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Journal of the American College of Cardiology Vol. 54, No. 1, Suppl S, 2009© 2009 by the American College of Cardiology Foundation ISSN 0735-1097/09/$36.00P

STATE-OF-THE-ART PAPERS

Development and Pathology of Pulmonary Hypertension

Rubin M. Tuder, MD,* Steven H. Abman, MD,† Thomas Braun, MD, PHD,‡Frédérique Capron, MD, PHD,§ Troy Stevens, PHD,� Patricia A. Thistlethwaite, MD, PHD,¶Sheila G. Haworth, MD#

Denver, Colorado; Bad Neuheim, Germany; Paris, France; Mobile, Alabama; San Diego, California;and London, United Kingdom

The Development and Pathology working group was charged with reviewing the present knowledge, gaps in un-derstanding, and areas for further studies in a broad range of themes. These themes in pulmonary vascular biol-ogy and pathobiology involved: 1) pulmonary vascular development; 2) pulmonary vascular disease accompany-ing fetal development and perinatal life; 3) properties of pulmonary vascular endothelial cells; 4) role of bonemarrow cells in pulmonary vascular disease; 5) insights into pulmonary thromboembolic disease; 6) role of pa-thology in the assessment of pulmonary vascular disease; and 7) considerations of tissue banking for researchin pulmonary hypertension. These important goals provide a blueprint for future research that may significantlyimpact our present and future understanding of pulmonary hypertension. (J Am Coll Cardiol 2009;54:S3–9)© 2009 by the American College of Cardiology Foundation

ublished by Elsevier Inc. doi:10.1016/j.jacc.2009.04.009

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ecent work has shown that in the normal lung, there-acinar arteries and post-acinar veins form by vasculo-enesis from the splanchnopleural mesoderm of the lungud (1). Serial reconstruction of human embryos indicateshat the pulmonary arteries and veins arise by the sustainedddition of newly formed coalescing endothelial tubes de-ived by vasculogenesis from the mesenchyme of the lunground the airway terminal buds. This occurs while pre-cinar airway branching continues. Angiogenesis appears toredominate after 15 to 17 weeks’ gestation during intra-cinar formation. Many of the signaling molecules ex-ressed in the early human embryo, such as endothelial cellitric oxide synthase (eNOS), vascular endothelial growthactor and its receptors, angiopoietins, the endothelial-pecific receptor TIE-2, transforming growth factorTGF)-�, hypoxia inducible factors 1� and 1�, and theone morphogenetic proteins (BMPs) and their receptors,ave been studied in vitro and in transgenic models, butow these and other molecules orchestrate developmentalrocesses in life is still unclear. Several important ques-ions remain to be addressed. These include: 1) thelucidation of the signaling processes responsible for the

rom the *Divisions of Pulmonary Sciences and Critical Care Medicine, and thePediatric Heart Lung Center, University of Colorado, Denver, Colorado; ‡Maxlanck Institute for Heart Lung Research, Bad Neuheim, Germany; §Department ofathology, Hôpital de la Pitié, Paris, France; �Center for Lung Biology, University ofouth Alabama College of Medicine, Mobile, Alabama; ¶Division of Cardiothoracicurgery, University of California, San Diego, San Diego, California; and theVascular Biology and Pharmacology Unit, Institute of Child Health, UCL, London,nited Kingdom. Please see the end of this article for each author’s conflict of interest

anformation.

