Prevention of crescentic glomerulonephritis by immunoneutralization of the fractalkine receptor CX3CR1 rapid communication

The role of chemokines and chemokine receptors in progressive renal diseases

Chemokines are members of a growing family of chemotactic cytokines, which induce recruitment of select leukocyte subpopulations. Additionally, they can activate leukocyte effector functions, are involved in the balance between proliferation and apoptosis, modulate angiogenesis, and regulate fibroblast functions. Most intrinsic renal cells can express chemokines on stimulation in vitro. Induction of chemokines and infiltration of chemokine receptor-bearing cells has been shown in a variety of animal models of renal diseases, as well as in human diseases and allograft rejection. This overview gives a short introduction into chemokines and current aspects of their role in progressive human kidney diseases.

Two polymorphisms in the fracalkine receptor CX3CR1 are not associated with peripheral arterial disease

OBJECTIVE

CX(3)CR1 is a novel chemokine receptor located on monocytes. Recently, two polymorphisms were linked to coronary artery disease (CAD), V249I and T280M. Carriers of at least one I-allele or one M-allele were found less frequently among patients with CAD compared to controls. The aim of the present study was to investigate the influence of these polymorphisms on the development of peripheral arterial disease (PAD).

METHODS

522 human subjects with documented PAD and 522 age and sex matched controls were genotyped by polymerase chain reaction followed by restriction digestion.

RESULTS

Adjusted odds ratio (OR) of carriers of the I-allele for PAD was 1.34 (95% confidential interval (CI) from 0.86 to 2.09; P=0.19). The OR associated with the M-allele for PAD was 0.65 (95% CI from 0.41 to 1.04; P=0.07), when tested in the same regression analysis with the V249I genotypes. The genotypes were not linked to age at onset or severity of the disease. A subgroup of 137 CAD patients of whom 131 could be genotyped and who did not differ in baseline parameters from the remaining PAD patients, showed VV-genotype in 52.0%, VI in 42.7% and II in 5.3% CAD (OR associated with the I-allele for CAD: 1.29; 95% CI: 0.66-2.51; P=0.46). The distribution of the T280M genotypes was 67.1, 29.8, 3.1% (TT, TM, MM) also showing no association with CAD (OR=0.77; 95% CI 0.36-1.46; P=0.37).

CONCLUSION

In this study we could not detect a difference in genotype frequencies of the V249I and T280M polymorphisms in CX(3)CR1 between PAD patients and controls. CAD concomitant with PAD was also not affected by the I- or the M-allele.

Tumor necrosis factor-alpha stimulates fractalkine production by mesangial cells and regulates monocyte transmigration: down-regulation by cAMP

BACKGROUND

Fractalkine is a CX3C chemokine for mononuclear cells that has been implicated in the recruitment and accumulation of monocytes seen in glomerular diseases. We investigated the mechanisms by which tumor necrosis factor (TNF)-alpha stimulates mesangial cell (MC) fractalkine expression, and the effects of MC-derived fractalkine on monocyte transmigration.

METHODS

Cultured rat MCs were incubated with TNF-alpha, with or without pretreatment with pharmacologic inhibitors of protein kinases or transcriptional factors downstream to TNF-alpha. Fractalkine mRNA and protein were analyzed by Northern and Western blotting. Translocation of nuclear factor (NF)-kappaB was evaluated by immunocytochemical staining. Monocyte transmigration was determined by in vitro chemotaxis assay.

RESULTS

TNF-alpha stimulated MC fractalkine mRNA as well as cell-bound and soluble protein expression in a dose- and time-dependent manner. The soluble fractalkine was shed from the cell-bound form via metalloproteinase-dependent cleavage, and mediated in part TNF-alpha-induced monocyte transmigration in vitro. The incubation of MCs with calphostin C [a selective inhibitor of protein kinase C (PKC)] or PD98059 [a selective inhibitor of p42/44 mitogen-activated protein kinase (MAPK) kinase] attenuated TNF-alpha-stimulated fractalkine mRNA and protein expression. Coincubation of MCs with calphostin C and PD98059 resulted in a synergistic inhibition of TNF-alpha-stimulated fractalkine mRNA and protein expression. Incubation of MCs with phorbol myristate acetate (PMA) for four hours resulted in an increase in fractalkine mRNA expression that could be suppressed by calphostin C or depletion of PKC by pretreatment with PMA for 24 hours. Further, activation of PKC-depleted MCs with TNF-alpha stimulated fractalkine mRNA expression that could be blocked by calphostin C. PD 98059, but not calphostin C, inhibited TNF-alpha-activated phospho-p42/44 MAPK and phospho-c-Jun levels, whereas only calphostin C inhibited TNF-alpha-activated phosphorylation of PKCzeta/iota. The incubation of MCs with MG132, a NF-kappaB inhibitor, abolished TNF-alpha-induced degradation of inhibitory protein of NF-kappaB (I-kappaB)alpha, nuclear translocation of NF-kappaB, and fractalkine expression, without affecting phospho-c-Jun levels. In contrast, curcumin, an activating protein (AP)-1 inhibitor, attenuated TNF-alpha-stimulated phospho-c-Jun levels and fractalkine expression without discernible effects on TNF-alpha-induced degradation of I-kappaBalpha or NF-kappaB nuclear translocation. Neither PD 98059 nor calphostin C affected TNF-alpha-induced degradation of I-kappaBalpha or NF-kappaB nuclear translocation. Additional experiments examining the role of cAMP on MC fractalkine expression showed that the incubation of MCs with TNF-alpha and either db-cAMP or forskolin attenuated TNF-alpha-stimulated fractalkine mRNA and protein expression, preceded by attenuation of TNF-alpha-activated phosphorylation of p42/44 MAPK, and c-Jun, but not phosphorylation of PKCzeta/iota or nuclear translocation of NF-kappaB.

