Vascular involvement in rheumatic diseases: 'vascular rheumatology'

The vasculature plays a crucial role in inflammation, angiogenesis, and atherosclerosis associated with the pathogenesis of inflammatory rheumatic diseases, hence the term 'vascular rheumatology'. The endothelium lining the blood vessels becomes activated during the inflammatory process, resulting in the production of several mediators, the expression of endothelial adhesion molecules, and increased vascular permeability (leakage). All of this enables the extravasation of inflammatory cells into the interstitial matrix. The endothelial adhesion and transendothelial migration of leukocytes is a well-regulated sequence of events that involves many adhesion molecules and chemokines. Primarily selectins, integrins, and members of the immunoglobulin family of adhesion receptors are involved in leukocyte 'tethering', 'rolling', activation, and transmigration. There is a perpetuation of angiogenesis, the formation of new capillaries from pre-existing vessels, as well as that of vasculogenesis, the generation of new blood vessels in arthritis and connective tissue diseases. Several soluble and cell-bound angiogenic mediators produced mainly by monocytes/macrophages and endothelial cells stimulate neovascularization. On the other hand, endogenous angiogenesis inhibitors and exogenously administered angiostatic compounds may downregulate the process of capillary formation. Rheumatoid arthritis as well as systemic lupus erythematosus, scleroderma, the antiphospholipid syndrome, and systemic vasculitides have been associated with accelerated atherosclerosis and high cardiovascular risk leading to increased mortality. Apart from traditional risk factors such as smoking, obesity, hypertension, dyslipidemia, and diabetes, inflammatory risk factors, including C-reactive protein, homocysteine, folate deficiency, lipoprotein (a), anti-phospholipid antibodies, antibodies to oxidized low-density lipoprotein, and heat shock proteins, are all involved in atherosclerosis underlying inflammatory rheumatic diseases. Targeting of adhesion molecules, chemokines, and angiogenesis by administering nonspecific immunosuppressive drugs as well as monoclonal antibodies or small molecular compounds inhibiting the action of a single mediator may control inflammation and prevent tissue destruction. Vasoprotective agents may help to prevent premature atherosclerosis and cardiovascular disease.


Introduction
Vessels and the vascular endothelium are involved in the pathogenesis of inflammatory rheumatic diseases. Rheumatoid arthritis (RA) serves as a prototype of these diseases as it is the most common type of arthritis and a great body of data is available regarding leukocyte recruitment into the synovium, angiogenesis, and accelerated atherosclerosis. The term 'vascular rheumatology' has been accepted by many investigators and includes both microvascular and macrovascular involvement in rheumatic diseases. Apart from RA, systemic lupus erythematosus (SLE), systemic sclerosis (SSc), the antiphospholipid syndrome (APS), and systemic vasculitides have been associated with vascular inflammation, altered angiogenesis, and increased cardiovascular morbidity and mortality. In this review, we will discuss the most relevant

Vascular involvement in rheumatic diseases: 'vascular rheumatology'
information on arthritis-related vascular inflammation, including the role of endothelial cells (ECs), endothelial adhesion molecules (CAMs) and chemokines, as well as the involvement of neovascularization and some aspects of accelerated atherosclerosis in rheumatic diseases. We will discuss RA in more detail, and other connective tissue diseases described above will also be mentioned. Finally, some aspects of vascular targeting in rheumatology will also be briefly summarized.

The process of leukocyte recruitment into inflamed tissues
Leukocyte adhesion to ECs occurs following a cascade of events. White blood cells in the bloodstream weakly adhere to the endothelium lining the inner vessel wall (tethering) followed by rolling of leukocytes on the endothelial layer. Tethering and rolling are mediated primarily by selectins and their ligands. These events are followed by leukocyte activation, which is dependent upon interactions between chemokine receptors expressed on leukocytes and proteoglycans on ECs. Activation-dependent firm adhesion occurs next, involving α 4 β 1 integrin/VCAM-1 (vascular cell adhesion molecule-1), β 2 integrin/ICAM-1, and JAM/integrin interactions. This is associated with the secretion of chemokines. These chemokines may also upregulate integrin expression on the adhering cells via PI3K (phosphatidylinositol 3-kinase)-Available online http://arthritis-research.com/content/10/5/224 Table 2 Relevant members of the selectin, integrin, and immunoglobulin adhesion molecule superfamilies  mediated pathways. Leukocyte diapedesis through the endothelial layer involving integrins occurs when chemokines bind to endothelial heparan sulphate. Chemokines preferentially chemoattract EC-adherent leukocytes. These processes lead to the transmigration of leukocytes into the inflamed tissue [1,8].

