Efficient Derivation of Functional Human Airway Epithelium from Pluripotent Stem Cells via Temporal Regulation of Wnt Signaling

CRISPR-Cas9 Genome Editing Allows Generation of the Mouse Lung in a Rat

Rationale: Recent efforts in bioengineering and embryonic stem cell (ESC) technology allowed the generation of ESC-derived mouse lung tissues in transgenic mice that were missing critical morphogenetic genes. Epithelial cell lineages were efficiently generated from ESC, but other cell types were mosaic. A complete contribution of donor ESCs to lung tissue has never been achieved. The mouse lung has never been generated in a rat. Objective: We sought to generate the mouse lung in a rat. Methods: Clustered regularly interspaced short palindromic repeat (CRISPR)-Cas9 genome editing was used to disrupt the Nkx2-1 gene in rat one-cell zygotes. Interspecies mouse-rat chimeras were produced by injection of wild-type mouse ESCs into Nkx2-1-deficient rat embryos with lung agenesis. The contribution of mouse ESCs to the lung tissue was examined by immunostaining, flow cytometry, and single-cell RNA sequencing. Measurements and Main Results: Peripheral pulmonary and thyroid tissues were absent in rat embryos after CRISPR-Cas9-mediated disruption of the Nkx2-1 gene. Complementation of rat Nkx2-1-/- blastocysts with mouse ESCs restored pulmonary and thyroid structures in mouse-rat chimeras, leading to a near-99% contribution of ESCs to all respiratory cell lineages. Epithelial, endothelial, hematopoietic, and stromal cells in ESC-derived lungs were highly differentiated and exhibited lineage-specific gene signatures similar to those of respiratory cells from the normal mouse lung. Analysis of receptor-ligand interactions revealed normal signaling networks between mouse ESC-derived respiratory cells differentiated in a rat. Conclusions: A combination of CRISPR-Cas9 genome editing and blastocyst complementation was used to produce mouse lungs in rats, making an important step toward future generations of human lungs using large animals as "bioreactors."

Building a human lung from pluripotent stem cells to model respiratory viral infections

To protect against the constant threat of inhaled pathogens, the lung is equipped with cellular defenders. In coordination with resident and recruited immune cells, this defence is initiated by the airway and alveolar epithelium following their infection with respiratory viruses. Further support for viral clearance and infection resolution is provided by adjacent endothelial and stromal cells. However, even with these defence mechanisms, respiratory viral infections are a significant global health concern, causing substantial morbidity, socioeconomic losses, and mortality, underlining the need to develop effective vaccines and antiviral medications. In turn, the identification of new treatment options for respiratory infections is critically dependent on the availability of tractable in vitro experimental models that faithfully recapitulate key aspects of lung physiology. For such models to be informative, it is important these models incorporate human-derived, physiologically relevant versions of all cell types that normally form part of the lungs anti-viral response. This review proposes a guideline using human induced pluripotent stem cells (iPSCs) to create all the disease-relevant cell types. iPSCs can be differentiated into lung epithelium, innate immune cells, endothelial cells, and fibroblasts at a large scale, recapitulating in vivo functions and providing genetic tractability. We advocate for building comprehensive iPSC-derived in vitro models of both proximal and distal lung regions to better understand and model respiratory infections, including interactions with chronic lung diseases.

Lung Organoids: Systematic Review of Recent Advancements and its Future Perspectives

Organoids are essentially an in vitro (lab-grown) three-dimensional tissue culture system model that meticulously replicates the structure and physiology of human organs. A few of the present applications of organoids are in the basic biological research area, molecular medicine and pharmaceutical drug testing. Organoids are crucial in connecting the gap between animal models and human clinical trials during the drug discovery process, which significantly lowers the time duration and cost associated with each stage of testing. Likewise, they can be used to understand cell-to-cell interactions, a crucial aspect of tissue biology and regeneration, and to model disease pathogenesis at various stages of the disease. Lung organoids can be utilized to explore numerous pathophysiological activities of a lung since they share similarities with its function. Researchers have been trying to recreate the complex nature of the lung by developing various "Lung organoids" models.This article is a systematic review of various developments of lung organoids and their potential progenitors. It also covers the in-depth applications of lung organoids for the advancement of translational research. The review discusses the methodologies to establish different types of lung organoids for studying the regenerative capability of the respiratory system and comprehending various respiratory diseases.Respiratory diseases are among the most common worldwide, and the growing burden must be addressed instantaneously. Lung organoids along with diverse bio-engineering tools and technologies will serve as a novel model for studying the pathophysiology of various respiratory diseases and for drug screening purposes.

Recent advances in cell-based in vitro models for predicting drug permeability across brain, intestinal, and pulmonary barriers

INTRODUCTION

Recent years have witnessed remarkable progress in the development of cell-based in vitro models aimed at predicting drug permeability, particularly focusing on replicating the barrier properties of the blood-brain barrier (BBB), intestinal epithelium, and lung epithelium.

AREA COVERED

This review provides an overview of 2D in vitro platforms, including monocultures and co-culture systems, highlighting their respective advantages and limitations. Additionally, it discusses tools and techniques utilized to overcome these limitations, paving the way for more accurate predictions of drug permeability. Furthermore, this review delves into emerging technologies, particularly microphysiological systems (MPS), encompassing static platforms such as organoids and dynamic platforms like microfluidic devices. Literature searches were performed using PubMed and Google Scholar. We focus on key terms such as in vitro permeability models, MPS, organoids, intestine, BBB, and lungs.

EXPERT OPINION

The potential of these MPS to mimic physiological conditions more closely offers promising avenues for drug permeability assessment. However, transitioning these advanced models from bench to industry requires rigorous validation against regulatory standards. Thus, there is a pressing need to validate MPS to industry and regulatory agency standards to exploit their potential in drug permeability prediction fully. This review underscores the importance of such validation processes to facilitate the translation of these innovative technologies into routine pharmaceutical practice.

