Differential expression of chemokine receptors and chemotactic responsiveness of type 1 T helper cells (Th1s) and Th2s

T helper cells type 1 (Th1s) that produce interferon-gamma predominantly mediate cellular immune responses and are involved in the development of chronic inflammatory conditions, whereas Th2s which produce large amounts of IL-4 and IL-5 upregulate IgE production and are prominent in the pathogenesis of allergic diseases. The precise factors determining whether Th1- or Th2-mediated immune responses preferentially occur at a peripheral site of antigen exposure are largely unknown. Chemokines, a superfamily of polypeptide mediators, are a key component of the leukocyte recruitment process. Here we report that among four CXC (CXCR1-4) and five CC (CCR1-5) chemokine receptors analyzed, CXCR3 and CCR5 are preferentially expressed in human Th1s. In contrast, Th2s preferentially express CCR4 and, to a lesser extent, CCR3. In agreement with the differential chemokine receptor expression, Th1s and Th2s selectively migrate in response to the corresponding chemokines. The differential expression of chemokine receptors may dictate, to a large extent, the migration and tissue homing of Th1s and Th2s. It may also determine different susceptibility of Th1s and Th2s to human immunodeficiency virus strains using different fusion coreceptors.

Neutrophil diversity and plasticity in tumour progression and therapy

Neutrophils play a key role in defence against infection and in the activation and regulation of innate and adaptive immunity. In cancer, tumour-associated neutrophils (TANs) have emerged as an important component of the tumour microenvironment. Here, they can exert dual functions. TANs can be part of tumour-promoting inflammation by driving angiogenesis, extracellular matrix remodelling, metastasis and immunosuppression. Conversely, neutrophils can also mediate antitumour responses by direct killing of tumour cells and by participating in cellular networks that mediate antitumour resistance. Neutrophil diversity and plasticity underlie the dual potential of TANs in the tumour microenvironment. Myeloid checkpoints as well as the tumour and tissue contexture shape neutrophil function in response to conventional therapies and immunotherapy. We surmise that neutrophils can provide tools to tailor current immunotherapy strategies and pave the way to myeloid cell-centred therapeutic strategies, which would be complementary to current approaches.

Chemokines and Chemokine Receptors: New Targets for Cancer Immunotherapy

Immunotherapy is a clinically validated treatment for many cancers to boost the immune system against tumor growth and dissemination. Several strategies are used to harness immune cells: monoclonal antibodies against tumor antigens, immune checkpoint inhibitors, vaccination, adoptive cell therapies (e.g., CAR-T cells) and cytokine administration. In the last decades, it is emerging that the chemokine system represents a potential target for immunotherapy. Chemokines, a large family of cytokines with chemotactic activity, and their cognate receptors are expressed by both cancer and stromal cells. Their altered expression in malignancies dictates leukocyte recruitment and activation, angiogenesis, cancer cell proliferation, and metastasis in all the stages of the disease. Here, we review first attempts to inhibit the chemokine system in cancer as a monotherapy or in combination with canonical or immuno-mediated therapies. We also provide recent findings about the role in cancer of atypical chemokine receptors that could become future targets for immunotherapy.

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.

Recruitment of immature plasmacytoid dendritic cells (plasmacytoid monocytes) and myeloid dendritic cells in primary cutaneous melanomas

The present study has analysed the distribution and phenotype of dendritic cells (DCs) in primary cutaneous melanomas and sentinel lymph nodes by immunohistochemistry. In primary melanomas, an increase of DCs was found in the epidermis and the peritumoural area. Intraepidermal DCs were mostly CD1a(+)/Langerin(+) Langerhans cells. Peritumoural DCs included a large population of DC-SIGN(+)/mannose-receptor(+)/CD1a(-) DCs, a small subset of CD1a(+) DCs, and, remarkably, plasmacytoid monocytes/plasmacytoid DCs (PM/PDCs). The PM/PDCs, most likely recruited by SDF-1 secreted by melanoma cells, produced type I interferon (IFN-I), but the expression of the IFN-alpha inducible protein MxA was extremely variable and very limited in the majority of cases. All DC subsets were predominantly immature. The peritumoural area also contained a minor subset of mature CD1a(+) DCs. However, the small amount of local interleukin (IL)-12 p40 mRNA and the naïve phenotype of 20-50% of peritumoural T-lymphocytes are consistent with poor T-cell stimulation or erroneous recruitment. In sentinel lymph nodes, notable expansion of mature CD1a(+)/Langerin(+) DCs was observed. The paucity of intratumoural DCs and the predominant immature phenotype of peritumoural dermal DCs indicate defective maturation of primary cutaneous melanoma-associated DCs, resulting in lack of T-cell priming. These results may explain why melanoma cells grow despite the presence of infiltrating immune cells.