CONCLUSION

The present data indicate that TNF-alpha activation of PKCzeta/iota, p42/44 MAPK, c-Jun/AP-1, and p65/NF-kappaB are involved in TNF-alpha-stimulated MC fractalkine expression, with the soluble fractalkine mediating in part the TNF-alpha-induced monocyte transmigration in vitro. Uncoupling of p42/44 MAPK or c-Jun/AP-1 signals may contribute to cAMP inhibition of MC fractalkine expression activated by TNF-alpha.

Chemokines and chemokine receptors are involved in the resolution or progression of renal disease

Locally secreted chemokines mediate leukocyte recruitment during the initiation and amplification phase of renal inflammation. In turn, the infiltrating leukocytes contribute to renal damage by releasing inflammatory and profibrotic factors. Rapid down modulation of the chemokine signal will support resolution of acute inflammation, whereas progression occurs if ongoing or repeated renal injury maintains continuous local chemokine secretion and leukocyte influx into the glomerulus or the interstitial space. In glomerular injury proteinuria itself as well as glomerular secreted cytokines stimulate downstream tubular epithelial cells to also secrete chemokines. During primary tubular injury, tubular epithelial cells directly become a major site of chemokine production. This in turn supports leukocyte infiltration and activation. Infiltrating leukocytes stimulate fibroblast proliferation and matrix synthesis, leading to widening of the interstitial space. The specific and intricate renal vascular architecture renders the organ susceptible to ischemic damage as interstitial volume increases. Ischemia in turn serves as a stimulus for chemokine and cytokine production and matrix synthesis. The mutual stimulation between fibroblasts and infiltrating leukocytes supports progressive tubular damage, renal fibrosis, and glomerulosclerosis. Potentially this vicious circle leading to progression of chronic nephropathies offers the opportunity for therapeutic intervention. Interfering with the chemokine network that mediates leukocyte recruitment may represent a promising therapeutic option for progressive renal disorders and renal fibrosis. This article summarizes the present data on the role of chemokines in acute and chronic renal disease with special emphasis on their potential role in mediating resolution or progression of renal disease as well as on therapeutic options.

Slit proteins, potential endogenous modulators of inflammation

Recent studies suggest that molecules important for guiding neuronal migration and axon path-finding also play a role in modulating leukocyte chemotaxis. Neuronal migration and leukocyte chemotaxis may share some common regulatory mechanisms. Intracellular signal transduction mechanisms guiding neuronal migration and leukocyte chemotaxis are beginning to be elucidated. Studying molecular mechanisms modulating cell migration may provide new insights into understanding of endogenous inhibitors of inflammation.

Fractalkine (CX3CL1) and brain inflammation: Implications for HIV-1-associated dementia

Leukocyte migration and activation play an important role in immune surveillance and the pathogenesis of a variety of neurodegenerative disorders, including human immunodeficiency virus (HIV)-1-associated dementia (HAD). A novel chemokine named fractalkine (FKN, CX3CL1), which exists in both membrane-anchored and soluble isoforms, has been proposed to participate in the generation and progression of inflammatory brain disorders. Upon binding to the CX3C receptor one (CX3CR1), FKN induces adhesion, chemoattraction, and activation of leukocytes, including brain macrophages and microglia (MP). Constitutively expressed in the central nervous system (CNS), mainly by neurons, FKN is up-regulated and released in response to proinflammatory stimuli. Importantly, FKN is up-regulated in the brain tissue and cerebrospinal fluid (CSF) of HAD patients. Together, these observations suggest that FKN and its receptor have a unique role in regulating the neuroinflammatory events underlying disease. This review will examine how FKN contributes to the recruitment and activation of CX3CR1-expressing MP, which are critical events in the neuropathogenesis of HAD.

Expression of the fractalkine receptor (CX3CR1) in human kidney diseases

BACKGROUND

CX3CL1 (fractalkine) is a membrane bound chemokine that can function as an adhesion molecule for cells expressing the receptor CX3CR1. This receptor is involved in the recruitment of inflammatory cells in a rat model of crescentic glomerulonephritis, where blockade of CX3CR1 has been shown to be of benefit. Here we describe the distribution of CX3CR1 positive cells in a variety of kidney diseases and renal development.

METHODS

A total of 84 formalin-fixed, paraffin-embedded specimens including fetal kidneys (N = 12), normal areas of kidneys uninvolved by neoplasia from tumor nephrectomies (N = 4), renal transplant nephrectomies (N = 5), renal transplant biopsies (N = 19), and kidney biopsies from patients with crescentic glomerulonephritis (N = 7), membranous nephropathy (N = 7), membranoproliferative glomerulonephritis (N = 8), focal and segmental glomerulosclerosis (N = 10), collapsing glomerulopathy (N = 6), and minimal change disease (N = 6) were studied. Immunohistochemistry was performed on consecutive tissue sections for CD3 positive T cells, CD68 positive monocyte/macrophages, CCR5 positive cells and CX3CR1 positive cells.

