Cellular immune responses in allogeneic and xenogeneic transplantation models

Abstract: The introduction of immunosuppressive agents and improvements in surgical techniques and ancillary care has made allotransplantation a routine procedure. However, improvements in short-term graft survival rates have not been accompanied by a similar improvement in long-term outcome due to the process of therapy-resistant chronic allograft rejection. The supply of human organs has, furthermore, become inadequate for the increasing number of patients thought likely to benefit from first set transplantation or re-transplantation. A partial solution to these problems could lie in the development of pre-transplantation protocols inducing donor-specific tolerance. A more general solution would be successful transplantation of grafts from other species, i.e. xenotransplantation. An improved knowledge about immune mechanisms that occur in allograft and xenograft rejection is however fundamental in order to find such solutions.<br/>The specific aims of the studies presented here were therefore: to perform a detailed phenotypic characterization of the different cells infiltrating the graft after transplantation of allogeneic rat islets or xenogeneic fetal porcine islet-cell clusters (ICC) in normal rats (Study I); to elucidate the role of the adaptive humoral and cellular immune response in the rejection process of xenogeneic porcine ICC in rats by studying the impact of active and passive immunization (Study II); to elucidate whether the immune response induced by direct allorecognition has any potential impact on indirect allosensitization in vitro (Study III); and to explore the effect of high dose UVB irradiation as a potential inducer of tolerance by unresponsiveness and/or immune deviation in a human allogeneic transfusion model in vitro (Study IV).<br/>The type, number, and spatial distribution of infiltrating cells were found to differ markedly between xeno- and allograft rejection. Macrophages with an activated phenotype (high expression of MHC class II, ED2 and CD8) were the dominating infiltrating cells in porcine ICC xenografts while T cells dominated in allografts. A rapid and intensified xenograft rejection with similarities to a delayed-type of hypersensitivity reaction was seen in rats pre-immunized with viable or necrotic ICC, indicating that T cells with indirect xenoreactivity are primed during primary xenograft rejection. Taken together, these findings identify macrophages, activated by T cells indirectly recognizing xenopeptides presented by these macrophages, as the primary effector cells in acute cellular xenorejection. <br/>The in vitro results in study III provide evidence that the inflammatory response that is induced during the different phases of direct allorecognition and that probably occurs at anatomical sites where immature host dendritic cells (DCs) capture donor-derived MHC products in vivo, induces production of monocyte and DC-recruiting CC-chemokines, promotes activation of immature bystander (host) DCs and primes these DCs for subsequent Th1-polarization. Such a mechanism could thus finally explain why the presence of passenger DCs in transplanted grafts not only promotes direct allosensitization and subsequent acute rejection but also promotes indirect alloimmunization that subsequently leads to chronic allograft rejection. <br/>The in vitro results in Study IV indicate that priming of T cells with UVB-irradiated allogeneic peripheral blood mononuclear cells (PBMCs) does not induce tolerance, neither by anergy nor immune deviation, to alloantigens presented by the direct pathway of allorecognition. High-dose UVB irradiation was further found to induce a rapid and massive apoptosis in human PBMCs. Taken together, these findings indicate that induction of allospecific regulatory T cells by transfusion of high-dose UVB-irradiated donor spleen cells, as shown by others to occur in vivo, is due to tolerance towards alloantigens presented by the indirect pathway of allorecognition. <br/>