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Inflammation, immunological reaction and role of infection in pulmonary hypertension
REVIEW
10.1111/j.1469-0691.2010.03285.x
Inflammation, immunological reaction and role of infection in pulmonary hypertension S. S. Pullamsetti1,2, R. Savai2, W. Janssen1, B. K. Dahal2, W. Seeger1,2, F. Grimminger2, H. A. Ghofrani2, N. Weissmann2 and R. T. Schermuly1,2 1) Department of Lung Development and Remodelling, Max-Planck Institute for Heart and Lung Research, Bad Nauheim and 2) Department of Internal Medicine, University of Giessen Lung Centre, Giessen, Germany
Abstract Inflammation underlies a wide variety of physiological and pathological processes. Acute inflammation is the initial response of the body to harmful stimuli. Chronic inflammation, by contrast, is a prolonged, dysregulated and maladaptive response that involves active inflammation, tissue destruction and attempts at tissue repair. Over the past few years, such persistent inflammation has been shown to be associated with pulmonary hypertension (PH). Substantial advances in basic and experimental science have illuminated the role of inflammation and the underlying cellular and molecular mechanisms that contribute to PH. This review summarizes the experimental and clinical evidence for inflammation in various types of PH. In addition, it assesses the current state of knowledge regarding the inducers/triggers of chronic inflammation and infection, as well as the inflammatory mediators and cells that are involved in PH. Infiltration of inflammatory cells, such as dendritic cells, macrophages, mast cells, T-lymphocytes and B-lymphocytes, in the vascular lesions and an elevation of serum/tissue concentrations of proinflammatory cytokines and chemokines and their contribution to pulmonary vascular remodelling are reported in detail. We review the data supporting the use of inflammatory markers as prognostic and predictive factors in PH. Finally, we consider how new insights into inflammation in PH may identify innovative therapeutic strategies. Keywords: Immunity, infection, inflammation, pulmonary hypertension, review Article published online: 8 June 2010 Clin Microbiol Infect 2011; 17: 7–14 Corresponding author: R. T. Schermuly, Max-Planck-Institute for Heart and Lung Research, Department of Lung Development and Remodelling, Parkstrasse 1, 61231 Bad Nauheim, Germany E-mail: [email protected]
Pathogenesis of Pulmonary Hypertension Pulmonary hypertension (PH) is a progressive disease with a poor prognosis that results in right heart dysfunction and right heart failure [1]. Characterized by increased pulmonary vascular resistance, PH includes a heterogeneous group of clinical entities that have been subcategorized into five groups on the basis of association with other diseases, according to the currently accepted PH classification [2]. Group 1, pulmonary arterial hypertension (PAH), is composed of a heterogeneous group of diseases that have been subcategorized as idiopathic PAH (IPAH), familial PAH, PH associated with other diseases, such as connective tissue
diseases (e.g. systemic sclerosis), portopulmonary hypertension, and PH related to human immunodeficiency virus (HIV) infection, drugs and toxins [2]. The pathogenesis of PH is a complex, multifactorial process. Increased pulmonary arterial pressure in PH patients may result from a combination of pulmonary vasoconstriction, inward vascular wall remodelling and in situ thrombosis, leading to increased vascular stiffness as well as narrowing of the vascular lumen [1]. However, the exact triggers of PH have not been identified yet. An initial insult, from chronic hypoxia, inflammation, viral infection, mechanical stretch, shear stress, and/or unknown factors, is considered to activate or affect the endothelium. This results in altered production of endothelial mediators and release of inflammatory mediators, which have
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been increasingly implicated in PH [3]. These inflammatory mediators, including cell adhesion molecules, cytokines, chemokines and growth factors, direct the recruitment of inflammatory cells and propagate a number of inflammatory pathways that lead to vascular cell proliferation, migration and extracellular matrix deposition, all of which contribute to the structural remodelling-characteristic of PH [4,5] (Fig. 1). Inflammatory processes are prominent in various forms of PH, and are increasingly recognized as major pathogenic components of pulmonary vascular remodelling in IPAH or PAH related to more classical forms of inflammatory syndromes, such as connective tissue diseases, HIV infection,
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or other viral aetiologies. Similarly, inflammation seems to play a significant role in experimental animal models of PH. This review will focus on inflammatory processes that are involved in the initiation or progression of PH, and the prognostic and therapeutic importance of inflammation in PH.
An Overview of Inflammation (Acute and Chronic) Inflammation is an adaptive response that is triggered by noxious stimuli and conditions, such as infection and tissue
Normal Inflammation
FIG. 1. Schematic illustration of infection-mediated and inflammation-mediated vascular remodelling: In response
Adventitia Media
to infection and inflammatory events,
Internal elastic lamina Endothelium
Infection (viral, parasitic)
lung vascular cells produce inflammatory mediators (chemokines and cytokines), thereby recruiting the inflammatory cells (macrophages, dendritic cells, mast cells,
Release of cytokines/chemokines CX3CL1, CCL5, CX3CR1, CCL2
B-cells, T-cells and regulatory T-cells). With the coordination of inflammatory mediators, inflammatory cells may per-
Inflammatory cells recruitment
petuate the release of cytokines, chemokines and growth factors. Finally, these
Treg cells
processes lead to vascular remodelling
Mast cells
Dendritic cells
B-cells
Macrophages
though
matrix
remodelling,
collagen
deposition, vascular cell proliferation,
T-cells
migration, and in situ thrombosis. CCL2, chemokine (C-C motif) ligand 2; CCL5, chemokine (C-C motif) ligand 5 or RANTES (regulated upon activation, normal T-cell expressed and secreted); Cytokines, chemokines, growth factors
CX3CL1, chemokine (C-X3-C motif) ligand 1 (fractalkine); CX3CR1, chemo-
IL-1, IL-6, TNF-α, CX3CL1, CCL5, MCP-1, SDF-1α PDGF, VEGF, FGF
kine (C-X3-C motif) receptor 1; EC, endothelial cells; FB, fibroblasts; FGF, fibroblast growth factor; IL-1, interleukin-1; IL-6, interleukin 6; MCP-1, monocyte Vascular Remodelling
chemotactic protein-1; PDGF, platelet-
Chronic vasoconstriction In-situ thrombosis SMC dysfunction FB dysfunction EC dysfunction Matrix remodelling/Collagen deposition
derived growth factor; PAH, pulmonary arterial hypertension; SDF-1a, stromal cell-derived factor 1a; SMC, smooth muscle cells; TNF-a, tumour necrosis factor-a; Treg cell, regulatory T-cell; VEGF,
PAH ª2010 The Authors Clinical Microbiology and Infection ª2010 European Society of Clinical Microbiology and Infectious Diseases, CMI, 17, 7–14
factor.
