The chemokine receptor D6 has opposing effects on allergic inflammation and airway reactivity

Bystander Expression of Atypical Chemokine Receptor 2 Protects T Cells from Chemoattraction towards Cancer-Associated Fibroblasts

Atypical chemokine receptors (ACKRs) are a subclass of chemokine receptors that internalise and degrade chemokines instead of eliciting chemotaxis. Scavenging by ACKRs reduces the local bioavailability of chemokines and can thus reshape chemokine gradients that direct leukocyte trafficking during inflammation and anticancer responses. In pancreatic ductal adenocarcinoma (PDAC), chemokine axes, such as CXCL12-CXCR4, are co-opted by cancer-associated fibroblasts (CAFs) for tumour growth and escape, and immunosuppression. Here, we explore the use of ACKRs to reshape chemokine gradients within the PDAC tumour microenvironment. ACKR2, previously only known to scavenge inflammatory CC chemokines, was recently shown to be able to interact with CXCL10 and CXCL14. Here, using a chemokine binding assay and cytometric bead arrays, we reveal that ACKR2 scavenges additional CXC chemokines CXCL12 and CXCL1. ACKR2 scavenges CXCL12 with reduced efficiency compared to ACKR3, previously reported to bind CXCL12. Finally, we demonstrate that the overexpression of ACKR2 on bystander cells protects primary murine cytotoxic T lymphocytes from PDAC CAF-mediated chemoattraction. These findings reveal new CXC chemokine ligands of ACKR2 and indicate that ACKR overexpression may protect T cells from misdirection by CAFs.

The Chemokine System as a Key Regulator of Pulmonary Fibrosis: Converging Pathways in Human Idiopathic Pulmonary Fibrosis (IPF) and the Bleomycin-Induced Lung Fibrosis Model in Mice

Idiopathic pulmonary fibrosis (IPF) is a chronic and lethal interstitial lung disease (ILD) of unknown origin, characterized by limited treatment efficacy and a fibroproliferative nature. It is marked by excessive extracellular matrix deposition in the pulmonary parenchyma, leading to progressive lung volume decline and impaired gas exchange. The chemokine system, a network of proteins involved in cellular communication with diverse biological functions, plays a crucial role in various respiratory diseases. Chemokine receptors trigger the activation, proliferation, and migration of lung-resident cells, including pneumocytes, endothelial cells, alveolar macrophages, and fibroblasts. Around 50 chemokines can potentially interact with 20 receptors, expressed by both leukocytes and non-leukocytes such as tissue parenchyma cells, contributing to processes such as leukocyte mobilization from the bone marrow, recirculation through lymphoid organs, and tissue influx during inflammation or immune response. This narrative review explores the complexity of the chemokine system in the context of IPF and the bleomycin-induced lung fibrosis mouse model. The goal is to identify specific chemokines and receptors as potential therapeutic targets. Recent progress in understanding the role of the chemokine system during IPF, using experimental models and molecular diagnosis, underscores the complex nature of this system in the context of the disease. Despite advances in experimental models and molecular diagnostics, discovering an effective therapy for IPF remains a significant challenge in both medicine and pharmacology. This work delves into microarray results from lung samples of IPF patients and murine samples at different stages of bleomycin-induced pulmonary fibrosis. By discussing common pathways identified in both IPF and the experimental model, we aim to shed light on potential targets for therapeutic intervention. Dysregulation caused by abnormal chemokine levels observed in IPF lungs may activate multiple targets, suggesting that chemokine signaling plays a central role in maintaining or perpetuating lung fibrogenesis. The highlighted chemokine axes (CCL8-CCR2, CCL19/CCL21-CCR7, CXCL9-CXCR3, CCL3/CCL4/CCL5-CCR5, and CCL20-CCR6) present promising opportunities for advancing IPF treatment research and uncovering new pharmacological targets within the chemokine system.

Homeostatic chemokines as putative therapeutic targets in idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a fatal chronic interstitial lung disease (ILD) that affects lung mechanical functions and gas exchange. IPF is caused by increased fibroblast activity and collagen deposition that compromise the alveolar-capillary barrier. Identifying an effective therapy for IPF remains a clinical challenge. Chemokines are key proteins in cell communication that have functions in immunity as well as in tissue homeostasis, damage, and repair. Chemokine receptor signaling induces the activation and proliferation of lung-resident cells, including alveolar macrophages (AMs) and fibroblasts. AMs are an important source of chemokines and cytokines during IPF. We highlight the complexity of this system and, based on insights from genetic and transcriptomic studies, propose a new role for homeostatic chemokine imbalance in IPF, with implications for putative therapeutic targets.

The role of DNA methylation in personalized medicine for immune-related diseases

Epigenetics functions as a bridge between host genetic & environmental factors, aiding in human health and diseases. Many immune-related diseases, including infectious and allergic diseases, have been linked to epigenetic mechanisms, particularly DNA methylation. In this review, we summarized an updated overview of DNA methylation and its importance in personalized medicine, and demonstrated that DNA methylation has excellent potential for disease prevention, diagnosis, and treatment in a personalized manner. The future implications and limitations of the DNA methylation study have also been well-discussed.

New pairings and deorphanization among the atypical chemokine receptor family - physiological and clinical relevance

Atypical chemokine receptors (ACKRs) form a small subfamily of receptors (ACKR1-4) unable to trigger G protein-dependent signaling in response to their ligands. They do, however, play a crucial regulatory role in chemokine biology by capturing, scavenging or transporting chemokines, thereby regulating their availability and signaling through classical chemokine receptors. ACKRs add thus another layer of complexity to the intricate chemokine-receptor interaction network. Recently, targeted approaches and screening programs aiming at reassessing chemokine activity towards ACKRs identified several new pairings such as the dimeric CXCL12 with ACKR1, CXCL2, CXCL10 and CCL26 with ACKR2, the viral broad-spectrum chemokine vCCL2/vMIP-II, a range of opioid peptides and PAMP-12 with ACKR3 as well as CCL20 and CCL22 with ACKR4. Moreover, GPR182 (ACKR5) has been lately proposed as a new promiscuous atypical chemokine receptor with scavenging activity notably towards CXCL9, CXCL10, CXCL12 and CXCL13. Altogether, these findings reveal new degrees of complexity of the chemokine network and expand the panel of ACKR ligands and regulatory functions. In this minireview, we present and discuss these new pairings, their physiological and clinical relevance as well as the opportunities they open for targeting ACKRs in innovative therapeutic strategies.

The atypical chemokine receptor-2 fine-tunes the immune response in herpes stromal keratitis

Herpes stromal keratitis (HSK) is a blinding corneal disease caused by herpes simplex virus-1 (HSV-1), a common pathogen infecting most of the world's population. Inflammation in HSK is chemokine-dependent, particularly CXCL10 and less so the CC chemokines. The atypical chemokine receptor-2 (ACKR2) is a decoy receptor predominantly for pro-inflammatory CC chemokines, which regulates the inflammatory response by scavenging inflammatory chemokines thereby modulating leukocyte infiltration. Deletion of ACKR2 exacerbates and delays the resolution of the inflammatory response in most models. ACKR2 also regulates lymphangiogenesis and mammary duct development through the recruitment of tissue-remodeling macrophages. Here, we demonstrate a dose-dependent upregulation of ACKR2 during corneal HSV-1 infection. At an HSV inoculum dose of 5.4 x 105 pfu, but not at higher dose, ACKR2 deficient mice showed prolonged clinical signs of HSK, increased infiltration of leukocytes and persistent corneal neovascularization. Viral clearance and T cell activation were similar in ACKR2-/- and wild type mice, despite a transient diminished expression of CD40 and CD86 in dendritic cells. The data suggest that ACKR2 fine-tunes the inflammatory response and the level of neovascularization in the HSK.

The Role of Atypical Chemokine Receptor D6 (ACKR2) in Physiological and Pathological Conditions; Friend, Foe, or Both?

