Inhibition of innate co-receptor TREM-1 signaling reduces CD4(+) T cell activation and prolongs cardiac allograft survival

Triggering receptor expressed on myeloid cells-1 aggravates obliterative bronchiolitis via enhancing the proinflammatory phenotype of macrophages

BACKGROUND

Triggering Receptor Expressed on Myeloid cells-1 (TREM-1) plays an important role in innate immune system. However, whether and how TREM-1 contributes to obliterative bronchiolitis (OB) progression remains unclear.

METHODS

A murine orthotopic tracheal transplantation model was constructed to mimic the pathogenesis of OB. qPCR and immunoblotting were used to measure TREM-1 expression. RNA sequencing was used to investigate the impact of TREM-1 on proinflammatory phenotype of macrophages. Trem-1 knockout mice and Nlrp3 knockout mice were generated to investigate the role of the TREM-1/NLRP3 pathway in the proinflammatory phenotype of macrophages. The infiltration of immune cells within the grafts was quantified using immunofluorescence staining. Flow cytometry was used to detect the proportion of different immune cells in mice spleen and the expression levels of iNOS and co-stimulatory molecules in macrophages.

RESULTS

The expression of TREM-1 was upregulated in the mouse OB model. Genetic ablation or pharmacological inhibition of TREM-1 ameliorated OB, whereas the stimulation of TREM-1 using anti-TREM-1 agonistic antibody exacerbated OB. Moreover, Trem-1 ablation reduced the infiltration of iNOS+ macrophages and limited the T cell responses. In vitro studies revealed that Trem-1 deletion impaired the proinflammatory function and antigen presentation ability of macrophages. Additionally, Trem-1 knockout inhibited the activation of NLRP3 signaling pathway. NLRP3 overexpression restored the proinflammatory phenotype of Trem-1 knockout macrophages.

CONCLUSIONS

These findings indicated that TREM-1 could promote the proinflammatory phenotype of macrophages through NLRP3 inflammasome activation, thereby exacerbating OB progression. These findings indicated that TREM-1 may serve as a therapeutic target for OB treatment.

TREM-1 Modulation Strategies for Sepsis

The triggering receptor expressed on myeloid cells-1 (TREM-1) is a pattern recognition receptor, which can be upregulated in inflammatory diseases as an amplifier of immune responses. Once activated, TREM-1 induces the production and release of pro-inflammatory cytokines and chemokines, in addition to increasing its own expression and circulating levels of the cleaved soluble extracellular portion of TREM-1 (sTREM-1). This amplification of the inflammatory response by TREM-1 has now been considered as a critical contributor to the dysregulated immune responses in sepsis. Studies have shown that in septic patients there is an elevated expression of TREM-1 on immune cells and increased circulating levels of sTREM-1, associated with increased mortality. As a result, a considerable effort has been made towards identifying endogenous ligands of TREM-1 and developing TREM-1 inhibitory peptides to attenuate the exacerbated inflammatory response in sepsis. TREM-1 modulation has proven a promising strategy for the development of therapeutic agents to treat sepsis. Therefore, this review encompasses the ligands investigated as activators of TREM-1 thus far and highlights the development and efficacy of novel inhibitors for the treatment of sepsis and septic shock.

Cross-presentation of dead-cell-associated antigens by DNGR-1+ dendritic cells contributes to chronic allograft rejection in mice

