Article by Ivana Matkovic & Roberta Lock
Repairing Damaged Airways via Cell Transplantation
Liang Ma, et al., Darrell Kotton Lab
Scientists are actively searching for reparative cells to enhance the process of lung regeneration. This quest is driven by the pressing need for more effective clinical therapies to address the challenges faced by patients afflicted with diseases stemming from lung tissue injury or degeneration. Current options, such as supportive care and, in severe cases, lung transplantation, fall short in providing effective treatments for acute damage to lung epithelia or the chronic degeneration of airway and alveolar tissues. Consequently, gaining a deeper understanding of the fundamental mechanisms that facilitate the self-renewal and differentiation of lung cells is imperative for unveiling innovative therapeutic strategies for lung diseases.
What did these researchers do?
Airway basal cells are the stem cells or progenitors of the airway epithelium and play a crucial role in replenishing all epithelial cells, which includes the ciliated cells, and secretory goblet cells, upon lung damage or injury. These researchers achieved successful engraftment of airway epithelial cells from both mouse and human primary basal cells, as well as engineered pluripotent stem cell (PSC) derived basal cells into an injured mouse lung. Importantly, regardless of whether the transplanted basal cells originated from primary sources or were engineered from pluripotent stem cells, they proficiently reconstructed the natural basal stem cell population within the pulmonary airways. The engrafted cells exhibited the ability to self-renew over the long term and maintained a durable capacity for versatile differentiation for at least 2 years in vivo.
Why is this important?
The maintenance of adult mammalian epithelial tissues relies on the continuous division of mature cells or the self-renewal and differentiation of resident stem cells within these tissues. While the successful engraftment of exogenous epithelial stem cells has been achieved for some ectoderm-derived tissues (e.g. skin tissue), applying the same approach to internal tissues has proven challenging, with only a few exceptions. This difficulty has hindered the long-term rescue of diseased internal epithelia, such as those in the respiratory airways. Theoretically, reinstating the stem cell population within internal epithelial tissues offers a promising solution, as it could lead to enduring and functional engraftment. This is because tissue-resident stem cells, once successfully engrafted, have the potential for multipotent differentiation (which means that they have the ability to differentiate into all cell types within a particular lineage) and self-renewal in vivo.
The different cell sources used in this study to test engraftment are also important. Primary cell sources can sometimes be challenging to obtain or grow from patients, whereas cells that are engineered from a PSC cell source provide a virtually unlimited source of cells, which is a much more accessible alternative. However, PSC-derived cells often do not behave exactly the same as primary cells, so it is always important to compare both to determine if one source is superior to the other or if they are functionally equivalent for their intended application.
How did the researchers do this?
For more than 20 years, the scientists leading this work have pursued a way to engraft cells into injured lung tissues with the goal of regenerating lung airways or alveoli. They concentrated on first developing methods for engineering each of the lung’s stem or progenitor cells “in a laboratory dish”. The first step was in vitro directed differentiation of murine pluripotent stem cells into airway basal epithelial-like cells in a special growth medium, followed by their expansion as monolayered epithelial spheres. The generated spheres were then analyzed by single cell RNA sequencing to confirm the presence of airway lineage signature genes. Once the basal airway cell identity was confirmed, mice were treated with polidocanol (PDOC), a reagent used for inducing lung injury in animal models. It temporarily removes the surface airway epithelial cells in mice and improves engraftment of stem cells. Basal cells grown in a dish were now ready for the transplantation. Recipient mice were sacrificed after several time points and transplantation efficiency was examined. Astonishingly, the results confirmed that mouse primary basal cells and engineered basal cells, when transplanted into syngeneic recipient mice without immunosuppression, replaced more than 50% of the endogenous tracheal epithelium after a single treatment.
Tracheal whole-mount fluorescence microscopy of green fluorescent protein labeled iPSC-derived basal stem cells 2 year post-transplantation
What comes next?
While this study demonstrates the potential for transplanting cells derived from both primary and pluripotent stem cells into the mouse trachea, further investigations are necessary to advance toward more clinically applicable methodologies. Particularly, emphasis should be placed on refining the epithelial injury model used prior to the transplantation. This current transplantation model relies on pre-exposing the murine tracheal epithelium to polidocanol, a method impractical for clinical implementation as it is toxic to humans and thus could not be used to condition human lungs. Ongoing efforts are directed at developing conditioning approaches that are more relevant to clinical settings. However, this study demonstrated successful in vivo engraftment of injured mouse airway epithelium with both mouse and human basal stem cells as well as engineered basal cells. The success with the engineered basal cells is especially exciting, as stem cell technology has come a long way in the last decade, and with gene editing technology, stem cells can be edited in the laboratory prior to transplantation. This means that for patients suffering from genetic lung diseases, like cystic fibrosis or primary ciliary dyskinesia, it may be possible in the future to cure their disease by transplanting cells that are genetically corrected for their disease-causing mutation. In conclusion, this breakthrough achievement is a very promising initial step towards developing cell-based treatments for diseases affecting the airway epithelium.