Inflammatory agents regulate in vivo expression of fractalkine in endothelial cells of the rat heart

The cellular microenvironment regulates CX3CR1 expression on CD8+ T cells and the maintenance of CX3CR1+ CD8+ T cells

Expression levels of the chemokine receptor CX3CR1 serve as high-resolution marker delineating functionally distinct antigen-experienced T-cell states. The factors that influence CX3CR1 expression in T cells are, however, incompletely understood. Here, we show that in vitro priming of naïve CD8+ T cells failed to robustly induce CX3CR1, which highlights the shortcomings of in vitro priming settings in recapitulating in vivo T-cell differentiation. Nevertheless, in vivo generated memory CD8+ T cells maintained CX3CR1 expression during culture. This allowed us to investigate whether T-cell receptor ligation, cell death, and CX3CL1 binding influence CX3CR1 expression. T-cell receptor stimulation led to downregulation of CX3CR1. Without stimulation, CX3CR1+ CD8+ T cells had a selective survival disadvantage, which was enhanced by factors released from necrotic but not apoptotic cells. Exposure to CX3CL1 did not rescue their survival and resulted in a dose-dependent loss of CX3CR1 surface expression. At physiological concentrations of CX3CL1, CX3CR1 surface expression was only minimally reduced, which did not hamper the interpretability of T-cell differentiation states delineated by CX3CR1. Our data further support the broad utility of CX3CR1 surface levels as T-cell differentiation marker and identify factors that influence CX3CR1 expression and the maintenance of CX3CR1 expressing CD8+ T cells.

Fractalkine (CX3CL1) and Its Receptor CX3CR1: A Promising Therapeutic Target in Chronic Kidney Disease?

Innate immune cells are key contributors to kidney inflammation and fibrosis. Infiltration of the renal parenchyma by innate immune cells is governed by multiple signalling pathways. Since the discovery of the chemokine fractalkine (CX3CL1) and its receptor, CX3CR1 over twenty years ago, a wealth of evidence has emerged linking CX3CL1-CX3CR1 signalling to renal pathologies in both acute and chronic kidney diseases (CKD). However, despite the extent of data indicating a pathogenic role for this pathway in kidney disease and its complications, no human trials of targeted therapeutic agents have been reported. Although acute autoimmune kidney disease is often successfully treated with immunomodulatory medications, there is a notable lack of treatment options for patients with progressive fibrotic CKD. In this article we revisit the CX3CL1-CX3CR1 axis and its functional roles. Furthermore we review the accumulating evidence that CX3CL1-CX3CR1 interactions mediate important events in the intra-renal pathophysiology of CKD progression, particularly via recruitment of innate immune cells into the kidney. We also consider the role that systemic activation of the CX3CL1-CX3CR1 axis in renal disease contributes to CKD-associated cardiovascular disease. Based on this evidence, we highlight the potential for therapies targeting CX3CL1 or CX3CR1 to benefit people living with CKD.

Fractalkine: an inflammatory chemokine elevated in subjects with polycystic ovary syndrome

PURPOSE

Fractalkine (FKN) is an inflammatory chemokine related to reproductive system and glucose metabolism. There is a link between FKN and steroidogenesis as FKN induces progesterone synthesis. Polycystic ovary syndrome (PCOS) is a common reproductive and metabolic disorder associated with low progesterone production and insulin resistance. We aimed to explore whether women with PCOS have any difference in FKN levels compared to women without PCOS. We also focused on determination of any association between FKN levels and hormonal-metabolic parameters in women with PCOS.

METHODS

The current research was designed as a case-control study. Eighty subjects with PCOS and 80 age- and body mass index (BMI)-matched subjects with normal menstrual cycle were taken into the study. We measured circulating FKN levels via ELISA methods.

RESULTS

Circulating FKN levels were higher in women with PCOS than controls (1.93 ± 0.61 vs. 1.22 ± 0.33 ng/ml, P< 0.001). FKN levels showed a positive correlation with body mass index (BMI), insulin resistance, inflammatory marker hs-CRP, total testosterone, and free-androgen index (FAI), whereas it showed a negative correlation with sex hormone-binding protein in women with PCOS. Linear regression analyses revealed that the link of FKN with BMI, insulin resistance, hs-CRP, and FAI was independent. Binary logistic regression analysis showed that the risk of having PCOS was associated with high levels of FKN.

CONCLUSIONS

Increased FKN levels related to insulin resistance, inflammation and androgens in women with PCOS. FKN may have an inter-related role in different pathophysiologic pathways of PCOS.

Fractalkine – a local inflammatory marker aggravating platelet activation at the vulnerable plaque

Chemokines play an important role in inducing chemotaxis of cells, piloting white blood cells in immune surveillance and are crucial parts in the development and progression of atherosclerosis. Platelets are mandatory players in the initiation of atherosclerotic lesion formation and are susceptible targets for and producers of chemokines. Several chemokine receptors on platelets have been described previously, amongst them CX3CR1, the receptor for fractalkine. The unique chemokine fractalkine (CX3CL1, FKN) exists as a soluble as well as a membrane-anchored glycoprotein. Its essential role in the formation of atherosclerotic lesions and atherosclerosis progression has been impressively described in mouse models. Moreover, fractalkine induces platelet activation and adhesion via a functional fractalkine receptor (CX3CR1) expressed on the platelet surface. Platelet activation via the FKN/CX3CR1-axis triggers leukocyte adhesion to activated endothelium, and fractalkine-induced platelet P-selectin is mandatory for leukocyte recruitment under arterial flow conditions. This review summarises the role of fractalkine as a potential local inflammatory mediator which influences platelet activation in the setting of atherosclerosis. Beyond that, aspects of a potential interaction between fractalkine and platelet responsiveness to antiplatelet drugs are described. Furthermore, the possible impact of high-density lipoprotein cholesterol (HDL-C) on atherosclerosis progression, platelet activation and fractalkine signalling are discussed.

Origin and production of inflammatory perivascular macrophages in pulmonary hypertension

Myeloid cells, including monocytes and macrophages participate in steady state immune homeostasis and help mount the adaptive immune response during infection. The function and production of these cells in sterile inflammation, such as pulmonary hypertension (PH), is understudied. Emerging data indicate that pulmonary inflammation mediated by lung perivascular macrophages is a key pathogenic driver of pulmonary remodeling leading to increased right ventricular systolic pressure (RVSP). However, the origin of these macrophages in pulmonary inflammation is unknown. Inflammatory monocytes, the precursors of pathogenic macrophages, are derived from hematopoietic stem and progenitor cells (HSPC) in the bone marrow and spleen during acute and chronic inflammation. Understanding the role of these organs in monocytopoiesis, and the mechanisms of HSPC proliferation and differentiation in PH are important to discover therapeutic targets curbing inflammation. This review will summarize the current limited knowledge of the origin of lung macrophage subsets and over-production of inflammatory monocytes in PH.

The therapeutic potential of targeting chemokine signalling in the treatment of chronic pain

Chronic pain is a distressing condition, which is experienced even when the painful stimulus, whether surgery or disease related, has subsided. Current treatments for chronic pain show limited efficacy and come with a host of undesirable side-effects, and thus there is a need for new, more effective therapies to be developed. The mechanisms underlying chronic pain are not fully understood at present, although pre-clinical models have facilitated the progress of this understanding considerably in the last decade. The mechanisms underlying chronic pain were initially thought to be neurocentric. However, we now appreciate that non-neuronal cells play a significant role in nociceptive signalling through their communication with neurons. One of the major signalling pathways, which mediates neuron/non-neuronal communication, is chemokine signalling. In this review, we discuss selected chemokines that have been reported to play a pivotal role in the mechanisms underlying chronic pain in a variety of pre-clinical models. Approaches that target each of the chemokines discussed in this review come with their advantages and disadvantages; however, the inhibition of chemokine actions is emerging as an innovative therapeutic strategy, which is now reaching the clinic, with the chemokine Fractalkine and its CX3 CR1 receptor leading the way. This article is part of the special article series "Pain".

