Lung pericytes as mediators of inflammation.

Pericytes are microvascular mural cells that directly contact endothelial cells. They have long been recognized for their roles in vascular development and homeostasis, but more recently have been identified as key mediators of the host response to injury. In this context, pericytes possess a surprising degree of cellular plasticity, behaving dynamically when activated and potentially participating in a range of divergent host responses to injury. Although there has been much interest in the role of pericytes in fibrosis and tissue repair, their involvement in the initial inflammatory process has been understudied and is increasingly appreciated. Pericytes mediate inflammation through leukocyte trafficking and cytokine signaling, respond to pathogen-associated molecular patterns and tissue damage-associated molecular patterns, and may drive vascular inflammation during human SARS-CoV-2 infection. In this review, we highlight the inflammatory phenotype of activated pericytes during organ injury, with an emphasis on novel findings relevant to pulmonary pathophysiology.

Nephronectin is required to maintain right lung lobar separation during embryonic development.

Nephronectin (NPNT) is a basement membrane (BM) protein and high-affinity ligand of integrin α8ß1 that is required for kidney morphogenesis in mice. In the lung, NPNT also localizes to BMs, but its potential role in pulmonary development has not been investigated. Mice with a floxed Npnt allele were used to generate global knockouts (KOs). Staged embryos were obtained by timed matings of heterozygotes and lungs were isolated for analysis. Although primary and secondary lung bud formation was normal in KO embryos, fusion of right lung lobes, primarily the medial and caudal, was first detected at E13.5 and persisted into adulthood. The lung parenchyma of KO mice was indistinguishable from wild-type (WT) and lobe fusion did not alter respiratory mechanics in adult KO mice. Interrogation of an existing single-cell RNA-seq atlas of embryonic and adult mouse lungs identified Npnt transcripts in mesothelial cells at E12.5 and into the early postnatal period, but not in adult lungs. KO embryonic lungs exhibited increased expression of laminin α5 and deposition of collagen IV in the mesothelial BM, accompanied by abnormalities in collagen fibrils in the adjacent stroma. Cranial and accessory lobes extracted from KO embryonic lungs fused ex vivo when cultured in juxtaposition, with the area of fusion showing loss of the mesothelial marker Wilms tumor 1. Because a similar pattern of lobe fusion was previously observed in integrin α8 KO embryos, our results suggest that NPNT signaling through integrin α8, likely in the visceral pleura, maintains right lung lobe separation during embryogenesis.

Effect of lung pericyte-like cell ablation on the bleomycin model of injury and repair.

We previously showed that pericyte-like cells derived from the FoxD1-lineage contribute to myofibroblasts following bleomycin-induced lung injury. However, their functional significance in lung fibrosis remains unknown. In this study, we used a model of lung pericyte-like cell ablation to test the hypothesis that pericyte-like cell ablation attenuates lung fibrosis in bleomycin-induced lung injury. Lung fibrosis was induced by intratracheal instillation of bleomycin. To ablate pericyte-like cells in the lung, diphtheria toxin (DT) was administered to Foxd1-Cre;Rosa26-iDTR mice at two different phases of bleomycin-induced lung injury. For early ablation, we coadministered bleomycin with DT and harvested mice at days 7 and 21. To test the effect of ablation after acute injury, we delivered DT 7 days after bleomycin administration. We assessed fibrosis by lung hydroxyproline content and semiquantitative analysis of picrosirius red staining. We performed bronchoalveolar lavage to determine cell count and differential. We also interrogated mRNA expression of fibrosis-related genes in whole lung RNA. Compared with DT-insensitive littermates where pericyte-like cells were not ablated, DT-sensitive animals exhibited no difference in fibrosis at day 21 both in the early and late pericyte ablation models. However, early ablation of pericytes reduced acute lung inflammation, as indicated by decreased inflammatory cells. Our data confirm a role for pericytes in regulating pulmonary inflammation in early lung injury.

