (CLL-1), a previously undescribed transmembrane glycoprotein. CLL-1 expression was analyzed with a human anti-CLL-1 antibody that was generated from the single chain Fv fragment. CLL-1 is restricted to the hematopoietic lineage, in particular to myeloid cells present in peripheral blood and bone marrow. CLL-1 is absent on uncommitted CD34(+)/CD38(-) or CD34(+)/CD33(-) stem cells and present on subsets of CD34(+)/CD38(+) or CD34(+)/CD33(+) progenitor cells. CLL-1 is not expressed in any other tissue. In contrast, analysis of primary AMLs demonstrated CLL-1 expression in 92% (68 of 74) of the samples. As an AML marker, CLL-1 was able to complement CD33, because 67% (8 of 12) of the CD33(-) AMLs expressed CLL-1. CLL-1 showed variable expression (10-60%) in CD34(+) cells in chronic myelogenous leukemia and myelodysplastic syndrome but was absent in 12 of 13 cases of acute lymphoblastic leukemia. The AML reactivity combined with the restricted expression on normal cells identifies CLL-1 as a novel potential target for AML treatment.

Post-transcriptional effects of phorbol 12-myristate 13-acetate on transcriptome of U937 cells

To identify post-transcriptionally modulated genes at the translational level by phorbol 12-myristate 13-acetate (PMA), we investigated mRNA profiles in the polysomal and the cytoplasmic fractions of U937 cells before and after PMA stimulation using microarrays with 15017 oligonucleotide probes. Global comparison of the profiles showed that the cytoplasmic distribution of mRNAs was considerably modulated upon PMA stimulation. The results also indicate that PMA post-transcriptionally regulated at least 0.7% of detectable genes in U937 cells. Thus, besides transcriptional modulation by PMA, changes in the translational state of transcripts seem to play a critical role in PMA-induced differentiation of U937 cells.

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

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

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

Prior studies have revealed that alternative mRNA splicing of the mouse NKG2D gene generates receptors that associate with either the DAP10 or DAP12 transmembrane adapter signaling proteins. We report that NKG2D function is normal in human patients lacking functional DAP12, indicating that DAP10 is sufficient for human NKG2D signal transduction. Further, we show that human NKG2D is incapable of associating with DAP12 and provide evidence that structural differences in the transmembrane of mouse and human NKG2D account for the species-specific difference for this immune receptor.

Activation of natural killer cells and dendritic cells upon recognition of a novel CD99-like ligand by paired immunoglobulin-like type 2 receptor

Paired receptors that consist of highly related activating and inhibitory receptors are widely involved in the regulation of the immune system. Here, we report a mouse orthologue of the human activating paired immunoglobulin-like type 2 receptor (PILR) β, which was cloned from a cDNA library of natural killer (NK) cells based on its ability to associate with the DAP12 signaling adaptor protein. The activating PILRβ was expressed not only on NK cells but also on dendritic cells and macrophages. Furthermore, we have identified a novel CD99-like molecule as a ligand for the activating PILRβ and inhibitory PILRα receptors. Transcripts of PILR ligand are present in many tissues, including some T cell lines. Cells expressing the PILR ligand specifically activated NK cells and dendritic cells that express the activating PILRβ. Our findings reveal a new regulatory mechanism of innate immunity by PILR and its CD99-like ligand.

IL-4 confers NK stimulatory capacity to murine dendritic cells: a signaling pathway involving KARAP/DAP12-triggering receptor expressed on myeloid cell 2 molecules

Dendritic cells (DC) regulate NK cell functions, but the signals required for the DC-mediated NK cell activation, i.e., DC-activated NK cell (DAK) activity, remain poorly understood. Upon acute inflammation mimicked by LPS or TNF-alpha, DC undergo a maturation process allowing T and NK cell activation in vitro. Chronic inflammation is controlled in part by Th2 cytokines. In this study, we show that IL-4 selectively confers to DC NK but not T cell stimulatory capacity. IL-4 is mandatory for mouse bone marrow-derived DC grown in GM-CSF (DC(GM/IL-4)) to promote NK cell activation in the draining lymph nodes. IL-4-mediated DAK activity depends on the KARAP/DAP12-triggering receptor expressed on myeloid cell 2 signaling pathway because: 1) gene targeting of the adaptor molecule KARAP/DAP12, a transmembrane polypeptide with an intracytoplasmic immunoreceptor tyrosine-based activation motif, suppresses the DC(GM/IL-4) capacity to activate NK cells, and 2) IL-4-mediated DAK activity is significantly blocked by soluble triggering receptor expressed on myeloid cell 2 Fc molecules. These data outline a novel role for Th2 cytokines in the regulation of innate immune responses through triggering receptors expressed on myeloid cells.