Plasmodium falciparum proteins involved in cytoadherence of infected erythrocytes to chemokine CX3CL1

Malaria caused by Plasmodium falciparum is associated with cytoadherence of infected red blood cells (iRBC) to endothelial cells. Numerous host molecules have been involved in cytoadherence, including the adhesive chemokine CX3CL1. Most of the identified parasite ligands are from the multigenic and hypervariable Plasmodium falciparum Erythrocyte Membrane Protein 1 (PfEMP1) family which makes them poor targets for the development of a broadly protective vaccine. Using proteomics, we have identified two 25-kDa parasite proteins with adhesive properties for CX3CL1, called CBP for CX3CL1 Binding Proteins. CBPs are coded by single-copy genes with little polymorphic variation and no homology with other P. falciparum gene products. Specific antibodies raised against epitopes from the predicted extracellular domains of each CBP efficiently stain the surface of RBC infected with trophozoites or schizonts, which is a strong indication of CBP expression at the surface of iRBC. These anti-CBP antibodies partially neutralize iRBC adherence to CX3CL1. This adherence is similarly inhibited in the presence of peptides from the CBP extracellular domains, while irrelevant peptides had no such effect. CBP1 and CBP2 are new P. falciparum ligands for the human chemokine CX3CL1. The identification of this non-polymorphic P. falciparum factors provides a new avenue for innovative vaccination approaches.

Analysis of the Role of CX3CL1 (Fractalkine) and Its Receptor CX3CR1 in Traumatic Brain and Spinal Cord Injury: Insight into Recent Advances in Actions of Neurochemokine Agents

CX3CL1 (fractalkine) is the only member of the CX3C (delta) subfamily of chemokines which is unique and combines the properties of both chemoattractant and adhesion molecules. The two-form ligand can exist either in a soluble form, like all other chemokines, and as a membrane-anchored molecule. CX3CL1 discloses its biological properties through interaction with one dedicated CX3CR1 receptor which belongs to a family of G protein-coupled receptors (GPCR). The CX3CL1/CX3CR1 axis acts in many physiological phenomena including those occurring in the central nervous system (CNS), by regulating the interactions between neurons, microglia, and immune cells. Apart from the role under physiological conditions, the CX3CL1/CX3CR1 axis was implied to have a role in different neuropathologies such as traumatic brain injury (TBI) and spinal cord injury (SCI). CNS injuries represent a serious public health problem, despite improvements in therapeutic management. To date, no effective treatment has been determined, so they constitute a leading cause of death and severe disability. The course of TBI and SCI has two consecutive poorly demarcated phases: the initial, primary injury and secondary injury. Recent evidence has implicated the role of the CX3CL1/CX3CR1 axis in neuroinflammatory processes occurring after CNS injuries. The importance of the CX3CL1/CX3CR1 axis in the pathophysiology of TBI and SCI in the context of systemic and direct local immune response is still under investigation. This paper, based on a review of the literature, updates and summarizes the current knowledge about CX3CL1/CX3CR1 axis involvement in TBI and SCI pathogenesis, indicating possible molecular and cellular mechanisms with a potential target for therapeutic intervention.

Ly6Chigh Monocytes Protect against Kidney Damage during Sepsis via a CX3CR1-Dependent Adhesion Mechanism

Monocytes have a crucial role in both proinflammatory and anti-inflammatory phenomena occurring during sepsis. Monocyte recruitment and activation are orchestrated by the chemokine receptors CX3CR1 and CCR2 and their cognate ligands. However, little is known about the roles of these cells and chemokines during the acute phase of inflammation in sepsis. Using intravital microscopy in a murine model of polymicrobial sepsis, we showed that inflammatory Ly6C(high) monocytes infiltrated kidneys, exhibited altered motility, and adhered strongly to the renal vascular wall in a chemokine receptor CX3CR1-dependent manner. Adoptive transfer of Cx3cr1-proficient monocyte-enriched bone marrow cells into septic Cx3cr1-depleted mice prevented kidney damage and promoted mouse survival. Modulation of CX3CR1 activation in septic mice controlled monocyte adhesion, regulated proinflammatory and anti-inflammatory cytokine expression, and was associated with the extent of kidney lesions such that the number of lesions decreased when CX3CR1 activity increased. Consistent with these results, the pro-adhesive I249 CX3CR1 allele in humans was associated with a lower incidence of AKI in patients with sepsis. These data show that inflammatory monocytes have a protective effect during sepsis via a CX3CR1-dependent adhesion mechanism. This receptor might be a new therapeutic target for kidney injury during sepsis.

Fractalkine/CX3CR1 signaling during neuropathic pain

Chronic pain represents a major problem in clinical medicine. Whilst the acute pain that is associated with tissue injury is a protective signal that serves to maintain homeostasis, chronic pain is a debilitating condition that persists long after the inciting stimulus subsides. Chronic neuropathic pain that develops following damage or disease of the nervous system is partially treated by current therapies, leaving scope for new therapies to improve treatment outcome. Peripheral nerve damage is associated with alterations to the sensory neuroaxis that promote maladaptive augmentation of nociceptive transmission. Thus, neuropathic pain patients exhibit exaggerated responses to noxious stimuli, as well as pain caused by stimuli which are normally non-painful. Increased nociceptive input from the periphery triggers physiological plasticity and long lasting transcriptional and post-translational changes in the CNS defined as central sensitization. Nerve injury induces gliosis which contributes to central sensitization and results in enhanced communication between neurons and microglial cells within the dorsal horn. Thus, identification of mechanisms regulating neuro-immune interactions that occur during neuropathic pain may provide future therapeutic targets. Specifically, chemokines and their receptors play a pivotal role in mediating neuro-immune communication which leads to increased nociception. In particular, the chemokine Fractalkine (FKN) and the CX3CR1 receptor have come to light as a key signaling pair during neuropathic pain states.

Fractalkine promotes platelet activation and vascular dysfunction in congestive heart failure

Endothelial dysfunction and enhanced platelet reactivity in congestive heart failure (CHF) contribute to poor prognosis. CHF patients display an impaired responsiveness to clopidogrel. Fractalkine activates platelets and elevated plasma levels of this chemokine are a feature of CHF. We here addressed the interrelation of fractalkine, platelet reactivity and clopidogrel efficacy in humans and rats with CHF. Fractalkine serum levels determined by ELISA were increased in CHF patients (CHF: 1548 ± 650 pg/ml;

CONTROL

968 ± 575 pg/ml, p<0.01) and following CHF induction in rats (CHF: 1509 ± 753 pg/ml; Sham: 1181 ± 275 pg/ml, p<0.05). Expression of fractalkine and its receptor CX3CR1 was enhanced in aortas of CHF rats as determined by immunofluorescence microscopy and molecular analysis. Fractalkine significantly aggravated endothelial dysfunction and augmented P-selectin expression on platelets from CHF rats. Platelet surface expression of CX3CR1 was increased in CHF rats, who displayed an impaired response to clopidogrel (platelet reactivity to ADP: CHF 30 ± 22%; Sham: 8 ± 5%, p<0.05). Similarly in humans with CHF, elevated fractalkine levels were accompanied by reduced clopidogrel responsiveness. Patients with high on-clopidogrel treatment platelet P2Y12 reactivity displayed higher fractalkine levels (1525 ± 487 pg/ml) than those with sufficient clopidogrel response (684 ± 315 pg/ml, p<0.01). In conclusion, in CHF fractalkine was increased on the endothelium and in blood serum, and platelet surface-expression of CX3CR1 was enhanced. Fractalkine diminished endothelial function beyond the impairment already observed in CHF and was associated with a reduced responsiveness to the platelet inhibitor clopidogrel. These findings may indicate a novel pathophysiological mechanism contributing to impaired clopidogrel responsiveness in CHF.

Type I interferon induces CX3CL1 (fractalkine) and CCL5 (RANTES) production in human pulmonary vascular endothelial cells

Type I interferon (IFN) medications cause various adverse reactions, including vascular diseases. Although an association between chemokines and vascular diseases has also been reported, the relationship between type I IFN and chemokines in vascular endothelial cells (VEC) remains unclear. To provide clues to pathogenesis of the diseases, we analysed the effects of type I IFN on chemokine production in human VEC. Type I IFN induced higher CX3CL1 (fractalkine) mRNA expression and protein secretion in pulmonary arterial VEC than in umbilical vein VEC. Type I IFN also induced CCL5 [regulated upon activation normal T cell expressed and secreted (RANTES)] production in VEC, especially in lung micro-VEC. IFN-β induced much higher chemokine production than IFN-α, and Janus protein tyrosine kinase (JAK) inhibitor I prevented type I IFN-induced chemokine secretion. Type I IFN-induced chemokines may be involved in the pathophysiology of pulmonary vascular diseases, and the JAK inhibitor may serve as a therapeutic option for these diseases.