Chemokines exert crucial roles in inducing immune responses through ligation to their canonical receptors. Besides these receptors, there are other atypical chemokine receptors (ACKR1–4) that can bind to a wide range of chemokines and carry out various functions in the body. ACKR2, due to its ability to bind various CC chemokines, has attracted much attention during the past few years. ACKR2 has been shown to be expressed in different cells, including trophoblasts, myeloid cells, and especially lymphoid endothelial cells. In terms of molecular functions, ACKR2 scavenges various inflammatory chemokines and affects inflammatory microenvironments. In the period of pregnancy and fetal development, ACKR2 plays a pivotal role in maintaining the fetus from inflammatory reactions and inhibiting subsequent abortion. In adults, ACKR2 is thought to be a resolving agent in the body because it scavenges chemokines. This leads to the alleviation of inflammation in different situations, including cardiovascular diseases, autoimmune diseases, neurological disorders, and infections. In cancer, ACKR2 exerts conflicting roles, either tumor-promoting or tumor-suppressing. On the one hand, ACKR2 inhibits the recruitment of tumor-promoting cells and suppresses tumor-promoting inflammation to blockade inflammatory responses that are favorable for tumor growth. In contrast, scavenging chemokines in the tumor microenvironment might lead to disruption in NK cell recruitment to the tumor microenvironment. Other than its involvement in diseases, analyzing the expression of ACKR2 in body fluids and tissues can be used as a biomarker for diseases. In conclusion, this review study has tried to shed more light on the various effects of ACKR2 on different inflammatory conditions.

Role of Atypical Chemokines and Chemokine Receptors Pathways in the Pathogenesis of COPD

Chronic obstructive pulmonary disease (COPD) represents a heightened inflammatory response in the lung generally resulting from tobacco smoking-induced recruitment and activation of inflammatory cells and/or activation of lower airway structural cells. Several mediators can modulate activation and recruitment of these cells, particularly those belonging to the chemokines (conventional and atypical) family. There is emerging evidence for complex roles of atypical chemokines and their receptors (such as high mobility group box 1 (HMGB1), antimicrobial peptides, receptor for advanced glycosylation end products (RAGE) or toll-like receptors (TLRs)) in the pathogenesis of COPD, both in the stable disease and during exacerbations. Modulators of these pathways represent potential novel therapies for COPD and many are now in preclinical development. Inhibition of only a single atypical chemokine or receptor may not block inflammatory processes because there is redundancy in this network. However, there are many animal studies that encourage studies for modulating the atypical chemokine network in COPD. Thus, few pharmaceutical companies maintain a significant interest in developing agents that target these molecules as potential antiinflammatory drugs. Antibody-based (biological) and small molecule drug (SMD)-based therapies targeting atypical chemokines and/or their receptors are mostly at the preclinical stage and their progression to clinical trials is eagerly awaited. These agents will most likely enhance our knowledge about the role of atypical chemokines in COPD pathophysiology and thereby improve COPD management.

ACKR2 contributes to pulmonary dysfunction by shaping CCL5:CCR5-dependent recruitment of lymphocytes during influenza A infection in mice

Inflammation triggered by influenza A virus (IAV) infection is important for viral clearance, induction of adaptive responses, and return to lung homeostasis. However, an exaggerated immune response, characterized by the overproduction of chemokines, can lead to intense lung injury, contributing to mortality. Chemokine scavenger receptors, such as ACKR2, control the levels of CC chemokines influencing the immune responses. Among the chemokine targets of ACKR2, CCL5 is important to recruit and activate lymphocytes. We investigated the role of ACKR2 during IAV infection in mice. Pulmonary ACKR2 expression was increased acutely after IAV infection preceding the virus-induced lung dysfunction. ACKR2-knockout (ACKR2−/−) mice were protected from IAV, presenting decreased viral burden and lung dysfunction. Mechanistically, the absence of ACKR2 resulted in augmented airway CCL5 levels, secreted by mononuclear and plasma cells in the lung parenchyma. The higher chemokine gradient led to an augmented recruitment of T and B lymphocytes, formation of inducible bronchus-associated lymphoid tissue and production of IgA in the airways of ACKR2−/− mice post-IAV. CCL5 neutralization in ACKR2−/− mice prevented lymphocyte recruitment and increased bronchoalveolar lavage fluid protein levels and pulmonary dysfunction. Finally, CCR5−/− mice presented increased disease severity during IAV infection, displaying increased neutrophils, pulmonary injury and dysfunction, and accentuated lethality. Collectively, our data showed that ACKR2 dampens CCL5 levels and the consequent recruitment of CCR5+ T helper 1 (Th1), T regulatory cells (Tregs), and B lymphocytes during IAV infection, decreasing pathogen control and promoting lung dysfunction in wild type mice. Therefore, ACKR2 is detrimental and CCR5 is protective during IAV infection coordinating innate and adaptive immune responses in mice.

Analysis of lung stromal expression of the atypical chemokine receptor ACKR2 reveals unanticipated expression in murine blood endothelial cells

Analysis of chemokine receptor, and atypical chemokine receptor, expression is frequently hampered by the lack of availability of high‐quality antibodies and the species specificity of those that are available. We have previously described methodology utilizing Alexa‐Fluor‐labeled chemokine ligands as versatile reagents to detect receptor expression. Previously this has been limited to hematopoietic cells and methodology for assessing expression of receptors on stromal cells has been lacking. Among chemokine receptors, the ones most frequently expressed on stromal cells belong to the atypical chemokine receptor subfamily. These receptors do not signal in the classic sense in response to ligand but scavenge their ligands and degrade them and thus sculpt in vivo chemokine gradients. Here, we demonstrate the ability to use either intratracheal or intravenous, Alexa‐Fluor‐labeled chemokine administration to detect stromal cell populations expressing the atypical chemokine receptor ACKR2. Using this methodology, we demonstrate, for the first time, expression of ACKR2 on blood endothelial cells. This observation sets the lung aside from other tissues in which ACKR2 is exclusively expressed on lymphatic endothelial cells and suggest unique roles for ACKR2 in the pulmonary environment.

Differential DNA methylation in bronchial biopsies between persistent asthma and asthma in remission

Approximately 40% of asthmatics experience remission of asthma symptoms. A better understanding of biological pathways leading to asthma remission may provide insight into new therapeutic targets for asthma. As an important mechanism of gene regulation, investigation of DNA methylation provides a promising approach. Our objective was to identify differences in epigenome wide DNA methylation levels in bronchial biopsies between subjects with asthma remission and subjects with persistent asthma or healthy controls.We analysed differential DNA methylation in bronchial biopsies from 26 subjects with persistent asthma, 39 remission subjects and 70 healthy controls, using the limma package. The comb-p tool was used to identify differentially methylated regions. DNA methylation of CpG-sites was associated to expression of nearby genes from the same biopsies to understand function.Four CpG-sites and 42 regions were differentially methylated between persistent asthma and remission. DNA methylation at two sites was correlated i n cis with gene expression at ACKR2 and DGKQ Between remission subjects and healthy controls 1163 CpG-sites and 328 regions were differentially methylated. DNA methylation was associated with expression of a set of genes expressed in ciliated epithelium.CpGs differentially methylated between remission and persistent asthma identify genetic loci associated with resolution of inflammation and airway responsiveness. Despite the absence of symptoms, remission subjects have a DNA methylation profile that is distinct from that of healthy controls, partly due to changes in cellular composition, with a higher gene expression signal related to ciliated epithelium in remission versus healthy controls.

G-Protein Coupled Receptor Targeting on Myeloid Cells in Atherosclerosis

Atherosclerosis, the underlying cause of the majority of cardiovascular diseases (CVDs), is a lipid-driven, inflammatory disease of the large arteries. Gold standard therapy with statins and the more recently developed proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors have improved health conditions among CVD patients by lowering low density lipoprotein (LDL) cholesterol. Nevertheless, a substantial part of these patients is still suffering and it seems that 'just' lipid lowering is insufficient. The results of the Canakinumab Anti-inflammatory Thrombosis Outcome Study (CANTOS) have now proven that inflammation is a key driver of atherosclerosis and that targeting inflammation improves CVD outcomes. Therefore, the identification of novel drug targets and development of novel therapeutics that block atherosclerosis-specific inflammatory pathways have to be promoted. The inflammatory processes in atherosclerosis are facilitated by a network of immune cells and their subsequent responses. Cell networking is orchestrated by various (inflammatory) mediators which interact, bind and induce signaling. Over the last years, G-protein coupled receptors (GPCRs) emerged as important players in recognizing these mediators, because of their diverse functions in steady state but also and specifically during chronic inflammatory processes - such as atherosclerosis. In this review, we will therefore highlight a selection of these receptors or receptor sub-families mainly expressed on myeloid cells and their role in atherosclerosis. More specifically, we will focus on chemokine receptors, both classical and atypical, formyl-peptide receptors, the chemerin receptor 23 and the calcium-sensing receptor. When information is available, we will also describe the consequences of their targeting which may hold promising options for future treatment of CVD.

