Fractalkine Targeting with a Receptor-Mimicking Peptide-Amphiphile

In this study we have designed the NTFR peptide-amphiphile that mimics a fragment of the N-terminus of the fractalkine receptor (CX(3)CR1) and specifically targets fractalkine, a novel adhesion molecule expressed on the surface of inflamed endothelial cells. Bioartificial membranes were constructed from mixtures of NTFR peptide-amphiphiles and DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) phospholipids, and the affinity and specificity of fractalkine for the synthetic NTFR was investigated with an atomic force microscope (AFM). Fractalkine was immobilized onto the AFM tips, and forces were collected between fractalkine and the bioartificial membranes. The adhesive interactions were studied at the collective level, when each adhesion event corresponded to the rupture of multiple biomolecular bonds. Retraction force profiles for the fractalkine-NTFR system exhibited single or multiple peaks and a small percentage of the force curves demonstrated stretching of the fractalkine-NTFR complex. Strong adhesion was measured when both DPPC and NTFR were present, compared to pure NTFR surfaces. This may be due to the fact that the DPPC molecule is shorter, and thus it can provide more space for the peptide headgroup to bend and expose its sequence at the interface. Specificity was demonstrated by comparing the NTFR-fractalkine adhesion to the forces between the alpha(5)beta(1) integrin (an adhesion receptor expressed on the surface of endothelial cells) and other surfaces such as GRGDSP (the specific ligand for alpha(5)beta(1)), GRGESP (an inactive sequence), and NTFR.

Up regulated expression of fractalkine/CX3CL1 and CX3CR1 in patients with systemic sclerosis

BACKGROUND

Fractalkine expressed on endothelial cells mediates activation and adhesion of leucocytes expressing its receptor, CX(3)CR1. Soluble fractalkine exhibits chemotactic activity for leucocytes expressing CX(3)CR1.

OBJECTIVE

To determine the role of fractalkine and its receptor in systemic sclerosis (SSc) by assessing their expression levels in patients with this disease.

METHODS

The expression of fractalkine and CX(3)CR1 in the skin and lung tissues was immunohistochemically examined. Circulating soluble fractalkine levels were examined by enzyme linked immunosorbent assay (ELISA). Blood samples from patients with SSc were stained for CX(3)CR1 with flow cytometric analysis.

RESULTS

CX(3)CR1 levels on peripheral monocytes/macrophages and T cells were found to be raised in patients with diffuse cutaneous SSc. The numbers of cells expressing CX(3)CR1, including monocytes/macrophages, were increased in the lesional skin and lung tissues from patients with diffuse cutaneous SSc. Fractalkine was strongly expressed on endothelial cells in the affected skin and lung tissues. Soluble fractalkine levels were significantly raised in sera and were associated with raised erythrocyte sedimentation rates, digital ischaemia, and severity of pulmonary fibrosis.

CONCLUSIONS

Up regulated expression of fractalkine and CX(3)CR1 cooperatively augments the recruitment of mononuclear cells expressing CX(3)CR1 into the affected tissue of SSc, leading to inflammation and vascular injury.

CX3CR1-mediated dendritic cell access to the intestinal lumen and bacterial clearance

Dendritic cells (DCs) and macrophages are critical to innate and adaptive immunity to the intestinal bacterial microbiota. Here, we identify a myeloid-derived mucosal DC in mice, which populates the entire lamina propria of the small intestine. Lamina propria DCs were found to depend on the chemokine receptor CX3CR1 to form transepithelial dendrites, which enable the cells to directly sample luminal antigens. CX3CR1 was also found to control the clearance of entero-invasive pathogens by DCs. Thus, CX3CR1-dependent processes, which control host interactions of specialized DCs with commensal and pathogenic bacteria, may regulate immunological tolerance and inflammation.

