Telomere damage-mediated senescence in alveolar epithelial type II cells but not in macrophages aggravates inflammation in acute lung injury

Severity of acute lung injury (ALI) increases with age and is associated with enhanced inflammation in older individuals (inflamm-aging), characterized by increased expression of pro-inflammatory cytokines. Telomere damage can cause irreversible cell cycle arrest, known as cellular senescence, that may promote pro-inflammatory signaling and exacerbate ALI. Therefore, we investigated the role of telomere damage-mediated senescence in alveolar epithelial type II (AEII) cells and macrophages in injury and inflammation of lipopolysaccharide (LPS)-induced ALI in mice. Cell type-specific deletion of Trf1 (telomeric repeat-binding factor 1) was achieved using tamoxifen-inducible Cre-loxP mouse models driven by the Sftpc (surfactant protein c) promoter for AEII cells or Lyz2 (lysozyme 2) promoter for macrophages to induce telomere damage in respective cell types. Mice without Trf1 deletion were used for comparison, as well as naturally aged mice. After tamoxifen injections, ALI was induced by intranasal application of 2.5 mg LPS/kg body weight. Control mice received saline. The severity of ALI, as well as cell type-specific molecular responses, were assessed using functional, immunological, histological and transcriptomic investigations. Telomere damage-mediated senescence in AEII cells resulted in an increase in inflammatory BALF cells and cytokines and a decline in lung function 72 h after LPS exposure, compared to mice without Trf1 deletion in AEII cells. Transcriptomic analysis revealed that Trf1-deleted AEII cells exhibited specific cell cycle inhibition, as well as an elevated DNA damage response (DDR), in both control and LPS-treated Sftpc-Cre mice. Trf1 deletion in macrophages triggered an augmented DDR without affecting LPS-driven lung inflammation compared to mice without Trf1 deletion. Control mice with Trf1 deletion of both Cre-loxP mouse models did not show signs of inflamm-aging. Additional comparison with old mice revealed that global aging causes a much greater severity in ALI than Trf1 deletion in lung cells. The findings demonstrate that telomere damage-mediated senescence in AEII cells promotes inflammation in the late exudative phase of ALI, while Trf1 deletion in macrophages did not cause an aggravation of ALI. The results also suggest that additional mechanisms of aging beyond pulmonary telomere damage and cellular senescence, such as global inflamm-aging, contribute to the severity of ALI in old age.