Regulation of TREM expression in hepatic macrophages and endothelial cells during acute endotoxemia

Epithelial barrier hypothesis in the context of nutrition, microbial dysbiosis, and immune dysregulation in metabolic dysfunction-associated steatotic liver

In recent years, the prevalence of chronic liver diseases, particularly Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD), has increased significantly. This upward trend is largely associated with lifestyle-related factors such as unhealthy dietary habits, physical inactivity, and various environmental influences. Among the key elements contributing to the pathogenesis of MASLD, the integrity of the intestinal epithelial barrier emerges as a critical determinant, given its central role in maintaining immune homeostasis along the gut-liver axis. Disruption of this barrier, often driven by excessive consumption of saturated fats and refined carbohydrates in combination with low dietary fiber intake, can lead to microbial dysbiosis. This imbalance in the gut microbiota triggers immune dysregulation and promotes systemic inflammation, thereby exacerbating hepatic injury. This review discusses the contribution of epithelial barrier dysfunction to the development and progression of MASLD, with a particular focus on how increased intestinal permeability may initiate and sustain chronic liver inflammation. Additionally, the influence of dietary and environmental factors on epithelial integrity, immune responses, and the inflammatory cascade is addressed. A better understanding of the complex interplay between gut barrier impairment, immune modulation, and liver pathology may offer valuable insights into MASLD pathophysiology and contribute to the development of more targeted therapeutic strategies.

TREM2-expressing macrophages in liver diseases

Metabolic dysfunction-associated steatotic liver disease (MASLD) affects over 30% of the global population and spans a spectrum of liver abnormalities, including simple steatosis, inflammation, fibrosis, cirrhosis, and hepatocellular carcinoma (HCC). Recent studies have identified triggering receptors expressed on myeloid cells 2 (TREM2)-expressing macrophages as key regulators of MASLD progression. TREM2 plays a pivotal role in regulating macrophage-mediated processes such as efferocytosis, inflammatory control, and fibrosis resolution. Additionally, soluble TREM2 (sTREM2) was proposed as a noninvasive biomarker for diagnosing and monitoring MASLD progression. However, the molecular mechanisms through which TREM2 influences MASLD pathogenesis remain incompletely understood. This review summarizes the current understanding of TREM2-expressing macrophages in MASLD, with the goal of illuminating future research and guiding the development of innovative therapeutic strategies targeting TREM2 signaling pathways.

Targeting TREM-1 receptors with metformin and pravastatin modulate monosodium iodoacetate-induced osteoarthritis

Osteoarthritis (OA), a degenerative joint condition, affects entire joints, including the tissues around it, causing pain, swelling and stiffness. This study explored the combined therapeutic effects of metformin and pravastatin on knee OA induced by monosodium iodoacetate (MIA) in rats. The study encompassed five animal groups (n = 10 per group) which were allocated as follows: group 1 received 100 µL of sterile saline injected intra-articularly into the left knee joint (control group) while groups 2-5 were injected with 2 mg MIA in 100µL normal saline into the intraarticular space. After 2 weeks, group 3 received oral metformin (100 mg/kg) for 2 weeks, group 4 received oral pravastatin (10 mg/kg) for 2 weeks and group 5 received both metformin 100 mg/kg and pravastatin 10 mg/kg orally for 2 weeks. The combination of metformin and pravastatin showed to be the most effective in terms of improving radiologic and histologic findings in knee OA. This combination also reduced both serum C reactive protein (CRP) and cartilage oligomeric matrix protein (COMP), while increasing serum collagen type II (COL2). Combining metformin and pravastatin also reduced AKT1, PI3K, STAT3 and IL-6 in the knee joint homogenate. TREM-1 levels showed the greatest reduction with the combination therapy. Furthermore, improvement was noted in oxidative stress markers with an increase in non-protein thiols and a decrease in malondialdehyde. In conclusion, the current study shows a therapeutic potential effect of combining metformin and pravastatin in OA, which could be partly attributed to TREM-1 signaling pathway.

Role of triggering receptor expressed on myeloid cells 2 in the pathogenesis of non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is a progressive disease. Without effective interventions, NAFLD can gradually develop to non-alcoholic steatohepatitis, fatty liver fibrosis, liver cirrhosis and even hepatocellular carcinoma. It is still to investigate the precise molecular mechanism behind the pathophysiology of NAFLD. Triggering receptor expressed on myeloid cells 2 (TREM2) can sense tissue injury and mediate immune remodeling, thereby inducing phagocytosis, lipid metabolism, and metabolic transfer, promoting cell survival and combating inflammatory activation. NAFLD might develop as a result of TREM2's regulatory role. We here briefly summarize the biological characteristics of TREM2 and its functions in the disease progression of NAFLD. Moreover, we propose to broaden the therapeutic strategy for NAFLD by targeting TREM2.

Infiltrating peripheral monocyte TREM-1 mediates dopaminergic neuron injury in substantia nigra of Parkinson's disease model mice

Neuroinflammation is a key factor in the pathogenesis of Parkinson's disease (PD). Activated microglia in the central nervous system (CNS) and infiltration of peripheral immune cells contribute to dopaminergic neuron loss. However, the role of peripheral immune responses, particularly triggering receptor expressed on myeloid cells-1 (TREM-1), in PD remains unclear. Using a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine hydrochloride (MPTP)-induced PD mouse model, we examined TREM-1 expression and monocyte infiltration in the substantia nigra pars compacta (SNpc). We found that MPTP increased peripheral monocytes, and deletion of peripheral monocytes protected against MPTP neurotoxicity in the SNpc. TREM-1 inhibition, both genetically and pharmacologically, reduced monocyte infiltration, alleviated neuroinflammation, and preserved dopaminergic neurons, resulting in improved motor function. Furthermore, adoptive transfer of TREM-1-expressing monocytes from PD model mice to naive mice induced neuronal damage and motor deficits. These results underscore the critical role of peripheral monocytes and TREM-1 in PD progression, suggesting that targeting TREM-1 could be a promising therapeutic approach to prevent dopaminergic neurodegeneration and motor dysfunction in PD. Schematic diagram of monocyte TREM-1-mediated dopaminergic neuron damage. The figure illustrates that in experimental MPTP-induced PD model mice, the number of inflammatory monocytes in the peripheral blood increases, after which the monocytes infiltrate the CNS through the Blood-Brain Barrier(BBB). These infiltrating monocytes increase the release of inflammatory cytokines and eventually cause neuronal injury. TREM-1 gene deletion and pharmacological blockade limit inflammatory monocyte recruitment into the SNpc and ameliorate neuroinflammatory events and the loss of dopaminergic neurons.

Transcriptomics changes of calcitonin gene-related peptide in mitigating lipopolysaccharide-induced septic cardiomyopathy

Septic cardiomyopathy (SCM), a complication initiated by sepsis, presents a significant clinical challenge, leading to increased mortality rates. However, the mechanisms of SCM have not been fully uncovered. Our study involved analyzing RNA sequencing (RNA-seq) data from rat heart tissue, along with utilizing molecular docking and molecular dynamics (MD) simulations, to discover key targets and potential pharmacological actions of the calcitonin gene-related peptide (CGRP) against SCM. A lipopolysaccharide-induced SCM model was established in rats (LPS 10 mg/kg, intraperitoneal (i.p.)). Thereafter, the myocardial tissues from the three groups of rats (Ctrl group, LPS group, and CGRP group) (n = 5) were extracted and underwent RNA-seq, followed by bioinformatics analyses. The qPCR-validated hub targets potentially interacting with CGRP were identified. Following this, homology modeling was utilized to obtain the 3D structure of hub targets, and molecular docking was conducted to evaluate the interaction between CGRP and hub targets. MD simulations (300 ns) were performed to confirm the findings further. Our findings demonstrated that CGRP significantly lowered mortality in SCM rats. 633 DEGs were affected by LPS, contrasted with the Ctrl group. 96 DEGs were affected by CGRP compared to the LPS group. In total, ten fully annotated CGRP-triggered hub genes were obtained. The molecular docking and MD simulations indicate that the relationship between CGRP and eight hub genes is extremely strong. This research offers a thorough examination of the possible objectives and fundamental molecular processes of CGRP in combating SCM, laying the groundwork for investigating the potential protective mechanisms of CGRP against SCM.

