Repopulation of a human alveolar matrix by adult rat type II pneumocytes in vitro. A novel system for type II pneumocyte culture

Liver tissue engineering using decellularized scaffolds: Current progress, challenges, and opportunities

Liver transplantation represents the only definitive treatment for patients with end-stage liver disease. However, the shortage of liver donors provokes a dramatic gap between available grafts and patients on the waiting list. Whole liver bioengineering, an emerging field of tissue engineering, holds great potential to overcome this gap. This approach involves two main steps; the first is liver decellularization and the second is recellularization. Liver decellularization aims to remove cellular and nuclear materials from the organ, leaving behind extracellular matrices containing different structural proteins and growth factors while retaining both the vascular and biliary networks. Recellularization involves repopulating the decellularized liver with appropriate cells, theoretically from the recipient patient, to reconstruct the parenchyma, vascular tree, and biliary network. The aim of this review is to identify the major advances in decellularization and recellularization strategies and investigate obstacles for the clinical application of bioengineered liver, including immunogenicity of the designed liver extracellular matrices, the need for standardization of scaffold fabrication techniques, selection of suitable cell sources for parenchymal repopulation, vascular, and biliary tree reconstruction. In vivo transplantation models are also summarized for evaluating the functionality of bioengineered livers. Finally, the regulatory measures and future directions for confirming the safety and efficacy of bioengineered liver are also discussed. Addressing these challenges in whole liver bioengineering may offer new solutions to meet the demand for liver transplantation and improve patient outcomes.

Mesenchymal cells support the early retention of primary alveolar type 2 cells on acellular mouse lung scaffolds

Objectives

Tissue engineering approaches via repopulation of acellular biological grafts provide an exciting opportunity to generate lung grafts for transplantation. Alveolar type 2 (AT2) cells are a promising cell source for re-epithelialization. There are however inherent limitations with respect to their survival and growth, thus impeding their usability for tissue engineering applications. This study investigates the use of mesenchymal stromal cells to support primary AT2 cells for recellularization of mouse lung scaffolds.

Methods

AT2 cells and bone marrow-derived mesenchymal cells (BMC) were co-delivered to decellularized mouse lung scaffolds. Recellularized lungs were evaluated for cell surface coverage, viability, and differentiation at 1 and 4 days after cell seeding. Recellularization was evaluated via histological analysis and immunofluorescence.

Results

Simultaneous delivery of AT2 and BMC into acellular lung scaffolds resulted in enhanced cell surface coverage and reduced AT2 cell apoptosis in the recellularized scaffolds at Day 1 but not Day 4. AT2 cell number decreased after 4 days in both of AT2 only and codelivery groups suggesting limited expansion potential in the scaffold. After retention in the scaffold, AT2 cells differentiated into Aqp5-expressing cells.

Conclusions

Our results indicate that BMC support AT2 cell survival during the initial attachment and engraftment phase of recellularization. While our findings suggest only a short-term beneficial effect of BMC, our study demonstrates that AT2 cells can be delivered and retained in acellular lung scaffolds; thus with preconditioning and supporting cells, may be used for re-epithelialization. Selection and characterization of appropriate cell sources for use in recellularization, will be critical for ultimate clinical application.

Rational engineering of lung alveolar epithelium

Engineered whole lungs may one day expand therapeutic options for patients with end-stage lung disease. However, the feasibility of ex vivo lung regeneration remains limited by the inability to recapitulate mature, functional alveolar epithelium. Here, we modulate multimodal components of the alveolar epithelial type 2 cell (AEC2) niche in decellularized lung scaffolds in order to guide AEC2 behavior for epithelial regeneration. First, endothelial cells coordinate with fibroblasts, in the presence of soluble growth and maturation factors, to promote alveolar scaffold population with surfactant-secreting AEC2s. Subsequent withdrawal of Wnt and FGF agonism synergizes with tidal-magnitude mechanical strain to induce the differentiation of AEC2s to squamous type 1 AECs (AEC1s) in cultured alveoli, in situ. These results outline a rational strategy to engineer an epithelium of AEC2s and AEC1s contained within epithelial-mesenchymal-endothelial alveolar-like units, and highlight the critical interplay amongst cellular, biochemical, and mechanical niche cues within the reconstituting alveolus.

What is the need and why is it time for innovative models for understanding lung repair and regeneration?

Advances in tissue engineering continue at a rapid pace and have provided novel methodologies and insights into normal cell and tissue homeostasis, disease pathogenesis, and new potential therapeutic strategies. The evolution of new techniques has particularly invigorated the field and span a range from novel organ and organoid technologies to increasingly sophisticated imaging modalities. This is particularly relevant for the field of lung biology and diseases as many lung diseases, including chronic obstructive pulmonary disease (COPD) and idiopathic fibrosis (IPF), among others, remain incurable with significant morbidity and mortality. Advances in lung regenerative medicine and engineering also offer new potential avenues for critical illnesses such as the acute respiratory distress syndrome (ARDS) which also continue to have significant morbidity and mortality. In this review, an overview of lung regenerative medicine with focus on current status of both structural and functional repair will be presented. This will serve as a platform for surveying innovative models and techniques for study, highlighting the need and timeliness for these approaches.

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.

Increased expression and accumulation of GDF15 in IPF extracellular matrix contribute to fibrosis

Idiopathic pulmonary fibrosis (IPF) is a chronic disease of unmet medical need. It is characterized by formation of scar tissue leading to a progressive and irreversible decline in lung function. IPF is associated with repeated injury, which may alter the composition of the extracellular matrix (ECM). Here, we demonstrate that IPF patient–derived pulmonary ECM drives profibrotic response in normal human lung fibroblasts (NHLF) in a 3D spheroid assay. Next, we reveal distinct alterations in composition of the diseased ECM, identifying potentially novel associations with IPF. Growth differentiation factor 15 (GDF15) was identified among the most significantly upregulated proteins in the IPF lung–derived ECM. In vivo, GDF15 neutralization in a bleomycin-induced lung fibrosis model led to significantly less fibrosis. In vitro, recombinant GDF15 (rGDF15) stimulated α smooth muscle actin (αSMA) expression in NHLF, and this was mediated by the activin receptor-like kinase 5 (ALK5) receptor. Furthermore, in the presence of rGDF15, the migration of NHLF in collagen gel was reduced. In addition, we observed a cell type–dependent effect of GDF15 on the expression of cell senescence markers. Our data suggest that GDF15 mediates lung fibrosis through fibroblast activation and differentiation, implicating a potential direct role of this matrix-associated cytokine in promoting aberrant cell responses in disease.

