The TREM receptor family and signal integration

Bilateral Alterations in Corneal Nerves, Dendritic Cells, and Tear Cytokine Levels in Ocular Surface Disease

This review summarizes the recent literature regarding corneal imaging in human subjects using in vivo confocal microscopy. It also covers the recent literature on corneal immune cells, nerves, and tear cytokine levels in ocular surface diseases as well as corneal immune privilege. The significance of interactions between corneal immune cells and nerves in health, neurotrophic keratopathy, and infectious keratitis is discussed. Furthermore, bilateral alterations of immune cells and nerves in clinically unilateral corneal diseases and the link to changes of tear cytokines or neuropeptide levels in contralateral eyes are described. Recent studies reported increased density and morphologic changes of corneal dendritic cells in ocular surface disease that correlated with a decrease in subbasal nerve and corneal nerve density, suggesting potential interactions between the immune and nervous systems in the cornea. Although the relevance of tear cytokines is poorly understood, tear cytokines might have an important role in the pathogenesis of ocular surface diseases. In humans and experimental animal models, alterations in immune cells, cytokines, and immunomodulatory neuropeptide levels in contralateral eyes might mediate the incidence of bilateral infectious keratitis and loss of immune privilege of the cornea in bilateral corneal transplantation or neurotrophic keratopathy cases. The discovery of bilateral alterations of immune cells and nerves in ocular surface diseases is considered the missing link between the immune and nervous systems in the cornea, and demonstrates how studies of animal models and humans aid our understanding of human corneal disease phenomena.

TREM1: A Potential Therapeutic Target For Alzheimer's Disease

Immunity has been suggested to play crucial roles in the pathogenesis of Alzheimer's disease (AD). The triggering receptor expressed on myeloid cells-1 (TREM1), a member of the immunoglobulin superfamily of receptors, is widely expressed in monocytes and microglia. On the other hand, TREM1 variant, rs6910730^G, is reported to associate with AD pathology; however, the exact mechanism is not yet clear. Since phagocytosis of Aβ by monocytes enhances Aβ clearance and attenuates AD pathogenesis, Jiang et al. has investigated if TREM1 can modulate Aβ phagocytosis and degradation by monocytes in the central nervous system (CNS). They found that TREM1 facilitates microglial Aβ phagocytosis while rs6910730^G impairs this function and exacerbates AD pathogenesis. These findings suggest that TREM1 can be implemented investigated as a potential therapeutic target in AD.

Divergent Neuroinflammatory Regulation of Microglial TREM Expression and Involvement of NF-κB

The triggering receptor expressed on myeloid cells (TREM) family of proteins are cell surface receptors with important roles in regulation of myeloid cell inflammatory activity. In the central nervous system, TREM2 is implicated in further roles in microglial homeostasis, neuroinflammation and neurodegeneration. Different TREM receptors appear to have contrasting roles in controlling myeloid immune activity therefore the relative and co-ordinated regulation of their expression is important to understand but is currently poorly understood. We sought to determine how microglial TREM expression is affected under neuroinflammatory conditions in vitro and in vivo. Our data show that microglial Trem1 and Trem2 gene expression are regulated in an opposing manner by lipopolysaccharide (LPS) in vitro in both adult murine and human microglia. LPS caused a significant induction of Trem1 and a contrasting suppression of Trem2 expression. We also observed similar divergent Trem1 and Trem2 responses in vivo in response to acute brain inflammation and acute cerebral ischaemia. Our data show that inhibition of NF-κB activation prevents the LPS-induced alterations in both Trem1 and Trem2 expression in vitro indicating NF-κB as a common signaling intermediate controlling these divergent responses. Distinct patterns of microglial Trem1 induction and Trem2 suppression to different Toll-like receptor (TLR) ligands were also evident, notably with Trem1 induction restricted to those ligands activating TLRs signaling via TRIF. Our data show co-ordinated but divergent regulation of microglial TREM receptor expression with a central role for NF-κB. Neuroinflammatory conditions that alter the balance in TREM expression could therefore be an important influence on microglial inflammatory and homeostatic activity with implications for neuroinflammatory and neurodegenerative disease.

Modular Activating Receptors in Innate and Adaptive Immunity

Triggering of cell-mediated immunity is largely dependent on the recognition of foreign or abnormal molecules by a myriad of cell surface-bound receptors. Many activating immune receptors do not possess any intrinsic signaling capacity but instead form noncovalent complexes with one or more dimeric signaling modules that communicate with a common set of kinases to initiate intracellular information-transfer pathways. This modular architecture, where the ligand binding and signaling functions are detached from one another, is a common theme that is widely employed throughout the innate and adaptive arms of immune systems. The evolutionary advantages of this highly adaptable platform for molecular recognition are visible in the variety of ligand-receptor interactions that can be linked to common signaling pathways, the diversification of receptor modules in response to pathogen challenges, and the amplification of cellular responses through incorporation of multiple signaling motifs. Here we provide an overview of the major classes of modular activating immune receptors and outline the current state of knowledge regarding how these receptors assemble, recognize their ligands, and ultimately trigger intracellular signal transduction pathways that activate immune cell effector functions.

Psychosocial stress on neuroinflammation and cognitive dysfunctions in Alzheimer's disease: the emerging role for microglia?

Chronic psychosocial stress is increasingly recognized as a risk factor for late-onset Alzheimer's disease (LOAD) and associated cognitive deficits. Chronic stress also primes microglia and induces inflammatory responses in the adult brain, thereby compromising synapse-supportive roles of microglia and deteriorating cognitive functions during aging. Substantial evidence demonstrates that failure of microglia to clear abnormally accumulating amyloid-beta (Aβ) peptide contributes to neuroinflammation and neurodegeneration in AD. Moreover, genome-wide association studies have linked variants in several immune genes, such as TREM2 and CD33, the expression of which in the brain is restricted to microglia, with cognitive dysfunctions in LOAD. Thus, inflammation-promoting chronic stress may create a vicious cycle of aggravated microglial dysfunction accompanied by increased Aβ accumulation, collectively exacerbating neurodegeneration. Surprisingly, however, little is known about whether and how chronic stress contributes to microglia-mediated neuroinflammation that may underlie cognitive impairments in AD. This review aims to summarize the currently available clinical and preclinical data and outline potential molecular mechanisms linking stress, microglia and neurodegeneration, to foster future research in this field.

Biomarkers of multiple sclerosis: current findings

Multiple sclerosis (MS) is an autoimmune disease affecting the brain and spinal cord that is associated with chronic inflammation leading to demyelination and neurodegeneration. With the recent increase in the number of available therapies for MS, optimal treatment will be based on a personalized approach determined by an individual patient's prognosis and treatment risks. An integral part of such therapeutic decisions will be the use of molecular biomarkers to predict disability progression, monitor ongoing disease activity, and assess treatment response. This review describes current published findings within the past 3 years in biomarker research in MS, specifically highlighting recent advances in the validation of cerebrospinal fluid biomarkers such as neurofilaments (light and heavy chains), chitinases and chitinase 3-like proteins, soluble surface markers of innate immunity, and oligoclonal immunoglobulin M antibodies. Current research in circulating miRNAs as biomarkers of MS is also discussed. Continued validation and testing will be required before MS biomarkers are routinely applied in a clinical setting.

The APOE ε4 genotype modulates CSF YKL-40 levels and their structural brain correlates in the continuum of Alzheimer's disease but not those of sTREM2

INTRODUCTION

Among other metabolic functions, the apolipoprotein E (APOE) plays a crucial role in neuroinflammation. We aimed at assessing whether APOE ε4 modulates levels of glial cerebrospinal fluid (CSF) biomarkers and their structural cerebral correlates along the continuum of Alzheimer's disease (AD).

METHODS

Brain magnetic resonance imaging (MRI) scans were acquired in 110 participants (49 control; 19 preclinical; 27 mild cognitive impairment [MCI] due to AD; 15 mild AD dementia) and CSF concentrations of YKL-40 and sTREM2 were determined. Differences in CSF biomarker concentrations and interactions in their association with gray-matter volume according to APOE ε4 status were sought after.

