Cloning and characterization of a novel human chemokine receptor

Exploring CCRL2 chemerin binding using accelerated molecular dynamics

Chemokine (C–C motif) receptor‐like 2 (CCRL2), is a seven transmembrane receptor closely related to the chemokine receptors CCR1, CCR2, CCR3, and CCR5. Nevertheless, CCRL2 is unable to activate conventional G‐protein dependent signaling and to induce cell directional migration. The only commonly accepted CCRL2 ligand is the nonchemokine chemotactic protein chemerin (RARRES2). The chemerin binding to CCLR2 does induce leukocyte chemotaxis, yet, genetic targeting of CCRL2 was shown to modulate the inflammatory response in different experimental models. This mechanism was shown to be crucial for lung dendritic cell migration, neutrophil recruitment, and Natural Killer cell‐dependent immune surveillance in lung cancer. To gain more insight in the interactions involved in the CCRL2‐chemerin, the binding complexes were generated by protein–protein docking, then submitted to accelerated molecular dynamics. The obtained trajectories were inspected by principal component analyses followed by kernel density estimation to identify the ligand‐receptor regions most frequently involved in the binding. To conclude, the reported analyses led to the identification of the putative hot‐spot residues involved in CCRL2‐chemerin binding.

Identification of Genes with Altered Methylation in Osteoclast Differentiation and Its Roles in Osteoporosis

Osteoporosis is one of the most common metabolic skeletal diseases, which affects more than 200 million people worldwide, especially elderly and postmenopausal women. One of the main processes of osteoporosis is attenuated bone formation. Abundant evidence has confirmed that overactivated osteoclasts are responsible for the attenuated bone formation. This study aims at identifying novel methylation-associated biomarkers and therapeutic targets in osteoclasts by integrally analyzing methylation profiles and gene expression data. DNA methylation profile and gene expression data were obtained from the Gene Expression Omnibus (GEO) database. Subsequently, we integrated the two sets of data to screen for differentially expressed genes with differential methylation level (DM-DEGs) between osteoclasts and CD14⁺ monocytes from donors. Then, Gene Ontology (GO) analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis were performed to uncover the enriched functions and pathways of identified DM-DEGs. In addition, by combining protein-protein interaction analysis and receiver-operator characteristic analysis, we finally identified four hub DM-DEGs. Gene Set Enrichment Analysis was utilized to validate and investigate the potential biological functions of the four hub DM-DEGs. Finally, Real-time quantitative PCR (QPCR) was performed to validate the mRNA expression level of the four identified hub DM-DEGs during osteoclast differentiation. CCRL2, CCL18, C1QB, and SELL were highly correlated with osteoclastic differentiation and osteoporosis phenotype. QPCR revealed that the expression of CCRL2, CCL18, and C1QB was increased during osteoclast differentiation, whereas the expression of SELL was decreased. The present study indicated a connection between gene expression and DNA methylation during osteoclast differentiation and that four hub DM-DEGs in osteoclastogenesis and osteoporosis pathogenesis might be potential candidates for intensive research and therapeutic targets for the treatment of osteoporosis.

CCRL2 Modulates Physiological and Pathological Angiogenesis During Retinal Development

Chemerin is a multifunctional protein involved in the regulation of inflammation, metabolism, and tumorigenesis. It binds to three receptors, CMKLR1, GPR1 and CCRL2. CMKLR1 is a fully functional receptor mediating most of the known activities of chemerin. CCRL2 does not seem to couple to any intracellular signaling pathway and is presently considered as an atypical receptor able to present the protein to cells expressing CMKLR1. CCRL2 is expressed by many cell types including leukocyte subsets and endothelial cells, and its expression is strongly upregulated by inflammatory stimuli. We recently reported that chemerin can negatively regulate the angiogenesis process, including during the development of the vascular network in mouse retina. The role of CCRL2 in angiogenesis was unexplored so far. In the present work, we demonstrate that mice lacking CCRL2 exhibit a lower density of vessels in the developing retina and this phenotype persists in adulthood, in a CMKLR1-dependent manner. Vascular sprouting was not affected, while vessel pruning, and endothelial cell apoptosis were increased. Pathological angiogenesis was also reduced in CCRL2^-/- mice in a model of oxygen-induced retinopathy. The phenotype closely mimics that of mice overexpressing chemerin, and the concentration of chemerin was found elevated in the blood of newborn mice, when the retinal vasculature develops. CCRL2 appears therefore to regulate the distribution and concentration of chemerin in organs, regulating thereby its bioactivity.