Differential regulation of DAP12 and molecules associated with DAP12 during host responses to mycobacterial infection

DAP12 and its associating molecules MDL-1, TREM-1, and TREM-2 are the recently identified immune regulatory molecules, expressed primarily on myeloid cells including monocytes/macrophages, dendritic cells, NK cells, and neutrophils. However, little is known about the regulation of their expression during host antimicrobial responses. We have investigated the effect of pulmonary mycobacterial infection and type 1 cytokines on the expression of these molecules both in vivo and in vitro. While DAP12 was constitutively expressed at high levels in the lungs, the MDL-1, TREM-1, and TREM-2 molecules were inducible during mycobacterial infection. Their kinetic expression was correlated with that of the type 1 cytokines tumor necrosis factor alpha (TNF-alpha) and gamma interferon (IFN-gamma). In primary lung macrophage cultures, high constitutive levels of DAP12 and TREM-2 were not modulated by mycobacterial or type 1 cytokine exposure. In contrast, expression of both MDL-1 and TREM-1 was markedly induced by mycobacterial infection and such induction was inhibited by concurrent exposure to IFN-gamma. On mycobacterial infection of TNF-alpha(-/-) and IFN-gamma(-/-) mice in vivo or their lung macrophages in vitro, TNF-alpha was found to be critical for mycobacterially induced MDL-1, but not TREM-1, expression whereas IFN-gamma negatively regulated mycobacterially induced MDL-1 and TREM-1 expression. Our findings thus suggest that DAP12 and its associating molecules are differentially regulated by mycobacterial infection and type 1 cytokines and that MDL-1- and TREM-1-triggered DAP12 signaling may play an important role in antimicrobial type 1 immunity.

OSCAR is an FcRgamma-associated receptor that is expressed by myeloid cells and is involved in antigen presentation and activation of human dendritic cells

We have isolated a novel cell surface molecule, the human homolog of osteoclast-associated receptor (OSCAR). Unlike mouse OSCAR, hOSCAR is widely transcribed in cells of the myeloid lineage. Notably, hOSCAR is expressed on circulating blood monocytes and CD11c(+) dendritic cells but not on T and B cells. hOSCAR is continually expressed during differentiation of CD14(+) monocytes into dendritic cells and maintained after maturation. hOSCAR associates with the FcRgamma as shown by translocation of FcRgamma to the cell surface in presence of hOSCAR and coimmunoprecipitation from transfected cell lines and ex vivo cells. Engagement of hOSCAR with specific mAb leads to Ca(2+) mobilization and cytokine release, indicators of cellular activation. Endocytosis of the receptor in dendritic cells was observed, followed by passage of the internalized material into Lamp-1(+) and HLA-DR(+) compartments, suggesting a role in antigen uptake and presentation. Dendritic cells were able to stimulate a T-cell clone specific for an epitope of mouse IgG1 after uptake and processing of the hOSCAR-specific antibody, demonstrating the capacity of this receptor to mediate antigen presentation. hOSCAR thus represents a novel class of molecule expressed by dendritic cells involved in the initiation of the immune response.

