BRD4 inhibition rewires cardiac macrophages toward a protective phenotype marked by low MHC class II expression
Bromodomain and extra-terminal domain (BET) proteins, including BRD4, bind acetylated chromatin and co-activate gene transcription.  A BET inhibitor, JQ1, prevents and reverses pathological cardiac remodeling in preclinical models of heart failure.  However, the underlying cellular mechanisms by which JQ1 improves cardiac structure and function remain poorly defined.  Here, we demonstrate that BRD4 knockdown reduced expression of genes encoding CC chemokines in cardiac fibroblasts, suggesting a role for this epigenetic reader in controlling fibroblast-immune cell crosstalk.  Consistent with this, JQ1 dramatically suppressed recruitment of monocytes to the heart in response to stress.  Normal mouse hearts were found to have approximately equivalent numbers of major histocompatibility complex (MHC-II)high and MHC-IIlow resident macrophages, while MHC-IIlow macrophages predominated following JQ1 treatment.  Single-cell RNA-seq data confirmed that JQ1 treatment or BRD4 knockout in CX3CR1+ cells reduced MHC-II gene expression in cardiac macrophages, and studies with cultured macrophages further illustrated a cell autonomous role for BET proteins in controlling the MHC-II axis. Bulk RNA-seq analysis demonstrated that JQ1 blocked proinflammatory macrophage gene expression through a mechanism that likely involves repression of NF-kB signaling. JQ1 treatment reduced cardiac infarct size in mice subjected to ischemia/reperfusion.  Our findings illustrate that BET inhibition affords a powerful pharmacological approach to manipulate monocyte-derived and resident macrophages in the heart.  Such an approach has the potential to enhance the reparative phenotype of macrophages to promote wound healing and limit infarct expansion following myocardial ischemia. Overall design: To address the impact of BRD4 inhibition on resident macrophage phenotypes in the heart, transcriptomic profiling by bulk RNA-seq was performed using purified cardiac macrophages from mice treated with JQ1 or vehicle control for three days.