RESULTS

The majority of inflammatory leukocytes infiltrating the kidney expressed CX3CR1. The distribution pattern was consistent with expression by both T cells and monocytes/macrophages. In contrast to the distribution of CCR5, which was expressed on a subset of infiltrating cells predominantly localized in the interstitium, CX3CR1 was present on both interstitial and glomerular infiltrating leukocytes. In developing kidneys CX3CR1 positive cells formed a small, scattered population of cells, consistent with the distribution of infiltrating leukocytes.

CONCLUSIONS

The high number of CX3CR1-positive inflammatory cells in various disease entities is consistent with its having a role in the accumulation of intrarenal inflammatory cells, but does not provide evidence of specificity of leukocytes bearing this receptor for specific types of injury. Other chemokine gradients, like those created by the ligands for the chemokine receptor CCR5, might subsequently guide leukocyte subsets to specific microenvironments.

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.

Up-regulated expression of fractalkine and its receptor CX3CR1 during liver injury in humans

BACKGROUND/AIMS

Little is known about the role of fractalkine (CX3CL1) in the liver. The aim of this study was to investigate the expression patterns of fractalkine and its receptor CX3CR1 in normal human liver and in conditions of injury.

METHODS

Distribution and expression of fractalkine and its receptor were investigated using immunohistochemistry, in situ hybridization, flow cytometry and reverse transcriptase-polymerase chain reaction. In vitro experiments were conducted in HepG2 cells.

RESULTS

Both fractalkine and CX3CR1 were up-regulated during chronic injury, in areas of portal and lobular inflammation. In severe acute hepatitis, fractalkine and CX3CR1 were expressed at high levels not only in areas of inflammation but also in regenerating epithelial cells within bile duct-like structures, which showed co-expression of fractalkine and cytokeratin-7 or CX3CR1. The human hepatocarcinoma cell line HepG2 expressed fractalkine at the gene and protein level, and HepG2-conditioned medium was chemotactic for cells overexpressing CX3CR1. Transcripts for CX3CR1 were detected in HepG2, and exposure of these cells to recombinant fractalkine induced cell migration.

CONCLUSIONS

This study shows that the fractalkine system is up-regulated during liver damage, and suggests that fractalkine may play a role in the recruitment and adhesion of inflammatory cells and in the biology of liver epithelial cells.

Chemokines and chemokine receptors in leukocyte trafficking

Chemokines regulate inflammation, leukocyte trafficking, and immune cell differentiation. The role of chemokines in homing of naive T lymphocytes to secondary lymphatic organs is probably the best understood of these processes, and information on chemokines in inflammation, asthma, and neurological diseases is rapidly increasing. Over the past 15 years, understanding of the size and functional complexity of the chemokine family of peptide chemoattractants has grown substantially. In this review, we first present information regarding the structure, expression, and signaling properties of chemokines and their receptors. The second part is a systems physiology-based overview of the roles that chemokines play in tissue-specific homing of lymphocyte subsets and in trafficking of inflammatory cells. This review draws on recent experimental findings as well as current models proposed by experts in the chemokine field.

Fractalkine expression and the recruitment of CX3CR1+ cells in the prolonged mesangial proliferative glomerulonephritis

BACKGROUND

We established the reversible and the prolonged models of mesangial proliferative glomerulonephritis (GN) with anti-Thy 1 antibody 1-22-3. However, the essential factors leading to the prolonged glomerular alterations have not been identified.

METHODS

The expressions of several chemokines and cytokines were compared in the reversible and the prolonged models. Expression of fractalkine and the number of the fractalkine receptor CX3CR1-positive cells in the glomeruli in the prolonged model were significantly higher than those in the reversible model. Then, the localization of fractalkine and the characteristics of CX3CR1+ cells were analyzed in glomeruli. To elucidate the significance of the fractalkine expression, we analyzed the expression in the model treated with angiotensin II receptor antagonist, candesartan.

RESULTS

Immunostaining of fractalkine was detected on endothelial cells on the fifth day, and fractalkine staining also was detected in the mesangial area on day 14. Major parts of the CX3CR1+ cells in the glomeruli were macrophages, especially ED3+ cells. Candesartan treatment ameliorated the glomerular morphological findings at six weeks after disease induction. Although the treatment did not ameliorate the morphological finding at two weeks, decreased expression of fractalkine and CX3CR1+ were already detected at two weeks in rats treated with candesartan.

CONCLUSIONS

Fractalkine expression and the recruitment of CX3CR1+ cells in glomeruli might play an important role in the development of the prolonged disease. These expressions could be predictors of the prolonged disease of the mesangial proliferative glomerulonephritis.

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.

