TREM-1 inhibition attenuates inflammation and tumor within the colon. Int Immunopharmacol. 2013;17:155-161. [PMID: 23810411 DOI: 10.1016/j.intimp.2013.06.009]

TREM1: Activation, signaling, cancer and therapy

Triggering receptor expressed on myeloid cells 1 (TREM1) is a cell surface receptor expressed on neutrophils, monocytes and some tissue macrophages, where it functions as an immunoregulator that controls myeloid cell responses. The activation of TREM1 is suggested to be an upregulation-based, ligands-induced and structural multimerization-mediated process, in which damage- and pathogen-associated molecular patterns play important roles. Activated TREM1 initiates an array of downstream signaling pathways that ultimately result in the production of pro-inflammatory cytokines and chemokines, whereby it functions as an amplifier of inflammation and is implicated in the pathogenesis of many inflammation-associated diseases. Over the past decade, there has been growing evidence for the involvement of TREM1 overactivation in tumor stroma inflammation and cancer progression. Indeed, it was shown that TREM1 promotes tumor progression, immunosuppression, and resistance to therapy by activating tumor-infiltrating myeloid cells. TREM1-deficiency or blockade provide protection against tumors and reverse the resistance to anti-PD-1/PD-L1 therapy and arginine-deprivation therapy in preclinical models. Here, we first review the structure, activation modes and signaling pathways of TREM1 and emphasize the role of soluble TREM1 as a biomarker of infection and cancer. We then focus on the role of TREM1 in cancer and systematically summarize its expression patterns, upregulation mechanisms and functions in tumor development and progression. Lastly, we discuss the therapeutic prospects of TREM1 inhibition, via effective pharmacological inhibitors, in treating cancer and other diseases.

Impaired lipophagy induced-microglial lipid droplets accumulation contributes to the buildup of TREM1 in diabetes-associated cognitive impairment

Neuroinflammation caused by microglial activation and consequent neurological impairment are prominent features of diabetes-associated cognitive impairment (DACI). Microglial lipophagy, a significant fraction of autophagy contributing to lipid homeostasis and inflammation, had mostly been ignored in DACI. Microglial lipid droplets (LDs) accumulation is a characteristic of aging, however, little is known about the pathological role of microglial lipophagy and LDs in DACI. Therefore, we hypothesized that microglial lipophagy could be an Achilles's heel exploitable to develop effective strategies for DACI therapy. Here, starting with characterization of microglial accumulation of LDs in leptin receptor-deficient (db/db) mice and in high-fat diet and STZ (HFD/STZ) induced T2DM mice, as well as in high-glucose (HG)-treated mice BV2, human HMC3 and primary mice microglia, we revealed that HG-dampened lipophagy was responsible for LDs accumulation in microglia. Mechanistically, accumulated LDs colocalized with the microglial specific inflammatory amplifier TREM1 (triggering receptor expressed on myeloid cells 1), resulting in the buildup of microglial TREM1, which in turn aggravates HG-induced lipophagy damage and subsequently promoted HG-induced neuroinflammatory cascades via NLRP3 (NLR family pyrin domain containing 3) inflammasome. Moreover, pharmacological blockade of TREM1 with LP17 in db/db mice and HFD/STZ mice inhibited accumulation of LDs and TREM1, reduced hippocampal neuronal inflammatory damage, and consequently improved cognitive functions. Taken together, these findings uncover a previously unappreciated mechanism of impaired lipophagy-induced TREM1 accumulation in microglia and neuroinflammation in DACI, suggesting its translational potential as an attractive therapeutic target for delaying diabetes-associated cognitive decline.Abbreviations: ACTB: beta actin; AIF1/IBA1: allograft inflammatory factor 1; ALB: albumin; ARG1: arginase 1; ATG3: autophagy related 3; Baf: bafilomycin A1; BECN1: beclin 1, autophagy related; BW: body weight; CNS: central nervous system; Co-IP: co-immunoprecipitation; DACI: diabetes-associated cognitive impairment; DAPI: 4',6-diamidino-2-phenylindole; DGs: dentate gyrus; DLG4/PSD95: discs large MAGUK scaffold protein 4; DMEM: Dulbecco's modified Eagle's medium; DSST: digit symbol substitution test; EDTA: ethylenedinitrilotetraacetic acid; ELISA: enzyme linked immunosorbent assay; GFAP: glial fibrillary acidic protein; HFD: high-fat diet; HG: high glucose; IFNG/IFN-γ: interferon gamma; IL1B/IL-1β: interleukin 1 beta; IL4: interleukin 4; IL6: interleukin 6; IL10: interleukin 10; LDs: lipid droplets; LPS: lipopolysaccharide; MAP2: microtubule associated protein 2; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; MWM: morris water maze; NFKB/NF-κB: nuclear factor of kappa light polypeptide gene enhancer in B cells; NLRP3: NLR family pyrin domain containing 3; NOS2/iNOS: nitric oxide synthase 2, inducible; NOR: novel object recognition; OA: oleic acid; PA: palmitic acid; PBS: phosphate-buffered saline; PFA: paraformaldehyde; PLIN2: perilipin 2; PLIN3: perilipin 3; PS: penicillin-streptomycin solution; RAPA: rapamycin; RBFOX3/NeuN: RNA binding protein, fox-1 homolog (C. elegans) 3; RELA/p65: RELA proto-oncogene, NF-kB subunit; ROS: reactive oxygen species; RT: room temperature; RT-qPCR: Reverse transcription quantitative real-time polymerase chain reaction; STZ: streptozotocin; SQSTM1/p62: sequestosome 1; SYK: spleen asociated tyrosine kinase; SYP: synaptophysin; T2DM: type 2 diabetes mellitus; TNF/TNF-α: tumor necrosis factor; TREM1: triggering receptor expressed on myeloid cells 1; TUNEL: terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling.

