Breathing New Life Into Lung Transplantation Therapy
Authors: Angela Panoskaltsis-Mortari, Daniel J Weiss
Journal: Molecular Therapy (2010)

Approximately 1,000–1,500 lung transplants per year are performed in the United States for various indications. However, there is a significant shortage of suitable donor lungs, and many on waiting lists die before a lung becomes available. In contrast to cadaveric kidneys, which have been successfully utilized for many years, the use of cadaveric lungs is not yet feasible. However, the decellularization of whole cadaveric lungs and their subsequent recellularization with fetal or neonatal cells in rodent models may provide an alternative approach for the generation of transplantable lungs or lung tissue. Such intact scaffolds can conceivably be seeded with embryonic stem cells (ESCs) or adult stem cells, including induced pluripotent stem cells, derived from individual patients and subsequently utilized for autologous transplantation. The decellularized lungs would thus lack donor cells that express immune epitopes that could potentially participate in immune rejection. This would provide an organ that could conceivably avoid both the problem of chronic immune rejection and the paucity of donor organs. However, the drawbacks to this approach are the difficulty of recapitulating the highly complex structure of lung tissue, which comprises more than 60 cell types whose activities must be carefully orchestrated and coordinated, providing that they can even be generated from stem cells, to obtain a functional lung. Nevertheless, the feasibility of using a whole decellularized lung as a scaffold for the generation of an experimental and functional lung replacement has now been addressed in four recent and exciting articles.1Price AP England KA Matson AM Blazar BR Panoskaltsis-Mortari A Development of a decellularized lung bioreactor system for bioengineering the lung: the matrix reloaded.Tissue Eng Part A. 2010; PubMed Google Scholar,2Cortiella J Niles J Cantu A Brettler A Pham A Vargas G et al.Influence of acellular natural lung matrix on murine embryonic stem cell differentiation and tissue formation.Tissue Eng Part A. 2010; PubMed Google Scholar,3Petersen TH Calle EA Zhao L Lee EJ Gui L Raredon MB et al.Tissue-engineered lungs for in vivo implantation.Science. 2010; Google Scholar,4Ott HC Clippinger B Conrad C Schuetz C Pomerantseva I Ikonomou L Regeneration and orthotopic transplantation of a bioartificial lung.Nat Med. 2010; Google Scholar Many lung diseases remain incurable. Notably, emphysema is a leading cause of death worldwide and the only major disease that is increasing in prevalence.5Chapman KR Mannino DM Soriano JB Vermeire PA Buist AS Thun MJ et al.Epidemiology and costs of chronic obstructive pulmonary disease.Eur Respir J. 2006; 27 (e-pub ahead of print 13 July 2010et al. e-pub ahead of print 28 June 2010 e-pub ahead of print 21 April 2010 e-pub ahead of print 26 May 2010): 188-207Crossref PubMed Scopus (572) Google Scholar Although medications can help alleviate symptoms, emphysema remains an inexorably fatal disease. Lung transplantation is the only curative option, but there will never be a sufficient number of available donor lungs to meet the needs for either emphysema or other less common but equally devastating and fatal lung diseases such as pulmonary fibrosis and cystic fibrosis. In addition, there are significant problems associated with current lung transplantation procedures. Recipients require lifelong immunosuppression, and the 5-year mortality rate is 50% with deaths resulting from infection or chronic rejection.6Trulock EP Edwards LB Taylor DO Boucek MM Keck BM Hertz MI The Registry of the International Society for Heart and Lung Transplantation: twenty-first official adult lung and heart-lung transplant report—2004.J Heart Lung Transplant. 