Endothelial RSPO3 mediates pulmonary endothelial regeneration by LGR4-dependent activation of β-catenin and ILK signaling pathways after inflammatory vascular injury

Endothelial repair is essential for restoring tissue fluid homeostasis following lung injury. R-spondin3 (RSPO3), a secreted protein mainly produced by endothelial cells (ECs), has shown its protective effect on endothelium. However, the specific mechanisms remain unknown. To explore whether and how RSPO3 regulates endothelial regeneration after inflammatory vascular injury, the role of RSPO3 in sepsis-induced pulmonary endothelial injury was investigated in EC-specific RSPO3 knockdown, inducible EC-specific RSPO3 deletion mice, EC-specific RSPO3 overexpression mice, systemic RSPO3-administration mice, in isolated mouse lung vascular endothelial cells (MLVECs), and in plasma from septic patients. Here we show that plasma RSPO3 levels are decreased in septic patients and correlated with endothelial injury markers and PaO2/FiO2 index. Both pulmonary EC-specific knockdown of RSPO3 and inducible EC-specific RSPO3 deletion inhibit pulmonary ECs proliferation and exacerbate ECs injury, whereas intra-pulmonary EC-specific RSPO3 overexpression promotes endothelial recovery and attenuates ECs injury during endotoxemia. We show that RSPO3 mediates pulmonary endothelial regeneration by a LGR4-dependent manner. Except for β-catenin, integrin-linked kinase (ILK)/Akt is also identified as a novel downstream effector of RSPO3/LGR4 signaling. These results conclude that EC-derived RSPO3 mediates pulmonary endothelial regeneration by LGR4-dependent activation of β-catenin and ILK signaling pathways after inflammatory vascular injury.

Molecular distinctions of bronchoalveolar and alveolar organoids under differentiation conditions

The bronchoalveolar organoid (BAO) model is increasingly acknowledged as an ex-vivo platform that accurately emulates the structural and functional attributes of proximal airway tissue. The transition from bronchoalveolar progenitor cells to alveolar organoids is a common event during the generation of BAOs. However, there is a pressing need for comprehensive analysis to elucidate the molecular distinctions characterizing the pre-differentiated and post-differentiated states within BAO models. This study established a murine BAO model and subsequently triggered its differentiation. Thereafter, a suite of multidimensional analytical procedures was employed, including the morphological recognition and examination of organoids utilizing an established artificial intelligence (AI) image tracking system, quantification of cellular composition, proteomic profiling and immunoblots of selected proteins. Our investigation yielded a detailed evaluation of the morphologic, cellular, and molecular variances demarcating the pre- and post-differentiation phases of the BAO model. We also identified of a potential molecular signature reflective of the observed morphological transformations. The integration of cutting-edge AI-driven image analysis with traditional cellular and molecular investigative methods has illuminated key features of this nascent model.

Essential Guidelines for Manufacturing and Application of Organoids

An organoid is a self-organized three-dimensional structure derived from stem cells that mimics the structure, cell composition, and functional characteristics of specific organs and tissues and is used for evaluating the safety and effectiveness of drugs and the toxicity of industrial chemicals. Organoid technology is a new methodology that could replace testing on animals testing and accelerate development of precision and regenerative medicine. However, large variations in production can occur between laboratories with low reproducibility of the production process and no internationally agreed standards for quality evaluation factors at endpoints. To overcome these barriers that hinder the regulatory acceptance and commercialization of organoids, Korea established the Organoid Standards Initiative in September 2023 with various stakeholders, including industry, academia, regulatory agencies, and standard development experts, through public and private partnerships. This developed general guidelines for organoid manufacturing and quality evaluation and for quality evaluation guidelines for organoid-specific manufacturing for the liver, intestines, and heart through extensive evidence analysis and consensus among experts. This report is based on the common standard guideline v1.0, which is a general organoid manufacturing and quality evaluation to promote the practical use of organoids. This guideline does not focus on specific organoids or specific contexts of use but provides guidance to organoid makers and users on materials, procedures, and essential quality assessment methods at end points that are essential for organoid production applicable at the current technology level.

Guidelines for Manufacturing and Application of Organoids: Lung

The objective of standard guideline for utilization of human lung organoids is to provide the basic guidelines required for the manufacture, culture, and quality control of the lung organoids for use in non-clinical efficacy and inhalation toxicity assessments of the respiratory system. As a first step towards the utilization of human lung organoids, the current guideline provides basic, minimal standards that can promote development of alternative testing methods, and can be referenced not only for research, clinical, or commercial uses, but also by experts and researchers at regulatory institutions when assessing safety and efficacy.

Generation of human alveolar epithelial type I cells from pluripotent stem cells

Alveolar epithelial type I cells (AT1s) line the gas exchange barrier of the distal lung and have been historically challenging to isolate or maintain in cell culture. Here, we engineer a human in vitro AT1 model system via directed differentiation of induced pluripotent stem cells (iPSCs). We use primary adult AT1 global transcriptomes to suggest benchmarks and pathways, such as Hippo-LATS-YAP/TAZ signaling, enriched in these cells. Next, we generate iPSC-derived alveolar epithelial type II cells (AT2s) and find that nuclear YAP signaling is sufficient to promote a broad transcriptomic shift from AT2 to AT1 gene programs. The resulting cells express a molecular, morphologic, and functional phenotype reminiscent of human AT1 cells, including the capacity to form a flat epithelial barrier producing characteristic extracellular matrix molecules and secreted ligands. Our results provide an in vitro model of human alveolar epithelial differentiation and a potential source of human AT1s.

Investigating pulmonary neuroendocrine cells in human respiratory diseases with airway models

Despite accounting for only ∼0.5% of the lung epithelium, pulmonary neuroendocrine cells (PNECs) appear to play an outsized role in respiratory health and disease. Increased PNEC numbers have been reported in a variety of respiratory diseases, including chronic obstructive pulmonary disease and asthma. Moreover, PNECs are the primary cell of origin for lung neuroendocrine cancers, which account for 25% of aggressive lung cancers. Recent research has highlighted the crucial roles of PNECs in lung physiology, including in chemosensing, regeneration and immune regulation. Yet, little is known about the direct impact of PNECs on respiratory diseases. In this Review, we summarise the current associations of PNECs with lung pathologies, focusing on how new experimental disease models, such as organoids derived from human pluripotent stem cells or tissue stem cells, can help us to better understand the contribution of PNECs to respiratory diseases.

Breathing new life into the study of COPD with genes identified from genome-wide association studies

COPD is a major cause of morbidity and mortality globally. While the significance of environmental exposures in disease pathogenesis is well established, the functional contribution of genetic factors has only in recent years drawn attention. Notably, many genes associated with COPD risk are also linked with lung function. Because reduced lung function precedes COPD onset, this association is consistent with the possibility that derangements leading to COPD could arise during lung development. In this review, we summarise the role of leading genes (HHIP, FAM13A, DSP, AGER and TGFB2) identified by genome-wide association studies in lung development and COPD. Because many COPD genome-wide association study genes are enriched in lung epithelial cells, we focus on the role of these genes in the lung epithelium in development, homeostasis and injury.

