An atypical addition to the chemokine receptor nomenclature: IUPHAR Review 15

Atypical chemokine receptors in the immune system

Leukocyte migration is a fundamental component of innate and adaptive immune responses as it governs the recruitment and localization of these motile cells, which is crucial for immune cell priming, effector functions, memory responses and immune regulation. This complex cellular trafficking system is controlled to a large extent via highly regulated production of secreted chemokines and the restricted expression of their membrane-tethered G-protein-coupled receptors. The activity of chemokines and their receptors is also regulated by a subfamily of molecules known as atypical chemokine receptors (ACKRs), which are chemokine receptor-like molecules that do not couple to the classical signalling pathways that promote cell migration in response to chemokine ligation. There has been a great deal of progress in understanding the biology of these receptors and their functions in the immune system in the past decade. Here, we describe the contribution of the various ACKRs to innate and adaptive immune responses, focussing specifically on recent progress. This includes recent findings that have defined the role for ACKRs in sculpting extracellular chemokine gradients, findings that broaden the spectrum of chemokine ligands recognized by these receptors, candidate new additions to ACKR family, and our increasing understanding of the role of these receptors in shaping the migration of innate and adaptive immune cells.

16S rRNA and transcriptome analysis of the FOS-mediated alleviation of Aeromonas hydrophila-induced intestinal damage in Megalobrama amblycephala

This study was conducted to elucidate the effects of FOS that alleviate Aeromonas hydrophila-induced intestinal damage. The results showed that A. hydrophila disrupted the intestinal structure and increased intestinal permeability, causing abnormalities in mucosal pathology. Additionally, A. hydrophila induced an imbalance in the intestinal flora and disturbed its stability. Dietary FOS ameliorated the injury to the intestinal structure of fish, but also in part improved the condition of the intestinal tight junction complex. Transcriptomic analysis showed that 120 genes were up-regulated and 320 genes were down-regulated. The intestinal immune network for the IgA production signalling pathway was enriched following A. hydrophila infection, and the change in the FOS group was mainly in the Tight junction signalling pathway. Similarly, dietary FOS reduced the disruption of the intestinal microbiota induced by A. hydrophila and improved the intestinal microbiota's stability; FOS was also partially implicated in the upregulation of Tight junction and Adhesion junction pathways by transcriptomic analysis. After further analysis, it was found that fish fed FOS had upregulated expression of genes related to apoptosis, antigen presentation, and the T-cell-mediated immune response in the intestine compared with those in the A. hydrophila group, which may be related to changes in the intestinal microbiome.

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.

CXCR1 drives the pathogenesis of EAE and ARDS via boosting dendritic cells-dependent inflammation

Chemokines secreted by dendritic cells (DCs) play a key role in the regulation of inflammation and autoimmunity through chemokine receptors. However, the role of chemokine receptor CXCR1 in inflammation-inducing experimental autoimmune encephalomyelitis (EAE) and acute respiratory distress syndrome (ARDS) remains largely enigmatic. Here we reported that compared with healthy controls, the level of CXCR1 was aberrantly increased in multiple sclerosis (MS) patients. Knockout of CXCR1 not only ameliorated disease severity in EAE mice but also suppressed the secretion of inflammatory factors (IL-6/IL-12p70) production. We observed the same results in EAE mice with DCs-specific deletion of CXCR1 and antibody neutralization of the ligand CXCL5. Mechanically, we demonstrated a positive feedback loop composed of CXCL5/CXCR1/HIF-1α direct regulating of IL-6/IL-12p70 production in DCs. Meanwhile, we found CXCR1 deficiency in DCs limited IL-6/IL-12p70 production and lung injury in LPS-induced ARDS, a disease model caused by inflammation. Overall, our study reveals CXCR1 governs DCs-mediated inflammation and autoimmune disorders and its potential as a therapeutic target for related diseases.

The Role of CCL3 in the Pathogenesis of Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a chronic autoimmune disease of unexplained causes. Its pathological features include synovial tissue hyperplasia, inflammatory cell infiltration in joint cavity fluid, cartilage bone destruction, and joint deformation. C-C motif chemokine ligand 3 (CCL3) belongs to inflammatory cell chemokine. It is highly expressed in inflammatory immune cells. Increasingly, studies have shown that CCL3 can promote the migration of inflammatory factors to synovial tissue, the destruction of bone and joint, angiogenesis, and participate in the pathogenesis of RA. These symptoms indicate that the expression of CCL3 is highly correlated with RA disease. Therefore, this paper reviews the possible mechanism of CCL3 in the pathogenesis of RA, which may provide some new insights for the diagnosis and treatment of RA.

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.