Dendritic cells as a major source of macrophage-derived chemokine/CCL22 in vitro and in vivo

Macrophage-derived chemokine (MDC)/CCL22 is a CC chemokine active on dendritic cells (DC), NK cells and Th2 lymphocytes. The present study was aimed at comprehensively investigating MDC production in vitro and in vivo. DC were the most potent producers of MDC among leukocytes tested. Endothelial cells did not produce MDC under a variety of conditions. Signals that induce maturation (lipopolysaccharide, IL-1, TNF, CD40 ligand, recognition of bacteria and yeast) dramatically augmented MDC production, and dexamethasone and vitamin D3 blocked it. Prostaglandin E(2), which blocked the acquisition of IL-12 production and the capacity to promote Th1 generation, did not affect MDC production. Using mass spectrometry-based techniques, DC supernatants were found to contain N-terminally truncated forms of MDC [MDC(3-69), MDC(5-69) and MD(C7-69)] as well as the full-length molecule. In vivo, CD1a(+), CD83(+), MDC(+) DC were found in reactive lymph nodes, and in Langerhans' cell histiocytosis. Skin lesions of atopic dermatitis patients showed that CD1a(+) or CD1b(+) DC, and DC with a CD83(+) phenotype were responsible for MDC production in this Th2-oriented disorder. Thus, DC are the predominant source of MDC in vitro and in vivo under a variety of experimental and clinical conditions. Processing of MDC to MDC(3-69) and shorter forms which do not recognize CCR4 is likely to represent a feedback mechanism of negative regulation.

Chemokines and chemokine receptors: an overview

Chemokines are chemotactic cytokines orchestrating leukocyte recruitment in physiological and pathological conditions. This complex system includes 42 molecules and 19 receptors and is subjected to different levels of regulation, including ligand production, post-translational modifications and degradation, as well as receptor expression and signaling activity. Here we analyze the chemokine system, with particular attention to available information on clinical situations in which chemokines or their receptors might assume diagnostic value.

Multiple Roles for Chemokines in Neutrophil Biology

Chemokines are recognized as the most critical mediators for selective neutrophil recruitment during inflammatory conditions. Furthermore, they are considered fundamental regulators of neutrophil mobilization from the bone marrow (BM) to the bloodstream and for their homing back at the end of their life for apoptosis and clearance. However, chemokines are also important mediators of neutrophil effector functions including oxidative burst, degranulation, neutrophil extracellular trap (NET)osis, and production of inflammatory mediators. Neutrophils have been historically considered as a homogeneous population. In recent years, several maturation stages and subsets with different phenotypic profiles and effector functions were described both in physiological and pathological conditions such as infections, autoimmunity, and cancer. The aim of this review is to give an overview of the current evidence regarding the role of chemokines and chemokine receptors in neutrophil biology, including their possible role in neutrophil maturation, differentiation, and in defining emerging neutrophil subsets.

Protection against inflammation- and autoantibody-caused fetal loss by the chemokine decoy receptor D6

Fetal loss in animals and humans is frequently associated with inflammatory conditions. D6 is a promiscuous chemokine receptor with decoy function, expressed in lymphatic endothelium, that recognizes and targets to degradation most inflammatory CC chemokines. Here, we report that D6 is expressed in placenta on invading extravillous trophoblasts and on the apical side of syncytiotrophoblast cells, at the very interface between maternal blood and fetus. Exposure of D6-/- pregnant mice to LPS or antiphospholipid autoantibodies results in higher levels of inflammatory CC chemokines and increased leukocyte infiltrate in placenta, causing an increased rate of fetal loss, which is prevented by blocking inflammatory chemokines. Thus, the promiscuous decoy receptor for inflammatory CC chemokines D6 plays a nonredundant role in the protection against fetal loss caused by systemic inflammation and antiphospholipid antibodies.