Mycobacterium bovis BCG cell wall-specific differentially expressed genes identified by differential display and cDNA subtraction in human macrophages

We have analyzed the gene expression profile of monocytes in response to a highly purified cell wall fraction of Mycobacterium bovis BCG, a clinically approved adjuvant known as BCG cell wall skeleton (BCG-CWS). It is composed of mycolic acid, arabinogalactan, and peptidoglycan and confers Toll-like receptor 2 (TLR2)- and TLR4-dependent signaling that induces monocytes to differentiate into antigen-presenting cells (APCs). Here we report differential gene expression analysis with BCG-CWS-stimulated versus nonstimulated monocytes. BCG-CWS exerted massive induction of genes regulated by TLR signaling. Marked gene regulatory characteristics in BCG-CWS-stimulated cells compared to lipopolysaccharide (LPS)-stimulated cells follow. (i) Spliced mRNAs encoding soluble forms of TREM-1 and TREM-2 (recently discovered inflammatory-signal-amplifying receptors) were regulated by BCG-CWS, resulting in their differential expression. (ii) The genes for zinc-iron transporter protein (ZIP)-like family proteins HKE-1 and LIV-1 were induced exclusively by BCG-CWS. (iii) Interleukin-23 (IL-23), rather than IL-12p70, was induced by BCG-CWS, while interferon-inducible genes were induced only by LPS. By Northern and reverse transcription-PCR analyses, we confirmed the differential expression of more than 30 BCG-CWS regulatory genes, and their expression was compared with that of LPS and other known TLR ligands. A battery of genes responded rapidly and for a short time to LPS but for a long time to BCG-CWS. Structural analysis of the identified novel or hypothetical proteins revealed that some are potential candidates as signaling mediators or transcriptional regulators. Hence, BCG-CWS may profoundly modulate APC responses in a way distinct from that of LPS, leading to possible advantages for its adjuvant-active therapeutic potential.

The signaling adapter protein DAP12 regulates multinucleation during osteoclast development

Deficiency of the signaling adapter protein DAP12 is associated with bony abnormalities in both mice and humans. We identify specific DAP12-associated receptors expressed by osteoclasts and examine function of DAP12 in murine osteoclasts in vivo and in vitro. These data show a new role for DAP12 signaling in regulating formation of multinucleated osteoclasts.

INTRODUCTION

Osteoclasts are bone-resorbing cells derived from hematopoietic precursors in the myeloid lineage. In other myeloid cell types, the signaling adapter protein DAP12 transmits activating signals on ligation of a DAP12-associated receptor (DAR). The aim of this study was to clarify the role of DAP12 signaling during osteoclast development.

MATERIALS AND METHODS

Osteoclasts from DAP12 -/- or control mice were analyzed in vitro for morphology, function, and for osteoclast markers. DARs were identified in osteoclast cultures through reverse transcriptase-polymerase chain reaction (RT-PCR). Bone density of DAP12 -/- and control mice were analyzed by microcomputed tomography. DAP12 -/- osteoclasts were retrovirally reconstituted with DAP12. RAW264.7 cells were transfected with FLAG-tagged DAP12 or TREM2 and stimulated by anti-FLAG antibody during in vitro osteoclastogenesis.

RESULTS

C57BL/6 DAP12-deficient mice have higher bone mass than C57BL/6 wildtype controls. We verified the presence of DAP12 in pre-osteoclasts and osteoclasts derived from C57BL/6 or the pre-osteoclast line RAW 264.7 and identified the DARs expressed. DAP12 -/- osteoclasts developed in vitro with macrophage colony-stimulating factor (M-CSF) and RANKL formed only intensely TRACP+ mononuclear cells and failed to generate multinuclear osteoclasts. These mononuclear cells are functional osteoclast-like cells because, by RT-PCR, they express other osteoclast markers and generate resorption pits on dentine slices, although quantitative assessment of bone resorption shows decreased resorption by DAP12 -/- osteoclasts compared with C57BL/6 osteoclasts. Restoration of DAP12 expression by retroviral transduction of DAP12 -/- osteoclast precursors rescued in vitro osteoclast multinucleation. Direct stimulation of DAP12 expressed in RAW264.7 during in vitro osteoclastogenesis led to a marked increase in the number of TRACP+ multinucleated osteoclast-like cells formed.

CONCLUSION

Our studies indicate that stimulation of the DAP12 adapter protein plays a significant role in formation of multinuclear osteoclasts and that DAP12 and DARs likely participate in the regulation of bony remodeling.