The high expression of Fractalkine results in a better prognosis for colorectal cancer patients

Local and systemic immune responses are impaired in patients with colorectal cancer (CRC) and it is known that the number of tumor infiltrating lymphocytes (TILs) is considerably few. On the other hand, some CRC cases in which many TIL were observed, survived longer than those cases with a small number of TIL. Considerable attention has been recently paid to the relationship between chemokines and tumor cells. Some chemokines recruit lymphocytes for tumor lesions. We made a hypothesis that Fractalkine, a CX3C chemokine, would recruit lymphocytes in the CRC and play an important role in anti-tumor immunity. We analyzed the expression level of Fractalkine in CRC cell lines as well as in clinical samples (n=80). The expression level of Fractalkine was thus found to correlate with the density of TIL (p<0.05). The CRC cases with a strong Fractalkine expression (n=50) showed a significantly better prognosis than those with a weak expression (n=30) (p<0.05). In addition, the Fractalkine expression was found to be an independent prognostic factor (p<0.05). We furthermore clarified that some of the tumor-infiltrating cells were natural killer cells and cytotoxic T cells expressed Fractalkine receptor. These data suggest that Fractalkine expressed in the tumor appears to recruit cytotoxic T cells and NK cells to the tumor site and these cytotoxic cells result in a better prognosis mediated by tumor cell cytotoxicity using a perforin and granzyme B mechanism. The expression level of Fractalkine was an essential biomarker for predicting the prognosis of patients with CRC. Fractalkine is considered to be one of the biomarkers for detecting patients with a high risk for recurrence, and who might therefore benefit from additional therapeutic strategies such as adjuvant therapy.

US28 actions in HCMV infection: lessons from a versatile hijacker

Mimicking host proteins is a strategy adopted by several herpesviruses to exploit the host cell for their own benefit. In this respect the human cytomegalovirus (HCMV) chemokine receptor homologue US28, has been extensively studied. Molecular pirates such as US28 can teach us about crucial events in HCMV infection and may either offer a potential target for antiviral therapy or provide an alternative strategy to immune suppression. Despite elaborate research into the chemokine binding affinity, signalling properties, intracellular trafficking and expression kinetics of US28, a solid hypothesis about the role of US28 in HCMV infection has not yet been proposed. It appears that US28 may behave as a molecular pirate that employs smart strategies for cell entry, host gene regulation and immune evasion. This review will elaborate on these aspects of US28 biology and discuss possible implications for HCMV infection.

Major reduction of atherosclerosis in fractalkine (CX3CL1)-deficient mice is at the brachiocephalic artery, not the aortic root

Fractalkine (CX3CL1) is of particular interest in atherogenesis because it can serve as an adhesion molecule and a chemokine. Fractalkine and its receptor CX3CR1 are expressed in atherosclerotic lesions of humans and mice. However, the effect of fractalkine deficiency on atherosclerosis susceptibility is unknown. Fractalkine-deficient mice on the C57BL/6 (B6) background were bred to the atherosclerosis-sensitizing B6.ApoE(-/-) and B6.LDLR(-/-) backgrounds. Compared with controls, aortic-root lesion area was unchanged in fractalkine-deficient male and female B6.ApoE(-/-) mice at 16 weeks of age and males at 12 weeks of age, but it was mildly reduced (30%, P = 0.005) in females at 12 weeks of age. In contrast, lesion area at the brachiocephalic artery (BCA) was reduced dramatically by approximately 85% in fractalkine-deficient females [42,251 +/- 26,136 microm(2) (n = 15) vs. 6,538 +/- 11,320 microm(2);(n = 24), P < 0.0001] and males [36,911 +/- 32,504 microm(2) (n = 24) vs. 6,768 +/- 8,595 microm(2) (n = 14); P = 0.001] at 16 weeks of age. Fractalkine-deficient B6.ApoE(-/-) mice were comparable with controls in body weight, plasma cholesterol, plasma high-density lipoprotein cholesterol and white blood cell counts. On the B6.LDLR(-/-) background, lesion areas were reduced by 35% at the aortic root (P < 0.01) and by 50% at the BCA (P < 0.05) in fractalkine-deficient females at 16 weeks of age. Lesions in fractalkine-deficient mice on the B6.ApoE(-/-) and B6.LDLR(-/-) backgrounds were less complex and contained significantly fewer macrophages than controls. In conclusion, the major reduction of atherosclerosis in fractalkine-deficient mice appears to be at the BCA rather than the aortic root.

