"Stuck in the Middle with You": intermediate cell states are not always in transition

Area of Science:

• Pulmonary Medicine
• Cell Biology
• Genomics

Background:

• Single-cell multiomics has revealed lung epithelial cells with mixed alveolar type 1 (AT1) and alveolar type 2 (AT2) pneumocyte features.
• These cells have been widely interpreted as a transitional cell type facilitating AT2 to AT1 cell differentiation.

Purpose of the Study:

• To investigate the functional role and developmental trajectory of cells exhibiting intermediate AT1/AT2 pneumocyte characteristics.
• To re-evaluate the classification of these cells beyond a simple transitional state.

Main Methods:

• Single-cell multiomics analysis
• Functional assays to assess contribution to lung repair
• Gene expression profiling

Main Results:

• Many cells identified as "transitional" do not actively participate in functional lung repair.
• These cells exist in a nondirectional, intermediate cell state rather than a transient developmental phase.
• Findings suggest a re-evaluation of cell state definitions in lung epithelial differentiation.

Conclusions:

• The concept of a distinct transitional AT2-to-AT1 cell type may be inaccurate.
• Lung epithelial cells with mixed AT1/AT2 features represent a stable intermediate state.
• Further research should focus on diverse cell state profiles and specific gene functions in lung repair.