Decellularized human-sized pulmonary scaffolds for lung tissue engineering: a comprehensive review

The ultimate goal of lung bioengineering is to produce transplantable lungs for human beings. Therefore, large-scale studies are of high importance. In this paper, we review the investigations on decellularization and recellularization of human-sized lung scaffolds. First, studies that introduce new ways to enhance the decellularization of large-scale lungs are reviewed, followed by the investigations on the xenogeneic sources of lung scaffolds. Then, decellularization and recellularization of diseased lung scaffolds are discussed to assess their usefulness for tissue engineering applications. Next, the use of stem cells in recellularizing acellular lung scaffolds is reviewed, followed by the case studies on the transplantation of bioengineered lungs. Finally, the remaining challenges are discussed, and future directions are highlighted.

Bioartificial lungs based on de- and recellularisation approaches: a historical perspective

A discussion of three landmark studies on bioartificial lungs published during 2010 that were instrumental in invigorating the lung regenerative medicine field https://bit.ly/31qQAEa.

Alveolar wars: The rise of in vitro models to understand human lung alveolar maintenance, regeneration, and disease

Diseases such as idiopathic pulmonary fibrosis, chronic obstructive pulmonary disease, and bronchopulmonary dysplasia injure the gas-exchanging alveoli of the human lung. Animal studies have indicated that dysregulation of alveolar cells, including alveolar type II stem/progenitor cells, is implicated in disease pathogenesis. Due to mouse-human differences, there has been a desperate need to develop human-relevant lung models that can more closely recapitulate the human lung during homeostasis, injury repair, and disease. Here we discuss how current single-cell RNA sequencing studies have increased knowledge of the cellular and molecular composition of human lung alveoli, including the identification of molecular heterogeneity, cellular diversity, and previously unknown cell types, some of which arise specifically during disease. For functional analysis of alveolar cells, in vitro human alveolar organoids established from human pluripotent stem cells, embryonic progenitors, and adult tissue from both healthy and diseased lungs have modeled aspects of the cellular and molecular features of alveolar epithelium. Drawbacks of such systems are highlighted, along with possible solutions. Organoid-on-a-chip and ex vivo systems including precision-cut lung slices can complement organoid studies by providing further cellular and structural complexity of lung tissues, and have been shown to be invaluable models of human lung disease, while the production of acellular and synthetic scaffolds hold promise in lung transplant efforts. Further improvements to such systems will increase understanding of the underlying biology of human alveolar stem/progenitor cells, and could lead to future therapeutic or pharmacological intervention in patients suffering from end-stage lung diseases.

Cardiac tissue-derived extracellular matrix scaffolds for myocardial repair: advantages and challenges

Decellularized extracellular matrix (dECM) derived from myocardium has been widely explored as a nature scaffold for cardiac tissue engineering applications. Cardiac dECM offers many unique advantages such as preservation of organ-specific ECM microstructure and composition, demonstration of tissue-mimetic mechanical properties and retention of biochemical cues in favor of subsequent recellularization. However, current processes of dECM decellularization and recellularization still face many challenges including the need for balance between cell removal and extracellular matrix preservation, efficient recellularization of dECM for obtaining homogenous cell distribution, tailoring material properties of dECM for enhancing bioactivity and prevascularization of thick dECM. This review summarizes the recent progresses of using dECM scaffold for cardiac repair and discusses its major advantages and challenges for producing biomimetic cardiac patch.

Lung tissue bioengineering for chronic obstructive pulmonary disease: overcoming the need for lung transplantation from human donors

Introduction: Chronic obstructive pulmonary disease (COPD) affects more than 380 million people, causing more than 3 million deaths annually worldwide. Despite this enormous burden, currently available therapies are largely limited to symptom control. Lung transplant is considered for end-stage disease but is severely limited by the availability of human organs. Furthermore, the pre-transplant course is a complex orchestration of locating and harvesting suitable lungs, and the post-transplant course is complicated by rejection and infection. Lung tissue bioengineering has the potential to relieve the organ shortage and improve the post-transplant course by generating patient-specific lungs for transplant. Additionally, emerging progenitor cell therapies may facilitate in vivo regeneration of pulmonary tissue, obviating the need for transplant. Areas Covered: We review several lung tissue bioengineering approaches including the recellularization of decellularized scaffolds, 3D bioprinting, genetically-engineered xenotransplantation, blastocyst complementation, and direct therapy with progenitor cells. Articles were identified by searching relevant terms (see Key Words) in the PubMed database and selected for inclusion based on novelty and uniqueness of their approach. Expert Opinion: Lung tissue bioengineering research is in the early stages. Of the methods reviewed, only direct cell therapy has been investigated in humans. We anticipate a minimum of 5-10 years before human therapy will be feasible.

