Thrombosis in vasculitis: from pathogenesis to treatment
The relationship between inflammation and thrombosis is not a recent concept 1], but it has been largely investigated only in recent years 2]. Nowadays inflammation-induced thrombosis is considered to be a feature of systemic
autoimmune diseases such as Systemic Lupus Erythematosus (SLE) 3], Rheumatoid Arthritis (RA) 4], or Sjögren Syndrome (SS) 5]. Moreover, both venous and arterial thrombosis represents a well known manifestation
of Behçet syndrome (BS) 6]; more recently accumulating data have demonstrated a significant increase in thrombo-embolic
events both in ANCA-associated vasculitis (AAV) and large-vessel vasculitis (LVV)
7], especially during active disease. These findings have important consequences in
terms of management and treatment; for example, BS requires immunosuppressive treatment
rather than anticoagulation for venous or arterial involvement 8], and perhaps one might speculate that also in AAV or LVV an aggressive anti-inflammatory
treatment during active phases could ameliorate vascular involvement especially in
early stages.
Here we will highlight some of the main pathogenetic and clinical aspects of thrombosis
in systemic vasculitis, and in particular in BS, AAV and LVV [Table 1].
Table 1. Summary of clinical and therapeutic features of thrombotic events in systemic vasculitis
Search strategy and selection criteria for review
We searched Pubmed matching the key search terms “thrombosis in vasculitisâ€, “Behçet
and thrombosisâ€, “ANCA-associated vasculitis and thrombosisâ€, “Large vessel vasculitis
and thrombosisâ€. Full texts, as well as abstracts of published articles were reviewed.
The search was limited to papers published in English language, and was conducted
through December 2014.
Behçet’s syndrome
Introduction
Behçet’s syndrome is a systemic vasculitis with a heterogeneous clinical phenotype
9], characterized by oral and genital ulcerations, uveitis, skin lesions and vascular,
neurological and gastrointestinal involvement. International diagnostic criteria for
BS, first published in 2006 and recently revised 10], have included vascular involvement as a diagnostic criterion. The term angio-Behçet
is used to define patients in whom large vessel lesions are the main feature. Both
arterial (e.g. aneurysms) and venous involvement (e.g. deep venous thrombosis) can
occur 11]. A peculiar feature of BS is the association between venous and arterial damage;
some authors have reported that pulmonary artery aneurysms and peripheral venous involvement
coexist in up to 90% of the patients 12].
Pathogenesis of (athero)thrombosis in Behçet’s syndrome
The pathophysiology of thrombosis in Behçet’s syndrome (BS) is not well known, but
systemic inflammation seems to play a major role whereas other thrombophilic factors
are less relevant 13]. However, it should be underlined that inflammation and haemostasis are closely linked
and that the immune system plays a role in the thrombotic process 14]; BS may thus be considered a model of inflammation-related thrombosis 15].
a) Immune system
A generalized derangement of CD4+ lymphocytes, monocytes and neutrophils and an overproduction
of proinflammatory cytokines related to Th1 cells, such as interferon-gamma (IFN?),
tumor necrosis factor (TNF)?, interleukin (IL)1, IL6, IL8 and IL12, have been observed
in BS 16]. Th17 cells along with their cytokines, IL17A, IL22, TNF?, also seem to be involved
in the inflammatory process, and so is IL21 which may promote Th1 and Th17 differentiation
and Treg cells suppression 17]. This condition is able to self-renew, so amplifying the proinflammatory environment
and promoting a prothrombotic state. Different mechanisms of inflammation may affect
endothelial cells; in particular, in BS anti-endothelial cell antibodies (AECA) have
been described as a possible link between immune response and endothelial dysfunction
18],19].
