Silencing Triggering Receptors Expressed on Myeloid Cells-1 Impaired the Inflammatory Response to Oxidized Low-Density Lipoprotein in Macrophages

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.

Role of triggering receptor expressed on myeloid cells-1 in the mechanotransduction signaling pathways that link low shear stress with inflammation

This study sought to investigate the role of triggering receptor expressed on myeloid cells-1 (TREM-1) in the mechanotransduction signaling pathways that link low shear stress with inflammation. Human coronary artery endothelial cells, human coronary artery smooth muscle cells, and THP-1 monocytes were co-cultured and exposed to varying endothelial shear stress (ESS) conditions: low (5 ± 3 dynes/cm²), medium (10 ± 3 dynes/cm²), and high (15 ± 3 dynes/cm²). We showed that low ESS increased the expression of TREM-1 by the cultured cells leading to increased production of inflammatory mediators and matrix-degrading enzymes, whereas high ESS did not have a significant effect in the expression of TREM-1 and inflammatory mediators. Furthermore, TREM-1 transcriptional inhibition with siRNA in endothelial cells, smooth muscle cells, and monocytes exposed to low ESS, led to a significant reduction in the production of vascular inflammatory mediators and matrix-degrading enzymes. Additionally, we identified the transcription factors that appear to upregulate the TREM-1 gene expression in response to low ESS. To the best of our knowledge, this is the first study to investigate the pathophysiologic association and molecular pathways that link low ESS, TREM-1, and inflammation using a sophisticated in-vitro model of atherosclerosis. Future studies on animals and humans are warranted to investigate the potential of TREM-1 inhibitors as adjunctive anti-atherosclerotic therapies.

Triggering receptor expressed on myeloid cells-1 deletion in mice attenuates high-fat diet-induced obesity

INTRODUCTION

The low-grade inflammatory state present in obesity leads to the development and perpetuation of comorbidities associated with obesity. Our laboratory has been working for several years on an amplification loop of the inflammatory response mediated by TREM-1 (Triggering Receptor of Expressed on Myeloid Cells-1). It is implicated in many acute (septic shock) and chronic (IBD) inflammatory diseases. Previously, TREM-1 has been shown to be overexpressed in adipose and liver tissue in obese and diabetic patients, but its impact has never been characterized in these pathologies.

METHODS

Our hypothesis is that TREM-1 plays a major role in the generation and perpetuation of inflammation during obesity and its associated complication (Insulin resistance and cardiac dysfunction). We assessed TREM-1 protein expression by western blot and immunofluorescence in omental and subcutaneous (pre-)adipocyte. Moreover, we submitted mice to a high-fat diet and investigated the effects of the genetic Trem1 deletion (trem1 KO mice).

RESULTS

We showed, for the first time, that TREM-1 is expressed and is functional in subcutaneous and omental (pre-)adipocytes. In the mouse model of high-fat diet-induced obesity, we found that Trem1 suppression limited weight gain, insulin resistance and inflammation in white adipose tissue and liver.

DISCUSSION/CONCLUSION

Our results reveal the trem1 KO model can be viewed as a preventive model and that TREM-1 seems to play an important role in the development of obesity and its associated complication. It could therefore be a new therapeutic target in this context.

Elevated Plasma Soluble Triggering Receptor Expressed on Myeloid Cells-1 Level in Patients with Acute Coronary Syndrome (ACS): A Biomarker of Disease Severity and Outcome

Background and Aims

Plasma levels of soluble triggering receptor expressed on myeloid cells (sTREM-1) reflect innate immune cell activation. We sought to evaluate sTREM-1 levels in patients with acute coronary syndrome (ACS) and their predictive value for disease severity and outcome.

Methods

Plasma sTREM-1 levels were prospectively measured by ELISA in 121 consecutive patients with new-onset (≤24 h) chest pain at arrival to the emergency department (ED) and 73 healthy controls. Secondary endpoints were the association of plasma levels of sTREM-1 with day 30 and month 6 major adverse cardiovascular events (MACE) defined as death, ACS, stroke, and need for coronary revascularization, as well as with CAD severity. The primary endpoint of the study was the association of plasma sTREM-1 level at the time of admission to the ED with a diagnosis of ACS at day 30.