Manuscript received February 6, 2009; accepted April 15, 2009.

nduction of the pulmonary capillary networks forminground each epithelial bud, the mechanisms regulatinghe coalescence of the endothelial tubes in the mesen-hyme, their fusion with the growing pulmonary artery, andhe organization of the randomly formed endothelial cellse.g., lining up alongside the developing airways, arteries onne side and veins on the other); 2) the precise role ofmbryonic and fetal lung signaling molecules and how theyelate to one another; 3) the role of oxygen-sensing mech-nisms, which have been described in the postnatal lung, intero, and at birth; 4) understanding the crosstalk betweenpithelial and vascular cells in this complex orchestratedevelopmental process; 5) clarifying the interplay betweenentral pulmonary arteries and the proximal intrapulmonaryrteries during development (e.g., whether they developimultaneously or independently); 6) determining the extento which extrapulmonary cells are incorporated into theeveloping pulmonary vasculature; 7) establishing the basisf the heterogeneity of all the structural components of theessel wall; 8) determining the potential for vascular cellaintenance of genetic memory that can influence cell

henotype and the response to injury in postnatal life; and) ascertaining whether the primordial human lung has aunctional circulation throughout its development, as haseen shown in the chick embryo.The developing pulmonary arteries and veins become

nvested by smooth muscle cells from different anatomicources. The smooth muscle cells show orderly acquisi-ion of specific cytoskeletal components, but we need tonderstand the expression patterns of other features, such

s K� channels, which would allow the identification of

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S4 Tuder et al. JACC Vol. 54, No. 1, Suppl S, 2009Development and Pathology June 30, 2009:S3–9

specific smooth muscle cellphenotypes. Furthermore, it re-mains unclear whether suchphenotypic features would per-tain to the specific origin of thesmooth muscle cells, eitherfrom the lung bud mesenchymeor from a specific extrapulmo-nary source. We strongly be-lieve that pulmonary vasculardevelopment should be consid-ered a critical determinant ofthe propensity to develop pul-monary vascular disease in laterlife.

Mechanisms RegulatingCrosstalk BetweenAirway and VasculatureDuring Development andthe Pathogenesis ofNeonatal Lung Diseases

In addition to being among theprimary causes of pulmonary hy-pertension (PH), pulmonary vas-cular diseases are strongly associ-ated with abnormalities of lungdevelopment. Normal lung vas-cular development is critical forsuccessful adaptation and survivalat birth and in postnatal life.Studies of mechanisms that reg-ulate development of the pulmo-nary circulation have been rela-

ively limited and largely descriptive in nature. However,ecent observations have challenged older notions that theevelopment of the blood vessels in the lung passivelyollows that of the airways. The mechanisms by which theung successfully achieves normal gas exchange require therowth and maintenance of an intricate system of airwaysnd vessels, including the establishment of a thin yet vastlood–gas interface, which is continuously modulated underoth physiologic and pathologic conditions (2,3).Increasing evidence suggests that lung blood vessels actively

romote normal alveolar growth during development andontribute to the maintenance of alveolar structures throughoutostnatal life (4–7). Disruption of angiogenesis during lungevelopment can impair alveolarization, and preservation ofascular growth and endothelial survival may promote lungrowth and structure of the distal airspace. Understanding howlveoli and the underlying capillary network develop and howhese mechanisms are disrupted in disease states is critical foreveloping efficient therapies for lung diseases characterized by

Abbreviationsand Acronyms

BMP � bonemorphogenetic protein

BMPR � bonemorphogenetic proteinreceptor

BPD � bronchopulmonarydysplasia

CDH � congenitaldiaphragmatic hernia

CTEPH � chronicthromboembolic pulmonaryhypertension

eNOS � endothelial nitricoxide synthase

EPC � endothelialprogenitor cell

IPAH � idiopathicpulmonary arterialhypertension

PAH � pulmonary arterialhypertension

PCH � pulmonary capillaryhemangiomatosis

PH � pulmonaryhypertension

PPHN � persistentpulmonary hypertension ofthe newborn

PVOD � pulmonary veno-occlusive disease

TGF � transforming growthfactor

WHO � World HealthOrganization

mpaired alveolar structure. C

Recent advances in the field of vascular biology haverovided novel experimental tools to probe how pulmonarylood vessels are assembled during early embryonic and fetalevelopment. These tools may provide important newnformation regarding pediatric lung diseases associatedith PH. Developmental abnormalities of the pulmonary

irculation contribute to the pathophysiology of such dis-ases as persistent PH of the newborn (PPHN), lungypoplasia, congenital diaphragmatic hernia (CDH), con-enital heart disease, and others. This may be especiallymportant in understanding the pathogenesis of broncho-ulmonary dysplasia (BPD), which is the chronic lungisease that follows premature birth. BPD is characterizedy arrested lung growth, which may play a central role in thensuing decreased alveolarization and a dysmorphic vascu-ature (8–10).

