Development of a decellularized lung bioreactor system for bioengineering the lung: the matrix reloaded

Adult lung stem cells and their contribution to lung tumourigenesis

The isolation and characterization of lung stem and progenitor cells represent an important step towards the understanding of lung repair after injury, lung disease pathogenesis and the identification of the target cells of transformation in lung carcinogenesis. Different approaches using prospective isolation of progenitor cells by flow cytometry or lineage-tracing experiments in mouse models of lung injury have led to the identification of distinct progenitor subpopulations in different morphological regions of the adult lung. Genetically defined mouse models of lung cancer are offering new perspectives on the cells of origin of different subtypes of lung cancer. These mouse models pave the way to further investigate human lung progenitor cells at the origin of lung cancers, as well as to define the nature of the lung cancer stem cells. It will be critical to establish the link between oncogenic driver mutations recently discovered in lung cancers, target cells of transformation and subtypes of lung cancers to enable better stratification of patients for improved therapeutic strategies.

Perfusion-decellularized pancreas as a natural 3D scaffold for pancreatic tissue and whole organ engineering

Approximately 285 million people worldwide suffer from diabetes, with insulin supplementation as the most common treatment measure. Regenerative medicine approaches such as a bioengineered pancreas has been proposed as potential therapeutic alternatives. A bioengineered pancreas will benefit from the development of a bioscaffold that supports and enhances cellular function and tissue development. Perfusion-decellularized organs are a likely candidate for use in such scaffolds since they mimic compositional, architectural and biomechanical nature of a native organ. In this study, we investigate perfusion-decellularization of whole pancreas and the feasibility to recellularize the whole pancreas scaffold with pancreatic cell types. Our result demonstrates that perfusion-decellularization of whole pancreas effectively removes cellular and nuclear material while retaining intricate three-dimensional microarchitecture with perfusable vasculature and ductal network and crucial extracellular matrix (ECM) components. To mimic pancreatic cell composition, we recellularized the whole pancreas scaffold with acinar and beta cell lines and cultured up to 5 days. Our result shows successful cellular engraftment within the decellularized pancreas, and the resulting graft gave rise to strong up-regulation of insulin gene expression. These findings support biological utility of whole pancreas ECM as a biomaterials scaffold for supporting and enhancing pancreatic cell functionality and represent a step toward bioengineered pancreas using regenerative medicine approaches.

Evaluation of small intestine grafts decellularization methods for corneal tissue engineering

Advances in the development of cornea substitutes by tissue engineering techniques have focused on the use of decellularized tissue scaffolds. In this work, we evaluated different chemical and physical decellularization methods on small intestine tissues to determine the most appropriate decellularization protocols for corneal applications. Our results revealed that the most efficient decellularization agents were the SDS and triton X-100 detergents, which were able to efficiently remove most cell nuclei and residual DNA. Histological and histochemical analyses revealed that collagen fibers were preserved upon decellularization with triton X-100, NaCl and sonication, whereas reticular fibers were properly preserved by decellularization with UV exposure. Extracellular matrix glycoproteins were preserved after decellularization with SDS, triton X-100 and sonication, whereas proteoglycans were not affected by any of the decellularization protocols. Tissue transparency was significantly higher than control non-decellularized tissues for all protocols, although the best light transmittance results were found in tissues decellularized with SDS and triton X-100. In conclusion, our results suggest that decellularized intestinal grafts could be used as biological scaffolds for cornea tissue engineering. Decellularization with triton X-100 was able to efficiently remove all cells from the tissues while preserving tissue structure and most fibrillar and non-fibrillar extracellular matrix components, suggesting that this specific decellularization agent could be safely used for efficient decellularization of SI tissues for cornea TE applications.

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.

Generation of Lung Epithelium from Pluripotent Stem Cells

The understanding of key processes and signaling mechanisms in lung development has been mainly demonstrated through gain and loss of function studies in mice, while human lung development remains largely unexplored due to inaccessibility. Several recent reports have exploited the identification of key signaling mechanisms that regulate lineage commitment and restriction in mouse lung development, to direct differentiation of both mouse and human pluripotent stem cells towards lung epithelial cells. In this review, we discuss the recent advances in the generation of respiratory epithelia from pluripotent stem cells and the potential of these engineered cells for novel scientific discoveries in lung diseases and future translation into regenerative therapies.

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.

Lung transplantation

Lung transplantation may be the only intervention that can prolong survival and improve quality of life for those individuals with advanced lung disease who are acceptable candidates for the procedure. However, these candidates may be extremely ill and require ventilator and/or circulatory support as a bridge to transplantation, and lung transplantation recipients are at risk of numerous post-transplant complications that include surgical complications, primary graft dysfunction, acute rejection, opportunistic infection, and chronic lung allograft dysfunction (CLAD), which may be caused by chronic rejection. Many advances in pre- and post-transplant management have led to improved outcomes over the past decade. These include the creation of sound guidelines for candidate selection, improved surgical techniques, advances in donor lung preservation, an improving ability to suppress and treat allograft rejection, the development of prophylaxis protocols to decrease the incidence of opportunistic infection, more effective therapies for treating infectious complications, and the development of novel therapies to treat and manage CLAD. A major obstacle to prolonged survival beyond the early post-operative time period is the development of bronchiolitis obliterans syndrome (BOS), which is the most common form of CLAD. This manuscript discusses recent and evolving advances in the field of lung transplantation.

