Chronic rejection in rat aortic allografts. An experimental model for transplant arteriosclerosis.

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Abstract
Chronic rejection has several histological appearances, depending on the type of organ graft. Common to all of them is transplant arteriosclerosis associated with an ongoing inflammatory response in the transplanted graft. To the contrary of classical atherosclerosis, in which the manifestations are mostly focal, proximal, and asymmetric, transplant arteriosclerosis is generalized, and the intimal thickening is concentric. In this article, we describe an experimental animal model whereby transplant arteriosclerosis may be investigated in the inbred rat. Aortic allografts were transplanted from DA (RTIa) to major histocompatibility complex-incompatible WF (RTIv) rats or, for control, to rats of the DA strain. Transplantation was followed by an acute inflammation episode in the aortic adventitia of the allograft, largely lacking in the syngeneic graft, with a prominence of lymphoid activation markers (Cd25) in the cells of the inflammatory infiltrate. The inflammation episode peaked at 2 months after transplantation, became attenuated, and was followed by a proliferative response of myocytes in the allograft media. An increase in the migration of myocytes to the subendothelial space (presumably through small breaks generated in the internal elastic lamina) was observed thereafter, and myocyte proliferation continued in the intima with some intermingled macrophages. Finally, necrosis and disappearance of myocytes and their replacement by fibrous tissue were observed in the media. These alterations are virtually identical with the vascular lesion of chronically rejecting parenchymal organ transplants in human subjects. We suggest that aortic allografts exchanged between histoincompatible rat strains may be used as an experimental model for transplant arteriosclerosis.