Experimental data further suggest a potential therapeuticole for modulation of angiogenesis for lung diseases that areharacterized by arrested alveolar growth, such as BPD. Thepidemiology and risk factors for PPHN, BPD, and CDHemain to be defined, along with the gene–environmentnteractions underlying their pathobiology and biomarkersmaternal, neonatal) to identify at-risk infants. Clinicalrials are needed to improve therapeutic interventions toreat PH and enhance distal lung growth and function.oninvasive methods to assess pulmonary hemodynamics

nd lung vascular growth and structure are also needed.urthermore, advances in stem cell biology suggest at leastotential roles for endothelial progenitor cells (EPCs) andesenchymal stem cells in the pathogenesis or treatment of

ung vascular disease, especially in experimental models ofPD (11,12).Future work aimed at better defining the basic mecha-

isms of lung vascular growth and development will likelyead to novel therapeutic approaches to diseases associatedith impaired vascular growth or PH. In addition, there isclear need to better understand the developmental phys-

ology of the lung circulation, especially regarding perinatalmaternal, placental, fetal, and neonatal) mechanisms that:) regulate vascular tone and reactivity in utero and matu-ational changes during normal development; 2) alter vas-ular tone, reactivity, and function in models of PPHN,ncluding disruption of growth factors, cytokines, and re-ated signaling pathways; and 3) disrupt lung vascularrowth and structure. In addition, further studies are neededo clarify the normal physiology and pathobiology ofPCs. Naive or transfected EPC strategies may have a

ritical role in the restoration of vascular growth andunction with impact on maintenance of alveolar struc-ure. EPCs may serve as potential biomarkers used toiagnose neonatal pulmonary vascular disease over the

ifetime of an individual.For the realization of these goals, there is a pressing need

or multicenter interventional studies aimed at the preven-ion of BPD and the treatment of severe PH in BPD and

DH. These approaches should be spearheaded by inter-

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S5JACC Vol. 54, No. 1, Suppl S, 2009 Tuder et al.June 30, 2009:S3–9 Development and Pathology

ational working groups of pathologists with special exper-ise in lung development and PH for the diagnostic reviewf cases. Consortia are needed to collect, process, and studyung tissues in order to enhance clinical research.

ulmonary Endothelial Cell Biologyn Health and Disease

ndothelial cells display remarkable heterogeneity in struc-ure and function, all along the pulmonary arterial-capillary-enous axis. This segment-specific heterogeneity can bellustrated by examining lectin-binding patterns within theulmonary circulation (13,14). Extra-alveolar endothelialells (artery and vein) interact with the lectins Helix pomatia,ut not with Griffonia simplicifolia, whereas microvascularndothelial cells (capillary) interact with Griffonia simplici-olia, but not with Helix pomatia. Using lectin binding asne approach to guide the study of pulmonary endothelialell function, it has become apparent that microvascularndothelial cells possess greater adhesion strength, uniqueechanosensing properties, different organization of signal-

ng networks (e.g., cAMP, calcium, and oxidants), highlycolytic flux, and a discrete distribution of organelles,ompared with pulmonary artery endothelial cells (13,14). Its interesting that the site-specific functions of pulmonaryrtery and microvascular endothelial cells are retained whenopulations of these cells are studied in culture, providinghe opportunity to dissect mechanisms underlying pheno-ypic heterogeneity. Recent in vitro studies have revealedhat endothelial cell populations are enriched with progen-tor cells, which account for the cells’ growth and angiogen-c/vasculogenic potential (15). Whereas EPCs comprisenly 5% to 10% of the pulmonary artery endothelial cellolony, nearly 50% of microvascular endothelial cells areade up of progenitor cells. EPCs may play a key role in

ascular development, maintenance in the post-natal period,nd repair following injury. However, progenitor cells thateside within the vascular wall may also play unappreciatedoles in vascular disease.