Molecular Basis for CCRL2 Regulation of Leukocyte Migration

CCRL2 is a seven-transmembrane domain receptor that belongs to the chemokine receptor family. At difference from other members of this family, CCRL2 does not promote chemotaxis and shares structural features with atypical chemokine receptors (ACKRs). However, CCRL2 also differs from ACKRs since it does not bind chemokines and is devoid of scavenging functions. The only commonly recognized CCRL2 ligand is chemerin, a non-chemokine chemotactic protein. CCRL2 is expressed both by leukocytes and non-hematopoietic cells. The genetic ablation of CCRL2 has been instrumental to elucidate the role of this receptor as positive or negative regulator of inflammation. CCRL2 modulates leukocyte migration by two main mechanisms. First, when CCRL2 is expressed by barrier cells, such endothelial, and epithelial cells, it acts as a presenting molecule, contributing to the formation of a non-soluble chemotactic gradient for leukocytes expressing CMKLR1, the functional chemerin receptor. This mechanism was shown to be crucial in the induction of NK cell-dependent immune surveillance in lung cancer progression and metastasis. Second, by forming heterocomplexes with other chemokine receptors. For instance, CCRL2/CXCR2 heterodimers were shown to regulate the activation of β2-integrins in mouse neutrophils. This mini-review summarizes the current understanding of CCRL2 biology, based on experimental evidence obtained by the genetic deletion of this receptor in in vivo experimental models. Further studies are required to highlight the complex functional role of CCRL2 in different organs and pathological conditions.

Involvement of Novel Adipokines, Chemerin, Visfatin, Resistin and Apelin in Reproductive Functions in Normal and Pathological Conditions in Humans and Animal Models

It is well known that adipokines are endocrine factors that are mainly secreted by white adipose tissue. Their central role in energy metabolism is currently accepted. More recently, their involvement in fertility regulation and the development of some reproductive disorders has been suggested. Data concerning the role of leptin and adiponectin, the two most studied adipokines, in the control of the reproductive axis are consistent. In recent years, interest has grown about some novel adipokines, chemerin, visfatin, resistin and apelin, which have been found to be strongly associated with obesity and insulin-resistance. Here, we will review their expression and role in male and female reproduction in humans and animal models. According to accumulating evidence, they could regulate the secretion of GnRH (Gonadotropin-Releasing Hormone), gonadotropins and steroids. Furthermore, their expression and that of their receptors (if known), has been demonstrated in the human and animal hypothalamo-pituitary-gonadal axis. Like leptin and adiponectin, these novel adipokines could thus represent metabolic sensors that are able to regulate reproductive functions according to energy balance changes. Therefore, after investigating their role in normal fertility, we will also discuss their possible involvement in some reproductive troubles known to be associated with features of metabolic syndrome, such as polycystic ovary syndrome, gestational diabetes mellitus, preeclampsia and intra-uterine growth retardation in women, and sperm abnormalities and testicular pathologies in men.

Adipokine Contribution to the Pathogenesis of Osteoarthritis

Recent studies have shown that overweight and obesity play an important role in the development of osteoarthritis (OA). However, joint overload is not the only risk factor in this disease. For instance, the presence of OA in non-weight-bearing joints such as the hand suggests that metabolic factors may also contribute to its pathogenesis. Recently, white adipose tissue (WAT) has been recognized not only as an energy reservoir but also as an important secretory organ of adipokines. In this regard, adipokines have been closely associated with obesity and also play an important role in bone and cartilage homeostasis. Furthermore, drugs such as rosuvastatin or rosiglitazone have demonstrated chondroprotective and anti-inflammatory effects in cartilage explants from patients with OA. Thus, it seems that adipokines are important factors linking obesity, adiposity, and inflammation in OA. In this review, we are focused on establishing the physiological mechanisms of adipokines on cartilage homeostasis and evaluating their role in the pathophysiology of OA based on evidence derived from experimental research as well as from clinical-epidemiological studies.

Signaling Properties of Chemerin Receptors CMKLR1, GPR1 and CCRL2

Chemerin is a small chemotactic protein originally identified as the natural ligand of CMKLR1. More recently, two other receptors, GPR1 and CCRL2, have been reported to bind chemerin but their functional relevance remains poorly understood. In this study, we compared the binding and signaling properties of the three human chemerin receptors and showed differences in mode of chemerin binding and receptor signaling. Chemerin binds to all three receptors with low nanomolar affinities. However, the contribution of the chemerin C-terminus to binding efficiency varies greatly amongst receptors. By using BRET-based biosensors monitoring the activation of various G proteins, we showed that binding of chemerin and the chemerin 9 nonapeptide (¹⁴⁹YFPGQFAFS¹⁵⁷) to CMKLR1 activates the three G_αi subtypes (G_αi1, G_αi2 and G_αi3) and the two G_αo isoforms (G_αoa and G_αob) with potencies correlated to binding affinities. In contrast, no significant activation of G proteins was detected upon binding of chemerin to GPR1 or CCRL2. Binding of chemerin and the chemerin 9 peptide also induced the recruitment of β-arrestin1 and 2 to CMKLR1 and GPR1, though to various degree, but not to CCRL2. However, the propensity of chemerin 9 to activate β-arrestins relative to chemerin is higher when bound to GPR1. Finally, we showed that binding of chemerin to CMKLR1 and GPR1 promotes also the internalization of the two receptors and the phosphorylation of ERK1/2 MAP kinases, although with a different efficiency, and that phosphorylation of ERK1/2 requires both G_αi/o and β-arrestin2 activation but not β-arrestin1. Collectively, these data support a model in which each chemerin receptor displays selective signaling properties.