Nasu-Hakola disease (polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy--PLOSL): a dementia associated with bone cystic lesions. From clinical to genetic and molecular aspects

The authors review the clinical, radiological, electrophysiological, pathological, and molecular aspects of Nasu-Hakola disease (polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy or PLOSL). Nasu-Hakola disease is a unique disease characterized by multiple bone cysts associated with a peculiar form of neurodegeneration that leads to dementia and precocious death usually during the fifth decade of life. The diagnosis can be established on the basis of clinical and radiological findings. Recently, molecular analysis of affected families revealed mutations in the DAP12 (TYROBP) or TREM2 genes, providing an interesting example how mutations in two different subunits of a multi-subunit receptor complex result in an identical human disease phenotype. The association of PLOSL with mutations in the DAP12 or TREM2 genes has led to improved diagnosis of affected individuals. Also, the possible roles of the DAP12/TREM2 signaling pathway in microglia and osteoclasts in humans are just beginning to be elucidated. Some aspects of this peculiar signaling pathway are discussed here.

Characterization of the CD200 receptor family in mice and humans and their interactions with CD200

CD200 (OX2) is a broadly distributed cell surface glycoprotein that interacts with a structurally related receptor (CD200R) expressed on rodent myeloid cells and is involved in regulation of macrophage function. We report the first characterization of human CD200R (hCD200R) and define its binding characteristics to hCD200. We also report the identification of a closely related gene to hCD200R, designated hCD200RLa, and four mouse CD200R-related genes (termed mCD200RLa-d). CD200, CD200R, and CD200R-related genes were closely linked in humans and mice, suggesting that these genes arose by gene duplication. The distributions of the receptor genes were determined by quantitative RT-PCR, and protein expression was confirmed by a set of novel mAbs. The distribution of mouse and human CD200R was similar, with strongest labeling of macrophages and neutrophils, but also other leukocytes, including monocytes, mast cells, and T lymphocytes. Two mCD200 receptor-like family members, designated mCD200RLa and mCD200RLb, were shown to pair with the activatory adaptor protein, DAP12, suggesting that these receptors would transmit strong activating signals in contrast to the apparent inhibitory signal delivered by triggering the CD200R. Despite substantial sequence homology with mCD200R, mCD200RLa and mCD200RLb did not bind mCD200, and presently have unknown ligands. The CD200 receptor gene family resembles the signal regulatory proteins and killer Ig-related receptors in having receptor family members with potential activatory and inhibitory functions that may play important roles in immune regulation and balance. Because manipulation of the CD200-CD200R interaction affects the outcome of rodent disease models, targeting of this pathway may have therapeutic utility.

The contribution of glycoprotein VI to stable platelet adhesion and thrombus formation illustrated by targeted gene deletion

Platelet interaction with exposed adhesive ligands at sites of vascular injury is required to initiate a normal hemostatic response and may become a pathogenic factor in arterial diseases leading to thrombosis. We report a targeted disruption in a key receptor for collagen-induced platelet activation, glycoprotein (GP) VI. The breeding of mice with heterozygous GP VI alleles produced the expected frequency of wild-type, heterozygous, and homozygous genotypes, indicating that these animals had no reproductive problems and normal viability. GP VInull platelets failed to aggregate in response to type I fibrillar collagen or convulxin, a snake venom protein and known platelet agonist of GP VI. Nevertheless, tail bleeding time measurements revealed no severe bleeding tendency as a consequence of GP VI deficiency. Ex vivo platelet thrombus formation on type I collagen fibrils was abolished using blood from either GP VInull or FcR-gammanull animals. Reflection interference contrast microscopy revealed that the lack of thrombus formation by GP VInull platelets could be linked to a defective platelet activation following normal initial tethering to the surface, visualized as lack of spreading and less stable adhesion. These results illustrate the role of GP VI in postadhesion events leading to the development of platelet thrombi on collagen fibrils.

