- Email: [email protected]
The pig as a potential xenograft donor
e,,~o"~,
,~. ~~ E LS EV I ER
Veterinary
and immunopathology Veterinary Immunology and Immunopathology 43 (1994) 185-191
The pig as a potential xenograft donor D a v i d H. Sachs* Harvard Medical School and the Transplantation Biology Research Center, Massachusetts General Hospital, MGH-East, Building 149-9019, 13th Street, Boston, MA 02129, USA
Abstract
Miniature swine have several advantages over other potential donor species as a xenograft donor for clinical use. Among these advantages are: ( 1 ) unlimited availability; (2) size (similar to human beings); (3) breeding characteristics; (4) physiologic and immunologic similarities to humans. Because of the genetic disparity between these two species, routine immunosuppression will probably not suffice for the long-term survival of pig to primate xenografts. Studies are therefore underway to induce tolerance across this species barrier, utilizing a mixed chimerism approach which has previously been successful for allogeneic and concordant xenogeneic combinations. Hyperacute rejection has been eliminated by an absorption technique and pig kidney xenograft survivals up to 13 days have been achieved.
1. Introduction: The need for xenografts Donor organ availability has recently become a factor limiting progress in the field of transplantation. Paradoxically, the problem exists largely because of the success of transplantation as a treatment modality. While it is clear that increasing efforts at donor procurement are still needed, it is also clear that the number of cadaver organs available will never be sufficient to provide organs for all patients who could benefit from a transplant. In the case of heart transplants, the number of transplants performed per year, which rose exponentially in the early 1980s following the introduction of Cyclosporine A, has now reached a plateau at approximately 2000 per year in the USA. With essentially all functioning cadaver hearts now being utilized, there are thousands of patients dying each year *Tel. 617-726-4065; fax 617-726-4067. 0165-2427/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0165-2427 ( 94 )06023-S
186
D.H. Sachs / Veterinary lmmunology and hnrnunopathology 43 (1994) 185-191
while waiting for a transplant. Therefore, the need for another source of donor organs has become apparent. Similar shortages exist for many other organs, including the kidney, the liver, the pancreas and the lung. The problem for lung transplants is perhaps the most acute, since most cadaver donors have been maintained on a ventilator prior to organ harvest, and have therefore frequently developed pulmonary infections, which limit the potential for donation of lung transplants.
2. Choice of a donor species
For all of these reasons, there has been renewed interest in the past few years in the xenograft as a potential source of donor organs. For the purposes of clinical xenotransplantation, the most appropriate potential donor species is a matter of controversy. Clearly, the most similar, or 'concordant' donors would be non-human primates (Calne, 1970; Sachs, 1990). However, there are serious problems in considering such animals as donors. The closest non-human primates phylogenetically would be chimpanzees, but these are far too rare to be considered seriously. The nonhuman primate species which has been most commonly suggested as a donor in terms of potential availability is the baboon. However, in addition to availability issues, the baboon is too small to be an appropriate donor for most organ transplants. Even the largest baboons weigh less than 40 kg, which would be inadequate as a heart donor for most adult human beings. In addition, questions about transmission of pathogenic viruses from non-human primates have been raised as a potential concern.
