4th National Symposium on Biosafety

PHS Perspective on Xenograft Transplantation

Louisa Chapman, MD
Medical Epidemiologist
Centers for Disease Control and Prevention
1600 Clifton Road
Atlanta, GA 30333
404-639-1028

Progress in any area of scientific or medical knowledge is marked by transition periods when new understandings emerge and result in the framing of new questions. We are at such a point in the field of xenotransplantation. The chronic shortage of human organ and tissue donors coupled with immunologic and biotechnical advances have catalyzed new attempts to use animal tissue in a wide variety of applications in humans. At the same time, our experience with zoonotic diseases, in particular our developing understanding of the origins of the human AIDS pandemic, have lead to the framing of new questions regarding the potential for the use of animal tissues in humans to introduce new infections of public health importance into the human community. Little scientific data exists to assist in addressing these questions. Developing a systematic factual basis for substantiating and quantifying the infectious risks associated with specific xenotransplantation applications or alleviating these concerns will require cooperative work by scientists and clinicians working broadly across expertise lines. It has been interesting to me to listen to my predecessors. Through the changes in their talks it is clear that this sort of cooperative work and scientific progress has already begun out of the dialogue that's been going on nationally for less than a year now.

Most physicians practice medicine retail, concerning themselves with the health of individual patients. Public health physicians practice medicine wholesale, concerning themselves with the health of populations. Four questions are currently important from a public health perspective:

(1) Do xenogeneic infections pose a risk for xenotransplant recipients?

Experience with zoonotic diseases suggests that mechanisms of evolutionary adaptation exist in both microbes and their natural hosts. The disease-producing potential of an infection is a function of the relationship between the host and the infecting agent and not of the microbe alone. As a result, the pathogenic potential of an infection can change in an unpredictable fashion when the infecting microbe is transmitted from its natural host into a new species.

For example, in its natural host, the macaque monkey, Cercopithecine herpesvirus 1 (B virus) has a clinical profile very similar to herpes simplex in humans. However, B virus infection of humans or other non-macaque primates results in an encephalitis with a mortality rate of about 70%. This failure of the pathogenic potential of a microbe in its host species to reliably predict the pathology that will result when it is introduced into another species is evident with many additional zoonotic agents and diseases.

(2) Do xenogeneic infections constitute a threat to the health of the greater community?

If the risk of xenotransplant-associated infections is restricted to the recipient, it simply constitutes one factor affecting the risks and benefits of transplantation. However, the historic experience with zoonotic disease suggests that xenogeneic infections have implications for the community that extend beyond the individual transplant recipient. Once introduced into humans, zoonotic viruses that are not particularly pathogenic in their host species have resulted in noteworthy outbreaks.

For example, the hospital admissions of patients infected with Ebola, a zoonotic filovirus, resulted in large nosocomial outbreaks with multiple generations of human-to-human transmission and case-fatality rates of 80-90 percent documented in Sudan in 1976 and in Zaire in 1976 and 1995. The hospital admissions of patients infected with Crimean-Congo hemorrhagic fever virus (CCHF), a zoonotic Bunyavirus, resulted in outbreaks involving up to three generations of human-to-human transmission and case fatality rates of around 25% reported from Pakistan in 1976, the United Arab Emirates in 1979, and South Africa in 1984. In all of these outbreaks the first generation of human-to-human transmission was invariably from the patient to the medical staff and family, with subsequent generations of transmission extending into the greater community. Thus, the world experience with zoonotic viruses provides substantial evidence for the potential of epidemic human infection due to xenogeneic infections. It is this potential that makes them a public health concern.

The initial drama of a zoonotic outbreak predicts the likelihood that it will receive attention, but not its eventual public health importance. Indeed the public health consequences of xenogeneic infections may be most significant when the immediate pathogenicity is least evident. The public health impact of the filo- and bunyavirus outbreaks that I just described was limited because these infections caused acute clinical illness followed by recovery or death, limiting the potential for human-to-human transmission. In contrast, consider the quiet but pandemic spread of the human immunodeficiency viruses.

Compelling arguments suggest that the HIV epidemics resulted from the adaptation of simian retroviruses introduced across species lines into humans. Existing data suggest that the HIV-2 epidemic in est Africa began with the horizontal transmission of simian immunodeficiency virus (SIV) from a sooty mangabey monkey into a human with subsequent transmission through the human population. In Central Africa horizontal cross-species transmission of SIV from a different primate species, probably chimpanzee, resulted in the HIV-1 pandemic. Initial human infections prior to 1970 resulted in more than a decade of insidious human-to-human transmission before AIDS was first suspected as a public health problem in the 1980s.