The Atypical Chemokine Receptor 2 Limits Progressive Fibrosis after Acute Ischemic Kidney Injury

Following renal ischemia-reperfusion injury (IRI), resolution of inflammation allows tubular regeneration, whereas ongoing inflammatory injury mediated by infiltrating leukocytes leads to nephron loss and renal fibrosis, typical hallmarks of chronic kidney disease. Atypical chemokine receptor 2 (ACKR2) is a chemokine decoy receptor that binds and scavenges inflammatory CC chemokines and reduces local leukocyte accumulation. We hypothesized that ACKR2 limits leukocyte infiltration, inflammation, and fibrotic tissue remodeling after renal IRI, thus preventing progression to chronic kidney disease. Compared with wild type, Ackr2 deficiency increases CC chemokine ligand 2 levels in tumor necrosis factor-stimulated tubulointerstitial tissue in vitro. In Ackr2-deficient mice with early IRI 1 or 5 days after transient renal pedicle clamping, tubular injury was similar to wild type, although accumulation of mononuclear phagocytes increased in postischemic Ackr2^-/- kidneys. Regarding long-term outcomes, Ackr2^-/- kidneys displayed more tubular injury 5 weeks after IRI, which was associated with persistently increased renal infiltrates of mononuclear phagocytes, T cells, Ly6C^high inflammatory macrophages, and inflammation. Moreover, Ackr2 deficiency caused substantially aggravated renal fibrosis in Ackr2^-/- kidneys 5 weeks after IRI, shown by increased expression of matrix molecules, renal accumulation of α-smooth muscle actin-positive myofibroblasts, and bone marrow-derived fibrocytes. ACKR2 is important in limiting persistent inflammation, tubular loss, and renal fibrosis after ischemic acute kidney injury and, thus, can prevent progression to chronic renal disease.

The atypical chemokine receptor ACKR2 drives pulmonary fibrosis by tuning influx of CCR2+ and CCR5+ IFNγ-producing γδT cells in mice

Chemokines coordinate lung inflammation and fibrosis by acting on chemokine receptors expressed on leukocytes and other cell types. Atypical chemokine receptors (ACKRs) bind, internalize, and degrade chemokines, tuning homeostasis and immune responses. ACKR2 recognizes and decreases the levels of inflammatory CC chemokines. The role of ACKR2 in fibrogenesis is unknown. The purpose of the study was to investigate the role of ACKR2 in the context of pulmonary fibrosis. The effects of ACKR2 expression and deficiency during inflammation and fibrosis were analyzed using a bleomycin-model of fibrosis, ACKR2-deficient mice, bone marrow chimeras, and antibody-mediated leukocyte depletion. ACKR2 was upregulated acutely in response to bleomycin and normalized over time. ACKR2^-/- mice showed reduced lethality and lung fibrosis. Bone marrow chimeras showed that lethality and fibrosis depended on ACKR2 expression in pulmonary resident (nonhematopoietic) cells but not on leukocytes. ACKR2^-/- mice exhibited decreased expression of tissue-remodeling genes, reduced leukocyte influx, pulmonary injury, and dysfunction. ACKR2^-/- mice had early increased levels of CCL5, CCL12, CCL17, and IFNγ and an increased number of CCR2⁺ and CCR5⁺ IFNγ-producing γδT cells in the airways counterbalanced by low Th17-lymphocyte influx. There was reduced accumulation of IFNγ-producing γδT cells in CCR2^-/- and CCR5^-/- mice. Moreover, depletion of γδT cells worsened the clinical symptoms induced by bleomycin and reversed the phenotype of ACKR2^-/- mice exposed to bleomycin. ACKR2 controls the CC chemokine expression that drives the influx of CCR2⁺ and CCR5⁺ IFNγ-producing γδT cells, tuning the Th17 response that mediated pulmonary fibrosis triggered by bleomycin instillation.

The atypical chemokine receptor 2 limits renal inflammation and fibrosis in murine progressive immune complex glomerulonephritis

The atypical chemokine receptor 2 (ACKR2), also named D6, regulates local levels of inflammatory chemokines by internalization and degradation. To explore potential anti-inflammatory functions of ACKR2 in glomerulonephritis, we induced autologous nephrotoxic nephritis in C57/BL6 wild-type and Ackr2-deficient mice. Renal ACKR2 expression increased and localized to interstitial lymphatic endothelium during nephritis. At two weeks Ackr2^-/-mice developed increased albuminuria and urea levels compared to wild-type mice. Histological analysis revealed increased structural damage in the glomerular and tubulointerstitial compartments within Ackr2^-/- kidneys. This correlated with excessive renal leukocyte infiltration of CD4⁺ T cells and mononuclear phagocytes with increased numbers in the tubulointerstitium but not glomeruli in knockout mice. Expression of inflammatory mediators and especially markers of fibrotic tissue remodeling were increased along with higher levels of ACKR2 inflammatory chemokine ligands like CCL2 in nephritic Ackr2^-/- kidneys. In vitro, Ackr2 deficiency in TNF-stimulated tubulointerstitial tissue but not glomeruli increased chemokine levels. These results are in line with ACKR2 expression in interstitial lymphatic endothelial cells, which also assures efflux of activated leukocytes into regional lymph nodes. Consistently, nephritic Ackr2^-/- mice showed reduced adaptive cellular immune responses indicated by decreased regional T-cell activation. However, this did not prevent aggravated injury in the kidneys of Ackr2^-/- mice with nephrotoxic nephritis due to simultaneously increased tubulointerstitial chemokine levels, leukocyte infiltration and fibrosis. Thus, ACKR2 is important in limiting renal inflammation and fibrotic remodeling in progressive nephrotoxic nephritis. Hence, ACKR2 may be a potential target for therapeutic interventions in immune complex glomerulonephritis.

Mast Cell-Dependent CD8+ T-cell Recruitment Mediates Immune Surveillance of Intestinal Tumors in ApcMin/+ Mice

The presence of mast cells in some human colorectal cancers is a positive prognostic factor, but the basis for this association is incompletely understood. Here, we found that mice with a heterozygous mutation in the adenomatous polyposis coli gene (Apc^Min/+) displayed reduced intestinal tumor burdens and increased survival in a chemokine decoy receptor, ACKR2-null background, which led to discovery of a critical role for mast cells in tumor defense. ACKR2^-/-Apc^Min/+ tumors showed increased infiltration of mast cells, their survival advantage was lost in mast cell-deficient ACKR2^-/-SA^-/-Apc^Min/+ mice as the tumors grew rapidly, and adoptive transfer of mast cells restored control of tumor growth. Mast cells from ACKR2^-/- mice showed elevated CCR2 and CCR5 expression and were also efficient in antigen presentation and activation of CD8⁺ T cells. Mast cell-derived leukotriene B₄ (LTB₄) was found to be required for CD8⁺ T lymphocyte recruitment, as mice lacking the LTB₄ receptor (ACKR2^-/-BLT1^-/-Apc^Min/+) were highly susceptible to intestinal tumor-induced mortality. Taken together, these data demonstrate that chemokine-mediated recruitment of mast cells is essential for initiating LTB₄/BLT1-regulated CD8⁺ T-cell homing and generation of effective antitumor immunity against intestinal tumors. We speculate that the pathway reported here underlies the positive prognostic significance of mast cells in selected human tumors. Cancer Immunol Res; 6(3); 332-47. ©2018 AACR.

The scavenging chemokine receptor ACKR2 has a significant impact on acute mortality rate and early lesion development after traumatic brain injury

The atypical chemokine receptor ACKR2 promotes resolution of acute inflammation by operating as a scavenger receptor for inflammatory CC chemokines in several experimental models of inflammatory disorders, however its role in the brain remains unclear. Based on our previous reports of increased expression of inflammatory chemokines and their corresponding receptors following traumatic brain injury (TBI), we hypothesised that ACKR2 modulates neuroinflammation following brain trauma and that its deletion exacerbates cellular inflammation and chemokine production. We demonstrate increased CCL2 and ACKR2 mRNA expression in post-mortem human brain, whereby ACKR2 mRNA levels correlated with later times post-TBI. This data is consistent with the transient upregulation of ACKR2 observed in mouse brain after closed head injury (CHI). As compared to WT animals, ACKR2^-/- mice showed a higher mortality rate after CHI, while the neurological outcome in surviving mice was similar. At day 1 post-injury, ACKR2^-/- mice displayed aggravated lesion volume and no differences in CCL2 expression and macrophage recruitment relative to WT mice. Reciprocal regulation of ACKR2 and CCL2 expression was explored in cultured astrocytes, which are recognized as the major source of CCL2 and also express ACKR2. ACKR2 mRNA increased as early as 2 hours after an inflammatory challenge in WT astrocytes. As expected, CCL2 expression also dramatically increased at 4 hours in WT astrocytes but was significantly lower in ACKR2^-/- astrocytes, possibly indicating a co-regulation of CCL2 and ACKR2 in these cells. Conversely, in vivo, CCL2 mRNA/protein levels were increased similarly in ACKR2^-/- and WT brains at 4 and 12 hours after CHI, in line with the lack of differences in cerebral macrophage recruitment and neurological recovery. In conclusion, ACKR2 is induced after TBI and has a significant impact on mortality and lesion development acutely following CHI, while its role in chemokine expression, macrophage activation, brain pathology, and neurological recovery at later time-points is minor. Concordant to evidence in multiple sclerosis experimental models, our data corroborate a distinct role for ACKR2 in cerebral inflammatory processes compared to its reported functions in peripheral tissues.

