Skip Navigation



Rheumatology Advance Access published online on May 21, 2008
Rheumatology, doi:10.1093/rheumatology/ken197
This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow All Versions of this Article:
47/10/1446    most recent
ken197v1
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrowRequest Permissions
Right arrow Disclaimer
Google Scholar
Right arrow Articles by Murphy, G.
Right arrow Articles by Molloy, M.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Murphy, G.
Right arrow Articles by Molloy, M.
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?

© The Author 2008. Published by Oxford University Press on behalf of the British Society for Rheumatology. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Review

Fractalkine in rheumatoid arthritis: a review to date

G. Murphy1, N. Caplice2 and M. Molloy1

1Department of Rheumatology, Cork University Hospital and 2Centre for Research in Vascular Biology, University College Cork, Cork, Ireland

Correspondence to: G. Murphy, Department of Rheumatology, Cork University Hospital, Wilton, Cork, Ireland. E-mail: grainne.murphy{at}ucc.ie


   Abstract
Top
 Abstract
Introduction
 FKN: structure
 FKN: biological actions relevant...
 Cells of the synovium...
 Angiogenesis
 Therapeutics and diagnostics
 Conclusion
 References
 
Rheumatoid arthritis (RA) is characterized by the expansion of the synovium, with infiltration of pro-inflammatory cells, neovascularization and an abundance of pro-inflammatory cytokines resulting in tissue destruction and bone erosion. Fractalkine (FKN), a recently described chemokine, possesses chemotactic, angiogenic and adhesive functions that associates it with all of these destructive processes. In this review, we describe the research to date, which implicates FKN and its receptor in the pathogenesis of RA and propose that this molecule may represent a future therapeutic target for RA.

KEY WORDS: Fractalkine, Angiogenesis, Chemotaxis, Rheumatoid


   Introduction
Top
 Abstract
 Introduction
FKN: structure
 FKN: biological actions relevant...
 Cells of the synovium...
 Angiogenesis
 Therapeutics and diagnostics
 Conclusion
 References
 
Rheumatoid arthritis (RA) is the most common of the inflammatory arthropathies, affecting up to 1% of the population. Histologically, it is characterized by hyperplasia and chronic inflammatory changes within the synovium. The inflammatory infiltrate that develops is the result of an aberrancy in both T cell and macrophage function. Macrophages are the primary source of pro-inflammatory cytokines that contribute locally to joint destruction, as well as to systemic inflammation [1]. In recent years, research has turned to the involvement of chemokines, pro-inflammatory molecules with potentially important roles in the pathogenesis of inflammatory arthritis.

Chemokines
The chemokines comprise a family of small 8–10 kDa proteins that were first described as chemoattractant cytokines. They are synthesized at the site of inflammation and play a role in leucocyte migration and trafficking [2]. Chemokines are named according to their structure and are, thus, subdivided into four families (CC, C, CXC and CX3C) based on the number and spacing of the first two cysteines within a conserved cysteine motif [3]. In RA, a number of these molecules have been isolated from the synovium, for example, IL-8. This is a CXC chemokine that has a role in angiogenesis and chemotaxis [4]. Fractalkine (FKN), a more recently described chemokine, shares a number of these attributes. It is a potent chemoattractant and angiogenic agent, while possessing unique adhesive properties. FKN–receptor interactions are present at the level of synovial macrophages and stimulated fibroblast-like synoviocytes (FLS). Soluble FKN levels are also increased in synovial fluid when compared with blood, thus generating a chemokine gradient that leads to the influx of pro-inflammatory cells along this gradient into the rheumatoid joint.

This review focuses on the role of FKN in RA, outlining its multiple pro-inflammatory effects, particularly at the synovial level. We also describe its role as a potential facilitator of angiogenesis in rheumatoid synovium. Finally, we assess the concept of selecting FKN as a novel therapeutic target in RA.


   FKN: structure
Top
 Abstract
 Introduction
 FKN: structure
FKN: biological actions relevant...
 Cells of the synovium...
 Angiogenesis
 Therapeutics and diagnostics
 Conclusion
 References
 
FKN, also known as CX3CL1, is the sole member of the unique family of CX3C chemokines. It is also the largest of the chemokine family, consisting of 373 amino acids [5]. FKN is a transmembrane spanning molecule expressed on the cell surface and exists in both soluble and membrane-bound forms. Structurally, it consists of a 76 amino acid extracellular chemokine domain that contains the novel cysteine motif. This can be cleaved by proteolysis (possibly mediated by TNF-{alpha}-converting enzyme and ADAM 10) [6] yielding the soluble molecule FKN. This 95 kDa glycoprotein has efficient chemotactic activity for both monocytes and T cells [7]). The chemokine domain is attached to a mucin-like stalk that extends it away from the cell surface, allowing presentation to leucocytes and cell adhesion. This, in turn, leads to a single transmembrane spanning domain and, finally, a 37 amino acid intracellular tail [8]. FKN binds to a receptor known as CX3CR1 that is similar in structure to other known chemokine receptors, signalling primarily via the G protein pathway [9]. This has to date been well described on most CD16+ NK cells, CD14+ macrophages and a proportion of CD3+ T cells; outlined in more detail later in this review [10]. Previous studies on chemokine–receptor interactions indicated a certain promiscuity in ligand–receptor interaction. Recent evidence suggests that certain chemokines are more selective. Amongst these are CXCL12-CXCR4 and FKN-CX3CR1. The latter pair has been demonstrated, both in vitro and in vivo, to be a highly selective pairing. In the original description of the CX3CR1 receptor, FKN was found to bind with both high affinity and specificity to this partner. There was no such interaction with the other known chemokine receptors identified at that time (CCR1-7 and two orphan receptors) [10]. This has been confirmed with further studies including additional in vitro specificity assays and an in vivo model of atherosclerosis [11, 12].


