Local lung hypoxia determines epithelial fate decisions during alveolar regeneration

Interplay between pulmonary epithelial stem cells and innate immune cells contribute to the repair and regeneration of ALI/ARDS

Mammalian lung is the important organ for ventilation and exchange of air and blood. Fresh air and venous blood are constantly delivered through the airway and vascular tree to the alveolus. Based on this, the airways and alveolis are persistently exposed to the external environment and are easily suffered from toxins, irritants and pathogens. For example, acute lung injury/acute respiratory distress syndrome (ALI/ARDS) is a common cause of respiratory failure in critical patients, whose typical pathological characters are diffuse epithelial and endothelial damage resulting in excessive accumulation of inflammatory fluid in the alveolar cavity. The supportive treatment is the main current treatment for ALI/ARDS with the lack of targeted effective treatment strategies. However, ALI/ARDS needs more targeted treatment measures. Therefore, it is extremely urgent to understand the cellular and molecular mechanisms that maintain alveolar epithelial barrier and airway integrity. Previous researches have shown that the lung epithelial cells with tissue stem cell function have the ability to repair and regenerate after injury. Also, it is able to regulate the phenotype and function of innate immune cells involving in regeneration of tissue repair. Meanwhile, we emphasize that interaction between the lung epithelial cells and innate immune cells is more supportive to repair and regenerate in the lung epithelium following acute lung injury. We reviewed the recent advances in injury and repair of lung epithelial stem cells and innate immune cells in ALI/ARDS, concentrating on alveolar type 2 cells and alveolar macrophages and their contribution to post-injury repair behavior of ALI/ARDS through the latest potential molecular communication mechanisms. This will help to develop new research strategies and therapeutic targets for ALI/ARDS.

Vascular endothelial-derived SPARCL1 exacerbates viral pneumonia through pro-inflammatory macrophage activation

Inflammation induced by lung infection is a double-edged sword, moderating both anti-viral and immune pathogenesis effects; the mechanism of the latter is not fully understood. Previous studies suggest the vasculature is involved in tissue injury. Here, we report that expression of Sparcl1, a secreted matricellular protein, is upregulated in pulmonary capillary endothelial cells (EC) during influenza-induced lung injury. Endothelial overexpression of SPARCL1 promotes detrimental lung inflammation, with SPARCL1 inducing 'M1-like' macrophages and related pro-inflammatory cytokines, while SPARCL1 deletion alleviates these effects. Mechanistically, SPARCL1 functions through TLR4 on macrophages in vitro, while TLR4 inhibition in vivo ameliorates excessive inflammation caused by endothelial Sparcl1 overexpression. Finally, SPARCL1 expression is increased in lung ECs from COVID-19 patients when compared with healthy donors, while fatal COVID-19 correlates with higher circulating SPARCL1 protein levels in the plasma. Our results thus implicate SPARCL1 as a potential prognosis biomarker for deadly COVID-19 pneumonia and as a therapeutic target for taming hyperinflammation in pneumonia.

Nrf-2 as a novel target in radiation induced lung injury

Radiation-induced lung injury (RILI) is a common and fatal complication of chest radiotherapy. The underlying mechanisms include radiation-induced oxidative stress caused by damage to the deoxyribonucleic acid (DNA) and production of reactive oxygen species (ROS), resulting in apoptosis of lung and endothelial cells and recruitment of inflammatory cells and myofibroblasts expressing NADPH oxidase to the site of injury, which in turn contribute to oxidative stress and cytokine production. Nuclear factor erythroid 2-related factor 2 (Nrf-2) is a vital transcription factor that regulates oxidative stress and inhibits inflammation. Studies have shown that Nrf-2 protects against radiation-induced lung inflammation and fibrosis. This review discusses the protective role of Nrf-2 in RILI and its possible mechanisms.

Repair and regeneration of the alveolar epithelium in lung injury

Considerable progress has been made in understanding the function of alveolar epithelial cells in a quiescent state and regeneration mechanism after lung injury. Lung injury occurs commonly from severe viral and bacterial infections, inhalation lung injury, and indirect injury sepsis. A series of pathological mechanisms caused by excessive injury, such as apoptosis, autophagy, senescence, and ferroptosis, have been studied. Recovery from lung injury requires the integrity of the alveolar epithelial cell barrier and the realization of gas exchange function. Regeneration mechanisms include the participation of epithelial progenitor cells and various niche cells involving several signaling pathways and proteins. While alveoli are damaged, alveolar type II (AT2) cells proliferate and differentiate into alveolar type I (AT1) cells to repair the damaged alveolar epithelial layer. Alveolar epithelial cells are surrounded by various cells, such as fibroblasts, endothelial cells, and various immune cells, which affect the proliferation and differentiation of AT2 cells through paracrine during alveolar regeneration. Besides, airway epithelial cells also contribute to the repair and regeneration process of alveolar epithelium. In this review, we mainly discuss the participation of epithelial progenitor cells and various niche cells involving several signaling pathways and transcription factors.

Cell competition drives bronchiolization and pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a progressive scarring disease arising from the maladaptive differentiation of lung stem cells into bronchial epithelial cells rather than into alveolar type 1 (AT1) cells, which are responsible for gas exchange. Here, we report that healthy lungs maintain their stem cells through tonic Hippo and β-catenin signaling, which promote Yap/Taz degradation and allow for low level expression of the Wnt target gene Myc. Inactivation of upstream activators of the Hippo pathway in lung stem cells inhibits this tonic β-catenin signaling and Myc expression and promotes their Taz mediated differentiation into AT1 cells. Vice versa, increased Myc in collaboration with Yap promotes the differentiation of lung stem cells along the basal and myoepithelial like lineages allowing them to invade and bronchiolize the lung parenchyma in a process reminiscent of submucosal gland development. Our findings indicate that stem cells exhibiting the highest Myc levels become supercompetitors that drive remodeling, whereas loser cells with lower Myc levels terminally differentiate into AT1 cells.

Lung repair and regeneration: Advanced models and insights into human disease

The respiratory system acts as both the primary site of gas exchange and an important sensor and barrier to the external environment. The increase in incidences of respiratory disease over the past decades has highlighted the importance of developing improved therapeutic approaches. This review will summarize recent research on the cellular complexity of the mammalian respiratory system with a focus on gas exchange and immunological defense functions of the lung. Different models of repair and regeneration will be discussed to help interpret human and animal data and spur the investigation of models and assays for future drug development.

Epithelial stem and progenitor cells of the upper airway

The upper airway acts as a conduit for the passage of air to the respiratory system and is implicated in several chronic diseases. Whilst the cell biology of the distal respiratory system has been described in great detail, less is known about the proximal upper airway. In this review, we describe the relevant anatomy of the upper airway and discuss the literature detailing the identification and roles of the progenitor cells of these regions.

An injury-induced tissue niche shaped by mesenchymal plasticity coordinates the regenerative and disease response in the lung

Severe lung injury causes basal stem cells to migrate and outcompete alveolar stem cells resulting in dysplastic repair and a loss of gas exchange function. This "stem cell collision" is part of a multistep process that is now revealed to generate an injury-induced tissue niche (iTCH) containing Keratin 5+ epithelial cells and plastic Pdgfra+ mesenchymal cells. Temporal and spatial single cell analysis reveals that iTCHs are governed by mesenchymal proliferation and Notch signaling, which suppresses Wnt and Fgf signaling in iTCHs. Conversely, loss of Notch in iTCHs rewires alveolar signaling patterns to promote euplastic regeneration and gas exchange. The signaling patterns of iTCHs can differentially phenotype fibrotic from degenerative human lung diseases, through apposing flows of FGF and WNT signaling. These data reveal the emergence of an injury and disease associated iTCH in the lung and the ability of using iTCH specific signaling patterns to discriminate human lung disease phenotypes.

