Purpose: Xenotransplantation from genetically modified swine has been proposed to address the shortage of hearts for transplantation in patients with end-stage heart failure. Here, we evaluated if triple carbohydrate deletion is sufficient in protecting the cardiac xenograft from recipient complement and antibody deposition or additional expression of human transgenes is required for xenograft protection. Additionally, we investigate the evidence of thrombotic complications in both groups. 

Methods: Baboons weighing 15-30 kg were used as recipients for life-supporting cardiac xenografts. Weight-matched swine with GTKO.hCD46.hTBM (Group 1) or triple carbohydrate knockout-GTKO.B4GalKO.CMAHKO constructs (Group 2), with or without hCD46 and hDAF, were used as donors. Histopathologic evaluation was performed on postmortem xenograft samples and stained for IgG, IgM and C4d complement deposition.

Results: Group 1 cardiac xenografts (n=4) were found to have survival up to 57 days post-transplant with minimal deposition of complement (figure 1). However, Group 2 grafts (n=4) did not survive long term and showed increased deposition of complement on explanted xenografts. Antibody deposition was variable but increased in triple knockout xenografts. Additionally, microscopic and macroscopic thrombotic phenomena occurred in 2 of these recipients. Group 2 grafts containing hCD46 and hDAF, even in the absence of hTBM, did have a protective effect against graft complement deposition. However, overall survival was still limited.

Conclusion: While hyperacute immunologic rejection has been circumvented with novel immunosuppression regimens and GTKO, it is likely that hTBM and complement regulatory transgenes are needed for improved xenograft protection and recipient survival.