Triggering Receptor Expressed On Myeloid Cells-1 Expression Research Articles

Knockdown of TREM-1 suppresses IL-1β-induced chondrocyte injury via inhibiting the NF-κB pathway

Triggering receptor expressed on myeloid cells 1 (TREM-1) is a recently discovered molecule that modulates inflammatory responses. This study aimed to investigate the specific function of TREM-1 in chondrocytes and its association with the pathophysiology of osteoarthritis (OA). We observed upregulation of TREM-1 in OA cartilage compared to normal tissues. Knockdown of TREM-1 suppressed interleukin 1 beta (IL-1β)-induced extracellular matrix (ECM) metabolic imbalance, pro-inflammatory cytokine production, decrease in cell viability and apoptosis. Mechanistic analyses further revealed that IL-1β-induced activation of the NF-κB pathway is suppressed upon TREM-1 knockdown, similar to the effect of pyrrolidine dithiocarbamate (PDTC), an inhibitor of NF-κB. TREM-1 expression was consistently increased in a mouse OA model in vivo, and its silencing led to inhibition of matrix metallopeptidase-13 (MMP-13) production, increased collagen type II synthesis and decreased NF-κB signaling. Our data collectively suggest that TREM-1 plays a critical in OA development through regulation of NF-κB signaling. Pharmacological inhibition of TREM-1 may therefore present an effective novel therapeutic approach for OA.

Expression of TREM-1 in maternal leukocytes in preterm, prelabour rupture of the membranes

Studies have shown that triggering receptor expressed on myeloid cells-1 (TREM-1) is the mediator and activator of neutrophils and monocytes after stimulation with lipopolysaccharide (LPS), heat-inactivated Gram (−) bacteria, Gram (+) bacteria or fungi. Different studies have measured the expression of TREM-1 in patients with bacterial infections and critical states. The purpose of this study was to evaluate the expression of TREM-1 in circulating maternal leukocytes in premature rupture of the membranes (PRM). Two groups of patients were included in this case control study: pregnant women with PRM and healthy controls. All patients were free of any infection, including cervix and urinary tract. Although all patients expressed TREM-1 to some extent, there was no statistically significant difference in the expression of different cellularities in both groups; except for the mononuclear leukocytes (p < 0.05). In this study, TREM-1 was not altered in PRM.

An Antibody Against Triggering Receptor Expressed on Myeloid Cells 1 (TREM-1) Dampens Proinflammatory Cytokine Secretion by Lamina Propria Cells from Patients with IBD.

Triggering receptor expressed on myeloid cells 1 (TREM-1) is a potent amplifier of inflammation. Recently, the antimicrobial peptide PGLYRP-1 was shown to be the ligand of TREM-1. Here, the ability of an anti-TREM-1 antibody to dampen the release of proinflammatory cytokines by colon lamina propria cells (LPCs) from patients with IBD was investigated and correlated with PGLYRP-1 levels. Biopsies from patients with ulcerative colitis (UC, n = 45) or Crohn's disease (CD, n = 26) were compared with those from individuals undergoing colonoscopy for other reasons (n = 17). TREM-1 expression was analyzed on myeloid cells by flow cytometry. Cell culture experiments with LPCs were used to analyze PGLYRP-1 and inflammatory cytokine levels and assess the effect of anti-TREM-1 on cytokine secretion. The frequency of TREM-1-expressing neutrophils and recruited macrophages was higher in inflamed than in noninflamed biopsies. The PGLYRP-1 level in inflamed tissue was higher than in noninflamed tissue; it was produced primarily by neutrophils, and its level correlated with the secretion of proinflammatory cytokines. Secretion of myeloperoxidase, tumor necrosis factor-α, interleukin-1β, and interleukin-8 by LPCs stimulated with the potent TREM-1 agonist consisting of PGLYRP-1 complexed with peptidoglycan was reduced in the presence of anti-TREM-1. Moreover, a blocking effect of anti-TREM-1 was apparent when LPCs from a subset of inflamed individuals with elevated PGLYRP-1 were stimulated with killed bacteria. An anti-TREM-1 antibody can dampen secretion of proinflammatory cytokines in inflamed patients with elevated PGLYRP-1. Moreover, PGLYRP-1 + myeloperoxidase is a potential biomarker for predicting the effect of anti-TREM-1 therapy.

