Xenotransplantation and Infectious Diseases

S.S. Kalter, Ph.D., is President of Virus Reference Laboratory Inc. (VRL), in San Antonio, Texas. R.L. Heberling, Ph.D., is Vice President of VRL. S.S. Kalter, Ph.D., is President of Virus Reference Laboratory Inc. (VRL), in San Antonio, Texas. R.L. Heberling, Ph.D., is Vice President of VRL. For want of a nail the shoe was lost... Herbert, G. 1651. Organ transplantation is now an accepted, and in many instances necessary, mechanism for the preservation of human life when disease has resulted in the loss of organ function. The exact number of organs required for transplants is probably unknown, but there is unquestionably an insufficient supply of human organs, and another source or method is required. Hilts (1993 ) estimated that approximately 60,000-80,000 lives could be saved if organs were available. According to the United Network for Organ Sharing, in 1994 there were over 37,000 registrants for organ transplants; approximately half will die for lack of an available organ. It has been suggested that improving the logistics of delivery and matching of supply and need would be helpful in supplying human organs. Mechanical replacement is currently an inadequate option, and it is not a permanent solution. With limited human organ sources, what are the alternatives? Leaving social and ethical aspects aside, animals could create a potential supply of temporary or perhaps permanent organs for transplantation. Assuming that xenotransplantation is viable, numerous queries enter into such a decision. For example, are infectious diseases acquired from transplanted animal organs and are body fluids the major conceivable danger? Humans are susceptible to animal-associated diseases (zoonoses). As it would appear that there is little choice between human and animal organs for transplants, and that there is an insufficient supply of human organs, problems with animal organs must be resolved. Of the animals under consideration as organ donors, nonhuman primates and swine are most frequently suggested. Currently, nonhuman primates, because of their phylogenetic relationship to humans, are preeminent as suitable organ donors. When contemplating the nonhuman primate, anxieties that continue to counter their usage are fears engendered by the possible presence of infectious agents. It would appear, however, that “;the jury is still out” with regard to this concept. Brack (1987 ) provides a background of infectious agents present in nonhuman primates. A potential for humans to become infected as a result of heterotransplantation exists and has been discussed in numerous reports (see references). Conceivable transfer of infectious agents as a result of xenotransplantation using nonhuman primates or pig sources is reviewed in cooper Cooper and others (1991 ), Kalter (1991 ), and more recently by Michaels and Simmons (1994 ). Hardy (1989 ) does not consider infectious diseases. If animals are to be a source of organs, it has been suggested that the spread of infectious diseases may be minimized by employing germfree (gnotobiotic) or specific pathogen-flee (SPF) animals for xenotransplants. Because of the numbers needed, the use of germfree or SPF animals would be impractical. Further, such animals still harbor endogenous agents. Limited experience suggests that well-characterized animals born in captivity and allowed no contact with animals introduced from the wild may be a more practical source of organs for xenotransplants. When considering the choice of an animal, disease potential is a moot point. Infectious agents are unfortunately universal. A precise understanding of the specificity of infectious agents is often lacking and in many instances is not clearly defined. An alien species infected by certain agents may change the virulence of that agent, such as B virus infection of humans (Herpesvirus simiae from Macaca sp.). Many agents are host-specific and not infectious when crossing species barriers. Two recently described highly fatal simian viruses, Ebola-Reston and Simian Hemorrhagic fever do not cause human disease. The simian immunodeficiency viruses (SIVs) are not known to cause human disease although infection is suspected ( Khabbaz and others 1994 ). Other agents, such as measles and tuberculosis are not influenced by species barriers and can cause infection or disease in diverse host species. Although the exact epidemiology of the Marburg virus is unknown, it was fatal for both human and nonhuman primates ( Kalter 1986 ). The human immunodeficiency viruses are considered to be derived from simian sources, and others may be forthcoming. It is not known if natural immunodepression is a cause of altered virulence of infection or disease in a new host. Many mammals are also infected by agents of nonmammalian origin (such as arboviruses, bacteria, parasites, and fungi). Arboviruses require a vector for transmission from one host to another and are not a transplant problem. Infection and disease are complicated by a number of extraneous considerations: immunological status, age, nutrition, socioeconomic factors, and geographic locale. If history foretells the future, “new” strains of infectious organisms most certainly will be forthcoming and could be a source of human infection and disease. Such occurrences have been observed in the past. Infectious agents that can cause zoonotic disease are frequently of little hazard in the host of origin; infection usually results in subclinical disease with antibody production. Zoonoses are generally initiated in immunologically naive individuals and are devastating as such to any population. A transplant patient, because of immunosuppression, is immunologically naive. Because overtly sick