3. The pig as a xenograft donor
In our laboratory, we have chosen the pig as a potential xenograft donor. We and others have noted the large number of similarities between swine and human beings with respect to parameters of importance to transplantation, some of which are summarized in Table 1 (Tumbleson, 1986). The cardiovascular system of the pig is remarkably similar to that of man, and, as shown in Table 2, many functional parameters of this system are essentially identical in the two species (Cooper et al., 1991 ). The same has been shown for renal function (Kirkman, 1989 ) (see Table 3 ), and is likely to be true for additional organs as well. Over the past 20 years, we have developed miniature swine as a large animal model for studies of transplantation biology. We chose miniature swine both because of the similarities indicated in Table 1, and because of a variety of additional advantages summarized in Table 4. The chief advantage of these animals over domestic swine involves their size. While domestic swine can attain weights in excess of 1000 lb., fully adult miniature swine weigh 250-300 lb., a size much more similar to that of human beings, and also more consistent with use as a laboratory animal. In terms of xenotransplantation, one can envision obtaining a xenograft organ of
D.H. Sachs / Veterinary Immunology and Immunopathology 43 (I 994) 185-191
187
Table 1 Similarities between swine and human beings ( 1 ) Size (2) Digestive physiology ( 3 ) Dietary habits (4) Kidney structure and function ( 5 ) Pulmonary vascular bed structure (6) Coronary artery distribution (7) Respiratory rates (8) Cardiovascular anatomy and physiology From Tumbleson ( 1986 ). Table 2 Similarities between swine and human beings: cardiovascular system parameters Parameter
Human
Swine
Cardiac output (1 rain -~ m -2) Right atrial pressure (mmHg) Right ventricular pressure (mmHg) Pulmonary arterial pressure (mmHg) Left ventricular pressure (mmHg) Aortic pressure (mmHg)
2.5-3.5 0-8 15-30 15-30 100-140 70-105
2.0-2.5 1-9 24-30 11-24 116 114-126
From Cooper etal. ( 1991 ), with modification. Table 3 Similarities between swine and human beings: parameters of renal function Parameter
Human
Swine
Maximal concentration (mOsm l- 1) Maximal urine/plasma osmol, ratio GFR (ml min -I per 70 kg) Total renal blood flow (ml rain-i g-~
l 160 4.0 130 4
1080 3.3 126-175 3.0-4.4
From Kirkman (1989). a p p r o p r i a t e size for a n y p o t e n t i a l h u m a n recipient, f r o m a n e w b o r n b a b y to the largest adult. T h e b r e e d i n g characteristics o f swine are particularly favorable for d e v e l o p m e n t as a large a n i m a l m o d e l . T h e s e a n i m a l s h a v e large litter sizes (three to ten piglets per litter), m a k i n g it possible to select a p p r o p r i a t e offspring for breeding. In a d d i t i o n , t h e y reach sexual m a t u r i t y relatively early ( 4 - 5 m o n t h s ) , a n d h a v e an estrus cycle every 3 weeks. T h u s , it has b e e n possible to i n b r e e d these a n i m a l s to h o m o z y g o s i t y for the m a j o r h i s t o c o m p a t i b i l i t y c o m p l e x ( M H C ) within a relatively short t i m e ( w i t h respect to the lifetime o f an investigator! ). As s h o w n in Fig. l, we h a v e d e v e l o p e d three lines o f m i n i a t u r e swine, each h o m o z y g o u s for a
188
D.H. Sachs / Veterinary lmmunology and lmmunopathology 43 (1994) 185-191
Table 4 Advantages of miniature swine as a large animal model A. Breeding characteristics ( 1 ) Reproducible MHC genetics (2) Backcrossing possible B. Similarities to man ( 1 ) Size (2) Tissue expression of MHC antigens ( 3 ) Response to ablative radiation (4) Cardiovascular anatomy and physiology
Haplotype
.
Origin of Regions Class II Class I
I
I
~
I
e
d f
g h
J
I
Fig. 1. Origin of the SLA haplotypes of partially inbred miniature swine (Sachs et al., 1992 ).
different set of alleles at the M H C (termed SLA in swine) (Sachs et al., 1992). In addition, during our subsequent breeding studies we have identified and bred to homozygosity the four intra-MHC recombinant haplotypes also illustrated in Fig. 1. The availability of these recombinants makes miniature swine the only large animal model in which one can reproducibly study the effects of transplantation across selective MHC barriers. In addition, the inbreeding of these animals makes it possible to develop new treatment regimens which are directed toward induction of specific tolerance to a set of pig M H C genes, as opposed to the prospect of tailoring therapy for each particular donor to be used as the source of a xenograft (Sachs et al., 1993).