   FKN: biological actions relevant to RA
Top
 Abstract
 Introduction
 FKN: structure
 FKN: biological actions relevant...
Cells of the synovium...
 Angiogenesis
 Therapeutics and diagnostics
 Conclusion
 References
 
There are a number of key functional aspects to CX3CL1 biology that make it attractive to study in the pathogenesis of RA. RA is characterized by synovial infiltration with pro-inflammatory cells, neovascularization and joint destruction [13]. FKN is a powerful chemoattractant agent, mediating adhesion, facilitating transmigration, and thus the synovial influx of pro-inflammatory cells (Fig. 1). It possesses significant angiogenic capability and can lead to the up-regulation of MMPs that are implicated in cartilage degradation. Thus, FKN may play an important role in the initiation and/or propagation of the inflammatory insult in RA.


Figure 1
View larger version (69K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		FIG.1. FKN function in RA. Membrane-bound FKN is cleaved by T.A.C.E, yielding the soluble form of the molecule. This, in turn, chemoattracts inflammatory cells that undergo adhesion and transmigration in response to the membrane-bound form. At the synovial-level FKN contributes to activation of macrophages and T cells, enhancing their effector function and perpetuating the inflammatory response.

 
Adhesion properties
FKN has a unique mechanism of facilitating adhesion. Most chemokines require association with glycosaminoglycans after secretion for retention at the cell surface and subsequent adhesion [14]. The chemokine domain of FKN, when presented at the end of its mucin-like stalk, functions itself as an adhesion molecule, circumventing the need for association with tissue matrix components [15]. The binding of CX3CR1+ cells to FKN occurs at high affinity under both static and flow conditions, with a higher avidity than the more well-known vascular cell adhesion molecule-1(VCAM-1)- very late antigen (VLA)-4 interaction [16]. Furthermore, the activation of both systems, i.e. the integrin system and FKN-CX3CR1, leads to the generation of a far greater response than either component individually. This co-operative relationship appears to be mediated by G-protein associated enhancement of the surface integrin's ligand avidity following CX3CR1 activation [17]. In RA, characterized by the influx of a large number of pro-inflammatory cells, this first step of leucocyte transmigration from peripheral blood to the synovium is an important regulatory checkpoint. FKN, which is markedly up-regulated in the presence of pro-inflammatory cytokines [8], may play a key role in mediating leucocyte extravasation facilitating both initial tethering and final transmigration steps. At the synovial level, the adhesive aspect of FKN function has also been demonstrated, in particular in FLS. These cells, like endothelium, demonstrate substantial up-regulation of surface FKN on stimulation with TNF-{alpha} or IFN-{gamma}, thus enhancing their adhesion to synovial CX3CR1+ T cells [18]. These two cell types interact to perpetuate the local inflammatory response by the up-regulation of T cell effector functions by FKN expressed on FLS. This is mediated by the membrane-bound form of FKN and appears to be facilitated by cell adhesion.

Leucocyte trafficking
The migration of leucocytes into extravascular tissue, including synovium, involves a sequence of molecular events [19–21]. These include the production and secretion of chemotactic substances, the adhesion of leucocytes to endothelial cells, (ECs) and finally their transmigration through the blood vessel wall. All of the above may be facilitated by FKN.

Soluble FKN (sFKN) is a potent chemoattractant molecule for both monocytes and T cells [7]. This aspect of function may be an important mediator of the migration of these cells into extra-vascular tissue; essentially controlling the recruitment of monocytes and CX3CR1+ T cells into inflamed synovium. It has been shown that depleting rheumatoid SF of FKN results in a 32% reduction in chemotactic ability [22]. As corrobatory in vivo evidence, the monocytic inflammatory infiltrate within synovium is significantly reduced in a murine model of arthritis upon administration of a systemic anti-FKN monoclonal antibody [23].

Like the soluble form of the molecule, it appears that membrane-bound FKN may also play a role in cell recruitment. It has previously been shown that CX3CR1+ T cells co-express CCR5 and this expression profile is typical of the polarized Th1 cells found in rheumatoid synovium [24]. Interestingly, membrane-bound FKN (mbFKN) enhances the migration of peripheral T cells to CCL5/RANTES in vitro [24]. It is therefore suggested that mbFKN, expressed on EC surfaces in RA, may contribute to the recruitment of leucocytes mediated by secondary cytokines, i.e. CCL5. In this scenario, migrating CD3+ CX3CR1+ T cells which produce significant levels of the pro-inflammatory cytokines, TNF-{alpha} and IFN-{gamma}, have the potential to form both autocrine and paracrine feedback loops. The TNF-{alpha} that they produce leads to up-regulation of FKN on ECs. This further attracts CX3CR1 + cells, both alone and in conjunction with other chemokines. These cells then produce further TNF-{alpha} setting up a local feedback network promoting inflammatory cell influx.