Autologous transplantation of P63+ lung progenitor cells for chronic obstructive pulmonary disease therapy

Adult lung resident stem/progenitor cells, including P63+ progenitor cells, have demonstrated the capacity for regeneration of lung epithelium in preclinical models. Here, we report a clinical trial of intrapulmonary P63+ progenitor cell transplantation in 28 participants with stage II to IV chronic obstructive pulmonary disease (COPD). Autologous P63+ progenitor cells were isolated from the airway basal layer of participants in the intervention group via bronchoscopic brushing, cultured for 3 to 5 weeks, and then transplanted back into the lungs via bronchoscopy at 0.7 × 106 to 5.2 × 106 cells per kilogram of body weight. Twenty patients were evaluable at the end of the study (intervention group, n = 17; control group, n = 3). No grade 3 to 5 adverse events (AEs) or serious AEs occurred. Although bronchoscopy-associated AEs were recorded in participants in the intervention group, other AEs were not substantial different between groups. Twenty-four weeks after transplantation, participants in the intervention group displayed improvement in gas transfer capacity [diffusing capacity of the lung for carbon monoxide (DLCO) change from baseline: +18.2%], whereas the control group experienced a decrease (DLCO change from baseline: -17.4%; P = 0.008). Furthermore, participants in the intervention group showed >30-meter increase in walking distance within 6 minutes. Transcriptomic analysis of progenitor cells isolated from responding and nonresponding individuals in the intervention group showed that higher expression of P63 was associated with treatment efficacy. In conclusion, transplantation of cultured P63+ lung progenitor cells was safe and might represent a potential therapeutic strategy for COPD.

HIF-1α promotes virus replication and cytokine storm in H1N1 virus-induced severe pneumonia through cellular metabolic reprogramming

The mortality of patients with severe pneumonia caused by H1N1 infection is closely related to viral replication and cytokine storm. However, the specific mechanisms triggering virus replication and cytokine storm are still not fully elucidated. Here, we identified hypoxia inducible factor-1α (HIF-1α) as one of the major host molecules that facilitates H1N1 virus replication followed by cytokine storm in alveolar epithelial cells. Specifically, HIF-1α protein expression is upregulated after H1N1 infection. Deficiency of HIF-1α attenuates pulmonary injury, viral replication and cytokine storm in vivo. In addition, viral replication and cytokine storm were inhibited after HIF-1α knockdown in vitro. Mechanistically, the invasion of H1N1 virus into alveolar epithelial cells leads to a shift in glucose metabolism to glycolysis, with rapid production of ATP and lactate. Inhibition of glycolysis significantly suppresses viral replication and inflammatory responses. Further analysis revealed that H1N1-induced HIF-1α can promote the expression of hexokinase 2 (HK2), the key enzyme of glycolysis, and then not only provide energy for the rapid replication of H1N1 virus but also produce lactate, which reduces the accumulation of the MAVS/RIG-I complex and inhibits IFN-α/β production. In conclusion, this study demonstrated that the upregulation of HIF-1α by H1N1 infection augments viral replication and cytokine storm by cellular metabolic reprogramming toward glycolysis mainly through upregulation of HK2, providing a theoretical basis for finding potential targets for the treatment of severe pneumonia caused by H1N1 infection.

Lung regeneration: diverse cell types and the therapeutic potential

Lung tissue has a certain regenerative ability and triggers repair procedures after injury. Under controllable conditions, lung tissue can restore normal structure and function. Disruptions in this process can lead to respiratory system failure and even death, causing substantial medical burden. The main types of respiratory diseases are chronic obstructive pulmonary disease (COPD), idiopathic pulmonary fibrosis (IPF), and acute respiratory distress syndrome (ARDS). Multiple cells, such as lung epithelial cells, endothelial cells, fibroblasts, and immune cells, are involved in regulating the repair process after lung injury. Although the mechanism that regulates the process of lung repair has not been fully elucidated, clinical trials targeting different cells and signaling pathways have achieved some therapeutic effects in different respiratory diseases. In this review, we provide an overview of the cell type involved in the process of lung regeneration and repair, research models, and summarize molecular mechanisms involved in the regulation of lung regeneration and fibrosis. Moreover, we discuss the current clinical trials of stem cell therapy and pharmacological strategies for COPD, IPF, and ARDS treatment. This review provides a reference for further research on the molecular and cellular mechanisms of lung regeneration, drug development, and clinical trials.

Linking Gene Fusions to Bone Marrow Failure and Malignant Transformation in Dyskeratosis Congenita

Dyskeratosis Congenita (DC) is a multisystem disorder intrinsically associated with telomere dysfunction, leading to bone marrow failure (BMF). Although the pathology of DC is largely driven by mutations in telomere-associated genes, the implications of gene fusions, which emerge due to telomere-induced genomic instability, remain unexplored. We meticulously analyzed gene fusions in RNA-Seq data from DC patients to provide deeper insights into DC's progression. The most significant DC-specific gene fusions were subsequently put through in silico assessments to ascertain biophysical and structural attributes, including charge patterning, inherent disorder, and propensity for self-association. Selected candidates were then analyzed using deep learning-powered structural predictions and molecular dynamics simulations to gauge their potential for forming higher-order oligomers. Our exploration revealed that genes participating in fusion events play crucial roles in upholding genomic stability, facilitating hematopoiesis, and suppressing tumors. Notably, our analysis spotlighted a particularly disordered polyampholyte fusion protein that exhibits robust higher-order oligomerization dynamics. To conclude, this research underscores the potential significance of several high-confidence gene fusions in the progression of BMF in DC, particularly through the dysregulation of genomic stability, hematopoiesis, and tumor suppression. Additionally, we propose that these fusion proteins might hold a detrimental role, specifically in inducing proteotoxicity-driven hematopoietic disruptions.

Airway epithelial cell identity and plasticity are constrained by Sox2 during lung homeostasis, tissue regeneration, and in human disease

Maintenance of the cellular boundary between airway and alveolar compartments during homeostasis and after injury is essential to prohibit pathological plasticity which can reduce respiratory function. Lung injury and disease can induce either functional alveolar epithelial regeneration or dysplastic formation of keratinized epithelium which does not efficiently contribute to gas exchange. Here we show that Sox2 preserves airway cell identity and prevents fate changes into either functional alveolar tissue or pathological keratinization following lung injury. Loss of Sox2 in airway epithelium leads to a loss of airway epithelial identity with a commensurate gain in alveolar and basal cell identity, in part due to activation of Wnt signaling in secretory cells and increased Trp63 expression in intrapulmonary basal-like progenitors. In idiopathic pulmonary fibrosis, loss of SOX2 expression correlates with increased WNT signaling activity in dysplastic keratinized epithelium. SOX2-deficient dysplastic epithelial cells are also observed in COVID-19 damaged lungs. Thus, Sox2 provides a molecular barrier that suppresses airway epithelial plasticity to prevent acquisition of alveolar or basal cell identity after injury and help guide proper epithelial fate and regeneration.