Pericyte-like cells undergo transcriptional reprogramming and distinct functional adaptations in acute lung injury.

We previously reported on the role of pericyte-like cells as functional sentinel immune cells in lung injury. However, much about the biological role of pericytes in lung injury remains unknown. Lung pericyte-like cells are well-positioned to sense disruption to the epithelial barrier and coordinate local inflammatory responses due to their anatomic niche within the alveoli. In this report, we characterized transcriptional responses and functional changes in pericyte-like cells following activation by alveolar components from injured and uninjured lungs in a mouse model of acute lung injury (ALI). Purified pericyte-like cells from lung digests using PDGFRß as a selection marker were expanded in culture as previously described (1). We induced sterile acute lung injury in mice with recombinant human Fas ligand (rhFasL) instillation followed by mechanical ventilation (1). We then collected bronchoalveolar lavage fluid (BALF) from injured and uninjured mice. Purified pericyte-like cells in culture were exposed to growth media only (control), BALF from uninjured mice, and BALF from injured mice for 6 and 24 hours. RNA collected from these treatment conditions were processed for RNAseq. Targets of interest identified by pathway analysis were validated using in vitro and in vivo assays. We observed robust global transcriptional changes in pericyte-like cells following treatment with uninjured and injured BALF at 6 hours, but this response persisted for 24 hours only after exposure to injured BALF. Functional enrichment analysis of pericytes treated with injured BALF revealed the activation of pro-inflammatory, cell migration, and angiogenesis-related pathways, whereas processes associated with tissue development and cell differentiation were down-regulated. We validated select upregulated targets in the inflammatory, angiogenic, and cell migratory pathways using functional biological assays in vitro and in vivo. We conclude that lung pericyte-like cells are highly responsive to alveolar compartment content from both uninjured and injured lungs, but injured BALF elicits a more sustained response. The inflammatory, angiogenic, and migratory changes exhibited by activated pericyte-like cells underscore the phenotypic plasticity of these specialized stromal cells in the setting of acute lung injury.

Pericytes in the Lung.

The lung has numerous roles, including gas exchange, immune surveillance, and barrier function. Being a highly vascularized organ, the lung receives dual blood supply from both the pulmonary and bronchial circulation. Therefore, pericytes likely play a prominent role in lung physiology given their localization in the perivascular niche. New genetic approaches have increased our understanding of the origin and the diverse functions of lung pericytes. Lung pericytes are myofibroblast progenitors, contributing to development of fibrosis in mouse models. Lung pericytes are also capable of responding to danger signals and amplify the inflammatory response through elaboration of cytokines and adhesion molecules. In this chapter, we describe the molecular, anatomical, and phenotypical characterization of lung pericytes. We further highlight their potential roles in the pathogenesis of lung diseases including pulmonary fibrosis, asthma, and pulmonary hypertension. Finally, current gaps in knowledge and areas of ongoing investigation in lung pericyte biology are also discussed.

Characterization of human PDGFR-ß-positive pericytes from IPF and non-IPF lungs.

Pericytes are key regulators of the microvasculature through their close interactions with the endothelium. However, pericytes play additional roles in tissue homeostasis and repair, in part by transitioning into myofibroblasts. Accumulation of myofibroblasts is a hallmark of fibrotic diseases such as idiopathic pulmonary fibrosis (IPF). To understand the contribution and role of pericytes in human lung fibrosis, we isolated these cells from non-IPF control and IPF lung tissues based on expression of platelet-derived growth factor receptor-ß (PDGFR-ß), a common marker of pericytes. When cultured in a specialized growth medium, PDGFR-ß+ cells retain the morphology and marker profile typical of pericytes. We found that IPF pericytes migrated more rapidly and invaded a basement membrane matrix more readily than control pericytes. Exposure of cells to transforming growth factor-ß, a major fibrosis-inducing cytokine, increased expression of α-smooth muscle actin and extracellular matrix genes in both control and IPF pericytes. Given that pericytes are uniquely positioned in vivo to respond to danger signals of both systemic and tissue origin, we stimulated human lung pericytes with agonists having pathogen-associated molecular patterns (PAMPs) or damage-associated molecular patterns (DAMPs). Both control and IPF lung pericytes increased expression of proinflammatory chemokines in response to specific PAMPs and DAMPs released from necrotic cells. Our results suggest that control and IPF lung pericytes are poised to react to tissue damage, as well as microbial and fibrotic stimuli. However, IPF pericytes are primed for migration and matrix invasion, features that may contribute to the function of these cells in lung fibrosis.