Chemerin as an independent predictor of cardiovascular event risk

Currently, coronary artery disease (CAD) is considered a major ailment in humans with widespread prevalence. CAD also accounts for high mortality rates around the world that involves several known risk factors. Chemerin is a novel adipokinine that is associated with inflammation and adipogenesis. Furthermore, experimental and clinical data indicate that localized as well as circulating chemerin expression and activation are elevated in numerous metabolic and inflammatory diseases including psoriasis, obesity, type 2 diabetes, metabolic syndrome and cardiovascular disease. Chemerin is accepted as being a strong marker because the serum chemerin levels are increased in a CAD condition. However, the chimeric characteristics of chemerin have not been fully investigated. Although chemerin is known to be responsible for CAD development among other factors, authors still investigate it at the marker level. This review focuses on chemerin expression, processing, biological function and relevance to human diseases, and on the role of chemerin in the maintenance of a cardiovascular disease.

Chemerin: A comprehensive review elucidating the need for cardiovascular research

When chemerin was discovered in 1997, it was relegated to being a protein associated with the normal skin function contrasting the setting of psoriasis. However, with the discovery of multiple receptors for the chemerin protein and a vast collection of associations with various pathologies, chemerin has global influence capable of regulating chemotactic, adipokine, autocrine/paracrine, adipogenic, angiogenic, and reproductive functions. These individual abilities of chemerin are important for understanding its basic pharmacology and physiology, but application of these principles to human pathology relies on the ability of scientists and physicians to view this protein from a much wider, all-encompassing angle. A global participant in the action of chemerin is the cardiovascular system (CVS). Although the CVS may not have as many direct interactions (e.g. smooth muscle in endothelium) with chemerin as it does indirect (e.g. chemerin activation in the lumen by proteases), our basic understanding of the CVS and its relation to chemerin is necessary to form a proper grasp of its individual actions and make the applications to pathology. This review provides a fundamental, yet comprehensive review of chemerin that inherently identifies the CVS as a necessary link between chemerin and its associated pathologies, but also calls for basic cardiovascular research as the solution to this chasm between knowledge and application.

Processing, signaling, and physiological function of chemerin

Chemerin is an immunomodulating factor secreted predominantly by adipose tissue and skin. Processed by a variety of proteases linked to inflammation, it activates the G-protein coupled receptor chemokine-like receptor 1 (CMKLR1) and induces chemotaxis in natural killer cells, macrophages, and immature dendritic cells. Recent developments revealed the role of the nonsignaling chemerin receptor C-C chemokine receptor-like 2 (CCRL2) in inflammation. Besides further research establishing its link to inflammatory skin conditions such as psoriasis, functions in healthy skin have also been reported. Here, the current understanding of chemerin processing, signaling and physiological function has been summarized, focusing on the regulation of its activity, its different receptors and its controversially discussed role in diseases.

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.

CCRL2, a fringe member of the atypical chemoattractant receptor family

The term atypical chemoattractant receptors is generally used to refer to a subset of G-protein-coupled receptors devoid of chemotactic activity and characterized by the ability to scavenge chemotactic factors from the inflammatory milieu. However, emerging evidence suggests that this class of receptors is heterogeneous in function. In this Viewpoint, we discuss the properties of CCRL2, a molecule devoid of ligand scavenging functions and suggested to regulate leukocyte recruitment by alternative mechanisms.

Chemerin and vaspin: possible targets to treat obesity?

Obesity is one of the main human epidemics today. The increase in fat accumulation, which is associated with obesity, may significantly change the expression of several bioactive molecules known as adipokines. These adipokines interact not only with adipose tissue, but also with metabolically relevant organs such as liver and muscle. Understanding the molecular basics of potential novel targets might help to improve the therapeutic treatment of people who suffer from obesity. Herein we summarize the state of the art of two novel adipokines and their impaired or protective action in obesity: chemerin and vaspin. Their expression patterns, signal transduction activity, and resulting functions within the human body are introduced. We also discuss various possibilities to target these adipokines, which may represent promising new targets for the treatment of obesity by small and synthetic compounds.