Expression of fractalkine and its receptor, CX3CR1, in response to ischaemia-reperfusion brain injury in the rat

Fractalkine is a neuronally expressed chemokine that acts through its G-protein-coupled receptor CX3CR1, localized on microglial and immune cells. Fractalkine might be involved in neuroinflammatory processes secondary to neuronal damage, which normally occur in a time frame of days after ischaemia. We evaluated by in situ hybridization and immunohistochemistry the expression of fractalkine and CX3CR1 in the rat brain, after a transient occlusion of the middle cerebral artery. We found that at 12 h after ischaemia neuronal fractalkine expression was transiently increased in scattered necrotic neurons of the cortex and lost from the ischaemic striatum. At 24 and 48 h after ischaemia, fractalkine immunoreactivity was strongly increased in morphologically intact cortical neurons of the ischaemic penumbra where also the stress-inducible HSP-72 was strongly up-regulated. The intensity of fractalkine immunoreactivity of neurons in the penumbra returned to basal levels at 7 days after ischaemia. Fractalkine synthesis was also induced in endothelial cells of the infarcted area, at 48 h and 7 days after ischaemia. CX3CR1 expression was detected in the activated microglial cells of the ischaemic tissue 24 and 48 h after ischaemia, and became strongly up-regulated in macrophages/phagocytic microglia inside the infarcted tissue 7 days after ischaemia. These data suggest that fractalkine may participate in the activation and chemoattraction of microglia into the infarcted tissue, and contribute to the control of leucocyte trafficking from blood vessels into the injured area.

Fractalkine is expressed by smooth muscle cells in response to IFN-gamma and TNF-alpha and is modulated by metalloproteinase activity

Fractalkine/CX3C-chemokine ligand 1 is expressed as a membrane-spanning adhesion molecule that can be cleaved from the cell surface to produce a soluble chemoattractant. Within the vasculature, fractalkine is known to be generated by endothelial cells, but to date there are no reports describing its expression by smooth muscle cells (SMC). In this study we demonstrate that IFN-gamma and TNF-alpha, but not IL-1beta, cooperate synergistically to induce fractalkine mRNA and protein expression in cultured aortic SMC. We also report the release of functional, soluble fractalkine from the membranes of stimulated SMC. This release is inhibited by the zinc metalloproteinase inhibitor batimastat, resulting in the accumulation of membrane-associated fractalkine on the SMC surface. Therefore, an SMC-derived metalloproteinase activity is involved in fractalkine shedding. While soluble fractalkine present in SMC-conditioned medium is capable of inducing calcium transients in cells expressing the fractalkine receptor (CX3CR1), blocking experiments using neutralizing Abs reveal that it can be inactivated without affecting the chemotactic activity of SMC-conditioned media on monocytes. However, membrane-bound fractalkine plays a major role in promoting adhesion of monocytic cells to activated SMC. This fractalkine-mediated adhesion is further enhanced in the presence of batimastat, indicating that shedding of fractalkine from the cell surface down-regulates the adhesive properties of SMC. Hence, during vascular inflammation, the synergistic induction of fractalkine by IFN-gamma and TNF-alpha together with its metalloproteinase-mediated cleavage may finely control the recruitment of monocytes to SMC within the blood vessel wall.

Fractalkine expression in human renal inflammation

Immune and inflammatory human renal disease is associated with heavy mononuclear cell infiltration. The trafficking of these cells to extravascular sites is directed by local production of chemokines. Fractalkine is the first described cell-surface anchored chemokine and has potent mononuclear cell-directed adhesion and chemotactic properties. The purpose of this study was to analyse the expression and distribution of fractalkine in human renal inflammation. In situ hybridization and immunohistochemistry were used to study renal biopsies from 15 patients with predominant glomerular inflammation (vasculitic glomerulonephritis) and 15 with predominant tubular and interstitial inflammation (acute renal allograft rejection). Controls comprised non-inflammatory glomerulonephritis and normal tissue. Fractalkine mRNA was predominantly expressed in the major compartment, glomerular or tubulointerstitial, affected by disease and with the strongest expression localized to vascular sites local to inflammation. In acute renal allograft rejection, there was increased expression of fractalkine mRNA by tubular epithelial cells. There was no expression of fractalkine by infiltrating leukocytes and there was only sparse expression in control tissue. Fractalkine mRNA expression correlated with infiltrating leukocyte subsets. Immunohistochemistry confirmed this pattern of expression, with serial section co-localization showing fractalkine expression in areas with macrophage (CD68+) and T cell (CD3+) infiltrates. These expression patterns show that fractalkine is a strong candidate for directing mononuclear cell infiltration in human renal inflammation.

Chemokine receptors

Although chemokines were originally defined as host defense proteins it is now clear that their repertoire of functions extend well beyond this role. For example chemokines such as MGSA have growth regulatory properties while members of the CXC chemokine family can be mediators or inhibitors of angiogenesis and may be important targets for oncology. Recent work shows that the chemokine receptor CXCR4 and its cognate ligand SDF play important roles in the development of the immune, circulatory and central nervous systems. In addition, chemokine receptors play an important role in the pathogenesis of the AIDS virus, HIV-1. Taken together these findings expand the biological importance of chemokines from that of simple immune modulators to a much broader biological role than was at first appreciated and these and other properties of the chemokine receptor family are discussed in detail in this review.

Fractalkine and vascular injury

The vascular endothelium plays a central role in the recruitment and migration of circulating effector cells into sites of inflammation and immune responses. The unique CX(3)C-chemokine, fractalkine, is expressed on activated endothelial cells, and its receptor, CX(3)CR1, is expressed on natural killer cells, monocytes and some CD8+ T cells, all of which possess cytolytic function. Accumulating evidence that fractalkine is expressed on endothelial cells during glomerulonephritis and cardiac allograft rejection, as well as on cardiac endothelial cells activated by pro-inflammatory cytokines, might provide insight into the pathogenesis of vascular injury. Here, we propose a model in which fractalkine mediates vascular injury through the accumulation and activation of killer cells.

