Pulmonary arterial hypertension is a progressive, fatal disease driven by pathologic vascular remodeling including arterial medial hypertrophy, occlusive neointimal lesion formation, and venous muscularization. Current vasodilatory therapies improve hemodynamics but do not reverse established remodeling. Imatinib mesylate, a tyrosine kinase inhibitor targeting the PDGF-PDGFR signaling axis, has been proposed as an anti-remodeling therapy for pulmonary arterial hypertension and has demonstrated hemodynamic benefit in both preclinical models and clinical trials. However, prior preclinical models lack the neointimal lesions characteristic of human disease, effects on venous remodeling have not been examined, and direct histologic assessment in human trials is precluded by the invasiveness of serial lung biopsy. Here, leveraging the house dust mite mouse model of pulmonary hypertension, which recapitulates medial thickening, neointimal lesion formation, and venous muscularization, we rigorously evaluate the anti-remodeling and hemodynamic effects of imatinib during two defined remodeling stages: neointimal lesion growth and neointimal lesion maintenance. Imatinib treatment significantly reduced right ventricular systolic pressure at both stages. Despite this hemodynamic improvement, quantitative vessel-level analysis of over 1,700 arteries and 1,200 veins revealed no significant effect of imatinib on arterial medial thickness, neointimal lesion growth, neointimal lesion maintenance, or venous muscularization across any vessel size class. These findings dissociate imatinib's hemodynamic benefit from structural vascular remodeling and suggest that imatinib functions primarily as a pulmonary vasodilator rather than an anti-remodeling agent.

Human lung development is governed by complex gene regulatory networks that orchestrate cellular differentiation and organogenesis. We present a single cell multiomic atlas of human pulmogenesis, simultaneously capturing both the chromatin accessibility profile and the transcriptome from each cell across fetal lungs spanning from post-conception weeks (PCW) 12 to 23. We identified 44 distinct developing cell clusters and mapped 581,745 candidate cis-regulatory elements and nominated 121,486 non-redundant peak-to-gene linkages. We identify highly regulated genes (HRGs) and the cognate highly regulating peaks (HRPs) that describe the most salient regulatory gene programs and developmental enhancer sites for each cell type. Trajectory analysis along with interpretable cell type specific convolutional neural network models were developed to delineate dynamic regulatory programs driving key developmental transitions, including aerocyte and arterial differentiation and alveolar formation. Furthermore, we identified distinct vascular smooth muscle subpopulations with unique spatial associations to either arterial or venous structures with reciprocal signaling within each niche. We also uncovered the regulatory modules of surfactant production in alveolar progenitors, implicating a direct role for the glucocorticoid receptor alongside novel transcription factors. Finally, using cell type specific models linking DNA sequence to chromatin accessibility we prioritize variants associated with impaired pulmonary function or disease and nominate mechanisms of motif disruption. Overall, our multiomic atlas deepens our understanding of the gene-regulatory architecture underlying human lung development and provides a valuable resource for the community to dissect the cellular and molecular programs of pulmonary physiology and disease at the cellular and nucleotide precision. ### Competing Interest Statement W.J.G. is a consultant and equity holder for 10x Genomics, Guardant Health, Quantapore, and Ultima Genomics and cofounder of Protillion Biosciences and is named on patents describing ATAC-seq. All other authors declare no competing interests. Arc Research Institute, https://ror.org/00wra1b14 Chan Zuckerberg Biohub San Francisco, https://ror.org/00knt4f32 Maternal and Child Health Research Institute Paul & Daisy Soros Fellowships for New Americans, https://ror.org/02320dz84

PURPOSE OF REVIEW:Pulmonary vascular disease is more common in certain genetic developmental lung disorders. This review synthesizes clinical descriptions, molecular analyses, and single-cell transcriptional data to build a conceptual framework to help understand why some variants affect the vasculature while others primarily manifest with parenchymal disease. RECENT FINDINGS:Genes predominantly expressed in endothelial and mesenchymal compartments ( TBX4 , FGF10 , FOXF1 , KDR ) commonly present with both parenchymal and pulmonary vascular disease, while epithelial-restricted genes ( SFTPC , ABCA3 , NKX2.1 ) typically manifest as parenchymal disease. Single-cell analyses reveal that compartment-specific expression patterns correlate with clinical phenotypes. Phenotypic variability, even among individuals sharing identical variants, suggests complex interactions between genetic modifiers, epigenetic factors, and developmental processes that remain poorly understood. SUMMARY:Compartment-specific gene expression patterns fundamentally underlie the differential presence of vascular phenotypes in DEVLDs. Genetic advances and single cell technologies have revolutionized our understanding of these disorders, but we are in the early stages of translating this knowledge into meaningful clinical advances. Future efforts must bridge this gap to transform clinical care from supportive to targeted, disease-modifying treatment based on cell-specific molecular mechanisms.

