Pericytes and Lung Vascular Remodeling

Abstract

Pericytes are mural cells that line capillaries throughout the brain, retina, lung, and other organs, where they support capillary homeostasis through direct contact and paracrine crosstalk with capillary endothelium. Despite being described more than a century ago, their contributions to health and vascular diseases remain unclear, largely due to the difficulty of definitive identification. Their inherent plasticity, as well as shared markers and close lineage relationships with other mural cells, result in overlap in identification and underrepresentation in single-cell datasets. Emerging evidence reveals that pericytes play a critical role in the vascular remodeling characteristics of pulmonary hypertension, via mechanisms involving smooth-muscle-like phenotypic switching and morphologic changes influenced by hypoxia signaling, transforming growth factor-β, cyclic GMP modulation, and disrupted pericyte-endothelial communication (e.g., Wnt5a). Recent single-cell RNA sequencing enabled the identification of a novel and specific pericyte marker, HIG1 hypoxia inducible domain family member 1B (Higd1b), thereby improving pericyte identification and revealing novel pericyte subtypes. In this review, we summarize historical and recent insights into pericyte morphology and function, their increasingly recognized role in PH pathobiology, and the potential to unlock novel therapeutic avenues targeting pericytes.

Pulmonary pericytes, mesenchymal-derived cells located within the basement membrane of capillaries, stabilize the alveolar-capillary barrier, regulate blood flow, and participate in tissue repair and angiogenesis ^1,2. Although these homeostatic functions in the lung microvasculature are well recognized, the contribution of pericytes to the pathophysiology of pulmonary hypertension (PH) has only recently begun to be elucidated. This is in part due to the lack of highly specific markers that distinguish pericytes from other mural cells, making it difficult to identify and isolate lung pericytes. In this review, we discuss (1) pericyte basic biology and function; (2) canonical markers of pericytes in the lungs and recent advances using state-of-the-art methodologies; (3) dysregulated pericyte behavior that contributes to PH pathobiology; (4) the intimate nature of pericyte-endothelial cell (EC) interactions and the role of pericytes in immune function; and (5) ongoing challenges and opportunities in the study of pericytes and their contribution to lung vascular remodeling.

Pericyte morphology and function

Pericytes are among the earliest recruited mesenchymal cells of the developing lung vasculature and critically influence alveologenesis¹. This is mediated by several signaling pathways, including platelet-derived growth factor receptor beta (Pdgfrβ), transforming growth factor beta (Tgf-β), angiopoietin, and other pericyte-associated molecules. For example, nerve/glial antigen-2 (Ng2, also called chondroitin sulfate proteoglycan 4/Cspg4) mediates pericyte-endothelial crosstalk via integrin signaling, where its activity promotes endothelial cell-cell junction integrity, thus providing a scaffold for coordinated angiogenesis in the developing vasculature ³. Beyond direct contact, tip ECs recruit pericytes to promote capillary maturation during angiogenesis, and in fully formed vessels pericyte presence restrains angiogenesis by inhibiting endothelial proliferation ^4–6.

Electron microscopy of pericytes in the cerebral cortex reveals an oval cell body and thin, long cytoplasmic processes that partially envelop ECs in capillaries ⁷. In vivo, pericytes display a stellate-shaped morphology with multiple thin protrusions and processes, exhibiting a fractal-like branching pattern ² (Figure 1). Pericytes establish several types of contacts directly with ECs, including gap junctions ^8,9, adhesion plaques ¹⁰, and “peg-and-socket” structures ¹ connecting the pericyte and EC membranes that stabilize the capillary tube assembly, allow for chemical and mechanical signaling between the cell types, and support barrier integrity. 3D electron microscopy of the peg-and-socket junctions reveals the ultrastructure of these connections between pericytes and ECs in mouse brain capillaries ¹¹. The pericyte pegs appear as bouton-shaped swellings with claw-like edges that protrude into the EC sockets, often positioned near EC nuclei and tight junctions with neighboring ECs, suggesting a physically robust interface for intercellular signaling. To a lesser extent, ECs also extend their pegs into pericyte sockets, further strengthening the bidirectional nature of these contacts. This unique structure may allow for synapse-like junctions that enable abundant and rapid signaling. Thousands of these peg-and-socket interactions can link a pericyte to multiple ECs, integrating a consistent EC signaling along a vessel and supporting endothelial barrier integrity ¹². Additionally, N-cadherin-mediated adhesion plaques are located at the ends of the pericyte processes in both adhesions and gap junctions, further facilitating mutual communication between pericytes and ECs ^8,12.