In vitro platform to model the function of ionocytes in the human airway epithelium

BACKGROUND

Pulmonary ionocytes have been identified in the airway epithelium as a small population of ion transporting cells expressing high levels of CFTR (cystic fibrosis transmembrane conductance regulator), the gene mutated in cystic fibrosis. By providing an infinite source of airway epithelial cells (AECs), the use of human induced pluripotent stem cells (hiPSCs) could overcome some challenges of studying ionocytes. However, the production of AEC epithelia containing ionocytes from hiPSCs has proven difficult. Here, we present a platform to produce hiPSC-derived AECs (hiPSC-AECs) including ionocytes and investigate their role in the airway epithelium.

METHODS

hiPSCs were differentiated into lung progenitors, which were expanded as 3D organoids and matured by air-liquid interface culture as polarised hiPSC-AEC epithelia. Using CRISPR/Cas9 technology, we generated a hiPSCs knockout (KO) for FOXI1, a transcription factor that is essential for ionocyte specification. Differences between FOXI1 KO hiPSC-AECs and their wild-type (WT) isogenic controls were investigated by assessing gene and protein expression, epithelial composition, cilia coverage and motility, pH and transepithelial barrier properties.

RESULTS

Mature hiPSC-AEC epithelia contained basal cells, secretory cells, ciliated cells with motile cilia, pulmonary neuroendocrine cells (PNECs) and ionocytes. There was no difference between FOXI1 WT and KO hiPSCs in terms of their capacity to differentiate into airway progenitors. However, FOXI1 KO led to mature hiPSC-AEC epithelia without ionocytes with reduced capacity to produce ciliated cells.

CONCLUSION

Our results suggest that ionocytes could have role beyond transepithelial ion transport by regulating epithelial properties and homeostasis in the airway epithelium.

Nuclear size-regulated emergence of topological packing order on growing human lung alveolospheres

Within multicellular living systems, cells coordinate their positions with spatiotemporal accuracy to form various structures, setting the clock to control developmental processes and trigger maturation. These arrangements can be regulated by tissue topology, biochemical cues, as well as mechanical perturbations. However, the fundamental rules of how local cell packing order is regulated in forming three-dimensional (3D) multicellular architectures remain unclear. Furthermore, how cellular coordination evolves during developmental processes, and whether this cell patterning behavior is indicative of more complex biological functions, is largely unknown. Here, using human lung alveolospheres as a model system, by combining experiments and numerical simulations, we find that, surprisingly, cell packing behavior on alveolospheres resembles hard-disk packing but with increased randomness; the stiffer cell nuclei act as the hard disks surrounded by deformable cell bodies. Interestingly, we observe the emergence of topological packing order during alveolosphere growth, as a result of increasing nucleus-to-cell size ratio. Specifically, we find more hexagon-concentrated cellular packing with increasing bond orientational order, indicating a topological gas-to-liquid transition. Additionally, by osmotically changing the compactness of cells on alveolospheres, we observe that the variations in packing order align with the change of nucleus-to-cell size ratio. Together, our findings reveal the underlying rules of cell coordination and topological phases during human lung alveolosphere growth. These static packing characteristics are consistent with cell dynamics, together suggesting that better cellular packing stabilizes local cell neighborhoods and may regulate more complex biological functions such as organ development and cellular maturation.

Unlocking the Future: Pluripotent Stem Cell-Based Lung Repair

The human respiratory system is susceptible to a variety of diseases, ranging from chronic obstructive pulmonary disease (COPD) and pulmonary fibrosis to acute respiratory distress syndrome (ARDS). Today, lung diseases represent one of the major challenges to the health care sector and represent one of the leading causes of death worldwide. Current treatment options often focus on managing symptoms rather than addressing the underlying cause of the disease. The limitations of conventional therapies highlight the urgent clinical need for innovative solutions capable of repairing damaged lung tissue at a fundamental level. Pluripotent stem cell technologies have now reached clinical maturity and hold immense potential to revolutionize the landscape of lung repair and regenerative medicine. Meanwhile, human embryonic (HESCs) and human-induced pluripotent stem cells (hiPSCs) can be coaxed to differentiate into lung-specific cell types such as bronchial and alveolar epithelial cells, or pulmonary endothelial cells. This holds the promise of regenerating damaged lung tissue and restoring normal respiratory function. While methods for targeted genetic engineering of hPSCs and lung cell differentiation have substantially advanced, the required GMP-grade clinical-scale production technologies as well as the development of suitable preclinical animal models and cell application strategies are less advanced. This review provides an overview of current perspectives on PSC-based therapies for lung repair, explores key advances, and envisions future directions in this dynamic field.

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.

Glycogen Synthase Kinase-3 Inhibition by CHIR99021 Promotes Alveolar Epithelial Cell Proliferation and Lung Regeneration in the Lipopolysaccharide-Induced Acute Lung Injury Mouse Model

Acute respiratory distress syndrome (ARDS) is a life-threatening lung injury that currently lacks effective clinical treatments. Evidence highlights the potential role of glycogen synthase kinase-3 (GSK-3) inhibition in mitigating severe inflammation. The inhibition of GSK-3α/β by CHIR99021 promoted fetal lung progenitor proliferation and maturation of alveolar epithelial cells (AECs). The precise impact of CHIR99021 in lung repair and regeneration during acute lung injury (ALI) remains unexplored. This study intends to elucidate the influence of CHIR99021 on AEC behaviour during the peak of the inflammatory phase of ALI and, after its attenuation, during the repair and regeneration stage. Furthermore, a long-term evaluation was conducted post CHIR99021 treatment at a late phase of the disease. Our results disclosed the role of GSK-3α/β inhibition in promoting AECI and AECII proliferation. Later administration of CHIR99021 during ALI progression contributed to the transdifferentiation of AECII into AECI and an AECI/AECII increase, suggesting its contribution to the renewal of the alveolar epithelial population and lung regeneration. This effect was confirmed to be maintained histologically in the long term. These findings underscore the potential of targeted therapies that modulate GSK-3α/β inhibition, offering innovative approaches for managing acute lung diseases, mostly in later stages where no treatment is available.

