4th National Symposium on Biosafety

Infectious Risks in Using Baboons

Jonathan Allan, DVM
Scientist, Southwest Foundation for Biomedical Research
Department of Virology and Immunology
7620 NW Loop 410 at Military Drive
San Antonio, TX 78228
210-670-3275

(Reprinted electronically with permission Nature Medicine, Volume 2, Number 1, January 1996)

Xenotransplantation at a crossroads: Prevention versus progress

The infectious disease risks associated with baboon-to-human transplants may represent an insurmountable hurdle in the race to save lives. Yet, public health agencies are reluctant to regulate xenotransplantation in spite of those risks.

Two public meetings were recently held to address the scientific, ethical and the infectious disease issues involved in the practice of cross-species transplantation to treat human disease (see Nature Medicine 1, 728; 1995) Beyond this, a joint committee of the Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) has now wrestled with the unenviable task of formulating and publishing guidelines that address the public health safety risks surrounding cross-species transplantation in humans. A provisional draft of the guidelines, to be published in the Federal Register (a U S government publication of both regulatory and legal notices), represents more than six month's effort to navigate through a complex problem. Having been a member of the FDA panel at which the guidelines were discussed and having spoken at the both the IOM and FDA meetings about the infectious disease risks associated with baboon transplants, I remain concerned that the proposed guidelines will not significantly reduce the risk of introducing a new human disease through xenotransplantation.

There are at least three main avenues that could have been aken in dealing with the infectious disease risks surrounding xenotransplantation. First, all cross-species transplants could have been banned, although a preferable option would have been to permit only pig organ transplants. The decision to proceed with both species means that our federal agencies have decided that the potential medical beneflts outweigh the infectious disease risks and that any dangers represent "manageable"risks. Second, having approved these procedures on principle, it seems reasonable to develop federal regulations much the same as those that have been developed by the FDA to oversee gene therapy and other novel therapeutic approaches in humans. One important reason for developing such strict regulations has been to ensure that any preparations such as monoclonal antibody treatments or gene delivery vectors be free of replicationcompetent retroviruses. (Retroviruses can be a serious problem because of their inherent ability to integrate into human chromosomes with the potential for inducing cancer).The FDA/CDC working group appears to have instead opted for a third alternative—a series of guidelines to be played out at the local level. Apparentlv there will be little federal oversight once the transplant communitv has begun xenotransplantation in earnest. It is also of concern that these guidelines provide few real safeguards against the introduction and spread of new infectious diseases in the human population.

Xenotransplantation's risks

The most pressing concern with cross-species transplants is the inherent risk of transimitting animal viruses to humans. Several recent reviews have addressed this risk ^1-4. The infectious disease community has been on alert for the appearance of new emerging diseases such as the recent Ebola virus outbreak in Africa, the hantavirus outbreak in the Southwest United States, and the reemergence of dengue fever that currently threatens Texas. It is also well established that most new emerging human infectious diseases generally have their origins in other species ^5-7. A direct method of establishing new human infectious disease would be to implant infected tissues from a nonhuman species into humans thus allowing viruses direct access to human tissues. Baboons carry several known viruses that have the potential to infect humans; these include baboon herpesviruses (SA8, herpes papio and CMV) and retroviruses (STLV, baboon endogenous virus and foamy virus) to name but a few ^8-l2. To complicate matters, the natural host for some infectious agents may also be naturally resistant to any disease associated with infection, which means that there is no way of knowing from any outward appearance whether or not a baboon, for example, carries a potential human pathogen.

How likely is it that a baboon virus could infect and establish itself in humans? Both the FDA and CDC have decided that, while baboon xenotransplantation poses a risk, we cannot quantify that risk, and not to proceed would put an end to scientific research dedicated toward using baboons as donors for human transplantation. However, neither of these agencies have stated that these procedures are safe and several of their scientists have also raised some of the same concerns ^l2. In my opinion, the risks associated with baboon donor tissue are significant and preclude their use as donors. As stated, baboons are known carriers of several chronic latent viuses that can infect human cells and might be a threat if they were inoculated into people. Organ transplantation would circumvent most natural barriers (skin, mucosal surfaces and the acid environment of the stomach), which means that baboon viruses not typically thought to be infectious for humans such as blood-borne or sexually transmitted pathogens would now have access to human organ systems.