The potential of targeting TREM-1 in IBD

Innate immune dysfunction is a hallmark of the pathogenesis of Inflammatory Bowel Disease, both in Crohn's disease and ulcerative colitis. Despite considerable efforts in research to better understand the pathophysiology of IBD and for the development of new therapeutic modalities for IBD patients, there is no therapy specifically targeting the dysregulations of the innate immune response available today in that field. TREM-1 is exclusively expressed by innate immune cells and is an immune amplifier. Its engagement following the primary activation of Pattern Recognition Receptors, including Toll-Like Receptors, triggers the development of a dysregulated and sustained innate immune response, promoting the perpetuation of the inflammatory response in the mucosa of IBD patients, microscopic mucosal tissue alterations, impaired autophagy, impaired epithelial barrier integrity and function, ulcerations, and mucosal damages. In patients, TREM-1 activation is associated with the active status of the disease as well as with severity. Blocking TREM-1 in experimental colitis attenuates the dysregulated innate immune response leading to improved clinical signs. Anti-TREM-1 approaches have the potential of controlling the pathogenic dysregulation of the immune response in IBD by targeting an upstream amplification loop of the activation of innate immunity.

Endothelial dysfunction: Pathophysiology and therapeutic targets for sepsis-induced multiple organ dysfunction syndrome

Sepsis and septic shock are critical medical conditions characterized by a systemic inflammatory response to infection, significantly contributing to global mortality rates. The progression to multiple organ dysfunction syndrome (MODS) represents the most severe complication of sepsis and markedly increases clinical mortality. Central to the pathophysiology of sepsis, endothelial cells play a crucial role in regulating microcirculation and maintaining barrier integrity across various organs and tissues. Recent studies have underscored the pivotal role of endothelial function in the development of sepsis-induced MODS. This review aims to provide a comprehensive overview of the pathophysiology of sepsis-induced MODS, with a specific focus on endothelial dysfunction. It also compiles compelling evidence regarding potential small molecules that could attenuate sepsis and subsequent multi-organ damage by modulating endothelial function. Thus, this review serves as an essential resource for clinical practitioners involved in the diagnosing, managing, and providing intensive care for sepsis and associated multi-organ injuries, emphasizing the importance of targeting endothelial cells to enhance outcomes of the patients.

Lipid-associated macrophages' promotion of fibrosis resolution during MASH regression requires TREM2

While macrophage heterogeneity during metabolic dysfunction-associated steatohepatitis (MASH) has been described, the fate of these macrophages during MASH regression is poorly understood. Comparing macrophage heterogeneity during MASH progression vs regression, we identified specific macrophage subpopulations that are critical for MASH/fibrosis resolution. We elucidated the restorative pathways and gene signatures that define regression-associated macrophages and establish the importance of TREM2+ macrophages during MASH regression. Liver-resident Kupffer cells are lost during MASH and are replaced by four distinct monocyte-derived macrophage subpopulations. Trem2 is expressed in two macrophage subpopulations: i) monocyte-derived macrophages occupying the Kupffer cell niche (MoKC) and ii) lipid-associated macrophages (LAM). In regression livers, no new transcriptionally distinct macrophage subpopulation emerged. However, the relative macrophage composition changed during regression compared to MASH. While MoKC was the major macrophage subpopulation during MASH, they decreased during regression. LAM was the dominant macrophage subtype during MASH regression and maintained Trem2 expression. Both MoKC and LAM were enriched in disease-resolving pathways. Absence of TREM2 restricted the emergence of LAMs and formation of hepatic crown-like structures. TREM2+ macrophages are functionally important not only for restricting MASH-fibrosis progression but also for effective regression of inflammation and fibrosis. TREM2+ macrophages are superior collagen degraders. Lack of TREM2+ macrophages also prevented elimination of hepatic steatosis and inactivation of HSC during regression, indicating their significance in metabolic coordination with other cell types in the liver. TREM2 imparts this protective effect through multifactorial mechanisms, including improved phagocytosis, lipid handling, and collagen degradation.

Role of triggering receptor expressed on myeloid cells-1 in kidney diseases: A biomarker and potential therapeutic target

ABSTRACT

Triggering receptor expressed on myeloid cells-1 (TREM-1) is a member of the immunoglobulin superfamily. As an amplifier of the inflammatory response, TREM-1 is mainly involved in the production of inflammatory mediators and the regulation of cell survival. TREM-1 has been studied in infectious diseases and more recently in non-infectious disorders. More and more studies have shown that TREM-1 plays an important pathogenic role in kidney diseases. There is evidence that TREM-1 can not only be used as a biomarker for diagnosis of disease but also as a potential therapeutic target to guide the development of novel therapeutic agents for kidney disease. This review summarized molecular biology of TREM-1 and its signaling pathways as well as immune response in the progress of acute kidney injury, renal fibrosis, diabetic nephropathy, immune nephropathy, and renal cell carcinoma.

Current understanding on TREM-2 molecular biology and physiopathological functions

Triggering receptor expressed on myeloid cells 2 (TREM-2), a glycosylated receptor belonging to the immunoglobin superfamily and especially expressed in the myeloid cell lineage, is frequently explained as a reminiscent receptor for both adaptive and innate immunity regulation. TREM-2 is also acknowledged to influence NK cell differentiation via the PI3K and PLCγ signaling pathways, as well as the partial activation or direct inhibition of T cells. Additionally, TREM-2 overexpression is substantially linked to cell-specific functions, such as enhanced phagocytosis, reduced toll-like receptor (TLR)-mediated inflammatory cytokine production, increased transcription of anti-inflammatory cytokines, and reshaped T cell function. Whereas TREM-2-deficient cells exhibit diminished phagocytic function and enhanced proinflammatory cytokines production, proceeding to inflammatory injuries and an immunosuppressive environment for disease progression. Despite the growing literature supporting TREM-2+ cells in various diseases, such as neurodegenerative disorders and cancer, substantial facets of TREM-2-mediated signaling remain inadequately understood relevant to pathophysiology conditions. In this direction, herein, we have summarized the current knowledge on TREM-2 biology and cell-specific TREM-2 expression, particularly in the modulation of pivotal TREM-2-dependent functions under physiopathological conditions. Furthermore, molecular regulation and generic biological relevance of TREM-2 are also discussed, which might provide an alternative approach for preventing or reducing TREM-2-associated deformities. At last, we discussed the TREM-2 function in supporting an immunosuppressive cancer environment and as a potential drug target for cancer immunotherapy. Hence, summarized knowledge of TREM-2 might provide a window to overcome challenges in clinically effective therapies for TREM-2-induced diseases in humans.

TREM-2: friend or foe in infectious diseases?

The triggering receptor expressed on myeloid cells-2 (TREM-2) is an immune receptor expressed on immune and non-immune cells, more frequently investigated in neurodegenerative disorders and considered a marker for microglia activation. In infectious diseases, the receptor was initially believed to be an anti-inflammatory molecule, opposing the inflammation triggered by TREM-1. Currently, TREM-2 is associated with different aspects in response to infectious stimuli, including the induction of bacterial phagocytosis and clearance, containment of exacerbated pro-inflammatory responses, induction of M2 differentiation and activation of Th1 lymphocytes, besides of neurological damage after viral infection. Here, we present and discuss results published in the last two decades regarding the expression, activation and functions of TREM-2 during the course of bacterial, viral, fungal and parasitic infections. A surprisingly plasticity was observed regarding the roles of the receptor in the aforementioned contexts, which largely varied according to the cell/organ and pathogen type, besides influencing disease outcome. Therefore, our review aimed to critically overview the role of TREM-2 in infectious diseases, highlighting its potential to be used as a clinical biomarker or therapeutic target.

TREM-1 induces pyroptosis in cardiomyocytes by activating NLRP3 inflammasome through the SMC4/NEMO pathway

Sepsis often causes cell death via pyroptosis and hence results in septic cardiomyopathy. Triggering receptors expressed in myeloid cells-1 (TREM-1) may initiate cellular cascade pathways and, in turn, induce cell death and vital organ dysfunction in sepsis, but the evidence is limited. We set to investigate the role of TREM-1 on nucleotide-binding oligomerization domain-like receptors with pyrin domain-3 (NLRP3) inflammasome activation and cardiomyocyte pyroptosis in sepsis models using cardiac cell line (HL-1) and mice. In this study, TREM-1 was found to be significantly increased in HL-1 cells challenged with lipopolysaccharide (LPS). Pyroptosis was also significantly increased in the HL-1 cells challenged with lipopolysaccharide and an NLRP3 inflammasome activator, nigericin. The close interaction between TREM-1 and structural maintenance of chromosome 4 (SMC4) was also identified. Furthermore, inhibition of TREM-1 or SMC4 prevented the upregulation of NLRP3 and decreased Gasdermin-D, IL-1β and caspase-1 cleavage. In mice subjected to caecal ligation and puncture, the TREM-1 inhibitor LR12 decreased the expression of NLRP3 and attenuated cardiomyocyte pyroptosis, leading to improved cardiac function and prolonged survival of septic mice. Our work demonstrates that, under septic conditions, TREM-1 plays a critical role in cardiomyocyte pyroptosis. Targeting TREM-1 and its associated molecules may therefore lead to novel therapeutic treatments for septic cardiomyopathy.