RESULTS

Preclinical and MCI carriers showed higher YKL-40 levels. There was a significant interaction in the association between YKL-40 levels and gray-matter volume according to ε4 status. No similar effects could be detected for sTREM2 levels.

DISCUSSION

Our findings are indicative of an increased astroglial activation in APOE ε4 carriers while both groups displayed similar levels of CSF AD core biomarkers.

Enhanced expression of TREM-1 in splenic cDCs in lupus prone mice and it was modulated by miRNA-150

Over activation of conventional dendritic cells (cDCs) contributes to the development of systemic lupus erythematosus (SLE). Triggering receptor expressed on myeloid cells 1 (TREM-1) is emerging as a potent amplifier of the inflammatory responses. We sought to determine the expression level of TREM-1 on cDCs in a mice model of SLE and to identify miRNA which could modulate TREM-1 expression. In the present study, TREM-1 expression in splenocytes and on cDCs was strongly up-regulated in vivo, and was enhanced with LPS stimulation in vitro. Blockade of TREM-1 signal impaired the TLR4-induced cytokines production. These indicated that TREM-1 potently amplified the function of TLR4 which enhanced the inflammation responses. A common set of dysregulated miRNAs (miRNA-98, -150 and -494) were identified in splenocytes of mice. Moreover, the results of bioinformatics and the immunoblotting, demonstrated that miRNA-150 inhibited the expression of TREM-1. Together, these data suggested that TREM-1 signaling pathway may be a therapeutic target to prevent the effects of the inflammatory cDCs in SLE and miRNA-150 serves as the important regulator.

Expression and processing analyses of wild type and p.R47H TREM2 variant in Alzheimer's disease brains

Background

Genetic analyses showed that the triggering receptor expressed in myeloid cells 2 (TREM2) p.R47H variant increases the risk for Alzheimer’s disease (AD). The question of whether the p.R47H mutation affects expression or function of the receptor remains unanswered. To address this question we quantified mRNA and analyzed protein profiles of WT and p.R47H TREM2 in human brains.

Methods

Quantitative real-time PCR (qPCR) was performed using 2 sets of primers one that detects all TREM2 mRNA isoforms and one specific for the alternative spliced isoform (TREM2alt) that encodes for the extracellular domain (soluble TREM2). Because in the brain TREM2 is expressed primarily in microglial cells, we also assessed the levels of IBA1 to control for microglial variability across samples. For TREM2 protein quantitation and N-glycosylation processing, RIPA brain extracts were analyzed by Western blot before and after EndoH and PNGaseF treatments.

Results

We identified statistically significant increased levels of TREM2 transcripts in the temporal cortex of AD subjects when compared with controls; TREM2alt was likewise higher in AD cases, but was not significant after adjustment for covariates. Quantitative analysis of TREM2 protein confirmed qPCR results that showed higher levels in AD than in control brains. Among AD subjects, we observed a trend towards higher mRNA and protein TREM2 levels in carriers of the p.R47H risk allele. Analysis of individual TREM2 species found no difference in the relative amounts of mature and immature species, and carboxyl terminal fragments between non carriers and p.R47H samples. Furthermore, TREM2 species from either non carriers or p.R47H brains were equally susceptible to EndoH and PNGaseF treatments.

Conclusions

Our results suggest that TREM2 expression is increased in AD. Furthermore, we provide evidence indicating that p.R47H mutation does not affect the levels of TREM2 either directly by altering expression or indirectly by affecting processing of the protein. Our data support previous findings that suggest that p.R47H variant affects TREM2 function by altering binding properties of the receptor rather than expression.

Triggering receptor expressed on myeloid cells and 5'adenosine monophosphate-activated protein kinase in the inflammatory response: a potential therapeutic target

INTRODUCTION

The events in the cellular and molecular signaling triggered during inflammation mitigate tissue healing. The metabolic check-point control mediated by 5'-adenosine monophosphate-activated protein kinase (AMPK) is crucial for switching the cells into an activated state capable of mediating inflammatory events. The cell metabolism involved in the inflammatory response represents a potential therapeutic target for the pharmacologic management of inflammation. Areas covered: In this article, a critical review is presented on triggering receptor expressed on myeloid cell (TREM) receptors and their role in the inflammatory responses, as well as homeostasis between different TREM molecules and their regulation. Additionally, we discussed the relationship between TREM and AMPK to identify novel targets to limit the inflammatory response. Literature search was carried out from the National Library of Medicine's Medline database (using PubMed as the search engine) and Google Scholar and identified relevant studies up to 30 March 2016 using inflammation, TREM, AMPK, as the key words. Expert commentary: The prevention of phenotype switching of immune cells during inflammation by targeting AMPK and TREM-1 could be beneficial for developing novel management strategies for inflammation and associated complications.

TREM1 facilitates microglial phagocytosis of amyloid beta

As the most common type of neurodegenerative disease, Alzheimer's disease (AD) is characterized by the accumulation of amyloid-β peptide (Aβ) within the brain. Triggering receptor expressed on myeloid cells (TREM) 1 is an immune receptor expressed by mononuclear phagocytes including monocytes and microglia, coupling with TYRO protein tyrosine kinase binding protein to regulate immune reactions. Emerging evidence indicates that rs6910730^G, an intronic variant of TREM1, is associated with an increased Aβ neuropathology in the brains of elderly subjects, but the underlying mechanisms remain unclear. Here, using two independent cohorts of healthy individuals, we provided evidence that rs6910730^G reduced the ability of human monocytes for Aβ phagocytosis, and this reduction was likely attributed to a decreased monocytic TREM1 expression. By knockdown and overexpression of Trem1 in mouse primary microglia, we showed that TREM1 facilitated microglial phagocytosis of Aβ. In support of this finding, knockdown of Trem1 in the brains of APP/PSEN1 mice increased Aβ_1-42 levels and total amyloid burden, whereas selective overexpression of Trem1 on microglia or activation of Trem1 signaling by an agonistic antibody ameliorated Aβ neuropathology and rescued AD-related spatial cognitive impairments. Altogether, these findings uncover the role of TREM1 in microglial Aβ clearance, and establish TREM1 as a potential therapeutic target for AD.

TREM-1 links dyslipidemia to inflammation and lipid deposition in atherosclerosis

Triggering receptor expressed on myeloid cells-1 (TREM-1) is a potent amplifier of pro-inflammatory innate immune responses, but its significance in non-infectious diseases remains unclear. Here, we demonstrate that TREM-1 promotes cardiovascular disease by exacerbating atherosclerosis. TREM-1 is expressed in advanced human atheromas and is highly upregulated under dyslipidemic conditions on circulating and on lesion-infiltrating myeloid cells in the Apoe^−/− mouse model. TREM-1 strongly contributes to high-fat, high-cholesterol diet (HFCD)-induced monocytosis and synergizes with HFCD serum-derived factors to promote pro-inflammatory cytokine responses and foam cell formation of human monocyte/macrophages. Trem1^−/−Apoe^−/− mice exhibit substantially attenuated diet-induced atherogenesis. In particular, our results identify skewed monocyte differentiation and enhanced lipid accumulation as novel mechanisms through which TREM-1 can promote atherosclerosis. Collectively, our findings illustrate that dyslipidemia induces TREM-1 surface expression on myeloid cells and subsequently synergizes with TREM-1 to enhance monopoiesis, pro-atherogenic cytokine production and foam cell formation.

TREM-1 is a receptor that amplifies acute pro-inflammatory responses in infection. Here the authors show that TREM-1 plays an important role in atherosclerosis, a chronic and non-infectious disease, by critically skewing myelopoiesis towards preferential monocyte differentiation and by contributing to CD36-driven cellular lipid accumulation.