TREM1 activation of myeloid cells promotes antitumor immunity

Myeloid cells in the tumor microenvironment (TME) can exist in immunosuppressive and immunostimulatory states that impede or promote antitumor immunity, respectively. Blocking suppressive myeloid cells or increasing stimulatory cells to enhance antitumor immune responses is an area of interest for therapeutic intervention. Triggering receptor expressed on myeloid cells-1 (TREM1) is a proinflammatory receptor that amplifies immune responses. TREM1 is expressed on neutrophils, subsets of monocytes and tissue macrophages, and suppressive myeloid populations in the TME, including tumor-associated neutrophils, monocytes, and tumor-associated macrophages. Depletion or inhibition of immunosuppressive myeloid cells, or stimulation by TREM1-mediated inflammatory signaling, could be used to promote an immunostimulatory TME. We developed PY159, an afucosylated humanized anti-TREM1 monoclonal antibody with enhanced FcγR binding. PY159 is a TREM1 agonist that induces signaling, leading to up-regulation of costimulatory molecules on monocytes and macrophages, production of proinflammatory cytokines and chemokines, and enhancement of T cell activation in vitro. An antibody against mouse TREM1, PY159m, promoted antitumor efficacy in syngeneic mouse tumor models. These results suggest that PY159-mediated agonism of TREM1 on tumoral myeloid cells can promote a proinflammatory TME and offer a promising strategy for immunotherapy.

The microglial innate immune receptors TREM-1 and TREM-2 in the anterior cingulate cortex (ACC) drive visceral hypersensitivity and depressive-like behaviors following DSS-induced colitis

Inflammatory bowel disease (IBD) is a chronic condition with a high recurrence rate. To date, the clinical treatment of IBD mainly focuses on inflammation and gastrointestinal symptoms while ignoring the accompanying visceral pain, anxiety, depression, and other emotional symptoms. Evidence is accumulating that bi-directional communication between the gut and the brain is indispensable in the pathophysiology of IBD and its comorbidities. Increasing efforts have been focused on elucidating the central immune mechanisms in visceral hypersensitivity and depression following colitis. The triggering receptors expressed on myeloid cells-1/2 (TREM-1/2) are newly identified receptors that can be expressed on microglia. In particular, TREM-1 acts as an immune and inflammatory response amplifier, while TREM-2 may function as a molecule with a putative antagonist role to TREM-1. In the present study, using the dextran sulfate sodium (DSS)-induced colitis model, we found that peripheral inflammation induced microglial and glutamatergic neuronal activation in the anterior cingulate cortex (ACC). Microglial ablation mitigated visceral hypersensitivity in the inflammation phase rather than in the remission phase, subsequently preventing the emergence of depressive-like behaviors in the remission phase. Moreover, a further mechanistic study revealed that overexpression of TREM-1 and TREM-2 remarkably aggravated DSS-induced neuropathology. The improved outcome was achieved by modifying the balance of TREM-1 and TREM-2 via genetic and pharmacological means. Specifically, a deficiency of TREM-1 attenuated visceral hyperpathia in the inflammatory phase, and a TREM-2 deficiency improved depression-like symptoms in the remission phase. Taken together, our findings provide insights into mechanism-based therapy for inflammatory disorders and establish that microglial innate immune receptors TREM-1 and TREM-2 may represent a therapeutic target for the treatment of pain and psychological comorbidities associated with chronic inflammatory diseases by modulating neuroinflammatory responses.

Triggering Receptors Expressed on Myeloid Cells 1 : Our New Partner in Human Oncology?

Inflammation is recognized as one of the hallmarks of cancer. Indeed, strong evidence indicates that chronic inflammation plays a major role in oncogenesis, promoting genome instability, epigenetic alterations, proliferation and dissemination of cancer cells. Mononuclear phagocytes (MPs) have been identified as key contributors of the inflammatory infiltrate in several solid human neoplasia, promoting angiogenesis and cancer progression. One of the most described amplifiers of MPs pro-inflammatory innate immune response is the triggering receptors expressed on myeloid cells 1 (TREM-1). Growing evidence suggests TREM-1 involvement in oncogenesis through cancer related inflammation and the surrounding tumor microenvironment. In human oncology, high levels of TREM-1 and/or its soluble form have been associated with poorer survival data in several solid malignancies, especially in hepatocellular carcinoma and lung cancer. TREM-1 should be considered as a potential biomarker in human oncology and could be used as a new therapeutic target of interest in human oncology (TREM-1 inhibitors, TREM-1 agonists). More clinical studies are urgently needed to confirm TREM-1 (and TREM family) roles in the prognosis and the treatment of human solid cancers.

Design of the lentivirus-driven sustained LR12 delivery system for TREM-1 inhibition for palliating atherosclerosis

Triggering Receptor Expressed on Myeloid cells-1 (TREM-1) has been a potential target in the management of pathophysiology and clinical sequelae of atherosclerosis. LR12 peptide effectively blocks ligand-TREM-1 interaction; however, the short half-life of LR12 is a major hurdle in its translational application in atherosclerosis management warranting new methods for sustained bioavailability in clinical applications. The present study reports a novel method of packing the coding sequence of LR12 in a lentiviral system to ensure a sustained expression and bioavailability for effective TREM-1 inhibition. Lentivirus vector systems (LV-LR12 and LV-SP) for the expression of LR12 peptide and SP (scrambled peptide) were successfully designed, constructed, and tested in vitro in smooth muscle cells (SMCs). Viral amounts obtained were 703.6 ± 145.12 and 609.3 ± 145.93 ng/ml p24 for LV-LR12 and LV-SP, respectively which correspond to ~ 107 IFU/ml for both vectors. Dot blot assay revealed significantly increased expression of LR12-FLAG and SP-FLAG in 125 μg total protein which was doubled in 250 μg protein with respect to un-transduced SMCs suggesting the sustained release of LR12/SP as confirmed by ELISA. Cellular expression of LR12-FLAG and SP-FLAG displayed 8.44-fold and 7.55-fold increase, respectively compared to the control SMCs. The findings demonstrated a promising strategy for packing the LR12 coding sequence in lentiviral vector for TREM-1 inhibition for the management of atherosclerosis and other inflammatory diseases.

