Implications of polymorphonuclear neutrophils for ischemic stroke and intracerebral hemorrhage: Predictive value, pathophysiological consequences and utility as therapeutic target

Polymorphonuclear neutrophil granulocytes (PMN) orchestrate the removal of cell debris in ischemic stroke and intracerebral hemorrhage. In both pathologies, high neutrophil to lymphocyte ratios in peripheral blood are predictive of poor outcome in human stroke patients. Following earlier studies indicating that the cerebral microvasculature forms an efficient barrier that impedes neutrophil brain entry, intravital microscopy and immunohistochemistry in the meantime unequivocally revealed the accumulation of PMN in the ischemic and hemorrhagic brain parenchyma. These studies provide definite evidence that PMN contribute to the degradation of the blood-brain barrier, predisposing the brain to secondary injury, edema, hemorrhage formation, hemorrhage growth and poor neurological recovery. Recent studies demonstrated the role of pro-inflammatory N1 neutrophils in brain edema and neurotoxicity, whereas anti-inflammatory N2 neutrophils were found to limit this excessive immune response, promoting neuronal survival and successful brain remodeling. In view of the recent failure of anti-inflammatory immunotherapies in clinical trials, strategies specifically modulating the brain accumulation, differentiation and action of PMN may open promising perspectives for stroke treatment.