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

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

beta-Arrestin-dependent constitutive internalization of the human chemokine decoy receptor D6

Seven transmembrane receptors mediate diverse physiological responses including hormone action, olfaction, neurotransmission, and chemotaxis. Human D6 is a non-signaling seven-transmembrane receptor expressed on lymphatic endothelium interacting with most inflammatory CC-chemokines resulting in their rapid internalization. Here, we demonstrate that this scavenging activity is mediated by continuous internalization and constant surface expression of the receptor, a process involving the clathrin-coated pit-dependent pathway. D6 constitutively associates with the cytoplasmic adaptor beta-arrestin, and this interaction is essential for D6 internalization. An acidic region, but not the putative phosphorylation sites in the cytoplasmic tail of D6, is critical for receptor interaction with beta-arrestin and subsequent internalization. Neither the native D6 nor mutants uncoupled from beta-arrestin activate any G-protein-mediated signaling pathways. Therefore, D6 may be considered a decoy receptor structurally adapted to perform chemokine scavenging.

Unique regulation of CCL18 production by maturing dendritic cells

Dendritic cells (DC) orchestrate the trafficking of lymphocytes by secreting chemokines with different specificity and function. Chemokines are produced at higher levels by mature DC. This study shows that CCL18 is one of the most abundant chemokines produced by immature DC. In contrast to all other chemokines investigated to date, CCL18 was selectively down-regulated during the maturation process induced by LPS, TNF, CD40 ligand, Staphylococcus aureus Cowan I, Candida albicans, and influenza virus. IL-10 and vitamin D(3), two known inhibitors of DC differentiation and function, strongly promoted CCL18 secretion, whereas IFN-gamma, a costimulator of DC function, inhibited its production. IL-10 also induced CCL18 secretion in blood myeloid DC. No CCL18 secretion was observed in blood plasmacytoid DC. The opposite pattern of regulation was observed for CCL20, a prototypic inflammatory chemokine. CCL18 was found to be a chemotactic factor for immature DC. Therefore, CCL18 may act as a chemotactic signal that promotes the colocalization of immature DC with naive T lymphocytes in an IL-10-dominated environment with the consequent generation of T regulatory cells. These characteristics suggest that CCL18 may be part of an inhibitory pathway devoted to limiting the generation of specific immune responses at peripheral sites.

Differential responsiveness to constitutive vs. inducible chemokines of immature and mature mouse dendritic cells

Upon exposure to immune or inflammatory stimuli, dendritic cells (DC) migrate from peripheral tissues to lymphoid organs, where they present antigen. The molecular basis for the peculiar trafficking properties of DC is largely unknown. In this study, mouse DC were generated from CD34+ bone marrow precursors and cultured with granulocyte-macrophage-CSF and Flt3 ligand for 9 days. Chemokines active on immature DC include MIP1alpha, RANTES, MIP1beta, MCP-1, MCP-3, and the constitutively expressed SDF1, MDC, and ELC. TNF-alpha-induced DC maturation caused reduction of migration to inducible chemokines (MIP1alpha, RANTES, MIP1beta, MCP-1, and MCP-3) and increased migration to SDF1, MDC, and ELC. Similar results were obtained by CD40 ligation or culture in the presence of bacterial lipopolysaccharide. TNF-alpha down-regulated CC chemokine receptor (CCR)1, CCR2, and CCR5 and up-regulated CCR7 mRNA levels, in agreement with functional data. This study shows that selective responsiveness of mature and immature DC to inducible vs. constitutively produced chemokines can contribute to the regulated trafficking of DC.

The lymphatic system controls intestinal inflammation and inflammation-associated Colon Cancer through the chemokine decoy receptor D6

BACKGROUND AND AIMS

Inflammatory CC chemokines have long been associated with cancer, but unequivocal evidence of a role in clinically relevant models of carcinogenesis is lacking. D6, a promiscuous decoy receptor that scavenges inflammatory CC chemokines, plays a non-redundant role in reducing the inflammatory response in various organs. As inflammation is a key player in the development of inflammatory bowel disease (IBD) and IBD-associated colorectal cancer, we investigated D6 expression in human colitis and colon cancer, and its role in experimental colitis and inflammation-associated colon cancer.