The role of triggering receptor expressed on myeloid cells-1 (TREM-1) in central nervous system diseases

Triggering receptor expressed on myeloid cells-1 (TREM-1) is a member of the immunoglobulin superfamily and is mainly expressed on the surface of myeloid cells such as monocytes, macrophages, and neutrophils. It plays an important role in the triggering and amplification of inflammatory responses, and it is involved in the development of various infectious and non-infectious diseases, autoimmune diseases, and cancers. In recent years, TREM-1 has also been found to participate in the pathological processes of several central nervous system (CNS) diseases. Targeting TREM-1 may be a promising strategy for treating these diseases. This paper aims to characterize TREM-1 in terms of its structure, signaling pathway, expression, regulation, ligands and pathophysiological role in CNS diseases.

Diagnostic and prognostic predictive values of triggering receptor expressed on myeloid cell-1 expression in neonatal sepsis: A meta-analysis and systematic review

OBJECTIVE

The purpose of this systematic review was to explore the value of the expression level of the triggering receptor expressed on myeloid cell-1 (TREM-1) in the diagnosis and prognosis of neonatal sepsis.

METHODS

A comprehensive search was performed to identify the diagnostic and prognostic predictive values of the TREM-1 expression level in neonatal sepsis. Based on the retrieval strategy, Cochrane Library, Embase, Ovid, ProQuest, PubMed, Scopus, and Web of Science databases were searched from inception to February 2022. Studies were included if they assessed the accuracy of TREM-1 expression in the diagnosis of neonatal sepsis and distinguished survival and death in neonatal sepsis. Two authors independently evaluated the study and extracted the data, including the first author of the literature, country, total study population, basic population characteristics of the study group and the control group, study design (observational studies), type of sample, sepsis onset, type of biomarker, assay method, cut-off, sensitivity, specificity, true positives (TP), false positives (FP), false negatives (FN), and true negatives (TN). A third party will be consulted if disputed. The accuracy of TREM-1 expression in the diagnosis and prognostic prediction of neonatal sepsis was evaluated by a bivariate mixed-effects model. The source of heterogeneity was explored through meta-regression analysis.

RESULTS

Thirteen articles that met the research criteria were included in qualitative analysis, and 11 of them were included in quantitative analysis. The pooled sensitivity, specificity, positive likelihood ratio (PLR), negative likelihood ratio (NLR), diagnostic odds ratio (DOR), and the area under the summary receiver operator characteristic (SROC) curve of soluble TREM-1 (sTREM-1) were 0.94 (95% CI: 0.82, 0.98), 0.87 (95% CI: 0.70, 0.95), 7.36 (95% CI: 2.75, 19.74), 0.07 (95% CI: 0.02, 0.24), 111.71 (95% CI: 13.24, 942.92), and 0.96 (95% CI: 0.94, 0.98), respectively. Meta-regression and subgroup analysis were used to investigate the heterogeneity, owing to non-threshold effects caused by types of test sample and research design. sTREM-1 as a biomarker for distinguishing survival and death in neonates with sepsis had pooled sensitivity, specificity, area under the SROC curve, PLR, NLR, and DOR of 0.95 (95% CI: 0.83, 0.99), 0.98 (95% CI: 0.68, 1.00), 0.99 (95% CI: 0.97, 0.99), 39.28 (95% CI: 2.13, 723.99), 0.05 (95% CI: 0.01, 0.19), and 789.61 (95% CI: 17.53, 35,560.72), respectively.

CONCLUSION

The study showed that TREM-1 was a potential biomarker for the diagnosis and prognosis of neonatal sepsis. The biggest advantage of this study is that it is the first to comprehensively explore the role of TREM-1 expression in the diagnosis and prognosis of neonatal sepsis. However, there are some limitations in this study, such as the reduced number of clinical studies on TREM-1 expression as a biomarker of neonatal sepsis, regional bias, and differences in detection methods. Hence, more large-scale and high-quality studies are needed to improve diagnostic accuracy.

SYSTEMATIC REVIEW REGISTRATION

https://www.crd.york.ac.uk/PROSPERO/, identifier: CRD42022338041.

sTREM-1 Predicts Disease Severity and Mortality in COVID-19 Patients: Involvement of Peripheral Blood Leukocytes and MMP-8 Activity

Uncontrolled inflammatory responses play a critical role in coronavirus disease (COVID-19). In this context, because the triggering-receptor expressed on myeloid cells-1 (TREM-1) is considered an intrinsic amplifier of inflammatory signals, this study investigated the role of soluble TREM-1 (sTREM-1) as a biomarker of the severity and mortality of COVID-19. Based on their clinical scores, we enrolled COVID-19 positive patients (n = 237) classified into mild, moderate, severe, and critical groups. Clinical data and patient characteristics were obtained from medical records, and their plasma inflammatory mediator profiles were evaluated with immunoassays. Plasma levels of sTREM-1 were significantly higher among patients with severe disease compared to all other groups. Additionally, levels of sTREM-1 showed a significant positive correlation with other inflammatory parameters, such as IL-6, IL-10, IL-8, and neutrophil counts, and a significant negative correlation was observed with lymphocyte counts. Most interestingly, sTREM-1 was found to be a strong predictive biomarker of the severity of COVID-19 and was related to the worst outcome and death. Systemic levels of sTREM-1 were significantly correlated with the expression of matrix metalloproteinases (MMP)-8, which can release TREM-1 from the surface of peripheral blood cells. Our findings indicated that quantification of sTREM-1 could be used as a predictive tool for disease outcome, thus improving the timing of clinical and pharmacological interventions in patients with COVID-19.

Trem1 Induces Periodontal Inflammation via Regulating M1 Polarization

Periodontitis is a chronic inflammatory condition characterized by destruction of nonmineralized and mineralized connective tissues. This study evaluated the role of Trem1 (triggering receptors expressed on myeloid cells 1) in periodontitis by influencing polarization of M1 macrophages through the STAT3/HIF-1α signaling pathway. Trem1 was significantly upregulated in the gingival tissues of patients with periodontitis, as identified by high-throughput RNA sequencing, and positively correlated with levels of M1 macrophage-associated genes. The results of flow cytometry, Western blotting, and reverse transcription quantitative polymerase chain reaction showed that knockdown of Trem1 in RAW 264.7 cells decreased polarization of M1 macrophages and increased polarization of M2 macrophages, while overexpression of Trem1 exerted an opposite effect. Furthermore, a mouse model of Trem1 knockout periodontitis exhibited limited infiltration of macrophages and decreased expression levels of M1 macrophage-associated genes in periodontitis lesions and bone marrow-derived macrophages. Importantly, we found that Trem1 could regulate polarization of M1 macrophages through STAT3/HIF-1α signaling as evidenced by RNA sequencing. Moreover, inhibition of Trem1 and HIF-1α could suppress the expression level of proinflammatory cytokine (interleukin 1β) and upregulate the expression level of anti-inflammatory cytokine (interleukin 10) in periodontitis. Collectively, we identified that the Trem1/STAT3/HIF-1α axis could regulate polarization of M1 macrophages and is a potential candidate in the treatment of periodontitis.

Evaluation of the relationship between TREM-1/TREM-2 ratio and clinical course in COVID-19 pneumonia

OBJECTIVE

The inflammatory/anti-inflammatory balance has an important role in the clinical course of SARS-CoV-2 infection (COVID-19), which has affected over 100 million people since it first appeared in China in December 2019. The aim of this study was to investigate the relationship between triggering receptor expressed on myeloid cells (TREM)-1/TREM-2 ratio and COVID-19 severity.

METHODS

A total of 171 individuals were included in the study: 121 patients who were admitted to the chest diseases department and intensive care unit of our hospital and diagnosed with COVID-19 by real-time PCR of nasopharyngeal swab samples from December 2020 to March 2021 and a control group consisting of 50 asymptomatic health workers in our hospital who had negative real-time PCR results during routine COVID-19 screening.