2004; 23: 804-815Abstract Full Text Full Text PDF PubMed Scopus (186) Google Scholar,7Orens JB Garrity Jr ER General overview of lung transplantation and review of organ allocation.Proc Am Thorac Soc. 2009; 6: 13-19Crossref PubMed Scopus (87) Google Scholar Rapid advances in tissue engineering have demonstrated the potential of developing functional tissue following decellularization and subsequent recellularization of organs such as heart and liver.8Ott HC Matthiesen TS Goh S-K Black LD Kren SM Netoff TI et al.Perfusion-decellularized matrix: using nature's platform to engineer a bioartificial heart.Nat Med. 2008; 14: 213-221Crossref PubMed Scopus (2023) Google Scholar,9Uygun BE Soto-Gutierrez A Yagi H Izamis M-L Guzzardi MA Shulman C et al.Organ reengineering through development of a transplantable recellularized liver graft using decellularized liver matrix.Nat Med. 2010; 16: 814-820Crossref PubMed Scopus (1013) Google Scholar However, given the complexity of the structure and the structure–function relationship of the respiratory system, particularly the lung itself, bioengineering the lung has proved to be a daunting task. Until recently, the most significant advances in bioengineering of the respiratory system have been made in engineering trachea ex vivo. Several investigative groups have been able to create bioengineered trachea using both artificial and decellularized, cadaveric scaffolds. This has culminated in successful clinical use of bioengineered airway in Spain.10Macchiarini P Jungebluth P Go T Asnaghi MA Rees LE Cogan TA et al.Clinical transplantation of a tissue-engineered airway.Lancet. 2008; 372: 2023-2030Abstract Full Text Full Text PDF PubMed Scopus (1175) Google Scholar,11Macchiarini P Walles T Biancosino C Mertsching H First human transplantation of a bioengineered airway tissue.J Thorac Cardiovasc Surg. 2005; 128: 638-641Abstract Full Text Full Text PDF Scopus (145) Google Scholar Decellularized trachea is used as the biological scaffold onto which cultured tracheal epithelial cells and cartilage derived from bone marrow stem cells were seeded and placed in a custom bioreactor. With this engineered construct, 5 cm of the left bronchus was replaced, with restoration of normal function. The trachea is a simple structure whose function is predominantly to conduct air into the lungs. The lungs themselves are much more complex organs with numerous functions, the most critical of which is to exchange gas with the blood that provides oxygen to all the tissues of the body. However, steady and significant progress has been made utilizing three-dimensional scaffolds engineered from synthetic and natural materials such as polyglycolic acid and collagen onto which lung cells can be seeded in attempts to create ex vivo functioning lung tissue (reviewed in ref. 12Weiss DJ Kolls JK Ortiz LA Panoskaltsis-Mortari A Prockop DJ Stem cells and cell therapies in lung biology and lung diseases.Proc Am Thorac Soc. 2008; 5: 637-667Crossref PubMed Scopus (146) Google Scholar). For example, a recent study demonstrated that fetal rat lung cells cultured in a widely used surgical sponge, Gelfoam, comprising a biodegradable gelatin matrix, and subsequently injected into normal rat lungs induced formation of tissue that resembled normal lung.13Andrade CF Wong AP Waddell TK Keshavjee S Liu M Cell-based tissue engineering for lung regeneration.Am J Physiol Lung Cell Mol Physiol. 2007; 292: L510-L518Crossref PubMed Scopus (101) Google Scholar However, there are fewer published studies evaluating the ability of stem or progenitor cells to generate lung tissue in biosynthetic scaffolds. For example, studies in which ESCs have been cultured in two- and three-dimensional culture systems with synthetic scaffolds can result in generation of one of the major cells that make up the alveoli (small air sacs or gas exchange units) of the lungs, type 2 alveolar epithelial cells.14Rippon HJ Polak JM Qin M Bishop AE Derivation of distal lung epithelial progenitors from murine embryonic stem cells using a novel three-step differentiation protocol.Stem Cells. 2006; 24: 1389-1398Crossref PubMed Scopus (109) Google Scholar,15Wang D Haviland DL Burns AR Zsigmond E Wetsel RA A pure population of lung alveolar epithelial type II cells derived from human embryonic stem cells.Proc Natl Acad Sci U S A. 2007; 104: 4449-4454Crossref PubMed Scopus (188) Google Scholar,16Mondrinos MJ Koutzaki S Jiwanmall E Li M Dechadarevian JP Lelkes PI et al.Engineering three-dimensional pulmonary tissue constructs.Tissue Eng. 2006; 12: 717-728Crossref PubMed Scopus (126) Google Scholar,17Roszell B Mondrinos MJ Seaton A Simons DM Koutzaki SH Fong GH et al.Efficient derivation of alveolar type II cells from embryonic stem cells for in vivo application.Tissue Eng Part A. 2009; 15: 3351-3365Crossref PubMed Scopus (74) Google Scholar However, it is not clear whether these cells were bona fide functional type 2 cells. Furthermore, these studies were not designed to demonstrate functionality of the engineered lung tissue, such as the potential for ventilation and gas exchange. It is also not clear whether artificial scaffolds provide the best framework