b) Coagulation system
In BS, the coagulation system may promote inflammation and thrombosis through multiple
factors such as the tissue factor (TF) pathway, thrombin and the protein C system
along with an impaired fibrinolysis 13]. Endothelial cell dysfunction, resulting from immunological and inflammatory factors,
seems to be a characteristic feature of BS and plays a key role in the pathogenesis
of thrombotic manifestations. A decreased production of nitric oxide (NO), a prominent
marker of endothelial dysfunction, was reported in some patients with active BS 20], and interestingly a reduction in asymmetric dimethylarginine, the endogenous inhibitor
of NO synthase, has also been observed 21]. Moreover, high levels of other endothelial injury markers, such as circulating von
Willebrand factor 22] and thrombomodulin 23] were found in patients with active BS. Increased serum levels of vascular endothelial
growth factor (VEGF), a marker of angiogenesis, and of some adhesion molecules such
as intercellular adhesion molecule 1 and E-selectin, produced by activated endothelial
cells, were reported in BS patients. These markers were increased particularly during
the active stage of the disease, thus underlying the close relationship between endothelial
cells, leukocytes and autoimmune mechanisms 24]. Another adhesion molecule, P selectin, was found to be elevated in the plasma of
BS patients. This molecule, located in the Weibel-Palade bodies of endothelial cells
and in the granules of platelets and released into the plasma during platelet activation,
promotes inflammatory reactions by facilitating leukocyte recruitment at the site
of injury 25]. Some studies have also reported signs of enhanced platelet activation including
higher concentrations of platelet microparticles (MPs) in BS patients compared with
healthy controls 26]. Moreover, several studies have reported high plasma levels of homocysteine in BS
patients with a history of thrombosis, especially in the active phase of disease 27], while data are conflicting on the possible correlation between homocysteine and
HLA-B51 28]; endothelial function, tested by flow-mediated dilatation of the brachial artery
was found to be significantly impaired in BS patients 29]. Different haemostatic factors have been investigated in BS with discordant results.
Controversial data were reported about the role of some procoagulant factors, such
as coagulation factor V G1691A (factor V Leiden mutation) and prothrombin G20210A
polymorphisms in BS, suggesting that they might be an additional risk factor for thrombosis
in certain populations. Factor V Leiden mutation is reported to be more prevalent
in Turkish 30],31], but not in Italian, Spanish and Israelian patients 32]-34]. Prothrombin gene mutation was not reported to be relevant in several studies 35], but a meta-analysis showed a significant association between the presence of prothrombin
G20210A mutation and thrombosis in BS, when Turkish patients were excluded 33]. Instead, deficiencies of natural anticoagulant proteins including protein C, protein
S and antithrombin have not been associated with thrombosis in BS patients 36]. High levels of Lipoprotein(a) were found in BS patients and might be involved in
the pathogenesis of thrombosis by impairing fibrinolysis 15]. Furthermore, high plasma levels of thrombin-activable fibrinolysis inhibitor were
reported in BS patients which could result in a significant reduction of clot lysis
process 37].
Venous thrombosis in Behçet’s syndrome
Thrombosis is the most frequent vascular manifestation in BS patients, its prevalence
ranges from 14% to 39% and venous involvement is characteristically more common and
makes up 75% of all vascular complications 38]. Venous thrombosis occurs more frequently in males with active disease during the
early years, sometimes at the onset of disease, and tends to recur 39],40]. Deep vein thrombosis (DVT) and superficial vein thrombophlebitis (SVT) of lower
extremities are the typical manifestations, but thrombosis may occur anywhere in the
venous system and the involvement of atypical sites such as hepatic veins, superior
and inferior vena cava and cerebral sinus veins are also observed 6]. BS should be always considered in the differential diagnosis of venous thrombosis
in unusual sites in young individuals.
In some studies SVT have been reported as the most frequent lesions. It usually occurs
in the lower limbs as painful nodules similar to erythema nodosum, but in some cases
it may be a complication of venipuncture reflecting a pathergy-like effect in the
venous wall 41].
Arterial involvement in Behçet’s syndrome
Arterial involvement is present between in 1 to 7% of the patients 6]. The most characteristic arterial manifestations in BS patients are aneurysms whereas
arterial thrombosis is less common 11]. These complications may remain asymptomatic or result in life- or organ-threatening
infarctions such as acute myocardial infarction, stroke, intestinal infarction, intermittent
claudication or gangrene of the lower extremities 13]. Arterial occlusions and venous thromboses sometimes coexist in the same patient
and may be associated with aneurysms 42],43]. Thus, the coexistence of thrombosis and aneurysms is a peculiar feature of BS. Overall,
cardiac manifestations are rare in BS patients, with a reported incidence between
1% and 6%, and are mainly represented by intracardiac thrombosis and coronary artery
disease 44].