TREM-1 expression in craniopharyngioma and Rathke's cleft cyst: its possible implication for controversial pathology.

Whether a mixed type of craniopharyngioma (CP) exists and whether papillary craniopharyngioma (pCP) is on a histopathological continuum with Rathke's cleft cyst (RCC) remain controversial. Herein, we examined the expression and localization of β-catenin, BRAF p.V600E (V600E), and triggering receptor expressed on myeloid cells-1 (TREM-1) in 58 samples including 20 pCPs, 26 adamantinomatous craniopharyngiomas (aCP), and 12 RCCs. Five aCPs were diagnosed with mixed type CPs and the remaining 21 cases were pure aCPs. Four of the 12 RCCs presented with significant squamous epithelium (SE). V600E immunoreactivity was observed in all pCPs in the cytoplasm, but not in the nuclei. aCPs and RCCs, including mixed type CP, did not express V600E. Nuclear β-catenin translocation was detected exclusively in aCPs. TREM-1 was expressed in pCPs. Additionally, TREM-1 expression was detected in the SE of 5 “mixed type” CPs, while it was absent in pure aCPs. TREM-1 was expressed in 4 RCCs with SE, but not in the remaining 8 RCCs. TREM-1 mRNA levels were compared in cultured pCP and aCP cells. TREM-1 mRNA level was significantly (p < 0.001; up to 4.045 fold) higher in pCPs than in aCPs. Western blotting revealed a significantly (p < 0.001; up to 7.19 fold) lower level of TREM-1 expression in aCP cells compared to that in pCP cells. Our findings further supported that RCC and pCP may represent two ends of a morphological spectrum. A variant showing overlapping histological features of aCP and pCP should not be considered as a mixed type.

TREM-1 activation modulates dsRNA induced antiviral immunity with specific enhancement of MAPK signaling and the RLRs and TLRs on macrophages

Triggering receptor expressed on myeloid cells 1(TREM-1) is a newly identified member of the immunoglobulin superfamily and is extensively involved in the regulation of innate immunity. To determine the role of TREM-1 in innate antiviral immunity, we investigated TREM-1 expression and its downstream signaling effect in the murine bone marrow-derived macrophages or RAW264.7 macrophage-like mouse cell line by double-stranded RNA (dsRNA) stimulation. The level of TREM-1 expression was low at the baseline and could up-regulate markedly in dose- and time-dependent manners upon stimulation by dsRNA/poly IC. Inhibitor studies disclosed mitogen-activated protein kinase (MAPK) p38 and PI3K pathways were involved in dsRNA-induced up-regulation of TREM-1. Compared with lipopolysaccharide (LPS), the peak response of poly IC-induced TREM-1 expression is delayed, and cells pretreated with scrambled RNA presented higher expression of TREM-1 upon LPS challenge. After ligation with the agonist antibody, TREM-1 can potentiate type I interferon (IFN) production and antiviral inflammation induced by dsRNA, which is ralated to the enhanced phosphorylation of MAPKs and expression of RLRs and TLRs by TREM-1 ligation. This study is the first to show the regulatory role of TREM-1 in RLRs and TLRs expression, and these findings might enrich the understanding of the up-regulation mechanism and the function of TREM-1.

TGF-β1 Upregulates the Expression of Triggering Receptor Expressed on Myeloid Cells 1 in Murine Lungs.

