A brief review of the current status of pig islet xenotransplantation

Abstract

An estimated 1.5 million Americans suffer from Type I diabetes mellitus, and its incidence is increasing worldwide. Islet allotransplantation offers a treatment, but the availability of deceased human donor pancreases is limited. The transplantation of islets from gene-edited pigs, if successful, would resolve this problem. Pigs are now available in which the expression of the three known xenoantigens against which humans have natural (preformed) antibodies has been deleted, and in which several human 'protective' genes have been introduced. The transplantation of neonatal pig islets has some advantages over that of adult pig islets. Transplantation into the portal vein of the recipient results in loss of many islets from the instant blood-mediated inflammatory reaction (IBMIR) and so the search for an alternative site continues. The adaptive immune response can be largely suppressed by an immunosuppressive regimen based on blockade of the CD40/CD154 T cell co-stimulation pathway, whereas conventional therapy (e.g., based on tacrolimus) is less successful. We suggest that, despite the need for effective immunosuppressive therapy, the transplantation of 'free' islets will prove more successful than that of encapsulated islets. There are data to suggest that, in the absence of rejection, the function of pig islets, though less efficient than human islets, will be sufficient to maintain normoglycemia in diabetic recipients. Pig islets transplanted into immunosuppressed nonhuman primates have maintained normoglycemia for periods extending more than two years, illustrating the potential of this novel form of therapy.

Keywords: diabetes; genetically-engineered; islets; nonhuman primates; pancreatic; pig; xenotransplantation.

Figure 1

Adult pig islets after isolation. Adult pig islets stained in red with dithizone after isolation and purification (magnification 40x). (Reproduced with permission from 6).

Figure 2

Human serum antibody binding to WT and TKO pig red blood cells (pRBCs). Correlation between human serum antibody binding to pig RBCs, by relative geometric mean [rGM]) and age. Human serum (A) IgM and (B) IgG antibody binding to wild-type (WT) pRBCs (top) and to Triple-knockout (TKO) pRBCs (bottom). The dotted lines indicate no IgM or IgG binding. (Note the great difference in the scale on the Y axis between A and B.) There is almost no anti-TKO pig antibody production during the first year of life and very low levels in adults compared to antibody against WT pig cells. (Reproduced with permission from 40).

Figure 3

Human and Old World monkey serum antibody binding and cytotoxicity to WT, GTKO, and TKO pig peripheral blood mononuclear cells (PBMCs). Human (top) and Old World monkey (OWNHPs) (bottom) IgM (left) and IgG (middle) binding and complement-dependent cytotoxicity (CDC, at 25% serum concentration) (right) to WT, GTKO, and TKO pig PBMCs. Results are expressed as mean ± SEM. (*p<0.05, **p<0.01; N.S. = not significant). On the y axis, the dotted line represents cut-off value of binding (relative geometric mean [GM]: IgM 1.2, IgG 1.1), below which there is no binding. For CDC on the y axis, the dotted line represents cut-off value of cytotoxicity (6.4%), below which there is no cytotoxicity. (Note the difference in scale on the y axis between IgM and IgG.) Although there is reduced antibody binding and cytotoxicity to GTKO PBMCs in both humans and monkeys, there is an increase in antibody binding and cytotoxicity to TKO PBMCs in monkeys. (Reproduced with permission from 40).

Figure 4

Post-transplant course of an immunosuppressed diabetic monkey following hCD46 pig islet transplantation. Blood glucose (blue) and pig C-peptide levels (red) in a streptozotocin-induced diabetic cynomolgus monkey before and after intraportal transplantation of islets from a pig expressing the human complement-regulatory protein, CD46. No exogenous insulin was administered after the transplant. The normoglycemic monkey was electively euthanized after 12 months. Day 0 = day of islet transplantation. (Reproduced with permission from 43).

Figure 5

GTKO pig kidney survival in baboons receiving US FDA-approved immunosuppressive agents (Group A, in red) was much shorter than in those receiving an anti-CD40mAb-based regimen (Group B, in black). (Reproduced with permission from 56).

Figure 6

Binding of human IgM and IgG antibody to pig islets (xenogeneic) (A, B) and to human islets (allogeneic) (C, D). IgM (green, A, C), IgG (green, B, D), insulin (red), nucleus (DAPI/blue). Yellow indicates colocalization of insulin and IgM/IgG. The greatly increased binding of human IgM and IgG to pig islets (compared to human islets) is obvious. (Reproduced with permission from 63).

Figure 7

Healthy islet in the liver of an immunosuppressed cynomolgus monkey 12 months after hCD46-transgenic pig islet transplantation. (Reproduced with permission from 69).