COVID-19 and influenza infections mediate distinct pulmonary cellular and transcriptomic changes

SARS-CoV-2 infection can cause persistent respiratory sequelae. However, the underlying mechanisms remain unclear. Here we report that sub-lethally infected K18-human ACE2 mice show patchy pneumonia associated with histiocytic inflammation and collagen deposition at 21 and 45 days post infection (DPI). Transcriptomic analyses revealed that compared to influenza-infected mice, SARS-CoV-2-infected mice had reduced interferon-gamma/alpha responses at 4 DPI and failed to induce keratin 5 (Krt5) at 6 DPI in lung, a marker of nascent pulmonary progenitor cells. Histologically, influenza- but not SARS-CoV-2-infected mice showed extensive Krt5+ "pods" structure co-stained with stem cell markers Trp63/NGFR proliferated in the pulmonary consolidation area at both 7 and 14 DPI, with regression at 21 DPI. These Krt5+ "pods" structures were not observed in the lungs of SARS-CoV-2-infected humans or nonhuman primates. These results suggest that SARS-CoV-2 infection fails to induce nascent Krt5+ cell proliferation in consolidated regions, leading to incomplete repair of the injured lung.

Proximal immune-epithelial progenitor interactions drive chronic tissue sequelae post COVID-19

The long-term physiological consequences of SARS-CoV-2, termed Post-Acute Sequelae of COVID-19 (PASC), are rapidly evolving into a major public health concern. The underlying cellular and molecular etiology remain poorly defined but growing evidence links PASC to abnormal immune responses and/or poor organ recovery post-infection. Yet, the precise mechanisms driving non-resolving inflammation and impaired tissue repair in the context of PASC remain unclear. With insights from three independent clinical cohorts of PASC patients with abnormal lung function and/or viral infection-mediated pulmonary fibrosis, we established a clinically relevant mouse model of post-viral lung sequelae to investigate the pathophysiology of respiratory PASC. By employing a combination of spatial transcriptomics and imaging, we identified dysregulated proximal interactions between immune cells and epithelial progenitors unique to the fibroproliferation in respiratory PASC but not acute COVID-19 or idiopathic pulmonary fibrosis (IPF). Specifically, we found a central role for lung-resident CD8+ T cell-macrophage interactions in maintaining Krt8hi transitional and ectopic Krt5+ basal cell progenitors, thus impairing alveolar regeneration and driving fibrotic sequelae after acute viral pneumonia. Mechanistically, CD8+ T cell derived IFN-γ and TNF stimulated lung macrophages to chronically release IL-1β, resulting in the abnormal accumulation of dysplastic epithelial progenitors and fibrosis. Notably, therapeutic neutralization of IFN-γ and TNF, or IL-1β after the resolution of acute infection resulted in markedly improved alveolar regeneration and restoration of pulmonary function. Together, our findings implicate a dysregulated immune-epithelial progenitor niche in driving respiratory PASC. Moreover, in contrast to other approaches requiring early intervention, we highlight therapeutic strategies to rescue fibrotic disease in the aftermath of respiratory viral infections, addressing the current unmet need in the clinical management of PASC and post-viral disease.

Proximal immune-epithelial progenitor interactions drive chronic tissue sequelae post COVID-19

The long-term physiological consequences of SARS-CoV-2, termed Post-Acute Sequelae of COVID-19 (PASC), are rapidly evolving into a major public health concern. The underlying cellular and molecular etiology remain poorly defined but growing evidence links PASC to abnormal immune responses and/or poor organ recovery post-infection. Yet, the precise mechanisms driving non-resolving inflammation and impaired tissue repair in the context of PASC remain unclear. With insights from three independent clinical cohorts of PASC patients with abnormal lung function and/or viral infection-mediated pulmonary fibrosis, we established a clinically relevant mouse model of post-viral lung sequelae to investigate the pathophysiology of respiratory PASC. By employing a combination of spatial transcriptomics and imaging, we identified dysregulated proximal interactions between immune cells and epithelial progenitors unique to the fibroproliferation in respiratory PASC but not acute COVID-19 or idiopathic pulmonary fibrosis (IPF). Specifically, we found a central role for lung-resident CD8+ T cell-macrophage interactions in maintaining Krt8hi transitional and ectopic Krt5+ basal cell progenitors, thus impairing alveolar regeneration and driving fibrotic sequelae after acute viral pneumonia. Mechanistically, CD8+ T cell derived IFN-γ and TNF stimulated lung macrophages to chronically release IL-1β, resulting in the abnormal accumulation of dysplastic epithelial progenitors and fibrosis. Notably, therapeutic neutralization of IFN-γ and TNF, or IL-1β after the resolution of acute infection resulted in markedly improved alveolar regeneration and restoration of pulmonary function. Together, our findings implicate a dysregulated immune-epithelial progenitor niche in driving respiratory PASC. Moreover, in contrast to other approaches requiring early intervention, we highlight therapeutic strategies to rescue fibrotic disease in the aftermath of respiratory viral infections, addressing the current unmet need in the clinical management of PASC and post-viral disease.

Alveolar epithelial regeneration in the aging lung

Advancing age is the most important risk factor for the development of and mortality from acute and chronic lung diseases, including pneumonia, chronic obstructive pulmonary disease, and pulmonary fibrosis. This risk was manifest during the COVID-19 pandemic, when elderly people were disproportionately affected and died from SARS-CoV-2 pneumonia. However, the recent pandemic also provided lessons on lung resilience. An overwhelming majority of patients with SARS-CoV-2 pneumonia, even those with severe disease, recovered with near-complete restoration of lung architecture and function. These observations are inconsistent with historic views of the lung as a terminally differentiated organ incapable of regeneration. Here, we review emerging hypotheses that explain how the lung repairs itself after injury and why these mechanisms of lung repair fail in some individuals, particularly the elderly.

Lung extracellular matrix modulates KRT5+ basal cell activity in pulmonary fibrosis

Aberrant expansion of KRT5+ basal cells in the distal lung accompanies progressive alveolar epithelial cell loss and tissue remodelling during fibrogenesis in idiopathic pulmonary fibrosis (IPF). The mechanisms determining activity of KRT5+ cells in IPF have not been delineated. Here, we reveal a potential mechanism by which KRT5+ cells migrate within the fibrotic lung, navigating regional differences in collagen topography. In vitro, KRT5+ cell migratory characteristics and expression of remodelling genes are modulated by extracellular matrix (ECM) composition and organisation. Mass spectrometry- based proteomics revealed compositional differences in ECM components secreted by primary human lung fibroblasts (HLF) from IPF patients compared to controls. Over-expression of ECM glycoprotein, Secreted Protein Acidic and Cysteine Rich (SPARC) in the IPF HLF matrix restricts KRT5+ cell migration in vitro. Together, our findings demonstrate how changes to the ECM in IPF directly influence KRT5+ cell behaviour and function contributing to remodelling events in the fibrotic niche.

CTGF promotes the repair and regeneration of alveoli after acute lung injury by promoting the proliferation of subpopulation of AEC2s

BACKGROUND

Functional alveolar regeneration is essential for the restoration of normal lung homeostasis after acute lung injury (ALI) and acute respiratory distress syndrome (ARDS). Lung is a relatively quiescent organ and a variety of stem cells are recruited to participate in lung repair and regeneration after lung tissue injury. However, there is still no effective method for promoting the proliferation of endogenous lung stem cells to promote repair and regeneration.