RESULTS

In humans, D6 was mainly expressed by lymphatic vessels and leukocytes in the mucosa of individuals with IBD and colon cancer, as well as the mucosa of control individuals. Mice lacking expression of D6 were significantly more susceptible to experimental colitis than wild-type mice and failed to resolve colitis, with significantly higher levels of several pro-inflammatory chemokines. In bone marrow chimeric mice, the ability of D6 to regulate colitis was tracked to the stromal/lymphatic compartment, with no contribution of haemopoietic cells. Finally, after administration of the carcinogen azoxymethane, D6(-/-) mice showed increased susceptibility to colitis-associated cancer in the distal segment of the colon compared with wild-type mice.

CONCLUSIONS

D6 expressed on lymphatic vessels plays a key role in the control of intestinal inflammation and the development of inflammation-associated colon cancer. Our results reveal a new unexpected role for the lymphatic system in the pathogenesis of IBD and intestinal cancer, and candidate chemokines as novel players in tumour promotion and progression.

Increased inflammation in mice deficient for the chemokine decoy receptor D6

Chemokines are chemotactic cytokines with a key role in the control of cell trafficking and positioning under homeostatic and inflammatory conditions. D6 is a promiscuous 7-transmembrane-domain receptor expressed on lymphatic vessels which recognizes most inflammatory, but not homeostatic, CC chemokines. In vitro experiments demonstrated that D6 is unable to signal after ligand engagement, and it is structurally adapted to sustain rapid and efficient ligand internalization and degradation. These unique functional properties lead to the hypothesis that D6 may be involved in the control of inflammation by acting as a decoy and scavenger receptor for inflammatory chemokines. Consistent with this hypothesis, here we report that D6(-/-) mice showed an anticipated and exacerbated inflammatory response in a model of skin inflammation. Moreover, the absence of D6 resulted in increase cellularity and inflammatory-chemokine levels in draining lymph nodes. Thus, D6 is a decoy receptor structurally adapted and strategically located to tune tissue inflammation and control transfer of inflammatory chemokines to draining lymph nodes.

Differential recognition and scavenging of native and truncated macrophage-derived chemokine (macrophage-derived chemokine/CC chemokine ligand 22) by the D6 decoy receptor

The promiscuous D6 receptor binds several inflammatory CC chemokines and has been recently proposed to act as a chemokine-scavenging decoy receptor. The present study was designed to better characterize the spectrum of CC chemokines scavenged by D6, focusing in particular on CCR4 ligands and analyzing the influence of NH(2)-terminal processing on recognition by this promiscuous receptor. Using D6 transfectants, it was found that D6 efficiently bound and scavenged most inflammatory CC chemokines (CCR1 through CCR5 agonists). Homeostatic CC chemokines (CCR6 and CCR7 agonists) were not recognized by D6. The CCR4 agonists CC chemokine ligand 17 (CCL17) and CCL22 bound to D6 with high affinity. CCL17 and CCL22 have no agonistic activity for D6 (chemotaxis and calcium fluxes), but were rapidly scavenged, resulting in reduced chemotactic activity on CCR4 transfectants. CD26 mediates NH(2) terminus processing of CCL22, leading to the production of CCL22 (3-69) and CCL22 (5-69) that do not interact with CCR4. These NH(2)-terminal truncated forms of CCL22 were not recognized by D6. The results presented in this study show that D6 recognizes and scavenges a wide spectrum of inflammatory CC chemokines, including the CCR4 agonists CCL22 and CCL17. However, this promiscuous receptor is not engaged by CD26-processed, inactive, CCL22 variants. By recognizing intact CCL22, but not its truncated variants, D6 expressed on lymphatic endothelial cells may regulate the traffic of CCR4-expressing cells, such as dendritic cells.