Using extracellular matrix for regenerative medicine in the spinal cord

Regeneration within the mammalian central nervous system (CNS) is limited, and traumatic injury often leads to permanent functional motor and sensory loss. The lack of regeneration following spinal cord injury (SCI) is mainly caused by the presence of glial scarring, cystic cavitation and a hostile environment to axonal growth at the lesion site. The more prominent experimental treatment strategies focus mainly on drug and cell therapies, however recent interest in biomaterial-based strategies are increasing in number and breadth. Outside the spinal cord, approaches that utilize the extracellular matrix (ECM) to promote tissue repair show tremendous potential for various application including vascular, skin, bone, cartilage, liver, lung, heart and peripheral nerve tissue engineering (TE). Experimentally, it is unknown if these approaches can be successfully translated to the CNS, either alone or in combination with synthetic biomaterial scaffolds. In this review we outline the first attempts to apply the potential of ECM-based biomaterials and combining cell-derived ECM with synthetic scaffolds.

The effect of age and emphysematous and fibrotic injury on the re-cellularization of de-cellularized lungs

Use of de-cellularized cadaveric lungs as 3-dimensional scaffolds for ex vivo lung tissue generation offers a new potential therapeutic approach for clinical lung transplantation. However, it is likely that some of the available cadaveric human lungs may be from older donors or from donors with previously existing structural lung diseases such as emphysema or pulmonary fibrosis. It is not known whether these lungs will be suitable for either de-cellularization or re-cellularization. To investigate this, we assessed the effects of advanced age, representative emphysematous and fibrotic injuries, and the combination of advanced age and emphysematous injury and found significant differences both in histologic appearance and in the retention of extracellular matrix (ECM) and other proteins, as assessed by immunohistochemistry and mass spectrometry, between the different conditions. However, despite these differences, binding, retention and growth of bone marrow-derived mesenchymal stromal cells (MSCs) over a 1-month period following intratracheal inoculation were similar between the different experimental conditions. In contrast, significant differences occurred in the growth of C10 mouse lung epithelial cells between the different conditions. Therefore, age, lung injury, and the cell type used for re-cellularization may significantly impact the usefulness of de-cellularized whole lungs for ex vivo lung tissue regeneration.

The effects of storage and sterilization on de-cellularized and re-cellularized whole lung

Despite growing interest on the potential use of de-cellularized whole lungs as 3-dimensional scaffolds for ex vivo lung tissue generation, optimal processing including sterilization and storage conditions, are not well defined. Further, it is unclear whether lungs need to be obtained immediately or may be usable even if harvested several days post-mortem, a situation mimicking potential procurement of human lungs from autopsy. We therefore assessed effects of delayed necropsy, prolonged storage (3 and 6 months), and of two commonly utilized sterilization approaches: irradiation or final rinse with peracetic acid, on architecture and extracellular matrix (ECM) protein characteristics of de-cellularized mouse lungs. These different approaches resulted in significant differences in both histologic appearance and in retention of ECM and intracellular proteins as assessed by immunohistochemistry and mass spectrometry. Despite these differences, binding and proliferation of bone marrow-derived mesenchymal stromal cells (MSCs) over a one month period following intratracheal inoculation was similar between experimental conditions. In contrast, significant differences occurred with C10 mouse lung epithelial cells between the different conditions. Therefore, delayed necropsy, duration of scaffold storage, sterilization approach, and cell type used for re-cellularization may significantly impact the usefulness of this biological scaffold-based model of ex vivo lung tissue regeneration.

Bioreactor design for perfusion-based, highly-vascularized organ regeneration

Bioartificial or laboratory-grown organs is a growing field centered on developing replacement organs and tissues to restore body function and providing a potential solution to the shortage of donor organs for transplantation. With the entry of engineered planar tissues, such as bladder and trachea, into clinical studies, an increasing focus is being given to designing complex, three-dimensional solid organs. As tissues become larger, thicker and more complex, the vascular network becomes crucial for supplying nutrients and maintaining viability and growth of the neo-organ. Perfusion decellularization, the process of removing cells from an entire organ, leaves the matrix of the vascular network intact. Organ engineering requires a delicate process of decellularization, sterilization, reseeding with appropriate cells, and organ maturation and stimulation to ensure optimal development. The design of bioreactors to facilitate this sequence of events has been refined to the extent that some bioartificial organs grown in these systems have been transplanted into recipient animals with sustained, though limited, function. This review focuses on the state-of-art in bioreactor development for perfusion-based bioartificial organs and highlights specific design components in need of further refinement.

