Since hepatocellular carcinomas (HCCs) develop from transformed hepatocytes, sometimes in a multicentrical manner, immunological deletion of such small intrahepatic regions should be an important strategy to prevent HCC development. The liver contains abundant innate cell lineages including natural killer (NK) cells and natural killer T (NKT) cells, the latter of which become activated in a CD1d-restricted manner by alpha-galactosylceramide (alpha-GalCer). In our study, we investigated the anti-tumor effect elicited by alpha-GalCer administration against transplanted hepatoma cells in the liver, in comparison with that in extrahepatic sites. alpha-GalCer administration completely suppressed the growth of BNL 1MEA.7R.1 (BNL) hepatoma cells disseminated in the liver of syngeneic BALB/c mouse but had no anti-tumor effect on subcutaneously implanted BNL cells. Hepatic NKT cells became rapidly activated after alpha-GalCer administration compared to splenic NKT cells and then disappeared. Hepatic NK cells substantially increased their population as well as up-regulated their cytotoxic activity against BNL cells, but NK cells in other tissues, including the spleen, blood and lymph node, did not. Anti-asialo GM1 antibody treatment, which depleted NK cells in vivo, resulted in hepatic tumor formation in alpha-GalCer-treated mice, indicating the critical involvement of NK cells in the alpha-GalCer-induced anti-tumor effect in the liver. In conclusion, our study demonstrates clear differences in NK cell activation and anti-tumor effect through stimulation of NKT cells by alpha-GalCer between the liver and extrahepatic tissues. Sequential activation of these innate cell lineages may be an attractive strategy for controlling micro-disseminated hepatoma cells in the liver.
Copyright 2003 Wiley-Liss, Inc.