vascular
endothelial
growth
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injury [6]. It is characterized by the sequential release of cytokines, chemokines and growth factors that regulate increased vascular permeability and recruitment of leukocytes. Increased vascular permeability also results in extravasation of plasma proteins, which further amplify the inflammatory reaction. Importantly, regardless of the cause, inflammation presumably evolved as an adaptive response for restoring homeostasis. However, when inflammation persists (chronic inflammation), it can cause tissue damage and loss of function. Chronic inflammation may occur because of the persistence of infection or antigen, recurring tissue injury, or a failure of endogenous anti-inflammatory mechanisms that drive the resolution of inflammation [6]. Notably, altered interactions between adaptive and innate immune cells can lead to chronic inflammation. It is generally thought that, during acute inflammation, innate immune cells, such as dendritic cells (DCs), natural killer cells, macrophages, neutrophils, basophils, eosinophils and mast cells, form the first line of immune defence and regulate the activation of adaptive immune responses. By contrast, during chronic inflammation, these roles can be reversed—adaptive immune responses by adaptive immune cells, such as B-lymphocytes, CD4+ helper T-lymphocytes and CD8+ cytotoxic T-lymphocytes, can cause ongoing and excessive activation of innate immune cells [7].
The Scientific Basis of Inflammation in PH The concept that immunological reactions in PH play a significant role in the development and worsening of the disease is now well accepted. Autoimmune infiltration of immune cells and inflammatory reactions have been shown to contribute to the pathogenesis of IPAH [4,5,8].
Inflammatory and Immune Cell Component of PH The inflammatory and immune component of structurally altered vessels includes circulating monocytes, neutrophils, DCs, macrophages and lymphocytes, as well as fibroblasts, resident endothelial cells and smooth muscle cells. Presumably, circulating cells are directed to sites of injury, adhere to or come close to endothelial cells, invade the internal elastic lamina, and release a variety of substances that act on the local environment and promote chemotaxis. Released cytokines and growth factors produce the effects of inflammation by mediating communication between and among circulating and resident vascular cells. Finally, all of these
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interactions result in vascular remodelling through matrix remodelling, collagen deposition, proliferation and migration of all vascular cell types [9] (Fig. 1).
Innate Immune Cells DCs
DCs, which are crucial for the activation of naive T-cells, play an important role as professional antigen-presenting cells in the immune system. Perros et al. [10] demonstrated immature DC population infiltrating vascular lesions in both IPAH patients and in experimental PH. Accumulation of these professional antigen-presenting cells may involve presentation of antibodies to endothelial cells, fibroblasts and nuclear antigens that are found in the serum of patients with IPAH and collagen vascular disease-associated PAH [11,12]. Recently, Wang et al. [13] reported a decrease in the percentage of monocytederived DCs in the peripheral blood of IPAH patients, suggesting a Th1 immune reaction in the pathogenesis of IPAH. Mast cells
Mast cells are major effector cells in allergy and host defence responses. Upon activation, mast cells release a broad spectrum of proinflammatory cytokines, growth factors, vasoactive substances and proteases (tryptase and chymase). Although vascular mast cells are rare in the normal vasculature, an increase in the number of vascular mast cells was found in patients with PH as well as in experimental models of PH [14,15]. It is postulated that mast cells are a rich source of interleukin (IL)-4, as well as other factors that are capable of stimulating autoreactive B-cells to secrete autoantibodies, including anti-endothelial cell antibodies, and mast cell degranulation through regulation of matrix metalloprotease activity appears to be an important factor in the initial phase of pulmonary vascular remodelling [16]. Macrophages/monocytes
Recruitment of macrophages, along with other mononuclear cells, has been reported in primary PH as well as in secondary forms of PH, including high-flow congenital heart disease, scleroderma, and PH-associated acute respiratory distress syndrome [17,18]. Similarly, increased numbers of macrophages and neutrophils have been described in murine and rat lungs in response to chronic hypoxia and monocrotaline (MCT) [19,20]. Although the molecular mechanism of monocyte/macrophage infiltration and its roles remain elusive, macrophage accumulation and activation in remodelled pulmonary vessels may lead to the release of a variety of proinflammatory cytokines, (e.g. monocyte chemotactic
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protein (MCP-I) and matrix metalloproteases) that are capable of causing vasoconstriction, increasing vascular permeability and inducing proliferation [21].
Adaptive Immune Cells B-lymphocytes
B-lymphocytes (B-cells) are fundamental to the humoral immune response, because of their potential to differentiate into antibody-producing plasma B-cells. B-cells also play a crucial role in cell-mediated immune regulation through antigen presentation, production of various cytokines, differentiation of T-effector cells, and collaboration with antigen-presenting DCs [8]. Antibodies directed against pulmonary endothelial cells and fibroblasts have been found in PAH, suggesting a role of B-cells in PAH [11,12]. In corroboration, activation of B-cells in peripheral blood B-lymphocytes and perivascular infiltration of B-lymphocytes has been observed in patients with IPAH [17,22]. T-lymphocytes (cytotoxic T-cells, helper T-cells and regulatory T-cells (Treg cells))
Infiltration of T-lymphocytes has been previously noted in lungs of IPAH patients [17]. Bonnet et al. [23] reported CD3+ T-lymphocytes with activated NFATc2 (a nuclear Ca2+/ calceneurin-sensitive transcription factor found in T-lymphocytes) in the walls of resistance arteries in IPAH patients. Recent studies by Austin et al. [24] support these findings, suggesting a preponderance of CD3+ and CD8+ T-lymphocytes in the peripheral lung of PAH patients. Animal studies also implicate the lymphocyte in the pathogenesis of PAH. Athymic rats, which lack T-cells, were shown to develop obliterative vascular lesions when treated with a vascular endothelial growth factor receptor blocker under normoxic conditions, suggesting a protective role of T-lymphocytes in pulmonary vascular remodelling [25]. On the other hand, Daley et al. [26] demonstrated that depletion of CD4+ T-cells, the antigen-specific T-helper type 2 (Th2) response or the pathogenic Th2 cytokine IL-13 significantly ameliorated pulmonary arterial muscularization. Treg cells control the balance between Th1 and Th2 responses, and are implicated in the maintenance of self-tolerance and control of autoimmunity. In addition, Treg cells can influence B-cell responses. Recent studies have shown that IPAH patients have a higher proportion of circulating Treg cells than controls, suggesting altered immune control by CD4+ T-lymphocytes [27]. However, the proportion of these cells in the hypertensive pulmonary vasculature remains to be investigated.