Figure 1: Heterogeneity of pericytes and smooth muscle cells in the pulmonary vasculature under normal conditions and hypoxia (Hx).

Top panel of illustration depicts mural cell distribution from pulmonary arterioles through capillary beds to venules, emphasizing distinct subtypes of pericytes (PCs) and vSMCs. Arterioles > 25μm diameter are covered by vSMCs exhibiting glove-like morphology and expressing Pdgfrb, Sma, and Smmhc. Transition zones 10–25 μm contain Type 2 pericytes with pro-remodeling characteristics expressing mRNA levels of Higd1b^low, Cspg4^high, Pdgfrb^high. Within alveolar capillaries 10–15μm, Type 1 pericytes show long thin processes and homeostatic phenotype expressing Higd1b^high, Cspg4^low, Pdgfrb^low. Venular SMC displays elongated spindle-like morphology and likewise express Pdgfrb, Sma, and Smmhc. After exposure to hypoxia (Hx, lower panel), Type 2 PCs migrate towards proximal arterioles and express vSMC markers Sma and Smmhc. Type 1 PCs develop longer and thinner processes, potentially in order to compensate for reduced Type 2 PC coverage.

Pericyte markers

Since their first description in 1873, definitive identification of pericytes has remained a major limitation to studying their roles in PH and other vascular diseases. Historically, pericyte identification has relied on nonspecific markers in combination with characteristic morphology and capillary location ^2,13. The mouse-derived monoclonal antibody 3G5 recognizes a sialylated glycosphingolipid restricted to pericytes and absent from ECs, vascular smooth muscle cells (vSMCs), fibroblasts, and myofibroblasts ^14,15 (Figure 2a). As the first precise in vitro pericyte marker, 3G5 has been an invaluable tool for the pericyte isolation workflow ². However, because the 3G5 epitope is not a gene or protein, it cannot be readily leveraged for advanced biologic approaches such as cell-specific transgenic mouse models. On the other hand, traditionally utilized mural markers, including Ng2 (Cspg4), Pdgfrβ (Cd140b/Pdgfrb), Sm22α (Transgelin/Tagln) and smooth muscle myosin heavy chain (Smmhc/Myh11), lack the specificity and accuracy to identify pericytes, as they are also expressed in vSMCs and other mural cells ^16,17. Therefore, utilizing these markers as Cre-expressing murine genetic tools to conduct pericyte lineage tracing studies cannot fully rule out contamination from other mural cells ¹⁸. Defining pericyte-specific markers is critical before delineating their roles in pathology and harnessing their plasticity therapeutically in pulmonary vascular disease ¹⁹.

Figure 2: Identification of pulmonary pericytes in human and mice and pericyte plasticity in hypoxia-induced mouse pulmonary hypertension model.

A) Staining with antibodies against the 3G5 epitope (red) identifies pericytes in close proximity to the CD31-positive endothelium (green). Scale bars = 100 μm. B) Representative image from the lung of a Higd1b-CreERT2::Ai14 mouse, in which pericytes were labeled by tdTomato (tdT). Scale bar = 500 μm. C) After 3 weeks of hypoxic conditions, tdT+ pericytes (red) translocated from capillaries to muscularized arterioles, as highlighted by positive Sma staining (white). Scale bars = 100 μm.