The purpose of this study was to elucidate whether DC NK lectin group receptor-1 (DNGR-1)-dependent cross-presentation of dead-cell-associated antigens occurs after transplantation and contributes to CD8⁺ T cell responses, chronic allograft rejection (CAR), and fibrosis. BALB/c or C57BL/6 hearts were heterotopically transplanted into WT, Clec9a^-/- , or Batf3^-/- recipient C57BL/6 mice. Allografts were analyzed for cell infiltration, CD8⁺ T cell activation, fibrogenesis, and CAR using immunohistochemistry, Western blot, qRT² -PCR, and flow cytometry. Allografts displayed infiltration by recipient DNGR-1⁺ DCs, signs of CAR, and fibrosis. Allografts in Clec9a^-/- recipients showed reduced CAR (p < 0.0001), fibrosis (P = 0.0137), CD8⁺ cell infiltration (P < 0.0001), and effector cytokine levels compared to WT recipients. Batf3-deficiency greatly reduced DNGR-1⁺ DC-infiltration, CAR (P < 0.0001), and fibrosis (P = 0.0382). CD8 cells infiltrating allografts of cytochrome C treated recipients, showed reduced production of CD8 effector cytokines (P < 0.05). Further, alloreactive CD8⁺ T cell response in indirect pathway IFN-γ ELISPOT was reduced in Clec9a^-/- recipient mice (P = 0.0283). Blockade of DNGR-1 by antibody, similar to genetic elimination of the receptor, reduced CAR (P = 0.0003), fibrosis (P = 0.0273), infiltration of CD8⁺ cells (p = 0.0006), and effector cytokine levels. DNGR-1-dependent alloantigen cross-presentation by DNGR-1⁺ DCs induces alloreactive CD8⁺ cells that induce CAR and fibrosis. Antibody against DNGR-1 can block this process and prevent CAR and fibrosis.

TREM-1 and TREM-2 Expression on CD14+ Cells in Bronchoalveolar Lavage Fluid in Pulmonary Sarcoidosis and Hypersensitivity Pneumonitis in the Context of T Cell Immune Response

Background

Sarcoidosis and hypersensitivity pneumonitis (HP) are immunologically mediated processes caused by hypersensitivity reaction accompanied by similar features including lymphocytic alveolitis and granuloma formation. Recent studies describe the role of TREM receptors in T cell activation, differentiation, and granuloma formation. Alveolar macrophages activation via TREM receptors may be the key factor mediating subsequent immune response. The aim of the study was to analyse TREM-1 and TREM-2 expression to identify further molecular mechanisms participating in the immunopathogenesis of sarcoidosis and HP.

Methods

Flow cytometry was performed to analyse TREM-1 and TREM-2 expression on CD14⁺ cells in bronchoalveolar lavage fluid from patients having sarcoidosis or HP and a control group.

Results

The study proved increased TREM-1 expression on alveolar macrophages in pulmonary sarcoidosis and diminished TREM-1 expression in HP-Sarcoidosis: median: 76.7; HP: median: 29.9; control: median: 53.3, (sarcoidosis versus HP: p < 0.001; sarcoidosis versus control: p < 0.05). TREM-2 expression was increased in both, sarcoidosis and HP-sarcoidosis: median: 34.79; HP: median: 36.00; control: median: 12.98, (sarcoidosis versus control: p < 0.05; HP versus control: p < 0.05). Correlation analysis showed negative correlation between TREM-1 and total number of CD8⁺ cytotoxic T cells. In sarcoidosis TREM-1 expression decreased with changes of HRCT image, decrease in CD4/CD8 ratio and decrease in DLCO.

Conclusions

Differences in TREM receptor expression in sarcoidosis (increase in TREM-1 and TREM-2) and HP (increase in TREM-2) and correlation analysis suggests that activation via TREM may participate in typical immunological characteristics of sarcoidosis and HP.

Autoantibody production significantly decreased with APRIL/BLyS blockade in murine chronic rejection kidney transplant model

