Both cell fusion and transdifferentiation account for the transformation of human peripheral blood CD34-positive cells into cardiomyocytes in vivo

Background - Adult human peripheral blood CD34-positive (CD34(+)) cells appear to transform into cardiomyocytes in the injured hearts of severe combined immunodeficient mice. It remains unclear, however, whether the apparent transformation is the result of transdifferentiation of the donor stem cells or of fusion of the donor cell with the cardiomyocyte of the recipients. Methods and Results - We performed flow cytometry analyses of cells isolated from the hearts of mice that received human CD34(+) cells. Human HLA-ABC antigen and cardiac troponin T or Nkx2.5 were used as markers for cardiomyocytes derived from human CD34(+) cells, and HLA- ABC and VE-cadherin were used to identify the transformed endothelial cells. The double-positive cells were collected and interphase fluorescence in situ hybridization was used to detect the expression of human and mouse X chromosomes in these cells. We found that 73.3% of nuclei derived from HLA(+) and troponin T+ or Nkx2.5(+) cardiomyocytes contain both human and mouse X chromosomes and 23.7% contain only human X chromosome. In contrast, the nuclei of HLA(-), troponin T+ cells contain only mouse X chromosomes. Furthermore, 97.3% of endothelial cells derived from CD34(+) cells contained human X chromosome only. Conclusions - Thus, both cell fusion and transdifferentiation may account for the transformation of peripheral blood CD34(+) cells into cardiomyocytes in vivo.