Targeting of cell adhesion, chemokines, and leukocyte recruitment
Inhibition of cell adhesion, chemokines, and migration using specific antibodies or purified ligands has provided an important perspective on the molecular pathogenesis of RA.

Angiogenesis and vasculogenesis in rheumatic diseases
The processes of angiogenesis and vasculogenesis Angiogenesis is the formation of new capillaries from preexisting vessels, whereas vasculogenesis involves circulating endothelial progenitor cells (EPCs) [14,16,[23][24][25][26][27]. Angiogenesis involves cell surface-bound and soluble angiogenic mediators, which activate vascular ECs (Table 4). In response, ECs release MMPs, which digest the underlying basal membrane and the ECM enabling the emigration of ECs. Single ECs will then gather to form capillary sprouts. Lumen formation within the sprouts leads to capillary loops. Finally, the synthesis of new basement membrane leads to the formation of new capillaries [23]. Regarding vasculogenesis, a subpopulation of circulating CD34 + cells expressing the vascular endothelial growth factor-2 (VEGF-2) receptor has been identified and characterized as functional EPCs. Decreased numbers of EPCs as well as impaired vasculogenesis have been associated with arthritis [27,28]. Table 3 Chemokine receptors with ligands relevant for arthritis and angiogenesis The major chemoattractant that drives EPCs is the SDF-1/CXCL12 chemokine and its receptor, CXCR4 [29]. In arthritis, proinflammatory cytokines stimulate the production of SDF-1/CXCL12 and thus tissue vasculogenesis by recruiting CXCR4 + EPCs [14,17,29].

Angiogenesis in other types of arthritis and connective tissue diseases
Differential vascular morphology may exist in the synovia of RA versus psoriatic arthritis (PsA) patients [16,25]. Furthermore, VEGF production may be associated with increased disease activity and accelerated angiogenesis in PsA and ankylosing spondylitis [16]. In SLE, angiogenic EGF, FGF, and IL-18 as well as angiostatic endostatin have been detected in the sera of patients. Serum VEGF levels were correlated with the SLAM (systemic lupus activity measure) activity score [16,25]. Angiogenesis in SSc is somewhat controversial. On one hand, there is significant loss of vessels in scleroderma despite severe tissue hypoxia associated with increased concentrations of the angiostatic endostatin [16,28]. On the other hand, SSc skin biopsy explants stimulated neovascularization and there is increased production of VEGF in the sera and skin of scleroderma patients [16,28]. Thus, hypoxia may induce angiogenesis in SSc but this is transient and the newly formed vessels are rather unstable in this disease [28]. Furthermore, sustained production of VEGF results in the formation of giant capillaries seen using capillaroscopy in SSc [16,28]. Similarly to SSc, in inflammatory myopathies, expression of hypoxia-associated increased HIF-1, α V β 3 integrin, and VEGF receptor in muscle biopsies was not sufficient to compensate the loss of blood vessels [16,25]. Regarding systemic vasculitides, abundant production of angiogenic VEGF and TGF-β has been associated with Kawasaki syndrome [16]. Increased serum levels of TGF-β were found in ANCA (antineutrophil cytoplasmic antibody)-associated vaculitides, including Wegener granulomatosis, Churg-Strauss syndrome, and microscopic polyangiitis [16,25].