CD200 maintains microglial potential to migrate in adult human retinal explant model

PURPOSE

Retinal microglia (MG) migrate in response to injury, degeneration and inflammation dependent upon both soluble and cognate signals they receive. Previously we found that lipopolysaccharide/interferon-gamma (LPS/IFNgamma) stimulation induces a paradoxical IL-10 mediated suppression of MG migration from retinal explants. Given the high expression of neuronal CD200, which can induce down regulation of CD200 receptor-positive MG activation and neuronal fractalkine expression potentially stimulating MG migration, we wished to further examine their respective roles in the maintenance of MG activation and migration.

METHODS

A human retinal explant model of MG migration was used. CD200 receptor and fractalkine receptor stimulation was achieved by addition to explants of CD200:Fc fusion protein and recombinant cytokine respectively, with or without LPS-IFNgamma stimulation that is known to suppress migration. Cell migration and cell activation (iNOS expression) was counted and assessed by numbers of CD45+ cells by immunofluorescence and standardised flow cytometric bead array analysis was performed for cytokine production.

RESULTS

Retinal explants expressed fractalkine and CX3CR1 immunohistochemically and by PCR. Addition of Fractalkine and not CD200:Fc induced MG migration from retinal explants. However LPS/IFNgamma-induced suppression of MG migration could only be restored in the presence of CD200:Fc, whilst MG remained iNOS-negative and generated IL-10.

CONCLUSIONS

Microglial responses are tightly governed within retina. Although MG do not classically activate following LPS/IFNgamma stimulation, their migration is sustained via CD200R stimulation maintaining their potential to migrate in response to injury.

Coexpression of fractalkine and its receptor in normal human endometrium and in endometrium from users of progestin-only contraception supports a role for fractalkine in leukocyte recruitment and endometrial remodeling

Leukocytes are critical mediators of endometrial remodeling, but the mechanisms by which leukocyte subpopulations enter the uterus are currently unknown. Endometrial leukocytes have no genomic progesterone receptors; thus, we hypothesized that leukocyte migration is induced indirectly by progesterone-regulated chemokines. Fractalkine (CX3CL1), a chemotactic membrane-bound adhesion factor, and its receptor (CX3CR1) were assessed by immunohistochemistry in endometrial samples across the menstrual cycle, in early pregnancy, and in women using progestin-only contraceptives. Fractalkine was localized predominantly to glandular epithelial and decidualized stromal cells, with the highest staining intensity in the secretory phase and early pregnancy. It was also detected in subpopulations of endometrial leukocytes (macrophages and uterine NK cells), with maximal numbers during the proliferative phase and early pregnancy. CX3CR1 was similarly colocalized to the glandular epithelium and decidualized stromal cells, with the highest expression in the secretory phase. CX3CR1-positive leukocytes (macrophages and neutrophils) were in greatest abundance during the menstrual phase. In the endometrium of women using progestin-only contraceptives, immunoreactive fractalkine was markedly reduced in the glandular epithelium, but was increased in decidualized stroma and infiltrating leukocytes. These findings support a number of roles for fractalkine in the endometrium, in the secretory phase, in early pregnancy, and when influenced by progestin-only contraceptives.

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.

Fractalkine (CX3CL1) and fractalkine receptor (CX3CR1) distribution in spinal cord and dorsal root ganglia under basal and neuropathic pain conditions

Fractalkine is a unique chemokine reported to be constitutively expressed by neurons. Its only receptor, CX3CR1, is expressed by microglia. Little is known about the expression of fractalkine and CX3CR1 in spinal cord. Given that peripheral nerve inflammation and/or injury gives rise to neuropathic pain, and neuropathic pain may be partially mediated by spinal cord glial activation and consequent glial proinflammatory cytokine release, there must be a signal released by affected neurons that triggers the activation of glia. We sought to determine whether there is anatomical evidence implicating spinal fractalkine as such a neuron-to-glia signal. We mapped the regional and cellular localization of fractalkine and CX3CR1 in the rat spinal cord and dorsal root ganglion, under basal conditions and following induction of neuropathic pain, employing both an inflammatory (sciatic inflammatory neuropathy; SIN) as well as a traumatic (chronic constriction injury; CCI) model. Fractalkine immunoreactivity and mRNA were observed in neurons, but not glia, in the rat spinal cord and dorsal root ganglia, and levels did not change following either CCI or SIN. By contrast, CX3CR1 was expressed by microglia in the basal state, and the microglial cellular concentration was up-regulated in a regionally specific manner in response to neuropathy. CX3CR1-expressing cells were identified as microglia by their cellular morphology and positive OX-42 and CD4 immunostaining. The cellular distribution of fractalkine and CX3CR1 in the spinal circuit associated with nociceptive transmission supports a potential role in the mechanisms that contribute to the exaggerated pain state in these models of neuropathy.

