Human CD55 Expression Blocks Hyperacute Rejection and Restricts Complement Activation in Gal Knockout Cardiac Xenografts

Abstract

Background Transgenic expression of human complement regulatory proteins (hCRPs) reduces the frequency of hyperacute rejection (HAR) in Gal-positive cardiac xenotransplantation. In this study we examine the impact of human CD55 (hCD55) expression on a Gal knock-out (GTKO) background using pig-to-primate heterotopic cardiac xenotransplantation. Methods Cardiac xenotransplantation was performed with GTKO (Group 1; n=6) and GTKO.hCD55 (Group 2; n=5) donor pigs using similar immunosuppression. Cardiac biopsies were obtained 30 minutes after organ reperfusion. Rejection was characterized by histology and immunohistology. Intragraft gene expression, serum non-Gal antibody and antibody recovered from rejected hearts were analyzed. Results HAR of a GTKO heart was observed. Remaining grafts developed delayed xenograft rejection. Median survival was 21 and 28 days for Groups 1 and 2 respectively. Vascular antibody deposition was uniformly detected 30 minutes after organ reperfusion and at explant. A higher frequency of vascular C5b deposition was seen in GTKO organs at explant. Serum non-Gal antibody, antibody recovered from the graft and intragraft gene expression were similar between the groups. Conclusion HAR of GTKO hearts without hCD55 may occur. Expression of hCD55 appeared to restrict local complement activation, but did not improve graft survival. Chronic vascular antibody deposition with evidence of protracted endothelial cell activation was seen. These observations suggest that non-Gal antibody-induced chronic endothelial cell activation coupled to possible haemostatic incompatibilities may be the primary stimulus for DXR of GTKO hearts. To avoid possible HAR, future clinical studies should employ donors expressing hCRPs in the GTKO background.

Current status of pig heart xenotransplantation

“…Expression of multiple hCRPs increases in vitro resistance to complement mediated lysis assays, however, in hCXTx a prospective comparison of single (CD55) and double (CD55:CD46) transgenic donors found no additional survival benefit from the double transgenic donor hearts [50]. In the GTKO background a head-to-head comparisons of GTKO and GTKO:CD55 donors transplanted under identical immune suppression regimens shows GTKO:CD55 donor hearts have improved complement regulation and prevent early graft failure but do not prolong graft survival [51]. The early protective effects of hCRP expression in hCXTx was recently confirmed by an aggregate analysis of GTKO, GTKO:CD55 or GTKO:CD46 transplants from three transplant centers [26].…”

Recent advances in understanding xenotransplantation: implications for the clinic

“…The combination, therefore, of a graft from a pig that does not express the important Gal epitopes, but does express one or more human complement-regulatory proteins provides considerable protection against the primate innate immune response [21,22]. If, in addition, an effective immunosuppressive regimen is administered that prevents a T cell response to the graft, then prolonged graft survival has been obtained [23-29].…”

Multi‐transgenic pigs for xenotransplantation

“…A combination of complement regulatory transgenes and GGTA1 gene inactivation can effectively control hyperacute rejection (Van Denderen et al, 1997;McGregor et al, 2012). Although the complement system and coagulation system are closely connected (Amara et al, 2010), additional genes regulating the coagulation system are necessary to inhibit AVR and AHXR.…”

“…To provide further protection in vivo, the GGTA1 gene was subsequently inactivated. The combination of complement regulatory genes with GGTA1 knockout has already shown to very efficiently prolong xenograft survival (Van Denderen et al, 1997;McGregor et al, 2012). To facilitate transplantation into human patients, the gene CMAH, responsible for the major non-Gal antigen Neu5Gc will be inactivated.…”