Coy decoy with a new ploy: interceptor controls the levels of homeostatic chemokines

Atypical chemokine receptors in cancer

Atypical chemokine receptors (ACKRs) are a group of seven-transmembrane spanning serpentine receptors that are structurally homologous to classical G-protein-coupled receptors and bind cognate chemokines with high affinities but do not signal via G-proteins or mediate cell migration. However, ACKRs efficiently modify the availability and function of chemokines in defined microanatomical environments, can signal via intracellular effectors other than G-proteins, and play complex roles in physiology and disease, including in cancer. In this review, we summarize the findings on the diverse contributions of individual ACKRs to cancer development, progression, and tumor-host interactions. We discuss how changes in ACKR expression within tumor affect cancer growth, tumor vascularization, leukocyte infiltration, and metastasis formation, ultimately resulting in differential disease outcomes. Across many studies, ACKR3 expression was shown to support tumor growth and dissemination, whereas ACKR1, ACKR2, and ACKR4 in tumors were more likely to contribute to tumor suppression. With few notable exceptions, the insights on molecular and cellular mechanisms of ACKRs activities in cancer remain sparse, and the intricacies of their involvement are not fully appreciated. This is particularly true for ACKR1, ACKR2 and ACKR4. A better understanding of how ACKR expression and functions impact cancer should pave the way for their future targeting by new and effective therapies.

Expression of ACKR4 demarcates the "peri-marginal sinus," a specialized vascular compartment of the splenic red pulp

The spleen comprises defined microanatomical compartments that uniquely contribute to its diverse host defense functions. Here, we identify a vascular compartment within the red pulp of the spleen delineated by expression of the atypical chemokine receptor 4 (ACKR4) in endothelial cells. ACKR4-positive vessels form a three-dimensional sinusoidal network that connects via shunts to the marginal sinus and tightly surrounds the outer perimeter of the marginal zone. Endothelial cells lining this vascular compartment express ACKR4 as part of a distinct gene expression profile. We show that T cells enter the spleen largely through this peri-marginal sinus and initially localize extravascularly around these vessels. In the absence of ACKR4, homing of T cells into the spleen and subsequent migration into T cell areas is impaired, and organization of the marginal zone is severely affected. Our data delineate the splenic peri-marginal sinus as a compartment that supports spleen homing of T cells.

CCL19 and CCR7 Expression, Signaling Pathways, and Adjuvant Functions in Viral Infection and Prevention

Chemokine (C–C motif) ligand 19 (CCL19) is a critical regulator of the induction of T cell activation, immune tolerance, and inflammatory responses during continuous immune surveillance, homeostasis, and development. Migration of CC-chemokine receptor 7 (CCR7)-expressing cells to secondary lymphoid organs is a crucial step in the onset of adaptive immunity, which is initiated by a complex interaction between CCR7 and its cognate ligands. Recent advances in knowledge regarding the response of the CCL19-CCR7 axis to viral infections have elucidated the complex network of interplay among the invading virus, target cells and host immune responses. Viruses use various strategies to evade or delay the cytokine response, gaining additional time to replicate in the host. In this review, we summarize the impacts of CCL19 and CCR7 expression on the regulation of viral pathogenesis with an emphasis on the corresponding signaling pathways and adjuvant mechanisms. We present and discuss the expression, signaling adaptor proteins and effects of CCL19 and CCR7 as these molecules differentially impact different viral infections and viral life cycles in host homeostatic strategies. The underlying mechanisms discussed in this review may assist in the design of novel agents to modulate chemokine activity for viral prevention.