HomeCirculationVol. 146, No. 5von Willebrand Factor Is Produced Exclusively by Endothelium, Not Neointima, in Occlusive Vascular Lesions in Both Pulmonary Hypertension and Atherosclerosis Free AccessLetterPDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citationsPermissions ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toFree AccessLetterPDF/EPUBvon Willebrand Factor Is Produced Exclusively by Endothelium, Not Neointima, in Occlusive Vascular Lesions in Both Pulmonary Hypertension and Atherosclerosis Lea C. Steffes, MD, Paul Cheng, MD, Thomas Quertermous, MD and Maya E. Kumar, PhD Lea C. SteffesLea C. Steffes Department of Pediatrics, Division of Pulmonary Medicine (L.C.S., M.E.K.), Stanford University School of Medicine. , Paul ChengPaul Cheng Department of Medicine, Division of Cardiovascular Medicine (P.C., T.Q.), Stanford University School of Medicine. , Thomas QuertermousThomas Quertermous https://orcid.org/0000-0002-7645-9067 Department of Medicine, Division of Cardiovascular Medicine (P.C., T.Q.), Stanford University School of Medicine. and Maya E. KumarMaya E. Kumar Correspondence to: Maya E. Kumar, PhD, 240 Pasteur Dr, Biomedical Innovations Building, Rm 2300, Stanford University School of Medicine, Stanford, CA 94305. Email E-mail Address: [email protected] https://orcid.org/0000-0001-6754-5660 Department of Pediatrics, Division of Pulmonary Medicine (L.C.S., M.E.K.), Stanford University School of Medicine. Originally published1 Aug 2022https://doi.org/10.1161/CIRCULATIONAHA.121.058427Circulation. 2022;146:429–431vWF (von Willebrand Factor) is a large, secreted, multimeric glycoprotein transcribed by vascular endothelial cells and, to a lesser extent, megakaryocytes throughout the body. vWF is an integral part of the coagulation cascade and is important in hemostasis, vessel wall homeostasis, and repair after injury. After endothelial insult, including increased shear stress or inflammation, vWF is released from storage vesicles within endothelial cells and is both secreted directly into circulation and maintained locally, binding to extracellular matrix (ECM) components in the area of vessel injury, resulting in local platelet aggregation.1 Given its role in vascular injury and repair, vWF has been implicated as a biomarker of endothelial dysfunction, with elevated plasma levels of vWF identified in multiple vascular disease processes, including atherosclerosis, cerebral vascular disease, and pulmonary hypertension (PH), with a direct correlation between increasing vWF levels and risk of adverse events and death.2Pulmonary vascular remodeling, including the formation of occlusive neointimal lesions throughout the pulmonary vascular bed, is a hallmark of advanced PH, and the cellular origin of these lesions in human disease is a topic of ongoing debate. Neointimal lesions are composed of longitudinally oriented mesenchymal cells located directly beneath the artery endothelium that grow to occlude the vessel lumen and are characterized by damaged elastic laminae and increased ECM deposition.3 Endothelial to mesenchymal transition, where endothelial cells undergo a phenotypic and transcriptional conversion to a mesenchymal cell type, has been proposed as a mechanism by which neointimal cells are generated in PH. Colocalization of vWF protein and other canonical endothelial cell markers, with neointimal cell markers (ie, Acta2) by immunohistochemistry within neointimal lesions from patient tissue and animal models, has been cited as evidence of an endothelial origin for neointimal cells through endothelial to mesenchymal transition.4 The origin of neointima cells in humans has yet to be definitively determined. We recently demonstrated in a mouse model of PH that neointima arises not from the endothelium but rather from Notch3-expressing smooth muscle cells of the arterial media, leading us to reexamine the use of the endothelial protein vWF as evidence of the neointima's endothelial origin.5 Because (1) vWF is an ECM-binding secreted molecule, (2) ECM is abundant within neointimal lesions, and (3) endothelial damage and dysfunction, including vWF secretion, is a feature of PH,3 the presence of vWF protein in neointimal lesions does not necessarily mean that neointima cells themselves are producing vWF but rather may reflect