Tumor necrosis factor-alpha-converting enzyme (ADAM17) mediates the cleavage and shedding of fractalkine (CX3CL1)

Fractalkine (CX3CL1) is an unusual member of the chemokine family that is synthesized with its chemokine domain at the end of a mucin-rich, transmembrane stalk. This membrane-bound localization allows fractalkine to function as an adhesion molecule for cells bearing its receptor, CX3CR1. In addition, fractalkine can be proteolytically released from the cell surface, generating a soluble molecule that functions as a chemoattractant similar to the other members of the chemokine family. In this study, we have examined the mechanisms that regulate the conversion between these two functionally distinct forms of fractalkine. We demonstrate that under normal conditions fractalkine is synthesized as an intracellular precursor that is rapidly transported to the cell surface where it becomes a target for metalloproteinase-dependent cleavage that causes the release of a fragment containing the majority of the fractalkine extracellular domain. We show that the cleavage of fractalkine can be markedly enhanced by stimulating cells with phorbol 12-myristate 13-acetate (PMA), and we identify tumor necrosis factor-alpha converting enzyme (TACE; ADAM17) as the protease responsible for this PMA-induced fractalkine release. In addition, we provide data showing that TACE-mediated fractalkine cleavage occurs at a site distinct from the dibasic juxtamembrane motif that had been suggested previously based on protein sequence homologies. The identification of TACE as a major protease responsible for the conversion of fractalkine from a membrane-bound adhesion molecule to a soluble chemoattractant will provide new information for understanding the physiological function of this chemokine.

Targeted deletion of CX(3)CR1 reveals a role for fractalkine in cardiac allograft rejection

Fractalkine (Fk) is a structurally unusual member of the chemokine family. To determine its role in vivo, we generated mice with a targeted disruption of CX(3)CR1, the receptor for Fk. CX(3)CR1(-/-) mice were phenotypically indistinguishable from wild-type mice in a pathogen-free environment. In response to antibody-induced glomerulonephritis, CX(3)CR1(-/-) and CX(3)CR1(+/+) mice had similar levels of proteinuria and injury. CX(3)CR1(-/-) and CX(3)CR1(+/+) mice also developed similar levels of disease in myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis. We performed heterotopic MHC class I/II cardiac transplants from BALB/c mice into C57BL/6 mice. In the absence of cyclosporin A (CsA), there was no difference in graft survival time between CX(3)CR1(-/-) and CX(3)CR1(+/+) recipient mice. However, in the presence of subtherapeutic levels of CsA, graft survival time was significantly increased in the CX(3)CR1(-/-) mice. Characterization of cells infiltrating the grafts revealed a selective reduction in natural killer cells in the CX(3)CR1(-/-) recipients in the absence of CsA and a reduction in macrophages, natural killer cells, and other leukocytes in the presence of CsA. We conclude that Fk plays an important role in graft rejection. The development of CX(3)CR1 antagonists may allow reductions in the doses of immunosuppressive drugs used in transplantation.

Phosphatidylinositol 3-kinase-dependent extracellular calcium influx is essential for CX(3)CR1-mediated activation of the mitogen-activated protein kinase cascade

Fractalkine, the first member of the CX(3)C chemokine family, induces leukocyte chemotaxis through activation of its high affinity receptor, CX(3)CR1. Like other chemokine receptors, CX(3)CR1 is coupled to a pertussis toxin-sensitive heterotrimeric G(i) protein, which is necessary for rapid rise in the concentration of intracellular calcium. Using a Chinese hamster ovary cell line stably transfected with the CX(3)CR1 receptor, we show that the source of calcium mobilized by fractalkine stimulation is the extracellular pool. Calcium influx is blocked by extracellular calcium chelators, as well as by divalent heavy metals such as Ni(2+), Co(2+), and Cd(2+) without affecting the integrity of intracellular stores. Remarkably, selective phosphoinositide 3-kinase (PI3K) inhibitors, wortmannin and LY294002, abolish the wave extracellular calcium, suggesting that an active PI3K is necessary for this event. The influx of extracellular calcium is in turn required to trigger the activation of the p42/44 mitogen-activated protein/extracellular signal-regulated kinase pathway, but is not necessary for other signals downstream to PI3K, such as phosphorylation of Akt. The potential role of this signaling cascade in fractalkine-mediated chemotaxis is discussed.