Decellularized bovine intervertebral disc as a natural scaffold for xenogenic cell studies

Low back pain that is associated with disc degeneration contributes to a huge economic burden in the worldwide healthcare system. Traditional methods, such as spinal fusion, have been adopted to relieve mechanical back pain, but this is compromised by decreased spinal motion. Tissue engineering has attracted much attention, and aims to correct the changes fundamentally occurring in the discs by a combination of cell biology, molecular biology and engineering. Synthetic materials including poly(l-lactic acid) or poly(glycolic acid) and biomolecules like hyaluronic acid or collagen have been adopted in the development of disc scaffolds for studying therapeutic approaches. Nevertheless, the complex biological and mechanical environment of the intervertebral disc (IVD) makes the synthesis of an artificial IVD with biomaterials a difficult task. Thus the aim of this study was to develop a natural disc scaffold for culturing disc cells for future development of biological disc constructs. We adopted a combination of currently used decellularization techniques to decellularize bovine IVD to create a complete endplate-to-endplate IVD scaffold. By altering the chemical and physical decellularization parameters, we reported the removal of up to 70% of the endogenous cells, and were able to preserve the glycosaminoglycan content, collagen fibril architecture and mechanical properties of the discs. The reintroduction of nucleus pulposus cells into the scaffold indicated a high survival rate over 7days, with cell penetration. We have shown here that conventional methods used for decellularizing thin tissues can also be applied to large organs, such as IVD. Our findings suggest the potential of using decellularized IVD as a scaffold for IVD bioengineering and culturing of cells in the context of the IVD niche.

Decellularization for whole organ bioengineering

Organ transplantation in an orthotopic location is the current treatment for end-stage organ failure. However, the need for transplantable organs far exceeds the number of available donor organs. As a result, new options, such as tissue engineering and regenerative medicine, have been explored to achieve functional organ replacement. Although there have been many advances in the laboratory leading to the reconstruction of tissue and organ structures in vitro, these efforts have fallen short of producing organs that contain intact vascular networks capable of nutrient and gas exchange and are suitable for transplantation. Recently, advances in whole organ decellularization techniques have enabled the fabrication of scaffolds for engineering new organs. These scaffolds, consisting of naturally-derived extracellular matrix (ECM), provide biological signals and maintain tissue microarchitecture, including intact vascular systems that could integrate into the recipient's circulatory system. The decellularization techniques have led to the development of scaffolds for multiple organs, including the heart, liver, lung and kidney. While the experimental studies involving the use of decellularized organ scaffolds are encouraging, the translation of whole organ engineering into the clinic is still distant. This paper reviews recently described techniques used to decellularize whole organs such as the heart, lung, liver and kidney and describes possible methods for using these matrices for whole organ engineering.

Comparison of methods for whole-organ decellularization in tissue engineering of bioartificial organs

Organ transplantation is now a well-established procedure for the treatment of end-stage organ failure due to various causes, but is a victim of its own success in that there is a growing disparity in numbers between the donor organ pool available for transplantation and the patients eligible for such a procedure; hence, an alternative solution to the limited donor organ pool is both desirable and necessary. Tissue engineering is an interdisciplinary field that applies the principles of engineering and life sciences toward the development of functional replacement tissues for clinical use. A recent innovation in tissue and organ engineering is the technique of whole-organ decellularization, which allows the production of complex three-dimensional extracellular matrix (ECM) bioscaffolds of the entire organ with preservation of the intrinsic vascular network. These bioscaffolds can then be recellularized to create potentially functional organ constructs as a regenerative medicine strategy for organ replacement. We review the current applications and methods in using xenogeneic whole-organ ECM scaffolds to create potentially functional bioartificial organ constructs for surgical implantation, and present a comparison of specific trends within this new and developing technique.

Regenerative medicine for the respiratory system: distant future or tomorrow's treatment?

Regenerative medicine (RM) is a new field of biomedical science that focuses on the regeneration of tissues and organs and the restoration of organ function. Although regeneration of organ systems such as bone, cartilage, and heart has attracted intense scientific research over recent decades, RM research regarding the respiratory system, including the trachea, the lung proper, and the diaphragm, has lagged behind. However, the last 5 years have witnessed novel approaches and initial clinical applications of tissue-engineered constructs to restore organ structure and function. In this regard, this article briefly addresses the basics of RM and introduces the key elements necessary for tissue regeneration, including (stem) cells, biomaterials, and extracellular matrices. In addition, the current status of the (clinical) application of RM to the respiratory system is discussed, and bottlenecks and recent approaches are identified. For the trachea, several initial clinical studies have been reported and have used various combinations of cells and scaffolds. Although promising, the methods used in these studies require optimization and standardization. For the lung proper, only (stem) cell-based approaches have been probed clinically, but it is becoming apparent that combinations of cells and scaffolds are required to successfully restore the lung's architecture and function. In the case of the diaphragm, clinical applications have focused on the use of decellularized scaffolds, but novel scaffolds, with or without cells, are clearly needed for true regeneration of diaphragmatic tissue. We conclude that respiratory treatment with RM will not be realized tomorrow, but its future looks promising.

Application of whole-organ tissue engineering in hepatology

Initially hailed as the ultimate solution to organ failure, engineering of vascularized tissues such as the liver has stalled because of the need for a well-structured circulatory system that can maintain the cells seeded inside the construct. A new approach has evolved to overcome this obstacle. Whole-organ decellularization is a method that retains most of the native vascular structures of the organ, providing microcirculatory support and structure, which can be anastomosed with the recipient circulation. The technique was first applied to the heart and then adapted for the liver. Several studies have shown that cells can be eliminated, the extracellular matrix and vasculature are reasonably preserved and, after repopulation with hepatocytes, these grafts can perform hepatic functions in vitro and in vivo. Progress is rapidly being made as researchers are addressing several key challenges to whole-organ tissue engineering, such as ensuring correct cell distribution, nonparenchymal cell seeding, blood compatibility, immunological concerns, and the source of cells and matrices.