binding of vWF protein released by neighboring endothelium. To resolve this controversy, we set out to characterize VWF/Vwf transcription and protein localization within neointimal lesions of both patients with PH and a range of established animal models of PH. We then performed a similar analysis of aortic root neointimal lesions in the ApoE–/– atherosclerosis mouse model to see whether vWF localization and Vwf transcription in systemic arteries parallels that seen in the pulmonary circulation.Using recently developed multiplexed in situ hybridization techniques with single-cell resolution, we show that VWF/Vwf is exclusively transcribed by vascular endothelial cells within remodeled pulmonary arteries (Figure) in tissue isolated from human PH, Sugen/Hypoxia-exposed rat, the endothelial deletion of PHD2 mouse model of PH, and the inflammation-driven house dust mite mouse model of PH with no evidence of transcription in Acta2-positive neointima cells (Figure [B–D, F, and G]). In contrast, vWF protein is detected by immunohistochemistry in both endothelial cells and Acta2-positive neointimal lesions from human and mouse PH tissue (Figure [A and E]). The same is found in atherosclerosis neointimal lesions (Figure [H and I]), suggesting that this pattern of vWF localization and VWF/Vwf transcription is a general characteristic of neointimal lesions in both the systemic and pulmonary vascular beds. This supports a model (Figure [J]) in which vWF protein is produced exclusively in endothelial cells that go on to secrete the protein in the setting of endothelial dysfunction.3 Secreted vWF then binds to the abundant ECM surrounding neointimal cells, making neointimal lesions falsely appear to share Vwf expression with endothelial cells. These findings highlight that in situ vWF antibody staining should not be used as evidence of endothelial to mesenchymal transition and, more generally, that caution is warranted when inferring cellular lineage from canonical marker expression. The cellular origin of the neointima in human PH remains undetermined.Download figureDownload PowerPointFigure. von Willebrand Factor is produced exclusively by endothelium in neointimal lesions of both pulmonary arteries and atherosclerosis aortic root. vWF protein (red) is detected in both the endothelium (white) and neointima (green) of pulmonary arteries with neointimal lesions (bounded by yellow dashed lines) in rodent models of PH (A), human pulmonary arterial hypertension (PAH, E), and in the aortic root of the ApoE–/- mouse model of atherosclerosis (H) by immunohistochemistry. Neointimal vWF protein is indicated by white arrowheads. In situ hybridization identifies transcripts of the VWF/Vwf gene (red) only in endothelial cells (CLDN5/Cldn5 mRNA, white) and not neointima cells (green) in multiple animal models of PH (B, HDM mouse; C, Sugen-hypoxia (Su/Hx) rat; D, Tie2-Cre PHD2 mouse), human PAH pulmonary arteries (F and G), and aortic root atherosclerosis (I). The diameters of pulmonary arteries evaluated for mouse and rat were 20 to 150 µm, and human 50 to 200 µm. J, Schematic of artery cross section with neointimal lesion showing proposed model in which vWF protein (red) is secreted from endothelial cells and bound to neointimal ECM. Neointima cells and smooth muscle media, green; endothelium, red; elastic laminae, heavy gray lines. The atherosclerosis mouse model is a constitutive knockout of ApoE transitioned onto a high-fat (Western) diet at 8 weeks old that was then sustained for 16 weeks. Neointima and vascular smooth muscle cells identified by antibody staining for Acta2/ACTA2 (clone 1A4; Sigma F3777) for A through G and by Myh11-CreER;tdTomato lineage trace for H and I. Endothelium in A and H identified by antibody staining for CD31 (BD Pharmingen, 550274) and in E for VE-cadherin (R&D Systems, AF938). vWF antibody in A, E, and H from AbCam (ab6994). In situ probes (all RNAscope from ACD): human VWF (560461), human CLDN5 (517141), mouse Vwf (858851), mouse Cldn5 (491611), rat Vwf (413401), and rat Cldn5 (1055891). DAPI indicates 4′,6-diamidino-2-phenylindole; ECM, extracellular matrix; HDM, house dust mite; PH, pulmonary hypertension; PHD2, prolyl hydroxylase domain protein 2; and vWF, von Willebrand Factor.All animal experiments received approval from the Stanford