RESULTS

TREM-1 level was significantly higher in patients with severe disease compared with the moderate and control groups (P = .003, P = .001). TREM-2 levels did not differ significantly in moderate and severe patients (P = .36) but were significantly higher in both patient groups compared with the control group (P = .001 for both). TREM-1/TREM-2 ratio was significantly higher in the severe patient group than in the moderate and control groups (P = .001 for both). In receiver operating characteristic curve analysis of TREM-1/TREM-2 ratio in patients with moderate and severe COVID-19, the area under the curve was 0.723. Using a cut-off value of 0.125 for TREM-1/TREM-2 ratio in the Youden index calculation, the sensitivity was 60% and specificity was 71%.

CONCLUSION

Experience with the positive effects of medical treatments to restore inflammatory balance in the course of COVID-19 is steadily increasing. TREM-1 and TREM-2 have an important role in inflammation and may serve as biomarkers and therapeutic targets in the early treatment and follow-up of COVID-19.

Reduced uterine tissue damage during Chlamydia muridarum infection in TREM-1,3 deficient mice

Genital infections with Chlamydia trachomatis can lead to uterine and oviduct tissue damage in the female reproductive tract. Neutrophils are strongly associated with tissue damage during chlamydial infection, while an adaptive CD4 T cell response is necessary to combat infection. Activation of triggering receptor expressed on myeloid cells-1 (TREM-1) on neutrophils has previously been shown to induce and/or enhance degranulation synergistically with TLR-signaling. Additionally, TREM-1 can promote neutrophil transepithelial migration. In this study, we sought to determine the contribution of TREM-1,3 in immunopathology in the female mouse genital tract during Chlamydia muridarum infection. Relative to control mice, trem1,3^-/- mice had no difference in chlamydial burden or duration of lower genital tract infection. We also observed a similar incidence of oviduct hydrosalpinx 45 days post-infection in trem1,3^-/- compared to WT mice. However, compared to WT, trem1,3^-/- mice developed significantly fewer uterine horn hydrometra. Early in infection, trem1,3^-/- mice displayed a notable decrease in the number of uterine glands containing polymorphonuclear cells and uterine horn lumens had fewer neutrophils, with increased G-CSF. Trem1,3^-/- mice also had reduced erosion of the luminal epithelium. These data indicate TREM-1,3 contributes to transepithelial neutrophil migration in the uterus and uterine glands, promoting the development of uterine hydrometra in infected mice.

Role of triggering receptor expressed on myeloid cells 2 (TREM2) in neurodegenerative dementias

Neurodegeneration is a debilitating condition that causes nerve cell degeneration or death. Neurodegenerative diseases (NDDs) such as Alzheimer's disease (AD), Parkinson's disease (PD), frontotemporal dementia (FTD), and Lewy body dementia (LBD) are posing a larger population burden of dementia worldwide. Neurodegenerative dementia is one of the main challenges in public health with its main characteristics being permanent loss of memory, impairment in cognition, and impaired daily functions. The published literature about genetic studies of these disorders suggests genetic underpinning in the pathogenesis of neurodegenerative dementia. In the process of underlining the pathogenesis of NDD, growing evidence has related genetic variations in the triggering receptor expressed on myeloid cells 2 (TREM2). This review paper aims to provide a detailed information regarding the association of TREM2 and NDDs leading to dementia. A central consideration is AD that accounts for almost 50%-70% of all late-life dementias alone or in combination with other neurological disorders. Other prevalent neurodegenerative conditions that lead to dementia are also discussed. Such studies are important as they can give a comprehensive knowledge of TREM2's role in various NDDs, in order to maximize the potential for developing new therapeutic approaches.

Role of TREM-1 in the development of early brain injury after subarachnoid hemorrhage

Triggering receptor expressed on myeloid cells-1 (TREM-1) was found to be induced in the context of subarachnoid hemorrhage (SAH) before. This study further investigates its role in the development of SAH-induced early brain injury (EBI). Firstly, rats were randomly divided into Sham and SAH groups for analysis of temporal patterns and cellular localization of TREM-1. Secondly, TREM-1 intervention was administrated to produce Sham, vehicle, antagonist and agonist groups, for analyzing TREM-1, Toll-like receptor 4 (TLR4), myeloid differentiation factor 88 (MyD88) and NF-κB expressions at 24 h post-modeling, and EBI assessment at 24 h and 72 h. Thirdly, TLR4 inhibitor (TAK-242) was exploited to produce Sham, Sham+TAK-242, SAH, and SAH + TAK-242 groups to analyze the effects of TLR4 inhibition on TREM-1 induction and EBI evaluation at 72 h. Fourthly, the relationship of soluble TREM-1 (sTREM-1) levels in cerebrospinal fluid of SAH patients with Hunt-Hess grades were explored. The results showed that TREM-1 increased in the brain after experimental SAH (eSAH) early at 6 h and peaked at 48 h, which was found to be located in microglia and endothelial cells. TREM-1 inhibition attenuated EBI associated with TLR4/MyD88/NF-κB suppression, while enhancement had the opposite effects. Contrarily, TLR4 inhibition prevented TREM-1 induction and ameliorated EBI. In addition, sTREM-1 levels in SAH patients positively correlated with Hunt-Hess grades. Overall, the present study provides new evidence that TREM-1 increases dynamically in the brain after eSAH and it is located in microglia and endothelial cells, which may aggravate EBI by interacting with TLR4 pathway. And sTREM-1 in patients might act as a monitoring biomarker of EBI, providing new insights for future studies.

Novel ligands and modulators of triggering receptor expressed on myeloid cells receptor family: 2015-2020 updates

Introduction: Triggering receptors expressed on myeloid cells (TREMs) are inflammatory amplifiers with defined pathophysiological role in various infectious diseases, acute and chronic aseptic inflammations, and a variety of cancers, depicting TREMs as prominent therapeutic targets.Areas covered: Herein, updates from 2015 to 2020 are discussed to divulge the TREM ligands, as well as their peptide blockers, claimed to modulate their expression. The article also presents different strategies employed during the last five years to block interactions between TREMs and their ligands to treat various disease conditions by modulating their expression and activity.Expert opinion: There has been significant progress in the discovery of novel ligands and modulators of TREMs in the last five years that mainly revolved around the function of TREM molecules. A few peptides showed encouraging results to modulate the expression and activity of TREMs in preclinical studies, and these peptides are currently under clinical investigation. Based on the findings so far in several careful studies, we expect novel therapeutics in the near future which could have the ability to treat various disease conditions associated with TREM expression.

TREM-1 isoforms in bacterial infections: to immune modulation and beyond

The triggering receptor expressed on myeloid cells 1 (TREM-1) is an innate immunity receptor associated with the amplification of inflammation in sterile and non-sterile inflammatory disorders. Since its first description, the two isoforms of the receptor, membrane and soluble (mTREM-1 and sTREM-1, respectively) have been largely explored in the immunopathogenesis of several bacterial diseases and sepsis. The role of the receptor in these scenarios seems to be at least partly dependent on the source/type of bacteria, host and context. As uncontrolled inflammation is a result of several bacterial infections, the inhibition of the receptor has been considered as a promising approach to treat such conditions. Further, sTREM-1 has been explored as a biomarker for diagnosis and/or prognosis of several bacterial diseases. Therefore, this review aims to provide an updated insight into how the receptor influences and is influenced by bacterial infections, highlighting the advances regarding the use/manipulation of TREM-1 isoforms in biomedical research and clinical practice.

Function of TREM1 and TREM2 in Liver-Related Diseases

TREM1 and TREM2 are members of the triggering receptors expressed on myeloid cells (TREM) family. Both TREM1 and TREM2 are immunoglobulin superfamily receptors. Their main function is to identify foreign antigens and toxic substances, thereby adjusting the inflammatory response. In the liver, TREM1 and TREM2 are expressed on non-parenchymal cells, such as liver sinusoidal endothelial cells, Kupffer cells, and hepatic stellate cells, and cells which infiltrate the liver in response to injury including monocyte-derived macrophages and neutrophils. The function of TREM1 and TREM2 in inflammatory response depends on Toll-like receptor 4. TREM1 mainly augments inflammation during acute inflammation, while TREM2 mainly inhibits chronic inflammation to protect the liver from pathological changes. Chronic inflammation often induces metabolic abnormalities, fibrosis, and tumorigenesis. The above physiological changes lead to liver-related diseases, such as liver injury, nonalcoholic steatohepatitis, hepatic fibrosis, and hepatocellular carcinoma. Here, we review the function of TREM1 and TREM2 in different liver diseases based on inflammation, providing a more comprehensive perspective for the treatment of liver-related diseases.

