Histone neutralization protects the ischemic brain against the consequences of stroke-associated pneumonia

Bacterial pneumonia aggravates ischemic stroke via mechanisms that still remain to be determined. In ischemic stroke patients, we show that stroke-associated pneumonia is associated with poor clinical outcome and long-term neutrophil deregulation. In mice exposed to transient middle cerebral artery occlusion (MCAO), experimental S. pneumoniae pneumonia induced at 3 days after stroke impaired neurological recovery and increased blood-brain barrier breakdown, brain neutrophil infiltrates, cerebral microvascular thrombosis, and progressive brain atrophy. The antibiotic amoxicillin only partially ameliorated pneumonia-associated neurological deficits and neutrophil infiltrates. Proteome analysis revealed that pneumonia induced a degranulation, platelet activation, and NETosis signature in peripheral blood neutrophils of MCAO mice, which was partly reversed by amoxicillin treatment. Subsequent studies showed that neutrophils were critical mediators of pneumonia-associated blood-brain barrier breakdown and microvascular thrombosis. Notably, administration of an antibody neutralizing extracellular histone proteins during pneumonia reduced ischemic injury, restored neurological recovery and prevented long-term brain atrophy in stroke-associated pneumonia mice. These protective effects were not observed after degradation of neutrophil extracellular traps by DNase-I, blockade of neutrophil extracellular trap formation by LDC7559 or myeloperoxidase inhibition by AZD4831, when these treatments were administered on occasion of pneumonia. This study identifies extracellular histones as key drivers of inflammatory brain injury and establishes histone neutralization as a strategy that protects against the deleterious effects of pneumonia after stroke.