Results

Fifty-nine patients (48.7%) were diagnosed with ACS and 62 (51.3%) with nonspecific chest pain (NSCP). Median plasma sTREM-1 level at admission was significantly higher in the ACS group than the NSCP group and the control group (539.4 ± 330.3 pg/ml vs. 432.5 ± 196.4 pg/ml vs. 230.1 ± 85.5 pg/ml, respectively; P < 0.001) and positively correlated with the number of stenosed/occluded coronary arteries on angiography (P < 0.001). On logistic regression analysis, higher sTREM-1 levels predicted definite ACS vs. NSCP determined on day 30 (OR 1.29, 95% CI 1.07-1.54, P = 0.01) as well as with recurrent ACS (P = 0.04) and stroke (P = 0.02) at 6 months.

Conclusions

Plasma sTREM-1 levels are significantly elevated in patients with ACS and might serve as a biomarker differentiating ACS from NSCP in the ED as well as an inflammatory biomarker for coronary artery disease severity and outcome.

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.

Innate signaling within the central nervous system recruits protective neutrophils

There is great interest in understanding how the central nervous system (CNS) communicates with the immune system for recruitment of protective responses. Infiltrating phagocytic monocytes and granulocytes are implicated in neuroinflammation in multiple sclerosis and its animal model experimental autoimmune encephalomyelitis (EAE). To investigate how CNS endogenous signals can be harnessed to promote anti-inflammatory programs, we have used a particulate Toll-like receptor 9 and nucleotide-oligomerization domain 2 bispecific innate ligand (MIS416), to address whether its phagocytosis within the CNS recruits protective myeloid cells. We find that MIS416 injected intrathecally into the cerebrospinal fluid via the cisterna magna induced a local chemokine response that recruited blood-derived monocytes and neutrophils to the CNS. These cells phagocytosed MIS416. The increase in EAE severity normally seen from time of onset did not occur in mice receiving MIS416. This suppression of disease symptoms was dependent on expression of the type I interferon receptor (IFNAR). Transfer of intrathecal MIS416-induced neutrophils suppressed EAE in recipient mice, while monocytes did not transfer protection. MIS416-induced neutrophils showed increased IL-10 expression that was IFNAR1-driven. In contrast to intrathecal administration, intravenous administration of MIS416 led to monocyte but not neutrophil infiltration to the CNS. We thus identify a CNS-intrinsic and -specific phagocytosis-induced recruitment of anti-inflammatory neutrophils that contribute to CNS homeostasis and may have therapeutic potential.

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.

TREM-1; Is It a Pivotal Target for Cardiovascular Diseases?

Cardiovascular diseases (CVDs) are as menacing as ever and still continue to kill adults worldwide, notwithstanding tremendous efforts to decrease their consequent mortality and morbidity. Lately, a growing body of research indicated that inflammation plays a pivotal role in the pathogenesis and complications of CVDs. A receptor of the immunoglobulin superfamily, triggering receptors expressed on myeloid cells-1 (TREM-1) was shown to induce and amplify the inflammation in both acute and chronic disease’ pathogenesis and progression, which hence makes it one of the most important complication factors of CVDs. Thus, studies endeavored to investigate the role played by TREM-1 in CVDs with respect to their etiologies, complications, and possible therapeutics. We examined here, for the first time, the most relevant studies regarding TREM-1 involvement in CVDs. We critically analyzed and summarized our findings and made some suggestions for furtherance of the investigations with the aim to utilize TREM-1 and its pathways for diagnostic, management, and prognosis of CVDs. Overall, TREM-1 was found to be involved in the pathogenesis of acute and chronic cardiovascular conditions, such as acute myocardial infarction (AMI) and atherosclerosis. Although most therapeutic approaches are yet to be elucidated, our present research outcome displays a promising future to utilizing the TREM-1 pathway as a potential target for understanding and managing CVDs.