Human Pluripotent Stem Cell-Derived Alveolar Organoids: Cellular Heterogeneity and Maturity

Chronic respiratory diseases such as idiopathic pulmonary fibrosis, chronic obstructive pulmonary disease, and respiratory infections injure the alveoli; the damage evoked is mostly irreversible and occasionally leads to death. Achieving a detailed understanding of the pathogenesis of these fatal respiratory diseases has been hampered by limited access to human alveolar tissue and the differences between mice and humans. Thus, the development of human alveolar organoid (AO) models that mimic in vivo physiology and pathophysiology has gained tremendous attention over the last decade. In recent years, human pluripotent stem cells (hPSCs) have been successfully employed to generate several types of organoids representing different respiratory compartments, including alveolar regions. However, despite continued advances in three-dimensional culture techniques and single-cell genomics, there is still a profound need to improve the cellular heterogeneity and maturity of AOs to recapitulate the key histological and functional features of in vivo alveolar tissue. In particular, the incorporation of immune cells such as macrophages into hPSC-AO systems is crucial for disease modeling and subsequent drug screening. In this review, we summarize current methods for differentiating alveolar epithelial cells from hPSCs followed by AO generation and their applications in disease modeling, drug testing, and toxicity evaluation. In addition, we review how current hPSC-AOs closely resemble in vivo alveoli in terms of phenotype, cellular heterogeneity, and maturity.

Advanced lung organoids for respiratory system and pulmonary disease modeling

Amidst the recent coronavirus disease 2019 (COVID-19) pandemic, respiratory system research has made remarkable progress, particularly focusing on infectious diseases. Lung organoid, a miniaturized structure recapitulating lung tissue, has gained global attention because of its advantages over other conventional models such as two-dimensional (2D) cell models and animal models. Nevertheless, lung organoids still face limitations concerning heterogeneity, complexity, and maturity compared to the native lung tissue. To address these limitations, researchers have employed co-culture methods with various cell types including endothelial cells, mesenchymal cells, and immune cells, and incorporated bioengineering platforms such as air-liquid interfaces, microfluidic chips, and functional hydrogels. These advancements have facilitated applications of lung organoids to studies of pulmonary diseases, providing insights into disease mechanisms and potential treatments. This review introduces recent progress in the production methods of lung organoids, strategies for improving maturity, functionality, and complexity of organoids, and their application in disease modeling, including respiratory infection and pulmonary fibrosis.

Prime editing-mediated correction of the CFTR W1282X mutation in iPSCs and derived airway epithelial cells

A major unmet need in the cystic fibrosis (CF) therapeutic landscape is the lack of effective treatments for nonsense CFTR mutations, which affect approximately 10% of CF patients. Correction of nonsense CFTR mutations via genomic editing represents a promising therapeutic approach. In this study, we tested whether prime editing, a novel CRISPR-based genomic editing method, can be a potential therapeutic modality to correct nonsense CFTR mutations. We generated iPSCs from a CF patient homozygous for the CFTR W1282X mutation. We demonstrated that prime editing corrected one mutant allele in iPSCs, which effectively restored CFTR function in iPSC-derived airway epithelial cells and organoids. We further demonstrated that prime editing may directly repair mutations in iPSC-derived airway epithelial cells when the prime editing machinery is efficiently delivered by helper-dependent adenovirus (HDAd). Together, our data demonstrated that prime editing may potentially be applied to correct CFTR mutations such as W1282X.

Human pluripotent stem cell fate trajectories toward lung and hepatocyte progenitors

In this study, we interrogate molecular mechanisms underlying the specification of lung progenitors from human pluripotent stem cells (hPSCs). We employ single-cell RNA-sequencing with high temporal precision, alongside an optimized differentiation protocol, to elucidate the transcriptional hierarchy of lung specification to chart the associated single-cell trajectories. Our findings indicate that Sonic hedgehog, TGF-β, and Notch activation are essential within an ISL1/NKX2-1 trajectory, leading to the emergence of lung progenitors during the foregut endoderm phase. Additionally, the induction of HHEX delineates an alternate trajectory at the early definitive endoderm stage, preceding the lung pathway and giving rise to a significant hepatoblast population. Intriguingly, neither KDR+ nor mesendoderm progenitors manifest as intermediate stages in the lung and hepatic lineage development. Our multistep model offers insights into lung organogenesis and provides a foundation for in-depth study of early human lung development and modeling using hPSCs.

Recent advances in lung organoid development and applications in disease modeling

Over the last decade, several organoid models have evolved to acquire increasing cellular, structural, and functional complexity. Advanced lung organoid platforms derived from various sources, including adult, fetal, and induced pluripotent stem cells, have now been generated, which more closely mimic the cellular architecture found within the airways and alveoli. In this regard, the establishment of novel protocols with optimized stem cell isolation and culture conditions has given rise to an array of models able to study key cellular and molecular players involved in lung injury and repair. In addition, introduction of other nonepithelial cellular components, such as immune, mesenchymal, and endothelial cells, and employment of novel precision gene editing tools have further broadened the range of applications for these systems by providing a microenvironment and/or phenotype closer to the desired in vivo scenario. Thus, these developments in organoid technology have enhanced our ability to model various aspects of lung biology, including pathogenesis of diseases such as chronic obstructive pulmonary disease, pulmonary fibrosis, cystic fibrosis, and infectious disease and host-microbe interactions, in ways that are often difficult to undertake using only in vivo models. In this Review, we summarize the latest developments in lung organoid technology and their applicability for disease modeling and outline their strengths, drawbacks, and potential avenues for future development.

Alveolar macrophages in tissue homeostasis, inflammation, and infection: evolving concepts of therapeutic targeting

Alveolar macrophages (AMs) are the sentinel cells of the alveolar space, maintaining homeostasis, fending off pathogens, and controlling lung inflammation. During acute lung injury, AMs orchestrate the initiation and resolution of inflammation in order to ultimately restore homeostasis. This central role in acute lung inflammation makes AMs attractive targets for therapeutic interventions. Single-cell RNA-Seq and spatial omics approaches, together with methodological advances such as the generation of human macrophages from pluripotent stem cells, have increased understanding of the ontogeny, function, and plasticity of AMs during infectious and sterile lung inflammation, which could move the field closer to clinical application. However, proresolution phenotypes might conflict with proinflammatory and antibacterial responses. Therefore, therapeutic targeting of AMs at vulnerable time points over the course of infectious lung injury might harbor the risk of serious side effects, such as loss of antibacterial host defense capacity. Thus, the identification of key signaling hubs that determine functional fate decisions in AMs is of the utmost importance to harness their therapeutic potential.

Application of colloidal photonic crystals in study of organoids

As alternative disease models, other than 2D cell lines and patient-derived xenografts, organoids have preferable in vivo physiological relevance. However, both endogenous and exogenous limitations impede the development and clinical translation of these organoids. Fortunately, colloidal photonic crystals (PCs), which benefit from favorable biocompatibility, brilliant optical manipulation, and facile chemical decoration, have been applied to the engineering of organoids and have achieved the desirable recapitulation of the ECM niche, well-defined geometrical onsets for initial culture, in situ multiphysiological parameter monitoring, single-cell biomechanical sensing, and high-throughput drug screening with versatile functional readouts. Herein, we review the latest progress in engineering organoids fabricated from colloidal PCs and provide inputs for future research.

