Role of complement regulatory protein expression and CD154 blockade in Gal‐independent xenograft rejection

Abstract

IntroductionHyperacute (HAR) and delayed xenograft (DXR) rejection are well defined barriers following discordant transplantation of porcine organs into primate recipients. Significant progress has been made in recent years to overcome these immunological barriers, based on the development of pigs genetically modified to reduce immunogenicity: [1,2] A major step has been the generation of pigs lacking expression of Gala1,3Gal (GalTKO), the principal carbohydrate target of human anti‐pig antibody. [3,4] However, both in vitro and in vivo studies demonstrated that non‐gal antibodies have the ability to activate endothelial cells, and thus, amplify procoagulant molecular incompatibilities between pig and primate.[5,6] Based on our prior work with pigs expressing the Gal epitope, we tested the hypothesis that over‐expression of human complement regulatory proteins (hCRP) and/or blockade of the CD154/CD40 costimulation pathway will result in reduced graft injury, anti‐pig responses and coagulation dysregulation. Briefly, twenty two organs (13 hearts, 9 kidneys) from GalT‐KO pigs (9 expressing hCRP) were transplanted into baboons that received no immunosuppressive therapy, or a therapeutic regimen based on costimulation blockade. The incidence of HAR was decreased in GalTKO organs expressing hCRP as compared to GalTKO. Pre‐transplant anti‐non‐Gal antibody titers were inconsistently associated with early GalT‐KO xenograft failure. In contrast, coagulation cascade activation and platelet activation correlated closely with incidence of HAR of GalTKO organs. Strong elicited anti‐non‐gal antibody responses were detected after transplantation of a GalTKO organ. Delayed xenograft rejection was associated with coagulation dysregulation and elicited anti‐non‐gal antibody production. Future work in xenotransplantation should place emphasis on further targeting of anti‐non gal humoral immune responses and inhibiting coagulation activation.