Type 1 diabetes has been treated successfully by transplanting islets of Langerhans, the endocrine tissue that releases insulin[1
]. Despite the clinical efficacy of islet transplantation, serious issues preclude its broad clinical application, including the side effects of chronic immunosuppressive regimens and a shortage of human donors. For example, it was recently estimated that only one pancreas is available per 333 patients with type 1 diabetes in the United States[2
]. This situation is aggravated by the need of each recipient undergoing transplantation for 2-4 pancreases[1,3
]. This shortage of pancreas donors therefore justifies the search for alternative sources of insulin-producing cells. Swine appear to be the major candidates for islet procurement because: (1) humans have been treated with porcine insulin for > 40 years (pig and human insulin differ by only one amino acid); (2) pigs have large litters with offspring that rapidly attain adult size and are therefore amenable to genetic engineering; (3) pig pancreases contain large islets that respond to glucose stimulation; and (4) since pigs are widely bred and slaughtered for food, the use of their islets to restore human health may be an option that could satisfy sociocultural and ethical concerns[4-6
Unlike primarily vascularized organs, pancreatic islets are implanted without direct connection to the host vascular network, with 7 to 14 d needed to re-establish blood flow[7-12
]. Thus, it was thought that pig islet xenografts could escape typical hyperacute rejection and acute vascular rejection[13
]. In vivo
, however, pig islets in non-immunosuppressed nonhuman primates are rejected by both humoral and cellular immune reactions[14-16
]. Diffuse, presumably nonspecific IgG deposits were observed within islet-associated accumulations of platelets 12 h after transplantation. Deposits of large amounts of IgM and moderate to large amounts of C3, C5, and C9 were present on islet surfaces 2 to 3 d after xenografting[15-17
]. Anti-galactosyl (anti-Gal) and non-Gal[18,19
] IgM antibodies bind to islet surfaces soon after transplantation and activate the classical complement pathway, as well as promoting neutrophil infiltration. These humoral immune responses to pig islets are consistent with early T-cell-independent immune system activation and are reminiscent of mechanisms that operate during the hyperacute rejection of solid-organ xenografts[20
]. Humans and nonhuman primates have preformed anti-pig antibodies that rapidly recognize the Gal epitope on islet endothelial cells. During pig islet-to-primate xenotransplantation, however, the expression of Gal epitopes is influenced by the age of the pig. Gal residues are present on 20% of fetal, but on only 5.1% of adult, islet β cells[21-23
]. Since Gal expression persists after islet isolation[24,25
], Gal remains a target for humoral xenorejection.
Islet xenografts that survive immediate blood-mediated inflammatory reactions and additional humoral damage will be subject to acute cellular xenograft rejection. Following transplantation of fetal pig islets under the kidney capsule, the cellular infiltrate in primates has been found to consist mainly of CD8 T cells (implicating the indirect pathway), whereas the cellular infiltrate in rodents was dominated by macrophages. T-cell infiltration precedes macrophage influx, with small numbers of CD3+
T cells observed 12 h after transplantation[14
]. After 24 h, equal numbers of CD3+
T cells and neutrophils were observed, and after 72 h, CD3+
T cells dominated, representing 50% to 80% of all infiltrating cells. After 72 h, large numbers of macrophages were observed, with T cells localized at the periphery of and within transplanted islets. In addition, increased E-selectin expression on portal vein endothelial cells correlated with the infiltration of neutrophils, which caused tissue damage by releasing enzymes, active oxygen intermediates, and proinflammatory cytokines, and produced chemokines that attracted dendritic cells and T lymphocytes. Pig islet xenorejection seems theoretically easier to overcome, but because hyperacute and acute vascular rejections do not occur, the rapid destruction of pig islets within 72 h of transplantation into nonhuman primates demonstrates the strength of xenorejection.
Thus, an immunosuppressive regimen is mandatory for the long-term survival of pig islets in primates[26,27
]. Although several immunosuppressive strategies have successfully suppressed alloimmune responses, T-cell-mediated xenoimmune responses have proven more resistant to immunosuppressive therapy[28,29
]. This may be due to the greater molecular incompatibility between donor and recipient, which activates particularly the innate immune response[30
Until recently, the maximum reported duration of pig islet survival (insulin-positive cells, no function) following transplantation under the kidney capsule in nondiabetic cynomolgus monkeys and immunosuppression with anti-thymocyte globulin, anti-interleukin-2R mAb, cyclosporine, and steroids was 53 d[31
]. In March 2006, however, two studies reported that neonate or adult pig islets xenotransplanted into primates survived and functioned for > 180 d[26,27
]. More recently, the transgenic expression of a human complement-regulatory protein (hCD46) on porcine islets was shown to enhance the survival of islets xenotransplanted into cynomolgus monkeys with streptozotocin (STZ)-induced diabetes for > 12 mo[32
]. In addition, transplantation of galactosyl knock-out neonate pig islets was found to significantly enhance normoglycemia rates in diabetic primates, likely due to the decreased susceptibility of these xenografts to innate immunity mediated by complement and preformed xenoantibodies[33
]. These results, however, required treatment with a heavy immunosuppressive regimen, in particular an anti-CD154-specific mAb, an antibody that induced thromboembolic events precluding its clinical use[34
]. Despite the unacceptability of these immunosuppressive regimens in humans, these results are very encouraging since an alternative, nontoxic regimen combined with xenotransplantation of pig islets may induce normoglycemia in diabetic patients.
A bioartificial pancreas, in which islets of Langerhans are encapsulated within a semipermeable membrane, may be an alternative therapeutic device for patients with insulin-dependent (type 1) diabetes mellitus. It may constitute a safe and simple method of transplanting islets without the need for immunosuppressive therapy. Since the semipermeable membrane protects the islets from the host immune system, the islets are likely to survive and release insulin for a long period of time, thereby controlling glucose metabolism in the absence of immunosuppressive medication. Nevertheless, several important questions are associated with the transplantation of immunoisolated adult pig islets as a “bioartificial pancreas”[35
] (Table ).
Bioartificial device configurations for encapsulation of pig islets