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Mediators and Effectors of Inflammation Inducers of inflammation trigger the production of numerous inflammatory mediators, which in turn alter the function of many tissues and organs. Inflammatory mediators can be derived from plasma proteins or secreted by circulating and resident cells of the vasculature. These mediators can be vasoactive amines, vasoactive peptides, fragments of complement components, lipid mediators, cytokines, chemokines or proteolytic enzymes [7,9]. Chemotactic cytokines play a role in leukocyte recruitment and trafficking in PH, such as rolling, activation, adhesion and extravasation into the inflamed tissue along a chemoattractant gradient involving chemokines (soluble, secreted basic proteins). An elevation of serum concentrations of proinflammatory cytokines and local expression of chemokines can be found in IPAH patients. IL-1, IL-6 and tumour necrosis factor-a (TNF-a)
Patients with idiopathic or associated PAH exhibit higher circulating levels and pulmonary expression of IL-1b, IL-6 and TNF-a than healthy controls [28]. Voelkel et al. studied the role of IL-1, a strong proinflammatory cytokine, in MCTinduced PH as compared with chronic hypoxia PH. IL-1 was produced in excessive amounts in the lungs of rats treated with MCT [29]. Repeated injections of an IL-1 receptor antagonist reduced PH and right heart hypertrophy in the MCT model but not in the chronic hypoxia model. Humbert et al. [28] found elevated circulating levels of IL-1 and IL-6 in serum of patients displaying severe PAH, and therefore postulated a possible role for mediators of inflammation in some forms of PH. Recently, transgenic mice overexpressing IL-6 have been shown to develop spontaneous pulmonary vascular remodelling and PH [30]. It was also reported that transgenic mice overexpressing TNF-a develop severe PH and right ventricular hypertrophy [31]. Fractalkine
Fractalkine (CX3CL1) exists in a membrane-bound form (as a cell adhesion molecule on endothelial cells) or a soluble form (as a chemotactic protein) after shedding from the cell surface, and is therefore a structurally distinct chemokine. It mediates its effects through CX3CR1, a seven-transmembrane receptor that is expressed by monocytes, DCs, microglial cells, neurons, natural killer cells, mast cells, and subpopulations of T-lymphocytes. Balabanian et al. [32] found not only that fractalkine is upregulated in circulating CD4+ and CD8+ T-lymphocytes from PAH patients as compared with control subjects, but also that these patients displayed elevated soluble fractalkine plasma concentrations.
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Furthermore, PAH lung samples showed increased fractalkine mRNA expression as compared with controls, and pulmonary artery endothelial cells from PAH patients expressed fractalkine [32]. Perros et al. [33] demonstrated that fractalkine was expressed by inflammatory cells surrounding plexiform lesions, and that pulmonary smooth muscle cells from these vessels exhibited increased fractalkine expression. Furthermore, it was shown that cultured rat pulmonary artery smooth muscle cells expressed CX3CR1 and that fractalkine induced proliferation, but not migration, of these cells, indicating that fractalkine may act as a growth factor for pulmonary artery smooth muscle cells [33]. RANTES
RANTES (regulated upon activation, normal T-cell expressed and secreted; CCL5) recruits leukocytes, including T-cells and monocytes, through different chemokine receptors (CCRs) (e.g. CCR1, CCR3 and CCR5). In lung samples from PAH patients, the mRNA expression of RANTES was increased as compared with control subjects, and Dorfmu¨ller et al. [34] proposed that it might originate from endothelial cells. Further investigations are needed to assess the exact relevance of these findings for the pathophysiology of PAH. MCP-1/CCR2
MCP-1 (CCL2) is one of the most relevant cytokines synthesized by vascular cells, and is a potent mediator of the activation of monocytes and macrophages. Its effects are mediated by CCR2, which is expressed by various cell types, including monocytes and vascular smooth muscle cells. MCP-1 activates monocytes and macrophages, thereby leading to induction of cytokine secretion and expression of adhesion molecules. As compared with healthy controls, IPAH patients exhibit increased levels of MCP-1 in the plasma as well as in lung tissue [21]. In addition, elevated release of MCP-1 by pulmonary endothelial cells and pulmonary arterial smooth muscle cells has been demonstrated [35]. The migration rate of monocytes was significantly increased in the absence of pulmonary endothelial cells (particularly in IPAH patients) and was significantly decreased by MCP-1-blocking antibodies [35]. Finally, pulmonary arterial smooth muscle cells of PAH patients display an increased migratory and proliferative response to stimulation with MCP-1 as compared with healthy controls, suggesting an important role of MCP-1 in PH.
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receptor CXCR4 in neonatal hypoxia-induced PH was described by Young et al. [36]. They demonstrated that inhibition of the SDF-1a–CXCR4 axis decreased the expression of progenitor cells in the lungs and right ventricles of treated mice, as well as decreasing pulmonary vascular cell proliferation and apoptosis and preventing the development of hypoxia-induced pulmonary vascular remodelling in neonatal mice [36].
Triggers for Inflammation—role of Infection Although the concept that inflammation occupies a central position in the pathophysiology of PH has gained considerable currency, knowledge of the inciting factors remains remarkably sketchy. Apart from exogenous and endogenous inducers of inflammation, infectious agents (e.g. viruses and parasites) might also conceivably furnish inflammatory stimuli in PH. Viral infection
HIV. HIV infection is one of the risk factors for PH. The literature suggests that the inflammatory and immune cells are major players in pulmonary vascular pathogenesis, where cytokines, growth factors and viral proteins may act as potential mediators [37]. However, the underlying pathomechanism is not precisely known. Human herpes virus (HHV)-8. HHV-8, a vasculotropic c-herpes virus, is associated with angioproliferative disorders such as Kaposi’s sarcoma and multicentric Castleman’s disease. Cool et al. suggested a potential association of HHV-8 infection with the development of the plexiform lesion of pulmonary vessels, but this relationship was contradicted by others [38,39]. Although it is still controversial, recent in vitro studies have demonstrated that HHV-8 infection significantly changes the gene expression pattern of pulmonary vascular endothelial cells in the direction of inflammation, proliferation, angiogenesis, matrix remodelling and apoptosis [40]. Such alterations in cell signalling as are induced by HHV-8 infection are implicated in pulmonary vascular remodelling. Nevertheless, caution must be exercised until these findings have been confirmed in vivo in animal model studies, and the links between HHV-8 infection and pulmonary vascular pathogenesis have been established.