Evidence that exogenous and endogenous fractalkine can induce spinal nociceptive facilitation in rats

Recent evidence suggests that spinal cord glia can contribute to enhanced nociceptive responses. However, the signals that cause glial activation are unknown. Fractalkine (CX3C ligand-1; CX3CL1) is a unique chemokine expressed on the extracellular surface of spinal neurons and spinal sensory afferents. In the dorsal spinal cord, fractalkine receptors are primarily expressed by microglia. As fractalkine can be released from neurons upon strong activation, it has previously been suggested to be a neuron-to-glial signal that induces glial activation. The present series of experiments provide an initial investigation of the spinal pain modulatory effects of fractalkine. Intrathecal fractalkine produced dose-dependent mechanical allodynia and thermal hyperalgesia. In addition, a single injection of fractalkine receptor antagonist (neutralizing antibody against rat CX3C receptor-1; CX3CR1) delayed the development of mechanical allodynia and/or thermal hyperalgesia in two neuropathic pain models: chronic constriction injury (CCI) and sciatic inflammatory neuropathy. Intriguingly, anti-CX3CR1 reduced nociceptive responses when administered 5-7 days after CCI, suggesting that prolonged release of fractalkine may contribute to the maintenance of neuropathic pain. Taken together, these initial investigations of spinal fractalkine effects suggest that exogenous and endogenous fractalkine are involved in spinal sensitization, including that induced by peripheral neuropathy.

A perforin/granzyme-positive MDS-derived T cell line, K2-MDS, induces apoptosis in CD34+ cells through the fractalkine?CX3CR1 system

Fractalkine (CX3CL1) and its receptor CX3CR1 play an important role in natural killer (NK) cell- and cytotoxic T cell-mediated endothelium damage. Here we describe the cytotoxicity of myelodysplastic syndrome (MDS)-derived T cell line, K2-MDS, through the fractalkine-CX3CR1 system. K2-MDS cells induced apoptosis against CD34(+) cells from normal bone marrow (BM) in a direct cell contact manner. K2-MDS cells expressed perforin and granzyme B, but they lacked Fas ligand expression. A specific inhibitor for perforin, concanamycin A, blocked K2-MDS-dependent cytotoxicity. Furthermore, a CX3C-chemokine, fractalkine, was expressed in CD34(+) cells, and its receptor, CX3CR1, was expressed on K2-MDS cells. The neutralizing monoclonal antibody (MoAb) for fractalkine and soluble fractalkine significantly inhibited K2-MDS-dependent cytotoxicity. K2-MDS cells also induced the cytotoxicity against human umbilical cord endothelial cells (HUVECs) expressing fractalkine. These data indicate that K2-MDS may be a perforin-granzyme-positive T cell line that exerts a cytotoxic effect on CD34(+) cells mediated through the fractalkine-CX3CR1 system.

Protein overload induces fractalkine upregulation in proximal tubular cells through nuclear factor kappaB- and p38 mitogen-activated protein kinase-dependent pathways

Investigated was the effect of high albumin concentrations on proximal tubular cell expression of fractalkine. Human proximal tubular cells (HK-2) were incubated with human serum albumin (HSA), which induced a dose-dependent increase in fractalkine mRNA associated with increased levels of both membrane-bound and soluble forms of the protein. To evaluate the role of nuclear factor kappaB (NF-kappaB) activation in HSA-induced fractalkine mRNA, HK-2 cells were infected with a recombinant adenovirus encoding the natural inhibitor of NF-kappaB, IkBalpha; a 43% reduction of fractalkine mRNA levels resulted. Similarly, when cells were infected with the recombinant adenovirus expressing dominant negative mutant of the IkB kinase 2, a 55% inhibition of fractalkine mRNA was achieved. p38 mitogen-activated protein kinase was activated by HSA and was involved in NF-kappaB-dependent transcription of fractalkine. In kidneys of mice with bovine serum albumin overload proteinuria, fractalkine mRNA levels were 2.3-fold greater than those of controls. Fractalkine expression was also induced in tubular epithelial cells in this model. Anti-CXCR1 antibody treatment limited interstitial accumulation of mononuclear cells. Protein overload is a promoter of fractalkine gene induction mediated by NF-kappaB and p38 activation in proximal tubular cells. Fractalkine might contribute to direct mononuclear cells into peritubular interstitium and enhance their adhesion property, which in turn would favor inflammation and disease progression.