To put the potential risk into perspective, there have been several instances where new emerging human viral diseases have been traced to nonhuman primates. The best examples are the simian immunodeficiency viruses (SIV) found in African nonhuman primates, two of which (HIV-1 and HIV-2) have crossed into humans and are responsible for an estimated 4.5 million AIDS cases worldwide ¹³. Of interest, these simian or monkey viruses may have been harbored in African primates for thousands of years without causing any disease in that host ^l4. Although a baboon-specific SIV has yet to be identified, these monkeys do carry viruses in persistent and latent forms, some of which can remain with the baboon throughout its life, increasing the likelihood of its transmission to humans. The first retrovirus discovered in humans, human T-cell lymphotropic virus (HTLV), also appears to have arisen by cross-species transmission of its close cousin (STLV) from African nonhuman primates ^15,16. HTLV and STLV induce leukemia and lymphoma in humans and baboons, respectively, and like HIV they can be transmitted through blood transfusion or sexual contact. Another dramatic example is the monkey equivalent of human herpes simplex virus (herpes B virus) that is found naturally in macaques. This virus causes only minor discomfort including conjunctivitis in monkeys, yet humans that are infrequently infected through close contact with infected monkeys invariably suffer a rapid and fatal encephalomyelitis⁸. However, the most insidious emerging viruses are not those such as Ebola-like viruses or herpes B virus that have immediate and dramatic effects--they are acutely lethal and as such can be effectively contained by quarantine--but viruses including retroviruses that may have extended clinical latency following infection and are therefore difficult to identify in the absence of demonstrable disease. For a new emerging viral infection to become established in humans, not only must a recipient become persistently infected with a baboon virus but there also needs to be transmission from that recipient to one or more contacts. Based on past experience with new infectious diseases in humans and our inability to prevent the worldwide spread of HIV infection, it comes as no surprise then that any measures put in place to contain the spread of a new human infection, should it arise, are unlikely to significantly curb the spread of a new baboon virus in the human population.

The most immediate need for baboon tissues comes from the AIDS research community. Recent proposals to attempt immune reconstitution of HlV-infected patients using baboon-derived bone marrow have been widely publicized. Whether or not bone marrow will be effective in reversing or ameliorating the effects of HIV-1 on the immune system is still unknown and some believe that this type of experiment may even hasten the death of the recipients ^l7. If the patient has little chance of long-term survival, the new viral pathogen is unlikely to be transmitted to others. The real hazard results from the possibility of the recipient transmitting these viruses to their contacts after leaving the hospital and hence the public health significance of these procedures is directly related to the success of these surgical procedures.

The risks are not limited to bone marrow transplants. Several transplant groups have proposed and will soon initiate clinical studies using baboon hearts as a temporary measure for those awaiting suitable human donors. Here, the patients' chances for survival are dramatically improved since the intention is that they will ultimately receive human organs. Of course, any baboon viruses implanted with the baboon heart will not be reoved when the baboon heart is. This stop-gap baboon heart transplantation protocol may therefore be an ideal method of introducing and establishing a zoonotic disease in the human population.

Having studied simian retroviruses for more than 11 years, I am acutely aware of the damage that a supposedly innocuous virus might do, given the right host. A public health disaster was narrowly averted during an Ebola virus outbreak at a primate facility in Reston, Virginia. Not only were macaques infected with an Ebola virus but they also harbored simian hemorraghic fever virus (SHFV), a virus found naturally in African species but which also causes acute illness in macaques. The success in containing these viruses from being disseminated into the human population had more to do with the biology of the viruses than any intervention on the part of public health agencies. In fact several individuals were infected with the Reston Ebola virus but luckily did not develop disease and did not spread the virus to others. Thus, dodging the bullet in this case in no way should be regarded as a success in terms of the measures used to contain such outbreaks. The Ebola virus is an acute pathogen in monkeys and is not normally persistent. Federal regulations prohibit housing Old World monkeys with New World species, or Asian monkeys with African ones, precisely because of the fear of an infectious disease debacle. As an example, squirrel monkeys harbor a herpesvirus (H. saimiri) that causes no overt symptoms in that host, yet when other species are infected they rapidly develop cancer.

Our experience with monkey viruses suggests it is difficult to predict the outcome when a virus from one animal finds its way into another. This is essentially the crux of the problem because a baboon virus is likely to be passed to the recipient. The unquantifiable nature of that risk comes with the outcome. However, simply because we can't measure the risk does not mean that we should proceed. Once the door is opened and a new virus is unleashed, it will be a monumental task to identify a new pathogen, develop adequate screening tests and prevent the spread of that new infection.

Guidelines versus regulations?

Implicit in the development of the FDA/CDC guidelines is the notion that xenotransplantation may become an acceptable surgical procedure for the forseeable future. The most important question then becomes how can a virus,once transmitted from a donor baboon to human, be limited or contained? In choosing voluntary guidelines to be enforced at a local level, rather than federal regulations, the FDA/CDC committee has chosen the least stringent and possibly least successful method of policing these transplants procedures. As a member of the FDA advisory panel that met recently to analyze a protocol for using baboon bone marrow for treating an AIDS patient, I believe we were heavily (and understandably) influenced by emotional pleas on the part of the family of the recipient and by the local community. Knowing that one and only one procedure was being voted upon, this transplant was approved. Unfortunately, having opened the door, it will be difficult to close it again. And if a federal panel can be so heavily influenced by political expediency or desperation on the part of a patient or community, it is not difficult to imagine that guidelines played out on the local level may be similarly influenced by the urgency of these life-or-death dramas. The nature of new infectious diseases necessitates that public health agencies remain vigilant in preventing the introduction of both known and unknown pathogens into a population.