Stromal cell-derived factor-1a (SDF-1a)
SDF-1a (CXCL12) is a small cytokine belonging to the chemokine family. SDF-1a activates leukocytes, and is often induced by proinflammatory stimuli such as lipopolysaccharides, TNF-a and IL-1. Recently, the role of SDF-1a and its
Hepatitis C virus (HCV). HCV infection is a leading cause of chronic liver disease. The infection is associated with multiple extrahepatic manifestations, including PH. PH complicates HCV infection in 1–5% of patients with chronic liver
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diseases, and the majority of the patients are diagnosed with portal hypertension before PH [41,42]. Although the pathogenesis of portopulmonary hypertension is poorly understood, the histopathology of pulmonary vessels is comparable with that of IPAH [41]. It is as yet unknown whether there is a direct role of viral factors, or whether the vascular lesions develop secondarily to increased vasoconstrictive, proliferative or inflammatory mediators owing to the infection-induced impaired hepatic metabolism and portosystemic shunting. Moreover, in the context of HCV infection being common among the population living with HIV infection and the progression of HCV disease being accelerated in HIV–HCV co-infection [43], future studies are warranted to elucidate the role of HCV infection as well as HIV–HCV co-infection in pulmonary vascular pathology. Parasitic infection
Schistosoma infection. The pulmonary vascular pathology characterized by intimal, medial and adventitial thickening, perivascular inflammatory cells and complex plexiform lesions is observed in chronic schistosomiasis, a disease caused by an infection with a parasitic trematode of the genus Schistosoma. The parasitic egg burden on the lung tissue and the consequent induction of inflammatory and immune processes have been implicated in the pathogenesis [44,45]. However, the precise pathomechanism is as yet incompletely understood. Other parasitic infections. Associations of Wuchereria bancrofti infection and of pulmonary embolism caused by the cystic stage of the tapeworm Echinococcus granulosus (hydatid cyst) with PH have been reported [46]. Studies on prevalence and pathogenesis in endemic regions will unveil further detail in the future. In summary, it is clear that there is an association of infections with pulmonary vascular pathology. Although advances continue to be made in understanding HIV-associated and Schistosomiasis-associated PAH, the precise pathomechanism is as yet unclear, and future investigations should focus more on cellular and molecular mechanisms. Other infections that have potential associations with pulmonary vascular diseases deserve further study with regard to their natural history and pathobiology.
Inflammation as a Therapeutic Target in PH The connection between inflammation and PH is now generally accepted, but several questions remain. Several
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important issues have to be resolved in the future: first, it is unclear whether inflammation is sufficient for the development of PH; second, what are the reasons for this high and/or low frequency of immune and inflammatory cells in IPAH tissue; and third, does this have pathological and/or clinical implications? A better understanding of the mechanisms that link immune and inflammatory cells, abnormal angiogenesis and pulmonary artery remodelling in PH is needed. Our understanding of the pivotal role of inflammation in the pathogenesis of PH opens opportunities in biomarker assessment and therapy for this disease. For example, levels of circulating inflammatory markers such as C-reactive protein are increased in chronic thrmoboembolic PH and PAH patients as compared with control subjects, predicting the outcome and response to therapy in PAH [47]. The relevance of inflammatory mechanisms in some individuals with severe PAH has been further suggested by the clinical improvement of a subset of patients after corticosteroid or immunosuppressive therapy [48]. In addition, some classes of drugs, such as prostacyclins, approved for the treatment of PAH, and inhibitors of hydroxymethylglutaryl coenzyme A (statins), recently used in clinical trials, were shown to have anti-inflammatory and immunomodulatory functions [49,50]. These examples provide illustrations of unexpected anti-inflammatory effects of existing therapies for PH. Uncovering inflammatory pathways has raised the possibility that future treatments may target effectors of inflammation directly, to add to the benefit of current treatments.
Conclusions It is now evident that inflammation plays a prominent role in the initiation/development of PH. The inflammatory cells, and the chemokines and cytokines that they produce, influence the pulmonary vasculature, and regulate the growth, migration and differentiation of all cell types, thus leading to vascular remodelling. However, many of the molecular and cellular mechanisms remain unresolved. New insights into inflammation in PH may identify innovative therapeutic strategies for the treatment of this devastating disease.
Transparency Declaration The authors do not have any dual/conflicting interests.
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47. Quarck R, Nawrot T, Meyns B, Delcroix M. C-reactive protein: a new predictor of adverse outcome in pulmonary arterial hypertension. J Am Coll Cardiol 2009; 53: 1211–1218. 48. Sanchez O, Sitbon O, Jais X, Simonneau G, Humbert M. Immunosuppressive therapy in connective tissue diseases-associated pulmonary arterial hypertension. Chest 2006; 130: 182–189. 49. Gomberg-Maitland M, Olschewski H. Prostacyclin therapies for the treatment of pulmonary arterial hypertension. Eur Respir J 2008; 31: 891–901. 50. Wilkins MR, Ali O, Bradlow W et al. Simvastatin as a treatment for pulmonary hypertension trial (SIPHT). Am J Respir Crit Care Med 2010 [Epub ahead of print].