Nishimura et al. [24] have also demonstrated an increase in the transmigration of CD8+ T cells and NK cells in the presence of membrane-bound FKN in response to secondary cytokines (in this case, MIP 1β as well as IL8). This led to the proposition that FKN, when expressed on inflamed endothelium, may act as a ‘vascular gateway’ for cytotoxic effector cells that express CX3CR1. FKN captures these cells from blood and facilitates their transmigration into tissue where ‘Th1 polarization may be occurring through IFNy production’. In RA, like many organ-specific auto-immune conditions, the Th1 response has been implicated in its pathogenesis. FKN itself may play a role in the generation of this response [25, 26]. IFN-{gamma}, which is produced by NK cells, {gamma}{delta} T cells and Th1 cells plays a role in Th1 polarization. Yoneda et al. [26] have shown that stimulation of NK cells with immobilized FKN markedly increases IFNy production, thus potentially contributing to Th1 polarization. IFN, in turn, leads to increased expression of FKN on ECs, implying a form of paracrine feedback.

Cytotoxic function
The expression of the CX3CR1 on circulating T cells has been suggested as a marker of cytotoxic activity in this cell population. In RA, cytotoxic effector functions including granule expulsion, perforin and granzyme expression are increased on the engagement of CX3CR1 by membrane-bound FKN [27]. It has also been demonstrated that the expression of this chemokine receptor on CD8 and CD4 T cells is associated with a surface expression profile indicative of a cytotoxic phenotype, including CD11b and CD57 expression [27]. The role of cytotoxicity in the pathogenesis of RA is not entirely clear. Apoptosis has the potential to be protective, in the form of eliminating auto-reactive T cells. Alternatively, it may have the opposing effect, leading to enhanced tissue destruction. In a murine model of arthritis, it has been shown that transgenic animals with perforin deficiency developed a milder form of arthritis than their wild-type counterparts [28]. Perforin has also been shown to be up-regulated in the inflamed joints of mouse models of arthritis and in patients with RA [29]. Hence, the increased expression of perforin seen in CX3CR1+ T cells may contribute to the invasive and destructive infiltrate seen in RA synovium. Thus FKN, in this context, may play a role in the retention of cells with an increased tissue-destructive potential at the synovial level.


   Cells of the synovium in RA
Top
 Abstract
 Introduction
 FKN: structure
 FKN: biological actions relevant...
 Cells of the synovium...
Angiogenesis
 Therapeutics and diagnostics
 Conclusion
 References
 
The synovium in RA is a micro-environment rich in pro-inflammatory cells and cytokines, interacting in numerous amplification loops to sustain local cell activation. Subsequently, we outline the various cells of the synovium and their relevance to CX3CR1–FKN biology, with a comparative description of peripheral blood (PB) findings.

T cells
T cells occupy primarily the sublining region in RA, forming organized structures resembling lymph nodes. These cells represent ~50% or more of the inflammatory cell population in RA, with the majority accounted for by Th1 cytokine producing CD4+ cells [30]. In synovial tissue from patients with RA undergoing elective procedures, there is low to moderate expression of the receptor, CX3CR1, on synovial T cells. These CX3CR1+ T cells are found particularly in the perivascular region, with neglible staining of CD3+ T cells elsewhere within the lymphoid follicles. In SF samples from a similar patient group, flow cytometry demonstrated that only 3% of SF T cells carry the receptor, in contrast to >8% of peripheral blood T cells [22]. Given the intense immunostaining in perivascular regions and the discrepancy between SF and peripheral blood compartments, it is tempting to speculate whether FKN mediates receptor down-regulation following cell recruitment and extravasation. Ruth et al. [31] have demonstrated that memory T cells express CX3CR1 to a lesser extent than their naïve counterparts in the rheumatoid joint. These investigators suggest that reduced CX3CR1 expression may be due to receptor down-regulation following lymphocyte activation [31]. This may explain the discrepancy noted between synovial and peripheral lymphocyte CX3CR1 expression with local activation leading to this phenomenon. Notably, this has not been demonstrated in other cell types, in particular macrophages, suggesting cell-specific pathways for CX3CR1 regulation.