Triggering receptor expressed on myeloid cells 1 (TREM-1) increases the expression of TGF-β family genes, which are known as profibrogenic cytokines in the pathogenesis of pulmonary fibrosis. In this study, we determined whether TGF-β1 regulated the expression of TREM-1 in a mouse model of pulmonary fibrosis. The expression of TGF-β1 and TREM-1 was increased on day 7, 14, and 21 after single intratracheal injection of bleomycin (BLM). And there was positive correlation between the expression of TGF-β1 and TREM-1. TGF-β1 increased expression of TREM-1 mRNA and protein in a time- and dose-dependent manner in mouse macrophages. The expression of the activator protein 1 (AP-1) was increased in lung tissues from mouse after BLM injection and in mouse macrophages after TGF-β1 treatment, respectively. TGF-β1 significantly increased the relative activity of luciferase in the cells transfected with plasmid contenting wild type-promoter of TREM-1. But TGF-β1 had no effect on the activity of luciferase in the cells transfected with a mutant-TREM1 plasmid carrying mutations in the AP-1 promoter binding site. In conclusion, we found the expression of TREM-1 was increased in lung tissues from mice with pulmonary fibrosis. TGF-β1 increased the expression of TREM-1 in mouse macrophages partly via the transcription factor AP-1.

Monosodium urate crystal-induced triggering receptor expressed on myeloid cells 1 is associated with acute gouty inflammation.

Triggering receptor expressed on myeloid cells 1 (TREM-1), which amplifies the inflammation elicited by the Toll-like receptor pathway, was originally implicated in sepsis and bacterial infection. However, it has been suggested that TREM-1 may also play an important role in non-infectious inflammation. The present study was conducted to investigate whether TREM-1 is involved in human acute gouty inflammation. A total of 37 gout patients were recruited between March 2011 and January 2014 from Seoul St Mary's Hospital. The expression of TREM-1 on mononuclear cells was assessed using FACS analysis, immunostaining and real-time RT-PCR. To block the TREM-1 signal, soluble TREM-1 (sTREM-1) or the synthetic blocking peptide LP17 was used. The concentration of sTREM-1 was assessed by ELISA. FACS analysis and real-time RT-PCR demonstrated that TREM-1 expression was higher in the SF mononuclear cells of acute gouty arthritis patients than in peripheral blood mononuclear cells (PBMCs). Immunohistochemical staining of tophi tissues revealed TREM-1 expression, with confocal microscopy demonstrating TREM-1 expression on tophi tissue macrophages. We also demonstrated that MSU treatment induced TREM-1 expression on the PBMCs of acute gout patients in vitro. Although blockade of TREM-1 did not directly suppress MSU-induced IL-1β production of PBMCs in vitro, the concentration of soluble TREM-1 was higher in the SF of gout vs OA patients and was positively correlated with serum CRP. TREM-1 is induced by MSU and is associated with the inflammation of human acute gouty arthritis.

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

Atherosclerosis is a chronic progressive inflammatory disease characterized by the accumulation of lipid contents in arterial walls. Previous studies suggest participation of Toll-like receptors (TLRs) in lipid deposition and inflammatory response in vascular wall. The triggering receptor expressed on myeloid cells 1 (TREM-1) is a cell surface receptor of the immunoglobulin superfamily, which amplifies signal transduction of TLR pathway and enhances immune response to microbial infections. The aim of the present study was to investigate the effect of the oxidized low-density lipoprotein (oxLDL) on the expression of the TREM-1, as well as its engagement in proinflammatory cytokine production and foam cell formation in RAW264.7 mice macrophages. oxLDL enhanced TREM-1 and TLR-4, but not TLR-2 gene expression in macrophages; furthermore, silencing TREM-1 expression by short hairpin interfering RNA inhibited lipid phagocytosis and proinflammatory tumor necrosis factor-α (TNF-α) as well as interleukin-6 (IL-6) production in macrophages; moreover, application of synthetic antagonist, LP-17 polypeptide, reduced IL-6 production upon oxLDL stimulation in vitro and in vivo. In conclusion, in macrophages, oxLDL enhanced expression of TREM-1, which amplifies the innate immune response of TLR pathway; activation of TREM-1 contributes to atherogenesis process by enhancing proinflammatory cytokine production and foam cell formation.