Innovative three-dimensional models for understanding mechanisms underlying lung diseases: powerful tools for translational research

Chronic lung diseases result from alteration and/or destruction of lung tissue, inevitably causing decreased breathing capacity and quality of life for patients. While animal models have paved the way for our understanding of pathobiology and the development of therapeutic strategies for disease management, their translational capacity is limited. There is, therefore, a well-recognised need for innovative in vitro models to reflect chronic lung diseases, which will facilitate mechanism investigation and the advancement of new treatment strategies. In the last decades, lungs have been modelled in healthy and diseased conditions using precision-cut lung slices, organoids, extracellular matrix-derived hydrogels and lung-on-chip systems. These three-dimensional models together provide a wide spectrum of applicability and mimicry of the lung microenvironment. While each system has its own limitations, their advantages over traditional two-dimensional culture systems, or even over animal models, increases the value of in vitro models. Generating new and advanced models with increased translational capacity will not only benefit our understanding of the pathobiology of lung diseases but should also shorten the timelines required for discovery and generation of new therapeutics. This article summarises and provides an outline of the European Respiratory Society research seminar "Innovative 3D models for understanding mechanisms underlying lung diseases: powerful tools for translational research", held in Lisbon, Portugal, in April 2022. Current in vitro models developed for recapitulating healthy and diseased lungs are outlined and discussed with respect to the challenges associated with them, efforts to develop best practices for model generation, characterisation and utilisation of models and state-of-the-art translational potential.

Opposing roles for TGFβ- and BMP-signaling during nascent alveolar differentiation in the developing human lung

Alveolar type 2 (AT2) cells function as stem cells in the adult lung and aid in repair after injury. The current study aimed to understand the signaling events that control differentiation of this therapeutically relevant cell type during human development. Using lung explant and organoid models, we identified opposing effects of TGFβ- and BMP-signaling, where inhibition of TGFβ- and activation of BMP-signaling in the context of high WNT- and FGF-signaling efficiently differentiated early lung progenitors into AT2-like cells in vitro. AT2-like cells differentiated in this manner exhibit surfactant processing and secretion capabilities, and long-term commitment to a mature AT2 phenotype when expanded in media optimized for primary AT2 culture. Comparing AT2-like cells differentiated with TGFβ-inhibition and BMP-activation to alternative differentiation approaches revealed improved specificity to the AT2 lineage and reduced off-target cell types. These findings reveal opposing roles for TGFβ- and BMP-signaling in AT2 differentiation and provide a new strategy to generate a therapeutically relevant cell type in vitro.

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.

Airway stem cell reconstitution by the transplantation of primary or pluripotent stem cell-derived basal cells

Life-long reconstitution of a tissue's resident stem cell compartment with engrafted cells has the potential to durably replenish organ function. Here, we demonstrate the engraftment of the airway epithelial stem cell compartment via intra-airway transplantation of mouse or human primary and pluripotent stem cell (PSC)-derived airway basal cells (BCs). Murine primary or PSC-derived BCs transplanted into polidocanol-injured syngeneic recipients give rise for at least two years to progeny that stably display the morphologic, molecular, and functional phenotypes of airway epithelia. The engrafted basal-like cells retain extensive self-renewal potential, evident by the capacity to reconstitute the tracheal epithelium through seven generations of secondary transplantation. Using the same approach, human primary or PSC-derived BCs transplanted into NOD scid gamma (NSG) recipient mice similarly display multilineage airway epithelial differentiation in vivo. Our results may provide a step toward potential future syngeneic cell-based therapy for patients with diseases resulting from airway epithelial cell damage or dysfunction.

Restoring Ciliary Function: Gene Therapeutics for Primary Ciliary Dyskinesia

Primary ciliary dyskinesia (PCD) is a genetic disease characterized by defects in motile cilia, which play an important role in several organ systems. Lung disease is a hallmark of PCD, given the essential role of cilia in airway surface defense. Diagnosis of PCD is complicated due to its reliance on complex tests that are not utilized by every clinic and also its phenotypic overlap with several other respiratory diseases. Nonetheless, PCD is increasingly being recognized as more common than once thought. The disease is genetically complex, with several genes reported to be associated with PCD. There is no cure for PCD, but gene therapy remains a promising therapeutic strategy. In this review, we provide an overview of the clinical symptoms, diagnosis, genetics, and current treatment regimens for PCD. We also describe PCD model systems and discuss the therapeutic potential of different gene therapeutics for targeting the intended cellular target, the ciliated cells of the airway.

Multiomic Investigations into Lung Health and Disease

Diseases of the lung account for more than 5 million deaths worldwide and are a healthcare burden. Improving clinical outcomes, including mortality and quality of life, involves a holistic understanding of the disease, which can be provided by the integration of lung multi-omics data. An enhanced understanding of comprehensive multiomic datasets provides opportunities to leverage those datasets to inform the treatment and prevention of lung diseases by classifying severity, prognostication, and discovery of biomarkers. The main objective of this review is to summarize the use of multiomics investigations in lung disease, including multiomics integration and the use of machine learning computational methods. This review also discusses lung disease models, including animal models, organoids, and single-cell lines, to study multiomics in lung health and disease. We provide examples of lung diseases where multi-omics investigations have provided deeper insight into etiopathogenesis and have resulted in improved preventative and therapeutic interventions.

Human Lung Organoids-A Novel Experimental and Precision Medicine Approach

The global burden of respiratory diseases is very high and still on the rise, prompting the need for accurate models for basic and translational research. Several model systems are currently available ranging from simple airway cell cultures to complex tissue-engineered lungs. In recent years, human lung organoids have been established as highly transferrable three-dimensional in vitro model systems for lung research. For acute infectious and chronic inflammatory diseases as well as lung cancer, human lung organoids have opened possibilities for precise in vitro research and a deeper understanding of mechanisms underlying lung injury and regeneration. Human lung organoids from induced pluripotent stem cells or from adult stem cells of patients' samples introduce tools for understanding developmental processes and personalized medicine approaches. When further state-of-the-art technologies and protocols come into use, the full potential of human lung organoids can be harnessed. High-throughput assays in drug development, gene therapy, and organoid transplantation are current applications of organoids in translational research. In this review, we emphasize novel approaches in translational and personalized medicine in lung research focusing on the use of human lung organoids.

