Clinical Microbiology Newsletter Vol. 20, No. 17 September 1, 1998

Infection and Xenotransplantation: Assessing the Risks Jay A. Fishman, M.D. Clinical Directol; Transplantation

Infectious Diseases Infectious Disease and Transplantation Units Massachusetts General Hospital and

Harvard Medical School 55 Fruit Street Boston. Massachusetts 02114

The success of human-to-human organ transplantation in the treatment of a variety of types of organ failure has resulted in a growing shortage in the availability of human organs for such procedures. Xenotransplantation, or transplantation of organs or tissues from non-human species into human

recipients, has been proposed as a poten- tial solution to the shortage of human organ and tissue donors. As is the case for allotransplantation, the need for long-term immune suppression for the prevention of graft rejection has as a major side effect the development of opportunistic infections and malignancy in the xenotransplant recipient. The use of non-human species as organ donors carries a risk in that organisms derived from another species and possibly not normally associated with human disease may gain entry and cause infection in the new human host. The unique aspect of this risk is that organisms from other

ANNOUNCEMENT The risk of the transfer of animal-

Scherago-Rubin Award Available for Travel to ASM General Meeting

The Scherago-Rubin award, estab- lished by the late Dr. Sally Jo Rubin, will be presented again in 1999. The award provides $1,500 to defray costs associated with attendance at the General Meeting of the American Society for Microbiology (ASM). Eligible individ- uals include non-doctoral level clinical microbiologists who are involved pri- marily in routine diagnostic work, rather than in research, and who have distinguished themselves by excellent performance in the clinical laboratory. Nominations must consist of a nomina- tion cover page, a curriculum vitae, a statement from the nominee describing his or her previous work experience and achievements in the field and indi- eating how attendance at the General Meeting will enhance attainment of his or her career goals in clinical microbi- ology. In addition, two supporting let-

ters, at least one of which must be from his or her supervisor or mentor must be provided. The other supporting letter may be from an academic advisor or colleague. Further information and the nomination cover page may be found online at www.asmusa.org. Submit the original and five copies to arrive no later than October 1, 1998 to: Awards Committee, American Academy of Microbiology, 1325 Massachusetts Avenue, N.W., Washington, DC 20005-4171 (tel. 202-942-9226).

The award honors both Dr. Sally Jo Rubin and her late grandfather, Dr. Morris Scherago. Dr. Rubin was a for- mer editor of the Clinical Microbiology Newslettel; the Director of the Micro- biology Division at St. Francis Hospital and Medical Center in Hartford, Con- necticut, and an active member of the ASM Clinical Microbiology Division.

In This Issue

Infection and Xenotransplantation: Assessing the Risks . . . . . . . . . . . . 141

The implications of using donor organs from non-human species for human transplantation reach beyond the clinic and the laboratory

Updates from the NCCLS Microbiology Area Committee . . 144 Clinical microbiologists benefit from volunteer efforts of subcommittees to develop and update consensus standards and guidelines for a wide

variety of laboratory areas.

Community-Acquired Burkholderiu cepacia Sepsis in Children . . . . . . . . . . . . . . . . . . 147

A case report

species, or organisms altered in the human host by genetic recombination or mutation, may also gain access to the general human population via the social and sexual partners of the xenograft recipient. A series of unanswered ques- tions emerge: What is the level of infec- tious risk to the xenograft recipient? Are these organisms of unique patho- genicity? How likely is it that these organisms will gain access to other individuals? How should we look for such organisms? These questions must be carefully balanced against the needs and rights of the patient with organ failure who is waiting for a transplant, and who may die while waiting for a human allograft.

CMNEEJ 20(17)141-148.1998 0 1998 Elsevier Science Inc. 0196-4399/98/$0.00 + 19.00 141

derived, infectious pathogens to humans via xenotransplants must be assessed in advance of such transplants. In addition, the risk of xenosis must also be evaluat- ed in the context of the potential spread of these novel pathogens to the commu- nity at large. To achieve this goal, data on the nature, transmissibility, and bio- logical behavior of potential pathogens between the source species and humans must be developed.

Xenosis The term “xenosis” was coined to

describe the “transplantation” of infec- tion with xenogeneic transplants (1). In contrast with the natural spread of infection between species (“zoonosis”), xenosis poses unique epidemiologic hazards due to the efficiency of trans- mission of pathogens, particularly viruses, with viable, cellular grafts. Experience with allotransplantation can be used to guide thinking about this problem:

i.

ii.

. . . 111

All organisms can potentially cause infection in any species; the behav- ior of a specific infectious agent in a novel host species cannot be predicted with assurance.

The risk of infection is related to the overall level of immune suppression needed to maintain graft function. Any manipulation of the donor or recipient (e.g., “tolerance” induction) that reduces the need for exogenous immune suppression will also reduce the risk of opportunistic infection.