Idiopathic pulmonary arterial hypertension (IPAH) is arominently pre-capillary disease that is characterized by

arge and intermediate-sized pulmonary arteries/arteriolesn which there is intimal hyperplasia with medial anddventitial hypertrophy and hyperplasia. In advanced stagesf PH, cells originating within the vessel wall (smoothuscle cells, endothelial cells, and fibroblasts), and poten-

ially cells from the circulation, assemble in a “plexiform”esion (16–18). Disordered endothelial cell growth haseen documented in patients with IPAH, even in pul-onary artery endothelial cells that are isolated from the

atients and grown in culture (16). These findingsupport the idea that endothelial cells acquire a pro-roliferative, apoptotic-resistant phenotype that, in thease of the plexiform lesion, contributes to the loss of the

ndothelial cell monolayer (17). b

We have only a rudimentary understanding of howndothelia contribute to the vascular pathology in IPAH.t present, the origin of endothelial cells within thelexiform lesion is not resolved; it may involve thearticipation of large or small pulmonary artery endothe-

ial cells, with variable contribution of bone marrowrecursors. However, cells within the lesion interact withriffonia simplicifolia, consistent with a microvascularhenotype. It is not clear whether PH selects for progen-tor cells within the vessel wall and whether cells con-ributing to either the intimal or plexiform lesion repre-ent an overgrowth of EPCs. It is also not clear whetherpigenetic modifications or somatic mutations contributeo the uncontrolled growth of endothelial cells in IPAHatients. Indeed, considerable work is needed to addresshese and related concerns regarding fundamental endo-helial cell biology in IPAH.

In summary, questions that remain to be answered are:) What are the molecular determinants of an EPC?) What is the unique cell biology of an EPC? 3) Is there anncreased propensity for progenitor cells to cause an endo-helial lesion? 4) Do hyperproliferative endothelial cells arisey somatic mutations, epigenetic modifications, or selectionf progenitor cells (e.g., apoptosis resistance)? 5) Where arehe EPCs located in vivo, and which signals activate theirrowth?

ole of Bone Marrow Cells inulmonary Vascular Structure and Remodeling

he term “bone marrow-derived cells” characterizes a wideariety of cell populations that differ with respect to theiriological characteristics, expression of marker molecules,nd biological functions. These cells range from macro-hages to inflammatory cells, EPCs, and fibrocytes.

hereas some of these cell populations may representargets for potential treatments (e.g., macrophages, inflam-atory cells, and fibrocytes), others, such as EPCs, might be

tilized as therapeutic agents. However, the rationale for theherapeutic use of such cells is unclear, and the evidence foreneficial effects is still limited. In general, it is accepted thatirculating EPCs and fibrocytes play important roles inngiogenesis and vascular remodeling (19,20). It should beointed out, however, that the literature is replete withonflicting data reporting significant differences in theontribution of EPCs to neoangiogenesis. Since EPCs wererst described (21), their identity and relative contributiono neovasculature formation have remained controversial.onflicting reports of the extent of the contribution of these

ells to new blood vessel formation can be ascribed to aimited analysis of the EPC phenotype in each study and aack of more definitive methods for distinguishing vessel-ncorporated bone marrow-derived endothelial cells andntimately associated perivascular cells. Yet another sourcef variability may result from the specific disease processes

eing investigated (tumor neovascularization, hypoxia-

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S6 Tuder et al. JACC Vol. 54, No. 1, Suppl S, 2009Development and Pathology June 30, 2009:S3–9

nduced neovascularization, PAH, cardiac ischemia, limbschemia, and others).

