Role of ATII cell senescence in influenza pathogenesis in aging

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Alveolar type II (ATII) cells synthesize, secrete, and recycle surfactant proteins and lipids and regulate alveolar lining fluid depth by alveolar fluid clearance. Because both processes require large amounts of energy, ATII cells contain large numbers of mitochondria and mainly generate ATP by oxidative phosphorylation (OXPHOS). ATII cells are the primary site for influenza A virus (IAV) replication in the distal lung and central players in the pathogenesis of IAV-induced ARDS. Importantly, the elderly are over-represented in influenza-related fatalities. However, there is limited understanding of the impact of either aging or IAV infection on ATII cell function, senescence, and energy metabolism. ATII cells isolated from lungs of young (2-3 month-old) C57BL/6 mice primarily generate ATP by OXPHOS. In contrast, preliminary studies show ATII cells from aging (27 month-old) mice undergo a glycolytic shift and downregulate OXPHOS, possibly as a result of senescence. IAV infection of young mice causes a glycolytic shift and a decrease in OXPHOS which is reversed by CDP- choline treatment, resulting in a net increase in total ATP production, attenuated hypoxemia, and reduced pulmonary inflammation. IAV infection of aging mice causes more severe hypoxemia and further reduces OXPHOS without any compensatory increase in glycolysis, resulting in a net decrease in total ATP production despite the increased energy demands imposed by viral replication. Hence, it is hypothesized that influenza is more severe in the elderly because IAV infection imposes additional energetic demands for viral replication on ATII cells that lack inherent metabolic flexibility due to aging-associated senescence. By inhibiting de novo phospholipid synthesis, IAV also induces further mt dysfunction. Together, these effects provoke an energy crisis and render ATII cells unable to perform their normal physiologic functions (alveolar fluid clearance and surfactant synthesis), which results in progression to ARDS. It is further proposed that CDP- choline treatment improves OXPHOS in ATII cells and increases their functional capacity, thereby improving influenza outcomes. This hypothesis will be tested in two Specific Aims. Aim 1 will use a robust, reproducible, and relevant model of IAV-induced ARDS in 21-24 month-old C57BL/6 and BALB/c mice to define effects of aging, IAV infection, and CDP- choline treatment on ATII cell physiologic functions (surfactant production and alveolar fluid clearance), whole body metabolism (by open circuit calorimetry), lung glucose uptake (by PET/CT), lung inflammation, and viral replication. Aim 2 will use a comprehensive battery of flow cytometric assays and Western blot to quantify the level of senescence in ATII cells isolated from the lungs of mock- and IAV-infected young and aging mice and will analyze the impact of aging, IAV infection, and CDP-choline treatment on ATII cell energetics by extracellular flux analysis. Proposed experiments will provide novel mechanistic insights into the contribution of ATII cell dysfunction and senescence to development of more severe influenza in the elderly and will generate fundamental new information relevant to many pulmonary diseases of aging, such as COPD, IPF, and non-small cell lung cancer.