Depletion of the triggering receptor expressed on myeloid cells 2 inhibits progression of renal cell carcinoma via regulating related protein expression and PTEN-PI3K/Akt pathway

The triggering receptor expressed on myeloid cells 2 (TREM-2) is suggested to be involved in the development of certain human malignancies. However, the functions of TREM-2 in renal cell carcinoma (RCC) are still less known. To reveal the effects of TREM-2 on the RCC progression, we examined the TREM-2 expression in RCC tumor tissues. Then, we analyzed the cell proliferation, cell apoptosis, cell cycle and expression of the relative factors in two selected RCC cell lines post RNA interference. We also analyzed the functions of TREM-2 in an in vivo nude mouse model. We found that, the expression of TREM-2 was abnormally elevated in RCC tumor tissues. Silencing TREM-2 inhibited cell growth, induced G1 phase arrest of cell cycle and cell apoptosis in RCC cells. In vivo, the results showed that depletion of TREM-2 significantly inhibited the ACHN tumor growth in the nude mouse model. The analysis of relative protein factors suggested that silencing TREM-2 downregulated the expression levels of Bcl2 and PCNA, and upregulated the expression levels of Bax and caspase-3 in RCC cell lines. Depletion of TREM-2 inactivated PI3K/Akt pathway through increasing the expression of PTEN. Taken together, TREM-2 acts as an oncogene in the development of RCC and can be considered as a novel therapeutic factor in the treatment of RCC.

Impact of aging on TREM-1 responses in the periodontium: a cross-sectional study in an elderly population

Background

Aging is associated with altered immune response, which increases susceptibility to infections. sTREM-1 is involved in the amplification of the inflammatory response to bacterial infection. The present cross-sectional study aims to investigate local sTREM-1 levels in gingival crevicular fluid (GCF) as well as key periodontal pathogen levels in the subgingival plaque in an elderly cohort with periodontal health, gingivitis, and chronic periodontitis (CP).

Methods

Subjects were 51 systemically healthy, elderly individuals (mean age, 68 ± 4.5 years) who had undergone full-mouth periodontal examinations. Subgingival plaque and GCF samples were collected from the healthy sites of participants without periodontal disease (n = 17), the sites with gingival inflammation from patients with gingivitis (n = 19), and the periodontitis sites of patients with CP (n = 15). GCF volumes were measured by an electronic impedance device, and total protein levels were assessed by a flouremetric assay. sTREM-1 levels in GCF were measured by enzyme-linked immunosorbent assay. The subgingival plaque total bacteria, Porphyromonas gingivalis, Tannerella forsythia, Treponema denticola, Fusobacterium nucleatum, and Prevotella intermedia levels were determined by quantitative real-time polymerase chain reaction. Statistical analysis was performed using nonparametric methods.

Results

GCF volume, total protein concentrations, and sTREM-1 levels in GCF were similar among the groups (p > 0.05). Significantly higher T. forsythia levels were observed in subgingival plaque samples harvested from patients with gingivitis and CP, than in those from healthy participants (p < 0.05). However, the subgingival levels of the other four periodontal pathogens and total bacteria were not statistically different among the groups (p > 0.05).

Conclusions

Our findings suggest that there are no differences in GCF volume, total protein, and sTREM-1 levels between healthy and periodontally diseased elderly adults. We found only limited differences in the studied subgingival microbial profile. This finding indicates an already deregulated, local inflammatory response in this elderly cohort, on which bacterial biofilm challenge may have a limited further impact.

Mechanisms of Aβ Clearance and Degradation by Glial Cells

Glial cells have a variety of functions in the brain, ranging from immune defense against external and endogenous hazardous stimuli, regulation of synaptic formation, calcium homeostasis, and metabolic support for neurons. Their dysregulation can contribute to the development of neurodegenerative disorders, including Alzheimer’s disease (AD). One of the most important functions of glial cells in AD is the regulation of Amyloid-β (Aβ) levels in the brain. Microglia and astrocytes have been reported to play a central role as moderators of Aβ clearance and degradation. The mechanisms of Aβ degradation by glial cells include the production of proteases, including neprilysin, the insulin degrading enzyme, and the endothelin-converting enzymes, able to hydrolyse Aβ at different cleavage sites. Besides these enzymes, other proteases have been described to have some role in Aβ elimination, such as plasminogen activators, angiotensin-converting enzyme, and matrix metalloproteinases. Other relevant mediators that are released by glial cells are extracellular chaperones, involved in the clearance of Aβ alone or in association with receptors/transporters that facilitate their exit to the blood circulation. These include apolipoproteins, α2macroglobulin, and α1-antichymotrypsin. Finally, astrocytes and microglia have an essential role in phagocytosing Aβ, in many cases via a number of receptors that are expressed on their surface. In this review, we examine all of these mechanisms, providing an update on the latest research in this field.

Attenuated viral hepatitis in Trem1-/- mice is associated with reduced inflammatory activity of neutrophils

TREM1 (Triggering Receptor Expressed on Myeloid Cells 1) is a pro-inflammatory receptor expressed by phagocytes, which can also be released as a soluble molecule (sTREM1). The roles of TREM1 and sTREM1 in liver infection and inflammation are not clear. Here we show that patients with hepatitis B virus (HBV) or hepatitis C virus (HCV) infection manifest elevated serum levels of sTREM1. In mice, experimental viral hepatitis induced by infection with Lymphocytic Choriomeningitis Virus (LCMV)-WE was likewise associated with increased sTREM1 in serum and urine, and with increased TREM1 and its associated adapter molecule DAP12 in the liver. Trem1−/− mice showed accelerated clearance of LCMV-WE and manifested attenuated liver inflammation and injury. TREM1 expression in the liver of wild-type mice was mostly confined to infiltrating neutrophils, which responded to LCMV by secretion of CCL2 and TNF-α, and release of sTREM1. Accordingly, the production of CCL2 and TNF-α was decreased in the livers of LCMV-infected Trem1−/− mice, as compared to LCMV-infected wildtype mice. These findings indicate that TREM1 plays a role in viral hepatitis, in which it seems to aggravate the immunopathology associated with viral clearance, mainly by increasing the inflammatory activity of neutrophils.

Triggering Receptor Expressed on Myeloid Cells (TREM)-2 Impairs Host Defense in Experimental Melioidosis

Background

Triggering receptor expressed on myeloid cells (TREM) -1 and TREM-2 are key regulators of the inflammatory response that are involved in the clearance of invading pathogens. Melioidosis, caused by the "Tier 1" biothreat agent Burkholderia pseudomallei, is a common form of community-acquired sepsis in Southeast-Asia. TREM-1 has been suggested as a biomarker for sepsis and melioidosis. We aimed to characterize the expression and function of TREM-1 and TREM-2 in melioidosis.

Methodology/Principal Findings

Wild-type, TREM-1/3 (Trem-1/3^-/-) and TREM-2 (Trem-2^-/-) deficient mice were intranasally infected with live B. pseudomallei and killed after 24, and/or 72 h for the harvesting of lungs, liver, spleen, and blood. Additionally, survival studies were performed. Cellular functions were further analyzed by stimulation and/or infection of isolated cells. TREM-1 and TREM-2 expression was increased both in the lung and liver of B. pseudomallei-infected mice. Strikingly, Trem-2^-/-, but not Trem-1/3^-/-, mice displayed a markedly improved host defense as reflected by a strong survival advantage together with decreased bacterial loads, less inflammation and reduced organ injury. Cellular responsiveness of TREM-2, but not TREM-1, deficient blood and bone-marrow derived macrophages (BMDM) was diminished upon exposure to B. pseudomallei. Phagocytosis and intracellular killing of B. pseudomallei by BMDM and alveolar macrophages were TREM-1 and TREM-2-independent.

Conclusions/Significance

We found that TREM-2, and to a lesser extent TREM-1, plays a remarkable detrimental role in the host defense against a clinically relevant Gram-negative pathogen in mice: TREM-2 deficiency restricts the inflammatory response, thereby decreasing organ damage and mortality.

