The pathogenic role of angiogenesis in rheumatoid arthritis

Angiogenesis is the formation of new capillaries from pre-existing vasculature, which plays a critical role in the pathogenesis of several inflammatory autoimmune diseases such as rheumatoid arthritis (RA), spondyloarthropathies, psoriasis, systemic lupus erythematosus, systemic sclerosis, and atherosclerosis. In RA, excessive migration of circulating leukocytes into the inflamed joint necessitates formation of new blood vessels to provide nutrients and oxygen to the hypertrophic joint. The dominance of the pro-angiogenic factors over the endogenous angiostatic mediators triggers angiogenesis. In this review article, we highlight the underlying mechanisms by which cells present in the RA synovial tissue are modulated to secrete pro-angiogenic factors. We focus on the significance of pro-angiogenic factors such as growth factors, hypoxia-inducible factors, cytokines, chemokines, matrix metalloproteinases, and adhesion molecules on RA pathogenesis. As pro-angiogenic factors are primarily produced from RA synovial tissue macrophages and fibroblasts, we emphasize the key role of RA synovial tissue lining layer in maintaining synovitis through neovascularization. Lastly, we summarize the specific approaches utilized to target angiogenesis. We conclude that the formation of new blood vessels plays an indispensable role in RA progression. However, since the function of several pro-angiogenic mediators is cross regulated, discovering novel approaches to target multiple cascades or selecting an upstream cascade that impairs the activity of a number of pro-angiogenic factors may provide a promising strategy for RA therapy.

Spatial relationships in early signaling events of flow-mediated endothelial mechanotransduction

Blood flow interactions with the vascular endothelium represent a specialized example of mechanical regulation of cell function that has important physiological and pathological cardiovascular consequences. The endothelial monolayer in vivo acts as a signal transduction interface for forces associated with flowing blood (hemodynamic forces) in the acute regulation of artery tone and chronic structural remodeling of arteries, including the pathology of atherosclerosis. Mechanisms related to spatial relationships at the cell surfaces and throughout the cell that influence flow-mediated endothelial mechanotransduction are discussed. In particular, flow-mediated ion channel activation and cytoskeletal dynamics are considered in relation to topographic analyses of the luminal and abluminal surfaces of living endothelial cells.

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.

IL-17 contributes to angiogenesis in rheumatoid arthritis

Angiogenesis is an early and a critical event in the pathogenesis of rheumatoid arthritis (RA). Neovascularization is dependent on endothelial cell activation, migration and proliferation, and inhibition of angiogenesis may provide a novel therapeutic approach in RA. In this study, we document a novel role of IL-17 in mediating angiogenesis. Local expression of IL-17 in mouse ankles increases vascularity. We further demonstrate that IL-17 is angiogenic by showing its ability to promote blood vessel growth in Matrigel plugs in vivo. Additionally, IL-17, in concentrations present in the RA joint, induces human lung microvascular endothelial cell (HMVEC) migration mediated through the PI3K/AKT1 pathway. Furthermore, suppression of the PI3K pathway markedly reduces IL-17-induced tube formation. We also show that both IL-17-induced HMVEC chemotaxis and tube formation are mediated primarily through IL-17 receptor C. Neutralization of either IL-17 in RA synovial fluids or IL-17 receptor C on HMVECs significantly reduces the induction of HMVEC migration by RA synovial fluid. Finally, RA synovial fluid immunoneutralized with anti-IL-17 and antivascular endothelial growth factor does not reduce HMVEC migration beyond the effect detected by immunodepleting each factor alone. These observations identify a novel function for IL-17 as an angiogenic mediator in RA, supporting IL-17 as a therapeutic target in RA.

Temporal expression of inflammatory cytokines and chemokines in rat adjuvant-induced arthritis

OBJECTIVE

To examine cytokine and chemokine production during the evolution of rat adjuvant-induced arthritis (AIA), a model of rheumatoid arthritis.

METHODS

Clinical and laboratory assessment of the course of AIA was performed over a 47-day period. Levels of the cytokines tumor necrosis factor a (TNFalpha), interleukin-1beta (IL-1beta), and IL-6, as well as levels of the chemokines macrophage inflammatory protein 1alpha (MIP-1alpha) and JE, the murine homolog of monocyte chemoattractant protein 1, were determined by enzyme-linked immunosorbent assay in the sera and joints of AIA and control rats. Synovia from AIA rats were (immuno)histochemically analyzed. Results of cytokine and chemokine measurements were correlated with clinical and laboratory markers of inflammation and histology.