METHODS

Using protein mass spectrometry analysis, we analyzed the microenvironment after acute lung injury. RNA sequencing and image cytometry were used in the alveolar epithelial type 2 cells (AEC2s) subgroup identification. Then we used Sftpc+AEC2 lineage tracking mice and purified AEC2s to further elucidate the molecular mechanism by which CTGF regulates AEC2s proliferation both in vitro and in vivo. Bronchoalveolar lavage fluid (BALF) from thirty ARDS patients who underwent bronchoalveolar lavage was collected for the analysis of the correlation between the expressing of Krt5 in BALF and patients' prognosis.

RESULTS

Here, we elucidate that AEC2s are the main facultative stem cells of the distal lung after ALI and ARDS. The increase of connective tissue growth factor (CTGF) in the microenvironment after ALI promoted the proliferation of AEC2s subpopulations. Proliferated AEC2s rapidly expanded and differentiated into alveolar epithelial type 1 cells (AEC1s) in the regeneration after ALI. CTGF initiates the phosphorylation of LRP6 by promoting the interaction between Krt5 and LRP6 of AEC2s, thus activating the Wnt signaling pathway, which is the molecular mechanism of CTGF promoting the proliferation of AEC2s subpopulation.

CONCLUSIONS

Our study verifies that CTGF promotes the repair and regeneration of alveoli after acute lung injury by promoting the proliferation of AEC2s subpopulation.

Epithelial plasticity and innate immune activation promote lung tissue remodeling following respiratory viral infection

Epithelial plasticity has been suggested in lungs of mice following genetic depletion of stem cells but is of unknown physiological relevance. Viral infection and chronic lung disease share similar pathological features of stem cell loss in alveoli, basal cell (BC) hyperplasia in small airways, and innate immune activation, that contribute to epithelial remodeling and loss of lung function. We show that a subset of distal airway secretory cells, intralobar serous (IS) cells, are activated to assume BC fates following influenza virus infection. Injury-induced hyperplastic BC (hBC) differ from pre-existing BC by high expression of IL-22Ra1 and undergo IL-22-dependent expansion for colonization of injured alveoli. Resolution of virus-elicited inflammation results in BC to IS re-differentiation in repopulated alveoli, and increased local expression of protective antimicrobial factors, but fails to restore normal alveolar epithelium responsible for gas exchange.

Durable alveolar engraftment of PSC-derived lung epithelial cells into immunocompetent mice

Durable reconstitution of the distal lung epithelium with pluripotent stem cell (PSC) derivatives, if realized, would represent a promising therapy for diseases that result from alveolar damage. Here, we differentiate murine PSCs into self-renewing lung epithelial progenitors able to engraft into the injured distal lung epithelium of immunocompetent, syngeneic mouse recipients. After transplantation, these progenitors mature in the distal lung, assuming the molecular phenotypes of alveolar type 2 (AT2) and type 1 (AT1) cells. After months in vivo, donor-derived cells retain their mature phenotypes, as characterized by single-cell RNA sequencing (scRNA-seq), histologic profiling, and functional assessment that demonstrates continued capacity of the engrafted cells to proliferate and differentiate. These results indicate durable reconstitution of the distal lung's facultative progenitor and differentiated epithelial cell compartments with PSC-derived cells, thus establishing a novel model for pulmonary cell therapy that can be utilized to better understand the mechanisms and utility of engraftment.

Circadian regulation of lung repair and regeneration

Optimal lung repair and regeneration are essential for recovery from viral infections, including influenza A virus (IAV). We have previously demonstrated that acute inflammation and mortality induced by IAV is under circadian control. However, it is not known whether the influence of the circadian clock persists beyond the acute outcomes. Here, we utilize the UK Biobank to demonstrate an association between poor circadian rhythms and morbidity from lower respiratory tract infections, including the need for hospitalization and mortality after discharge; this persists even after adjusting for common confounding factors. Furthermore, we use a combination of lung organoid assays, single-cell RNA sequencing, and IAV infection in different models of clock disruption to investigate the role of the circadian clock in lung repair and regeneration. We show that lung organoids have a functional circadian clock and the disruption of this clock impairs regenerative capacity. Finally, we find that the circadian clock acts through distinct pathways in mediating lung regeneration - in tracheal cells via the Wnt/β-catenin pathway and through IL-1β in alveolar epithelial cells. We speculate that adding a circadian dimension to the critical process of lung repair and regeneration will lead to novel therapies and improve outcomes.

Senescence of alveolar epithelial progenitor cells: a critical driver of lung fibrosis

Pulmonary fibrosis comprises a range of chronic interstitial lung diseases (ILDs) that impose a significant burden on patients and public health. Among these, idiopathic pulmonary fibrosis (IPF), a disease of aging, is the most common and most severe form of ILD and is treated largely by lung transplantation. The lack of effective treatments to stop or reverse lung fibrosis-in fact, fibrosis in most organs-has sparked the need to understand causative mechanisms with the goal of identifying critical points for potential therapeutic intervention. Findings from many groups have indicated that repeated injury to the alveolar epithelium-where gas exchange occurs-leads to stem cell exhaustion and impaired alveolar repair that, in turn, triggers the onset and progression of fibrosis. Cellular senescence of alveolar epithelial progenitors is a critical cause of stemness failure. Hence, senescence impairs repair and thus contributes significantly to fibrosis. In this review, we discuss recent evidence indicating that senescence of epithelial progenitor cells impairs alveolar homeostasis and repair creating a profibrotic environment. Moreover, we discuss the impact of senescent alveolar epithelial progenitors, alveolar type 2 (AT2) cells, and AT2-derived transitional epithelial cells in fibrosis. Emerging evidence indicates that transitional epithelial cells are prone to senescence and, hence, are a new player involved in senescence-associated lung fibrosis. Understanding the complex interplay of cell types and cellular regulatory factors contributing to alveolar epithelial progenitor senescence will be crucial to developing targeted therapies to mitigate their downstream profibrotic sequelae and to promote normal alveolar repair.NEW & NOTEWORTHY With an aging population, lung fibrotic diseases are becoming a global health burden. Dysfunctional repair of the alveolar epithelium is a key causative process that initiates lung fibrosis. Normal alveolar regeneration relies on functional progenitor cells; however, the senescence of these cells, which increases with age, hinders their ability to contribute to repair. Here, we discuss studies on the control and consequence of progenitor cell senescence in fibrosis and opportunities for research.

The lung mesenchyme in development, regeneration, and fibrosis

Mesenchymal cells are uniquely located at the interface between the epithelial lining and the stroma, allowing them to act as a signaling hub among diverse cellular compartments of the lung. During embryonic and postnatal lung development, mesenchyme-derived signals instruct epithelial budding, branching morphogenesis, and subsequent structural and functional maturation. Later during adult life, the mesenchyme plays divergent roles wherein its balanced activation promotes epithelial repair after injury while its aberrant activation can lead to pathological remodeling and fibrosis that are associated with multiple chronic pulmonary diseases, including bronchopulmonary dysplasia, idiopathic pulmonary fibrosis, and chronic obstructive pulmonary disease. In this Review, we discuss the involvement of the lung mesenchyme in various morphogenic, neomorphogenic, and dysmorphogenic aspects of lung biology and health, with special emphasis on lung fibroblast subsets and smooth muscle cells, intercellular communication, and intrinsic mesenchymal mechanisms that drive such physiological and pathophysiological events throughout development, homeostasis, injury repair, regeneration, and aging.