Viral Macrophage Inflammatory Protein-II and Fractalkine (CX3CL1) Chimeras Identify Molecular Determinants of Affinity, Efficacy, and Selectivity at CX3CR1

Fractalkine (FKN/CX3CL1) is a cell surface-expressed chemokine involved in many aspects of leukocyte trafficking and activation. The various structural domains of FKN play distinct roles in its ability to bind and activate its receptor, CX3CR1. A human herpesvirus 8-encoded chemokine, termed viral macrophage inflammatory protein (vMIP)-II, is structurally similar to FKN; vMIP-II is a nonselective chemokine receptor antagonist (binding multiple chemokine receptors, including CX3CR1). The goal of this study was to identify FKN determinants of selectivity for its receptor and to further refine domains important in affinity and efficacy at CX3CR1. Chimeric and insertional mutagenesis was used to generate mutants of both vMIP-II and FKN, and the expressed proteins were evaluated for chemokine receptor binding affinities and efficacy at CX3CR1. Modification of the intervening amino acids between the first two conserved cysteine residues of FKN or vMIP-II indicated a role of the X3 bulge of FKN in affinity and selectivity for CX3CR1. Substitution of the vMIP-II N terminus with that of FKN created an agonist that was just as potent and efficacious as FKN for binding and stimulating CX3CR1, whereas replacement of the FKN N terminus with the cognate domain of vMIP-II disrupted the ability of FKN to bind CX3CR1. Furthermore, the entire N terminus of FKN was necessary for the high-affinity and full agonist properties of FKN at CX3CR1. These results refine the pharmacophore for chemokine binding to and activation of CX3CR1 and demonstrate the usefulness of modified virally encoded chemokines as templates for the development of selective chemokine receptor antagonists.

Transendothelial migration of CD16+ monocytes in response to fractalkine under constitutive and inflammatory conditions

CD16+ monocytes represent 5-10% of circulating monocytes in healthy individuals and are dramatically expanded in several pathological conditions including AIDS and HIV-1-associated dementia (HAD). CD16+ monocytes constitutively produce high levels of pro-inflammatory cytokines and neurotoxic factors that may contribute to the pathogenesis of these disorders. Monocyte recruitment into the central nervous system (CNS) and other peripheral tissues in response to locally produced chemokines is a critical event in immune surveillance and inflammation and involves monocyte arrest onto vascular beds and subsequent diapedesis. Here we investigate the ability of CD16+ monocytes to undergo transendothelial migration (TEM) under constitutive and inflammatory conditions. CD16+ monocytes underwent TEM across unstimulated human umbilical vascular (HUVEC) and brain microvascular endothelial (BMVEC) cell monolayers in response to soluble fractalkine (FKN/CX3CL1). Stimulation with tumor necrosis factor (TNF) and interferon-gamma (IFN-gamma) induced high and low expression of membrane-bound FKN on HUVEC and BMVEC, respectively, together with expression of VCAM-1 and intercellular adhesion molecule-1 (ICAM)-1. By contrast, only HUVEC expressed CD62E while BMVEC remained negative. Both CD16- and CD16+ monocyte subsets adhered to TNF/IFN-gamma-stimulated HUVEC with higher frequency than to unstimulated HUVEC. Monocyte chemoattractant protein-1 (MCP-1) triggered efficient TEM of CD16- monocytes across TNF/IFN-gamma-stimulated HUVEC, whereas soluble FKN failed to induce TEM of CD16+ monocytes across stimulated HUVEC. These results demonstrate that stimulation with TNF and IFN-gamma triggers expression of membrane-bound FKN on both HUVEC and BMVEC, but prevents TEM of CD16+ monocytes in response to soluble FKN. Thus, pro-inflammatory CD16+ monocytes may contribute to the pathogenesis of HAD and other inflammatory CNS diseases by affecting the integrity of the blood-brain barrier as a consequence of their massive accumulation onto inflamed brain vascular endothelial cells expressing FKN and other adhesion molecules.

