Xenotransplantation – introduction

Meeting demand for cells, tissues and organs

Since the 1960s, transplants of human cells, tissues or organs from deceased donors have been used successfully to treat disease. However, this has resulted in a demand for donated cells, tissues and organs that far exceeds supply. Because people are living longer and the population is increasing, this need is only likely to increase. Researchers are investigating whether xenotransplants – transplants between animals and humans – may help to solve this shortfall.

History of xenotransplantation

Animal to human transplants were first attempted in the early 1900s, but all of these xenotransplants failed. Over the last century, our increasing knowledge of the immune system’s role in rejection is now making xenotransplantation a real possibility. This article has more information about the history of xenotransplantation.

Uses of xenotransplantation

Xenotransplantation could benefit thousands of people by providing an unlimited supply of cells, tissues and organs with many uses:

• Organ transplants – replacing diseased organs, such as hearts, lungs, livers, pancreases and kidneys.
• Cell transplants – replacing damaged or destroyed cells in diseases such as diabetes, Alzheimer’s and Parkinson’s disease
• Tissue transplants – skin grafts, cornea transplants or bone transplants.
• Bridging transplants – providing organ function externally to patients with organ failure.

Preventing xenotransplant rejection

Our immune system specialises in recognising and attacking foreign cells and tissues – helping us to fight infections and stay healthy. It also recognises transplanted tissues as foreign. Transplanted animal tissues, in particular, are rapidly rejected by a person’s immune system – this is called hyperacute rejection.

Researchers are investigating several ways of preventing xenotransplant rejection:

• Suppressing the recipient’s immune system: The immune system’s response can be suppressed with drugs, although these can be toxic and affect the recipient’s ability to fight off infections. Alternatively, the recipient’s immune system can be manipulated by removing antibodies to the xenotransplant or adding immune cells from the donor animal.
• Modifying the genetic make-up of donor animals: Using modern DNA technologies, donor animals can be genetically modified so their tissues are no longer recognised as foreign by the transplant recipient’s immune system.
• Protecting cells from the immune system: This is only possible for cells, not whole organs. Transplanted cells can be protected by coating them in substances that prevent them being recognised by the recipient’s immune system. For example, in New Zealand, Living Cell Technologies (LCT) protects its pig cell transplants with a seaweed-based coating that prevents the immune system from detecting the cells. These encapsulated cells are being trialled as treatments for type 1 diabetes and Parkinson’s disease. Find out more about Pig cell transplants.

Risk of disease transmission

One of the main concerns raised by xenotransplantation is the risk of an infectious disease or zoonosis spreading from animals to humans. Examples of zoonotic diseases include human immunodeficiency virus (HIV), bird flu and bovine spongiform encephalopathy. The article Ethics of xenotransplantation discusses these issues more.

Choosing animal donors

While it may seem that animals similar to humans, such as chimpanzees and baboons, would be the best source of xenotransplants, their similarities mean that there is a greater risk of cross-species infection. The use of these animals also raises significant ethical issues.

Pigs are the current preferred donor species because their organs are a similar size to human organs, they have large litters and they are easy to rear. Fewer ethical concerns are raised about using pigs as they are already killed for food. However, all pigs carry a virus called porcine endogenous retrovirus (PERV). This virus has never been shown to infect recipients of pig tissue, but it could potentially release infectious particles and cause a new disease epidemic.

Alternatives to xenotransplantation

Some conditions that require a transplant are brought about by lifestyle. For example, heart transplants are often required by people who have damaged their hearts through poor diet, insufficient exercise and/or smoking. Perhaps if people had healthier lifestyles, the demand for donors would decrease?

The shortfall in human donors could also be addressed by increasing the number of willing donors. But how do you do this, and will it be enough? Use the article Ethics of organ donation to get further information.

Mechanical artificial body parts may provide alternative solutions to some disorders currently requiring a transplant. Pacemakers are a very successful example of this.

Finally, stem cell research might find ways to replace more complex tissues, although many people still consider this work to be highly controversial.

Use this activity, Q&A: Xenotransplantation in your classroom to check your students understanding of xenotransplantation.