Clinical Pig Kidney Xenotransplantation: How Close Are We?

Abstract

Patients with ESKD who would benefit from a kidney transplant face a critical and continuing shortage of kidneys from deceased human donors. As a result, such patients wait a median of 3.9 years to receive a donor kidney, by which time approximately 35% of transplant candidates have died while waiting or have been removed from the waiting list. Those of blood group B or O may experience a significantly longer waiting period. This problem could be resolved if kidneys from genetically engineered pigs offered an alternative with an acceptable clinical outcome. Attempts to accomplish this have followed two major paths: deletion of pig xenoantigens, as well as insertion of "protective" human transgenes to counter the human immune response. Pigs with up to nine genetic manipulations are now available. In nonhuman primates, administering novel agents that block the CD40/CD154 costimulation pathway, such as an anti-CD40 mAb, suppresses the adaptive immune response, leading to pig kidney graft survival of many months without features of rejection (experiments were terminated for infectious complications). In the absence of innate and adaptive immune responses, the transplanted pig kidneys have generally displayed excellent function. A clinical trial is anticipated within 2 years. We suggest that it would be ethical to offer a pig kidney transplant to selected patients who have a life expectancy shorter than the time it would take for them to obtain a kidney from a deceased human donor. In the future, the pigs will also be genetically engineered to control the adaptive immune response, thus enabling exogenous immunosuppressive therapy to be significantly reduced or eliminated.

Keywords: clinical trial; genetically-engineered; kidney; nonhuman primates; patients; pigs; selection; xenotransplantation.

Figure 1.

Percentage survival of patients with ESKD by treatment modality in 2010 (modified from reference , with permission, and on the basis of data from two sources: US Renal Data System [USRDS] and Orandi et al.).

Figure 2.

Reported maximum survivals of nonhuman primates with life-supporting pig kidney grafts, 1989–2017. Details can be found in Lambrigts et al., and Cooper et al. In some years, no results were reported.

Figure 3.

Macroscopic appearances of a wildtype (i.e., genetically unmodified) pig kidney transplanted into a baboon (A) immediately after reperfusion, and (B) 10 minutes later, showing the typical features of hyperacute rejection. Modified from reference , with permission.

Figure 4.

Rapid development of thrombocytopenia (consumptive coagulopathy, a reliable indicator of graft rejection/failure) in two baboons with life-supporting GTKO/hCD46 pig kidney grafts (indicated in red), and maintenance of normal platelet counts in two baboons (treated identically) with life-supporting GKTO/hCD46/hTBM pig kidney grafts (indicated in black). Modified from reference , with permission.

Figure 5.

(A) Pig kidney graft survival in baboons receiving either conventional (tacrolimus-based; group A) or anti-CD40 mAb–based (group B) immunosuppressive therapy. Median pig kidney graft survival in group B (186 days) was significantly longer than in group A (13 days) (P<0.01). Reproduced from reference , with permission). (B) Histopathology of thrombotic glomerulopathy in a pig kidney of group A. Glomerular histopathology included thrombotic microangiopathic glomerulopathy, glomerular thrombi, mesangial thickening, and glomerular edema (expansion of Bowman’s space). Hematoxylin and eosin stain, original magnification ×400. Reproduced from reference , with permission.

Figure 6.

(A) Human IgM (left) and IgG (right) antibody binding to wildtype (WT), GTKO, double knockout (DKO; i.e., knockout of Gal and Sda), and triple knockout (TKO) pig red blood cells (RBCs). Human serum antibody binding to pig RBCs (n=14) was measured by flow cytometry using the relative geometric mean (GM), which was calculated by dividing the GM value for each sample by the negative control (see Gao et al., or Li et al.). Negative controls were obtained by incubating the cells with secondary anti-human antibodies only (with no serum). Binding to TKO pig RBCs was not significantly different from human IgM and IgG binding to human RBCs of blood type O. **P<0.01. Modified and reproduced from reference , with permission. (B) Pooled human serum complement-dependent cytotoxicity (hemolysis) to WT, GTKO, and TKO (GTKO/β4GalKO/CMAHKO) pig RBCs was performed (Cooper et al.). Briefly, RBCs were incubated with diluted serum for 30 minutes at 37°C. After washing, RBCs were incubated with rabbit complement (Sigma, St. Louis, MO) (final concentration 20%) for 150 minutes at 37°C. After centrifugation, supernatant was collected, and hemolysis was evaluated using a Multi-Label Microplate Reader (Victor3; Perkin Elmer, Waltham, MA). The absorbance of each sample at 541 nm was measured. Cytotoxicity of the same serum to autologous human blood type O RBCs was tested as a control. Serum cytotoxicity to TKO pig cells was not significantly different to human O blood type cells. Reproduced from reference , with permission. (C) Human (left) and baboon (right) IgM and IgG binding to TKO pig red blood cells. Baboon sera contain significantly more IgM directed to TKO pig cells than human sera. Modified from reference , with permission.

Figure 7.

Increases in the lengths of the kidneys in four baboons with genetically engineered pig kidney grafts that functioned for 90, >136, >237, and >260 days, respectively. There were similar increases in the width and depth of the kidneys. A 2-month-old pig weighing 20 kg will grow rapidly to approximately 75–90 kg by 6 months of age, whereas a recipient 8 kg baboon may grow by only 1 kg within the following 6 months.