Aims/Objectives:  Treatment of myocardial infarction (MI) focuses on the rapid re-establishment of regional myocardial perfusion following a blockage in one or more of the coronary arteries. Although the culprit artery can be re-perfused, a significant proportion of patients still incur extensive myocardial damage resulting in heart failure. This is partly due to reperfusion paradoxically leading to additional tissue damage, a condition called ischaemia-reperfusion (IR) injury that has been ascribed to inadequate coronary microcirculatory perfusion. Nevertheless, the degree of microvascular perfusion is unknown, limited by an inability to clinically image coronary microvessels. We have previously shown using intravital microscopy that myocardial IR injury induces thromboinflammation and reduces functional capillary density (FCD) in the adult mouse beating heart microcirculation in vivo. [1] The newly discovered and inflammatory cytokine, interleukin-36 (IL-36), could potentially mediate these disturbances.  However, its role in myocardial IR injury is not known. This study firstly aimed to determine whether the coronary microcirculatory disturbances and infarct size post-IR injury was modified by age and gender. Secondly, we investigated whether an IL-36 receptor antagonist (IL-36Ra) could confer vasculoprotection and reduce infarct size Methods:  Adult (3-months) and aged (>18-months) male and female C57BL/6 mice (n=5/group) were anaesthetised using intraperitoneal administration of ketamine hydrochloride (100mg/kg) and medetomidine hydrochloride (100mg/kg).  IR injury was induced by reversibly suture ligating the left anterior descending coronary artery for 45 minutes with reperfusion mediated for 2 hours.  A custom designed 3D-printed stabliser was attached to the left ventricle downstream of the ligation site to permit intravital imaging.  In some studies, recombinant mouse IL-36Ra (15ug/mouse) was injected intra-arterially at 5 minutes pre-reperfusion and 60 minutes post-reperfusion.  Beating heart coronary microcirculation was imaged in real-time intravitally and also ex vivo using multiphoton microscopy.  Infarct size was measured using dual TTC/Evans Blue staining.  Experiments were conducted in accordance with the Animals (Scientific Procedures) Act of 1986 (Project licence: P552D4447). Results: Significantly increased basal (p<0.0001) and IR injury-induced (p<0.0001) neutrophil recruitment, and greater decreases in FCD, was observed in aged mice compared to adults (ANOVA + Tukey’s post hoc test).  Neutrophils primarily adhered within coronary capillaries although in aged hearts remarkable venular adhesion was also identified.  These events were mirrored in deeper myocardial layers when imaged using multiphoton microscopy.  Interesting gender-dependent perturbations were noted with neutrophil recruitment dominating in IR injured female hearts whilst male hearts demonstrated a greater presence of occlusive platelet microthrombi. IL-36Ra significantly reduced inflammation (p<0.0001) and infarct size (p<0.0001) in both adult and aged mice. Conclusion: Our novel findings of enhanced coronary microcirculatory perturbations associated with age may explain the poorer outcomes in elderly patients following MI. Furthermore, the cellular nature of the thromboinflammatory response may explain the gender-related differences in outcome after MI. Importantly, we are the first to demonstrate that targeting IL-36 may be a potential novel therapy for treatment of myocardial IR injury.