RESULTS

Early (before day 14 post adjuvant injection) and later phases of AIA could be distinguished. Cytokine and chemokine production was increased in AIA versus control rats. The production of TNFalpha, IL-1beta, MIP-1alpha, and, as determined earlier, epithelial neutrophil-activating peptide 78-like protein was abundant prior to and during the course of AIA, while that of IL-6 and JE was elevated in the late phase of AIA. Cytokine and chemokine levels were correlated with the clinical symptoms of arthritis and blood neutrophil counts. Joint levels of IL-1beta showed correlation with synovial lining proliferation and neutrophil ingress into AIA synovium.

CONCLUSION

Cytokines and chemokines are involved in the clinical, laboratory, and histologic changes underlying AIA. The production of these mediators may be temporally and spatially regulated. These findings may be important for the optimal timing of cytokine and chemokine targeting.

Fractalkine: a novel angiogenic chemokine in rheumatoid arthritis

Angiogenesis is an important aspect of the vasculoproliferation found in the rheumatoid arthritic (RA) pannus. We have previously implicated members of the CXC chemokine family as potent angiogenic mediators in RA. We investigated the possibility that the sole member of the CX(3)C chemokine family, fractalkine (fkn), induces angiogenesis and that fkn might mediate angiogenesis in RA. Recombinant human fkn significantly induced migration of human dermal microvascular endothelial cells (HMVECs), a facet of the angiogenic response, in the pmol/L range in a concentration-dependent manner (P < 0.05). Fkn also induced the formation of significantly more endothelial tubes on Matrigel than did a negative control (P < 0.05). Fkn significantly induced 2.3-fold more blood vessel growth than control in the in vivo Matrigel plug assays (P < 0.05). We identified HMVEC expression of the fkn receptor, CX(3)CR1. Next, we determined if RA synovial fluid (SF)-induced angiogenesis was fkn-dependent. SFs from six RA patients immunodepleted of soluble fkn induced 56% less migration of HMVECs than did sham-depleted RA SFs (P < 0.05). In vivo, immunodepletion of fkn from six RA SFs significantly inhibited their angiogenic activity in Matrigel plug assays (P < 0.05). Immunodepletion of fkn from five RA synovial tissue homogenates inhibited their ability to induce angiogenesis in in vivo Matrigel plug assays (P < 0.05). These results establish a new function for fkn as an angiogenic mediator and suggest that it may mediate angiogenesis in RA.

IL-4 adenoviral gene therapy reduces inflammation, proinflammatory cytokines, vascularization, and bony destruction in rat adjuvant-induced arthritis

IL-4 is a cytokine with anti-inflammatory properties on activated macrophages. Rheumatoid arthritis, an autoimmune inflammatory disease, is characterized by a paucity of IL-4 and an abundance of synovial macrophage-derived mediators. Herein, the effect of a single injection of adenovirus-producing rat IL-4 (AxCAIL-4) or a control virus with no inserted gene was compared with the effect of PBS injection into rat ankles. Ankles were injected before arthritis onset or at maximal inflammation. Preventatively, AxCAIL-4 reduced adjuvant-induced arthritis (AIA)- and/or AIA/adenoviral-induced ankle inflammation, decreasing articular index scores, ankle circumferences, paw volumes, radiographic scores, mean levels of monocyte chemoattractant protein-1, the number of inflammatory cells, and the number of synovial blood vessels. Therapeutically, AxCAIL-4 also decreased ankle circumferences and paw volumes in comparison with a control virus with no inserted gene and PBS groups. After arthritis onset, mean levels of TNF-alpha, IL-1beta, macrophage inflammatory protein-2, and RANTES were decreased in AxCAIL-4 rat ankle homogenates compared with PBS-treated homogenates. Thus, increased expression of IL-4 via gene therapy administered in a preventative and/or therapeutic manner reduced joint inflammation, synovial cellularity, levels of proinflammatory cytokines, vascularization, and bony destruction in rat AIA, suggesting that a similar treatment in humans may be beneficial.

Characterization of CCL19 and CCL21 in rheumatoid arthritis

OBJECTIVE

To characterize the expression of CCL19 and CCL21 in rheumatoid arthritis (RA) synovial tissue (ST) and to examine their regulation and pathogenetic role in macrophages and RA ST fibroblasts.

METHODS

Expression of CCL19 and CCL21 in RA and normal ST was demonstrated by immunohistochemistry analysis. CCL19 and CCL21 levels in synovial fluid (SF) from patients with osteoarthritis (OA), juvenile idiopathic arthritis, psoriatic arthritis (PsA), and RA were quantified by enzyme-linked immunosorbent assay (ELISA). Regulation of CCL19 and CCL21 expression in in vitro-differentiated RA peripheral blood macrophages as well as RA ST fibroblasts was determined by real-time reverse transcription-polymerase chain reaction. Proangiogenic factor production in CCL19- and CCL21-activated in vitro-differentiated peripheral blood macrophages and RA ST fibroblasts was examined by ELISA.

