Abundant progenitor cells in the adventitia contribute to atherosclerosis of vein grafts in ApoE-deficient mice

Evidence suggests that haematopoietic stem cells might have unexpected developmental plasticity, highlighting therapeutic potential. For example, bone-marrow-derived hepatocytes can repopulate the liver of mice with fumarylacetoacetate hydrolase deficiency and correct their liver disease. To determine the underlying mechanism in this murine model, we performed serial transplantation of bone-marrow-derived hepatocytes. Here we show by Southern blot analysis that the repopulating hepatocytes in the liver were heterozygous for alleles unique to the donor marrow, in contrast to the original homozygous donor cells. Furthermore, cytogenetic analysis of hepatocytes transplanted from female donor mice into male recipients demonstrated 80,XXXY (diploid to diploid fusion) and 120,XXXXYY (diploid to tetraploid fusion) karyotypes, indicative of fusion between donor and host cells. We conclude that hepatocytes derived form bone marrow arise from cell fusion and not by differentiation of haematopoietic stem cells.

Results from several experimental systems suggest that cells from one tissue type can form other tissue types after transplantation. This could be due to the presence of multipotential or several types of adult stem cells in donor tissues, or alternatively, to fusion of donor and recipient cells. In a model of tyrosinaemia type I, mice with mutations in the fumarylacetoacetate hydrolase gene (Fah-/-) regain normal liver function after transplantation of Fah+/+ bone marrow cells, and form regenerating liver nodules with normal histology that express Fah. Here we show that these hepatic nodules contain more mutant than wild-type Fah alleles, and that their hepatocytes express both donor and host genes, consistent with polyploid genome formation by fusion of host and donor cells. Using bone marrow cells marked with integrated foamy virus vectors that express green fluorescent protein, we identify common proviral junctions in hepatic nodules and haematopoietic cells. We also show that the haematopoietic donor genome adopts a more hepatocyte-specific expression profile after cell fusion, as the wild-type Fah gene was activated and the pan-haematopoietic CD45 marker was no longer expressed.

Previous studies have shown that during avian heart development, epicardial and coronary vascular smooth muscle precursors are derived from the proepicardium, a derivative of the developing liver. This finding led to a model of coronary vascular development in which epicardial cells migrate over the postlooped heart, followed by migration of committed endothelial and smooth muscle precursors from the proepicardium through the subepicardial matrix where the coronary arteries develop. Here we show that epicardial cells undergo epithelial-mesenchymal transformation to become coronary vascular smooth muscle, perivascular fibroblasts, and intermyocardial fibroblasts. We began by establishing primary cultures of quail epicardial cells that retain morphologic and antigenic identity to epicardial cells in vivo. Quail epicardial monolayers stimulated with serum or vascular growth factors produced invasive mesenchyme in collagen gels. Chick epicardial cells labeled in ovo with DiI invaded the subepicardial extracellular matrix, demonstrating that mesenchymal transformation of epicardium occurs in vivo. To determine the fates of epicardially derived mesenchymal cells, quail epicardial cells labeled in vitro with LacZ were grafted into the pericardial space of E2 chicks. These cells attached to the heart, formed a chimeric epicardium, invaded the subepicardial matrix and myocardial wall, and became coronary vascular smooth muscle, perivascular fibroblasts, and intermyocardial fibroblasts, demonstrating the common epicardial origin of these cell types. A general model of coronary vascular development should now include epicardial-mesenchymal transformation and direct participation of mesenchyme derived from the epicardium in coronary morphogenesis. Copyright 1998 Academic Press.