Ablation of Pericyte-Like Cells in Lungs by Oropharyngeal Aspiration of Diphtheria Toxin.

We demonstrated previously that FoxD1-derived cells in the lung are enriched in pericyte-like cells in mouse lung. These cells express the common pericyte markers and are located adjacent to endothelial cells. In this study, we demonstrate the feasibility of administering diphtheria toxin (DT) by oropharyngeal aspiration as an approach to ablating FoxD1-derived cells. We crossed mice expressing Cre-recombinase under the FoxD1 promoter to Rosa26-loxP-STOP-loxP-iDTR mice and generated a bitransgenic line (FoxD1-Cre;Rs26-iDTR) in which FoxD1-derived cells heritably express simian or human diphtheria toxin receptor and are sensitive to DT. We delivered low-dose (0.5 ng/g) and high-dose (1ng/g × 2) to FoxD1-Cre;Rs26-iDTR mice and littermate control mice by oropharyngeal aspiration and evaluated ablation by flow cytometry and immunohistochemistry. FoxD1-Cre mice showed a 40-50% reduction in PDGFRß+ cells by flow cytometry at Days 2 and 7 after DT administration, with a return of PDGFRß+ cells at Day 28. Confocal microscopy revealed an observable reduction in pericyte markers. Bronchoalveolar lavage fluid analysis revealed no significant differences in total protein, bronchoalveolar lavage fluid red blood cell, or white blood cell counts at low dose. However, at high-dose DT, there was a proinflammatory effect in the control mice and increased mortality associated with systemic toxicity in Cre+ mice. Low-dose DT reduced lung PDGFRß+ stromal cells in the FoxD1-Cre;iDTR transgenic model without a differential effect on lung inflammation in DT-sensitive and DT-insensitive animals. Low-dose DT is a viable method for transient lineage-specific stromal cell ablation in the lung that minimizes systemic toxicity.

Lung pericyte-like cells are functional interstitial immune sentinel cells.

Pericytes are perivascular PDGF receptor-ß+ (PDGFRß+) stromal cells required for vasculogenesis and maintenance of microvascular homeostasis in many organs. Because of their unique juxtaposition to microvascular endothelium, lung PDGFRß+ cells are well situated to detect proinflammatory molecules released following epithelial injury and promote acute inflammatory responses. Thus we hypothesized that these cells represent an unrecognized immune surveillance or injury-sentinel interstitial cell. To evaluate this hypothesis, we isolated PDGFRß+ cells from murine lung and demonstrated that they have characteristics consistent with a pericyte population (referred to as pericyte-like cells for simplicity hereafter). We showed that pericyte-like cells expressed functional Toll-like receptors and upregulated chemokine expression following exposure to bronchoalveolar lavage fluid (BALF) collected from mice with sterile lung injury. Interestingly, BALF from mice without lung injury also induced chemokine expression in pericyte-like cells, suggesting that pericyte-like cells are primed to sense epithelial injury (permeability changes). Following LPS-induced lung inflammation, increased numbers of pericyte-like cells expressed IL-6, chemokine (C-X-C motif) ligand-1, chemokine (C-C motif) ligand 2/ monocyte chemotactic protein-1, and ICAM-1 in vivo. Sterile lung injury in pericyte-ablated mice was associated with decreased inflammation compared with normal mice. In summary, we found that pericyte-like cells are immune responsive and express diverse chemokines in response to lung injury in vitro and in vivo. Furthermore, pericyte-like cell ablation attenuated inflammation in sterile lung injury, suggesting that these cells play an important functional role in mediating lung inflammatory responses. We propose a model in which pericyte-like cells function as interstitial immune sentinels, detecting proinflammatory molecules released following epithelial barrier damage and participating in recruitment of circulating leukocytes.

