TRANSPLANTED AUTOLOGOUS HEPATOCYTES ARE ELIMINATED BY LIVER NK CELLS EXPRESSING TRAIL WHICH LACK EXPRESSION OF INHIBITORY RECEPTORS RECOGNIZING SELF MHC CLASS I MOLECULES:

P1046 Natural killer (NK) cells are thought to provide a first line of defense against invading infectious microbes by exerting an effector function without the necessity for priming. Activated liver NK cells target not only infected or transformed hepatocytes but also intact hepatocytes in a nonspecific fashion. But the mechanisms underlying NK cell-induced killing of self-hepatocytes are not clear. In the present study, we demonstrated that liver NK cells have cytotoxicity against normal syngeneic hepatocytes in mice. Polyinosinic-polycytidylic acid (poly I:C) treatment enhanced hepatocyte toxicity of liver NK cells but not that of spleen NK cells. Unlike NK cells in other tissues, approximately 30-40% of liver NK cells constitutively express tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). An in vitro NK cell cytotoxic assay revealed that hepatocyte toxicity of liver NK cells from both naïve and poly I:C-treated mice was inhibited partially by an anti-TRAIL mAb alone and completely by the combination with an anti-FasL mAb and perforin inhibitor, concanamycin A, indicating contribution of TRAIL to NK cell-mediated hepatocyte toxicity. Liver TRAIL+ NK cells expressed high levels of CD69, indicating their naturally activating state. The majority of TRAIL+ NK cells lacked expression of Ly-49 inhibitory receptors (Ly-49A, Ly-49C/I and Ly-49G2) recognizing self-major histocompatibility complex (MHC) class I, indicating a propensity to targeting self-hepatocytes. Poly I:C treatment significantly upregulated the expression of Ly-49 inhibitory receptors on TRAIL- NK cells. This might be a compensatory mechanism to protect self-class I-expressing cells from activated NK cell-mediated killing. However, such compensatory alteration was not seen at all in the TRAIL+ NK cell fraction. Thus, TRAIL+ liver NK cells have less capacity for self-recognition, and this might be involved in NK cell-dependent self-hepatocyte toxicity. In the case of hepatocyte transplantation, NK cell-mediated cytotoxicity would be a major obstacle to engraftement of autologous hepatocytes, because direct interaction of the hepatocytes transplanted via the portal vein with liver NK cells is inevitable. To address this possibility, CFSE-labeled syngeneic hepatocytes were infused into the liver via the portal vein in normal B6 mice. Seven days after hepatocyte transplantation, the fluorescence of the CFSE-labeled hepatocytes was undetectable in the liver by fluorescent microscopic analysis, indicating failure in engraftment of inoculated hepatocytes. In contrast, when the recipient mice were treated with anti-AsGM1 sera prior to transplantation, hepatocyte grafts were clearly detected in the perisinusoidal area of the liver. These findings indicate that liver NK cell-mediated hepatocyte toxicity causes insufficient intrahepatic engraftment of transplanted hepatocytes.