Fractalkine activates a signal transduction pathway similar to P2Y12 and is associated with impaired clopidogrel responsiveness

OBJECTIVE

Fractalkine (FKN) activates a G(αi) protein-coupled signaling pathway similar to the one activated by ADP via P2Y(12), which is the drug target of clopidogrel. FKN levels are increased under several disease conditions associated with impaired clopidogrel responsiveness.

METHODS AND RESULTS

Blood samples were obtained from healthy volunteers and from 40 patients under chronic clopidogrel treatment. FKN reduced prostaglandin E1-induced vasodilator-stimulated phosphoprotein phosphorylation by ≈ 25% (P<0.01) at least partially mimicking the effect of ADP via P2Y(12). In vitro, FKN increased platelet reactivity index in clopidogrel-treated patients indicating potential activation of downstream targets of P2Y(12). When stratifying patients by their FKN levels, patients within the highest quartile of FKN (2042 ± 25 pg/mL) had the weakest response to clopidogrel (platelet reactivity index, 68 ± 4%), and patients within the lowest quartile (479 ± 50 pg/mL) had the strongest response (platelet reactivity index, 48 ± 7%; P=0.0106). FKN by itself induced phosphoinositide 3-kinase activation leading to Akt phosphorylation at Ser(473) (P<0.01 versus basal).

CONCLUSIONS

In addition to desensitizing platelets to prostaglandin E1 via G(αi), FKN induces phosphoinositide 3-kinase-dependent Akt phosphorylation via a G(βγ) protein similar to ADP signaling through P2Y(12). FKN increased the platelet ADP response in clopidogrel-treated patients. Once released from an atherosclerotic lesion, this mechanism could contribute locally to impaired clopidogrel responsiveness at the vulnerable plaque.

Fractalkine (CX3CL1) levels in patients with Behçet's disease and neuro-Behçet's disease

OBJECTIVE

The aim of the present study was to assess the role of CX3CL1 in patients with active and inactive Behçet's Disease (BD), Neuro-Behçet's Disease (NBD) and control subjects.

METHODS

Fifty-six patients admitted to the BD and NBD Outpatient Clinics, and 30 healthy controls were enrolled in the study. Serum CX3CL1 levels were measured by an enzyme linked immunosorbent assay (ELISA).

RESULTS

No significant difference was found between the serum CX3CL1 levels of control subjects, and patients with active and inactive BD or NBD, regardless of treatment.

CONCLUSION

To our knowledge, this is the first study analyzing CX3CL1 levels in patients with BD and NBD. Our results demonstrated that serum CX3CL1 levels were not changed in active and inactive BD and NBD. However, further large-scale studies are needed to confirm our results.

CX(3)CR1 drives cytotoxic CD4(+)CD28(-) T cells into the brain of multiple sclerosis patients

Immunosenescence, or ageing of the immune system, contributes to the increased morbidity and mortality seen in the elderly population. Premature immunosenescence is shown to occur in a subgroup of patients with autoimmune diseases. One of the main characteristics of immunosenescence is the expansion of CD4(+)CD28(-) T cells in the blood. In this study, we investigate the potential contribution of these cells to disease processes in a subgroup of multiple sclerosis (MS) and rheumatoid arthritis (RA) patients. Characterization of CD4(+)CD28(-) T cells in patients and healthy controls reveals that they have an inflammation-seeking effector-memory T cell phenotype with cytotoxic properties, as they expel cytotoxic granules in response to a polyclonal stimulus or MS-related autoantigens. We identify CX(3)CR1, the fractalkine receptor, as a selective marker to discriminate CD4(+)CD28(-) T cells from their CD4(+)CD28(+) counterparts. CX(3)CR1 expression enables CD4(+)CD28(-) T cells to migrate towards a fractalkine gradient in vitro. In addition, we find increased levels of fractalkine in the cerebrospinal fluid and inflammatory lesions of MS patients. We demonstrate for the first time that CD4(+)CD28(-) T cells accumulate in MS lesions of a subgroup of patients. Moreover, we have indications that these cells are cytotoxic in the target tissue. Overall, our findings suggest that CD4(+)CD28(-) T cells migrate in response to a chemotactic gradient of fractalkine to sites of inflammation, where they contribute to the inflammatory processes in a subgroup of patients with MS and RA.

Synergistic induction of CX3CL1 by interleukin-1β and interferon-γ in human lung fibroblasts: involvement of signal transducer and activator of transcription 1 signaling pathways

CX3CL1 (fractalkine), a membrane-bound chemokine that induces both the adhesion and the migration of leukocytes, is involved in the recruitment of cells into tissues undergoing inflammatory responses. To explore the regulation of CX3CL1 in pulmonary inflammation and fibrosis, CX3CL1 expression in lung fibroblasts was examined. Normal human fibroblasts were obtained from Promocell (Lonza Walkersville Inc, Md) and were incubated in the presence or absence of various inflammatory stimuli. Culture supernatants were collected, and the soluble CX3CL1 levels were determined with an enzyme-linked immunosorbent assay. The expression of CX3CL1 mRNA transcripts in lung fibroblasts was assessed using quantitative TaqMan real-time polymerase chain reaction. Interleukin (IL)-1β or interferon (IFN)-γ individually induced negligible soluble CX3CL1 secretion by human lung fibroblasts after 24 h. However, the combination of IL-1β and IFN-γ induced dramatic increases in both soluble CX3CL1 protein and mRNA transcripts in a dose- and time-dependent manner. Synergistic up-regulation of cell-associated CX3CL1 protein also was observed after treatment with IL-1β and IFN-γ. The secretion and expression of lung fibroblast-derived CX3CL1 were markedly reduced by specific inhibitors of the STAT-1 transcription factor. These findings suggest that lung fibroblasts are an important cellular source of CX3CL1 and may play a role in pulmonary inflammation and fibrosis.

Patients with insulin-dependent diabetes or coronary heart disease following rehabilitation express serum fractalkine levels similar to those in healthy control subjects

The chemokine and adhesion molecule fractalkine and its receptor CX₃CR1 have emerged as interesting regulators in inflammation and related atherosclerosis. The pro-inflammatory status may be counteracted by appropriate treatment, such as in rehabilitation. We compared serum fractalkine concentrations of 46 patients with coronary heart disease (CHD) and 47 insulin-dependent diabetic patients (IDDM) following rehabilitation with those of 50 control subjects. Following rehabilitation serum fractalkine levels (477 ± 225 pg/mL) in CHD patients were similar to those in control subjects (572 ± 205 pg/mL; P = 0.303), whereas fractalkine levels were lower in IDDM patients (430 ± 256 pg/mL; P = 0.042). No significant difference between CHD and IDDM patients was present (P = 0.319). Postprandial hyperlipemia may influence inflammation; thus, we investigated fractalkine levels four and eight hours after inducing postprandial hyperlipemia. However, we did not find any significant alterations in CHD and diabetic patients, whereas the fractalkine levels in controls were reduced. In vitro, lipofundin used as a hyperlipemic stimulus was added to vessel wall cells and reduced fractalkine levels. Low fractalkine levels in patients attending rehabilitation indicate a beneficial effect of the rehabilitation procedure on innate inflammatory pathways, such as the chemokine and adhesion molecule fractalkine.

Constitutive endocytosis of the chemokine CX3CL1 prevents its degradation by cell surface metalloproteases

CX(3)CL1, a chemokine with transmembrane and soluble species, plays a key role in inflammation by acting as both chemoattractant and adhesion molecule. CX(3)CL1 is the only chemokine known to undergo constitutive internalization, raising the possibility that dynamic equilibrium between the endocytic compartment and the plasma membrane critically regulates the availability and processing of CX(3)CL1 at the cell surface. We therefore investigated how transmembrane CX(3)CL1 is internalized. Inhibition of dynamin using a nonfunctional allele or of clathrin using specific small interfering RNA prevented endocytosis of the chemokine in CX(3)CL1-expressing human ECV-304 cells. Perusal of the cytoplasmic domain of CX(3)CL1 revealed two putative adaptor protein-2 (AP-2)-binding motifs. Accordingly, CX(3)CL1 co-localized with AP-2 at the plasma membrane. We generated a mutant allele of CX(3)CL1 lacking the cytoplasmic tail. Deletion of the cytosolic tail precluded internalization of the chemokine. We used site-directed mutagenesis to disrupt AP-2-binding motifs, singly or in combination, which resulted in diminished internalization of CX(3)CL1. Although CX(3)CL1 was present in both superficial and endomembrane compartments, ADAM10 (a disintegrin and metalloprotease 10) and tumor necrosis factor-converting enzyme, the two metalloproteases that cleave CX(3)CL1, localized predominantly to the plasmalemma. Inhibition of endocytosis using the dynamin inhibitor, Dynasore, promoted rapid metalloprotease-dependent shedding of CX(3)CL1 from the cell surface into the surrounding medium. These findings indicate that the cytoplasmic tail of CX(3)CL1 facilitates its constitutive clathrin-mediated endocytosis. Such regulation enables intracellular storage of a sizable pool of presynthesized CX(3)CL1 that protects the chemokine from degradation by metalloproteases at the plasma membrane.