Breast Cancer: An Examination of the Potential of ACKR3 to Modify the Response of CXCR4 to CXCL12

Upon binding with the chemokine CXCL12, the chemokine receptor CXCR4 has been shown to promote breast cancer progression. This process, however, can be affected by the expression of the atypical chemokine receptor ACKR3. Given ACKR3's ability to form heterodimers with CXCR4, we investigated how dual expression of both receptors differed from their lone expression in terms of their signalling pathways. We created single and double CXCR4 and/or ACKR3 Chinese hamster ovary (CHO) cell transfectants. ERK and Akt phosphorylation after CXCL12 stimulation was assessed and correlated with receptor internalization. Functional consequences in cell migration and proliferation were determined through wound healing assays and calcium flux. Initial experiments showed that CXCR4 and ACKR3 were upregulated in primary breast cancer and that CXCR4 and ACKR3 could form heterodimers in transfected CHO cells. This co-expression modified CXCR4's Akt activation after CXCL12's stimulation but not ERK phosphorylation (p < 0.05). To assess this signalling disparity, receptor internalization was assessed and it was observed that ACKR3 was recycled to the surface whilst CXCR4 was degraded (p < 0.01), a process that could be partially inhibited with a proteasome inhibitor (p < 0.01). Internalization was also assessed with the ACKR3 agonist VUF11207, which caused both CXCR4 and ACKR3 to be degraded after internalization (p < 0.05 and p < 0.001), highlighting its potential as a dual targeting drug. Interestingly, we observed that CXCR4 but not ACKR3, activated calcium flux after CXCL12 stimulation (p < 0.05) and its co-expression could increase cellular migration (p < 0.01). These findings suggest that both receptors can signal through ERK and Akt pathways but co-expression can alter their kinetics and internalization pathways.

Homing in hematopoietic stem cells: focus on regulatory role of CXCR7 on SDF1a/CXCR4 axis

Hematopoietic stem cells (HSCs) form a rare population of multipotent stem cells, which give rise to all hematopoietic lineages. HSCs home to bone marrow niches and circulate between blood and bone marrow. Many factors, especially SDF1a, affect the circulation of HSCs, but these have not been fully recognized. SDF1a has been shown to bind CXCR7 in addition to CXCR4 and can also function as SDF1a/CXCR4 modulator. CXCR7 plays a role in HSCs homing via SDF1a gradient and is a mediator of CXCR4/SDF1a axis. This review describes the current concepts and questions concerning CXCR7/CXCR4/SDF1a axis as an important key in hematopoietic stem cells homing with particular emphasis on CXCR7 receptor. Homing of HSCs is an essential step for successful hematopoietic stem cell transplantation.

Monitoring Scavenging Activity of Chemokine Receptors

Migration and positioning of cells is fundamental for complex functioning of multicellular organisms. During an immune response, cells are recruited from remote distances to a distinct location. Cells that are passively transported leave the circulation stimulated by locally produced signals and follow chemotactic cues to reach specific destinations. Such gradients are short (<150 μm) and require a source of production where the concentration is the highest and a sink in apposition where the attractant dissipates and the concentration is the lowest. Several straight forward methods exist to identify in vitro and in vivo cells producing chemoattractants. This can be achieved at the transcriptional level and by measuring secreted proteins. However, to demonstrate the activity of sinks in vitro and in vivo is more challenging. Cell-mediated dissipation of an attractant must be revealed by measuring its uptake and subsequent destruction. Elimination of chemoattractants such as chemokines can be monitored in vitro using radiolabeled ligands or more elegantly with fluorescent-labeled chemoattractants. The latter method can also be used in vivo and enables to monitor the process in real time using time-lapse video microscopy. In this chapter, we describe methods to produce fluorescently labeled chemokines either as fusion proteins secreted from insect cells or as recombinant bacterial proteins that can enzymatically be labeled. We discuss methods that were successfully used to demonstrate sink activities of scavenger receptors. Moreover, fluorescent chemokines can be used to noninvasively analyze receptor expression and activity in living cells.

Solution Structure of CCL19 and Identification of Overlapping CCR7 and PSGL-1 Binding Sites

CCL19 and CCL21 are chemokines involved in the trafficking of immune cells, particularly within the lymphatic system, through activation of CCR7. Concurrent expression of PSGL-1 and CCR7 in naive T-cells enhances recruitment of these cells to secondary lymphoid organs by CCL19 and CCL21. Here the solution structure of CCL19 is reported. It contains a canonical chemokine domain. Chemical shift mapping shows the N-termini of PSGL-1 and CCR7 have overlapping binding sites for CCL19 and binding is competitive. Implications for the mechanism of PSGL-1's enhancement of resting T-cell recruitment are discussed.