Tissue-Engineered Grafts from Human Decellularized Extracellular Matrices: A Systematic Review and Future Perspectives

Tissue engineering and regenerative medicine involve many different artificial and biologic materials, frequently integrated in composite scaffolds, which can be repopulated with various cell types. One of the most promising scaffolds is decellularized allogeneic extracellular matrix (ECM) then recellularized by autologous or stem cells, in order to develop fully personalized clinical approaches. Decellularization protocols have to efficiently remove immunogenic cellular materials, maintaining the nonimmunogenic ECM, which is endowed with specific inductive/differentiating actions due to its architecture and bioactive factors. In the present paper, we review the available literature about the development of grafts from decellularized human tissues/organs. Human tissues may be obtained not only from surgery but also from cadavers, suggesting possible development of Human Tissue BioBanks from body donation programs. Many human tissues/organs have been decellularized for tissue engineering purposes, such as cartilage, bone, skeletal muscle, tendons, adipose tissue, heart, vessels, lung, dental pulp, intestine, liver, pancreas, kidney, gonads, uterus, childbirth products, cornea, and peripheral nerves. In vitro recellularizations have been reported with various cell types and procedures (seeding, injection, and perfusion). Conversely, studies about in vivo behaviour are poorly represented. Actually, the future challenge will be the development of human grafts to be implanted fully restored in all their structural/functional aspects.

How to build a lung: latest advances and emerging themes in lung bioengineering

Chronic respiratory diseases remain a major cause of morbidity and mortality worldwide. The only option at end-stage disease is lung transplantation, but there are not enough donor lungs to meet clinical demand. Alternative options to increase tissue availability for lung transplantation are urgently required to close the gap on this unmet clinical need. A growing number of tissue engineering approaches are exploring the potential to generate lung tissue ex vivo for transplantation. Both biologically derived and manufactured scaffolds seeded with cells and grown ex vivo have been explored in pre-clinical studies, with the eventual goal of generating functional pulmonary tissue for transplantation. Recently, there have been significant efforts to scale-up cell culture methods to generate adequate cell numbers for human-scale bioengineering approaches. Concomitantly, there have been exciting efforts in designing bioreactors that allow for appropriate cell seeding and development of functional lung tissue over time. This review aims to present the current state-of-the-art progress for each of these areas and to discuss promising new ideas within the field of lung bioengineering.

Acellular human lung scaffolds to model lung disease and tissue regeneration

Recent advances in whole lung bioengineering have opened new doors for studying lung repair and regeneration ex vivo using acellular human derived lung tissue scaffolds. Methods to decellularise whole human lungs, lobes or resected segments from normal and diseased human lungs have been developed using both perfusion and immersion based techniques. Immersion based techniques allow laboratories without access to intact lobes the ability to generate acellular human lung scaffolds. Acellular human lung scaffolds can be further processed into small segments, thin slices or extracellular matrix extracts, to study cell behaviour such as viability, proliferation, migration and differentiation. Recent studies have offered important proof of concept of generating sufficient primary endothelial and lung epithelial cells to recellularise whole lobes that can be maintained for several days ex vivo in a bioreactor to study regeneration. In parallel, acellular human lung scaffolds have been increasingly used for studying cell–extracellular environment interactions. These studies have helped provide new insights into the role of the matrix and the extracellular environment in chronic human lung diseases such as chronic obstructive pulmonary disease and idiopathic pulmonary fibrosis. Acellular human lung scaffolds are a versatile new tool for studying human lung repair and regeneration ex vivo.

Acellular human lung scaffolds can be used as diverse tools to study lung disease and tissue regeneration ex vivohttp://ow.ly/ZS0l30k9MEH

Bio-scaffolds in organ-regeneration: Clinical potential and current challenges

Cadaveric organ transplantation represents the definitive treatment option for end-stage disease but is restricted by the shortage of clinically-viable donor organs. This limitation has, in part, driven current research efforts for in vitro generation of transplantable tissue surrogates. Recent advances in organ reconstruction have been facilitated by the re-purposing of decellularized whole organs to serve as three-dimensional bio-scaffolds. Notably, studies in rodents indicate that such scaffolds retain native extracellular matrix components that provide appropriate biochemical, mechanical and physical stimuli for successful tissue/organ reconstruction. As such, they support the migration, adhesion and differentiation of reseeded primary and/or pluripotent cell populations, which mature and achieve functionality through short-term conditioning within specialized tissue bioreactors. Whilst these findings are encouraging, significant challenges remain to up-scale the present technology to accommodate human-sized organs and thereby further the translation of this approach towards clinical use. Of note, the diverse structural and cellular composition of large mammalian organ systems mean that a "one-size fits all" approach cannot be adopted either to the methods used for their decellularization or the cells required for subsequent re-population, to create fully functional entities. The present review seeks to highlight the clinical potential of decellularized organ bio-scaffolds as a route to further advance the field of tissue- and organ-regeneration, and to discuss the challenges which are yet to be addressed if such a technology is ever to become a credible rival to conventional organ allo-transplantation.

Biocompatibility evaluation of tissue-engineered decellularized scaffolds for biomedical application

Biomaterials based on seeding of cells on decellularized scaffolds have gained increasing interest in the last few years and suggested to serve as an alternative approach to bioengineer artificial organs and tissues for transplantation. The reaction of the host toward the decellularized scaffold and transplanted cells depends on the biocompatibility of the construct. Before proceeding to the clinical application step of decellularized scaffolds, it is greatly important to apply a number of biocompatibility tests in vitro and in vivo. This review describes the different methodology involved in cytotoxicity, pathogenicity, immunogenicity and biodegradability testing for evaluating the biocompatibility of various decellularized matrices obtained from human or animals.

What We Talk About When We Talk About Evolution

Currently, the biologic sciences are a Tower of Babel, having become so highly specialized that one discipline cannot effectively communicate with another. A mechanism for evolution that integrates development and physiologic homeostasis phylogenetically has been identified—cell-cell interactions. By reducing this process to ligand-receptor interactions and their intermediate down-stream signaling partners, it is possible, for example, to envision the functional homologies between such seemingly disparate structures and functions as the lung alveolus and kidney glomerulus, the skin and brain, or the skin and lung. For example, by showing the continuum of the lung phenotype for gas exchange at the cell-molecular level, being selected for increased surface area by augmenting lung surfactant production and function in lowering surface tension, we have determined an unprecedented structural-functional continuum from proximate to ultimate causation in evolution. It is maintained that tracing the changes in structure and function that have occurred over both the short-term history of the organism (as ontogeny), and the long-term history of the organism (as phylogeny), and how the mechanisms shared in common can account for both biologic stability and novelty, will provide the key to understanding the mechanisms of evolution. We need to better understand evolution from its unicellular origins as the Big Bang of biology.