Significance of triggering receptor expressed on myeloid cells-1 of cord blood leukocytes in neonates

Objective To investigate the triggering receptor expressed on myeloid cells-1(TREM-1) of cord blood leukocytes in neonates and the transcription level of mRNA, and analyze its promoting function of inflammatory cytokine secretion. Methods During the period from September 2013 to March 2014, cord blood was collected from 20 term neonates at the time of birth, and peripheral blood was collected from 20 healthy adults. The expression of TREM-1 and TREM-1 mRNA on leukocytes was observed using flow cytometry and real-time reverse transeription-polymerase chain reaction, respectively. After the whole cord blood was stimulated by lipopolysaccharide (LPS) or LP17 plus LPS, the contents of interleukin (IL)-6, tumor necrosis factor-α (TNF-α), IL-8 and soluble TREM-1 (sTREM-1) in the supernatant were analyzed by enzyme linked immunosorbent assay. The statistical significance was determined using the one-way ANOVA test, t test, q test and Pearson correlation coefficient. Results The mean fluorescence intensity of TREM-1 on leukocytes of newborns was not different compared with healthy adults (P>0.05), while the percentage of TREM-1 positive on polymorphonuclear cells was lower than that of healthy adults [(82.3±7.1)% vs (98.6±4.8)%, P<0.05]. The level of TREM-1 mRNA in newborns was lower than in healthy adults (1.16±0.13 vs 1.63±0.24, t=7.714, P<0.01). The LPS treatment significantly increased sTREM-1 in newborn whole blood compared with the control treatment [(156.7±36.3) vs (34.6±6.1) pg/ml, t=13.623, P<0.01]. The concentration of IL-6, TNF-α and IL-8 decreased significantly when TREM-1 was blocked by LP17. In addition, the concentration of sTREM-1 showed a positive correlation with the levels of TNF-α (r=0.519, P<0.05), IL-6 (r=0.507, P<0.05) and IL-8 (r=0.538, P<0.05). Conclusions Healthy newborns exhibit expression of TREM-1 on monocytes similar to healthy adults, and most PMNs express TREM-1 at the newborn stage. Blocking the TREM-1 signal transduction pathway may reduce inflammatory responses of neonate leukocytes. Key words: Receptors, immunologic; Leukocytes; Fetal blood; Sepsis; Infant, newborn

TREM1 intervention does not affect renal pathology upon ischemia/reperfusion injury (TRAN1P.929)

Abstract Rationale:Following transplantation, renal ischemia is a major cause of Acute Kidney injury(AKI). TREM1(Triggering receptor expressed on myeloid cells 1) is an innate immune receptor expressed on neutrophils and monocytes/macrophages, the most common cells infiltrating the kidney upon IR. TREM1 enhances Toll-like receptor (TLR)-induced inflammatory response in several models of infection and sterile inflammation.TREM1 inhibition reduced disease severity in mesenteric ischemia-reperfusion induced injury (IRI).We hypothesized that TREM1 intervention will also reduce renal IRI. Methods:Mice were subjected to renal IRI induced by occlusion of the renal artery for 25’ and reperfused for 24h. Mice were treated i.p. with different doses of LP17, LR12, TREM1 fusion protein and control peptides.Renal TREM1 protein expression and plasma sTREM1 were measured. TREM1 expression on circulating granulocyte and monocyte, renal function (creatinine, urea), damage (PASD) and inflammation (KC and LY6G-positive cell infiltration) were evaluated. Results:IRI was associated with increased renal and circulating TREM1 protein levels. TREM1 levels were similar on circulating granulocyte but decrease on monocyte upon IR. TREM1 intervention with LP17, LR12 and TREM1 fusion protein did not reduce renal dysfunction, injury or inflammation following IRI. Conclusion: The modulation of the TREM-1 pathway by means of synthetic peptides and fusion protein does not represent a useful tool to prevent renal IRI.

Abstract 370: Dendritic Cells Express Triggering Receptor Expressed on Myeloid Cells-1 and Correlate with Plaque Stability in Symptomatic and Asymptomatic Patients with Carotid Stenosis