D.H. Sachs / Veterinary Immunology and Immunopathology 43 (1994) 185-191
189
4. Problems to be faced in pig xenografting It has been relatively easy to establish long-term xenografts between such closely related (i.e. concordant) species combinations as rat and mouse (Zoalberg, 1957; Lance et al., 1969; Burdick et al., 1979; Muller-Rucholtz et al., 1979; Ildstad and Sachs, 1984; Sharabi et al., 1990). Most experimental approaches to such xenograft tolerance have involved establishment of lymphohematopoietic chimerism, which, if stable, carries with it tolerance to most other tissues from the same donor. Work from this laboratory has demonstrated that the establishment of mixed chimerism (i.e. survival of both host and donor lymphohematopoietic elements ) produces tolerance and also maintains full immunocompetence, both for allogeneic and concordant r a t ~ m o u s e xenogeneic systems (Ildstad and Sachs, 1984; Sharabi and Sachs, 1989; Sharabi et al., 1990, 1992 ). We have recently begun studies attempting to extend our results for nonmyeloablative induction of transplantation tolerance in the concordant rat-,mouse system to a discordant pig~non-human primate system as a preclinical model. There are a variety of differences between these models which need to be taken into account. Most importantly, in this discordant system, natural antibodies exist which cause hyperacute rejection of a pig organ (Calne, 1970; Auchincloss, 1988). Such hyperacute rejection has in fact prevented progress in the field of discordant xenografting for many years. However, there are several procedures which have been suggested as potential ways to eliminate natural antibodies. A1exandre and colleagues have pioneered the use of extensive plasmapheresis as a means of removing these antibodies (Alexandre et al., 1989). In our own laboratory we have chosen an absorption technique, utilizing a pig liver, to absorb antibodies in vivo prior to the xenotransplant. We have recently reported the preliminary results of our first studies utilizing this model (Latinne et al., 1993). Recipient cynomolgus monkeys were treated with either anti-T cell monoclonal antibodies or ATG to remove mature T cell subsets and natural killer (NK) cells. They received similar sublethal irradiation to that utilized in our previous r a t ~ m o u s e studies, and also received bone marrow from the pig donor. At the time of operation, the recipient's blood was perfused from the aorta, through a freshly isolated pig liver, and back to the recipient vena cava, using Silastic catheters. A l h perfusion was found to be sufficient to remove the vast majority of natural antibodies. Following this procedure, a pig renal allograft was transplanted as a test organ for the induction of transplant tolerance. Our results to date have shown elimination of hyperacute rejection by this absorption, and we have achieved kidney graft survival times of up to 13 days. During this time, the kidney functioned normally, and maintained a normal creatinine and acid-base balance. However, we have not yet achieved persistence of mixed xenogeneic chimerism in this model, nor has long-term tolerance yet been established. Clearly, a variety of parameters will need to be refined to achieve this goal. Among the approaches being tested are the use of columns bearing specific sugars to absorb natural antibodies, rather than the use of a whole liver, which is a major stress to the cardiovascular system of the host. Production and use of specific monoclonal
190
D.H. Sachs/Veterinarylmmunologyandlmmunopathology43
(1994) 185-191
antibodies rather than the use of ATG to eliminate host immune cells is also in progress. We are encouraged by the survivals achieved without additional immunosuppression, and by the fact that the recipients have maintained normal renal function during this time with only a functioning pig xenograft kidney.
5. Conclusions The pig has numerous advantages as a potential xenograft donor. Miniature swine, in particular, are of exactly the right size to provide organs to human beings of all ages and weights. In addition, the breeding characteristics of swine have made it possible to produce genetically defined lines, and should make it possible in the future to produce appropriate genetic alterations to favor xenograft acceptance. These animals can be produced under conditions of well-controlled flora, which will hopefully make it possible to avoid introduction of dangerous pathogens along with the xenogeneic tissues and organs. The major obstacle to discordant xenotransplantation, that of preformed natural antibodies, can probably be overcome by absorption techniques. It will remain to be seen whether or not the use of standard immunosuppressive agents will be sufficient to prevent cellular rejection, the return of natural antibodies, or the production of immune antibodies. It seems likely, however, that the amount of immunosuppression which will be required to prevent rejection of xenografls will be higher than that needed for mismatched allografts, the complications of which already provide limitations to the field of transplantation. In this case, the induction of specific tolerance may be of even greater importance to the future of this field.
