Xenotransplantation and Inter-Species Kidney Transplants

Could xenotransplantation be the answer to shortage of human organs?

Human kidney cross section
HYWARDS / istockphoto

In 1997, an Indian heart surgeon gained notoriety after he transplanted a pig's heart into a human. The patient died a week later due to complications from infections. However, the incident did bring back focus on a lesser-known field of organ transplantation, in this case from animals to humans. In medical terms, this is referred to as xenotransplantation.

As per the official World Health Organization definition, xenotransplantation refers to:

  • Transplantation of living cells, tissues, or organs of animal origin into humans.
  • Transplantation of human body fluid or cells that have had contact with live non-human cells or tissues.

Imagine the prospect: a future where human organ failure is no longer a dreaded problem. Where a ready, "on-demand" supply of organs obtained from animals is available to be transplanted into people with kidney failure, heart failure, liver failure, etc. The possibilities could be endless. But are we there yet? Is it even possible? And what about the ethical issues?

Xenotransplantation in History

Enhancing the human form and function has been a fantasy that humans have harbored since antiquity. The familiar tale of Icarus and Daedalus attaching birds' wings in their vain attempt to fly across the sea from Crete to Greece is well-known. The popular Hindu God, Ganesha has an elephant's head transplanted on a human form. Some of these symbols date back to over 2000 years prior to Christ. Therefore, it might be safe to say that humans have been toying with the idea of xenotransplantation for over four millennia.

Prior to the misadventures of the Indian surgeon mentioned above, there have been reports of chimpanzee-to-human heart transplant, which was conducted in 1964 (the patient's survival was again very short).

Why Would We Need Animals for Organ Transplants at All?

The short and dispassionate answer is that xenotransplantation could be the answer to the current mismatch between demand and supply. As per the FDA, ten patients die every day in the United States alone waiting for life-saving organ transplants.

USRDS data report that the list of patients waiting for a kidney transplant as of December 31st, 2013 had over 86,000 candidates. This is over four times the number of kidney transplants that were performed in the U.S. in the same year (about 17,600), a stark reminder of the mismatch between the number of donors available and the people waiting on organ transplant waiting lists.

Beyond these life-saving scenarios, treatment of chronic diseases like diabetes has the potential to be a revolutionized due to transplantation of cells and tissues from non-human sources (think pancreatic transplantation in an insulin-dependent diabetic).

Which Animals Could Be Used for Non-Human Kidney Transplants?

Intuitively, it might seem like our closest cousins on the evolutionary chain—"non-human" primates like chimpanzees—would be the best source of such organs. However, these primates are relatively rare and are typically not "reared" on a large scale. Non-primates like pigs, therefore, are preferred because easy availability in practically unlimited numbers makes them a cost-effective source. Specifically, as far as the kidneys are concerned, kidneys derived from pigs are very close in size to a human kidney.

Barriers and Risks

Xenotransplantation has not yet taken off on a large scale because of certain barriers. Here are some of the issues we still face when it comes to transplanting organs from animals into humans:

  • The risk of our immune system rejecting the transplanted organ obtained from an animal.
  • The risk of transmitting infections (known and unknown) from animals into a human: At first glance, this seems like a big risk. In practice, the fact that most potential sources of human organs would be animals raised in controlled and isolated conditions makes for a drastic reduction in this risk.
  • Physiological limitations of the transplant: In the failed chimpanzee-to-human heart transplantation mentioned above, for instance, the smaller size of the chimp's heart being insufficient to take care of a human body's circulatory needs was cited as a possible reason for the patient's death.
  • Ethical issues: Should we take an animal's life to save our own? The public health impact of xenotransplantation is an ethical issue as well. One of the common questions facing transplant physicians today is about what would happen if an infectious agent is inadvertently introduced into human society because of xenotransplantation. This would be something similar to the hypothesis regarding the AIDS virus and its "jump" into humans.

Xenotransplantation and Reality

It is now a commonly held belief that transplantation of non-human organs into people is a matter of when, rather than if. Issues regarding the rejection of such organs might be addressed by potentially having donor animals be genetically engineered to express human genes. If this is successful, a human's immune system is less likely to reject that animal organ. Issues about infection and ethics still require more research.

The first "baby step" towards xenotransplantation might be in the form of a temporizing role in patients with organ failure, where it could be used as a bridge to the final therapy. A plausible scenario could be a patient with fulminant liver failure who does not have a human liver available for transplantation and would otherwise die waiting. In this case, a non-human liver could buy that patient precious time until a human liver is available. We call this the "something is better than nothing" scenario!

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Article Sources

  • Human Organ and Tissue Transplantation. World Health Organization International Xenotransplantation Information. http://www.who.int/transplantation/xeno/en/
  • Cooper D. A brief history of cross-species organ transplantation. Proc (Bayl Univ Med Cent). 2012 Jan; 25(1): 49–57. PMCID: PMC3246856
  • US Food & Drug Administration. Xenotransplantation. https://www.fda.gov/BiologicsBloodVaccines/Xenotransplantation/