CCX-CKR expression in colorectal cancer and patient survival

Colorectal cancer is one of the most common malignant cancers, with bad prognosis when distal metastasis occurs. The current study aimed to investigate the potential value of using CCX-CKR expression for the prognosis of colorectal cancer patients. The results showed that CCX-CKR expression was a negative predictor of cancer metastasis, and that it was positively correlated to the patients&rsquo; survival rate. Finally, we found that CCX-CKR expression in vitro could modulate cellular migration and invasion abilities, potentially via the regulation of other chemotactic factors/receptors.

β-Arrestin recruitment and G protein signaling by the atypical human chemokine decoy receptor CCX-CKR

Chemokine receptors form a large subfamily of G protein-coupled receptors that predominantly activate heterotrimeric Gi proteins and are involved in immune cell migration. CCX-CKR is an atypical chemokine receptor with high affinity for CCL19, CCL21, and CCL25 chemokines, but is not known to activate intracellular signaling pathways. However, CCX-CKR acts as decoy receptor and efficiently internalizes these chemokines, thereby preventing their interaction with other chemokine receptors, like CCR7 and CCR9. Internalization of fluorescently labeled CCL19 correlated with β-arrestin2-GFP translocation. Moreover, recruitment of β-arrestins to CCX-CKR in response to CCL19, CCL21, and CCL25 was demonstrated using enzyme-fragment complementation and bioluminescence resonance energy transfer methods. To unravel why CCX-CKR is unable to activate Gi signaling, CCX-CKR chimeras were constructed by substituting its intracellular loops with the corresponding CCR7 or CCR9 domains. The signaling properties of chimeric CCX-CKR receptors were characterized using a cAMP-responsive element (CRE)-driven reporter gene assay. Unexpectedly, wild type CCX-CKR and a subset of the chimeras induced an increase in CRE activity in response to CCL19, CCL21, and CCL25 in the presence of the Gi inhibitor pertussis toxin. CCX-CKR signaling to CRE required an intact DRY motif. These data suggest that inactive Gi proteins impair CCX-CKR signaling most likely by hindering the interaction of this receptor with pertussis toxin-insensitive G proteins that transduce signaling to CRE. On the other hand, recruitment of the putative signaling scaffold β-arrestin to CCX-CKR in response to chemokines might allow activation of yet to be identified signal transduction pathways.

The biochemistry and biology of the atypical chemokine receptors

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

Chemokine CXCL12 and its receptors in the developing central nervous system: emerging themes and future perspectives

Homeostatic chemokine CXCL12 (also known as SDF-1) and its receptor CXCR4 are indispensable for the normal development of the nervous system. This chemokine system plays a plethora of functions in numerous neural developmental processes, from which the underlying molecular and cellular mechanisms are beginning to be unravelled. Recent identification of CXCR7 as a second receptor for CXCL12 provides opportunities to gain deeper insights into how CXCL12 operates in the nervous system. Here, we review the diverse roles of CXCL12 in the developing central nervous system, summarize the recent progress in uncovering CXCR7 functions, and discuss the emerging common themes from these works and future perspectives.