Acknowledgments This work was supported by a Sponsored Research Agreement from BioTransplant Inc.
References G.P.J. Alexandre et al. ( 1989 ), in Xenografl 25, M.A. Hardy, Ed. (Excerpta Medica, New York), pp. 259-266. H. Auchincloss, Jr. ( 1988 ), Transplantation 46, 1. J.F. Burdick, P.S. Russell, H.J. Winn (1979), J. Immunol. 123, 1732. R.Y. Calne (1970), Transplant. Proc. 2, 550. D.K.C. Cooper, Y. Ye, L.L Rolf, N. Zuhdi ( 1991 ), in Xenotransplantation: The Transplantation of Organs and Tissues Between Species, D.K.C. Cooper, E. Kemp, K. Reemtsma, D.J.G. White, Eds. (Springer, Berlin), pp. 481-500. S.T. Ildstad and D.H. Sachs (1984), Nature 307 (5947), 168. R.L. Kirkman (1989), in Xenograft 25, M.A. Hardy, Ed. (Excerpta Medica, Amsterdam), pp. 125132.
D.H. Sachs / Veterinary lmmunology and lmmunopathology 43 (1994) 185-191
191
E.M. Lance, R.H. Levey, P.B. Medawar, M. Ruszkiewicz (1969), Proc. Natl. Acad. Sci. USA 64, 1356. D. Latinne et al. (1993), Transplant. Proc. 25,336. W. Muller-Rucholtz, H.K. Muller-Hermelink, H.U. Wottge (1979), Transplant. Proc. 11, 517. D.H. Sachs and F.H. Bach (1990), Hum. Immunol. 28, 245. D.H. Sachs, in M.M. Swindle, D.C. Moody, L.D. Phillips (1992), Eds. (Iowa State University Press, Ames), pp. 3-15. D.H. Sachs et al. (1993), Exp. Nephrol. 1, 128. Y. Sharabi and D.H. Sachs (1989), J. Exp. Med. 169, 493. Y. Sharabi, I. Aksentijevich, T.M. Sundt III, D.H. Sachs, M. Sykes (1990), J. Exp. Med. 172, 195. Y. Sharabi, V.S. Abraham, M. Sykes, D.H. Sachs (1992), Bone Marrow Transplant. 9, 91. M.E. Tumbleson ( 1986 ), Swine in Biomedical Research (Plenum Press, New York). O.B. Zoalberg, O. Vos, D.W. van Bekkum (1957), Nature 180, 238.
,~. ~~ E LS EV I ER
Veterinary
and immunopathology Veterinary Immunology and Immunopathology 43 (1994) 185-191
The pig as a potential xenograft donor D a v i d H. Sachs* Harvard Medical School and the Transplantation Biology Research Center, Massachusetts General Hospital, MGH-East, Building 149-9019, 13th Street, Boston, MA 02129, USA
Abstract
Miniature swine have several advantages over other potential donor species as a xenograft donor for clinical use. Among these advantages are: ( 1 ) unlimited availability; (2) size (similar to human beings); (3) breeding characteristics; (4) physiologic and immunologic similarities to humans. Because of the genetic disparity between these two species, routine immunosuppression will probably not suffice for the long-term survival of pig to primate xenografts. Studies are therefore underway to induce tolerance across this species barrier, utilizing a mixed chimerism approach which has previously been successful for allogeneic and concordant xenogeneic combinations. Hyperacute rejection has been eliminated by an absorption technique and pig kidney xenograft survivals up to 13 days have been achieved.
