Excessive cardiac fibrosis is central to adverse cardiac remodeling and dysfunction leading to heart failure in many cardiac diseases. Histone methylation plays a crucial role in various pathophysiological events. However, the role of histone methylation modification enzymes in pathological cardiac fibrosis needs to be fully elucidated. Here, we identified lysine demethylase 5B (KDM5B), a histone H3K4me2/me3 demethylase, as a key epigenetic mediator of pathological cardiac fibrosis. KDM5B expression was upregulated in cardiac fibroblasts and myocardial tissues in response to pathological stress. KDM5B deficiency markedly ameliorated cardiac fibrosis, improved cardiac function, and prevented adverse cardiac remodeling following myocardial infarction (MI) or pressure overload. KDM5B knockout or inhibitor treatment constrained the transition of cardiac fibroblasts to profibrogenic myofibroblasts and suppressed fibrotic responses. KDM5B deficiency also facilitated the transformation of cardiac fibroblasts to endothelial-like cells and promoted angiogenesis in response to myocardial injury. Mechanistically, KDM5B bound to the promoter of activating transcription factor 3 (Atf3), an antifibrotic regulator of cardiac fibrosis, and inhibited ATF3 expression by demethylating the activated H3K4me2/3 modification, leading to the enhanced activation of TGF-β signaling and excessive expression of profibrotic genes. Our study indicates that KDM5B drives pathological cardiac fibrosis and represents a candidate target for intervention in cardiac dysfunction and heart failure.

Messenger RNA (mRNA) vaccines are promising alternatives to conventional vaccines in many aspects. We previously developed a lipopolyplex (LPP)‐based mRNA vaccine (SW0123) that demonstrated robust immunogenicity and strong protective capacity against severe acute respiratory syndrome coronavirus‐2 (SARS‐CoV‐2) infection in mice and rhesus macaques. However, the immune profiles and mechanisms of pulmonary protection induced by SW0123 remain unclear. Through high‐resolution single‐cell analysis, we found that SW0123 vaccination effectively suppressed SARS‐CoV‐2‐induced inflammatory responses by inhibiting the recruitment of proinflammatory macrophages and increasing the frequency of polymorphonuclear myeloid‐derived suppressor cells. In addition, the apoptotic process in both lung epithelial and endothelial cells was significantly inhibited, which was proposed to be one major mechanism contributing to vaccine‐induced lung protection. Cell−cell interaction in the lung compartment was also altered by vaccination. These data collectively unravel the mechanisms by which the SW0123 protects against lung damage caused by SARS‐CoV‐2 infection.