The incorporation of endothelial or smooth muscle cellsrom bone marrow into the growing, adult, or aging lungsppears to be negligible (22). Circulating fibrocytes, hybridells expressing myeloid and fibroblast markers, may beecruited from the bloodstream to promote tissue remodel-ng during organ and vascular fibrosis (19,23). These fibro-ytes may represent a new target to prevent tissue remod-ling, but their precise role in the pathogenesis of PAHemains unclear. It will be interesting to determine whetherPCs contribute to this process via integration into the

ndothelium and whether the same process also occurs inhe lung. The finding that TGF-�1 induces endothelialells to undergo endothelial-to-mesenchymal transforma-ion, whereas BMP-7 has been found to preserve thendothelial phenotype, may offer an interesting opportunityo intervene in this process.

There is still uncertainty about the therapeutic andathogenetic impact of bone marrow cells in PH. Thera-eutic infusion of in vitro cultured and eNOS-transfectedPCs increased survival and reduced right ventricular pres-

ure and hypertrophy in rats with monocrotaline-inducedH (24). In mice, bone marrow injection attenuatedonocrotaline-induced PH but aggravated chronic hypoxicH (25). An open, placebo-controlled pilot study on theffect of autologous EPC infusion in IPAH reported anmprovement in exercise capacity and hemodynamics (26).

owever, compelling evidence is still lacking that autologousPC infusion has decisive effects in the development of PH.ore studies are required to come to definitive conclusions and

o elucidate pathogenetic mechanisms that might explain theffects of circulating bone marrow-derived cells.

In summary, there is a need to clarify: 1) whether bonearrow and mononuclear cells are of potential therapeu-

ic value with respect to pathogenic mechanisms andlinical efficacy; 2) whether improvement of cell-basedherapies will have an impact on clinical outcome;) whether the extent of the contribution of EPCs fromifferent organs plays a role in the pathogenesis of PH;) whether the contribution of different types of EPCsffects the pathogenesis of PH; and 5) whether there isndeed a relatively low-level contribution of bone marrow-erived EPCs and potentially of smooth muscle cell precursors

n pulmonary hypertensive vasculopathy, which emphasizes theequirement for accurate quantitative assessments. All clinicaltudies should be performed using precisely defined precursorell populations that have been isolated according to acceptednd reproducible protocols.

ellular and Molecular Underpinningsf Large Pulmonary Arteries andicrocirculation in Thromboembolic Disease

hronic thromboembolic pulmonary hypertension (CTEPH)

s a rare disease that is estimated to result in approximately a

.8% of all cases of acute pulmonary embolism (27). Severalechanisms have been postulated to cause CTEPH after an

cute embolic event, including recurrence of embolism in 2.5%o 7% of adequately treated pulmonary embolic events, in situhrombus propagation into branch pulmonary vessels, andailure to resolve the initial embolus, leading to large- andmall-vessel vasculopathy (28). In the proximal pulmonaryrterial tree, unresolved pulmonary emboli cause vascular ob-truction of the vessel lumen by 2 mechanisms: 1) directcclusion of the vessel lumen; and 2) induction of secondaryndothelial changes of cellular hyperplasia, webbing, and in-omplete clot remodeling. In a subset of patients withTEPH, small pulmonary arterioles also manifest a pathologicrocess similar to that seen in PAH, whereby these vesselsecome excessively thickened by muscular hypertrophy andbrointimal hyperplasia, leading to eventual occlusion (29,30).he importance of pulmonary arteriolar/capillary remodeling

n the development of CTEPH is supported by the followingacts: 1) there is lack of correlation between elevated pulmonaryrterial pressure and the degree of angiographic pulmonaryascular bed obstruction in humans; 2) PH can progress in thebsence of recurrent venous thromboembolism; and 3) totalulmonary vascular resistance is still significantly higher inTEPH patients than in acute pulmonary embolism patientsith a similar percentage of vascular bed obstruction (31,32).In addition to physically occluding the vessel lumen,