Accelerated atherosclerosis in rheumatic diseases
The basis of atherosclerosis and increased vascular risk Accelerated atherosclerosis and increased cardiovascular morbidity and mortality have been associated with RA, SLE, APS, and SSc [36][37][38][39][40][41]. Cardiovascular disease (CVD) causes reduced life expectancy and became a major mortality factor in these diseases [36][37][38][39][40][41]. Atherosclerosis is also considered an inflammatory disease; thus, it may share common pathogenic mechanisms with rheumatic diseases [36,42,43] (Table 6). Numerous studies have demonstrated the role of traditional, Framingham, and inflammation-associated risk factors in atherosclerosis associated with arthritis [36][37][38]44]. Among traditional risk factors, cigarette smoking not only is a major risk factor for CVD but has recently been implicated in tissue citrullination, the production of anti-cyclic citrullinated peptide (anti-CCP) antibodies, and thus susceptibility to RA [36,38]. In addition to smoking, physical inactivity, obesity, hypertension, dyslipidemia, and diabetes mellitus may be implicated in accelerated atherosclerosis [36][37][38]44]. Yet excess CVD mortality occurs predominantly in RA patients with a higher degree of systemic inflammation [36]; therefore, accelerated atherosclerosis cannot be fully explained on the basis of traditional risk factors [42,43].
Indeed, several inflammatory and atherogenic mediators, including homocysteine, lipoprotein (a), C-reactive protein (CRP), hyperhomocysteinemia, and folate, and vitamin B 12 deficiency and decreased paraoxonase-1 activity are strongly associated with atherosclerosis and CVD [36,42,43]. Atherosclerotic plaques, similarly to the RA joint, are characterized by enhanced accumulation of inflammatory monocytes/ macrophages and T cells. These inflammatory leukocytes abundantly produce proinflammatory cytokines, chemokines, and MMPs [42,43]. CD4 + T cells, especially the CD4 + / CD28 -T-cell subset, have been associated with both arthritis and inflammation-related vascular damage [37,38,43]. Regarding proinflammatory cytokines, TNF-α and IL-6 play an important role in atherosclerosis as well as in RA [31,36,43]. Increased production of TNF-α and IL-6 has been associated with heart failure as well as with insulin resistance, dyslipidemia, and obesity [36,43]. In contrast, IL-4 and IL-10 may exert an anti-inflammatory role during the development of atherosclerosis by driving Th2 responses [31,43] (Table 6).

Vascular involvement in various rheumatic diseases
In RA, age, gender, ethnicity, traditional risk factors described above as well as (among RA-related risk factors) disease duration, activity, and severity, functional impairment, rheumatoid factor and anti-CCP status, CRP, radiographic indicators, presence of the shared epitope, and treatment modalities have been implicated in the development of accelerated atherosclerosis [36][37][38]44]. We have recently assessed common carotid intima-media thickness (ccIMT) indicating atherosclerosis and flow-mediated vasodilation (FMD), a marker of endothelial dysfunction in RA. Increased ccIMT and impaired FMD have been associated with age, disease duration, and anti-CCP, CRP, and IL-6 production [44]. In SLE, primary APS (PAPS) and secondary APS associated with SLE, traditional, and autoimmune-inflammatory factors are involved [40]. Among these factors, longer disease duration and cumulative corticosteroid dose seem to be the major predictors of clinical atherosclerosis [37,38,40,41]. Additional inflammatory risk factors include CRP, fibrinogen, IL-6, costimulatory molecules (CD40/CD40L), CAMs, anti-phospholipid antibodies (APAs), including anticardiolipin and anti-β2 glycoprotein I (anti-β2GPI), antioxidized low-density lipoprotein (anti-oxLDL), anti-oxidized palmitoyl arachidonoyl phosphocholine (anti-oxPAPC), anti-HDL and anti-hsp antibodies, homocysteine, and lipoprotein (a) [37,40,41]. APAs are of importance in both SLE and APS. APAs may bind to neoepitopes of oxLDL as well as to oxLDL-β2GPI complexes, and both APA and anti-oxLDL antibodies have been implicated in the pathogenesis of atherosclerosis associated with SLE and APS [37,38,40,41]. Autoantibodies against oxLDL-β2GPI complexes have been detected in SLE and PAPS patients [40,41]. Both APA and anti-oxLDL may account for increased mortality in CVD [41]. The β2GPI phospholipid cofactor has been detected in the wall of large arteries in the vicinity of CD4 + T-cell infiltrates. Macrophages and ECs bind to β2GPI during the atherosclerotic process [37,38,41]. Atherosclerosis is the most pronounced in lupusassociated secondary APS, in which traditional and nontraditional risk factors are multiplied and atherosclerosis occurs more prematurely [40,41]. SSc is associated with both macrovascular disease (including CVD, pulmonary hypertension, and peripheral arterial occlusion) and microvascular disease (including Raynaud phenomenon) [37-39, 45,46]. Pathogenic factors involved in SSc-associated vascular damage include increased LDL, homocysteine, and CRP production [37,39,46]. We recently described the association of 5,10-methylene-tetrahydrofolate reductase (MTHFR) C677T polymorphism with homocysteine, vitamin B 12 production, and macrovascular abnormalities in SSc [46]. Increased arterial stiffness and ccIMT as well as impaired FMD have been detected by us [39,45] and others [37] in scleroderma.