Impaired Antitumor Immune Response in MYCN-amplified Neuroblastoma Is Associated with Lack of CCL2 Secretion and Poor Dendritic Cell Recruitment

In neuroblastoma, MYCN amplification is associated with sparse immune infiltrate and poor prognosis. Dendritic cells (DC) are crucial immune sentinels but their involvement in neuroblastoma pathogenesis is poorly understood. We observed that the migration of monocytes, myeloid and plasmacytoid DC induced by MYCN-nonamplified neuroblastoma supernatants was abrogated by the addition of anti-CCL2 antibodies, demonstrating the involvement of the CCR2/CCL2 axis in their recruitment by these tumors. Using public RNA sequencing and microarray datasets, we describe lower level of expression of CCL2 in MYCN-amplified neuroblastoma tumors, and we propose a working model for T-cell recruitment in neuroblastoma tumors in which CCL2 produced by neuroblastoma cells initiates the recruitment of monocytes, myeloid and plasmacytoid DCs. Among these cells, the CD1c⁺ subset may recruit T cells by means of CCL19/CCL22 secretion. In vitro, supernatants from DCs cocultured with neuroblastoma cell lines and activated contain CCL22 and CCL19, and are chemotactic for both CD4⁺ and CD8⁺ T cells. We also looked at immunomodulation induced by neuroblastoma cell lines, and found MYCN-nonamplified neuroblastoma cell lines were able to create a microenvironment where DC activation is enhanced. Overall, our findings highlight a major role for CCL2/CCR2 axis in monocytes, myeloid and plasmacytoid cells recruitment toward MYCN-nonamplified neuroblastoma, allowing further immune cell recruitment, and show that these tumors present a microenvironment that can favor DC responses.

Significance

In MYCN-nonamplified neuroblastoma, CCL2 produced by neuroblastoma cells induces the recruitment of antigen-presenting cells (DCs and monocytes/macrophages), allowing infiltration by T cells, in link with CCL19 and CCL22 production, hence favoring immune responses.

The Extended N-Terminal Domain Confers Atypical Chemokine Receptor Properties to CXCR3-B

The chemokine receptor CXCR3 plays a critical role in immune cell recruitment and activation. CXCR3 exists as two main isoforms, CXCR3-A and CXCR3-B, resulting from alternative splicing. Although the two isoforms differ only by the presence of an N-terminal extension in CXCR3-B, they have been attributed divergent functional effects on cell migration and proliferation. CXCR3-B is the more enigmatic isoform and the mechanisms underlying its function and signaling remain elusive. We therefore undertook an in-depth cellular and molecular comparative study of CXCR3-A and CXCR3-B, investigating their activation at different levels of the signaling cascades, including G protein coupling, β-arrestin recruitment and modulation of secondary messengers as well as their downstream gene response elements. We also compared the subcellular localization of the two isoforms and their trafficking under resting and stimulated conditions along with their ability to internalize CXCR3-related chemokines. Here, we show that the N-terminal extension of CXCR3-B drastically affects receptor features, modifying its cellular localization and preventing G protein coupling, while preserving β-arrestin recruitment and chemokine uptake capacities. Moreover, we demonstrate that gradual truncation of the N terminus leads to progressive recovery of surface expression and G protein coupling. Our study clarifies the molecular basis underlying the divergent effects of CXCR3 isoforms, and emphasizes the β-arrestin-bias and the atypical nature of CXCR3-B.

The Chemokine System in Oncogenic Pathways Driven by Viruses: Perspectives for Cancer Immunotherapy

Simple Summary

Oncoviruses are viruses with oncogenic potential, responsible for almost 20% of human cancers worldwide. They are from various families, some of which belong to the microbial communities that inhabit several sites in the body of healthy humans. As a result, they most often establish latent infections controlled by the arsenal of human host responses that include the chemokine system playing key roles at the interface between tissue homeostasis and immune surveillance. Yet, chemokines and their receptors also contribute to oncogenic processes as they are targeted by the virus-induced deregulations of host responses and/or directly encoded by viruses. Thus, the chemokine system offers a strong rationale for therapeutic options, some few already approved or in trials, and future ones that we are discussing in view of the pharmacological approaches targeting the different functions of chemokines operating in both cancer cells and the tumor microenvironment.

Abstract

Chemokines interact with glycosaminoglycans of the extracellular matrix and activate heptahelical cellular receptors that mainly consist of G Protein-Coupled Receptors and a few atypical receptors also with decoy activity. They are well-described targets of oncogenic pathways and key players in cancer development, invasiveness, and metastasis acting both at the level of cancer cells and cells of the tumor microenvironment. Hence, they can regulate cancer cell proliferation and survival and promote immune or endothelial cell migration into the tumor microenvironment. Additionally, oncogenic viruses display the potential of jeopardizing the chemokine system by encoding mimics of chemokines and receptors as well as several products such as oncogenic proteins or microRNAs that deregulate their human host transcriptome. Conversely, the chemokine system participates in the host responses that control the virus life cycle, knowing that most oncoviruses establish asymptomatic latent infections. Therefore, the deregulated expression and function of chemokines and receptors as a consequence of acquired or inherited mutations could bias oncovirus infection toward pro-oncogenic pathways. We here review these different processes and discuss the anticancer therapeutic potential of targeting chemokine availability or receptor activation, from signaling to decoy-associated functions, in combination with immunotherapies.

