transplantation via the portal vein to the liver resulted in failure (see Figure 4.3). These experiments give important pointers as to the ways in which pigs might be genetically engineered to reduce the immune responses described previously. The failure of the human transgene to be expressed on the islet b cells also highlights the importance of selecting the appropriate promoter when constructing animals specifically designed to provide therapeutic cells. At present, the only animals that have been genetically engineered with the specific aim of providing cells for xenotransplantation are tilapia (a fish) into which the human insulin gene has been introduced [60].
Clinical Islet Xenotransplantation
Dr. Rafael Valdes at Hopital de Infantil de Mexico in Mexico City has performed a small controversial study of porcine to human islet transplantation in type 1 diabetic patients. Twelve patients with C-peptide negative type1diabetesweretransplantedwithneonatalpigislets (5,000/kg) and Sertoli cells (35 to 100/islet) derived from an SPF herd of pigs. The cells were inserted into neovascularized collagen tubes created by implanting a closed stainless steel mesh containing an inner Teflon stent subcutaneously 8 weeks prior to transplantation [80]. No immunosuppression was given. Of the12 transplanted patients, 6 have functioning grafts as defined by reduction of exogenous insulin requirement HbA1c < 8% and production of porcine insulin
in response to IV glucose challenge. Biopsies of the grafts revealed the presence of insulin- and glucagonpositive cells, although the overall architecture of the islets was destroyed. Follow-up extends to more than 4 years, with two patients currently insulin independent. The presumption is that these cells survive in this subcutaneous environment because the Sertoli cells induce a T-cell deficit via Fas-Fas ligand interactions [8]. The ability of Sertoli cells to provide tropic and nutritive factors that stimulate germ cells and support islets in vitro [47, 75] must also be taken into account when interpreting the overall results of this study. However, it remains unclear how these islets can survive in the presence of both preexisting and elicited anti-pig antibody. It seems possible that the artificially created collagen chamber may provide a protective environment comparable to that of the kidney capsule in nude mice and rats. This chamber cannot, however, be considered an immunologically privileged site, nor can it be compared with encapsulation technology. Although the Sertoli cells may prevent T-cell-mediated rejection, there are clearly other factors that protect the islets from destruction by antibody and complement. There is a temptation to speculate that graft accommodation [1, 3] may be taking place. However, there is currently no evidence to support (or refute) this hypothesis. Curiously, in all of the reported studies in the literature involving the transplantation of porcine islets under the kidney capsule of nude mice not a single investigator has published an analysis of the antibody responses produced by these animals. Given the recent elegant studies of