Elevated chemokine CC-motif receptor-like 2 (CCRL2) promotes cell migration and invasion in glioblastoma

Chemokine CC-motif receptor-like 2 (CCRL2) is a 7-transmembrane G protein-coupled receptor which plays a key role in lung dendritic cell trafficking to peripheral lymph nodes. The function and expression of CCRL2 in cancer is not understood at present. Here we report that CCRL2 expression level is elevated in human glioma patient samples and cell lines. The magnitude of increase is positively associated with increasing tumor grade, with the highest level observed in grade IV glioblastoma. By gain-of-function and loss-of-function studies, we further showed that CCRL2 did not regulate the growth of human glioblatoma U87 and U373 cells. Importantly, we demonstrated that over-expression of CCRL2 significantly enhanced the migration rate and invasiveness of the glioblastoma cells. Taken together, these results suggest for the first time that elevated CCRL2 in glioma promotes cell migration and invasion. The potential roles of CCRL2 as a novel therapeutic target and biomarker warrant further investigations.

Expression, regulation, and function of atypical chemerin receptor CCRL2 on endothelial cells

Chemokine (CC motif) receptor-like 2 (CCRL2) binds leukocyte chemoattractant chemerin and can regulate local levels of the attractant, but does not itself support cell migration. In this study, we show that CCRL2 and VCAM-1 are upregulated on cultured human and mouse vascular endothelial cells (EC) and cell lines by proinflammatory stimuli. CCRL2 induction is dependent on NF-κB and JAK/STAT signaling pathways, and activated endothelial cells specifically bind chemerin. In vivo, CCRL2 is constitutively expressed at high levels by lung endothelial cells and at lower levels by liver endothelium; and liver but not lung EC respond to systemic LPS injection by further upregulation of the receptor. Plasma levels of total chemerin are elevated in CCRL2(-/-) mice and are significantly enhanced after systemic LPS treatment in CCRL2(-/-) mice compared with wild-type mice. Following acute LPS-induced pulmonary inflammation in vivo, chemokine-like receptor 1 (CMKLR1)(+) NK cell recruitment to the airways is significantly impaired in CCRL2(-/-) mice compared with wild-type mice. In vitro, chemerin binding to CCRL2 on endothelial cells triggers robust adhesion of CMKLR1(+) lymphoid cells through an α(4)β(1) integrin/VCAM-1-dependent mechanism. In conclusion, CCRL2 is expressed by EC in a tissue- and activation-dependent fashion, regulates circulating chemerin levels and its bioactivity, and enhances chemerin- and CMKLR1-dependent lymphocyte/EC adhesion in vitro and recruitment to inflamed airways in vivo. Its expression and/or induction on EC by proinflammatory stimuli provide a novel and specific mechanism for the local enrichment of chemerin at inflammatory sites, regulating the recruitment of CMKLR1(+) cells.

Atypical chemokine receptors

Atypical chemokine receptors (ACRs) are cell surface receptors with seven transmembrane domains structurally homologous to chemokine G-protein coupled receptors (GPCRs). However, upon ligation by cognate chemokines, ACRs fail to induce classical signaling and downstream cellular responses characteristic for GPCRs. Despite this, by affecting chemokine availability and function, ACRs impact on a multitude of pathophysiological events and have emerged as important molecular players in health and disease. This review discusses individual characteristics of the currently known ACRs, highlights their similarities and differences and attempts to establish their group identity. It summarizes the progress made in mapping ACR expression, understanding their diverse in vitro and in vivo functions of ACRs and uncovering their contributions to disease pathogeneses.

Chemokine-like receptor 1 (CMKLR1) and chemokine (C-C motif) receptor-like 2 (CCRL2); two multifunctional receptors with unusual properties