Androgen receptor signalling in macrophages promotes TREM-1-mediated prostate cancer cell line migration and invasion

The androgen receptor (AR) is the master regulator of prostate cancer (PCa) development, and inhibition of AR signalling is the most effective PCa treatment. AR is expressed in PCa cells and also in the PCa-associated stroma, including infiltrating macrophages. Macrophages have a decisive function in PCa initiation and progression, but the role of AR in macrophages remains largely unexplored. Here, we show that AR signalling in the macrophage-like THP-1 cell line supports PCa cell line migration and invasion in culture via increased Triggering Receptor Expressed on Myeloid cells-1 (TREM-1) signalling and expression of its downstream cytokines. Moreover, AR signalling in THP-1 and monocyte-derived macrophages upregulates IL-10 and markers of tissue residency. In conclusion, our data suggest that AR signalling in macrophages may support PCa invasiveness, and blocking this process may constitute one mechanism of anti-androgen therapy.

Anti-androgen therapy inhibits prostate cancer (PC) progression, and is thought to act directly on cancer cells. Here the authors show that androgen receptor is expressed on normal and PC-associated macrophages, and its stimulation alters macrophage secretome to promote migration of cultured PC cell lines.

Extracellular CIRP Induces Inflammation in Alveolar Type II Cells via TREM-1

Extracellular cold-inducible RNA-binding protein (eCIRP) induces acute lung injury (ALI) in sepsis. Triggering receptor expressed on myeloid cells-1 (TREM-1) serves as a receptor for eCIRP to induce inflammation in macrophages and neutrophils. The effect of eCIRP on alveolar epithelial cells (AECs) remains unknown. We hypothesize that eCIRP induces inflammation in AECs through TREM-1. AECs were isolated from C57BL/6 mice and freshly isolated AECs were characterized as alveolar type II (ATII) cells by staining AECs with EpCAM, surfactant protein-C (SP-C), and T1 alpha (T1α) antibodies. AECs were stimulated with recombinant murine (rm) CIRP and assessed for TREM-1 by flow cytometry. ATII cells from WT and TREM-1^-/- mice were stimulated with rmCIRP and assessed for interleukin-6 (IL-6) and chemokine (C-X-C motif) ligand 2 (CXCL2) in the culture supernatants. ATII cells from WT mice were pretreated with vehicle (PBS), M3 (TREM-1 antagonist), and LP17 (TREM-1 antagonist) and then after stimulating the cells with rmCIRP, IL-6 and CXCL2 levels in the culture supernatants were assessed. All of the freshly isolated AECs were ATII cells as they expressed EpCAM and SP-C, but not T1α (ATI cells marker). Treatment of ATII cells with rmCIRP significantly increased TREM-1 expression by 56% compared to PBS-treated ATII cells. Stimulation of WT ATII cells with rmCIRP increased IL-6 and CXCL2 expression, while the expression of IL-6 and CXCL2 in TREM-1^-/- ATII cells were reduced by 14 and 23%, respectively. Pretreatment of ATII cells with M3 and LP17 significantly decreased the expression of IL-6 by 30 and 47%, respectively, and CXCL2 by 27 and 34%, respectively, compared to vehicle treated ATII cells after stimulation with rmCIRP. Thus, eCIRP induces inflammation in ATII cells via TREM-1 which implicates a novel pathophysiology of eCIRP-induced ALI and directs a possible therapeutic approach targeting eCIRP-TREM-1 interaction to attenuate ALI.

Therapeutic Trem2 activation ameliorates amyloid-beta deposition and improves cognition in the 5XFAD model of amyloid deposition

BACKGROUND

Triggering receptor expressed on myeloid cell-2 (TREM2) is a lipid and lipoprotein binding receptor expressed by cells of myeloid origin. Homozygous TREM2 mutations cause early onset progressive presenile dementia while heterozygous, point mutations triple the risk of Alzheimer's disease (AD). Although human genetic findings support the notion that loss of TREM2 function exacerbates neurodegeneration, it is not clear whether activation of TREM2 in a disease state would result in therapeutic benefits. To determine the viability of TREM2 activation as a therapeutic strategy, we sought to characterize an agonistic Trem2 antibody (AL002a) and test its efficacy and mechanism of action in an aggressive mouse model of amyloid deposition.

METHODS

To determine whether agonism of Trem2 results in therapeutic benefits, we designed both intracranial and systemic administration studies. 5XFAD mice in the intracranial administration study were assigned to one of two injection groups: AL002a, a Trem2-agonizing antibody, or MOPC, an isotype-matched control antibody. Mice were then subject to a single bilateral intracranial injection into the frontal cortex and hippocampus and euthanized 72 h later. The tissue from the left hemisphere was histologically examined for amyloid-beta and microglia activation, whereas the tissue from the right hemisphere was used for biochemical analyses. Similarly, mice in the systemic administration study were randomized to one of the aforementioned injection groups and the assigned antibody was administered intraperitoneally once a week for 14 weeks. Mice underwent behavioral assessment between the 12- and 14-week timepoints and were euthanized 24 h after their final injection. The tissue from the left hemisphere was used for histological analyses whereas the tissue from the right hemisphere was used for biochemical analyses.

RESULTS

Here, we show that chronic activation of Trem2, in the 5XFAD mouse model of amyloid deposition, leads to reversal of the amyloid-associated gene expression signature, recruitment of microglia to plaques, decreased amyloid deposition, and improvement in spatial learning and novel object recognition memory.

CONCLUSIONS

These findings indicate that Trem2 activators may be effective for the treatment of AD and possibly other neurodegenerative disorders.

Triggering receptor expressed on myeloid cells-1 (TREM-1) as a therapeutic target in infectious and noninfectious disease: a critical review

The triggering receptor expressed on myeloid cells-1 (TREM-1) is an innate immune receptor found in the surface of several immune and non-immune cells. Since its first description in 2000, this molecule and its soluble form (sTREM-1) have been implicated in many diseases with infectious and noninfectious origins. As an amplifier of inflammation, the membrane-associated TREM-1 (mTREM-1) isoform induces the production of pro-inflammatory mediators, thus contributing to the pathogenesis of diseases such as sepsis, arthritis, colitis and infections. In this context, many studies have used molecules capable of inhibiting TREM-1 activity as anti-inflammatory drugs. In this regard, a few peptides have been showing promising results in the amelioration of detrimental immune responses. Some commercially available drugs, including corticosteroids and antibiotics, with known anti-inflammatory effects, have also shown activity in TREM-1 signaling. Therefore, considering the potential of this receptor as a therapeutic target, the present review encompasses the main compounds explored so far in TREM-1 modulation, highlighting and critically discussing its effects and major drawbacks of such approaches.

The role of the triggering receptor expressed on myeloid cells-1 (TREM-1) in non-bacterial infections

The triggering receptor expressed on myeloid cells 1 (TREM-1) is a receptor of the innate immune system, expressed mostly by myeloid cells and primarily associated with pro- inflammatory responses. Although the exact nature of its ligands has not yet been fully elucidated, many microorganisms or danger signals have been proposed as inducers of its activation or the secretion of sTREM-1, the soluble form with putative anti-inflammatory effects. In the course of the 20 years since its first description, several studies have investigated the involvement of TREM-1 in bacterial infections. However, the number of studies describing the role of TREM-1 in fungal, viral and parasite-associated infections has only increased in the last few years, showing a diverse contribution of the receptor in these scenarios, with beneficial or detrimental activities depending on the context. Therefore, this review aims to discuss how TREM-1 may influence viral, fungal and parasitic infection outcomes, highlighting its potential as a therapeutic target and biomarker for diagnosis and prognosis of non-bacterial infectious diseases.

