Acute lung injury (ALI) is characterized by hypoxemia, enhanced permeability of the air-blood barrier, and pulmonary edema. Particularly in the elderly, ALI is associated with increased morbidity and mortality. The reasons for this, however, are poorly understood. We hypothesized that age-related changes in pulmonary structure, function, and inflammation lead to a worse prognosis in ALI. ALI was induced in young (10 wk old) and old (18 mo old) male C57BL/6 mice by intranasal application of 2.5 mg lipopolysaccharide (LPS)/kg body wt or saline (control mice). After 24 h, lung function was assessed, and lungs were either processed for stereological or inflammatory analysis, such as bronchoalveolar lavage fluid (BALF) cytometry and qPCR. Both young and old mice developed severe signs of ALI, including alveolar and septal edema and enhanced inflammatory BALF cells. However, the pathology of ALI was more pronounced in old compared with young mice with nearly sixfold higher BALF protein concentration, twice the number of neutrophils, and significantly higher expression of neutrophil chemokine Cxcl1, adhesion molecule Icam-1, and metalloprotease-9, whereas the expression of tight junction protein occludin significantly decreased. The old LPS mice had thicker alveolar septa attributable to higher volumes of interstitial cells and extracellular matrix. Tissue resistance and elastance reflected observed changes at the ultrastructural level in the lung parenchyma in ALI of young and old mice. In summary, the pathology of ALI with advanced age in mice is characterized by a greater neutrophilic inflammation, leakier air-blood barrier, and altered lung function, which is in line with findings in elderly patients.

Acute lung injury (ALI) is associated with increased morbidity and mortality in the elderly (> 65 years), but the knowledge about origin and effects of immunosenescence in ALI is limited. Here, we investigated the immune response at pulmonary, systemic and cellular level in young (2-3 months) and old (18-19 months) C57BL/6J mice to localize and characterize effects of immunosenescence in ALI. ALI was induced by intranasal lipopolysaccharide (LPS) application and the animals were sacrificed 24 or 72 h later. Pulmonary inflammation was investigated by analyzing histopathology, bronchoalveolar lavage fluid (BALF) cytometry and cytokine expression. Systemic serum cytokine expression, spleen lymphocyte populations and the gut microbiome were analyzed, as well as activation of alveolar and bone marrow derived macrophages (BMDM) in vitro. Pulmonary pathology of ALI was more severe in old compared with young mice. Old mice showed significantly more inflammatory cells and pro-inflammatory cyto- or chemokines (TNFα, IL-6, MCP-1, CXCL1, MIP-1α) in the BALF, but a delayed expression of cytokines associated with activation of adaptive immunity and microbial elimination (IL-12 and IFNγ). Alveolar macrophages, but not BMDM, of old mice showed greater activation after in vivo and in vitro stimulation with LPS. No systemic enhanced pro-inflammatory cytokine response was detected in old animals after LPS exposure, but a delayed expression of IL-12 and IFNγ. Furthermore, old mice had less CD8+ T-cells and NK cells and more regulatory T-cells in the spleen compared with young mice and a distinct gut microbiome structure. The results of our study show an increased alveolar macrophage activation and pro-inflammatory signaling in the lungs, but not systemically, suggesting a key role of senescent alveolar macrophages in ALI. A decrease in stimulators of adaptive immunity with advancing age might further promote the susceptibility to a worse prognosis in ALI in elderly.

No abstract

Pulmonary alveolar septa are thought to contain at least two types of fibroblasts that are termed myofibroblasts and lipofibroblasts based on their morphological characteristics. Lipofibroblasts possess cytoplasmic lipid inclusions (lipid bodies or droplets) and are involved in several important functions, such as surfactant synthesis, development, vitamin A storage and presumably regeneration. As vitamin A was shown to reduce pulmonary emphysema in several but not all mouse and rat strains, we hypothesized that these strain differences might be explained by a differential occurrence of lipofibroblasts and their lipid bodies in various mouse strains. Therefore, mouse lungs of six strains (NMRI, BALB/c, C3H/HeJ, C57BL/6J, C57BL/6N and FVB/N) were investigated by light and electron microscopic stereology to quantify the amount of lipid bodies and the composition of alveolar septa. Lipofibroblasts were observed qualitatively by transmission electron microscopy in every investigated mouse strain. The total volume and the volume-weighted mean volume of lipid bodies were similar in all mouse strains. The results on the composition of the interalveolar septa did not show major differences between the groups. The only mouse strain that differed significantly from the other strains was the NMRI strain because the lungs had a higher volume and consequently many of the morphological parameters were also larger than in the other groups. In conclusion, the present study showed that lipofibroblasts are a common cell type in the mouse lung across various strains. Therefore, the mere presence or absence of lipofibroblasts does not explain differences in the pulmonary regenerative potential among mouse strains.