Sentinelp16INK4a+ cells in the basement membrane form a reparative niche in the lung

Senescent cells are recognized drivers of aging-related decline in organ function, but deciphering the biology of senescence in vivo has been hindered by the paucity of tools to track and isolate senescent cells in tissues 1–4 . Deleting senescent cells from transgenic murine models have demonstrated therapeutic benefits in numerous age-related diseases 5–11 , but the identity, behavior, and function of the senescent cells deleted in vivo remain elusive. We engineered an ultra-sensitive reporter of p16 INK4a , a biomarker of senescence 12 , to isolate and track p16 INK4a + cells in vivo . Surprisingly, p16 INK4a + mesenchymal cells appear in the basement membrane adjacent to epithelial progenitors in the lung shortly after birth, and these cells demonstrate senescent characteristics in vivo and ex vivo . Transcriptomic analysis of p16 INK4a + mesenchymal cells from non-aged lungs demonstrates a transition to a secretory phenotype upon airway epithelial injury. Heterotypic 3D organoid assays show that injured p16 INK4a + mesenchymal cells enhance epithelial progenitor proliferation, and we identified EREG as a novel airway progenitor mitogen produced by the secretory p16 INK4a + mesenchymal cells. Mesenchymal-specific deletion of the p16 INK4a gene abrogates features of senescence in vivo , but also attenuates normal epithelial repair. Thus, p16 INK4a + mesenchymal cells can act as sentinels for the airway epithelial stem cell niche, poised to transition to a senescence-associated secretory phenotype to support barrier repair. Our data identify possible cellular targets in vivo for a rapidly growing list of senolytic therapies, but also raises important questions about the hidden cost of targeting senescent cells present in normal organs.