Targeted alveolar regeneration with Frizzled-specific agonists

Wnt ligands oligomerize Frizzled (Fzd) and Lrp5/6 receptors to control the specification and activity of stem cells in many species. How Wnt signaling is selectively activated in different stem cell populations, often within the same organ, is not understood. In lung alveoli, we show that distinct Wnt receptors are expressed by epithelial (Fzd5/6), endothelial (Fzd4), and stromal (Fzd1) cells. Fzd5 is uniquely required for alveolar epithelial stem cell activity, whereas fibroblasts utilize distinct Fzd receptors. Using an expanded repertoire of Fzd-Lrp agonists, we could activate canonical Wnt signaling in alveolar epithelial stem cells via either Fzd5 or, unexpectedly, non-canonical Fzd6. A Fzd5 agonist (Fzd5ag) or Fzd6ag stimulated alveolar epithelial stem cell activity and promoted survival in mice after lung injury, but only Fzd6ag promoted an alveolar fate in airway-derived progenitors. Therefore, we identify a potential strategy for promoting regeneration without exacerbating fibrosis during lung injury.

Regulatory Functions of Hypoxia in Host-Parasite Interactions: A Focus on Enteric, Tissue, and Blood Protozoa

Body tissues are subjected to various oxygenic gradients and fluctuations and hence can become transiently hypoxic. Hypoxia-inducible factor (HIF) is the master transcriptional regulator of the cellular hypoxic response and is capable of modulating cellular metabolism, immune responses, epithelial barrier integrity, and local microbiota. Recent reports have characterized the hypoxic response to various infections. However, little is known about the role of HIF activation in the context of protozoan parasitic infections. Growing evidence suggests that tissue and blood protozoa can activate HIF and subsequent HIF target genes in the host, helping or hindering their pathogenicity. In the gut, enteric protozoa are adapted to steep longitudinal and radial oxygen gradients to complete their life cycle, yet the role of HIF during these protozoan infections remains unclear. This review focuses on the hypoxic response to protozoa and its role in the pathophysiology of parasitic infections. We also discuss how hypoxia modulates host immune responses in the context of protozoan infections.

Role of heparanase in ARDS through autophagy and exosome pathway (review)

Acute respiratory distress syndrome (ARDS) is the most common respiratory disease in ICU. Although there are many treatment and support methods, the mortality rate is still high. The main pathological feature of ARDS is the damage of pulmonary microvascular endothelium and alveolar epithelium caused by inflammatory reaction, which may lead to coagulation system disorder and pulmonary fibrosis. Heparanase (HPA) plays an significant role in inflammation, coagulation, fibrosis. It is reported that HPA degrades a large amount of HS in ARDS, leading to the damage of endothelial glycocalyx and inflammatory factors are released in large quantities. HPA can aggrandize the release of exosomes through syndecan-syntenin-Alix pathway, leading to a series of pathological reactions; at the same time, HPA can cause abnormal expression of autophagy. Therefore, we speculate that HPA promotes the occurrence and development of ARDS through exosomes and autophagy, which leads to a large amount of release of inflammatory factors, coagulation disorder and pulmonary fibrosis. This article mainly describes the mechanism of HPA on ARDS.

Temporal and spatial staging of lung alveolar regeneration is determined by the grainyhead transcription factor Tfcp2l1

Alveolar epithelial type 2 (AT2) cells harbor the facultative progenitor capacity in the lung alveolus to drive regeneration after lung injury. Using single-cell transcriptomics, software-guided segmentation of tissue damage, and in vivo mouse lineage tracing, we identified the grainyhead transcription factor cellular promoter 2-like 1 (Tfcp2l1) as a regulator of this regenerative process. Tfcp2l1 loss in adult AT2 cells inhibits self-renewal and enhances AT2-AT1 differentiation during tissue regeneration. Conversely, Tfcp2l1 blunts the proliferative response to inflammatory signaling during the early acute injury phase. Tfcp2l1 temporally regulates AT2 self-renewal and differentiation in alveolar regions undergoing active regeneration. Single-cell transcriptomics and lineage tracing reveal that Tfcp2l1 regulates cell fate dynamics across the AT2-AT1 differentiation and restricts the inflammatory program in murine AT2 cells. Organoid modeling shows that Tfcp2l1 regulation of interleukin-1 (IL-1) receptor expression controlled these cell fate dynamics. These findings highlight the critical role Tfcp2l1 plays in balancing epithelial cell self-renewal and differentiation during alveolar regeneration.

Vascular Endothelial-derived SPARCL1 Exacerbates Viral Pneumonia Through Pro-Inflammatory Macrophage Activation

Inflammation upon infectious lung injury is a double-edged sword: while tissue-infiltrating immune cells and cytokines are necessary to control infection, these same factors often aggravate injury. Full appreciation of both the sources and targets of inflammatory mediators is required to facilitate strategies to maintain antimicrobial effects while minimizing off-target epithelial and endothelial damage. Recognizing that the vasculature is centrally involved in tissue responses to injury and infection, we observed that pulmonary capillary endothelial cells (ECs) exhibit dramatic transcriptomic changes upon influenza injury punctuated by profound upregulation of Sparcl1 . Endothelial deletion and overexpression of SPARCL1 implicated this secreted matricellular protein in driving key pathophysiologic symptoms of pneumonia, which we demonstrate result from its effects on macrophage polarization. SPARCL1 induces a shift to a pro-inflammatory "M1-like" phenotype (CD86 + CD206 - ), thereby increasing associated cytokine levels. Mechanistically, SPARCL1 acts directly on macrophages in vitro to induce the pro-inflammatory phenotype via activation of TLR4, and TLR4 inhibition in vivo ameliorates inflammatory exacerbations caused by endothelial Sparcl1 overexpression. Finally, we confirmed significant elevation of SPARCL1 in COVID-19 lung ECs in comparison with those from healthy donors. Survival analysis demonstrated that patients with fatal COVID-19 had higher levels of circulating SPARCL1 protein compared to those who recovered, indicating the potential of SPARCL1 as a biomarker for prognosis of pneumonia and suggesting that personalized medicine approaches might be harnessed to block SPARCL1 and improve outcomes in high-expressing patients.

ABL kinases regulate the stabilization of HIF-1α and MYC through CPSF1

The hypoxia-inducible factor 1-α (HIF-1α) enables cells to adapt and respond to hypoxia (Hx), and the activity of this transcription factor is regulated by several oncogenic signals and cellular stressors. While the pathways controlling normoxic degradation of HIF-1α are well understood, the mechanisms supporting the sustained stabilization and activity of HIF-1α under Hx are less clear. We report that ABL kinase activity protects HIF-1α from proteasomal degradation during Hx. Using a fluorescence-activated cell sorting (FACS)-based CRISPR/Cas9 screen, we identified HIF-1α as a substrate of the cleavage and polyadenylation specificity factor-1 (CPSF1), an E3-ligase which targets HIF-1α for degradation in the presence of an ABL kinase inhibitor in Hx. We show that ABL kinases phosphorylate and interact with CUL4A, a cullin ring ligase adaptor, and compete with CPSF1 for CUL4A binding, leading to increased HIF-1α protein levels. Further, we identified the MYC proto-oncogene protein as a second CPSF1 substrate and show that active ABL kinase protects MYC from CPSF1-mediated degradation. These studies uncover a role for CPSF1 in cancer pathobiology as an E3-ligase antagonizing the expression of the oncogenic transcription factors, HIF-1α and MYC.