Chemokine expression precedes inflammatory cell infiltration and chemokine receptor and cytokine expression during the initiation of murine lupus nephritis

Lupus nephritis is characterized by immune complex deposition and inflammatory cell infiltration. Therefore, the initiation and progression of lupus nephritis in MRL/MpJ Fas(lpr/lpr) (MRL/lpr) mice were investigated, with a focus on the expression of several chemokines and chemokine receptors. Mice were monitored for proteinuria from 6 to 20 wk of age, and kidneys were examined every 2 wk by light microscopy, electron microscopy, and immunohistologic analyses. Furthermore, the expression of chemokines, chemokine receptors, and proinflammatory cytokines was analyzed in ribonuclease protection assays. MRL/lpr mice demonstrated increased expression of monocyte chemoattractant protein-1, regulated upon activation, normal T cell-expressed and -secreted protein, inducible protein of 10 kD, and macrophage inflammatory protein-1beta at week 8. At that time point, levels of circulating and glomerular immune complexes were increased, and no proteinuria or histopathologic signs of renal damage could be observed. As assessed in immunohistochemical and in situ hybridization analyses, monocyte chemoattractant protein-1 and regulated upon activation, normal T cell-expressed and -secreted protein expression was preferentially located in the glomeruli and interstitium. Mononuclear cell infiltration of the kidney was observed by weeks 10 to 12. At week 12, the renal expression of chemokine receptor 1 (CCR1), CCR2, and CCR5 was increased, mice became proteinuric, and renal damage was histologically evident. Finally, the expression of proinflammatory cytokines was detected (weeks 12 to 14). In summary, (1) chemokines are upregulated before inflammatory cell infiltration, proteinuria, and kidney damage are observed; (2) chemokine generation is restricted to sites of subsequent inflammatory cell infiltration, i.e., glomeruli and interstitium; (3) chemokine receptor expression parallels mononuclear cell infiltration; and (4) proinflammatory cytokines are upregulated later, in parallel with inflammatory cell infiltration and the onset of proteinuria. These results support the hypothesis that chemokines initiate leukocyte infiltration and precede proteinuria and renal damage in MRL/lpr mice.

Fractalkine, a novel chemokine in rheumatoid arthritis and in rat adjuvant-induced arthritis

OBJECTIVE

To examine the expression of the novel CX3C chemokine fractalkine (Fkn) and its receptor (CX3CR1) in rheumatoid arthritis (RA) and rat adjuvant-induced arthritis (AIA), a model of RA.

METHODS

Immunohistochemistry, flow cytometry, enzyme-linked immunosorbent assay (ELISA), reverse transcriptase-polymerase chain reaction (RT-PCR), and chemotaxis assays were used.

RESULTS

In rat AIA, synovial tissue (ST) macrophages, fibroblasts, endothelial cells, and dendritic cells were Fkn immunopositive, whereas lymphocytes did not significantly express Fkn. Significant staining for CX3CR1 was found in ST macrophages, fibroblasts, and dendritic cells, whereas only a small percentage of endothelial cells stained for CX3CR1 in rat AIA. We immunolocalized Fkn to RA ST macrophages, fibroblasts, endothelial cells, and dendritic cells. We also found intense ST macrophage and dendritic cell staining for CX3CR1 in RA ST. Flow cytometry analysis of RA synovial fluid (SF) and peripheral blood revealed a greater percentage of monocytes expressing Fkn and CX3CR1 compared with T cells. By ELISA, we found significantly elevated soluble Fkn (sFkn) levels in RA SF compared with SF from patients with osteoarthritis or other forms of arthritis. By RT-PCR, we found enhanced expression of Fkn and CX3CR1 mRNA on day 18 in rat AIA, a time of pronounced inflammation in the rat joint. Soluble Fkn-depleted RA SF showed significantly decreased chemotactic activity for monocytes compared with sham-depleted RA SF.

CONCLUSION

These results indicate that Fkn and its receptor are both expressed in RA and in rat AIA, and that sFkn is up-regulated in RA SF. Furthermore, our data suggest a new role for Fkn in monocyte chemotaxis in the inflamed RA joint.

Macrophages in renal inflammation

This review describes recent advances in macrophage biology in the context of renal inflammation. It highlights the importance of the activated macrophage for the progression and resolution of renal disease, and discusses recent and potential future approaches to modify macrophage function selectively within the kidney to activate them specifically to promote the healing of kidney disease.

Combinatorial model of chemokine involvement in glomerular monocyte recruitment: role of CXC chemokine receptor 2 in infiltration during nephrotoxic nephritis

A sequential model involving chemokines has been proposed for leukocyte extravasation into areas of inflammation; however, site-specific aspects remain to be elucidated. Hence, we studied the role of chemokines produced by mesangial (MC) or glomerular endothelial cells (GEC) and their receptors in glomerular recruitment of monocytes. Stimulation of MC with TNF-alpha up-regulated mRNA and protein of CC and CXC chemokines but not constitutive expression of the CX(3)C chemokine fractalkine. While growth-related activity (GRO)-alpha was immobilized to MC proteoglycans, monocyte chemotactic protein (MCP)-1 was secreted into the soluble phase. Firm adhesion and sequestration of monocytes on activated MC was supported by the GRO-alpha receptor CXCR2 and to a lesser extent by CX(3)CR, whereas the MCP-1 receptor CCR2 contributed to their transendothelial chemotaxis toward activated MC. In contrast, fractalkine mRNA and protein was induced by TNF-alpha in transformed rat GEC, and both CXCR2 and CX(3)CR mediated monocyte arrest on GEC in shear flow. The relevance of these mechanisms was confirmed in a rat nephrotoxic nephritis model where acute glomerular macrophage recruitment was profoundly inhibited by blocking CXCR2 or CCR2. In conclusion, our results epitomize a combinatorial model in which chemokines play specialized roles in driving glomerular monocyte recruitment and emphasize an important role for CXCR2 in macrophage infiltration during early phases of nephrotoxic nephritis.