Recent technological advances, including single-cell RNA sequencing (scRNA-seq) and spatial transcriptomics, have enabled the identification of mural cell sub-populations at the transcriptional level and have yielded pericyte-specific cell markers. In the Tabula Muris Senis dataset, pericytes (coexpressing both Pdgfrb and Cspg4) were reannotated using a more stringent classification and found to express numerous organ-conserved marker signatures in the heart, lung, kidney, and bladder ²⁰. Of the 18 identified lung pericyte-specific markers, four (Higd1b, Ndulfa4l2, Kcnk3, and Cox4i2) were also enriched in annotated pericytes from the Human Lung Cell Atlas ²¹. Among them, HIG1 hypoxia inducible domain family member 1B (Higd1b) emerged as the most promising marker for lung pericytes. While its exact function is yet to be defined, Higd1b has been implicated in promoting cardiomyocyte survival via modulation of caspase-3 and -9 activity ²² and is highly enriched in cerebral pericytes ²³. In human lung tissue, spatial transcriptomics further demonstrated that HIGD1B had a higher positive predictive value and specificity compared to CSPG4 or PDGFRB, albeit with a lower sensitivity ²⁴. This high specificity makes HIGD1B a superior marker for studying gene expression changes under pathological conditions, with the caveat that some pericytes may be missed if used alone. Leveraging these insights, a tamoxifen-inducible knock-in mouse line Higd1b-CreERT2 was generated to effectively label pericytes in the lung, heart, kidney, retina, and other organ systems ²⁴. This model enabled the identification of pericyte subtypes in the distal vasculature under physiological and pathological conditions (Figure 1, bottom panel; 3-week hypoxia mouse PH model, Figure 2), highlighting their multipotency and contribution to vascular remodeling ²⁴.

Pericyte contribution to vascular remodeling in PH

PH comprises a heterogeneous group of disorders characterized by pathologically elevated pressures in the pulmonary circulation. The World Symposium classification defines five subclasses of PH: Group I, also termed pulmonary arterial hypertension (PAH), is driven by molecular and structural abnormalities in pulmonary arteries and includes idiopathic, heritable, and drug/toxin-induced PH; Group II PH is due to left heart failure and other chronic cardiac conditions that increase vascular pressure via overcirculation or congestion of the pulmonary vasculature; Group III PH is due to chronic lung disease and hypoxia, which cause chronic pulmonary vasoconstriction; Group IV is due to chronic thromboembolic disease or obstruction of the pulmonary vascular outlet, as in pulmonary vein stenosis; and Group V describes PH due to multisystem or inflammatory disorders or unknown etiologies ²⁵. Prognosis remains poor for patients with PH and disease-modifying therapies are limited ^26,27. Current medications are mostly vasodilators that directly target the pulmonary vasculature and do not prevent disease progression, but show mixed efficacy and many safety concerns ²⁷.

The prevalence of PH is increasing, partially due to the aging of the world population and the increasing rate of heart and lung disease that underlie Group II and III PH, respectively ²⁸. Importantly, PH is not confined to older adults. As neonatal care and survival rates for extremely preterm infants improve, the prevalence of PH among survivors of bronchopulmonary dysplasia is rising ^29,30. In infants, PH is associated with poorer neurocognitive outcomes and increased morbidity and mortality despite vasodilator therapy ³⁰. Thus, there is an urgent need for new mechanistic insights and therapeutic approaches across PH subtypes.

Substantial prior work has focused on pulmonary vSMCs and ECs as key contributors to vascular remodeling in several PH models. For example, vSMC precursors migrate distally to the small arterioles and contribute to arteriolar muscularization in the hypoxia-induced mouse PH model ^31,32 under the influence of macrophage- ³³ and endothelial-derived ³⁴ signaling and growth factors. Recently, smooth muscle Cxcl12 was shown to play a role in vSMC proliferation and accumulation on distal arterioles in SMC lineage-tracing mice subjected to left pneumonectomy and hypoxia, a model that potentially represents Group II PH ³⁵. In 1980, evidence from lung biopsies of children with congenital heart defects first demonstrated that pericytes may also contribute to pathologic muscularization of the distal arterioles ³⁶. However, progress in investigating the pericyte contribution to vascular remodeling in PH has been hindered by the scarcity of highly specific pericyte markers. Nevertheless, recent evidence highlights pericytes as active migratory and phenotype-switching participants in PH pathobiology, and ongoing molecular studies are beginning to define the drivers of this process.

