Modification of donor MHC antigens with ribozyme RNA transfer to prevent rejection of graft liver

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In our first study, we examined whether insertion of ribozyme RNA into the allograft could down-regulate the expression of the target antigens and prevent the rejection of transplanted organs. However, at present, significant down-regulation of donor MHC class I antigen expression on the donor vascular endothelial cells was not observed after insertion of ribozyme RNA even in vitro. Therefore, immunological response to these cells was not significantly reduced. Probably, structure of ribozyme RNA was not suitable for these target antigens. Now we are developing new ribozyme RNA to provide for efficient gene therapy.In our second study, we investigated the expression of angiopoietin (Ang)-1, Ang-2, Tie-2, and vascular endothelial growth factor (VEGF) in both regenerating liver tissue and isolated liver cells after 70% hepatectomy in rats. Proliferation and apoptosis of sinusoidal endothelial cells (SECs) were also evaluated with proliferating cell nuclear antigen (PCNA) and TUNEL staining, respectively. Results : Expression of VEGF rRNA increased, with a peak at 72 h after hepatectomy, decreasing thereafter. Ang-1 mRNA was present at detectable levels before hepatectomy and increased slowly with a peak at 96 h. Meanwhile, Ang-2 mRNA was hardly detected before hepatectomy, but increased remarkably at 120 h. In isolated cells, VEGF mRNA expression was limited to hepatocytes, but Ang-2 mRNA was found in sinusoidal cells, mainly in hepatic stellate cells (HSCs). The PCNA labeling index of SECs increased slowly, reaching a peak at 72 h, whereas apoptotic SECs were detected at 120 h. Conclusions : VEGF secreted by hepatocytes may play a key role in SEC proliferation, followed by increased Ang-1, presumably promoting sinusoidal maturation. Thereafter, Ang-2 mainly released from HSCs in the absence of VEGF may contribute to apoptosis of superfluous SECs for cessation of regenerative process.