Therapeutic considerations
Anti-inflammatory treatment used in inflammatory rheumatic diseases may be either proatherogenic or antiatherogenic [37,47]. Corticosteroids are atherogenic by augmenting dyslipidemia, hypertension, and diabetes mellitus [36,47]. In autopsy studies, long exposure to corticosteroid therapy was associated with the development of atherosclerosis. However, other clinical studies could not confirm this association [36,47]. Glucocorticoids may exert a bimodal action as they are atherogenic but, on the other hand, also anti-inflammatory. There is evidence that the above-described inflammatory factors associated with more active disease may exert higher risk for atherosclerosis than anti-inflammatory treatment [37,47]. In contrast to corticosteroids, antimalarial drugs such as chloroquine and hydroxychloroquine may exert evident antiatherogenic properties. Antimalarials may reduce LDL cholesterol, very LDL cholesterol, and (in corticosteroidtreated patients) triglyceride production [36,37,47]. Methotrexate (MTX) exerts bipolar effects on atherosclerosis in RA: on one hand, MTX treatment increases plasma levels of homocysteine, but, on the other hand, MTX controls several other mediators of inflammation and thus may beneficially influence the net outcome of CVD in RA [36,47]. Concomitant folate supplementation prevented the increase of homocysteine production and reduced CVD mortality in MTX-treated patients [36]. Among biologic agents, TNF-α Table 6 Common risk factors in the pathogenesis of atherosclerosis underlying rheumatic diseases anti-CCP, anti-cyclic citrullinated peptide; anti-oxLDL, anti-oxidized low-density lipoprotein.
blockers may have significant effects on the vasculature [48]. In RA, infliximab treatment reduced endothelial dysfunction and ccIMT [48]. We recently proposed that rituximab may also exert favorable effects on FMD, ccIMT, and dyslipidemia [49]. Atherosclerosis treatment strategies in rheumatic diseases should include an aggressive control of all traditional risk factors, including hyperlipidemia, hypertension, smoking, obesity, and diabetes mellitus. Both pharmacological treatment and changes in lifestyle should be introduced in these patients [47]. There is very little solid evidence from randomized controlled trials indicating the preventative action of any drugs in arthritis-associated CVD [47]. Drug therapy may include the use of antiplatelet agents, statins, folic acid, B vitamins, and (as described above) possibly antimalarials [36,47]. A recommendation from the European League Against Rheumatism for the prevention and management of CVD in arthritis is about to be published [50].

Summary
In this review, we discussed the putative role of leukocyte-EC adhesion, chemokines, and angiogenesis in leukocyte recruitment underlying the pathogenesis of inflammatory synovitis. A number of CAMs are involved in this process. These CAMs interact with soluble inflammatory mediators such as cytokines and chemokines. The presence of various CAM pairs and the existence of distinct steps of rolling, activation, adhesion, and migration account for the diversity and specificity of leukocyte-EC interactions. Chemokines and their receptors drive inflammatory leukocytes into the synovium. A number of soluble and cell-bound factors may stimulate or inhibit angiogenesis. The outcome of inflammatory and other 'angiogenic diseases' such as various forms of arthritis depends on the imbalance between angiogenic and angiostatic mediators. There have been several attempts to therapeutically interfere with the cellular and molecular mechanisms described above. Specific targeting of leukocyte adhesion, CAMs, chemokines, chemokine receptors, and/or angiogenesis, primarily by using agents with multiple actions, may be useful for the future management of inflammatory rheumatic diseases.