THE IMPACT OF AGING AND MECHANICAL INJURY ON ALVEOLAR EPITHELIAL AND MACROPHAGE RESPONSES IN ACUTE LUNG INJURY AND INFLAMMATION

Patients with severe lung pathologies, such as Acute Respiratory Distress Syndrome (ARDS), often require mechanical ventilation as a clinical intervention; however, this procedure frequently exacerbates the original pulmonary issue and produces an exaggerated inflammatory response that potentially leads to sepsis, multisystem organ failure, and mortality. This acute lung injury (ALI) condition has been termed Ventilator-Induced Lung Injury (VILI). Alveolar overdistension, cyclic atelectasis, and biotrauma are the primary injury mechanisms in VILI that lead to the loss of alveolar barrier integrity and pulmonary inflammation. Stress and strains during mechanical ventilation are believed to initiate alveolar epithelial mechanotransduction signaling mechanisms that contribute to injury and repair responses and lead to the direct activation of resident lung and recruited macrophages. These types of cells, alveolar and interstitial macrophages, have various polarization states, such as M1 and M2, which are believed to play significant roles in tissue homeostasis and inflammatory regulation. Epidemiology studies have also suggested that age influences lung function and is a predictive factor in the severity of VILI; however, the mechanisms of aging that influence the progression or increased susceptibility of VILI in the elderly are still unknown. Aging also critically impacts immune system function and may increase inflammation in healthy individuals, which is known as inflammaging. Disruption to Endoplasmic Reticulum (ER) homeostasis results in a condition known as ER stress that leads to disruption of cellular homeostasis, apoptosis, and inflammation. ER stress is increased with aging and other pathological stimuli. We hypothesized that age and mechanical stretch increase alveolar epithelial cells’ pro-inflammatory responses that are mediated by ER stress. Furthermore, we hypothesized that inhibition of this upstream mechanism with 4PBA, an ER stress reducer, alleviates subsequent inflammation and monocyte recruitment. Type II alveolar epithelial cells (ATII) were harvested from C57Bl6/J mice 2 months (young) and 20 months (old) of age. The cells were cyclically stretched at 15% change in surface area for up to 24 hours. Prior to stretch, groups were administered 4PBA or vehicle as a control. Mechanical stretch and age upregulated ER stress and proinflammatory signaling expression in ATIIs. Age increases susceptibility to stretch-induced ER stress and downstream inflammatory gene expression in a primary ATII epithelial cell model. Administration of 4PBA attenuated the increased ER stress and proinflammatory responses from stretch and/or age and significantly reduced monocyte migration to ATII conditioned media. Recent studies also suggest a critical, protective role for the bioactive sphingolipid mediator sphingosine-1-phosphate (S1P) signaling in several lung pathologies and macrophage differentiation and function. It is unknown whether aging alters S1P signaling that appears involved in lung inflammation, injury, and apoptosis. We postulated that aging and injurious mechanical ventilation synergistically impair macrophage polarization in the lung that is associated with dysfunctional S1P signaling and produces amplified alveolar barrier damage and diminished pulmonary function. Young (2-3mo) and old (20-25mo) C57BL/6 mice were mechanically ventilated for 2 hours using pressure-controlled mechanical ventilation (PCMV). We assessed tissue mechanics, lung injury/repair responses, macrophage polarization, and S1P/S1PL lung activity. PCMV exacerbated lung injury in old mice. CD80 and CD206, classical and alternative macrophage markers, were both elevated in old alveolar and interstitial macrophages that also further increased due to PCMV. S1P lung levels were elevated in the young ventilated mice compared to the control group, which was not observed with the old mice. S1P lyase expression increased in the young and old ventilated mice and the old nonventilated group. 2-Acetyl-4- tetrahydroxybutyl Imidazole (THI) administration reduced indications of ALI in both young and old mice and altered macrophage polarization. The structural and cellular implications in injury responses of an aging lung more accurately represent clinical conditions and warrant further study at a cellular level. We found that aging significantly impacts alveolar epithelial and lung macrophage signaling and polarization; moreover, these aging disparities may result from elevated ER stress and/or a loss of protective S1P signaling in response to mechanical stretch that further contribute to the age-associated susceptibility and alveolar barrier dysfunction. Furthermore, administration of 4PBA, an ER stress inhibitor, and THI, an S1PL inhibitor, attenuated ER stress and S1PL activity, respectively, as well as several indications of ALI