Nature Communications, Published online: 29 August 2026; doi:10.1038/s41467-026-77219-3 Atrial fibrillation involves systemic inflammatory activation that remodel the atrial endocardium. Here, the authors show that gut microbiota activate PLCy2 positive monocytes, promoting endothelial-to-mesenchymal transition and atrial fibrillation through a microbiota–monocyte–endocardial axis.