Novel bio-genetic predictors of response to biologic treatment in inflammatory bowel diseases

Despite the evolving therapeutic armamentarium, the treatment of IBD patients remains challenging and many patients fail to respond to biologic agents. With the limited yield of clinical factors to predict the outcome of biologic treatments, studies have focused on identifying genetic alterations and circulating or tissue biomarkers to identify patients who are likely to respond to therapy. In this review, we examine the current knowledge and status of genetic, expression biomarkers, and microbiome predictors. The search for genetic predictors has yielded many genetic loci variants, but few were reproducible. Expression studies of putative biomarkers show promising results, especially with TREM1, oncostatin M and TNF biomarkers, but confirmatory studies are warranted. Finally, the microbiome is emerging as an important player with specific taxa and functional pathways differentially abundant and enriched in responders versus non-responders to certain biologics. Integrating different factors into a robust predictive model, which is both reproducible, accurate and affordable, remains the main challenge before these individualized strategies can reach clinical use.

Dietary Oat Bran Reduces Systemic Inflammation in Mice Subjected to Pelvic Irradiation

Patients undergoing radiotherapy to treat pelvic-organ cancer are commonly advised to follow a restricted fiber diet. However, reducing dietary fiber may promote gastrointestinal inflammation, eventually leading to deteriorated intestinal health. The goal of this study was to evaluate the influence of dietary fiber on radiation-induced inflammation. C57BL/6J male mice were fed a High-oat bran diet (15% fiber) or a No-fiber diet (0% fiber) and were either irradiated (32 Gy delivered in four fractions) to the colorectal region or only sedated (controls). The dietary intervention started at 2 weeks before irradiation and lasted for 1, 6, and 18 weeks after irradiation, at which time points mice were sacrificed and their serum samples were assayed for 23 cytokines and chemokines. Our analyses show that irradiation increased the serum cytokine levels at all the time points analyzed. The No-fiber irradiated mice had significantly higher levels of pro-inflammatory cytokines than the High-oat irradiated mice at all time points. The results indicate that a fiber-rich oat bran diet reduces the intensity of radiation-induced inflammation, both at an early and late stage. Based on the results, it seems that the advice to follow a low-fiber diet during radiotherapy may increase the risk of decreased intestinal health in cancer survivors.

Targeting Mononuclear Phagocyte Receptors in Cancer Immunotherapy: New Perspectives of the Triggering Receptor Expressed on Myeloid Cells (TREM-1)

Inflammatory cells are major players in the onset of cancer. The degree of inflammation and type of inflammatory cells in the tumor microenvironment (TME) are responsible for tilting the balance between tumor progression and regression. Cancer-related inflammation has also been shown to influence the efficacy of conventional therapy. Mononuclear phagocytes (MPs) represent a major component of the inflammatory circuit that promotes tumor progression. Despite their potential to activate immunosurveillance and exert anti-tumor responses, MPs are subverted by the tumor to support its growth, immune evasion, and spread. MP responses in the TME are dictated by a network of stimuli integrated through the cross-talk between activatory and inhibitory receptors. Alterations in receptor expression/signaling can create excessive inflammation and, when chronic, promote tumorigenesis. Research advances have led to the development of new therapeutic strategies aimed at receptor targeting to induce a tumor-infiltrating MP switch from a cancer-supportive toward an anti-tumor phenotype, demonstrating efficacy in different human cancers. This review provides an overview of the role of MP receptors in inflammation-mediated carcinogenesis and discusses the most recent updates regarding their targeting for immunotherapeutic purposes. We focus in particular on the TREM-1 receptor, a major amplifier of MP inflammatory responses, highlighting its relevance in the development and progression of several types of inflammation-associated malignancies and the promises of its inhibition for cancer immunotherapy.

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.

TREM Receptors Connecting Bowel Inflammation to Neurodegenerative Disorders

Alterations in Triggering Receptors Expressed on Myeloid cells (TREM-1/2) are bound to a variety of infectious, sterile inflammatory, and degenerative conditions, ranging from inflammatory bowel disease (IBD) to neurodegenerative disorders. TREMs are emerging as key players in pivotal mechanisms often concurring in IBD and neurodegeneration, namely microbiota dysbiosis, leaky gut, and inflammation. In conditions of dysbiosis, compounds released by intestinal bacteria activate TREMs on macrophages, leading to an exuberant pro-inflammatory reaction up to damage in the gut barrier. In turn, TREM-positive activated macrophages along with inflammatory mediators may reach the brain through the blood, glymphatic system, circumventricular organs, or the vagus nerve via the microbiota-gut-brain axis. This leads to a systemic inflammatory response which, in turn, impairs the blood-brain barrier, while promoting further TREM-dependent neuroinflammation and, ultimately, neural injury. Nonetheless, controversial results still exist on the role of TREM-2 compared with TREM-1, depending on disease specificity, stage, and degree of inflammation. Therefore, the present review aimed to provide an update on the role of TREMs in the pathophysiology of IBD and neurodegeneration. The evidence here discussed the highlights of the potential role of TREMs, especially TREM-1, in bridging inflammatory processes in intestinal and neurodegenerative disorders.

Inhibition of TREM-1 attenuates inflammation and lipid accumulation in diet-induced nonalcoholic fatty liver disease

In the liver tissues of obese diabetic or nondiabetic patients, triggering receptor expressed on myeloid cells-1 (TREM-1) is usually found to be upregulated, thus leading to upregulation of various inflammatory cytokines and lipid accumulation. On the other hand, nonalcoholic fatty liver disease (NAFLD), characterized by excess lipid accumulation, and inflammatory injury in liver, is becoming an epidemic disease, globally. In the present study, we aimed to investigate the biological role and the underlying mechanisms of TREM-1 in NAFLD. upregulation of TREM-1 occurred in high-fat diet (HFD)-induced mice NAFLD model and oleic acid-treated HepG2 and primary mouse hepatocytes cell model at messenger RNA and protein levels. Functional studies established that overexpression of TREM-1 displayed hyperlipidemia, and increased in inflammatory indicators and lipid accumulation-related genes, which was ameliorated by knockdown of TREM-1. Our results also showed that obvious lipid accumulation and inflammatory injury occurred in the liver tissue of HFD-fed mice, while treatment with lentiviral vector short hairpin TREM showed marked improvement in tissue morphology and architecture and less lipid accumulation, thus deciphering the mechanism through which knockdown of TREM-1 ameliorated the inflammatory response and lipid accumulation of NAFLD mice through inactivation of the nuclear factor-κB (NF-κB) and PI3K/AKT signal pathways, respectively. In conclusion, TREM-1/NF-κB and TREM-1/PI3K/AKT axis could be an important mechanism in ameliorating the inflammatory response and lipid accumulation, respectively, thus shedding light on the development of novel therapeutics to the treatment of NAFLD.