RESULTS

CCL19 and CCL21 were elevated in RA ST compared to tissue from normal controls. Levels of CCL19 and CCL21 were greatly increased in RA and PsA SF versus OA SF. In RA macrophages and fibroblasts, expression of CCL19 was increased by stimulation with lipopolysaccharide, tumor necrosis factor α (TNFα), and interleukin-1β (IL-1β). However, CCL21 expression was modulated only by IL-1β in RA fibroblasts, and by TNFα and RA SF in RA macrophages. CCL19 and CCL21 activation induced vascular endothelial growth factor and angiotensin I (Ang I) production in RA ST fibroblasts and secretion of IL-8 and Ang I from macrophages.

CONCLUSION

The findings of the present study identify, for the first time, regulators of CCL19 and CCL21 in RA fibroblasts and in vitro-differentiated RA peripheral blood macrophages and demonstrate a novel role of CCL19/CCL21 in angiogenesis in RA.

Regulation of angiogenesis by the C-X-C chemokines interleukin-8 and epithelial neutrophil activating peptide 78 in the rheumatoid joint

OBJECTIVE

Angiogenesis, the growth of new blood vessels, is vital to the ingress of inflammatory leukocytes in rheumatoid arthritis (RA) synovial tissue and to the growth and proliferation of RA pannus. The factors that mediate the growth of new blood vessels have not been completely defined. This study examined the ability of Glu-Leu-Arg (ELR)-containing chemokines to induce angiogenesis in the RA joint.

METHODS

To reflect angiogenic activity in vivo, we selected a model using whole human synovial tissue rather than isolated cells. Tissues were examined by immunohistochemistry and enzyme-linked immunosorbent assay, and tissue homogenates were immunoneutralized and assayed for their ability to induce endothelial cell chemotaxis and rat corneal neovascularization.

RESULTS

Cells expressing interleukin-8 (IL-8) and epithelial neutrophil activating peptide 78 (ENA-78) were located in proximity to factor VIII-related antigen-immunopositive endothelial cells. RA homogenates produced more IL-8 and ENA-78 compared with normal synovial tissue homogenates. Moreover, homogenates from RA synovial tissue produced significantly more chemotactic activity for endothelial cells in vitro and angiogenic activity in the rat cornea in vivo than did normal synovial tissue homogenates. The effects of IL-8 and ENA-78 accounted for a significant proportion of the chemotactic activity of endothelial cells and angiogenic activity found in RA synovial tissue homogenates.

CONCLUSION

These results indicate that the ELR-containing chemokines IL-8 and ENA-78 are important contributors to the angiogenic activity found in the inflamed RA joint. It is possible that efforts aimed at down-regulating these chemokines offer a novel targeted therapy for the treatment of RA.

Chemokine receptor CXCR4 expression in endothelium

The expression of chemokine receptor and viral coreceptor CXCR4 is reported in cultured endothelial cells and in arterial endothelium. A 1.9 kb transcript was cloned from cultured bovine aortic (BAEC) and human umbilical vein endothelial cells (HUVEC). CXCR4 mRNA was expressed at high levels in BAEC and HUVEC but was not expressed by cultured bovine arterial smooth muscle cells (BASM) or human umbilical vein smooth muscle cells (HUVSM). Western blotting with polyclonal antibodies demonstrated an approximate 46KD protein in endothelial cells only. In situ hybridization and immunocytochemistry (anti-CXCR4 monoclonal antibody 12G5) revealed both transcript and protein expression in cultured endothelial cells, and in the endothelium of normal aorta but not in aortic smooth muscle. The ligand for CXCR4, stromal cell derived factor 1 (SDF-1) stimulated mobilization of intracellular calcium at a moderate level (37% of the peak response to thrombin), confirming the expression of functional receptor at the endothelial surface. The involvement of CXCR4 in chemokine signaling, chemoattraction (through SDF-1), and its potential viral coreceptor activity suggest a multifunctional role in vascular homeostasis and pathophysiology.

RANTES expression and contribution to monocyte chemotaxis in arthritis

Rheumatoid arthritis (RA) is characterized by recruitment of leukocytes from the vasculature into inflamed synovial tissue (ST) and synovial fluid (SF), which depends, in part, upon the continued maintenance of chemotactic stimuli. RANTES is a potent chemoattractant for leukocytes including monocytes and CD45RO+ memory T lymphocytes. The aim of this study was to determine the production, the source, and the function of antigenic RANTES in arthritis. We detected antigenic RANTES in SFs from RA and OA patients (100 +/- 22.7 and 72 +/- 30.7 pg/ml, respectively). CM from RA ST fibroblasts stimulated with interleukin-1beta or tumor necrosis factor-alpha contained significantly more antigenic RANTES than unstimulated CM (452 +/- 181.6 and 581 +/- 200.2 pg/ml, respectively, versus 12 +/- 4.4 pg/ml, P < 0.05). PHA-stimulated RA SF mononuclear cells secreted 5- to 15-fold more antigenic RANTES than did nonstimulated mononuclear cells, while LPS induced secretion up to 4-fold. We immunolocalized antigenic RANTES to sublining macrophages (28 +/- 3.7 and 8 +/- 2.0% immunopositive cells), perivascular macrophages (56 +/- 6.9 and 19 +/- 3.4%), and synovial lining cells (37 +/- 5.8 and 60 +/- 10.4%) in RA and OA tissue, respectively. Anti-RANTES neutralized 20.2 +/- 1.3% of the RA SF chemotactic activity for normal peripheral blood monocytes (P < 0.05). These results demonstrate antigenic RANTES in RA and OA ST and SF and identify RANTES as a chemoattractant for monocytes in the RA joint.

