[Table of Contents]

Volume: 27S3 • September 2001

Viral Hepatitis and Emerging Bloodborne Pathogens in Canada

Xenotransplantation Surveillance in Canada

Marian P. Laderoute

Xenotransplantation is the transplantation, implantation, or infusion into a human recipient of either (a) live cells, tissues, or organs from a non-human animal source or (b) human body fluids, cells, tissues, or organs that have had ex vivo contact with live, non-human animal cells, tissues, or organs⁽¹⁾. Fixed pig heart valves that have been commonly used as medical devices in Canada do not qualify as xenotransplantation products or "xenografts", as this tissue is not alive and viruses have been destroyed by the fixation process.

A common animal species being explored worldwide for source materials for xenotransplantation is the pig, although in the past non-human primates and other sources have been occasionally tried. Mouse fibroblasts have been used in some cases in the U.S. as a feeder layer for supporting the expansion of human (autologous) skin for burn patients. This has prompted the inclusion of such products under the jurisdiction and the definition of xenotransplantation (given above) of the United States Public Health Service (PHS)⁽¹⁾. Although some countries, such as the U.S., have authorized or allowed xenotransplantation clinical trials, to date such trials have not been authorized in Canada, nor are applications for xenotransplantation considered under the Special Access Program, even under extenuating circumstances⁽²⁾.

Surveillance measures for xenotransplantation clinical trials

Efforts are under way to set up surveillance measures for xenotransplantation in Canada. HC is responsible for such surveillance, specifically through the Centre for Infectious Disease Prevention and Control within the Public Health Agency of Canada, and the regulatory bodies within the Health Products and Food Branch.

Although xenotransplantation products would be subject to drug and/or medical device regulations, including the need for HC authorization to proceed with clinical trials, xenotransplantation protocols are thought to present a biologic hazard meriting enhanced surveillance measures^(3,4). These risks have also been viewed as warranting the establishment of minimal operating criteria to deal with animal husbandry issues, source animal procurement, archiving of samples, etc. The draft standard on xenotransplatation is available for comment on the HC website⁽⁵⁾. Enhanced surveillance, however, goes well beyond the usual adverse event reporting and safety assessments required for all clinical trials. These additional concerns primarily relate to the largely unknown but theoretic risks of transmission of infectious agents from the non-human source, such as pigs, to human populations⁽⁶⁾. Many exogenous viruses that cause frank diseases in pigs can be screened or bred out of herds biologically sequestered from other animals, but there are three categories of virus that are more problematic. They are the endogenous, endemic and, as yet, unknown viruses of pigs.

Transmission of endogenous viruses

Endogenous viruses include porcine endogenous retroviruses (PERVs). All pig herds studied to date express one or more PERV subtypes, some which have been shown to infect human cells in vitro⁽⁴⁾. Preliminary studies on human populations exposed to xenografts seem to suggest that transpecies infections do not commonly occur, if at all, although these trials did not evaluate transmission from transgenic sources, from which the risks are thought to be somewhat higher. However, in some cases pig DNA and pig PERV DNA have been found in human recipients, but without evidence of viremia or infection. This condition has been referred to as "microchimerism", in which it is assumed that pig cells have survived and are circulating in the host. Whether microchimerism actually represents free-floating pig cells in human recipients or not, the important question becomes, are these recipients at an increased or decreased risk of infection from PERVs and other infectious agents⁽⁶⁾? Furthermore, what are the risks of transmission of these viruses, if they become replication competent, to other humans?

Transmission of endemic viruses

Endemic pig viruses are detailed more extensively elsewhere in this supplement and have been recently reviewed⁽⁷⁾. Most endemic viruses are present in all herds, have little or no known disease association in pigs, but have the potential to be transmitted and cause disease in humans. This risk is probably increased if the host is immunosuppressed or other barriers to cross-species transmission are lacking, which occurs when transgenic pig materials are implanted. Although it is thought that these viruses can be bred out, details on the successes of such programs have yet to be published. For example, a porcine cytomegalovirus (pCMV)-free herd has not been described to date. Whether pCMV or any of the other pig endemic viruses will or can cause communicable diseases in humans remains to be established. There is a large gap in knowledge about the transmission of these endemic viruses and the potential for human diseases.

Screening for unknown viruses

A third category and the most contentious one is that of unknown viruses for which specific screening methods are currently not available. However, modern technology does allow the screening for certain types of viruses, even if the virus has not yet been characterized. For example, assays for reverse transcriptase activity in which the output is amplified by PCR (the PERT assay) provide a very sensitive means of detecting retroviruses (DNA or RNA). As well, primers for PCR can be designed that will detect most herpesviruses, including those that are unknown. Nevertheless, the challenge remains of how to protect Canadians from unknown pig infectious agents that theoretically could become new emerging bloodborne pathogens. It is important to note that this risk is present regardless of whether xenotransplantation actually occurs on Canadian soil or not. In this regard, HC is currently considering adding xenograft recipients to blood donor exclusion criteria, despite the fact that xenotransplantation clinical trials are not under way in Canada.