The species disparity between a non-human donor and recipient may provide some advantages in terms of the relative resistance of the xenografted cells and tissues to infection by human organisms. This benefit of using more highly disparate species as donors (e.g., swine over non-human primates) must be balanced against the poten- tial for altered function of the host’s immune system (which depends, in part, upon identity of the major his- tocompatibility antigens) in regard to resisting infections derived from

iv.

V.

xenograft tissues.

The risk of xenotransplantation will never be zero. The measure of risk is incompletely assessable prior to human trials.

The risks are not abstract: xeno- transplants in humans have already been performed and more are planned in FDA-approved clinical trials.

Are the Risks Greater Than for Allotransplantation?

Despite advances in the prevention of xenograft rejection in animal models, it is likely that xenotransplantation will require a level of exogenous immune suppression equal to or greater than cur- rently in use in clinical allotransplanta- tion. This suggests that there will be a comparable or increased risk of infec- tion in xenotransplantation. A number of other factors contribute to the poten- tially increased risk of infection includ- ing: (i) the xenograft itself serves as a nidus or “culture plate” from which such organisms can spread in the human host avoiding the need for a “vector” to achieve disease transmission; (ii) clinical laboratory testing for most organisms derived from non-human species are not generally available (e.g., antibodies, molecular probes, or culture systems for species-specific organisms, or sero- logic tests for human antibodies against animal pathogens); (iii) the migration of cells from the graft to other sites in the host may carry cell-associated infection throughout the host ; (iv) the inability to recognize clinical syndromes caused by new pathogens (Table 1); (v) the lack of knowledge about the behavior of poten- tial pathogens from the donor species in humans; (vi) graft rejection and immune suppression are stimuli for the activation of many organisms from latency; (vii) recombination or interactions between infecting and/or endogenous organisms that may mask or alter the expected manifestations of disease; and as was noted, (viii) the ability of the host to respond immunologically to organisms derived from xenograft tissues may be

Table 1. Non-recognition of infection associated with xenograft transplantation

l New pathogens . New syndrome or altered manifestation . Low incidence, scattered, or sporadic cases l Co-infection with other opportunistic

pathogens . Absence of diagnostic tests for animal-

derived organisms . High background rate of infection in

transplant recipients e Common syndromes (respiratory, liver, GI) l Latency l Carrier state

reduced to a greater degree in the xeno- graft recipient than in the allograft recipient. Host responsiveness may be decreased by the disparity between the major histocompatibility antigens between the recipient and donor needed for the function of the host’s cellular immune response to organisms within the xenograft; the absence of pre-existing immunity in the recipient to novel, animal-derived organisms; and by the ability of some pathogens (e.g., viruses) to change by mutation, recombination, or through the acquisition of host- derived characteristics (e.g., carbo- hydrate molecules).

Source Animal Selection Because tests for many organisms

are available in veterinary clinical labo- ratories, it is possible to exclude them prospectively from isolated, closed- donor herds. By contrast, the pathogens carried by wild-caught animals cannot be predicted with assurance. Thus, swine or primates maintained outdoors will have unregulated contact with insects, rodents, amphibians, farm animals, and humans. Meat by-products must be excluded from animal diets to avoid the potential for the spread of prions.

Unknown Pathogens and Viruses Including the Retroviruses

Immunocompromised hosts have served as sentinels for the recognition of many new pathogens and clinical

NOTE: No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products. instructions or ideas contained in the material herein. No suggested test or procedure should be carried out unless, in the reader’s judgment. its risk 1s justified. Because of rapid advances in the medical sciences, we recommend that the independent verification of diagnoses and drug doses should be made. Discussions, views and recommendations as to medical procedures, choice of drugs and drug dosages are the responsibility of the authors.

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syndromes in infectious disease. Thus, AIDS patients have developed infec- tions due to newly described protozoa (microsporidia, Cryptosporidium), viruses (JK, human herpes virus 8, cytomegalovirus), bacteria (Bartonella, Rhodococcus), and fungi (Penicillium), and infections of enhanced severity com- pared with normal hosts. The greatest potential threat to the general popula- tion is the silent spread of organisms that cause few symptoms in either the transplant recipient or in normal hosts, or of organisms of prolonged latency. Concerns about these potential agents have recently focused on the retro- viruses. Recent viral epidemics and outbreaks (i.e., AIDS, Ebola) may have contributed to this focus. The activation and behavior of the retroviruses in transplantation have not been well stud- ied in vivo, largely due to the absence of data to support the existence of func- tional endogenous human retroviruses. However, conditions present in trans- plantation may increase the likelihood of retroviral activation if any intact and infectious proviruses are present.

