The anti-nonGal xenoantibody response to alpha1,3-galactosyltransferase gene knockout pig xenografts

Abstract

Purpose of review: Anti-nonGal xenoantibodies are a major barrier to the survival of genetically modified porcine xenografts. This review summarizes the contribution of anti-nonGal xenoantibodies to the activation of porcine endothelial cells and graft rejection, and further provides an update on recent advancements in defining the unique features of anti-nonGal xenoantibody structure.

Recent findings: Anti-nonGal xenoantibodies pre-exist at low levels in humans and nonhuman primates, and are notably absent in neonates. Exposure of nonhuman primates to alpha1,3-galactosyltransferase gene knockout endothelial cells initiates an induced xenoantibody response that is restricted and encoded by the germline immunoglobulin heavy chain gene IGHV3-21. The target xenoantigen remains undetermined, but several candidate targets have been proposed, including carbohydrate xenoantigens. New advancements in molecular modeling provide insight on the mechanism by which xenoantibodies bind to structurally related carbohydrates.

Summary: Genetic manipulation of porcine donors has significantly prolonged the survival of grafts placed into nonhuman primate recipients, but anti-nonGal xenoantibodies and thrombosis limit the ability of these grafts to function on a long-term basis. Recent developments defining pre-existing anti-nonGal xenoantibody levels, the restriction in the anti-nonGal xenoantibody response and the identification of key sites defining xenoantibody-carbohydrate interactions now provide the information necessary to develop new approaches to preventing xenoantibody-mediated rejection.

Figure 1. Key contact sites within the CDR1 and CDR2 regions of anti-gal and anti-non-gal xenoantibodies predicted by computer-simulated modeling

The amino acid translation of the complementarity determining regions of the IgV_H genes encoding induced anti-gal and anti-non-gal IgM xenoantibodies in monkeys are aligned with their closest human germline progenitor. Anti-gal xenoantibodies were induced after transplantation of wild type pig grafts [31]. Anti-non-gal xenoantibodies (hDAF non-gal) were induced after transplantation of hDAF transgenic pig grafts in non-human primates treated with GAS914 [29]. The amino acid sequence of the CDR regions of anti-non-gal xenoantibodies induced after immunization of non-human primates with GalTKO endothelial cells are identified as GalTKO non-gal [11]. Amino acids located at contact sites predicted by computer-simulated models to be relevant for xenoantibody/carbohydrate interaction are identified in bold lettering [32].