Induction of immune tolerance toward tumor-associated-antigens enables growth of human hepatoma in mice

Adoptive transfer of immunity against hepatitis B surface antigen (HBsAg) was previously shown to facilitate suppression of experimental human hepatocellular carcinoma (HCC) expressing HBsAg in athymic mice. We have shown that oral tolerance induces antigen-specific immune suppression of HBsAg by feeding hepatitis B virus (HBV) antigens. In the present study we evaluated the effect of oral tolerance induction toward HBV or HCC antigens on the growth of experimental HCC-expressing HBsAg in mice. Tolerance induction was induced in mice by 5 oral feedings of I μg HBV antigens or HCC-extracted proteins (50 μg protein) before vaccination with recombinant HBsAg. Splenocytes (2 × 10⁶) from these mice were transferred to sublethally irradiated athymic BALB/c mice previously transplanted subcutaneously with 10⁷ human hepatoma Hep3B cells. Adoptive transfer of splenocytes immunized toward HBsAg prevented tumor growth. At 4 weeks after splenocyte transplantation, tumor volume and serum alpha-fetoprotein (AFP) levels in athymic mice transplanted with splenocytes immunized to HBsAg were undetectable as compared with 1,048 ± 738 mm³ and 2,500 ± 1,431 ng/ml in recipients of naïve splenocytes (p < 0.0001). Mice receiving splenocytes tolerized toward Hep3B cells, as manifested by reduced serum HBs antibody levels, reduced HBV-specific stimulation index and reduced HBV-specific-IFN-γ spot-forming cells, had early tumor growth evident by elevated AFP serum levels, weight loss and mortality, which were suppressed at 6 weeks. Mice transplanted with splenocytes tolerized toward HBV antigens did not have direct evidence of tumor growth. Induction of oral tolerance toward HCC-extracted proteins enabled transient tumor growth in this model. This effect was mediated through downregulation of the anti-HBV immune response.