Elastin expression in a model of acute arterial graft rejection

Elastin is an important component of normal blood vessels and the extracellular matrix of atherosclerotic plaques, but its role in intimal thickening in the arteries of transplanted organs has not been defined. We have looked at elastin gene expression (by in situ mRNA hybridization) in an animal model using an abdominal aortic transplant between 2 strains of rats disparate for MHC class I antigens. The normal aortic wall of adult rats lacks elastin mRNA. Aortic allografts at 7 days after transplantation exhibit increased elastin mRNA in the medial vascular smooth muscle cells. This medial elastin mRNA expression is present only until 20 days after transplantation, and at later times, only the juxtaluminal cells of the neointima express elastin mRNA. Stainable elastin is detectable only in regions that previously demonstrated high levels of elastin mRNA. Combined in situ hybridization and immunocytochemistry reveals that most elastin mRNA-expressing cells in the media are alpha-actin-positive smooth muscle cells. In the neointima, elastin mRNA-expressing cells do not stain with antibodies to either smooth muscle alpha-actin or macrophage proteins. This cell population may represent a "synthetic" phenotype of vascular smooth muscle cell lacking alpha-actin protein. We presume there is immune cell-mediated injury leading to a vascular smooth muscle cell response and part of the vascular smooth muscle cell response may be increased elastin mRNA expression and elastin deposition in the allografts.