CCR5 is a potential therapeutic target for cancer

INTRODUCTION

Chemokines and their cognate receptors play a major role in modulating inflammatory responses. Depending on their ligand binding, chemokine receptors can stimulate both immune activating and inhibitory signaling pathways. The CC chemokine receptor 5 (CCR5) promotes immune responses by recruiting immune cells to the sites of inflammation/tumor, and is involved in stimulating tumor cell proliferation, invasion and migration through various mechanisms. Moreover, CCR5 also contributes to an immune-suppressive tumor microenvironment by recruiting regulatory T cells and myeloid-derived suppressor cells facilitating tumor development and progression. In summary, cells expressing CCR5 modulate immune response and tumor progression. Expression of CCR5 is increased in various malignancies and associated with poor outcome. Experimental data show promising efficacy signals with CCR5 antagonists in preclinical tumor models. Therefore, CCR5 has been recognized as a potential therapeutic target for cancer.

AREAS COVERED

In this review, we focus on the role of CCR5 in cancer progression and discuss its impact and potential as a therapeutic target for cancer.

EXPERT OPINION

Beyond immune-checkpoint inhibitors, potentially synergistic immune-modulatory drugs such as CCR5 antagonists are a promising approach to enlarge our treatment armamentarium against cancer.

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.

GPR182 is an endothelium-specific atypical chemokine receptor that maintains hematopoietic stem cell homeostasis

G protein–coupled receptors (GPCRs) are important regulators of cellular and biological functions and are primary targets of therapeutic drugs. About 100 mammalian GPCRs are still considered orphan receptors because they lack a known endogenous ligand. We report the deorphanization of GPR182, which is expressed in endothelial cells of the microvasculature. We show that GPR182 is an atypical chemokine receptor, which binds CXCL10, 12, and 13. However, binding does not induce downstream signaling. Consistent with a scavenging function of GPR182, mice lacking GPR182 have increased plasma levels of chemokines. In line with the crucial role of CXCL12 in hematopoietic stem cell homeostasis, we found that loss of GPR182 results in increased egress of hematopoietic stem cells from the bone marrow.

G protein–coupled receptor 182 (GPR182) has been shown to be expressed in endothelial cells; however, its ligand and physiological role has remained elusive. We found GPR182 to be expressed in microvascular and lymphatic endothelial cells of most organs and to bind with nanomolar affinity the chemokines CXCL10, CXCL12, and CXCL13. In contrast to conventional chemokine receptors, binding of chemokines to GPR182 did not induce typical downstream signaling processes, including G_q- and G_i-mediated signaling or β-arrestin recruitment. GPR182 showed relatively high constitutive activity in regard to β-arrestin recruitment and rapidly internalized in a ligand-independent manner. In constitutive GPR182-deficient mice, as well as after induced endothelium-specific loss of GPR182, we found significant increases in the plasma levels of CXCL10, CXCL12, and CXCL13. Global and induced endothelium-specific GPR182-deficient mice showed a significant decrease in hematopoietic stem cells in the bone marrow as well as increased colony-forming units of hematopoietic progenitors in the blood and the spleen. Our data show that GPR182 is a new atypical chemokine receptor for CXCL10, CXCL12, and CXCL13, which is involved in the regulation of hematopoietic stem cell homeostasis.

Control of Cytoskeletal Dynamics by β-Arrestin1/Myosin Vb Signaling Regulates Endosomal Sorting and Scavenging Activity of the Atypical Chemokine Receptor ACKR2