Elevated ACKR2 expression is a common feature of inflammatory arthropathies

Objectives

Chemokines are essential contributors to leucocyte accumulation at sites of inflammatory pathology. Interfering with chemokine or chemokine receptor function therefore represents a plausible therapeutic option. However, our currently limited understanding of chemokine orchestration of inflammatory responses means that such therapies have not yet been fully developed. We have a particular interest in the family of atypical chemokine receptors that fine-tune, or resolve, chemokine-driven responses. In particular we are interested in atypical chemokine receptor 2 (ACKR2), which is a scavenging receptor for inflammatory CC-chemokines and that therefore helps to resolve in vivo inflammatory responses. The objective of the current study was to examine ACKR2 expression in common arthropathies.

Methods

ACKR2 expression was measured by a combination of qPCR and immuno-histochemistry. In addition, circulating cytokine and chemokine levels in patient plasma were assessed using multiplexing approaches.

Results

Expression of ACKR2 was elevated on peripheral blood cells as well as on leucocytes and stromal cells in synovial tissue. Expression on peripheral blood leucocytes correlated with, and could be regulated by, circulating cytokines with particularly strong associations being seen with IL-6 and hepatocyte growth factor. In addition, expression within the synovium was coincident with aggregates of lymphocytes, potentially atopic follicles and sites of high inflammatory chemokine expression. Similarly increased levels of ACKR2 have been reported in psoriasis and SSc.

Conclusion

Our data clearly show increased ACKR2 in a variety of arthropathies and taking into account our, and others', previous data we now propose that elevated ACKR2 expression is a common feature of inflammatory pathologies.

ACKR2: An Atypical Chemokine Receptor Regulating Lymphatic Biology

The lymphatic system plays an important role in the induction of the immune response by transporting antigens, inflammatory mediators, and leukocytes from peripheral tissues to draining lymph nodes. It is emerging that lymphatic endothelial cells (LECs) are playing an active role in this context via the expression of chemokines, inflammatory mediators promoting cell migration, and chemokine receptors. Particularly, LECs express atypical chemokine receptors (ACKRs), which are unable to promote conventional signaling and cell migration while they are involved in the regulation of chemokine availability. Here, we provide a summary of the data on the role of ACKR2 expressed by lymphatics, indicating an essential role for this ACKRs in the regulation of the inflammation and the immune response in different pathological conditions, including infection, allergy, and cancer.

Atypical Chemokine Receptors and Their Roles in the Resolution of the Inflammatory Response

Chemokines and their receptors are key mediators of the inflammatory process regulating leukocyte extravasation and directional migration into inflamed and infected tissues. The control of chemokine availability within inflamed tissues is necessary to attain a resolving environment and when this fails chronic inflammation ensues. Accordingly, vertebrates have adopted a number of mechanisms for removing chemokines from inflamed sites to help precipitate resolution. Over the past 15 years, it has become apparent that essential players in this process are the members of the atypical chemokine receptor (ACKR) family. Broadly speaking, this family is expressed on stromal cell types and scavenges chemokines to either limit their spatial availability or to remove them from in vivo sites. Here, we provide a brief review of these ACKRs and discuss their involvement in the resolution of inflammatory responses and the therapeutic implications of our current knowledge.

Renal Protection by Genetic Deletion of the Atypical Chemokine Receptor ACKR2 in Diabetic OVE Mice

In diabetic nephropathy (DN) proinflammatory chemokines and leukocyte infiltration correlate with tubulointerstitial injury and declining renal function. The atypical chemokine receptor ACKR2 is a chemokine scavenger receptor which binds and sequesters many inflammatory CC chemokines but does not transduce typical G-protein mediated signaling events. ACKR2 is known to regulate diverse inflammatory diseases but its role in DN has not been tested. In this study, we utilized ACKR2(-/-) mice to test whether ACKR2 elimination alters progression of diabetic kidney disease. Elimination of ACKR2 greatly reduced DN in OVE26 mice, an established DN model. Albuminuria was significantly lower at 2, 4, and 6 months of age. ACKR2 deletion did not affect diabetic blood glucose levels but significantly decreased parameters of renal inflammation including leukocyte infiltration and fibrosis. Activation of pathways that increase inflammatory gene expression was attenuated. Human biopsies stained with ACKR2 antibody revealed increased staining in diabetic kidney, especially in some tubule and interstitial cells. The results demonstrate a significant interaction between diabetes and ACKR2 protein in the kidney. Unexpectedly, ACKR2 deletion reduced renal inflammation in diabetes and the ultimate response was a high degree of protection from diabetic nephropathy.

Chemokine decoy receptor D6 mimicking trap (D6MT) prevents allosensitization and immune rejection in murine corneal allograft model

Although corneal allotransplantation is performed in the immune-privileged cornea, many grafts are still rejected after transplantation. This study examined the role of chemokine receptor D6 expression in a corneal allograft rejection, investigated the modulation of D6 expression in cells, and determined the effect of D6 on graft survival. Interestingly, D6 was highly expressed in CD45 -: cells and the corneal epithelium of accepted corneal allografts. From the mouse corneal allograft model, TGF-β was found to play a key role in D6 up-regulation, leading to reduced CCL2, CCL5, and CCL3. To modulate D6 chemokine binding, a D6MT was developed and showed effective chemokine trapping through SPR and FACS assays. By treating corneal allografts with D6MT, the allograft survival rate was improved, and (lymph) angiogenesis was reduced. Direct allosensitization and DC LN homing was drastically reduced in the mouse corneal allograft model. These findings suggest that TGF-β is a positive regulator of D6 expression, and it is a potential therapeutic target to enhance the survival of corneal allografts.

The atypical chemokine receptor ACKR2 suppresses Th17 responses to protein autoantigens

Chemokine-directed leukocyte migration is a critical component of all innate and adaptive immune responses. The atypical chemokine receptor ACKR2 is expressed by lymphatic endothelial cells and scavenges pro-inflammatory CC chemokines to indirectly subdue leukocyte migration. This contributes to the resolution of acute inflammatory responses in vivo. ACKR2 is also universally expressed by innate-like B cells, suppressing their responsiveness to the non-ACKR2 ligand CXCL13, and controlling their distribution in vivo. The role of ACKR2 in autoimmunity remains relatively unexplored, although Ackr2 deficiency reportedly lessens the clinical symptoms of experimental autoimmune encephalomyelitis induced by immunization with encephalogenic peptide (MOG_35–55). This was attributed to poor T-cell priming stemming from the defective departure of dendritic cells from the site of immunization. However, we report here that Ackr2-deficient mice, on two separate genetic backgrounds, are not less susceptible to autoimmunity induced by immunization, and in some cases develop enhanced clinical symptoms. Moreover, ACKR2 deficiency does not suppress T-cell priming in response to encephalogenic peptide (MOG_35–55), and responses to protein antigen (collagen or MOG_1–125) are characterized by elevated interleukin-17 production. Interestingly, after immunization with protein, but not peptide, antigen, Ackr2 deficiency was also associated with an increase in lymph node B cells expressing granulocyte-macrophage colony-stimulating factor (GM-CSF), a cytokine that enhances T helper type 17 (Th17) cell development and survival. Thus, Ackr2 deficiency does not suppress autoreactive T-cell priming and autoimmune pathology, but can enhance T-cell polarization toward Th17 cells and increase the abundance of GM-CSF⁺ B cells in lymph nodes draining the site of immunization.

