Schuwei Cai, Cathy Ross, Steve Wong
Approximately 48,000 people are on the waiting list for organs; contrast this with an average of 4,800 people per year donating their organs after death and there exists a huge discrepency in the number of people requiring organ transplants and the number of organs available. Consequently, an estimate 60% of all patients have died waiting for transplants, thus stimulating the need for an alternative source of donor organs. This limited availability of human organs and tissues for allografts has been the initiating force behind xenografts, the organ or tissue transplantation between members of different species.
The first animal-to-human transplant was in 1906, when a pig's kidney was implanted into a woman. Neither survived. In 1984 at the Loma Linda University Medical Center in CA, a heart from a 7-month-old baboon was transplanted into a newborn human; in 1992 at the University of Pittsburgh Medical Center, the liver of a baboon was transplanted into a critically-ill patient; in 1992 at the Cedars-Sinai Medical Center the liver of a pig was transplanted into a woman dying of hepatitis; in 1993 the University of Pittsburgh transplanted a baboon's liver into a man with hepatitis. Although all the patients died within weeks after their operations, they did not die of organ rejection but rather of infections common to patients on immunosuppressive drugs; and so the feasibility of using animals as a source of organs for transplantation continues to be researched. Certain findings, however, suggested a similarity in the rejection process in both allografts and xenografts.
Complement System Key Obstacle to Xenotransplantation.
The major immune rejection response involves the production of humoral antibody and complement with the development of a hyperacute rejection reaction (Kuby). The complement system is a key obstacle to plans to transplant organs between species.
Complement is the defense mechanism that uses at least 30 proteins in the bloodstream. It marks any cell which does not have certain protective proteins, presumably non-self cells, on their membrane surfaces. Two major proteins functioning as protective proteins are MCP, preventing complement molecules from attaching to the cell membrane of self-cells; and DAF, Decay-Accelerating Factor, which destroys complement that does manage to bind to the cell. Once bound to the cell membrane, these complement markers attract certain white blood cells, phagocytes, which mediate the cell-mediated immune response.
Hyperacute Rejection Caused by Complement.
The major problem with xenotransplants is that the immune rejection is quite vigorous, a consequence due primarily of the hyperacute rejection caused by complement which destroys foreign bodies much faster that a cell-mediated immune attack. Larkin et al examined the mechanisms of graft rejection in the rat. Xenograft failure here was characterized by early cell damage followed by infiltration with T cells, macrophages and eosinophils. An accelerated pattern of guinea pig graft failure suggested that preformed anti-donor antibody and complement were responsible for early graft damage. The paper generalized but further substantiates that xenotransplantation invariably results in hyperacute antibody-mediated rejection within minutes or hours of the graft and the roles of natural antibody and complement in initiating this type of rejection.
Allografts and Xenografts Share Similar Path of Rejection.
An important finding that has emerged from experimental transplant models is that the critical period for graft rejection is from 2 to 4 weeks after grafting. This similarity in rejection of allografts and xenografts therefore suggests that the rejection of non-human organs can be reversed by much the same techniques used with human organs - the overall goal being to over-ride the body's recognition, and subsequent rejection, of the foreign tissue as non-self.
Approaches to Limiting the Destruction Mediated by Complement.
Of all the animals, baboons and pigs are the favored xenotransplant donors. Baboons because they are so genetically similar to humans that their cells display similar protective proteins; and pigs because they share a considerable number of anatomic and physiologic similarities with humans. In addition to exploiting these natural similarities, one possible approach for limiting the destruction mediated by antibody and complement is to genetically engineer donor animals (transgenic) expressing DAF.
Byrne et al characterized a line of transgenic pigs by their expression of human complement-regulating proteins and DAF (decay-accelerating factor). They showed that the exprpession of these proteins is sufficient to block the complement-mediated damage and that this results suggests that transgenic organs with high levels of human complement protein expression will be sufficient to alleviate the humoral immunological barriers that currently block the use of xenogeneic organs for human transplantation. Suppression of the immune response in the recipient was sufficient to prevent donor graft rejection in these experiments, and Khan et al assert that thymic transplantation can induce donor-specific permanent tolerance to xenogeneic donors.
Theoretical Risk of the Introduction of Dangerous New Viruses.
It is herein these successful xenograft experiments, whereby suppression of the immune response in the recipient is sufficient to prevent donor graft rejection, that the issue of the theoretical risk of the introduction of dangerous new viruses into the human population surfaces. There is great risk involved in suppressing the immune response of the recipient; they would not have mounted an immune response to the donor's tissue, nor to possibly any viruses or other pathogens of the donor's which may have been introduced via the transplantation. The concern is nicely framed in Science News where mention of the discovery of a pig retrovirus that can infect human cells is brought to light.
Baboons and pigs both cary a myriad of transmittable agents that are known, and perhaps many more which are not known. These bacteria and fungi may be fairly harmless in their natural host, yet an infecting microbe may change when it is transmitted into a new species; causing disease, or possibly being fatal to a human recipient. HIV, for example, is a retrovirus that many researchers believe was transmitted to humans from monkeys.
Transplanted cells or organs are the ideal pathway for infection since any virus they contain would bypass the mucosal and skin barriers to infection. Through xenotransplantation, the virus circumvents most of the natural barriers and, in essence, is given a free ride into the body. For several reasons, there are several concerns about the possible risk of retroviruses. The chance of cross-species infection is presumably increased by such factors as: the time and closeness of in vivo contact between cells; the susceptibility of the human cells and the competence of the immune system. Notably, situations have been described in the literature in which retroviral sequences of one animal species are believed to have led to disease conditions in another species. To date, no human disease has been traced to a xenotransplant. Interesting to note, however, is the charcteristically long latent period (up to many years) which can exist between initial exposure and manifestation of infectious viruses.
Conservative Approach to Xenotransplantation fo Avoid Opening Pandora's
Box.
According to Greek mythology, it was Pandora, who, after continuous warning cautioning her not to open the box, was unable to resist her curiosity and having doing so, released upon the world all that has come to plague mankind. Although there have been no such consequences resulting from xenografts, it would only take one incidence of infection to spread a highly infectious, readily transmittable and devastating disease to mankind. That there exists the theoretical possibility for xenografts to create a pathway for infection by, as yet, some unknown virus would suggest that research should include extra precautions to ensure that Pandora's Box is not opened.
Byrne,G. et al. Transgenic Pigs Expressing Human CD59 and DAF Produce an Intrinsic Barrier to Complement-Mediate Damage. Transplantation. 63 (1997):149-155
Khan,A et al. Discordant Xenogenic Neonatal Thymic Transplantation Can Induce Donor-Specific Tolerance. Transplantation. 63 (1997):124-131
Kuby, Janice. Immunology. 3rd Edition. New York: W.H. Freeman and Co., 1997
Larkin,F. et al. Experimental Orthotopic Corneal Xenotransplantation in the Rat. Transplantation. 60 (1995):491-497