Dendritic cell immunoactivating receptor, a novel C-type lectin immunoreceptor, acts as an activating receptor through association with Fc receptor gamma chain

An increasing number of C-type lectin receptors are being discovered on dendritic cells, but their signaling abilities and underlying mechanisms require further definition. Among these, dendritic cell immunoreceptor (DCIR) induces negative signals through an inhibitory immunoreceptor tyrosine-based inhibitory motif (ITIM) in its cytoplasmic tail. Here we identify a novel C-type lectin receptor, dendritic cell immunoactivating receptor (DCAR), whose extracellular lectin domain is highly homologous to that of DCIR. DCAR is expressed similarly in tissues to DCIR, but its short cytoplasmic portion lacks signaling motifs like ITIM. However, a positively charged arginine residue is present in the transmembrane region of the DCAR, which may explain its association with Fc receptor gamma chain and its stable expression on the cell surface. Furthermore, cross-linking of DCAR in the presence of gamma chain activates calcium mobilization and tyrosine phosphorylation of cellular proteins. These signals are mediated by the immunoreceptor tyrosine-based activating motif (ITAM) of the gamma chain. Thus, DCAR is closely related to DCIR, but it introduces activating signals into antigen-presenting cells through its physical and functional association with ITAM-bearing gamma chain. The identification of this activating immunoreceptor provides an example of signaling via a dendritic cell-expressed C-type lectin receptor.

Paired activating and inhibitory immunoglobulin-like receptors, MAIR-I and MAIR-II, regulate mast cell and macrophage activation

Immune responses are regulated by opposing positive and negative signals triggered by the interaction of activating and inhibitory cell surface receptors with their ligands. Here, we describe novel paired activating and inhibitory immunoglobulin-like receptors, designated myeloid-associated immunoglobulin-like receptor (MAIR) I and MAIR-II, whose extracellular domains are highly conserved by each other. MAIR-I, expressed on the majority of myeloid cells, including macrophages, granulocytes, mast cells, and dendritic cells, contains the tyrosine-based sorting motif and the immunoreceptor tyrosine-based inhibitory motif-like sequences in the cytoplasmic domain and mediates endocytosis of the receptor and inhibition of IgE-mediated degranulation from mast cells. On the other hand, MAIR-II, expressed on subsets of peritoneal macrophages and B cells, associates with the immunoreceptor tyrosine-based activation motif-bearing adaptor DAP12 and stimulates proinflammatory cytokines and chemokine secretions from macrophages. Thus, MAIR-I and MAIR-II play important regulatory roles in cell signaling and immune responses.

TREMs in the immune system and beyond

Triggering receptors expressed by myeloid cells (TREMs) belong to a rapidly expanding family of receptors that include activating and inhibitory isoforms encoded by a gene cluster linked to the MHC. TREM1 and TREM2 activate myeloid cells by signalling through the adaptor protein DAP12. TREM1 triggers phagocyte secretion of pro-inflammatory chemokines and cytokines, amplifying the inflammation that is induced by bacteria and fungi. TREM2 activates monocyte-derived dendritic cells and regulates osteoclast development. Remarkably, TREM2 deficiency leads to a severe disease that is characterized by bone cysts and demyelination of the central nervous system, which results in dementia, implying that the function of TREM2 extends beyond the immune system.

Modulation of hepatic granulomatous responses by transgene expression of DAP12 or TREM-1-Ig molecules

DAP12 (also known as KARAP) is a novel ITAM-bearing transmembrane adapter molecule that is expressed on the cell surface of natural killer cells, monocytes, dendritic cells, and macrophages. Several myeloid cell-specific DAP12-associating receptors, such as TREM receptor family, SIRP-beta1, and MDL-1 have been identified. The in vivo function of DAP12 and its associating molecules in inflammation has remained primarily unknown. To investigate DAP12 signaling during chronic inflammation, we constructed two adenoviral gene transfer vectors to express FLAG/DAP12 (Ad-FDAP12) and the extracellular domain of mouse TREM-1 and the Fc portion of human IgG1 (Ad-TREM-1 Ig), respectively, and observed their modulatory activities in a mouse model of hepatic granulomatous inflammation elicited by zymosan A. Mice were injected with zymosan A intravenously and 24 hours after zymosan A, they were injected with Ad-FDAP12 or Ad-TREM-1 Ig. Zymosan A-induced hepatic granuloma formation peaked at day 7 and markedly declined by day 10. Although adenoviral-mediated DAP12 gene transfer did not enhance granuloma formation by day 7, it sustained and enhanced granuloma formation beyond day 7. However, an anti-FLAG monoclonal antibody used to potentiate the signaling of adenoviral-derived DAP12, enhanced granuloma formation at day 7. In sharp contrast to the effect by Ad-FDAP12, transgene expression in the liver of soluble form of extracellular domain of TREM-1 as an antagonist of DAP12 signaling, remarkably inhibited zymosan A-induced granuloma formation at all time points examined. Our findings thus suggest that both DAP12 and TREM-1 are involved in the development of granulomatous responses in the liver.