Role of TREM2 in Alzheimer's Disease and its Consequences on β- Amyloid, Tau and Neurofibrillary Tangles

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Alzheimer's Disease (AD) is age-related neurodegenerative disorder recognized by a steadily gradual cognitive decline that has devastating personal and socioeconomic implications. Recently, some genetic factors for AD have been identified which attracted wide attention of researchers in different areas of AD biology and possible new therapeutic targets. Alternative forms of triggering receptor expressed on myeloid cells 2 (TREM2) genes are examples of such risk factors, which contribute higher risk for developing AD. Comprehending TREM2 function pledge to provide salient insight into how neuroinflammation contributes to AD pathology. The dearth of microglial TREM2 shepherd to augmented tau pathology is couple with frequent enhancement of activated neuronal stress kinases. The involvement of TREM2 in the regulation of tau-associated innate immune response of the CNS has clearly demonstrated through these findings. However, whether decrease level of TREM2 assists pathology of tau through changed clearance and pathological escalation of tau or through direct contact between microglia and neuron and any alternative possible mechanisms need to examine. This review briefly summarizes distinct functional roles of TREM2 in AD pathology and highlights the TREM2 gene regulation. We have also addressed the impact of TREM2 on β-amyloid plaques and tau pathology in Alzheimer’s disease.

TREM-1 Attenuates RIPK3-mediated Necroptosis in Hyperoxia-induced Lung Injury in Neonatal Mice

Hyperoxia-induced injury to the developing lung, impaired alveolarization, and dysregulated vascularization are critical factors in the pathogenesis of bronchopulmonary dysplasia (BPD); however, mechanisms for hyperoxia-induced development of BPD are not fully known. In this study, we show that TREM-1 (triggering receptor expressed on myeloid cells 1) is upregulated in hyperoxia-exposed neonatal murine lungs as well as in tracheal aspirates and lungs of human neonates with respiratory distress syndrome and BPD as an adaptive response to survival in hyperoxia. Inhibition of TREM-1 function using an siRNA approach or deletion of the Trem1 gene in mice showed enhanced lung inflammation, alveolar damage, and mortality of hyperoxia-exposed neonatal mice. The treatment of hyperoxia-exposed neonatal mice with agonistic TREM-1 antibody decreased lung inflammation, improved alveolarization, and was associated with diminished necroptosis-regulating protein RIPK3 (receptor-interacting protein kinase 3). Mechanistically, we show that TREM-1 activation alleviates lung inflammation and improves alveolarization through downregulating RIPK3-mediated necroptosis and NLRP3 (nucleotide-binding oligomerization domain-like receptor containing pyrin domain 3) inflammasome activation in hyperoxia-exposed neonatal mice. These data show that activating TREM-1, enhancing angiopoietin 1 signaling, or blocking the RIPK3-mediated necroptosis pathway may be used in new therapeutic interventions to control adverse effects of hyperoxia in the development of BPD.

Triggering Receptor Expressed on Myeloid Cells-1 Inhibitor Targeted to Endothelium Decreases Cell Activation

TREM-1 (Triggering Receptor Expressed on Myeloid cells-1) is an immunoreceptor expressed on neutrophils, monocytes/macrophages, and endothelial cells. It amplifies the inflammatory response driven by Toll-Like Receptors (TLR) engagement. The pharmacological inhibition of TREM-1 confers protection in several pre-clinical models of acute inflammation. In this study, we aimed to investigate the role of TREM-1 in endothelial cells using a sneaking ligand construct (SLC) inhibiting TREM-1 in the endothelium. The SLC was made of 3 modules: an E-selectin targeting domain, a Pseudomonas aeruginosa exotoxin a translocation domain, and a 7 aa peptide (LSKSLVF) that contains the interaction site between TREM-1 and its adaptor protein DAP-12. SLC peptide was effectively picked up by endothelial cells following LPS stimulation. It decreased LPS induced TREM-1 up-regulation and cell activation, neutrophils extravasation, and improved median survival time during experimental peritonitis in mice. We reported that a targeted endothelial TREM-1 inhibition is able to dampen cell activation and to confer protection during septic shock in mice. The use of such cell-specific, ligand- independent TREM-1 inhibitors deserve further investigations during acute or chronic inflammatory disorders.

TREM2 Acts as a Tumor Suppressor in Colorectal Carcinoma through Wnt1/β-catenin and Erk Signaling

TREM2 (triggering receptor expressed on myeloid cells) is involved in the development of malignancies. However, the function of TREM2 in colorectal cancer has not been clearly elucidated. Here, we investigated TREM2 function for the first time in colorectal epithelial cancer cells and demonstrated that TREM2 is a novel tumor suppressor in colorectal carcinoma. Blockade of TREM2 significantly promoted the proliferation of HT29 colorectal carcinoma cells by regulating cell cycle-related factors, such as p53 phosphorylation and p21 and cyclin D1 protein levels. HT29 cell migration was also increased by TREM2 inhibition via MMP9 (matrix metalloproteinase 9) expression upregulation. Furthermore, we found that the tumor suppressor effects of TREM2 were associated with Wnt/β-catenin and extracellular signal-regulated kinase (ERK) signaling. Importantly, the effect of TREM2 in the suppression of tumor development was demonstrated by in vivo and in vitro assays, as well as in human colon cancer patient tissue arrays. Overall, our results identify TREM2 as a potential prognostic biomarker and therapeutic target for colorectal cancer.

Targeted endothelial gene deletion of triggering receptor expressed on myeloid cells-1 protects mice during septic shock

Aims

TREM-1 (Triggering Receptor Expressed on Myeloid cells-1) is an immunoreceptor expressed on neutrophils and monocytes/macrophages whose role is to amplify the inflammatory response driven by Toll-Like Receptors engagement. The pharmacological inhibition of TREM-1 confers protection in several pre-clinical models of acute inflammation. In this study, we aimed to decipher the role of TREM-1 on the endothelium.

Methods and results

We first showed by qRT-PCR, flow cytometry and confocal microscopy that TREM-1 was expressed in human pulmonary microvascular endothelial cells as well as in mouse vasculature (aorta, mesenteric artery, and pulmonary vessels). TREM-1 expression was upregulated following septic insult. We next observed that TREM-1 engagement impaired mouse vascular reactivity and promoted vascular inflammation. The pharmacological inhibition of TREM-1 (using the synthetic inhibitory peptide LR12) prevented these disorders both in vitro and in vivo. We generated endothelium-conditional Trem-1 ko mice (EndoTREM-1-/-) and submitted them to a caecal ligation and puncture-induced septic shock. As compared with wild-type littermates, targeted endothelial Trem-1 deletion conferred protection during septic shock in modulating inflammatory cells mobilization and activation, in restoring vasoreactivity, and in improving the survival.

Conclusion

We reported that TREM-1 is expressed and inducible in endothelial cells and plays a direct role in vascular inflammation and dysfunction. The targeted deletion of endothelial Trem-1 conferred protection during septic shock in modulating inflammatory cells mobilization and activation, restoring vasoreactivity, and improving survival. The effect of TREM-1 on vascular tone, while impressive, deserves further investigations including the design of endothelium-specific TREM-1 inhibitors.

Non-parenchymal TREM-2 protects the liver from immune-mediated hepatocellular damage

OBJECTIVE

Liver injury impacts hepatic inflammation in part via Toll-like receptor (TLR) signalling. Triggering receptor expressed on myeloid cells 2 (TREM-2) modulates TLR4-mediated inflammation in bone marrow (BM)-derived macrophages but its function in liver injury is unknown. Here we hypothesised that the anti-inflammatory effects of TREM-2 on TLR signalling may limit hepatic injury.

DESIGN

TREM-2 expression was analysed in livers of humans with various forms of liver injury compared with control individuals. Acute and chronic liver injury models were performed in wild type and Trem-2-/- mice. Primary liver cells from both genotypes of mice were isolated for in vitro experiments.

RESULTS

TREM-2 was expressed on non-parenchymal hepatic cells and induced during liver injury in mice and man. Mice lacking TREM-2 exhibited heightened liver damage and inflammation during acute and repetitive carbon tetrachloride and acetaminophen (APAP) intoxication, the latter of which TREM-2 deficiency was remarkably associated with worsened survival. Liver damage in Trem-2-/- mice following chronic injury and APAP challenge was associated with elevated hepatic lipid peroxidation and macrophage content. BM transplantation experiments and cellular reactive oxygen species assays revealed effects of TREM-2 in the context of chronic injury depended on both immune and resident TREM-2 expression. Consistent with effects of TREM-2 on inflammation-associated injury, primary hepatic macrophages and hepatic stellate cells lacking TREM-2 exhibited augmented TLR4-driven proinflammatory responses.

CONCLUSION

Our data indicate that by acting as a natural brake on inflammation during hepatocellular injury, TREM-2 is a critical regulator of diverse types of hepatotoxic injury.