Dewaxed ECM: A simple method for analyzing cell behaviour on decellularized extracellular matrices

Decellularization techniques have been used on a wide variety of tissues to create cell-seedable scaffolds for tissue engineering. Finding a suitable decellularization protocol for a certain type of tissue can be laborious, especially when organ perfusion devices are needed. In this study, we report a quick and simple method for comparing decellularization protocols combining the use of paraffin slices and two-dimensional cell cultures. We developed three decellularization protocols for adult murine kidney that yielded decellularized extracellular matrices (ECMs) with varying histological properties. The resulting paraffin-embedded ECM slices were deparaffinized and reseeded with murine embryonic stem cells (mESCs). We analyzed cell attachment four days post seeding via determination of cell numbers, and used quantitative Real-Time PCR 13 days post seeding to measure gene expression levels of two genes associated with renal development, Pax2 and Pou3f3. The three decellularization protocols produced kidney-matrices that showed clearly distinguishable results. We demonstrated that formerly paraffin-embedded decellularized ECMs can effectively influence differentiation of stem cells. This method can be used to identify optimal decellularization protocols for recellularization of three-dimensional tissue-scaffolds with embryonic stem cells and other tissue-specific cell types.

Hydrogels derived from central nervous system extracellular matrix

Biologic scaffolds composed of extracellular matrix (ECM) are commonly used repair devices in preclinical and clinical settings; however the use of these scaffolds for peripheral and central nervous system (CNS) repair has been limited. Biologic scaffolds developed from brain and spinal cord tissue have recently been described, yet the conformation of the harvested ECM limits therapeutic utility. An injectable CNS-ECM derived hydrogel capable of in vivo polymerization and conformation to irregular lesion geometries may aid in tissue reconstruction efforts following complex neurologic trauma. The objectives of the present study were to develop hydrogel forms of brain and spinal cord ECM and compare the resulting biochemical composition, mechanical properties, and neurotrophic potential of a brain derived cell line to a non-CNS-ECM hydrogel, urinary bladder matrix. Results showed distinct differences between compositions of brain ECM, spinal cord ECM, and urinary bladder matrix. The rheologic modulus of spinal cord ECM hydrogel was greater than that of brain ECM and urinary bladder matrix. All ECMs increased the number of cells expressing neurites, but only brain ECM increased neurite length, suggesting a possible tissue-specific effect. All hydrogels promoted three-dimensional uni- or bi-polar neurite outgrowth following 7 days in culture. These results suggest that CNS-ECM hydrogels may provide supportive scaffolding to promote in vivo axonal repair.

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.

Strategies for tissue and organ decellularization

Decellularized tissues have been successfully used in a variety of tissue engineering/regenerative medicine applications, and more recently decellularized organs have been utilized in the first stages of organ engineering. The protocols used to decellularize simple tissues versus intact organs differ greatly. Herein, the most commonly used decellularization methods for both surgical mesh materials and whole organs are described, with consideration given to how these different processes affect the extracellular matrix and the host response to the scaffold.

The future of tissue-engineered organs for transplant? It's here!

The first-in-man bioengineered artificial tracheobronchial transplant consisting of a synthetic scaffold and autologous bone marrow-derived mesenchymal cells was recently reported. Extensive evaluation of the patient before and after transplant provides some insight into the potential mechanisms of stem cell mobilization and tracheal tissue regeneration.

Advances in biomimetic regeneration of elastic matrix structures

Elastin is a vital component of the extracellular matrix, providing soft connective tissues with the property of elastic recoil following deformation and regulating the cellular response via biomechanical transduction to maintain tissue homeostasis. The limited ability of most adult cells to synthesize elastin precursors and assemble them into mature crosslinked structures has hindered the development of functional tissue-engineered constructs that exhibit the structure and biomechanics of normal native elastic tissues in the body. In diseased tissues, the chronic overexpression of proteolytic enzymes can cause significant matrix degradation, to further limit the accumulation and quality (e.g., fiber formation) of newly deposited elastic matrix. This review provides an overview of the role and importance of elastin and elastic matrix in soft tissues, the challenges to elastic matrix generation in vitro and to regenerative elastic matrix repair in vivo, current biomolecular strategies to enhance elastin deposition and matrix assembly, and the need to concurrently inhibit proteolytic matrix disruption for improving the quantity and quality of elastogenesis. The review further presents biomaterial-based options using scaffolds and nanocarriers for spatio-temporal control over the presentation and release of these biomolecules, to enable biomimetic assembly of clinically relevant native elastic matrix-like superstructures. Finally, this review provides an overview of recent advances and prospects for the application of these strategies to regenerating tissue-type specific elastic matrix structures and superstructures.

Acellular normal and fibrotic human lung matrices as a culture system for in vitro investigation

RATIONALE

Extracellular matrix (ECM) is a dynamic tissue that contributes to organ integrity and function, and its regulation of cell phenotype is a major aspect of cell biology. However, standard in vitro culture approaches are of unclear physiologic relevance because they do not mimic the compositional, architectural, or distensible nature of a living organ. In the lung, fibroblasts exist in ECM-rich interstitial spaces and are key effectors of lung fibrogenesis.