TREM-1-dependent M1 macrophage polarization restores intestinal epithelium damaged by DSS-induced colitis by activating IL-22-producing innate lymphoid cells

BACKGROUND

Triggering receptor expressed on myeloid cells-1 (TREM-1) is highly expressed on macrophages in inflamed intestines and reportedly promotes inflammatory bowel disease (IBD) by augmenting pro-inflammatory responses. To study the mechanism mediated by TREM-1 on macrophages, we generated an independent TREM-1 deficient mouse.

METHODS

Acute colitis was induced in C57BL/6 and TREM-1-deficient mice by the administration of dextran sodium sulfate (DSS). Colonic lamina propria immune cell composition and cytokines were analyzed. An innate lymphoid cell (ILC) co-culture experiment with macrophages was used to analyze IL-22 levels. Exogenous IL-22 and TREM-1-expressing macrophages were supplied to TREM-1-deficient mice for examining their effects on intestinal barrier integrity.

RESULTS

In inflamed colons, TREM-1 loss compromised the activation of ILC3 and their production of IL-22, which is required for intestinal barrier integrity. ILC3-mediated IL-22 production depends on IL-1β secreted by M1-polarized macrophages, and we found that TREM-1 deficiency results in a decreased number of IL-1β producing-M1 macrophages in colons exposed to DSS. Accordingly, DSS-mediated damage was ameliorated by supplying exogenous IL-22 and TREM-1-expressing macrophages to TREM-1-deficient mice.

CONCLUSIONS

TREM-1 plays a crucial role in regulating IL-22 production by ILC3 through modulating M1-macrophage polarization during DSS-induced acute colitis.

Clinical assessment and identification of immuno-oncology markers concerning the 19-gene based risk classifier in stage IV colorectal cancer

BACKGROUND

Genomic profiling of tumors has contributed to the understanding of colorectal cancer (CRC), facilitating diagnosis, prognosis and selection of treatments, including targeted regimens. A report suggested that a 19-gene-based risk classifier (TCA19) was a prognostic tool for patients with stage III CRC. The survival outcomes in patients with stage IV CRC are still poor and appropriate selection of targeted therapies and immunotherapies is challenging.

AIM

To assess clinical implication of TCA19 in patients with stage IV CRC, and to identify TCA19 with involvement in immune-oncology.

METHODS

A retrospective review of the medical records of 60 patients with stage IV CRC was conducted, assessing clinicopathological variables and progression-free survival (PFS). TCA19 gene expression was determined by quantitative polymerase chain reaction (qPCR) in matched normal and tumor tissues taken from the study cohort. Expression of potential immune-oncology regulatory proteins and targets was examined by immunohistochemistry (IHC), western blot, immunofluorescence staining in tissues from a validation cohort of 10 patients, and in CRC cell lines co-cultured with monocyte in vitro.

RESULTS

In the patients with TCA19 score higher than the median, the PFS rates of eight patients who received the targeted regimens were significantly higher than the PFS rates of four patients who received 5-fluorouracil-based regimen (P = 0.041). In multivariate analysis, expression of signaling lymphocytic activation molecule family, member 7 (SLAMF7) and triggering receptor expressed on myeloid cells 1 (TREM1) was associated with PFS in the 60-patient cohort. After checking another 10 validate set, the expression of the IHC, the level of real-time qPCR, and the level of western blot were lower for SLAMF7 and higher for TREM7 in primary and metastatic tumors than in normal tissues. In CRC cells expressing SLAMF7 that were co-cultured with a monocytic cell line, levels of CD 68 and CD 73 were significantly lower at day 5 of co-culture than at day 0.

CONCLUSION

The TCA19 score might be prognostic for target-regimen-specific PFS in stage IV CRC. Down-regulation of SLAMF7 and up-regulation of TREM1 occur in primary and metastatic tumor tissues.

Nuclear receptor ligands induce TREM-1 expression on dendritic cells: analysis of their role in tumors

Dendritic cells (DCs) initiate adaptive immune responses after their migration to secondary lymphoid organs. The LXR ligands/oxysterols and the RXR ligand 9-cis Retinoic Acid (9-cis RA) were shown to dampen DC migration to lymphoid organs through the inhibition of CCR7 expression. We performed transcriptomics of DCs undergoing maturation in the presence of the LXR ligand 22R-Hydroxycholesterol (22R-HC). The analysis highlighted more than 1500 genes modulated by 22R-HC treatment, including the triggering receptor expressed on myeloid cells (TREM)-1, which was found markedly up-regulated. We tested the effect of other nuclear receptor ligands (NRL) and we reported the induction of TREM-1 following RXR, RAR and VDR activation. From a functional point of view, triggering of TREM-1 induced by retinoids increased TNFα and IL-1β release, suggesting an active role of NRL-activated TREM-1⁺ DCs in inflammation-driven diseases, including cancer. Consistently with this hypothesis we detected DCs expressing TREM-1 in pleural effusions and ascites of cancer patients, an observation validated by the induction of TREM-1, LXR and RAR target genes when monocyte-DCs were activated in the presence of tumor-conditioned fluids. Finally, we observed a better control of LLC tumor growth in Trem-1^-/- bone marrow chimera mice as compared to wild type chimera mice. Future studies will be necessary to shed light on the mechanism of TREM-1 induction by distinct NRL, and to characterize the role of TREM-1⁺ DCs in tumor growth.