Leukocyte recruitment to atherosclerotic lesions

Atherosclerosis is a complex disease process in which monocytes and lymphocytes are recruited from the blood into the arterial intima. Mouse models of atherosclerosis have been developed, carefully characterized and used to elucidate molecular mechanisms of atherosclerotic lesion formation. Deficiency of various chemokines, chemokine receptors and leukocyte adhesion molecules that are known to participate in mononuclear leukocyte emigration, such as monocyte chemoattractant protein-1 and its receptor chemokine (CC motif) receptor 2, CX3C chemokine receptor 1 and vascular cell adhesion molecule 1, results in decreased formation of atherosclerotic lesions. In these studies, analysis was usually limited to assessment of lesion size, cellular composition and histological features. An assumption is often made that leukocyte recruitment is diminished if a reduction in lesions is found in chemokine- or adhesion molecule-deficient animals. However, direct quantification of leukocyte recruitment to atherosclerotic lesions is lacking and there is a need for practical recruitment assays that have the potential to provide precise and novel insights. For example, insights may be gained to distinguish the contribution of chemokines, chemokine receptors and adhesion molecules to the recruitment, survival or proliferation of different leukocyte types in atherosclerotic lesions.

Regulation of CX3CL1/fractalkine expression in endothelial cells

CX3CL1/fractalkine is a chemokine with a unique CX3C motif. Fractalkine is synthesized in endothelial cells as a membrane protein, and the N-terminal domain containing a CX3C motif is cleaved and secreted. CX3CR1, the specific receptor for fractalkine, is expressed in monocytes and lymphocytes. Membrane-bound fractalkine works as an adhesion molecule for these leukocytes and the secreted form as a chemotactic factor. Fractalkine is produced by endothelial cells stimulated with tumor necrosis factor-alpha, interleukin-1 (IL-1), lipopolysaccharide and interferon-gamma. Expression of fractalkine in endothelial cells is inhibited by the soluble form of IL-6 receptor-alpha, 15-deoxy-Delta(12,14)-prostaglandin J(2), and hypoxia. The expression of fractalkine is tightly regulated and fractalkine plays an important role in the interaction between leukocytes and endothelial cells.

Effect of hypoxia on the expression of fractalkine in human endothelial cells

CX3CL1/fractalkine is a chemokine with a unique CX3C motif. Hypoxia mediates the expression of various genes, such as vascular endothelial growth factor (VEGF), cyclooxygenase-2, and plasminogen-activator inhibitor-1, in vascular endothelial cells. We studied the effect of hypoxia on the expression of fractalkine induced by interferon-gamma (IFN-gamma) in endothelial cells. Human umbilical vein endothelial cells were cultured, and the stimulation of the cells with IFN-gamma was found to induce the expression of fractalkine. Hypoxia inhibited the expression of fractalkine mRNA and protein by IFN-gamma, and this effect was observed with concomitant increase in VEGF expression. Desferrioxamine, an iron chelator that mimics hypoxia in vitro, also inhibited the fractalkine production induced by IFN-gamma. Hypoxia did not affect the degradation of fractalkine mRNA. The inhibition of fractalkine expression by hypoxia was reversed on returning the cultures to reoxygenation condition. Inhibition of IFN-induced fractalkine expression by hypoxia was not affected by the presence of a radical scavenger, N-acetyl-L-cysteine, and the involvement of reactive oxygen species may be excluded. Inhibition of fractalkine expression by hypoxia may be involved in the pathophysiology of ischemic diseases.

Expression of fractalkine and its receptor, CX3CR1, in atopic dermatitis: possible contribution to skin inflammation

BACKGROUND

Fractalkine (FKN) induces activation and adhesion of leukocytes expressing its receptor, CX(3)CR1. FKN is released from the cell surface through proteolytic cleavage as soluble FKN (sFKN).

OBJECTIVE

We sought to assess FKN and CX(3)CR1 expression in the skin, serum sFKN levels, and CX(3)CR1 expression on blood leukocytes in patients with atopic dermatitis (AD).