In peripheral blood, the population of CX3CR1-positive T cells is amplified in rheumatoid populations compared with healthy controls. On analysis of CD4- and CD8-positive T cells, Nanki et al. [27] found that a substantial proportion of CD8 T cells bear surface CX3CR1 when compared with cells from a healthy population (53.4% vs 19.2%). There was also a significant difference between RA and healthy donor CD4 cells (7.1% vs 3.8%) [27]. Although in the minority, these CD4 cells may still be important. A subgroup of CD4 + cells in RA lack CD 28, an important co-stimulatory receptor, the loss of which occurs in the process of T-cell senescence. This CD4+ CD28- population has previously been shown to undergo oligoclonal expansion in RA at both peripheral blood and synovial level [32]. These cells have a number of properties that differ from their counterparts and which may be of biological relevance in RA pathogenesis. They show resistance to apoptosis, overexpress IFN-{gamma} and acquire a number of regulatory receptors, including killer cell immunoglobulin receptor [33]. Indeed, it is these senescent T cells that bear surface CX3CR1 in RA and their destructive properties are enhanced by interaction with membrane-bound FKN. One study showed that although CD4 + 28+ cells were consistently negative for CX3CR1, up to 60% of senescent cells bore this chemokine receptor in RA populations [34]. Notably, in the presence of membrane-bound FKN, the conditions for cell cycle entry for this cell subset are enhanced. Taken together this suggests that mbFKN may both enhance the cytotoxic ability and improve the replicative potential of this specific subset of immune cells in RA, contributing to a breakdown in self-tolerance and tissue damage. CX3CR1 expression is also correlated closely with enhanced secretion of TNF-{alpha} from CD4+ cells, as well as IFN-{gamma} from both CD4 and CD8 cells. CX3CR1 also predicts an increase in the presence of cytoplasmic granules within T cells [27]. Hence, CX3CR1 may be a marker for a more pro-inflammatory T-cell population.

Macrophages
Macrophages are expanded in both the lining and sublining region of the synovium and are the primary source of pro-inflammatory cytokines. A high percentage of macrophages within rheumatoid synovium express the CX3CR1 receptor. This has been demonstrated in both human and animal models of disease [22, 35]. Both CD68- and CD16-positive monocytes bearing surface CX3CR1 have been identified and soluble FKN shown to be efficiently chemotactic for these cell populations [22, 35]. In the work published last year, Yano et al. [35] looked at the co-expression of CD14+ CD16+ cells in RA. This group demonstrated co-localization of CD16 and CX3CR1 in the lining and sublining regions of the synovium. They postulated that the recruitment of this cell type may be the result of the chemoattractant properties of FKN. If this is borne out by additional studies, then CD14/16 dual positive cells may be at the vanguard of tissue destruction, as they express higher levels of MHC Class 2 molecules, adhesion molecules, chemokine receptors and pro-inflammatory cytokines [36]. Yano's group also demonstrated that soluble FKN induced IL-1β and IL-6 secretion from monocytes in a dose-dependent manner. This supports a crucial pro-inflammatory effect of FKN on monocyte function. In peripheral blood, a significant proportion of monocytes bear surface CX3CR1 (~80%) and it is suggested that FKN recruits these cells to the synovium where they become the main driving force of the inflammatory response [22]. Further studies are required to characterize the relationship between CX3CR1-positive monocyte subsets in PB and FKN, as well as the dynamic relationship between receptor expression and monocyte function.

FLS
FLS, that are resident in the sublining region, and markedly expanded in RA synovium have been linked with a number of important functions in RA. They secrete pro-inflammatory cytokines (e.g. GM-CSF), have antigen-presentation capabilities and can induce T-cell expansion [37]. In RA, they stain intensely for surface CX3CL1, a characteristic that is amplified by stimulation with pro-inflammatory cytokines, in particular TNF-{alpha} [34]. In vitro, membrane-bound FKN on FLS has been shown to have a number of important effects on senescent T cells. Upon interaction, T cells showed augmentation of esterase granule expulsion, IFN-{gamma} release, enhanced adhesion and both early (CD69 up-regulation) and late (IFN-{gamma}, TNF-{alpha} release and clonal burst) activation events. Thus, membrane-bound FKN may act as a co-stimulatory signal for CD4 + 28- T cells in RA synovium [34].

Sawai et al. [34] found that a number of these functions—adhesion and early activation—were inhibited by soluble FKN. The biological relevance of this inhibition remains uncertain. Further research is required to investigate whether this inhibition is related to receptor tachyphylaxis with an excess of ligand, or whether FKN-mediated inhibition is due to a differential effect of the membrane-bound form (e.g. alternative intracellular signalling events). In vivo, it may be that the balance between the stimulatory and inhibitory capabilities of this chemokine determine the biological effects of its up-regulation.

B Cells
The recognition of the importance of B cells in RA pathogenesis has lead to the use of rituximab, a CD20 inhibitor, as a treatment for refractory disease. B cells function in RA as producers of autoantibodies, antigen-presenting cells and as costimulatory cells [38]. The earlier studies on FKN and CX3CR expression identified only a minute proportion of B cells that bear the FKN receptor. B cells also showed no chemotactic response in vitro to FKN stimulation [10]. No further studies have been carried out on the presence or FKN or its receptor on these cells in the context of RA.