Cytokine expression profiling of the myocardium reveals a role for CX3CL1 (fractalkine) in heart failure

Several lines of evidence suggest that inflammatory processes mediated by cytokines are involved in the pathogenesis of heart failure (HF). However, the regulation of cytokine expression and the role of cytokines during HF development are not well understood. To address this issue, we have examined alterations in gene expression during HF progression by microarray technology in non-infarcted left ventricular (LV) murine tissue at various time points after myocardial infarction (MI). The highest number of regulated genes was found five days after MI. In total, we identified 14 regulated genes encoding cytokines with no previous association to HF. The strongest up-regulation was found for the chemokine fractalkine (CX3CL1). In human failing hearts we detected a 3-fold increase in CX3CL1 protein production, and both cardiomyocytes and fibrous tissue revealed immunoreactivity for CX3CL1 and its specific receptor CX3CR1. We also found that the circulating level of CX3CL1 was increased in patients with chronic HF in accordance with disease severity (1.6-fold in NYHA II, 2.2-fold in NYHA III and 2.9-fold in NYHA IV). In vitro experiments demonstrated that CX3CL1 production could be induced by inflammatory cytokines known to be highly expressed in HF. CX3CL1 itself induced the expression of markers of cardiac hypertrophy and protein phosphatases in neonatal cardiomyocytes. Given the increased CX3CL1 production in both an experimental HF model and in patients with chronic HF as well as its direct effects on cardiomyocytes, we suggest a role for CX3CL1 and its receptor CX3CR1 in the pathogenesis of HF.

Thrombin-induced expression of endothelial CX3CL1 potentiates monocyte CCL2 production and transendothelial migration

CX3CL1 (fractalkine, neurotactin) is the sole CX3C chemokine. It induces monocyte locomotion in its cleaved form, but in its membrane-anchored form, it also acts as an adhesion molecule. The expression of CX3CL1 is up-regulated in endothelial cells by proinflammatory cytokines such as IL-1 or TNF-alpha. Here, we studied the effect of the serine protease thrombin on endothelial CX3CL1 induction and its putative relevance for monocyte function. In HUVEC, thrombin triggered a time- and concentration-dependent expression of CX3CL1 at the mRNA and the protein level as shown by RT-PCR, Western immunoblotting, and flow cytometric analysis. Thrombin induced CX3CL1 by activating protease-activated receptor 1 (PAR1) as demonstrated by the use of PAR1-activating peptide and the PAR1-specific antagonist SCH 79797. The thrombin-induced CX3CL1 expression was NF-kappaB-dependent, as shown by EMSA, ELISA, and by inhibition of the NF-kappaB signaling pathway by the IkappaB kinase inhibitor acety-11-keto-beta-boswellic acid or by transient overexpression of a transdominant-negative form of IkappaBalpha. Upon cocultivation of human monocytes with HUVEC, the thrombin-dependent induction of membrane-anchored CX3CL1 in HUVEC triggered monocyte adhesion and an enhanced release of the MCP-1/CCL2 by monocytes and potentiated the monocyte transendothelial migration. Accordingly, the recombinant extracellular domain of CX3CL1 induced CCL2 release by monocytes. Thus, the thrombin-induced monocyte/endothelial cell cross-talk mediated by increased CX3CL1 expression potentiates the CCL2 chemokine generation that might contribute to the recruitment of monocytes into inflamed areas.

CX3CR1 deficiency impairs dendritic cell accumulation in arterial intima and reduces atherosclerotic burden

OBJECTIVE

Dendritic cells (DCs) have recently been found in atherosclerosis-predisposed regions of arteries and have been proposed to be causal in atherosclerosis. The chemokine receptor CX3CR1 is associated with arterial injury and atherosclerosis. We sought to determine whether a link exists between arterial DC accumulation, CX3CR1, and atherosclerosis.

METHODS AND RESULTS

Mouse aortas were isolated and subjected to en face immunofluorescence analysis. We found that DCs were located predominantly in the intimal regions of arterial branch points and curvatures. Consistent with the increased accumulation of intimal DCs in aged and ApoE-/- aortas compared with young WT aortas (P=0.004 and 0.05, respectively), the incidence of atherosclerosis was 88.9% for aged WT and 100% for ApoE-/- mice compared with 0% for young WT mice. CX3CR1 was expressed on intimal DCs and DC numbers were decreased in CX3CR1-deficient aortas of young, aged, and ApoE-/- mice (P=0.0008, 0.013, and 0.0099). The reduced DC accumulation in CX3CR1-deficiency was also correlated with decreased atherosclerosis in these animals.

CONCLUSIONS

The accumulation of intimal DC increases in aged and ApoE-/- aortas and correlates with the generation of atherosclerosis. CX3CR1-deficiency impairs the accumulation of DC in the aortic wall and markedly reduces the atherosclerotic burden.

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.

Involvement of the fractalkine pathway in the pathogenesis of childhood hemolytic uremic syndrome

Thrombotic microangiopathy and acute renal failure are cardinal features of postdiarrheal hemolytic uremic syndrome (HUS). These conditions are related to endothelial and epithelial cell damage induced by Shiga toxin (Stx) through the interaction with its globotriaosyl ceramide receptor. However, inflammatory processes contribute to the pathogenesis of HUS by sensitizing cells to Stx fractalkine (FKN), a CX(3)C transmembrane chemokine expressed on epithelial and endothelial cells upon activation, is involved in the selective migration and adhesion of specific leukocyte subsets to tissues. Here, we demonstrated a selective depletion of circulating mononuclear leukocytes expressing the receptor for FKN (CX(3)CR1) in patients with HUS. We found a unique phenotype in children with HUS distinct from that seen in healthy, uremic, or infected controls, in which monocytes lost CX(3)CR1, down-modulated CD62L, and increased CD16. In addition, the CD56(dim) natural killer (NK) subpopulation was decreased, leading to an altered peripheral CD56(dim)/CD56(bright) ratio from 10.0 to 4.5. It is noteworthy that a negative correlation existed between the percentage of circulating CX(3)CR1(+) leukocytes and the severity of renal failure. Finally, CX(3)CR1(+) leukocytes were observed in renal biopsies from patients with HUS. We suggest that the interaction of CX(3)CR1(+) cells with FKN present on activated endothelial cells may contribute to renal injury in HUS.

CX 3 CR1 Deficiency Confers Protection From Intimal Hyperplasia After Arterial Injury

Objective— A functional polymorphism in the chemokine receptor CX 3 CR1 is associated with protection from vascular diseases including coronary artery disease and internal carotid artery occlusive disease. We investigated the mechanisms by which CX 3 CR1 may be involved by evaluating the inflammatory response to arterial injury in CX 3 CR1-deficient animals.

Methods and Results— Femoral arteries of CX 3 CR1 −/− and wild-type (WT) mice were injured with an angioplasty guide wire. After 1, 5, 14, and 28 days, arteries were harvested and evaluated by histology, morphometry, and immunohistochemistry. Arterial injury upregulated the CX 3 CR1 ligand CX 3 CL1. In CX 3 CR1 −/− compared with WT animals, the incidence of neointima formation was 58% lower ( P =0.0017), accompanied by no difference in the area of platelet accumulation at day 1 ( P =0.48) but a significant decrease in intimal monocyte infiltration at day 5 ( P =0.006), vascular smooth muscle cell (VSMC) proliferation at days 5 and 14, and intimal area at day 28 ( P =0.009).

Conclusions— In an endothelial denudation injury model, CX 3 CR1 deficiency protects animals from developing intimal hyperplasia as a result of decreased monocyte trafficking to the lesion. CX 3 CR1 deficiency decreases VSMC proliferation and intimal accumulation either directly or indirectly as a result of defective monocyte infiltration.

The expression of fractalkine in the endometrium during the menstrual cycle

OBJECTIVE

The objective of the study was to evaluate the presence of fractalkine in the endometrium of the uterus and the change of fractalkine protein levels during menstrual cycle.