Aim: Atherosclerosis is a chronic inflammatory disease with atherosclerotic plaques containing inflammatory infiltrates. Immune cells, including T-lymphocytes, dendritic cells (DCs) and macrophages recruited from blood stream into atherosclerotic lesions, are responsible for progression and destabilization of atherosclerotic plaques. Stressed cells undergoing necrosis release molecules that act as endogenous danger signals to alert and activate innate immune cells. Upon activation, DCs, macrophages, and neutrophils produce several inflammatory cytokines and growth factors. In atherosclerotic tissue, the number of DCs increases with the progression of the lesion. In this study, we examined the expression of triggering receptor expressed on myeloid cells-1 (TREM-1) in plaque DCs and compared between the plaques from symptomatic (S) and asymptomatic (AS) patients with carotid stenosis. Methods and Results: Surgical specimens of human atherosclerotic plaques were obtained from patients undergoing carotid endarterectomy. The expression patterns of TREM-1, HLA-DR, and fascin (DC) were determined by immunofluorescence with α-actin as specific marker to identify smooth muscle cells (VSMCs). There was a marked decrease in VSMCs and an increased density of HLA-DR+ cells in the plaques of S than AS patients. Fascin expression to show DCs was undetectable in normal carotid artery specimens but significantly increased in the carotid plaques of S compared to AS. TREM-1 expression was increased in S and localized with HLA-DR and fascin. However, TREM-1 was increased in S plaques containing less number of VSMCs. The mRNA expression of MMP-9, MMP-1 was increased while Col 1 (α1) and Col III (α1) mRNA transcripts were decreased in S plaques. Conclusions: Increased expression of TREM-1 in plaque DCs of S compared to AS patients suggests a potential role of TREM-1 in atherosclerotic plaque vulnerability. Collectively, these data demonstrate that selective blockade of TREM-1 may contribute to the development of new therapies and promising targets for stabilizing atherosclerotic plaques.

Abstract 372: Tumor Necrosis Factor-α Regulates Triggering Receptor Expressed on Myeloid Cells-1-dependent Matrix Metalloproteinases in the Carotid Plaques of Symptomatic Patients with Carotid Stenosis

Aims: Unstable atherosclerotic plaques in carotid artery are characterized by rupture of their fibrous cap, leading to transient ischemic attack or stroke. However, the underlying mechanisms are unclear. Our aim was to investigate the expression and function of triggering receptor expressed on myeloid cells-1 (TREM-1) on plaque stability involving matrix metalloproteinases (MMPs) and examine the effect of inflammatory cytokine TNF-α in symptomatic (S) compared to asymptomatic (AS) patients with carotid stenosis. Methods and Results: The mRNA transcripts for TREM-1, MMPs (MMP-1, MMP-3, and MMP-9) and collagen type I and III were analyzed by qPCR and immunofluorescence, respectively in whole plaque and plaque smooth muscle cells (pSMCs) isolated from carotid endarterectomy tissues of patients with carotid stenosis. The mRNA transcripts of TREM-1, MMP-1 and MMP-9 were increased while Col I and Col III mRNA transcripts were decreased in pSMCs of S compared to AS patients. Stimulation of pSMCs with TNF-α further increased the mRNA transcripts of TREM-1 and MMPs without any additional effect on Col I (α1) or Col III (α1) mRNA levels. The inhibitors of NF-kB (BAY11-7085), JNK (SP600125) and PI3K (LY294002) signaling pathways attenuated the effect of TNF-α. Modulation of TREM-1 in TNF-α-treated pSMCs by the administration of TREM-1 decoy receptor rTREM-1/Fc, monoclonal antibodies, or siRNA to TREM-1 attenuated the expression of MMPs and Col I (α1) and Col III (α1) in both S and AS groups. Conclusion: Increased expression of TREM-1 in pSMCs of symptomatic than in asymptomatic patients suggest a potential role of TREM-1 in plaque destabilization. This is further supported by the pronounced effect of inflammatory cytokine, TNF-α, to increase of mRNA transcripts for TREM-1 and MMPs. Selective blockade of TREM-1 may contribute to the development of new therapies and promising targets for stabilizing vulnerable atherosclerotic plaques.

TREM-1 expression in rat corneal epithelium with Aspergillus fumigatus infection.