The characteristics and pivotal roles of triggering receptor expressed on myeloid cells-1 in autoimmune diseases

Triggering receptor expressed on myeloid cells-1 (TREM-1) engagement can directly trigger inflammation or amplify an inflammatory response by synergizing with TLRs or NLRs. Autoimmune diseases are a family of chronic systemic inflammatory disorders. The pivotal role of TREM-1 in inflammation makes it important to explore its immunological effects in autoimmune diseases. In this review, we summarize the structural and functional characteristics of TREM-1. Particularly, we discuss recent findings on TREM-1 pathway regulation in various autoimmune diseases, including rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), inflammatory bowel disease (IBD), type 1 diabetes (T1D), and psoriasis. This receptor may potentially be manipulated to alter the inflammatory response to chronic inflammation and possible therapies are explored in this review.

Genome-wide association studies for tick resistance in Bos taurus × Bos indicus crossbred cattle: A deeper look into this intricate mechanism

Rhipicephalus (Boophilus) microplus is the main cattle ectoparasite in tropical areas. Gir × Holstein crossbred cows are well adapted to different production systems in Brazil. In this context, we performed genome-wide association study (GWAS) and post-GWAS analyses for R. microplus resistance in an experimental Gir × Holstein F₂ population. Single nucleotide polymorphisms (SNP) identified in GWAS were used to build gene networks and to investigate the breed of origin for its alleles. Tick artificial infestations were performed during the dry and rainy seasons. Illumina BovineSNP50 BeadChip (Illumina Inc., San Diego, CA) and single-step BLUP procedure was used for GWAS. Post-GWAS analyses were performed by gene ontology terms enrichment and gene transcription factors networks, generated from enriched transcription factors, identified from the promoter sequences of selected gene sets. The genetic origin of marker alleles in the F₂ population was assigned using the breed of origin of alleles approach. Heritability estimates for tick counts were 0.40 ± 0.11 in the rainy season and 0.54 ± 0.11 in the dry season. The top ten 0.5-Mbp windows with the highest percentage of genetic variance explained by SNP markers were found in chromosomes 10 and 23 for both the dry and rainy seasons. Gene network analyses allowed the identification of genes involved with biological processes relevant to immune system functions (TREM1, TREM2, and CD83). Gene-transcription factors network allowed the identification of genes involved with immune functions (MYO5A, TREML1, and PRSS16). In resistant animals, the average proportion of animals showing significant SNPs with paternal and maternal alleles originated from Gir breed was 44.8% whereas the proportion of animals with both paternal and maternal alleles originated from Holstein breed was 11.3%. Susceptible animals showing both paternal and maternal alleles originated from Holstein breed represented 44.6% on average, whereas both paternal and maternal alleles originated from Gir breed animals represented 9.3%. This study allowed us to identify candidate genes for tick resistance in Gir × Holstein crossbreds in both rainy and dry seasons. According to the origin of alleles analysis, we found that most animals classified as resistant showed 2 alleles from Gir breed, while the susceptible ones showed alleles from Holstein. Based on these results, the identified genes may be thoroughly investigated in additional experiments aiming to validate their effects on tick resistance phenotype in cattle.

Vitamin D suppresses macrophage infiltration by down-regulation of TREM-1 in diabetic nephropathy rats

This study intends to investigate the effect of active vitamin D (VD) on the expression of triggering receptor expressed on myeloid cells-1 (TREM-1) in the renal tissues of diabetic nephropathy (DN) rats and to explore the impact of TREM-1 on macrophage adhesion and migration. We find that the expressions of TREM-1 and CD68 protein are higher in DN rats compared with rats in the normal control group and that these changes are decreased in the DN + VD group. In vitro, the capacity for macrophage adhesion and migration and the expression of TREM-1 are increased under high-glucose conditions, but VD inhibits this progress. TREM-1 siRNA decreases high-glucose-induced macrophage adhesion and migration, whereas over-expression of TREM-1 inhibits its action. However, VD cannot suppress high glucose-induced TREM-1 expression and macrophage adhesion and migration when TREM-1 is over-expressed. These results demonstrate that VD can suppress macrophage adhesion and migration by reducing the expression of TREM-1.

Soluble Triggering Receptor Expressed on Myeloid Cells 1 in lung cancer

Soluble Triggering Receptor Expressed on Myeloid Cells 1 (sTREM-1) can be found in the sera of patients with infectious, autoimmune and malignant diseases. The primary objective of this study was to investigate the prognostic significance of sTREM-1 in lung cancer patients. We analyzed the sera of 164 patients with lung cancer of all histologies and all stages at the time of diagnosis. We employed an ELISA using the anti-TREM-1 clone 6B1.1G12 mAb and recombinant human TREM-1. Patient data was collected retrospectively by chart review. In ROC-analysis, a sTREM-1 serum level of 163.1 pg/ml showed the highest Youden-Index. At this cut-off value sTREM-1 was a marker of short survival in patients with NSCLC (median survival 8.5 vs. 13.3 months, p = 0.04). A Cox regression model showed stage (p < 0.001) and sTREM-1 (p = 0.011) to indicate short survival. There were no differences in sTREM-1 serum values among patients with or without infection, pleural effusion or COPD. sTREM-1 was not associated with metastasis at the time of diagnosis and was not a predictor of subsequent metastasis. In SCLC patients sTREM-1 levels were lower than in NSCLC patients (p = 0.001) and did not predict survival. sTREM-1 did not correlate with CRP or the number of neutrophils. In non-small cell lung cancer patients, sTREM-1 in serum has prognostic significance.

TREM-1 and its potential ligands in non-infectious diseases: from biology to clinical perspectives

Triggering receptor expressed on myeloid cells-1 (TREM-1) is expressed on the majority of innate immune cells and to a lesser extent on parenchymal cells. Upon activation, TREM-1 can directly amplify an inflammatory response. Although it was initially demonstrated that TREM-1 was predominantly associated with infectious diseases, recent evidences shed new light into its role in sterile inflammatory diseases. Indeed, TREM-1 receptor and its signaling pathways contribute to the pathology of several non-infectious acute and chronic inflammatory diseases, including atherosclerosis, ischemia reperfusion-induced tissue injury, colitis, fibrosis and cancer. This review, aims to give an extensive overview of TREM-1 in non-infectious diseases, with the focus on the therapeutic potential of TREM-1 intervention strategies herein. In addition, we provide the reader with a functional enrichment analysis of TREM-1 signaling pathway and potential TREM-1 ligands in these diseases, obtained via in silico approach. We discuss pre-clinical studies which show that TREM-1 inhibition, via synthetic soluble TREM-1 protein mimickers, is effective in treating (preventing) specific inflammatory disorders, without significant effects on antibacterial response. Further research aimed at identifying specific TREM-1 ligands, in different inflammatory disorders, is required to further unravel the role of this receptor, and explore new avenues to modulate its function.

Effect of TREM-1 blockade and single nucleotide variants in experimental renal injury and kidney transplantation

Renal ischemia reperfusion (IR)-injury induces activation of innate immune response which sustains renal injury and contributes to the development of delayed graft function (DGF). Triggering receptor expressed on myeloid cells-1 (TREM-1) is a pro-inflammatory evolutionary conserved pattern recognition receptor expressed on a variety of innate immune cells. TREM-1 expression increases following acute and chronic renal injury. However, the function of TREM-1 in renal IR is still unclear. Here, we investigated expression and function of TREM-1 in a murine model of renal IR using different TREM-1 inhibitors: LP17, LR12 and TREM-1 fusion protein. In a human study, we analyzed the association of non-synonymous single nucleotide variants in the TREM1 gene in a cohort comprising 1263 matching donors and recipients with post-transplant outcomes, including DGF. Our findings demonstrated that, following murine IR, renal TREM-1 expression increased due to the influx of Trem1 mRNA expressing cells detected by in situ hybridization. However, TREM-1 interventions by means of LP17, LR12 and TREM-1 fusion protein did not ameliorate IR-induced injury. In the human renal transplant cohort, donor and recipient TREM1 gene variant p.Thr25Ser was not associated with DGF, nor with biopsy-proven rejection or death-censored graft failure. We conclude that TREM-1 does not play a major role during experimental renal IR and after kidney transplantation.