Comparative modeling of CCRL1, a key protein in masked immune diseases and virtual screening for finding inhibitor of this protein

Human CCRL1 belongs to the family of silent chemokine receptors. This transmembrane protein plays a role in blunting function of chemokines through binding to them. This will attenuate immune responses. Interaction between CCRL1 and CCL21 determines this immune extinction. Thus inhibiting the action of this atypical chemokine seems to stimulate immune responses especially in the case of suppressed and immune deficient conditions. In this study we predicted 3D structure of CCRL1 using comparative modeling and Hiddebn Markov Model algorithm. Final predicted model optimized by Modeller v9.8 and minimized regarding energy level using UCSF chimera candidate version1.5.3. ClasPro webserver was used to find interacting residues between CCRL1 and CCL21. Interacting residues were used as target for chemical inhibitors by simulated docking study. For finding potential inhibitors, library of KEGG compounds screened and 97 obtained chemicals docked against interacting residues between CCRL1- CCL21 and MolDock was used as docking scoring function. Results indicated that Hexadecanal is a potential inhibitor of CCRL1- CCL21 interaction. Inhibition of this interaction will increase intercellular level of CCl21 and interaction between CCL21 and CCR7 causes immune potentiaiton.

IL-13 antibodies influence IL-13 clearance in humans by modulating scavenger activity of IL-13Rα2

Human studies using Abs to two different, nonoverlapping epitopes of IL-13 suggested that epitope specificity can have a clinically significant impact on clearance of IL-13. We propose that Ab modulation of IL-13 interaction with IL-13Rα2 underlies this effect. Two Abs were administered to healthy subjects and mild asthmatics in separate dose-ranging studies and allergen-challenge studies. IMA-638 allows IL-13 interaction with IL-13Rα1 or IL-13Rα2 but blocks recruitment of IL-4Rα to the IL-13/IL-13Rα1 complex, whereas IMA-026 competes with IL-13 interaction with IL-13Rα1 and IL-13Rα2. We found ∼10-fold higher circulating titer of captured IL-13 in subjects treated with IMA-026 compared with those administered IMA-638. To understand how this difference could be related to epitope, we asked whether either Ab affects IL-13 internalization through cell surface IL-13Rα2. Humans inducibly express cell surface IL-13Rα2 but lack the soluble form that regulates IL-13 responses in mice. Cells with high IL-13Rα2 expression rapidly and efficiently depleted extracellular IL-13, and this activity persisted in the presence of IMA-638 but not IMA-026. The potency and efficiency of this clearance pathway suggest that cell surface IL-13Rα2 acts as a scavenger for IL-13. These findings could have important implications for the design and characterization of IL-13 antagonists.

Chemokine triggered integrin activation and actin remodeling events guiding lymphocyte migration across vascular barriers

Chemokine signals activate leukocyte integrins and actin remodeling machineries critical for leukocyte adhesion and motility across vascular barriers. The arrest of leukocytes at target blood vessel sites depends on rapid conformational activation of their α4 and β2 integrins by the binding of endothelial-displayed chemokines to leukocyte Gi-protein coupled receptors (GPCRs). A universal regulator of this event is the integrin-actin adaptor, talin1. Chemokine-stimulated GPCRs can transmit within fractions of seconds signals via multiple Rho GTPases, which locally raise plasma membrane levels of the talin activating phosphatidyl inositol, PtdIns(4,5)P2 (PIP2). Additional pools of GPCR stimulated Rac-1 and Rap-1 GTPases together with GPCR stimulated PLC and PI3K family members regulate the turnover of focal contacts of leukocyte integrins, induce the collapse of leukocyte microvilli, and promote polarized leukocyte crawling in search of exit cues. Concomitantly, other leukocyte GTPases trigger invasive protrusions into and between endothelial cells in search of basolateral chemokine exit cues. We will review here major findings and open questions related to these sequential guiding activities of endothelial presented chemokines, focusing mainly on lymphocyte-endothelial interactions as a paradigm for other leukocytes.

Receptor oligomerization: a pivotal mechanism for regulating chemokine function

Since the first reports on chemokine function, much information has been generated on the implications of these molecules in numerous physiological and pathological processes, as well as on the signaling events activated through their binding to receptors. Despite these extensive studies, no chemokine-related drugs have yet been approved for use in patients with inflammatory or autoimmune diseases. This discrepancy between efforts and results has forced a re-evaluation of the chemokine field. We have explored chemokine receptor conformations at the cell surface and found that, as is the case for other G protein-coupled receptors, chemokine receptors are not isolated entities that are activated following ligand binding; rather, they are found as dimers and/or higher order oligomers at the cell surface, even in the absence of ligands. These complexes form organized arrays that can be modified by receptor expression and ligand levels, indicating that they are dynamic structures. The way in which these receptor complexes are stabilized modulates ligand binding, as well as their pharmacological properties and the signaling events activated. These conformations thus represent a mechanism that increases the broad variety of chemokine functions. Understanding these receptor interactions and their dynamics at the cell surface is thus critical for influencing chemokine function and could open up new possibilities for drug design.

Atypical chemokine receptors

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

Coexpression of atypical chemokine binders (ACBs) in breast cancer predicts better outcomes

Some evidence suggests that atypical chemokine binders (ACBs) including DARC, D6, and CCX-CKR play an important role in inhibiting invasion and metastasis of cancer cells; however, their expression in breast cancer has not been well characterized. The purpose of this study was to determine the predictive value of ACBs for relapse-free survival and overall survival in breast cancer. The expressions of the three molecules were analyzed immunohistochemically in a total of 558 consecutive breast specimens comprising 12 normal breast tissues, 29 noninvasive (carcinoma in situ), and 517 invasive breast carcinoma and their relationships to clinicopathological features and survival were investigated in invasive breast cancer. Coexpression of ACBs in invasive breast carcinoma (55.9%) was much lower that of noninvasive breast carcinoma (93.1%) and normal breast tissue (100.0%), P = 0.0004, 0.0096, respectively. Their separate stainings in invasive cancer were significantly conversely correlated with lymph node status and tumor stage. In univariate analysis, the three proteins and their coexpression were significantly associated with higher relapse-free survival and overall survival. In multivariate analysis, each of these molecules was favorable for relapse-free survival, but not overall survival. Surprisingly, their coexpression was not only independently prognostic factor for relapse-free survival (RR = 0.182, 95% CI: 0.101-0.327, P < 0.001), but also for overall survival (RR = 0.271, 95% CI: 0.081-0.910, P = 0.035). These findings highlight that the multiple loss of ACBs may occur during the development of tumorigenesis and their coexpression in breast cancer is predictive of favorable outcomes.

Overlapping and distinct role of CXCR7-SDF-1/ITAC and CXCR4-SDF-1 axes in regulating metastatic behavior of human rhabdomyosarcomas

We have demonstrated that the α-chemokine stromal-derived factor (SDF)-1-CXCR4 axis plays an important role in rhabdomyosarcoma (RMS) metastasis. With the recent description of CXCR7, a new receptor for SDF-1 that also binds the interferon-inducible T-cell α chemoattractant (ITAC) chemokine, we became interested in the role of the CXCR7-SDF-1/ITAC axis in RMS progression. To address this issue, we evaluated 6 highly metastatic alveolar (A)RMS and 3 less metastatic embryonal (E)RMS cell lines and found that all these cell lines express CXCR7. Although CXCR4 was expressed at a much higher level by highly metastatic ARMS lines, CXCR7 was present at a high level on ERMS lines. We also noticed that CXCR7 expression on RMS cells was downregulated in hypoxic conditions. More importantly, the CXCR7 receptor on RMS cell lines was functional after stimulation with ITAC and SDF-1 as evidenced by mitogen-activated protein kinase (MAPK)p42/44 and AKT phosphorylation as well as CXCR7 internalization, chemotaxis, cell motility and adhesion assays. Similarly to CXCR4, signaling from activated CXCR7 was not associated with increased RMS proliferation or cell survival. Moreover, CXCR7(+) RMS cells responded to SDF-1 and I-TAC in the presence of CXCR4 antagonists (T140, AMD3100). Furthermore, while intravenous injection of RMS cells with overexpressed CXCR7 resulted in increased seeding efficiency of tumor cells to bone marrow, CXCR7 downregulation showed the opposite effect. In conclusion, the CXCR7-SDF-1/ITAC axis is involved in the progression of RMS; targeting of the CXCR4-SDF-1 axis alone without simultaneous blockage of CXCR7 will be an inefficient strategy for inhibiting SDF-1-mediated prometastatic responses of RMS cells.

Macrophage migration inhibitory factor is secreted by rhabdomyosarcoma cells, modulates tumor metastasis by binding to CXCR4 and CXCR7 receptors and inhibits recruitment of cancer-associated fibroblasts

The overexpression of macrophage migration inhibitory factor (MIF) has been observed in many tumors and is implicated in oncogenic transformation and tumor progression. MIF activates CXCR2 and CD74 receptors and, as recently reported, may also bind to the stromal-derived factor-1 (SDF-1)-binding receptor CXCR4. Here, we report that human rhabdomyosarcoma (RMS) cell lines secrete MIF and that this chemokine (a) induces phosphorylation of mitogen-activated protein kinase (MAPK) p42/44 and AKT, (b) stimulates RMS cell adhesion, (c) enhances tumor vascularization, but surprisingly (d) decreases recruitment of cancer-associated fibroblasts (CAF). Because RMS cells used in our studies do not express CXCR2 and CD74 receptors, the biological effects of MIF on RMS cells depend on its interaction with CXCR4, and as we report here for the first time, MIF may also engage another SDF-1-binding receptor (CXCR7) as well. Interestingly, downregulation of MIF in RMS cells inoculated into immunodeficient mice led to formation of larger tumors that displayed higher stromal cell support. Based on these observations, we postulate that MIF is an important autocrine/paracrine factor that stimulates both CXCR4 and CXCR7 receptors to enhance the adhesiveness of RMS cells. We also envision that when locally secreted by a growing tumor, MIF prevents responsiveness of RMS to chemoattractants secreted outside the growing tumor (e.g., SDF-1) and thereby prevents release of cells into the circulation. On the other hand, despite its obvious proangiopoietic effects, MIF inhibits in CXCR2/CD74-dependent manner recruitment of CAFs to the growing tumor. Our data indicate that therapeutic inhibition of MIF in RMS may accelerate metastasis and tumor growth.

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.

Chemokine homeostasis vs. chemokine presentation during severe acute lung injury: the other side of the Duffy antigen receptor for chemokines

Acute lung injury (ALI) still poses a major challenge in critical care medicine. Neutrophils, platelets, and chemokines are all considered key components in the development of ALI. The Duffy antigen receptor for chemokines (DARC) is thought to be involved in scavenging, transendothelial transport, and presentation of neutrophil-specific chemokines. DARC is expressed on endothelial cells and erythrocytes but not on leukocytes. Here, we show that DARC is crucial for chemokine-mediated leukocyte recruitment in vivo. However, we also demonstrate that changes in chemokine and chemokine receptor homeostasis, associated with Darc gene deficiency, exert strong anti-inflammatory effects. Neutrophils from Darc gene-deficient (Darc(-/-)) mice display a more prolonged downregulation of CXCR2 during severe inflammation than neutrophils from wild-type mice. In a CXCR2-dependent model of acid-induced ALI, Darc gene deficiency prevents ALI. Darc(-/-) mice demonstrate fully preserved oxygenation, only a small increase in vascular permeability, and a complete lack of pulmonary neutrophil recruitment. Further analysis reveals that only neutrophils but neither endothelial cells nor erythrocytes from Darc(-/-) mice confer protection from ALI. The protection appears to be due to abolished pulmonary recruitment of neutrophils from Darc(-/-) mice. The generation of neutrophil-platelet aggregates, a key mechanism in both pulmonary neutrophil recruitment and thrombus formation, is also affected by altered CXCR2 homeostasis in Darc(-/-) mice. CXCR2 blockade enhances the formation of platelet-neutrophil aggregates and thereby corrects a formerly unknown bleeding defect in Darc(-/-) mice. In summary, our study suggests that chemokine/chemokine receptor homeostasis plays a previously unrecognized and crucial role in severe ALI.

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

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

The role of stromal-derived factor-1--CXCR7 axis in development and cancer

Cancer metastasis is a major clinical problem that contributes to unsuccessful therapy. Augmenting evidence indicates that metastasizing cancer cells employ several mechanisms that are involved in developmental trafficking of normal stem cells. Stromal-derived factor-1 (SDF-1) is an important alpha-chemokine that binds to the G-protein-coupled seven-transmembrane span CXCR4. The SDF-1-CXCR4 axis regulates trafficking of normal and malignant cells. SDF-1 is an important chemoattractant for a variety of cells including hematopoietic stem/progenitor cells. For many years, it was believed that CXCR4 was the only receptor for SDF-1. However, several reports recently provided evidence that SDF-1 also binds to another seven-transmembrane span receptor called CXCR7, sharing this receptor with another chemokine family member called Interferon-inducible T-cell chemoattractant (I-TAC). Thus, with CXCR7 identified as a new receptor for SDF-1, the role of the SDF-1-CXCR4 axis in regulating several biological processes becomes more complex. Based on the available literature, this review addresses the biological significance of SDF-1's interaction with CXCR7, which may act as a kind of decoy or signaling receptor depending on cell type. Augmenting evidence suggests that CXCR7 is involved in several aspects of tumorogenesis and could become an important target for new anti-metastatic and anti-cancer drugs.

Involvement of a novel chemokine decoy receptor CCX-CKR in breast cancer growth, metastasis and patient survival

PURPOSE

The biological axes of chemokines and chemokine receptors, such as CXCR4/CXCL12, CCR7/CCL19 (CCL21), CCR9/CCL25, and CXCR5/CXCL13, are involved in cancer growth and metastasis. This study is aimed at the potential regulatory role of atypical chemokine binder CCX-CKR, as a scavenger of CCL19, CCL21, CCL25, and CXCL13, in human breast cancer.

EXPERIMENTAL DESIGN

The role of CCX-CKR in human breast cancer was investigated in cell lines, animal models, and clinical samples.

RESULTS

Overexpression of CCX-CKR inhibited cancer cell proliferation and invasion in vitro and attenuated xenograft tumor growth and lung metastasis in vivo. CCX-CKR can be regulated by cytokines such as interleukin-1beta, tumor necrosis factor-alpha, and IFN-gamma. Lack or low expression of CCX-CKR correlated with a poor survival rate in the breast cancer patients. A significant correlation between CCX-CKR and lymph node metastasis was observed in human breast cancer tissues. CCX-CKR status was an independent prognostic factor for disease-free survival in breast cancer patients.

CONCLUSION

We showed for the first time that CCX-CKR is a negative regulator of growth and metastasis in breast cancer mainly by sequestration of homeostatic chemokines and subsequent inhibition of intratumoral neovascularity. This finding may lead to a new therapeutic strategy against breast cancer.

Comparison of immune cell recruitment and function in endometrium during development of epitheliochorial (pig) and hemochorial (mouse and human) placentas

The role of maternal immune cells in early implantation sites has received special attention from reproductive biologists because immune cells participate in tissue transplant rejection. During normal pregnancy, endometrial immune cells differ from those in blood by subset distribution and appear to be activated but non-destructive of conceptuses. The immune system evolved well before placental mammals. By comparing the regulation and functions of endometrial immune cells between species in two phylogenetic clades that model differently evolved placental types (pig (Sus scrofa) versus mouse (Mus musculus) and human (Homo sapiens)), we seek to understand how "non-self" trophoblast cells thrive in most pregnancies. Our studies suggest recruitment of specific immune cells to conceptus-associated endometrium and immune cell-promoted endometrial angiogenesis are of key importance for mammalian conceptus well-being.

How chemokines invite leukocytes to dance

A prominent activity of the chemokine system is the regulation of leukocyte trafficking. Here we summarize recent findings on the initial steps in chemokine receptor-induced signal transduction in leukocytes. In particular, we discuss the potential influences of the formation of oligomers of ligand and receptor and of coupling between chemokine signals and regulators of the cytoskeleton, such as small GTPases.

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

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

CXCR7, CXCR4 and CXCL12: an eccentric trio?

CXCR7, formerly called RDC1 is a recently deorphanized G-protein coupled receptor which binds with high affinity the inflammatory and homing chemokines CXCL11/ITAC and CXCL12/SDF-1. Despite its phylogenetic relation and ligand binding properties CXCR7 does not mediate typical chemokine receptor responses such as leukocyte trafficking. Recent findings in zebrafish indicate that a critical activity of the receptor is scavenging of CXCL12 thereby generating guidance cues for CXCR4-dependent migration. The observations do not exclude the possibility that the receptor is capable of inducing signal transduction which is suggestive from studies of tumor growth and survival. The pronounced expression in central and peripheral nervous tissue and the absence of a brain phenotype in CXCR7(-/-) mice suggest a subtle activity of the receptor.

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.

Opposing roles of murine duffy antigen receptor for chemokine and murine CXC chemokine receptor-2 receptors in murine melanoma tumor growth

The Duffy antigen receptor for chemokines (DARC) has been classified as a "silent" receptor, as it can bind CXC and CC chemokines to undergo ligand-induced receptor internalization, but is not coupled to trimeric G proteins required for the classic G protein-coupled receptor-mediated signaling. CXC chemokine receptor-2 (CXCR2) has been shown to play a major role in tumor angiogenesis. To test the hypothesis that these two chemokine receptors might play opposing roles in the growth of melanoma tumors, we developed a transgenic mouse model, where the preproendothelin promoter/enhancer (PPEP) is used to drive expression of either murine DARC (mDARC) or murine CXCR2 (mCXCR2) in endothelial cells. We show herein that the growth of melanoma tumor xenografts, established from s.c. injection of immortalized murine melanocytes overexpressing macrophage inflammatory protein-2, was inhibited or enhanced in the PPEP-mDARC and PPEP-mCXCR2 transgenic mice, respectively, compared with control mice. The early tumors formed in mDARC transgenic mice exhibited a significantly higher number of infiltrating leukocytes compared with either the control or mCXCR2 transgenic mice, suggesting a potential role for DARC expressed on endothelial cells in leukocyte migration. In addition, the tumor-associated angiogenesis in mDARC transgenic mice was reduced when compared with the control. Conversely, tumor angiogenesis was significantly increased in mCXCR2 transgenic mice. Results indicate that endothelial cell overexpression of mDARC increased leukocyte trafficking to the tumor, reduced the growth of blood vessels into the tumor, and reduced the growth rate of the tumor, whereas endothelial cell overexpression of mCXCR2 had the reverse effect on tumor angiogenesis and growth.

Disrupted cardiac development but normal hematopoiesis in mice deficient in the second CXCL12/SDF-1 receptor, CXCR7

Chemotactic cytokines (chemokines) attract immune cells, although their original evolutionary role may relate more closely with embryonic development. We noted differential expression of the chemokine receptor CXCR7 (RDC-1) on marginal zone B cells, a cell type associated with autoimmune diseases. We generated Cxcr7(-/-) mice but found that CXCR7 deficiency had little effect on B cell composition. However, most Cxcr7(-/-) mice died at birth with ventricular septal defects and semilunar heart valve malformation. Conditional deletion of Cxcr7 in endothelium, using Tie2-Cre transgenic mice, recapitulated this phenotype. Gene profiling of Cxcr7(-/-) heart valve leaflets revealed a defect in the expression of factors essential for valve formation, vessel protection, or endothelial cell growth and survival. We confirmed that the principal chemokine ligand for CXCR7 was CXCL12/SDF-1, which also binds CXCR4. CXCL12 did not induce signaling through CXCR7; however, CXCR7 formed functional heterodimers with CXCR4 and enhanced CXCL12-induced signaling. Our results reveal a specialized role for CXCR7 in endothelial biology and valve development and highlight the distinct developmental role of evolutionary conserved chemokine receptors such as CXCR7 and CXCR4.