for generating functional lung tissues. Artificial scaffolds neither fully replicate the complexity of the lung architecturally, nor do they contain all the matrix components essential for normal lung development and function. In the new studies, the workers show the feasibility of using a whole decellularized lung as a scaffold for the generation of an experimental and functional lung replacement.1Price AP England KA Matson AM Blazar BR Panoskaltsis-Mortari A Development of a decellularized lung bioreactor system for bioengineering the lung: the matrix reloaded.Tissue Eng Part A. 2010; PubMed Google Scholar,2Cortiella J Niles J Cantu A Brettler A Pham A Vargas G et al.Influence of acellular natural lung matrix on murine embryonic stem cell differentiation and tissue formation.Tissue Eng Part A. 2010; PubMed Google Scholar,3Petersen TH Calle EA Zhao L Lee EJ Gui L Raredon MB et al.Tissue-engineered lungs for in vivo implantation.Science. 2010; Google Scholar,4Ott HC Clippinger B Conrad C Schuetz C Pomerantseva I Ikonomou L Regeneration and orthotopic transplantation of a bioartificial lung.Nat Med. 2010; Google Scholar The use of decellularized lungs for growing alveolar cells in culture was first described 25 years ago,18Lwebuga-Mukasa JS Ingbar DH Madri JA Repopulation of a human alveolar matrix by adult rat type II pneumocytes in vitro. A novel system for type II pneumocyte culture.Exp Cell Res. 1986; 162: 423-435Crossref PubMed Scopus (84) Google Scholar and in that sense has now been essentially “rediscovered” for testing in in vivo applications. Decellularization is achieved primarily through the use of detergents that dissolve the lipid membrane bilayer of the cells. The advantages of a fully decellularized lung are significant in that it should retain its native architecture, including both the key airway and vasculature structures that are critical to normal functioning of the lung. Keeping the trachea and large vessels intact within the lung scaffold ensures the ability to ventilate and reperfuse the matrix once it has been recellularized.1Price AP England KA Matson AM Blazar BR Panoskaltsis-Mortari A Development of a decellularized lung bioreactor system for bioengineering the lung: the matrix reloaded.Tissue Eng Part A. 2010; PubMed Google Scholar Mechanical ventilation of the engineered lung provides mechanical stretch that simulates normal breathing movements. Importantly, as demonstrated in the first of the four recent articles, distribution of extracellular matrix components critical for the lung, such as collagen, elastin, and laminin, are all maintained in the engineered organ1Price AP England KA Matson AM Blazar BR Panoskaltsis-Mortari A Development of a decellularized lung bioreactor system for bioengineering the lung: the matrix reloaded.Tissue Eng Part A. 2010; PubMed Google Scholar (Figure 1). In another of the recently published studies, injection of decellularized lung with mouse ESCs resulted in apparent differentiation and recellularization of several epithelial and vascular structures.2Cortiella J Niles J Cantu A Brettler A Pham A Vargas G et al.Influence of acellular natural lung matrix on murine embryonic stem cell differentiation and tissue formation.Tissue Eng Part A. 2010; PubMed Google Scholar However, neither of these first two articles tested gas exchange functionality. A further significant advance has been achieved in the article by Petersen et al., published in Science,3Petersen TH Calle EA Zhao L Lee EJ Gui L Raredon MB et al.Tissue-engineered lungs for in vivo implantation.Science. 2010; Google Scholar which tested the function of an implanted decellularized rat lung that had been recellularized with neonatal rat lung cells. In this model, the recellularized lung was surgically connected to both the airway and vascular systems in the transplant recipient rat, which resulted in blood flow through the lung for as long as 2 hours. Importantly, the authors were able to show that the lungs participated in gas exchange. The recipient animal, although mechanically ventilated, did appear to survive the 2-hour time period following transplantation. However, there was significant hemorrhage in the lung (Figure 2), and it was not clear that the implanted lung would have functioned if not mechanically ventilated. Remarkably, a similar study by Ott and colleagues, recently published in Nature Medicine,4Ott HC Clippinger B Conrad C Schuetz C Pomerantseva I Ikonomou L Regeneration and orthotopic transplantation of a bioartificial