METHODS

Twelve samples of endometrium of the uterus were obtained from gynecological patients who underwent total hysterectomy. Western blotting, RT-PCR and immunohistochemistry were performed.

RESULTS

Fractalkine protein was detected in the endometrium of the uterus. Positive staining was confirmed in the epithelial cells and grandular cells in the endometrium. Expression levels of fractalkine protein and mRNA in the endometrium during secretory phase were significantly higher than those during proliferative phase. Immunohistochemical analysis using an anti-CX3CR1 antibody demonstrated positive staining in the glandular cells of the endometrium of the uterus.

CONCLUSION

Fractalkine was expressed in the endometrium and its production was up-regulated during secretory phase.

Fractalkine in the peritoneal fluid of women with endometriosis

OBJECTIVE

The objective of this study was to evaluate the presence of fractalkine in the ascites and the association between fractalkine levels in the ascites and endometriosis.

METHODS

Peritoneal fluids and peripheral blood samples were obtained from patients undergoing laparoscopy for infertility work-up or laparoscopic cystectomy. Three samples of peritoneum were obtained from patients undergoing hysterectomy. Western blotting, RT-PCR and immunohistochemistry were performed.

RESULTS

Fractalkine protein was detected in the ascites. Positive staining was confirmed in peritoneal surface cells and perivascular cells of the peritoneum. CX3CR1 positive cells were present in the cells in the peritoneal fluid. The fractalkine concentrations in the ascites of patients with endometriosis were lower than those without endometriosis. There was no significant difference between serum fractalkine levels in patients with and without endometriosis.

CONCLUSION

The decreased level of fractalkine found in the peritoneal fluid of patients with endometriosis may contribute to the pathogenesis of endometriosis.

Increased expression of the chemokine fractalkine in Crohn's disease and association of the fractalkine receptor T280M polymorphism with a fibrostenosing disease Phenotype

BACKGROUND

The fractalkine receptor CX3CR1 has been shown to be involved in inflammation and immune response. Recently, two polymorphisms of CX3CR1 (V249I and T280M) were reported.

AIMS

Our aim was to analyze fractalkine expression and the role of CX3CR1 polymorphisms in Crohn's disease (CD).

METHODS

We determined fractalkine mRNA expression in the intestinal epithelial cell (IEC) line SW480 after stimulation with proinflammatory cytokines as well as in inflamed (n = 14) and noninflamed (n = 14) CD lesions by quantitative PCR. By restriction fragment length polymorphism analysis, genomic DNA from 206 patients with CD and 211 unrelated controls was analyzed for the two single nucleotide polymorphisms in the CX3CR1 gene, which result in the V249I and T280M substitutions.

RESULTS

All proinflammatory stimuli (TNF-alpha, IL-1beta, LPS) significantly increased fractalkine mRNA expression in IEC. There was also a significant increase in fractalkine mRNA expression in inflamed lesions of CD patients when compared to noninflamed colonic mucosa (p = 0.02). Intestinal fractalkine mRNA levels correlated highly with IL-8 mRNA expression levels (r = 0.931). However, there was no difference in the V249I and T280M genotype frequencies between CD patients and the control group. In the CD group, 33.0% were heterozygous and 8.3% homozygous for the V249I polymorphism, while 23.3% were heterozygous and 4.4% homozygous for the T280M polymorphism. All T280M homozygotes were diagnosed of intestinal stenosis (p = 0.03 vs wildtype and heterozygous carriers) and had significantly more often ileocolonic involvement more often than patients with wildtype and heterozygous genotypes (p = 0.01). These associations were independent of the CARD15 genotype status.

CONCLUSIONS

The expression of the chemokine fractalkine is upregulated by proinflammatory cytokines and enhanced in inflamed CD lesions. The CX3CR1 T280M polymorphism appears to influence CD phenotype and localization.

Protective effect of alpha-lipoic acid in lipopolysaccharide-induced endothelial fractalkine expression

Fractalkine is a unique chemokine that functions as a chemoattractant as well as an adhesion molecule on endothelial cells activated by proinflammatory cytokines. Alpha-lipoic acid (LA), a naturally occurring dithiol compound, is an essential cofactor for mitochondrial bioenergetic enzymes. LA improves glycemic control, reduces diabetic polyneuropathies, and mitigates toxicity associated with heavy metal poisoning. The effects of LA on processes associated with sepsis, however, are unknown. We evaluated the antiinflammatory effect of LA on fractalkine expression in a lipopolysaccharide-induced endotoxemia model. Tumor necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta) significantly induced fractalkine mRNA and protein expression in endothelial cells. LA strongly suppressed TNF-alpha- or IL-1beta-induced fractalkine expression in endothelial cells by suppressing the activities of nuclear factor-kappaB and specificity protein-1. LA also decreased TNF-alpha- or IL-1beta-stimulated monocyte adhesion to human umbilical vein endothelial cells. As shown by immunohistochemistry, fractalkine protein expression was markedly increased by treatment with lipopolysaccharide in arterial endothelial cells, endocardium, and endothelium of intestinal villi. LA suppressed lipopolysaccharide-induced fractalkine protein expression and infiltration of endothelin 1-positive cells into the heart and intestine in vivo. LA protected against lipopolysaccharide-induced myocardial dysfunction and improved survival in lipopolysaccharide-induced endotoxemia. These results suggest that LA could be an effective agent to reduce fractalkine-mediated inflammatory processes in endotoxemia.

CX3CL1 (fractalkine) and CX3CR1 expression in myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis: kinetics and cellular origin

BACKGROUND

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS). It is associated with local activation of microglia and astroglia, infiltration of activated macrophages and T cells, active degradation of myelin and damage to axons and neurons. The proposed role for CX3CL1 (fractalkine) in the control of microglia activation and leukocyte infiltration places this chemokine and its receptor CX3CR1 in a potentially strategic position to control key aspects in the pathological events that are associated with development of brain lesions in MS. In this study, we examine this hypothesis by analyzing the distribution, kinetics, regulation and cellular origin of CX3CL1 and CX3CR1 mRNA expression in the CNS of rats with an experimentally induced MS-like disease, myelin oligodendrocyte glycoprotein (MOG)-induced autoimmune encephalomyelitis (EAE).

METHODS

The expression of CX3CL1 and its receptor CX3CR1 was studied with in situ hybridization histochemical detection of their mRNA with radio labeled cRNA probes in combination with immunohistochemical staining of phenotypic cell markers. Both healthy rat brains and brains from rats with MOG EAE were analyzed. In defined lesional stages of MOG EAE, the number of CX3CR1 mRNA-expressing cells and the intensity of the in situ hybridization signal were determined by image analysis. Data were statistically evaluated by ANOVA, followed by Tukey\primes multiple comparison test.

RESULTS

Expression of CX3CL1 mRNA was present within neuronal-like cells located throughout the neuraxis of the healthy rat. Expression of CX3CL1 remained unaltered in the CNS of rats with MOG-induced EAE, with the exception of an induced expression in astrocytes within inflammatory lesions. Notably, the brain vasculature of healthy and encephalitic animals did not exhibit signs of CX3CL1 mRNA expression. The receptor, CX3CR1, was expressed by microglial cells in all regions of the healthy brain. Induction of MOG-induced EAE was associated with a distinct accumulation of CX3CR1 mRNA expressing cells within the inflammatory brain lesions, the great majority of which stained positive for markers of the microglia-macrophage lineage. Analysis in time-staged brain lesions revealed elevated levels of CX3CR1 mRNA in microglia in the periplaque zone, as well as a dramatically enhanced accumulation of CX3CR1 expressing cells within the early-active, late-active and inactive, demyelinated lesions.

CONCLUSION

Our data demonstrate constitutive and regulated expression of the chemokine CX3CL1 and its receptor CX3CR1 by neurons/astrocytes and microglia, respectively, within the normal and inflamed rat brain. Our findings propose a mechanism by which neurons and reactive astrocytes may control migration and function of the surrounding microglia. In addition, the accumulation of CX3CR1 expressing cells other than microglia within the inflammatory brain lesions indicate a possible role for CX3CL1 in controlling invasion of peripheral leucocytes to the brain.