METHODS

FKN and CX(3)CR1 expression in the skin was examined immunohistochemically. mRNA expression of FKN, thymus and activation-regulated chemokine, and macrophage-derived chemokine in the skin was assessed by means of real-time RT-PCR. Serum sFKN levels were assessed by using ELISA. Blood leukocytes were stained for CX(3)CR1 by means of flow cytometric analysis.

RESULTS

FKN was strongly expressed on endothelial cells in skin lesions of patients with AD and psoriasis but not in normal skin. FKN mRNA levels in AD lesional skin increased to a similar extent to thymus and activation-regulated chemokine and macrophage-derived chemokine mRNA levels. CX(3)CR1-expressing cells in the affected skin of patients with AD or psoriasis increased compared with those in normal skin. Serum sFKN levels were increased in patients with AD but not in patients with psoriasis relative to levels in healthy control subjects. Serum sFKN levels were associated with the disease severity and decreased with the improvement of skin lesions in patients with AD. CX(3)CR1(+) cell frequencies and CX(3)CR1 expression levels were decreased in CD8(+) T cells, monocytes, and natural killer cells from patients with AD, but this was not observed in patients with psoriasis.

CONCLUSIONS

These results suggest that through functions in both membrane-bound and soluble forms, FKN plays an important role in the trafficking of CX(3)CR1(+) leukocytes during the inflammation caused by AD.

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.

Respiratory syncytial virus (RSV) G glycoprotein is not necessary for vaccine-enhanced disease induced by immunization with formalin-inactivated RSV

Following respiratory syncytial virus (RSV) challenge, mice immunized with RSV G or with formalin-inactivated RSV (FI-RSV) exhibit severe disease associated with type 2 cytokine production and pulmonary eosinophilia. This has led to the proposal that the presence of RSV G is the factor in FI-RSV that induces disease-enhancing T-cell responses. Therefore, we evaluated the role of RSV G and its immunodominant region in the induction of aberrant immune responses during FI-RSV immunization. BALB/c mice were immunized with FI preparations of wild-type (wt) RSV or recombinant RSV (rRSV) containing deletions of (i) the entire G gene, (ii) the region of the G gene encoding amino acids 187 to 197 of the immunodominant region, or (iii) the entire SH gene. After challenge, illness, RSV titers, cytokine levels, and pulmonary eosinophilia were measured. Peak RSV titers postchallenge were significantly greater in mice immunized with FI preparations of the deletion viruses than in those immunized with FI-rRSV wt, suggesting that the absence of G or SH in FI-RSV reduced its protective efficacy. Deletion of G or its epitope did not reduce illness, cytokine production, or eosinophilia relative to that in mice immunized with FI-rRSV wt. While cytokine levels and eosinophilia were similar, illness was reduced in mice immunized with SH-deleted FI-RSV. These data suggest that G-specific immune responses may be important for vaccine-induced protection and are not solely the basis for FI-RSV vaccine-enhanced illness. These data suggest that the method of RSV antigen delivery, rather than the protein composition, influences the phenotype of the induced immune responses and that RSV G should not necessarily be excluded from potential vaccine strategies.

Receptor−Ligand Binding in the Cell−Substrate Contact Zone: A Quantitative Analysis Using CX3CR1 and CXCR1 Chemokine Receptors†

Receptor-ligand binding analyses have generally used soluble components to measure thermodynamic binding constants. In their biological context, adhesion receptors bind to an immobile ligand and the binding reaction is confined to the cell-substrate contact zone. We have developed a new procedure based on the spinning disk technology to measure the number of receptor-ligand bonds in the contact zone. Application of this methodology to the CX3CR1-fractalkine and the CXCR1-IL-8 receptor-ligand systems demonstrated that the level of binding to an immobilized ligand is reduced by several orders of magnitude in comparison to solution binding. A comparison of the solution binding and contact zone binding constants shows that the effect of ligand immobilization was similar for each system. In contrast, although the CXCR1-IL-8 bond had the higher affinity, the average bond strength was only 10% of that for the CX3CR1 bond. Because fractalkine can be expressed as a cell surface-bound protein, CX3CR1 has been proposed to function as an adhesion receptor. The higher bond strength suggests that the bond architecture has also evolved to serve an adhesion function.