Chronic antibody mediated rejection (cAMR) remains a significant barrier to achieving long-term graft survival in kidney transplantation, which results from alloantibody production from B lymphocytes and plasma cells. APRIL (A proliferation-inducing ligand) and BLyS (B lymphocyte stimulator) are critical survival factors for B lymphocytes and plasma cells. Here we describe the results of APRIL/BLyS blockade in a murine cAMR kidney transplant model. c57/B6 mice underwent kidney transplantation with Bm12 kidneys (minor MHC mismatch), a well-described model for chronic rejection where animals cannot make donor specific antibody but rather make antinuclear antibody (ANA). Following transplantation, animals received TACI-Ig (to block APRIL and BLyS) or no treatment. Animals were continued on treatment until harvest 4 weeks following transplant. Serum was analyzed for circulating anti-nuclear autoantibodies using HEp-2 indirect immunofluorescence. Spleen and transplanted kidneys were analyzed via H&E. ANA production was significantly decreased in APRIL/BLyS blockade treated animals (p<0.0001). No significant difference in autoantibody production was found between syngeneic transplant control (B6 to B6) and APRIL/BLyS blockade treated animals (p = 0.90). Additionally, disruption of splenic germinal center architecture was noted in the APRIL/BLyS blockade treated animals. Despite the significant decrease in autoantibody production and germinal center disruption, no significant difference in lymphocyte infiltration was noted in the transplanted kidney. APRIL/BLyS blockade resulted in a significant decrease of autoantibody production and disrupted splenic germinal center formation in a chronic kidney transplant model, however in this model no difference in kidney transplant pathology was seen, which may have to do with the absence of any T cell centric immunosuppression. Regardless, these findings suggest that APRIL/BLyS blockade may play a role in decreasing antibody formation long-term in kidney transplantation. Future investigations will use APRIL/BLyS blockade in conjunction with T lymphocyte depleting agents to determine its efficacy in chronic rejection.

Inhibition of Lung Tumor Development in ApoE Knockout Mice via Enhancement of TREM-1 Dependent NK Cell Cytotoxicity

Apolipoprotein E (ApoE) is known to regulate lipid homeostasis and associated with atherosclerogenesis. Eventhough atherosclerogenesis is associated with tumor development, the role of ApoE in lung tumorigenesis and metastasis is not clear. Thus, the tumor growth and metastasis were compared in WT and ApoE knockout (KO) mice. Urethane-induced lung tumor incidence and B16F10 lung metastasis in ApoE knockout (KO) mice were significantly reduced in comparison to that in WT mice. Knockdown of ApoE expression in lung cancer cells and B16F10 cells also decreased cancer cell growth and metastasis. The inhibitory effect of ApoE KO on tumor development and metastasis was associated with increase of infiltration of NK cells. NK cells derived from ApoE KO mice showed much greater cytotoxicity than those from WT mice. These cytotoxic effect of NK cells derived from ApoE KO mice was associated with higher expression of Granzyme B, Fas Ligand, IFN-γ, TNF-α, NKG2D, NKp46, and DNAM-1 expression. Triggering receptor expressed on myeloid cell (TREM)-1 is a proinflammatory mediator expressed on NK cells, and is known to be associated with NK cell cytotoxicity. Thus, we investigated the role of TREM-1 on ApoE KO mice originated NK cell mediated cytotoxicity for cancer cells. Blockade of TREM-1 expression with a TREM-1 antagonist prevented NK cell-mediated cytotoxicity. TREM-1 antibody recovered cytotoxic effect of NK cells derived from KO mice of T-bet, which upregulating gene for TREM-1. These data indicate that ApoE KO suppressed lung tumor development and metastasis via increase of TREM-1-dependent anti-tumor activity of NK cells.

The heterotopic heart transplantation in mice as a small animal model to study mechanical unloading - Establishment of the procedure, perioperative management and postoperative scoring

BACKGROUND

Unloading of failing hearts by left ventricular assist devices induces an extensive cardiac remodeling which may lead to a reversal of the initial phenotype-or to its deterioration. The mechanisms underlying these processes are unclear.

HYPOTHESIS

Heterotopic heart transplantion (hHTX) is an accepted model for the study of mechanical unloading in rodents. The wide variety of genetically modified strains in mice provides an unique opportunity to examine remodeling pathways. However, the procedure is technically demanding and has not been extensively used in this area. To support investigators adopting this method, we present our experience establishing the abdominal hHTX in mice and describe refinements to the technique.

METHODS

In this model, the transplanted heart is vascularised but implanted in series, and therefore does not contribute to systemic circulation and results in a complete mechanical unloading of the donor heart. Training followed a systematic program using a combination of literature, video tutorials, cadaveric training, direct observation and training in live animals.

RESULTS

Successful transplantation was defined as a recipient surviving > 24 hours with a palpable, beating apex in the transplanted heart and was achieved after 20 transplants in live animals. A success rate of 90% was reached after 60 transplants. Operative time was shown to decrease in correlation with increasing number of procedures from 200 minutes to 45 minutes after 60 operations. Cold/warm ischemia time improved from 45/100 to 10/20 minutes. Key factors for success and trouble shootings were identified.

