Molecular and Cellular Hurdles to Xenotransplantation

The transplantation of organs, tissues, or cells between individuals of different species has been of increasing interest in recent years because the use of animals as organ and tissue donors as a source of transplants would overcome the severe and worsening shortage of human organs available for transplantation. This shortage restricts the application of organ transplantation to 5–15% of the patients who might benefit in the United States (1). Indeed, the shortage of donor organs is now widely acknowledged to be the major limitation of transplantation. Interest in xenotransplantation also arises because a xenotransplant might, in principle, be less susceptible to infection by viruses or other agents that caused the primary disease. Avoiding viral infection was the rationale for several baboon-to-human liver transplants (2) and for the transplantation of baboon bone marrow in a human patient with AIDS. Interest in xenotransplantation may further arise because the animal source can be subjected to genetic engineering, and such engineering might provide a vehicle for delivery of genes or enduring expression of genes (3). For example, genetic engineering might be used to express therapeutic genes in stem cells.