Immunophenotyping of macrophages in human pulmonary tuberculosis and sarcoidosis

Classic studies of tuberculosis (TB) revealed morphologic evidence of considerable heterogeneity of macrophages (MØs), but the functional significance of this heterogeneity remains unknown. We have used newly available specific antibodies for selected membrane and secretory molecules to examine the phenotype of MØs in situ in a range of South African patients with TB, compared with sarcoidosis. Patients were human immunodeficiency virus-negative adults and children, and the examined biopsy specimens included lung and lymph nodes. Mature pulmonary MØs (alveolar, interstitial, epithelioid and multinucleated giant cells) selectively expressed scavenger receptor type A and a novel carboxypeptidase-like antigen called carboxypeptidase-related vitellogenin-like MØ molecule (CPVL). CPVL did not display enhanced expression in sarcoidosis, vs. TB patients, as observed with angiotensin-converting enzyme (ACE), a related molecule. Immunocytochemical studies with surfactant proteins (SP)-A and -D showed that type II alveolar cells expressed these collectins, as did MØs, possibly after binding of secreted proteins. Studies with an antibody specific for the C-terminus of fractalkine, a tethered CX3C chemokine, confirmed synthesis of this molecule by bronchiolar epithelial cells and occasional endothelial cells. These studies provide new marker antigens and extend previous studies on MØ differentiation, activation and local interactions in chronic human granulomatous inflammation in the lung.

Two Novel Fully Functional Isoforms of CX3CR1 Are Potent HIV Coreceptors

We identified two novel isoforms of the human chemokine receptor CX3CR1, produced by alternative splicing and with N-terminal regions extended by 7 and 32 aa. Expression of the messengers coding these isoforms, compared with that of previously described V28 messengers, is lower in monocytes and NK cells, but higher in CD4(+) T lymphocytes. CX3CR1 and its extended isoforms were expressed in HEK-293 cells and compared for expression, ligand binding, and cellular responses. In steady state experiments, all three CX3CR1 isoforms bound CX3CL1 with similar affinity. In kinetic binding studies, however, k(on) and k(off) were significantly greater for the extended CX3CR1 isoforms, thereby suggesting that the N-terminal extensions may alter the functions induced by CX3CL1. In signaling studies, all three CX3CR1 isoforms mediated agonist-dependent calcium mobilization, but the EC(50) was lower for the extended than for the standard isoforms. In addition, chemotactic responses for these extended isoforms shifted left, also indicating a more sensitive response. Finally, the longer variants appeared to be more potent HIV coreceptors when tested in fusion and infection assays. In conclusion, we identified and characterized functionally two novel isoforms of CX3CR1 that respond more sensitively to CX3CL1 and HIV viral envelopes. These data reveal new complexity in CX3CR1 cell activation and confirm the critical role of the N-terminal domain of the chemokine receptors in ligand recognition and cellular response.

Smooth muscle cells in human atherosclerotic plaques express the fractalkine receptor CX3CR1 and undergo chemotaxis to the CX3C chemokine fractalkine (CX3CL1)

BACKGROUND

Chemokines are important mediators of inflammatory cell recruitment that play a significant role in atherosclerosis. Fractalkine (CX3CL1) is an unusual membrane-bound chemokine that mediates chemotaxis through the CX3CR1 receptor. Recently, functional polymorphisms in the human CX3CR1 gene have been described that are associated with coronary artery disease.

METHODS AND RESULTS

We investigated the expression of the CX3C chemokine fractalkine and its receptor CX3CR1 in human coronary artery plaques by immunocytometry. We show that a subset of mononuclear cells expresses high levels of fractalkine in human coronary atherosclerotic plaques and that smooth muscle cells within the neointima express the fractalkine receptor CX3CR1. There is a positive correlation between the number of fractalkine-expressing cells and the number of CX3CR1-positive cells in human atherosclerotic plaques (r=0.70, n=15 plaques). Furthermore, we demonstrate that cultured vascular smooth muscle cells express the CX3CR1 receptor and undergo chemotaxis to fractalkine that can be inhibited by G protein inactivation by pertussis toxin.

CONCLUSIONS

These results suggest that in human atherosclerosis, fractalkine, rather than mediating inflammatory cell recruitment, can act as a mediator of smooth muscle cell migration.