The atypical chemokine receptor ACKR2, formerly named D6, is a scavenger chemokine receptor with a non-redundant role in the control of inflammation and immunity. The scavenging activity of ACKR2 depends on its trafficking properties, which require actin cytoskeleton rearrangements downstream of a β-arrestin1-Rac1-PAK1-LIMK1-cofilin-dependent signaling pathway. We here demonstrate that in basal conditions, ACKR2 trafficking properties require intact actin and microtubules networks. The dynamic turnover of actin filaments is required to sustain ACKR2 constitutive endocytosis, while both actin and microtubule networks are involved in processes regulating ACKR2 constitutive sorting to rapid, Rab4-dependent and slow, Rab11-dependent recycling pathways, respectively. After chemokine engagement, ACKR2 requires myosin Vb activity to promote its trafficking from Rab11-positive recycling endosomes to the plasma membrane, which sustains its scavenging activity. Other than cofilin phosphorylation, induction of the β-arrestin1-dependent signaling pathway by ACKR2 agonists also leads to the rearrangement of microtubules, which is required to support the myosin Vb-dependent ACKR2 upregulation and its scavenging properties. Disruption of the actin-based cytoskeleton by the apoptosis-inducing agent staurosporine results in impaired ACKR2 internalization and chemokine degradation that is consistent with the emerging scavenging-independent activity of the receptor in apoptotic neutrophils instrumental for promoting efficient efferocytosis during the resolution of inflammation. In conclusion, we provide evidence that ACKR2 activates a β-arrestin1-dependent signaling pathway, triggering both the actin and the microtubule cytoskeletal networks, which control its trafficking and scavenger properties.

The Role of CCL21/CCR7 Chemokine Axis in Breast Cancer Progression

Breast cancer is a leading cause of cancer-related deaths worldwide, predominantly caused by metastasis. It is generally accepted that the pattern of breast cancer metastasis is largely determined by the interaction between the chemokine receptors on cancer cells and the chemokines expressed at the sites of metastatic disease. Chemokine receptors belong to the G-protein-coupled receptors (GPCRs) family that appear to be implicated in inflammatory diseases, tumor growth and metastasis. One of its members, C-C Chemokine receptor 7 (CCR7), binds chemokines CCL19 and CCL21, which are important for tissue homeostasis, immune surveillance and tumorigenesis. These receptors have been shown to induce the pathobiology of breast cancer due to their ability to induce cellular proliferation and migration upon the binding of the cognate chemokine receptors. The underlying signaling pathways and exact cellular interactions within this biological system are not fully understood and need further insights. Thus, in this review, we summarize the essential roles of CCR7 and its receptors in breast cancer progression. Furthermore, we discuss the mechanisms of regulation that may lead to novel opportunities for therapeutic intervention. Despite the enormous advances in our knowledge of the nature of the chemokines in breast cancer metastasis, research about the involvement of CCR7 in cancer progression is still limited. Therefore, further studies are essential to illustrate the distinct roles of CCR7 in cancer progression and validate its potential as a preventive bio-factor for human breast cancer metastasis by targeting chemokine receptor genes.

Neutrophil chemoattractant receptors in health and disease: double-edged swords

Neutrophils are frontline cells of the innate immune system. These effector leukocytes are equipped with intriguing antimicrobial machinery and consequently display high cytotoxic potential. Accurate neutrophil recruitment is essential to combat microbes and to restore homeostasis, for inflammation modulation and resolution, wound healing and tissue repair. After fulfilling the appropriate effector functions, however, dampening neutrophil activation and infiltration is crucial to prevent damage to the host. In humans, chemoattractant molecules can be categorized into four biochemical families, i.e., chemotactic lipids, formyl peptides, complement anaphylatoxins and chemokines. They are critically involved in the tight regulation of neutrophil bone marrow storage and egress and in spatial and temporal neutrophil trafficking between organs. Chemoattractants function by activating dedicated heptahelical G protein-coupled receptors (GPCRs). In addition, emerging evidence suggests an important role for atypical chemoattractant receptors (ACKRs) that do not couple to G proteins in fine-tuning neutrophil migratory and functional responses. The expression levels of chemoattractant receptors are dependent on the level of neutrophil maturation and state of activation, with a pivotal modulatory role for the (inflammatory) environment. Here, we provide an overview of chemoattractant receptors expressed by neutrophils in health and disease. Depending on the (patho)physiological context, specific chemoattractant receptors may be up- or downregulated on distinct neutrophil subsets with beneficial or detrimental consequences, thus opening new windows for the identification of disease biomarkers and potential drug targets.

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.

The diverse and complex roles of atypical chemokine receptors in cancer: From molecular biology to clinical relevance and therapy

Chemokines regulate directed cell migration, proliferation and survival and are key components in cancer biology. They exert their functions by interacting with seven-transmembrane domain receptors that signal through G proteins (GPCRs). A subgroup of four chemokine receptors known as the atypical chemokine receptors (ACKRs) has emerged as essential regulators of the chemokine functions. ACKRs play diverse and complex roles in tumor biology from tumor initiation to metastasis, including cancer cell proliferation, adherence to endothelium, epithelial-mesenchymal transition (EMT), extravasation from blood vessels, tumor-associated angiogenesis or protection from immunological responses. This chapter gives an overview on the established and emerging roles that the atypical chemokine receptors ACKR1, ACKR2, ACKR3 and ACKR4 play in the different phases of cancer development and dissemination, their clinical relevance, as well as on the hurdles to overcome in ACKRs targeting as cancer therapy.