Pericyte migration and phenotype switching

A variety of preclinical models are used to study PH, each capturing only some degree of human disease. Rat monocrotaline and SU5416 (Sugen)/hypoxia models develop medial thickening and varying degrees of neointimal remodeling but still fail to recapitulate clinical PAH ³⁷. In mice, chronic hypoxia produces a Group III-like PH phenotype with relatively mild, largely reversible remodeling upon return to normoxia. Fibrosis-associated PH can be modeled as Group III disease using bleomycin combined with hypoxia or monocrotaline-based protocols. Although none of these models fully reproduces human pathology, murine systems uniquely enable precise cell labeling and lineage tracing, providing powerful tools to interrogate cell function and fate during vascular remodeling.

Regardless of etiology, a hallmark of PH is the combined loss of microvessels driven by the apoptosis of endothelial cells and muscularization of microvessels due to abnormal proliferation of mural cells ¹⁸. Distal pulmonary arteries demonstrate increased pericyte coverage compared to controls in both human PAH tissues and hypoxia-induced PH in Ng2DsRedBAC transgenic mice, which label Ng2+ mural cells ³⁸. In the mouse model, fibroblast growth factor 2 promotes pericyte proliferation and migration, while interleukin 6 amplifies Tgf-β-driven differentiation of pericytes toward a contractile phenotype with upregulation of smooth muscle alpha-actin (Sma), Calponin, and Sm22α.

Studies of PDGFRB+ pericyte-like cells from patients with idiopathic pulmonary fibrosis show increased levels of SMA and upregulation of fibrosis-associated extracellular matrix genes ^39,40. Pericytes from patients with idiopathic pulmonary fibrosis closely resemble myofibroblasts, underscoring pericyte plasticity and implicating phenotype switching as a key pathologic mechanism. Consistent with this finding, lung tissues from affected patients exhibit fibroblastic foci within the interstitial space between the alveolar and capillary basement membranes, where pericytes reside, suggesting that pericytes contribute to fibrogenesis ⁴¹. In cultured human lung pericytes, TGF-β is a potent trigger for the pericyte-myofibroblast transition, whereas withdrawal of TGF-β reverses the process and restores a more canonical pericyte morphology and function ³⁹. Finally, increased basement membrane stiffness promotes pericyte switching and SMA expression in vitro ⁴⁰. Together, the literature points to a key role for pericytes in the pathogenesis of pulmonary fibrosis and downstream consequences such as PH.

Mouse Ng2CreER/mTmG fate mapping reveals increased GFP+ cells within remodeled vessels and a progressive rise in Sma expression in hypoxia and Sugen-hypoxia PH mouse models ⁴². In this study, GFP+ cells exhibit increased proliferation, contractility, and motility in PH compared to control conditions. These findings mirror results from human PAH-derived lung pericytes, which show enhanced migration and expression of SMC/myofibroblast markers calponin, SMA, and collagen 1A in culture. Signaling via the chemokine Cxcl12 and its receptors Cxcr4 and −7 is necessary for pericyte migration in PH: chemical blockade of Cxcl12 binding significantly reduces GFP+ cell accumulation in the distal arterioles of hypoxia-exposed mice, while Cxcr7 agonism enhances pericyte migration in vitro. TGF-β receptor II is upregulated in human and mouse PH pericytes, and responses to TGF-β stimulation are enhanced in PH pericytes in vitro, highlighting the greater myogenic potential of pericytes in PH. In another recent study using Ng2 as a pericyte marker, pericytes under hypoxia phenotype switch to Sma+, vSMC-like cells and migrate away from pulmonary capillaries and towards the larger arterioles ⁴³. Notably, because Ng2 is a marker shared by multiple mural cell populations, these studies cannot exclusively attribute the observed effects to pericytes. However, they provide valuable insights into the intrinsic plasticity of pericytes during vascular remodeling.