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10.1111/j.1469-0691.2010.03285.x
Inflammation, immunological reaction and role of infection in pulmonary hypertension S. S. Pullamsetti1,2, R. Savai2, W. Janssen1, B. K. Dahal2, W. Seeger1,2, F. Grimminger2, H. A. Ghofrani2, N. Weissmann2 and R. T. Schermuly1,2 1) Department of Lung Development and Remodelling, Max-Planck Institute for Heart and Lung Research, Bad Nauheim and 2) Department of Internal Medicine, University of Giessen Lung Centre, Giessen, Germany
Abstract Inflammation underlies a wide variety of physiological and pathological processes. Acute inflammation is the initial response of the body to harmful stimuli. Chronic inflammation, by contrast, is a prolonged, dysregulated and maladaptive response that involves active inflammation, tissue destruction and attempts at tissue repair. Over the past few years, such persistent inflammation has been shown to be associated with pulmonary hypertension (PH). Substantial advances in basic and experimental science have illuminated the role of inflammation and the underlying cellular and molecular mechanisms that contribute to PH. This review summarizes the experimental and clinical evidence for inflammation in various types of PH. In addition, it assesses the current state of knowledge regarding the inducers/triggers of chronic inflammation and infection, as well as the inflammatory mediators and cells that are involved in PH. Infiltration of inflammatory cells, such as dendritic cells, macrophages, mast cells, T-lymphocytes and B-lymphocytes, in the vascular lesions and an elevation of serum/tissue concentrations of proinflammatory cytokines and chemokines and their contribution to pulmonary vascular remodelling are reported in detail. We review the data supporting the use of inflammatory markers as prognostic and predictive factors in PH. Finally, we consider how new insights into inflammation in PH may identify innovative therapeutic strategies. Keywords: Immunity, infection, inflammation, pulmonary hypertension, review Article published online: 8 June 2010 Clin Microbiol Infect 2011; 17: 7–14 Corresponding author: R. T. Schermuly, Max-Planck-Institute for Heart and Lung Research, Department of Lung Development and Remodelling, Parkstrasse 1, 61231 Bad Nauheim, Germany E-mail: [email protected]
Pathogenesis of Pulmonary Hypertension Pulmonary hypertension (PH) is a progressive disease with a poor prognosis that results in right heart dysfunction and right heart failure [1]. Characterized by increased pulmonary vascular resistance, PH includes a heterogeneous group of clinical entities that have been subcategorized into five groups on the basis of association with other diseases, according to the currently accepted PH classification [2]. Group 1, pulmonary arterial hypertension (PAH), is composed of a heterogeneous group of diseases that have been subcategorized as idiopathic PAH (IPAH), familial PAH, PH associated with other diseases, such as connective tissue
diseases (e.g. systemic sclerosis), portopulmonary hypertension, and PH related to human immunodeficiency virus (HIV) infection, drugs and toxins [2]. The pathogenesis of PH is a complex, multifactorial process. Increased pulmonary arterial pressure in PH patients may result from a combination of pulmonary vasoconstriction, inward vascular wall remodelling and in situ thrombosis, leading to increased vascular stiffness as well as narrowing of the vascular lumen [1]. However, the exact triggers of PH have not been identified yet. An initial insult, from chronic hypoxia, inflammation, viral infection, mechanical stretch, shear stress, and/or unknown factors, is considered to activate or affect the endothelium. This results in altered production of endothelial mediators and release of inflammatory mediators, which have
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been increasingly implicated in PH [3]. These inflammatory mediators, including cell adhesion molecules, cytokines, chemokines and growth factors, direct the recruitment of inflammatory cells and propagate a number of inflammatory pathways that lead to vascular cell proliferation, migration and extracellular matrix deposition, all of which contribute to the structural remodelling-characteristic of PH [4,5] (Fig. 1). Inflammatory processes are prominent in various forms of PH, and are increasingly recognized as major pathogenic components of pulmonary vascular remodelling in IPAH or PAH related to more classical forms of inflammatory syndromes, such as connective tissue diseases, HIV infection,
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or other viral aetiologies. Similarly, inflammation seems to play a significant role in experimental animal models of PH. This review will focus on inflammatory processes that are involved in the initiation or progression of PH, and the prognostic and therapeutic importance of inflammation in PH.
An Overview of Inflammation (Acute and Chronic) Inflammation is an adaptive response that is triggered by noxious stimuli and conditions, such as infection and tissue
Normal Inflammation
FIG. 1. Schematic illustration of infection-mediated and inflammation-mediated vascular remodelling: In response
Adventitia Media
to infection and inflammatory events,
Internal elastic lamina Endothelium
Infection (viral, parasitic)
lung vascular cells produce inflammatory mediators (chemokines and cytokines), thereby recruiting the inflammatory cells (macrophages, dendritic cells, mast cells,
Release of cytokines/chemokines CX3CL1, CCL5, CX3CR1, CCL2
B-cells, T-cells and regulatory T-cells). With the coordination of inflammatory mediators, inflammatory cells may per-
Inflammatory cells recruitment
petuate the release of cytokines, chemokines and growth factors. Finally, these
Treg cells
processes lead to vascular remodelling
Mast cells
Dendritic cells
B-cells
Macrophages
though
matrix
remodelling,
collagen
deposition, vascular cell proliferation,
T-cells
migration, and in situ thrombosis. CCL2, chemokine (C-C motif) ligand 2; CCL5, chemokine (C-C motif) ligand 5 or RANTES (regulated upon activation, normal T-cell expressed and secreted); Cytokines, chemokines, growth factors
CX3CL1, chemokine (C-X3-C motif) ligand 1 (fractalkine); CX3CR1, chemo-
IL-1, IL-6, TNF-α, CX3CL1, CCL5, MCP-1, SDF-1α PDGF, VEGF, FGF
kine (C-X3-C motif) receptor 1; EC, endothelial cells; FB, fibroblasts; FGF, fibroblast growth factor; IL-1, interleukin-1; IL-6, interleukin 6; MCP-1, monocyte Vascular Remodelling
chemotactic protein-1; PDGF, platelet-
Chronic vasoconstriction In-situ thrombosis SMC dysfunction FB dysfunction EC dysfunction Matrix remodelling/Collagen deposition
derived growth factor; PAH, pulmonary arterial hypertension; SDF-1a, stromal cell-derived factor 1a; SMC, smooth muscle cells; TNF-a, tumour necrosis factor-a; Treg cell, regulatory T-cell; VEGF,
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factor.
vascular
endothelial
growth
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injury [6]. It is characterized by the sequential release of cytokines, chemokines and growth factors that regulate increased vascular permeability and recruitment of leukocytes. Increased vascular permeability also results in extravasation of plasma proteins, which further amplify the inflammatory reaction. Importantly, regardless of the cause, inflammation presumably evolved as an adaptive response for restoring homeostasis. However, when inflammation persists (chronic inflammation), it can cause tissue damage and loss of function. Chronic inflammation may occur because of the persistence of infection or antigen, recurring tissue injury, or a failure of endogenous anti-inflammatory mechanisms that drive the resolution of inflammation [6]. Notably, altered interactions between adaptive and innate immune cells can lead to chronic inflammation. It is generally thought that, during acute inflammation, innate immune cells, such as dendritic cells (DCs), natural killer cells, macrophages, neutrophils, basophils, eosinophils and mast cells, form the first line of immune defence and regulate the activation of adaptive immune responses. By contrast, during chronic inflammation, these roles can be reversed—adaptive immune responses by adaptive immune cells, such as B-lymphocytes, CD4+ helper T-lymphocytes and CD8+ cytotoxic T-lymphocytes, can cause ongoing and excessive activation of innate immune cells [7].
The Scientific Basis of Inflammation in PH The concept that immunological reactions in PH play a significant role in the development and worsening of the disease is now well accepted. Autoimmune infiltration of immune cells and inflammatory reactions have been shown to contribute to the pathogenesis of IPAH [4,5,8].