Revascularization of decellularized lung scaffolds: principles and progress

There is a clear unmet clinical need for novel biotechnology-based therapeutic approaches to lung repair and/or replacement, such as tissue engineering of whole bioengineered lungs. Recent studies have demonstrated the feasibility of decellularizing the whole organ by removal of all its cellular components, thus leaving behind the extracellular matrix as a complex three-dimensional (3D) biomimetic scaffold. Implantation of decellularized lung scaffolds (DLS), which were recellularized with patient-specific lung (progenitor) cells, is deemed the ultimate alternative to lung transplantation. Preclinical studies demonstrated that, upon implantation in rodent models, bioengineered lungs that were recellularized with airway and vascular cells were capable of gas exchange for up to 14 days. However, the long-term applicability of this concept is thwarted in part by the failure of current approaches to reconstruct a physiologically functional, quiescent endothelium lining the entire vascular tree of reseeded lung scaffolds, as inferred from the occurrence of hemorrhage into the airway compartment and thrombosis in the vasculature in vivo. In this review, we explore the idea that successful whole lung bioengineering will critically depend on 1) preserving and/or reestablishing the integrity of the subendothelial basement membrane, especially of the ultrathin respiratory membrane separating airways and capillaries, during and following decellularization and 2) restoring vascular physiological functionality including the barrier function and quiescence of the endothelial lining following reseeding of the vascular compartment. We posit that physiological reconstitution of the pulmonary vascular tree in its entirety will significantly promote the clinical translation of the next generation of bioengineered whole lungs.

Homeostasis as the Mechanism of Evolution

Homeostasis is conventionally thought of merely as a synchronic (same time) servo-mechanism that maintains the status quo for organismal physiology. However, when seen from the perspective of developmental physiology, homeostasis is a robust, dynamic, intergenerational, diachronic (across-time) mechanism for the maintenance, perpetuation and modification of physiologic structure and function. The integral relationships generated by cell-cell signaling for the mechanisms of embryogenesis, physiology and repair provide the needed insight to the scale-free universality of the homeostatic principle, offering a novel opportunity for a Systems approach to Biology. Starting with the inception of life itself, with the advent of reproduction during meiosis and mitosis, moving forward both ontogenetically and phylogenetically through the evolutionary steps involved in adaptation to an ever-changing environment, Biology and Evolution Theory need no longer default to teleology.

Coming to terms with tissue engineering and regenerative medicine in the lung

Lung diseases such as emphysema, interstitial fibrosis, and pulmonary vascular diseases cause significant morbidity and mortality, but despite substantial mechanistic understanding, clinical management options for them are limited, with lung transplantation being implemented at end stages. However, limited donor lung availability, graft rejection, and long-term problems after transplantation are major hurdles to lung transplantation being a panacea. Bioengineering the lung is an exciting and emerging solution that has the ultimate aim of generating lung tissues and organs for transplantation. In this article we capture and review the current state of the art in lung bioengineering, from the multimodal approaches, to creating anatomically appropriate lung scaffolds that can be recellularized to eventually yield functioning, transplant-ready lungs. Strategies for decellularizing mammalian lungs to create scaffolds with native extracellular matrix components vs. de novo generation of scaffolds using biocompatible materials are discussed. Strengths vs. limitations of recellularization using different cell types of various pluripotency such as embryonic, mesenchymal, and induced pluripotent stem cells are highlighted. Current hurdles to guide future research toward achieving the clinical goal of transplantation of a bioengineered lung are discussed.

Fate of distal lung epithelium cultured in a decellularized lung extracellular matrix

Type II cells are the defenders of the alveolus. They produce surfactant to prevent alveolar collapse, they actively transport water to prevent filling of the air sacs that would otherwise prevent gas exchange, and they differentiate to type I epithelial cells. They are an indispensable component of functional lung tissue. To understand the functionality of type II cells in isolation, we sought to track their fate in decellularized matrices and to assess their ability to contribute to barrier function by differentiation to type I alveolar epithelial cells. Rat type II cells were isolated from neonatal rat lungs by labeling with the RTII-70 surface marker and separation using a magnetic column. This produced a population of ∼50% RTII-70-positive cells accompanied by few type I epithelial cells or α-actin-positive mesenchymal cells. This population was seeded into decellularized rat lung matrices and cultured for 1 or 7 days. Culture in Dulbecco's modified Eagle's medium +10% fetal bovine serum (FBS) resulted in reduced expression of epithelial markers and increased expression of mesenchymal markers. By 7 days, no epithelial markers were visible by immunostaining; nearly all cells were α-actin positive. Gene expression for the mesenchymal markers, α-actin, vimentin, and TGF-βR, was significantly upregulated on day 1 (p=0.0005, 0.0005, and 2.342E-5, respectively). Transcript levels of α-actin and TGF-βR remained high at 7 days (p=1.364E-10 and 0.0002). Interestingly, human type II cells cultured under the same conditions showed a similar trend in the loss of epithelial markers, but did not display high expression of mesenchymal markers. Rat cells additionally showed the ability to produce and degrade the basement membrane and extracellular matrix components, such as fibronectin, collagen IV, and collagen I. Quantitative real-time reverse transcription polymerase chain reaction (RT-PCR) showed significant increases in expression of the fibronectin and matrix metalloprotease-2 (MMP-2) genes after 1 day in culture (p=0.0135 and 0.0128, respectively) and elevated collagen I expression at 7 days (p=0.0016). These data suggest that the original type II-enriched population underwent a transition to increased expression of mesenchymal markers, perhaps as part of a survival or wound-healing program. These results suggest that additional medium components and/or the application of physiologically appropriate stimuli such as ventilation may be required to promote lung-specific epithelial phenotypes.