Chemokines in renal diseases

The interaction of activated leukocytes and renal resident cells is thought to play a crucial role in the pathogenesis of renal diseases. Recent investigations of the pathophysiological roles of chemokines and their cognate receptors have shed light on the detailed molecular mechanisms of leukocyte trafficking and activation in the diseased kidneys. Chemokine/chemokine receptor systems may be essentially involved in the pathogenesis of phase-specific renal disorders and the measurement of urinary levels of chemokines may be clinically useful for monitoring the different disease phases and activities. In addition, chemokine receptors expressed in renal resident cells may be involved in proliferation, fibrogenesis, as well as chemotaxis. The selective intervention of chemokine/chemokine receptor systems (anti-chemokine therapy) may have the potential as the particular therapeutic strategies for renal diseases in future.

Macrophages transfected with adenovirus to express IL-4 reduce inflammation in experimental glomerulonephritis

Nephrotoxic nephritis (NTN) is characterized by acute macrophage-dependent inflammation and serves as a model of human glomerulonephritis. In this study we have transfected rat macrophages with recombinant adenovirus expressing IL-4 (Ad-IL4) and demonstrated that these transfected macrophages develop fixed properties as a result of transfection, as shown by reduced NO production in response to IFN-gamma and TNF. Ad-IL4-transfected macrophages localized with enhanced efficiency to inflamed glomeruli after renal artery injection in rats with NTN compared with adenovirus expressing beta-galactosidase (Ad-beta gal)-transfected macrophages and produced elevated levels of the cytokine in glomeruli in vivo for up to 4 days. The delivery of IL-4-expressing macrophages produced a marked reduction in the severity of albuminuria (day 2 albuminuria, 61 +/- 15 mg/24 h) compared with unmodified NTN (day 2 albuminuria, 286 +/- 40 mg/24 h; p < 0.01), and this was matched by a reduction in the number of ED1-positive macrophages infiltrating the glomeruli. Interestingly, the injection of IL-4-expressing macrophages into single kidney produced a marked reduction in the numbers of ED1-positive macrophages in the contralateral noninjected kidney, an effect that could not be mimicked by systemic delivery of IL-4-expressing macrophages. This implies that the presence of IL-4-expressing macrophages in a single kidney can alter the systemic development of the inflammatory response. Macrophage transfection and delivery provide a valuable system to study and modulate inflammatory disease and highlight the feasibility of macrophage-based gene therapy.

The transmembrane form of the CX3CL1 chemokine fractalkine is expressed predominantly by epithelial cells in vivo

Fractalkine (CX3CL1) is synthesized as a type I transmembrane protein. Its unique CX(3)C chemokine domain is attached to a 241-amino acid mucin stalk, a 19-amino acid transmembrane domain, and a 37-amino acid intracellular domain of unknown function. A soluble form of fractalkine can be generated by proteolytic cleavage at the base of the mucin stalk. Novel monoclonal and polyclonal antibodies that specifically recognize only the amino- or carboxyl-terminal ends of the human fractalkine molecule have revealed that epithelial cells are the predominant cell type expressing transmembrane forms of fractalkine in human skin, the tonsil, and the large intestine. Using these specific anti-fractalkine reagents we do not detect high-level expression of fractalkine on endothelial cells in normal or inflamed colon samples obtained from patients with Crohn's disease or ulcerative colitis. In contrast to previous reports we do not detect fractalkine expression by Langerhans cells or immature dendritic cells in mucosal-associated lymphoid tissues in vivo. We show that the reagent used in previous studies, an anti-fractalkine N-terminal peptide antisera, cross-reacts with human CD84. Finally we discuss potential roles for fractalkine in constitutive leukocyte trafficking based on its observed pattern of expression in epithelia.

A role for fractalkine and its receptor (CX3CR1) in cardiac allograft rejection

The hallmark of acute allograft rejection is infiltration of the inflamed graft by circulating leukocytes. We studied the role of fractalkine (FKN) and its receptor, CX(3)CR1, in allograft rejection. FKN expression was negligible in nonrejecting cardiac isografts but was significantly enhanced in rejecting allografts. At early time points, FKN expression was particularly prominent on vascular tissues and endothelium. As rejection progressed, FKN expression was further increased, with prominent anti-FKN staining seen around vessels and on cardiac myocytes. To determine the capacity of FKN on endothelial cells to promote leukocyte adhesion, we performed adhesion assays with PBMC and monolayers of TNF-alpha-activated murine endothelial cells under low-shear conditions. Treatment with either anti-FKN or anti-CX(3)CR1-blocking Ab significantly inhibited PBMC binding, indicating that a large proportion of leukocyte binding to murine endothelium occurs via the FKN and CX(3)CR1 adhesion receptors. To determine the functional significance of FKN in rejection, we treated cardiac allograft recipients with daily injections of anti-CX(3)CR1 Ab. Treatment with the anti-CX(3)CR1 Ab significantly prolonged allograft survival from 7 +/- 1 to 49 +/- 30 days (p < 0.0008). These studies identify a critical role for FKN in the pathogenesis of acute rejection and suggest that FKN may be a useful therapeutic target in rejection.