Inflammatory and Immune Cell Component of PH The inflammatory and immune component of structurally altered vessels includes circulating monocytes, neutrophils, DCs, macrophages and lymphocytes, as well as fibroblasts, resident endothelial cells and smooth muscle cells. Presumably, circulating cells are directed to sites of injury, adhere to or come close to endothelial cells, invade the internal elastic lamina, and release a variety of substances that act on the local environment and promote chemotaxis. Released cytokines and growth factors produce the effects of inflammation by mediating communication between and among circulating and resident vascular cells. Finally, all of these
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interactions result in vascular remodelling through matrix remodelling, collagen deposition, proliferation and migration of all vascular cell types [9] (Fig. 1).
Innate Immune Cells DCs
DCs, which are crucial for the activation of naive T-cells, play an important role as professional antigen-presenting cells in the immune system. Perros et al. [10] demonstrated immature DC population infiltrating vascular lesions in both IPAH patients and in experimental PH. Accumulation of these professional antigen-presenting cells may involve presentation of antibodies to endothelial cells, fibroblasts and nuclear antigens that are found in the serum of patients with IPAH and collagen vascular disease-associated PAH [11,12]. Recently, Wang et al. [13] reported a decrease in the percentage of monocytederived DCs in the peripheral blood of IPAH patients, suggesting a Th1 immune reaction in the pathogenesis of IPAH. Mast cells
Mast cells are major effector cells in allergy and host defence responses. Upon activation, mast cells release a broad spectrum of proinflammatory cytokines, growth factors, vasoactive substances and proteases (tryptase and chymase). Although vascular mast cells are rare in the normal vasculature, an increase in the number of vascular mast cells was found in patients with PH as well as in experimental models of PH [14,15]. It is postulated that mast cells are a rich source of interleukin (IL)-4, as well as other factors that are capable of stimulating autoreactive B-cells to secrete autoantibodies, including anti-endothelial cell antibodies, and mast cell degranulation through regulation of matrix metalloprotease activity appears to be an important factor in the initial phase of pulmonary vascular remodelling [16]. Macrophages/monocytes
Recruitment of macrophages, along with other mononuclear cells, has been reported in primary PH as well as in secondary forms of PH, including high-flow congenital heart disease, scleroderma, and PH-associated acute respiratory distress syndrome [17,18]. Similarly, increased numbers of macrophages and neutrophils have been described in murine and rat lungs in response to chronic hypoxia and monocrotaline (MCT) [19,20]. Although the molecular mechanism of monocyte/macrophage infiltration and its roles remain elusive, macrophage accumulation and activation in remodelled pulmonary vessels may lead to the release of a variety of proinflammatory cytokines, (e.g. monocyte chemotactic
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protein (MCP-I) and matrix metalloproteases) that are capable of causing vasoconstriction, increasing vascular permeability and inducing proliferation [21].
Adaptive Immune Cells B-lymphocytes
B-lymphocytes (B-cells) are fundamental to the humoral immune response, because of their potential to differentiate into antibody-producing plasma B-cells. B-cells also play a crucial role in cell-mediated immune regulation through antigen presentation, production of various cytokines, differentiation of T-effector cells, and collaboration with antigen-presenting DCs [8]. Antibodies directed against pulmonary endothelial cells and fibroblasts have been found in PAH, suggesting a role of B-cells in PAH [11,12]. In corroboration, activation of B-cells in peripheral blood B-lymphocytes and perivascular infiltration of B-lymphocytes has been observed in patients with IPAH [17,22]. T-lymphocytes (cytotoxic T-cells, helper T-cells and regulatory T-cells (Treg cells))
Infiltration of T-lymphocytes has been previously noted in lungs of IPAH patients [17]. Bonnet et al. [23] reported CD3+ T-lymphocytes with activated NFATc2 (a nuclear Ca2+/ calceneurin-sensitive transcription factor found in T-lymphocytes) in the walls of resistance arteries in IPAH patients. Recent studies by Austin et al. [24] support these findings, suggesting a preponderance of CD3+ and CD8+ T-lymphocytes in the peripheral lung of PAH patients. Animal studies also implicate the lymphocyte in the pathogenesis of PAH. Athymic rats, which lack T-cells, were shown to develop obliterative vascular lesions when treated with a vascular endothelial growth factor receptor blocker under normoxic conditions, suggesting a protective role of T-lymphocytes in pulmonary vascular remodelling [25]. On the other hand, Daley et al. [26] demonstrated that depletion of CD4+ T-cells, the antigen-specific T-helper type 2 (Th2) response or the pathogenic Th2 cytokine IL-13 significantly ameliorated pulmonary arterial muscularization. Treg cells control the balance between Th1 and Th2 responses, and are implicated in the maintenance of self-tolerance and control of autoimmunity. In addition, Treg cells can influence B-cell responses. Recent studies have shown that IPAH patients have a higher proportion of circulating Treg cells than controls, suggesting altered immune control by CD4+ T-lymphocytes [27]. However, the proportion of these cells in the hypertensive pulmonary vasculature remains to be investigated.
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Mediators and Effectors of Inflammation Inducers of inflammation trigger the production of numerous inflammatory mediators, which in turn alter the function of many tissues and organs. Inflammatory mediators can be derived from plasma proteins or secreted by circulating and resident cells of the vasculature. These mediators can be vasoactive amines, vasoactive peptides, fragments of complement components, lipid mediators, cytokines, chemokines or proteolytic enzymes [7,9]. Chemotactic cytokines play a role in leukocyte recruitment and trafficking in PH, such as rolling, activation, adhesion and extravasation into the inflamed tissue along a chemoattractant gradient involving chemokines (soluble, secreted basic proteins). An elevation of serum concentrations of proinflammatory cytokines and local expression of chemokines can be found in IPAH patients. IL-1, IL-6 and tumour necrosis factor-a (TNF-a)
Patients with idiopathic or associated PAH exhibit higher circulating levels and pulmonary expression of IL-1b, IL-6 and TNF-a than healthy controls [28]. Voelkel et al. studied the role of IL-1, a strong proinflammatory cytokine, in MCTinduced PH as compared with chronic hypoxia PH. IL-1 was produced in excessive amounts in the lungs of rats treated with MCT [29]. Repeated injections of an IL-1 receptor antagonist reduced PH and right heart hypertrophy in the MCT model but not in the chronic hypoxia model. Humbert et al. [28] found elevated circulating levels of IL-1 and IL-6 in serum of patients displaying severe PAH, and therefore postulated a possible role for mediators of inflammation in some forms of PH. Recently, transgenic mice overexpressing IL-6 have been shown to develop spontaneous pulmonary vascular remodelling and PH [30]. It was also reported that transgenic mice overexpressing TNF-a develop severe PH and right ventricular hypertrophy [31]. Fractalkine
Fractalkine (CX3CL1) exists in a membrane-bound form (as a cell adhesion molecule on endothelial cells) or a soluble form (as a chemotactic protein) after shedding from the cell surface, and is therefore a structurally distinct chemokine. It mediates its effects through CX3CR1, a seven-transmembrane receptor that is expressed by monocytes, DCs, microglial cells, neurons, natural killer cells, mast cells, and subpopulations of T-lymphocytes. Balabanian et al. [32] found not only that fractalkine is upregulated in circulating CD4+ and CD8+ T-lymphocytes from PAH patients as compared with control subjects, but also that these patients displayed elevated soluble fractalkine plasma concentrations.