The simian immunodeficiency viruses are only examples of an expanding number of exogenous retroviruses recognized to infect non-human primates. The potential for the introduction of exogenous simian retroviruses such as SIV, primate T-lymphotropic viruses, and primate foamy retroviruses, into human hosts via implanted xenogeneic tissue is of public health concern due to the uncertainty regarding their pathogenic potential and the long period of clinical latency associated with all known retrovirus diseases. Primate foamy viruses are of additional concern because they are known to transmit among baboons by respiratory spread. Standardized diagnostic testing is not available for most of these retroviruses.

Endogenous retroviruses exist as part of the genomic material of most if not all mammalian species, including humans. Endogenous retroviruses cause equal concern and greater uncertainty than the exogenous retroviruses. Endogenous retroviruses presumably originated as exogenous viruses that became permanently integrated into the host germ line and are transmitted vertically through inheritance. In the host species, they are benign. However, endogenous viruses are frequently xenotropic; although the original host is refractory to infection, the viruses are able to infect related species.

For example, baboon endogenous retrovirus (BaEV) proviral DNA can be detected in the tissues of all baboon species. This xenotropic virus can be readily isolated by co-cultivation of baboon tissue with human cells in vitro. While the direct risk for human infection with BaEV in vivo is unknown, in the laboratory the virus can not only infect human tissue but amplify to high titers. BaEV is a type C retrovirus. However, sequencing of BaEV suggests that it is a chimeric virus; the gag-pol region of the genome is closely related to that of murine type C retrovirus while the env region is related to simian type D retrovirus, suggesting recombination events occurred during the evolution of the virus. What is the potential for recombination or complementation of BaEV with viruses latent in human tissue? Phenotypic mixing of BaEV with other primate viruses occurs under appropriate experimental conditions, suggesting that endogenous retroviruses might recombine in humans after xenotransplantation. Whether viral mixing could create a hybrid better adapted to survival, replication, and pathogenicity in the human host is unknown.

It has been widely presumed that the increased phylogenetic distance between swine and humans makes them safer donors than non-human primates. However, this presumption has been incompletely explored. The biology and pathogenicity of a type-C retrovirus identified in the blood of leukemic or irradiated swine are incompletely characterized. The short life expectancy of the average swine minimizes the opportunity to observe clinical manifestations that might result from infections with agents characterized by long periods of clinical latency such as retroviruses or prions. The potential for endogenous retroviruses in transplanted porcine tissue to infect immunosuppressed human recipients, rescue replication-defective viruses or recombine with latent human viruses to create a pathogenic hybrid is unknown. We were only framing these questions, the science hasn't been done to give answers to them. The development of encapsulation techniques intended for the immunoisolation of xenogeneic tissue and the creation of transgenic animals whose organs are intended to survive immune surveillance after transplantation may increae the potential for viral recombination or reassortment. All of this work is aimed at allowing xenogeneic tissue to go into humans and to survive immune surveillance after transplantation but it may have the side effect of also increasing the potential for viral replication or viral recombination or allowing porcine origin viruses to infect humans.

Little research exists into the presence or absence of prion-associated disease in potential donor animal species. Like retroviruses, prion encephalopathies have protracted clinical latency. No diagnostic tests are available. Transmission has been documented both between species and through transplanted tissue. However, prion transmission has no epidemic potential.

Lastly, the oncogenic potential of animal DNA viruses introduced into immunosuppressed humans remains undefined.

(3) How can public health threats associated with the use of xenogeneic tissue in humans be minimized?

The lessons learned from the experience with zoonotic disease argue that there are sufficient infectious risks inherent in xenotransplantation to justify a reasonable degree of public concern. Planning for these infectious risks should be incorporated into the development of any clinical strategy involving the use of xenogeneic tissue in humans in the interest of the patient, the medical staff and the public. Public health guidelines intended to minimize the risk of transmission of known pathogens through human-to-human transplantation exist, and similar guidelines addressing xenotransplantation are undergoing clearance at the three public health agencies responsible for generating them, the NIH, FDA and CDC, and will be available for 90 days of public comment through the Federal Register in the immediate future.

I want to stop at this point and say that CDC's contribution to these PHS guidelines is the intellectual product of an exceptionally good working group of 11 of my colleagues at least two of whom are participating in this meeting, Bobby Brown and Jim Ebert. The members of CDC's working group were selected on the basis of epidemiologic, public health or microbiologic expertise, but if you described the group by degrees, the largest proportion of group were veterinarians. The final PHS guideline is a joint product of our working group and a similar but larger working group based at the FDA. The FDA based group was composed largely of FDA and NIH employees but also with expertise representative of federal employees in the Department of the Defense and the USDA. The guidelines will be available through the Federal Register for public comment for a period of 90 days because we are actively interested in input from the concerned community.

