Post-translational control of chemokines: a role for decoy receptors?

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

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

Homeostatic chemokines as putative therapeutic targets in idiopathic pulmonary fibrosis

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

Prospects for targeting ACKR1 in cancer and other diseases

The chemokine network is comprised of a family of signal proteins that encode messages for cells displaying chemokine G-protein coupled receptors (GPCRs). The diversity of effects on cellular functions, particularly directed migration of different cell types to sites of inflammation, is enabled by different combinations of chemokines activating signal transduction cascades on cells displaying a combination of receptors. These signals can contribute to autoimmune disease or be hijacked in cancer to stimulate cancer progression and metastatic migration. Thus far, three chemokine receptor-targeting drugs have been approved for clinical use: Maraviroc for HIV, Plerixafor for hematopoietic stem cell mobilization, and Mogalizumab for cutaneous T-cell lymphoma. Numerous compounds have been developed to inhibit specific chemokine GPCRs, but the complexity of the chemokine network has precluded more widespread clinical implementation, particularly as anti-neoplastic and anti-metastatic agents. Drugs that block a single signaling axis may be rendered ineffective or cause adverse reactions because each chemokine and receptor often have multiple context-specific functions. The chemokine network is tightly regulated at multiple levels, including by atypical chemokine receptors (ACKRs) that control chemokine gradients independently of G-proteins. ACKRs have numerous functions linked to chemokine immobilization, movement through and within cells, and recruitment of alternate effectors like β-arrestins. Atypical chemokine receptor 1 (ACKR1), previously known as the Duffy antigen receptor for chemokines (DARC), is a key regulator that binds chemokines involved in inflammatory responses and cancer proliferation, angiogenesis, and metastasis. Understanding more about ACKR1 in different diseases and populations may contribute to the development of therapeutic strategies targeting the chemokine network.

The DARC Side of Inflamm-Aging: Duffy Antigen Receptor for Chemokines (DARC/ACKR1) as a Potential Biomarker of Aging, Immunosenescence, and Breast Oncogenesis among High-Risk Subpopulations

The proclivity of certain pre-malignant and pre-invasive breast lesions to progress while others do not continues to perplex clinicians. Clinicians remain at a crossroads with effectively managing the high-risk patient subpopulation owing to the paucity of biomarkers that can adequately risk-stratify and inform clinical decisions that circumvent unnecessary administration of cytotoxic and invasive treatments. The immune system mounts the most important line of defense against tumorigenesis and progression. Unfortunately, this defense declines or "ages" over time-a phenomenon known as immunosenescence. This results in "inflamm-aging" or the excessive infiltration of pro-inflammatory chemokines, which alters the leukocyte composition of the tissue microenvironment, and concomitant immunoediting of these leukocytes to diminish their antitumor immune functions. Collectively, these effects can foster the sequelae of neoplastic transformation and progression. The erythrocyte cell antigen, Duffy antigen receptor for chemokines(DARC/ACKR1), binds and internalizes chemokines to maintain homeostatic levels and modulate leukocyte trafficking. A negative DARC status is highly prevalent among subpopulations of West African genetic ancestry, who are at higher risk of developing breast cancer and disease progression at a younger age. However, the role of DARC in accelerated inflamm-aging and malignant transformation remains underexplored. Herein, we review compelling evidence suggesting that DARC may be protective against inflamm-aging and, therefore, reduce the risk of a high-risk lesion progressing to malignancy. We also discuss evidence supporting that immunotherapeutic intervention-based on DARC status-among high-risk subpopulations may evade malignant transformation and progression. A closer look into this unique role of DARC could glean deeper insight into the immune response profile of individual high-risk patients and their predisposition to progress as well as guide the administration of more "cyto-friendly" immunotherapeutic intervention to potentially "turn back the clock" on inflamm-aging-mediated oncogenesis and progression.

Association between ABO and Duffy blood types and circulating chemokines and cytokines

Blood group antigens are inherited traits that may play a role in immune and inflammatory processes. We investigated associations between blood groups and circulating inflammation-related molecules in 3537 non-Hispanic white participants selected from the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial. Whole-genome scans were used to infer blood types for 12 common antigen systems based on well-characterized single-nucleotide polymorphisms. Serum levels of 96 biomarkers were measured on multiplex fluorescent bead-based panels. We estimated marker associations with blood type using weighted linear or logistic regression models adjusted for age, sex, smoking status, and principal components of population substructure. Bonferroni correction was used to control for multiple comparisons, with two-sided p values < 0.05 considered statistically significant. Among the 1152 associations tested, 10 were statistically significant. Duffy blood type was associated with levels of CXCL6/GCP2, CXCL5/ENA78, CCL11/EOTAXIN, CXCL1/GRO, CCL2/MCP1, CCL13/MCP4, and CCL17/TARC, whereas ABO blood type was associated with levels of sVEGFR2, sVEGFR3, and sGP130. Post hoc pairwise t-tests showed that individuals with type Fy(a+b−) had the lowest mean levels of all Duffy-associated markers, while individuals with type A blood had the lowest mean levels of all ABO-associated markers. Additional work is warranted to explore potential clinical implications of these differences.

Chemokine Signatures of Pathogen-Specific T Cells I: Effector T Cells

The choreography of complex immune responses, including the priming, differentiation, and modulation of specific effector T cell populations generated in the immediate wake of an acute pathogen challenge, is in part controlled by chemokines, a large family of mostly secreted molecules involved in chemotaxis and other patho/physiological processes. T cells are both responsive to various chemokine cues and a relevant source for certain chemokines themselves; yet, the actual range, regulation, and role of effector T cell-derived chemokines remains incompletely understood. In this study, using different in vivo mouse models of viral and bacterial infection as well as protective vaccination, we have defined the entire spectrum of chemokines produced by pathogen-specific CD8⁺ and CD4⁺T effector cells and delineated several unique properties pertaining to the temporospatial organization of chemokine expression patterns, synthesis and secretion kinetics, and cooperative regulation. Collectively, our results position the "T cell chemokine response" as a notably prominent, largely invariant, yet distinctive force at the forefront of pathogen-specific effector T cell activities and establish novel practical and conceptual approaches that may serve as a foundation for future investigations into the role of T cell-produced chemokines in infectious and other diseases.