Toll-interacting protein inhibits transforming growth factor beta signaling in mouse lung fibroblasts.

Variations in the Toll-interacting protein (TOLLIP) gene have been identified in genome-wide association studies to correlate with risk of disease, mortality, and response to N-acetylcysteine therapy in idiopathic pulmonary fibrosis. Although TOLLIP is known to modulate innate immune responses, its relevance in organ fibrogenesis remains unknown. Prior work in the literature suggests TOLLIP dampens transforming growth factor beta (TGFß) signaling in human cell lines. In this study, we examined the role of TOLLIP in mouse lung fibroblast (MLF) responses to TGFß and in the bleomycin model of experimental lung fibrosis using Tollip-/- mice. We hypothesize that if TOLLIP negatively regulates TGFß signaling, then Tollip-/- mouse lung fibroblasts (MLFs) would have enhanced response to TGFß treatment, and Tollip-/- mice would develop increased fibrosis following bleomycin challenge. Primary MLFs were stimulated with TGFß (1 ng/mL) for 24 h. RNA was obtained to assess global transcriptional responses by RNA-seq and markers of myofibroblast transition by qPCR. Functional assessment of TGFß-stimulated MLFs included cell migration by scratch assay, cell proliferation, and matrix invasion through Matrigel. In the in vivo model of lung fibrosis, Tollip-/- mice and wild-type (WT) littermates were administered bleomycin intratracheally and assessed for fibrosis. We further examined TGFß signaling in vivo after bleomycin injury by SMAD2, ERK1/2, and TGFßR1 Western blot. In response to TGFß treatment, both WT and Tollip-/- MLFs exhibited global transcriptional changes consistent with myofibroblast differentiation. However, Tollip-/- MLFs showed greater number of differentially expressed genes compared to WT MLFs and greater upregulation of Acta2 by qPCR. Functionally, Tollip-/- MLFs also exhibited increased migration and Matrigel invasiveness compared to WT. We found evidence of enhanced TGFß signaling in Tollip-/- through SMAD2 in vitro and in vivo. Tollip-/- mice experienced lower survival using a standard weight-adjusted dosing without evidence of differences in fibrosis at Day 21. With adjustment of dosing for sex, no differences were observed in fibrosis at Day 21. However, Tollip-/- mice had greater weight loss and increased bronchoalveolar lavage fluid total protein during early resolution at Day 14 compared to WT without evidence of differences in acute lung injury at Day 7, suggesting impaired resolution of lung injury.

TOLLIP inhibits lipid accumulation and the integrated stress response in alveolar macrophages to control Mycobacterium tuberculosis infection.

A polymorphism causing deficiencies in Toll-interacting protein (TOLLIP), an inhibitory adaptor protein affecting endosomal trafficking, is associated with increased tuberculosis (TB) risk. It is, however, unclear how TOLLIP affects TB pathogenesis. Here we show that TB severity is increased in Tollip-/- mice, characterized by macrophage- and T cell-driven inflammation, foam cell formation and lipid accumulation. Tollip-/- alveolar macrophages (AM) specifically accumulated lipid and underwent necrosis. Transcriptional and protein analyses of Mycobacterium tuberculosis (Mtb)-infected, Tollip-/- AM revealed increased EIF2 signalling and downstream upregulation of the integrated stress response (ISR). These phenotypes were linked, as incubation of the Mtb lipid mycolic acid with Mtb-infected Tollip-/- AM activated the ISR and increased Mtb replication. Correspondingly, the ISR inhibitor, ISRIB, reduced Mtb numbers in AM and improved Mtb control, overcoming the inflammatory phenotype. In conclusion, targeting the ISR offers a promising target for host-directed anti-TB therapy towards improved Mtb control and reduced immunopathology.

