Consequences of experimentally induced Streptococcus pneumoniae pneumonia for post-ischemic neurological recovery, ischemic injury and brain remodeling

Pneumonia is one of the most common complications following ischemic stroke. It is widely recognized that stroke-associated pneumonia significantly worsens outcomes of stroke patients. However, the underlying mechanisms of its influence remain unclear. In this study male C57Bl6/j mice underwent transient middle cerebral artery occlusion (MCAO). Three days after MCAO, mice received an intratracheal injection of bacterial suspension, containing approximately 1 x 108 colony-forming units (CFUs) of Streptococcus pneumoniae to induce pneumonia. Pneumonia significantly deteriorated neurological deficits and rotarod test performance, exacerbated brain edema and atrophy, enhanced IgG extravasation, inflammation, and microthrombosis within the infarct, and increased the neutrophil number in blood. Amoxicillin was used to treat pneumonia. Our studies showed that amoxicillin treatment attenuated the deleterious impacts of pneumonia on stroke recovery, notably diminishing the incidence of thrombi within the infarct and reducing neutrophil counts in the blood. Considering that microthrombosis is associated to neutrophils, neutrophil depletion was conducted. Neutrophil depletion more effectively improved stroke outcome than amoxicillin treatment. We also assessed the effect of DNase I and the gasdermin D inhibitor LDC7559 to prevent the formation of neutrophil extracellular traps (NETs), which are released by activated neutrophils and associated with thrombosis. DNase I treatment failed to mitigate the adverse effects of post-stroke pneumonia in the acute phase, but increased the risk of hemorrhagic transformation. LDC7559 treatment successfully reduced NET formation, microvascular thrombus, and brain edema in mice with post-stroke pneumonia. In conclusion, post-stroke pneumonia impaired the recovery from stroke, potentially through the increase of circulating neutrophils and microthrombus formation. According to our studies, gasdermin D inhibition by LDC7559 might represent a promising strategy for the treatment of stroke-associated pneumonia.