Triggering receptor expressed on myeloid cells (TREM)-1 and -2 are receptors on immune cells that act as mediators of the innate immune response. It is thought that TREM-1 amplifies the immune response, while TREM-2 acts as a negative regulator. Previously, we found that TREM-1 is upregulated in melioidosis patients. In contrast, nothing is known on TREM-2 expression and its role in melioidosis. In this study we examined the expression and functional role of both TREM-1 and -2 in a murine melioidosis model. We found that TREM-1 and-2 expression was upregulated during melioidosis. Using our experimental melioidosis model, we observed that Trem-2^-/- mice were protected against B.pseudomallei-induced lethality. Trem-2^-/- mice demonstrated reduced bacterial loads, inflammation and organ damage compared to wild-type mice in experimental melioidosis. Despite reduced bacterial dissemination of B.pseudomallei to distant organs in Trem-1/3^-/ mice^-, no differences in survival were found between Trem-1/3^-/- and wild-type mice during melioidosis. Lastly, we investigated cellular functions of TREM-1 and TREM-2 and found that TREM-2 deficiency led to decreased cellular responsiveness to B. pseudomallei infection. In conclusion, we found that TREM-2 plays an important role during experimental murine melioidosis. TREM-2-deficiency reduces inflammation and organ damage, thereby improving survival.

Blockade of CD354 (TREM-1) Ameliorates Anti-GBM-Induced Nephritis

CD354, Triggering Receptor of Myeloid Cells-1 (TREM-1), is a potent amplifier of myeloid immune responses. Our goal was to determine the expression and function of TREM-1 in immune-mediated nephritis. An anti-glomerular basement membrane antibody (anti-GBM)-induced nephritis model was employed, where mice were sensitized with rabbit IgG followed by anti-GBM serum to induce disease. Anti-GBM-treated 129x1/svJ mice developed severe nephritis whereas C57BL/6 (B6) mice were resistant to disease. Anti-GBM disease resulted in elevated renal TREM-1 messenger RNA (mRNA) and protein levels and increased urine TREM-1 levels in 129x1/svJ. TREM-1 blockade with an inhibitory peptide, LP17, inhibited proteinuria and renal disease as measured by glomerulonephritis class, severity of tubulointerstitial disease, crescent formation, and inflammatory cell infiltrates. In sum, TREM-1 is upregulated in renal inflammation and plays a vital role in driving disease. Thus, TREM-1 blockade emerges as a potential therapeutic avenue for immune-mediated renal diseases such as lupus nephritis.

Dendritic Cells Expressing Triggering Receptor Expressed on Myeloid Cells-1 Correlate with Plaque Stability in Symptomatic and Asymptomatic Patients with Carotid Stenosis

Atherosclerosis is a chronic inflammatory disease with atherosclerotic plaques containing inflammatory cells, including T-lymphocytes, dendritic cells (DCs) and macrophages that are responsible for progression and destabilization of atherosclerotic plaques. Stressed cells undergoing necrosis release molecules that act as endogenous danger signals to alert and activate innate immune cells. In atherosclerotic tissue the number of DCs increases with the progression of the lesion and produce several inflammatory cytokines and growth factors. Triggering receptor expressed on myeloid cells (TREM)-1 plays a crucial role in inflammation. However, relationship of DCs and the role of TREM-1 with the stability of atherosclerotic plaques have not been examined. In this study, we investigated the heterogeneity of the plaque DCs, myeloid (mDC1 and mDC2) and plasmacytoid (pDCs), and examined the expression of TREM-1 and their co-localization with DCs in the plaques from symptomatic (S) and asymptomatic (AS) patients with carotid stenosis. We found increased expression of HLA-DR, fascin, and TREM-1 and decreased expression of TREM-2 and α-smooth muscle actin in S compared to AS atherosclerotic carotid plaques. Both TREM-1 and fascin were co-localized suggesting increased expression of TREM-1 in plaque DCs of S compared to AS patients. These data were supported by increased mRNA transcripts of TREM-1 and decreased mRNA transcripts of TREM-2 in carotid plaques of S compared to AS patients. There was higher density of both CD1c+ mDC₁ and CD141+ mDC₂ in the carotid plaques from AS compared to S patients, where as the density of CD303+ pDCs were higher in the carotid plaques of S compared to AS patients. These findings suggest a potential role of pDCs and TREM-1 in atherosclerotic plaque vulnerability. Thus, newer therapies could be developed to selectively block TREM-1 for stabilizing atherosclerotic plaques.

Double-stranded RNA analog and type I interferon regulate expression of Trem paired receptors in murine myeloid cells

Background

Triggering receptors expressed on myeloid cells (Trem) proteins are a family of cell surface receptors used to control innate immune responses such as proinflammatory cytokine production in mice. Trem genes belong to a rapidly expanding family of receptors that include activating and inhibitory paired-isoforms.

Results

By comparative genomic analysis, we found that Trem4, Trem5 and Trem-like transcript-6 (Treml6) genes typically paired receptors. These paired Trem genes were murine-specific and originated from an immunoreceptor tyrosine-based inhibition motif (ITIM)-containing gene. Treml6 encoded ITIM, whereas Trem4 and Trem5 lacked the ITIM but possessed positively-charged residues to associate with DNAX activating protein of 12 kDa (DAP12). DAP12 was directly associated with Trem4 and Trem5, and DAP12 coupling was mandatory for their expression on the cell surface. In bone marrow-derived dendritic cells (BMDCs) and macrophages (BMDMs), and splenic DC subsets, polyinosinic-polycytidylic acid (polyI:C) followed by type I interferon (IFN) production induced Trem4 and Treml6 whereas polyI:C or other TLR agonists failed to induce the expression of Trem5. PolyI:C induced Treml6 and Trem4 more efficiently in BMDMs than BMDCs. Treml6 was more potentially up-regulated in conventional DC (cDCs) and plasmacytoid DC (pDCs) than Trem4 in mice upon in vivo stimulation with polyI:C.

Discussion

Treml6-dependent inhibitory signal would be dominant in viral infection compared to resting state. Though no direct ligands of these Trem receptors have been determined, the results infer that a set of Trem receptors are up-regulated in response to viral RNA to regulate myeloid cell activation through modulation of DAP12-associated Trem4 and ITIM-containing Treml6.

Electronic supplementary material

The online version of this article (doi:10.1186/s12865-016-0147-y) contains supplementary material, which is available to authorized users.

sTREM2 cerebrospinal fluid levels are a potential biomarker for microglia activity in early-stage Alzheimer's disease and associate with neuronal injury markers

TREM2 is an innate immune receptor expressed on the surface of microglia. Loss‐of‐function mutations of TREM2 are associated with increased risk of Alzheimer's disease (AD). TREM2 is a type‐1 protein with an ectodomain that is proteolytically cleaved and released into the extracellular space as a soluble variant (sTREM2), which can be measured in the cerebrospinal fluid (CSF). In this cross‐sectional multicenter study, we investigated whether CSF levels of sTREM2 are changed during the clinical course of AD, and in cognitively normal individuals with suspected non‐AD pathology (SNAP). CSF sTREM2 levels were higher in mild cognitive impairment due to AD than in all other AD groups and controls. SNAP individuals also had significantly increased CSF sTREM2 compared to controls. Moreover, increased CSF sTREM2 levels were associated with higher CSF total tau and phospho‐tau_181P, which are markers of neuronal degeneration and tau pathology. Our data demonstrate that CSF sTREM2 levels are increased in the early symptomatic phase of AD, probably reflecting a corresponding change of the microglia activation status in response to neuronal degeneration.

Diagnostic value of soluble triggering receptor expressed on myeloid cells in paediatric sepsis: a systematic review

Background

Differential diagnosis between sepsis and non-infectious inflammatory disorders demands improved biomarkers. Soluble Triggering Receptor Expression on Myeloid cells (sTREM-1) is an activating receptor whose role has been studied throughout the last decade. We performed a systematic review to evaluate the accuracy of plasma sTREM-1 levels in the diagnosis of sepsis in children with Systemic Inflammatory Response Syndrome (SIRS).

