Dysregulated alveolar epithelial maintenance in Hermansky-Pudlak syndrome pulmonary fibrosis

Hermansky-Pudlak syndrome (HPS) is a genetic disorder of intracellular endosomal trafficking defects associated with pulmonary fibrosis. Double mutant HPS1/2 mice exhibit spontaneous fibrosis and single mutant HPS1 and HPS2 mice display increased fibrotic sensitivity. To identify mechanisms of AT2 cell dysfunction contributing to fibrosis in HPS, we examined lungs from aged HPS1/2 mice and observed regions of AT2 cell loss adjacent to areas of marked AT2 cell hyperplasia as compared to wild type (WT). Overall, there was accelerated loss of AT2 cells in HPS1/2, HPS1, and HPS2 mice with aging based on immunohistochemistry and lineage tracing studies. Lung organoids generated with HPS2 AT2 cells with WT fibroblasts were smaller in size and displayed significantly decreased colony forming efficiency compared to organoids with WT AT2 cells. Utilizing an H1N1 PR8 influenza model to examine AT2 cell regeneration after injury, there was a significant decrease in the percentage of proliferating AT2 cells in HPS1 and HPS2 mice compared to WT mice. RNA sequencing of AT2 cells isolated from unchallenged mice demonstrated upregulation of genes associated with proliferation and cellular senescence in HPS AT2 cells. Collectively, AT2 cells in HPS mice exhibit dysregulated proliferation with transcriptomic features suggesting subpopulations of senescent and hyperproliferative cells. Dysregulated maintenance of the alveolar epithelium with accelerated senescence and aberrant proliferation of AT2 cells appears to be a shared pathway underlying HPS and other sporadic and genetic forms of pulmonary fibrosis.