Chemokine Signatures of Pathogen-Specific T Cells II: Memory T Cells in Acute and Chronic Infection

Pathogen-specific memory T cells (T_M) contribute to enhanced immune protection under conditions of reinfection, and their effective recruitment into a recall response relies, in part, on cues imparted by chemokines that coordinate their spatiotemporal positioning. An integrated perspective, however, needs to consider T_M as a potentially relevant chemokine source themselves. In this study, we employed a comprehensive transcriptional/translational profiling strategy to delineate the identities, expression patterns, and dynamic regulation of chemokines produced by murine pathogen-specific T_M CD8⁺T_M, and to a lesser extent CD4⁺T_M, are a prodigious source for six select chemokines (CCL1/3/4/5, CCL9/10, and XCL1) that collectively constitute a prominent and largely invariant signature across acute and chronic infections. Notably, constitutive CCL5 expression by CD8⁺T_M serves as a unique functional imprint of prior antigenic experience; induced CCL1 production identifies highly polyfunctional CD8⁺ and CD4⁺T_M subsets; long-term CD8⁺T_M maintenance is associated with a pronounced increase of XCL1 production capacity; chemokines dominate the earliest stages of the CD8⁺T_M recall response because of expeditious synthesis/secretion kinetics (CCL3/4/5) and low activation thresholds (CCL1/3/4/5/XCL1); and T_M chemokine profiles modulated by persisting viral Ags exhibit both discrete functional deficits and a notable surplus. Nevertheless, recall responses and partial virus control in chronic infection appear little affected by the absence of major T_M chemokines. Although specific contributions of T_M-derived chemokines to enhanced immune protection therefore remain to be elucidated in other experimental scenarios, the ready visualization of T_M chemokine-expression patterns permits a detailed stratification of T_M functionalities that may be correlated with differentiation status, protective capacities, and potential fates.

Comparative Transcriptomic Analysis of Rectal Tissue from Beef Steers Revealed Reduced Host Immunity in Escherichia coli O157:H7 Super-Shedders

Super-shedder cattle are a major disseminator of E. coli O157:H7 into the environment, and the terminal rectum has been proposed as the primary E. coli O157:H7 colonization site. This study aimed to identify host factors that are associated with the super-shedding process by comparing transcriptomic profiles in rectal tissue collected from 5 super-shedder cattle and 4 non-shedder cattle using RNA-Seq. In total, 17,859 ± 354 genes and 399 ± 16 miRNAs were detected, and 11,773 genes were expressed in all animals. Fifty-eight differentially expressed (DE) genes (false discovery rate < 0.05) including 11 up-regulated and 47 down-regulated (log 2 (fold change) ranged from -5.5 to 4.2), and 2 up-regulated DE miRNAs (log 2 (fold change) = 2.1 and 2.5, respectively) were identified in super-shedders compared to non-shedders. Functional analysis of DE genes revealed that 31 down-regulated genes were potentially associated with reduced innate and adaptive immune functions in super-shedders, including 13 lymphocytes membrane receptors, 3 transcription factors and 5 cytokines, suggesting the decreased key host immune functions in the rectal tissue of super-shedders, including decreased quantity and migration of immune cells such as lymphocytes, neutrophils and dendritic cells. The up-regulation of bta-miR-29d-3p and the down regulation of its predicted target gene, regulator of G-protein signaling 13, suggested a potential regulatory role of this miRNA in decreased migration of lymphocytes in super-shedders. Based on these findings, the rectal tissue of super-shedders may inherently exhibit less effective innate and adaptive immune protection. Further study is required to confirm if such effect on host immunity is due to the nature of the host itself or due to actions mediated by E. coli O157:H7.

A combination of in vitro techniques for efficient discovery of functional monoclonal antibodies against human CXC chemokine receptor-2 (CXCR2)

BACKGROUND

Development of functional monoclonal antibodies against intractable GPCR targets.

RESULTS

Identification of structured peptides mimicking the ligand binding site, their use in panning to enrich for a population of binders, and the subsequent challenge of this population with receptor overexpressing cells leads to functional monoclonal antibodies.

CONCLUSION

The combination of techniques provides a successful strategic approach for the development of functional monoclonal antibodies against CXCR2 in a relatively small campaign.

SIGNIFICANCE

The presented combination of techniques might be applicable for other, notoriously difficult, GPCR targets.

SUMMARY

The CXC chemokine receptor-2 (CXCR2) is a member of the large 'family A' of G-protein-coupled-receptors and is overexpressed in various types of cancer cells. CXCR2 is activated by binding of a number of ligands, including interleukin 8 (IL-8) and growth-related protein α (Gro-α). Monoclonal antibodies capable of blocking the ligand-receptor interaction are therefore of therapeutic interest; however, the development of biological active antibodies against highly structured GPCR proteins is challenging. Here we present a combination of techniques that improve the discovery of functional monoclonal antibodies against the native CXCR2 receptor. The IL-8 binding site of CXCR2 was identified by screening peptide libraries with the IL-8 ligand, and then reconstructed as soluble synthetic peptides. These peptides were used as antigens to probe an antibody fragment phage display library to obtain subpopulations binding to the IL-8 binding site of CXCR2. Further enrichment of the phage population was achieved by an additional selection round with CXCR2 overexpressing cells as a different antigen source. The scFvs from the CXCR2 specific phage clones were sequenced and converted into monoclonal antibodies. The obtained antibodies bound specifically to CXCR2 expressing cells and inhibited the IL-8 and Gro-α induced ß-arrestin recruitment with IC50 values of 0.3 and 0.2 nM, respectively, and were significantly more potent than the murine monoclonal antibodies (18 and 19 nM, respectively) obtained by the classical hybridoma technique, elicited with the same peptide antigen. According to epitope mapping studies, the antibody efficacy is largely defined by N-terminal epitopes comprising the IL-8 and Gro-α binding sites. The presented strategic combination of in vitro techniques, including the use of different antigen sources, is a powerful alternative for the development of functional monoclonal antibodies by the classical hybridoma technique, and might be applicable to other GPCR targets.

Chemokines and atherosclerosis: focus on the CX3CL1/CX3CR1 pathway

Atherosclerosis is currently considered an inflammatory disease. Much attention has been focused on the potential role of inflammatory mediators as prognostic/diagnostic markers or therapeutic targets of atherosclerotic cardiovascular disease. CX3CL1 (or fractalkine) is a structurally and functionally unique chemokine with a well documented role in atherosclerosis. In its membrane bound form it promotes the firm adhesion of rolling leucocytes onto the vessel wall, while in its soluble form it serves as a potent chemoattractant for CX3CR1-expressing cells. Additionally, CX3CL1 exerts cytotoxic effects on the endothelium as well as anti-apoptotic and proliferative effects on vascular cells, affecting the context and stability of the atherosclerotic plaque. Studies on animal models have shown that the blockade of the CX3CL1/CX3CR1 pathway ameliorates the severity of atherosclerosis, while genetic epidemiology has confirmed that a genetically-defined less active CX3CL1/CX3CR1 pathway is associated with a reduced risk of atherosclerotic disease in humans. Although several studies support an important pathogenic role of CX3CL1/CX3CR1 in atherogenesis and plaque destabilization, this does not necessarily suggest that this pathway is a suitable therapeutic target or that CX3CL1 can serve as a prognostic/diagnostic biomarker. Further studies on the CX3CL1/CX3CR1 chemokine pathway are clearly warranted to justify the clinical relevance of its role in atherosclerosis.