SF
A number of investigators have demonstrated that, in subjects with RA, there are elevated levels of soluble FKN in SF compared with subjects with OA, other rheumatic diseases or healthy controls [35, 39]. In peripheral blood, however, there is debate as to whether sFKN is elevated or suppressed in disease states. Both Yano et al. [35] and Matsunawa et al. [40] found increased circulating sFKN while Ruth et al. found near undetectable levels. It is hard to interpret these findings without assessing the contribution of disease-modifying drugs or arthritis activity and it is certain that larger studies are needed. The physiological significance of FKN within these two biological compartments (synovium and PB) is, also, not yet fully characterized. Soluble FKN in SF is chemotactic for human monocytes, with a 32% reduction in the chemotactic function of SF immunodepleted of FKN. At this level, it may contribute to the generation of local chemokine gradients. It may also play a part in tissue destruction, with stimulation of cultured FLSs by FKN resulting in up-regulation of matrix metalloproteinase 2 [39]. At a diagnostic level, this discrepancy may be exploited to distinguish between RA, OA and other inflammatory arthropathies. At a peripheral blood level further studies are required to determine the amount of circulating FKN and the relationship, if any, between serum FKN and RA disease activity.


   Angiogenesis
Top
 Abstract
 Introduction
 FKN: structure
 FKN: biological actions relevant...
 Cells of the synovium...
 Angiogenesis
Therapeutics and diagnostics
 Conclusion
 References
 
Angiogenesis is the growth and proliferation of new blood vessels and this process plays a key role in many chronic inflammatory states including RA. The biology of angiogenesis involves a number of steps including the induction of EC migration, proliferation, elongation, orientation and differentiation resulting in lumen formation, re-establishment of a basement membrane and anastamosis with other blood vessels [41]. Molecules that contribute to this process include members of the adhesion molecule superfamily, such as VCAM1 and E-selectin, as well as chemokines, including IL-8 [42].

It was previously reported that the CXC chemokines with pro-angiogenic effects contained a conserved structural motif. This ELR motif is composed of a glutamic acid–leucine–arginine combination preceding the cysteine domain [43]. Since then, a number of additional chemokines with angiogenic effect have been described, which lack the ELR motif and one of these is FKN. In RA, synovial vascularity correlates with more severe clinical and inflammatory scores [44]. The possible role of FKN in this process has been recently studied. Volin et al. [45] found that soluble FKN was both chemotactic and chemokinetic for HMVECs at picomoles per litre and nanomoles per litre concentrations. This effect was completely inhibited by immunodepletion of FKN from the assay. This was achieved using a chemokine domain-specific antibody, demonstrating that this domain is essential for EC migration. Using SF from RA patients, HMVEC chemotaxis was assayed before and after immunodepletion of soluble FKN and the potent chemotactic ability of SF was reduced by >50% in the immunodepleted group.

Although it appears that FKN is chemotactic for ECs, the same group were unable to demonstrate any mitogenic activity of the chemokine [45]. They did, however, demonstrate further angiogenic activity of FKN in SF using both in vitro and in vivo models. To achieve this they examined the effect of FKN on EC tube formation in vitro and matrigel plug neovascularization in vivo. In comparison with a negative control, FKN induced significantly more endothelial tube formation, an effect that was mediated, at least in part, by its chemokine domain. Moreover, FKN-loaded matrigel plugs implanted subcutaneously in mice showed significantly more new vessel formation than negative controls and comparable responses with other known angiogenic chemokines (IL-8). This activity was markedly reduced using FKN-depleted SF from patients with RA.

It has been well-described that ECs express surface FKN and this is enhanced by the pro-inflammatory cytokines, TNF-{alpha} and IL-1. A number of studies have also shown that ECs express surface CX3CR1 [45, 46]. Thus, in a condition such as RA, pro-inflammatory cytokines may up-regulate EC FKN that upon cleavage might attract ECs bearing the surface receptor, leading to the initiation of neovascularization.


   Therapeutics and diagnostics
Top
 Abstract
 Introduction
 FKN: structure
 FKN: biological actions relevant...
 Cells of the synovium...
 Angiogenesis
 Therapeutics and diagnostics
Conclusion
 References
 
To date there has been only one study on the therapeutic effect of inhibiting FKN in an experimental animal model of arthritis [23]. This study was performed by Nanki et al. [23] who administered an anti-FKN monoclonal antibody to mice with CIA. This group found that clinical measures of arthritis as well as the inflammatory infiltrate on histological sections were reduced 2 weeks after the booster injection. In addition, there was a 73% reduction in the incidence of arthritis and a reduction in the trafficking of cells of the monocytic lineage to the synovium.

It is unknown whether these results can be replicated at later time points, given the expected time course of CIA. The in vivo effect on angiogenesis of FKN inhibition would also be of great interest. Regarding the latter, Volin et al. [45] have demonstrated that SF depleted of FKN has a diminished angiogenic effect compared with control SF when tested in vivo in a mouse matrigel model of angiogenesis. This suggests a potential clinical effect that should be pursued in more physiological models of disease, for example, CIA or antibody-induced arthritis in mice.

Given the multitude of functions of FKN and its receptor that have been outlined, FKN has the potential to play an important part in many aspects of RA pathogenesis. As it appears to have relatively less redundancy than some of its chemokine counterparts [47], the inhibition of FKN or CX3CR1 may well become a new and effective therapeutic target in RA. Practically, this may consist of monotherapy with a receptor inhibitor or inhibitor of the soluble molecule. Inhibition of this pathway should also be studied with regard to angiogenesis and aberrant neovascularization in rheumatoid arthritis. Although not discussed within the framework of this review, FKN is currently receiving significant attention in the field of atherosclerosis [12, 48]. Given the premature atherosclerosis seen in RA, it would be interesting to study the effect of FKN inhibition on cardiovascular disease activity in this cohort of patients.