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.

Effects of Porphyromonas gingivalis LPS and LR12 peptide on TREM-1 expression by monocytes

Periodontal disease involves the activation of host immune response, acting not only as defender of periodontal tissues against bacterial aggression but also as mediator of tissue destruction. Triggering receptor expressed on myeloid cells 1 (TREM-1) is an immune receptor that synergizes with Toll-like receptors in amplifying the inflammatory response mediated by microbial molecules.

AIM

To investigate the role of P. gingivalis lipopolysaccharide (LPS) and the effect of LR12, a TREM-1 inhibitory peptide, on the expression of membrane-bound and soluble form of TREM-1 on human primary monocytes, as well as the production of proinflammatory cytokines.

MATERIAL AND METHODS

Cells were stimulated with 1 μg/ml of LPS with or without LR12. PCR, flow cytometry and ELISA were used to determine TREM-1 expressions and cytokines release by monocytes.

RESULTS

P. gingivalis LPS can induce a significant increase in TREM-1 expression (mRNA, membrane-bound and soluble form, p < 0.001) as well as cytokines (IL-1β, TNFα) and chemokines (IL-8) production by monocytes. This monocytes' activation was partly prevented by LR12.

CONCLUSIONS

TREM-1 inhibitors such as LR12 could be interesting for the modulation of the excessive inflammatory response that occurs during periodontal disease.

Genetic variants of TREML2 are associated with HLA-B27-positive ankylosing spondylitis

Although ankylosing spondylitis (AS) is a common, highly heritable arthropathy, the precise genetic mechanism underlying the disease remains elusive. Here, we investigate the disease-causing mutations in a large AS family with distinguished complexity, consisting of 23 patients covering four generations and exhibiting a mixed HLA-B27 (+) and (-) status. Linkage analysis with 32 members using three methods and whole-exome sequencing analysis with three HLA-B27 (+) patients, one HLA-B27 (-) patient, and one healthy individual did not identify a mutation common to all of the patients, strongly suggesting the existence of genetic heterogeneity in this large pedigree. However, if only B27-positive patients were analyzed, the linkage analysis located a 22-Mb region harboring the HLA gene cluster in chromosome 6 (LOD = 4.2), and the subsequent exome analysis identified two non-synonymous mutations in the TREML2 and IP6K3 genes. These genes were resequenced among 370 sporadic AS patients and 487 healthy individuals. A significantly higher mutation frequency of TREML2 was observed in AS patients (1.51% versus 0.21%). The results obtained for the AS pedigree and sporadic patients suggest that mutation of TREML2 is a major factor leading to AS for HLA-B27 (+) members in this large family and that TREML2 is also a susceptibility gene promoting the development of ankylosing spondylitis in HLA-B27 (+) individuals.

Assessment of Serum sTREM-1 as a Marker of Subclinical Inflammation in Diarrhea-Predominant Patients with Irritable Bowel Syndrome

BACKGROUND

Irritable bowel disease (IBS) is viewed upon as a functional disorder of subclinical inflammatory changes in recent years, and there is no reliable biomarker. Triggering receptor expressed on myeloid cells 1 (TREM-1), also produced in a soluble form (sTREM-1), is involved in the activation of inflammatory cascades of intracellular events and may play a role in pathogenesis of IBS.

AIM

To investigate whether serum sTREM-1 level can be used as a marker of subclinical inflammation in D-IBS.

METHODS

Abdominal pain was quantified by a validated questionnaire. Expression level of TREM-1 in colonic mucosa as well as sTREM-1 level in serum was also detected. Furthermore, we investigated the involvement of TREM-1-associated macrophage activation in IBS-like visceral hypersensitivity.

RESULTS

No evidence for obvious inflammation was found in D-IBS patients. Serum sTREM-1 level in D-IBS patients was significantly higher than that in HCs, which was also significantly correlated with abdominal pain scores. We showed a marked increase in the proportion of TREM-1-expressing macrophages in D-IBS, which was significantly correlated with abdominal pain scores. Functionally, gadolinium chloride (GdCl₃), a macrophage selective inhibitor, or LP17, the TREM-1-specific peptide, significantly suppressed the visceral hypersensitivity in trinitrobenzene sulfonic acid (TNBS)-treated mice with IBS-like visceral hypersensitivity.

CONCLUSIONS

Serum sTREM-1 level is significantly higher in D-IBS patients and positively correlates with abdominal pain, which may be initiated by TREM-1-associated macrophage activation, indicating the existence of subclinical inflammation in D-IBS. Therefore, serum sTREM-1 is a potential marker of subclinical inflammation in D-IBS.

TREM-1 Inhibition Restores Impaired Autophagy Activity and Reduces Colitis in Mice

BACKGROUND AND AIMS

Triggering receptor expressed on myeloid cells-1 [TREM-1] is known to amplify inflammation in several diseases. Autophagy and endoplasmic reticulum [ER] stress, which activate the unfolded protein response [UPR], are closely linked and defects in these pathways contribute to the pathogenesis of inflammatory bowel disease [IBD]. Both autophagy and UPR are deeply involved in host-microbiota interactions for the clearance of intracellular pathogens, thus contributing to dysbiosis. We investigated whether inhibition of TREM-1 would prevent aberrant inflammation by modulating autophagy and ER stress and preventing dysbiosis.

METHODS

An experimental mouse model of colitis was established by dextran sulphate sodium treatment. TREM-1 was inhibited, either pharmacologically by LR12 peptide or genetically with Trem-1 knock-out [KO] mice. Colon tissues and faecal pellets of control and colitic mice were used. Levels of macroautophagy, chaperone-mediated autophagy [CMA], and UPR proteins were evaluated by western blotting. The composition of the intestinal microbiota was assessed by MiSeq sequencing in both LR12-treated and KO animals.