Neutralizing nanobodies targeting diverse chemokines effectively inhibit chemokine function

Chemokine receptors and their ligands play a prominent role in immune regulation but many have also been implicated in inflammatory diseases such as multiple sclerosis, rheumatoid arthritis, allograft rejection after transplantation, and also in cancer metastasis. Most approaches to therapeutically target the chemokine system involve targeting of chemokine receptors with low molecular weight antagonists. Here we describe the selection and characterization of an unprecedented large and diverse panel of neutralizing Nanobodies (single domain camelid antibodies fragment) directed against several chemokines. We show that the Nanobodies directed against CCL2 (MCP-1), CCL5 (RANTES), CXCL11 (I-TAC), and CXCL12 (SDF-1α) bind the chemokines with high affinity (at nanomolar concentration), thereby blocking receptor binding, inhibiting chemokine-induced receptor activation as well as chemotaxis. Together, we show that neutralizing Nanobodies can be selected efficiently for effective and specific therapeutic treatment against a wide range of immune and inflammatory diseases.

Inhibition of CXCR3-mediated chemotaxis by the human chemokine receptor-like protein CCX-CKR

BACKGROUND AND PURPOSE

Induction of cellular migration is the primary effect of chemokine receptor activation. However, several chemokine receptor-like proteins bind chemokines without subsequent induction of intracellular signalling and chemotaxis. It has been suggested that they act as chemokine scavengers, which may control local chemokine levels and contribute to the function of chemokines during inflammation. This has been verified for the chemokine-like receptor proteins D6 and DARC as well as CCX-CKR. Here, we provide evidence for an additional biological function of human (h)CCX-CKR.

EXPERIMENTAL APPROACH

We used transfection strategies in HEK293 and human T cells.

KEY RESULTS

Co-expression of hCCX-CKR completely inhibits hCXCR3-induced chemotaxis. We found that hCCX-CKR forms complexes with hCXCR3, suggesting a relationship between CCX-CKR heteromerization and inhibition of chemotaxis. Moreover, negative binding cooperativity induced by ligands both for hCXCR3 and hCCX-CKR was observed in cells expressing both receptors. This negative cooperativity may also explain the hCCX-CKR-induced inhibition of chemotaxis.

CONCLUSIONS AND IMPLICATIONS

These findings suggest that hCCX-CKR prevents hCXCR3-induced chemotaxis by heteromerization thus representing a novel mechanism of regulation of immune cell migration.

Function, diversity and therapeutic potential of the N-terminal domain of human chemokine receptors

Chemokines and their receptors play fundamental roles in many physiological and pathological processes such as leukocyte trafficking, inflammation, cancer and HIV-1 infection. Chemokine-receptor interactions are particularly intricate and therefore require precise orchestration. The flexible N-terminal domain of human chemokine receptors has regularly been demonstrated to hold a crucial role in the initial recognition and selective binding of the receptor ligands. The length and the amino acid sequences of the N-termini vary considerably among different receptors but they all show a high content of negatively charged residues and are subject to post-translational modifications such as O-sulfation and N- or O-glycosylation. In addition, a conserved cysteine that is most likely engaged in a receptor-stabilizing disulfide bond delimits two functionally distinct parts in the N-terminus, characterized by specific molecular signatures. Structural analyses have shown that the N-terminus of chemokine receptors recognizes a groove on the chemokine surface and that this interaction is stabilized by high-affinity binding to a conserved sulfotyrosine-binding pocket. Altogether, these data provide new insights on the chemokine-receptor molecular interplay and identify the receptor N-terminus-binding site as a new target for the development of therapeutic molecules. This review presents and discusses the diversity and function of human chemokine receptor N-terminal domains and provides a comprehensive annotated inventory of their sequences, laying special emphasis on the presence of post-translational modifications and functional features. Finally, it identifies new molecular signatures and proposes a computational model for the positioning and the conformation of the CXCR4 N-terminus grafted on the first chemokine receptor X-ray structure.

CXCL10: a candidate biomarker in transplantation

Interferon (IFN) γ-induced protein 10 kDa (IP-10) or C-X-C motif chemokine 10 (CXCL10) is a small cytokine belonging to the CXC chemokine family. This family of signaling molecules is known to control several biological functions and to also play pivotal roles in disease initiation and progression. By binding to its specific cognate receptor CXCR3, CXCL10 critically regulates chemotaxis during several immune-inflammatory processes. In particular, this chemokine controls chemotaxis during the inflammatory response resulting from allograft rejection after transplantation. Interestingly, a strong association has been described between CXCL10 production, immune response and the fate of the graft following allotransplantation. Enhanced CXCL10 production has been observed in recipients of transplants of different organs. This enhanced production likely comes from either the graft or the immune cells and is correlated with an increase in the concentration of circulating CXCL10. Because CXCL10 can be easily measured in the serum and plasma from a patient, the detection and quantitation of circulating CXCL10 could be used to reveal a transplant recipient's immune status. The purpose of this review is to examine the critical role of CXCL10 in the pathogenesis of allograft rejection following organ transplantation. This important role highlights the potential utilization of CXCL10 not only as a therapeutic target but also as a biomarker to predict the severity of rejection, to monitor the inflammatory status of organ recipients and, hopefully, to fine-tune patient therapy in transplantation.

Erythrocyte Duffy antigen receptor for chemokines (DARC): diagnostic and therapeutic implications in atherosclerotic cardiovascular disease

Atherosclerosis is an inflammatory disease. The last three decades efforts have been made to elucidate the biochemical pathways that are implicated in the process of atherogenesis and plaque development. Chemokines are crucial mediators in every step of this process. Additionally, cellular components of the peripheral blood have been proved important mediators in the formation and progression of atherosclerotic lesions. However, until recently data were mostly focusing on leukocytes and platelets. Erythrocytes were considered unreceptive bystanders and limited data supported their importance in the progression and destabilization of the atherosclerotic plaque. Recently erythrocytes, through their Duffy antigen receptor for chemokines (DARC), have been proposed as appealing regulators of chemokine-induced pathways. Dissimilar to every other chemokine receptor DARC possesses high affinity for several ligands from both CC and CXC chemokine sub-families. Moreover, DARC is not coupled to a G-protein or any other intracellular signalling system; thus it is incapable of generating second messages. The exact biochemical role of erythrocyte DARC remains to be determined. It is however challenging the fact that DARC is a regulator of almost every CC and CXC chemokine ligand and therefore DARC antagonism could effectively block the complex pre-inflammatory chemokine network. In the present review we intent to provide recent evidence supporting the role of erythrocytes in atherosclerosis focusing on the erythrocyte-chemokine interaction through the Duffy antigen system.

Finding their niche: chemokines directing cell migration in the thymus

T lymphocytes are generated throughout life, arising from bone marrow-derived progenitors that complete an essential developmental process in the thymus. Thymic T cell education leads to the generation of a self-restricted and largely self-tolerant peripheral T-cell pool and is facilitated by interactions with thymic stromal cells residing in distinct supportive niches. The signals governing thymocyte precursor migration into the thymus, directing thymocyte navigation through thymic microenvironments and mature T-cell egress into circulation were, until recently, largely unknown, but presumed to be mediated to a large extent by chemokine signalling. Recent studies have now uncovered various specific functions for members of the chemokine superfamily in the thymus. These studies have not only revealed distinct but also in some cases overlapping roles for several chemokine family members in various thymocyte migration events and have also shown that homing and positioning of other cells in the thymus, such as dendritic cells and natural killer T cells is also chemokine-dependent. Here, we discuss current understanding of the role of chemokines in the thymus and highlight key future avenues for investigation in this field.