OBJECTIVES

To better address how ECM influences fibroblast phenotype in a disease-specific manner, we developed a culture system using acellular human normal and fibrotic lungs.

METHODS

Decellularization was achieved using treatment with detergents, salts, and DNase. The resultant matrices can be sectioned as uniform slices within which cells were cultured.

MEASUREMENTS AND MAIN RESULTS

We report that the decellularization process effectively removes cellular and nuclear material while retaining native dimensionality and stiffness of lung tissue. We demonstrate that lung fibroblasts reseeded into acellular lung matrices can be subsequently assayed using conventional protocols; in this manner we show that fibrotic matrices clearly promote transforming growth factor-β-independent myofibroblast differentiation compared with normal matrices. Furthermore, comprehensive analysis of acellular matrix ECM details significant compositional differences between normal and fibrotic lungs, paving the way for further study of novel hypotheses.

CONCLUSIONS

This methodology is expected to allow investigation of important ECM-based hypotheses in human tissues and permits future scientific exploration in an organ- and disease-specific manner.

A nonhuman primate model of lung regeneration: detergent-mediated decellularization and initial in vitro recellularization with mesenchymal stem cells

Currently, patients with end-stage lung disease are limited to lung transplantation as their only treatment option. Unfortunately, the lungs available for transplantation are few. Moreover, transplant recipients require life-long immune suppression to tolerate the transplanted lung. A promising alternative therapeutic strategy is decellularization of whole lungs, which permits the isolation of an intact scaffold comprised of innate extracellular matrix (ECM) that can theoretically be recellularized with autologous stem or progenitor cells to yield a functional lung. Nonhuman primates (NHP) provide a highly relevant preclinical model with which to assess the feasibility of recellularized lung scaffolds for human lung transplantation. Our laboratory has successfully accomplished lung decellularization and initial stem cell inoculation of the resulting ECM scaffold in an NHP model. Decellularization of normal adult rhesus macaque lungs as well as the biology of the resulting acellular matrix have been extensively characterized. Acellular NHP matrices retained the anatomical and ultrastructural properties of native lungs with minimal effect on the content, organization, and appearance of ECM components, including collagen types I and IV, laminin, fibronectin, and sulfated glycosaminoglycans (GAG), due to decellularization. Proteomics analysis showed enrichment of ECM proteins in total tissue extracts due to the removal of cells and cellular proteins by decellularization. Cellular DNA was effectively removed after decellularization (∼92% reduction), and the remaining nuclear material was found to be highly disorganized, very-low-molecular-weight fragments. Both bone marrow- and adipose-derived mesenchymal stem cells (MSC) attach to the decellularized lung matrix and can be maintained within this environment in vitro, suggesting that these cells may be promising candidates and useful tools for lung regeneration. Analysis of decellularized lung slice cultures to which MSC were seeded showed that the cells attached to the decellularized matrix, elongated, and proliferated in culture. Future investigations will focus on optimizing the recellularization of NHP lung scaffolds toward the goal of regenerating pulmonary tissue. Bringing this technology to eventual human clinical application will provide patients with an alternative therapeutic strategy as well as significantly reduce the demand for transplantable organs and patient wait-list time.

Endogenous regeneration of critical-size chondral defects in immunocompromised rat xiphoid cartilage using decellularized human bone matrix scaffolds

Clinical efforts to repair cartilage defects delivering cells or engineered cartilage implants into the lesions have met with limited success. This study used a critical-size chondral defect model in immunocompromised rat xiphoid cartilage to test whether endogenous chondrogenesis could be achieved using human bone matrix scaffolds to deliver human cartilage particles and/or a variant isoform of fibroblast growth factor-2 (FGF2-variant). Seventy-two male athymic RNU rats were enrolled in this study with eight rats per experimental group. Decellularized and demineralized human bone matrix scaffolds loaded with human articular cartilage particles or heat-inactivated cartilage particles were combined with different doses of the FGF2-variant. Scaffolds were implanted into 3-mm-diameter critical-size defects prepared using a biopsy punch through the center of the xiphoid. The samples were evaluated 28 days postsurgery using X-ray, equilibrium partitioning of ionic contrast microcomputed tomography, and safranin O-stained histological sagittal sections. Scaffolds containing cartilage particles plus the FGF2-variant induced dose-dependent increases in the formation of neocartilage (p<0.05), which was distributed homogeneously throughout the defects in comparison to scaffolds containing only the FGF2-variant. These effects were less pronounced when scaffolds with heat-inactivated cartilage particles were used. These results demonstrate that endogenous repair of chondral defects can be achieved in the absence of exogenous cells or bone marrow, suggesting that a similar approach may be successful for treating chondral lesions clinically.