Leukocyte chemoattractant receptors in human disease pathogenesis

Combinations of leukocyte attractant ligands and cognate heptahelical receptors specify the systemic recruitment of circulating cells by triggering integrin-dependent adhesion to endothelial cells, supporting extravasation, and directing specific intratissue localization via gradient-driven chemotaxis. Chemoattractant receptors also control leukocyte egress from lymphoid organs and peripheral tissues. In this article, we summarize the fundamental mechanics of leukocyte trafficking, from the evolution of multistep models of leukocyte recruitment and navigation to the regulation of chemoattractant availability and function by atypical heptahelical receptors. To provide a more complete picture of the migratory circuits involved in leukocyte trafficking, we integrate a number of nonchemokine chemoattractant receptors into our discussion. Leukocyte chemoattractant receptors play key roles in the pathogenesis of autoimmune diseases, allergy, inflammatory disorders, and cancer. We review recent advances in our understanding of chemoattractant receptors in disease pathogenesis, with a focus on genome-wide association studies in humans and the translational implications of mechanistic studies in animal disease models.

The N-terminal region of the atypical chemokine receptor ACKR2 is a key determinant of ligand binding

The atypical chemokine receptor, ACKR2 is a pivotal regulator of chemokine-driven inflammatory responses and works by binding, internalizing, and degrading inflammatory CC-chemokines. ACKR2 displays promiscuity of ligand binding and is capable of interacting with up to 14 different inflammatory CC-chemokines. Despite its prominent biological role, little is known about the structure/function relationship within ACKR2, which regulates ligand binding. Here we demonstrate that a conserved tyrosine motif at the N terminus of ACKR2 is essential for ligand binding, internalization, and scavenging. In addition we demonstrate that sulfation of this motif contributes to ligand internalization. Furthermore, a peptide derived from this region is capable of binding inflammatory chemokines and inhibits their interaction with their cognate signaling receptors. Importantly, the peptide is only active in the sulfated form, further confirming the importance of the sulfated tyrosines for function. Finally, we demonstrate that the bacterial protease, staphopain A, can cleave the N terminus of ACKR2 and suppress its ligand internalization activity. Overall, these results shed new light on the nature of the structural motifs in ACKR2 that are responsible for ligand binding. The study also highlights ACKR2-derived N-terminal peptides as being of potential therapeutic significance.

Lymphatic Regulation of Cellular Trafficking

Lymphatic vessels play vital roles in immune surveillance and immune regulation by conveying antigen loaded dendritic cells, memory T cells, macrophages and neutrophils from the peripheral tissues to draining lymph nodes where they initiate as well as modify immune responses. Until relatively recently however, there was little understanding of how entry and migration through lymphatic vessels is organized or the specific molecular mechanisms that might be involved. Within the last decade, the situation has been transformed by an explosion of knowledge generated largely through the application of microscopic imaging, transgenic animals, specific markers and function blocking mAbs that is beginning to provide a rational conceptual framework. This article provides a critical review of the recent literature, highlighting seminal discoveries that have revealed the fascinating ultrastructure of leucocyte entry sites in lymphatic vessels, as well as generating controversies over the involvement of integrin adhesion, chemotactic and haptotactic mechanisms in DC entry under normal and inflamed conditions. It also discusses the major changes in lymphatic architecture that occur during inflammation and the different modes of leucocyte entry and trafficking within inflamed lymphatic vessels, as well as presenting a timely update on the likely role of hyaluronan and the major lymphatic endothelial hyaluronan receptor LYVE-1 in leucocyte transit.

Microarray analyses demonstrate the involvement of type I interferons in psoriasiform pathology development in D6-deficient mice

The inflammatory response is normally limited by mechanisms regulating its resolution. In the absence of resolution, inflammatory pathologies can emerge, resulting in substantial morbidity and mortality. We have been studying the D6 chemokine scavenging receptor, which played an indispensable role in the resolution phase of inflammatory responses and does so by facilitating removal of inflammatory CC chemokines. In D6-deficient mice, otherwise innocuous cutaneous inflammatory stimuli induce a grossly exaggerated inflammatory response that bears many similarities to human psoriasis. In the present study, we have used transcriptomic approaches to define the molecular make up of this response. The data presented highlight potential roles for a number of cytokines in initiating and maintaining the psoriasis-like pathology. Most compellingly, we provide data indicating a key role for the type I interferon pathway in the emergence of this pathology. Neutralizing antibodies to type I interferons are able to ameliorate the psoriasis-like pathology, confirming a role in its development. Comparison of transcriptional data generated from this mouse model with equivalent data obtained from human psoriasis further demonstrates the strong similarities between the experimental and clinical systems. As such, the transcriptional data obtained in this preclinical model provide insights into the cytokine network active in exaggerated inflammatory responses and offer an excellent tool to evaluate the efficacy of compounds designed to therapeutically interfere with inflammatory processes.

Immune regulation by atypical chemokine receptors

Chemokines have fundamental roles in regulating immune and inflammatory responses, primarily through their control of leukocyte migration and localization. The biological functions of chemokines are typically mediated by signalling through G protein-coupled chemokine receptors, but chemokines are also bound by a small family of atypical chemokine receptors (ACKRs), the members of which are unified by their inability to initiate classical signalling pathways after ligand binding. These ACKRs are emerging as crucial regulatory components of chemokine networks in a wide range of developmental, physiological and pathological contexts. In this Review, we discuss the biochemical and immunological properties of ACKRs and the potential unifying themes in this family, and we highlight recent studies that identify novel roles for these molecules in development , homeostasis, inflammatory disease, infection and cancer.

Expression of the chemokine decoy receptor D6 mediates dendritic cell function and promotes corneal allograft rejection

PURPOSE

To identify the role of chemokine receptor 6 (D6) expression by dendritic cells (DCs) and its role in corneal transplant immunity.

METHODS

Flow cytometry analysis was used to assess the expression level of the D6 chemokine receptor in different leukocyte populations and DC maturation following lipopolysaccharides (LPS) stimulation of bone marrow-derived DCs isolated from wild-type (WT) or D6(-/-) mice (C57BL/6 background). Mixed-lymphocyte reactions and delayed-type hypersensitivity assays were performed with bone marrow-derived DCs from WT or D6(-/-) mice to evaluate T-cell alloreactivity. Adoptive transfer experiments with T cells from WT or D6(-/-) hosts with BALB/c corneal allografts were performed. Graft opacity was assessed over an 8-week period, and graft survival was plotted using Kaplan-Meier survival curves.

RESULTS

Expression of the D6 chemokine receptor was significantly higher in DCs compared to other leukocyte subpopulations, including neutrophils, lymphocytes, and monocytes/macrophages. LPS challenge of D6(-/-) bone marrow-derived DCs elicited significantly lower levels of major histocompatibility complex II and costimulatory molecules (CD40, CD80, and CD86) compared to WT bone marrow-derived DCs, indicating the role of the D6 chemokine receptor in DC maturation. Further, DCs isolated from D6(-/-) mice induced less T-cell proliferation (p≤0.001) and interferon-gamma production in T cells of draining lymph nodes compared to WT mice following corneal transplantation (p≤0.001). Moreover, adoptively transferred T cells from D6(-/-) corneal transplanted mice to WT mice led to impaired graft rejection, compared to the hosts that received T cells from the WT transplanted mice.

CONCLUSIONS

We demonstrated D6 chemokine receptor expression by DCs and identified its critical function in multiple aspects of DC biology, including maturation and consequent elicitation of alloreactive T-cell responses that are responsible for corneal allograft rejection.

International Union of Basic and Clinical Pharmacology. [corrected]. LXXXIX. Update on the extended family of chemokine receptors and introducing a new nomenclature for atypical chemokine receptors

Sixteen years ago, the Nomenclature Committee of the International Union of Pharmacology approved a system for naming human seven-transmembrane (7TM) G protein-coupled chemokine receptors, the large family of leukocyte chemoattractant receptors that regulates immune system development and function, in large part by mediating leukocyte trafficking. This was announced in Pharmacological Reviews in a major overview of the first decade of research in this field [Murphy PM, Baggiolini M, Charo IF, Hébert CA, Horuk R, Matsushima K, Miller LH, Oppenheim JJ, and Power CA (2000) Pharmacol Rev 52:145-176]. Since then, several new receptors have been discovered, and major advances have been made for the others in many areas, including structural biology, signal transduction mechanisms, biology, and pharmacology. New and diverse roles have been identified in infection, immunity, inflammation, development, cancer, and other areas. The first two drugs acting at chemokine receptors have been approved by the U.S. Food and Drug Administration (FDA), maraviroc targeting CCR5 in human immunodeficiency virus (HIV)/AIDS, and plerixafor targeting CXCR4 for stem cell mobilization for transplantation in cancer, and other candidates are now undergoing pivotal clinical trials for diverse disease indications. In addition, a subfamily of atypical chemokine receptors has emerged that may signal through arrestins instead of G proteins to act as chemokine scavengers, and many microbial and invertebrate G protein-coupled chemokine receptors and soluble chemokine-binding proteins have been described. Here, we review this extended family of chemokine receptors and chemokine-binding proteins at the basic, translational, and clinical levels, including an update on drug development. We also introduce a new nomenclature for atypical chemokine receptors with the stem ACKR (atypical chemokine receptor) approved by the Nomenclature Committee of the International Union of Pharmacology and the Human Genome Nomenclature Committee.