Treatment
Currently, the management of vascular thrombosis in BS patients is based on immunosuppressive
therapy to reduce the inflammation of the vessel wall. Anti-inflammatory treatments
are able to promote a rapid and effective regression of the vascular lesions, to prevent
the extension of thrombosis and its recurrence. The European League Against Rheumatism
(EULAR) recommendations suggest immunosuppressive treatment with agents such as corticosteroids
(CS), azathioprine (AZA), cyclophosphamide (CYC) or cyclosporine A (CsA) 8].
AZA and CsA in association with low dose CS are usually the first choice in the treatment
of DVT and SVT. In some serious cases such as Budd-Chiari Syndrome or superior and
inferior vena cava thrombosis, treatment with pulse CYC is suggested 45]. CYC is also the recommended treatment in BS patients with arterial involvement.
Usually, anticoagulants alone are not recommended in BS patients 8]. Actually, only for CNS venous thrombosis some authors suggest anticoagulation, with
or without corticosteroids 46],47]. The pathophysiology of thrombosis in BS, where systemic inflammation promotes the
prothrombotic state leading to the formation of a thrombus tightly adherent to the
vessel wall with a low rate of embolism 13], the discordant data on coagulation abnormalities, the possibility of the coexistence
of PAA and thrombosis and the low efficacy of the anticoagulants reported in several
studies are the main reasons that support the treatment with immunosuppressive agents
and not with anticoagulants in BS patients. However, the role of anticoagulants continues
to be an open question.
Sometimes thrombosis in BS as well as other manifestations are refractory to traditional
immunosuppressive therapy and tend to recur, so more effective therapeutic options
are required. According to the hypothesis that inflammatory cells and proinflammatory
cytokines, including gamma-delta T cells (?? T cells) and TNF?, play a major role
in the development of thrombosis 48], a successful use of anti-TNF? agents, especially for uveitis, neurological and gastrointestinal
manifestations, has been increasingly reported in BS patients 48].
Cases of angio-Behçet patients resistant or intolerant to conventional immunosuppressive
therapy successfully treated with anti-TNF? agents have been increasingly reported
over the last years. However, the experience with anti-TNF? agents for major vessel
involvement is limited to case reports. An analysis of 369 BS patients treated with
anti-TNF? agents in 20 different countries has recently been reported 49], but only few cases of treatment with anti-TNF? agents in BS patients with vascular
complications have been described 50],13].
Interestingly, among conventional agents used in cardiovascular diseases, only atorvastatin
and lisinopril have been investigated, with results showing a possible improvement
in endothelial function in BS patients 51].
ANCA-associated vasculitis
Introduction
Anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) comprises
a group of disorders characterized by necrotizing inflammation of small vessels and
the presence of ANCA directed to specific antigens, particularly proteinase 3 (PR3-ANCA)
and myeloperoxidase (MPO-ANCA) 52]. The main clinical entities within the AAV spectrum are Granulomatosis with Polyangitiis
(GPA, formerly Wegener’s Granulomatosis), Microscopic Polyangitiis (MPA) and Eosinophilic
Granulomatosis with Polyangitiis (EGPA, formerly Churg-Strauss syndrome) 53].
Pathogenesis of (athero)thrombosis in ANCA-associated vasculitis
a) Immune system
Endothelial cell dysfunction is a feature of AAV and is probably caused by the interaction
between neutrophils (activated by TNF? and ANCA) and endothelial cells, with consequent
massive oxidative stress finally leading to atherothrombotic complications 54].
Recently, an additional mechanism of neutrophil activation has been described, termed
NETosis; neutrophils are able to release extracellular nucleic acids associated with
histones and granular proteins capable of entrapping bacterial agents. These neutrophil
extracellular traps (NETs) have been also implicated in thrombotic events and seem
to be a potential bridge between autoimmunity and coagulation. In particular, neutrophils
primed by ANCA degranulate and release NETs, which in turn contain MPO and PR3, that
act as autoantigens, thus creating a self-amplyfing process 55]. In active AAV, neutrophils release high levels of TF-expressing NETs 56]; moreover NETs are able to promote thrombosis by inhibiting the TF pathway inhibitor
and by recruiting platelets 14]. Finally, an intriguing in vivo model in which dendritic cells primed by NETs are able to induce the production of
ANCA in mice has been recently proposed, thereby strengthening the role of NETs in
AAV 57].