Chemokine-like receptor 1 (CMKLR1), also known as ChemR23, and chemokine (C-C motif) receptor-like 2 (CCRL2) are 7-transmembrane receptors that were cloned in the late 1990s based on their homology to known G-protein-coupled receptors. They were previously orphan receptors without any known biological roles; however, recent studies identified ligands for these receptors and their functions have begun to be unveiled. The plasma protein-derived chemoattractant chemerin is a ligand for CMKLR1 and activation of CMKLR1 with chemerin induces the migration of macrophages and dendritic cells (DCs) in vitro, suggesting a proinflammatory role. However, in vivo studies using CMKLR-deficient mice suggest an anti-inflammatory role for this receptor, possibly due to the recruitment of tolerogenic plasmacytoid DCs. Chemerin/CMKLR1 interaction also promotes adipogenesis and angiogenesis. The anti-inflammatory lipid mediator, resolving E1, is another CMKLR1 ligand and it inhibits leukocyte infiltration and proinflammatory gene expression. These divergent results suggest that CMKLR1 is a multifunctional receptor. The chemokine CCL5 and CCL19 are reported to bind to CCRL2. Like Duffy antigen for chemokine receptor (DARC), D6 and CCX-CKR, CCRL2 does not signal, but it constitutively recycles, potentially reducing local concentration of CCL5 and CCL19 and subsequent immune responses. Surprisingly, chemerin, a ligand for CMKLR1, is a ligand for CCRL2. CCRL2 binds chemerin and increases local chemerin concentration to efficiently present it to CMKLR1 on nearby cells, providing a link between CCRL2 and CMKLR1. Although these findings suggest an anti-inflammatory role, a recent study using CCRL2-deficient mice indicates a proinflammatory role; thus, CCRL2 may also be multifunctional. Further studies using CMKLR1- or CCRL2-deficient mice are needed to further define the role of these receptors in immune responses and other cellular processes.

Chemokine decoy receptors: structure-function and biological properties

Chemokines induce cell migration through the activation of a distinct family of structurally related heterotrimeric G protein-coupled receptors (GPCR). Over the last few years, several receptors in this family that recognize chemokines but do not induce cell migration have been identified. These "atypical" chemokine receptors are unable to activate transduction events that lead directly to cell migration, but appear nonetheless to play a nonredundant role in the control of leukocyte recruitment at inflammatory sites and in tumors by shaping the chemoattractant gradient, either by removing, transporting, or concentrating their cognate ligands.

CCL19 is a specific ligand of the constitutively recycling atypical human chemokine receptor CRAM-B

The human chemokine receptor CRAM (chemokine receptor on activated macrophages), encoded by the gene CCRL2, is a new candidate for the atypical chemokine receptor family that includes the receptors DARC, D6 and chemocentryx chemokine receptor (CCX-CKR). CRAM is maturation-stage-dependently expressed on human B lymphocytes and its surface expression is up-regulated upon short-term CCL5 exposure. Here, we demonstrate that the homeostatic chemokine CCL19 is a specific ligand for CRAM. In radioactive labelling studies CCL19 bound to CRAM-expressing cells with an affinity similar to the described binding of its other receptor CCR7. In contrast to the known CCL19/CCR7 ligand/receptor pair, CRAM stimulation by CCL19 did not result in typical chemokine-receptor-dependent cellular activation like calcium mobilization or migration. Instead, we demonstrate that CRAM is constitutively recycling via clathrin-coated pits and able to internalize CCL19 as well as anti-CRAM antibodies. As this absence of classical chemokine receptor responses and the recycling and internalization features are characteristic for non-classical chemokine receptors, we suggest that CRAM is the newest member of this group. As CCL19 is known to be critically involved in lymphocyte and dendritic cell trafficking, CCL19-binding competition by CRAM might be involved in modulating these processes.

Chemerin reveals its chimeric nature

Chemerin is a proinflammatory plasma protein that binds to the GPCR ChemR23/CMKLR1 on macrophages and plasmacytoid dendritic cells, and promotes chemotaxis. An orphan GPCR, CCRL2, has now been identified as an additional receptor for chemerin, providing a unique mechanism by which chemerin enhances inflammation. Furthermore, because recent data shows that chemerin-derived peptides possess antiinflammatory properties, chemerin may be involved in both the initiation and resolution of inflammation.

Mast cell-expressed orphan receptor CCRL2 binds chemerin and is required for optimal induction of IgE-mediated passive cutaneous anaphylaxis

Mast cells contribute importantly to both protective and pathological IgE-dependent immune responses. We show that the mast cell–expressed orphan serpentine receptor mCCRL2 is not required for expression of IgE-mediated mast cell–dependent passive cutaneous anaphylaxis but can enhance the tissue swelling and leukocyte infiltrates associated with such reactions in mice. We further identify chemerin as a natural nonsignaling protein ligand for both human and mouse CCRL2. In contrast to other “silent” or professional chemokine interreceptors, chemerin binding does not trigger ligand internalization. Rather, CCRL2 is able to bind the chemoattractant and increase local concentrations of bioactive chemerin, thus providing a link between CCRL2 expression and inflammation via the cell-signaling chemerin receptor CMKLR1.