lung.Nat Med. 2010; Google Scholar has shown survival of the transplant recipients without mechanical ventilation. Using similar techniques, the authors of the latter study seeded decellularized lung matrices with fetal rat lung cells and human umbilical cord endothelial cells (which line the interior of the blood vessels). That study demonstrated the survival of a group of rats that received the engineered, recellularized lung transplant for 6 hours after they had been taken off mechanical ventilation (Figure 3).Figure 3Orthotopic transplantation and in vivo function. (a) Photograph of left rat chest after left anterior thoracotomy, left pneumonectomy, and orthotopic transplantation of a regenerated left lung construct. Recipient's left pulmonary artery (A), left main bronchus (B), and left pulmonary vein (V) are connected to regenerated left lung pulmonary artery (a), bronchus (b), and pulmonary vein (v). White arrowheads, the recipient's right lung (infracardiac and right lower lobe); black arrowheads, the regenerated left lung construct. (b) Radiograph of rat chest after left pneumonectomy and orthotopic transplantation of a regenerated left lung construct. White arrowheads, recipient's right lung; black arrowheads, regenerated left lung construct. Reprinted from ref. 4Ott HC Clippinger B Conrad C Schuetz C Pomerantseva I Ikonomou L Regeneration and orthotopic transplantation of a bioartificial lung.Nat Med. 2010; Google Scholar.View Large Image Figure ViewerDownload Hi-res image Download (PPT) Although the hemorrhaging and thrombus formation observed in the Petersen study were not observed by Ott and colleagues, the lungs did develop substantial edema causing fluid retention in the lungs, necessitating that the animals be returned to mechanical ventilation. Further, in both the Petersen and Ott studies, the reconstituted lungs had areas that either were not completely recellularized/regenerated or consisted of cells that were not fully mature. Nonetheless, these groundbreaking studies set the stage for further investigations utilizing decellularized lungs as scaffolds for bioengineering functional lung tissue with both adult differentiated respiratory cells as well as stem and progenitor cells that can be obtained from patients. The future challenges in developing complex three-dimensional functional lung tissues ex vivo will be in recapitulating the normal dynamic integrated three-dimensional network of epithelial cells, endothelial cells, fibroblasts, and neuronal and inflammatory cells in the appropriate environment and architecture with the correct effector molecules and mechanical forces, all of which are vital for proper function. This is a mammoth task that will involve concerted, coordinated, interdisciplinary effort between lung biologists and bioengineers as well as dedicated funding from the National Institutes of Health and respiratory disease foundations. A thorough understanding of the synergistic interactions between cells in physiologically relevant conditions is critical for development of tissues that recapitulate the structure and function of the parental tissue in vivo. A significant advance in this area was recently demonstrated by Huh et al., who were able to coat human lung cells and human blood capillaries on chips mimicking the alveoli of the lungs.19Huh D Matthews BD Mammoto A Montoya-Zavala M Hsin HY Ingber DE Reconstituting organ-level lung functions on a chip.Science. 2010; 328: 1662-1668Crossref PubMed Scopus (2375) Google Scholar This “lung-on-a-chip” device was tested by the authors to evaluate how nanoparticles and bacteria enter the lungs and may also be useful for high-throughput screening of drugs. Despite the four recent articles demonstrating the use of decellularized lungs as model systems for ex vivo lung tissue generation, many hurdles remain, including the ability to appropriately seed these scaffolds with the combination of cells that will result in normal recellularization of the lung and also generation of lungs with the full range of normal functions. In particular, to generate autologous lung tissue for use in clinical transplantation, methodologies will need to be developed to induce ESCs and adult stem cells to differentiate in these scaffolds into the appropriate cell types, organized in the correct manner to resemble normal lung. These goals are probably many years away. Nonetheless, generation of functional lung tissue ex vivo, although a daunting, formidable task, seems closer to reality.