Increased Expressions and Roles of CC Chemokine Ligand 21 and CC Chemokine Ligand 25 in Chronic Rhinosinusitis with Nasal Polyps

INTRODUCTION

Chronic rhinosinusitis (CRS) is a local inflammation of the nasal mucosa and sinus that persists for >12 weeks. As CC chemokine ligand (CCL) 19 expression is known to be elevated in CRS, and CCL 19, CCL21, and CCL25 share the same atypical chemokine receptor 4, so we focused on CCL21 and CCL25.

OBJECTIVES

To investigate the expression of CCL21 and CCL25 in different types of CRS and their significance in CRS development.

METHODS

A total of 116 patients participated in the study, and uncinate process mucosa or nasal polyp (NP) specimens were collected during surgery. Western blotting and immunohistochemistry were performed to detect the expression of CCL21 and CCL25, respectively, in the nasal mucosa. Immunofluorescence was used to determine their cellular localization in NPs, whereas macrophage culture was used to determine their relationships with macrophages.

RESULTS

Immunohistochemistry revealed that the expressions of CCL21 and CCL25 were increased in NPs only. Western blotting revealed that these expressions were gradually increased in control, CRS without NP and CRS with NP groups and were positively correlated with disease severity. Furthermore, increased expressions of CCL21 and CCL25 in NPs were not related to eosinophil infiltration. Immunofluorescence results demonstrated colocalization of CCL25+ cells and CD68+ macrophages. CCL25 expression was increased in macrophage culture, especially in M1 macrophages, while CCL21 expression was not significantly associated with macrophages.

CONCLUSIONS

CCL21 and CCL25 were significantly upregulated in NPs and positively correlated with disease severity. CCL25 upregulation was related to M1 macrophages.

Frontline Science: Antagonism between regular and atypical Cxcr3 receptors regulates macrophage migration during infection and injury in zebrafish

The CXCR3‐CXCL11 chemokine‐signaling axis plays an essential role in infection and inflammation by orchestrating leukocyte trafficking in human and animal models, including zebrafish. Atypical chemokine receptors (ACKRs) play a fundamental regulatory function in signaling networks by shaping chemokine gradients through their ligand scavenging function, while being unable to signal in the classic G‐protein‐dependent manner. Two copies of the CXCR3 gene in zebrafish, cxcr3.2 and cxcr3.3, are expressed on macrophages and share a highly conserved ligand‐binding site. However, Cxcr3.3 has structural characteristics of ACKRs indicative of a ligand‐scavenging role. In contrast, we previously showed that Cxcr3.2 is an active CXCR3 receptor because it is required for macrophage motility and recruitment to sites of mycobacterial infection. In this study, we generated a cxcr3.3 CRISPR‐mutant to functionally dissect the antagonistic interplay among the cxcr3 paralogs in the immune response. We observed that cxcr3.3 mutants are more susceptible to mycobacterial infection, whereas cxcr3.2 mutants are more resistant. Furthermore, macrophages in the cxcr3.3 mutant are more motile, show higher activation status, and are recruited more efficiently to sites of infection or injury. Our results suggest that Cxcr3.3 is an ACKR that regulates the activity of Cxcr3.2 by scavenging common ligands and that silencing the scavenging function of Cxcr3.3 results in an exacerbated Cxcr3.2 signaling. In human, splice variants of CXCR3 have antagonistic functions and CXCR3 ligands also interact with ACKRs. Therefore, in zebrafish, an analogous regulatory mechanism appears to have evolved after the cxcr3 gene duplication event, through diversification of conventional and atypical receptor variants.

Design, Synthesis, and Characterization of Ogerin-Based Positive Allosteric Modulators for G Protein-Coupled Receptor 68 (GPR68)

G protein-coupled receptor 68 (GPR68) is an understudied orphan G protein-coupled receptor (GPCR). It is expressed most abundantly in the brain, potentially playing important roles in learning and memory. Pharmacological studies with GPR68 have been hindered by lack of chemical tools that can selectively modulate its activity. We previously reported the first small-molecule positive allosteric modulator (PAM), ogerin (1), and showed that 1 can potentiate proton activity at the GPR68-G_s pathway. Here, we report the first comprehensive structure-activity relationship (SAR) study on the scaffold of 1. Our lead compound resulted from this study, MS48107 (71), displayed 33-fold increased allosteric activity compared to 1. Compound 71 demonstrated high selectivity over closely related proton GPCRs and 48 common drug targets, and was bioavailable and brain-penetrant in mice. Thus, our SAR study has resulted in an improved GPR68 PAM for investigating the physiological and pathophysiological roles of GPR68 in vitro and in vivo.

