**2. Characteristics of vascular damage in GCA arteries**

Healthy arteries have a large lumen, the arterial wall histology comprises three distinct layers. The adventitia is the external layer, separated from the medial layer by the external elastic lamina, the medial muscular layer is separated from the internal layer by the internal elastic lamina and the intima is composed of unistratificated endothelial cells (ECs) [3]. Positive temporal artery biopsy (TAB) is the gold standard of GCA diagnosis [31], however due to the segmented pattern in which the lesions occur a TAB in a segment without lesion will generate a false negative GCA test. The typical histopathology of GCA arteries highlights transmural granulomatous inflammation. It consists of transmural mononuclear cell infiltration, mostly T cells, macrophages, numerous multinucleated giant cells (seen in 75% of cases), surrounded by low number B cells in all the layers of the artery [16]. The destruction of the internal elastic lamina, vascular smooth muscle cells (VSMCs) from the media to the intimal layer, and intima hyperplasia (IH) [1] are pathognomonic for GCA. IH protrudes in the lumen, progressively causing the occlusion of the vessel. Intraluminal thrombosis is reportedly seen in about 10–20% of GCA cases, probably occurring more frequently in the conditions of high shear in artery stenosis in GCA ischemic complications. A negative TAB does not exclude GCA [1].

The kind of inflammation pattern the vascular wall will take depends entirely on the initial injury events. In GCA, the site of inflammation is restricted the blood vessel wall [4, 11]. Typical GCA pathological findings involve local vascular inflammation, granulomatous infiltrate, and segmented lesions alternating with healthy segments [16]. GCA is characterized by a highly specific tropism to medium and large arteries of the upper extremities, neck and head in older people [1, 11, 32]. On the other hand, the fact that PMR and GCA have similar clinical presentation and a percentage of patients with GCA also have PMR suggest that these two clinical presentations might be two distinct phenotypes of the same pathological process [23].

Intriguingly, topography and histological structure dictate whether GCA lesions are likely to occur. GCA develops mainly in the 2nd-5th extracranial branches of the carotid arteries and upper extremities branches of the proximal aorta that have a highly visible internal elastic membrane and vasa vasorum. Temporal arteries are the most affected [1], and the vertebrobasilar [28] and the ophthalmic arteries can be affected as well [33]. Intracranial branches of the cervical arteries are small vessels that are not prone to GCA as their elastic membranes are very thin to inexistent and lack vasa vasorum because they are nourished through diffusion not through vasa vasorum [34, 35].

#### **2.1 Vasculitis initial events**

Under physiological conditions, the vascular wall mainly acts as a barrier between circulating immune cells and surrounding tissues. Besides its role in blood transport, blood gas regulation and preservation of wall integrity, the arterial network also has an immunosurveillance function [4]. The only immune cell known to be present in the healthy vascular wall is the intramural DCs, found in an immature state, characterized by the expression of C-C motif chemokine receptor 6 (CCR6) and a low density of Toll-like receptors (TLRs) [14].

Under pathological conditions, the large arteries turn into a target of autoimmune disease [4]. The main immunopathogenic mechanisms in the GCA arteries are progressing from the outside to the luminal side of the arterial wall, from the adventitia, where mural DCs reside, towards the intima [4].

#### *2.1.1 The crucial role of dendritic cells in GCA pathogenesis*

To date it is well known that DCs are the antigen-presenting cells (APCs) belonging to the vessel wall-responsible for the initial steps in GCA pathogenesis [14]. Arteries use their wall-embedded sentinel cells, the dendritic cells (DCs) [14], to intervene and generate inflammatory responses [11]. This ability of large arteries to control localized and systemic inflammation using indigenous cell populations is a critical trigger element in this primary vasculitis [11, 12, 14]. Vascular DCs are mostly localized at adventitia/media border [12, 14]. In the presence of unknown, diverse, and non-specific PAMPs, resident DCs activate and break the "immune privilege" [4] in GCA arteries [12], DCs initiate the pathogenic cascade [11] and DCs sense the danger from a distance through PAMPs interactions with their specific toll-ligand-receptors (TLRs) [11]. In contrast to physiological conditions, in GCA arteries, activated DCs at the adventitial/media border fail to leave the artery lesion site, meaning they don't migrate to a lymph node, are retained in the granulomatous infiltrates in the wall of the arteries [14], amplifying a mispositioned inflammation reaction [14].