Methods

A literature search of PubMed, Cochrane Central Register of Controlled Trials, Cumulative Index to Nursing and Allied Health Literature (CINAHL) and ISI Web of Knowledge databases was performed using specific search terms. Studies were included if they assessed the diagnostic accuracy of plasma sTREM-1 for sepsis in paediatric patients with SIRS. Data on sensitivity, specificity, positive predictive value, negative predictive value, area under receiver operating characteristic curve were extracted. The methodological quality of each study was assessed using a checklist based on the Quality Assessment Tool for Diagnostic Accuracy Studies.

Results

Nine studies comprising 961 patients were included, four of which were in newborns, three in children and two in children with febrile neutropenia. Some data from single studies support a role of sTREM-1 as a diagnostic tool in pediatric sepsis, but cannot be considered conclusive, because a quantitative synthesis was not possible, due to heterogeneity in studies design.

Conclusions

This systematic review suggests that available data are insufficient to support a role for sTREM in the diagnosis and follow-up of paediatric sepsis.

Electronic supplementary material

The online version of this article (doi:10.1186/s13052-016-0242-y) contains supplementary material, which is available to authorized users.

sTREM-1 in patients with chronic kidney disease on hemodialysis

The triggering receptor expressed on myeloid cells-1 (TREM-1) is a member of the immunoglobulin superfamily. TREM-1 has been implicated as an amplifier of inflammation. Soluble TREM-1 (sTREM-1) was investigated in different clinical conditions, but not in hemodialysis (HD) patients. We aimed to investigate sTREM-1 as a marker of inflammation in HD patients. We investigated 40 CKD patients undergoing chronic HD treatment and 15 controls. Routine laboratory investigations in addition to CRP measured by immunoturbidimetry, TNF- α, and sTREM-1 measured by ELISA were assayed in post-hemodialysis patients' blood samples and in controls' blood samples. CRP, TNF-α, and sTREM-1 levels were significantly higher in HD patients than in controls (p < 0.001 for all). sTREM-1 was positively correlated with CRP and TNF-α (r = +0.50, p < 0.001 and r = +0.53, p < 0.001 respectively). It was negatively correlated with hemoglobin concentration (r = -0.69, p < 0.001). Hemoglobin concentration was the significant predictor of sTREM-1 level (p < 0.001). In conclusion, sTREM-1 level is significantly increased in HD patients as are other pro-inflammatory markers.

Tumor necrosis factor-α regulates triggering receptor expressed on myeloid cells-1-dependent matrix metalloproteinases in the carotid plaques of symptomatic patients with carotid stenosis

OBJECTIVE

To determine the relationship between increased triggering receptor expressed on myeloid cells (TREM)-1 and plaque stability in atherosclerotic carotid stenosis.

METHODS

The mRNA transcripts and protein for TREM-1, MMP-1, MMP-9, collagen type I (COL1A1) and collagen type III (COL3A1) were analyzed by qPCR and immunofluorescence in both tissues and VSMCs isolated from atherosclerotic carotid plaques of symptomatic and asymptomatic patients with carotid stenosis.

RESULTS

The TREM-1, MMP-1 and MMP-9 mRNA transcripts were significantly increased (TREM-1, p < 0.01; MMP-1, p < 0.01 and MMP-9, p < 0.001) while COL1A1 and COL3A1 mRNA transcripts were decreased (p < 0.001) in VSMCs isolated from carotid plaques of symptomatic (S) than asymptomatic (AS) patients. Stimulation of cells with TNF-α further increased the mRNA transcripts of TREM-1, MMPs, COL1A1 and COL3A1. Modulation of TREM-1 by treatment with TREM-1 decoy receptor rTREM-1/Fc, and either TREM-1 antibodies or TREM-1 siRNA attenuated the TNF-α-induced expression of MMP-1 and MMP-9 (p < 0.01) and COL1A1 and COL3A1 (p < 0.01) in S compared to AS VSMCs isolated from carotid plaques. Inhibition of NF-kB (BAY 11-7085), JNK (SP600125) and PI3K (LY294002) signaling pathways decreased the expression of TREM-1 (p < 0.01), MMP-1 (p < 0.001) and MMP-9 (p < 0.01) in TNF-α-treated VSMCs isolated from S carotid plaques compared to AS patients.

CONCLUSION

Increased expression of TREM-1 in S compared to AS patients involving MMP-1 and MMP-9 suggest a potential role of TREM-1 in plaque destabilization. Selective blockade of TREM-1 may contribute to the development of new therapies and promising targets for stabilizing vulnerable atherosclerotic plaques.

Opposing roles of the triggering receptor expressed on myeloid cells 2 and triggering receptor expressed on myeloid cells-like transcript 2 in microglia activation

Mutations in triggering receptor expressed on myeloid cells 2 (TREM2), which has been proposed to regulate the inflammatory responses and the clearance of apoptotic neurons and/or amyloid-β, are genetically linked to increased risk for late-onset Alzheimer's disease (AD). Interestingly, a missense variant in TREM-like transcript 2 (TREML2), a structurally similar protein encoded by the same gene cluster with TREM2 on chromosome 6, has been shown to protect against AD. However, the molecular mechanisms by which TREM2 and TREML2 regulate the pathogenesis of AD, and their functional relationship, if any, remain unclear. Here, we show that lipopolysaccharide (LPS) stimulation significantly suppressed TREM2 but increased TREML2 expression in mouse brain. Consistent with this in vivo result, LPS or oligomeric amyloid-β treatment down regulated TREM2 but up-regulated TREML2 expression in primary microglia. Most important, modulation of TREM2 or TREML2 levels had opposing effects on inflammatory responses with enhancement or suppression of LPS-induced proinflammatory cytokine gene expression observed on TREM2 or TREML2 down regulation, respectively. In addition, the proliferation of primary microglia was significantly decreased when TREM2 was down regulated, whereas it was increased on TREML2 knockdown. Together, our results suggest that several microglial functions are strictly regulated by TREM2 and TREML2, whose dysfunctions likely contribute to AD pathogenesis by impairing brain innate immunity. Our findings provide novel mechanistic insights into the functions of TREM2 and TREML2 in microglia and have implications on designing new therapeutic strategies to treat AD.

Immunoreceptors on neutrophils

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Neutrophil activities must be tightly controlled to maintain immune homeostasis.

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Activating and inhibitory receptors balance the outcome of immune cell activation.

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Immunoreceptors contain Ig-like extracellular domains and signal via ITAMs or ITIMs.

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Syk or SHP/SHIP mediate downstream signaling after immunoreceptor activation.

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Targeting immunoreceptors provides opportunities for therapeutic interventions.

Neutrophils play a critical role in the host defense against infection, and they are able to perform a variety of effector mechanisms for this purpose. However, there are also a number of pathological conditions, including autoimmunity and cancer, in which the activities of neutrophils can be harmful to the host. Thus the activities of neutrophils need to be tightly controlled. As in the case of other immune cells, many of the neutrophil effector functions are regulated by a series of immunoreceptors on the plasma membrane. Here, we review what is currently known about the functions of the various individual immunoreceptors and their signaling in neutrophils. While these immunoreceptors allow for the recognition of a diverse range of extracellular ligands, such as cell surface structures (like proteins, glycans and lipids) and extracellular matrix components, they commonly signal via conserved ITAM or ITIM motifs and their associated downstream pathways that depend on the phosphorylation of tyrosine residues in proteins and/or inositol lipids. This allows for a balanced homeostatic regulation of neutrophil effector functions. Given the number of available immunoreceptors and their fundamental importance for neutrophil behavior, it is perhaps not surprising that pathogens have evolved means to evade immune responses through some of these pathways. Inversely, some of these receptors evolved to specifically recognize these pathogens. Finally, some interactions mediated by immunoreceptors in neutrophils have been identified as promising targets for therapeutic intervention.