RESULTS

We confirmed that inhibition of TREM-1 attenuates the severity of colitis clinically, endoscopically and histologically. We observed an increase in macroautophagy [ATG1/ULK-1, ATG13, ATG5, ATG16L1, and MAP1LC3-I/II] and in CMA [HSPA8 and HSP90AA1], whereas there was a decrease in the UPR [PERK, IRE-1α, and ATF-6α] protein expression levels in TREM-1 inhibited colitic mice. TREM-1 inhibition prevented dysbiosis.

CONCLUSIONS

TREM-1 may represent a novel drug target for the treatment of IBD, by modulating autophagy activity and ER stress.

[Research progress on the relationship between triggering receptor expressed on myeloid cells 1 and 2 and malignant tumors]

Increasing scientific evidence supports the positive relationship between inflammation and cancer development. The immune response initiated by pattern recognition receptors is critical to triggering of tumor-associated inflammation. Triggering receptor expressed on myeloid cells (TREM) is an immunoglobulin of the super transmembrane glycoprotein family, which is mainly expressed on select groups of myeloid cells. The most important members of TREM comprise TREM-1 and TREM-2. Activation of TREM-1 and TREM-2 signaling is initiated upon binding of their ligands. Subsequently, cross-linking reactions of downstream effectors occur, resulting in inflammation regulation. Recently, the connection between TREM and malignant tumors has been widely noticed and studied. This review summarizes studies of association between TREM-1, TREM-2, and malignant tumors in the medical field to provide new ideas for study on the correlation between periodontitis and oral cavity cancer.

TREM-1 promotes intestinal tumorigenesis

Triggering receptor expressed on myeloid cells-1 (TREM-1) is a potent amplifier of pro-inflammatory innate immune responses. Increasing evidence suggests a role for TREM-1 not only in acute pathogen-induced reactions but also in chronic and non-infectious inflammatory disorders, including various types of cancer. Here, we demonstrate that genetic deficiency in Trem1 protects from colorectal cancer. In particular, Trem1^−/− mice exhibited reduced tumor numbers and load in an experimental model of inflammation-driven tumorigenesis. Gene expression analysis of Trem1^−/− versus Trem1^+/+ tumor tissue demonstrated distinct immune signatures. Whereas Trem1^−/− tumors showed an increased abundance of transcripts linked to adaptive immunity, Trem1^+/+ tumors were characterized by overexpression of innate pro-inflammatory genes associated with tumorigenesis. Compared to adjacent tumor-free colonic mucosa, expression of Trem1 was increased in murine and human colorectal tumors. Unexpectedly, TREM-1 was not detected on tumor-associated Ly6C⁻ MHC class II⁺ macrophages. In contrast, TREM-1 was highly expressed by tumor-infiltrating neutrophils which represented the predominant myeloid population in Trem1^+/+ but not in Trem1^−/− tumors. Collectively, our findings demonstrate a clear role of TREM-1 for intestinal tumorigenesis and indicate TREM-1-expressing neutrophils as critical players in colorectal tumor development.

Butyrate modulating effects on pro-inflammatory pathways in human intestinal epithelial cells

Butyrate acts as energy source for intestinal epithelial cells and as key mediator of several immune processes, modulating gene expression mainly through histone deacetylation inhibition. Thanks to these effects, butyrate has been proposed for the treatment of many intestinal diseases. Aim of this study was to investigate the effect of butyrate on the expression of a large series of target genes encoding proteins involved in pro-inflammatory pathways. We performed quantitative real-time-PCR analysis of the expression of 86 genes encoding proteins bearing to pro-inflammatory pathways, before and after butyrate exposure, in primary epithelial cells derived from human small intestine and colon. Butyrate significantly down-regulated the expression of genes involved in inflammatory response, among which nuclear factor kappa beta, interferon-gamma, Toll like 2 receptor and tumour necrosis factor-alpha. Further confirmations of these data, including studies at protein level, would support the use of butyrate as effective therapeutic strategy in intestinal inflammatory disorders.

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.

α-Mangostin inhibits DMBA/TPA-induced skin cancer through inhibiting inflammation and promoting autophagy and apoptosis by regulating PI3K/Akt/mTOR signaling pathway in mice

Skin cancer is the most common form of cancer responsible for considerable morbidity and mortality, the treatment progress of which remains slow though. Therefore, studies identifying anti-skin cancer agents that are innocuous are urgently needed. α-Mangostin, a natural product isolated from the pericarp of mangosteen fruit, has potent anti-cancer activity. However, its role in skin cancer remains unclear. The aim of this study was to evaluate the treatment effect of α-mangostin on skin tumorigenesis induced by 9,10-dimethylbenz[a]anthracene (DMBA)/TPA in mice and the potential mechanism. Treatment with α-mangostin significantly suppressed tumor formation and growth, and markedly reduced the incidence rate. α-Mangostin not only inhibited the expressions of pro-inflammatory factors, but also promoted the production of anti-inflammatory factors in tumor and blood. It induced autophagy of skin tumor and regulated the expressions of autophagy-related proteins. The protein expressions of LC3, LC3-II and Beclin1 increased whereas those of LC3-I and p62 decreased after treatment with α-mangostin. Moreover, α-mangostin promoted the apoptosis of skin tumor dose-dependently by up-regulating of Bax, cleaved caspase-3, cleaved PARP and Bad, and down-regulating of Bcl-2 and Bcl-xl. Furthermore, showed α-mangostin inhibited the PI3K/AKT/mTOR (mammalian target of rapamycin) signaling pathway, as evidenced by decreased expressions of phospho-PI3K (p-PI3K), p-Akt and p-mTOR, but did not affect the expressions of t-PI3K, t-Akt or t-mTOR. Collectively, α-mangostin suppressed murine skin tumorigenesis induced by DMBA/TPA through inhibiting inflammation and promoting autophagy and apoptosis by regulating the PI3K/Akt/mTOR signaling pathway, as a potential candidate for future clinical therapy.