In addition to its potential for therapeutic targeting, the significant discrepancies between healthy and disease populations at SF and peripheral blood level suggest that measurement of serum and SF FKN may be exploited in disease diagnosis and quantification of response to treatment. Prior to consideration of this, larger scale and stratified studies will be required to demonstrate whether this is independent of disease-modifying treatment and whether the FKN concentration correlates with either clinical or other laboratory measures of disease activity. Given the present knowledge, it would appear that SF measurements allow a more definitive definition of a disease population. Soluble FKN can be measured following arthrocentesis using a simple ELISA assay and in conjunction with clinical measures may allow a more accurate diagnosis. Analysis of surface CX3CR1 expression on SF CD3+ cells may also be helpful in this regard. Ruth et al., for example, demonstrated that CX3CR1 expression on SF CD3+ T cells was correlated with swollen joint count in a RA population [17]. Hence, flow cytometry of this cellular subset may be a useful additional diagnostic tool.


   Conclusion
Top
 Abstract
 Introduction
 FKN: structure
 FKN: biological actions relevant...
 Cells of the synovium...
 Angiogenesis
 Therapeutics and diagnostics
 Conclusion
References
 
RA is a debilitating disease characterized histologically by synovial proliferation and infiltration by a variety of immune cells. FKN is a chemo-attractant chemokine that, together with its cognate receptor CX3CR1, has a multitude of functions implicated in RA. This suggests that this chemokine–receptor pathway may lie at the core of disease pathophysiology. It has been demonstrated to play a part in the influx of immune cells, in particular monocytes, it enhances the function of senescent T cells, mediating their effector function and it may also participate in angiogenesis. In addition to the above, FKN has the ability to amplify its function through both autocrine and paracrine pathways. Given the demonstrated effects of FKN-CX3CR1 in human models of disease, this pathway remains of major interest in RA therapeutics and may also have potential as a target for diagnostic screening of rheumatoid disease activity.

Formula

Disclosure statement: The authors have declared no conflicts of interest.


   References
Top
 Abstract
 Introduction
 FKN: structure
 FKN: biological actions relevant...
 Cells of the synovium...
 Angiogenesis
 Therapeutics and diagnostics
 Conclusion
 References
 

