Immunity against porcine islet xenografts in man
Abstract: Due to the lack of organs for clinical transplantation, alternative sources have been investigated. One solution to this shortage could be xenotransplantation, i.e. the transplantation of organs/cells across species barriers. Ethical, physiological, infectious and financial reasons make pigs the most suitable species for clinical xenotransplantation. In the 1990's, ten type 1 diabetic patients were transplanted with fetal pig islet-like cellclusters (ICC) as an attempt to cure the disease. None of the patients became insulinindependent after the transplantation. However, in five cases, signs of engrafted pig islets were detected for up to 14 months. The early xenoimmune antibody response in these patients was shown to be homogenous and mainly directed against the carbohydrate epitope Gala 1,3Gal, an epitope that is present in all mammals except for humans, apes and old world monkeys. Instead, these species have preformed natural antibodies against the Gala 1,3Galepitope. Here we have investigated the long term antibody response following ICC xenotransplantation, with focus being on the IgG antibody response. Most patients continue to produce xenoimmune antibodies for extended periods after transplantation. Interestingly, we found that some patients produced IgG1 antibodies, at 6-8 years after transplantation, which were not directed against the Galalpha 1,3Gal-epitope. These IgG1 antibodies were not present at one year after xenotransplantation. The biological function of the long term xenoimmune antibodies was investigated, showing that the sera containing the non-Galalpha 1,3Gal-reactive IgG1 antibodies mediate antibody dependent cellular cytotoxicity against pig islet cells. The immunological specificity of the non-Galalpha 1,3Gal-reactive IgG 1 antibodies was not revealed but we showed that the antigen is present on endocrine pig islet cells. We also investigated whether the xenotransplanted fetal islets could cause an increase/recurrence of autoantibody titers. Analysis of IgG antibody titers against GAD and IA-2 was performed, comparing sera from before, 30 days after and 7-9 years after xenotransplantation. However, no serum samples contained any elevated autoantibody titers following xenotransplantation. IgG1 antibody formation is known to be dependent on T cell help. Therefore the human T cell reactivity against pig islet cells was investigated. Endocrine islet cells are not professional antigen presenting cells. Thus, T cell reactivity against such grafts is most likely the result of indirect antigen presentation. We have produced and characterized a number of T cell clones indirectly activated against pig islets. All clones but one were CD4+ and expressed conventional alphabeta T cell receptors (TcR). The TcR usage was heterogeneous, a finding with implications for future xenotransplantation. Furthermore we did not detect any activation of NKT cells or gammadelta T cells, arguing against glycolipids as major T cell xenoimmunogens. The immunological specificities varied among clones with some specifically reactive against pig islet cells and others exhibiting a broader xenoreactivity. All clones produced a wide variety of cytokines with no bias towards so called Th1 or Th2 patterns. None of the CD4+ T cell clones were cytotoxic against pig islet cells, but two clones killed pig lymphoblasts. Taken together, these results demonstrate that the immune response against pig islet cells is heterogeneous and not restricted to one dominating epitope. This is true for both T and B cell reactivity. The biological importance of xenoimmune antibodies needs to be emphasized and in any future clinical trials, the immunosuppressive treatment needs to be directed against the B cell reactivity not only in the initial post-transplant period but also during maintenance therapy.
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