Immunologic Considerations for Therapeutic Strategies Utilizing Allogeneic Hepatocytes: Hepatocyte–Expressed Membrane–Bound Major Histocompatibility Complex Class I Antigen Sensitizes While Soluble Antigen Suppresses the Immune Response in Rats

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Abstract
Understanding the immunologic effects of hepatocytes is critical because of the potential to use these cells for bioartificial livers, as a vehicle for gene transfer, and as a means to induce donor–specific immunosuppression in organ transplantation. However, this understanding is complicated by the fact that hepatocytes express membrane–bound and soluble forms of major histocompatibility complex (MHC) class I antigen, each with the potential to induce different immune responses. In the present study we first determined the immunologic effect of normal donor–derived hepatocytes in a rat heart transplant model. We then used ex vivo hepatocyte gene transfer to examine the immunologic effects of different forms of hepatocyte–expressed MHC class I antigen. Results showed that intrasplenic injection of purified, donor–strain-specific hepatocytes into recipients primes alloimmunity, as evidenced by acceleration of heart allograft rejection. Interestingly, injection of autologous hepatocytes transfected ex vivo with DNA encoding only membrane–bound donor MHC class I antigen (RT1.Aa) also accelerated allograft rejection. However, hepatocytes transfected to express only secreted donor MHC antigen prolonged transplant survival. Limiting–dilution analysis of lymphocytes from animals treated with hepatocytes producing only secreted alloantigen showed an antigen–specific reduction in cytotoxic T lymphocyte (CTL) and helper T lymphocyte (HTL) precursors. Further analysis of CTL populations by flow cytometry revealed a relatively high percentage of nonviable cells, implying that soluble antigen promotes allospecific CTL death. In summary, this study suggests that hepatocyte–expressed MHC class I molecules have opposing immunologic effects, with the membrane–bound antigen inducing immunologic sensitization, and the soluble antigen promoting donor–specific immunosuppression.