Tissue engineering and regenerative medicine: history, progress, and challenges

The past three decades have seen the emergence of an endeavor called tissue engineering and regenerative medicine in which scientists, engineers, and physicians apply tools from a variety of fields to construct biological substitutes that can mimic tissues for diagnostic and research purposes and can replace (or help regenerate) diseased and injured tissues. A significant portion of this effort has been translated to actual therapies, especially in the areas of skin replacement and, to a lesser extent, cartilage repair. A good amount of thoughtful work has also yielded prototypes of other tissue substitutes such as nerve conduits, blood vessels, liver, and even heart. Forward movement to clinical product, however, has been slow. Another offshoot of these efforts has been the incorporation of some new exciting technologies (e.g., microfabrication, 3D printing) that may enable future breakthroughs. In this review we highlight the modest beginnings of the field and then describe three application examples that are in various stages of development, ranging from relatively mature (skin) to ongoing proof-of-concept (cartilage) to early stage (liver). We then discuss some of the major issues that limit the development of complex tissues, some of which are fundamentals-based, whereas others stem from the needs of the end users.

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.

Human lung cancer cells grown on acellular rat lung matrix create perfusable tumor nodules

BACKGROUND

Extracellular matrix allows lung cancer to form its shape and grow. Recent studies on organ reengineering for orthotopic transplantation have provided a new avenue for isolating purified native matrix to use for growing cells. Whether human lung cancer cells grown in a decellularized rat lung matrix would create perfusable human lung cancer nodules was tested.

METHODS

Rat lungs were harvested and native cells were removed using sodium dodecyl sulfate and Triton X-100 in a decellularization chamber to create a decellularized rat lung matrix. Human A549, H460, or H1299 lung cancer cells were placed into the decellularized rat lung matrix and grown in a customized bioreactor with perfusion of oxygenated media for 7 to 14 days.

RESULTS

Decellularized rat lung matrix showed preservation of matrix architecture devoid of all rat cells. All three human lung cancer cell lines grown in the bioreactor developed tumor nodules with intact vasculature. Moreover, the lung cancer cells developed a pattern of growth similar to the original human lung cancer.

CONCLUSIONS

Overall, this study shows that human lung cancer cells form perfusable tumor nodules in a customized bioreactor on a decellularized rat lung matrix created by a customized decellularization chamber. The lung cancer cells grown in the matrix had features similar to the original human lung cancer. This ex vivo model can be used potentially to gain a deeper understanding of the biologic processes involved in human lung cancer.

Engineered whole organs and complex tissues

End-stage organ failure is a key challenge for the medical community because of the ageing population and the severe shortage of suitable donor organs available. Equally, injuries to or congenital absence of complex tissues such as the trachea, oesophagus, or skeletal muscle have few therapeutic options. A new approach to treatment involves the use of three-dimensional biological scaffolds made of allogeneic or xenogeneic extracellular matrix derived from non-autologous sources. These scaffolds can act as an inductive template for functional tissue and organ reconstruction after recellularisation with autologous stem cells or differentiated cells. Such an approach has been used successfully for the repair and reconstruction of several complex tissues such as trachea, oesophagus, and skeletal muscle in animal models and human beings, and, guided by appropriate scientific and ethical oversight, could serve as a platform for the engineering of whole organs and other tissues.

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.

Comparative assessment of detergent-based protocols for mouse lung de-cellularization and re-cellularization

Several different detergent-based methods are currently being explored for de-cellularizing whole lungs for subsequent use as three-dimensional scaffolds for ex vivo lung tissue generation. However, it is not yet clear which of these methods may provide a scaffold that best supports re-cellularization and generation of functional lung tissue. Notably, the detergents used for de-cellularization activate matrix metalloproteinases that can potentially degrade extracellular matrix (ECM) proteins important for subsequent binding and growth of cells inoculated into the de-cellularized scaffolds. We assessed gelatinase activation and the histologic appearance, protein composition, and lung mechanics of the end product scaffolds produced with three different detergent-based de-cellularization methods utilizing either Triton-X 100/sodium deoxycholate (Triton/SDC), sodium dodecyl sulfate (SDS), or 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS). There were significant differences both in gelatinase activation and in the retention of ECM and other intracellular proteins, assessed by immunohistochemistry, mass spectrometry, and western blotting as well as in airways resistance and elastance of lungs de-cellularized with the different methods. However, despite these differences, binding and initial growth following intratracheal inoculation with either bone marrow-derived mesenchymal stromal cells or with C10 mouse lung epithelial cells was similar between lungs de-cellularized with each method. Therefore despite differences in the structural composition of the de-cellularized lungs, initial re-cellularization does not appear significantly different between the three de-cellularization approaches studied.

What do we know about mechanical strain in lung alveoli?

The pulmonary alveolus, terminal gas-exchange unit of the lung, is composed of alveolar epithelial and endothelial cells separated by a thin basement membrane and interstitial space. These cells participate in the maintenance of a delicate system regulated not only by biological factors but also by the mechanical environment of the lung, which undergoes dynamic deformation during breathing. Clinical and animal studies as well as cell culture studies point toward a strong influence of mechanical forces on lung cells and tissues including effects on growth and repair, surfactant release, injury, and inflammation. However, despite substantial advances in our understanding of lung mechanics over the last half century, there are still many unanswered questions regarding the micromechanics of the alveolus and how it deforms during lung inflation. Therefore, the aims of this review are to draw a multidisciplinary account of the mechanics of the alveolus on the basis of its structure, biology, and chemistry and to compare estimates of alveolar deformation from previous studies.

