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Background and Aims Survivors of acute coronary syndromes face an elevated risk of recurrent atherosclerosis-related vascular events despite advanced medical treatments. The underlying causes remain unclear. This study aims to investigate whether myocardial infarction (MI)-induced trained immunity in monocytes could sustain proatherogenic traits and expedite atherosclerosis. Methods Apolipoprotein-E deficient (ApoE−/−) mice and adoptive bone marrow transfer chimeric mice underwent MI or myocardial ischaemia–reperfusion (IR). A subsequent 12-week high-fat diet (HFD) regimen was implemented to elucidate the mechanism behind monocyte trained immunity. In addition, classical monocytes were analysed by flow cytometry in the blood of enrolled patients. Results In MI and IR mice, blood monocytes and bone marrow-derived macrophages exhibited elevated spleen tyrosine kinase (SYK), lysine methyltransferase 5A (KMT5A), and CCHC-type zinc finger nucleic acid-binding protein (CNBP) expression upon exposure to a HFD or oxidized LDL (oxLDL) stimulation. MI-induced trained immunity was transmissible by transplantation of bone marrow to accelerate atherosclerosis in naive recipients. KMT5A specifically recruited monomethylation of Lys20 of histone H4 (H4K20me) to the gene body of SYK and synergistically transactivated SYK with CNBP. In vivo small interfering RNA (siRNA) inhibition of KMT5A or CNBP potentially slowed post-MI atherosclerosis. Sympathetic denervation with 6-hydroxydopamine reduced atherosclerosis and inflammation after MI. Classical monocytes from ST-elevation MI (STEMI) patients with advanced coronary lesions expressed higher SYK and KMT5A gene levels. Conclusions The findings underscore the crucial role of monocyte trained immunity in accelerated atherosclerosis after MI, implying that SYK in blood classical monocytes may serve as a predictive factor for the progression of atherosclerosis in STEMI patients. [ABSTRACT FROM AUTHOR]