Role of chemokines in inflammation and immunoregulation

Chemokines are first noted for their ability to attract and activate leukocytes, as well as their potential role as mediators of inflammation. However, emerging data have shown that various chemokines may exert other biologic effects both inside and outside the immune system. Inducible chemokines participate primarily in inflammatory responses and comprise the bulk of the chemokine family. Constitutive chemokines are expressed primarily in secondary lymphoid organs and some nonlymphoid organs, where they play a major role in lymphocyte homing. Studies expanding to areas beyond inflammatory leukocyte recruitment will likely give us a more complete picture of chemokine function, its regulation in lymphoid and nonlymphoid tissues, and ways of utilizing endogenous chemokines to intervene with immune and inflammatory reactions.

Chemokines as therapeutic targets in renal disease

Increased understanding of the fundamental importance of the role of chemokines and their receptors in inflammation, together with the demonstration of their involvement in human and experimental inflammatory renal disease, make these molecules potential therapeutic targets. A number of recent studies using genetically deficient mice and chemokine receptor antagonists in animal models have demonstrated that chemokine inhibition can attenuate experimental renal injury. Because there is simultaneous expression of multiple chemokines and receptors in disease, strategies that are aimed at antagonizing multiple chemokines receptor interactions are likely to be more effective than therapies that target a single chemokine. It is also now recognized that chemokines are involved in normal immune development and immune regulation. These observations, together with the results of studies that have demonstrated deleterious effects of chemokine receptor antagonism in experimental renal disease, highlight the need for thorough understanding of the role of individual chemokines in the pathogenesis of different types of renal disease before optimal therapeutic interventions may be achieved.

Chemokine blockade as a therapy for renal disease

Chemokines have been verified as inflammatory mediators of experimental nephritis by using anti-chemokine antibodies, chemokine receptor antagonists or targeted chemokine gene disruption. In human disease, chemokines are expressed in the kidney in association with inflammatory infiltrates. Chemokine blockade is thus a potentially novel approach to therapy of inflammatory renal disease. Several anti-chemokine strategies are under development. Effective use of anti-chemokine therapy for renal disease will require a precise understanding of the unique roles that individual chemokines have in renal pathology, and the genetic factors that regulate chemokine expression.

The role of fractalkine in the recruitment of monocytes to the endothelium

Recombinant fractalkine possesses both chemoattractive and adhesive properties in vitro. Previous studies have demonstrated an upregulation of this molecule on the membranes of activated human endothelial cells and hypothesised that fractalkine plays a role in the recruitment and adherence of monocytes to the activated endothelium. Here we present data analysing both the adhesive and chemoattractive properties of this chemokine expressed by activated human umbilical vein endothelial cells. We demonstrate that both recombinant fractalkine and endogenously produced fractalkine function as adhesion molecules, tethering monocytes to the endothelium. However, our data demonstrate that although recombinant fractalkine has the potential to function as a potent monocyte chemoattractant, the endogenous fractalkine cleaved from activated human umbilical vein endothelial cells is not responsible for the observed chemotaxis in this model. Instead, we show that monocyte chemoattractant protein-1 (MCP-1), secreted from the activated human umbilical vein endothelial cells, is responsible for the chemotaxis of these monocytes.

Ultrastructure and function of the fractalkine mucin domain in CX(3)C chemokine domain presentation

Fractalkine (FKN), a CX(3)C chemokine/mucin hybrid molecule on endothelium, functions as an adhesion molecule to capture and induce firm adhesion of a subset of leukocytes in a selectin- and integrin-independent manner. We hypothesized that the FKN mucin domain may be important for its function in adhesion, and tested the ability of secreted alkaline phosphatase (SEAP) fusion proteins containing the entire extracellular region (FKN-SEAP), the chemokine domain (CX3C-SEAP), or the mucin domain (mucin-SEAP) to support firm adhesion under flow. CX3C-SEAP induced suboptimal firm adhesion of resting peripheral blood mononuclear cells, compared with FKN-SEAP, and mucin-SEAP induced no firm adhesion. CX3C-SEAP and FKN-SEAP bound to CX(3)CR1 with similar affinities. By electron microscopy, fractalkine was 29 nm in length with a long stalk (mucin domain), and a globular head (CX(3)C). To test the function of the mucin domain, a chimeric protein replacing the mucin domain with a rod-like segment of E-selectin was constructed. This chimeric protein gave the same adhesion of peripheral blood mononuclear cells as intact FKN, both when immobilized on glass and when expressed on the cell surface. This implies that the function of the mucin domain is to provide a stalk, extending the chemokine domain away from the endothelial cell surface to present it to flowing leukocytes.

Chemokines, chemokine receptors, and renal disease: from basic science to pathophysiologic and therapeutic studies

Leukocyte trafficking from peripheral blood into affected tissues is an essential component of the inflammatory reaction to virtually all forms of injury and is an important factor in the development of many kidney diseases. Advances in the past few years have highlighted the central role of a family of chemotactic cytokines called chemokines in this process. Chemokines help to control the selective migration and activation of inflammatory cells into injured renal tissue. Chemokines and their receptors are expressed by intrinsic renal cells as well as by infiltrating cells during renal inflammation. This study summarizes the in vitro and in vivo data on chemokines and chemokine receptors in renal diseases with a special focus on potential therapeutic effects on inflammatory processes.