Expression of the chemokine decoy receptor D6 is decreased in colon adenocarcinomas

Recruitment of immune cells to tumors is a complex process crucial for both inflammation-driven tumor progression and specific anti-tumor cytotoxicity. Chemokines control the directed migration of immune cells, and their actions are partly controlled by nonsignaling chemokine decoy receptors. The role of the receptors such as D6, Duffy antigen receptor for chemokines and ChemoCentryx chemokine receptor in immunity to tumors is still unclear. Using real-time PCR, we detected significantly decreased expression of D6 mRNA in colon tumors compared to unaffected mucosa. D6 protein was expressed by lymphatic endothelium and mononuclear cells in the colon lamina propria and detected by immunohistochemistry in two out of six tissue samples containing high D6 mRNA levels, whereas no staining was observed in any tissue samples expressing low mRNA levels. When examining the density of lymphatic vessels in colon tumors, we detected a marked increase in vessels identified by the lymphatic endothelial marker Lyve-1, excluding passive regulation of D6 due to decreased lymphatic vessel density. In parallel, the Treg-recruiting chemokine CCL22, which is sequestered by D6, was threefold increased in tumor tissue. Furthermore, we could show that low D6 expression correlated to more invasive tumors and that tumor location influences D6 expression, which is lower in the more distal parts of the colon. The data support that regulation of D6 by colon tumors results in altered levels of proinflammatory CC chemokines, thereby shaping the local chemokine network to favor tumor survival. This may have implications for the design of future immunotherapy for colon cancer.

Role of Abl in airway hyperresponsiveness and airway remodeling

BACKGROUND

Asthma is a chronic disease that is characterized by airway hyperresponsiveness and airway remodeling. The underlying mechanisms that mediate the pathological processes are not fully understood. Abl is a non-receptor protein tyrosine kinase that has a role in the regulation of smooth muscle contraction and smooth muscle cell proliferation in vitro. The role of Abl in airway hyperresponsiveness and airway remodeling in vivo is largely unknown.

METHODS

To evaluate the role of Abl in asthma pathology, we assessed the expression of Abl in airway tissues from the ovalbumin sensitized and challenged mouse model, and human asthmatic airway smooth muscle cells. In addition, we generated conditional knockout mice in which Abl expression in smooth muscle was disrupted, and then evaluated the effects of Abl conditional knockout on airway resistance, smooth muscle mass, cell proliferation, IL-13 and CCL2 in the mouse model of asthma. Furthermore, we determined the effects of the Abl pharmacological inhibitors imatinib and GNF-5 on these processes in the animal model of asthma.

RESULTS

The expression of Abl was upregulated in airway tissues of the animal model of asthma and in airway smooth muscle cells of patients with severe asthma. Conditional knockout of Abl attenuated airway resistance, smooth muscle mass and staining of proliferating cell nuclear antigen in the airway of mice sensitized and challenged with ovalbumin. Interestingly, conditional knockout of Abl did not affect the levels of IL-13 and CCL2 in bronchoalveolar lavage fluid of animals treated with ovalbumin. However, treatment with imatinib and GNF-5 inhibited the ovalbumin-induced increase in IL-13 and CCL2 as well as airway resistance and smooth muscle growth in animals.

CONCLUSIONS

These results suggest that the altered expression of Abl in airway smooth muscle may play a critical role in the development of airway hyperresponsiveness and airway remodeling in asthma. Our findings support the concept that Abl may be a novel target for the development of new therapy to treat asthma.

Review: Structure-function and biological properties of the atypical chemokine receptor D6

The atypical chemokine receptor D6 was initially called "silent" on the basis of lack of conventional signaling events that lead to directional cell migration. It has emerged that D6 is able to bind and drive to degradative compartments most inflammatory CC chemokines and that is able to convey G-protein independent signaling events to optimize its scavenging activity. We here summarize the knowledge available today on D6 structural and signaling properties and its essential role for the control of inflammatory cells traffic and proper development of the adaptive immune response.

Dietary fish oil reduces the acute inflammatory response and enhances resolution of antigen-induced peritonitis

Dietary n-3 polyunsaturated fatty acids (PUFA) influence the inductive phase of inflammation but less is known about their effects on the resolution phase. This study examined the effects of dietary fish oil on induction and resolution of antigen-induced inflammation in mice. Mice were fed a control diet with or without 2.8% fish oil, immunized twice with methylated BSA (mBSA) and inflammation induced by intraperitoneal injection of mBSA. Prior to and at different time points after mBSA administration, peritoneal cells were analyzed and expression of surface molecules determined by flow cytometry. Concentration of chemokines, cytokines and soluble cytokine receptors was determined by ELISA. Mice fed the fish oil diet had fewer peritoneal neutrophils, shorter resolution interval and lower levels of pro-inflammatory cytokines and chemokines than mice fed the control diet. In mice fed the fish oil diet there was an early peak in peritoneal levels of the immunosuppressive molecules sIL-6R and TGF-β, that was not seen in mice fed the control diet. In the resolution phase, peritoneal macrophages from mice fed the fish oil diet expressed more of the atypical chemokine receptor D6 and peritoneal TGF-β levels were higher than that in mice fed the control diet. Furthermore, in the late-resolution phase there were more peritoneal eosinophils and macrophages in mice fed the fish oil diet than in mice fed the control diet. These results demonstrate a suppressive effect of n-3 PUFA on the inductive phase of inflammation and indicate an enhancing effect of n-3 PUFA on resolution of inflammation.

Cell-autonomous regulation of neutrophil migration by the D6 chemokine decoy receptor

Chemokines, acting on their cognate receptors on infiltrating leukocytes, drive the inflammatory response. We have been interested in determining roles and potential mechanisms for the atypical chemokine-scavenging receptor D6 in the regulation of inflammation. In this study, we show that a psoriasis-like pathology that arises in inflamed skins of D6-deficient mice is characterized by a massive and aberrant localization of neutrophils to the dermal/epidermal junction, which is associated with development of the pathology. Such misplacement of neutrophils is also seen with D6-deficient mice in other inflammatory models, suggesting a role for D6 in the spatial positioning of neutrophils within inflamed sites. We further show that D6 functions cell autonomously in this context and that D6, expressed by neutrophils, limits their migrational responses to CCR1 ligands such as CCL3. Our data therefore indicate that D6 is able to play a cell-autonomous role as a migratory rheostat restricting migration of D6-expressing cells such as neutrophils toward ligands for coexpressed inflammatory chemokine receptors. These data have important implications for our understanding of the roles for D6 in regulating inflammation and for our understanding of the control of spatial positioning of leukocytes at inflamed sites.

Absence of CC chemokine receptors 2a and 2b from human adipose lineage cells

Previous results have suggested the existence of receptors for monocyte chemoattractant protein-1 (MCP-1), CC chemokine receptors 2 (CCR2), in human adipocytes and their involvement in mediating effects of MCP-1 on adipocyte functions. However, the presence of CCR2 present on non-adipose-lineage cells of adipose tissue has not been excluded. We have used human Simpson-Golabi-Behmel-Syndrome (SGBS) preadipocytes and in-vitro-differentiated mature adipocytes to investigate the expression of CCR2 in human (pre)adipocytes. We found that the cells are devoid of CCR2 receptor protein and mRNA expression and fail to respond to treatment with all known CCR2 chemokine agonists. CCR2 is also absent from (pre)adipocytes prepared in vitro from human multipotent adipose-derived stem cells, bone-marrow-derived mesenchymal stem cells, or from primary (pre)adipocytes. Conditions mimicking proinflammatory changes in adipose tissue did not induce CCR2 receptor expression. We conclude that CCR2 is absent from human adipose lineage cells. Functional effects previously described for MCP-1 in human adipose tissue may be mediated indirectly through paracrine effects on non-adipose-lineage cells or by a (pre)adipocyte receptor for MCP-1 distinct from CCR2.

