Structural biology of carbohydrate xenoantigens

Abstract

Transplantation of organs across species (xenotransplantation) is being considered to overcome the shortage of human donor organs. However, unmodified pig organs undergo an antibody-mediated hyperacute rejection that is brought about by the presence of natural antibodies to Galα(1,3)Gal, which is the major carbohydrate xenoantigen. Genetic modification of pig organs to remove most of the Galα(1,3)Gal epitopes has been achieved, but the human immune system may still recognize residual lipid-linked Galα(1,3)Gal carbohydrates, new (cryptic) carbohydrates or additional non-Galα(1,3)Gal carbohydrate xenoantigens. The structural basis for lectin and antibody recognition of Galα(1,3)Gal carbohydrates is starting to be understood and is discussed in this review. Antibody binding to Galα(1,3)Gal carbohydrates is predicted to primarily involve end-on insertion of the terminal αGal residue, but it is possible that groove-type binding can occur, as for some lectins. It is likely that similar antibody and lectin recognition will occur with other non-Galα(1,3)Gal xenoantigens, which potentially represent new barriers for pig-to-human xenotransplantation.

Keywords::

• antibody structure–function relationships
• carbohydrate antigens
• lectin recognition
• xenotransplantation