1. Introduction: The need for xenografts Donor organ availability has recently become a factor limiting progress in the field of transplantation. Paradoxically, the problem exists largely because of the success of transplantation as a treatment modality. While it is clear that increasing efforts at donor procurement are still needed, it is also clear that the number of cadaver organs available will never be sufficient to provide organs for all patients who could benefit from a transplant. In the case of heart transplants, the number of transplants performed per year, which rose exponentially in the early 1980s following the introduction of Cyclosporine A, has now reached a plateau at approximately 2000 per year in the USA. With essentially all functioning cadaver hearts now being utilized, there are thousands of patients dying each year *Tel. 617-726-4065; fax 617-726-4067. 0165-2427/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0165-2427 ( 94 )06023-S
186
D.H. Sachs / Veterinary lmmunology and hnrnunopathology 43 (1994) 185-191
while waiting for a transplant. Therefore, the need for another source of donor organs has become apparent. Similar shortages exist for many other organs, including the kidney, the liver, the pancreas and the lung. The problem for lung transplants is perhaps the most acute, since most cadaver donors have been maintained on a ventilator prior to organ harvest, and have therefore frequently developed pulmonary infections, which limit the potential for donation of lung transplants.
2. Choice of a donor species
For all of these reasons, there has been renewed interest in the past few years in the xenograft as a potential source of donor organs. For the purposes of clinical xenotransplantation, the most appropriate potential donor species is a matter of controversy. Clearly, the most similar, or 'concordant' donors would be non-human primates (Calne, 1970; Sachs, 1990). However, there are serious problems in considering such animals as donors. The closest non-human primates phylogenetically would be chimpanzees, but these are far too rare to be considered seriously. The nonhuman primate species which has been most commonly suggested as a donor in terms of potential availability is the baboon. However, in addition to availability issues, the baboon is too small to be an appropriate donor for most organ transplants. Even the largest baboons weigh less than 40 kg, which would be inadequate as a heart donor for most adult human beings. In addition, questions about transmission of pathogenic viruses from non-human primates have been raised as a potential concern.
3. The pig as a xenograft donor
In our laboratory, we have chosen the pig as a potential xenograft donor. We and others have noted the large number of similarities between swine and human beings with respect to parameters of importance to transplantation, some of which are summarized in Table 1 (Tumbleson, 1986). The cardiovascular system of the pig is remarkably similar to that of man, and, as shown in Table 2, many functional parameters of this system are essentially identical in the two species (Cooper et al., 1991 ). The same has been shown for renal function (Kirkman, 1989 ) (see Table 3 ), and is likely to be true for additional organs as well. Over the past 20 years, we have developed miniature swine as a large animal model for studies of transplantation biology. We chose miniature swine both because of the similarities indicated in Table 1, and because of a variety of additional advantages summarized in Table 4. The chief advantage of these animals over domestic swine involves their size. While domestic swine can attain weights in excess of 1000 lb., fully adult miniature swine weigh 250-300 lb., a size much more similar to that of human beings, and also more consistent with use as a laboratory animal. In terms of xenotransplantation, one can envision obtaining a xenograft organ of
D.H. Sachs / Veterinary Immunology and Immunopathology 43 (I 994) 185-191
187
Table 1 Similarities between swine and human beings ( 1 ) Size (2) Digestive physiology ( 3 ) Dietary habits (4) Kidney structure and function ( 5 ) Pulmonary vascular bed structure (6) Coronary artery distribution (7) Respiratory rates (8) Cardiovascular anatomy and physiology From Tumbleson ( 1986 ). Table 2 Similarities between swine and human beings: cardiovascular system parameters Parameter
Human
Swine
Cardiac output (1 rain -~ m -2) Right atrial pressure (mmHg) Right ventricular pressure (mmHg) Pulmonary arterial pressure (mmHg) Left ventricular pressure (mmHg) Aortic pressure (mmHg)
2.5-3.5 0-8 15-30 15-30 100-140 70-105
2.0-2.5 1-9 24-30 11-24 116 114-126