The atypical chemokine receptor CCX-CKR scavenges homeostatic chemokines in circulation and tissues and suppresses Th17 responses

Our previous in vitro studies led to proposals that the atypical chemokine receptor CCX-CKR is a scavenger of CCR7 ligand homeostatic chemokines. In the present study, we generated CCX-CKR−/− mice and confirm this scavenger function in vivo. Compared with wild-type mice, CCX-CKR−/− have a 5-fold increase in the level of CCL21 protein in blood, and 2- to 3-fold increases in CCL19 and CCL21 in peripheral lymph nodes. The effect of these protein increases on immunity was investigated after immunization with MOG35-55 peptide emulsified in complete Freund adjuvant (CFA). The subsequent characteristic paralysis develops with enhanced kinetics and severity in CCX-CKR−/− versus wild-type mice. Despite this effect, antigen-specific immune responses in the draining lymph nodes are diminished in CCX-CKR−/− mice. Instead, the earlier onset of disease is associated with enhanced T-cell priming in the CCX-CKR−/− spleen and a skewing of CD4+ T-cell responses toward Th17 rather than Th1. This observation correlates with increased expression of IL-23 in the CCX-CKR−/− spleen and increased CCL21 levels in the central nervous system postimmunization. The early onset of disease in CCX-CKR−/− mice is reversed by systemic administration of neutralizing anti-CCL21 antibodies. Thus, by regulating homeostatic chemokine bioavailability, CCX-CKR influences the localization, kinetics, and nature of adaptive immune responses in vivo.

A recombinant dromedary antibody fragment (VHH or nanobody) directed against human Duffy antigen receptor for chemokines

Fy blood group antigens are carried by the Duffy antigen receptor for chemokines (DARC), a red cells receptor for Plasmodium vivax broadly implicated in human health and diseases. Recombinant VHHs, or nanobodies, the smallest intact antigen binding fragment derivative from the heavy chain-only antibodies present in camelids, were prepared from a dromedary immunized against DARC N-terminal extracellular domain and selected for DARC binding. A described VHH, CA52, does recognize native DARC on cells. It inhibits P. vivax invasion of erythrocytes and displaces interleukin-8 bound to DARC. The targeted epitope overlaps the well-defined DARC Fy6 epitope. K (D) of CA52-DARC equilibrium is sub-nanomolar, hence ideal to develop diagnostic or therapeutic compounds. Immunocapture by immobilized CA52 yielded highly purified DARC from engineered K562 cells. This first report on a VHH with specificity for a red blood cell protein exemplifies VHHs' potentialities to target, to purify, and to modulate the function of cellular markers.

CCL21-induced calcium transients and proliferation in primary mouse astrocytes: CXCR3-dependent and independent responses

CCL21 is a homeostatic chemokine that is expressed constitutively in secondary lymph nodes and attracts immune cells via chemokine receptor CCR7. In the brain however, CCL21 is inducibly expressed in damaged neurons both in vitro and in vivo and has been shown to activate microglia in vitro, albeit not through CCR7 but through chemokine receptor CXCR3. Therefore, a role for CCL21 in CXCR3-mediated neuron-microglia signaling has been proposed. It is well established that human and mouse astrocytes, like microglia, express CXCR3. However, effects of CCL21 on astrocytes have not been investigated yet. In this study, we have examined the effects of CCL21 on calcium transients and proliferation in primary mouse astrocytes. We show that similar to CXCR3-ligand CXCL10, CCL21 (10(-9) M and 10(-8) M) induced calcium transients in astrocytes, which were mediated through CXCR3. However, in response to high concentrations of CCL21 (10(-7) M) calcium transients persisted in CXCR3-deficient astrocytes, whereas CXCL10 did not have any effect in these cells. Furthermore, prolonged exposure to CXCL10 or CCL21 promoted proliferation of wild type astrocytes. Although CXCL10-induced proliferation was absent in CXCR3-deficient astrocytes, CCL21-induced proliferation of these cells did not significantly differ from wild type conditions. It is therefore suggested that primary mouse astrocytes express an additional (chemokine-) receptor, which is activated at high CCL21 concentrations.

Cardiac allograft rejection: examination of the expression and function of the decoy chemokine receptor D6

BACKGROUND

Inflammatory cell recruitment during allograft rejection is driven by a group of inflammatory cytokines termed chemokines. Chemokines are presented on the surface of the vascular endothelium where they ligate specific receptors expressed on the surface of leukocytes. Recently, a group of nonsignaling chemokine receptors have been described. These bind and internalize chemokines but do not drive leukocyte migration. It is believed that these compete with classical signaling receptors to modulate inflammation.

METHODS

This study describes the first examination of the human decoy chemokine receptor D6 during rejection; D6 binds at least 12 potent proinflammatory chemokines. The expression of D6 by graft infiltrating leukocytes was examined in cardiac allografts by confocal microscopy on biopsy sections (n=19). Cytokine regulation of D6 was examined in vitro, and a chemokine scavenging assay was performed using the prototypical transplant-associated chemokine CCL5/RANTES.

RESULTS

D6 expression was found to be higher in the biopsies taken from more severe cardiac allograft rejection (P<0.01) and was predominantly localized to graft infiltrating CD45(+)CD68(+) leukocytes. In vitro studies demonstrated that the transforming growth factor-beta strongly increased the expression of D6 by monocytes, which significantly enhanced D6-mediated chemokine scavenging (by 85%, P<0.05).

CONCLUSIONS

We present the first examination of the biology of D6 during rejection and identify a transplant-associated cytokine that is able to regulate its expression. These data suggest an exciting new mechanism for the antiinflammatory actions of transforming growth factor-beta. Understanding the expression patterns of D6 may provide important insight into the regulation and control of inflammatory cell recruitment during allograft rejection.

Optimized procedures for producing biologically active chemokines

We describe here two strategies to produce biologically active chemokines with authentic N-terminal amino acid residues. The first involves producing the target chemokine with an N-terminal 6xHis-SUMO tag in Escherichia coli as inclusion bodies. The fusion protein is solubilized and purified with Ni-NTA-agarose in denaturing reagents. This is further followed by tag removal and refolding in a redox refolding buffer. The second approach involves expressing the target chemokine with an N-terminal 6xHis-Trx-SUMO tag in an engineered E. coli strain that facilitates formation of disulfide bonds in the cytoplasm. Following purification of the fusion protein via Ni-NTA and tag removal, the target chemokine is refolded without redox buffer and purified by reverse phase chromatography. Using the procedures, we have produced more than 15 biologically active chemokines, with a yield of up to 15 mg/L.