Chicken TREM-B1, an Inhibitory Ig-Like Receptor Expressed on Chicken Thrombocytes

Triggering receptors expressed on myeloid cells (TREM) form a multigene family of immunoregulatory Ig-like receptors and play important roles in the regulation of innate and adaptive immunity. In chickens, three members of the TREM family have been identified on chromosome 26. One of them is TREM-B1 which possesses two V-set Ig-domains, an uncharged transmembrane region and a long cytoplasmic tail with one ITSM and two ITIMs indicating an inhibitory function. We generated specific monoclonal antibodies by immunizing a Balb/c mouse with a TREM-B1-FLAG transfected BWZ.36 cell line and tested the hybridoma supernatants on TREM-B1-FLAG transfected 2D8 cells. We obtained two different antibodies specific for TREM-B1, mab 7E8 (mouse IgG1) and mab 1E9 (mouse IgG2a) which were used for cell surface staining. Single and double staining of different tissues, including whole blood preparations, revealed expression on thrombocytes. Next we investigated the biochemical properties of TREM-B1 by using the specific mab 1E9 for immunoprecipitation of either lysates of surface biotinylated peripheral blood cells or stably transfected 2D8 cells. Staining with streptavidin coupled horse radish peroxidase revealed a glycosylated monomeric protein of about 50 kDa. Furthermore we used the stably transfected 2D8 cell line for analyzing the cytoplasmic tyrosine based signaling motifs. After pervanadate treatment, we detected phosphorylation of the tyrosine residues and subsequent recruitment of the tyrosine specific protein phosphatase SHP-2, indicating an inhibitory potential for TREM-B1. We also showed the inhibitory effect of TREM-B1 in chicken thrombocytes using a CD107 degranulation assay. Crosslinking of TREM-B1 on activated primary thrombocytes resulted in decreased CD107 surface expression of about 50-70%.

Serum sTREM-1 level is quite higher in Crimean Congo Hemorrhagic Fever, a viral infection

Members of triggering receptor expressed on myeloid cells (TREM) family are known as immunmodulators in several infectious or noninfectious inflammatory disorders. The information about their role in viral infections is very limited. To enlighten if there is a relation between soluble TREM-1(sTREM-1) and a viral infection, Crimean Congo Haemorrhagic Fever (CCHF), we investigated the levels of sTREM-1 in the sera of 39 CCHF patients both at admission and at recovery and compared with 40 healthy controls by using microELISA technique. Statistical analysis was made by using Statistical Package for Social Sciences (SPSS) for Windows 20 programme. Value of P < 0.05 was accepted as significant for statistical analyses. Median sTREM-1 level was higher in CCHF group when compared to the control group (1,961 vs. 151.1 pg/ml, respectively; P < 0.001). In CCHF patients, sTREM-1 levels were significantly decreased at recovery compared to initial level measured at hospital admission (1,961 vs. 948 pg/ml, respectively; P = 0.019). ΔsTREM-1 is correlated with ΔCRP, ΔWBC, and ΔPlt. We found that serum levels of sTREM-1 higher than 405.9 pg/ml existed as a cut off point for differentiating CCHF patients and control group with a sensitivity of 94.9% and specifity of 87.5%. It is proved that sTREM-1 is increased and correlates with the clinical and laboratory findings in CCHF, a viral infection characterized by activation of inflammation. This finding may lead new studies to enlighten the pathogenesis of infections developing by activation of inflammatory cascades and high level cytokine releases, especially. J. Med. Virol. 88:1473-1478, 2016. © 2016 Wiley Periodicals, Inc.

TREM2 variants: new keys to decipher Alzheimer disease pathogenesis

Genome-wide association studies have identified rare variants of the gene that encodes triggering receptor expressed on myeloid cells 2 (TREM2) - an immune receptor that is found in brain microglia - as risk factors for non-familial Alzheimer disease (AD). Furthermore, animal studies have indicated that microglia have an important role in the brain response to amyloid-β (Aβ) plaques and that TREM2 variants may have an impact on such a function. We discuss how TREM2 may control the microglial response to Aβ and its impact on microglial senescence, as well as the interaction of TREM2 with other molecules that are encoded by gene variants associated with AD and the hypothetical consequences of the cleavage of TREM2 from the cell surface.

Role of triggering receptor expressed on myeloid cells-1/3 in Klebsiella-derived pneumosepsis

Triggering receptor expressed on myeloid cells (TREM)-1 and -2 can affect Toll-like receptor-mediated activation of immune cells. Klebsiella pneumoniae is a common cause of pneumonia-derived sepsis. Here we studied the role of TREM-1/3 and TREM-2 in the host response during Klebsiella pneumonia. Macrophages lacking either TREM-1/3 or TREM-2 were tested for their responsiveness toward K. pneumoniae and for their capacity to internalize this pathogen in vitro. TREM-1/3- and TREM-2-deficient mice were infected with K. pneumoniae via the airways, and their responses were compared with those in wild-type mice. TREM-1/3-deficient macrophages produced lower cytokine levels upon exposure to K. pneumoniae, whereas TREM-2-deficient macrophages released higher cytokine concentrations. TREM-2-deficient, but not TREM-1/3-deficient, macrophages showed a reduced capacity to phagocytose K. pneumoniae. TREM-1/3-deficient mice showed an impaired host defense during Klebsiella pneumonia, as reflected by worsened survival and increased bacterial growth and dissemination. In contrast, TREM-2 deficiency did not affect disease outcome. Although TREM-1/3 and TREM-2 influence macrophage responsiveness to K. pneumoniae in vitro, only TREM-1/3 contribute to the host response during Klebsiella pneumonia in vivo, serving a protective role.

TREM-like transcript 2 is stored in human neutrophil primary granules and is up-regulated in response to inflammatory mediators

The triggering receptor expressed on myeloid cell locus encodes a family of receptors that is emerging as an important class of molecules involved in modulating the innate immune response and inflammation. Of the 4 conserved members, including triggering receptor expressed on myeloid cells 1 and 2 and triggering receptor expressed on myeloid cell-like transcripts 1 and 2, relatively little is known about triggering receptor expressed on myeloid cell-like transcript 2 expression and function, particularly in humans. In this study, experiments were performed to determine if triggering receptor expressed on myeloid cell-like transcript 2 expression is conserved between mouse and human, demonstrating that human triggering receptor expressed on myeloid cell-like transcript 2 is expressed on cells of the lymphoid, as well as myeloid/granuloid lineages, similar to murine triggering receptor expressed on myeloid cell-like transcript 2. Consistent with studies in the mouse, triggering receptor expressed on myeloid cell-like transcript 2 expression is up-regulated in response to inflammatory mediators on human neutrophils. Importantly, it was shown that triggering receptor expressed on myeloid cell-like transcript 2, in resting human neutrophils, is predominantly localized to intracellular vesicles, including secretory vesicles and primary granules; with the majority of triggering receptor expressed on myeloid cell-like transcript 2 stored in primary granules. In contrast to other primary granule proteins, triggering receptor expressed on myeloid cell-like transcript 2 is not expelled on neutrophil extracellular traps but is retained in the plasma membrane following primary granule exocytosis. In summary, these findings establish that triggering receptor expressed on myeloid cell-like transcript 2 expression is conserved between species and is likely to be important in regulating neutrophil antimicrobial function following primary granule exocytosis.

Soluble TREM-2 in cerebrospinal fluid from patients with multiple sclerosis treated with natalizumab or mitoxantrone

BACKGROUND

Microglia-mediated proteolysis of the triggering receptor expressed on myeloid cells-2 (TREM-2) produces soluble TREM-2 (sTREM-2) that can be measured in cerebrospinal fluid (CSF) samples. Loss-of-function mutations in TREM2 or in the gene encoding its adaptor protein cause the rare Nasu-Hakola disease (NHD). Multiple sclerosis (MS) is an autoimmune disease that in common with NHD is characterized by demyelination and microglial activation.

OBJECTIVE

To investigate the potential utility of sTREM-2 as a biomarker for MS and to follow treatment effects.

METHODS

sTREM-2 was analyzed in CSF samples from subjects with MS (N = 59); relapsing-remitting MS (RRMS) (N = 36), secondary progressive MS (SPMS) (N = 20) and primary progressive MS (PPMS) (N = 3), and controls (N = 27). CSF levels of sTREM-2 were also assessed before and after treatment of patients with natalizumab or mitoxantrone.