Lung development and regeneration: newly defined cell types and progenitor status

The lung is the most critical organ of the respiratory system supporting gas exchange. Constant interaction with the external environment makes the lung vulnerable to injury. Thus, a deeper understanding of cellular and molecular processes underlying lung development programs and evaluation of progenitor status within the lung is an essential part of lung regenerative medicine. In this review, we aim to discuss the current understanding of lung development process and regenerative capability. We highlight the advances brought by multi-omics approaches, single-cell transcriptome, in particular, that can help us further dissect the cellular player and molecular signaling underlying those processes.

IL-1β Induces a Proinflammatory Fibroblast Microenvironment that Impairs Lung Progenitors' Function

Chronic obstructive pulmonary disease (COPD) is characterized by a persistent inflammatory state in the lungs and defective tissue repair. Although the inflammatory response in patients with COPD is well characterized and known to be exaggerated during exacerbations, its contribution to lung injury and abnormal repair is still unclear. In this study, we aimed to investigate how the inflammatory microenvironment affects the epithelial progenitors and their supporting mesenchymal niche cells involved in tissue repair of the distal lung. We focused on IL-1β, a key inflammatory mediator that is increased during exacerbations of COPD, and used an organoid model of lung epithelial cells and fibroblasts to assess the effect of IL-1β treatment on these cells' transcriptome and secreted factors. Whereas direct treatment of the lung organoids with IL-1β promoted organoid growth, this switched toward inhibition when it was added as fibroblast pretreatment followed by organoid treatment. We then investigated the IL-1β-driven mechanisms in the fibroblasts and found an inflammatory response related to (C-X-C motif) ligand (CXCL) chemokines; we confirmed that these chemokines were responsible for the impaired organoid growth and found that targeting their C-X-C chemokine receptors 1/2 (CXCR1/2) receptors or the IL-1β intracellular signaling reduced the proinflammatory response and restored organoid growth. These data demonstrate that IL-1β alters the fibroblasts' state by promoting a distinct inflammatory response, switching their supportive function on epithelial progenitors toward an inhibitory one in an organoid assay. These results imply that chronic inflammation functions as a shift toward inhibition of repair, thereby contributing to chronic inflammatory diseases like COPD.

Basal Cell-derived WNT7A Promotes Fibrogenesis at the Fibrotic Niche in Idiopathic Pulmonary Fibrosis

Loss of epithelial integrity, bronchiolarization, and fibroblast activation are key characteristics of idiopathic pulmonary fibrosis (IPF). Prolonged accumulation of basal-like cells in IPF may impact the fibrotic niche to promote fibrogenesis. To investigate their role in IPF, basal cells were isolated from IPF explant and healthy donor lung tissues. Single-cell RNA sequencing was used to assess differentially expressed genes in basal cells. Basal cell and niche interaction was demonstrated with the sLP-mCherry niche labeling system. Luminex assays were used to assess cytokines secreted by basal cells. The role of basal cells in fibroblast activation was studied. Three-dimensional organoid culture assays were used to interrogate basal cell effects on AEC2 (type 2 alveolar epithelial cell) renewal capacity. Perturbation was used to investigate WNT7A function in vitro and in a repetitive bleomycin model in vivo. We found that WNT7A is highly and specifically expressed in basal-like cells. Proteins secreted by basal cells can be captured by neighboring fibroblasts and AEC2s. Basal cells or basal cell-conditioned media activate fibroblasts through WNT7A. Basal cell-derived WNT7A inhibits AEC2 progenitor cell renewal in three-dimensional organoid cultures. Neutralizing antibodies against WNT7A or a small molecule inhibitor of Frizzled signaling abolished basal cell-induced fibroblast activation and attenuated lung fibrosis in mice. In summary, basal cells and basal cell-derived WNT7A are key components of the fibrotic niche, providing a unique non-stem cell function of basal cells in IPF progression and a novel targeting strategy for IPF.

Hypoxia enhances IPF mesenchymal progenitor cell fibrogenicity via the lactate/GPR81/HIF1α pathway

Hypoxia is a sentinel feature of idiopathic pulmonary fibrosis (IPF). The IPF microenvironment contains high lactate levels, and hypoxia enhances cellular lactate production. Lactate, acting through the GPR81 lactate receptor, serves as a signal molecule regulating cellular processes. We previously identified intrinsically fibrogenic mesenchymal progenitor cells (MPCs) that drive fibrosis in the lungs of patients with IPF. However, whether hypoxia enhances IPF MPC fibrogenicity is unclear. We hypothesized that hypoxia increases IPF MPC fibrogenicity via lactate and its cognate receptor GPR81. Here we show that hypoxia promotes IPF MPC self-renewal. The mechanism involves hypoxia-mediated enhancement of LDHA function and lactate production and release. Hypoxia also increases HIF1α levels, and this increase in turn augments the expression of GPR81. Exogenous lactate operating through GPR81 promotes IPF MPC self-renewal. IHC analysis of IPF lung tissue demonstrates IPF MPCs expressing GPR81 and hypoxic markers on the periphery of the fibroblastic focus. We show that hypoxia enhances IPF MPC fibrogenicity in vivo. We demonstrate that knockdown of GPR81 inhibits hypoxia-induced IPF MPC self-renewal in vitro and attenuates hypoxia-induced IPF MPC fibrogenicity in vivo. Our data demonstrate that hypoxia creates a feed-forward loop that augments IPF MPC fibrogenicity via the lactate/GPR81/HIF1α pathway.

Unusual X chromosome inactivation maintenance in female alveolar type 2 cells is correlated with increased numbers of X-linked escape genes and sex-biased gene expression

Sex differences exist for many lung pathologies, including COVID-19 and pulmonary fibrosis, but the mechanistic basis for this remains unclear. Alveolar type 2 cells (AT2s), which play a key role in alveolar lung regeneration, express the X-linked Ace2 gene that has roles in lung repair and SARS-CoV-2 pathogenesis, suggesting that X chromosome inactivation (XCI) in AT2s might impact sex-biased lung pathology. Here we investigate XCI maintenance and sex-specific gene expression profiles using male and female AT2s. Remarkably, the inactive X chromosome (Xi) lacks robust canonical Xist RNA "clouds" and less enrichment of heterochromatic modifications in human and mouse AT2s. We demonstrate that about 68% of expressed X-linked genes in mouse AT2s, including Ace2, escape XCI. There are genome-wide expression differences between male and female AT2s, likely influencing both lung physiology and pathophysiologic responses. These studies support a renewed focus on AT2s as a potential contributor to sex-biased differences in lung disease.