A review of cellularization strategies for tissue engineering of whole organs

With the advent of whole organ decellularization, extracellular matrix scaffolds suitable for organ engineering were generated from numerous tissues, including the heart, lung, liver, kidney, and pancreas, for use as alternatives to traditional organ transplantation. Biomedical researchers now face the challenge of adequately and efficiently recellularizing these organ scaffolds. Herein, an overview of whole organ decellularization and a thorough review of the current literature for whole organ recellularization are presented. The cell types, delivery methods, and bioreactors employed for recellularization are discussed along with commercial and clinical considerations, such as immunogenicity, biocompatibility, and Food and Drug Administartion regulation.

Applying Biotechnology and Bioengineering to Pediatric Lung Disease: Emerging Paradigms and Platforms

Pediatric lung diseases remain a costly worldwide health burden. For many children with end-stage lung disease, lung transplantation remains the only therapeutic option. Due to the limited number of lungs available for transplantation, alternatives to lung transplant are desperately needed. Recently, major improvements in tissue engineering have resulted in newer technology and methodology to develop viable bioengineered lungs. These include critical advances in lung cell biology, stem cell biology, lung extracellular matrix, microfabrication techniques, and orthotopic transplantation of bioartificial lungs. The goal of this short review is to engage the reader's interest with regard to these emerging concepts and to stimulate their interest to learn more. We review the existing state of the art of lung tissue engineering, and point to emerging paradigms and platforms in the field. Finally, we summarize the challenges and unmet needs that remain to be overcome.

The clinical use of regenerative therapy in COPD

Regenerative or stem cell therapy is an emerging field of treatment based on stimulation of endogenous resident stem cells or administration of exogenous stem cells to treat diseases or injury and to replace malfunctioning or damaged tissues. Current evidence suggests that in the lung, these cells may participate in tissue homeostasis and regeneration after injury. Animal and human studies have demonstrated that tissue-specific stem cells and bone marrow-derived cells contribute to lung tissue regeneration and protection, and thus administration of exogenous stem/progenitor cells or humoral factors responsible for the activation of endogenous stem/progenitor cells may be a potent next-generation therapy for chronic obstructive pulmonary disease. The use of bone marrow-derived stem cells could allow repairing and regenerate the damaged tissue present in chronic obstructive pulmonary disease by means of their engraftment into the lung. Another approach could be the stimulation of resident stem cells by means of humoral factors or photobiostimulation.

Enhanced lung epithelial specification of human induced pluripotent stem cells on decellularized lung matrix

BACKGROUND

Whole-lung scaffolds can be created by perfusion decellularization of cadaveric donor lungs. The resulting matrices can then be recellularized to regenerate functional organs. This study evaluated the capacity of acellular lung scaffolds to support recellularization with lung progenitors derived from human induced pluripotent stem cells (iPSCs).

METHODS

Whole rat and human lungs were decellularized by constant-pressure perfusion with 0.1% sodium dodecyl sulfate solution. Resulting lung scaffolds were cryosectioned into slices or left intact. Human iPSCs were differentiated to definitive endoderm, anteriorized to a foregut fate, and then ventralized to a population expressing NK2 homeobox 1 (Nkx2.1). Cells were seeded onto slices and whole lungs, which were maintained under constant perfusion biomimetic culture. Lineage specification was assessed by quantitative polymerase chain reaction and immunofluorescent staining. Regenerated left lungs were transplanted in an orthotopic position.

RESULTS

Activin-A treatment, followed by transforming growth factor-β inhibition, induced differentiation of human iPSCs to anterior foregut endoderm as confirmed by forkhead box protein A2 (FOXA2), SRY (Sex Determining Region Y)-Box 17 (SOX17), and SOX2 expression. Cells cultured on decellularized lung slices demonstrated proliferation and lineage commitment after 5 days. Cells expressing Nkx2.1 were identified at 40% to 60% efficiency. Within whole-lung scaffolds and under perfusion culture, cells further upregulated Nkx2.1 expression. After orthotopic transplantation, grafts were perfused and ventilated by host vasculature and airways.

CONCLUSIONS

Decellularized lung matrix supports the culture and lineage commitment of human iPSC-derived lung progenitor cells. Whole-organ scaffolds and biomimetic culture enable coseeding of iPSC-derived endothelial and epithelial progenitors and enhance early lung fate. Orthotopic transplantation may enable further in vivo graft maturation.

Modeling the lung: Design and development of tissue engineered macro- and micro-physiologic lung models for research use

Respiratory tract specific cell populations, or tissue engineered in vitro grown human lung, have the potential to be used as research tools to mimic physiology, toxicology, pathology, as well as infectious diseases responses of cells or tissues. Studies related to respiratory tract pathogenesis or drug toxicity testing in the past made use of basic systems where single cell populations were exposed to test agents followed by evaluations of simple cellular responses. Although these simple single-cell-type systems provided good basic information related to cellular responses, much more can be learned from cells grown in fabricated microenvironments which mimic in vivo conditions in specialized microfabricated chambers or by human tissue engineered three-dimensional (3D) models which allow for more natural interactions between cells. Recent advances in microengineering technology, microfluidics, and tissue engineering have provided a new approach to the development of 2D and 3D cell culture models which enable production of more robust human in vitro respiratory tract models. Complex models containing multiple cell phenotypes also provide a more reasonable approximation of what occurs in vivo without the confounding elements in the dynamic in vivo environment. The goal of engineering good 3D human models is the formation of physiologically functional respiratory tissue surrogates which can be used as pathogenesis models or in the case of 2D screening systems for drug therapy evaluation as well as human toxicity testing. We hope that this manuscript will serve as a guide for development of future respiratory tract model systems as well as a review of conventional models.