animals would not be considered for organ donation, it is unlikely an agent would be transferred from them. However, latent infections of donor or recipient, which are not discernible, may be activated as a result of the transplant. Endogenous agents are also an unknown factor, and their presence may need consideration. In pursuing possible sources of transplant organs for humans from nonhuman primates and swine, human contact with these animals over the years has not been overshadowed by human infection and disease. However, these contacts have been by immunologically “normal”, not immunosuppressed, individuals. Immunosuppression increases the susceptibility of an organ recipient to infection and frequently permits nonpathogenic organisms to become pathogenic. As a consequence, activation of dormant or latent infectious agents in the recipient should be anticipated. Similarly, transferring a nonpathogenic or latent infection from an organ donor to an immunosuppressed individual may also result in enhanced pathogenesis. Past infections of both organ donor and recipient may be detected by antibody surveys. The presence of antibody may serve as a protective mechanism in transplants ( Peterson and others 1980 ). However, antibody surveys are not infallible as titers may fall below perceptible levels or inappropriate methodologies may be employed. While there is reason to be concerned about the transfer of infectious agents from an animal source, experience has indicated thus far that infection has not been a major problem in heterotransplants. Infections from animal donors do occur, but as in human-to-human transplants where infections are recognized ( Ho 1977 ; Michaels and Simmons 1994 ), careful selection of a donor may limit the spread of infectious agents. Human infection from swine has not been as extensively studied as from nonhuman primates. Of all the nonhuman primate viruses, B virus has received the most attention because of its pathogenicity tot humans. This virus, which is closely related antigenically to human herpes simplex, is common to Macaca sp. B virus infection in the macaque simulates human infection with herpes simplex, while infection in humans is fatal. Macaques, however, are now not considered for xenotransplants largely because they are hosts for B virus, so the question of B virus transfer is irrelevant. SA8, a herpesvirus found in baboons that is related to B virus but not considered a human pathogen, frequently causes misinterpretation of serologic results because of its antigenic cross reactivity with B virus. Among nonhuman-primate organ donors, apes and principally chimpanzees are most desirable. While apes other than chimpanzees (such as gorillas, orangutans, gibbons) have been considered, they are not available. Immunologically these apes are also more remote from humans than the chimpanzee and would be less enticing as donors. The availability of chimpanzees is extremely limited and for all practical purposes this animal cannot be considered as an organ donor because they are an endangered species. Development of chimpanzee breeding colonies, although intriguing, would be inappropriate because of cost as well as for ethical reasons. The baboon (Papio sp.), a monkey that is readily available and raised in captivity, is currently the most frequently used nonhuman primate considered for xenotransplantation. Of the nonprimates, considerable attention has been given to the pig (Sus scrofa). Both animals, the baboon and the pig, have a microflora. The baboon and pig meet the various criteria established for donor animals: size, ease of breeding in captivity, a source of food (pig), phylogenetic relationship (baboon), and many other physiological, anatomical, and other similarities to humans. The major negative feature shared by both animals, but also a problem in human-to-human transplants, is the continuing need for immunosuppression and the conceivable danger of infection. Baboons have been extensively studied, and preliminary data are available on their acceptance as organ donors for humans ( Starzl and others 1964 , 1993 ; Hitchcock and others 1964 ; Reemtsma and others 1964 , 1969 ; Murphy and others 1970 ; Brede and Murphy 1972 ; Van der Riet and others 1987 ; Bailey and others 1985 ; Human and others 1987 ). The baboon is currently the nonhuman primate of choice for xenotransplants in the United States and South Africa, the major areas of xenotransplantation activity. Other nonhuman primates, particularly the chimpanzee, have been used in limited instances over the years. In South Africa, the Chacma (Papio ursinus) baboon is locally available. In the United States the much smaller yellow baboon (P. cynocephalus), derived from Kenya, Tanzania, and Ethiopia, is used. Breeding of P. cynocephalus in the United States is well established ( Goodwin and Coelho 1982 ). For practical purposes there does not appear to be any major differences in the normal flora of both species ( Human and others 1987 ; Michaels and others 1994 ). Baboons breed well in captivity, do not appear to be endangered, and are readily available. In areas of origin, baboons are considered agricultural pests. An extensive bibliography is available on the baboon, and its overall microbiology has been reported ( Kalter 1967 ; Brede and Murphy 1968 , 1972 ; Kalter and Heberling 1971 ; Kalter 1973 , 1986 ). Noteworthy are the studies done on the baboon immediately following capture in Kenya providing information on natural infections ( Kalter 1973 ). As suggested above, the phylogenetic relationship of nonhuman-to-human primates emphasizes their common flora and fauna, including bacteria, fungi, parasites, and viruses and their enhanced susceptibility to many of these agents. In