Recycling of the Membrane-anchored Chemokine, CX3CL1

CX(3)CL1 (fractalkine) plays an important role in inflammation by acting as both chemoattractant and as an adhesion molecule. As for other chemokines, expression of CX(3)CL1 is known to be regulated at the level of transcription and translation. The unique transmembrane structure of CX(3)CL1 raises the possibility of additional functional regulation by altering its abundance at the cell surface. This could be accomplished in principle by changes in traffic between subcellular compartments. To analyze this possibility we examined the subcellular distribution of CX(3)CL1 in human ECV-304 cells stably expressing untagged or green fluorescent protein-tagged forms of the chemokine. CX(3)CL1 was present in two distinct compartments, diffusely on the plasma membrane and in a punctate juxtanuclear compartment. The latter shared some features with, yet was distinct from the conventional endocytic pathway and may represent a specialized recycling subcompartment. Accordingly, surface CX(3)CL1 was found to be in dynamic equilibrium with the juxtanuclear vesicular compartment. Intracellular CX(3)CL1 co-localized with the SNARE (soluble N-ethylmaleimide factor attachment protein receptor) proteins syntaxin-13 and VAMP-3. Cleavage of VAMP-3 by tetanus toxin or impairment of syntaxin-13 function by expression of a dominant-negative allele inhibited the ability of internalized CX(3)CL1 to traffic back to the plasma membrane. These data demonstrate the existence of a dynamic, SNARE-mediated recycling of CX(3)CL1 from the cell surface to and from an endomembrane storage compartment. The intracellular storage depot may serve as a source of the chemokine that could be rapidly mobilized by stimuli.

CC and CX3C chemokines differentially interact with the N terminus of the human cytomegalovirus-encoded US28 receptor

Human cytomegalovirus (HCMV) is the causative agent of life-threatening systemic diseases in immunocompromised patients as well as a risk factor for vascular pathologies, like atherosclerosis, in immunocompetent individuals. HCMV encodes a G-protein-coupled receptor (GPCR), referred to as US28, that displays homology to the human chemokine receptor CCR1 and binds several chemokines of the CC family as well as the CX3C chemokine fractalkine with high affinity. Most importantly, following HCMV infection, US28 activates several intracellular pathways, either constitutively or in a chemokine-dependent manner. In this study, our goal was to understand the molecular interactions between chemokines and the HCMV-encoded US28 receptor. To achieve this goal, a double approach has been used, consisting in the analysis of both receptor and ligand mutants. This approach has led us to identify several amino acids located in the N terminus of US28 that differentially contribute to the high affinity binding of CC versus CX3C chemokines. Additionally, our results highlight the importance of secondary modifications occurring at US28, such as sulfation, for ligand recognition. Finally, the effects of chemokine dimerization and interaction with glycosaminoglycans (GAGs) on chemokine binding and activation of US28 were investigated as well using CCL4 as model ligand. In line with the two-state model describing chemokine/receptor interaction, we show that an aromatic residue in the N-loop region of CCL4 promotes tight binding to US28, whereas receptor activation depends on the presence of the N terminus of CCL4, as shown previously for CCR5.

In Vivo Transcriptional Response of Cardiac Endothelium to Lipopolysaccharide

OBJECTIVE

Vascular endothelial cells must integrate stimuli from multiple sources, including plasma, leukocytes, and neighboring components of the vessel. These stimuli are difficult to recapitulate in vitro. We have developed a method to examine the in vivo regulation of gene expression in endothelial cells and have applied it to a model of sepsis.

METHODS AND RESULTS

We used fluorescent-activated cell sorting to isolate highly purified endothelial cells from the hearts of transgenic mice that express green fluorescent protein driven by the endothelial-specific promoter Tie2. We treated these mice with intraperitoneal lipopolysaccharide and identified those genes within cardiac endothelium that were >3-fold dysregulated 4 and 24 hours later by microarray analysis. These findings were confirmed by real-time polymerase chain reaction and compared with in vitro regulation in a murine endothelial cell line.

CONCLUSIONS

The in vivo regulation was distinct and, in general, more robust than that seen in vitro. We identified endothelial-expressed genes not previously recognized to be regulated in response to lipopolysaccharide. This approach provides insight into the cardiac-specific responses of the endothelium that contribute to the specific responses of the heart to sepsis, and can be generalized to the exploration of endothelial responses in any organ.

The chemokine fractalkine in patients with severe traumatic brain injury and a mouse model of closed head injury

The potential role of the chemokine Fractalkine (CX3CL1) in the pathophysiology of traumatic brain injury (TBI) was investigated in patients with head trauma and in mice after experimental cortical contusion. In control individuals, soluble (s)Fractalkine was present at low concentrations in cerebrospinal fluid (CSF) (12.6 to 57.3 pg/mL) but at much higher levels in serum (21,288 to 74,548 pg/mL). Elevation of sFractalkine in CSF of TBI patients was observed during the whole study period (means: 29.92 to 535.33 pg/mL), whereas serum levels remained within normal ranges (means: 3,100 to 59,159 pg/mL). Based on these differences, a possible passage of sFractalkine from blood to CSF was supported by the strong correlation between blood-brain barrier dysfunction (according to the CSF-/serum-albumin quotient) and sFractalkine concentrations in CSF (R = 0.706; P < 0.01). In the brain of mice subjected to closed head injury, neither Fractalkine protein nor mRNA were found to be augmented; however, Fractalkine receptor (CX3CR1) mRNA steadily increased peaking at 1 week postinjury (P < 0.05, one-way analysis of variance). This possibly implies the receptor to be the key factor determining the action of constitutively expressed Fractalkine. Altogether, these data suggest that the Fractalkine-CX3CR1 protein system may be involved in the inflammatory response to TBI, particularly for the accumulation of leukocytes in the injured parenchyma.

Immunohistochemical assessment of fractalkine, inflammatory cells, and human herpesvirus 7 in human salivary glands

Human fractalkine (CX3CL1), a delta-chemokine, is implicated in the mediation of multiple cell functions. In addition to serving as a chemotactic factor for mononuclear cell subtypes, membrane-bound fractalkine may promote viral infection by interacting with virions that encode putative fractalkine-binding proteins. Fractalkine expression in normal epithelial tissues studied to date is either constitutive or is upregulated with inflammation. In salivary glands, the expression of fractalkine is unknown. Moreover, salivary glands are a major site for the persistent and productive infection by human herpesvirus (HHV)-7, which encodes two putative fractalkine-binding gene products, U12 and U51. Surprisingly, the cellular distribution of HHV-7 in major salivary glands has not been explored. We therefore determined by immunohistochemistry the cellular localization of fractalkine in three different salivary glands: parotid, submandibular, and labial glands. Fractalkine expression was highly variable, ranging from high to undetectable levels. We further examined the association of fractalkine with inflammatory cell infiltration or HHV-7 infection of salivary epithelial cells. Inflammatory cells were always adjacent to epithelial cells expressing fractalkine, consistent with a function of fractalkine in inflammatory cell recruitment and/or retention in salivary glands. In contrast, HHV-7-infected epithelial cells did not always express fractalkine, suggesting that fractalkine may not be an absolute requirement for viral entry.

TNF-α and IL-4 regulate expression of fractalkine (CX3CL1) as a membrane-anchored proadhesive protein and soluble chemotactic peptide on human fibroblasts

The CX(3)C chemokine, fractalkine (FKN, CX(3)CL1), has multiple functions and exists as two distinct forms, a membrane-anchored protein and a soluble chemotactic peptide that cleaves from the cell surface FKN. In this study, we first demonstrated the expression of FKN in tumor necrosis factor (TNF)-alpha- and interleukin (IL)-4-stimulated human fibroblasts. The induction of FKN was observed for both forms. We also demonstrated monocyte chemotactic activity in the culture supernatant from the fibroblasts stimulated with these cytokines. These results suggest that TNF-alpha- and IL-4-stimulated fibroblasts may play an important role in accumulation of monocytes at inflammatory sites.

Novel role of the membrane-bound chemokine fractalkine in platelet activation and adhesion

Chemokines released by the endothelium have proaggregatory properties on platelets. Fractalkine, a recently discovered membrane-bound chemokine with a transmembrane domain, is expressed in vascular injury; however, the effects of fractalkine on platelets have not yet been investigated. Blood was taken from healthy Wistar-Kyoto rats and the expression of the fractalkine receptor on platelets was demonstrated. The modulation of surface expression of P-selectin was assessed by flow cytometry. P-selectin expression was significantly enhanced by in vitro stimulation with recombinant rat fractalkine compared with baseline levels. Selectively inhibiting the function of recombinant fractalkine by an antagonizing antibody or the disruption of the G-protein-coupled intracellular signaling cascade of the fractalkine receptor by pertussis toxin (PTX) completely prevented fractalkine-mediated platelet activation. Preincubation with apyrase significantly attenuated the fractalkine-induced degranulation. In a flow chamber model of platelet adhesion, stimulation with fractalkine significantly enhanced platelet adhesion to collagen and fibrinogen. Similar to P-selectin expression, enhanced adhesion could be prevented by the antagonizing antibody or preincubation of platelets with PTX. Fractalkine, which is overexpressed in atherosclerosis and vascular injury, contributes to platelet activation and adhesion and hence is likely to play a pathophysiologically important role for increased thrombogenesis in vascular diseases.