Human B cells express the orphan chemokine receptor CRAM-A/B in a maturation-stage-dependent and CCL5-modulated manner

Chemokines orchestrate the organization of leucocyte recruitment during inflammation and homeostasis. Despite growing knowledge of chemokine receptors, some orphan chemokine receptors are still not characterized. The gene CCRL2 encodes such a receptor that exists in two splice variants, CRAM-A and CRAM-B. Here, we report that CRAM is expressed by human peripheral blood and bone marrow B cells, and by different B-cell lines dependent on the B-cell maturation stage. Intriguingly, CRAM surface expression on the pre-B-cell lines Nalm6 and G2 is specifically upregulated in response to the inflammatory chemokine CCL5 (RANTES), a chemokine that is well known to play an important role in modulating immune responses. Although Nalm6 cells do not express any of the known CCL5 binding receptors, extracellular signal-regulated kinases 1 and 2 (ERK1/2) are phosphorylated upon CCL5 stimulation, suggesting a direct effect of CCL5 through the CRAM receptor. However, no calcium mobilization or migratory responses upon CCL5 stimulation are induced in B-cell lines or in transfected cells. Also, ERK1/2 phosphorylation cannot be inhibited by pertussis toxin, suggesting that CRAM does not couple to Gi proteins. Our results describe the expression of a novel, non-classical chemokine receptor on B cells that is potentially involved in immunomodulatory functions together with CCL5.

Tuning inflammation and immunity by chemokine sequestration: decoys and more

A set of chemokine receptors are structurally unable to elicit migration or conventional signalling responses after ligand engagement. These 'silent' (non-signalling) chemokine receptors regulate inflammatory and immune reactions in different ways, including by acting as decoys and scavengers. Chemokine decoy receptors recognize distinct and complementary sets of ligands and are strategically expressed in different cellular contexts. Importantly, viruses and parasites have evolved multiple strategies to elude chemokines, including the expression of decoy receptors. So, decoy receptors for chemokines represent a general strategy to tune, shape and temper innate and adaptive immunity.

Down regulation of 3p genes,LTF, SLC38A3 andDRR1, upon growth of human chromosome 3–mouse fibrosarcoma hybrids in severe combined immunodeficiency mice

We have applied a functional test for tumour antagonizing genes based on human chromosome 3 (chr3)-mouse fibrosarcoma A9 MCHs that were studied in vitro and after growth as tumours in severe combined immunodeficiency (SCID) mice. Previously, we reported that 9 out of the 36 SCID-tumours maintained the transferred chr3 ("chr3+" tumours), but lost the expression of the known human TSG fragile histidine triad gene (FHIT) in contrast to 14 other 3p-genes examined. Here we report the results of the duplex RT-PCR analysis of 9 "chr3+" tumours and 3 parental MCHs. We have examined the expression of 34 human 3p-genes from known cancer-related regions of instability, including 13 genes from CER1 defined by us previously at 3p21.33-p21.31 and 10 genes from the LUCA region at 3p21.31. We have found that in addition to FHIT, expression of the LTF gene from CER1 at 3p21.33-p21.31 was lost in all 9 tumours analyzed. The transcript of the solute carrier family 38 member 3 gene (SLC38A3) gene from LUCA region at 3p21.31 was not found in 8 and was greatly reduced in 1 out of these 9 tumours. Expression of the down-regulated in renal cell carcinoma gene (DRR1) gene at 3p14.2 was lost in 7 and down regulated in 2 "chr3+" tumours. In the SCID-tumour derived cell lines treatment with 5-aza-2'-deoxycytidine restored the mRNA expression of LTF, indicating the integrity of DNA sequences. Notably that transcription of the LTF and 2 flanking genes, LRRC2 and TMEM7, as well as transcription of the SLC38A3 gene, were also impaired in all 5 RCC cell lines analyzed. Our data indicate these genes as putative tumour suppressor genes.

Silent chemoattractant receptors: D6 as a decoy and scavenger receptor for inflammatory CC chemokines

The chemokine system includes at least three "silent" receptors, DARC, D6 and CCX CKR, with distinct specificity and tissue distribution. D6 binds most inflammatory, but not homeostatic, CC chemokines and shuttles in a ligand-independent way from the plasma membrane to endocytic compartments where chemokines are targeted to degradation. In vitro and in vivo evidence, including results with gene-targeted mice, is consistent with the view that D6 acts as a decoy and scavenger for inflammatory CC chemokines. Thus, D6 has unique functional and structural features, which make it ideally adapted to act as a chemokine decoy and scavenger receptor, strategically located on lymphatic endothelium to dampen inflammation in tissues and draining lymph nodes.