Identification of the genes differentially expressed in human dendritic cell subsets by cDNA subtraction and microarray analysis

Recent studies on dendritic cell (DC)–associated genes have been performed using monocyte-derived DCs (MoDCs) in different maturation stages. In our approach, to uncover the novel DC-associated genes and their expression profiles among the different DC subsets, we constructed a subtracted DC-cDNA library from CD1a+, CD14+, and CD11c− DCs by subtracting the genes shared with T cells, B cells, and monocytes, and we then screened the libraries with the aid of microarray technique. The genes showing remarkable specificity to DCs in the microarray analysis were selected and confirmed by semiquantitative reverse transcriptase–polymerase chain reaction. Our investigations revealed the following: (1) Genes highly expressed in myeloid DCs are those involved in antigen uptake/processing/presentation, cell metamorphosis, or chemotaxis. (2) Most of the genes previously identified in MoDCs, such as TARC, ferritin L-chain, lysosomal acid lipase, α- and β-tubulin, osteopontin (Eta-1), and others, are not markedly expressed in CD11c− DCs regardless of their maturation status. On the other hand, specific transcription factors and MHC class II molecules, such as interferon regulatory factor-4 (IRF4) and HLA-DR, are similarly expressed in both DC subsets. (3) CD14+ DCs retain unique features of tissue DCs, as evidenced by the gene expression profile of “no CCR7 but more CCR1” and “no TARC but abundant MCP1 and Eta-1.” (4) The genes for immunoglobulin (Ig) superfamily Z39Ig, CD20-like precursor, glycoprotein NMB (GPNMB), transforming growth factorβ (TGF-β)–induced protein (TGFBI), myeloid DAP12-associated lectin (MDL-1), and 6 novel genes are newly identified as being associated with the phenotypic expression of the DC subsets. These identifications provide important molecular information for further functional studies of the DC subsets.

Mutations in two genes encoding different subunits of a receptor signaling complex result in an identical disease phenotype

Polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy (PLOSL), also known as "Nasu-Hakola disease," is a globally distributed recessively inherited disease leading to death during the 5th decade of life and is characterized by early-onset progressive dementia and bone cysts. Elsewhere, we have identified PLOSL mutations in TYROBP (DAP12), which codes for a membrane receptor component in natural-killer and myeloid cells, and also have identified genetic heterogeneity in PLOSL, with some patients carrying no mutations in TYROBP. Here we complete the molecular pathology of PLOSL by identifying TREM2 as the second PLOSL gene. TREM2 forms a receptor signaling complex with TYROBP and triggers activation of the immune responses in macrophages and dendritic cells. Patients with PLOSL have no defects in cell-mediated immunity, suggesting a remarkable capacity of the human immune system to compensate for the inactive TYROBP-mediated activation pathway. Our data imply that the TYROBP-mediated signaling pathway plays a significant role in human brain and bone tissue and provide an interesting example of how mutations in two different subunits of a multisubunit receptor complex result in an identical human disease phenotype.

Ly-49s3 is a promiscuous activating rat NK cell receptor for nonclassical MHC class I-encoded target ligands