addition, most nonhuman primates have an extensive specific pattern of infectious agents that are related, but distinct from their human counterparts ( Kalter and others 1980 ). Widespread geographic and species differences in flora and fauna among nonhuman primate species is recognized. When contemplating nonhuman primates as a source of organs, it would appear that the choice is limited. Apes, as indicated, cannot be considered. South American monkeys are generally too small, many species are restricted in availability, and all species have a definite viral flora that includes several oncogenic viruses. Of the Asian and African nonhuman primates, macaques are not desirable because of the presence of Herpes B virus. Of the remaining African species, none offer any advantages over the baboon. Despite the large number of viruses and other organisms recovered from the nonhuman primate, as well as the recognition of disease among these animals ( Benirschke 1986 ), there is little evidence to indicate that extensive human infection, other than B virus, has been recognized. B virus infection of humans following contact with macaques is well described ( Keeble and others 1958 ; Hunt and Melendez 1969 ; Hull 1968 ). Information on human infection with nonhuman primate agents following xenotranplants is very limited and is unquestionably due to the restricted number of simian-to-human xenotransplants completed. If the practice of using nonhuman primates as organ donors on immunosuppressed recipients continues, infection and disease must increase. In nontransplant situations, hepatitis A contracted from chimpanzees is well substantiated ( Hillis 1961 ; Smetana and others 1970 ). In 1967 an outbreak of a highly fatal disease occurred among humans as a result of contact with African green monkeys (Cercopithecus aethiops) blood and tissues. Monkeys carrying the virus (Marburg virus) rapidly succumbed to the disease and would not have been considered as organ donors Kalter 1986 ). Monkeypox virus, a close relative of smallpox and vaccinia viruses, has infected humans following contact with monkeys ( Kalter 1986 ). There are other examples of human infections resulting from nonhuman primate contact. Human vaccines (such as polio and adenovirus) made in monkey tissues have resulted in the transfer of SV40 virus, a monkey papovavirus, to human recipients. This virus is oncogenic in experimental animals, but human disease has not been observed ( Shah and Nathanson 1976 ). Unquestionably, other viruses have also been transferred by this mechanism. Human-to-human organ transplants are known to result in infection (Fulginiti and others 1968; Ho 1977 ), principally with herpesviruses (cytomegalovirus). As a consequence, it must be assumed that similar infections may occur as a result of xenotransplants ( Michaels and Simmons 1994 ). Several antibody studies on donor baboons have been conducted and antibody to a number of viruses has been demonstrated ( Van der Riet and others 1987 ; Human and others 1987 ; Michaels and others 1994 ). In the limited number of instances where xenotransplants were conducted, few adverse effects from infectious agents were noted ( Hitchcock and others 1964 ; Starzl and others 1964 ; Reemtsma 1964 ; # Reemtsma 1969 ; Bailey and others 1985 ; Starzl and others 1993 ). Infections, possibly leading to have been observed as a result of agents other than viruses ( Starzl and others 1964 ; Reemtsma and others 1964 ; Brede and Murphy 1968 ; Michaels and Simmons 1994 ). It is that more xenotransplants have been infection and disease resulting from the donor organs will not be provides a of virus and the results of baboon for xenotransplants from the South African and transplant An consideration when animals on antibody is the source of In a in which the for it that there was a need to specific the animal species used in the ( Michaels and others 1994 ). It is that extensive not about the donor but about methodologies is required any on the of infection and disease following xenotransplants. on data studies on both of animals the Virus Reference The presence or of antibody a or no Other of baboon have been found with antibody to many of these viruses ( Kalter and Heberling 1971 ; Kalter 1986 ). The data were from the of were derived from nonhuman primate sources and were highly among the herpesviruses were but on data studies on both of animals the Virus Reference The presence or of antibody a or no Other of baboon have been found with antibody to many of these viruses ( Kalter and Heberling 1971 ; Kalter 1986 ). The data were from the of were derived from nonhuman primate sources and were highly among the herpesviruses were but A major virus that must be with and that is a possible cause for in is the a large of viruses into a number of all with and and the are considered to be with as well as human viruses are known to a disease in nonhuman primates similar to human Both endogenous and are and they may be or which have been recognized as and are with both human and simian have about the disease potential of The of human infection with is not The precise relationship between human and simian immunodeficiency viruses, particularly has also not been in human with other infection but no is of in this over by simian ( Khabbaz and others 1994 ). antibody lack of infection. of other strains with differences in pathogenicity need consideration and one may that human However, such pathogenicity is demonstrated or a has in the of the strains should not be considered as the cause of human disease. in to a is to be viruses another of are in nonhuman primates and other animal species. viruses have little that they are other than with no recognized disease production. presence