B cell tolerance to xenoantigens

Xenotransplantation of pig organs to humans is a possible solution to the shortage of donor organs for transplantation. Multiple immunologic barriers need to be overcome if pig-to-primate transplantation is to be successful. The presence, in humans, of natural antibodies (Abs) directed against Galalpha1-3Galbeta1-4GlcNAc epitopes on pig vascular endothelium provides the major barrier, as antibody-antigen binding initiates the process of hyperacute rejection. Even if hyperacute rejection is prevented, acute vascular rejection develops. Acute vascular rejection is also mediated, in part, by xenoreactive Abs and may be complement-independent. Efforts being made to overcome antibody-mediated rejection include depletion of antibody by extracorporeal immunoadsorption, prevention of an induced Ab response by pharmacologic reagents, B-cell and/or plasma cell depletion, depletion or inhibition of complement, and the use of organs from pigs transgenic for human complement regulatory proteins. The ultimate solution would be the induction of B-cell tolerance to xenogeneic antigens, which is being explored by attempting to induce xenogeneic hematopoietic chimerism. Here, we review the properties of the B cell types responding to xenoantigens and the strategies for tolerizing those B cells.