title: Neutrophil derived enzymes in acute ST elevation myocardial infarction
Creators: name: Marshall, Catriona J.
Publisher: name: University of Glasgow
Year: 2011
Subjects: subject: STEMI, Neutrophils, Myeloperoxidase, Leukocyte Elastase

description: Acute ST segment elevation myocardial infarction (STEMI) is the most life threatening presentation in patients with coronary heart disease (CHD). This typically occurs due to rupture of an unstable atherosclerotic plaque with thrombus formation, occlusion or partial occlusion of an epicardial vessel and the onset of myocardial ischaemia. The well established treatment of choice is early primary percutaneous intervention (PCI) to restore culprit coronary artery flow, halt ischaemia and prevent myocardial necrosis.

Historically, discussions of the inflammatory processes around the time of coronary plaque disruption and thrombotic occlusion have centred on intra-plaque macrophages and T lymphocytes. Although neutrophils are widely recognised as important mediators of myocardial and vascular injury following ischaemic reperfusion injury much less is currently known of their role prior to recannalization of the culprit vessel.

Previous studies have shown that neutrophils are activated systemically in unstable angina and myocardial infarction. It has also been suggested that in patients with unstable angina a trans-cardiac gradient exists, with up-regulation of neutrophil activation from the aorta to coronary sinus. The concept of widespread inflammation across the coronary vasculature independent of culprit lesion location has been considered, and neutrophil infiltrates have been found in culprit plaque specimens from patients who have died following acute myocardial infarction (MI).

Myeloperoxidase (MPO) is a heme enzyme, and leukocyte elastase (LE) a serine protease contained within the azurophilic granules of neutrophils. Upon neutrophil activation and degranulation these enzymes are released into the plasma. MPO has been considered a surrogate marker of neutrophil activation and plasma levels of the enzyme found to be elevated in patients with coronary heart disease (CHD) and acute coronary syndromes (ACS). Plasma MPO levels have also been investigated in
patients presenting with troponin negative ACS and found to be higher than controls suggesting the early activation of neutrophils. MPO has several potentially deletrious effects including the ability to catalyse the production of several reactive oxidant species such as hypochlorus acid which can chlorinate both proteins and lipids. It also consumes nitric oxide affecting endothelial function leading to vasoconstriction. Leukocyte elastase is associated with extracellular matrix breakdown and up-regulation of cytokines and metalloproteinases. Elastase has been found within atherosclerotic plaques and through its actions in matrix degradation has been associated with plaque rupture.

Activation of neutrophils at the culprit coronary lesion following acute plaque disruption in the very early stages of STEMI has not previously been reported. This thesis details a programme of original work exploring three different aspects of neutrophil activation. The first section covers the neutrophil derived enzyme MPO and explores the factors influencing systemic and local levels of the enzyme, at the culprit coronary lesion in patients with acute STEMI. The second section investigates neutrophil activation through intracellular degranulation and release of MPO using flow cytometry, and surface expression of the adhesion molecule CD11b. Finally plasma levels of LE, an alternative marker of neutrophil degranulation and one which is not affected by the delivery of unfractionated heparin are explored. The central hypothesis was that the neutrophil derived enzymes have an early role in events subsequent to plaque instability and prior to mechanical reperfusion therapy.

STEMI patients presenting to the emergency department at Christchurch Hospital who were eligible for treatment with primary PCI according to standard criteria were approached to participate. Exclusion criteria included left main stem disease, cardiogenic shock, stent thrombosis and rescue PCI post thrombolysis. Patients with class II-III angina symptoms and documented stable coronary artery disease undergoing elective, single vessel PCI were studied as a control. Blood was sampled peripherally from the femoral artery (FA) sheath prior to the delivery of weight adjusted heparin. Further sampling was carried out prior to angioplasty and stenting in the ascending aorta (Ao) at the coronary ostium from the guiding catheter and from the coronary sinus (CS) using a separate catheter introduced via the femoral vein. The culprit coronary lesion was sampled prior to PCI using a low profile, minimally disruptive sampling catheter. Following PCI, with the exception of the FA, blood was drawn from the aforementioned sites again, and at 24 hours after presentation from a forearm vein. Whole blood flow cytometry was performed immediately to determine the intracellular concentration of neutrophil MPO and the expression of the surface adhesion molecule CD11b. Plasma MPO and LE were measured using standardised enzyme linked immune-sorbent assays (ELISA).

We found that MPO levels at the FA were higher in STEMI patients compared to elective PCI controls. The delivery of unfractionated heparin but not bivalirudin lead to a marked increase in MPO levels at all sampling sites in both groups. Despite this we showed a further rise in MPO at the culprit coronary lesion in STEMI prior to PCI. MPO levels were higher in patients who presented earlier, and in those with baseline Thrombolysis in Myocardial Infarction (TIMI) 0-1 flow in the culprit vessel. These results were supported by the finding of increased neutrophil degranulation, with release of intracellular MPO at all sampling sites in STEMIs versus controls. A further subtle reduction in neutrophil intracellular MPO content in STEMI patients at the culprit lesion pre PCI and at the coronary ostium in the aortic root was demonstrated. In keeping with the MPO results, we found that LE was elevated in STEMI patients compared to controls and was highest locally at the culprit lesion pre PCI. The increase in local plasma elastase was seen predominantly in patients with TIMI 0-1 flow in the culprit vessel. Prior to PCI we showed a subtle gradient across the coronary vasculature from the aorta to the coronary sinus which was not present following re-instatement of coronary flow.

The studies contained within this thesis support recent suggestions that neutrophil activation is an early event in the pathophysiology of acute myocardial infarction, in this particular setting ST elevation MI, and can be detected prior to treatment in its current best form. Our results suggest that in addition to a systemic release of MPO
in patients presenting with acute STEMI, plasma levels of this neutrophil derived enzyme are higher at the culprit lesion in the coronary artery prior to primary PCI. We have also demonstrated that plasma levels of neutrophil enzymes at the culprit lesion are influenced by baseline TIMI flow in the coronary artery. Additionally we have confirmed that heparin, yet not bivalirudin causes an elevation in detectable MPO in the plasma and suggest that future work aimed at probing associations between MPO in ACS should acknowledge this.