The TLR cellular distribution in the GCA arteries is as follows: (1) immune cells: dendritic cells, T cells, monocytes, macrophages, and to a lower extent B cells (2) vascular cells: endothelial cells and vascular smooth muscle cells [11]. GCA artery

*Cellular and Molecular Characteristics of Vascular Damage in Giant Cell Arteritis, the 'Unmet… DOI: http://dx.doi.org/10.5772/intechopen.97933*

TLR fingerprint consist of high amounts of TLR2, TLR4 and TLR8, intermediate levels of the TLR1, TLR5, and TLR6 and lower or absent for TLR3 and TLR9 [11]. TLRs of vascular DCs are implicated in the strong tissue tropism of GCA [11].

In GCA, most severe inflammatory effects occur at the intima/media border, adjacent to the internal elastic lamina from the outside in, from the adventitia to the intima direction [14], but sometimes the inflammation is initiated in a tiny area in the vasa vasorum in intra-adventitial small vessels [31].

### *2.1.2 Recruitment, proliferation, and polarization of T cells into GCA arteries*

Already activated adventitial DCs produce cytokines and chemokines (C-C motif chemokine ligand) CCL 19, 20, and 21 that trigger the recruitment of CD4+ T cell subpopulation, in proximity to vascular DCs [4, 36]. They in turn proliferate and synthesize chemokines (including CD4, CD61), creating an inflammatory environment [12]. Under the influence of the DCs and modulated by immune checkpoints [36], CD 4+ T cells subpopulation [37] differentiate into T helper 1 (Th1) cells and T helper 17 (Th 17) cells [38]. Next, T cells polarize into two T cell lineages defined by the production of their marker cytokines Th1 cells start producing interferon gamma (IFNᵧ), while Th17 cells produce interleukins IL17 and IL21 [38]. IFNv-releasing T cells axis and IL17/21 T cells axis, respectively, have different immunomodulatory effects [4].

IL-12 and IL 23 stimulate Th1 and Th17 responses, respectively, both of which are believed to be involved in promoting systemic and vascular inflammation, progression, and maintenance of inflammation [39, 40].

Interleukin 6 is the cytokine that controls the balance between proinflammatory Th1 and Th17 cells and the regulatory T cells, particularly involved in GCA pathogenesis [4, 41]. Regulatory T cells normally ponder or inhibit the immune system response. The disturbance of T cell homeostasis is probably related to an imbalance in the amount of serum IL6 [42]. In patients with GCA, IL 6 is upregulated in the inflamed arteries and in circulation [5, 42]. Serum IL6 corelates with disease activity and is decreased when GCA is in remission [42]. T proinflammatory cells Th1 and Th17 are in excess [38], while regulatory T cell numbers are inhibited by excess interleukin 6 (IL 6) [42].

Upon T cell activation in presence of an antiself attack [14], inhibitory checkpoints such as T-cell–inducible immune checkpoint programmed death-1/programmed death ligand-1 (PD-1/PD-L1) pathway are instrumental to minimize potential immunopathology [36]. PD1/PD-L1 pathway has a role in maintenance of tolerance, protective immunity, preventing autoimmunity disease, and protection against collateral vascular damage [36]. PD-1 is expressed on activated T and DCs cells [36] and its coupling by its ligands PD-L1 or PD-L2 induces T cell receptor (TCR)-activation cascade in a Src homology region 2 domain-containing phosphatase 2 (SHP2) manner [36], resulting in immunosuppression. An aberrant PD-1/PD-L1 checkpoint in DCs (low amount of both PD1/PDL1) and T cells of GCA patients [36] is responsible for the observed DC-mediated hyperactivation of T cells, polarizing T cells to Th1 and Th17 [36]. The immunotolerance defect in regulatory T cells in GCA patients [41] is characterized by a stimulatory instead of inhibitory function of PD1-mediated immune checkpoint in GCA patients [36], followed downstream by a FOXP3 transcription factor defect in the regulatory T cells locally in GCA arterial lesions [41] which leads to decrease in the number of FoxP3+ T regulatory cells and hyperstimulation of proinflammatory T cells. In addition, the regulatory T cell population can be influenced with an IL6 receptor antagonist [41] which stresses the necessity of IL6 in these processes. Importantly, IL17 is controlled rapidly by glucocorticoids [4]. IFNỿ is resistant to corticoids, also to aspirin and NOTCH inhibitors [4, 43].