  1. Solau-Gervais E, Laxenaire N, Cortet B, Dubucquoi S, Duquesnoy B, Flipo RM. Lack of efficacy of a third tumour necrosis factor alpha antagonist after failure of a soluble receptor and a monoclonal antibody. Rheumatology (2006) 45:1121–4.[Abstract/Free Full Text]
  2. Wells TN, Proudfoot AE. Chemokine receptors and their antagonists in allergic lung disease. Inflamm Res (1999) 48:353–62.[CrossRef][Web of Science][Medline]
  3. Zlotnik A, Yoshie O. Chemokines: a new classification system and their role in immunity. Immunity (2000) 12:121–7.[CrossRef][Web of Science][Medline]
  4. Koch AE, Volin MV, Woods JM, et al. Regulation of angiogenesis by the C-X-C chemokines interleukin-8 and epithelial neutrophil activating peptide 78 in the rheumatoid joint. Arthritis Rheum (2001) 44:31–40.[CrossRef][Web of Science][Medline]
  5. Umehara H, Bloom E, Okazaki T, Nagano Y, Yoshie O, Imai T. Fractalkine in vascular biology: from basic research to clinical disease. Art Throm Vasc Biol (2004) 24:34.[CrossRef]
  6. Tsou C-L, Haskell CA, Charo IF. Tumor necrosis factor converting enzyme mediates the inducible cleavage of fractalkine. J Biol Chem (2001) 276:44622–26.[Abstract/Free Full Text]
  7. Chapman GA, Moores K, Harrison D, Campbell CA, Stewart BR, Strijbos PJ. Fractalkine cleavage from neuronal membranes represents an acute event in the inflammatory response to excitotoxic brain damage. J Neurosci (2000) 20:RC87.[Abstract/Free Full Text]
  8. Bazan JF, Bacon KB, Hardiman G, et al. A new class of membrane-bound chemokine with a CX3C motif. Nature (1997) 385:649–44.
  9. Al-Aoukaty A, Rolstad B, Giaid A, Maghazachi AA. MIP-3alpha, MIP-3beta and fractalkine induce the locomotion and the mobilization of intracellular calcium, and activate the heterotrimeric G proteins in human natural killer cells. Immunology (1998) 95:618–24.[CrossRef][Web of Science][Medline]
  10. Imai T, Hieshima K, Haskell C, et al. Identification and molecular characterization of Fractalkine receptor CX3CR1, which mediates both leukocyte migration and adhesion. Cell (1997) 91:521–30.[CrossRef][Web of Science][Medline]
  11. Saederup N, Chan L, Lira SA, Charo IF. Fractalkine deficiency markedly reduces macrophage accumulation and atherosclerotic lesion formation in CCR2–/– mice. Advance Access Published December 28, 2007. Evidence for independent chemokine functions in atherogenesis. Circulation 2007.
  12. Combadière C, Potteaux S, Gao J, et al. Decreased atherosclerotic lesion formation in CX3CR1/apolipoprotein E double knockout mice. Circulation (2003) 107:1009–16.[Abstract/Free Full Text]
  13. Youssef PP, Kraan M, Breedveld F, et al. Quantitative microscopic analysis of inflammation in rheumatoid arthritis synovial membrane samples selected at arthroscopy compared with samples obtained blindly by needle biopsy. Arthritis Rheum (1998) 41:663–9.[CrossRef][Web of Science][Medline]
  14. Johnson Z, Proudfoot AE, Handel TM. Interaction of chemokines and glycosaminoglycans: a new twist in the regulation of chemokine function with opportunities for therapeutic intervention. Cytokine Growth Factor Rev (2005) 16:625–36.[CrossRef][Web of Science][Medline]
  15. Kerfoot S, Lord S, Bell R, Gill V, Robbins S, Kubes P. Human fractalkine mediates leukocyte adhesion but not capture under physiological shear conditions: a mechanism for selective monocyte recruitment. Eur J Immunol (2003) 33:729–39.[CrossRef][Web of Science][Medline]
  16. Haskell CA, Cleary MD, Charo IF. Molecular uncoupling of fractalkine-mediated cell adhesion and signal transduction. Rapid flow arrest of CX3CR1-expressing cells is independent of G-protein activation. J Biol Chem (1999) 274:10053–8.[Abstract/Free Full Text]
  17. Goda S, Imai T, Yoshie O, et al. CX3C-chemokine, fractalkine enhanced adhesion of THP-1 cells to endothelial cells through integrin dependent and independent mechanisms. J Immunol (2000) 164:4313–20.[Abstract/Free Full Text]
  18. Yoshikawa M. TNF-alpha and IL-4 regulate expression of fractalkine (CX3CL1) as a membrane-anchored proadhesive protein and soluble chemotactic peptide on human fibroblasts. FEBS Lett (2004) 561:105–10.[CrossRef][Web of Science][Medline]
  19. Springer TA. Traffic signals on endothelium for lymphocyte recirculation and leukocyte emigration: the multistep paradigm. Annu Rev Physiol (1995) 57:827–72.[CrossRef][Web of Science][Medline]
  20. Zimmerman GA, McIntyre TM, Prescott SM. Adhesion and signaling in vascular cell-cell interactions. J Clin Invest (1996) 98:1699–702.[Web of Science][Medline]
  21. Albelda SM, Smith CW, Ward PA. Adhesion molecules and inflammatory injury. FASEB J (1994) 8:504–12.[Abstract]
  22. Ruth J, Volin M, Haines K, et al. Fractalkine, a novel chemokine in rheumatoid arthritis and in rat adjuvant induced arthritis. Arth Rheum (2001) 44:1568–81.[CrossRef][Web of Science][Medline]
  23. Nanki T, Urasaki Y, Imai T, et al. Inhibition of fractalkine ameliorates murine collagen-induced-arthritis. J Immunol (2004) 173:7010–16.[Abstract/Free Full Text]
  24. Nishimura M, Umehara H, Nakayama T, et al. Dual functions of fractalkine/CX3Cr1 in trafficking of circulating cytotoxic effector lymphocytes that are defined by CX3CR1 expression. J Immunol (2002) 168:6173–80.[Abstract/Free Full Text]
  25. Umehara H, Bloom ET, Okazaki T, Yoshie O, Imai T. Fractalkine and vascular injury. Trends Immunol (2001) 22:602–07.[CrossRef][Web of Science][Medline]