Anti-inflammatory therapy by intravenous delivery of non-heparan sulfate-binding CXCL12

Interaction between chemokines and heparan sulfate (HS) is essential for leukocyte recruitment during inflammation. Previous studies have shown that a non-HS-binding mutant form of the inflammatory chemokine CCL7 can block inflammation produced by wild-type chemokines. This study examined the anti-inflammatory mechanism of a non-HS-binding mutant of the homeostatic chemokine CXCL12. Initial experiments demonstrated that mutant CXCL12 was an effective CXCR4 agonist. However, this mutant chemokine failed to promote transendothelial migration in vitro and inhibited the haptotactic response to wild-type CCL7, CXCL12, and CXCL8, and naturally occurring chemoattractants in synovial fluid from the rheumatoid synovium, including CCL2, CCL7, and CXCL8. Notably, intravenous administration of mutant CXCL12 also inhibited the recruitment of leukocytes to murine air pouches filled with wild-type CXCL12. Following intravenous administration, wild-type CXCL12 was cleared from the circulation rapidly, while the mutant chemokine persisted for >24 h. Chronic exposure to mutant CXCL12 in the circulation reduced leukocyte-surface expression of CXCR4, reduced the chemotactic response of these cells to CXCL12, and inhibited normal chemokine-mediated induction of adhesion between the alpha4beta1 integrin, VLA-4, and VCAM-1. These data demonstrate that systemic administration of non-HS-binding variants of CXCL12 can mediate a powerful anti-inflammatory effect through chemokine receptor desensitization.

Soluble TLR2 reduces inflammation without compromising bacterial clearance by disrupting TLR2 triggering

TLR overactivation may lead to end organ damage and serious acute and chronic inflammatory conditions. TLR responses must therefore be tightly regulated to control disease outcomes. We show in this study the ability of the soluble form of TLR2 (sTLR2) to regulate proinflammatory responses, and demonstrate the mechanisms underlying sTLR2 regulatory capacity. Cells overexpressing sTLR2, or stimulated in the presence of the sTLR2 protein, are hyporesponsive to TLR2 ligands. Regulation was TLR2 specific, and affected NF-kappaB activation, phagocytosis, and superoxide production. Natural sTLR2-depleted serum rendered leukocytes hypersensitive to TLR2-mediated stimulation. Mice administered sTLR2 together with Gram-positive bacteria-derived components showed lower peritoneal levels of the neutrophil (PMN) chemoattractant, keratinocyte-derived chemokine; lower PMN numbers; and a reduction in late apoptotic PMN. Mononuclear cell recruitment remained unaffected, and endogenous peritoneal sTLR2 levels increased. Notably, the capacity of sTLR2 to modulate acute inflammatory parameters did not compromise the ability of mice to clear live Gram-positive bacteria-induced infection. Mechanistically, sTLR2 interfered with TLR2 mobilization to lipid rafts for signaling, acted as a decoy microbial receptor, and disrupted the interaction of TLR2 with its coreceptor, CD14, by associating with CD14. These findings establish sTLR2 as a regulator of TLR2-mediated inflammatory responses, capable of blunting immune responses without abrogating microbial recognition and may inform the design of novel therapeutics against acute and chronic inflammatory conditions.

Expression of chemokines and their receptors by human brain endothelium: implications for multiple sclerosis

Leukocyte migration into the central nervous system (CNS) is mediated by chemokines expressed on CNS endothelial cell surfaces. This study investigated the production of chemokines and expression of chemokine receptors by human brain endothelial cells (HBECs) in vitro and in situ. Four chemokines (CCL2, CCL5, CXCL8, and CXCL10) were demonstrated by immunohistochemistry in endothelial cells in brain samples from patients with multiple sclerosis. CXCL8 and CCL2 were constitutively released and increased by primary HBECs and the brain endothelial cell line hCEMC/D3 in response to tumor necrosis factor and/or interferon gamma. CXCL10 and CCL5 were undetectable in resting endothelial cells but were secreted in response to these proinflammatory cytokines. Tumor necrosis factor strongly increased the production of CCL2, CCL5, and CXCL8; interferon gamma upregulated CXCL10 exclusively. CCL3 was not secreted by HBECs and seemed to be confined to astrocytes in situ. The chemokine receptors CXCR1 and CXCR3 were expressed by HBECs both in vitro and in situ; CXCR3 was upregulated in response to cytokine stimulation in vitro. In contrast, CXCR3 expression was reduced in noninflammatory (silent) multiple sclerosis lesions. The particularly high levels of CXCL10 and CXCL8 expressed by brain endothelium may contribute to the predominant TH1-type inflammatory response observed in chronic inflammatory conditions such as multiple sclerosis.

Early immune response following Salmonella enterica subspecies enterica serovar Typhimurium infection in porcine jejunal gut loops

Salmonella enterica subspecies enterica serovar Typhimurium, commonly called S. Typhimurium, can cause intestinal infections in humans and various animal species such as swine. To analyze the host response to Salmonella infection in the pig we used an in vivo gut loop model, which allows the analysis of multiple immune responses within the same animal. Four jejunal gut-loops were each inoculated with 3×10⁸ cfu of S. Typhimurium in 3 one-month-old piglets and mRNA expressions of various cytokines, chemokines, transcription factors, antimicrobial peptides, toll like and chemokine receptors were assessed by quantitative real-time PCR in the Peyer’s patch and the gut wall after 24 h. Several genes such as the newly cloned CCRL1/CCX-CKR were assessed for the first time in the pig at the mRNA level. Pro-inflammatory and T-helper type-1 (Th1) cytokine mRNA were expressed at higher levels in infected compared to non-infected control loops. Similarly, some B cell activation genes, NOD2 and toll like receptor 2 and 4 transcripts were more expressed in both tissues while TLR5 mRNA was down-regulated. Interestingly, CCL25 mRNA expression as well as the mRNA expressions of its receptors CCR9 and CCRL1 were decreased both in the Peyer’s patch and gut wall suggesting a potential Salmonella strategy to reduce lymphocyte homing to the intestine. In conclusion, these results provide insight into the porcine innate mucosal immune response to infection with entero-invasive microorganisms such as S. Typhimurium. In the future, this knowledge should help in the development of improved prophylactic and therapeutic approaches against porcine intestinal S. Typhimurium infections.

Genetic variations of the chemokine scavenger receptor D6 are associated with liver inflammation in chronic hepatitis C

Chronic hepatitis C (HCV) represents one of the most common chronic infections worldwide and is a major indication for liver transplantation. Liver inflammation is the main predictor of advanced fibrosis in HCV. Inflammatory cells are recruited to the liver by chemokines. Recently, a novel class of chemokine receptors has been characterized that lack signaling functions and are termed scavenger receptors. We determine here whether genetic variations of the scavenger receptor D6 contribute to the grade of liver inflammation in HCV. Four haplotype tagging single nucleotide polymorphisms (SNPs) were identified from HapMap that cover the genetic information of D6 (CCBP2). Among these SNPs, rs4683336 was associated with liver inflammation in qualitative (p = 0.003) and quantitative (p = 0.0086) genotype analysis. This association was confirmed in an independent cohort of HCV-infected patients (p = 0.006 for qualitative and p = 0.0046 for quantitative analysis, respectively). Furthermore, the haplotype that is tagged by marker rs4683336 was significantly correlated with liver inflammation when compared with the most common D6 haplotype (p = 0.014). The importance of genetic variations in D6 was supported through the demonstration of an association of D6 mRNA expression with histologic inflammation in liver biopsies and a considerable range of D6 mRNA expression in isolated human hepatocytes. In conclusion, we demonstrate that variations in a chemokine scavenging receptor are significantly correlated with clinical inflammatory phenotypes such as HCV infection.

