Pig Liver Xenotransplantation: A Review of Progress Toward the Clinic

Abstract

Experience with clinical liver xenotransplantation has largely involved the transplantation of livers from nonhuman primates. Experience with pig livers has been scarce. This brief review will be restricted to assessing the potential therapeutic impact of pig liver xenotransplantation in acute liver failure and the remaining barriers that currently do not justify clinical trials. A relatively new surgical technique of heterotopic pig liver xenotransplantation is described that might play a role in bridging a patient with acute liver failure until either the native liver recovers or a suitable liver allograft is obtained. Other topics discussed include the possible mechanisms for the development of the thrombocytopenis that rapidly occurs after pig liver xenotransplantation in a primate, the impact of pig complement on graft injury, the potential infectious risks, and potential physiologic incompatibilities between pig and human. There is cautious optimism that all of these problems can be overcome by judicious genetic manipulation of the pig. If liver graft survival could be achieved in the absence of thrombocytopenia or rejection for a period of even a few days, there may be a role for pig liver transplantation as a bridge to allotransplantation in carefully selected patients.

Figure 1

Time-line in experimental pig liver xenotransplantation in NHPs. Abbreviations: a-CD154 = anti-CD154 monoclonal antibodies; ALG = antilymphocyte globulin; ATG = antithymocyte globulin; Aza = azathioprine; Bela = belatacept; BM = bone marrow; Cs = corticosteroids; CsA = cyclosporine; CVF = cobra venom factor; CyP = cyclophosphamide; FK = tacrolimus; Gal abs = extracorporeal anti-Gal antibody adsorption; GTKO = 1,3-galactosyltransferase gene-knockout; hCD46 = expression of the human regulator of complement, hCD46; hCD55 = expression of the human regulator of complement, CD55; HLT = heterotopic liver transplantation; HT = H-transferase; LoCD2b = anti-CD2 monoclonal antibody; MMF = mycophenolate mofetil; OLT = orthotopic liver transplantation; WBI = whole body irradiation; WT = wild-type.

Figure 2

Histopathology of (A) hyperacute rejection (<24 hours) in a wild-type pig liver transplanted orthotopically into a baboon, (B) a GTKO/hCD46 orthotopic pig liver graft in a baboon that survived for 6 days, and (C) a pig left liver lobe graft in a Tibetan monkey that survived for 14 days (A) WT pig-to-baboon liver xenotransplantation at 1 h (x200). Severe hepatocellular vacuolar change, focal hepatocyte necrosis, and few thrombi. (B) Vacuolar hepatocellular cytoplasmic change with minimal hepatocellular necrosis on postoperative day 6 (x200). (C) The graft shows some lymphocyte infiltration in the portal area, but no major features of antibody-mediated or cellular rejection (x100).

Figure 2

Histopathology of (A) hyperacute rejection (<24 hours) in a wild-type pig liver transplanted orthotopically into a baboon, (B) a GTKO/hCD46 orthotopic pig liver graft in a baboon that survived for 6 days, and (C) a pig left liver lobe graft in a Tibetan monkey that survived for 14 days (A) WT pig-to-baboon liver xenotransplantation at 1 h (x200). Severe hepatocellular vacuolar change, focal hepatocyte necrosis, and few thrombi. (B) Vacuolar hepatocellular cytoplasmic change with minimal hepatocellular necrosis on postoperative day 6 (x200). (C) The graft shows some lymphocyte infiltration in the portal area, but no major features of antibody-mediated or cellular rejection (x100).

Figure 2

Histopathology of (A) hyperacute rejection (<24 hours) in a wild-type pig liver transplanted orthotopically into a baboon, (B) a GTKO/hCD46 orthotopic pig liver graft in a baboon that survived for 6 days, and (C) a pig left liver lobe graft in a Tibetan monkey that survived for 14 days (A) WT pig-to-baboon liver xenotransplantation at 1 h (x200). Severe hepatocellular vacuolar change, focal hepatocyte necrosis, and few thrombi. (B) Vacuolar hepatocellular cytoplasmic change with minimal hepatocellular necrosis on postoperative day 6 (x200). (C) The graft shows some lymphocyte infiltration in the portal area, but no major features of antibody-mediated or cellular rejection (x100).

Figure 3

Platelet counts (A) after GTKO/hCD46 orthotopic pig liver transplantation in baboons (n=6) that survived from 4–7 days, and (B) after a GTKO Wu Zhishan miniature swine heterotopic left liver lobe transplant in Tibetan monkeys (n=3, mean ±SD) within the first 48 hours, and (C) in the Tibetan monkey that survived for 14 days. (A is reproduced with permission from Ekser et al. Transplantation 2010;90:483–493; B is reproduced with permission from Ji H, et al.

Figure 3

Platelet counts (A) after GTKO/hCD46 orthotopic pig liver transplantation in baboons (n=6) that survived from 4–7 days, and (B) after a GTKO Wu Zhishan miniature swine heterotopic left liver lobe transplant in Tibetan monkeys (n=3, mean ±SD) within the first 48 hours, and (C) in the Tibetan monkey that survived for 14 days. (A is reproduced with permission from Ekser et al. Transplantation 2010;90:483–493; B is reproduced with permission from Ji H, et al.

Figure 4

Surgical technique of pig left liver lobe transplantation in Tibetan monkeys. After native splenectomy, the pig liver graft was placed in the splenic recess. The recipient’s left renal vein was divided, the distal end being anastomosed to the graft portal vein, and the proximal end to the graft hepatic vein. The graft hepatic artery was anastomosed end-to-end to the recipient splenic artery (using a microvascular technique with an operating microscope). After reperfusion, the bile duct was drained through the abdominal wall to allow measurement of bile drainage. (Using the same technique in clinical cases of allotransplantation, the bile duct is drained into a Roux-en-Y jejunal loop.)

Figure 5

Potential mechanism of human platelet phagocytosis by pig livers - 1. Human platelets may be selectively phagocytosed by ASGR1-expressing cells, e.g., liver sinusoidal endothelial cells, as human platelets express higher levels of Galβ1,4N-acetylglucosamine (Galβ1,4GlcNAc).

Figure 6

Potential mechanism of human platelet phagocytosis by pig livers - 2. Schematic representation of CD47-SIRPα (signal regulatory protein-alpha) interaction in relation to natural expression of SIRPα on pig Kupffer cells; (Left) In the organ-source pig, there is a normal inhibitory signal of pig CD47 (in this example expressed on platelets) that is recognized by porcine SIRPα; (Center) After pig liver xenotransplantation, there could be a lack of recognition of human CD47 by pig SIRPα, resulting in phagocytosis of human platelets; (Right) Transgenic expression of human SIRPα on pig Kupffer cells would result in recognition of human CD47 on human platelets, thus inhibiting phagocytosis. (Reproduced with permission from Ekser et al. Expert Rev. Clin. Immunol. 2012;8:621–634)