Prognostic Utility of Soluble TREM-1 in Predicting Mortality and Cardiovascular Events in Patients With Acute Myocardial Infarction

BACKGROUND

Triggering receptor expressed on myeloid cells-1 (TREM-1) is thought to be critical for inflammatory signal amplification and involved in the development of atherosclerosis. TREM-1 is significantly increased in patients with myocardial infarction. The aim of this study was to investigate the association between soluble TREM-1 (sTREM-1) and mortality and cardiovascular events in patients with acute myocardial infarction.

METHODS AND RESULTS

We included 838 consecutive patients with acute myocardial infarction from October 7, 2012 to December 5, 2014. Blood samples were collected from patients with acute myocardial infarction immediately after diagnosis. During follow-up, 88 patients died, and 180 patients reached the combined end points of major adverse cardiovascular event (MACE). Patients with high sTREM-1 (higher than the median) had increased risk of all-cause mortality and MACE compared with those with low sTREM-1 (log-rank test, P<0.001). After adjustment for confounding risk factors by Cox regression analysis, high sTREM-1 remained an independent predictor of all-cause mortality (hazard ratio, 1.978; 95% confidence interval, 1.462-2.675; P<0.001) and MACE (hazard ratio, 2.413; 95% confidence interval, 2.022-2.879; P<0.001). After the addition of sTREM-1 to the reference model, the C-statistic for all-cause mortality increased from 0.86 to 0.89, and the difference was 0.023 (95% confidence interval, 0.0009-0.0477), and the C-statistic for MACE increased from 0.71 to 0.80, and the difference was 0.087 (95% confidence interval, 0.053-0.122). sTREM-1 levels were consistently positively associated with risks of all-cause mortality and MACE in various subpopulations, and there was no significant interaction among prespecified subgroups.

CONCLUSIONS

sTREM-1 was significantly associated with all-cause mortality and MACE, independent of established conventional risk factors in patients with acute myocardial infarction.

Evolving targets for the treatment of atherosclerosis

Atherosclerosis is a progressive disease of large arteries and a leading cause of cardiovascular diseases and stroke. Chronic inflammation, aberrant immune response, and disturbances to key enzymes involved with lipid metabolism are characteristic features of atherosclerosis. Apart from targeting the derangements in lipid metabolism, therapeutic modulation to regulate chronic inflammation and the immune system response may prove to be very promising strategies in the management of atherosclerosis. In recent years, various targets have been studied for the treatment of atherosclerosis. PCSK9, a serine protease, actively targets the LDL-R and causes lysosomal degradation, which leads to excessive accumulation of LDL-C. Regulatory T cells (Tregs) and Triggering Receptor Expressed on Myeloid cells-1 (TREM-1) affects the adaptive and innate immune response, respectively, and thus, therapeutic intervention of either of these targets would directly modulate disease progression. Advanced atherosclerotic lesions are characterized by an accumulation of apoptotic cells. Cluster of differentiation-47 (CD47), an anti-phagocytic known as the "don't eat me" signaling molecule, inhibits efferocytosis, which causes accumulation of cell debris in plaque. ADAMTS and Notch signaling potentially affect the formation of neointima by modulation of extracellular matrix components such as macrophages and vascular smooth muscle cells. This review provides insights on the molecular targets for therapeutic intervention of atherosclerosis, their effect at various stages of atherosclerosis development, and the therapies that have been designed and currently being evaluated in clinical trials.

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.

