Discordant Neural Xenografting: Analysis and Modulation of Immune Responses

Abstract: Transplantation of embryonic human neural tissue can restore function in patients with Parkinson’s disease, but there is a severe shortage of donor tissue which limits the applicability of the technique. Embryonic neural tissue from other species, xenogeneic tissue, would provide a readily accessible donor tissue with the potential to help patients if the immunological reactions in the host could be prevented. Even if the immune responses in the brain are restricted and xenografts placed in the brain survive longer than when placed in other parts of the body, neural xenografts are ultimately rejected in untreated hosts. In this thesis, we show that porcine dopamine neurons can restore functional deficits when transplanted to hemi-parkinsonian rats. Between 6 and 9 weeks after implantation, the porcine neurons had matured to express a TH+ phenotype, extend axons into the host brain and functional effects were seen in the rats. The optimal age for the donor tissue was found to be embryonic days 26-27. There was no proliferation of porcine TH+ neurons after implantation, although porcine glial cells were observed to proliferate in the grafts. Conventional immunosuppression with cyclosporin A (CyA) did not completely protect porcine neural xenografts in rats from infiltration of immune cells and rejection. We therefore analysed the components involved in the rejection process and demonstrate that the immune response to the discordant neural xenografts is dependent on lymphocytes, but that macrophages, microglia and immunoglobulins are also involved in the rejection process. However, Natural Killer cells that are crucially important in the rejection of organ xenografts do not seem to be important for neural xenografts in the brain, which is beneficial. We further show that human complement factors can lyse porcine neural tissue via antibody binding in vitro, but there was no clear protective effect when we grafted porcine neural tissue to complement deficient mice. The role of complement factors and antibodies in the rejection of discordant neural xenografts appears to be an enhancement of the cellular immune responses, in contrast to any direct lytic effects or hyper acute rejection. A combination of CyA or tacrolimus with prednisolone or mycophenolate mofetil (MMF), suppressing broader aspects of the immune response than CyA only, led to an unimpaired graft survival for 14 weeks in rats. We also tried an alternative method of manipulating the immune responses, short-term blockade of co-stimulatory signals, which potentially would be more specific and have less side-effects. Injections with CTLA4Ig and antibodies against LFA-1 during the first 10 days after grafting led to large grafts with minimal infiltration of immune cells at four weeks in mice lacking the co-stimulatory molecule CD40 ligand. In conclusion, embryonic porcine neural tissue is a promising donor tissue for transplantation in neurodegenerative disorders. The treatment of preference would be tacrolimus and MMF, in combination with short-term depletion of xenoreactive antibodies during the period that the blood-brain barrier is open. Before clinical trials are initiated, we suggest that the immunosuppressive regimens are tested in primate models and that the methods with co-stimulation blockade are investigated further.