Renal tissue engineering with decellularized rhesus monkey kidneys: age-related differences

New therapies for severely damaged kidneys are needed due to limited regenerative capacity and organ donor shortages. The goal of this study was to repopulate decellularized kidney sections in vitro and to determine the impact of donor age on recellularization. This was addressed by generating decellularized kidney scaffolds from fetal, juvenile, and adult rhesus monkey kidney sections using a procedure that removes cellular components while preserving the structural and functional properties of the native extracellular matrix (ECM). Kidney scaffolds were recellularized using explants from different age groups (fetal, juvenile, adult) and fetal renal cell fractions. Results showed vimentin+ cytokeratin+ calbindin+ cell infiltration and organization around the scaffold ECM. The extent of cellular repopulation was greatest with scaffolds from the youngest donors, and with seeding of mixed fetal renal aggregates that formed tubular structures within the kidney scaffolds. These findings suggest that decellularized kidney sections from different age groups can be effectively repopulated with donor cells and the age of the donor is a critical factor in repopulation efficiency.

Initial binding and recellularization of decellularized mouse lung scaffolds with bone marrow-derived mesenchymal stromal cells

Recellularization of whole decellularized lung scaffolds provides a novel approach for generating functional lung tissue ex vivo for subsequent clinical transplantation. To explore the potential utility of stem and progenitor cells in this model, we investigated recellularization of decellularized whole mouse lungs after intratracheal inoculation of bone marrow-derived mesenchymal stromal cells (MSCs). The decellularized lungs maintained structural features of native lungs, including intact vasculature, ability to undergo ventilation, and an extracellular matrix (ECM) scaffold consisting primarily of collagens I and IV, laminin, and fibronectin. However, even in the absence of intact cells or nuclei, a number of cell-associated (non-ECM) proteins were detected using mass spectroscopy, western blots, and immunohistochemistry. MSCs initially homed and engrafted to regions enriched in types I and IV collagen, laminin, and fibronectin, and subsequently proliferated and migrated toward regions enriched in types I and IV collagen and laminin but not provisional matrix (fibronectin). MSCs cultured for up to 1 month in either basal MSC medium or in a small airways growth media (SAGM) localized in both parenchymal and airway regions and demonstrated several different morphologies. However, while MSCs cultured in basal medium increased in number, MSCs cultured in SAGM decreased in number over 1 month. Under both media conditions, the MSCs predominantly expressed genes consistent with mesenchymal and osteoblast phenotype. Despite a transient expression of the lung precursor TTF-1, no other airway or alveolar genes or vascular genes were expressed. These studies highlight the power of whole decellularized lung scaffolds to study functional recellularization with MSCs and other cells.

Biomaterials to prevascularize engineered tissues

Tissue engineering promises to restore tissue and organ function following injury or failure by creating functional and transplantable artificial tissues. The development of artificial tissues with dimensions that exceed the diffusion limit (1-2 mm) will require nutrients and oxygen to be delivered via perfusion (or convection) rather than diffusion alone. One strategy of perfusion is to prevascularize tissues; that is, a network of blood vessels is created within the tissue construct prior to implantation, which has the potential to significantly shorten the time of functional vascular perfusion from the host. The prevascularized network of vessels requires an extracellular matrix or scaffold for 3D support, which can be either natural or synthetic. This review surveys the commonly used biomaterials for prevascularizing 3D tissue engineering constructs.

Whole-organ tissue engineering: decellularization and recellularization of three-dimensional matrix scaffolds

The definitive treatment for end-stage organ failure is orthotopic transplantation. However, the demand for transplantation far exceeds the number of available donor organs. A promising tissue-engineering/regenerative-medicine approach for functional organ replacement has emerged in recent years. Decellularization of donor organs such as heart, liver, and lung can provide an acellular, naturally occurring three-dimensional biologic scaffold material that can then be seeded with selected cell populations. Preliminary studies in animal models have provided encouraging results for the proof of concept. However, significant challenges for three-dimensional organ engineering approach remain. This manuscript describes the fundamental concepts of whole-organ engineering, including characterization of the extracellular matrix as a scaffold, methods for decellularization of vascular organs, potential cells to reseed such a scaffold, techniques for the recellularization process and important aspects regarding bioreactor design to support this approach. Critical challenges and future directions are also discussed.

Extracellular matrix powder protects against bleomycin-induced pulmonary fibrosis

Pulmonary fibrosis refers to a group of lung diseases characterized by inflammation, fibroblast proliferation, and excessive collagen deposition. Although the mechanisms underlying pulmonary fibrosis are poorly understood, current evidence suggests that epithelial injury contributes to the development of fibrosis. Regenerative medicine approaches using extracellular matrix (ECM) scaffolds have been shown to promote site-specific tissue remodeling. This led to the hypothesis that particulate ECM would promote normal tissue repair and attenuate bleomycin-induced pulmonary fibrosis. C57BL/6 mice were treated intratracheally with bleomycin or saline with or without a particulate form of ECM scaffold from porcine urinary bladder matrix (UBM-ECM) or enzymatically digested UBM-ECM. Mice were sacrificed 5 and 14 days after exposure. Compared to control mice, bleomycin-exposed mice had similar increases in inflammation in the bronchoalveolar lavage fluid regardless of UBM-ECM treatment. However, 14 days after exposure, lung histology and collagen levels revealed that mice treated with bleomycin and the particulate or digested UBM-ECM had negligible fibrosis, whereas mice given only bleomycin had marked fibrosis. Administration of the particulate UBM-ECM 24 h after bleomycin exposure also significantly protected against pulmonary injury. In vitro epithelial cell migration and wound healing assays revealed that particulate UBM-ECM promoted epithelial cell chemotaxis and migration. This suggests that promotion of epithelial wound repair may be one mechanism in which UBM-ECM limits pulmonary fibrosis.