Differential expression of the angiogenic Tie receptor family in arthritic and normal synovial tissue

Angiopoietins (Ang) are vascular endothelial cell-specific growth factors that play important roles principally during the later stages of angiogenesis. We have compared the distribution of the receptor tyrosine kinase (Tie) and the Ang ligands in synovial tissues from normal subjects and those with rheumatoid arthritis (RA) and osteoarthritis (OA). Immunohistochemical analysis was used to determine the expression of Ang-1, Ang-2, Tie1 and Tie2 in synovial tissue of normal subjects and those with RA and OA. Ang-1, Ang-2, Tie1 and Tie2 mRNA and protein expression were quantified in synovial tissues and RA synovial tissue fibroblasts with real-time reverse transcription polymerase chain reaction and western blot analysis. In RA, Ang-1 positive immunostaining on lining cells, macrophages and endothelial cells was significantly higher than in OA and normal synovial tissue. The expression pattern of Ang-2 in synovial tissue was similar in RA and OA, whereas the Ang-2 expression was low in normal tissue. Synovial tissue from subjects with RA and OA showed a significant upregulation of Tie1 on lining cells, macrophages and endothelial cells compared to that from normal subjects. Tie2 was significantly upregulated in the RA and OA synovial tissue lining cells, macrophages and smooth muscle cells compared to normal synovial tissue. Generally Ang-1, Ang-2, Tie1 and Tie2 mRNA levels were higher in RA synovial tissue compared to normal and OA synovial tissues, and RA synovial tissue fibroblasts. Western blot analysis also demonstrated greater Tie1 and Tie2 protein expression in RA and OA synovial tissue compared to RA synovial tissue fibroblasts. In conclusion, the dominance of Ang-1 mRNA and protein expression over Ang-2 is in agreement with an active neovascularization in RA synovial tissue.

Interleukin-18: a mediator of inflammation and angiogenesis in rheumatoid arthritis

Interleukin-18 (IL-18) is a highly regulated inflammatory cytokine that is elevated in synovial tissues and synovial fluids of patients with rheumatoid arthritis (RA) compared with patients with osteoarthritis (OA) and patients with other arthropathies. Within the RA joint, IL-18 can contribute to the inflammatory process by inducing leukocyte extravasation through upregulation of endothelial cell adhesion molecules, the release of chemokines from RA synovial fibroblasts, and directly as a monocytes, lymphocyte, and neutrophil chemoattractant. IL-18 can also help maintain and develop the inflammatory pannus by inducing endothelial cell migration and angiogenesis. IL-18 does this directly by binding and activating endothelial cells and indirectly by inducing RA synovial fibroblasts to produce angiogenic chemokines and vascular endothelial growth factor. IL-18 is present in RA synovial fluid in high levels, where it functions as an angiogenic mediator and leukocyte chemoattractant. IL-18 mediates all these inflammatory processes by binding to its receptor, IL-18 receptor, and initiating the activation of different signaling cascades leading to changes in target cells gene expression and behavior. IL-18 has been identified as a potential therapeutic target in the treatment of RA.

Inhibition of monocyte chemoattractant protein-1 ameliorates rat adjuvant-induced arthritis