The guidelines

Some of the proposed guidelines should lessen the infectious disease risk, but none of them, in my opinion, will prevent the introduction or possible spread of an unknown virus into the human population. There is little doubt that thorough screening of donor animals for known viruses will significantly decrease the possibility of transmitting a known virus to humans. In transplant situations, it is blood-borne or sexually transmitted viruses that are of most concern, and eliminating known infectious agents that are transmitted among baboons by sexual contact will by inference lessen the risk posed by uncharacterized viruses also harbored and transmitted by similar routes. Nevertheless, there is no way to screen for viruses that have yet to be discovered.

The guidelines also call for posttransplantation virological screening of the human recipients for evidence of both known and unknown baboon viral infections. Although this approach also has merit and should of course be instituted, in cases where the agent is uncharacterized, this search, like that in baboons, will be mostly ineffective. As an example, AIDS patients may have many opportunistic infections, so the ability to identify novel infectious agents from the list of known human pathogens would be no small feat. Direct virus isolation of new agents should also be attempted with the understanding that our ability to identify and isolate novel viruses may be very limited.

Long-term surveillance and recipient transplant registries will help in the tracing and containment of any new infections. Patients could also be advised on the risks of donating blood and the need for vigilance with regard to unexplained illnesses and unusual symptoms. However, these measures only allow one to intercede after a problem has arisen, with the responsibility directed toward containment rather than prevention. Also, leaving surveillance and its costs to local review boards and local public health officials may result in patients being lost to follow-up should they leave the locale. Regional differences in practices might also mean only variable success rates in containing a new infectious disease should it arise. Unfortunately, however, lessons from the AIDS epidemic tell us that these measures will be ineffective, because persons infected by contact with a recipient are not likely to be part of a surveillance program. It has been virtually impossible to stem the spread of HIV among the general population in spite of the wealth of knowledge about HlV's transmission. That virus has eluded some of the best and brightest for more than 15 years, a lesson that is sadly etched into the souls of those infected and those who care for them.

To ensure that baboons are free from the known viruses will require intensive screening and long-term breeding of baboons, much the same as has been established for rhesus monkey breeding colonies in AIDS research. Simply maintaining colonies in well-managed establishments without regard to the pathogens that they carry does not constitute clean animals. Rhesus macaque specific-pathogen-free (SPF) colonies have been developed for research purposes, and there is no reason why the same facilities could not be established for baboons. Not to establish government-sponsored primate breeding resources for human transplantation would be irresponsible. The necessary technology and the expertise is available. Having SPF animals is likely to do more to reduce the risk of introducing a baboon virus into humans than any other single measure. Obviously, we cannot screen for viruses that are yet to be discovered (and there may be several of these), but eliminating the known viruses that have various modes of transmission is also likely to reduce the risk of unknown viruses. If the resolve to build and fund baboon SPF colonies is not there, we should resolve to say "no" to xenotransplantation.

In many ways, the pig offers a more promising transplant organ source. Pigs are more amenable to SPF colonies, they have a much shorter generation time, they have been bred in domesticity for thousands of years, and methods are being developed to genetically manipulate this species to deal with hyperacute rejection. By comparison, the baboon has a long generation time (six years), high maintenance costs and overall a much higher risk of harbouring potential human pathogens.

The Alternatives

Seldom, if ever, have we had as much knowledge to prevent a future epidemic. What is lacking is the wisdom to act upon that knowledge. Yet, it is not too late to muster the will to halt baboon transplantation as an unacceptable risk to the general health of humankind. Common sense tells us that the focus should always be on prevention rather than containment. That means removing nonhuman primates from the list of acceptable organ donors. Discussions aimed at regulating xenotransplantation should consider the following points:

Although these measures deserve careful consideration if xenotransplants are to continue, at the same time it must be understood that even these measures are unlikely to eliminate the risk of propagating a new infectious disease, although they may reduce it significantly. In spite of the risk that might be ascribed to these procedures and my personal opposition to proceeding with xenotransplantation involving baboons, if the prevailing sentiment and political will allow these transplants to happen, then it is critical that strict regulations are fully implemented and that members of local boards have the will and strong administrative support to approve protocols strictly on the basis of their merit and adherence to policy. This level of concern and regulation is not without precedent. Federally funded gene therapy protocols undergo a strict scrutiny that aims to assess the balance between risk and likely efficacy but is heavily weighted on the side of public safety. Given that the risks from xenotransplantation may be far greater than those of genetic engineering and therapy, no less should be expected for this new and growing enterprise of crossspecies medicine.