Genetically engineered pigs and target-specific immunomodulation provide significant graft survival and hope for clinical cardiac xenotransplantation

Abstract

Objectives: Cardiac transplantation and available mechanical alternatives are the only possible solutions for end-stage cardiac disease. Unfortunately, because of the limited supply of human organs, xenotransplantation may be the ideal method to overcome this shortage. We have recently seen significant prolongation of heterotopic cardiac xenograft survival from 3 to 12 months and beyond.

Methods: Hearts from genetically engineered piglets that were alpha 1-3 galactosidase transferase knockout and expressed the human complement regulatory gene, CD46 (groups A-C), and the human thrombomodulin gene (group D) were heterotropically transplanted in baboons treated with antithymocyte globulin, cobra venom factor, anti-CD20 antibody, and costimulation blockade (anti-CD154 antibody [clone 5C8]) in group A, anti-CD40 antibody (clone 3A8; 20 mg/kg) in group B, clone 2C10R4 (25 mg/kg) in group C, or clone 2C10R4 (50 mg/kg) in group D, along with conventional nonspecific immunosuppressive agents.

Results: Group A grafts (n = 8) survived for an average of 70 days, with the longest survival of 236 days. Some animals in this group (n = 3) developed microvascular thrombosis due to platelet activation and consumption, which resulted in spontaneous hemorrhage. The median survival time was 21 days in group B (n = 3), 80 days in group C (n = 6), and more than 200 days in group D (n = 5). Three grafts in group D are still contracting well, with the longest ongoing graft survival surpassing the 1-year mark.

Conclusions: Genetically engineered pig hearts (GTKOhTg.hCD46.hTBM) with modified targeted immunosuppression (anti-CD40 monoclonal antibody) achieved long-term cardiac xenograft survival. This potentially paves the way for clinical xenotransplantation if similar survival can be reproduced in an orthotopic transplantation model.

Figure 1

Timeline of various immunosuppressive drugs: The red stop sign indicates that the drug was stopped at this point. The duration of antibody treatment in 4 different groups is shown. Antibody in groups A and D were given until the graft was rejected or explanted.

Figure 2

Graft survival: Figure 1a, survival days for grafts in all groups are shown. Fig 1b, Median survival time of xenografts in each group is shown. Fig 1c, longest survival time in each group is shown.

Figure 2

Graft survival: Figure 1a, survival days for grafts in all groups are shown. Fig 1b, Median survival time of xenografts in each group is shown. Fig 1c, longest survival time in each group is shown.

Figure 3

Complete blood counts: WBC and Platelet counts, hematocrit percentage and Hemoglobin levels were compared between all 4 experimental groups. Horizontal bars in each Figure indicate the median value that is also shown on the graph.

Figure 3

Complete blood counts: WBC and Platelet counts, hematocrit percentage and Hemoglobin levels were compared between all 4 experimental groups. Horizontal bars in each Figure indicate the median value that is also shown on the graph.

Figure 4

Graft histology: H&E staining of a biopsy sample from one representative long term surviving graft each from group A (a) & D (b) is shown. Most of the group A graft shows fibrosis with some areas of normal myocytes, whereas mostly normal myocytes along with a small area of fibrosis is evident in group D.