(A) Flow cytometric analysis of CD45⁺CD19⁺ myocardial cells in a C57/B6J mouse. Most myocardial B cells are CD11b^–. Among the CD19⁺CD11b⁺ myocardial B cells, the majority are IgM⁺CD5^–, and a small portion are IgM⁺CD5⁺. The flow cytometry plots are representative of 3 animals analyzed in different experiments. Mean percentage of total CD19⁺ cells ± SD is reported next to each gate. (B) Flow cytometric analysis of CD45⁺CD19⁺ myocardial cells in C57/B6J B cell–deficient mice (μMT) after BM transplant with WT BM. BM transplant replenished CD11b^– cells more efficiently than CD11b⁺ cells. In the CD11b⁺ compartment, BM transplant did not produce IgM⁺CD5⁺ cells. The flow cytometry plots are representative of 4 animals analyzed in different experiments. Mean percentage of total CD19⁺ cells ± SD is reported next to each gate. (C) Analysis of myocardial CD45⁺CD19⁺ cells from animals conjoined via parabiosis for 3 weeks. Both CD19⁺CD11b⁺ and CD19⁺CD11b^– cells showed 50% chimerism, a finding consistent with the observation that myocardial B cells moved freely between animals. Percent chimerism for Ly6G⁺ neutrophils, CD64⁺Ly6C^lo macrophages, and CD3⁺ T cells is shown for comparison. (D) Analysis of CD45⁺CD19⁺ myocardial B cells in recipient and donor hearts before transplant and on day 4 after heterotopic heart transplant. Before transplant, the heart of the CD45.1 recipient animal contains only CD45.1⁺ B cells, and the donor heart from a CD45.2 animal contains only CD45.2⁺ B cells. Four days after transplant, the recipient heart is mostly unchanged, though it contains a small population of CD45.2⁺ B cells derived from the transplanted heart (left side). The CD45.2 transplanted heart instead has lost almost all of its CD45.2⁺ B cells and now contains mostly CD45.1⁺ recipient-derived B cells. Representative flow cytometry plots from 3 independently transplanted animals. Percentage of total CD19⁺ cells is reported within each gate.