Previous studies of the rapid rejection of MHC-disparate lymphocytes in rats, named allogeneic lymphocyte cytotoxicity, have indicated that rat NK cells express activating receptors for nonclassical MHC class I allodeterminants from the RT1-C/E/M region. Using an expression cloning system that identifies activating receptors associated with the transmembrane adapter molecule DAP12, we have cloned a novel rat Ly-49 receptor that we have termed Ly-49 stimulatory receptor 3 (Ly-49s3). A newly generated anti-Ly-49s3 Ab, mAb DAR13, identified subpopulations of resting and IL-2-activated NK cells, but not T or B lymphocytes. Depletion of Ly-49s3-expressing NK cells drastically reduced alloreactivity in vitro, indicating that this subpopulation is responsible for a major part of the observed NK alloreactivity. DAR13-mediated blockade of Ly-49s3 inhibited killing of MHC-congenic target cells from the av1, n, lv1, and c haplotypes, but not from the u or b haplotypes. A putative ligand was mapped to the nonclassical MHC class I region (RT1-C/E/M) using intra-MHC recombinant strains. Relative numbers of Ly-49s3(+) NK cells were reduced, and surface levels of Ly-49s3 were lower, in MHC congenic strains expressing the putative Ly-49s3 ligand(s). In conclusion, we have identified a novel Ly-49 receptor that triggers rat NK cell-mediated responses.

Functional modulation of expanded CD8+ synovial fluid T cells by NK cell receptor expression in HLA-B27-associated reactive arthritis

The aim of this study was to determine whether NK cell receptor (NKR) expression could modulate cytotoxicity of oligoclonal CD8+ T cells present in the synovial fluid (SF) of HLA-B27-reactive arthritis (ReA) patients, especially in a TCRBV1 population shared among different patients and cytotoxic toward HLA-B27. A CD8+ T cell line, two TCRBV1 lines and clones were isolated from the SF of an HLA-B27+ ReA patient, and tested with mAb specific for Ig-like (KIR2DL1, KIR2DL2, KIR3DL1 and ILT2) and CD94 C-type lectin NKR. Transcripts for NKG2 subunits (NKG2A-2E) associated with CD94 were also evaluated. Function was tested in a 51Cr-release cytotoxic assay. We found stable but distinct levels of CD94/NKG2 complexes at the surface of T cell lines and clones. Different NKG2 members could be associated with CD94, either inhibitory (NKG2A/B) or activating (NKG2C). The inhibitory ILT2 receptor could also be differently expressed, but other Ig-like NKR were negative. Functionally, one TCRBV1 line and clones with a high CD94/NKG2A expression did not lyse B27+ targets. Another TCRBV1 line with the same TCRBV1 rearrangement had a low expression of CD94/NKG2A, but expressed NKG2C transcripts and was cytotoxic toward HLA-B27. HLA-B27 is a ligand for ILT2 and we observed an inhibitory effect of ILT2 engagement on B*2705 targets in blockade experiments. Altogether, these data indicate a high degree of heterogeneity in the expression of NKR by intrasynovial CD8+ T cells which could modulate their cytotoxicity and play a role in the control of this HLA class I-associated autoimmune disease.

TREM-1, MDL-1, and DAP12 expression is associated with a mature stage of myeloid development

The triggering receptor expressed on myeloid cells (TREM-1) and the myeloid DAP12-associating lectin (MDL-1) are two recently identified receptors which associate non-covalently with DAP12 to form receptor complexes involved in monocytic activation and inflammatory response. In this study, we investigated whether the expression of TREM-1, MDL-1, and DAP12 correlated with myelomonocytic differentiation. Northern and RT-PCR revealed a strong expression of TREM-1, MDL-1, and DAP12 in peripheral blood-derived CD14(+) mature monocytes in contrast to undifferentiated bone marrow CD34(+) stem cells, and in the differentiated versus undifferentiated U937 cells. TREM-1 and MDL-1 RNA expression was also more elevated in adult than fetal tissues and in normal than malignant cells. These findings suggest that the TREM-1/DAP12 and MDL-1/DAP12 signaling pathways are features of mature differentiated myelomonocytic cells. In addition, expression of an alternative mRNA TREM-1 splice variant (TREM-1sv) was detected that might translate into a soluble receptor with potential as a regulator of myeloid activation.