MPs are involved in many biological mechanisms, including thrombosis 58]. Neutrophil-derived MPs have been recently demonstrated in active AAV 56]; they contain inflammatory mediators such as platelet activating factor, adhesion
molecules and MPO, suggesting that they may activate and damage endothelial cells
59],60].
In vitro and in vivo models have shown also in AAV a possible role of AECA in inducing endothelial cell
dysfunction via an antibody-dependent cytotoxicity mechanism 18],19].
In EGPA, in addition to neutrophils, also eosinophils seem to promote vascular injury
via the release of preformed granules during active disease. In particular eosinophil
cationic protein and membrane basic protein can inhibit the activation of the protein
C system and, at the same time, induce platelets to produce platelet factor 4, able
to impair heparin function. TF and eosinophil peroxidase are also released by activated
eosinophils; the former activates factor X, while the latter induces endothelial cells
to express TF 61]-63].
b) Coagulation system
Plasminogen has been described as an autoantigen in PR3-ANCA patients, and its interaction
with autoantibodies directed towards complementary PR3, a recombinant protein translated
from the antisense strand of PR3 cDNA, is able to block its conversion to plasmin,
ultimately impairing fibrinolysis 64],65].
Boomsma et al. have shown that elevated levels of soluble thrombomodulin and plasma
endothelial protein C receptor, which are markers of endothelial cell damage were
increased in GPA patients and partly correlated with disease activity 66].
Patients with GPA have not been reported to have an increased prevalence of thrombophilic
factors such as Factor V Leiden and prothrombin gene mutations, while they were found
to have an increased prevalence of anticardiolipin antibodies (aCL), although no correlation
with thrombotic events was reported 67].
More recently a procoagulant state was reported in non active AAV as well: endogenous
thrombin potential and Factor VIII were found to be increased in patients in stable
remission compared to healthy controls 68].
Venous thrombosis in ANCA-associated vasculitis
In recent years, evidence supporting an increased frequency of venous thrombotic events
in AAV has arisen. The prevalence of venous thrombosis in AAV ranges between 5.8%
and 30% 61]. Relevant data came from the WGET trial (Wegener’s granulomatosis Etanercept Trial)
published in 2005 69]; in this study 180 patients with GPA followed for more than 2Â years had an increase
incidence of venous thromboembolism (VTE), especially during active disease. Subsequent
studies 70]-72] confirmed a high frequency of thrombotic events among patients with AAV, especially
during early and active disease stages. In a recent Australian case series of EGPA
patients 73] an increased incidence of VTE (both in typical and atypical sites) and pulmonary
embolism (PE) was reported. Very recently a retrospective study conducted in a Tertiary
Reference Center in Denmark has confirmed that patients diagnosed as having GPA have
a significant risk of VTE both early and late during the course of their follow-up
(median 7.2Â yrs) and are hospitalized several times for PE and DVT 74].
Arterial involvement in ANCA-associated vasculitis
An increased frequency of arterial events in AAV has been reported in the literature.
The estimated prevalence of arterial involvement in AAV is between 3.1% and 18.7%
75],76]. In a retrospective study, 113 patients with AAV were compared with a matched population
with non-inflammatory chronic kidney disease, showing a significant increase in cardiovascular
events (CVE) in the AAV group. Previous cardiovascular disease, dialysis dependence,
and smoking were the strongest predictors of CVE. Of note, only 2 patients with EGPA
were included, while the majority were patients diagnosed as GPA (n?=?65) and Microscopic
Polyangiitis (MPA, n?=?46) 75]. Another retrospective study was conducted using the Danish National Hospital Register,
on 293 patients with GPA; an increased risk of acute myocardial infarction was observed,
in particular in men aged 50Â yrs at the time of diagnosis and with a cumulative dose
of cyclophosphamide 36 grams. Interestingly, this GPA population had an increased
risk of CVE both in the early (within 5Â years of diagnosis) and in the late (after
10Â years of diagnosis) phase of the disease, suggesting that not only acute, but also
chronic inflammation may be implicated in this process 76].