Now as much as I can within the limits of not having the guidelines yet through clearance and publicly available I want to talk about some of the areas that are going to be covered in the guidelines and some of the issues that emerge out of them.

Research team expertise: Xenotransplantation proposals are complex protocols requiring complex expertise. Collaborative work between transplant surgeons, transplant physicians, and immunologists has brought the field to it current transitional position. Addressing concerns about the potential for xenogeneic infections will require the additional expertise of infectious disease physicians, veterinarians, epidemiologists and microbiologists.

The complexity of the issues raised by xenotransplant protocols merits review by a committee with expertise in epidemiology, risk assessment, human and veterinary infectious diseases including virology, and laboratory diagnostics. These reviews should assess the immediate and long term biologic safety hazards posed by the protocol, the likelihood of risk or benefit to the recipient, and epidemiologic concerns relating to the risk of infection for both recipients and the greater community.

Preclinical studies: Species-specific preclinical studies should be available in the literature or pre-clinical testing should be performed in conjunction with the development of specific clinical applications for the use of xenogeneic tissue in humans. These studies should attempt to identify the presence of xenotropic endogenous retroviruses or persistent viral infections in source animals, to characterize the potential for human pathogenicity associated with the identified agents, and to develop diagnostic tools needed to monitor for post-transplantation infection in humans. These studies will require forging new collaborations between the transplant community and molecular biologists, virologists, and other microbiologists who have the expertise and experience necessary to develop a body of scientific knowledge addressing these issues.

For a number of years the Retrovirus Diseases Branch at CDC has been conducting surveillance among animal workers and laboratory workers exposed to primate tissues for primate retro-viruses. This begin as SIV screening, and has been expanded to include screening for a number of other primate retroviruses. Many of the people in this room may have been involved this. It is still going on, we are interested in your involvement. If any of you want to be involved the occupational health officer at your institution can contact Bill Switzer, Retrovirus Diseases Branch, CDC, phone number 404-639-1024.

Prevention of recognized zoonoses: Infections with recognized zoonotic pathogens should be relatively preventable and identifiable when they occur. The risk of recipient infection can be decreased by quality control of the animal procurement source, screening and quarantine of the individual source animal, and aseptic tissue procurement.

Xenograft issue for transplantation should be procured only from animals bred and reared in captivity in biomedical facilities meeting standards of the US PHS Guidelines for the Care and Use of Laboratory Animals. These facilities should have closed herds and well-documented herd health and infectious disease surveillance systems that concentrate on infections known to exist in captive animals of the relevant species in North America.

The presence of acute infections in individual source animals can be controlled by herd surveillance indicating the presence or absence of infection in the herd from which the individual source animal is selected, by clinical examination and treatment of individual source animals, and by the use of appropriate individual quarantine periods that extend beyond the incubation period for pathogens of concern. Any transportation of source animals will compromise the protection insured by the closed colony and necessitate a more extensive period of quarantine and screening upon arrival at a new facility.

Monitoring for xenogeneic infections:

Lifelong clinical surveillance: Lifelong post-transplantation clinical surveillance of xenotransplant recipients is critical to monitor for events suggestive of xenogeneic infection.

Archiving of biologic specimens: Biologic specimens designated for PHS use should be collected from source animals and tissue recipients and archived to permit retrospective analysis if a public health need arises. Post-transplantation testing of archived biologic specimens should be conducted in response to a clinical episode possibly representing a xenogeneic infection and conducted in association with an epidemiologic investigation.

Investigation of acute infections: The majority of acute infectious episodes among the general population are never etiologically identified. Recipients of xenografts remain at risk for these infections. In the xenotransplant setting the diagnosis and management of familiar zoonoses may be complicated by immunologic manipulations that alter the clinical presentation of illness, the reliability of antibody testing, and the response to therapy. When the source of a significant illness remains obscure despite standard diagnostic testing, consideration must be given to the need for and the type of further diagnostic testing, infection control precautions, and public health investigations. Disease evaluations may require diagnostic tests based on nucleic acid detection or cell culture methodologies and other non-standard techniques. The development of algorithms for the identification of what may be unknown xenogeneic pathogens may require consultation with persons with experience in the laboratory identification of unknown infectious agents, including the appropriate management of biosafety hazards associated with such investigations.

Now every step of what I've described is going to be contingent on clinical judgment and that is an uncomfortable fact when we talk about the risks in this field. As Dr. Allan has argued persuasively you have to recognize that its not going to be possible to talk about putting animal tissues into humans, to accept that there's a risk involved and to have absolute confidence that the clinical judgment or the diagnostic tools available or any of the other safeguards that we've put into place are going to be adequate to catch the first infection before it spreads, so when we talk about safety we are really talking about calculated risks.