Repair of airway epithelia requires metabolic rewiring towards fatty acid oxidation

Epithelial tissues provide front-line barriers shielding the organism from invading pathogens and harmful substances. In the airway epithelium, the combined action of multiciliated and secretory cells sustains the mucociliary escalator required for clearance of microbes and particles from the airways. Defects in components of mucociliary clearance or barrier integrity are associated with recurring infections and chronic inflammation. The timely and balanced differentiation of basal cells into mature epithelial cell subsets is therefore tightly controlled. While different growth factors regulating progenitor cell proliferation have been described, little is known about the role of metabolism in these regenerative processes. Here we show that basal cell differentiation correlates with a shift in cellular metabolism from glycolysis to fatty acid oxidation (FAO). We demonstrate both in vitro and in vivo that pharmacological and genetic impairment of FAO blocks the development of fully differentiated airway epithelial cells, compromising the repair of airway epithelia. Mechanistically, FAO links to the hexosamine biosynthesis pathway to support protein glycosylation in airway epithelial cells. Our findings unveil the metabolic network underpinning the differentiation of airway epithelia and identify novel targets for intervention to promote lung repair.

Notch1 Induces Defective Epithelial Surfactant Processing and Pulmonary Fibrosis

Rationale: Although type II alveolar epithelial cells (AEC2s) are chronically injured in idiopathic pulmonary fibrosis (IPF), they contribute to epithelial regeneration in IPF. Objectives: We hypothesized that Notch signaling may contribute to AEC2 proliferation, dedifferentiation characterized by loss of surfactant processing machinery, and lung fibrosis in IPF. Methods: We applied microarray analysis, kinome profiling, flow cytometry, immunofluorescence analysis, western blotting, quantitative PCR, and proliferation and surface activity analysis to study epithelial differentiation, proliferation, and matrix deposition in vitro (AEC2 lines, primary murine/human AEC2s), ex vivo (human IPF-derived precision-cut lung slices), and in vivo (bleomycin and pepstatin application, Notch1 [Notch receptor 1] intracellular domain overexpression). Measurements and Main Results: We document here extensive SP-B and -C (surfactant protein-B and -C) processing defects in IPF AEC2s, due to loss of Napsin A, resulting in increased intra-alveolar surface tension and alveolar collapse and induction of endoplasmic reticulum stress in AEC2s. In vivo pharmacological inhibition of Napsin A results in the development of AEC2 injury and overt lung fibrosis. We also demonstrate that Notch1 signaling is already activated early in IPF and determines AEC2 fate by inhibiting differentiation (reduced lamellar body compartment, reduced capacity to process hydrophobic SP) and by causing increased epithelial proliferation and development of lung fibrosis, putatively via altered JAK (Janus kinase)/Stat (signal transducer and activator of transcription) signaling in AEC2s. Conversely, inhibition of Notch signaling in IPF-derived precision-cut lung slices improved the surfactant processing capacity of AEC2s and reversed fibrosis. Conclusions: Notch1 is a central regulator of AEC2 fate in IPF. It induces alveolar epithelial proliferation and loss of Napsin A and of surfactant proprotein processing, and it contributes to fibroproliferation.

Pulmonary endogenous progenitor stem cell subpopulation: Physiology, pathogenesis, and progress

Lungs are structurally and functionally complex organs consisting of diverse cell types from the proximal to distal axis. They have direct contact with the external environment and are constantly at risk of various injuries. Capable to proliferate and differentiate, pulmonary endogenous progenitor stem cells contribute to the maintenance of lung structure and function both under homeostasis and following injuries. Discovering candidate pulmonary endogenous progenitor stem cell types and underlying regenerative mechanisms provide insights into therapeutic strategy development for lung diseases. In this review, we reveal their compositions, roles in lung disease pathogenesis and injury repair, and the underlying mechanisms. We further underline the advanced progress in research approach and potential therapy for lung regeneration. We also demonstrate the feasibility and prospects of pulmonary endogenous stem cell transplantation for lung disease treatment.

After the Storm: Regeneration, Repair, and Reestablishment of Homeostasis Between the Alveolar Epithelium and Innate Immune System Following Viral Lung Injury

The mammalian lung has an enormous environmental-epithelial interface that is optimized to accomplish the principal function of the respiratory system, gas exchange. One consequence of evolving such a large surface area is that the lung epithelium is continuously exposed to toxins, irritants, and pathogens. Maintaining homeostasis in this environment requires a delicate balance of cellular signaling between the epithelium and innate immune system. Following injury, the epithelium can be either fully regenerated in form and function or repaired by forming dysplastic scar tissue. In this review, we describe the major mechanisms of damage, regeneration, and repair within the alveolar niche where gas exchange occurs. With a focus on viral infection, we summarize recent work that has established how epithelial proliferation is arrested during infection and how the innate immune system guides its reconstitution during recovery. The consequences of these processes going awry are also considered, with an emphasis on how this will impact postpandemic pulmonary biology and medicine.

Epithelial Endoplasmic Reticulum Stress Enhances the Risk of Muc5b-associated Lung Fibrosis

The gain-of-function minor allele of the MUC5B (mucin 5B, oligomeric mucus/gel-forming) promoter (rs35705950) is the strongest risk factor for idiopathic pulmonary fibrosis (IPF), a devastating fibrotic lung disease that leads to progressive respiratory failure in adults. We have previously demonstrated that Muc5b overexpression in mice worsens lung fibrosis after bleomycin exposure and have hypothesized that excess Muc5b promotes endoplasmic reticulum (ER) stress and apoptosis, stimulating fibrotic lung injury. Here, we report that ER stress pathway members ATF4 (activating transcription factor 4) and ATF6 coexpress with MUC5B in epithelia of the distal IPF airway and honeycomb cyst and that this is more pronounced in carriers of the gain-of-function MUC5B promoter variant. Similarly, in mice exposed to bleomycin, Muc5b expression is temporally associated with markers of ER stress. Using bulk and single-cell RNA sequencing in bleomycin-exposed mice, we found that pathologic ER stress-associated transcripts Atf4 and Ddit3 (DNA damage inducible transcript 3) were elevated in alveolar epithelia of SFTPC-Muc5b transgenic (SFTPC-Muc5bTg) mice relative to wild-type (WT) mice. Activation of the ER stress response inhibits protein translation for most genes by phosphorylation of Eif2α (eukaryotic translation initiation factor 2 alpha), which prevents guanine exchange by Eif2B and facilitates translation of Atf4. The integrated stress response inhibitor (ISRIB) facilitates interaction of phosphorylated Eif2α with Eif2B, overcoming translation inhibition associated with ER stress and reducing Atf4. We found that a single dose of ISRIB diminished Atf4 translation in SFTPC-Muc5bTg mice after bleomycin injury. Moreover, ISRIB resolved the exaggerated fibrotic response of SFTPC-Muc5bTg mice to bleomycin. In summary, we demonstrate that MUC5B and Muc5b expression is associated with pathologic ER stress and that restoration of normal translation with a single dose of ISRIB promotes lung repair in bleomycin-injured Muc5b-overexpressing mice.

Novel diagnostic techniques in interstitial lung disease

Research into novel diagnostic techniques and targeted therapeutics in interstitial lung disease (ILD) is moving the field toward increased precision and improved patient outcomes. An array of molecular techniques, machine learning approaches and other innovative methods including electronic nose technology and endobronchial optical coherence tomography are promising tools with potential to increase diagnostic accuracy. This review provides a comprehensive overview of the current evidence regarding evolving diagnostic methods in ILD and to consider their future role in routine clinical care.