Characterization of Human T Cells That Regulate Neutrophilic Skin Inflammation

It is unknown whether neutrophilic inflammations can be regulated by T cells. This question was analyzed by studying acute generalized exanthematous pustulosis (AGEP), which is a severe drug hypersensitivity resulting in intraepidermal or subcorneal sterile pustules. Recently, we found that drug-specific blood and skin T cells from AGEP patients secrete high levels of the potent neutrophil-attracting chemokine IL-8/CXCL8. In this study, we characterize the phenotype and function of CXCL8-producing T cells. Supernatants from CXCL8(+) T cells were strongly chemotactic for neutrophils, CXCR1, and CXCR2 transfectants, but not for transfectants expressing CXCR4, CX3CR1, human chemokine receptor, and RDC1. Neutralization experiments indicated that chemotaxis was mainly mediated by CXCL8, but not by granulocyte chemotactic protein-2/CXCL6, epithelial cell-derived neutrophil attractant-78/CXCL5, or growth-related oncogene-alpha,beta,gamma/CXCL1,2,3. Interestingly, approximately 2.5% of CD4(+) T cells in normal peripheral blood also produced CXCL8. In addition to CXCL8, AGEP T cells produced large amounts of the monocyte/neutrophil-activating cytokine GM-CSF, and the majority released IFN-gamma and the proinflammatory cytokine TNF-alpha. Furthermore, apoptosis in neutrophils treated with conditioned medium from CXCL8(+) T cells could be reduced by 40%. In lesional skin, CXCL8(+) T cells consistently expressed the chemokine receptor CCR6, suggesting a prominent role for CCR6 in early inflammatory T cell recruitment. Finally, our data suggest that CXCL8-producing T cells facilitate skin inflammation by orchestrating neutrophilic infiltration and ensuring neutrophil survival, which leads to sterile pustular eruptions found in AGEP patients. This mechanism may be relevant for other T cell-mediated diseases with a neutrophilic inflammation such as Behçet's disease and pustular psoriasis.

Up-regulated expression and activation of the orphan chemokine receptor, CCRL2, in rheumatoid arthritis

OBJECTIVE

Rheumatoid arthritis (RA) is a chronic inflammatory condition characterized by a cellular influx and destruction of the joint architecture. Chemokines characteristically regulate leukocyte recruitment and activation. Chemokine (CC motif) receptor-like 2 (CCRL2) is an orphan receptor with homology to other CC chemokine receptors. We undertook this study to examine CCRL2 expression in RA, cytokine regulation of expression, and the source of a putative ligand in an attempt to determine the role of this receptor during inflammation.

METHODS

Expression of CCRL2 on joint-infiltrating leukocytes was examined by immunocytochemistry. In vitro studies evaluated CCRL2 expression in primary neutrophils using Northern and Western blotting and reverse transcriptase-polymerase chain reaction. HEK 293 cells expressing two splice variants of CCRL2 (HEK/CCRL2A or HEK/CCRL2B) were generated with a retroviral expression system, and their migration in response to fractions of synovial fluid (SF) from RA patients was examined using a 48-well chamber.

RESULTS

CCRL2 expression was observed on all infiltrating neutrophils and on some macrophages obtained from the SF of 5 RA patients. In vitro studies of primary neutrophils revealed that CCRL2 messenger RNA (mRNA) was rapidly up-regulated following stimulation with lipopolysaccharide (1 microg/ml) or tumor necrosis factor (5 ng/ml). The mRNA for both CCRL2A and CCRL2B were expressed in cytokine-stimulated neutrophils. Cells expressing either of these splice variants migrated in response to a fraction of RA SF.

CONCLUSION

CCRL2 expression is up-regulated on synovial neutrophils of RA patients. Inflammatory products present in the SF activate this receptor, indicating that CCRL2 is a functional receptor that may be involved in the pathogenesis of RA.

Distribution and regulation of expression of the putative human chemokine receptor HCR in leukocyte populations

Human chemokine receptor (HCR) is a putative chemokine receptor sharing high similarity with CCR1, CCR2, CCR3 and CCR5. Its gene is located within the main cluster of CC-chemokine receptor genes, in the 3p21 region of the human genome. We generated monoclonal antibodies directed at human HCR, and studied its distribution in human leukocyte populations and cell lines, and its regulation following maturation or activation of these populations. In peripheral blood leukocytes, HCR is expressed on CD4+ and CD8+ T lymphocytes, including most memory and part of naive cells, but is absent from B cells. Expression of HCR was enhanced following stimulation of T cells by OKT3 and IL-2. HCR is present on monocytes and macrophages. Monocyte-derived dendritic cells harbored HCR, and expression was enhanced following stimulation by lipopolysaccharides, poly (I:C), IFN-gamma or CD40L. Neutrophils strongly expressed HCR. A similar distribution was found in bone marrow,and HCR was also expressed in CD34+ precursors. Expression of HCR and its regulation were confirmed by real-time PCR. In a panel of human tissues, we found abundant HCR transcripts in thymus, spleen, lymph nodes and lung. This large distribution across leukocyte populations, and the up-regulation during DC maturation, represent a new profile among chemokine receptors. We speculate that HCR responds to inflammatory chemokines, and might be involved in the interaction between antigen presenting and T cells, and in hematopoiesis.