Active post-transplantation laboratory surveillance: In addition to archiving biologic specimens, when xenogeneic agents of unknown pathogenicity for humans (e.g., baboon endogenous virus, foamy virus) are known or suspected to be present in the transplanted tissue an active post-transplantation laboratory surveillance program directed toward the detection of sentinel human infection with these agents prior to dissemination in the general population will be needed.

Health care worker surveillance: A comprehensive occupational health services program should be designed to educate workers about the risks associated with xenotransplantation and to conduct surveillance for the development of xenogeneic infections, including post-exposure evaluation and management. Health care workers and laboratory personnel who handle the animal tissues prior to transplantation will have a definable risk of infection that will not exceed that of animal care, veterinary, or abattoir workers routinely exposed to the source animal species provided equivalent biosafety standards are employed. The risk to health care workers who provide direct or indirect care to xenogeneic tissue recipients post-transplantation is undefined. Strict adherence to standard precautions will reduce the risk of transmission of xenogeneic infections as well as other blood-borne pathogens in hospital and other settings. Additional infection control or isolation precautions (e.g., airborne, droplet, contact) should be employed as clinically indicated in the judgement of the xenogeneic transplant team ID specialist and the hospital epidemiologist. Again we'recoming back to fallible human clinical judgment in situations of uncertainty as the best protection available.

Record Keeping: As the field progresses to the point that xenotransplant patients survive and return to the general population, surveillance of populations of xenogeneic tissue recipients for clustering of adverse health outcomes and monitoring of individual recipients for unexplained illnesses are clearly the most important public health tools available to address associated infectious risks. To be effective in the xenotransplant setting adequate surveillance would have to detect xenogeneic infections in recipients prior to spread into the general population and allow suspension of the specific associated procedure, donor species, or tissue pending the development of adequate methods for prevention and control and thereby eliminating risk of further infection. Systematic record maintenance linking records of the recipients, the source animals, the herd surveillance, and the occupational health program is important. A national registry would allow prospective monitoring of individual xenogeneic tissue recipients for unusual unexplained health events, monitoring of the recipient population to define the rates of occurrence of unusual events, and the retrospective tracking of epidemiologically linked recipients for investigations of concerning events. For this reason a national registry is the most important tool to assess long term safety.

(4) What constitutes an acceptable balance between caution and progress in xenotransplantation?

The extent to which the potential for medical progress is considered to justify acceptance of associated risks will always differ among individuals. Clinicians and policy makers alike must recognize that while xenotransplantation promises potential benefit for individual patients, that promise is accompanied by an unquantifiable but undeniable potential for harm to the greater community. Thus, the determination of what constitutes an "acceptable risk" in the balance between caution and progress in this arena is a public concern and not merely a private matter among individual scientists, physicians, and patients.

In weighing this balance it is important to remember that the complete elimination of smallpox stands alone as a public health triumph. In most instances, once a problem exists in the general population public health control measures decrease rather than eliminate risk. We haven't yet succeeded in eradicating any known infectious disease from the face of the earth other than smallpox. While investigations of individual instances of unusual illnesses can be revealing, usually a cluster of cases must occur before a problem can be recognized, a new etiologic agent identified, or associated risk factors delineated. For instance, although scattered individual cases had occurred throughout the country for over a decade, the AIDS epidemic was not suspected until the spring of 1981 when similar sexual activity histories were noted among seven young men with unexplained Pneumocystis carinii pneumonia (PCP) geographically clustered in the Los Angeles area. This is only one example of many new diseases in public health and I'm sure you can bring others to mind readily. Legionnaires disease 1976. Hantavirus Pulmonary Syndrome, 1993.

The public discussion; the decisins about what is appropriate in terms of risks versus progress; how much progress to sacrifice for how much protection; these are all public matters. These issues need the best thought and the best voice that all of us can bring to the table. We invite the greater community's critique of the draft guidelines that the public heath service will be making available through the Federal Register in the near future.

References:

Committee on Xenograft Transplantation: Ethical Issues and Public Policy, Institute of Medicine. Xenotransplantation: Science, Ethics, and Public Policy. National Academy Press, Washington, D.C. 1996.

Department of Health and Human Services, Draft Public Health Guideline on Infectious Disease Issues in Xenotransplantation, Notice, Federal Register, September 23, 1966 (Vol. 61, No. 185, page 49919).

Office of Health and Safety, Centers for Disease Control and Prevention, 1600 Clifton Road N.E., Mail Stop F05 Atlanta, Georgia 30333, USA Last Modified: 1/2/97
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PHS Perspective on Xenograft Transplantation

(1) Do xenogeneic infections pose a risk for xenotransplant recipients?

(2) Do xenogeneic infections constitute a threat to the health of the greater community?

Monitoring for xenogeneic infections:

References: