In vivo structure/function and expression analysis of the CX3C chemokine fractalkine.

The CX3C chemokine family is composed of only one member, CX3CL1, also known as fractalkine, which in mice is the sole ligand of the G protein-coupled, 7-transmembrane receptor CX3CR1. Unlike classic small peptide chemokines, CX3CL1 is synthesized as a membrane-anchored protein that can promote integrin-independent adhesion. Subsequent cleavage by metalloproteases, either constitutive or induced, can generate shed CX3CL1 entities that potentially have chemoattractive activity. To study the CX3C interface in tissues of live animals, we generated transgenic mice (CX3CL1cherry:CX3CR1gfp), which express red and green fluorescent reporter genes under the respective control of the CX3CL1 and CX3CR1 promoters. Furthermore, we performed a structure/function analysis to differentiate the in vivo functions of membrane-tethered versus shed CX3CL1 moieties by comparing their respective ability to correct established defects in macrophage function and leukocyte survival in CX3CL1-deficient mice. Specifically, expression of CX3CL1(105Δ), an obligatory soluble CX3CL1 isoform, reconstituted the formation of transepithelial dendrites by intestinal macrophages but did not rescue circulating Ly6Clo CX3CR1hi blood monocytes in CX3CR1gfp/gfp mice. Instead, monocyte survival required the full-length membrane-anchored CX3CL1, suggesting differential activities of tethered and shed CX3CL1 entities.

Inflammation, growth factors, and pulmonary vascular remodeling.

Inflammatory processes are prominent in various types of human and experimental pulmonary hypertension (PH) and are increasingly recognized as major pathogenic components of pulmonary vascular remodeling. Macrophages, T and B lymphocytes, and dendritic cells are present in the vascular lesions of PH, whether in idiopathic pulmonary arterial hypertension (PAH) or PAH related to more classical forms of inflammatory syndromes such as connective tissue diseases, human immunodeficiency virus (HIV), or other viral etiologies. Similarly, the presence of circulating chemokines and cytokines, viral protein components (e.g., HIV-1 Nef), and increased expression of growth (such as vascular endothelial growth factor and platelet-derived growth factor) and transcriptional (e.g., nuclear factor of activated T cells or NFAT) factors in these patients are thought to contribute directly to further recruitment of inflammatory cells and proliferation of smooth muscle and endothelial cells. Other processes, such as mitochondrial and ion channel dysregulation, seem to convey a state of cellular resistance to apoptosis; this has recently emerged as a necessary event in the pathogenesis of pulmonary vascular remodeling. Thus, the recognition of complex inflammatory disturbances in the vascular remodeling process offers potential specific targets for therapy and has recently led to clinical trials investigating, for example, the use of tyrosine kinase inhibitors. This paper provides an overview of specific inflammatory pathways involving cells, chemokines and cytokines, cellular dysfunctions, growth factors, and viral proteins, highlighting their potential role in pulmonary vascular remodeling and the possibility of future targeted therapy.

The roles of chemokines in leukocyte recruitment and fibrosis in systemic sclerosis.

Systemic sclerosis (SSc, scleroderma) is an autoimmune disease characterized by excessive extracellular matrix deposition and vascular injury in the skin and other visceral organs. Although the pathogenesis remains unclear, interactions among leukocytes, endothelial cells, and fibroblasts are likely to be central to the pathogenesis of the disease. Chemokines mediate the leukocyte chemotaxis and migration through endothelia into the organ tissues, leading to the interaction between leukocytes and fibroblasts. While amounts of literatures reported chemokine abnormalities in SSc, which might explain the altered accumulation of effector leukocyte subsets in the affected tissues. Among various chemokines, monocyte chemoattractant protein-1 (MCP-1/CCL2) likely has the most critical role for tissue fibrosis in SSc. Although therapeutic effect for targeting MCP-1 has been demonstrated in mouse models of SSc or fibrotic disorders, it is unknown whether this strategy is effective in human clinical trials. Here recent data will be reviewed on the pathogenic role of chemokines and their receptors in SSc.

An oxidized lipid-peroxisome proliferator-activated receptor gamma-chemokine pathway in the regulation of macrophage-vascular smooth muscle cell adhesion.

Recent genetic studies have implicated pro-inflammatory chemokines and chemokine receptors in atherogenesis. Studies at the molecular and cellular levels have suggested specific atherogenic mechanisms for two chemokine-chemokine receptor pairs, CCL2-CCR2 and CX3CL1-CX3CR1, involving differential receptor regulation by the transcription factor peroxisome proliferator-activated receptor gamma. This pathway is triggered by oxidized proatherogenic lipids, such as oxidized low-density lipoprotein and linoleic acid derivatives, which promote differentiation of CCR2(hi)CX3CR1(lo) human monocytes to CCR2(lo)CX3CR1(hi) macrophages that adhere to coronary artery smooth muscle cells in a CX3CR1- and peroxisome proliferator-activated receptor gamma-dependent manner. Switching CX3CR1 on and CCR2 off in vivo may result in cessation of CCR2-dependent migration and activation of CX3CR1-dependent retention that together may promote foam cell accumulation in the vessel wall.

Chemokines and chemokine receptors: multipurpose players in neuroinflammation.

Chemokines were detected by virtue of chemotactic effects toward neutrophils in the late 1970s. During subsequent decades, it has become clear that their primordial role in vertebrate biology was to facilitate organogenesis, with particularly important functions in the central nervous system (CNS). In common with other developmentally relevant factors, chemokines and their G-protein-coupled receptors continue to be expressed in the adult CNS as neuromodulators. In our progress toward chemokine receptor blockade for treatment of inflammatory and infectious diseases, the CNS physiology of the chemokine system will need to be a material consideration. In some cases, the dual functions of the chemokine system in the periphery and in the CNS offer unique possibilities for disease treatment.

Activation of human natural killer cells by recombinant membrane-expressed fractalkine on the surface of tumor cells.

Chemokine receptors are typically expressed on natural killer cells, which can be activated by membrane ligands including the membrane chemokine fractalkine (mFKN). This study investigated the function of mFKN on natural killer (NK) cell activation for interferon (IFN)-gamma production and cytotoxicity against tumors. HeLa cells were transfected with a membrane human fractalkine (mhFKN)-expressing vector, and the transcription and surface expression of mhFKN in transfected HeLa cells were confirmed by RT-PCR analysis and immunofluorescence assay, respectively. After co-culture of NK-92 cells with FKN-HeLa cells, the intracellular IFN-gamma in the NK-92 cells significantly increased compared to mock-HeLa cells. The concentration of IFN-gamma also increased in the supernatant of the NK-92 cells stimulated with FKN-HeLa cells. Moreover, the cytolytic activity of NK-92 cells against K562 target tumor cells was significantly enhanced at each effector:target ratio in 4-h (51)Cr-release assays when the NK-92 cells were pretreated with FKN-HeLa, indicating that membrane fractalkine activates the NK cells in the killing process. This study further confirms that membrane-expressed fractalkine plays a critical role in NK cell activation.

Fractalkine (CX3CL1)- and interleukin-2-enriched neuroblastoma microenvironment induces eradication of metastases mediated by T cells and natural killer cells.

Fractalkine (FKN) is a unique CX3C chemokine (CX3CL1) known to induce both adhesion and migration of leukocytes mediated by a membrane-bound and a soluble form, respectively. Its function is mediated through CX3C receptor (CX3CR), which is expressed by T(H)1 immune cells including T cells and natural killer (NK) cells. FKN was shown to be expressed in >90% of 68 neuroblastoma samples as determined by cDNA microarray analysis. Here, we characterized the effect of FKN in the neuroblastoma microenvironment using a syngeneic model genetically engineered to secrete FKN. We show FKN-mediated migration, adhesion, and IFN-gamma secretion of immune effector cells, but limited antineuroblastoma activity, in vitro and in vivo. Therefore, we tested the hypothesis that a combined increase of FKN and interleukin-2 (IL-2) in the neuroblastoma microenvironment induces an effective antitumor immune response. For this purpose, IL-2 was targeted to ganglioside GD2, which is highly expressed on neuroblastoma tissue, using an anti-GD2 antibody IL-2 immunocytokine (ch14.18-IL-2). Only mice bearing FKN- and IL-2-enriched neuroblastoma tumors exhibited a reduction in primary tumor growth and a complete eradication of experimental liver metastases. The depletion of T cells and NK cells in vivo abrogated the effect, and these effector cells showed the highest cytolytic activity in vitro. Finally, only the FKN- and IL-2-enriched neuroblastoma microenvironment resulted in T-cell activation and the release of proinflammatory cytokines. In summary, we showed for the first time the immunologic mechanisms by which targeted IL-2 treatment of neuroblastoma with an FKN-rich microenvironment induces an effective antitumor response.

Tissue-specific differential antitumour effect of molecular forms of fractalkine in a mouse model of metastatic colon cancer.

BACKGROUND AND AIMS: Fractalkine, a chemokine that presents as both a secreted and a membrane-anchored form, has been described as having tumour-suppressive activities in standard subcutaneous models. Here, we investigate the antitumour effect of fractalkine, in its three molecular forms, in two orthotopic models of metastatic colon cancer (liver and lung) and in the standard subcutaneous model. METHODS: We have developed models of skin tumours, liver and pulmonary metastasis and compared the extent of tumour development between C26 colon cancer cells expressing either the native, the soluble, the membrane-bound fractalkine or none. RESULTS: The native fractalkine exhibits the strongest antitumour effect, reducing the tumour size by 93% in the skin and by 99% in the orthotopic models (p<0.0001). Its overall effect results from a critical balance between the activity of the secreted and the membrane-bound forms, balance that is itself dependent on the target tissue. In the skin, both molecular variants reduce tumour development by 66% (p<0.01). In contrast, the liver and lung metastases are only significantly reduced by the soluble form (by 96%, p<0.002) whereas the membrane-bound variant exerts a barely significant effect in the liver (p = 0.049) and promotes tumour growth in the lungs. Moreover, we show a significant difference in the contribution of the infiltrating leukocytes to the tumour-suppressive activity of fractalkine between the standard and the orthotopic models. CONCLUSIONS: Fractalkine expression by C26 tumour cells drastically reduces their metastatic potential in the two physiological target organs. Both molecular forms contribute to its antitumour potential but exhibit differential effects on tumour development depending on the target tissue.

Intervention of MAdCAM-1 or fractalkine alleviates graft-versus-host reaction associated intestinal injury while preserving graft-versus-tumor effects.

Coincidence of the beneficial graft-vs.-tumor (GVT) effects and the detrimental graft-vs.-host disease (GVHD) remains the major obstacle against the widespread use of allogeneic bone marrow transplantation (BMT) as tumor immunotherapy. We here demonstrate that intervention of MAdCAM-1 (mucosal vascular addressin cell adhesion molecule-1) or fractalkine/CX3CL1 after the expansion of allo-reactive donor CD8 T cells selectively inhibits the recruitment of effector donor CD8 T cells to the intestine and alleviates the graft-vs.-host reaction (GVHR) associated intestinal injury without impairing GVT effects. In a nonirradiated acute GVHD model, donor CD8 T cells up-regulate the expression of intestinal homing receptor alpha4beta7 and chemokine receptors CXCR6 and CX3CR1, as they differentiate into effector cells and subsequently infiltrate into the intestine. Administration of anti-MAdCAM-1 antibody or anti-fractalkine antibody, even after the expansion of alloreactive donor CD8 T cells, selectively reduced the intestine-infiltrating donor CD8 T cells and the intestinal crypt cell apoptosis without affecting the induction of donor derived anti-host CTL or the infiltration of donor CD8 T cells in the hepatic tumor. Moreover, in a clinically relevant GVHD model with myeloablative conditioning, these antibodies significantly improved the survival and loss of weight without impairing the beneficial GVT effects. Thus, interruption of alpha4beta7-MAdCAM-1 or CX3CR1-fractalkine interactions in the late phase of GVHD would be a novel therapeutic approach for the separation of GVT effects from GVHR-associated intestinal injury.

The CX3C chemokine fractalkine induces vascular dysfunction by generation of superoxide anions.

OBJECTIVE: The chemokine fractalkine activates platelets and induces leukocyte adhesion to the endothelium. Expression of fractalkine and its receptor, CX3CR1, is elevated in coronary artery disease. We assessed the effects of fractalkine on vascular function in isolated rat aorta. METHODS AND RESULTS: CX3CR1 expression was demonstrated in rat aortic endothelial and smooth muscle cells by immunohistochemistry, Western blot, and polymerase chain reaction (PCR). Fractalkine (up to 1 microg/mL) did not directly induce contractile or relaxant responses when applied to rat aortic rings in organ baths. Short-term incubation with fractalkine (1 microg/mL) for 5 minutes did not affect vascular reactivity. Pretreatment of isolated rat aortic rings with fractalkine for 2 hours impaired acetylcholine-induced nitric oxide (NO)-mediated relaxation after preconstriction with phenylephrine in a concentration-dependent manner. The concentration response to the NO donor DEA-NONOate was significantly shifted to the right. The radical scavenger tiron normalized the attenuated acetylcholine-induced relaxation after fractalkine incubation. Aortic superoxide formation was enhanced by fractalkine, which was inhibited by diphenyleneiodonium but not by inhibitors of xanthine oxidase or NO synthase. CONCLUSIONS: In addition to its role as a chemokine and adhesion molecule, fractalkine induces vascular dysfunction by stimulating vascular reactive oxygen species resulting in reduced NO bioavailability.

Fractalkine stimulates angiogenesis by activating the Raf-1/MEK/ERK- and PI3K/Akt/eNOS-dependent signal pathways.

Fractalkine (FKN) has been implicated in modulation of angiogenesis and vascular inflammation, but the underlying mechanism has not been elucidated. We have investigated the molecular mechanism by which FKN regulates angiogenesis. We found that recombinant FKN increases in vitro proliferation, migration, and tube formation of human umbilical vein endothelial cells and stimulates in vivo angiogenesis. FKN-induced angiogenesis was accompanied by phosphorylation of ERK, Akt, and endothelial nitric oxide (NO) synthase (eNOS), as well as an increase in NO production. These biochemical events and angiogenesis were completely inhibited by the G protein-coupled receptor inhibitor pertussis toxin. Inhibitors of Raf-1, MEK, phosphatidylinositol 3-kinase (PI3K), and eNOS or transfection with dominant-negative forms of ERK and Akt significantly suppressed the angiogenic activity of FKN. However, inhibitors of Raf-1 and MEK or a dominant-negative ERK mutant blocked FKN-induced ERK, but not Akt and eNOS, phosphorylation. The PI3K inhibitor and a dominant-negative mutant of Akt suppressed Akt and eNOS phosphorylation and NO production. Our results demonstrated that FKN stimulated angiogenesis by activating the Raf-1/MEK/ERK and PI3K/Akt/eNOS/NO signal pathways via the G protein-coupled receptor CX3CR1, indicating that two pathways are required for full angiogenic activity of FKN. This study suggests that FKN may play an important role in the pathophysiological process of inflammatory angiogenesis.

Fractalkine in rheumatoid arthritis and allied conditions.

Leukocyte adhesion and trafficking at the endothelium requires both adhesion molecules and chemotactic factors. Fractalkine (CX3C) is a unique chemokine, and is expressed on tumor necrosis factor-alpha- and interleukin-1-activated endothelial cells (ECs). Fractalkine receptor, CX3CR1, is expressed on NK cells, monocytes, and some portion of CD4- and CD8-positive T cells. Interactions between fractalkine and CX3CR1 can mediate not only chemotaxis, but also cell adhesion in the absence of substrates for other adhesion molecules. Furthermore, fractalkine activates NK cells, leading to increased cytotoxicity and interferon-gamma production. Recently, accumulating evidence has shown that fractalkine is involved in the pathogenesis of rheumatoid arthritis and allied conditions. This review examines new concepts underlying fractalkine-mediated leukocyte migration and tissue damage, focusing primarily on the pathophysiological roles of fractalkine in rheumatic diseases.

The CC chemokine MCP-1 stimulates surface expression of CX3CR1 and enhances the adhesion of monocytes to fractalkine/CX3CL1 via p38 MAPK.

The membrane-anchored form of CX3CL1 has been proposed as a novel adhesion protein for leukocytes. This functional property of CX3CL1 is mediated through CX3CR1, a chemokine receptor expressed predominantly on circulating white blood cells. Thus far, it is still uncertain at what stage of the trafficking process CX3CR1 becomes importantly involved and how the CX3CR1-dependent adhesion of leukocytes is regulated during inflammation. The objective of this study was to examine the functional effects of chemokine stimulation on CX3CR1-mediated adhesion of human monocytes. Consistent with previous reports, our data indicate that the activity of CX3CR1 on resting monocytes is sufficient to mediate cell adhesion to CX3CL1. However, the basal, nonstimulated adhesion activity is low, and we hypothesized that like the integrins, CX3CR1 may require a preceding activation step to trigger firm leukocyte adhesion. Compatible with this hypothesis, stimulation of monocytes with MCP-1 significantly increased their adhesion to immobilized CX3CL1, under both static and physiological flow conditions. The increase of the adhesion activity was mediated through CCR2-dependent signaling and obligatory activation of the p38 MAPK pathway. Stimulation with MCP-1 also induced a rapid increase of CX3CR1 protein on the cell surface. Inhibition of the p38 MAPK pathway prevented this increase of CX3CR1 surface expression and blunted the effect of MCP-1 on cell adhesion, indicating a causal link between receptor surface density and adhesion activity. Together, our data suggest that a chemokine signal is required for firm CX3CR1-dependent adhesion and demonstrate that CCR2 is an important regulator of CX3CL1-dependent leukocyte adhesion.

Fraktalkine produced by airway smooth muscle cells contributes to mast cell recruitment in asthma.

Human airway smooth muscle cells (HASMC) secrete fractalkine (FKN), a chemokine the concentration of which is increased in asthmatic patients. HASMC also induce mast cell chemotaxis, as a component of asthma inflammation. We therefore evaluated the role of smooth muscle-derived FKN in mast cell migration. We assessed the capacity of recombinant FKN to induce human mast cell chemotaxis. This effect implicates a calcium-independent pathway involving actin reorganization and protein kinase C-delta. We found that HASMC constitutively produce FKN, the synthesis of which is reinforced upon proinflammatory stimulation. Under basal experimental conditions, FKN production by HASMC is not sufficient to induce mast cell chemotaxis. However, pretreatment of mast cells with the neuropeptide vasoactive intestinal peptide (VIP) increases FKN potency to attract mast cells. Since we observed, in asthmatic patients, an increase in both FKN and VIP expression by airway smooth muscle and a positive correlation between VIP staining and mast cell infiltration of the smooth muscle layer, we conclude that HASMC-derived FKN may contribute to mast cell recruitment in asthma.

Involvement of fractalkine/CX3CL1 expression by dendritic cells in the enhancement of host immunity against Legionella pneumophila.

Legionnaires' disease is clinically manifested as severe pneumonia caused by Legionella pneumophila. However, the dendritic cell (DC)-centered immunological framework of the host defense against L. pneumophila has not been fully delineated. For this study, we focused on a potent chemoattractant for lymphocytes, fractalkine/CX3CL1, and observed that the fractalkine expression of DCs was somewhat up-regulated when they encountered L. pneumophila. We therefore hypothesized that fractalkine expressed by Legionella-capturing DCs is involved in the induction of T-cell-mediated immune responses against Legionella, which would be enhanced by a genetic modulation of DCs to overexpress fractalkine. In vivo immunization-challenge experiments demonstrated that DCs modified with a recombinant adenovirus vector to overexpress fractalkine (AdFKN) and pulsed with heat-killed Legionella protected immunized mice from a lethal Legionella infection and that the generation of in vivo protective immunity depended on the host lymphocyte subsets, including CD4(+) T cells, CD8(+) T cells, and B cells. Consistent with this, immunization with AdFKN/Legionella/DC induced significantly higher levels of serum anti-Legionella antibodies of several isotypes than those induced by control immunizations. Further analysis of spleen cells from the immunized mice indicated that the AdFKN/Legionella/DC immunization elicited Th1-dominated immune responses to L. pneumophila. These observations suggest that fractalkine may play an important role in the DC-mediated host defense against intracellular pathogens such as L. pneumophila.

[Fractalkine and inflammatory diseases].