Increased expression of triggering receptor expressed on myeloid cells 1 and 2 in inflamed human gingiva

BACKGROUND AND OBJECTIVE

Periodontitis is an infectious disease in which the host immune and inflammatory responses play essential roles in resistance to bacterial infection, as well as the induction of tissue destruction if the immune response is dysregulated. The triggering receptor expressed on myeloid cells (TREMs) modulates inflammatory and innate immune signaling. TREM-1 is considered as an amplifier of the immune response, while TREM-2 is a negative regulator that has yet to be explored in periodontal disease before. We hypothesized that TREMs participated in the innate immune responses during the pathogenesis of periodontitis. Therefore, the aim of this study was to evaluate TREM-1 and TREM-2 expression in the gingival tissues from patients with chronic periodontitis and healthy subjects as well as their correlation with clinical periodontal parameters. This study is the first to identify TREM-2 in periodontal tissue, as well as the protein expression changes of TREM-1 and TREM-2 in periodontal tissues.

MATERIAL AND METHODS

Gingival tissue sections were collected from 31 healthy subjects and 53 patients with chronic periodontitis. Immunohistochemistry and quantitative real-time polymerase chain reaction were employed to evaluate the protein and mRNA expression of these receptors in gingival tissues. The recorded clinical parameters were probing depth, clinical attachment loss, plaque index and bleeding on probing.

RESULTS

In addition to myeloid cells in gingival connective tissues, TREM-1 and TREM-2 were also found expressed in gingival epithelial cells. In particular, TREM-1 was detected in almost all gingival epithelium from both healthy and inflamed biopsies. The expression levels of TREM-1 and TREM-2 were significantly increased in the periodontitis group compared to the healthy group. Increased levels of these receptors are to be positively correlated with site-specific periodontal parameters.

CONCLUSION

The increased expression of TREM-1 and TREM-2 levels in periodontitis may confer diagnostic and potential therapeutic targets as well as indicating their association with the clinical severity of the disease.

Association of decreased serum sTREM-1 level with the severity of coronary artery disease: Inhibitory effect of sTREM-1 on TNF-α- and oxLDL-induced inflammatory reactions in endothelial cells

Soluble triggering receptor expressed on myeloid cells 1 (sTREM-1) is closely involved in autoimmune diseases and inflammatory reactions. We aimed to investigate whether serum sTREM-1 is related to coronary artery disease (CAD) and to evaluate the biological effects of sTREM-1 in cell experiments.This cross-sectional study included 263 consecutive patients with angiographically documented CAD, who were admitted for diagnosis and interventional treatment of CAD (CAD group), with 162 participants without CAD serving as controls (control group). Serum levels of sTREM-1 and high sensitivity C reactive protein (hsCRP) were determined in all participants. In cell experiments, the influence of sTREM-1 on tumor necrosis factor-α (TNF-α)- or oxidized low-density lipoprotein (oxLDL)-induced inflammatory reactions was evaluated in human umbilical vein endothelial cells (HUVECs).Serum level of sTREM-1 was significantly lower in CAD patients than in controls (P < 0.001). sTREM-1 values were related to the number of diseased coronary arteries (Spearman r = -0.413, P < 0.001) and the severity represented by Gensini score (Pearson r = -0.336, P < 0.001). Multivariable logistic regression analysis revealed that decreased sTREM-1 were independent determinants of CAD (OR = 0.428, P < 0.001). In cell experiments, recombinant sTREM-1 protein concentration-dependently inhibited the expression of IL-1β, IL-6, TNF-α, VCAM-1, and ICAM-1 induced by TNF-α or oxLDL in HUVECs.This study demonstrates that decreased serum sTREM-1 level is significantly associated with the presence and severity of CAD. sTREM-1 restrains inflammatory reaction in endothelial cells, suggesting that it might be a potential vascular protective factor.

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.

GPR84 and TREM-1 Signaling Contribute to the Pathogenesis of Reflux Esophagitis

Gastro-esophageal reflux disease (GERD) is one of the most common disorders in gastroenterology. Patients present with or without increased acid exposure indicating a nonuniform etiology. Thus, the common treatment with proton pump inhibitors (PPIs) fails to control symptoms in up to 40% of patients. To further elucidate the pathophysiology of the condition and explore new treatment targets, transcriptomics, proteomics and histological methods were applied to a surgically induced subchronic reflux esophagitis model in Wistar rats after treatment with either omeprazole (PPI) or STW5, a herbal preparation shown to ameliorate esophagitis without affecting refluxate pH. The normal human esophageal squamous cell line HET-1A and human endoscopic biopsies were used to confirm our findings to the G-protein-coupled receptor (GPR) 84 in human tissue. Both treatments reduced reflux-induced macroscopic and microscopic lesions of the esophagi as well as known proinflammatory cytokines. Proteomic and transcriptomic analyses identified CINC1-3, MIP-1/3α, MIG, RANTES and interleukin (IL)-1β as prominent mediators in GERD. Most regulated cyto-/chemokines are linked to the TREM-1 signaling pathway. The fatty acid receptor GPR84 was upregulated in esophagitis but significantly decreased in treated groups, a finding supported by Western blot and immunohistochemistry in both rat tissue and HET-1A cells. GPR84 was also found to be significantly upregulated in patients with grade B reflux esophagitis. The expression of GPR84 in esophageal tissue and its potential involvement in GERD are reported for the first time. IL-8 (CINC1-3) and the TREM-1 signaling pathway are proposed, besides GPR84, to play an important role in the pathogenesis of GERD.org.

Ameliorative effects of glycine in an experimental nonalcoholic steatohepatitis and its correlation between TREM-1 and TREM-2

Inflammation plays an important role in Nonalcoholic Steatohepatitis (NASH), triggering receptor expressed on myeloid cells-1 and 2 (TREM-1 and TREM-2) modulates inflammatory and innate immune, they have been investigated in various inflammatory diseases, but not in NASH. Meanwhile we added glycine in HFO (HFOG) to investigate if the liver pathologic relief is related with TREM-1 and TREM-2. Liver tissue staining and serum indexes showed that the NASH was successful from the 4(th) weekend and glycine can improved many features of NASH. Results from Q-PCR and ELISA study showed that compareaded with control, TREM-1 is upregulated and TREM-2 is downregulated respectively in 4 and 8-week NASH model (TREM-1: p < 0.001; TREM-2: p < 0.001).Compared with HFO group, HFOG group with an extra 5% Glycine into the diet of NASH, we found that all model liver pathologic and serum indexes ameliorated in this group. Furthermore, Results from Q-PCR and ELISA study showed that compareaded with HFO group, TREM-2 of this group is upregulated and TREM-1 is downregulated respectively from the 4(th) weekend, which is more significant at the 8(th) weekend (TREM-1: p <0.001; TREM-2: p =0.048). Pearson correlation showed that TREM-1 and TREM-2 were closely associated with serum ET, TNF-α, TLR-4 and PC III. Besides, using multiple-stepwise regression analysis, we found that the ameliorative effects of glycine in HFOG was mainly related to its counteraction of PC III, TREM-1 and upregulation of TREM-2. Furthermore, we detected the expression of TREM-1 and TREM-2 in gall stone patients without drinking excessively before undergoing cholecystectomy, and found that the rise of TREM-1 and reduction of TREM-2 was close associated with the severity of fatty liver. To conclude, our results support the concept that TREM-1 and TREM-2 were close strongly linked to NASH and NALFD. Glycine can relieve NASH by its anti-fibrosis effect, and this ameliorative effect is related to the expression change of TREM-1/2 to some extent.

Osteoimmunology: Major and Costimulatory Pathway Expression Associated with Chronic Inflammatory Induced Bone Loss

The field of osteoimmunology has emerged in response to the range of evidences demonstrating the close interrelationship between the immune system and bone metabolism. This is pertinent to immune-mediated diseases, such as rheumatoid arthritis and periodontal disease, where there are chronic inflammation and local bone erosion. Periprosthetic osteolysis is another example of chronic inflammation with associated osteolysis. This may also involve immune mediation when occurring in a patient with rheumatoid arthritis (RA). Similarities in the regulation and mechanisms of bone loss are likely to be related to the inflammatory cytokines expressed in these diseases. This review highlights the role of immune-related factors influencing bone loss particularly in diseases of chronic inflammation where there is associated localized bone loss. The importance of the balance of the RANKL-RANK-OPG axis is discussed as well as the more recently appreciated role that receptors and adaptor proteins involved in the immunoreceptor tyrosine-based activation motif (ITAM) signaling pathway play. Although animal models are briefly discussed, the focus of this review is on the expression of ITAM associated molecules in relation to inflammation induced localized bone loss in RA, chronic periodontitis, and periprosthetic osteolysis, with an emphasis on the soluble and membrane bound factor osteoclast-associated receptor (OSCAR).