The lineage tracing study utilizing novel Higd1b-CreERT2 reporter mice provides a more precise view of the plasticity of pericytes as a contributor to the pathophysiology of PH ²⁴. Under normoxic conditions, a subset of pericytes termed Type 1 cells was predominantly located within parenchymal capillaries (diameter <10um), while Type 2 pericytes resided at the junction between larger arterioles and capillaries (diameter <20um). At baseline, both types of pericytes were quiescent and played a crucial role in maintaining capillary integrity and homeostasis. However, under hypoxic conditions, Type 2 cells migrated toward the large arterioles and co-expressed vSMC markers Sma, Smmhc, and the cytoskeletal protein Vimentin. In contrast, Type 1 cells remained in the parenchymal capillary without expressing vSMC markers, even under hypoxia, but their processes showed increased length and coverage, which may compensate for the reduced coverage resulting from Type 2 pericyte migration (Figure 1). These two pericyte subtypes were conserved in scRNA-seq data analysis of human PAH lungs, highlighting their biological relevance to human disease. However, further mechanistic studies are required to define unique markers for distinguishing Type 1 and Type 2 pericytes and to better characterize their function and plasticity.

Cyclic GMP signaling

Emerging data implicates cyclic GMP (cGMP) signaling in regulating pericyte function in PH. The regulator of G-protein signaling 5 (Rgs5) is significantly upregulated in the pericytes of patients with PAH compared to controls in the analysis of large gene expression datasets ⁴⁴. Although Rgs5 and cGMP signaling are well studied in vSMCs ⁴⁵, its activity in pericytes is a novel finding. Because Rgs5 typically exerts pro-apoptotic and anti-angiogenic effects in vascular cells, its enrichment in PAH pericytes appears paradoxical in light of prior reports that pericytes show increased proliferation and enhanced increased capillary coverage in PAH ³⁸. Another recent study shows preserved cGMP signaling in cultured pericytes isolated from PAH patients, and activation of cGMP signaling also attenuates growth factor-induced proliferation and migration as well as decreasing Sma expression in vitro ⁴⁶. Taken together, the results of these studies suggest that Rgs5 and cGMP signaling may counteract the pro-migration and proliferative capacities observed in pericytes in PH. If validated in vivo, therapeutic augmentation of pericyte cGMP signaling could restrain maladaptive pericyte activation, representing a novel therapeutic avenue to complement existing vasodilator strategies in treating PAH.

Hypoxia-inducible factor

Deletion of hypoxia-inducible factor 2α (Hif2α) across the Wt1 lineage, which includes cardiopulmonary pericytes, ECs, and vSMCs, demonstrates the importance of Hif2α in the cardiopulmonary response to chronic hypoxia ⁴⁷. Using Ng2 as a marker for pericytes, Hif2α signaling has been shown to activate pericyte migration and transition, thus contributing to vascular remodeling in hypoxia-induced PH ⁴³. Hif2α was also upregulated in pericytes and tissues from patients with idiopathic (Group I) PAH. Pericytes isolated from human PAH lungs and transfected with a Hif2α overexpression vector exhibited disorganized migration and impaired endothelial tube coverage under hypoxic conditions, indicating that Hif2α activation drives pericyte dysfunction. Consistent with these observations, Hif2α overexpression in hypoxic mice increased Sma expression in Ng2+ cells and was associated with significantly elevated right ventricular systolic pressure (RVSP) and right ventricular hypertrophy (RVH), whereas genetic or pharmacologic Hif2α inhibition attenuated PH. These findings corroborate an earlier study in which partial Hif2α deletion or Hif2α-antisense oligonucleotide treatment of hypoxia-exposed mice reduces capillary muscularization and decreases RVSP and RVH compared to controls ⁴⁸. Notably, that study focuses on the role of Hif2α in vascular ECs, where EC-specific knockout also mitigates many hypoxia-induced PH features in mice. Nevertheless, these findings implicate Hif2α as a shared driver of pericyte and EC dysfunction in PH, which could serve as a potential target for treatments for both Group I and Group III PH.

Pericyte-endothelial crosstalk in PH

Pericytes directly contact capillary ECs, where they provide structural and regulatory support ². In synergy with ECs, they regulate blood flow, maintain barrier integrity, and are critical for vessel formation and maturation ^49,50. Disruptions in this interaction are associated with various diseases including diabetes, Alzheimer’s disease, sepsis, and PH ^38,51.

