To: Richard Gold & Wendy Adams

MEMORANDUM

To: Richard Gold & Wendy Adams

From: Erin Rogozinski

Date: July 11, 2000

Re: Xenotransplantation Scientific Background

1. Summary

You have asked for a brief assessment of authoritative sources regarding the costs and benefits of xenotransplantation technology. In this memo I will define xenotransplantation, explain why it is relevant, and outline its historical development. I will then examine the problems in achieving xenotransplantation on three levels. First, I will discuss the physiological problems encountered because other species’ organs may work differently from our own. Next, I will discuss the immunological challenges that must to overcome for xenotransplants to survive in the human body. The body has three distinct types of rejection: hyperacute rejection, acute or delayed rejection, and cell-mediated rejection. Researchers will soon be able to practically control these reactions through recent developments. Finally, I will discuss the microbiological threat of zoonoses and the trade-off between individual and societal risk. Although xenotransplantation is less likely to result in individual infection, it creates the possibility of new infections that could be passed from animals to the human population.

2. Introduction

2.1 What is Xenotransplantation?

Xenotransplantation is the transfer of animal cells, tissues, and organs to humans. It is not entirely novel: pig heart valves have been used for many years without known ill effects; however, they are essentially inert tissue and seldom elicit rejection. The possibility is now developing for the transplantation of live animal tissues as a practical treatment option.[1]

2.2 The Need for Xenotransplants

Over the past eight years, the number of organ transplants performed annually has increased by about 30% while the number of candidates waiting for transplantation has increased almost 100%.[2] By far, the largest waiting list is for kidney transplants. The need for organ donors will continue to rise as more patients and new diseases are deemed eligible for treatment by transplantation.[3]

2.3 Historical Development

Efforts to accomplish xenotransplantation were first reported in the early 1960s, involving several chimpanzee-to-human kidney transplants. One patient suffered a rejection episode that was reversed by steroid therapy and another survived for nine months before dying from the side effects of immunosuppression. Subsequent attempts involved baboon-to-human liver transplants and cardiac transplants from several different species to human patients. Although none of these efforts achieved one year patient or graft survival, they showed that xenotransplantation was possible and could last for significant time periods. Research declined in the late 1960s; however, the increase in candidates this decade and ensuing shortage of donors has caused a resurgence of interest in xenotransplantation.[4]

3. Problems with Xenotransplantation

3.1 Physiological Level

One of the major barriers to xenotransplantation is that some types of organs from other species may not function adequately in human hosts. This is especially difficult to evaluate because so few xenotransplants have survived for prolonged time periods. We already know that chimpanzee kidneys can support human life and that porcine insulin can regulate blood sugar levels in humans, but there are reports that primates surviving with pig kidney transplants develop anemia.[5] There may be other discordant signals between pigs and humans. We do not yet know which tissues will function properly in the cross species setting; some functions of xenogeneic organs will remain intact but others may not.[6] It is reasonable to expect significant deficiencies if complex organs, such as the liver, are used for transplantation.[7]

3.2 Immunological Level

3.2.1 Hyperacute Rejection

Experimental studies in the 1960s established that xenografts between very different (discordant) species were immediately destroyed by hyperacute rejection. Hyperacute rejection results from antibodies in the host attacking foreign antigens from the xenograft. One of the major findings in recent years is that pigs express a blood group antigen lacking in humans, galactose-(1-3)galactose [Gal].[8] Natural human antibodies bind to the foreign Gal sugar on a pig organ, destroying the organ within minutes of exposure to human blood.

There has been significant progress in resolving the hyperacute rejection of transplanted pig cells. Although preformed human antibodies attack the Gal antigen, research has shown that the reaction may be overwhelmed by transplanting larger numbers of donor cells.[9] Another potential solution is to breed pigs that resemble humans in lacking the enzyme that synthesises Gal. Although the technology to “knockout” specific genes in pigs does not yet exist, scientists have recently achieved promising results by cloning two lambs to express a targeted gene.[10]

3.2.2 Acute Rejection

For discordant species it is now possible to prevent hyperacute rejection. However, even between similar species, vigorous rejection typically occurs within two to three days. This is much faster than most forms of allogeneic transplantation.[11] Little is known about acute rejection. We do know that it is similar to hyperacute rejection in that it mainly results from human Gal antibodies, but it involves a distinctly different process.

Attenuating the human B-cell response in order to prevent new antibody production is being evaluated as a way to overcome acute rejection. Alternate possibilities include breeding transgenic pigs that either make a competing sugar,  fucose, or possess human genes for certain cell membrane proteins that inhibit the chain of events resulting in acute rejection.[12]

3.2.3 Cell Mediated Rejection

Cell-based rejection is particularly strong in xenotransplants; however, it has been less intensively studied than the other forms of rejection. Until recently, researchers have had difficulty keeping xenografts alive long enough to study cell-mediated immune mechanisms. Most of the features of cell-mediated rejection are similar to allograft rejection only much stronger. Between humans and pigs, the most significant problem is that antigens on the surface of the engrafted cells are perceived as foreign and are attacked by human T-cells.