Neutrophils in Gliomas

Neutrophils are the most abundant white blood cells and are the first recruited to inflammatory sites. Neutrophils are an important component of the tumor stroma and can exert both anti-tumoral and pro-tumoral activities, depending on their maturation and activation state. In human gliomas, the number of circulating and infiltrating neutrophils correlates with the severity of the disease, indicating a prognostic and possible pro-tumoral role for these leukocytes. In glioma preclinical models, neutrophils promote tumor growth and orchestrate the resistance to anti-angiogenic therapies. Nevertheless, recent data indicate that neutrophils can be activated to directly kill tumor cells or to orchestrate the anti-tumoral response. Here, we review current knowledge about the role of neutrophils in glioma and their possible involvement in new strategies to improve current cancer therapies.

Induction of functional IL-8 receptors by IL-4 and IL-13 in human monocytes

IL-8 and related Glu-Leu-Arg (ELR+) CXC chemokines are potent chemoattractants for neutrophils but not for monocytes. IL-13 and IL-4 strongly increased CXCR1 and CXCR2 chemokine receptor expression in human monocytes, macrophages, and dendritic cells. The effect was receptor- and cell type-selective, in that CCRs were not increased and no augmentation was seen in neutrophils. The effect was rapid, starting at 4 h, and concentration dependent (EC50 = 6.2 and 8.3 ng/ml for CXCR1 and CXCR2, respectively) and caused by new transcriptional activity. IL-13/IL-4-treated monocytes showed increased CXCR1 and CXCR2 membrane expression. IL-8 and related ELR+ chemokines were potent and effective chemotactic agents for IL-13/IL-4-treated monocytes, but not for untreated mononuclear phagocytes, with activity comparable to that of reference monocyte attractants, such as MCP-1. In the same cells, IL-8 also caused superoxide release. Macrophages and dendritic cells present in biopsies from Omenn's syndrome and atopic dermatitis patients, two Th2 skewed pathologies, expressed IL-8 receptors by immunohistochemistry. These results show that IL-13 and IL-4 convert IL-8 and related ELR+ chemokines, prototypic neutrophil attractants, into monocyte chemotactic agonists, by up-regulating receptor expression. Therefore, IL-8 and related chemokines may contribute to the accumulation and positioning of mononuclear phagocytes in Th2-dominated responses.

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.

Regulation of D6 chemokine scavenging activity by ligand- and Rab11-dependent surface up-regulation

The decoy receptor D6 plays a nonredundant role in the control of inflammatory processes through scavenging of inflammatory chemokines. However it remains unclear how it is regulated. Here we show that D6 scavenging activity relies on unique trafficking properties. Under resting conditions, D6 constitutively recycled through both a rapid wortmannin (WM)-sensitive and a slower brefeldin A (BFA)-sensitive pathway, maintaining low levels of surface expression that required both Rab4 and Rab11 activities. In contrast to "conventional" chemokine receptors that are down-regulated by cognate ligands, chemokine engagement induced a dose-dependent BFA-sensitive Rab11-dependent D6 re-distribution to the cell membrane and a corresponding increase in chemokine degradation rate. Thus, the energy-expensive constitutive D6 cycling through Rab11 vesicles allows a rapid, ligand concentration-dependent increase of chemokine scavenging activity by receptor redistribution to the plasma membrane. D6 is not regulated at a transcriptional level in a variety of cellular contexts, thus ligand-dependent optimization of its scavenger performance represents a rapid and unique mechanism allowing D6 to control inflammation.

Atypical chemokine receptors in cancer: friends or foes?

The chemokine system is a fundamental component of cancer-related inflammation involved in all stages of cancer development. It controls not only leukocyte infiltration in primary tumors but also angiogenesis, cancer cell proliferation, and migration to metastatic sites. Atypical chemokine receptors are a new, emerging class of regulators of the chemokine system. They control chemokine bioavailability by scavenging, transporting, or storing chemokines. They can also regulate the activity of canonical chemokine receptors with which they share the ligands by forming heterodimers or by modulating their expression levels or signaling activity. Here, we summarize recent results about the role of these receptors (atypical chemokine receptor 1/Duffy antigen receptor for chemokine, atypical chemokine receptor 2/D6, atypical chemokine receptor 3/CXC-chemokine receptor 7, and atypical chemokine receptor 4/CC-chemokine receptor-like 1) on the tumorigenesis process, indicating that their effects are strictly dependent on the cell type on which they are expressed and on their coexpression with other chemokine receptors. Indeed, atypical chemokine receptors inhibit tumor growth and progression through their activity as negative regulators of chemokine bioavailability, whereas, on the contrary, they can promote tumorigenesis when they regulate the signaling of other chemokine receptors, such as CXC-chemokine receptor 4. Thus, atypical chemokine receptors are key components of the regulatory network of inflammation and immunity in cancer and may have a major effect on anti-inflammatory and immunotherapeutic strategies.

