Fibrosis in chronic rejection of human liver allografts - Expression patterns of transforming growth factor-TGF beta 1 and TGF-beta 3

Activation and transformation of lipocytes (Ito cells, stellate cells) into alpha-actin-positive myofibroblast-like cells is an essential step in the initiation of liver fibrosis, Transforming growth factor-beta (TGF-beta) 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 (alpha-actin), macrophages, and TGF-beta 1 and -beta 3. 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. Likewise, strong accumulation of alpha-actin-positive cells and TGF-beta 1-expressing macrophages was observed in the same fields, supporting the concept of lipocyte activation/transformation and subsequent ECM production fostered by macrophage-derived TGF-beta 1, In contrast, TGF-beta 3 was found to be mainly expressed by a markedly increased number of lipocytes, Interestingly, distribution of TGF-beta 3 corresponded to that of tenascin, an ECM molecule known to be involved in early matrix organization, suggesting that TGF-beta 3 may likewise act mainly in early stages of fibrogenesis, Furthermore, TGF-beta 3 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-beta isoforms 1 and 3 in the course of a multistep process leading to lipocyte transformation and ECM production.