CONCLUSION

Abdominal hHTX in the mouse may enable future examination of specific pathways in unloading induced myocardial remodeling. Establishment of the technique, however, is challenging. Structured training programs utilising a variety of training methods can help to expedite the process. Postoperative management, including daily scoring increases animal wellbeing and helps to predict survival.

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

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

Innate immune mechanisms in transplant allograft vasculopathy

PURPOSE OF REVIEW

Allograft vasculopathy is the leading cause of late allograft loss following solid organ transplantation. Ischemia reperfusion injury and donor-specific antibody-induced complement activation confer heightened risk for allograft vasculopathy via numerous innate immune mechanisms, including MyD88, high-mobility group box 1 (HMGB1), and complement-induced noncanonical nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) signaling.

RECENT FINDINGS

The role of MyD88, a signal adaptor downstream of the Toll-like receptors (TLR), has been defined in an experimental heart transplant model, which demonstrated that recipient MyD88 enhanced allograft vasculopathy. Importantly, triggering receptor on myeloid receptor 1, a MyD88 amplifying signal, was present in rejecting human cardiac transplant biopsies and enhanced the development of allograft vasculopathy in mice. HMGB1, a nuclear protein that activates Toll-like receptors, also enhanced the development of allograft vasculopathy. Complement activation elicits assembly of membrane attack complexes on endothelial cells which activate noncanonical NF-κB signaling, a novel complement effector pathway that induces proinflammatory genes and potentiates endothelial cell-mediated alloimmune T-cell activation, processes which enhance allograft vasculopathy.

SUMMARY

Innate immune mediators, including HMGB1, MyD88, and noncanonical NF-κB signaling via complement activation contribute to allograft vasculopathy. These pathways represent potential therapeutic targets to reduce allograft vasculopathy after solid organ transplantation.

Effect of Triggering Receptor Expressed on Myeloid Cells 1 (TREM-1) Blockade in Rats with Cecal Ligation and Puncture (CLP)-Induced Sepsis

Background

Blocking of TREM-1 signaling improves survival of mice with sepsis induced by Pseudomonas aeruginosa. However, whether TREM-1 blockade has beneficial effects in polymicrobial sepsis is poorly understood. Here, we aimed to investigate the effect of modulation of the TREM-1 pathway in rats with polymicrobial sepsis induced by cecal ligation and puncture (CLP).

Material/Methods

Normal Sprague-Dawley (SD) rats with sepsis induced by CLP were allocated randomly to received scramble peptide or LP17 via the jugular vein. Serum level of sTREM-1, IL6, TNF-α, and IL-1β were detected by ELISA assay. The mRNA and protein levels of JAK2 and STAT3 were detected by real-time PCR and Western blot analysis.

Results

STREM-1 concentration was greatly and progressively increased in rats with CLP-induced sepsis, and the increase was attenuated by TREM-1 inhibitory peptide LP17. More than 60% survival was observed in rats at the experiment endpoint after LP17 treatment. TREM-1 blockade also attenuated the increased level of pro-inflammatory cytokines TNF-α, IL-6, and IL-1β, and thus attenuated systematic and distant inflammatory responses. Furthermore, TREM-1 blockade significantly attenuated the increased levels of pJAK2 and pSTAT3.

Conclusions

TREM-1 blockade by the use of an inhibitory peptide LP17 could prolong survival of rats with polymicrobial sepsis and attenuate systematic inflammatory responses through the JAK2/STAT3 signaling pathway. Our results suggest that modulation of TREM-1 by a synthetic peptide might be a potential therapeutic option for polymicrobial sepsis.

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.

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

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

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.