Inhibition of triggering receptor expressed on myeloid cells-1 ameliorates experimental autoimmune neuritis

Experimental autoimmune neuritis (EAN) is a cluster of differentiation 4+ T helper 1 cell-mediated inflammatory demyelinating disease of the peripheral nervous system and serves as a useful animal model for Guillain‑Barré syndrome. Triggering receptor expressed on myeloid cells‑1 (TREM‑1) is an important receptor involved in sepsis and the innate inflammatory response. Linear plasmid 17 (LP 17) peptide is a competitive antagonist of TREM‑1. To investigate the role of TREM‑1 in EAN, 64 male Lewis rats were randomly divided into four groups: Normal saline, complete Freund's adjuvant, EAN and LP 17. The present study assessed the mRNA expression levels of TREM‑1, tumor necrosis factor‑α and interleukin‑1β in sciatic nerves and peripheral blood mononuclear cells. The results demonstrated that inhibiting TREM-1 by administering LP 17 ameliorated symptoms and reduced inflammation in EAN rats. The present study concluded that TREM‑1 may be involved in the pathogenesis of EAN, and that inhibition of TREM-1 may ameliorate EAN.

Overexpression of TREM2 enhances glioma cell proliferation and invasion: a therapeutic target in human glioma

Gliomas are the most common and aggressive type of primary adult brain tumors. Although TREM2 mutation is reported to be related to Nasu-Hakola disease and Alzheimer's disease, little is known about the association between TREM2 and gliomas. Here, we reported that TREM2 was significantly overexpressed in glioma tissues compared with non-tumorous brain tissues. Furthermore, TREM2 expression was closely related to pathological grade and overall survival of patients with gliomas. Down-regulation of TREM2 in two glioma cell lines, U87 and U373, resulted in a significant reduction in cell proliferation, migration and invasion and a dramatic increase in S phase arrest and apoptosis. In vivo tumorigenesis experiment also revealed that depletion of TREM2 expression inhibited U87 cell proliferation. Moreover, based on gene set enrichment analysis (GSEA) with The Cancer Genome Atlas (TCGA) dataset, we found that TREM2 was positive related to Kyoto Encyclopedia of Genes and Genomes (KEGG) apoptosis, Cromer metastasis and KEGG chemokine pathways, which was further validated by western blot in TREM2 knockdown glioma cells and indicated a possible mechanism underlying its effects on glioma. In summary, our study suggests that TREM2 may work as an oncogene and a new effective therapeutic target for glioma treatment.

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

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

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

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.

Dose-dependent role of claudin-1 in vivo in orchestrating features of atopic dermatitis

Atopic dermatitis (AD) is a chronic inflammatory skin disease in humans. It was recently noted that the characteristics of epidermal barrier functions critically influence the pathological features of AD. Evidence suggests that claudin-1 (CLDN1), a major component of tight junctions (TJs) in the epidermis, plays a key role in human AD, but the mechanism underlying this role is poorly understood. One of the main challenges in studying CLDN1's effects is that Cldn1 knock-out mice cannot survive beyond 1 d after birth, due to lethal dehydration. Here, we established a series of mouse lines that express Cldn1 at various levels and used these mice to study Cldn1's effects in vivo. Notably, we discovered a dose-dependent effect of Cldn1's expression in orchestrating features of AD. In our experimental model, epithelial barrier functions and morphological changes in the skin varied exponentially with the decrease in Cldn1 expression level. At low Cldn1 expression levels, mice exhibited morphological features of AD and an innate immune response that included neutrophil and macrophage recruitment to the skin. These phenotypes were especially apparent in the infant stages and lessened as the mice became adults, depending on the expression level of Cldn1 Still, these adult mice with improved phenotypes showed an enhanced hapten-induced contact hypersensitivity response compared with WT mice. Furthermore, we revealed a relationship between macrophage recruitment and CLDN1 levels in human AD patients. Our findings collectively suggest that CLDN1 regulates the pathogenesis, severity, and natural course of human AD.

Gut mucosal DAMPs in IBD: from mechanisms to therapeutic implications

Endogenous damage-associated molecular patterns (DAMPs) are released during tissue damage and have increasingly recognized roles in the etiology of many human diseases. The inflammatory bowel diseases (IBD), ulcerative colitis (UC) and Crohn's disease (CD), are immune-mediated conditions where high levels of DAMPs are observed. DAMPs such as calprotectin (S100A8/9) have an established clinical role as a biomarker in IBD. In this review, we use IBD as an archetypal common chronic inflammatory disease to focus on the conceptual and evidential importance of DAMPs in pathogenesis and why DAMPs represent an entirely new class of targets for clinical translation.

Chronic inflammation and cancer: emerging roles of triggering receptors expressed on myeloid cells

Inflammation is tightly regulated by a vast system that is intricately interconnected with innate immunity. Aberrations in expression or signaling, such as in innate immune receptors, can create excessive inflammation and, when chronic, often promote oncogenesis. The triggering receptor expressed on myeloid cells receptor family has been characterized as a major player in the amplification and signaling of the inflammatory response. In a number of chronic inflammatory conditions and malignancies, the triggering receptor expressed on myeloid cells has been implicated in disease severity and progression. In this article, the current understanding of triggering receptor expressed on myeloid cells function in pre-malignant, malignant and chronic inflammatory conditions is critically reviewed. The potential for therapeutic application is also discussed.

PF2405, standardized fraction of Scutellaria baicalensis, ameliorates colitis in vitro and in vivo

Standardized extraction procedures for herb are as important as their authentication to maintain their quality and ensure their safe use. We had prepared a standardized and purified Scutellaria baicalensis Georgi extract, PF2405, which was enriched with three major components, baicalein, oroxylin A and wogonin. In the present study, we investigated the potential anti-inflammatory effects of PF2405 in vitro and in two different experimental animal models of inflammatory bowel disease. Effect of PF2405 studied in tumor necrosis factor (TNF)-α-induced HT-29 cells in vitro. In vivo experimental colitis models were induced by administration of trinitrobenzene sulfonic acid (TNBS) or dextran sulfate sodium (DSS). PF2405 (50 μg/ml) decreased TNF-α-induced cyclooxygenase (COX)-2 expressions through inhibition of phosphorylation of c-Jun N-terminal kinases and p38 mitogen-activated protein kinase in HT-29 cells. Combination of baicalein (20 μg/ml), oroxylin A (8 μg/ml), and wogonin (2 μg/ml) markedly inhibits TNF-α-induced COX-2 expression when compared with individual components. PF2405 (25 mg/kg b.w.) treatment significantly reduced histopathological severity; suppressed expression of COX-2, TNF-α, and interleukin-1β in TNBS-induced mice. Moreover, PF2405 (25 mg/kg b.w.) has both potent preventive and therapeutic activities in DSS-induced colitis. Collectively, PF2405 shows prominent anti-inflammatory effect that can be used as a new therapeutic approach for intestinal inflammatory disorders.