The role of damage- and pathogen-associated molecular patterns in inflammation-mediated vulnerability of atherosclerotic plaques

Atherosclerosis is a chronic inflammatory disease resulting in the formation of the atherosclerotic plaque. Plaque formation starts with the inflammation in fatty streaks and progresses through atheroma, atheromatous plaque, and fibroatheroma leading to development of stable plaque. Hypercholesterolemia, dyslipidemia, and hyperglycemia are the risk factors for atherosclerosis. Inflammation, infection with viruses and bacteria, and dysregulation in the endothelial and vascular smooth muscle cells leads to advanced plaque formation. Death of the cells in the intima due to inflammation results in secretion of damage-associated molecular patterns (DAMPs) such as high mobility group box 1 (HMGB1), receptor for advanced glycation end products (RAGE), alarmins (S100A8, S100A9, S100A12, and oxidized low-density lipoproteins), and infection with pathogens leads to secretion of pathogen-associated molecular patterns (PAMPs) such as lipopolysaccharides, lipoteichoic acids, and peptidoglycans. DAMPs and PAMPs further activate the inflammatory surface receptors such as TREM-1 and toll-like receptors and downstream signaling kinases and transcription factors leading to increased secretion of pro-inflammatory cytokines such as tumor necrosis factor α, interleukin (IL)-1β, IL-6, and interferon-γ and matrix metalloproteinases (MMPs). These mediators and cytokines along with MMPs render the plaque vulnerable for rupture leading to ischemic events. In this review, we have discussed the role of DAMPs and PAMPs in association with inflammation-mediated plaque vulnerability.

Microbiota, Immune Subversion, and Chronic Inflammation

Several host-adapted pathogens and commensals have evolved mechanisms to evade the host innate immune system inducing a state of low-grade inflammation. Epidemiological studies have also documented the association of a subset of these microorganisms with chronic inflammatory disorders. In this review, we summarize recent studies demonstrating the role of the microbiota in chronic inflammatory diseases and discuss how specific microorganisms subvert or inhibit protective signaling normally induced by toll-like receptors (TLRs). We highlight our work on the oral pathogen Porphyromonas gingivalis and discuss the role of microbial modulation of lipid A structures in evasion of TLR4 signaling and resulting systemic immunopathology associated with atherosclerosis. P. gingivalis intrinsically expresses underacylated lipid A moieties and can modify the phosphorylation of lipid A, leading to altered TLR4 signaling. Using P. gingivalis mutant strains expressing distinct lipid A moieties, we demonstrated that expression of antagonist lipid A was associated with P. gingivalis-mediated systemic inflammation and immunopathology, whereas strains expressing agonist lipid A exhibited modest systemic inflammation. Likewise, mice deficient in TLR4 were more susceptible to vascular inflammation after oral infection with P. gingivalis wild-type strain compared to mice possessing functional TLR4. Collectively, our studies support a role for P. gingivalis-mediated dysregulation of innate and adaptive responses resulting in immunopathology and systemic inflammation. We propose that anti-TLR4 interventions must be designed with caution, given the balance between the protective and destructive roles of TLR signaling in response to microbiota and associated immunopathologies.

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.

Clinical Association of a Soluble Triggering Receptor Expressed on Myeloid Cells-1 (sTREM-1) in Patients with Systemic Lupus Erythematosus

A triggering receptor expressed on myeloid cells-1 (TREM-1) is a member of the immunoglobulin superfamily with an established role in innate and adaptive immune response. We aimed to determine the plasma concentrations and clinical association of sTREM-1 in Systemic Lupus Erythematosus (SLE) patients. Plasma from 79 SLE patients and 35 normal healthy subjects were assayed for sTREM-1 and IL-6 levels using Enzyme Linked Immunosorbant Assay (ELISA). The clinical disease characteristics and serological data were prospectively assessed. Disease activity was scored using the SLE disease activity index. We detected significantly higher levels of sTREM-1 in plasma of SLE patients than the healthy control group. We also detected high sTREM-1 levels in subgroups of patients with neuropsychiatric manifestations (NPLE) and patients with the total high disease activity and NPLE activity. In addition, sTREM-l levels were significantly correlated with parameters of disease activity, i.e. SLEDAI score, IL-6, hypoalbuminemia. On the other hand, we did not find significant differences in sTREM-1 levels in relation to age, disease duration, medications, ESR, other organ system involvement, or the presence of anti-dsDNA. Our preliminary data indicated that sTREM-1 levels may be an additional useful marker of disease activity in SLE. It also highlights its importance in patients with NPLE. An additional prospective longitudinal study should be carried out to support these findings.