University Institutional Animal Care and Use Committee (institutional review board 31869, 27636, and 10054). Patient tissues were obtained under a protocol approved by Stanford University's Human Subjects Research Compliance Office (institutional review board 54182), and informed consent was obtained from each patient before surgery. All experiments followed applicable regulations and guidelines. All image data, analytic methods, and study materials will be made available to other researchers on request.Article InformationAcknowledgmentsDrs Steffes and Kumar conceived, performed, and interpreted experiments, wrote the manuscript, and secured funding. Drs Cheng and Quertermous provided expertise and experimental samples, interpreted experiments, and reviewed and edited the manuscript. The authors wish to thank R. Metzger for providing PHD2 (prolyl hydroxylase domain protein 2) mouse tissue, E. Spiekerkoetter for providing Su/Hx rat tissue, A. Andruska for comments on the manuscript, and Stanford Medicine's Animal Histology Service for paraffin sample preparation.Sources of FundingThis work was supported by Stanford Maternal & Child Health Research Institute Ernest & Amelia Gallo Endowed Postdoctoral Fellowship, National Institutes of Health (NIH) 5T32HL129970-03 and a Parker B. Francis Fellowship to Dr Steffes; NIH K08HL153798 and American Heart Association 20CDA35310303 to Dr Cheng; R01HL134817, R01HL139478, R01HL156846, R01HL151535, R01HL145708, and UM1 HG011972 to Dr Quertermous; and NIH R01HL163013, American Heart Association 16SDG30030006, Stanford Spectrum-Child Health Research Institute seed grants, Bravo Family endowed faculty scholarship, and a Vera Moulton Wall Center for Pulmonary Vascular Disease research grant to Dr Kumar.Nonstandard Abbreviations and AcronymsECMextracellular matrixPHpulmonary hypertensionvWFvon Willebrand FactorDisclosures None.FootnotesCirculation is available at www.ahajournals.org/journal/circFor Sources of Funding and Disclosures, see page 431.Correspondence to: Maya E. Kumar, PhD, 240 Pasteur Dr, Biomedical Innovations Building, Rm 2300, Stanford University School of Medicine, Stanford, CA 94305. Email [email protected]eduReferences1. Wagner DD. Cell biology of von Willebrand factor.Annu Rev Cell Biol. 1990; 6:217–246. doi: 10.1146/annurev.cb.06.110190.001245CrossrefMedlineGoogle Scholar2. Lip G, Blann A. von Willebrand factor: a marker of endothelial dysfunction in vascular disorders?Cardiovasc Res. 1997; 34:255–265. doi: 10.1016/s0008-6363(97)00039-4CrossrefMedlineGoogle Scholar3. Humbert M, Guignabert C, Bonnet S, Dorfmüller P, Klinger JR, Nicolls MR, Olschewski AJ, Pullamsetti SS, Schermuly RT, Stenmark KR, et al. Pathology and pathobiology of pulmonary hypertension: state of the art and research perspectives.Eur Respir J. 2019; 53:1801887. doi: 10.1183/13993003.01887-2018CrossrefMedlineGoogle Scholar4. Good RB, Gilbane AJ, Trinder SL, Denton CP, Coghlan G, Abraham DJ, Holmes AM. Endothelial to mesenchymal transition contributes to endothelial dysfunction in pulmonary arterial hypertension.Am J Pathol. 2015; 185:1850–1858. doi: 10.1016/j.ajpath.2015.03.019CrossrefMedlineGoogle Scholar5. Steffes LC, Froistad AA, Andruska A, Boehm M, McGlynn M, Zhang F, Zhang W, Hou D, Tian X, Miquerol L, et al. A Notch3-marked subpopulation of vascular smooth muscle cells is the cell of origin for occlusive pulmonary vascular lesions.Circulation. 2020; 142:1545–1561. doi: 10.1161/circulationaha.120.045750LinkGoogle Scholar Previous Back to top Next FiguresReferencesRelatedDetails August 2, 2022Vol 146, Issue 5 Advertisement Article InformationMetrics © 2022 American Heart Association, Inc.https://doi.org/10.1161/CIRCULATIONAHA.121.058427PMID: 35914017 Originally publishedAugust 1, 2022 Keywordsmodels, animalvascular remodelinghypertension, pulmonaryvon Willebrand FactorPDF download Advertisement SubjectsAnimal Models of Human DiseaseAtherosclerosisBasic Science ResearchPulmonary BiologyVascular Disease