Atypical chemokine receptors-"chemokine PACMANs" as new therapeutic targets in glomerulonephritis

Inflammatory cells are recruited to sites of inflammation by chemokines. Atypical chemokine receptors regulate chemokine gradients, thereby limiting inflammation. In this issue of Kidney International, atypical chemokine receptor 2 knockouts were described to be increasingly susceptible to immune complex-mediated glomerulonephritis. By degrading CCL2, atypical chemokine receptor 2 limited the recruitment of immune cells and myofibroblasts to the renal interstitial compartment. Therefore, not only inflammation, but also fibrosis, was effectively inhibited, making atypical chemokine receptor 2 an attractive therapeutic target in glomerulonephritis.

Advances in Biomarker-Guided Therapy for Pediatric- and Adult-Onset Neuroinflammatory Disorders: Targeting Chemokines/Cytokines

The concept and recognized components of “neuroinflammation” are expanding at the intersection of neurobiology and immunobiology. Chemokines (CKs), no longer merely necessary for immune cell trafficking and positioning, have multiple physiologic, developmental, and modulatory functionalities in the central nervous system (CNS) through neuron–glia interactions and other mechanisms affecting neurotransmission. They issue the “help me” cry of neurons and astrocytes in response to CNS injury, engaging invading lymphoid cells (T cells and B cells) and myeloid cells (dendritic cells, monocytes, and neutrophils) (adaptive immunity), as well as microglia and macrophages (innate immunity), in a cascade of events, some beneficial (reparative), others destructive (excitotoxic). Human cerebrospinal fluid (CSF) studies have been instrumental in revealing soluble immunobiomarkers involved in immune dysregulation, their dichotomous effects, and the cells—often subtype specific—that produce them. CKs/cytokines continue to be attractive targets for the pharmaceutical industry with varying therapeutic success. This review summarizes the developing armamentarium, complexities of not compromising surveillance/physiologic functions, and insights on applicable strategies for neuroinflammatory disorders. The main approach has been using a designer monoclonal antibody to bind directly to the chemo/cytokine. Another approach is soluble receptors to bind the chemo/cytokine molecule (receptor ligand). Recombinant fusion proteins combine a key component of the receptor with IgG1. An additional approach is small molecule antagonists (protein therapeutics, binding proteins, and protein antagonists). CK neutralizing molecules (“neutraligands”) that are not receptor antagonists, high-affinity neuroligands (“decoy molecules”), as well as neutralizing “nanobodies” (single-domain camelid antibody fragment) are being developed. Simultaneous, more precise targeting of more than one cytokine is possible using bispecific agents (fusion antibodies). It is also possible to inhibit part of a signaling cascade to spare protective cytokine effects. “Fusokines” (fusion of two cytokines or a cytokine and CK) allow greater synergistic bioactivity than individual cytokines. Another promising approach is experimental targeting of the NLRP3 inflammasome, amply expressed in the CNS and a key contributor to neuroinflammation. Serendipitous discovery is not to be discounted. Filling in knowledge gaps between pediatric- and adult-onset neuroinflammation by systematic collection of CSF data on CKs/cytokines in temporal and clinical contexts and incorporating immunobiomarkers in clinical trials is a challenge hereby set forth for clinicians and researchers.

Chemokine and chemotactic signals in dendritic cell migration

Dendritic cells (DCs) are professional antigen-presenting cells responsible for the activation of specific T-cell responses and for the development of immune tolerance. Immature DCs reside in peripheral tissues and specialize in antigen capture, whereas mature DCs reside mostly in the secondary lymphoid organs where they act as antigen-presenting cells. The correct localization of DCs is strictly regulated by a large variety of chemotactic and nonchemotactic signals that include bacterial products, DAMPs (danger-associated molecular patterns), complement proteins, lipids, and chemokines. These signals function both individually and in concert, generating a complex regulatory network. This network is regulated at multiple levels through different strategies, such as synergistic interactions, proteolytic processing, and the actions of atypical chemokine receptors. Understanding this complex scenario will help to clarify the role of DCs in different pathological conditions, such as autoimmune diseases and cancers and will uncover new molecular targets for therapeutic interventions.

