Scientists have identified what they believe is a subtype of alveolar epithelial progenitor cells that can help repair lung tissue after injury in mice. The University of California, San Francisco (UCSF)-based team says the cells specifically express the integrin laminin receptor α6β4 but do not produce surfactant or the Clara cell secretory protein.
Studies in mice strongly suggested that the newly identified progenitors are directly involved in the replenishment of type II alveolar epithelial cells (AECs) after lung damage, a finding that contrasts with the traditionally held belief that lung repair involves the expansion of existing populations of type II cells.
Interestingly, while the UCSF team was carrying out its work, independent research had also isolated stem-like α6β4-expressing cells from adult mouse lungs. The USCF scientists claim the identification of a population of immature progenitor cells in adult lung parenchyma that appears to contribute to alveolar repair has implications for the pathogenesis of common lung diseases in which tissue remodeling is a dominant feature.
They report their findings in The Journal of Clinical Investigation in a paper titled “Integrin α6β4 identifies an adult distal lung epithelial population with regenerative potential in mice.”
The replenishment of damaged epithelial cells in lung parenchyma after injury has been thought to depend on the proliferation and differentiation of surfactant protein C (SPC)+ type II cells, report lead investigators Harold A. Chapman, Ph.D., and Thiennu H. Vu, at UCSF’s Cardiovascular Research Institute and Lung Biology Center. However, in their work to address this widely held tenet further, the USCF team has now revealed a role for α6β4-expressing progenitor cells in the response to injury.
The researchers developed a mouse model with selective lung epithelial loss of α6β4 (Fβ4SC mice). When these animals were injected with the potentially lung-damaging drug bleomycin, nearly all died within two weeks, whereas wild-type mice mostly survived.
Inspection of the lungs of the Fβ4SC mice showed marked sloughing of intact epithelial sheets from distal airways, an observation which is consistent with previously reported effects of integrin β4 deficiency in skin and nasal mucosa.
In the normal lung β4+ cells were located both in the bronchoalveolar junction regions and within alveoli. Cells purified from standard AEC preparations were found to proliferate in culture through a number of passages, with about 60% of cells remaining SPC- CC10-. However, after repeated passaging most colonies also contained cells that were SPC+ and CC10+, and about 10-15% of colony cells were β4-, indicating a tendency to lose the marker in culture.
The researchers went on to see whether the numbers of β4+ AECs in mice lungs changed in response to injury. Mice were exposed to bleomycin and AECs isolated after 14 days. Analysis showed that the numbers of β4+ AECs increased significantly after injury, from about 10% of the total AECs isolated from control-treated mice to up to 37% in bleomycin-injured mice.
The rise in β4+ expressing AECs corresponded with a rise in integrin β4 levels. The majority of recovered β4+ AECs after bleomycin injury remained low or negative for CC10 and SPC but, in contrast with cells isolated from normal lungs, some alveolar β4+ cells in injured lungs also appeared to express SPC.
The researchers went on to develop what they claim are unique single cell suspension-derived lung organoids that recapitulate lung development when implanted in mice and develop alveolar-like structures composed of interconnectd airspaces lined by flattened epithelial cells. Tests using this system and additional mouse models strongly suggested that the SPC+ type cells observed in bleomycin-damaged areas of lung were largely derived from the non-SPC-expressing progenitor cells.
“Our findings here showed that in the normal lung, α6β4 expression marked a strikingly fixed population of non-SPC-expressing AECs that possessed the capacity for proliferation and further differentiation ex vivo and regeneration of lung structures in vivo,” the authors conclude. “Although we cannot prove at this point that β4+ AECs within alveoli are the only progenitor source of type II AECs within the injured lung parenchyma, our data identify a strong candidate stem/progenitor cell that could account for these data.”