To investigate the expression of triggering receptor expressed on myeloid cells-1 (TREM-1) in the aberrant inflammation within the corneal epithelium at early period of fungal infection. A total of 65 Wistar rats were randomly divided into control group, sham group and fungal keratitis (FK) group, in which the cornea was infected by Aspergillus fumigatus (A. fumigatus). After executed randomly at 8, 16, 24, 48 and 72h after experimental model being established, the severity of keratomycosis in rats was scored visually with the aid of a dissecting microscope and slit lamp. Then corneas in three groups were collected to assess the expression of TREM-1 through quantitative reverse transcription-polymerase chain reaction (RT-PCR), immunofluorescence technique and Western blot analysis. The correlation between FK inflammation and expression of TREM-1 was also analyzed. Corneal inflammation scores increased with time after fungal infection (F=49.74, P=0.000). The inflammation scores in FK group were obviously higher than those in sham group on the whole (F=137.78, P=0.000). Levels of TREM-1 in the infected rat corneal epithelium had elevated at 8h and peaked at 48h (P<0.001, compared with control group). Western blot analysis also showed an obviously elevated TREM-1 level in rat corneal epithelium at 24h and 48h after fungal infection. Immunofluorescencetechnique showed that TREM-1 mainly existed in corneal epithelium and infected corneal stoma of rat. TREM-1 protein expression was enhanced after fungal infection. Moreover, severity of FK inflammation was significantly related to TREM-1 expression in FK (r=0.942, P=0.000). TREM-1 may contribute to amplify the inflammation in the cornea infected with A. fumigatus and play critical roles in the battle against A. fumigatus in the innate immune responses.

TREM-1 Mediates Inflammatory Injury and Cardiac Remodeling Following Myocardial Infarction.

Optimal outcome after myocardial infarction (MI) depends on a coordinated healing response in which both debris removal and repair of the myocardial extracellular matrix play a major role. However, adverse remodeling and excessive inflammation can promote heart failure, positioning leucocytes as central protagonists and potential therapeutic targets in tissue repair and wound healing after MI. In this study, we examined the role of triggering receptor expressed on myeloid cells-1(TREM-1) in orchestrating the inflammatory response that follows MI. TREM-1, expressed by neutrophils and mature monocytes, is an amplifier of the innate immune response. After infarction, TREM-1 expression is upregulated in ischemic myocardium in mice and humans. Trem-1 genetic invalidation or pharmacological inhibition using a synthetic peptide (LR12) dampens myocardial inflammation, limits neutrophils recruitment and monocyte chemoattractant protein-1 production, thus reducing classical monocytes mobilization to the heart. It also improves left ventricular function and survival in mice (n=20-22 per group). During both permanent and transient myocardial ischemia, Trem-1 blockade also ameliorates cardiac function and limits ventricular remodeling as assessed by fluorodeoxyglucose-positron emission tomographic imaging and conductance catheter studies (n=9-18 per group). The soluble form of TREM-1 (sTREM-1), a marker of TREM-1 activation, is detectable in the plasma of patients having an acute MI (n=1015), and its concentration is an independent predictor of death. These data suggest that TREM-1 could constitute a new therapeutic target during acute MI.

Tacrolimus (FK506) suppresses TREM-1 expression at an early but not at a late stage in a murine model of fungal keratitis.

PurposeTo investigate the efficacy and mechanism of tacrolimus(FK506), which is a novel macrolide immunosuppressant, in inhibiting triggering receptor expressed on myeloid cells-1 (TREM-1) expression in a murine keratitis model induced by Aspergillus fumigatus. MethodTREM-1 was detected in 11 fungus-infected human corneas by quantitative real-time PCR (qRT-PCR). RAW264.7 macrophages were divided into four groups, which received treatment with zymosan (100 µg/ml), zymosan (100 µg/ml) + mTREM-1/Fc protein (1 µg/ml), or zymosan (100 µg/ml) + FK506 (20 µM) or negative-control treatment. After this treatment, the expression of TREM-1, interleukin-1β (IL-1β) and tumor necrosis factor α (TNFα) was assayed using qRT-PCR and ELISA. The mouse model of fungal keratitis was created by intrastromal injection with Aspergillus fumigatus, and the mice were divided into 2 groups: group A received vehicle eye drops 4 times each day, and group B received 4 doses of FK506 eye drops each day. Corneal damage was evaluated by clinical scoring and histologic examination,and myeloperoxidase (MPO) protein levels were also detected by ELISA. The expression of TREM-1, IL-1β and TNFα was then determined at different time points using qRT-PCR and ELISA.ResultsTREM-1 expression dramatically increased in the human corneas with fungal keratitis. In contrast, FK506 reduced the expression of TREM-1, IL-1β and TNFα in RAW264.7 macrophages stimulated with zymosan. In the mouse model, at day 1 post-infection, the corneal score of the FK506-treated group was lower than that of the control, and polymorphonuclear neutrophil (PMN) infiltration was diminished. TREM-1, IL-1β and TNFα expression was significantly reduced at the same time point. However, the statistically significant differences in cytokine expression, clinical scores and infiltration disappeared at 5 days post-infection.ConclusionsFK506 may inhibit the inflammation induced by fungi and alleviate the severity of corneal damage at an early stage of fungal keratitis by downregulating TREM-1 expression.

