Dual effects of supplemental oxygen on pulmonary infection, inflammatory lung injury, and neuromodulation in aging and COVID-19.

Clinical studies have shown a significant positive correlation between age and the likelihood of being infected with SARS-CoV-2. This increased susceptibility is positively correlated with chronic inflammation and compromised neurocognitive functions. Postmortem analyses suggest that acute lung injury (ALI)/acute respiratory distress syndrome (ARDS), with systemic and lung hyperinflammation, can cause significant morbidity and mortality in COVID-19 patients. Supraphysiological supplemental oxygen, also known as hyperoxia, is commonly used to treat decreased blood oxygen saturation in COVID-19 patients. However, prolonged exposure to hyperoxia alone can cause oxygen toxicity, due to an excessive increase in the levels of reactive oxygen species (ROS), which can overwhelm the cellular antioxidant capacity. Subsequently, this causes oxidative cellular damage and increased levels of aging biomarkers, such as telomere shortening and inflammaging. The oxidative stress in the lungs and brain can compromise innate immunity, resulting in an increased susceptibility to secondary lung infections, impaired neurocognitive functions, and dysregulated hyperinflammation, which can lead to ALI/ARDS, and even death. Studies indicate that lung inflammation is regulated by the central nervous system, notably, the cholinergic anti-inflammatory pathway (CAIP), which is innervated by the vagus nerve and α7 nicotinic acetylcholine receptors (α7nAChRs) on lung cells, particularly lung macrophages. The activation of α7nAChRs attenuates oxygen toxicity in the lungs and improves clinical outcomes by restoring hyperoxia-compromised innate immunity. Mechanistically, α7nAChR agonist (e.g., GAT 107 and GTS-21) can regulate redox signaling by 1) activating Nrf2, a master regulator of the antioxidant response and a cytoprotective defense system, which can decrease cellular damage caused by ROS and 2) inhibiting the activation of the NF-κB-mediated inflammatory response. Notably, GTS-21 has been shown to be safe and it improves neurocognitive functions in humans. Therefore, targeting the α7nAChR may represent a viable therapeutic approach for attenuating dysregulated hyperinflammation-mediated ARDS and sepsis in COVID-19 patients receiving prolonged oxygen therapy.