Expression and functional analysis of chemokine receptors in human peripheral blood leukocyte populations

Emerging evidence indicates that chemokine receptor expression patterns are critical in determining the spectrum of action of the chemokines. We have analysed the expression patterns of 17 chemokine receptors and two orphan chemokine receptor-like genes in various freshly prepared human peripheral blood leucocyte populations, including neutrophils, lymphocytes, and naïve and differentiated monocytes using real-time quantitative polymerase chain reaction (TaqMan). This is the first comprehensive study of chemokine receptor expression in such a wide variety of cell types. Human peripheral blood leukocyte populations were found to express a wide range of chemokine receptors that varies depending on cell type and differentiation state. Novel expression patterns of certain chemokine receptors were seen during our analysis. For example, the orphan chemokine receptor HCR was expressed at very high levels by both primary neutrophils and primary monocytes, and was further upregulated on neutrophil activation and during monocyte to macrophage differentiation. When neutrophil calcium transients were measured in response to a panel of 30 different chemokines the results clearly correlated with the chemokine receptor expression profile. For example strong calcium responses were seen in neutrophils following stimulation with the CXCR1 and CXCR2 ligands, interleukin (IL-)8, GCP-2 and Gro-beta. These data have implications for the study of the functional responses of leukocytes to external stimuli and will aid in our understanding of general leukocyte biology.

Cutting edge: identification of the orphan receptor G-protein-coupled receptor 2 as CCR10, a specific receptor for the chemokine ESkine

A number of orphan G-protein coupled receptors (GPR) have been reported as putative chemokine receptors. One previously reported orphan receptor is an incomplete PCR clone, called GPR2. Here we report the cloning of full-length human (h)GPR2 and mouse (m)GPR2 cDNAs, and the identification of GPR2 as a receptor for a novel CC chemokine called ESkine. hGPR2 is expressed at high levels in testis and small intestine, and at lower levels in other tissues. mGPR2 was expressed at high levels in small intestine, colon, lymph nodes, and Peyer's patches and at lower levels in thymus and spleen. Stimulation of L1.2/hGPR2 transfectants with hESkine induced their migration and resulted in intracellular calcium mobilization. These results provide evidence that GPR2 is a specific receptor for ESkine. We propose that GPR2 be renamed as CCR10. The expression pattern of mGPR2/CCR10 suggests that it may play a role in the homing/trafficking of leukocytes within intestinal and lymphoid environments.

The Kaposi's sarcoma-related herpesvirus (KSHV)-encoded chemokine vMIP-I is a specific agonist for the CC chemokine receptor (CCR)8

The Kaposi's sarcoma-related herpesvirus (KSHV), also designated human herpesvirus 8, is the presumed etiologic agent of Kaposi's sarcoma and certain lymphomas. Although KSHV encodes several chemokine homologues (viral macrophage inflammatory protein [vMIP]-I, -II, and -III), only vMIP-II has been functionally characterized. We report here that vMIP-I is a specific agonist for the CC chemokine receptor (CCR)8 that is preferentially expressed on Th2 T cells. Y3 cells transfected with CCR8 produced a calcium flux in response to vMIP-I and responded vigorously in in vitro chemotaxis assays. In competition binding experiments, the interaction of vMIP-I with CCR8 was shown to be specific and of high affinity. In contrast to its agonist activity at CCR8, vMIP-I did not interact with CCR5 or any of 11 other receptors examined. Furthermore, vMIP-I was unable to inhibit CCR5-mediated HIV infection. These findings suggest that expression of vMIP-I by KSHV may influence the Th1/Th2 balance of the host immune response.

Cutting Edge: Identification of the Orphan Chemokine Receptor GPR-9-6 as CCR9, the Receptor for the Chemokine TECK

Thymus-expressed chemokine (TECK) has been reported to chemoattract dendritic cells, thymocytes, and activated macrophages. Here, we show that TECK is a specific agonist for a human orphan receptor called GPR-9-6. We have determined the cDNA sequence of human GPR-9-6 and cloned the corresponding murine cDNA. Human and murine GPR-9-6 expression is very high in the thymus and low in lymph nodes and spleen. RT-PCR analysis of murine GPR-9-6 expression on murine FACS-sorted thymocyte subpopulations showed that this gene is expressed in both immature and mature T cells. Additions of human or murine TECK to HEK 293/human GPR-9-6 and HEK 293/murine GPR-9-6 transfectants provoked intracytoplasmic calcium mobilization. Human TECK also induced the in vitro migration of HEK 293/human GPR-9-6 cells. These results confirm that GPR-9-6 is a specific receptor for TECK. According to the established nomenclature system, we propose to rename GPR-9-6 as CC chemokine receptor 9 (CCR9).