Activation of the Wnt/planar cell polarity pathway is essential for pericyte recruitment during angiogenesis ⁵². Lung pericytes isolated from PAH patients were co-cultured with pulmonary microvascular ECs where they exhibited reduced polarity and motility toward the ECs compared to pericytes isolated from control patients. This was due to the downregulation of Frizzled 7 (FZD7) and the downstream GTP-ase CDC42, key components of the Wnt pathway. Silencing both genes in healthy pericytes recapitulated the PAH phenotype, whereas restoring their expression in PAH pericytes rescued impaired interactions, improved pericyte motility, and enhanced EC tube formation. In vivo Matrigel plug assays in mice corroborated these findings, showing that the loss of Fzd7 and Cdc42 significantly reduced the size of blood-filled vessels and diminished pericyte coverage. Extending this work, Wnt5a, an EC-secreted ligand that activates Wnt through Fzd7, was found to be critical for establishing EC-pericyte interactions in the pulmonary microvasculature ⁵³. Healthy pulmonary microvascular ECs released Wnt5a via exosomes to facilitate pericyte polarization and recruitment. In PAH, Wnt5a expression was found remarkably reduced in pulmonary microvascular ECs from PAH patients and in Sugen/hypoxia rat PH. Moreover, EC-specific Wnt5a knockout mice (Cdh5-CreER::Wnt5a^fl/fl) showed persistent PH and RVH after recovery from hypoxia, along with significantly reduced pericyte coverage and increased microvascular muscularization. Other pathways likely intersect with Wnt pathways to regulate EC-pericyte interactions, such as Tgf-β/bone morphogenic protein signaling. Bone morphogenic protein receptor 2 mutations, among the most common causes of PAH ²⁵, may modulate Wnt activation in PAH pericytes, an avenue requiring further investigation.

As with pericytes, scRNA-seq technology has enabled the identification of pulmonary EC subtypes with distinct roles in healthy and inflammatory states ^54,55 as well as in PH ^56,57. Spatial transcriptomics studies show that TGF-β is upregulated in human PAH lesions ⁵⁸, and mouse pulmonary ECs undergo Hif2α-mediated capillary-to-arterial transition in PH ⁵⁹. As these same pathways are implicated in pericyte dysfunction in PH, coordinated EC-pericyte signaling may potentially vary across pericyte and EC subtypes. Determining which EC subsets engage specific pericyte subtypes and how these interactions evolve during disease progression remains an open area for future investigation.

Pericytes in lung inflammation and injury

Dysregulated inflammation underlies many pulmonary diseases, from common pathologies such as asthma and pneumonia to critical lung disease such as acute respiratory distress syndrome and insidious, progressive disease, including pulmonary fibrosis and PH. Pericytes are active regulators of these responses ^60,61. Pulmonary pericytes express functional toll-like receptors and directly sense damage- and pathogen-associated molecular patterns via nuclear factor-κB ⁶². The resultant release of cytokines and chemokines induces adhesion molecule expression on pericytes and ECs, thus facilitating immune cell entry ^60,63. This function of pericytes has prompted their characterization as interstitial immune sentinel cells. In vitro, pericyte-like cells exposed to bronchoalveolar lavage fluid from injured mice secrete chemokines, including Cxcl1 and Ccl2, and upregulate genes associated with inflammation, cell migration, and angiogenesis ^64,65. Consistent with these findings, injured mouse lungs exhibit increased pericyte numbers along with reduced EC abundance ⁶⁶. By contrast, ablation of Pdgfrβ+ cells in mice attenuates inflammation in experimental lung injury, underscoring a causal role for pericytes in modulating inflammatory responses ⁶⁴.

Challenges in studying pericytes

While scRNA-seq and advanced bioinformatics have improved pericyte identification and functional studies, significant challenges remain. Although pericytes are abundantly distributed throughout the lung microvasculature, technical limitations yield a relatively low number of annotated pericytes in publicly available datasets ¹⁹. Obtaining an enriched pericyte population for isolation may be challenging due to incomplete dissociation of the basement membrane, particularly the collagen IV matrix, which tightly connects pericytes to capillary ECs ¹². Although their multipotent-cell-like plasticity makes them an attractive candidate for disease-modifying cell therapies, this presents additional challenges for pericyte studies in vitro. Culture conditions must be tightly controlled since medium growth factor composition and oxygen tension are critical to prevent inadvertent phenotype switching ^67,68 and exposure to growth factors such as TGF-β can lead to phenotypic drift ⁴¹. Additional detailed mechanistic studies are required to fully understand the factors that govern pericyte phenotype switching, both in the context of lung pathologies and in cell culture models, to fully harness this feature of pericytes.