Basophils Trigger Fibroblast Activation in Cardiac Allograft Fibrosis Development

Fibrosis is a major component of chronic cardiac allograft rejection. Although several cell types are able to produce collagen, resident (donor-derived) fibroblasts are mainly responsible for excessive production of extracellular matrix proteins. It is currently unclear which cells regulate production of connective tissue elements in allograft fibrosis and how basophils, as potential producers of profibrotic cytokines, are involved this process. We studied this question in a fully MHC-mismatched model of heart transplantation with transient depletion of CD4(+) T cells to largely prevent acute rejection. The model is characterized by myocardial infiltration of leukocytes and development of interstitial fibrosis and allograft vasculopathy. Using depletion of basophils, IL-4-deficient recipients and IL-4 receptor-deficient grafts, we showed that basophils and IL-4 play crucial roles in activation of fibroblasts and development of fibrotic organ remodeling. In the absence of CD4(+) T cells, basophils are the predominant source of IL-4 in the graft and contribute to expansion of myofibroblasts, interstitial deposition of collagen and development of allograft vasculopathy. Our results indicated that basophils trigger the production of various connective tissue elements by myofibroblasts. Basophil-derived IL-4 may be an attractive target for treatment of chronic allograft rejection.

SDF-1/CXCR4/CXCR7 is pivotal for vascular smooth muscle cell proliferation and chronic allograft vasculopathy

Chronic rejection remains a major obstacle in transplant medicine. Recent studies suggest a crucial role of the chemokine SDF-1 on neointima formation after injury. Here, we investigate the potential therapeutic effect of inhibiting the SDF-1/CXCR4/CXCR7 axis with an anti-SDF-1 Spiegelmer (NOX-A12) on the development of chronic allograft vasculopathy. Heterotopic heart transplants from H-2bm12 to B6 mice and aortic transplants from Balb/c to B6 were performed. Mice were treated with NOX-A12. Control animals received a nonfunctional Spiegelmer (revNOX-A12). Samples were retrieved at different time points and analysed by histology, RT-PCR and proliferation assay. Blockade of SDF-1 caused a significant decrease in neointima formation as measured by intima/media ratio (1.0 ± 0.1 vs. 1.8 ± 0.1, P < 0.001 AoTx; 0.35 ± 0.05 vs. 1.13 ± 0.27, P < 0.05 HTx). In vitro treatment of primary vascular smooth muscle cells with NOX-A12 showed a significant reduction in proliferation (0.42 ± 0.04 vs. 0.24 ± 0.03, P < 0.05). TGF-β, TNF-α and IL-6 levels were significantly reduced under SDF-1 inhibition (3.42 ± 0.37 vs. 1.67 ± 0.33, P < 0.05; 2.18 ± 0.37 vs. 1.0 ± 0.39, P < 0.05; 2.18 ± 0.26 vs. 1.6 ± 0.1, P < 0.05). SDF-1/CXCR4/CXCR7 plays a critical role in the development of chronic allograft vasculopathy (CAV). Therefore, pharmacological inhibition of SDF-1 with NOX-A12 may represent a therapeutic option to ameliorate chronic rejection changes.

Triggering receptor expressed on myeloid cells-1 (TREM-1): a new player in antiviral immunity?

The triggering receptor expressed on myeloid cells (TREM) family of protein receptors is rapidly emerging as a critical regulator of a diverse array of cellular functions, including amplification of inflammation. Although the ligand(s) for TREM have not yet been fully identified, circumstantial evidence indicates that danger- and pathogen-associated molecular patterns (DAMPs and PAMPs) can induce cytokine production via TREM-1 activation. The discovery of novel functions of TREM, such as regulation of T-cell proliferation and activation of antigen-presenting cells, suggests a larger role of TREM proteins in modulation of host immune responses to microbial pathogens, such as bacteria and fungi. However, the significance of TREM signaling in innate immunity to virus infections and the underlying mechanisms remain largely unclear. The nature and intensity of innate immune responses, specifically production of type I interferon and inflammatory cytokines is a crucial event in dictating recovery vs. adverse outcomes from virus infections. In this review, we highlight the emerging roles of TREM-1, including synergy with classical pathogen recognition receptors. Based on the literature using viral PAMPs and other infectious disease models, we further discuss how TREM-1 may influence host-virus interactions and viral pathogenesis. A deeper conceptual understanding of the mechanisms associated with pathogenic and/or protective functions of TREM-1 in antiviral immunity is essential to develop novel therapeutic strategies for the control of virus infection by modulating innate immune signaling.

