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

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. Hepatocytes have the potential to be useful for a wide variety of therapeutic applications in modern medicine. First, hepatocytes may be transplanted or used in bioartificial devices to sustain adequate liver function during periods of organ failure. This type of "bridging" therapy can allow for adequate organ recovery following acute liver failure or may also extend the time possible for a patient to receive a liver transplant. Studies to sustain failed liver function with allogeneic hepatocytes are already underway and have shown promise over the short-term. 1 Second, hepatocytes are an attractive cellular vehicle for ex vivo gene delivery, as has already been shown in both animals and humans. 2-4 A third possible benefit is that major histocompatibility complex (MHC) class I molecules expressed by hepatocytes may have a donor-specific immunosuppressive effect with the potential to prevent rejection and possibly establish immunologic tolerance in organ transplantation. However, a key to being able to use hepatocytes safely and successfully for such applications will be a thorough understanding of the complex immunologic effects mediated by these cells.One well-documented observation that has particularly stimulated interest in hepatocyte immunology is that donorspecific immunosuppression is clearly associated with allogeneic liver transplantation. More specifically, a crossmatchpositive donor liver can be successfully transplanted, 5 and liver allografts protect simultaneously transplanted kidneys against hyperacute rejection in sensitized human recipients. 6 Also, compared with other organ allografts, liver transplants have fewer rejection episodes, and recipients have been successfully removed from immunosuppressive drug therapy. 7 In animal models, Dark Agouti (DA)-to-PVG rat liver transplants are spontaneously accepted, and allow for acceptance of subsequent DA kidney, heart, pancreas, or skin grafts. [8][9][10] Even more impressive is the observation that a DA liver transplant can stop the progression of ongoing DA heart allograft rejection in a PVG recipient. 11 Although it...

Apoptosis of Antigen-Specific T Lymphocytes upon the Engagement of CD8 by Soluble HLA Class I Molecules Is Fas Ligand/Fas Mediated: Evidence for the Involvement of p56lck, Calcium Calmodulin Kinase II, and Calcium-Independent Protein Kinase C Signaling Pathways and for NF-κB and NF-AT Nuclear Translocation

“…However, the percentage of apoptotic cells (70 -80%) induced by sHLA-I in bulk CD8 ϩ T cell lines is higher than the percentage of sHLA-I/Ag-specific cell complexes within the bulk population (14,19,20). Moreover, it has been reported that various sHLA-I isoforms may induce different immunoregulatory effects in animal models (53,54). Therefore, differences in sHLA-I preparations used in the experiments might explain the inconsistent results reported in the literature.…”

“…In this regard, Ͼ30 years ago, Balner and Van Rood (72) demonstrated that the injection of donor serum that contains sHLA-I molecules into the recipient before a skin transplant resulted in prolongation of graft survival. More recent studies in animals undergoing liver and heart allografts have provided convincing evidence that the injection into the portal vein of hepatocytes transfected with the soluble form of MHC molecules induces tolerance and transplant acceptance (53,54). To the best of our knowledge, no data are available for the human system.…”

Insulin expressing hepatocytes not destroyed in transgenic NOD mice

“…This fundamental question has not been addressed by previous studies with (pro)insulin producing hepatocytes as diabetes in these models was induced chemically rather than by autoimmune means [6-9]. The autoimmune destruction of insulin-producing hepatocytes is a possibility since hepatocytes express major histocompatibility complex (MHC) class I molecules [10], and constitutively express Fas [11], moreover the liver cells have antigen presenting activity [12] and are attacked in autoimmune diseases such as chronic active hepatitis. Furthermore (pro)insulin appears to be an autoantigen in the development of type 1 diabetes [13,14].…”

Immunological tolerance and liver transplantation

“…Indeed, early after liver transplantation most MHC class I molecules detected in peripheral blood are of donor origin, and these soluble antigens have been shown to induce apoptosis of alloreactive CD8CT cells both in vitro and in vivo, leading to prolonged transplant survival [43,44].…”