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Furthermore, PAH lung samples showed increased fractalkine mRNA expression as compared with controls, and pulmonary artery endothelial cells from PAH patients expressed fractalkine [32]. Perros et al. [33] demonstrated that fractalkine was expressed by inflammatory cells surrounding plexiform lesions, and that pulmonary smooth muscle cells from these vessels exhibited increased fractalkine expression. Furthermore, it was shown that cultured rat pulmonary artery smooth muscle cells expressed CX3CR1 and that fractalkine induced proliferation, but not migration, of these cells, indicating that fractalkine may act as a growth factor for pulmonary artery smooth muscle cells [33]. RANTES
RANTES (regulated upon activation, normal T-cell expressed and secreted; CCL5) recruits leukocytes, including T-cells and monocytes, through different chemokine receptors (CCRs) (e.g. CCR1, CCR3 and CCR5). In lung samples from PAH patients, the mRNA expression of RANTES was increased as compared with control subjects, and Dorfmu¨ller et al. [34] proposed that it might originate from endothelial cells. Further investigations are needed to assess the exact relevance of these findings for the pathophysiology of PAH. MCP-1/CCR2
MCP-1 (CCL2) is one of the most relevant cytokines synthesized by vascular cells, and is a potent mediator of the activation of monocytes and macrophages. Its effects are mediated by CCR2, which is expressed by various cell types, including monocytes and vascular smooth muscle cells. MCP-1 activates monocytes and macrophages, thereby leading to induction of cytokine secretion and expression of adhesion molecules. As compared with healthy controls, IPAH patients exhibit increased levels of MCP-1 in the plasma as well as in lung tissue [21]. In addition, elevated release of MCP-1 by pulmonary endothelial cells and pulmonary arterial smooth muscle cells has been demonstrated [35]. The migration rate of monocytes was significantly increased in the absence of pulmonary endothelial cells (particularly in IPAH patients) and was significantly decreased by MCP-1-blocking antibodies [35]. Finally, pulmonary arterial smooth muscle cells of PAH patients display an increased migratory and proliferative response to stimulation with MCP-1 as compared with healthy controls, suggesting an important role of MCP-1 in PH.
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receptor CXCR4 in neonatal hypoxia-induced PH was described by Young et al. [36]. They demonstrated that inhibition of the SDF-1a–CXCR4 axis decreased the expression of progenitor cells in the lungs and right ventricles of treated mice, as well as decreasing pulmonary vascular cell proliferation and apoptosis and preventing the development of hypoxia-induced pulmonary vascular remodelling in neonatal mice [36].
Triggers for Inflammation—role of Infection Although the concept that inflammation occupies a central position in the pathophysiology of PH has gained considerable currency, knowledge of the inciting factors remains remarkably sketchy. Apart from exogenous and endogenous inducers of inflammation, infectious agents (e.g. viruses and parasites) might also conceivably furnish inflammatory stimuli in PH. Viral infection
HIV. HIV infection is one of the risk factors for PH. The literature suggests that the inflammatory and immune cells are major players in pulmonary vascular pathogenesis, where cytokines, growth factors and viral proteins may act as potential mediators [37]. However, the underlying pathomechanism is not precisely known. Human herpes virus (HHV)-8. HHV-8, a vasculotropic c-herpes virus, is associated with angioproliferative disorders such as Kaposi’s sarcoma and multicentric Castleman’s disease. Cool et al. suggested a potential association of HHV-8 infection with the development of the plexiform lesion of pulmonary vessels, but this relationship was contradicted by others [38,39]. Although it is still controversial, recent in vitro studies have demonstrated that HHV-8 infection significantly changes the gene expression pattern of pulmonary vascular endothelial cells in the direction of inflammation, proliferation, angiogenesis, matrix remodelling and apoptosis [40]. Such alterations in cell signalling as are induced by HHV-8 infection are implicated in pulmonary vascular remodelling. Nevertheless, caution must be exercised until these findings have been confirmed in vivo in animal model studies, and the links between HHV-8 infection and pulmonary vascular pathogenesis have been established.
Stromal cell-derived factor-1a (SDF-1a)
SDF-1a (CXCL12) is a small cytokine belonging to the chemokine family. SDF-1a activates leukocytes, and is often induced by proinflammatory stimuli such as lipopolysaccharides, TNF-a and IL-1. Recently, the role of SDF-1a and its
Hepatitis C virus (HCV). HCV infection is a leading cause of chronic liver disease. The infection is associated with multiple extrahepatic manifestations, including PH. PH complicates HCV infection in 1–5% of patients with chronic liver
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diseases, and the majority of the patients are diagnosed with portal hypertension before PH [41,42]. Although the pathogenesis of portopulmonary hypertension is poorly understood, the histopathology of pulmonary vessels is comparable with that of IPAH [41]. It is as yet unknown whether there is a direct role of viral factors, or whether the vascular lesions develop secondarily to increased vasoconstrictive, proliferative or inflammatory mediators owing to the infection-induced impaired hepatic metabolism and portosystemic shunting. Moreover, in the context of HCV infection being common among the population living with HIV infection and the progression of HCV disease being accelerated in HIV–HCV co-infection [43], future studies are warranted to elucidate the role of HCV infection as well as HIV–HCV co-infection in pulmonary vascular pathology. Parasitic infection
Schistosoma infection. The pulmonary vascular pathology characterized by intimal, medial and adventitial thickening, perivascular inflammatory cells and complex plexiform lesions is observed in chronic schistosomiasis, a disease caused by an infection with a parasitic trematode of the genus Schistosoma. The parasitic egg burden on the lung tissue and the consequent induction of inflammatory and immune processes have been implicated in the pathogenesis [44,45]. However, the precise pathomechanism is as yet incompletely understood. Other parasitic infections. Associations of Wuchereria bancrofti infection and of pulmonary embolism caused by the cystic stage of the tapeworm Echinococcus granulosus (hydatid cyst) with PH have been reported [46]. Studies on prevalence and pathogenesis in endemic regions will unveil further detail in the future. In summary, it is clear that there is an association of infections with pulmonary vascular pathology. Although advances continue to be made in understanding HIV-associated and Schistosomiasis-associated PAH, the precise pathomechanism is as yet unclear, and future investigations should focus more on cellular and molecular mechanisms. Other infections that have potential associations with pulmonary vascular diseases deserve further study with regard to their natural history and pathobiology.