RESULTS

CSF levels of sTREM-2 were significantly increased in patients with RRMS, SPMS, and PPMS compared with controls. After natalizumab treatment, the levels of sTREM-2 were normalized to control levels. The levels of sTREM-2 were also reduced after mitoxantrone treatment.

CONCLUSION

Increased CSF levels of sTREM-2, a new marker of microglial activation, in MS and normalization upon treatment with either natalizumab or mitoxantrone support a role for microglial activation in active MS.

Pathogenesis of infection in surgical patients

Purpose of review

Despite the application of prophylactic antimicrobial therapy and advanced technologies, infection remains one of the most common causes of morbidity and mortality in surgical patients. Understanding the pathogenesis of surgical infection would offer new insights into the development of biomarkers to predict and stratify infection in patients, and to explore specific strategies to minimize this serious postoperative complication.

Recent findings

The acute nonspecific inflammatory response triggered by endogenous danger signals evoked by surgical insult is beneficial, while paradoxically associated with reduced resistance to infection. There is growing evidence indicating that primed inflammation by surgical insult exaggerates the dysregulation of the immune-inflammatory response to the invasion of pathogens postoperatively. Innate immune receptors, such as Toll-like receptors (TLRs), contribute to detecting both pathogen-associated molecular patterns and endogenous damage-associated molecular patterns, and to further amplifying inflammatory responses to infection. Current evidence shows the fascinating role of non-TLRs in the process of infection. Non-TLRs, such as membrane-associated triggering receptor expressed on myeloid cells family, cytosolic nucleotide-binding oligomerization domain-like receptors and nuclear receptor nuclear family 4 subgroup A receptors, are also crucial in triggering the immune responses and mounting an effective defense against surgical insults and the second hit of infection.

Summary

Understanding the pivotal role of non-TLRs in sensing exogenous and endogenous molecules, and the influence of primed systemic inflammation and depressed immune status on the defense against pathogen after surgical insult, would be helpful to fully explore the relevant sophisticated phenomena of surgical infection, and to elucidate the occurrence of heterogeneous constellations of clinical signs and symptoms among this special population.

Identification of a Novel Splice Variant Isoform of TREM-1 in Human Neutrophil Granules

Triggering receptor expressed on myeloid cells-1 (TREM-1) is critical for inflammatory signal amplification. Humans have two forms of TREM-1: a membrane receptor, associated with the adaptor DAP12, and a soluble receptor detected at times of infection. The membrane receptor isoform acts synergistically with the TLR pathway to promote cytokine secretion and neutrophil migration, whereas the soluble receptor functions as a counterregulatory molecule. In multiple models of sepsis, exogenous administration of soluble forms of TREM-1 attenuates inflammation and markedly improves survival. Despite intense interest in soluble TREM-1, both as a clinical predictor of survival and as a therapeutic tool, the origin of native soluble TREM-1 remains controversial. Using human neutrophils, we identified a 15-kDa TREM-1 isoform in primary (azurophilic) and secondary (specific) granules. Mass spectrometric analysis, ELISA, and immunoblot confirm that the 15-kDa protein is a novel splice variant form of TREM-1 (TREM-1sv). Neutrophil stimulation with Pseudomonas aeruginosa, LPS, or PAM(3)Cys4 resulted in degranulation and release of TREM-1sv. The addition of exogenous TREM-1sv inhibited TREM-1 receptor-mediated proinflammatory cytokine production. Thus, these data reveal that TREM-1 isoforms simultaneously activate and inhibit inflammation via the canonical membrane TREM-1 molecule and this newly discovered granular isoform, TREM-1sv.

A New Triggering Receptor Expressed on Myeloid Cells (TREM) Family Member, TLT-6, is Involved in Activation and Proliferation of Macrophages

The triggering receptor expressed on myeloid cells (TREM) family, which is abundantly expressed in myeloid lineage cells, plays a pivotal role in innate and adaptive immune response. In this study, we aimed to identify a novel receptor expressed on hematopoietic stem cells (HSCs) by using in silico bioinformatics and to characterize the identified receptor. We thus found the TREM-like transcript (TLT)-6, a new member of TREM family. TLT-6 has a single immunoglobulin domain in the extracellular region and a long cytoplasmic region containing 2 immunoreceptor tyrosine-based inhibitory motif-like domains. TLT-6 transcript was expressed in HSCs, monocytes and macrophages. TLT-6 protein was up-regulated on the surface of bone marrow-derived and peritoneal macrophages by lipopolysaccharide stimulation. TLT-6 exerted anti-proliferative effects in macrophages. Our results demonstrate that TLT-6 may regulate the activation and proliferation of macrophages.

Microglial genes regulating neuroinflammation in the progression of Alzheimer's disease

Neuroinflammation is a pathological hallmark of Alzheimer's disease (AD), and microglia, the brain's resident phagocyte, are pivotal for the immune response observed in AD. Microglia act as sentinel and protective cells, but may become inappropriately reactive in AD to drive neuropathology. Recent Genome Wide Association Studies (GWAS) have identified more than 20 gene variants associated with an increased risk of late-onset AD (LOAD), the most prevalent form of AD [1]. The findings strongly implicate genes related to the immune response (CR1, CD33, MS4A, CLU, ABCA7, EPHA1 and HLA-DRB5-HLA-DRB1), endocytosis (BIN1, PICALM, CD2AP, EPHA1 and SORL1) and lipid biology (CLU, ABCA7 and SORL1) [2-8], and many encode proteins which are highly expressed in microglia [1]. Furthermore, recent identification of a low frequency mutation in the gene encoding the triggering receptor expressed in myeloid cells 2 protein (TREM2) confers increased risk of AD in LOAD cohorts with an effect size similar to that for APOE, until recently the only identified genetic risk factor associated with LOAD [9,10(••)] (Figure 1). The present review summarises our current understanding of the probable roles of microglial genes in the regulation of neuroinflammatory processes in AD and their relation to other processes affecting the disease's progression.

IL-1R/TLR2 through MyD88 Divergently Modulates Osteoclastogenesis through Regulation of Nuclear Factor of Activated T Cells c1 (NFATc1) and B Lymphocyte-induced Maturation Protein-1 (Blimp1)

Toll-like receptors (TLR) and the receptor for interleukin-1 (IL-1R) signaling play an important role in bacteria-mediated bone loss diseases including periodontitis, rheumatoid arthritis, and osteomyelitis. Recent studies have shown that TLR ligands inhibit the receptor activator of NF-κB ligand (RANKL)-induced osteoclast differentiation from un-committed osteoclast precursors, whereas IL-1 potentiates RANKL-induced osteoclast formation. However, IL-1R and TLR belong to the same IL-1R/TLR superfamily, and activate similar intracellular signaling pathways. Here, we investigate the molecular mechanisms underlying the distinct effects of IL-1 and Porphyromonas gingivalis lipopolysaccharide (LPS-PG) on RANKL-induced osteoclast formation. Our results show that LPS-PG and IL-1 differentially regulate RANKL-induced activation of osteoclast genes encoding Car2, Ctsk, MMP9, and TRAP, as well as expression of NFATc1, a master transcription factor of osteoclastogenesis. Regulation of osteoclast genes and NFATc1 by LPS-PG and IL-1 is dependent on MyD88, an important signaling adaptor for both TLR and IL-1R family members. Furthermore, LPS-PG and IL-1 differentially regulate RANKL-costimulatory receptor OSCAR (osteoclast-associated receptor) expression and Ca(2+) oscillations induced by RANKL. Moreover, LPS-PG completely abrogates RANKL-induced gene expression of B lymphocyte-induced maturation protein-1 (Blimp1), a global transcriptional repressor of anti-osteoclastogenic genes encoding Bcl6, IRF8, and MafB. However, IL-1 enhances RANKL-induced blimp1 gene expression but suppresses the gene expression of bcl6, irf8, and mafb. Our study reveals the involvement of multiple signaling molecules in the differential regulation of RANKL-induced osteoclastogenesis by TLR2 and IL-1 signaling. Understanding the signaling cross-talk among TLR, IL-1R, and RANK is critical for identifying therapeutic strategies to control bacteria-mediated bone loss.