In 2011 a prognostic tool to define the 5-year cardiovascular risk was created for
AAV patients based on data from four European Vasculitis Study Group (EUVAS) trials
of WG and MPA considering a total population of 535 patients. The results indicated
that almost 12% of newly diagnosed GPA and 16% of MPA patients had presented at least
one CVE, defined as cardiovascular death, myocardial infarction, coronary artery bypass
graft/percutaneous coronary intervention or stroke. Interestingly in this risk algorithm,
while an older age and the presence of diastolic hypertension were associated with
an increased incidence of CVE, the positivity of PR3-ANCA was associated with a lower
cardiovascular risk 77].
Treatment
An in vitro model has demonstrated that simvastatin is able to significantly inhibit neutrophil
degranulation induced by ANCA 78], so suggesting its potential role in clinical practice beyond cardiovascular protection,
although some cases of AAV induced by statins have been reported 79],80]. Currently there are no significant data on the use of antiplatelet and/or anticoagulant
therapy in AAV.
Large vessel vasculitis
Introduction
Large vessel vasculitis (LVV) usually comprises Giant Cell Arteritis (GCA) and Takayasu
arteritis (TA) 52]. The histopathologic features of these two entities are similar, whilst they substantially
differ in the age range of the affected patients, since TA usually affects young women
and GCA predominates in the elderly 52].
Pathogenesis of (athero)thrombosis in large vessel vasculitis
The pathogenesis of atherothrombotic events in these inflammatory conditions is overlooked.
a) Immune system
As in BS and in AAV, even in LVV, and especially in TA, a possible role of AECA in
inducing endothelial damage has been suggested 18],19].
More interestingly, one of the possible vascular complications is the development
of aneurysms as a consequence of inflammatory damage. Vessel wall remodeling in LVV
starts from the adventitial layer, with an infiltrate mainly consisting of Th1/Th17
lymphocytes activated by resident dendritic cells 81], and macrophages producing pro-inflammatory cytokines such as IL1? and IL6; macrophages
of adventitial and medial layers are responsible for the production of growth factors,
such as platelet-derived growth factor and VEGF, which induce intimal hyperplasia
81],82]. Interestingly the innate immunity also contributes to vascular remodeling, via pentraxin
3 (an innate pattern recognition receptor), which accumulates at the site of active
remodeling both in GCA and TA vessels 83],84].
b) Coagulation system
Inheritable thrombophilia does not seem to have any role in GCA patients; a high prevalence
of antiphospholipid antibodies was observed, without any correlation with vascular
events 85]. High levels of homocysteine were reported in a single study in patients with polymyalgia
rheumatica (PMR) and GCA, probably related to corticosteroid treatment 86].
Venous thrombosis in large vessel vasculitis
Venous thrombosis in LVV has been poorly investigated. In GCA the incidence rate of
venous involvement is estimated to be 13.3/1000/year for VTE and 8.5/1000/year for
DVT 87]. A recent population-based study has tried to fill this gap; in a large cohort comprising
909 patients with GCA, an increased risk of VTE (both DVT and PE) in particular during
the first year after diagnosis has been observed 87]. Interestingly, similar findings were reported in a recent Swedish nationwide hospital-based
study in patients affected by PMR, a condition strictly related to GCA, who showed
an increased incidence of PE. Also in this population the risk was higher during the
first year after diagnosis, suggesting a possible role of inflammation in the pathogenesis
of the vascular events 88]. None of the traditional risk factors has been definitely linked so far to an increased
occurrence of venous events in GCA; in two previous reports of small populations,
a role for aCL antibodies was hypothesized 89],90].
Arterial involvement in large vessel vasculitis
Interesting data about arterial involvement in LVV are available. A recent cohort
study evaluating nearly 3500 patients with GCA has reported an increased risk of CVE,
especially in the first month after the diagnosis; the study was mainly limited by
the source of data (primary care database) and the lack of data about temporal artery
biopsies 91].
The influence of the traditional cardiovascular risk factors in GCA is far from being
established, however a retrospective Spanish study has reported several risk factors
for atherosclerosis at the time of diagnosis, and among them, especially hypertension
significantly enhances the risk of developing severe ischemic events 92].