Duffy blood group system genotyping in an urban Tunisian population

BACKGROUND

The Duffy blood group system, besides its relevance in transfusion medicine, is of major interest for population genetics. In fact, the Duffy molecule is the only red cell receptor for Plasmodium vivax, thus the fixation of FY*silent allele in western south-Saharan Africa resulted in the absence of this type of malaria in that area (for a review see Kwiatowski, Am J Hum Genet 77:171-192, 2005). For the Duffy functional role see, for example, Daniels (Vox Sanguinis 93:331-340, 2007).

METHODS

Duffy blood group distribution in 115 unrelated Tunisians was determined using the polymerase chain reaction with sequence specific primer (PCR-SSP) method detecting the five allelic versions of the FY gene. The red cell antigenic FY phenotype, for each donor, was deduced through DNA analysis. The blood samples of the positive FY*X alleles were investigated by serological methods, mainly the fixation-elution technique.

RESULTS

The following allele frequencies were found (after having excluded FY*X, which had frequency of 0.0174): FY*1 = 0.291 (expressed 0.260; silent 0.031); FY*2 = 0.709 (expressed 0.427; silent 0.282). The most surprising result in this work is the detection of the FY*1 silent allele, usually quite rare, in four samples (1.74%). For FY*2 silent, the predominant allele in Africans, genotyping results showed a prevalence of 29.57%. The FY locus was in Hardy-Weinberg equilibrium in the present sample.

CONCLUSION

When compared with European and African data, Tunisian samples demonstrated the presence of the common signs of these two ancestries (FY*2 and FY*X for the first population; and FY*2 silent for the last one). These data confirm the mixed roots of this urban Tunisian population already suggested by numerous studies on other haematological markers.

Duffy antigen receptor for chemokines mediates trans-infection of HIV-1 from red blood cells to target cells and affects HIV-AIDS susceptibility

Duffy antigen receptor for chemokines (DARC) expressed on red blood cells (RBCs) influences plasma levels of HIV-1-suppressive and proinflammatory chemokines such as CCL5/RANTES. DARC is also the RBC receptor for Plasmodium vivax. Africans with DARC -46C/C genotype, which confers a DARC-negative phenotype, are resistant to vivax malaria. Here, we show that HIV-1 attaches to RBCs via DARC, effecting trans-infection of target cells. In African Americans, DARC -46C/C is associated with 40% increase in the odds of acquiring HIV-1. If extrapolated to Africans, approximately 11% of the HIV-1 burden in Africa may be linked to this genotype. After infection occurs, however, DARC-negative RBC status is associated with slower disease progression. Furthermore, the disease-accelerating effect of a previously described CCL5 polymorphism is evident only in DARC-expressing and not in DARC-negative HIV-infected individuals. Thus, DARC influences HIV/AIDS susceptibility by mediating trans-infection of HIV-1 and by affecting both chemokine-HIV interactions and chemokine-driven inflammation.

Suppression of CCL2/MCP-1 and CCL5/RANTES expression by nociceptin in human monocytes

The receptor designated Opioid Receptor-Like 1 (ORL1) is abundantly expressed in the central nervous system (CNS) as well as by cells of the immune system. While much is known about the function of ORL1 in the CNS, there is little information in the literature about the role of ORL1 in the immune response. There have been numerous reports documenting the effects of GPCR activation on the expression of chemokines crucial in mediating inflammatory events in biological systems. The aim of the present work was to examine the effect of nociceptin administration on the pro-inflammatory chemokine expression of human monocytes. We report here that human CD14(+) monocytes expresses the mRNA for ORL1. Our results also demonstrate that nociceptin can suppress the production of CCL2/MCP-1 and CCL5/RANTES chemokine protein in both primary CD14(+) human monocytes and monocyte-like cell lines. However, nociceptin does not appear to regulate the expression of these chemokines at the level of transcription, as CCL2/MCP-1 and CCL5/RANTES mRNA levels following nociceptin treatment of monocytes were essentially normal. Although the mechanism of chemokine regulation by nociceptin is as yet unknown, it is evident that the ORL1/nociceptin system plays a role in regulating chemotactic responses of leukocytes through chemokine suppression. Finally, these data may provide the initial basis for the development of ORL1 agonists and antagonists for therapeutic treatment of inflammatory disease.

A review of molecular contrasts between arresting and viable porcine attachment sites

Significant spontaneous fetal loss of unknown cause occurs in North American commercial swine. About 30% of conceptuses, thought to be genetically normal, are lost during the peri-attachment period. An additional 20% are lost at mid-pregnancy. Littermate endometrial and trophoblast biopsies were studied by quantitative real-time PCR for gene expression, and immunohistochemistry for protein expression at gestation day (gd)15-23 and 50. RNA analyses were also conducted on endometrial lymphocytes and arterial endothelial cells removed from biopsies by laser capture microdissection. Genes were selected for study from human literature and cloned as required. As in humans, angiogenic, cytokine, chemokine and chemokine decoy receptor gene expression occurs at the porcine maternal-fetal interface. In each tissue studied, distinct patterns of expression are found between early and mid-pregnancy, as well as between viable and arresting conceptus attachment sites. These changes involve both endometrial lymphocytes and dendritic cells. Restriction in endometrial angiogenesis, reduction in expression of the chemokine decoy receptor D6, and reduction in dendritic cell numbers contribute to fetal arrest. In peri-attachment loss, interferon-gamma is more abundantly transcribed than tumor necrosis factor-alpha, but this ratio is reversed during midgestation failure. Further characterization of spontaneous fetal loss in pigs will identify targets for modification by hog producers and may provide a model for identification of antecedents to fetal loss in humans.

Chemokines and cancer: migration, intracellular signalling and intercellular communication in the microenvironment

Inappropriate chemokine/receptor expression or regulation is linked to many diseases, especially those characterized by an excessive cellular infiltrate, such as rheumatoid arthritis and other inflammatory disorders. There is now overwhelming evidence that chemokines are also involved in the progression of cancer, where they function in several capacities. First, specific chemokine-receptor pairs are involved in tumour metastasis. This is not surprising, in view of their role as chemoattractants in cell migration. Secondly, chemokines help to shape the tumour microenvironment, often in favour of tumour growth and metastasis, by recruitment of leucocytes and activation of pro-inflammatory mediators. Emerging evidence suggests that chemokine receptor signalling also contributes to survival and proliferation, which may be particularly important for metastasized cells to adapt to foreign environments. However, there is considerable diversity and complexity in the chemokine network, both at the chemokine/receptor level and in the downstream signalling pathways they couple into, which may be key to a better understanding of how and why particular chemokines contribute to cancer growth and metastasis. Further investigation into these areas may identify targets that, if inhibited, could render cancer cells more susceptible to chemotherapy.