#### *2.1.3 Monocytes differentiation/macrophages role in GCA-related vascular damage*

Under an increased level of interferon-gamma (IFNỿ) produced by CD+4 T cells, monocytes and macrophages are recruited in the arterial wall [44]. IFNv primarily targets macrophages leading to their fusion together into giant cells (GC) [4, 44]. IFNv concentration was found to be elevated in arterial tissue from GCA patients with ischemic disturbances, including visual loss [44]. In addition, they reported patients with PMR and fever had elevated IL2 production [44].

Using *ex vivo* cultures of temporal artery biopsies, Cid et al., 2006 demonstrated that the role of IFNỿ is to induce the production of C-C motif chemokine ligand 2 (CCL2) (ligand of CCR2 receptor) by VSMCs [45, 46]. CCR2, the corresponding receptor, is also expressed by monocytes, binding of CCR2 on monocytes leads to the recruitment of monocytes and their differentiation inside all layers of the arterial wall [46]. INFỿ induces the production of C-X-C motif chemokine ligand (CXCL9, 10, 11) by VSMCs [46], linked to the recruitment of cells expressing CXCR3 which is expressed by Th1 cells and CD8+ cells [4]. This will initiate a positive feedback loop since T cells are being activated and produce more IFNỿ [4, 46, 47].

Macrophages have different functions depending on which vascular layer they are going to be trapped in (1) adventitial CD 68+ TGF β1+ macrophages produce proinflammatory cytokines IL1β and IL 6 [4, 11, 48]; (2) intimal-media junction macrophages secrete metalloproteinases to clear cellular debris. They are also responsible for unintended, pathological elastic membranes digestion; (3) macrophages in the intima layer have roles in cellular outgrowth [49]. Intimalocated macrophages led to production platelet-derived growth factor (PDGF) [50] which is needed for dedifferentiation, proliferation and migration of VSMCs [50, 51] and vascular endothelial growth factor (VEGF) which is needed for neoangiogenesis [52].

IL1β and IL 6 levels are influenced by GS in GCA [43], in contrast, IL12 is mostly resistant to GS therapy [4, 18]. The lack of reaction to GS suggests a need for better therapeutic strategies to interfere with these pathologic cascades.

Monocytes and macrophages accumulate in high numbers generating granulomatous inflammation of large and medium arteries and, under stimulatory influence of INFv, they form giant cells (GC) by fusing together [4, 5].

In physiological conditions, GC are the body defense response against a foreign body or a kind of irritant, for instance, a splinter in the finger, the body will produce GCs to break up this irritant and remove it. In the case of GCA, 'the foreign body' is not known. It has been stated by some authors that it might be the arteriosclerotic plaque [53]. Macrophage multinucleated giant cells in pathological conditions of GCA are a unique cell population that produce different mediators leading to the destruction and faulty reconstruction of the arterial wall [4, 54].

#### **3. Vascular remodeling of healthy arteries to GCA arteries**

Regarding the GCA pathophysiology, studies have observed many interchangeable features dictated by the 'confused' immune system and by the cellular populations of the vascular wall itself mediated by blood factors. Upon the destructive actions of GCs, the vessel initiates a faulty PDGF- [50] and VEGF- [55] dependent maladaptive reparatory mechanism [55].

GCs' proteases digest the vascular wall at the level of the internal elastic lamina [54, 56, 57]. For instance, media-intima junction GCs are producing metalloproteinases (MMP-9 and MMP2) [57] as was demonstrated recently [54, 57], and also

#### *Cellular and Molecular Characteristics of Vascular Damage in Giant Cell Arteritis, the 'Unmet… DOI: http://dx.doi.org/10.5772/intechopen.97933*

other mediators. VEGF [55] is linked to neoangiogenesis and the recruitment of proinflammatory T cells via Notch/Jagged 1 dependent pathway [4, 5], reactive oxygen species (ROS) [54], and the other inflammatory mediators with proteolytic activity that cause breakage of the internal elastic lamina. The vascular response is a result of the two-way interaction between the hyperactive immune cells and the activated vascular cells. These processes lead to: (1) release of additional growth factors; (2) release of vWF from ECs Weibel-Palade bodies [19, 58–61]; (3) release of macrophage factors [13, 44, 48]; (4) media thickening in response to the immune insult [50] and deposition of extracellular matrix proteins (i.e. collagen); (5) intima myofibrotic hyperplasia (IH) [50]; (6) release of angiogenic factors in the vasa vasorum [62]; and finally, (7) upregulation of the proteinases [54] and downregulation of their inhibitors which causes the intima elastic membrane to tear [54], destroying locally the vessel wall.