D6: the 'crowd controller' at the immune gateway

The chemokine-scavenging receptor, D6, is reported to regulate resolution of inflammatory responses. However, recent data also point to an unanticipated role for D6 in coordinating innate and adaptive immune responses. Here, we propose that D6 is essential for preventing inflammatory leukocyte association with lymphatic vasculature. In the absence of D6, inappropriate inflammatory leukocyte accumulation around lymphatic endothelium congests the lymphatic system, impairing fluid and cellular flow from inflamed sites to lymph nodes and reducing efficiency of antigen presentation. Thus, the inability of D6-deficient mice to resolve inflammation may be a byproduct of impaired fluid drainage from inflamed sites and thus we provide a model unifying D6 function in innate and adaptive immune responses.

Expression of the atypical chemokine receptor D6 in human alveolar macrophages in COPD

BACKGROUND

D6 is an atypical chemokine receptor involved in chemokine degradation and resolution of acute inflammatory responses in mice. Emerging evidence suggests that D6 might behave differently in human chronic inflammatory conditions. We, therefore, investigated the involvement of D6 in the immune responses in COPD, a chronic inflammatory condition of the lung.

METHODS

D6 expression was quantified by immunohistochemistry in surgical resected lung specimens from 16 patients with COPD (FEV(1), 57% ± 6% predicted) and 18 control subjects with normal lung function (nine smokers and nine nonsmokers). BAL was also obtained and analyzed by flow cytometry, immunofluorescence, and molecular analysis for further assessment of D6 involvement.

RESULTS

D6 expression in the lung was mainly detected in alveolar macrophages (AMs). The percentage of D6(+) AMs was markedly increased in patients with COPD as compared with both smoker and nonsmoker control subjects (P < .0005 for both). D6 expression was detected at both transcript and protein level in AMs but not in monocyte-derived macrophages. Finally, D6 expression was positively correlated with markers of immune activation (CD8(+) T lymphocytes, IL-32, tumor necrosis factor-α, B-cell activating factor of the tumor necrosis factor family, phospho-p38 mitogen-activated protein kinase) and negatively with lung function (FEV(1), FEV(1)/FVC).

CONCLUSIONS

D6 is expressed in AMs from patients with COPD, and its expression correlates with the degree of functional impairment and markers of immune activation. Upregulation of D6 in AMs could indicate that, besides its known scavenger activity in acute inflammation, D6 may have additional roles in chronic inflammatory conditions possibly promoting immune activation.

Regulation of the immune and inflammatory responses by the 'atypical' chemokine receptor D6

Chemokines and their receptors are key regulators of leukocyte migration and intra-tissue accumulation under both homeostatic and inflammatory conditions. Regulation of chemokine-dependent responses, particularly those relating to inflammation, is essential to avoid the development of inflammatory and autoimmune pathologies. Recently, a new subfamily of chemokine receptors referred to as the 'atypical' chemokine receptors has emerged, members of which have been shown to play important roles in controlling in vivo chemokine biology. Here we review the basic biology of the chemokine and chemokine receptor family, introduce the topic of 'atypical' chemokine receptor biology and focus specifically on the best-characterized of the 'atypical' chemokine receptors, D6. D6 is a 'scavenging' receptor for inflammatory CC chemokines and plays a central role in the resolution of in vivo inflammatory responses. We describe the biology, biochemistry and pathological relevance of D6 and outline emerging data suggesting that it has additional important roles in integrating innate and adaptive immune responses.

The Toll-like receptor 5 ligand flagellin promotes asthma by priming allergic responses to indoor allergens

Allergic asthma is a complex disease characterized by eosinophilic pulmonary inflammation, mucus production and reversible airway obstruction¹. Exposure to indoor allergens is a clear risk factor for asthma, but this disease is also associated with high household levels of total and Gram-negative bacteria². The ability of bacterial products to act as adjuvants³ suggests they might promote asthma by priming allergic sensitization to inhaled allergens. In support of this idea, house dust extracts (HDEs) can activate antigen presenting dendritic cells (DC) in vitro and promote allergic sensitization to inhaled innocuous proteinsin vivo⁴. It is unknown which microbial products provide most of the adjuvant activity in HDEs. A screen of microbial products for their adjuvant activity in the airway revealed that the bacterial protein, flagellin (FLA) stimulated strong allergic responses to an innocuous inhaled protein. Moreover, toll-like receptor (TLR)5, the mammalian receptor for FLA^5,6, was required for priming strong allergic responses to natural indoor allergens present in HDEs. In addition, the incidence of human asthma was associated with high serum levels of FLA-specific antibodies. Together, these findings suggest that household FLA promotes the development of allergic asthma by TLR5-dependent priming of allergic responses to indoor allergens.

The biochemistry and biology of the atypical chemokine receptors

A subset of chemokine receptors, initially called "silent" on the basis of their apparent failure to activate conventional signalling events, has recently attracted growing interest due to their ability to internalize, degrade, or transport ligands and thus modify gradients and create functional chemokine patterns in tissues. These receptors recognize distinct and complementary sets of ligands with high affinity, are strategically expressed in different cellular contexts, and lack structural determinants supporting Gα(i) activation, a key signalling event in cell migration. This is in keeping with the hypothesis that they have evolved to fulfil fundamentally different functions to the classical signalling chemokine receptors. Based on these considerations, these receptors (D6, Duffy antigen receptor for chemokines (DARC), CCX-CKR1 and CXCR7) are now collectively considered as an emerging class of 'atypical' chemokine receptors. In this article, we review the biochemistry and biology of this emerging chemokine receptor subfamily.

Elevated expression of the chemokine-scavenging receptor D6 is associated with impaired lesion development in psoriasis

D6 is a scavenging-receptor for inflammatory CC chemokines that are essential for resolution of inflammatory responses in mice. Here, we demonstrate that D6 plays a central role in controlling cutaneous inflammation, and that D6 deficiency is associated with development of a psoriasis-like pathology in response to varied inflammatory stimuli in mice. Examination of D6 expression in human psoriatic skin revealed markedly elevated expression in both the epidermis and lymphatic endothelium in "uninvolved" psoriatic skin (ie, skin that was more than 8 cm distant from psoriatic plaques). Notably, this increased D6 expression is associated with elevated inflammatory chemokine expression, but an absence of plaque development, in uninvolved skin. Along with our previous observations of the ability of epidermally expressed transgenic D6 to impair cutaneous inflammatory responses, our data support a role for elevated D6 levels in suppressing inflammatory chemokine action and lesion development in uninvolved psoriatic skin. D6 expression consistently dropped in perilesional and lesional skin, coincident with development of psoriatic plaques. D6 expression in uninvolved skin also was reduced after trauma, indicative of a role for trauma-mediated reduction in D6 expression in triggering lesion development. Importantly, D6 is also elevated in peripheral blood leukocytes in psoriatic patients, indicating that upregulation may be a general protective response to inflammation. Together our data demonstrate a novel role for D6 as a regulator of the transition from uninvolved to lesional skin in psoriasis.

The atypical chemokine receptor D6 controls macrophage efferocytosis and cytokine secretion during the resolution of inflammation

The resolution of acute inflammation is hallmarked by the apoptotic death of inflammatory polymorphonuclear (PMN) cells, followed by their clearance by macrophages. In turn, resolution-phase macrophages exert reduced proinflammatory cytokine production, termed immune silencing. In this study, we found that the atypical chemokine receptor D6 plays an important and chemokine scavenging-independent role in promoting macrophage-mediated resolution. D6(-/-) mice displayed increased numbers of macrophages (2.2-fold increase), but not neutrophils, in their peritonea during the resolution of murine zymosan A-initiated peritonitis, in comparison to D6(+/+) animals. Moreover, D6-deficient macrophages engulfed higher numbers of apoptotic PMN cells in vivo (1.6-fold increase), and secreted higher amounts of TNF-α, CCL3, and CCL5 ex vivo than their wild-type (WT) counterparts. In addition, D6 was found to be expressed on apoptotic neutrophils from healthy humans and rodents. Moreover, the immune silencing of LPS-stimulated macrophages following their incubation with senescent PMN cells ex vivo (in terms of TNF-α, IL-1β, and CCL5 secretion) was diminished (50-65% decrease) when D6(-/-) PMN cells were applied. Accordingly, the adhesive responses induced by macrophage interactions with senescent PMN cells were reduced with D6-deficient PMN cells. Thus, our results indicate a novel mode of action for D6 during the resolution of inflammation that is instrumental to the shaping of resolving macrophage phenotypes and the completion of resolution.