hrombi act as a physical trap for circulating mitogenic,nflammatory, and vasoreactive factors, many of which cannterfere with or alter the function of the adjacent endothe-ium. Endothelial barrier dysfunction, leading to increasedndothelial permeability, is evident in response to baro-rauma, inflammation, acute lung injury, and clot retention33). Changes in shear stress due to vascular occlusion maylso influence endothelial cell function and production ofasoreactive and mitogenic factors. Because platelets are aource of numerous inflammatory, vasoactive, mitogenic,nd chemotactic factors (e.g., thromboxane A2, serotonin,latelet-activating factor, angiopoietin-1), their aggregationn the thrombus should influence endothelial function. Inummary, a retained clot in the pulmonary vascular tree ishought to be an instigator of endothelial permeability,esulting in access of growth factors, cytokines, mitogens,nd vasoreactive factors to both pulmonary artery endothe-ial cells and pulmonary artery smooth muscle cells. There islso preliminary evidence that thromboemboli in the prox-mal pulmonary arterial tree contains EPCs that migratento the vessel wall and contribute to the remodeling process34).

Although the clinical characteristics of CTEPH haveeen well defined, understanding of its cellular and molec-lar mechanisms is lacking. The prevailing opinion is thatH resulting from chronic thromboembolism is a conse-uence of unresolved pulmonary emboli with adjacentascular remodeling as well as a secondary vasculopathy inmall pulmonary arterioles and capillaries. Mechanistic par-

llels with other forms of PH (e.g., IPAH) have been

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S7JACC Vol. 54, No. 1, Suppl S, 2009 Tuder et al.June 30, 2009:S3–9 Development and Pathology

rawn, particularly in light of similar histopathology inome distal arteriolar beds (35). Many questions remain, theost fundamental of which deal with elucidation of risk

actors and pathogenesis of CTEPH. Future researchhould answer several questions, including: 1) why mostatients with pulmonary embolism do not developTEPH; 2) why some individuals fail to resolve acute

hromboembolic obstruction of the pulmonary vascular tree;) why some CTEPH patients have postoperative residualH; and 4) whether patients with unsatisfactory postoper-tive outcomes have cellular, molecular, and genetic abnor-alities in the pulmonary vasculature similar to those in

atients with IPAH. Furthermore, investigation is neededo define the role of: 1) inherited defects in coagulation,ncomplete/defective fibrinolytic pathways; 2) pulmonaryrtery endothelium contributing to the intimal changes thatccur adjacent to a thromboembolism in main, lobar, andegmental pulmonary arteries; 3) infiltrating cell types fromhe thromboemboli; cellular proliferation of fibromyocyte,yocyte, and endothelial subtypes within the vessel wall; 4)

one marrow progenitors as either bystanders or activeemodeling agents; and 5) bone morphogenetic proteineceptor (BMPR)-1A and angiopoeitin-1/TIE-2 signalingn CTEPH. From a scientific and diagnostic point of view,here is need: 1) to develop an animal model for CTEPH;) to design better diagnostic modalities to distinguishTEPH from IPAH; and 3) to study signaling pathways

nd patterns of gene expression and understand theirotential clinical significance in the treatment of patientsefore and after pulmonary endarterectomy.

athology of PH: Evian, Venice, and Beyond

he pathologic interpretation of pulmonary vascular remod-ling in PH has been central in studies related to the disease.s outlined by Zaiman et al. (36), the importance ofathology in the clinical management or even the diagnosisf PH has fallen behind the direct assessment of pulmonaryrtery pressures using pulmonary arterial catheterization orstimation of right ventricular pressures using echocardiog-aphy. Infrequently, lung biopsies have served as a basis forhe diagnosis of PH. Furthermore, the importance of properdentification of the different forms of pulmonary vascularemodeling, or so-called pulmonary vascular lesions, intudies of the pathogenesis of PH and the effects ofotential treatments cannot be underestimated. Key ques-ions concern the role of pathology of PH in the near future.