Adding to the complexity, recent studies have demonstrated organ-specific transcriptional differences among pericytes, reflecting divergent pericyte functions across tissues ^19,69. Consequently, a single, highly specific and sensitive marker that can reliably label pericytes across all physiological and pathological contexts is unlikely. For example, Higd1b, a robust marker in lung and heart pericytes, shows minimal expression in liver or spleen pericytes. Integrating large-scale bioinformatics (including pseudotime and trajectory analyses) across robust pericyte cohorts from healthy and diseased conditions and validating predictions in vitro with controlled cytokine and stimulus perturbations could yield a high-accuracy composite marker strategy across various organs.

From a therapeutic perspective, targeting vSMC proliferation in PAH remains challenging ⁷⁰. Although PDGFRβ inhibitors such as imatinib have shown promise, PDGFRB is expressed on both vSMCs and pericytes. Broad inhibition risks unintended disruption of pericyte function, potentially compromising capillary homeostasis and leading to vascular leakage and other adverse effects. Developing strategies that selectively target vSMCs while sparing pericytes by using more specific surface markers could provide safer and more effective treatments.

Conclusions and future directions

Pericytes are critical yet understudied components of the pulmonary vasculature. Recent work implicates pericytes in the pathologic small vessel muscularization that characterizes PH, through a variety of molecular mechanisms including metabolic pathways and hypoxia-inducible factors. Understanding how pericytes initiate early vascular remodeling and whether timely interventions can halt these early events remains a priority. Given the scarcity of early-stage clinical PH samples and pediatric PH specimens, complementary resources such as human induced pluripotent stem cell (iPSC)-derived vascular cells may provide crucial insight.

Pericytes demonstrate remarkable plasticity, translocation within vascular beds, and acquisition of fibroblasts and vSMC features under pathological conditions. Advances in large-scale data techniques have enabled the discovery of the pericyte-specific marker Higd1b, which has, in turn, facilitated the identification of pericyte subtypes in the hypoxia-induced PH lungs. The specificity of Higd1b offers a path toward more precise, mechanistic investigations of pericyte subsets and their roles in PH pathobiology.

Highlights.

• Pericytes are overlooked but critical components of the lung vasculature, maintaining vascular stability and homeostasis.

• Pericytes directly contact neighboring capillary endothelial cells, including peg-and-socket and adherens/gap junctions, with mutual mechanical and chemical signaling between the two cell types.

• Higd1b is a newly identified pericyte-specific marker that offers improved specificity and significant promise in future pericyte studies.

• In hypoxia induced PH, pericytes contribute to the vascular remodeling by translocating from distal capillaries to arterioles and gaining a contractile, smooth muscle-like phenotype. Elucidating the basic biology of pericytes and the molecular mechanisms that govern phenotype switching could prevent early vascular remodeling and halt subsequent arteriolar remodeling and disease exacerbation.

Abbreviation

Sma/SMA

smooth muscle alpha-actin

Cspg4/CSPG4

chondroitin sulfate proteoglycan

cGMP

cyclic GMP

EC

endothelial cell

Fzd7/FZD7

frizzled 7

Higd1b/HIGD1B

HIG1 hypoxia inducible domain family member 1B

Hif2α

hypoxia inducible factor 2-alpha

Smmhc/Myh11

smooth muscle myosin heavy chain 11

Ng2

nerve/glial antigen-2

Pdgfrβ/PDGFRB

platelet derived growth factor receptor beta

PAH

pulmonary arterial hypertension

PH

pulmonary hypertension

Rgs5

regulator of G-protein signaling 5

scRNA-seq

single-cell RNA sequencing

Tgf-β/TGF-β

transforming growth factor beta

vSMC

vascular smooth muscle cell