Summary Idiopathic pulmonary fibrosis (IPF) is a form of progressive interstitial lung disease with unknown etiology. Due to a lack of effective treatment, IPF is associated with a high mortality rate. The hallmark feature of this disease is the accumulation of activated myofibroblasts that excessively deposit extracellular matrix proteins, thus compromising lung architecture and function and hindering gas exchange. Here we investigated the origin of activated myofibroblasts and the molecular mechanisms governing fibrosis formation and resolution. Genetic engineering in mice enables the time-controlled labeling and monitoring of lipogenic or myogenic populations of lung fibroblasts during fibrosis formation and resolution. Our data demonstrate a lipogenic-to-myogenic switch in fibroblastic phenotype during fibrosis formation. Conversely, we observed a myogenic-to-lipogenic switch during fibrosis resolution. Analysis of human lung tissues and primary human lung fibroblasts indicates that this fate switching is involved in IPF pathogenesis, opening potential therapeutic avenues to treat patients.

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Pulmonary vascular remodeling is the main pathological hallmark of pulmonary hypertension disease. We undertook a comprehensive and multilevel approach to investigate the origin of smooth muscle actin‐expressing cells in remodeled vessels. Transgenic mice that allow for specific, inducible, and permanent labeling of endothelial (Cdh5‐tdTomato), smooth muscle (Acta2‐, Myh11‐tdTomato), pericyte (Cspg4‐tdTomato), and fibroblast (Pdgfra‐tdTomato) lineages were used to delineate the cellular origins of pulmonary vascular remodeling. Mapping the fate of major lung resident cell types revealed smooth muscle cells (SMCs) as the predominant source of cells that populate remodeled pulmonary vessels in chronic hypoxia and allergen‐induced murine models. Combining in vivo cell type‐specific, time‐controlled labeling of proliferating cells with a pulmonary artery phenotypic explant assay, we identified proliferation of SMCs as an underlying remodeling pathomechanism. Multicolor immunofluorescence analysis showed a preserved pattern of cell type marker localization in murine and human pulmonary arteries, in both donors and idiopathic pulmonary arterial hypertension (IPAH) patients. Whilst neural glial antigen 2 (chondroitin sulfate proteoglycan 4) labeled mostly vascular supportive cells with partial overlap with SMC markers, PDGFRα‐expressing cells were observed in the perivascular compartment. The luminal vessel side was lined by a single cell layer expressing endothelial markers followed by an adjacent and distinct layer defined by SMC marker expression and pronounced thickening in remodeled vessels. Quantitative flow cytometric analysis of single cell digests of diverse pulmonary artery layers showed the preserved separation into two discrete cell populations expressing either endothelial cell (EC) or SMC markers in human remodeled vessels. Additionally, we found no evidence of overlap between EC and SMC ultrastructural characteristics using electron microscopy in either donor or IPAH arteries. Lineage‐specific marker expression profiles are retained during pulmonary vascular remodeling without any indication of cell type conversion. The expansion of resident SMCs is the major underlying and evolutionarily conserved paradigm of pulmonary vascular disease pathogenesis. © 2018 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Extracellular high-mobility group box-1 (HMGB1) acts as a signalling molecule during inflammation, cell differentiation and angiogenesis. Increased abundance of HMGB1 is associated with several pathological disorders such as cancer, asthma and chronic obstructive pulmonary disease (COPD). In this study, we investigated the relevance of HMGB1 in the pathological remodelling present in patients with idiopathic pulmonary arterial hypertension (IPAH) and pulmonary hypertension (PH) associated with COPD. Remodelled vessels present in COPD with PH and IPAH lung samples were often surrounded by HMGB1-positive cells. Increased HMGB1 serum levels were detected in both patient populations compared to control samples. The effects of physiological HMGB1 concentrations were then examined on cellular responses in vitro. HMGB1 enhanced proliferation of pulmonary arterial smooth muscle cells (PASMC) and primary human arterial endothelial cells (PAEC). HMGB1 stimulated p38, extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase (JNK) phosphorylation. Furthermore, activation of the downstream AP-1 complex proteins c-Fos and c-Jun was observed. Silencing of c-Jun ablated the HMGB1-induced proliferation in PASMC. Thus, an inflammatory component such as HMGB1 can contribute to PASMC and PAEC proliferation and therefore potentially to vascular remodelling and PH pathogenesis.

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Rationale: Remodeling and fibrosis of the right ventricle (RV) may cause RV dysfunction and poor survival in patients with pulmonary hypertension. Objectives: To investigate the consequences of RV fibrosis modulation and the accompanying cellular changes on RV function. Methods: Expression of fibrotic markers was assessed in the RV of patients with pulmonary hypertension, the murine pulmonary artery banding, and rat monocrotaline and Sugen5416/hypoxia models. Invasive hemodynamic and echocardiographic assessment was performed on galectin-3 knockout or inhibitor-treated mice. Measurements and Main Results: Established fibrosis was characterized by marked expression of galectin-3 and an enhanced number of proliferating RV fibroblasts. Galectin-3 genetic and pharmacologic inhibition or antifibrotic treatment with pirfenidone significantly diminished RV fibrosis progression in the pulmonary artery banding model, without improving RV functional parameters. RV fibrotic regions were populated with mesenchymal cells coexpressing vimentin and PDGFRa (platelet-derived growth factor receptor-a), but generally lacked aSMA (a-smooth muscle actin) positivity. Serum levels of galectin-3 were increased in patients with idiopathic pulmonary arterial hypertension but did not correlate with cardiac function. No changes of galectin-3 expression were observed in the lungs. Conclusions: We identified extrapulmonary galectin-3 as an important mediator that drives RV fibrosis in pulmonary hypertension through the expansion of PDGFRa/vimentinexpressing cardiac fibroblasts. However, interventions effectively targeting fibrosis lack significant beneficial effects on RV function.

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Pulmonary fibrosis is characterized by pronounced collagen deposition and myofibroblast expansion, whose origin and plasticity remain elusive. We utilized a fate-mapping approach to investigate α-smooth muscle actin (αSMA)+ and platelet-derived growth factor receptor α (PDGFRα)+ cells in two lung fibrosis models, complemented by cell type-specific next-generation sequencing and investigations on human lungs. Our data revealed that αSMA+ and PDGFRα+ cells mark two distinct mesenchymal lineages with minimal transdifferentiation potential during lung fibrotic remodeling. Parenchymal and perivascular fibrotic regions were populated predominantly with PDGFRα+ cells expressing collagen, while αSMA+ cells in the parenchyma and vessel wall showed variable expression of collagen and the contractile protein desmin. The distinct gene expression profile found in normal conditions was retained during pathologic remodeling. Cumulatively, our findings identify αSMA+ and PDGFRα+ cells as two separate lineages with distinct gene expression profiles in adult lungs. This cellular heterogeneity suggests that anti-fibrotic therapy should target diverse cell populations.