Pulmonary arterial hypertension (PAH) is a rare, life-limiting disease marked by progressive remodeling of the pulmonary vascular bed that affects patients of all ages (1). Over the past two decades, the advent and increased use of pulmonary-specific vasodilator therapies has improved lung transplant–free survival in children with PAH (2–4). These therapies modulate vasoactive signaling to relax the pulmonary arterial bed through three distinct pathways: the nitric oxide, endothelin, and prostacyclin pathways (5). In recent years, much research has focused on optimization of timing, dosing, and defining the advantages of combination therapy in the treatment of PAH (6). Despite these improved therapeutic opportunities, the timing of drug initiation, transition, dosing, and benefits of additive therapy specific to the pediatric population remains incompletely understood. Credence for early aggressive pulmonary vasodilator therapy in patients with PAH started with clinician experience and observational data but was subsequently propelled by adult studies suggesting beneficial response (7). Over 15 years ago, the small BREATHE-2 (Bosentan Randomized trial of Endothelin Antagonist Therapy for PAH) trial was the first upfront combination randomized clinical trial, using intravenous epoprostenol and bosentan in some subjects and finding a nonsignificant improvement in hemodynamics for those on combination therapy (8). In 2015, the AMBITION (Ambrisentan and Tadalafil in Patients with Pulmonary Arterial Hypertension) trial, a double blinded randomized control trial in adult patients with functionally mild PAH (World Health Organization class I and II), studied the efficacy of monotherapy versus a more aggressive upfront dual therapy regimen with both ambrisentan and tadalafil. Results from the primary and associated studies demonstrated decreased death, disease progression, and hospitalization, and significant improvements in NT-proBNP and 6-minute-walk distance in patients treated with upfront dual therapy compared with either monotherapy alone (9–12). These publications, buoyed by clinical experience, supported the concept that instead of a sequential therapy approach, early synergistic targeting of multiple vasoactive pathways using existing FDA-approved therapeutics improves the care of patients with PAH, even those with mild disease. Subsequent adult studies evaluating the benefits of early implementation of three pulmonary hypertension–specific vasodilators (“upfront triple combination therapy”) suggested that adults with severe PAH showed a better clinical response, including improved hemodynamics, functional status, and longterm outcomes (13, 14). Although this increasingly robust adult data suggests an overall benefit from aggressive early upfront therapywith a two or three drug regimen at time of diagnosis, the optimal approach for the treatment of pediatric PAH remains unclear. Given the relative paucity of pediatric clinical trials in PAH in general, the issue of upfront combination therapy versus a sequential therapy approach has yet to be investigated in a formal randomized clinical trial. This leaves clinicians to approach pediatric care by extrapolating data and experience from adult patients, a process with both risks and benefits (15). Current pediatric guidelines distinguish between low and highrisk disease, suggesting a treatment strategy thatmay result in the avoidance of parental prostacyclin/prostacyclin derivatives (hereafter called “prostanoids”) in patients with less severe disease despite possible functional and survival benefits with early, aggressive treatment (6, 16). In addition, current recommendations lack direction for goal prostanoid dosing and safe parameters for therapy deescalation fromparental to oral/ inhaled prostanoid. Intriguingly, a recent small retrospective observational study of 21 childrenwith severe PAHbyHaarman and colleagues reported improved survival among individuals treatedwith upfront triple combination therapy, but the results were confounded by a large percentage of patients (43%) undergoing Potts shunt during the study period (17). In this issueofAnnalsATS,Douwes and colleagues (pp. 227–237) (18) for thefirst time provide a relatively large scale, international, multiinstitutional retrospective analysis of the long-termoutcomesof childrenwith varying baselinePAHseverity treatedwith intravenous or subcutaneous (IV/SQ)prostanoid therapy andaggressivedual and triple combination therapy.Notably, results demonstrate improved transplant-free survivalwithhigh dose (.25ng/kg/min epoprostenol and approx. 45ng/kg/min treprostinil) and early initiationof prostanoids in addition todual or triple combinationPAHtherapy, including a prostanoid regardless of baselinedisease severity, functional class, age, sex, andpresence of cardiac shunt.The authorsprovide an additional contribution, exploring the predictors for successful transition fromIV/SQ This article is open access and distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives License 4.0. For commercial usage and reprints, please e-mail Diane Gern (dgern@thoracic.org).

