Xenotransplantation

Introduction

Tragic shortage of donor organs has compelled the researchers to explore their alternatives. Among the several ways of approaching the alternatives are the development of artificial organs and the growth of replacement organs from human stem cells. But there is another possibility that is the use of organs from other species, viz. Xenotransplantation. Although the concept of trans-species organ transplants seems bizarre, the pace of developments may soon call for its practical implementation.

Surprisingly the pig, rather than other primates, is the best candidate for cross-species transplantation. That is because its organs are around the same size as those of humans. The major obstacle is not what the pig can offer, but what the human body is willing to accept. In the transplants from human donors, rejection by the recipient's immune system is the greatest obstacle. In the case of pigs, the substance that triggers the biggest response by the human immune system is a sugar named alpha-1-galactose, found on the surface of pig cells. Human body does not have this sugar and so its immune system treats it as a foreign antigen and mounts an immune response. The resulting attack by antibodies leads to the formation of blood clots that destroy the function of the transplanted organ.

Recent advances in cloning and genetic manipulation have made it possible to engineer pigs that lack this sugar, and their organs may be suitable for xenotransplantation without the fear of acute rejection. The British company PPL Therapeutic (linked with Roslin Institute) that cloned the sheep Dolly, and an American-South Korean group (linked with Novartis) have reported success with this strategy. Both announced the creation of piglets that lack the gene for the enzyme responsible for attaching the sugar cells. It is a major step forward in the development of xenotransplantation as a practical form or organ replacement therapy.

Japanese institute experimenting with pigs for xenotransplantation has been pursuing a different strategy to overcome the acute rejection response. The Animal Engineering Research Institute, Tsukuba, has created lots of cloned pigs that express certain human genes. But, now the strategy of using gene knock-out and cloning techniques to create useful animals for xenotransplantation has been validated and apparently this is going to be the process for the supply of organs for xenotransplantation, a field whose world market is expected to grow to $ 5 billion.

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