Islet xenograft rejection : Studies in the pig-to-rodents and pig-to-primate models

Sammanfattning: The aims of this thesis was: I) to compare islet allo- and xenograft rejectionin rats, using immunosupreesion with CsA in a mixed allogeneic-xenogeneic islet transplantaionmodel, 2) to evaluate the efficacy of various immunosuppressive drugs, with specialreference to Leflunomide, in preventing islet xenograft rejection in the pig-to-ratmodel, 3) to perform an immunohistochemical study of porcine islet xenograft rejecionin primates including primates treated with CsA+DSG, and 4) to study the metabolismand excretion of porcine C-peptide in mice, with special reference to the use ofdonor-specific C-peptide for monitoring islet xenograft function. In summary, theresults were as follows: I ). After transplantation of mixed allogeneic-xenogeneic islet grafts into ratstreated with CsA, a cell-mediated rejection destroyed the xenogeneic islets but leftthe allogeneic islets intact. Thus, the ongoing islet xenograft rejection did notinitiate rejection of the allogeneic islets as well. The xenogeneic islets were destroyedby a massive cellular infiltration, similar to that observed after transplantationof xenogeneic islets alone. We conclude that the xenogeneic islets were rejectedby a cell-mediated process, most likely qualitatively different from islet allograftrejection. 2). 12 days after transplantation, fetal porcine ICC transplanted into untreatedrats were destroyed by a massive cellular infiltration. Treatment with CsA+LEF+MMFprevented rejection of ICC's for up to 24 days after transplantation. Immunosuppressionwith CsA+DSG (10 mg/kg BW) prevented rejection for up to 12 days. It can be speculatedthat the preventive effect resulted from a suppressive effect on macrophage activationby xenoreactive T cells. However, a direct suppressive effect on macrophages cannotbe ruled out. CsA+LEF+RPM, CsA+CYP+plasmapheresis, or CsA+plasmapheresis also preventedICC xenograft rejection. However, these tbree protocols all caused significant toxicity.Treatment with CsA+LEF, CsA+RPM, and LEF+RPM also had an inhibitory effect but therejecion was not prevented. The protecive effect of CsA+LEF was probably not causedby inhibition of pynmidine nucleotide biosynthesis or from the suppression of xenoreactiveantibody production. 3). Fetal porcine ICC transplanted under the kidney capsule of cynomolgus monkeyswere rejected by a cell mediated process, during the first 6 days after transplantation.This process was not dependent on recipient xenoreactive antibodies or C3-bindingto the graft. There was no evidence of an ADCC since there were no deposits of IgM,IgG, C Iq or C3 in the grafts. The rejection of ICC was more vigorous in the primatesthan in rodents. In contrast to the finding in rodents, in which macrophages werethe main infiltrating cells, islet xenograft rejection in the primates was dominatedby infiltration with CD8-posiive T cells The rejection was delayed in cynomolgosmonkeys treated with CsA+DSG, but not as significant as in the pig-to-rat model.In these animals, there was a marked reducion in the infiltration of CD8-positiveT cells. However, no, or only a marginal, reduction of the macrophage infiltrationwas observed. . On the basis of these observations, we can speculate that rejectionof a fetal porcine ICC xenograft in the cynomolgus monkey depends on at least twoqualitatively different mechanisms. In non-immunosuppressed primates, a rapid andvigorous CTL-mediated rejection process dominated. When this mechanism was inhibitedin the primates immunosuppressed with CsA+DSG, a second mechanism far less sensitiveto treatment with CsA+DSG and dominated by a massive infiltration of macrophageswas revealed. 4). Injections of porcine C-peptide into mice did not result in the excretionof the C-peptide in the urine. In contrast, when porcine C-peptide was injected intonude mice, small amounts of the C-peptide were excreted. Presumably, this is becauseof more pronounced immunological reactivity to, and subsequent accelerated degradationof, the xenogeneic peptide in immunocompetent mice than T cell deficient mice. Afters.c. injection of radioactively labeled porcine C-peptide into mice, the radioactivityin urine was at background levels. The probable explanation of this finding is thatthe the xenogeneic C-peptide had been degraded and that the radioactive tracer hadbeen separated from the peptide during the degradation process. Furthermore, followinginjection of radioacively labeled porcine C-peptide, radioactive uptake in tissuesbelonging to the mononuclear phagocytic system, including Kupffer cells, was significantlyincreased in mice which had previously been injected with porcine C-peptide for severalweeks. This may reflect an increased immunological reactivity to, and a subsequentincreased degradation of xenogeneic C-peptide. Consequently, in islet xenotransplantrecipients, determinations of donor-specifie C-peptide may not properly refleet isletxenograft funetion. In fact, negative findings concerning xenogeneic C-peptide inthe urine may occur in spite of islet xenograft function. Key words: Xenotransplantation, islets, rejection, porcine, immunosuppression,C-peptide ISBN 91-628-2835-5

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