Inflammation as a Therapeutic Target in PH The connection between inflammation and PH is now generally accepted, but several questions remain. Several
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important issues have to be resolved in the future: first, it is unclear whether inflammation is sufficient for the development of PH; second, what are the reasons for this high and/or low frequency of immune and inflammatory cells in IPAH tissue; and third, does this have pathological and/or clinical implications? A better understanding of the mechanisms that link immune and inflammatory cells, abnormal angiogenesis and pulmonary artery remodelling in PH is needed. Our understanding of the pivotal role of inflammation in the pathogenesis of PH opens opportunities in biomarker assessment and therapy for this disease. For example, levels of circulating inflammatory markers such as C-reactive protein are increased in chronic thrmoboembolic PH and PAH patients as compared with control subjects, predicting the outcome and response to therapy in PAH [47]. The relevance of inflammatory mechanisms in some individuals with severe PAH has been further suggested by the clinical improvement of a subset of patients after corticosteroid or immunosuppressive therapy [48]. In addition, some classes of drugs, such as prostacyclins, approved for the treatment of PAH, and inhibitors of hydroxymethylglutaryl coenzyme A (statins), recently used in clinical trials, were shown to have anti-inflammatory and immunomodulatory functions [49,50]. These examples provide illustrations of unexpected anti-inflammatory effects of existing therapies for PH. Uncovering inflammatory pathways has raised the possibility that future treatments may target effectors of inflammation directly, to add to the benefit of current treatments.
Conclusions It is now evident that inflammation plays a prominent role in the initiation/development of PH. The inflammatory cells, and the chemokines and cytokines that they produce, influence the pulmonary vasculature, and regulate the growth, migration and differentiation of all cell types, thus leading to vascular remodelling. However, many of the molecular and cellular mechanisms remain unresolved. New insights into inflammation in PH may identify innovative therapeutic strategies for the treatment of this devastating disease.
Transparency Declaration The authors do not have any dual/conflicting interests.
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pulmonary arterial hypertension express a different RNA pattern compared with healthy controls: a cross sectional study. Respir Res 2008; 9: 20. Bonnet S, Rochefort G, Sutendra G et al. The nuclear factor of activated T cells in pulmonary arterial hypertension can be therapeutically targeted. Proc Natl Acad Sci USA 2007; 104: 11418– 11423. Austin ED, Rock MT, Mosse CA et al. T lymphocyte subset abnormalities in the blood and lung in pulmonary arterial hypertension. Respir Med 2009; 104: 454–462. Taraseviciene-Stewart L, Nicolls MR, Kraskauskas D et al. Absence of T cells confers increased pulmonary arterial hypertension and vascular remodeling. Am J Respir Crit Care Med 2007; 175: 1280–1289. Daley E, Emson C, Guignabert C et al. Pulmonary arterial remodeling induced by a Th2 immune response. J Exp Med 2008; 205: 361–372. Ulrich S, Nicolls MR, Taraseviciene L, Speich R, Voelkel N. Increased regulatory and decreased CD8+ cytotoxic T cells in the blood of patients with idiopathic pulmonary arterial hypertension. Respiration 2008; 75: 272–280. Humbert M, Monti G, Brenot F et al. Increased interleukin-1 and interleukin-6 serum concentrations in severe pulmonary hypertension. Am J Respir Crit Care Med 1995; 151: 1628–1631. Voelkel N, Tuder R, Bridges J, Arend W. Interleukin-1 receptor antagonist treatment reduces pulmonary hypertension generated in rats by monocrotaline. Am J Respir Cell Mol Biol 1994; 11: 664–675. Steiner MK, Syrkina OL, Kolliputi N, Mark EJ, Hales CA, Waxman AB. Interleukin-6 overexpression induces pulmonary hypertension. Circ Res 2009; 104: 236–244. Fujita M, Mason R, Cool C, Shannon J, Hara N, Fagan K. Pulmonary hypertension in TNF-a-overexpressing mice is associated with decreased VEGD gene expression. J Appl Physiol 2002; 93: 2162–2170. Balabanian K, Fousset A, Dorfmuller P et al. Cx(3)c chemokine fractalkine in pulmonary arterial hypertension. Am J Respir Crit Care Med 2002; 165: 1419–1425. Perros F, Dorfmuller P, Souza R et al. Fractalkine-induced smooth muscle cell proliferation in pulmonary hypertension. Eur Respir J 2007; 29: 937–943. Dorfmuller P, Zarka V, Durand-Gasselin I et al. Chemokine RANTES in severe pulmonary arterial hypertension. Am J Respir Crit Care Med 2002; 165: 534–539. Sanchez O, Marcos E, Perros F et al. Role of endothelium-derived CC chemokine ligand 2 in idiopathic pulmonary arterial hypertension. Am J Respir Crit Care Med 2007; 176: 1041–1047. Young KC, Torres E, Hatzistergos KE, Hehre D, Suguihara C, Hare JM. Inhibition of the SDF-1/CXCR4 axis attenuates neonatal hypoxiainduced pulmonary hypertension. Circ Res 2009; 104: 1293–1301. Humbert M. Mediators involved in HIV-related pulmonary arterial hypertension. AIDS 2008; 22 (suppl 3): S41–S47. Cool CD, Rai PR, Yeager ME et al. Expression of human herpesvirus 8 in primary pulmonary hypertension. N Engl J Med 2003; 349: 1113–1122. Henke-Gendo C, Mengel M, Hoeper MM, Alkharsah K, Schulz TF. Absence of Kaposi’s sarcoma-associated herpesvirus in patients with pulmonary arterial hypertension. Am J Respir Crit Care Med 2005; 172: 1581–1585. Bull TM, Meadows CA, Coldren CD et al. Human herpesvirus-8 infection of primary pulmonary microvascular endothelial cells. Am J Respir Cell Mol Biol 2008; 39: 706–716. Moorman J, Saad M, Kosseifi S, Krishnaswamy G. Hepatitis C virus and the lung: implications for therapy. Chest 2005; 128: 2882–2892. Robalino BD, Moodie DS. Association between primary pulmonary hypertension and portal hypertension: analysis of its pathophysiology and clinical, laboratory and hemodynamic manifestations. J Am Coll Cardiol 1991; 17: 492–498.
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ª2010 The Authors Clinical Microbiology and Infection ª2010 European Society of Clinical Microbiology and Infectious Diseases, CMI, 17, 7–14