Stem cells, cell therapies, and bioengineering in lung biology and diseases. Comprehensive review of the recent literature 2010-2012

A conference, "Stem Cells and Cell Therapies in Lung Biology and Lung Diseases," was held July 25 to 28, 2011 at the University of Vermont to review the current understanding of the role of stem and progenitor cells in lung repair after injury and to review the current status of cell therapy and ex vivo bioengineering approaches for lung diseases. These are rapidly expanding areas of study that provide further insight into and challenge traditional views of mechanisms of lung repair after injury and pathogenesis of several lung diseases. The goals of the conference were to summarize the current state of the field, to discuss and debate current controversies, and to identify future research directions and opportunities for basic and translational research in cell-based therapies for lung diseases. The goal of this article, which accompanies the formal conference report, is to provide a comprehensive review of the published literature in lung regenerative medicine from the last conference report through December 2012.

Can stem cells be used to generate new lungs? Ex vivo lung bioengineering with decellularized whole lung scaffolds

For patients with end-stage lung diseases, lung transplantation is the only available therapeutic option. However, the number of suitable donor lungs is insufficient and lung transplants are complicated by significant graft failure and complications of immunosuppressive regimens. An alternative to classic organ replacement is desperately needed. Engineering of bioartificial organs using either natural or synthetic scaffolds is an exciting new potential option for generation of functional pulmonary tissue for human clinical application. Natural organ scaffolds can be generated by decellularization of native tissues; these acellular scaffolds retain the native organ ultrastructure and can be seeded with autologous cells towards the goal of regenerating functional tissues. Several decellularization strategies have been employed for lungs; however, there is no consensus on the optimal approach. A variety of cell types have been investigated as potential candidates for effective recellularization of acellular lung scaffolds. Candidate cells that might be best utilized are those which can be easily and reproducibly isolated, expanded in vitro, seeded onto decellularized matrices, induced to differentiate into pulmonary lineage cells, and which survive to functional maturity. Whole lung cell suspensions, endogenous progenitor cells, embryonic and adult stem cells and induced pluripotent stem (iPS) cells have been investigated for their applicability to repopulate acellular lung matrices. Ideally, patient-derived autologous cells would be used for lung recellularization as they have the potential to reduce the need for post-transplant immunosuppression. Several studies have performed transplantation of rudimentary bioengineered lung scaffolds in animal models with limited, short-term functionality but much further study is needed.

Engineering tissue for the fetus: stem cells and matrix signalling

Congenital malformations are major causes of disease and death during the first years of life and, most of the time, functional replacement of the missing or damaged organs remains an unmet clinical need. Particularly relevant for the treatment of congenital malformation would be to collect the stem cells at diagnosis, before birth, to be able to intervene during the gestation or in the neonatal period. Human AFSCs (amniotic fluid stem cells), which have characteristics intermediate between those of embryonic and adult stem cells, have been isolated. c-Kit+Lin− cells derived from amniotic fluid display a multilineage haemopoietic potential and they can be easily reprogrammed to a pluripotent status. Although, in the future, we hope to use cells derived from the amniotic fluid, we and others have proved recently that simple organs such as the trachea can be engineered using adult progenitors utilizing decellularized cadaveric matrices. A similar approach could be used in the future for more complex organs such as the muscles, intestines or lungs.

Strategies for whole lung tissue engineering

Recent work has demonstrated the feasibility of using decellularized lung extracellular matrix scaffolds to support the engineering of functional lung tissue in vitro. Rendered acellular through the use of detergents and other reagents, the scaffolds are mounted in organ-specific bioreactors where cells in the scaffold are provided with nutrients and appropriate mechanical stimuli such as ventilation and perfusion. Though initial studies are encouraging, a great deal remains to be done to advance the field and transition from rodent lungs to whole human tissue engineered lungs. To do so, a variety of hurdles must be overcome. In particular, a reliable source of human-sized scaffolds, as well as a method of terminal sterilization of scaffolds, must be identified. Continued research in lung cell and developmental biology will hopefully help identify the number and types of cells that will be required to regenerate functional lung tissue. Finally, bioreactor designs must be improved in order to provide more precise ventilation stimuli and vascular perfusion in order to avoid injury to or death of the cells cultivated within the scaffold. Ultimately, the success of efforts to engineer a functional lung in vitro will critically depend on the ability to create a fully endothelialized vascular network that provides sufficient barrier function and alveolar-capillary surface area to exchange gas at rates compatible with healthy lung function.

Future prospects for tissue engineered lung transplantation: decellularization and recellularization-based whole lung regeneration

The shortage of donor lungs for transplantation causes a significant number of patient deaths. The availability of laboratory engineered, functional organs would be a major advance in meeting the demand for organs for transplantation. The accumulation of information on biological scaffolds and an increased understanding of stem/progenitor cell behavior has led to the idea of generating transplantable organs by decellularizing an organ and recellularizing using appropriate cells. Recellularized solid organs can perform organ-specific functions for short periods of time, which indicates the potential for the clinical use of engineered solid organs in the future.   The present review provides an overview of progress and recent knowledge about decellularization and recellularization-based approaches for generating tissue engineered lungs. Methods to improve decellularization, maturation of recellularized lung, candidate species for transplantation and future prospects of lung bioengineering are also discussed.

Fibrotic extracellular matrix activates a profibrotic positive feedback loop

Pathological remodeling of the extracellular matrix (ECM) by fibroblasts leads to organ failure. Development of idiopathic pulmonary fibrosis (IPF) is characterized by a progressive fibrotic scarring in the lung that ultimately leads to asphyxiation; however, the cascade of events that promote IPF are not well defined. Here, we examined how the interplay between the ECM and fibroblasts affects both the transcriptome and translatome by culturing primary fibroblasts generated from IPF patient lung tissue or nonfibrotic lung tissue on decellularized lung ECM from either IPF or control patients. Surprisingly, the origin of the ECM had a greater impact on gene expression than did cell origin, and differences in translational control were more prominent than alterations in transcriptional regulation. Strikingly, genes that were translationally activated by IPF-derived ECM were enriched for those encoding ECM proteins detected in IPF tissue. We determined that genes encoding IPF-associated ECM proteins are targets for miR-29, which was downregulated in fibroblasts grown on IPF-derived ECM, and baseline expression of ECM targets could be restored by overexpression of miR-29. Our data support a model in which fibroblasts are activated to pathologically remodel the ECM in IPF via a positive feedback loop between fibroblasts and aberrant ECM. Interrupting this loop may be a strategy for IPF treatment.