Control of murine Ly6C(high) monocyte traffic and immunosuppressive activities by atypical chemokine receptor D6

The atypical chemokine receptor D6 is a decoy and scavenger receptor for most inflammatory CC chemokines and prevents the development of exacerbated inflammatory reactions. Here we report that mice lacking D6 expression in the nonhematopoietic compartment have a selective increase in the number of Ly6C(high) monocytes in the circulation and in secondary lymphoid tissues. Under inflammatory conditions, Ly6C(high) monocytes accumulate in increased number in secondary lymphoid organs of D6(-/-) mice in a CCR2-dependent manner. Ly6C(high) monocytes derived from D6(-/-) mice have enhanced immunosuppressive activity, inhibit the development of adaptive immune responses, and partially protect mice from the development of GVHD. Thus, control of CCR2 ligands by D6 regulates the traffic of Ly6C(high) monocytes and controls their immunosuppressive potential.

CCL2 binding is CCR2 independent in primary adult human astrocytes

Chemokines are low relative molecular mass proteins, which have chemoattractant actions on many cell types. The chemokine, CCL2, has been shown to play a major role in the recruitment of monocytes in central nervous system (CNS) lesions in multiple sclerosis (MS). Since resident astrocytes constitute a major source of chemokine synthesis including CCL2, we were interested to assess the regulation of CCL2 by astrocytes. We showed that CCL2 bound to the cell surface of astrocytes and binding was not modulated by inflammatory conditions. However, CCR2 protein was not detected nor was activation of the classical CCR2 downstream signaling pathways. Recent studies have shown that non-signaling decoy chemokine receptors bind and modulate the expression of chemokines at site of inflammation. Here, we show that the D6 chemokine decoy receptor is constitutively expressed by primary human adult astrocytes at both mRNA and protein level. In addition, CCL3, which binds to D6, but not CCL19, which does not bind to D6, displaced CCL2 binding to astrocytes; indicating that CCL2 may bind to this cell type via the D6 receptor. Our results suggest that CCL2 binding to primary adult human astrocytes is CCR2-independent and is likely to be mediated via the D6 decoy chemokine receptor. Therefore we propose that astrocytes are implicated in both the establishment of chemokine gradients for the migration of leukocytes into and within the CNS and in the regulation of CCL2 levels at inflammatory sites in the CNS.

D6 facilitates cellular migration and fluid flow to lymph nodes by suppressing lymphatic congestion

Lymphatic endothelial cells are important for efficient flow of antigen-bearing fluid and antigen-presenting cells (APCs) from peripheral sites to lymph nodes (LNs). APC movement to LNs is dependent on the constitutive chemokine receptor CCR7, although how conflicting inflammatory and constitutive chemokine cues are integrated at lymphatic surfaces during this process is not understood. Here we reveal a previously unrecognized aspect of the regulation of this process. The D6 chemokine-scavenging receptor, which is expressed on lymphatic endothelial cells (LECs), maintains lymphatic surfaces free of inflammatory CC-chemokines and minimizes interaction of inflammatory leukocytes with these surfaces. D6 does not alter the level of CCR7 ligands on LECs, thus ensuring selective presentation of homeostatic chemokines for interaction with CCR7(+) APCs. Accordingly, in D6-deficient mice, inflammatory CC-chemokine adherence to LECs results in inappropriate perilymphatic accumulation of inflammatory leukocytes at peripheral inflamed sites and draining LNs. This results in lymphatic congestion and impaired movement of APCs, and fluid, from inflamed sites to LNs. We propose that D6, by suppressing inflammatory chemokine binding to lymphatic surfaces, and thereby preventing inappropriate inflammatory leukocyte adherence, is a key regulator of lymphatic function and a novel, and indispensable, contributor to the integration of innate and adaptive immune responses.

Contributions of vascular inflammation in the brainstem for neurogenic hypertension

Essential hypertension is idiopathic although it is accepted as a complex polygenic trait with underlying genetic components, which remain unknown. Our supposition is that primary hypertension involves activation of the sympathetic nervous system. One pivotal region controlling arterial pressure set point is nucleus tractus solitarii (NTS). We recently identified that pro-inflammatory molecules, such as junctional adhesion molecule-1, were over expressed in endothelial cells of the microvasculature supplying the NTS in an animal model of human hypertension (the spontaneously hypertensive rat: SHR) compared to normotensive Wistar Kyoto (WKY) rats. We have also shown endogenous leukocyte accumulation inside capillaries within the NTS of SHR but not WKY rats. Despite the inflammatory state in the NTS of SHR, transcripts of some inflammatory molecules such as chemokine (C-C motif) ligand 5 (Ccl5), and its receptors, chemokine (C-C motif) receptor 1 and 3 were down-regulated in the NTS of SHR compared to WKY rats. This may be compensatory to avoid further strong inflammatory activity. More importantly, we found that down-regulation of Ccl5 in the NTS of SHR may be pro-hypertensive since microinjection of Ccl5 into the NTS of SHR decreased arterial pressure but was less effective in WKY rats. Leukocyte accumulation of the NTS microvasculature may also induce an increase in vascular resistance and hypoperfusion within the NTS; the latter may trigger release of pro-inflammatory molecules which via paracrine signaling may affect central neural cardiovascular activity conducive to neurogenic hypertension. All told, we suggest that vascular inflammation within the brainstem contributes to neurogenic hypertension by multiple pathways.

An investigation of the inflammatory cytokine and chemokine network in systemic sclerosis

OBJECTIVES

Systemic sclerosis (SSc) is characterised by vasculopathy, an aberrantly activated immune system and excessive extracellular matrix deposition. Inflammatory chemokines control migration of cells to sites of tissue damage; their removal from inflamed sites is essential for resolution of the inflammatory response. The atypical chemokine receptor D6 has a critical role in this physiological balance. To explore potential deregulation of this system in SSc, inflammatory chemokine and D6 expression were compared with that in healthy controls (HC).

METHODS

Serum levels of inflammatory mediators were assessed by luminex analysis. Peripheral blood mononuclear cells (PBMCs) were used in molecular and immunocytochemical analysis. Platelet-rich plasma was collected and assessed by western blotting for D6 expression levels. Sex-matched HC were used for comparison.

RESULTS

72 patients with SSc and 30 HC were enrolled in the study. The chemokines MCP-1/CCL2, MIP-1α/CCL3, MIP-1β/CCL4 and IL-8/CXCL8 were significantly increased in patients with SSc, regardless of disease subtype and phase. Quantitative PCR analysis revealed a significant 10-fold upregulation of D6 transcripts in patients with SSc compared with controls, and this was paralleled by increased D6 protein expression in the PBMCs of patients with SSc. Platelet lysates also showed strong D6 expression in patients with SSc but not in controls. Importantly, high levels of D6 expression correlated with reduced levels of its ligands in serum.

CONCLUSIONS

Inflammatory chemokines and the regulatory receptor D6 are significantly upregulated in SSc and high D6 levels are associated with lower systemic chemokine levels, indicating that some patients control systemic chemokine levels using D6. These results suggest that chemokines may represent a therapeutic target in SSc.

Down-regulation of chemokine Ccl5 gene expression in the NTS of SHR may be pro-hypertensive

OBJECTIVES

Recent studies have demonstrated that pro-inflammatory molecules such as junctional adhesion molecules-1 are highly expressed in the nucleus tractus solitarii (NTS) of the spontaneously hypertensive rat (SHR), compared to normotensive rats (Wistar-Kyoto rats: WKY), suggesting that the NTS of SHR may exhibit an abnormal inflammatory state. In the present study, we tested whether gene expression of inflammatory markers such as cytokines and chemokines is altered in the NTS of SHR and whether this contributes to the hypertensive phenotype in the SHR.

METHODS

We have performed RT Profiler PCR arrays in the NTS of SHR and WKY, which were designed to specifically target major cytokines/chemokines and their receptors. To validate PCR array results quantitative RT-PCR was performed. Microinjection studies using anesthetized rats were also carried out to examine whether validated inflammatory molecules exhibit functional roles on cardiovascular regulation at the level of the NTS.

RESULTS

Five inter-related transcripts were identified to be differentially expressed between the NTS of SHR and WKY. They include chemokine (C-C motif) ligand 5 (Ccl5), and its receptors, chemokine (C-C motif) receptor 1 and 3. All of them were down-regulated in the NTS of SHR compared to WKY. Moreover, we found that the protein Ccl5 microinjected into the NTS significantly decreased baseline arterial pressure and that the response was greater in the SHR compared to the WKY (-33.2±3.2 vs. -8.8±1.6 mmHg, P<0.001), demonstrating that its down-regulation in the NTS may contribute to hypertension in the SHR.

CONCLUSION

We suggest that gene expression of specific chemokines may be down-regulated to protect further inflammatory reactions in the NTS of SHR at the expense of arterial hypertension.