Chemokine decoy receptors: new players in reproductive immunology

Chemokines are multifunctional molecules with roles in leukocyte trafficking and developmental processes. Both fetal and maternal components of the placenta produce chemokines, which control leukocyte trafficking observed in the placenta. Thus, chemokines play roles in the balance between protection of the developing embryo/fetus and tolerance of its hemiallogeneic tissues. Recently, a group of chemokine receptors, which include D6, DARC, and CCX-CKR, have been described as "silent" receptors by virtue of their inability to activate signal transduction events leading to cell chemoattraction. Here we review in vitro and in vivo evidence indicating that chemokine "silent" receptors regulate innate and adaptive immunity behaving as decoy receptors that support internalization and degradation of chemotactic factors, and discuss available information on their potential role in reproductive immunology.

The chemokine network. I. How the genomic organization of chemokines contains clues for deciphering their functional complexity

Chemokines are a superfamily of small structurally related cytokines that have evolved to form a complex network of proteins that typically regulate leucocyte traffic but also carry very diverse sets of immune and non-immune functions. Two general features of cytokines, redundancy and promiscuity, are particularly prominent in chemokines. In part, these properties result from repeated processes of gene duplication and diversification, which has led to the present complex genomic map of chemokines, which contains cases of non-allelic isoforms, copy number polymorphisms and classical allelic variation. This genomic complexity is compounded with pre-translational and post-translational mechanisms resulting in a complex network of proteins whose essential functions are maintained, constituting a remarkable case of robustness reminiscent of crucial metabolic pathways. This reflects the adaptation of a system under strong evolutive pressure, supporting the concept that the chemokine system is essential for the coordination, regulation and fine-tuning of the type of immune response. In this first review, we analyse currently available data on the chemokine superfamily, focusing on its complex genomic organization. Genes encoding essential inflammatory chemokines are grouped into defined chromosomal locations as clusters and miniclusters that, from the genetic point of view, can be considered single entities given their overall functions (many ligands of a cluster bind to a few shared receptors). We will try to interpret this genomic organization of chemokines in relation to the main functions acquired by each individual member or by each cluster. In a second review, we shall focus on the relationship of chemokine variability and disease susceptibility.

Professional and part-time chemokine decoys in the resolution of inflammation

Inflammation is essential for protection from infection and for the repair of damaged tissue. Much is now known about how inflammation is induced and maintained, but the processes underlying the resolution of inflammation are often overlooked. However, resolution is an essential component of all successful inflammatory responses because it ensures the restoration of tissue homeostasis and prevents immunopathology of the type seen in chronic inflammatory diseases and autoimmunity. Small secreted proteins called chemokines, acting through chemokine receptors, are known to be critical regulators of leukocyte recruitment and function during the initiation and maintenance of inflammation. Thus, their efficient removal would seem to be a prerequisite for successful resolution. In recent years, it has emerged that specialized chemokine "decoy" receptors exist that actively participate in this process. Moreover, other chemokine receptors have been proposed to lead a double life and perform opposing roles during inflammation: leukocyte recruitment (by signaling) and resolution (by chemokine sequestration). A recent study provides further support for this theory by showing that apoptotic inflammatory leukocytes increase the number of surface chemokine receptors and that these receptors can remove chemokines from inflamed tissue. Leukocyte apoptosis is already known to aid resolution, not just because it eliminates leukocytes from inflamed tissues, but also because their consumption by macrophages leads to the production of anti-inflammatory cytokines. The new work indicates that chemokine sequestration may be another mechanism exploited by dying cells to assist in the resolution of inflammation.

Proteolytic processing of CXCL11 by CD13/aminopeptidase N impairs CXCR3 and CXCR7 binding and signaling and reduces lymphocyte and endothelial cell migration

CXCR3 ligands were secreted by tissue fibroblasts and peripheral blood-derived mononuclear leukocytes in response to interferon-gamma (IFN-gamma) and Toll-like receptor (TLR) ligands. Subsequent purification and identification revealed the presence of truncated CXCL11 variants missing up to 6 amino acids. In combination with CD26/dipeptidyl peptidase IV, the metalloprotease aminopeptidase N (APN), identical to the myeloid cell marker CD13, rapidly processed CXCL11, but not CXCL8, to generate truncated CXCL11 forms. Truncated CXCL11 had reduced binding, signaling, and chemotactic properties for lymphocytes and CXCR3- or CXCR7-transfected cells. CD13/APN-truncated CXCL11 failed to induce an intracellular calcium increase but was still able to bind and desensitize CXCR3 for intact CXCL11 signaling. CXCL11 efficiently bound to CXCR7, but CXCL11 was not able to induce calcium signaling or ERK1/2 or Akt phosphorylation through CXCR7. CD26-truncated CXCL11 failed to attract lymphocytes but still inhibited microvascular endothelial cell (HMVEC) migration. However, further processing of CXCL11 by CD13 resulted in significant reduction of inhibition of HMVEC migration. Taken together, during inflammation or cancer, CXCL11 processing by CD13 may lead to a reduced number of tumor-infiltrating lymphocytes and in a more angiogenic environment.

Chemokine sequestration by atypical chemokine receptors

Leucocyte migration is essential for robust immune and inflammatory responses, and plays a critical role in many human diseases. Chemokines, a family of small secreted protein chemoattractants, are of fundamental importance in this process, directing leucocyte trafficking by signalling through heptahelical G-protein-coupled receptors expressed by the migrating cells. However, several mammalian chemokine receptors, including D6 and CCX-CKR (ChemoCentryx chemokine receptor), do not fit existing models of chemokine receptor function, and do not even appear to signal in response to chemokine binding. Instead, these 'atypical' chemokine receptors are biochemically specialized for chemokine sequestration, acting to regulate chemokine bioavailability and thereby influence responses through signalling-competent chemokine receptors. This is of critical importance in vivo, as mice lacking D6 show exaggerated cutaneous inflammatory responses and an increased susceptibility to the development of skin cancer. CCX-CKR, on the other hand, is predicted to modulate homoeostatic lymphocyte and dendritic cell trafficking, key migratory events in acquired immune responses that are directed by CCX-CKR-binding chemokines. Thus studies on 'atypical' chemokine receptors are revealing functional and biochemical diversity within the chemokine receptor family and providing insights into novel mechanisms of chemokine regulation.

Monocyte chemoattractant protein 1, macrophage inflammatory protein 1 alpha, and RANTES recruit macrophages to the kidney in a mouse model of hemolytic-uremic syndrome

The macrophage has previously been implicated in contributing to the renal inflammation associated with hemolytic-uremic syndrome (HUS). However, there is currently no in vivo model detailing the contribution of the renal macrophage to the kidney disease associated with HUS. Therefore, renal macrophage recruitment and inhibition of infiltrating renal macrophages were evaluated in an established HUS mouse model. Macrophage recruitment to the kidney was evident by immunohistochemistry 2 h after administration of purified Stx2 and peaked at 48 h postinjection. Mice administered a combination of Stx2 and lipopolysaccharide (LPS) showed increased macrophage recruitment to the kidney compared to mice treated with Stx2 or LPS alone. Monocyte chemoattractants were induced in the kidney, including monocyte chemoattractant protein 1 (MCP-1/CCL2), macrophage inflammatory protein 1alpha (MIP-1alpha/CCL3), and RANTES (CCL5), in a pattern that was coincident with macrophage infiltration as indicated by immunohistochemistry, protein, and RNA analyses. MCP-1 was the most abundant chemokine, MIP-1alpha was the least abundant, and RANTES levels were intermediate. Mice treated with MCP-1, MIP-1alpha, and RANTES neutralizing antibodies had a significant decrease in Stx2 plus LPS-induced macrophage accumulation in the kidney, indicating that these chemokines are required for macrophage recruitment. Furthermore, mice exposed to these three neutralizing antibodies had decreased fibrin deposition in their kidneys, implying that macrophages contribute to the renal damage associated with HUS.