ΔNp63 drives dysplastic alveolar remodeling and restricts epithelial plasticity upon severe lung injury

The lung exhibits a robust, multifaceted regenerative response to severe injuries such as influenza infection, during which quiescent lung-resident epithelial progenitors participate in two distinct reparative pathways: functionally beneficial regeneration via alveolar type 2 (AT2) cell proliferation and differentiation, and dysplastic tissue remodeling via intrapulmonary airway-resident basal p63+ progenitors. Here we show that the basal cell transcription factor ΔNp63 is required for intrapulmonary basal progenitors to participate in dysplastic alveolar remodeling following injury. We find that ΔNp63 restricts the plasticity of intrapulmonary basal progenitors by maintaining either active or repressive histone modifications at key differentiation gene loci. Following loss of ΔNp63, intrapulmonary basal progenitors are capable of either airway or alveolar differentiation depending on their surrounding environment both in vitro and in vivo. Uncovering these regulatory mechanisms of dysplastic repair and lung basal cell fate choice highlight potential therapeutic targets to promote functional alveolar regeneration following severe lung injuries.

Lung Tissue Engineering: Toward a More Deliberate Approach

Chronic lung disease remains a leading cause of morbidity and mortality. Given the dearth of definitive therapeutic options, there is an urgent need to augment the pool of donor organs for transplantation. One strategy entails building a lung ex vivo in the laboratory. The past decade of whole lung tissue engineering has laid a foundation of systems and strategies for this approach. Meanwhile, tremendous progress in lung stem cell biology is elucidating cues contributing to alveolar repair, and speaks to the potential of whole lung regeneration in the future. This perspective discusses the key challenges facing the field and highlights opportunities to combine insights from biology with engineering strategies to adopt a more deliberate, and ultimately successful, approach to lung engineering.

Wnt5a/β-catenin axis is involved in the downregulation of AT2 lineage by PAI-1

Failure to regenerate injured alveoli functionally and promptly causes a high incidence of fatality in coronavirus disease 2019 (COVID-19). How elevated plasminogen activator inhibitor-1 (PAI-1) regulates the lineage of alveolar type 2 (AT2) cells for re-alveolarization has not been studied. This study aimed to examine the role of PAI-1-Wnt5a-β catenin cascades in AT2 fate. Dramatic reduction in AT2 yield was observed in Serpine1Tg mice. Elevated PAI-1 level suppressed organoid number, development efficiency, and total surface area in vitro. Anti-PAI-1 neutralizing antibody restored organoid number, proliferation and differentiation of AT2 cells, and β-catenin level in organoids. Both Wnt family member 5A (Wnt5a) and Wnt5a-derived N-butyloxycarbonyl hexapeptide (Box5) altered the lineage of AT2 cells. This study demonstrates that elevated PAI-1 regulates AT2 proliferation and differentiation via the Wnt5a/β catenin cascades. PAI-1 could serve as autocrine signaling for lung injury repair.

Airway basal cells show regionally distinct potential to undergo metaplastic differentiation

Basal cells are multipotent stem cells of a variety of organs, including the respiratory tract, where they are major components of the airway epithelium. However, it remains unclear how diverse basal cells are and how distinct subpopulations respond to airway challenges. Using single cell RNA-sequencing and functional approaches, we report a significant and previously underappreciated degree of heterogeneity in the basal cell pool, leading to identification of six subpopulations in the adult murine trachea. Among these, we found two major subpopulations, collectively comprising the most uncommitted of all the pools, but with distinct gene expression signatures. Notably, these occupy distinct ventral and dorsal tracheal niches and differ in their ability to self-renew and initiate a program of differentiation in response to environmental perturbations in primary cultures and in mouse injury models in vivo. We found that such heterogeneity is acquired prenatally, when the basal cell pool and local niches are still being established, and depends on the integrity of these niches, as supported by the altered basal cell phenotype of tracheal cartilage-deficient mouse mutants. Finally, we show that features that distinguish these progenitor subpopulations in murine airways are conserved in humans. Together, the data provide novel insights into the origin and impact of basal cell heterogeneity on the establishment of regionally distinct responses of the airway epithelium during injury-repair and in disease conditions.

Contribution of Trp63CreERT2-labeled cells to alveolar regeneration is independent of tuft cells

Viral infection often causes severe damage to the lungs, leading to the appearance of ectopic basal cells (EBCs) and tuft cells in the lung parenchyma. Thus far, the roles of these ectopic epithelial cells in alveolar regeneration remain controversial. Here, we confirm that the ectopic tuft cells are originated from EBCs in mouse models and COVID-19 lungs. The differentiation of tuft cells from EBCs is promoted by Wnt inhibition while suppressed by Notch inhibition. Although progenitor functions have been suggested in other organs, pulmonary tuft cells don't proliferate or give rise to other cell lineages. Consistent with previous reports, Trp63CreERT2 and KRT5-CreERT2-labeled ectopic EBCs do not exhibit alveolar regeneration potential. Intriguingly, when tamoxifen was administrated post-viral infection, Trp63CreERT2 but not KRT5-CreERT2 labels islands of alveolar epithelial cells that are negative for EBC biomarkers. Furthermore, germline deletion of Trpm5 significantly increases the contribution of Trp63CreERT2-labeled cells to the alveolar epithelium. Although Trpm5 is known to regulate tuft cell development, complete ablation of tuft cell production fails to improve alveolar regeneration in Pou2f3-/- mice, implying that Trpm5 promotes alveolar epithelial regeneration through a mechanism independent of tuft cells.

An insight into the placental growth factor (PlGf)/angii axis in Covid-19: a detrimental intersection

Coronavirus disease 2019 (Covid-19) is a recent and current infectious pandemic caused by severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2). Covid-19 may lead to the development of acute lung injury (ALI), acute respiratory distress syndrome (ARDS), and extrapulmonary manifestations in severe cases. Down-regulation of angiotensin-converting enzyme (ACE2) by the SARS-CoV-2 increases the production of angiotensin II (AngII), which increases the release of pro-inflammatory cytokines and placental growth factor (PlGF). PlGF is a critical molecule involved in vasculogenesis and angiogenesis. PlGF is stimulated by AngII in different inflammatory diseases through a variety of signaling pathways. PlGF and AngII are interacted in SARS-CoV-2 infection resulting in the production of pro-inflammatory cytokines and the development of Covid-19 complications. Both AngII and PlGF are interacted and are involved in the progression of inflammatory disorders; therefore, we aimed in this review to highlight the potential role of the PlGF/AngII axis in Covid-19.

New Insights into the Alveolar Epithelium as a Driver of Acute Respiratory Distress Syndrome

The alveolar epithelium serves as a barrier between the body and the external environment. To maintain efficient gas exchange, the alveolar epithelium has evolved to withstand and rapidly respond to an assortment of inhaled, injury-inducing stimuli. However, alveolar damage can lead to loss of alveolar fluid barrier function and exuberant, non-resolving inflammation that manifests clinically as acute respiratory distress syndrome (ARDS). This review discusses recent discoveries related to mechanisms of alveolar homeostasis, injury, repair, and regeneration, with a contemporary emphasis on virus-induced lung injury. In addition, we address new insights into how the alveolar epithelium coordinates injury-induced lung inflammation and review maladaptive lung responses to alveolar damage that drive ARDS and pathologic lung remodeling.