BIOTECHNOLOGIES

Xenotransplantation: overcoming the obstacles

The key researchers for pork xenotransplants. Observation by IRM reveals the functioning of a pig’s heart, which could be useful for humans. By measuring the movement of microscopic water molecules in cardiac muscle fibre, this technique can reconstruct the organisation of these fibres (multicoloured spaghetti). Their orientation is a good indicator of integrity and functionality of the heart. The image can also show the rotation of fibre in the space between the pericardium (outer part of the myocardium) and endocardium (inner part). © CNRS Photothèque/Florent Goutailler
The key researchers for pork xenotransplants. Observation by IRM reveals the functioning of a pig’s heart, which could be useful for humans. By measuring the movement of microscopic water molecules in cardiac muscle fibre, this technique can reconstruct the organisation of these fibres (multicoloured spaghetti). Their orientation is a good indicator of integrity and functionality of the heart. The image can also show the rotation of fibre in the space between the pericardium (outer part of the myocardium) and endocardium (inner part).
© CNRS Photothèque/Florent Goutailler

Can animal organs be used to make up for the shortage of organs? The biotechnologies have permitted dramatic progress in transplanting pig hearts and kidneys to monkeys, but it is still too soon to envisage any attempts on man. In the meantime, it is the ethical implications that could be usefully explored.

A decade ago, the medical world had little faith in xenotransplantation, or the inter-species transplantation of organs. Few believed this technology had a future given the seemingly insurmountable immunity barriers and the fear of transmitting animal viruses to man. But progress in the biotechnologies has since brought some dramatic results. ‘On the road leading to clinical xenotransplantation,’ announced the headline in the June 2009 edition of the scientific journal Transplant Immunology. But it looks like a long road, with many obstacles along the way.

Of pigs and men

The most evident benefit of xenotransplantation is to help make up for the shortage of organs, although nobody sees this as the panacea and there remains considerable scope for progress in spreading the word on organ donation and improving the effectiveness of traditional transplants. Yet xenotransplantation has some unexpected virtues also. Medically, it permits better transplant planning and can provide organs of better quality than those currently taken from accident victims or suicides who fall into an irreversible coma. Then there is the ethical advantage of transplant patients being spared the psychological stress of living in the knowledge that they owe their own survival to the death of another human being.

Even among animal rights campaigners there are few who would oppose the use of pig organs to relieve human suffering. But why the pig, as it is indeed on this animal that current xenotransplantation research is concentrating exclusively. First, pig and human organs are of a comparable size. Cardiac surgeons already use pig valves as spare parts for the human heart (these operations not being regarded as xenotransplants as the valve is regarded as inert rather than living tissue). It is also because the gestation is short (115 days) and litters frequent, thereby ensuring a supply of large quantities of organs. Finally, the pig is easy to rear in sanitary conditions that reduce to a minimum the transmission of pathogens. When a sow is close to giving birth she is placed in a sterile bubble, the piglets are born by caesarean, subsequently transferred to a kind of incubator where they are fed on sterilised milk and then raised, after weaning, in a confined pigsty.

More than six months survival

It is from pigs reared in these very strict hygiene conditions that the organs were taken for use, in recent years, in the many transplant experiments involving the great apes. The most dramatic results were obtained with the transplantation of islets of Langherans (the pancreas cells that produce insulin) to diabetic monkeys. Encapsulated in a special material to limit the risk of immune rejection, these transplants enabled diabetic monkeys to regulate their glycaemia for more than six months, without even receiving an immunosuppressor. ‘These results are of the greatest importance as they are the first demonstration of the longterm survival in a primate of a xenotransplant that fulfils a vital biological role,’ explains Emanuele Cozzi of the Department of Medicine and Surgery at the University of Padua (IT), coordinator of the European programme Xenome, the principal European effort in the field of xenotransplantation. Dramatic progress has also been achieved with heart xenotransplants (2 to 6 months) and kidney xenotransplants (3 months) to monkeys. A decade ago the survival records here were no more than a few weeks. On the other hand, lung and liver xenotransplants continue to be rejected after a few days, for reasons that we do not yet fully understand.

The transgenic pig

The first obstacle is what is known as the hyperacute rejection of the transplanted organ, in which clots form that block off the blood supply to the transplanted organ. The recipient then dies within a few minutes or a few hours at the most. Genetic engineering has enabled researchers to partly overcome this obstacle that was regarded as insurmountable a decade ago. A study of the mechanism involved in hyperacute rejection has shown that the cause lies in incompatibilities between the proteins involved in the chemical reactions of the immune defence mechanism (complement cascade) and in blood clotting. The very presence of pig forms of these proteins in the primate’s body triggers these reactions and leads to the almost immediate rejection of the organ. The solution was therefore to genetically modify the pigs to remove these genes from their genome, or otherwise to replace them with genes producing ‘humanised’ forms of these proteins. It is this approach that the European Xenome project plans to pursue to overcome another obstacle, that of acute rejection. This occurs during the weeks following the transplant and is due to the antibodies of the recipient’s immune system attacking the transplanted organ. Xenome has set itself the goal of obtaining a ‘super transgenic’ pig whose genome will have been modelled so that the animal’s organs possess the antiinflammatory, anti-coagulant and immunosuppressive properties that will facilitate its acceptance by the recipient.

In addition to these gene additions, the future super transgenic pig will also possess fewer DNA sequences, namely those of the PERV (porcine endogenous retro-virus), a pig virus that has the formidable property of being able to infect, in vitro, human cells. In the midst of the BSE crisis and fears of pathologies crossing the species barrier, the discovery of this retrovirus in 1997 very nearly put an end to xenotransplantation research. This virus is in fact present in the genome of nearly all pig varieties and no extent of hygiene precautions can eliminate it. With hindsight, it seems that the threat is less worrying than was initially believed. No human contamination by the PERV has ever been identified, despite very advanced research. But this does not mean that the virus may not surface under certain conditions. At the time of the A/H1N1 flu virus pandemic, which in part originated in pigs, no precaution can be too much: the sequences that are essential to PERV reproduction will therefore be eliminated in the super transgenic pig on which the Xenome project is working.

The ethical before the technical

When this animal – which those involved in the project like to describe as ‘Europe’s contribution to xenotransplantation research’ – becomes available, will it then be time to start the clinical trials? Emanuele Cozzi is very cautious. ‘Research in recent years has concentrated mainly on the effectiveness of xenotransplantation and other fundamental questions must be studied before moving on to clinical trials: physiology and safety, but also ethics and the regulatory aspects of xenotransplantations.’ Even if the problem of immune rejection is resolved there is nothing to indicate that pig organs will be able to replace human organs. The blood cholesterol level of pigs is lower than that of man, for example. In the case of a pig heart transplant, the high cholesterol levels in man could block the arteries. Also, organs such as the liver, kidneys, lungs and pancreas are all subject to hormonal control and it is not known whether animal organs can be regulated by human hormones.

Even if these physiological problems could be fully resolved, an in-depth ethical reflection would be needed before xenotransplantation could become current practice. What would be the criteria, for example, in deciding whether a transplant patient will receive a human organ or a pig organ? As Emanuele Cozzi concludes, ‘until these questions are resolved in a satisfactory manner, I believe, and I am not alone, that it is not yet the time to begin clinical trials.’

Mikhaïl Stein

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