Neutrophils: Beneficial and Harmful Cells in Septic Arthritis

Septic arthritis is an inflammatory joint disease that is induced by pathogens such as Staphylococcus aureus. Infection of the joint triggers an acute inflammatory response directed by inflammatory mediators including microbial danger signals and cytokines and is accompanied by an influx of leukocytes. The recruitment of these inflammatory cells depends on gradients of chemoattractants including formylated peptides from the infectious agent or dying cells, host-derived leukotrienes, complement proteins and chemokines. Neutrophils are of major importance and play a dual role in the pathogenesis of septic arthritis. On the one hand, these leukocytes are indispensable in the first-line defense to kill invading pathogens in the early stage of disease. However, on the other hand, neutrophils act as mediators of tissue destruction. Since the elimination of inflammatory neutrophils from the site of inflammation is a prerequisite for resolution of the acute inflammatory response, the prolonged stay of these leukocytes at the inflammatory site can lead to irreversible damage to the infected joint, which is known as an important complication in septic arthritis patients. Thus, timely reduction of the recruitment of inflammatory neutrophils to infected joints may be an efficient therapy to reduce tissue damage in septic arthritis.

International Union of Basic and Clinical Pharmacology CIII: Chemerin Receptors CMKLR1 (Chemerin1) and GPR1 (Chemerin2) Nomenclature, Pharmacology, and Function

Chemerin, a chemoattractant protein and adipokine, has been identified as the endogenous ligand for a G protein–coupled receptor encoded by the gene CMKLR1 (also known as ChemR23), and as a consequence the receptor protein was renamed the chemerin receptor in 2013. Since then, chemerin has been identified as the endogenous ligand for a second G protein–coupled receptor, encoded by the gene GPR1. Therefore, the International Union of Basic and Clinical Pharmacology Committee on Receptor Nomenclature and Drug Classification recommends that the official name of the receptor protein for chemokine-like receptor 1 (CMKLR1) is chemerin receptor 1, and G protein–coupled receptor 1 is chemerin receptor 2 to follow the convention of naming the receptor protein after the endogenous ligand. Chemerin receptor 1 and chemerin receptor 2 can be abbreviated to Chemerin₁ and Chemerin₂, respectively. Chemerin requires C-terminal processing for activity, and human chemerin21–157 is reported to be the most active form, with peptide fragments derived from the C terminus biologically active at both receptors. Small-molecule antagonist, CCX832, selectively blocks CMKLR1, and resolvin E1 activation of CMKLR1 is discussed. Activation of both receptors by chemerin is via coupling to G_i/o, causing inhibition of adenylyl cyclase and increased Ca²⁺ flux. Receptors and ligand are widely expressed in humans, rats, and mice, and both receptors share ∼80% identity across these species. CMKLR1 knockout mice highlight the role of this receptor in inflammation and obesity, and similarly, GPR1 knockout mice exhibit glucose intolerance. In addition, the chemerin receptors have been implicated in cardiovascular disease, cancer, steroidogenesis, human immunodeficiency virus replication, and neurogenerative disease.

Ligand-specific conformational transitions and intracellular transport are required for atypical chemokine receptor 3-mediated chemokine scavenging

The atypical chemokine receptor ACKR3 contributes to chemotaxis by binding, internalizing, and degrading the chemokines CXCL11 and CXCL12 to shape and terminate chemotactic gradients during development and immune responses. Although unable to trigger G protein activation, both ligands activate G protein-independent ACKR3 responses and prompt arrestin recruitment. This offers a model to specifically study ligand-specific receptor conformations leading to G protein-independent signaling and to functional parameters such as receptor transport and chemokine degradation. We here show chemokine specificity in arrestin recruitment, by different effects of single amino acid substitutions in ACKR3 on arrestin in response to CXCL12 or CXCL11. Chemokine specificity in receptor transport was also observed, as CXCL11 induced faster receptor internalization, slower recycling, and longer intracellular sojourn of ACKR3 than CXCL12. Internalization and recycling rates of the ACKR3 R142^3.50A substitution in response to each chemokine were similar; however, ACKR3 R142^3.50A degraded only CXCL12 and not CXCL11. This suggests that ligand-specific intracellular receptor transport is required for chemokine degradation. Remarkably, the failure of ACKR3 R142^3.50A to degrade CXCL11 was not caused by the lack of arrestin recruitment; rather, arrestin was entirely dispensable for scavenging of either chemokine. This suggests the involvement of another, yet unidentified, ACKR3 effector in scavenging. In summary, our study correlates ACKR3 ligand-specific conformational transitions with chemokine-dependent receptor transport dynamics and points toward unexpected ligand specificity in the mechanisms of chemokine degradation.

Yeast surface display identifies a family of evasins from ticks with novel polyvalent CC chemokine-binding activities

Chemokines function via G-protein coupled receptors in a robust network to recruit immune cells to sites of inflammation. Due to the complexity of this network, targeting single chemokines or receptors has not been successful in inflammatory disease. Dog tick saliva contains polyvalent CC-chemokine binding peptides termed evasins 1 and 4, that efficiently disrupt the chemokine network in models of inflammatory disease. Here we develop yeast surface display as a tool for functionally identifying evasins, and use it to identify 10 novel polyvalent CC-chemokine binding evasin-like peptides from salivary transcriptomes of eight tick species in Rhipicephalus and Amblyomma genera. These evasins have unique binding profiles compared to evasins 1 and 4, targeting CCL2 and CCL13 in addition to other CC-chemokines. Evasin binding leads to neutralisation of chemokine function including that of complex chemokine mixtures, suggesting therapeutic efficacy in inflammatory disease. We propose that yeast surface display is a powerful approach to mine potential therapeutics from inter-species protein interactions that have arisen during evolution of parasitism in ticks.