Role of IGF-1 pathway in lung fibroblast activation.

BACKGROUND: IGF-1 is elevated in pulmonary fibrosis and acute lung injury, where fibroblast activation is a prominent feature. We previously demonstrated that blockade of IGF pathway in murine model of lung fibrosis improved outcome and decreased fibrosis. We now expand that study to examine effects of IGF pathway on lung fibroblast behaviors that could contribute to fibrosis. METHODS: We first examined mice that express αSMA promoter upstream of GFP reporter treated with A12, a blocking antibody to IGF-1 receptor, after bleomycin induced lung injury. We then examined the effect of IGF-1 alone, or in combination with the pro-fibrotic cytokine TGFß on expression of markers of myofibroblast activation in vitro, including αSMA, collagen α1, type 1, collagen α1, type III, and TGFß expression. RESULTS: After bleomycin injury, we found decreased number of αSMA-GFP + cells in A12 treated mice, validated by αSMA immunofluorescent staining. We found that IGF-1, alone or in combination with TGF-ß, did not affect αSMA RNA expression, promoter activity, or protein levels when fibroblasts were cultured on stiff substrate. IGF-1 stimulated Col1a1 and Col3a1 expression on stiff substrate. In contrast, IGF-1 treatment on soft substrate resulted in upregulation of αSMA gene and protein expression, as well as Col1a1 and Col3a1 transcripts. In conclusion, IGF-1 stimulates differentiation of fibroblasts into a myofibroblast phenotype in a soft matrix environment and has a modest effect on αSMA stress fiber organization in mouse lung fibroblasts.

Endotoxin-induced acute lung injury in mice with postnatal deletion of nephronectin.

Acute injury of the lung involves damage to the epithelium and its underlying extracellular matrix (ECM), the basement membrane (BM). How BMs contribute to injury resolution is poorly understood. Nephronectin (NPNT) is a high-affinity ligand for integrin α8ß1 and, although first identified in the mouse kidney, is prominently expressed in the lung, where it localizes to BMs in the alveoli. To determine if NPNT plays a role in acute injury and inflammation of the lung, we developed a model for postnatal deletion of NPNT using mice with a floxed allele of Npnt in combination with a tamoxifen-inducible Cre recombinase expressed at the ROSA locus. Expression of NPNT was substantially reduced in lungs from tamoxifen-treated Cre+ animals. Cre+ mice and Cre- controls were given E. coli LPS by oropharyngeal aspiration to induce injury and inflammation. In Cre- lungs, although both Npnt and Itga8 (integrin α8) transcripts were downregulated at the peak of inflammation, NPNT protein was still detectable. While the onset of inflammation was similar for Cre+ and Cre-, NPNT-deficient lungs still had thickened alveolar septa and there were increased macrophages in the bronchoalveolar lavage fluid (BALF) in the resolution phase. BALF from Cre+ lungs was more chemotactic for bone marrow-derived macrophages than Cre- in in vitro experiments, but there were no differences in the elaboration of chemokines in vivo. We speculate that absence of NPNT in BMs of the alveoli impairs or delays inflammatory and injury resolution in this model, but further studies are needed to establish the precise role of NPNT in tissue repair.

The bioactivity of soluble Fas ligand is modulated by key amino acids of its stalk region.