Chemokines, including RANTES/CCL5 and MCP-1/CCL2, are highly expressed in the joints of patients with rheumatoid arthritis, and they promote leukocyte migration into the synovial tissue. This study was conducted to determine whether the inhibition of RANTES and MCP-1 therapeutically was capable of ameliorating rat of adjuvant-induced arthritis (AIA). Postonset treatment of AIA using a novel inhibitor for endogenous MCP-1 (P8A-MCP-1) improved clinical signs of arthritis and histological scores measuring joint destruction, synovial lining, macrophage infiltration, and bone erosion. Using immunohistochemistry, ELISA, real-time RT-PCR, and Western blot analysis, we defined joint inflammation, bony erosion, monocyte migration, proinflammatory cytokines, and bone markers, and p-p38 levels were reduced in rat AIA treated with P8A-MCP-1. In contrast, neither the dominant-negative inhibitor for endogenous RANTES (44AANA47-RANTES) nor the CCR1/CCR5 receptor antagonist, methionylated-RANTES, had an effect on clinical signs of arthritis when administered after disease onset. Additionally, therapy with the combination of 44AANA47-RANTES plus P8A-MCP-1 did not ameliorate AIA beyond the effect observed using P8A-MCP-1 alone. Treatment with P8A-MCP-1 reduced joint TNF-alpha, IL-1beta, and vascular endothelial growth factor levels. P8A-MCP-1 also decreased p38 MAPK activation in the joint. Our results indicate that inhibition of MCP-1 with P8A-MCP-1 after the onset of clinically detectable disease ameliorates AIA and decreases macrophage accumulation, cytokine expression, and p38 MAPK activation within the joint.

Local expression of interleukin-27 ameliorates collagen-induced arthritis

OBJECTIVE

To determine the mechanism of action of interleukin-27 (IL-27) against rheumatoid arthritis (RA).

METHODS

Adenovirus containing IL-27 transcript was constructed and was locally delivered into the ankles of mice with collagen-induced arthritis (CIA). Progression of arthritis was determined in treated and untreated mice by measuring ankle circumference and through histologic analysis. IL-17 and its downstream targets as well as cytokines promoting Th17 cell differentiation were quantified by enzyme-linked immunosorbent assay in CIA mouse ankles locally expressing adenoviral IL-27 as well as in control-treated mouse ankles. Ankles from both treatment groups were immunostained for neutrophil and monocyte migration (macrophages in the tissue). Finally, vascularization was quantified by histology and by determining ankle hemoglobin levels.

RESULTS

Ectopic expression of IL-27 in CIA mice ameliorated inflammation, lining hypertrophy, and bone erosion as compared with control-treated CIA mice. Serum and joint levels of IL-17 were significantly reduced in the IL-27-treated group compared with the control-treated group. Two of the main cytokines that induce Th17 cell differentiation and IL-17 downstream target molecules were greatly down-regulated in CIA mouse ankles receiving forced expression of IL-27. The control mice had higher levels of vascularization and monocyte trafficking than did mice ectopically expressing IL-27.

CONCLUSION

Our results suggest that increased levels of IL-27 relieve arthritis in CIA mouse ankles. This amelioration of arthritis involves a reduction in CIA mouse serum and joint levels of IL-17 and results in decreased IL-17-mediated monocyte recruitment and angiogenesis. Hence, the use of IL-27 may be a strategy for treatment of patients with RA.

Characterization of interleukin-7 and interleukin-7 receptor in the pathogenesis of rheumatoid arthritis

OBJECTIVE

To characterize the expression of interleukin-7 (IL-7) and IL-7 receptor (IL-7R) in rheumatoid arthritis (RA) synovial tissue and to examine their regulation and pathogenic role in macrophages, endothelial cells, and synovial tissue fibroblasts in RA.

METHODS

Expression of IL-7 and IL-7R in RA and normal synovial tissue was demonstrated by immunohistochemistry. Expression and regulation of IL-7 and IL-7R in RA peripheral blood in vitro-differentiated macrophages, RA synovial tissue fibroblasts, and human microvascular endothelial cells (HMVECs) were determined by real-time reverse transcription-polymerase chain reaction and/or flow cytometry. Enzyme-linked immunosorbent assay was used to examine production of proangiogenic factors by IL-7-activated macrophages, RA fibroblasts, and endothelial cells.

RESULTS

IL-7 and IL-7R were coexpressed on RA synovial tissue lining and sublining macrophages and endothelial cells. Expression of IL-7 and its receptor was significantly elevated in RA synovial fluid and peripheral blood macrophages as well as RA fibroblasts, compared to normal cells. Toll-like receptor 4 ligation (with lipopolysaccharide) and tumor necrosis factor α (TNFα) stimulation modulated expression of IL-7 and IL-7R on RA macrophages and HMVECs. However, in RA fibroblasts, lipopolysaccharide and TNFα activation increased expression of IL-7R only. IL-7 also mediated RA pathogenesis by inducing production of potent proangiogenic factors from macrophages and endothelial cells.

CONCLUSION

We have identified, for the first time, regulators of IL-7 and IL-7R expression in RA fibroblasts, RA peripheral blood in vitro-differentiated macrophages, and endothelial cells. Our results document a novel role of IL-7 in RA angiogenesis.

