SUMMARY Organ-specific functions of tissue-resident macrophages in the steady-state heart are unknown. Here we show that cardiac macrophages facilitate electrical conduction through the distal atrioventricular node, where conducting cells densely intersperse with elongated macrophages expressing connexin 43. When coupled to spontaneously beating cardiomyocytes via connexin 43-containing gap junctions, cardiac macrophages have a negative resting membrane potential and depolarize in synchrony with cardiomyocytes. Conversely, macrophages render the resting membrane potential of cardiomyocytes more positive and, according to computational modeling, accelerate their repolarization. Photostimulation of channelrhodopsin 2-expressing macrophages improves atrioventricular conduction, while conditional deletion of connexin 43 in macrophages and congenital lack of macrophages delay atrioventricular conduction. In the Cd11bDTR mouse, macrophage ablation induces progressive atrioventricular block. These observations implicate macrophages in normal and aberrant cardiac conduction.

Background: Ibrutinib is a Bruton's tyrosine kinase (BTK) inhibitor with remarkable efficacy against B-cell cancers. Ibrutinib also increases the risk of atrial fibrillation (AF) which remains poorly understood. Methods: We performed electrophysiology studies on mice treated with ibrutinib to assess inducibility of AF. Chemoproteomic analysis of cardiac lysates identified candidate ibrutinib targets, which were further evaluated in genetic mouse models and additional pharmacologic experiments. The pharmacovigilance database, VigiBase ® was queried to determine whether drug inhibition of an identified candidate kinase was associated with increased reporting of AF. Results: We demonstrate that treatment of mice with ibrutinib for four weeks results in inducible AF, left atrial enlargement, myocardial fibrosis and inflammation. This effect was reproduced in mice lacking BTK but not in mice treated with four-weeks of acalabrutinib, a more specific BTK inhibitor, demonstrating that AF is an off-target side effect. Chemoproteomic profiling identified a short list of candidate kinases that was narrowed by additional experimentation leaving C-terminal src kinase (CSK) as the strongest candidate for ibrutinib induced AF. Cardiac specific Csk knockout in mice led to increased AF, left atrial enlargement, fibrosis, and inflammation, phenocopying ibrutinib treatment. Disproportionality analyses in Vigibase ® confirmed increased reporting of AF associated with kinase inhibitors blocking Csk vs non-Csk inhibitors, with a reporting odds-ratio of 8.0 [95% CI 7.3-8.7, p<0.0001]. Conclusions: These data identify Csk inhibition as the mechanism by which ibrutinib leads to AF. Clinical Trial Registration: URL: www.clinicaltrials.gov Unique Identifier: NCT03530215