NLRs: Nucleotide-Binding Domain and Leucine-Rich-Repeat-Containing Proteins

Eukaryotes have evolved strategies to detect microbial intrusion and instruct immune responses to limit damage from infection. Recognition of microbes and cellular damage relies on the detection of microbe-associated molecular patterns (MAMPs, also called PAMPS, or pathogen-associated molecular patterns) and so-called "danger signals" by various families of host pattern recognition receptors (PRRs). Members of the recently identified protein family of nucleotide-binding domain andleucine-rich-repeat-containing proteins (NLR), including Nod1, Nod2, NLRP3, and NLRC4, have been shown to detect specific microbial motifs and danger signals for regulating host inflammatory responses. Moreover, with the discovery that polymorphisms in NOD1, NOD2, NLRP1, and NLRP3 are associated with susceptibility to chronic inflammatory disorders, the view has emerged that NLRs act not only as sensors butalso can serve as signaling platforms for instructing and balancing host immune responses. In this chapter, we explore the functions of these intracellular innate immune receptors and examine their implication in inflammatory diseases.

CD33 modulates TREM2: convergence of Alzheimer loci

Here, we report results from a protein quantitative trait analysis in monocytes from 226 individuals to evaluate cross-talk between Alzheimer loci. We find that the NME8 locus influences PTK2B and that the CD33 risk allele leads to greater TREM2 expression. Further, we observe (1) a decreased TREM1/TREM2 ratio with a TREM1 risk allele, (2) decreased TREM2 expression with CD33 suppression, and (3) elevated cortical TREM2 mRNA expression with amyloid pathology.

The role of platelets in inflammation

There is growing recognition of the critical role of platelets in inflammation and immune responses. Recent studies have indicated that antiplatelet medications may reduce mortality from infections and sepsis, which suggests possible clinical relevance of modifying platelet responses to inflammation. Platelets release numerous inflammatory mediators that have no known role in haemostasis. Many of these mediators modify leukocyte and endothelial responses to a range of different inflammatory stimuli. Additionally, platelets form aggregates with leukocytes and form bridges between leukocytes and endothelium, largely mediated by platelet P-selectin. Through their interactions with monocytes, neutrophils, lymphocytes and the endothelium, platelets are therefore important coordinators of inflammation and both innate and adaptive immune responses.

Relationships between genetic polymorphisms of triggering receptor expressed on myeloid cells-1 and septic shock in a Chinese Han population

BACKGROUND

Triggering receptor expressed on myeloid cells-1 (TREM-1) is a cell surface receptor expressed on neutrophils and monocytes. TREM-1 acts to amplify inflammation and serves as a critical mediator of inflammatory response in the context of sepsis. To date, the predisposition of TREM-1 gene polymorphisms to septic shock has not been reported. This study was designed to investigate whether TREM-1 genomic variations are associated with the development of septic shock.

METHODS

We genotyped two TREM-1 single nucleotide polymorphisms (SNPs, rs2234237 and rs2234246) and evaluated the relationships between these SNPs and septic shock on susceptibility and prognosis.

RESULTS

TREM-1 rs2234246 A allele in the promoter region was significantly associated with the susceptibility of septic shock in recessive model (AA, OR=3.10, 95%CI 1.15 to 8.32, P=0.02), and in codominant model (AG, OR=0.72, 95%CI 0.43-1.19, P=0.02; AA, OR=2.71, 95%CI 1.00-7.42; P=0.03). However, in three inherited models (dominant model, recessive model, and codominant model), none of the assayed loci was significantly associated with the prognosis of septic shock. The non-survivor group demonstrated higher plasma IL-6 levels (99.7±34.7 pg/mL vs. 61.2±26.5 pg/mL, P<0.01) than the survivor group. Plasma concentrations of IL-6 among the three genotypes of rs2234246 were AA 99.4±48.9 pg/mL, AG 85.4±43 pg/mL, and GG 65.3±30.7 pg/mL (P<0.01). The plasma concentrations of IL-6 in patients with AA genotypes were significantly higher than those in patients with GG genotypes (P<0.01).

CONCLUSION

TREM-1 genetic polymorphisms rs2234246 may be significantly correlated only with susceptibility to septic shock in the Chinese Han population.

Soluble triggering receptor expressed on myeloid cells-1 is a biomarker of anti-CCP-positive, early rheumatoid arthritis

OBJECTIVES

To assess serum soluble triggering receptor expressed on myeloid cells-1 (sTREM-1) levels in disease-modifying antirheumatic drug (DMARD)-naïve early rheumatoid arthritis (ERA), to investigate the association of sTREM-1 levels with Disease Activity Score in 28 joints (DAS28) and seropositivity for anti-cyclic citrullinated peptide (CCP) antibody and to determine the predictive value of sTREM-1 with respect to clinical response to DMARD therapy.

METHODS

Twenty-two consecutive patients with DMARD-naïve ERA were prospectively evaluated for serum sTREM-1 by means of ELISA at diagnosis and at the following clinic visit after prednisone and/or DMARD has been administered, and related to DAS28 and serum level of anti-CCP antibody. We compared the sTREM-1 level to that of 31 patients with established RA as well as to 24 controls.

RESULTS

Serum sTREM-1 level was significantly higher in the DMARD-naïve ERA group (212.9 ± 388.9 ρg/mL) compared to established RA group (1478.0 ± 280.0 ρg/mL, P = 0.001) and normal control (34.4 ± 7.4 ρg/mL, P < 0.001). In the ERA group, elevated basal sTREM-1 level correlated with higher DAS28-CRP score (P = 0.001, HR 3.23, 95% CI 1.4-8.12), DAS28-ESR (P = 0.04, HR 2.34 95% CI 0.1-8.12), as well as predicted higher DAS28 score at the following encounter after DMARD treatment was administered (P = 0.001, HR 3.2 95% CI 1.1-7.2). Higher serum level of sTREM-1 correlated with higher titres of anti-CCP antibody (P < 0.001).

CONCLUSIONS

Our results suggest that serum sTREM-1 may provide a novel biomarker for DMARD-naïve ERA as well as for seropositivity for anti-CCP antibody and RA activity.

Diagnostic value of neutrophil gelatinase-associated lipocalin, cystatin C, and soluble triggering receptor expressed on myeloid cells-1 in critically ill patients with sepsis-associated acute kidney injury

INTRODUCTION

Neutrophil gelatinase-associated lipocalin (NGAL), cystatin C (Cys-C), and soluble triggering receptor expressed on myeloid cells-1 (sTREM-1) are novel diagnostic biomarkers of acute kidney injury (AKI). We aimed to determine the diagnostic properties of these biomarkers for detecting AKI in critically ill patients with sepsis.

METHODS

We divided 112 patients with sepsis into non-AKI sepsis (n = 57) and AKI sepsis (n = 55) groups. Plasma and urine specimens were collected on admission and every 24 hours until 72 hours and tested for NGAL, Cys-C, and TREM-1 concentrations. Their levels were compared on admission, at diagnosis, and 24 hours before diagnosis.

RESULTS

Both plasma and urine NGAL, Cys-C, and sTREM-1 were significantly associated with AKI development in patients with sepsis, even after adjustment for confounders by using generalized estimating equations. Compared with the non-AKI sepsis group, the sepsis AKI group exhibited markedly higher levels of these biomarkers at diagnosis and 24 hours before AKI diagnosis (P < 0.01). The diagnostic and predictive values of plasma and urine NGAL were good, and those of plasma and urine Cys-C and sTREM-1 were fair.

CONCLUSION

Plasma and urine NGAL, Cys-C, and sTREM-1 can be used as diagnostic and predictive biomarkers for AKI in critically ill patients with sepsis.