Most importantly, the increased production of such mediators as PDGF [50, 51] and endothelin-1 [63, 64], initiates a faulty vascular repair process, leading to the activation, dedifferentiation, proliferation, and migration of the VSMCs from the arterial media to the intima. This leads to myofibrous intimal hyperplasia and "nodular" media thickening with granulomatous giant multinucleated cells infiltration, and neoangiogenic vasa vasorum [44, 62] characteristic to GCA pathology [51]. Typically, VSMCs turn from a contractile cell into a dedifferentiated, secretory, and migratory cell [5, 65]. Activated and injured vascular smooth muscle cells (VSMCs) produce growth factors (including PDGFs [55], TGF-β and ET-1 [63, 64]) that promote further myofibroblast dedifferentiation, proliferation and migration [4]. Reversely, pharmacologic blockage of the PDGF receptor or blockage ET-1 receptors [64] results in reduced IH in cultured GCA arteries [51, 64].

In all, hyperplastic cell outgrowth in the lumen of medium sized artery through autoimmune vascular remodeling progressively narrows the lumen, resulting in vascular stenosis and ischemia in the distal organs these vessels normally irrigate. The eye and the brain are at highest risk. The invalidating or life-threating consequences are severe, possibly visual loss or even stroke. Patients with GCA who have ocular ischemic complications have higher blood concentrations of ET-1 and other EC biomarkers, highlighting these biomarker potential role in thromboembolic vascular disturbances [63, 64].

### **3.1 The crosstalk between immune cells/vascular cells, faulty vascular repair and thrombosis in primary vasculitis**

More and more evidence suggests the presence of crosstalk between mispositioned vascular inflammation [4] and thrombosis [5, 61, 66]. In the past the underlying molecular mechanisms of vasculitis have been overlooked, but more recently, it has become increasingly evident that inflammatory diseases of the blood vessels are associated with arterial [25, 67] and venous thrombosis [66]. In GCA, certain risk factors will seriously increase the likeliness of thrombotic events, including, but not limited to systemic inflammation, localized vascular inflammation, endothelial dysfunction and treatment-related complications [5, 68]. The higher risk of thrombosis during active disease underlines the role of inflammation in thrombogenesis [61, 68]. Furthermore, GCA patients have a greater risk of thromboembolic complications due to the advanced age, and to the other risk factors that they concomitantly have, such as hypertension, smoking, hypercholesterolemia, previous arterial thrombotic events, family history of thrombosis, and the presence of additional cardiovascular risk factors [25, 28, 61, 68]. This may contribute to the choice of administrating antithrombotic therapy: low dose aspirin 75–250 milligram per day prevents cerebrovascular events and ocular symptoms [69]. Antiplatelet therapy

influences arterial disease events [69], while anticoagulants and immunosuppressive medication have a debatable effect [21, 70].

### *3.1.1 Immune cells response in vasculitis*

A growing body of evidence supports the above-mentioned 'outside-in' hypothesis that vascular inflammation is initiated and perpetuated in the adventitia in GCA and contributes progressively to medial and intimal remodeling [14]. Once the immune barrier is broken [12], the vessel expresses cell surface adhesion molecules in the vasa vasorum [55] and inflammatory mediators [5]. In result, the monocytes migrate towards the intima of the blood vessels [53]. Th1 and Th17 T-cells pathogenic pathways promote the production of IFN-ỿ and, respectively IL-17 [38, 38]. Th17 differentiation via its effector IL17 pathways induce chronic inflammation [71]. B cell differentiation contribute less to the formation of granulomatous structures [4, 5]. Cytokines like TNF alpha, interleukins IL1β and IL6 are promoting not only systemic signs in primary vasculitis (fever, malaise, weight loss, fatigue) but they also favor a prothromboembolic state [61, 67]. GCA complications comprise arterial [67] and, although less frequent, venous involvement, both DVT and PE [66]. In mouse models of DVT, it has been demonstrated high IL 6 during thrombogenesis [72], while inhibition of IL6 reduces expression of CCL2 which leads to a low recruitment of monocytes at the site of thrombosis of the vessel wall and the post-thrombotic syndrome [72]. IL6 is triggering an amplification and recruitment of monocytes which is in relation to the ability to express in excess cell adhesion molecules in vasa vasorum [4]. One puzzling observation from published data is that IL 6 pleiotropic effects on immune and vascular cells assert disease activity in GCA, however, when IL6 expression in the temporal artery is low, the IL6-induced angiogenic response is decreased without a protection mechanism against ischemic events in GCA patient [42]. Acute phase proteins are elevated: serum amyloid A (SAA), CRP, ESR, WBC count and platelet count [1]. GCA patients having ocular complications have significantly decreased level of SAA, CRP and ESR [27] as well as high VCAM1 levels [73]. GCA patients during relapses had significantly higher levels of SAA, CRP, ESR and WBC counts [73, 74].

