Involvement of Neutrophils in the Pathogenesis of Lethal Myocardial Reperfusion Injury

Neutrophils respond to myocardial ischemia-reperfusion in a manner similar to the bacterial invasion of a host. The inflammatory-like response that follows the onset of reperfusion involves intense interactions with the coronary vascular endothelium, arterial wall, and cardiomyocytes in a very well-choreographed manner. Neutrophils have been implicated as primary and secondary mediators of lethal injury after reperfusion to coronary vascular endothelium and cardiomyocytes. The involvement of neutrophils in the pathogenesis of lethal myocardial injury has been inferred from (1) their presence and accumulation in reperfused myocardium in temporal agreement with injury induced, (2) the armamentarium of toxic agents such as oxidants and proteases that are released by neutrophils in reperfused myocardium, (3) responsivity to (recruitment by and/or activation by) inflammatory factors released by reperfused myocardium, and (4) inhibition of lethal post-ischemic myocyte or endothelial cell injury by strategies that interdict neutrophil interactions at any number of stages. However, whether neutrophils are directly involved in the pathogenesis of lethal reperfusion injury in the myocardium, are just pedestrian (first) responders to inflammatory signals released after the onset of reperfusion, or are important to an early but not clinically important phase of pathology are still points of controversy. As with the general area of myocardial protection itself, the failure to reproduce the salubrious effects of anti-neutrophil therapeutic strategies and to successfully translate these strategies into clinical practice has not only fueled the debate, but has jeopardized the further pursuit of myocardial protection therapeutics to improve post-ischemic outcomes. This review will describe the molecular responses of neutrophils to ischemia-reperfusion, discuss the cellular and tissue damage inflicted either directly or indirectly by these white cells, and discuss the physiological impact of interdiction of neutrophil-mediated interactions with myocardial cells at various levels on lethal post-ischemic injury. In addition, it will discuss the arguments for and against the involvement of neutrophils in responses to ischemia-reperfusion in experimental models, and the failure to translate experimentally successful therapy into clinical practice.