Perturbations of mucosal homeostasis through interactions of intestinal microbes with myeloid cells

Mucosal surfaces represent the largest areas of interactions of the host with its environment. Subsequently, the mucosal immune system has evolved complex strategies to maintain the integrity of the host by inducing protective immune responses against pathogenic and tolerance against dietary and commensal microbial antigens within the broad range of molecules the intestinal epithelium is exposed to. Among many other specialized cell subsets, myeloid cell populations - due to their strategic location in the subepithelial lamina propria - are the first ones to scavenge and process these intestinal antigens and to send consecutive signals to other immune and non-immune cell subsets. Thus, myeloid cell populations represent attractive targets for clinical intervention in chronic inflammatory bowel diseases (IBDs) such as ulcerative colitis (UC) and Crohn's disease (CD) as they initiate and modulate inflammatory or regulatory immune response and shape the intestinal T cell pool. Here, we discuss the interactions of the intestinal microbiota with dendritic cell and macrophage populations and review in this context the literature on four promising candidate molecules that are critical for the induction and maintenance of intestinal homeostasis on the one hand, but also for the initiation and propagation of chronic intestinal inflammation on the other.

The triggering receptor expressed on myeloid cells (TREM) in inflammatory bowel disease pathogenesis

The Triggering Receptors Expressed on Myeloid cells (TREM) are a family of cell-surface molecules that control inflammation, bone homeostasis, neurological development and blood coagulation. TREM-1 and TREM-2, the best-characterized receptors so far, play divergent roles in several infectious diseases. In the intestine, TREM-1 is highly expressed by macrophages, contributing to inflammatory bowel disease (IBD) pathogenesis. Contrary to current understanding, TREM-2 also promotes inflammation in IBD by fueling dendritic cell functions. This review will focus specifically on recent insights into the role of TREM proteins in IBD development, and discuss opportunities for novel treatment approaches.

A novel ligand-independent peptide inhibitor of TREM-1 suppresses tumor growth in human lung cancer xenografts and prolongs survival of mice with lipopolysaccharide-induced septic shock

Triggering receptor expressed on myeloid cells-1 (TREM-1) amplifies the inflammatory response and plays a role in cancer and sepsis. Inhibition of TREM-1 by short hairpin RNA (shRNA) in macrophages suppresses cancer cell invasion in vitro. In the clinical setting, high levels of TREM-1 expression on tumor-associated macrophages are associated with cancer recurrence and poor survival of patients with non-small cell lung cancer (NSCLC). TREM-1 upregulation on peritoneal neutrophils has been found in human sepsis patients and in mice with experimental lipopolysaccharide (LPS)-induced septic shock. However, the precise function of TREM-1 and the nature of its ligand are not yet known. In this study, we used the signaling chain homooligomerization (SCHOOL) model of immune signaling to design a novel, ligand-independent peptide-based TREM-1 inhibitor and demonstrated that this peptide specifically silences TREM-1 signaling in vitro and in vivo. Utilizing two human lung tumor xenograft nude mouse models (H292 and A549) and mice with LPS-induced sepsis, we show for the first time that blockade of TREM-1 function using non-toxic and non-immunogenic SCHOOL peptide inhibitors: 1) delays tumor growth in xenograft models of human NSCLC, 2) prolongs survival of mice with LPS-induced septic shock, and 3) substantially decreases cytokine production in vitro and in vivo. In addition, targeted delivery of SCHOOL peptides to macrophages utilizing lipoprotein-mimicking nanoparticles significantly increased peptide half-life and dosage efficacy. Together, the results suggest that ligand-independent modulation of TREM-1 function using small synthetic peptides might be a suitable treatment for sepsis and NSCLC and possibly other types of inflammation-associated disorders.

Triggering receptor expressed on myeloid cells 1 (TREM-1)-mediated Bcl-2 induction prolongs macrophage survival

Triggering receptor expressed on myeloid cells 1 (TREM-1) is a superimmunoglobulin receptor expressed on myeloid cells that plays an important role in the amplification of inflammation. Recent studies suggest a role for TREM-1 in tumor-associated macrophages with relationship to tumor growth and progression. Whether the effects of TREM-1 on inflammation and tumor growth are mediated by an alteration in cell survival signaling is not known. In these studies, we show that TREM-1 knock-out macrophages exhibit an increase in apoptosis of cells in response to lipopolysaccharide (LPS) suggesting a role for TREM-1 in macrophage survival. Specific ligation of TREM-1 with monoclonal TREM-1 (mTREM-1) or overexpression of TREM-1 with adeno-TREM-1 induced B-cell lymphoma-2 (Bcl-2) with depletion of the key executioner caspase-3 prevents the cleavage of poly(ADP-ribose) polymerase. TREM-1 knock-out cells showed lack of induction of Bcl2 with an increase in caspase-3 activation in response to lipopolysaccharide. In addition overexpression of TREM-1 with adeno-TREM-1 led to an increase in mitofusins (MFN1 and MFN2) and knockdown of TREM-1 decreased the expression of mitofusins suggesting that TREM-1 contributes to the maintenance of mitochondrial integrity favoring cell survival. Investigations into potential mechanisms by which TREM-1 alters cell survival showed that TREM-1-induced Bcl-2 in an Egr2-dependent manner. Furthermore, our data shows that expression of Egr2 in response to specific ligation of TREM-1 is ERK mediated. These data for the first time provide novel mechanistic insights into the role of TREM-1 as an anti-apoptotic protein that prolongs macrophage survival.