The migration of leukocytes into inflamed peripheral tissues and lymphoid organs involves a cascade of molecular events finely regulated by cell adhesion molecules and chemokines. Fractalkine/CX3CL1 is a membrane-bound chemokine that functions not only as a chemoattractant but also as an adhesion molecule, and is expressed on endothelial cells activated by proinflammatory cytokines. The fractalkine receptor, CX3CR1, is expressed on cytotoxic effector lymphocytes including NK cells and cytotoxic effector T cells (T(CE)), mature monocytes/macrophages, and mucosal dendritic cells, all of which play important roles in elimination of pathogens and cancer cells. Recently, accumulating evidence in both clinical studies and animal disease models has shown that fractalkine is also involved in the pathogenesis of various chronic inflammatory diseases, such as rheumatoid arthritis and atherosclerosis. This article reviews the unique functions of fractalkine and its pathophysiological roles in various clinical conditions.

Chemokine CX3CL1 protects rat hippocampal neurons against glutamate-mediated excitotoxicity.

Excitotoxicity is a cell death caused by excessive exposure to glutamate (Glu), contributing to neuronal degeneration in many acute and chronic CNS diseases. We explored the role of fractalkine/CX3CL1 on survival of hippocampal neurons exposed to excitotoxic doses of Glu. We found that: CX3CL1 reduces excitotoxicity when co-applied with Glu, through the activation of the ERK1/2 and PI3K/Akt pathways, or administered up to 8 h after Glu insult; CX3CL1 reduces the Glu-activated whole-cell current through mechanisms dependent on intracellular Ca2+; CX3CL1 is released from hippocampal cells after excitotoxic insult, likely providing an endogenous protective mechanism against excitotoxic cell death.

Role of fractalkine (CX3CL1) in regulating neuron-microglia interactions: development of viral-based CX3CR1 antagonists.

Blocking the effects of fractalkine therapeutically may regulate microglia cell activation and provide neuroprotection in the AD brain. A human herpesvirus 8-encoded chemokine, termed vMIP-II is a non-selective chemokine receptor antagonist (binding multiple chemokine receptors, including CX3CR1). By comparing vMIP-II and FKN, we have generated molecules that selectively antagonize CX3CR1 activation. The results from these studies will guide future development of therapeutic agents designed to modulate microglial activation with the goal of preventing or slowing the progression of AD.

T cell costimulation by fractalkine-expressing synoviocytes in rheumatoid arthritis.

OBJECTIVE: Patients with rheumatoid arthritis (RA) accumulate prematurely aged T cells that have acquired a new profile of regulatory receptors. Many of the de novo-expressed receptors are typically found on natural killer cells, including CX(3)CR1, the receptor for the chemokine fractalkine (FKN). This study explored whether interactions between CX(3)CR1 and FKN are relevant for T cell functions in rheumatoid synovitis. METHODS: FKN expression was examined by real-time polymerase chain reaction and immunohistochemistry. CX(3)CR1 expression on peripheral blood T cells was analyzed by flow cytometry. T cell activation was quantified by determining proliferative responses, interferon-gamma (IFNgamma) secretion, and granule release. Fibroblast-like synoviocyte (FLS)/T cell adhesion was measured by the retention of 5-carboxyfluorescein diacetate succinimidyl ester-labeled T cells on FLS monolayers. RESULTS: FKN was expressed on cultured synovial fibroblasts and hyperplastic synoviocytes in the rheumatoid tissue. Among CD4+ T cells, only senescent CD28- T cells were positive for CX(3)CR1 (P < 0.001). Such CD4+,CD28-,CX(3)CR1+ T cells strongly adhered to FLS, with soluble FKN blocking the interaction. FKN expressed on FLS costimulated T cell-activating signals and amplified proliferation, IFNgamma production, and expulsion of cytoplasmic granules. CONCLUSION: Senescent CD4+ T cells that accumulate in rheumatoid arthritis aberrantly express CX(3)CR1. FKN, which is membrane-anchored on synoviocytes, enhances CD4+ T cell adhesion, provides survival signals, and costimulates the production of proinflammatory cytokines as well as the release of granules. By virtue of their altered receptor profile, senescent CD4+ T cells receive strong stimulatory signals from nonprofessional antigen-presenting cells in the synovial microenvironment.