(A) Flow cytometry analysis of myocardial leucocytes in WT and μMT mice. μMT mice have less Ly6C⁺ cells and more CD4⁺ and CD8⁺ T cells. (B). Flow cytometric analysis of WT animals depleted of B cell through administration of anti-CD20 antibody or isotype control. Antibody-mediated B cell depletion is associated with an increase in Ly6G⁺ cells (P = 0.056), CD4⁺ and CD8⁺ cells. (C) Analysis of hearts from 27-week-old C57/B6J mice and syngeneic age/sex-matched μMT mice showed that μMT mice have myocardial fibers with smaller cross-sectional area as assessed by WGA staining. Representative images of WGA staining for each group are shown and cumulative data are reported. (D) μMT mice have lower myocardial mass as assessed by heart weight/tibia length (mg/mm). (E) Echocardiographic analysis of left ventricular function in WT mice and matched μMT mice. μMT mice have higher left ventricular ejection fraction (LVEF) and faster ventricular relaxation (dV/dT-d, volume change per unit of time during diastolic relaxation; ESV, end-systolic volume). (F) Flow cytometry analysis of a sample of human myocardium collected at the time of implantation of a left ventricular assist device and dissociated enzymatically. A clear population of CD19⁺ cells is shown. Representative plot from 3 heart samples analyzed. Percentage of myocardial CD45⁺ cells is reported next to the gate. (G) Histological analysis of human myocardium collected at the time of LVAD placement shows that, similarly to the murine heart, the human heart harbors B cells in the intravascular space, in intimate contact with the endothelium. Magnification, 40×. Pairwise comparisons were performed with 2-tailed t test. Data are expressed as average ± SD. The WT and μMT groups were compared using unpaired 2-tailed t test adjusted for multiple comparisons with the FDR method in A and B. In C, the 2 groups were compared using nested t test *P < 0.05. **P < 0.01.