Molecular characterization of cell types using single-cell transcriptome sequencing is revolutionizing cell biology and enabling new insights into the physiology of human organs. We created a human reference atlas comprising nearly 500,000 cells from 24 different tissues and organs, many from the same donor. This atlas enabled molecular characterization of more than 400 cell types, their distribution across tissues, and tissue-specific variation in gene expression. Using multiple tissues from a single donor enabled identification of the clonal distribution of T cells between tissues, identification of the tissue-specific mutation rate in B cells, and analysis of the cell cycle state and proliferative potential of shared cell types across tissues. Cell type-specific RNA splicing was discovered and analyzed across tissues within an individual.

Pulmonary hypertension (PH) is a heterogenous and incurable disease marked by varying degrees of pulmonary vascular remodeling. This vascular remodeling, which includes thickening of the smooth muscle layer (an early finding) and formation of occlusive neointimal lesions (a late finding) in the pulmonary arteries, is a major driver of morbidity and mortality in PH. Available PH therapies consist of vasodilators that do not specifically target lesion formation or expansion and neither prevent progression nor reverse disease. This paucity of curative treatments highlights the need for new drug discovery targeting crucial steps of artery remodeling in PH. The cell dynamics and molecular signals driving neointimal lesion formation have been difficult to elucidate as classic mouse models of PH do not develop neointima. Here, we detail the methods to generate a robust and non-genetic mouse model of PH with medial thickening and neointimal lesion formation in the pulmonary arteries, through chronic exposure to an inflammatory stimulus-house dust mite (HDM). This model rapidly generates human-like pulmonary arterial lesions following a reproducible time course, allowing scrutiny of the cellular and molecular mechanisms controlling each stage of artery remodeling. Further, we outline optimal tissue handling, sectioning, and staining methodologies for detailed quantitative analysis of artery medial thickening and neointimal lesion formation and expansion. Finally, we present a method for staged pharmacologic intervention to identify molecules and pathways required at each step of the pulmonary arterial remodeling process. The advantages of this mouse model of PH over currently available animal models are five-fold. (i) It allows the use of the full range of genetic and single cell tools available in mice to manipulate and study the process of vascular remodeling seen in human disease, including the formation of neointimal lesions in a controlled and cell specific manner. (ii) The vascular lesions develop in a stereotyped manner with predictable timing, allowing for pharmacologic manipulation at discrete stages of vessel remodeling. (iii) It is rapid, with development of PH and vascular remodeling in a timeframe of two to eight weeks. (iv) It uses simple techniques and requires neither surgery, unusual equipment, or extensive personnel training. (v) The staining and quantitation methodologies we present are a significant improvement over those currently in use in the field. We hope that dissemination of this model and the associated detailed methods will speed up the development of novel and more effective PH therapeutics. Graphic abstract: Chronic perivascular inflammation induces medial thickening and neointima formation in pulmonary arteries, following a stereotyped time course, and allowing staged pharmacologic intervention during specific remodeling events, as well as quantitative assessment of vascular changes.