Regulation of chemotactic networks by ‘atypical’ receptors

Directed cell migration is a fundamental component of numerous biological systems and is critical to the pathology of many diseases. Although the importance of secreted chemoattractant factors in providing navigational cues to migrating cells bearing specific chemoattractant receptors is now well-established, how the function of these factors is regulated is not so well understood and may be of key importance to the design of new therapeutics for numerous human diseases. While regulation of migration clearly takes place on a number of different levels, it is becoming clear that so-called 'atypical' receptors play a role in scavenging, or altering the localisation of, chemoattractant molecules such as chemokines and complement components. These receptors do this through binding and/or internalising their chemoattractant ligands without activating signal transduction cascades leading to cell migration. The atypical chemokine receptor family currently comprises the receptors D6, DARC and CCX-CKR. In this review, we discuss the evidence from in vitro and in vivo studies that these receptors play a role in regulating cell migration, and speculate that other orphan receptors may also belong to this family. Furthermore, with the advent of gene therapy on the horizon, the therapeutic potential of these receptors in human disease is also considered.

Leucocyte expression of the chemokine scavenger D6

Selective sequestration of inflammatory chemokines is critical for the successful resolution of inflammatory responses in vivo. D6 is an atypical chemokine receptor that scavenges inflammatory chemokines and is pivotal in resolving models of chemokine-driven cutaneous inflammation. We provide evidence that expression of D6 is not limited to the lymphatic endothelium at sites of inflammation as previously believed. Instead we postulate that D6 expression in leucocytes may have a significant impact upon chemokine bioavailability during the resolution phase of inflammation. D6 expressed on the lymphatic endothelia may instead have complementary roles in preventing inappropriate leucocyte migration to the lymph node by keeping the endothelium free from inflammatory chemokines.

The chemokine receptor CCX-CKR mediates effective scavenging of CCL19 in vitro

The chemokines CCL19, CCL21 and CCL25, by signalling through the receptors CCR7 or CCR9, play critical roles in leukocyte homing. They also bind another heptahelical surface protein, CCX-CKR. CCX-CKR cannot couple to typical chemokine receptor signalling pathways or mediate chemotaxis, and its function remains unclear. We have proposed that it controls chemokine bioavailability. Here, using transfected HEK293 cells, we have shown that both CCX-CKR and CCR7 mediate rapid CCL19 internalisation upon initial chemokine exposure. However, internalised CCL19 was more efficiently retained and degraded after uptake via CCX-CKR. More importantly, CCR7 rapidly became refractory for CCL19 uptake, but the sequestration activity of CCX-CKR was enhanced. These properties endowed CCX-CKR with an impressive ability to mediate progressive sequestration and degradation of large quantities of CCL19, and conversely, prevented CCR7-expressing cells from extensively altering their chemokine environment. These differences may be linked to the routes of endocytosis used by these receptors. CCX-CKR, unlike CCR7, was not critically dependent on beta-arrestins or clathrin-coated pits. However, over-expression of caveolin-1, which stabilises caveolae, blocked CCL19 uptake by CCX-CKR while having no impact on other chemokine receptors, including CCR7. These data predict that CCX-CKR scavenges extracellular chemokines in vivo to modify responses through CCR7.

Chemokine scavenging by D6: a movable feast?

The atypical chemokine receptor, D6, is efficient at sequestering and scavenging inflammatory CC chemokines. The absence of D6 blocks the successful resolution of immune responses in models of inflammation, suggesting that CC-chemokine scavenging by D6 is an important component of the resolution phase of in vivo inflammatory responses. Most studies have suggested that lymphatic endothelial cells are the main vehicles for D6 function in vivo. Here, we propose that leukocytes, which also express D6, could be more-effective vehicles for D6 scavenging function. Thus, leukocytes might be the primary cell type that removes inflammatory chemokines from inflamed tissues. We also propose that lymphatic endothelial cell-expressed D6 might have a distinct but complementary role in restricting inflammatory leukocyte access to the lymphatic vasculature.

Enhanced expression of Duffy antigen receptor for chemokines by breast cancer cells attenuates growth and metastasis potential

In addition to the role in regulating leukocyte trafficking, chemokines recently have been shown to be involved in cancer growth and metastasis. Chemokine network in tumor neovascularity may be regulated by decoy receptors. Duffy antigen receptor for chemokines (DARC) is a specific decoy receptor binding with the angiogenic CC and CXC chemokines. To investigate the effects of DARC on the tumorigenesis and the metastasis potential of human breast cancer cells, human DARC cDNA was reintroduced into the MDA-MB-231 and MDA-MB-435HM cells which have a high capability of spontaneous pulmonary metastasis. We demonstrated that DARC overexpression induced inhibition of tumorigenesis and/or metastasis through interfering with the tumor angiogenesis in vivo. This inhibition is associated with decreasing CCL2 protein levels, and MVD and MMP-9 expression in xenograft tumors. In human breast cancer samples, we also demonstrated that low expression of the DARC protein is significantly associated with estrogen receptor (ER) status, MVD, lymph node metastasis, distant metastasis and poor survival. Our results suggest for the first time that DARC is a negative regulator of growth in breast cancer, mainly by sequestration of angiogenic chemokines and subsequent inhibition of tumor neovascularity.

Costimulation of Chemokine Receptor Signaling by Matrix Metalloproteinase-9 Mediates Enhanced Migration of IFN-α Dendritic Cells

Type I IFNs induce differentiation of dendritic cells (DCs) with potent Ag-presenting capacity, termed IFN-alpha DCs, that have been implicated in the pathogenesis of systemic lupus erythematosus. In this study, we found that IFN-alpha DCs exhibit enhanced migration across the extracellular matrix (ECM) in response to chemokines CCL3 and CCL5 that recruit DCs to inflammatory sites, but not the lymphoid-homing chemokine CCL21. IFN-alpha DCs expressed elevated matrix metalloproteinase-9 (MMP-9), which mediated increased migration across ECM. Unexpectedly, MMP-9 and its cell surface receptors CD11b and CD44 were required for enhanced CCL5-induced chemotaxis even in the absence of a matrix barrier. MMP-9, CD11b, and CD44 selectively modulated CCL5-dependent activation of JNK that was required for enhanced chemotactic responses. These results establish the migratory phenotype of IFN-alpha DCs and identify an important role for costimulation of chemotactic responses by synergistic activation of JNK. Thus, cell motility is regulated by integrating signaling inputs from chemokine receptors and molecules such as MMP-9, CD11b, and CD44 that also mediate cell interactions with inflammatory factors and ECM.