Another Spanish study on 287 GCA patients reported that stroke occurred in almost
3% of them, mostly within 1Â month of diagnosis; most of the patients were male, smokers
and with arterial hypertension, and permanent visual loss was the best predictor of
stroke occurrence 93]. A recent population-based study has confirmed these data, except for a higher incidence
of stroke (7%) 94].
An Italian study conducted by Salvarani et al. demonstrated that the PlA2/A2 homozygosity
of the GPIIIa gene is associated with anterior ischemic optic neuritis in GCA and
could partly explain the reduced capacity of aspirin to prevent cerebral events in
this population 95]. Another study by the same group 96] interestingly showed that, together with a history of arterial hypertension and previous
CVE, also low levels of acute phase reactants were associated with occurrence of cerebral
accidents; this evidence is not surprising, since the data published by Hernández-RodrÃguez
and colleagues demonstrated that the angiogenic properties of IL6 could compensate
ischemic injury in GCA patients 97]. Curiously, a recent retrospective study on 245 patients with GCA has shown that
the risk of acute coronary syndromes in patients with GCA was comparable with that
of non-GCA patients and that some cardiovascular risk factors, such as diabetes, were
found to be protective in the GCA population 98]. As previously reported, GCA patients with coronary artery disease however demonstrated
an increased risk of aortic aneurysm and arterial thrombosis 99].
In a Korean retrospective study on Takayasu arteritis, 21 of 190 (11%) patients, almost
all young (mean age 40Â years) female had presented a stroke 100]. In this population, risk factors were comparable between Takayasu-stroke and Takayasu
non-stroke patients, while transient monocular blindness (considered a warning bell
of cerebral events) occurred at low frequency.
Treatment
A recently published comprehensive meta-analysis has clearly shown that in GCA the
use of antiplatelet/anticoagulation therapy is not effective for primary prophylaxis,
whilst it could be beneficial as combination therapy with corticosteroids in established
GCA, without an increased risk of bleeding 101]. In TA the use of antiplatelet treatment appears to have a protective effect against
ischemic events, while neither anticoagulants, nor corticosteroids/immunosuppressants
seem to be able to prevent CVE 102].
A retrospective recent study has shown that patients using statins were less likely
to develop GCA compared to patients not using them 103]. Interestingly, a recent association study performed using the World Health Organization
database, has suggested that the occurrence of PMR could be correlated with statin
use 104].
Others systemic vasculitis
Polyarteritis nodosa
Polyarteritis nodosa (PAN) is a multisystemic necrotizing vasculitis of medium-sized
arteries, not associated with glomerulonephritis, nor with ANCA positivity 52]. Few conflicting results about thrombotic events in PAN exist; indeed one study on
285 PAN patients has reported a much lower incidence of VTE compared to AAV patients
72], whilst a more recent Swedish population-based study has suggested an increased risk
of thrombotic events 88]; of note, the latter study included subjects with different autoimmune diseases observed
from 1964 to 2008, thus it is unclear whether or not patients with MPA were included
together with PAN patients. Finally, a certain relation seems to exist between PAN
and antiphospholipid antibody syndrome, so complicating the scenario 105].
Henoch-Schönlein purpura
Henoch-Schönlein purpura (HSP) is a systemic vasculitis of the small vessels mainly
affecting children. Thrombotic events are rare complications of HSP, and to date only
case reports are reported 106].
Kawasaki disease
Kawasaki disease is a systemic vasculitis and represents the most common cause of
acquired heart disease in childhood. Sometimes, despite appropriate treatment with
acetylsalicylic acid and intravenous immunoglobulins, coronary aneurysms occur; the
formation of a thrombus at this level could lead to vascular occlusion and consequent
myocardial infarction 107].
Retroperitoneal fibrosis
Retroperitoneal fibrosis (RPF) is a rare fibroinflammatory disorder characterized
by the presence of a retroperitoneal mass, that could be associated with abdominal
aorta aneurysms and/or vasculitis of the thoracic aorta; RPF could be primary or secondary,
mainly to neoplastic or infectious diseases. Venous thrombosis could be a presentation
symptom in RPF due to the compression of vascular structures, in particular of the
inferior vena cava and iliac veins 108].