Enhanced surveillance measures

To determine what enhanced surveillance measures may be appropriate or feasible, the Blood-Borne Pathogens Division, Centre for Infectious Disease Prevention and Control, held a 1-day workshop on xenotransplantation surveillance in March 2000⁽⁴⁾. An important problem to emerge was that any symptom or even no obvious symptom could be associated with a xenozoonosis (transmission of an infectious agent from one species to another associated with xenotransplantation). Thus, relying on clinical symptoms to decide whether to investigate for a known or unknown infectious agent was concluded to be likely relatively ineffective as an early warning system. No particular surrogate marker for a xenozoonosis was identified, although any abnormal laboratory test might provide the first hint of an infectious disease problem. From the analysis of the workshop survey, it was strongly suggested that active screening in xenograft recipients for viral agents known to be in pig herds, such as PERVs and endemic viruses, will be necessary to assess whether they cross the species barriers and/or have the potential to be associated with disease⁽⁴⁾. In the revised U.S. PHS guideline, a similar recommendation was made⁽¹⁾.

As well from the workshop, a full infectious disease investigation on major autopsy tissues for deceased xenograft recipients was judged to be very important in assessing the ability of endemic and endogenous pig viruses to cross the species barrier to human populations. To date these investigations, if conducted, have not been published, despite the fact that deaths have occurred within xenotransplantation clinical trials⁽⁴⁾ and, at least for one individual, autopsy samples were taken⁽⁸⁾. A major conclusion of the workshop was the need for international collaboration, including the harmonization of definitions, the standardization of adverse event reporting and screening methods, and the sharing of data, experience, and outcomes. A similar conclusion and a specific call for enhanced xenotransplantation surveillance at the international level was recently reached at the OECD/WHO consultation meeting on xenotransplantation surveillance, held October 4-6, 2000, in Paris.

A number of activities to address xenotransplantation surveillance are in the works. Plans are being developed to first provide a science-based risk assessment on the potential for cross-species transfer of endogenous and endemic pig infectious agents to humans. Several hundred high-risk individuals who are occupationally or medically exposed to pig blood or materials will be tested for the presence of the various known endogenous and endemic pig viruses using modern PCR technology and, when practical, for evidence of exposure by serologic means. The results will be compared with those of individuals at low risk who have not been exposed to pig blood, aside from the preparation and consumption of pork. In parallel, an attempt will be made to establish a sensitive method to determine whether a person has, in fact, been exposed to pig blood. It is hoped that all individuals showing the presence of pig infectious agents or previous exposure would all be positive for the pig blood exposure test. If this can be confirmed and demonstrated in the laboratory, then it is possible that this "pig blood exposure test" might be used as a surrogate marker for known and unknown pig viruses in the general population, for example in the screening of blood and organ donors. The development of a single screening assay rather than testing for a dozen or more specific pig viruses, is not only more cost-effective and plausible, but for blood donor screening, it would have the added benefit of reducing the risks from unknown viruses for which specific detection methods have not yet been developed.

Since the methods for screening for endogenous and endemic pig viruses along with non-specific tests for other viruses would be set up and validated on humans samples, this would help furnish HC with the testing capacity necessary to prepare for outbreaks in human populations associated with pig blood-borne pathogens, whether they were associated with a xenotransplantation clinical trial or not.

In summary, new developments in xenotransplantation and infectious disease risks potentially associated with the procedure are closely monitored by HC. The framework for international surveillance is being established and laboratory detection capacity is being developed for known and unknown pig infectious agents. HC is committed to its mandate to prevent and control infectious disease risks in the medical setting.

References

1. United States Public Health Service. Guideline on infectious disease issues in xenotransplantation. May 26, 2000. See Website http://www.fda.gov/cber/gdlns/xeno0500.pdf

2. Health Canada, Therapeutic Products Program. Notice to hospitals from the Therapeutic Products Program, March 29, 1999: The clinical use of viable animal cells, tissues, or organs to treat patients. See Website http://www/hc-sc.gc.ca/hpb-dgps/therapeut/zfiles/english/btox/
   notices/noticetohospitals_e.html

3. Health Canada, Therapeutic Products Program. National Forum on Xenotransplantation: Clinical, Ethical and Regulatory Issues, November 1997. See Website http://www/hc-sc.gc.ca/hpb-dgps/therapeut/
   zfiles/english/btox/reports/frmrptx_e.html

4. Health Canada, Centre for Infectious Disease Prevention and
   Control. Xenotransplantation Surveillance Workshop I: Infection Control Database and Sample Archiving, March 31, 2000 (submitted for the Public Health Agency of Canada website).

5. Proposed Canadian Standard for Xenotransplantation, see Website http://www.hc-sc.gc.ca/hpb-dgps/therapeut/zfiles/english/btox/Standard/xeno_std_e.html

6. Tackaberry ES, Ganz PR. Xenotransplantation: assessing the unknown. Can Med Assoc J 1998;159:41-3.

7. Yoo D, Giulivi A. Xenotransplantation and the potential risk of porcine viruses for xenogeneic transmission. Can J Veterinary Res 2000;64:193-203.

8. Deacon T, Schumacher J, Dinsmore J et al. Histological evidence of fetal pig neural cell survival after transplantation into a patient with Parkinson's disease. Nature Medicine 1997;3:350-53.

[Previous] [Table of Contents] [Next]