1α,25-Dihydroxyvitamin D3 upregulates HIF-1 and TREM-1 via mTOR signaling

Triggering receptor expressed on myeloid cells-1 (TREM-1) is induced by 1α,25-dihydroxyvitamin D3 (1,25(OH)2D3) in human monocytes/macrophages and epithelial cells. However, little information is available regarding the mechanism of 1,25(OH)2D3-induced TREM-1 expression in human monocytes/macrophages. In this study, 1,25(OH)2D3 was shown to strongly upregulate hypoxia-inducible transcription factor (HIF) in PMA-differentiated U937 cells. However, HIF was not mainly involved in 1,25(OH)2D3-induced TREM-1 expression. Instead, 1,25(OH)2D3-induced expression of TREM-1 was inhibited by rapamycin, a specific inhibitor of the mammalian target of rapamycin (mTOR) signaling pathway, indicating the involvement of mTOR. Induction of HIF proteins by 1,25(OH)2D3 was also inhibited by rapamycin. In addition, 1,25(OH)2D3 induced the phosphorylation of p70S6 kinase, a target of mTOR complex 1 (mTORC1). Our results suggest that 1,25(OH)2D3 induces the expression of TREM-1 through the mTOR signaling pathway in human macrophages.

Role of TREM-1 in response to Aspergillus fumigatus infection in corneal epithelial cells.

Triggering receptor expressed on myeloid cells-1 (TREM-1) is a cell surface receptor that is highly expressed in inflammatory lesions caused by infectious agents such as bacteria and fungi and amplifies immune responses. The aim of the current study was to investigate TREM-1 expression in corneal epithelial cells infected by Aspergillus fumigatus (A. fumigatus) and evaluate its role. In this study, infection with A. fumigatus upregulated TREM-1 expression in corneal epithelial cells both in vitro and in vivo. Furthermore, treatment with the antagonistic peptide of TREM-1 decreased the levels of inflammatory cytokines that were enhanced by the fungal infection. We speculated that cross-talk occurs between TREM-1 and Toll-like receptor-4 (TLR-4) in cornea fungal infection. Inhibitors of TLR-4 and myeloid differentiation factor 88 (MyD88) could partially inhibit the upregulation of TREM-1 induced by A. fumigatus respectively. In addition, TLR-4 blockade enhanced the inhibitory effect of the antagonistic peptide of TREM-1 on A. fumigatus-induced inflammation. These findings suggest that TREM-1 plays critical roles in fungal infection, and targeting it may represent a novel therapeutic strategy for patients with fungal keratitis.

Expression and regulation of triggering receptor expressed on myeloid cells 1 in periodontal diseases.