  26. Yoneda O, Imai T, Inoue H, et al. Membrane bound form of fractalkine induces IFN-{gamma} production by NK cells: a role for the TH1 response. Eur J Immunol (2003) 33:53–8.[CrossRef][Web of Science][Medline]
  27. Nanki T, Imai T, Nagasaka K, et al. Migration of CX3CR1-positice T cells producing type 1 cytokines and cytotoxic molecules into the synovium of patients with rheumatoid arthritis. Arthritis Rheum (2002) 46:2878–83.[CrossRef][Web of Science][Medline]
  28. Bauer K, Knipper A, Tu-Rapp H, et al. Perforin deficiency attenuates collagen-induced arthritis. Arthritis Res Ther (2005) 7:877–84.[CrossRef]
  29. Gulan G, Ravlic-Gulan J, Strbo N, et al. Systemic and local expression of perforin in lymphocyte subsets in acute and chronic rheumatoid arthritis. J Rheumatol (2003) 30:660–70.[Abstract/Free Full Text]
  30. Morita Y, Yamamura M, Kawashima M, et al. Flow cytometric single-cell analysis of cytokine production by CD4+ T cells in synovial tissue and peripheral blood from patients with rheumatoid arthritis. Arthritis Rheum (1998) 41:1669–76.[CrossRef][Web of Science][Medline]
  31. Ruth JH, Rottman JB, Katschke KJ Jr, et al. Selective lymphocyte chemokine receptor expression in the rheumatoid joint. Arthritis Rheum (2001) 44:2750–60.[CrossRef][Web of Science][Medline]
  32. Pawlick A, Ostanek L, Brzosoko I, et al. The expansion of CD4+ CD28- T cells in patients with rheumatoid arthritis. Arthritis Res Ther (2003) 5:210–3.[CrossRef][Web of Science][Medline]
  33. Groh V, Bruhl A, El-Gabalway H, Nelson JL, Spies T. Stimulation of T cell autoreactivity by anomalous expression of NKG2D and its MIC ligands in rheumatoid arthritis. Proc Natl Acad Sci USA (2003) 100:9452–7.[Abstract/Free Full Text]
  34. Sawai H, Park Y, Roberson J, Imai T, Goronzy J, Weyand C. T cell costimulation by fractalkine-expressing synoviocytes in rheumatoid arthritis. Arthritis Rheum (2005) 52:1392–401.[CrossRef][Web of Science][Medline]
  35. Yano R, Yamamura M, Sunahori K, et al. Recruitment of CD 16+ monocytes into synovial tissues is mediated by fractalkine and CX3CR1 in rheumaotid arthritis patients. Acta Med Okayama (2007) 61:89–98.[Medline]
  36. Kawanaka N, Yamamura M, Aita T, et al. CD14+,CD16+ blood monocytes and joint inflammation in rheumatoid arthritis. Arthritis Rheum (2002) 46:2578–86.[CrossRef][Web of Science][Medline]
  37. McInnes IB, Leung BP, Liew FY. Cell-cell interactions in synovitis. Interactions between T lymphocytes and synovial cells. Arthritis Res (2000) 2:374–8.[CrossRef][Web of Science][Medline]
  38. Nandakumar K, Holmdahl R. Antibody induced arthritis: disease mechanisms and genes involved at the effector phase of arthritis. Arthritis Res Ther (2006) 8:223.[CrossRef][Medline]
  39. Blaschke S, Koziolek M, Schwarz A, et al. Proinflammatory role of fractalkine (CX3CL1) in rheumatoid arthritis. J Rheumatol (2003) 30:1918–27.[Abstract/Free Full Text]
  40. Matsunawa M, Isozaki T, Odai T, et al. Increased serum levels of soluble fractalkine(CX3CL1) correlate with disease activity in rheumatoid vaculitis. Arthritis Rheum (2006) 54:3408–16.[CrossRef][Web of Science][Medline]
  41. Augustin HG. Tubes, branches, and pillars: the many ways of forming a new vasculature. Circ Res (2001) 89:645–7.[Free Full Text]
  42. Koch AE, Halloran MM, Haskell CJ, Shah MR, Polverini PJ. Angiogenesis mediated by soluble forms of E-selectin and vascular adhesion molecule 1. Nature (1995) 376:517–9.[CrossRef][Medline]
  43. Strieter RM, Polverini PJ, Kunkel SL, et al. The functional role of the ELR motif in CXC chemokine-mediated angiogenesis. J Biol Chem (1995) 270:27348–57.[Abstract/Free Full Text]
  44. Baeten D, Houbiers J, Kruithof E, et al. Synovial inflammation does not change in the absence of effective treatment: implications for the use of synovial histopathology as biomarker in early phase clinical trials in rheumatoid arthritis. Ann Rheum Dis (2006) 65:990–7.[Abstract/Free Full Text]
  45. Volin M, Woods J, Amin A, Connors M, Harlow L, Koch A. Fractalkine: a novel angiogenic chemokine in rheumatoid arthritis. Am J Path (2001) 159:1521–30.[Abstract/Free Full Text]
  46. Ryu J, Lee, Hong KH, et al. Activation of Fractalkine/CX3CR1 by vascular endothelial cells induces angiogenesis through VEGF-A/KDR and reverses hindlimb ischemia. Cardiovasc Res (2008) 78:333–40.[Abstract/Free Full Text]
  47. Stievano L, Piovan E, Amadori A. C and CX3C chemokines: cell sources and physiopathological implications. Crit Rev Immunol (2004) 24:205–28.[CrossRef][Web of Science][Medline]
  48. Lesnik P, Haskell CA, Charo IF. Decreased atherosclerosis in CX3Cr1–/– mice reveals a role for fractalkine in atherogenesis. J Clin Invest (2003) 11:333–40.
  49. Combadiere C, Potteaux S, Gao J-L, et al. Decreased atherosclerotic lesion formation in VX3CR1-apolipoprotein E double knockout mice. Circulation (2003) 107:1009–16.[Abstract/Free Full Text]
Submitted 7 January 2008; revised version accepted 15 April 2008.
Add to CiteULike CiteULike   Add to Connotea Connotea   Add to Del.icio.us Del.icio.us    What's this?



This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow All Versions of this Article:
47/10/1446    most recent
ken197v1
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrowRequest Permissions
Right arrow Disclaimer
Google Scholar
Right arrow Articles by Murphy, G.
Right arrow Articles by Molloy, M.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Murphy, G.
Right arrow Articles by Molloy, M.
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?