Neural tissue xenografting

Neural transplantation may become an important treatment alternative for focal brain disorders. To date, the most successful grafts have been obtained in patients with Parkinson's disease. Completely normalized dopamine production and reduction of Parkinsonian symptoms have been demonstrated 10 years after grafting. However, the allogeneic donor tissue has to be obtained from induced abortions, and there are logistical difficulties, risks of infection, and ethical constraints limiting a wider clinical use. Xenografting is an alternative that could bridge these limitations if immunological rejection could be prevented. Pig embryonic neural tissue has been grafted to patients with Parkinson's disease, but no functional benefits have clinically been proven so far. The immune reactions to neural xenografts were incompletely characterized at the time of these early clinical trials, and it is likely that the treatments used were insufficient and that the grafts were rejected. In this article we will review new experiments addressing the immune responses against porcine neural tissue grafted to the adult brain, including the role of antibodies, complement, natural killer (NK) cells, lymphocytes, as well as the effects of immunosuppressive drugs and donor tissue modifications.