in host animals, however, suggests that their continue to be studied and generally are the endogenous Both and endogenous viruses have been usually in or by but in of these viruses probably all animals, but from to While such are to may be of in their Although a number of and viruses have been little is known of their pathogenicity with the of the baboon This virus, as a the disease in and nonhuman primates ( Heberling and others 1976 ). The potential for the baboon endogenous virus, which can in human to a present in the transplant is In to viruses, other organisms have been recognized in nonhuman ( Brede and Murphy 1968 ; and others 1986 ), ( 1986 ), and ( 1986 ). agents may have a on nonhuman primates in the is an of animals and transmission to humans. in disease do occur, and require and precise to the devastating effects of disease Many of these organisms are recognized as infectious for the human and a cause for in xenotransplantation ( Michaels and others ). donor is in contact with human agent transmission is require careful animals would be as donor should most infections and disease including due to agents other than viruses. is on available but more so on including recognized While may indicate infection by agents other than in the such a is extremely limited and to disease cannot be in the agent is in to antibody the need for in disease is The effects of immunosuppression on infection were in the cause of was organ need for infections in the of Starzl and others 1964 ). infectious agents detected and The epidemiology of these infections it by the baboon or activation of recipients Reemtsma and others ) that their chimpanzee been immunosuppressed, of infection evidence of organ infection will be a problem in xenotransplantation to be Several animals have been suggested as organ donors and a number have been used. have received considerable attention and are in a number of studies ( Cooper and others ). The pig a number of with humans including size, and As would be the pig also has a which includes bacteria, parasites, fungi, and viruses. Many of these are known to be with human infection and disease ( Cooper and others ). Endogenous viruses are suspected in swine but have not been As with nonhuman primate use of the pig is of a method for Breeding is more and a than breeding nonhuman primates, and their usage as a source of food is another possible The availability of large numbers of permits into the of swine that could be immunologically to humans ( ). the mechanism be to the nonhuman ethical is another major when contemplating the pig as an organ The need for a source of organs as for humans in the is The supply of organs from human donors is extremely and organ are currently impractical. The into using animals as organ donors, with recognition of their is on the that there is an insufficient supply of human organs and no In this the nonhuman primate and the pig are as potential organ donors. For practical animal would be or the problem of is the phylogenetic of the baboon that of the pig to the baboon over the pig as the donor of offer a number of considerations: they are to are a source of many with humans, and their use ethical than the use of suggests that infection and while they occur, thus far have been a in the few xenotransplants However, the of immunosuppression must be used to is the major cause for infection as a result of or enhanced In human immunosuppression and infection are closely # ) that infection in of all transplant recipients and is the leading cause of Both baboons and are known to harbor agents of human infection and and careful may this problem. endogenous in the nonhuman primate and perhaps in the pig could be a source of disease. agents, from to are as and are observed recovered from tissues. Endogenous viruses would not be by use of or SPF limited data an oncogenic for the baboon endogenous viruses, but more information is are the potential to organ transplant recipients with regard to Such diseases have been observed in nonhuman primates including baboons ( Kalter ). The of a herpesvirus closely related to human herpes and the macaque B virus, that is SA8, may baboon However, human disease with has not been recognized. While the question of has been are so far given the to using animals social and ethical particularly if a nonhuman question and particularly would be supply is limited in the wild and are to numbers in use of present of the would not be but would an the result from the choice of the baboon or with a of of in an There is evidence to indicate that immunosuppressed individuals organs from donors have a of disease ( Peterson and others 1980 ). vaccines require availability of are would be limited to in the Of would be the to the It has been suggested that the use of have enhanced the of recipients ( and others ). The major in xenotransplants still to in the of the human body to a in to by of has been and to ( Starzl and others 1993 ). and of the donor animal the of animals could be providing organs that would be immunologically to humans ( ). However, will of donors result in the of new from which human may infections are under ethical and social about the use of animal organs for xenotransplants be by the problem of organ infection, an animal host other than a primate and that also as a source of Because of the limited supply of human organs, more than half of the individuals organs from animal sources for the approximately for transplants, if not a permanent should any to the use of an animal source of organs. reported on serologic studies were in by from the and also to to the many studies were not in this is an This is an to indicate the potential and problems of infectious diseases and xenotransplants.