DAP12 ITAM motif regulates differentiation and apoptosis in M1 leukemia cells

DAP12 is an immunoreceptor tyrosine-based activation motif (ITAM)-bearing transmembrane adapter molecule that is associated with the NK-activating receptors. DAP12 is expressed not only in NK cells, but also in myeloid cells. Previously, we reported that DAP12 was likely to be involved in monocyte differentiation to macrophage. In this study, we established the mutant DAP12-M1 transfectants (Y76F-M1) that have mutation at their ITAM motifs. We observed that Y76F-M1 cells could not differentiate to macrophages by stimulation via DAP12, whereas wild type DAP12 transfectants (FDAP-M1) could. Furthermore, we demonstrated that the apoptosis signal mediated by LPS was inhibited in Y76F-M1 cells, but was augmented in FDAP-M1 cells. In contrast to the LPS-mediated apoptosis, the combination of LPS and DAP12 stimulation showed good cell viability in FDAP-M1 cells. Collectively our studies demonstrated that DAP12 has a critical role for macrophage differentiation and LPS induced apoptosis in M1 leukemia cells.

HIV gp120 receptors on human dendritic cells

Dendritic cells (DCs) are important targets for human immunodeficiency virus (HIV) because of their roles during transmission and also maintenance of immune competence. Furthermore, DCs are a key cell in the development of HIV vaccines. In both these settings the mechanism of binding of the HIV envelope protein gp120 to DCs is of importance. Recently a single C-type lectin receptor (CLR), DC-SIGN, has been reported to be the predominant receptor on monocyte-derived DCs (MDDCs) rather than CD4. In this study a novel biotinylated gp120 assay was used to determine whether CLR or CD4 were predominant receptors on MDDCs and ex vivo blood DCs. CLR bound more than 80% of gp120 on MDDCs, with residual binding attributable to CD4, reconfirming that CLRs were the major receptors for gp120 on MDDCs. However, in contrast to recent reports, gp120 binding to at least 3 CLRs was observed: DC-SIGN, mannose receptor, and unidentified trypsin resistant CLR(s). In marked contrast, freshly isolated and cultured CD11c(+ve) and CD11c(-ve) blood DCs only bound gp120 via CD4. In view of these marked differences between MDDCs and blood DCs, HIV capture by DCs and transfer mechanisms to T cells as well as potential antigenic processing pathways will need to be determined for each DC phenotype.

A DAP12-mediated pathway regulates expression of CC chemokine receptor 7 and maturation of human dendritic cells

Gene targeting of the adaptor molecule DAP12 in mice caused abnormal distribution and impaired antigen presentation capacity of dendritic cells (DCs). However, the DAP12-associated receptors expressed on DCs and their functions have not been identified yet. Here we show that the triggering receptor expressed on myeloid cells-2 (TREM-2) is a cell surface receptor on human monocyte-derived DCs, which is associated with DAP12. TREM-2/DAP12 promotes upregulation of CC chemokine receptor 7, partial DC maturation, and DC survival through activation of protein tyrosine kinases and extracellular signal-regulated kinase. In contrast to Toll-like receptor-mediated signaling, TREM2/DAP12 stimulation is independent of nuclear factor-kappaB and p38 stress-activated protein kinase. This novel DC activation pathway may regulate DC homeostasis and amplify DC responses to pathogens, explaining the phenotype observed in DAP12-deficient mice.

Bitter-sweet symphony: defining the role of dendritic cell gp120 receptors in HIV infection

BACKGROUND

Dendritic cells (DC) are believed to be one of the first cell types infected during HIV transmission. Recently a single C-type lectin receptor (CLR), DC-SIGN, has been reported to be the predominant receptor on monocyte derived DC (MDDC) rather than CD4. The role of other CLRs in HIV binding and HIV binding by CLRs on other types of DC in vivo is largely unknown.

OBJECTIVES AND STUDY DESIGN

Review HIV binding to DC populations, both in vitro and in vivo, in light of the immense interest of a recently re-identified CLR called DC-SIGN.

RESULTS AND CONCLUSIONS

From recent work, it is clear that immature MDDC have a complex pattern of HIV gp120 binding. In contrast to other cell types gp120 has the potential to bind to several receptors on DC including CD4 and several types of C type lectin receptor, not just exclusively DC-SIGN. Given the diverse types of DC in vivo future work will need to focus on defining the receptors for HIV binding to these different cell types. Mucosal transmission of HIV in vivo targets immature sessile DCs, including Langerhans cells which lack DC-SIGN. The role of CLRs and DC-SIGN in such transmission remains to be defined.