Developmental expression of mouse muscleblind genes Mbnl1, Mbnl2 and Mbnl3

The RNA-mediated pathogenesis model for the myotonic dystrophies DM1 and DM2 proposes that mutant transcripts from the affected genes sequester a family of double-stranded RNA-binding factors, the muscleblind proteins MBNL1, MBNL2 and MBNL3, in the nucleus. These proteins are homologues of the Drosophila muscleblind proteins that are required for the terminal differentiation of muscle and photoreceptor tissues, and thus nuclear sequestration of the human proteins might impair their normal function in muscle and eye development and maintenance. To examine this model further, we analyzed the expression pattern of the mouse Mbnl1, Mbnl2, and Mbnl3 genes during embryonic development and compared muscleblind gene expression to Dmpk since the RNA pathogenesis model for DM1 requires the coordinate synthesis of mutant Dmpk transcripts and muscleblind proteins. Our studies reveal a striking overlap between the expression of Dmpk and the muscleblind genes during development of the limbs, nervous system and various muscles, including the diaphragm and tongue.

Fractalkine is not a major chemoattractant for the migration of neutrophils across microvascular endothelium

Inflammatory responses during sepsis are determined by leucocyte recruitment into inflamed tissues. Both chemokines and adhesion molecules are believed to be involved in this process. As fractalkine exists as transmembrane protein with cell adhesion properties and as soluble chemotactic factor, the present study was conducted to study the role of fractalkine, produced by microvascular and macrovascular endothelial cells, in neutrophil recruitment. Lung microvascular endothelial cells (LMVECs) stimulated with lipopolysaccharide, tumour necrosis factor-alpha or interleukin-1 (IL-1) produced much more fractalkine compared with the macrovascular human umbilical vein endothelial cells (HUVECs). No differences were found between microvascular endothelial cells of different organs. Chemotactic activity in supernatants was significantly stronger in stimulated LMVEC when compared with HUVEC. Although recombinant fractalkine induced migration of neutrophils, IL-8 and monocyte chemoattractant protein-1 were found to be more strictly required. In vivo fractalkine was strongly upregulated in septic lung and kidney. Our data suggest that fractalkine production per se does not explain the preference for inflammation in the lung of septic patients.

Fractalkine (CX3CL1) stimulated by nuclear factor kappaB (NF-kappaB)-dependent inflammatory signals induces aortic smooth muscle cell proliferation through an autocrine pathway

Fractalkine (also known as CX3CL1), a CX3C chemokine, activates and attracts monocytes/macrophages to the site of injury/inflammation. It binds to CX3C receptor 1 (CX3CR1), a pertussis toxin-sensitive G-protein-coupled receptor. In smooth muscle cells (SMCs), fractalkine is induced by proinflammatory cytokines [tumour necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma)], which may mediate monocyte adhesion to SMCs. However, the mechanisms underlying its induction are unknown. In addition, it is unlear whether SMCs express CX3CR1. TNF-alpha activated nuclear factor kappaB (NF-kappaB) and induced fractalkine and CX3CR1 expression in a time-dependent manner in rat aortic SMCs. Transient transfections with dominant-negative (dn) inhibitory kappaB (IkappaB)-alpha, dnIkappaB-beta, dnIkappaB kinase (IKK)-gamma, kinase-dead (kd) NF-kappaB-inducing kinase (NIK) and kdIKK-beta, or pretreatment with wortmannin, Akt inhibitor, pyrrolidinecarbodithioc acid ammonium salt ('PDTC') or MG-132, significantly attenuated TNF-alpha-induced fractalkine and CX3CR1 expression. Furthermore, expression of dn TNF-alpha-receptor-associated factor 2 (TRAF2), but not dnTRAF6, inhibited TNF-alpha signal transduction. Pretreatment with pertussis toxin or neutralizing anti-CX3CR1 antibodies attenuated TNF-alpha-induced fractalkine expression, indicating that fractalkine autoregulation plays a role in TNF-alpha-induced sustained fractalkine expression. Fractalkine induced its own expression, via pertussis toxin-sensitive G-proteins, phosphoinositide 3-kinase (PI 3-kinase), phosphoinositide-dependent kinase 1 (PDK1), Akt, NIK, IKK and NF-kappaB activation, and induced SMC cell-cell adhesion and cellular proliferation. Taken together, our results demonstrate that TNF-alpha induces the expression of fractalkine and CX3CR1 in rat aortic SMCs and that this induction is mediated by NF-kappaB activation. We also show that fractalkine induces its own expression, which is mediated by the PI 3-kinase/PDK1/Akt/NIK/IKK/NF-kappaB signalling pathway. More importantly, fractalkine increased cell-cell adhesion and aortic SMC proliferation, indicating a role in initiation and progression of atherosclerotic vascular disease.

Fractalkine protein localization and gene expression in mouse brain

Few chemokines are expressed constitutively in the brain at detectable levels; amongst them is fractalkine. We analyzed the distribution of fractalkine in the mouse brain with the aim of giving a neuroanatomical support to the study of its physiological function. To this end, we carried out an analysis of fractalkine protein localization and gene expression. An anti-fractalkine antibody was produced and used to perform an immunohistochemical study. The results indicated a high level of fractalkine protein in cortex, hippocampus, basal ganglia, and olfactory bulb. In particular, the presence of abundant immunoreactive neurons was observed in layers II, III, V, and VI of the cortex. In the hippocampus, the CA1 region was the most intensely labeled, but immunoreactive neurons were present also in CA2 and CA3, whereas in the basal ganglia, immunoreactive cells were observed in the caudate putamen. Other brain structures such as the brainstem showed a few scattered immunoreactive cells. The presence of fractalkine immunoreactive fibers was revealed only in the olfactory bulb and in the anterior olfactory nuclei. Gene expression study results, obtained by both semiquantitative PCR and in situ hybridization, matched protein localization with the highest levels of fractalkine transcript detected in the hippocampus, cortex, and striatum. The present study showed that fractalkine protein and mRNA are constitutively expressed at a high level in forebrain structure, but are almost absent in the hindbrain. Furthermore, localization at the cellular body level would suggest a paracrine or cell-to-cell interaction role for fractalkine more than a neurotransmission modulatory function.

Chemokine receptor mutant CX3CR1-M280 has impaired adhesive function and correlates with protection from cardiovascular disease in humans

The chemokine receptor CX3CR1 is a proinflammatory leukocyte receptor specific for the chemokine fractalkine (FKN or CX3CL1). In two retrospective studies, CX3CR1 has been implicated in the pathogenesis of atherosclerotic cardiovascular disease (CVD) based on statistical association of a common receptor variant named CX3CR1-M280 with lower prevalence of atherosclerosis, coronary endothelial dysfunction, and acute coronary syndromes. However, the general significance of CX3CR1-M280 and its putative mechanism of action have not previously been defined. Here we show that FKN-dependent cell-cell adhesion under conditions of physiologic shear is severely reduced in cells expressing CX3CR1-M280. This was associated with marked reduction in the kinetics of FKN binding as well as reduced FKN-induced chemotaxis of primary leukocytes from donors homozygous for CX3CR1-M280. We also show that CX3CR1-M280 is independently associated with a lower risk of CVD (adjusted odds ratio, 0.60, P = 0.008) in the Offspring Cohort of the Framingham Heart Study, a long-term prospective study of the risks and natural history of this disease. These data provide mechanism-based and consistent epidemiologic evidence that CX3CR1 may be involved in the pathogenesis of CVD in humans, possibly by supporting leukocyte entry into the coronary artery wall. Moreover, they suggest that CX3CR1-M280 is a genetic risk factor for CVD.