It is possible to control cell-mediated rejection with standard immunosuppressive drugs, but the doses and combinations needed are extremely toxic. Xenotransplant recipients might need lifelong treatment with these drugs, leaving them perpetually at risk of viruses normally controlled by T-cell immunity.[13] Another treatment involves creating transplant tolerance through a mixed bone marrow chimerism approach. Pig bone marrow is transplanted to the recipient in order to reeducate the recipient’s immune system to accept the donor organ as self.[14]

3.3 Microbiological Level

Even if all of the barriers to xenotransplantation are overcome, the prospect of zoonoses still remains. Xenotransplantation may afford easy passage for animal viruses to infect human recipients and consequently enter the human population.[15] Some supporters argue that since pigs have lived with humans for so long, we would already have picked up any of their microbes capable of infecting us. However, there are three main reasons why infection would be more likely through xenotransplantation. First, the physical barriers to infection are broken if pig tissue is placed within the human body. Secondly, the immune suppression needed to prevent rejection by the host will help viruses to propagate and adapt. Finally, human genes bred into transgenic donor pigs could promote preadaptation of animal viruses for human infection.[16]

The best way to eliminate the risk of zoonoses would be to ensure that no pig viruses were present in the transgenic animals used for xenotransplantation. Transgenic pigs will be screened to eliminate known viruses, but we cannot screen for viruses yet to be discovered. Moreover, pigs contain inherited retrovirus genomes that cannot be eliminated. These viral sequences can be activated in host DNA to produce infectious viruses related to leukaemia and HIV.[17] In 1997, researchers discovered that two of three pig retroviruses infect human cells in culture.[18] A final concern is that pig viruses may not be recognised if they do not cause disease in pigs.[19]

Despite the uncertain risk of viral infection from xenotransplantation, there will almost certainly be less transmission of known infections than by allotransplantation. Human transplantation and blood transfusions have transmitted many life-threatening infections.[20] Whereas researchers have years available to screen animals and to raise them in relative isolation, there are only hours to screen potential human cadavers. In addition, many pathogens are considered acceptable for human donors even though they are known to frequently infect and even kill recipients.[21] However, the benefit of xenotransplantation to the individual is offset by the potential risk of a new infection to the entire human population.

4. Conclusion

Although there are many physical barriers to be overcome before xenotransplantation becomes a viable treatment option, recent developments suggest that the technology will be available in the near future. The most important scientific focus now will be on quantifying the risk of zoonoses and weighing the individual risk against that posed to society at large.

[1] R. A. Weiss, “Xenotransplantation” (1998) 317 BMJ 931-934 [hereinafter Weiss].

[2] H.G. Auchincloss & D.H. Sachs, “Xenogeneic Transplantation” (1998) 16 AR Immunol. 433-470 [hereinafter Auchincloss]; A. Persidis, “Xenotransplantation” (1999) 17 Nature Biotechnology 205-206 [hereinafter Persidis].

[3] Weiss, supra note 1.

[4] The decline is largely attributed to two factors. First, hemodialysis technology became widely available. Secondly, the public started to accept the notion of brain death, thus making a larger supply of human organs available. See Auchincloss and Persidis, supra note 2.

[5] Kidneys synthesise a hormone called erythropoietin that is essential for regulating the production of red blood cells. Porcine erythropoietin does not function in humans, thus human recipients of pig kidneys would need to be treated with recombinant human erythropoietin. See Weiss, supra note 1 and Auchincloss, supra note 2.

[6] Weiss, supra note 1.

[7] Auchincloss, supra note 2.

[8] Weiss, supra note 1.

[9] Auchincloss, supra note 2.

[10] See e.g.: Reuters, “Cloning Produces Sheep With Modified Cells” NY Times (29 June 2000) at (last visited 29 June 2000); “Cloning gets specific” BBC News (29 June 2000) at (last visited 10 July 2000); K.J. McCreath et al., “Production of gene-targeted sheep by nuclear transfer from cultured somatic cells” (2000) 405 Nature 1066 – 1069.

[11] Allogeneic transplantation (allotransplantation) transfers organs from within the same species ie: between a human donor and recipient. See Auchincloss, supra note 2.

[12] Weiss, supra note 1; Persidis, supra note 2

[13] Weiss, supra note 1; Auchincloss, supra note 2.

[14] Auchincloss, supra note 2.

[15] A prime example is the HIV-1 virus which probably came from chimpanzees. The worldwide AIDS epidemic may have been started by a single cross-species event or by the use of chimpanzee kidneys in Africa to create batches of poliovirus vaccine. R.A. Weiss, “Xenografts and Retroviruses” (1999) 285 Science 1221-1222.

[16] Weiss, supra note 1.

[17] Weiss, supra note 1; Every potential donor species also carries one or more herpes virus, usually silently by a high percentage of individuals. These viruses could cause severe disease in humans. F.A. Murphy, “The Public Health Risk of Animal Organ and Tissue Transplantation into Humans” (1996) 273 Science 746-747.

[18] C. Patience, Y. Takeuchi & R.A. Weiss, “Infection of human cells by an endogenous retrovirus of pigs” (1997) 3 Nature Med 282-286; P. Le Tissier et al., “Two sets of human-tropic pig retrovirus” (1997) 389 Nature 681-682.

[19]For example, a pig calcivirus related to hepatitis E was discovered in 1997 that might cause no harm to pigs but be pathogenic to humans. Similarly, macaques herpes virus B only gives monkeys coldsores but causes encephalitis in humans. See X-J Meng et al., “A novel virus in swine is closely related to the human hepatitis E virus” (1997) 94 Proc Natl Acad Sci USA 9860-9865; Weiss, supra note 1.

[20] These viruses include HIV, hepatitis B and C, various herpes viruses, tuberculosis, and Creutzfeldt-Jakob disease. See supra note 15.

[21] Auchincloss, supra note 2.