Recent work in this laboratory and in others has demonstrated the presence and replication of C-type retroviruses in normal swine tissues (6). Our studies do not demonstrate that these viruses will cause disease in xenograft recipients, but only that it is likely that there is a real risk of exposure to retroviruses in recipients of tissues from swine. Recent studies (7) also demonstrate that some of these viruses are infective for human cells in vitro. The cloning and sequenc- ing of these viruses has provided the tools to investigate the activation and the risk of infection associated with the transplantation of various tissues from

Table 2. Advantages of xenotransplantation

Size-to-fit organs to recipient

Unlimited supply No allocation scheme needed Timing of transplantation by clinical criteria: patient optimization Decreased hospitalization time prior to surgery: - Less nosocomial pathogen exposure - Decreased rehabilitation time after surgery Characterized microbial flora of source animal Potential resistance by xenograft to infection with human pathogens

swine. What is the clinical appearance of disease due to porcine or primate retroviruses? The activation of latent virus and the development of clinical manifestations, if any, may be delayed for over a decade and may not occur during the life span of the recipient. Furthermore, the manifestations of retroviral infection, commonly includ- ing immune suppression, altered gene regulation, oncogenesis, or recombi- nation with other viruses, may be clinically inapparent.

The Advantages of Xenotransplantation

Despite the potential risks, clinical xenotransplantation may provide some tangible benefits in the clinical care of patients with progressive organ failure (Table 2). Many patients die while awaiting cadaver donor organs. In addi- tion to an unlimited supply of properly sized organs, xenogenic organs may be resistant to infection with viral patho- gens of humans including HIV (1 and 2), HTLV, hepatitis viruses, and herpes viruses including human CMV. This “species specificity” may reflect the absence of receptors or of cellular “machinery” necessary for viral replication in a novel host.

Most cadaver donor organs are derived from hospitalized patients potentially infected with nosocomial pathogens. Similarly, transplant recipi- ents often have prolonged in-hospital waiting times for allografts. During this time, they are colonized with nosocomial organisms and may develop infection related to intravenous and urinary catheters or respiratory and cardiac assist devices. Patients receiving xeno- grafts can receive their transplants at the time of greatest clinical need; surgery can be timed to avoid infections, to vac- cinate against potential pathogens, to store blood products, and to optimize the patient physically. The source animal can be derived free of such increased infectious hazards.

Strategies for the Future: Minimizing the Risks

Do the benefits of xenotransplan- tation justify the potential risks to the recipient and the community? Some of the potential areas for study include: (i) Studies of infection performed in the appropriate models. Thus, viral activa- tion and susceptibility to infection can

be assessed only in xenograft models. (ii) The presence of unknown patho- gens must be considered and strategies developed to uncover potential human pathogens. (iii) Diagnostic assays, preferably molecular or antigen-based, must be developed for source animal- derived organisms. (iv) Stringent peer review and oversight of clinical proto- cols are needed to assure the protection of the patient and appropriate attention to public health concerns. (v) Clinical trials must include careful archiving of source animal and recipient sera and tissues to aid in microbiologic and epidemiologic studies of emerging infections in xenograft recipients. (vi) Efforts must be made to minimize the immune suppression needed to maintain graft function. (vii) Source animals must be protected from human pathogens.

The risk of xenosis will never be zero. Significant dangers of infection exist for all transplant recipients given the relatively crude, though improving, immune suppressive regimens that are currently available. Careful source ani- mal selection may allow the risks to the recipient and to the community to be reduced, and may confer substantial benefits when the potential of xeno- transplantation is recognized.

References

1.

2.

3.

4.

5

6.

7.

Fishman, J.A. 1994. Miniature swine as organ donors for man: strategies for prevention of xenotransplant-associated infections. Xenotransplantation 1:47-57.

Fishman, J.A. 1997. Xenosis and xeno- transplantation: addressing the infectious risks posed by an emerging technology. Kidney International Sl(supp): 41-45.

Michaels, M.G. and R.L. Simmons. 1994. Xenotransplant-associated zoonoses: strategies for prevention. Transplantation 57: l-7.

Ye, Y. et al. 1994. A practical study of zoonoses that could complicate pig-to-man organ transplantation. Transplantation Proceedings 26: 13 12.

Ye, Y. et al. 1994. The pig as a potential organ donor for man. a study of potentially transferable disease from donor pig to recipient man. Transplantation 57:694-703.

Akiyoshi D.E. et al. 1998. Identification of a full-length cDNA for an endogenous retrovirus of miniature swine. J. Virology 7214503-4507.

Patience, C., Y. Takeuchi, and R.A. Weiss. 1997. Infection of human cells by an endogenous retrovirus of pigs. Nat. Med. 3:276-282.

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