Modeling fibrotic alveolar transitional cells with pluripotent stem cell-derived alveolar organoids

Repeated injury of the lung epithelium is proposed to be the main driver of idiopathic pulmonary fibrosis (IPF). However, available therapies do not specifically target the epithelium and human models of fibrotic epithelial damage with suitability for drug discovery are lacking. We developed a model of the aberrant epithelial reprogramming observed in IPF using alveolar organoids derived from human-induced pluripotent stem cells stimulated with a cocktail of pro-fibrotic and inflammatory cytokines. Deconvolution of RNA-seq data of alveolar organoids indicated that the fibrosis cocktail rapidly increased the proportion of transitional cell types including the KRT5 - /KRT17 + aberrant basaloid phenotype recently identified in the lungs of IPF patients. We found that epithelial reprogramming and extracellular matrix (ECM) production persisted after removal of the fibrosis cocktail. We evaluated the effect of the two clinically approved compounds for IPF, nintedanib and pirfenidone, and found that they reduced the expression of ECM and pro-fibrotic mediators but did not completely reverse epithelial reprogramming. Thus, our system recapitulates key aspects of IPF and is a promising system for drug discovery.

A single-cell atlas of in vitro multiculture systems uncovers the in vivo lineage trajectory and cell state in the human lung

We present an in-depth single-cell atlas of in vitro multiculture systems on human primary airway epithelium derived from normal and diseased lungs of 27 individual donors. Our large-scale single-cell profiling identified new cell states and differentiation trajectories of rare airway epithelial cell types in human distal lungs. By integrating single-cell datasets of human lung tissues, we discovered immune-primed subsets enriched in lungs and organoids derived from patients with chronic respiratory disease. To demonstrate the full potential of our platform, we further illustrate transcriptomic responses to various respiratory virus infections in vitro airway models. Our work constitutes a single-cell roadmap for the cellular and molecular characteristics of human primary lung cells in vitro and their relevance to human tissues in vivo.

The Transcriptome Landscape of the In Vitro Human Airway Epithelium Response to SARS-CoV-2

Airway-liquid interface cultures of primary epithelial cells and of induced pluripotent stem-cell-derived airway epithelial cells (ALI and iALI, respectively) are physiologically relevant models for respiratory virus infection studies because they can mimic the in vivo human bronchial epithelium. Here, we investigated gene expression profiles in human airway cultures (ALI and iALI models), infected or not with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), using our own and publicly available bulk and single-cell transcriptome datasets. SARS-CoV-2 infection significantly increased the expression of interferon-stimulated genes (IFI44, IFIT1, IFIT3, IFI35, IRF9, MX1, OAS1, OAS3 and ISG15) and inflammatory genes (NFKBIA, CSF1, FOSL1, IL32 and CXCL10) by day 4 post-infection, indicating activation of the interferon and immune responses to the virus. Extracellular matrix genes (ITGB6, ITGB1 and GJA1) were also altered in infected cells. Single-cell RNA sequencing data revealed that SARS-CoV-2 infection damaged the respiratory epithelium, particularly mature ciliated cells. The expression of genes encoding intercellular communication and adhesion proteins was also deregulated, suggesting a mechanism to promote shedding of infected epithelial cells. These data demonstrate that ALI/iALI models help to explain the airway epithelium response to SARS-CoV-2 infection and are a key tool for developing COVID-19 treatments.

Air-Liquid interface cultures to model drug delivery through the mucociliary epithelial barrier

Epithelial cells from mucociliary portions of the airways can be readily grown and expanded in vitro. When grown on a porous membrane at an air-liquid interface (ALI) the cells form a confluent, electrically resistive barrier separating the apical and basolateral compartments. ALI cultures replicate key morphological, molecular and functional features of the in vivo epithelium, including mucus secretion and mucociliary transport. Apical secretions contain secreted gel-forming mucins, shed cell-associated tethered mucins, and hundreds of additional molecules involved in host defense and homeostasis. The respiratory epithelial cell ALI model is a time-proven workhorse that has been employed in various studies elucidating the structure and function of the mucociliary apparatus and disease pathogenesis. It serves as a critical milestone test for small molecule and genetic therapies targeting airway diseases. To fully exploit the potential of this important tool, numerous technical variables must be thoughtfully considered and carefully executed.

A distal lung organoid model to study interstitial lung disease, viral infection and human lung development

Organoids have been an exciting advancement in stem cell research. Here we describe a strategy for directed differentiation of human pluripotent stem cells into distal lung organoids. This protocol recapitulates lung development by sequentially specifying human pluripotent stem cells to definitive endoderm, anterior foregut endoderm, ventral anterior foregut endoderm, lung bud organoids and finally lung organoids. The organoids take ~40 d to generate and can be maintained more than 180 d, while progressively maturing up to a stage consistent with the second trimester of human gestation. They are unique because of their branching morphology, the near absence of non-lung endodermal lineages, presence of mesenchyme and capacity to recapitulate interstitial lung diseases. This protocol can be performed by anyone familiar with cell culture techniques, is conducted in serum-free conditions and does not require lineage-specific reporters or enrichment steps. We also provide a protocol for the generation of single-cell suspensions for single-cell RNA sequencing.

Directed differentiation of mouse pluripotent stem cells into functional lung-specific mesenchyme

While the generation of many lineages from pluripotent stem cells has resulted in basic discoveries and clinical trials, the derivation of tissue-specific mesenchyme via directed differentiation has markedly lagged. The derivation of lung-specific mesenchyme is particularly important since this tissue plays crucial roles in lung development and disease. Here we generate a mouse induced pluripotent stem cell (iPSC) line carrying a lung-specific mesenchymal reporter/lineage tracer. We identify the pathways (RA and Shh) necessary to specify lung mesenchyme and find that mouse iPSC-derived lung mesenchyme (iLM) expresses key molecular and functional features of primary developing lung mesenchyme. iLM recombined with engineered lung epithelial progenitors self-organizes into 3D organoids with juxtaposed layers of epithelium and mesenchyme. Co-culture increases yield of lung epithelial progenitors and impacts epithelial and mesenchymal differentiation programs, suggesting functional crosstalk. Our iPSC-derived population thus provides an inexhaustible source of cells for studying lung development, modeling diseases, and developing therapeutics.

