Since the first attempt in 1968, survival following pig-to-primate liver xenotransplantation (LXT) has been limited. We evaluated a model utilizing α-1,3-galactosyltransferase knockout donors, continuous post-transplant infusion of human prothrombin concentrate complex and immunosuppression including anti-thymocyte globulin, FK-506, methylprednisone and co-stimulation blockade (belatacept, n=3 or anti-CD40mAb, n=1) to extend survival. Baboon #1 remained well until POD25 when euthanasia was required due to cholestasis and plantar ulcers. Baboon #2 was euthanized following a seizure on POD5, despite normal LFTs and no apparent pathology. Baboon # 3 demonstrated initial stable liver function, but was euthanized on POD8 due to worsening LFTs. Pathology revealed C4d positivity, extensive hemorrhagic necrosis and a focal CMV inclusion. Baboon # 4 was clinically well with stable LFTs until POD29, when euthanasia was again necessitated by plantar ulcerations as well as rising LFTs. Final pathology was C4d negative and without evidence of rejection, inflammation or TMA. Thus, nearly one-month rejection-free survival has been achieved following LXT in 2 of 4 continuous recipients, demonstrating that the porcine liver can support life in primates for several weeks and is encouraging for potential clinical application of LXT as a bridge to allotransplantation for patients with acute-on-chronic or fulminant hepatic failure.

In our studies of life-supporting GalT-KO pig-to-baboon kidneys, we have found that some recipients developed increased serum creatinine with growth of the grafts, without histologic or immunologic evidence of rejection. We hypothesized that the rapid growth of orthotopic pig grafts in smaller baboon recipients may have led to deterioration of organ function. To test this hypothesis for both kidneys and lungs, we have assessed whether the growth of outbred (Yorkshire) organ transplants in miniature swine is regulated by intrinsic (graft) or extrinsic (host environment) factors. Yorkshire kidneys exhibited persistent growth in miniature swine, reaching 3.7x their initial volume over 3 months vs. 1.2x for miniature swine kidneys over the same time period. Similar rapid early growth of lung allografts was observed and, in this case, led to organ dysfunction. For xenograft kidneys, a review of our results suggests that there is a threshold of 25cm3/kg for kidney graft volume/recipient body weight that induces cortical ischemia in transplanted GalT-KO kidneys in baboons. These results suggest that intrinsic factors are responsible, at least in part, for growth of donor organs and that this property should be taken into consideration for growth-curve mismatched transplants, especially for life-supporting organs transplanted into a limited recipient space.

We have previously reported that co-transplantation of the kidney with vascularized donor thymus from α-1,3-galactosyltransferase gene knockout pigs with an anti-CD154 with rituximab-based regimen led to improved xenograft survival in baboons with donor-specific unresponsiveness. However, nephrotic syndrome emerged as a complication in which the glomeruli showed mild mesangial expansion with similarities to minimal change disease (MCD) in humans. Since MCD is associated with CD80 expression in glomeruli and elevated urinary excretion, we evaluated a potential role for CD80 in xenograft nephropathy. Study 1 confirmed high urinary CD80 excretion in nephrotic animals with renal xenografts showing CD80 expression in glomeruli. In Study 2, baboons receiving xenografts received CTLA4-Ig once a week from the second postoperative week or no CTLA4-Ig. The non-CTLA4-Ig group developed severe proteinuria with modest mesangial expansion with high urinary excretion of CD80 and documented CD80 expression in glomerular podocytes. All of the recipients in non-CTLA4-Ig groups had to be euthanized before POD 60. In contrast, CTLA4-Ig group showed a marked reduction in proteinuria and survived significantly longer, up to 193 days. These results demonstrate that anti-CD80 targeted therapy represents a promising strategy for reduction of proteinuria following renal xeno-transplantation with improved survival.

We sought to determine the effects of exogenous administration of human coagulation factors following pig-to-baboon liver xenotransplantation (LXT) using GalT-KO donors. Post-LXT, baboons received either: no coagulation factors (historical control, n=1), bolus administration of a human prothrombin concentrate complex (hPCC) (2.5 mL/kg, n = 2), continuous infusion of hPCC (1.0 ml/hr, n=1) or a continuous infusion of human recombinant Factor VIIa (1 mcg/kg/hr, n=3). The historical control recipient demonstrated persistent thrombocytopenia despite platelet administration post-transplant along with widespread thrombotic microangiopathy (TMA). In contrast, platelet levels were maintained in bolus hPCC recipients, however these animals quickly developed large vessel thrombosis and TMA leading to graft failure with shortened survival. Recipients of continuous coagulation factor administration experienced either stabilization or an increase in their circulating platelets with escalating doses. Furthermore, transfusion requirements were decreased and hepatic TMA was noticeably absent in recipients of continuous coagulation factor infusions when compared to the historical control and bolus hPCC recipients. This effect was most profound with a continuous, escalating dose of Factor VIIa. Further studies are warranted as this regimen may allow for prolonged survival following LXT.