From an engineering and materials science perspective, the lung is a paradigm of design efficiency. A gas transfer surface area of approximately 70 m2 is packed into an elastic, dynamic structure to accomplish efficient oxygen and carbon dioxide transfer. There has been modest success in organizing cells into small-scale structures that mimic pulmonary tissue, but the question of how to scale up and effectively connect such structures has loomed large. Two studies by Petersen et al. on page 538 of this issue (1) and by Ott et al. (2) describe an alternative approach by which the structural efficiencies of native lung tissue can be captured while potentially avoiding the immunogenicity barriers associated with nonautologous tissue transplantation.
Temporary reproduction of the gas transfer function of the lungs does not depend on an organized and viable pulmonary tissue, despite early clinical attempts to do this, such as the perfusion of rhesus monkey lungs with patient's blood during open-heart surgery (3). Rather, mechanical blood oxygenator designs have advanced to make cardiopulmonary support a routine aspect of such procedures. However, biocompatible artificial lung technology to provide chronic aid for patients in end-stage pulmonary failure remains elusive. Support by synthetic membrane–based lung devices remains limited to a period of days or weeks, requires aggressive therapy to prevent blood clotting (anticoagulation), greatly restricts patient mobility and quality of life, and has high mortality.
The limits of current technology are found at the interface where oxygen and carbon dioxide pass between the blood and gas sides of synthetic hollow-fiber membranes. Blood proteins adsorb to the polymeric membrane surfaces, which can trigger the activation immune cells and deposition of clots onto the fibers. This can result in immune responses and a tendency toward bleeding, respectively, when the blood reenters the patient. Further, to achieve adequate gas transfer, membrane surface areas on the order of 1 m2 are required. The size of current membrane oxygenators is incompatible with placement within the body. This is in stark contrast to the inherently anticoagulant surfaces of endothelial cell–lined capillaries that provide the 70 m2 of gas transfer area in the native human lung (see the figure).
Recent approaches to create a more biocompatible artificial lung for extended support have sought to address the surface area per volume limitations of current devices as well as the blood interface bioincompatibility. By reducing fluid boundary layers and optimizing blood flow patterns, blood-contacting synthetic surface areas can be minimized and inhibition of cellular deposition can be improved (4). Smaller oxygenators can also be designed to accommodate patients who do not require complete respiratory support (5). Microfabrication techniques have generated both branched and densely packed blood-carrying polymeric channels with high gas diffusivity that can be stacked in layers near gas channels (6, 7). The blood pathways of these devices can be lined with autologous endothelial cells to provide a surface that mimics that of blood vessels and minimizes the need for anticoagulants. In a cell-seed approach, microporous hollow-fiber membranes are chemically modified to support endothelial cell adhesion. These endothelial cell–covered fibers are bundled and rotated in an oxygenator to create mixing and reduce boundary layer limits on gas transfer efficiency (8). Such devices create a "bio-hybrid" lung that is primarily synthetic but incorporates patient endothelial cells. If they can simulate native thromboresistant endothelial surfaces, a major goal will be achieved
Artificial and limited. Membrane oxygenators currently in clinical use (left) commonly use packed hollow-fiber membranes across which blood flows. Protein and cellular deposits on the synthetic surfaces lead to patient complications and the need for anticoagulation therapy. The native lung (right) provides approximately 70 m2 of gas transfer area in alveoli with actively anticoagulant endothelial surfaces lining the blood
Petersen et al. and Ott et al. apply whole-organ decellularization (removal of all cellular constituents) to the lung, an approach recently explored for heart, liver, and kidney (9–11). This method has conceptual roots in clinically successful bone marrow transplantation. In vivo tissue devitalization, followed by introduction of a regenerating cell population to reconstruct function, was pioneered by E. Donnall Thomas and colleagues in the 1950s and was the basis of the 1990 Nobel Prize in Physiology or Medicine. Petersen et al. and Ott et al. removed the cells from isolated adult rat lungs in a manner that preserved the structural characteristics of the organ, notably the vasculature and the airways (including alveoli, the tiny air sacs in which gas transfer occurs). The remaining lung "scaffolds" were maintained in a bioreactor designed to mimic the developmental environment of lung tissue, and repopulated with epithelial and endothelial cells. Lung tissue was successfully regenerated that mimicked native tissue in appearance and could facilitate gas exchange in vitro and when grafted into rodents.
The promise of this tissue-engineering approach lies in its potential to effectively expand the pool of donor organs, particularly if lungs unsuitable for transplant can be processed for seeding with autologous cells for a given patient while the patient is supported with artificial lung technology. More important, if a tissue-engineered lung had the functional profile of an allograft lung, the removal of pharmacologic therapy to combat organ rejection would have a profound effect on the prognosis and quality of life for lung recipients. Other possible applications of this technique include processing of a patient's diseased tissue to remove problematic cellular components followed by regeneration to a healthy state.
Although the reports of Petersen et al. and Ott et al. open possibilities for treating pulmonary failure and studying mechanisms underlying cell–extracellular matrix interactions, many issues remain to be addressed. Identifying cell sources from the patient that are most effective in repopulating the decellularized lung, achieving a sound blood-gas barrier and a completely endothelialized blood pathway, and providing long-term evaluation of cellularization and differentiation in situ are considerations. The use of extracellular matrix for connective tissue repair is effective in many applications, but there can be failures associated with tissue remodeling that result in mechanically inadequate structures (12). How will the lung extracellular matrix ultimately be remodeled? If replacement lung tissue is inappropriately fibrous or weak, long-term outcomes will be insufficient. Also, the level of phenotypic organ recapitulation that can be achieved and is functionally adequate remains open. Like the native organ, will the regenerated lung recruit vascular beds that will permit increasing blood flows with low resistance? The organ decellularization paradigm has opened a breach where multidisciplinary teams of biologists, clinicians, and engineers can explore new ways to engineer complex tissues. The next generation of artificial or bio-hybrid organs may provide temporary support for patients while patient-specific regenerative solutions are prepared and implanted.
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