Periodontitis is an inflammatory infectious disease that destroys the tooth-supporting tissues. It is caused by multi-species subgingival biofilms that colonize the tooth surface. Porphyromonas gingivalis, Treponema denticola and Tannerella forsythia (i.e. 'red complex' bacteria) are characteristic subgingival biofilm species. The triggering receptor expressed on myeloid cells 1 (TREM-1) is a cell surface receptor of the immunoglobulin superfamily, with a role in the amplification of proinflammatory cytokine production during infection. This study aimed to investigate TREM-1 mRNA expression in gingival tissues from patients with chronic periodontitis, generalized aggressive periodontitis and healthy subjects and its correlation with the levels of periodontal pathogens in the tissue. A further aim was to investigate the regulation of TREM-1 in human monocytic cells (MM6) challenged with an in-vitro subgingival biofilm model. Gingival tissue TREM-1 expression was increased in both chronic and aggressive periodontitis, compared to health, and correlated with the levels of the 'red complex' species in the tissue. No significant differences were detected between the two forms of periodontitis. Biofilm-challenged MM6 cells exhibited higher TREM-1 expression and secretion compared to controls, with partial involvement of the 'red complex'. Engagement or inhibition of TREM-1 affected the capacity of the biofilms to stimulate interleukin (IL)-1β, but not IL-8, secretion by the cells. In conclusion, this study reveals that TREM-1 tissue expression is enhanced in periodontal disease, and correlates with the level of periodontal pathogens. It also provides a mechanistic insight into the regulation of TREM-1 expression and the associated IL-1β production in biofilm-challenged monocytes.

Triggering receptor expressed on myeloid cells-1 (TREM-1) improves host defence in pneumococcal pneumonia.

Streptococcus (S.) pneumoniae is a common Gram-positive pathogen in community-acquired pneumonia and sepsis. Triggering receptor expressed on myeloid cells-1 (TREM-1) is a receptor on phagocytes known to amplify inflammatory responses. Previous studies showed that TREM-1 inhibition protects against lethality during experimental Gram-negative sepsis. We here aimed to investigate the role of TREM-1 in an experimental model of pneumococcal pneumonia, using TREM-1/3-deficient (Trem-1/3(-/-) ) and wild-type (Wt) mice. Additionally ex vivo responsiveness of Trem-1/3(-/-) neutrophils and macrophages was examined. S. pneumoniae infection resulted in a rapid recruitment of TREM-1-positive neutrophils into the bronchoalveolar space, while high constitutive TREM-1 expression on alveolar macrophages remained unchanged. TREM-1/3 deficiency led to increased lethality, accompanied by enhanced growth of S. pneumoniae at the primary site of infection and increased dissemination to distant organs. Within the first 3-6 h of infection, Trem-1/3(-/-) mice demonstrated a strongly impaired innate immune response in the airways, as reflected by reduced local release of cytokines and chemokines and a delayed influx of neutrophils. Trem-1/3(-/-) alveolar macrophages produced fewer cytokines upon exposure to S. pneumoniae in vitro and were less capable of phagocytosing this pathogen. TREM-1/3 deficiency did not influence neutrophil responsiveness to S. pneumoniae. These results identify TREM-1 as a key player in protective innate immunity during pneumococcal pneumonia, most likely by enhancing the early immune response of alveolar macrophages.

TREM-1 Is Induced in Tumor Associated Macrophages by Cyclo-Oxygenase Pathway in Human Non-Small Cell Lung Cancer

It is increasingly recognized that the tumor microenvironment plays a critical role in the initiation and progression of lung cancer. In particular interaction of cancer cells, macrophages, and inflammatory response in the tumor microenvironment has been shown to facilitate cancer cell invasion and metastasis. The specific molecular pathways in macrophages that immunoedit tumor growth are not well defined. Triggering receptor expressed on myeloid cells 1 (TREM-1) is a member of the super immunoglobulin family expressed on a select group of myeloid cells mainly monocyte/macrophages. Recent studies suggest that expression of TREM-1 in tumors may predict cancer aggressiveness and disease outcomes in liver and lung cancer however the mechanism of TREM-1 expression in the setting of cancer is not defined. In this study we demonstrate that tumor tissue from patients with non-small cell lung cancer show an increased expression of TREM-1 and PGE2. Immunohistochemistry and immunofluorescence confirmed that the expression of TREM-1 was selectively seen in CD68 positive macrophages. By employing an in vitro model we confirmed that expression of TREM-1 is increased in macrophages that are co-cultured with human lung cancer cells. Studies with COX-2 inhibitors and siCOX-2 showed that expression of TREM-1 in macrophages in tumor microenvironment is dependent on COX-2 signaling. These studies for the first time define a link between tumor COX-2 induction, PGE2 production and expression of TREM-1 in macrophages in tumor microenvironment and suggest that TREM-1 might be a novel target for tumor immunomodulation.