CF Alters Cells of Airways’ Lining, Including Certain Stem Cells

Cystic fibrosis (CF) is linked with changes in the types of cells — including stem cells — that compose the tissue lining the airways, a study reported.

Specifically, CF patients show an exhaustion in certain basal cells, progenitor-like cells with the capacity to repair and regenerate damaged lung tissue.

These findings may help researchers pinpoint those cells that are potential targets for CF therapies offering a “long-term correction,” its scientists said.

The study, “Transcriptional analysis of cystic fibrosis airways at single-cell resolution reveals altered epithelial cell states and composition,” was published as a letter in the journal Nature Medicine.

CF is caused by mutations in the CFTR gene, which contains instructions to make a protein with the same name that is responsible for regulating salt and water balance on cell membranes.

In the airways, a defective CFTR protein allows a thick and sticky mucus to accumulate, affecting lung function.

Increasing evidence suggest that the epithelial (surface) tissue of the airways is made of different cell types, and these types can be identified by analyzing their transcriptome — the entire set of messenger RNA molecules made from DNA, and used as templates to build proteins.

“If you can understand how things work in a state of health, it becomes easier to see what cellular and molecular changes occur in a disease state,” Brigitte Gomperts, MD, a member of the Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research at UCLA, and one of the study’s co-senior authors, said in a press release.

Scientists with UCLA, Cedars-Sinai, and the Cystic Fibrosis Foundation — brought together through the foundation’s Epithelial Stem Cell Consortium — used an advanced molecular technique, called single-cell RNA sequencing, to analysis the transcriptome  of healthy people and CF patients at the level of a unique, single cell.

“The process [single-cell RNA sequencing] is analogous to taking a smoothie and ‘un-blending’ it to discover all the ingredients it contains and measure how much of each ingredient was used,” said Kathrin Plath, PhD, another co-senior author and a member of the UCLA Broad Stem Cell Research Center.

Overall, the researchers analyzed epithelial lung tissue from 19 CF patients who underwent lung transplant due to end-stage lung disease, and 19 donors who had died of causes other than lung disease. Tissues were broken down to single cells and processed for single-cell RNA sequencing.

Computer analyses revealed a catalog of cells: the main cell types of  the airway’s epithelium — basal, secretory, and ciliated cells — as well as some rarer types, including a cellular subset called ionocytes previous linked with CF.

Among the most common cell types, investigators were able to distinguish three types of ciliated cells, five of secretory cells, and five types of basal cells.

Basal cells act as stem cells, meaning they can give rise to other cell types. Secretory cells include the main producers of mucus in the lungs, while ciliated cells use their projections to help clear mucus and push invaders from the airways.

Ionocytes had higher levels of CFTR RNA, with more than 30% of these cells showing activity in this gene. However, most CFTR-expressing (activating) cells were secretory cells, followed by basal cells.

Basal cells of people with CF had a lower proliferative capacity, indicative cell exhaustion due to constant inflammation and airway injury, the researchers noted.

More basal cells were also seen to be transitioning into specialized ciliated cells in CF lung tissue, possibly in an attempt to improve mucus clearance.

“We were surprised to find that the airways of people with cystic fibrosis showed differences in the types and proportions of basal cells, a cell category that includes stem cells responsible for repairing and regenerating upper airway tissue, compared with airways of people without this disease,” said Barry R. Stripp, PhD, also a co-senior study author and director of the lung stem cell program at a Cedars-Sinai institute.

“We suspect that changes the basal cells undergo to replenish ciliated cells represent an unsuccessful attempt to clear mucus that typically accumulates in airways of patients with cystic fibrosis,” he added.

Targeting a specific group of key stem cells in the airways may help in therapy development that’s not tied to specific disease-causing mutations, its scientists said.

“This catalog [of cellular makeup in the airways] represents an important step toward identifying those rare stem cells that regenerate the airways over a person’s lifetime,” Gompert said. “We suspect these cells could be targets for future therapies that may provide a long-term correction to the mutation that causes this disease.”