Functional Lung Tissue-Engineered in Rat Model
Tissue engineering and cell therapy are an essential plank in the Strategies for Engineered Negligible Senescence (SENS) platform of regenerative engineering. These biotechnologies are most obviously central for direct clinical use in repairing and replacing cells and tissues "injured by trauma, damaged by disease or worn by time" (as William Haseltine first defined regenerative medicine (1)). But additionally, mature tissue and organ engineering are a prerequisite for the building of impenetrable defenses against malignant disease in the negligibly-aging body, via the Whole-body Interdiction of Lengthening of Telomeres (WILT, or OncoSENS) strategy.(2,3) The main and earliest targets for the use of tissue engineering in WILT will be in major epithelial tissues such as the gut, skin, and lung; progress toward tissue engineering of these organs is therefore of higher priority for SENS Foundation than would be expected from direct projections from existing clinical needs for transplant medicine.
A recent report from researchers at Yale and Duke Universities (4) heralds a significant advance toward the tissue engineering of the lung.
.... we treated lungs from adult rats using a procedure that removes cellular components but leaves behind a scaffold of extracellular matrix that retains the hierarchical branching structures of airways and vasculature. [Compare the pioneering work of Dr. Doris Taylor's group in the rat heart,(5) and of Macchiarini et al in the bronchus (6) -MR] We then used a bioreactor to culture [neonatal] pulmonary epithelium and vascular endothelium on the acellular lung matrix. The seeded epithelium displayed remarkable hierarchical organization within the matrix, and the seeded endothelial cells efficiently repopulated the vascular compartment. In vitro, the mechanical characteristics of the engineered lungs were similar to those of native lung tissue ...
[W]hen implanted into rats in vivo for short time intervals (45 to 120 min), the engineered lungs participated in gas exchange. ... In all cases, the engineered lungs were easily suturable to the recipient and they were ventilated with no visible air leak from the parenchyma. All engineered lungs became perfused with blood over a period of seconds to minutes, with blood visibly turning from dark to bright red as the hemoglobin became oxygenated. ... After perfusion and ventilation, blood gas samples were drawn from the pulmonary artery, left and right pulmonary veins [individually], ... and from the unclamped pulmonary vein, to document the extent of gas exchange ... [T]he engineered lung was inflated with air, but the level of inflation was less than that of the native right lung. ... Partial pressures of oxygen increased from 27±7 mmHg in the pulmonary artery, to 283±48 mmHg in the left pulmonary vein ... Although the partial pressure of oxygen in the right pulmonary vein was higher [634±69 mmHg], ... this difference may not be of substantial physiological consequence, since hemoglobin saturation is complete above oxygen pressures of 100 mm [and was 100% for both venous samples in this study] ... In addition, carbon dioxide removal was efficient, with CO2 falling from 41±13 mmHg in the pulmonary artery to 11±5mmHg in the left, engineered pulmonary vein. [As with p02, pCO2 of the native right lung's pulmonary vein was approximately half of that in the left, engineered one; the mixed venous blood was roughly halfway between that of the right and left pulmonary veins considered in isolation. As a nonspecialist, I do not feel qualified to speculate on the functional significance of these nominal deficiencies beyond the authors' cautious reasurrances -MR]. ...
Although representing only an initial step toward the ultimate goal of generating fully functional lungs in vitro, these results suggest that repopulation of lung matrix is a viable strategy for lung regeneration.
This represents a substantial advance for tissue engineering via the repopulated stromal scaffold approach, executed in a tissue far more complex in structure and function than the bronchus (6) and actually shown to function, albeit only for a brief window, in vivo rather than demonstrated ex vivo (5). And as noted, the engineering of functional lungs with autologous cells is of particular importance to WILT (2,3). The broad outlines of the path ahead for clinical use to replace whole lung transplantation in injury and pulmonary disorders is reasonably clear, moving through ongoing refinement of the protocol and its demonstration for progressively longer periods of time, to translating the technique first to large mammal models, and later to human patients. The latter might initially be achieved via the use of decellularized porcine or other lung tissue as a xenoscaffold with autologous patient cells. Later, further experience and innovation in tissue engineering, along with greater understanding of the stroma (including its development, the "body as best bioreactor," and its interactions with lung parenchyma) should allow for completely engineered scaffolds to take the place of biologically-sourced ones. The eventual engineering of cancer-impervious lung tissue will require, in addition, the generation of suitable patient-derived engineered cells, with the telomere maintenance machinery deleted and telomeres lengthened to youthful physiological levels ex vivo, and , and their use in the seeding of such scaffolds. If scaffold technology is sufficiently sophisticated at that time, the engineering of aditional cell populations responsible for the ongoing physiological maintenance of the engineered stroma in situ may also be desirable. This landmark report is a step change along that path.
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