Comparative investigation of Cx3cr1-Expressing Cardiac Macrophages in Atrioventricular Nodes of Wild-Type and...

Cardiac macrophages were recently discovered to regulate cardiomyocyte function at the atrioventricular (AV) nodes of healthy murine and human hearts. Macrophages and immune mediators have been implicated in ischemia-induced cardiac damage and arrhythmia; however, the relevance of macrophages in congenital arrhythmia pathogenesis remains unclear. Here, the wild-type (WT) mice and Calsequestrin2 gene null mutant (Casq2^-/-) transgenic model of catecholaminergic polymorphic ventricular tachycardia (CPVT) were comparatively evaluated for cardiac macrophage population at the histological level. The localization and density of the chemokine receptor, Cx3cr1-expressing cardiac macrophages were investigated ex vivo in heart sections from WT and Casq2^-/- mice at the AV node region using immunofluorescence, Masson's Trichrome, and Hematoxylin-Eosin staining. Cx3cr1⁺ cardiac macrophages were localized in all cardiac layers and chambers of the heart, as well as septum and valve roots. Cx3cr1⁺ macrophages were coimmunostained and confirmed for expression of CD68 as a pan-macrophage marker. Macrophages were detected in close proximity to the cTnT⁺ cardiomyocytes in the myocardium of both WT and Casq2^-/- transgenic mice. Macrophage clusters were abundantly observed in the Hcn4 immunoreactive AV node region in healthy murine hearts. Subsequent quantification of signal intensities of Cx3cr1⁺ cardiac macrophages in the AV nodes in the Casq2^-/- transgenic mice was significantly lower than in WT mice. These results demonstrated a decline in Cx3cr1⁺ cardiac macrophages in AV node regions of CPVT mouse hearts, which could imply a potential contribution to arrhythmia. Our findings could serve as a valuable source for future functional investigations of macrophages in the pathogenesis of congenital arrhythmia.