Hyperactivated T cells and macrophages organize the granulomatous lesions in the vessel wall [44], destroy the media layer, the inflamed artery initiates an abnormal vascular repair program [11], inducing ischemic organ damage through intimal hyperplasia and luminal occlusion changes [5, 36]. Elevated IFNv was demonstrated to be correlated with neoangiogenesis [4, 62], and IH [46], two critical processes in vascular remodeling in GCA artery. Persistance of IFNv correlates with chronical arterial inflammatory disease [4]. Clinical variations in GCA are correlated with local expression of cytokine mRNA (elevated IL1β, IL6, TGF, IL2, INFỿ, IL17, 13). Heterogeneity of immune response and targeting of the arterial tissue in a TLR-specific manner explain the diverse clinical manifestations inside GCA spectrum of diseases [4, 11].

#### *3.1.2 Vascular wall response to vasculitis*

In GCA pathogenesis it is thought that the vascular wall response has the same impact as the immune cell response [4].

#### *3.1.2.1 Arterial luminal vs. vasa vasorum endothelial cells response in GCA*

Vascular endothelial dysfunction was previously reported in GCA [53]. It is associated with elevated blood levels of proinflammatory endothelial factors that

#### *Cellular and Molecular Characteristics of Vascular Damage in Giant Cell Arteritis, the 'Unmet… DOI: http://dx.doi.org/10.5772/intechopen.97933*

have important roles in the pathogenesis of GCA: endothelin 1 (ET-1, 64], and cell adhesion molecules [19, 64, 75], and von Willebrand factor (VWF) [19, 20, 53, 59]. The presence of the proinflammatory and procoagulant factors at GCA lesions sites is indicative of the extensive crosstalk between immune system and vascular cells [61]. The contribution of the blood vessel wall to the GCA pathogenesis is stressed by the fact that symmetrical, collateral vessels are much more likely to be affected (implicating GCA strong tissue tropism [4] in contrast to, for instance, atherosclerotic diffuse display which also manifests in this age group [53]) as demonstrated in these patients by PET scans/CT angiography imaging [17, 73].

Thrombin, the main protease in the coagulation cascade, also has numerous effects on the endothelium, i.e. thrombin-induced expression of chemokines that trigger binding of platelets and monocytes to the endothelial surface [76] and increased permeability across endothelium [77]. By these and many other mechanisms [78], thrombin is coupling coagulation and inflammation [61, 78, 79]. The stimulatory effects of thrombin on ECs and platelets occur mainly through activation of the protease-activated receptors (PARs) [79]. PARs are seven-transmembrane G protein coupled signaling proteins [80]. PAR1 is the prototype for a family of four related receptors [79]. PAR1 is the key mediator of thrombin's effect on human ECS [79]. Thrombin activates PAR1 receptors which couple to Gαq/11 and Gα12/13 that upregulate the vWF and P-selectin secretion from the ECs storage granules named Weibel Palade bodies (WPBs) [79, 81, 82]. Soluble P-selectin is an adhesive molecule also stored in WPBs [81]. Relevantly, P-selectin secreted by luminal endothelium of the carotid artery in a murine vascular damage model was reported to be involved in monocyte trafficking and neointima formation [65].