Merits and challenges of iPSC-derived organoids for clinical applications

Induced pluripotent stem cells (iPSCs) have entered an unprecedented state of development since they were first generated. They have played a critical role in disease modeling, drug discovery, and cell replacement therapy, and have contributed to the evolution of disciplines such as cell biology, pathophysiology of diseases, and regenerative medicine. Organoids, the stem cell-derived 3D culture systems that mimic the structure and function of organs in vitro, have been widely used in developmental research, disease modeling, and drug screening. Recent advances in combining iPSCs with 3D organoids are facilitating further applications of iPSCs in disease research. Organoids derived from embryonic stem cells, iPSCs, and multi-tissue stem/progenitor cells can replicate the processes of developmental differentiation, homeostatic self-renewal, and regeneration due to tissue damage, offering the potential to unravel the regulatory mechanisms of development and regeneration, and elucidate the pathophysiological processes involved in disease mechanisms. Herein, we have summarized the latest research on the production scheme of organ-specific iPSC-derived organoids, the contribution of these organoids in the treatment of various organ-related diseases, in particular their contribution to COVID-19 treatment, and have discussed the unresolved challenges and shortcomings of these models.

Opposing roles for TGFβ- and BMP-signaling during nascent alveolar differentiation in the developing human lung

Alveolar type 2 (AT2) cells function as stem cells in the adult lung and aid in repair after injury. The current study aimed to understand the signaling events that control differentiation of this therapeutically relevant cell type during human development. Using lung explant and organoid models, we identified opposing effects of TGFβ- and BMP-signaling, where inhibition of TGFβ- and activation of BMP-signaling in the context of high WNT- and FGF-signaling efficiently differentiated early lung progenitors into AT2-like cells in vitro . AT2-like cells differentiated in this manner exhibit surfactant processing and secretion capabilities, and long-term commitment to a mature AT2 phenotype when expanded in media optimized for primary AT2 culture. Comparing AT2-like cells differentiated with TGFβ-inhibition and BMP-activation to alternative differentiation approaches revealed improved specificity to the AT2 lineage and reduced off-target cell types. These findings reveal opposing roles for TGFβ- and BMP-signaling in AT2 differentiation and provide a new strategy to generate a therapeutically relevant cell type in vitro .

Human pluripotent-stem-cell-derived organoids for drug discovery and evaluation

Human pluripotent stem cells (hPSCs) and three-dimensional organoids have ushered in a new era for disease modeling and drug discovery. Over the past decade, significant progress has been in deriving functional organoids from hPSCs, which have been applied to recapitulate disease phenotypes. In addition, these advancements have extended the application of hPSCs and organoids for drug screening and clinical-trial safety evaluations. This review provides an overview of the achievements and challenges in using hPSC-derived organoids to conduct relevant high-throughput, high-contentscreens and drug evaluation. These studies have greatly enhanced our knowledge and toolbox for precision medicine.

The developmental toxicity of PM2.5 on the early stages of fetal lung with human lung bud tip progenitor organoids

Exposure to air pollution has been proven to be associated with impaired fetal lung development. However, due to the lack of reliable human source models, it is still challenging to deeply understand the human fetal lung development under PM2.5 exposure. Here, we utilized human embryonic stem cell (hESC) line H9 to generate lung bud tip progenitor organoids (LPOs), a process that mimics early stages of fetal lung development including definitive endoderm (DE) formation, anterior foregut endoderm (AFE) differentiation and lung progenitor cell specification, to evaluate potential pulmonary developmental toxicity of PM2.5. We demonstrated that PM2.5 exposure the entire LPOs induction from hESCs significantly affected cellular proliferation of LPOs, and altered the expression of lung progenitor cell markers NKX2.1, SOX2 and SOX9, which are canonically defined subsequently proximal-distal airways specification. To explore the dynamic influences of PM2.5 exposure at different stages of LPOs specification, we also found that PM2.5 exposure significantly affected the expression of several transcriptional factors that are important for the differentiation of DE and AFE. Mechanistically, we suggested PM2.5-induced developmental toxicity to LPOs was partially linked with the Wnt/β-catenin signaling pathway. Therefore, our findings further emphasize the substantial health risks in the development of respiratory system associated with prenatal exposure to PM2.5.

Label-free intratissue activity imaging of alveolar organoids with dynamic optical coherence tomography

An organoid is a three-dimensional (3D) in vitro cell culture emulating human organs. We applied 3D dynamic optical coherence tomography (DOCT) to visualize the intratissue and intracellular activities of human induced pluripotent stem cells (hiPSCs)-derived alveolar organoids in normal and fibrosis models. 3D DOCT data were acquired with an 840-nm spectral domain optical coherence tomography with axial and lateral resolutions of 3.8 µm (in tissue) and 4.9 µm, respectively. The DOCT images were obtained by the logarithmic-intensity-variance (LIV) algorithm, which is sensitive to the signal fluctuation magnitude. The LIV images revealed cystic structures surrounded by high-LIV borders and mesh-like structures with low LIV. The former may be alveoli with a highly dynamics epithelium, while the latter may be fibroblasts. The LIV images also demonstrated the abnormal repair of the alveolar epithelium.

The interplay of cells, polymers, and vascularization in three-dimensional lung models and their applications in COVID-19 research and therapy

Millions of people have been affected ever since the emergence of the corona virus disease of 2019 (COVID-19) outbreak, leading to an urgent need for antiviral drug and vaccine development. Current experimentation on traditional two-dimensional culture (2D) fails to accurately mimic the in vivo microenvironment for the disease, while in vivo animal model testing does not faithfully replicate human COVID-19 infection. Human-based three-dimensional (3D) cell culture models such as spheroids, organoids, and organ-on-a-chip present a promising solution to these challenges. In this report, we review the recent 3D in vitro lung models used in COVID-19 infection and drug screening studies and highlight the most common types of natural and synthetic polymers used to generate 3D lung models.

Organotypic human lung bud microarrays identify BMP-dependent SARS-CoV-2 infection in lung cells

Although lung disease is the primary clinical outcome in COVID-19 patients, how SARS-CoV-2 induces lung pathology remains elusive. Here we describe a high-throughput platform to generate self-organizing and commensurate human lung buds derived from hESCs cultured on micropatterned substrates. Lung buds resemble human fetal lungs and display proximodistal patterning of alveolar and airway tissue directed by KGF. These lung buds are susceptible to infection by SARS-CoV-2 and endemic coronaviruses and can be used to track cell type-specific cytopathic effects in hundreds of lung buds in parallel. Transcriptomic comparisons of infected lung buds and postmortem tissue of COVID-19 patients identified an induction of BMP signaling pathway. BMP activity renders lung cells more susceptible to SARS-CoV-2 infection and its pharmacological inhibition impairs infection by this virus. These data highlight the rapid and scalable access to disease-relevant tissue using lung buds that recapitulate key features of human lung morphogenesis and viral infection biology.

