The Role of Non-Classical Monocytes and the CX3CL1/CX3CR1 Axis in the Immune Response to Myocardial Ischemia-Reperfusion

Abstract

Myocardial Infarction is a leading cause of morbidity and mortality worldwide. Over the past decade, STEMI patient prognosis has improved in line with the increased clinical use of reperfusion therapy by primary percutaneous coronary intervention. Despite advances in treatment however, the mortality rate following STEMI remains up to 43.1%, due to a range of short and long term complications. Novel treatment options are therefore required to improve outcome following MI. In the immune response to MI, monocytes are rapidly recruited to the injured tissue where they contribute to the inflammatory milieu. While the function of classical monocytes in this response has been well described, the role of the non-classical monocyte subset is poorly defined. These cells express high levels of the fractalkine receptor, CX3CR1, which has increasingly been implicated in cardiac ischemia-reperfusion (I/R) injury. Expression of the fractalkine ligand, CX3CL1, also known as Fractalkine (FKN) is elevated in mouse models of MI, and neutralization of the ligand leads to improved survival. Similarly, in MI and heart failure patients, plasma levels of the soluble form of CX3CL1 are increased. The aim of this project was therefore to investigate non-classical monocyte behaviour and function following cardiac I/R, with relation to CX3CR1 expression, in both STEMI patients and a mouse model of cardiac I/R. The research presented in this thesis describes the distinct dynamics of CX3CR1hi non-classical monocytes in STEMI patients immediately following reperfusion. These cells show an increased depletion in patients with larger infarcts and lower left ventricular ejection fraction, thereby acting as a potential prognostic biomarker of myocardial outcome. In complementary pre-clinical mouse studies of cardiac I/R, this project established that monocyte adherence in the coronary vessel endothelium occurs over a post-reperfusion timeframe which corresponds approximately to the 90 min post-reperfusion drop in circulating non-classical monocytes in STEMI patients. Investigation of CX3CR1 function using a Cx3cr1 knockout mouse demonstrated that infarct-triggered monocyte recruitment and later decrease in myocardial function are not significantly affected by loss of the Cx3cr1 gene. Research into the signalling pathways downstream of CX3CR1 activation identified the pro-inflammatory NFkB pathway as a possible contributor to non-classical monocyte function. Based on this study, the CX3CL1/CX3CR1 axis appears to have an important role in mediating the acute post-reperfusion dynamics and function of circulating non-classical monocytes following human cardiac I/R. Unexpectedly, however, loss of Cx3cr1 in pre-clinical mouse models does not appear to compromise intra-myocardial monocyte recruitment or heart function following cardiac I/R.