FIBROSIS IN CHRONIC REJECTION OF HUMAN LIVER ALLOGRAFTS

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Abstract
Activation and transformation of lipocytes (Ito cells, stellate cells) into α-actin-positive myofibroblast-like cells is an essential step in the initiation of liver fibrosis. Transforming growth factor-β(TGF-β) is considered an important mediator of this process. In order to determine mechanisms of fibrotic deposition in a hepatic transplant setting, we analyzed 10 chronically rejected human liver allografts for the expression of extracellular matrix (ECM) molecules, myofibroblast-like cells(α-actin), macrophages, and TGF-β1 and -β3. Using single- and double-immunohistochemical staining techniques, all specimens investigated showed increased deposition of the ECM proteins fibronectin, tenascin, undulin, and collagen VI with a characteristic densification especially in pericentral areas. Like-wise, strong accumulation of α-actin-positive cells and TGF-β1-expressing macrophages was observed in the same fields, supporting the concept of lipocyte activation/transformation and subsequent ECM production fostered by macrophage-derived TGF-β1. In contrast, TGF-β3 was found to be mainly expressed by a markedly increased number of lipocytes. Interestingly, distribution of TGF-β3 corresponded to that of tenascin, an ECM molecule known to be involved in early matrix organization, suggesting that TGF-β3 may likewise act mainly in early stages of fibrogenesis. Furthermore, TGF-β3 restriction to high numbers of a single cell type (i.e., lipocytes) implied a possible role in cell proliferation through autocrine loops. In conclusion, fibrosis in chronic rejection seems to follow similar mechanisms as in non-transplanted livers but additionally suggests differential temporal and functional roles for the TGF-β isoforms 1 and 3 in the course of a multistep process leading to lipocyte transformation and ECM production.