Structural Analysis of Chemokine Receptor-Ligand Interactions

This review focuses on the construction and application of structural chemokine receptor models for the elucidation of molecular determinants of chemokine receptor modulation and the structure-based discovery and design of chemokine receptor ligands. A comparative analysis of ligand binding pockets in chemokine receptors is presented, including a detailed description of the CXCR4, CCR2, CCR5, CCR9, and US28 X-ray structures, and their implication for modeling molecular interactions of chemokine receptors with small-molecule ligands, peptide ligands, and large antibodies and chemokines. These studies demonstrate how the integration of new structural information on chemokine receptors with extensive structure–activity relationship and site-directed mutagenesis data facilitates the prediction of the structure of chemokine receptor–ligand complexes that have not been crystallized. Finally, a review of structure-based ligand discovery and design studies based on chemokine receptor crystal structures and homology models illustrates the possibilities and challenges to find novel ligands for chemokine receptors.

Chemokine Receptor Signaling and the Hallmarks of Cancer

The chemokines are a family of chemotactic cytokines that mediate their activity by acting on seven-transmembrane-spanning G protein-coupled receptors. Both the ability of the chemokines and their receptors to form homo- and heterodimers and the promiscuity of the chemokine-chemokine receptor interaction endow this protein family with enormous signaling plasticity and complexity that are not fully understood at present. Chemokines were initially identified as essential regulators of homeostatic and inflammatory trafficking of innate and adaptive leucocytes from lymphoid organs to tissues. Chemokines also mediate the host response to cancer. Nevertheless, chemokine function in this response is not limited to regulating leucocyte infiltration into the tumor microenvironment. It is now known that chemokines and their receptors influence most-if not all-hallmark processes of cancer; they act on both neoplastic and untransformed cells in the tumor microenvironment, including fibroblasts, endothelial cells (blood and lymphatic), bone marrow-derived stem cells, and, obviously, infiltrating leucocytes. This review begins with an overview of chemokine and chemokine receptor structure, to better define how chemokines affect the proliferation, survival, stemness, and metastatic potential of neoplastic cells. We also examine the main mechanisms by which chemokines regulate tumor angiogenesis and immune cell infiltration, emphasizing the pro- and antitumorigenic activity of this protein superfamily in these interrelated processes.

Overview and potential unifying themes of the atypical chemokine receptor family

Chemokines modulate immune responses through their ability to orchestrate the migration of target cells. Chemokines directly induce cell migration through a distinct set of 7 transmembrane domain G protein-coupled receptors but are also recognized by a small subfamily of atypical chemokine receptors, characterized by their inability to support chemotactic activity. Atypical chemokine receptors are now emerging as crucial regulatory components of chemokine networks in a wide range of physiologic and pathologic contexts. Although a new nomenclature has been approved recently to reflect their functional distinction from their conventional counterparts, a systematic view of this subfamily is still missing. This review discusses their biochemical and immunologic properties to identify potential unifying themes in this emerging family.

Teleost Chemokines and Their Receptors

Chemokines are a superfamily of cytokines that appeared about 650 million years ago, at the emergence of vertebrates, and are responsible for regulating cell migration under both inflammatory and physiological conditions. The first teleost chemokine gene was reported in rainbow trout in 1998. Since then, numerous chemokine genes have been identified in diverse fish species evidencing the great differences that exist among fish and mammalian chemokines, and within the different fish species, as a consequence of extensive intrachromosomal gene duplications and different infectious experiences. Subsequently, it has only been possible to establish clear homologies with mammalian chemokines in the case of some chemokines with well-conserved homeostatic roles, whereas the functionality of other chemokine genes will have to be independently addressed in each species. Despite this, functional studies have only been undertaken for a few of these chemokine genes. In this review, we describe the current state of knowledge of chemokine biology in teleost fish. We have mainly focused on those species for which more research efforts have been made in this subject, specifically zebrafish (Daniorerio), rainbow trout (Oncorhynchusmykiss) and catfish (Ictaluruspunctatus), outlining which genes have been identified thus far, highlighting the most important aspects of their expression regulation and addressing any known aspects of their biological role in immunity. Finally, we summarise what is known about the chemokine receptors in teleosts and provide some analysis using recently available data to help characterise them more clearly.