Fractalkine/CX3CL1 production by human aortic smooth muscle cells impairs monocyte procoagulant and inflammatory responses

Expression of membrane-bound CX3CL1, a CX(3)C chemokine, can be strongly induced by inflammatory cytokines in primary endothelial cells, mediating capture and tight adhesion of cells, such as monocytes, that carry the CX(3)CR1 receptor. Here, we measured CX3CL1 mRNA and protein induction by human aortic smooth muscle cells (SMCs), another major component of vessel walls, in response to inflammatory stimuli, and analyzed the effect of membrane-bound CX3CL1 on monocyte adhesion, tissue factor (TF) expression, and tumor necrosis factor-alpha (TNF-alpha) released. In human vascular SMCs, CX3CL1 transcripts were induced after 4h of stimulation with a combination of TNF-alpha and interferon-gamma. Cell-associated and shedded CX3CL1 were measured with a specific ELISA, showing that only 30% of the protein was cleaved from the membrane. Expression of CX3CL1 by SMC increased adhesion of monocytic cells, an effect, which was blocked by soluble CX3CL1. Interestingly, monocyte adhesion to CX3CL1-coated plates partially inhibited lipopolysaccharide-induced TF expression and TNF-alpha release. Thus, CX3CL1, in addition to its adhesive/chemotactic functions, directly promotes monocyte antiinflammatory and antiprocoagulant responses. This could have important implications in clinical settings such as atherosclerosis, in which SMCs and monocytic cells are in close proximity.

Decreased atherosclerosis in CX3CR1-/- mice reveals a role for fractalkine in atherogenesis

The hallmark of early atherosclerosis is the accumulation of lipid-laden macrophages in the subendothelial space. Circulating monocytes are the precursors of these "foam cells," and recent evidence suggests that chemokines play important roles in directing monocyte migration from the blood to the vessel wall. Fractalkine (FK) is a structurally unusual chemokine that can act either as a soluble chemotactic factor or as a transmembrane-anchored adhesion receptor for circulating leukocytes. A polymorphism in the FK receptor, CX(3)CR1, has been linked to a decrease in the incidence of coronary artery disease. To determine whether FK is critically involved in atherogenesis, we deleted the gene for CX(3)CR1 and crossed these mice into the apoE(-/-) background. Here we report that FK is robustly expressed in lesional smooth muscle cells, but not macrophages, in apoE(-/-) mice on a high-fat diet. CX(3)CR1(-/-) mice have a significant reduction in macrophage recruitment to the vessel wall and decreased atherosclerotic lesion formation. Taken together, these data provide strong evidence that FK plays a key role in atherogenesis.

Characterization of fractalkine (CX3CL1) and CX3CR1 in human coronary arteries with native atherosclerosis, diabetes mellitus, and transplant vascular disease

BACKGROUND

Fractalkine is a novel chemokine that mediates both firm adhesion of leukocytes to the endothelium via CX3CR1 and leukocyte transmigration out of the bloodstream. Fractalkine has recently been shown to play a role in the pathogenesis of acute organ rejection. Since its expression is regulated by inflammatory agents such as LPS, IL-1, and TNF-alpha, fractalkine involvement in atherosclerosis and transplant vascular disease (TVD) is of particular interest. In this study, we characterized the presence of fractalkine and its receptor CX3CR1 in human coronary arteries from normal, atherosclerotic, diabetic, and TVD settings.

METHOD

Polyclonal rabbit antibodies were used to immunostain human fractalkine and CX3CR1 to localize their presence in transverse sections of the proximal left anterior descending and/or right coronary arteries. Slides were scored in a blinded fashion for intensity of staining (0 to 4+) and for localization in vessel walls.

RESULTS

Normal coronary arteries showed no fractalkine staining. In atherosclerotic coronary arteries, staining was localized to the intima, media, and adventitia. Within the media, fractalkine expression was seen in macrophages, foam cells, and smooth muscle cells (SMCs). Diabetic vessels showed similar staining patterns to atherosclerotic coronaries, with much stronger staining in the deep intima. Transplanted coronaries showed staining in the endothelium, intima, and adventitia in early disease, and intimal, medial, and adventitial staining in late disease. CX3CR1 staining was seen in the coronary arteries of all cases, with specific localization to regions with fractalkine staining.

CONCLUSION

The distinctive staining patterns in native atherosclerosis, diabetes mellitus with atherosclerosis, and TVD indicate that the expression of fractalkine and CX3CR1 may be important in the pathogenesis of these diseases.

Fractalkine expression on human renal tubular epithelial cells: potential role in mononuclear cell adhesion

Fractalkine (CX3CL1) is a transmembrane molecule with a CX3C chemokine domain attached to an extracellular mucin stalk which can induce both adhesion and migration of leucocytes. Mononuclear cell infiltration at renal tubular sites and associated tubular epithelial cell damage are key events during acute renal inflammation following renal allograft transplantation. Using northern and Western blot analysis, we have demonstrated the expression of fractalkine message and protein by renal tubular epithelial cells in vitro. The expression was up-regulated by TNF-alpha, a key proinflammatory cytokine in acute rejection. Investigation of surface expression of fractalkine on cultured proximal tubular epithelial cells revealed only a subpopulation of positively staining cells. Immunohistochemistry revealed that only a proportion of tubules in renal allograft biopsies showed induction of fractalkine expression. Studies using a static model of adhesion demonstrated CX3CR1/fractalkine interactions accounted for 26% of monocytic THP-1 cell and 17% of peripheral blood natural killer cell adhesion to tubular epithelial cells, suggesting that fractalkine may have a functional role in leucocyte adhesion and retention, at selected tubular sites in acute renal inflammation. Thus, fractalkine blockade strategies could reduce mononuclear cell mediated tubular damage and improve graft survival following kidney transplantation.

CX(3)C chemokine fractalkine in pulmonary arterial hypertension

Perivascular infiltrates composed of macrophages and lymphocytes have been described in lung biopsies of patients displaying pulmonary arterial hypertension (PAH), suggesting that circulating inflammatory cells can be recruited in affected vessels. CX(3)C chemokine fractalkine is produced by endothelial cells and promotes leukocyte recruitment, but unlike other chemokines, it can capture leukocytes rapidly and firmly in an integrin-independent manner under high blood flow. We therefore hypothesized that fractalkine may contribute to pulmonary inflammatory cell recruitment in PAH. Expression and function of the fractalkine receptor (CX(3)CR1) were studied by use of triple-color flow cytometry on circulating T-lymphocyte subpopulations in freshly isolated peripheral blood mononuclear cells from control subjects and patients with PAH. Plasma-soluble fractalkine concentrations were measured by enzyme-linked immunosorbent assay. Finally, fractalkine mRNA and protein expression were analyzed in lung samples by reverse transcriptase-polymerase chain reaction or in situ hybridization and immunohistochemistry, respectively. In patients with PAH, CX(3)CR1 expression and function are upregulated in circulating T-lymphocytes, mostly of the CD4+ subset, and plasma soluble fractalkine concentrations are elevated, as compared with control subjects. Fractalkine mRNA and protein product are expressed in pulmonary artery endothelial cells. We conclude that inflammatory mechanisms involving chemokine fractalkine and its receptor CX(3)CR1 may have a role in the natural history of PAH.

Fractalkine-mediated signals regulate cell-survival and immune-modulatory responses in intestinal epithelial cells

BACKGROUND & AIMS

In this study, we determined the signal transduction and functional consequences after ligand-specific activation of the fractalkine receptor CX3CR1 in human intestinal epithelial cells.

METHODS

CX3CR1 expression in human colonic tissues and intestinal epithelial cell lines was determined by immunohistochemistry, immunoblotting, and reverse-transcription polymerase chain reaction. The regulation of mitogen-activated protein kinase (MAPK) activation was assessed by immunoblotting. Regulation of chemokine messenger RNA (mRNA) expression was determined by Northern blotting. NF-kappa B and p53 activation was assessed by electromobility shift assays.

RESULTS

Fractalkine mediated the MEK-1 and G alpha i-dependent but phosphatidylinositol-3-kinase-independent activation of extracellular signal-regulated kinase-MAPK. Fractalkine activated NF-kappa B and p53 resulting in interleukin 8 and fractalkine mRNA expression. CX3CR1-mediated activation of intestinal epithelial cells was able to induce migration of human neutrophils into but not through the intestinal epithelial cell monolayer.

CONCLUSIONS

CX3CR1 mediates distinct functional responses in intestinal epithelial cells, which include the autocrine regulation of cell-survival signals and activation of immune modulators, indicating a role of CX3CR1 in host defense mechanisms originating from the intestinal epithelium.