Detection, assessment and modulation of myocardial inflammation
Alam, Syed Shirjel Rizwan
Coronary atherosclerosis and plaque rupture leads to acute coronary thrombosis and myocardial infarction. Current treatment involves re-establishing vessel patency, but no treatments have been developed to target post-infarction inflammatory pathways. Such treatments may reduce cardiomyocyte injury, attenuate adverse remodelling and improve clinical outcome. Inflammation within the infarcted myocardium is associated with chemotaxis of neutrophils and monocytes to the site of injury. Early reperfusion therapy amplifies this inflammatory cell influx. Neutrophil release a variety of pro-inflammatory factors, including human neutrophil elastase (HNE). HNE has a wide range of substrates. Preclinical studies have demonstrated that neutrophil depletion or inhibition of neutrophil elastase attenuates post-ischemic inflammatory reperfusion injury within the myocardium. Recruitment of monocytes into the infarcted myocardium is followed by maturation and differentiation into macrophages. Macrophages play a key role in orchestrating inflammation and repair. Therapeutic manipulation of this healing process will only come from understanding mechanisms and targeting reparative pathways. “Ultrasmall superparamagnetic iron oxide particles” (USPIOs) extravasate through capillaries and are phagocytosed by tissue inflammatory cells. These cells are predominately macrophages, but neutrophils have also been shown to take up USPIOs. USPIO-enhanced MRI can identify areas of inflammation in models inflammation in various tissues. Therefore we hypothesised that USPIO enhanced MRI could identify and assess cellular inflammation of the myocardium. During coronary artery bypass graft surgery (CABG), the myocardium receives an immediate ischaemic insult that is exacerbated by post-ischaemic reperfusion inflammatory responses leading to increased myocardial injury. CABG surgery can therefore be used as a clinical model of myocardial infarction and inflammation. We investigated this with blood markers of inflammation, MRI scanning and USPIO. Elafin inhibits the destructive and inflammatory HNE enzyme. Beyond this elafin inhibits inflammatory cytokines and modulates the innate and adaptive immune systems. In preclinical studies elafin treatment is associated with reduced myocardial injury. As such, elafin has a marked potential for the treatment of cardiovascular disease involving inflammation. Therefore, we hypothesised that elafin will reduce perioperative ischaemic myocardial injury and inflammation in patients undergoing elective coronary artery bypass graft surgery. We demonstrated for the first time that USPIOs are taken up by the infarct tissue in patients with recent myocardial infarction and by the peri-infarct myocardium to a lesser degree. This represents a novel non-invasive method to further study cardiac inflammation and therapeutic interventions. All patients undergoing CABG surgery demonstrated >10-fold elevation above the 99th centile of cardiac troponin by high sensitivity assay (hs-cTnI) indicating the current universal definition of type 5 myocardial infarction lacks specificity. A peak hs-cTnI at 6 hours following CABG surgery appears to be related to the surgical process and non-specific myocardial injury whilst a continuing increase at 24 hours suggests myocardial infarction. We would suggest hs-cTnI sampling at 6 and 24 hours post CABG surgery together with ECG assessment for the routine detection and diagnosis of type 5 MI. Differing levels of humoral makers inflammation post CABG surgery occurred, and did not correlate directly with the length of cardiopulmonary bypass time or hs-cTnI release. For the first time we identified differing levels of inflammatory cell infiltrate into the myocardium post CABG. This varied from none to levels similar to infarcted myocardial tissues. Elafin did not attenuate myocardial ischemia-reperfusion injury and inflammation. Post-hoc analysis identified reduced cTnI concentrations at 6 hours in Elafin treated patients and it is possible that a bigger dose would have conferred protection out to 48 hours. Elafin did not attenuate the cellular infiltration into the myocardium post CABG surgery, but did appear to reduce inflammation in renal tissue. USPIO enhanced CMR holds major promise in the non-invasive assessment of myocardial inflammation post surgery.