Introduction Aside from their patrolling behaviour, the early functions of CX3CR1hi non-classical (NC) monocytes following acute myocardial ischemia-reperfusion (I/R) are largely unknown. NC monocyte behaviour is orchestrated in part by their high expression of the fractalkine receptor, CX3CR1, however the signalling pathways activated by fractalkine stimulation remain largely undefined. Increasing evidence has now implicated the CX3CR1/CX3CL1 system in myocardial I/R injury, which conclusively suggest that fractalkine has a detrimental effect on myocardial ischemia and accelerates the progress of heart failure. Neutralization of the receptor is successful in improving cardiac function in a mouse model of MI, and STEMI patient CX3CL1 concentration is a prognostic marker following MI. The first aim of this project was therefore to characterise circulating CX3CR1hi NC monocyte dynamics in ST elevation MI (STEMI) patients immediately following reperfusion, and evaluate their prognostic value. Using a mouse model of cardiac I/R, we evaluated the effect of genetic CX3CR1 knockout on non-classical monocyte recruitment into the injured myocardium at 2h and 24h post-I/R. Since contemporary efforts to promote myocardial repair have failed to translate into clinical therapies thus far, we hypothesize that CX3CR1 may be a prime candidate to target myocardial repair following I/R.

Method Flow Cytometry (FC) quantified monocyte subpopulations in the blood of 59 STEMI patients pre and post PPCI. Infarct size and microvascular obstruction (MVO) were assessed using cardiac MRI. Monocytes were quantified in a second study of 14 STEMI patients. A mouse model of cardiac I/R injury was used to study monocyte infiltration following MI. The recruitment of classical monocytes [CD11b+ GFPmid CCR2hi] and NC monocytes [CD11b+ GFPhi CCR2lo] into the injured myocardium at 2h and 24h post-IR was studied by immunofluorescence in CX3CR1+/GFP CX3CR1GFP/GFP mice. FC quantified absolute monocyte subset counts in mouse blood at 2h and 24h post-IR.

Results STEMI patients showed a 46%(±4) drop in circulating CX3CR1hi CD16++ NC monocytes (p<0.0001) at pre-90 min post-reperfusion. This was not observed for classical and intermediate monocytes (7% and 20%, respectively). STEMI patients with larger infarcts and reduced LVEF post-MI showed a significantly greater decline (p=0.039 & p=0.043, respectively) in CX3CR1hi CD16++ NC monocytes. These findings were confirmed by enhanced monocyte subset quantification in a second STEMI cohort (52%(±6) NC drop, p=0.0008). Intra-myocardial infiltration of NC monocytes was determined in a CX3CR1+/GFP mouse model of cardiac I/R. At 24h post-I/R, NC counts significantly increased from 2h by approximately 5-fold (p<0.0001). In CX3CR1GFP/GFP mice, NC monocyte infiltration was 2-fold lower compared to CX3CR1+/GFP mice, suggesting that NC monocytes may rely partly on the fractalkine receptor for myocardial infiltration post-I/R. In the circulation, there was a 70% reduction in NC monocytes from baseline to 2h post-I/R, suggesting that NC mono may originate from the marginating pool of monocytes in the peripheral blood.

Conclusion & Future Work: NC monocytes appear to specifically respond to cardiac I/R, and may be predictive of cardiac function following STEMI. The 2-fold reduction in CX3CR1hi LY6Clo NC monocyte infiltration in CX3CR1GFP/GFP mice suggests that CX3CR1 has an important function in the NC monocyte response to cardiac I/R.

Conflict of Interest non