Influence of glycan structure on the colonization of Streptococcus pneumoniae on human respiratory epithelial cells

Virtually all living cells are encased in glycans. They perform key cellular functions such as immunomodulation and cell-cell recognition. Yet, how their composition and configuration affect their functions remains enigmatic. Here, we constructed isogenic capsule-switch mutants harboring 84 types of capsular polysaccharides (CPSs) in Streptococcus pneumoniae. This collection enables us to systematically measure the affinity of structurally related CPSs to primary human nasal and bronchial epithelial cells. Contrary to the paradigm, the surface charge does not appreciably affect epithelial cell binding. Factors that affect adhesion to respiratory cells include the number of rhamnose residues and the presence of human-like glycomotifs in CPS. Besides, pneumococcal colonization stimulated the production of interleukin 6 (IL-6), granulocyte-macrophage colony-stimulating factor (GM-CSF), and monocyte chemoattractantprotein-1 (MCP-1) in nasal epithelial cells, which also appears to be dependent on the serotype. Together, our results reveal glycomotifs of surface polysaccharides that are likely to be important for colonization and survival in the human airway.

Airway and Lung Organoids from Human-Induced Pluripotent Stem Cells Can Be Used to Assess CFTR Conductance

Airway and lung organoids derived from human-induced pluripotent stem cells (hiPSCs) are current models for personalized drug screening, cell–cell interaction studies, and lung disease research. We analyzed the existing differentiation protocols and identified the optimal conditions for obtaining organoids. In this article, we describe a step-by-step protocol for differentiating hiPSCs into airway and lung organoids. We obtained airway and lung organoids from a healthy donor and from five donors with cystic fibrosis. Analysis of the cellular composition of airway and lung organoids showed that airway organoids contain proximal lung epithelial cells, while lung organoids contain both proximal and distal lung epithelial cells. Forskolin-induced swelling of organoids derived from a healthy donor showed that lung organoids, as well as airway organoids, contain functional epithelial cells and swell after 24 h exposure to forskolin, which makes it a suitable model for analyzing the cystic fibrosis transmembrane conductance regulator (CFTR) channel conductance in vitro. Thus, our results demonstrate the feasibility of generating and characterizing airway and lung organoids from hiPSCs, which can be used for a variety of future applications.

Modeling of Respiratory Diseases Evolving with Fibrosis from Organoids Derived from Human Pluripotent Stem Cells

Respiratory disease is one of the leading causes of morbidity and mortality worldwide. There is no cure for most diseases, which are treated symptomatically. Hence, new strategies are required to deepen the understanding of the disease and development of therapeutic strategies. The advent of stem cell and organoid technology has enabled the development of human pluripotent stem cell lines and adequate differentiation protocols for developing both airways and lung organoids in different formats. These novel human-pluripotent-stem-cell-derived organoids have enabled relatively accurate disease modeling. Idiopathic pulmonary fibrosis is a fatal and debilitating disease that exhibits prototypical fibrotic features that may be, to some extent, extrapolated to other conditions. Thus, respiratory diseases such as cystic fibrosis, chronic obstructive pulmonary disease, or the one caused by SARS-CoV-2 may reflect some fibrotic aspects reminiscent of those present in idiopathic pulmonary fibrosis. Modeling of fibrosis of the airways and the lung is a real challenge due to the large number of epithelial cells involved and interaction with other cell types of mesenchymal origin. This review will focus on the status of respiratory disease modeling from human-pluripotent-stem-cell-derived organoids, which are being used to model several representative respiratory diseases, such as idiopathic pulmonary fibrosis, cystic fibrosis, chronic obstructive pulmonary disease, and COVID-19.

hPSC-derived lung organoids: Potential opportunities and challenges

Three-dimensional hPSC-derived lung organoids resemble the fetal lung stage, making them an excellent model for studying human lung development. However, current hPSC-derived lung organoids remain incomplete as they lack native lung components such as vasculature, neurons and immune cells. This highlights the need to generate more complex hPSC-derived lung organoids that can faithfully mimic native human lungs for studying human lung development, regeneration, disease modeling and drug screen. In this review, we will discuss the current studies related to the generation of hPSC-derived lung organoids, highlighting how hPSC-derived lung organoids can contribute to the understanding of human lung development. We further focus on potential approaches to generate more complex hPSC-derived lung organoids containing native cellular components. Finally, we discuss the present limitations and potential applications of hPSC-derived lung organoids in the future.

iPSC-Derived Airway Epithelial Cells: Progress, Promise, and Challenges

Induced pluripotent stem cells (iPSCs) generated from somatic cell sources are pluripotent and capable of indefinite expansion in vitro. They provide an unlimited source of cells that can be differentiated into lung progenitor cells for potential clinical use in pulmonary regenerative medicine. This review gives a comprehensive overview of recent progress toward the use of iPSCs to generate proximal and distal airway epithelial cells and mix lung organoids. Furthermore, their potential applications and future challenges for the field are discussed, with a focus on the technological hurdles that must be cleared before stem cell therapeutics can be used for clinical treatment.

Open questions in human lung organoid research

Organoids have become a prominent model system in pulmonary research. The ability to establish organoid cultures directly from patient tissue has expanded the repertoire of physiologically relevant preclinical model systems. In addition to their derivation from adult lung stem/progenitor cells, lung organoids can be derived from fetal tissue or induced pluripotent stem cells to fill a critical gap in modelling pulmonary development in vitro. Recent years have seen important progress in the characterisation and refinement of organoid culture systems. Here, we address several open questions in the field, including how closely organoids recapitulate the tissue of origin, how well organoids recapitulate patient cohorts, and how well organoids capture diversity within a patient. We advocate deeper characterisation of models using single cell technologies, generation of more diverse organoid biobanks and further standardisation of culture media.

Organoid modeling of human fetal lung alveolar development reveals mechanisms of cell fate patterning and neonatal respiratory disease

Variation in lung alveolar development is strongly linked to disease susceptibility. However, underlying cellular and molecular mechanisms are difficult to study in humans. We have identified an alveolar-fated epithelial progenitor in human fetal lungs, which we grow as self-organizing organoids that model key aspects of cell lineage commitment. Using this system, we have functionally validated cell-cell interactions in the developing human alveolar niche, showing that Wnt signaling from differentiating fibroblasts promotes alveolar-type-2 cell identity, whereas myofibroblasts secrete the Wnt inhibitor, NOTUM, providing spatial patterning. We identify a Wnt-NKX2.1 axis controlling alveolar differentiation. Moreover, we show that differential binding of NKX2.1 coordinates alveolar maturation, allowing us to model the effects of human genetic variation in NKX2.1 on alveolar differentiation. Our organoid system recapitulates key aspects of human fetal lung stem cell biology allowing mechanistic experiments to determine the cellular and molecular regulation of human development and disease.