Stem cells and cell therapies in lung biology and lung diseases

The University of Vermont College of Medicine and the Vermont Lung Center, with support of the National Heart, Lung, and Blood Institute (NHLBI), the Alpha-1 Foundation, the American Thoracic Society, the Emory Center for Respiratory Health,the Lymphangioleiomyomatosis (LAM) Treatment Alliance,and the Pulmonary Fibrosis Foundation, convened a workshop,‘‘Stem Cells and Cell Therapies in Lung Biology and Lung Diseases,’’ held July 26-29, 2009 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 approaches for lung diseases. These are rapidly expanding areas of study that provide further insight into and challenge traditional views of the 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, discuss and debate current controversies, and identify future research directions and opportunities for both basic and translational research in cell-based therapies for lung diseases.

Organ-specific scaffolds for in vitro expansion, differentiation, and organization of primary lung cells

In light of the increasing need for differentiated primary cells for cell therapy and the rapid dedifferentiation occurring during standard in vitro cultivation techniques, there is an urgent need for developing three-dimensional in vitro systems in which expanded cells display in vivo-like differentiated phenotypes. It is becoming clear that the natural microenvironment provides the optimal conditions for achieving this aim. To this end, we prepared natural decellularized scaffolds of microscopic dimensions that would allow appropriate diffusion of gases and nutrients to all seeded cells. Scaffolds from either the lung or the liver were analyzed for their ability to support growth and differentiation of progenitor alveolar cells and hepatocytes. We observed that progenitor alveolar cells that have been expanded on plastic culture and thus dedifferentiated grew within the lung-derived scaffolds into highly organized structures and regained differentiation markers classical for type I and type II alveolar cells. The cells generated proper alveolar structures, and only 15%-30% of them secreted surfactant proteins in a localized manner for extended periods. Vice versa, liver-derived scaffolds supported the differentiation state of primary hepatocytes. We further demonstrate that the natural scaffolds are organ specific, that is, only cells derived from the same organ become properly differentiated. A proteomic analysis shows significant different composition of lung and liver scaffolds, for example, decorin, thrombospondin 1, vimentin, and various laminin isoforms are especially enriched in the lung. Altogether, our data demonstrate that complex interactions between the seeded cells and a highly organized, organ-specific stroma are required for proper localized cell differentiation. Thus, our novel in vitro culture system can be used for ex vivo differentiation and organization of expanded primary cells.

Matrix composition and mechanics of decellularized lung scaffolds

The utility of decellularized native tissues for tissue engineering has been widely demonstrated. Here, we examine the production of decellularized lung scaffolds from native rodent lung using two different techniques, principally defined by use of either the detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS) or sodium dodecyl sulfate (SDS). All viable cellular material is removed, including at least 99% of DNA. Histochemical staining and mechanical testing indicate that collagen and elastin are retained in the decellularized matrices with CHAPS-based decellularization, while SDS-based decellularization leads to loss of collagen and decline in mechanical strength. Quantitative assays confirm that most collagen is retained with CHAPS treatment but that about 80% of collagen is lost with SDS treatment. In contrast, for both detergent methods, at least 60% of elastin content is lost along with about 95% of native proteoglycan content. Mechanical testing of the decellularized scaffolds indicates that they are mechanically similar to native lung using CHAPS decellularization, including retained tensile strength and elastic behavior, demonstrating the importance of collagen and elastin in lung mechanics. With SDS decellularization, the mechanical integrity of scaffolds is significantly diminished with some loss of elastic function as well. Finally, a simple theoretical model of peripheral lung matrix mechanics is consonant with our experimental findings. This work demonstrates the feasibility of producing a decellularized lung scaffold that can be used to study lung matrix biology and mechanics, independent of the effects of cellular components.

Molecular basis of lung tissue regeneration

Recent advances have expanded our understanding of lung endogenous stem cells, and this knowledge provides us with new ideas for future regenerative therapy for lung diseases. In studies using animal models for lung regeneration, compensatory lung growth, and lung repair, promising reagents for lung regeneration have been discovered. Stem or progenitor cells are needed for alveolar regeneration, lung growth, and lung repair after injury. Endogenous progenitor cells mainly participate in alveologenesis. However, human lung endogenous progenitor cells have not yet been clearly defined. Recently discovered human alveolar epithelial progenitor cells may give us a new perspective for understanding the pathogenesis of lung diseases. In parallel with such basic research, projects geared toward clinical application are proceeding. Cell therapy using mesenchymal stem cells to treat acute lung injury is one of the promising areas for this research. The creation of bioartificial lungs, which are based on decellularized lungs, is another interesting approach for future clinical applications. Although lungs are the most challenging organ for regenerative medicine, our cumulative knowledge of lung regeneration and of endogenous progenitor cells makes clear the possibilities and limitations of regenerative medicine for lung diseases.