Automated procedure for biomimetic de-cellularized lung scaffold supporting alveolar epithelial transdifferentiation

The optimal method for creating a de-cellularized lung scaffold that is devoid of cells and cell debris, immunologically inert, and retains necessary extracellular matrix (ECM) has yet to be identified. Herein, we compare automated detergent-based de-cellularization approaches utilizing either constant pressure (CP) or constant flow (CF), to previously published protocols utilizing manual pressure (MP) to instill and rinse out the de-cellularization agents. De-cellularized lungs resulting from each method were evaluated for presence of remaining ECM proteins and immunostimulatory material such as nucleic acids and intracellular material. Our results demonstrate that the CP and MP approaches more effectively remove cellular materials but differentially retain ECM proteins. The CP method has the added benefit of being a faster, reproducible de-cellularization process. To assess the functional ability of the de-cellularized scaffolds to maintain epithelial cells, intra-tracheal inoculation with GFP expressing C10 alveolar epithelial cells (AEC) was performed. Notably, the CP de-cellularized lungs were able to support growth and spontaneous differentiation of C10-GFP cells from a type II-like phenotype to a type I-like phenotype.

Production and assessment of decellularized pig and human lung scaffolds

The authors have previously shown that acellular (AC) trachea-lung scaffolds can (1) be produced from natural rat lungs, (2) retain critical components of the extracellular matrix (ECM) such as collagen-1 and elastin, and (3) be used to produce lung tissue after recellularization with murine embryonic stem cells. The aim of this study was to produce large (porcine or human) AC lung scaffolds to determine the feasibility of producing scaffolds with potential clinical applicability. We report here the first attempt to produce AC pig or human trachea-lung scaffold. Using a combination of freezing and sodium dodecyl sulfate washes, pig trachea-lungs and human trachea-lungs were decellularized. Once decellularization was complete we evaluated the structural integrity of the AC lung scaffolds using bronchoscopy, multiphoton microscopy (MPM), assessment of the ECM utilizing immunocytochemistry and evaluation of mechanics through the use of pulmonary function tests (PFTs). Immunocytochemistry indicated that there was loss of collagen type IV and laminin in the AC lung scaffold, but retention of collagen-1, elastin, and fibronectin in some regions. MPM scoring was also used to examine the AC lung scaffold ECM structure and to evaluate the amount of collagen I in normal and AC lung. MPM was used to examine the physical arrangement of collagen-1 and elastin in the pleura, distal lung, lung borders, and trachea or bronchi. MPM and bronchoscopy of trachea and lung tissues showed that no cells or cell debris remained in the AC scaffolds. PFT measurements of the trachea-lungs showed no relevant differences in peak pressure, dynamic or static compliance, and a nonrestricted flow pattern in AC compared to normal lungs. Although there were changes in content of collagen I and elastin this did not affect the mechanics of lung function as evidenced by normal PFT values. When repopulated with a variety of stem or adult cells including human adult primary alveolar epithelial type II cells both pig and human AC scaffolds supported cell attachment and cell viability. Examination of scaffolds produced using a variety of detergents indicated that detergent choice influenced human immune response in terms of T cell activation and chemokine production.

Whole-organ re-engineering: a regenerative medicine approach to digestive organ replacement

Recovery from end-stage organ failure presents a challenge for the medical community, considering the limitations of extracorporeal assist devices and the shortage of donors when organ replacement is needed. There is a need for new methods to promote recovery from organ failure and regenerative medicine is an option that should be considered. Recent progress in the field of tissue engineering has opened avenues for potential clinical applications, including the use of microfluidic devices for diagnostic purposes, and bioreactors or cell/tissue-based therapies for transplantation. Early attempts to engineer tissues produced thin, planar constructs; however, recent approaches using synthetic scaffolds and decellularized tissue have achieved a more complex level of tissue organization in organs such as the urinary bladder and trachea, with some success in clinical trials. In this context, the concept of decellularization technology has been applied to produce whole organ-derived scaffolds by removing cellular content while retaining all the necessary vascular and structural cues of the native organ. In this review, we focus on organ decellularization as a new regenerative medicine approach for whole organs, which may be applied in the field of digestive surgery.

Preservation of micro-architecture and angiogenic potential in a pulmonary acellular matrix obtained using intermittent intra-tracheal flow of detergent enzymatic treatment

Tissue engineering of autologous lung tissue aims to become a therapeutic alternative to transplantation. Efforts published so far in creating scaffolds have used harsh decellularization techniques that damage the extracellular matrix (ECM), deplete its components and take up to 5 weeks to perform. The aim of this study was to create a lung natural acellular scaffold using a method that will reduce the time of production and better preserve scaffold architecture and ECM components. Decellularization of rat lungs via the intratracheal route removed most of the nuclear material when compared to the other entry points. An intermittent inflation approach that mimics lung respiration yielded an acellular scaffold in a shorter time with an improved preservation of pulmonary micro-architecture. Electron microscopy demonstrated the maintenance of an intact alveolar network, with no evidence of collapse or tearing. Pulsatile dye injection via the vasculature indicated an intact capillary network in the scaffold. Morphometry analysis demonstrated a significant increase in alveolar fractional volume, with alveolar size analysis confirming that alveolar dimensions were maintained. Biomechanical testing of the scaffolds indicated an increase in resistance and elastance when compared to fresh lungs. Staining and quantification for ECM components showed a presence of collagen, elastin, GAG and laminin. The intratracheal intermittent decellularization methodology could be translated to sheep lungs, demonstrating a preservation of ECM components, alveolar and vascular architecture. Decellularization treatment and methodology preserves lung architecture and ECM whilst reducing the production time to 3 h. Cell seeding and in vivo experiments are necessary to proceed towards clinical translation.