hich recommendations regarding pathology of PH willest serve clinicians and, ultimately, patients? Is there alassification that should be added to or substituted forrior classifications?To answer these questions, it is useful to review the

ifferent classifications used for the last 50 years. Theseriginated from international conferences, that is, the 1973

orld Health Organization (WHO) symposium, the 1998

vian, France, international meeting, and the 2003 inter- n

ational meeting held in Venice, Italy. The 1973 WHOymposium stressed the contribution of pulmonary vascularathology in the classification of the disease, an emphasishat was eventually superseded by developments in hemo-ynamic assessment and novel clinical algorithms. Thevian meeting recommended a descriptive approach, basedn acknowledged limitations in interpreting and correlatingathology with specific causes, severity grades, and out-omes (37). This descriptive approach emphasized a com-ination of the classic microanatomic and lesional nomen-lature with the underlying cellular components thatharacterize the lesion. It therefore followed the recommen-ation that lesional descriptors, such as eccentric or concen-ric, plexiform or dilation lesions, be identified based onheir location in the intimal, medial, and/or adventitialegions. An attempt to complement this approach with oneased on the recognition of particular cells involved in thetructure of a lesion implied that investigation of theaseline abnormalities of pulmonary vascular cells mightffer an insight into the underlying diagnosis and potentialherapeutic responses of different forms of PH. This ap-roach attempted to incorporate the use of immunohisto-hemical markers and opened the possibility that functionalarkers related to disease pathogenesis might serve as

otential tools for enhancing the importance of pathology inhe diagnosis and prognosis of PH. As these dysfunctionsre segregated by vascular cell types, most prominentlyndothelial (38) or smooth muscle cells, the application ofhese tools is expected to provide a much-needed insertionf pathology into the clinical workup of patients with PH.he identification of mutations of receptors in the TGF-�

amily, particularly of BMPR-2, further highlights theverall need for such an approach.The Venice symposium (39) made several recommenda-

ions, the intent of which was to “provide a more descriptivepproach to . . . the main vascular changes and the associ-ted pathological alterations.” The report discussed theescriptions of intimal, medial, and adventitial thickening.he intimal lesions were segregated into concentric cellular,

oncentric acellular, and eccentric lesions. Complex vascularesions were categorized as an individual category of lesions.hese included plexiform and dilation lesions, arteritis,cclusive venous thrombotic lesions, and pulmonary micro-asculopathy (also known as pulmonary alveolar capillaryemangiomatosis). Given the common denominator ofMPR-2 mutations in some forms of pulmonary veno-cclusive disease and pulmonary microvasculopathy, theecommendations leave open a potential continuum be-ween pre-capillary PH and venous disease associatedith PH.In the present discussions, it was agreed that pathology

hould have a primary role in documenting the types andxtent of vascular lesions and associated morphologic alter-tions in lung tissue from patients and animals with PH.his systematic approach should serve to correlate pulmo-

ary vascular pathology with clinical presentation/outcome.

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systematic approach based a multilevel analysis should beutlined, primarily at the examination of hematoxylin– andosin–stained slides, further supported by cell structuralmmunohistochemical markers, and potentially in the fu-ure, complemented by the detection of pathobiologicallyelevant markers. These recommendation are very much inine with those made in the Evian meeting (37).

ulmonary Vascular Pathology:ecommendation on Reporting andxamination of Pulmonary Venopathy

he proper interpretation of pulmonary vascular remodelingequires that the pulmonary vessels be correctly identified,ith determination of topography of the vascular lesions.rteries have to be named according to their location and

ccompanying airway structure, with approximate estima-ion of diameter. For example, pulmonary arteries can beroperly labeled as intralobular arteries if they are seenccompanying terminal bronchioles. A similar approach cane applied to veins, including their location in the lobuleintralobular), or in pre-septal or intraseptal locations. Pul-onary arterial lesions may involve isolated medial hyper-

rophy, medial hypertrophy and intimal thickening, concen-ric lamellar, eccentric, concentric nonlaminar, complexesions with plexiform parts, and arteritis. Plexiform/omplex lesions are usually similar in the lungs of individualatients. Plexiform lesions with venous and capillaryhanges may coexist in similar parts of the lung. Vascularhanges can be segmental.