The chemokine decoy receptor D6 prevents excessive inflammation and adverse ventricular remodeling after myocardial infarction

OBJECTIVE

Leukocyte infiltration in ischemic areas is a hallmark of myocardial infarction, and overwhelming infiltration of innate immune cells has been shown to promote adverse remodeling and cardiac rupture. Recruitment of inflammatory cells in the ischemic heart depends highly on the family of CC-chemokines and their receptors. Here, we hypothesized that the chemokine decoy receptor D6, which specifically binds and scavenges inflammatory CC-chemokines, might limit inflammation and adverse cardiac remodeling after infarction.

METHODS AND RESULTS

D6 was expressed in human and murine infarcted myocardium. In a murine model of myocardial infarction, D6 deficiency led to increased chemokine (C-C motif) ligand 2 and chemokine (C-C motif) ligand 3 levels in the ischemic heart. D6-deficient (D6(-/-)) infarcts displayed increased infiltration of pathogenic neutrophils and Ly6Chi monocytes, associated with strong matrix metalloproteinase-9 and matrix metalloproteinase-2 activities in the ischemic heart. D6(-/-) mice were cardiac rupture prone after myocardial infarction, and functional analysis revealed that D6(-/-) hearts had features of adverse remodeling with left ventricle dilation and reduced ejection fraction. Bone marrow chimera experiments showed that leukocyte-borne D6 had no role in this setting, and that leukocyte-specific chemokine (C-C motif) receptor 2 deficiency rescued the adverse phenotype observed in D6(-/-) mice.

CONCLUSIONS

We show for the first time that the chemokine decoy receptor D6 limits CC-chemokine-dependent pathogenic inflammation and is required for adequate cardiac remodeling after myocardial infarction.

Chemokine receptors intracellular trafficking

Chemokines coordinate leukocyte recruitment during inflammatory and immune responses through the interaction with a distinct subfamily of G protein-coupled receptors. The magnitude of the cellular response elicited by chemokines is dictated by the level of receptor expression at the plasma membrane, which is the balance of finely tuned endocytic and recycling pathways. Recent data have revealed that receptor trafficking properties can drive chemokine receptors to lysosomal degradation or recycling pathways, producing opposite effects on the strength of the intracellular signaling cascade. This review will cover recent advances on the molecular mechanisms underlying chemokine receptor internalization, recycling and degradation pathways, with particular attention to structural motifs present in receptor intracellular domains and their interacting adaptor proteins that modulate receptor trafficking and dictate proper biological response.

Chemokines and cancer: a fatal attraction

Chemokines are important components of cancer-related inflammation. In this issue of Cancer Cell, Chen et al. report that the chemokine CCL18, produced by tumor-associated macrophages, promotes malignant behavior and correlates with metastasis in human breast cancer. Unexpectedly, PITPNM3/Nir1, a molecule unrelated to chemokine receptors, was identified as its elusive receptor.

β-arrestin-dependent activation of the cofilin pathway is required for the scavenging activity of the atypical chemokine receptor D6

Chemokines promote the recruitment of leukocytes to sites of infection and inflammation by activating conventional heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs). Chemokines are also recognized by a set of atypical chemokine receptors (ACRs), which cannot induce directional cell migration but are required for the generation of chemokine gradients in tissues. ACRs are presently considered "silent receptors" because no G protein-dependent signaling activity is observed after their engagement by cognate ligands. We report that engagement of the ACR D6 by its ligands activates a β-arrestin1-dependent, G protein-independent signaling pathway that results in the phosphorylation of the actin-binding protein cofilin through the Rac1-p21-activated kinase 1 (PAK1)-LIM kinase 1 (LIMK1) cascade. This signaling pathway is required for the increased abundance of D6 protein at the cell surface and for its chemokine-scavenging activity. We conclude that D6 is a signaling receptor that exerts its regulatory function on chemokine-mediated responses in inflammation and immunity through a distinct signaling pathway.