BMP4 and myocarditis: Further cell–cell communication analysis of fibroblasts interacting with neighboring cells predicted that MHC class II-dependent cellular interactions of cardiac fibroblasts occur almost exclusively in inflamed hearts, whereas the binding of Bmp4 to the heterotetrameric BMP receptor 1a and 2 (Bmpr1a–Bmpr2) as ligand–receptor interaction showed the strongest relative decrease during myocarditis compared to homeostatic hearts (Fig. 1i).