One of the mechanisms that may cause endothelial dysfunction in primary vasculitis is the excess of proinflammatory cytokines that are depressing endothelial function [31]. Also, the inflammatory microenvironment is directly leading t0 endothelial cell toxicity. In addition, healthy and pathologically damaged cells are intercommunicating and interconnecting, for instance monocytes and GC interactions close to the vessel wall trigger endothelial damage. Another role of the endothelium in inflammation is in leukocyte trafficking (P-selectin, E-selectins expression) [55] and the expression of cell adhesion molecules in the vasa vasorum [55]. Transcript levels for markers of endothelial activation: VWF, ICAM1, VCAM1, CD31, VE-cadherin (and of myofibroblasts smooth muscle cells actin (SMA)) measured by RT-PCR in the tissues in a GCA mouse model [36] were found to be elevated by up to fourfold when PD-1 was blocked with anti-PD-1 Ab when compared with control IgG or vehicle-treated [36] immune checkpoint inhibition led to intimal hyperplasia, angiogenesis and nodular thickening of the media. Inflammation-induced effects caused by the endothelium in systemic inflammation disease include but are not limited to: (1) increased expression of procoagulant factors: VWF, plasminogen activator inhibitor 1 (PAI1), platelets activated factor (PAF), vascular cell adhesion molecule 1 (VCAM1), intercellular adhesion molecule (ICAM1) and tissue factor (TF) and (2) and inhibition of anticoagulation pathways and fibrinolysis activity: endothelial protein C receptor (EPCR), tissue plasminogen activator (tPA), thrombomodulin (TM), prostaglandin I 2 (PGI2), that are causing thrombotic tendency [61, 83]. In patients with visual disturbances there were reported high VCAM1 levels compared with GCA patients that did not have visual disturbances [73]. VCAM1 was also significantly correlated with large vessel envolvement [73].

Weibel-Palade bodies (WPBs) are the secretory granules of vascular ECs [81]. The main resident of WPBs is vWF [81, 84]. vWF is pro-inflammatory and prothrombotic agent which plays a central role in morbidity and mortality associated with systemic inflammation and cardiovascular disease. The fact that elevated vWF in the circulation is a marker of inflammation-induced activation of ECs is well established [82, 85], but why do activated endothelial cell release von Willebrand factor in the context of the pro-inflammatory microenvironment in vasculitis, in particular in GCA, by which mechanism, is not completely understood; it was suggested it is part of the reparatory process. When an immune checkpoint is inhibited, endothelial cells are bigger in size and increased VWF expression and secretion was reported [36], indicative of endothelial cell maladaptive reaction. Moreover, VEGF derived from macrophages and the other immune cells have a stimulatory influence on Weibel-Palade bodies' secretion of VWF and angiopoetin-2 [86] involved in neointima formation. Intriguingly, it was reported that PDGF released by vascular dendritic cells [55] -on top of the above-described proinflammatory features- regulates vWF gene promoter.

vWF is an important molecular link coupling thrombosis and inflammation. It was found to contribute to systemic and vascular inflammatory manifestations of GCA. VWF levels are elevated in GCA patients circulation up to three-folds [19]. Highest vWF values are recorded at the onset of the disease [19], high blood VWF levels are persistent throughout the active disease period and remain elevated in some patients a long time after corticosteroid treatment [19, 59]. According to Persellin et al. elevated blood vWF values are not due to impairment of vWF formation or storage but to increased ECs secretion of normal von Willebrand factor, as shown by electrophoretic analysis of high molecular weight vWF polymers pattern [20]. High active VWF levels reflect vascular distress that predicts the course of the disease towards vasoocclusive problems [20]. Intraluminal thromboses were seen in 10% in TAB+ GCA, but in fact, the rate might be higher and hidden by the concomitant hyperplastic reaction in the intima. Several studies proposed VWF could be a parameter to monitor treatment or a parameter for diagnosis when the acute phase reactants (ESR) are normal in treated GCA patients. The fact that elevation of VWF is persistent throughout the steroid treatment suggests that GS treatment has little effect on the underlying endothelial disease. A significant percentage of patients receiving steroid treatment are developing irreversible vascular occlusive complications episodes (%) even after receiving GS treatment [67].

Several studies investigated the role of vWF in the formation of the hyperplasic intima (IH) [58, 60]. In one study, matching TABs and matching blood collection was done for VWF measurement. Increased vWF deposition in hyperplastic neointima mirrored high plasma vWF levels [19]. These data and other published data- that increased levels of vWF are associated with hyperplasia in grafts [87], and no occurrence of atherosclerotic plaques in vWF deficient pigs [88] -suggests a role for vWF in vascular remodeling and faulty vacular repair [60, 75]. Indeed, in a recent study by Lagrange et al. [58] it was found that vWF/LRP4/integrin α vβ3 axis stimulates proliferation of VSMCs: (1) vWF binds through its A2 domain to the VSMCs LRP4 receptor; (2) crosstalk LRP4 receptor-integrin α vβ3; (3) integrin α vβ3 activates Src signaling leading to vWF-dependent VSMCs proliferation [58]. Relevant to the aim of this review, their new findings provide new insights into the pathogenic mechanisms that drive pathological hyperplasia of the GCA arterial vessel wall. Moreover, the vWF/LRP4/integrin α vβ3 axis may represent a novel therapeutic target to inhibit VSMC proliferation, and, at least partially, prevent the maladaptive reparatory process in GCA [58].