CCL21/CCR7 signaling in macrophages promotes joint inflammation and Th17-mediated osteoclast formation in rheumatoid arthritis

In rheumatoid arthritis (RA), synovial tissue abundantly expresses CCL21, a chemokine strongly associated with RA susceptibility. In this study, we aimed to characterize the functional significance of CCL21/CCR7 signaling in different phases of RA pathogenesis. We determined that CCR7 is a hallmark of RA M1 synovial fluid (SF) macrophages, and its expression in RA monocytes and in vitro differentiated macrophages is closely associated with disease activity score (DAS28). In early stages of RA, monocytes infiltrate the synovial tissue. However, blockade of SF CCL21 or CCR7 prevents RA SF-mediated monocyte migration. CCR7 expression in the newly migrated macrophages can be accentuated by LPS and IFNγ and suppressed by IL-4 treatment. We also uncovered that CCL21 stimulation increases the number of M1-polarized macrophages (CD14+CD86+), resulting in elevated transcription of IL-6 and IL-23. These CCL21-induced M1 cytokines differentiate naïve T cells to Th17 cells, without affecting Th1 cell polarization. In the erosive stages of disease, CCL21 potentiates RA osteoclastogenesis through M1-driven Th17 polarization. Disruption of this intricate crosstalk, by blocking IL-6, IL-23, or IL-17 function, impairs the osteoclastogenic capacity of CCL21. Consistent with our in vitro findings, we establish that arthritis mediated by CCL21 expands the joint inflammation to bone erosion by connecting the differentiation of M1 macrophages with Th17 cells. Disease progression is further exacerbated by CCL21-induced neovascularization. We conclude that CCL21 is an attractive novel target for RA therapy, as blockade of its function may abrogate erosive arthritis modulated by M1 macrophages and Th17 cell crosstalk.

Identification of a Novel Toll-like Receptor 7 Endogenous Ligand in Rheumatoid Arthritis Synovial Fluid That Can Provoke Arthritic Joint Inflammation

OBJECTIVE

Levels of Toll-like receptor 7 (TLR-7) are elevated in rheumatoid arthritis (RA), but the impact on RA is unknown because the endogenous ligand for TLR-7 has not been identified. The aim of this study was to identify a TLR-7 endogenous ligand and to determine its role in the pathogenesis of RA.

METHODS

The presence of an endogenous TLR-7 ligand, microRNA let-7b (miR-let-7b), was examined by real-time polymerase chain reaction (PCR) analysis. Using RA knockdown cells, TLR-7-knockout mice, or antagonist, the specificity of miR-let-7b as a potential ligand for TLR-7 was tested. The mechanism by which ligation of miR-let-7b to TLR-7 promotes disease was investigated in RA myeloid cells by real-time PCR, enzyme-linked immunosorbent assay, and fluorescence-activated cell sorting. We also established the effect of ectopic miR-let-7b expression on arthritic joint inflammation.

RESULTS

We found that a TLR-7 endogenous ligand resides mainly in RA synovial fluid macrophages. The GU-rich domain in miR-let-7b was found to be essential for TLR-7 ligation, since miR-147, the positive control for GU, was able to stimulate TLR-7+ myeloid cells, whereas miR-124, the negative, non-GU, control, was not. We demonstrated that miR-let-7b or exosomes containing miR-let-7b could transform the RA and/or mouse naive or antiinflammatory macrophages into inflammatory M1 macrophages via TLR-7 ligation. Consistently, we showed that miR-let-7b provokes arthritis by remodeling naive myeloid cells into M1 macrophages via TLR-7 ligation, since joint swelling and M1 macrophages are absent in TLR-7-deficient mice.

CONCLUSION

The results of this study underscore the importance of miR-let-7b ligation to TLR-7 in the joint during the effector phase of RA.

TLR5, a novel and unidentified inflammatory mediator in rheumatoid arthritis that correlates with disease activity score and joint TNF-α levels

The innate immune system plays an important role in rheumatoid arthritis (RA) pathogenesis. Previous studies support the role of TLR2 and 4 in RA and experimental arthritis models; however, the regulation and pathogenic effect of TLR5 is undefined in RA. In this study, we show that TLR5 is elevated in RA and osteoarthritis ST lining and sublining macrophages and endothelial cells compared with normal individuals. Furthermore, expression of TLR5 is elevated in RA synovial fluid macrophages and RA peripheral blood monocytes compared with RA and normal peripheral blood in vitro-differentiated macrophages. We also found that TLR5 on RA monocytes is an important modulator of TNF-α in RA synovial fluid and that TLR5 expression on these cells strongly correlates with RA disease activity and TNF-α levels. Interestingly, TNF-α has a feedback regulation with TLR5 expression in RA monocytes, whereas expression of this receptor is regulated by IL-17 and IL-8 in RA macrophages and fibroblasts. We show that RA monocytes and macrophages are more responsive to TLR5 ligation compared with fibroblasts despite the proinflammatory response being mediated through the same signaling pathways in macrophages and fibroblasts. In conclusion, we document the potential role of TLR5 ligation in modulating transcription of TNF-α from RA synovial fluid and the strong correlation of TLR5 and TNF-α with each other and with disease activity score in RA monocytes. Our results suggest that expression of TLR5 may be a predictor for RA disease progression and that targeting TLR5 may suppress RA.