Coexisting venous-venular changes should be noted. Thesenclude several potential occlusive lesions, such as intimalhickening/obstruction (fibrosis, cells), luminal septa, recanali-ation, adventitial thickening, muscularization, iron and cal-ium incrustation, and foreign body reaction. Associated cap-llary changes may be present with multiplication androliferation with variable dilatation. Angioma-like lesions andrimary capillary angiomatosis may be present. Most if not allases have associated variable arterial changes. Other changesnclude dilated lymphatics, hemosiderin-laden alveolar macro-hages, and type II cell hyperplasia.In addition to an extensive literature on the arterial

ndings in PH (40,41), there is increasing awareness ofompromise of the venous circulation in PAH. Pulmonaryeno-occlusive disease (PVOD) now belongs together withulmonary capillary hemangiomatosis (PCH) to the PAHroup, with predominance of vascular disease at the post-apillary level of the pulmonary vasculature. There is aossible overlap between PVOD and cases of PCH, aisease classically characterized by an aggressive, patchlike,apillary angioproliferation, as outlined in a report of 35ases of PVOD and PCH sharing similar histologic patterns42). The observed post-capillary lesions involve septal veinsnd pre-septal venules and frequently consist of a loose,brous remodelling of the intima that may totally occlude

he lumen. The involvement of pre-septal venules should be

onsidered as necessary for the histologic diagnosis ofVOD, as fibrous occlusion of large septal veins may beeen in many forms of pulmonary venous hypertension,ncluding a frequently reported obstruction of large pulmo-ary veins following catheter ablation for cardiac atrialbrillation (43).

issue Banking for Pulmonary Vascular Research

urrently, there are some initiatives to bank human lung tissueor pulmonary vascular research. In North America, the Car-iovascular Medical Research and Educational Fund has set upmulti-institutional effort to collect diseased lung tissue for

esearch in IPAH, under the Pulmonary Hypertension Break-hrough Initiative (44,45). This network interfaces multipleransplant centers with tissue processing sites, genomics, cellrocessing, and proteomics centers. The process of organiza-ion and operational infrastructure has proved to be complexut, since the network has succeeded in its goals, it provides anxample that can be followed in the future. Furthermore, it hasighlighted the complexities of setting up a bank of tissue,

ncluding delicate regulatory issues that have to be methodi-ally addressed prior to setting up this type of enterprise. Athis time, local or regional tissue banks involving variousnstitutions may be more feasible. However, exchange ofaraffin blocks of diseased lungs with PH among centers thattudy the disease is highly desirable. More importantly, thisffort may offer a unique opportunity to better define theatural history of IPAH and potentially to identify earlyascular lesions indicative of the disease.

uthor Disclosures

r. Tuder has received grant support from the Cardiovas-ular Medical Research Fund and the National Heart,ung, and Blood Institute. Dr. Abman has received re-

earch grant support from Bayer HealthCare. Dr. Thistle-hwaite has received grant support from the Nationalnstitutes of Health and the Center for Medical Researchnd Education Fund. Dr. Haworth has received educationalrant support and consulting fees from Actelion, Pfizer, andlaxoSmithKline. Drs. Braun, Capron, and Stevens report

o conflicts of interest.

eprint requests and correspondence: Dr. Rubin M. Tuder,rogram in Translational Lung Research, Division of Pulmonaryciences and Critical Care Medicine, Department of Medicine,niversity of Colorado at Denver, School of Medicine, Anschutzedical Campus Research 2, Room 9001, 12700 East 19thvenue, Aurora, Colorado 80045. E-mail: Rubin.Tuder@Cdenver.edu.

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ey Words: pulmonary hypertension y pulmonary circulation y

ngiogenesis y endothelial cell progenitor y pathology.