New ideas in lung regeneration: a personal view from Estoril

10th ERS Lung Science Conference--rebuilding a diseased lung: repair and regeneration Estoril, Portugal, 30 March-1 April 2012 The 10th ERS Lung Science Conference, held in Estoril, Portugal, focused on rebuilding a diseased lung: repair and regeneration, seeking to understand, with some amount of precision, how the processes by which the vastly complex self-assembling, self-repairing machine that is a human lung actually develops from a few cells in the embryo, repairs itself or fails to and succumbs to disease. Thus, the major research themes focused on lung development, lung stem and progenitor cell populations, regenerative signaling mechanisms and tissue engineering and transplantation.

Perspectives on whole-organ assembly: moving toward transplantation on demand

There is an ever-growing demand for transplantable organs to replace acute and chronically damaged tissues. This demand cannot be met by the currently available donor organs. Efforts to provide an alternative source have led to the development of organ engineering, a discipline that combines cell biology, tissue engineering, and cell/organ transplantation. Over the last several years, engineered organs have been implanted into rodent recipients and have shown modest function. In this article, we summarize the most recent advances in this field and provide a perspective on the challenges of translating this promising new technology into a proven regenerative therapy.

Production and utilization of acellular lung scaffolds in tissue engineering

Pulmonary disease is a worldwide public health problem that reduces the quality of life and increases the need for hospital admissions as well as the risk for premature death for those affected. For many patients, lung transplantation is the only chance for survival. Unfortunately, there is a significant shortage of lungs for transplantation and since the lung is the most likely organ to be damaged during procurement many lungs deemed unacceptable for transplantation are simply discarded. Rather than discarding these lungs they can be used to produce three-dimensional acellular (AC) natural lung scaffolds for the generation of engineered lung tissue. AC scaffolds are lungs whose original cells have been destroyed by exposure to detergents and physical methods of removing cells and cell debris. This creates a lung scaffold from the skeleton of the lungs themselves. The scaffolds are then used to support adult, stem or progenitor cells which can be grown into functional lung tissue. Recent studies show that engineered lung tissues are capable of surviving after in vivo transplantation and support limited gas exchange. In the future engineered lung tissue has the potential to be used in clinical applications to replace lung functions lost following injury or disease. This manuscript discusses recent advances in development and use of AC scaffolds to support engineering of lung tissues.

Regenerative medicine as applied to general surgery

The present review illustrates the state of the art of regenerative medicine (RM) as applied to surgical diseases and demonstrates that this field has the potential to address some of the unmet needs in surgery. RM is a multidisciplinary field whose purpose is to regenerate in vivo or ex vivo human cells, tissues, or organs to restore or establish normal function through exploitation of the potential to regenerate, which is intrinsic to human cells, tissues, and organs. RM uses cells and/or specially designed biomaterials to reach its goals and RM-based therapies are already in use in several clinical trials in most fields of surgery. The main challenges for investigators are threefold: Creation of an appropriate microenvironment ex vivo that is able to sustain cell physiology and function in order to generate the desired cells or body parts; identification and appropriate manipulation of cells that have the potential to generate parenchymal, stromal and vascular components on demand, both in vivo and ex vivo; and production of smart materials that are able to drive cell fate.

A rapid lung de-cellularization protocol supports embryonic stem cell differentiation in vitro and following implantation

Pulmonary diseases represent a large portion of neonatal and adult morbidity and mortality. Many of these have no cure, and new therapeutic approaches are desperately needed. De-cellularization of whole organs, which removes cellular elements but leaves intact important extracellular matrix (ECM) proteins and three-dimensional architecture, has recently been investigated for ex vivo generation of lung tissues. As specific cell culture surfaces, including ECM composition, profoundly affect cell differentiation, this approach offers a potential means of using de-cellularized lungs to direct differentiation of embryonic and other types of stem/progenitor cells into lung phenotypes. Several different methods of whole-lung de-cellularization have been reported, but the optimal method that will best support re-cellularization and generation of lung tissues from embryonic stem cells (ESCs) has not been determined. We present a 24-h approach for de-cellularizing mouse lungs utilizing a detergent-based (Triton-X100 and sodium deoxycholate) approach with maintenance of three-dimensional lung architecture and ECM protein composition. Predifferentiated murine ESCs (mESCs), with phenotypic characteristics of type II alveolar epithelial cells, were seeded into the de-cellularized lung scaffolds. Additionally, we evaluated the effect of coating the de-cellularized scaffold with either collagen or Matrigel to determine if this would enhance cell adhesion and affect mechanics of the scaffold. Finally, we subcutaneously implanted scaffolds in vivo after seeding them with mESCs that are predifferentiated to express pro-surfactant protein C (pro-SPC). The in vivo environment supported maintenance of the pro-SPC-expressing phenotype and further resulted in vascularization of the implant. We conclude that a rapid detergent-based de-cellularization approach results in a scaffold that can maintain phenotypic evidence of alveolar epithelial differentiation of ESCs and support neovascularization after in vivo implantation.

Stem cells and regenerative medicine in lung biology and diseases

A number of novel approaches for repair and regeneration of injured lung have developed over the past several years. These include a better understanding of endogenous stem and progenitor cells in the lung that can function in reparative capacity as well as extensive exploration of the potential efficacy of administering exogenous stem or progenitor cells to function in lung repair. Recent advances in ex vivo lung engineering have also been increasingly applied to the lung. The current status of these approaches as well as initial clinical trials of cell therapies for lung diseases are reviewed below.