The novel role of IL-7 ligation to IL-7 receptor in myeloid cells of rheumatoid arthritis and collagen-induced arthritis

Although the role of IL-7 and IL-7R has been implicated in the pathogenesis of rheumatoid arthritis (RA), the majority of the studies have focused on the effect of IL-7/IL-7R in T cell development and function. Our novel data, however, document that patients with RA and greater disease activity have higher levels of IL-7, IL-7R, and TNF-α in RA monocytes, suggesting a feedback regulation between IL-7/IL-7R and TNF-α cascades in myeloid cells that is linked to chronic disease progression. Investigations into the involved mechanism showed that IL-7 is a novel and potent chemoattractant that attracts IL-7R(+) monocytes through activation of the PI3K/AKT1 and ERK pathways at similar concentrations of IL-7 detected in RA synovial fluid. To determine whether ligation of IL-7 to IL-7R is a potential target for RA treatment and to identify their mechanism of action, collagen-induced arthritis (CIA) was therapeutically treated with anti-IL-7 Ab or IgG control. Anti-IL-7 Ab treatment significantly reduces CIA monocyte recruitment and osteoclast differentiation as well as potent joint monocyte chemoattractants and bone erosion markers, suggesting that both direct and indirect pathways might contribute to the observed effect. We also demonstrate that reduction in joint MIP-2 levels is responsible for suppressed vascularization detected in mice treated with anti-IL-7 Ab compared with the control group. To our knowledge, we show for the first time that expression of IL-7/IL-7R in myeloid cells is strongly correlated with RA disease activity and that ligation of IL-7 to IL-7R contributes to monocyte homing, differentiation of osteoclasts, and vascularization in the CIA effector phase.

Ley/H: an endothelial-selective, cytokine-inducible, angiogenic mediator

Endothelial cells (ECs) are key participants in angiogenic processes that characterize tumor growth, wound repair, and inflammatory diseases, such as human rheumatoid arthritis (RA). We and others have shown that EC molecules, such as soluble E-selectin, mediate angiogenesis. Here we describe an EC molecule, Lewisy-6/H-5-2 glycoconjugate (Ley/H), that shares some structural features with the soluble E-selectin ligand, sialyl Lewisx (sialyl Lex). One of the main previously recognized functions of Lewisy is as a blood group glycoconjugate. Here we show that Ley/H is rapidly cytokine inducible, up-regulated in RA synovial tissue, where it is cell-bound, and up-regulated in the soluble form in angiogenic RA compared with nonangiogenic osteoarthritic joint fluid. Soluble Ley/H also has a novel function, for it is a potent angiogenic mediator in both in vitro and in vivo bioassays. These results suggest a novel paradigm of soluble blood group Ags as mediators of angiogenic responses and suggest new targets for therapy of diseases, such as RA, that are characterized by persistent neovascularization.

Role of the CCL21 and CCR7 pathways in rheumatoid arthritis angiogenesis

OBJECTIVE

To determine the role of CCL21 and its receptor CCR7 in the pathogenesis of rheumatoid arthritis (RA).

METHODS

Histologic studies were performed to compare the expression of CCR7 and CCL21 in RA synovial tissue. Next, the role of CCL21 and/or CCR7 in angiogenesis was examined using in vitro chemotaxis, tube formation, and in vivo Matrigel plug assays. Finally, the mechanism by which CCL21 mediates angiogenesis was determined by Western blot analysis and endothelial cell chemotaxis and tube formation assays.

RESULTS

CCL21, but not CCL19, at concentrations present in the RA joint, induced human microvascular endothelial cell (HMVEC) migration that was mediated through CCR7 ligation. Suppression of the phosphatidylinositol 3-kinase pathway markedly reduced CCL21-induced HMVEC chemotaxis and tube formation; however, suppression of the ERK and JNK pathways had no effect on these processes. Neutralization of either CCL21 in RA synovial fluid or CCR7 in HMVECs significantly reduced the induction of HMVEC migration and/or tube formation by RA synovial fluid. We further demonstrated that CCL21 is angiogenic, by showing its ability to promote blood vessel growth in Matrigel plugs in vivo at concentrations that are present in RA joints.

CONCLUSION

Angiogenesis is dependent on endothelial cell activation, migration, and proliferation, and inhibition of angiogenesis may provide a novel therapeutic approach in RA. This study identified a novel function of CCL21 as a mediator of RA angiogenesis, supporting CCL21/CCR7 as a therapeutic target in RA.