At high shear-rates-which is the case in our pathogenetic context in the artery stenosis provoked by vascular remodeling in select ischemic complications of GCA, the inactive, globular circulating vWF unfolds into a highly active HMW elongated conformation. The active, elongated vWF can bind platelets via its repetitive A1 domain forming'beads on a string' conformations the incipient steps of thrombus

#### *Cellular and Molecular Characteristics of Vascular Damage in Giant Cell Arteritis, the 'Unmet… DOI: http://dx.doi.org/10.5772/intechopen.97933*

formation [82]. von Willebrand factor, then, has a two-way pathogenic mechanism to actively participate in GCA artery occlusion, on one hand, because at the site of the autoimmune vascular lesion, VWF, released from systemic inflammation thrombin-activated vascular ECs, initiates platelet adhesion, and changes the thrombotic propensity [19, 59], and on the other hand, amplifies maladaptive vascular response via vWF/integrin axis [58], based on these published data it is highly probably that the rate of intraluminal thrombotic events is higher than expected and the rate of TIA in GCA patients is also probably to be higher. It is worth mentioning here that episodes of transient visual loss precede permanent visual loss in 44% of cases [27]. To conclude, these two roles of vWF in GCA arteries are most likely associated with poor outcome of cerebral, coronary, or ocular ischemic complications of GCA disease [67]; leading in some people to type 2 GCA-related myocardial infarction [25, 26]. The highest VWF levels were recorded in GCA patients with positive temporal arteries biopsy when associating ocular symptoms [20], further studies are needed for vWF role in the ischemic complications of GCA. Reversely, in PMR, they reported lower levels of VWF than in GCA, which might be indicative of a lower degree of endothelial dysfunction, in PMR it correlates with the severity of the clinical signs, "more fuel on the fire" in the course of the disease [20]. For all these purposes, accurate criteria for active GCA disease are needed.

GCA relapsing cases or cases unresponsive to corticosteroid therapy that also have high blood VWF levels, high ESR, eye symptoms, would be candidates for testing new prospective therapeutics that either block vWF release from ECs activated by inflammatory cytokines in GCA [19] or block the mitogenic effect of vWF on IH [58].

#### *3.1.2.2 Vasa vasorum ECs response to vasculitis*

In response to activated vascular DCs in the adventitial layer, the invasion of multiple types of immune cells is currently thought to occur through the vasa vasorum endothelium [4]. The question to ask is why and how endothelial cells of the tiny adventitial vessels allow the immune cells to break into the vascular wall of the GCA arteries. The role of vasa vasorum ECs is major in GCA pathogenesis though the molecular details are still cryptic. In PMR and GCA as well, adventitial macrophages stimulated by DCs produce IL6 and IL1 which are detected from the early stages, when temporal artery is histologically apparent normal and the INFỿ expressing T cells are still absent from the vascular wall [11, 13]. At these early stages it is expected that vasa vasorum ECs are activated, express selectins and have a role in increasing wall permeability. In the same time, new vasa vasorum are formed, not only in the adventitia, but across all layers of arterial wall. Their role is to transport the invading immune cells.

### *3.1.3 Platelets response to vascular inflammation*

Platelets are activated by the following activated EC factors: (1) increased thromboxane A2; (2) increased von Willebrand factor and (3) decreased prostaglandin I2 [83]. Platelets are activated by the pro-inflammatory cytokines expressed by ECs and immune cells, by PAF, and by thrombin [83]. When activated, platelets secrete P-selectin and directly interact with endothelial cells [83]. Activated platelets interact with monocytes and neutrophils through the NF-KB mediated pathway [4, 83]. Activated platelets release pro-inflammatory cytokines or chemokines (like IL1 and CD 40) [4, 83]. Activated platelets are involved in microparticle-mediated inflammation [83].