Purpose of review Pediatric coronavirus disease 2019 (COVID-19) respiratory disease is a distinct entity from adult illness, most notable in its milder phenotype. This review summarizes the current knowledge of the clinical patterns, cellular pathophysiology, and epidemiology of COVID-19 respiratory disease in children with specific attention toward factors that account for the maturation-related differences in disease severity. Recent findings Over the past 14 months, knowledge of the clinical presentation and pathophysiology of COVID-19 pneumonia has rapidly expanded. The decreased disease severity of COVID-19 pneumonia in children was an early observation. Differences in the efficiency of viral cell entry and timing of immune recognition and response between children and adults remain at the center of ongoing research. Summary The clinical spectrum of COVID-19 respiratory disease in children is well defined. The age-related differences protecting children from severe disease and death remain incompletely understood.

ABCA3 deficiency is a rare cause of neonatal respiratory failure. Biallelic complete loss of function variants lead to neonatal demise without lung transplantation, but children with partial function variants have variable outcomes. The favorable clinical course of 3 such infants presenting with respiratory distress at birth is described.

Background: Pulmonary arterial hypertension (PAH) is a fatal disease characterized by profound vascular remodeling in which pulmonary arteries narrow because of medial thickening and occlusion by neointimal lesions, resulting in elevated pulmonary vascular resistance and right heart failure. Therapies targeting the neointima would represent a significant advance in PAH treatment; however, our understanding of the cellular events driving neointima formation, and the molecular pathways that control them, remains limited. Methods: We comprehensively map the stepwise remodeling of pulmonary arteries in a robust, chronic inflammatory mouse model of pulmonary hypertension. This model demonstrates pathological features of the human disease, including increased right ventricular pressures, medial thickening, neointimal lesion formation, elastin breakdown, increased anastomosis within the bronchial circulation, and perivascular inflammation. Using genetic lineage tracing, clonal analysis, multiplexed in situ hybridization, immunostaining, deep confocal imaging, and staged pharmacological inhibition, we define the cell behaviors underlying each stage of vascular remodeling and identify a pathway required for neointima formation. Results: Neointima arises from smooth muscle cells (SMCs) and not endothelium. Medial SMCs proliferate broadly to thicken the media, after which a small number of SMCs are selected to establish the neointima. These neointimal founder cells subsequently undergoing massive clonal expansion to form occlusive neointimal lesions. The normal pulmonary artery SMC population is heterogeneous, and we identify a Notch3-marked minority subset of SMCs as the major neointimal cell of origin. Notch signaling is specifically required for the selection of neointimal founder cells, and Notch inhibition significantly improves pulmonary artery pressure in animals with pulmonary hypertension. Conclusions: This work describes the first nongenetically driven murine model of pulmonary hypertension (PH) that generates robust and diffuse occlusive neointimal lesions across the pulmonary vascular bed and does so in a stereotyped timeframe. We uncover distinct cellular and molecular mechanisms underlying medial thickening and neointima formation and highlight novel transcriptional, behavioral, and pathogenic heterogeneity within pulmonary artery SMCs. In this model, inflammation is sufficient to generate characteristic vascular pathologies and physiological measures of human PAH. We hope that identifying the molecular cues regulating each stage of vascular remodeling will open new avenues for therapeutic advancements in the treatment of PAH.