We have previously reported that the 26-amino acid N-terminus stalk region of soluble Fas ligand (sFasL), which is separate from its binding site, is required for its biological function. Here we investigate the mechanisms that link the structure of the sFasL stalk region with its function. Using site-directed mutagenesis we cloned a mutant form of sFasL in which all the charged amino acids of the stalk region were changed to neutral alanines (mut-sFasL). We used the Fas-sensitive Jurkat T-cell line and mouse and human alveolar epithelial cells to test the bioactivity of sFasL complexes, using caspase-3 activity and Annexin-V externalization as readouts. Finally, we tested the effects of mut-sFasL on lipopolysaccharide-induced lung injury in mice. We found that mutation of all the 8 charged amino acids of the stalk region into the non-charged amino acid alanine (mut-sFasL) resulted in reduced apoptotic activity compared to wild type sFasL (WT-sFasL). The mut-sFasL attenuated WT-sFasL function on the Fas-sensitive human T-cell line Jurkat and on primary human small airway epithelial cells. The inhibitory mechanism was associated with the formation of complexes of mut-sFasL with the WT protein. Intratracheal administration of the mut-sFasL to mice 24 hours after intratracheal Escherichia coli lipopolysaccharide resulted in attenuation of the inflammatory response 24 hours later. Therefore, the stalk region of sFasL has a critical role on bioactivity, and changes in the structure of the stalk region can result in mutant variants that interfere with the wild type protein function in vitro and in vivo.

TOLLIP deficiency is associated with increased resistance to Legionella pneumophila pneumonia.

Legionella pneumophila (Lp) is a flagellated, intracellular bacterium that can cause Legionnaires' disease (LD). Lp activates multiple innate immune receptors, and TOLLIP dampens MyD88-dependent signaling and may influence susceptibility to LD. We evaluated the effect of TOLLIP on innate immunity, pneumonia severity, and LD susceptibility in mouse lungs and human populations. To accomplish this, we evaluated the effect of TOLLIP on lung-specific Lp control and immune response and associated a common functional TOLLIP variant with Lp-induced innate immune responses and LD susceptibility in humans. After aerosol Lp infection, Tollip-/- mice demonstrated significantly fewer bacterial colony-forming unit and increased cytokine responses from BAL fluid. Tollip-/- macrophages also suppressed intracellular Lp replication in a flagellin-independent manner. The presence of a previously characterized, functionally active SNP associated with decreased TOLLIP mRNA transcript in monocytes was associated with increased TNF and IL-6 secretion after Lp stimulation of PBMC ex vivo. This genotype was separately associated with decreased LD susceptibility (309 controls, 88 cases, p = 0.008, OR 0.36, 95% CI 0.16-0.76) in a candidate gene association study. These results suggest that TOLLIP decreases lung-specific TLR responses to increase LD susceptibility in human populations. Better understanding of TOLLIP may lead to novel immunomodulatory therapies.

Role of integrin alpha8 in murine model of lung fibrosis.

BACKGROUND: Integrin α8 (ITGA8) heterodimerizes with integrin ß1 and is highly expressed in stromal cells of the lung. Platelet-derived growth factor receptor beta (PDGFRß+) cells constitute a major population of contractile myofibroblasts in the lung following bleomycin-induced fibrosis. Integrin α8ß1 is upregulated in fibrotic foci in bleomycin-induced lung injury. However, the functional role of ITGA8 in fibrogenesis has not been characterized. In this study, we examined whether genetic deletion of ITGA8 from PDGFRß+ cells in the lung altered fibrosis. METHODS: Pdgfrb-Cre/+;Itga8flox/- or Pdgfrb-Cre/+;Itga8flox/flox (Cre+) and control mice (Cre-) were used for in vitro and in vivo studies. Primary cultures of PDGFRß+ cells were exposed to TGFß, followed by RNA isolation for qPCR. For in vivo studies, Cre+ and Cre- mice were characterized at baseline and after bleomycin-induced fibrosis. RESULTS: PDGFRß-selected cells from Cre+ animals showed higher levels of Col1a1 expression after treatment with TGFß. However, Cre- and Cre+ animals showed no significant difference in measures of acute lung injury or fibrosis following bleomycin challenge. CONCLUSION: While ITGA8 deletion in lung PDGFRß+ stromal cells showed evidence of greater Col1a1 mRNA expression after TGFß treatment in vitro, no functional difference was detected in vivo.