Methods & Results:  To identify circRNAs which play important roles in macrophages’ physiology during heart failure progression, we have performed in depth RNA sequencing comparing heart samples from healthy donors and patients with ischemic HF. We identified circRNA MJ1 (circMJ1) as an important regulator of macrophages polarization and related inflammatory response. CircMJ1 is highly expressed in cardiac macrophages and significantly dysregulated in pro-inflammatory macrophages both in in vitro (LPS-induced) and ex vivo (d1 post-MI) settings. CircMJ1 knockdown by specific siRNAs targeting the backsplicing site significantly inhibited activation of macrophages towards the M1 phenotype as well as blocked secretion of pro-inflammatory cytokines. The secretome collected from circMJ1-silenced cardiac macrophages significantly reduced the migration of cardiac fibroblasts and inhibited apoptosis of cardiomyocytes. These results strongly suggested that macrophage specific inhibition of circMJ1 might be a novel therapeutic tool to regulate inflammatory response following MI. Next, to investigate the therapeutic effects of circMJ1-silenced macrophages on cardiac remodeling, we co-cultured modified macrophages (scr siR- or circMJ1 siR-transfected) on a novel multicellular ex vivo model, living myocardial slices (LMS). LMS are ultrathin cardiac tissue slices and can be prepared from small and large animal as well as from human specimens, especially HF patients. More importantly, with cryo-injury, we can mimic pathological model of ischemic injury like MI. The addition of circMJ1-silenced macrophages led to improved cardiac function as well as reduced inflammation and fibrosis in living myocardial slices from rats and humans.

Conclusion: Overall, our results showed circMJ1 is a crucial “switch” for macrophages’ dynamics and demonstrate a therapeutic potential of engineered circMJ1-silenced macrophages to improve post-MI healing.