DAP12 deficiency in liver allografts results in enhanced donor DC migration, augmented effector T cell responses and abrogation of transplant tolerance

Liver interstitial dendritic cells (DC) have been implicated in immune regulation and tolerance induction. We found that the transmembrane immuno-adaptor DNAX-activating protein of 12 kDa (DAP12) negatively regulated conventional liver myeloid (m) DC maturation and their in vivo migratory and T cell allostimulatory ability. Livers were transplanted from C57BL/6(H2(b) ) (B6) WT or DAP12(-/-) mice into WT C3H (H2(k) ) recipients. Donor mDC (H2-K(b+) CD11c(+) ) were quantified in spleens by flow cytometry. Anti-donor T cell reactivity was evaluated by ex vivo carboxyfluorescein diacetate succinimidyl ester-mixed leukocyte reaction and delayed-type hypersensitivity responses, while T effector and regulatory T cells were determined by flow analysis. A threefold to fourfold increase in donor-derived DC was detected in spleens of DAP12(-/-) liver recipients compared with those given WT grafts. Moreover, pro-inflammatory cytokine gene expression in the graft, interferon gamma (IFNγ) production by graft-infiltrating CD8(+) T cells and systemic levels of IFNγ were all elevated significantly in DAP12(-/-) liver recipients. DAP12(-/-) grafts also exhibited reduced incidences of CD4(+) Foxp3(+) cells and enhanced CD8(+) T cell IFNγ secretion in response to donor antigen challenge. Unlike WT grafts, DAP12(-/-) livers failed to induce tolerance and were rejected acutely. Thus, DAP12 expression in liver grafts regulates donor mDC migration to host lymphoid tissue, alloreactive T cell responses and transplant tolerance.

Emerging roles for triggering receptor expressed on myeloid cells receptor family signaling in inflammatory diseases

Innate immune receptors represent important therapeutic targets for inflammatory disorders. In particular, the Toll-like receptor (TLR) family has emerged as a promoter of chronic inflammation that contributes to obesity, insulin resistance and atherosclerosis. Importantly, triggering receptor expressed on myeloid cells-1 (TREM-1) has been characterized as an 'amplifier' of TLR2 and TLR4 signaling. TREM-1- and TREM-2-dependent signaling, as opposed to TREM-like transcript-1 (TLT-1 or TREML1), are mediated through association with the transmembrane adaptor DNAX activation protein of 12 kDa (DAP12). Recessive inheritance of rare mutations in DAP12 or TREM-2 results in a disorder called polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy, and surprisingly these subjects are not immunocompromised. Recent progress into the roles of TREM/DAP12 signaling is critically reviewed here with a focus on metabolic, cardiovascular and inflammatory diseases. The expanding repertoire of putative ligands for TREM receptors is also discussed.

NKG2D blockade attenuated cardiac allograft vasculopathy in a mouse model of cardiac transplantation

A previous paper has reported that blockade of NKG2D was effective in protecting allograft in murine models of cardiac transplantation, but the mechanism of NKG2D blockade on attenuated cardiac allograft vasculopathy (CAV) was still unknown. In our current study, we found that wild-type recipients treated with anti-NKG2D monoclonal antibody (mAb) plus cytotoxic T lymphocyte antigen (CTLA)-4-immunoglobulin (I)g showed prolonged allograft survivals (>90 days, P < 0·001) significantly and attenuated CAV. These in-vivo results correlated with reduced alloantibody production, low expression of interleukin (IL)-17 and IL-6, while infiltration of regulatory T cells increased. IL-6 administration induced shorter allograft survival and higher CAV grade in CTLA-4-Ig plus anti-NKG2D mAb-treated recipients, whereas IL-17 had no significant effect on allograft survival and CAV grade in CTLA-4-Ig plus anti-NKG2D mAb-treated recipients. Furthermore, the prolonged allograft survival induced by NKG2D blockade was abrogated partially with depletion of regulatory T cells. In conclusion, blockade of NKG2D combined with CTLA-4-Ig attenuated CAV and this effect was associated with lower alloantibody production, inhibited IL-6 expression and enhanced expansion of regulatory T cells.