Ligation of TLR7 by rheumatoid arthritis synovial fluid single strand RNA induces transcription of TNFα in monocytes

OBJECTIVE

The aim of the study was to characterise the expression, regulation and pathogenic role of toll-like receptor 7 (TLR7) and TLR8 in rheumatoid arthritis (RA).

METHODS

Expression of TLR7 and TLR8 was demonstrated in RA, osteoarthritis (OA) and normal (NL) synovial tissues (STs) employing immunohistochemistry. The authors next examined the mechanism by which TLR7 and TLR8 ligation mediates proinflammatory response by Western blot analysis and ELISA. Expression of TLR7 and TLR8 in RA monocytes was correlated to disease activity score (DAS28) and tumour necrosis factor α (TNFα) levels. Further, the effect of TLR7 ligation in RA monocytes was determined on synovial fluid (SF)-mediated TNFα transcription.

RESULTS

TLR7/8 are predominately expressed in RA ST lining and sublining macrophages. The authors show that NF-κB and/or PI3K pathways are essential for TLR7/8 induction of proinflammatory factors in RA peripheral blood (PB)-differentiated macrophages. Expression of TLR7 in RA monocytes shows a strong correlation with DAS28 and TNFα levels. By contrast, expression of TLR8 in these cells does not correlate with DAS28, TLR7 or TNFα levels. The authors further demonstrate that RNA from RA SF, but not RA or NL plasma, could modulate TNFα transcription from RA monocytes that can be downregulated by antagonising TLR7 ligation or degradation of single stand (ss) RNA. Thus, ssRNA present in RA SF may function as a potential endogenous ligand for TLR7.

CONCLUSIONS

These results suggest that expression of TLR7, but not TLR8, may be a predictor for RA disease activity and anti-TNFα responsiveness, and targeting TLR7 may suppress chronic progression of RA.

IL-11 facilitates a novel connection between RA joint fibroblasts and endothelial cells

IL-11 has been detected in inflamed joints; however, its role in the pathogenesis of arthritis is not yet clear. Studies were conducted to characterize the expression and functional significance of IL-11 and IL-11Rα in rheumatoid arthritis (RA). IL-11 levels were elevated in RA synovial fluid (SF) compared to osteoarthritis (OA) SF and plasma from RA, OA and normal individuals (NLs). Morphologic studies established that IL-11 was detected in lining fibroblasts and macrophages in addition to sublining endothelial cells and macrophages at higher levels in RA compared to NL synovial tissues. Since IL-11Rα was exclusively expressed in RA fibroblasts and endothelial cells, macrophages were not involved in IL-11 effector function. Ligation of IL-11 to IL-11Rα strongly provoked fibroblast infiltration into RA joint, while cell proliferation was unaffected by this process. Secretion of IL-8 and VEGF from IL-11 activated RA fibroblasts was responsible for the indirect effect of IL-11 on endothelial cell transmigration and tube formation. Moreover, IL-11 blockade impaired RA SF capacity to elicit endothelial cell transmigration and tube formation. We conclude that IL-11 binding to endothelial IL-11Rα can directly induce RA angiogenesis. In addition, secretion of proangiogenic factors from migrating fibroblasts potentiated by IL-11 can indirectly contribute to RA neovascularization.

Ligation of TLR5 promotes myeloid cell infiltration and differentiation into mature osteoclasts in rheumatoid arthritis and experimental arthritis

Our aim was to examine the impact of TLR5 ligation in rheumatoid arthritis (RA) and experimental arthritis pathology. Studies were conducted to investigate the role of TLR5 ligation on RA and mouse myeloid cell chemotaxis or osteoclast formation, and in addition, to uncover the significance of TNF-α function in TLR5-mediated pathogenesis. Next, the in vivo mechanism of action was determined in collagen-induced arthritis (CIA) and local joint TLR5 ligation models. Last, to evaluate the importance of TLR5 function in RA, we used anti-TLR5 Ab therapy in CIA mice. We show that TLR5 agonist, flagellin, can promote monocyte infiltration and osteoclast maturation directly through myeloid TLR5 ligation and indirectly via TNF-α production from RA and mouse cells. These two identified TLR5 functions are potentiated by TNF-α, because inhibition of both pathways can more strongly impair RA synovial fluid-driven monocyte migration and osteoclast differentiation compared with each factor alone. In preclinical studies, flagellin postonset treatment in CIA and local TLR5 ligation in vivo provoke homing and osteoclastic development of myeloid cells, which are associated with the TNF-α cascade. Conversely, CIA joint inflammation and bone erosion are alleviated when TLR5 function is blocked. We found that TLR5 and TNF-α pathways are interconnected, because TNF-α is produced by TLR5 ligation in RA myeloid cells, and anti-TNF-α therapy can markedly suppress TLR5 expression in RA monocytes. Our novel findings demonstrate that a direct and an indirect mechanism are involved in TLR5-driven RA inflammation and bone destruction.