From Cooper etal. ( 1991 ), with modification. Table 3 Similarities between swine and human beings: parameters of renal function Parameter
Human
Swine
Maximal concentration (mOsm l- 1) Maximal urine/plasma osmol, ratio GFR (ml min -I per 70 kg) Total renal blood flow (ml rain-i g-~
l 160 4.0 130 4
1080 3.3 126-175 3.0-4.4
From Kirkman (1989). a p p r o p r i a t e size for a n y p o t e n t i a l h u m a n recipient, f r o m a n e w b o r n b a b y to the largest adult. T h e b r e e d i n g characteristics o f swine are particularly favorable for d e v e l o p m e n t as a large a n i m a l m o d e l . T h e s e a n i m a l s h a v e large litter sizes (three to ten piglets per litter), m a k i n g it possible to select a p p r o p r i a t e offspring for breeding. In a d d i t i o n , t h e y reach sexual m a t u r i t y relatively early ( 4 - 5 m o n t h s ) , a n d h a v e an estrus cycle every 3 weeks. T h u s , it has b e e n possible to i n b r e e d these a n i m a l s to h o m o z y g o s i t y for the m a j o r h i s t o c o m p a t i b i l i t y c o m p l e x ( M H C ) within a relatively short t i m e ( w i t h respect to the lifetime o f an investigator! ). As s h o w n in Fig. l, we h a v e d e v e l o p e d three lines o f m i n i a t u r e swine, each h o m o z y g o u s for a
188
D.H. Sachs / Veterinary lmmunology and lmmunopathology 43 (1994) 185-191
Table 4 Advantages of miniature swine as a large animal model A. Breeding characteristics ( 1 ) Reproducible MHC genetics (2) Backcrossing possible B. Similarities to man ( 1 ) Size (2) Tissue expression of MHC antigens ( 3 ) Response to ablative radiation (4) Cardiovascular anatomy and physiology
Haplotype
.
Origin of Regions Class II Class I
I
I
~
I
e
d f
g h
J
I
Fig. 1. Origin of the SLA haplotypes of partially inbred miniature swine (Sachs et al., 1992 ).
different set of alleles at the M H C (termed SLA in swine) (Sachs et al., 1992). In addition, during our subsequent breeding studies we have identified and bred to homozygosity the four intra-MHC recombinant haplotypes also illustrated in Fig. 1. The availability of these recombinants makes miniature swine the only large animal model in which one can reproducibly study the effects of transplantation across selective MHC barriers. In addition, the inbreeding of these animals makes it possible to develop new treatment regimens which are directed toward induction of specific tolerance to a set of pig M H C genes, as opposed to the prospect of tailoring therapy for each particular donor to be used as the source of a xenograft (Sachs et al., 1993).
D.H. Sachs / Veterinary Immunology and Immunopathology 43 (1994) 185-191
189
4. Problems to be faced in pig xenografting It has been relatively easy to establish long-term xenografts between such closely related (i.e. concordant) species combinations as rat and mouse (Zoalberg, 1957; Lance et al., 1969; Burdick et al., 1979; Muller-Rucholtz et al., 1979; Ildstad and Sachs, 1984; Sharabi et al., 1990). Most experimental approaches to such xenograft tolerance have involved establishment of lymphohematopoietic chimerism, which, if stable, carries with it tolerance to most other tissues from the same donor. Work from this laboratory has demonstrated that the establishment of mixed chimerism (i.e. survival of both host and donor lymphohematopoietic elements ) produces tolerance and also maintains full immunocompetence, both for allogeneic and concordant r a t ~ m o u s e xenogeneic systems (Ildstad and Sachs, 1984; Sharabi and Sachs, 1989; Sharabi et al., 1990, 1992 ). We have recently begun studies attempting to extend our results for nonmyeloablative induction of transplantation tolerance in the concordant rat-,mouse system to a discordant pig~non-human primate system as a preclinical model. There are a variety of differences between these models which need to be taken into account. Most importantly, in this discordant system, natural antibodies exist which cause hyperacute rejection of a pig organ (Calne, 1970; Auchincloss, 1988). Such hyperacute rejection has in fact prevented progress in the field of discordant xenografting for many years. However, there are several procedures which have been suggested as potential ways to eliminate natural antibodies. A1exandre and colleagues have pioneered the use of extensive plasmapheresis as a means of removing these antibodies (Alexandre et al., 1989). In our own laboratory we have chosen an absorption technique, utilizing a pig liver, to absorb antibodies in vivo prior to the xenotransplant. We have recently reported the preliminary results of our first studies utilizing this model (Latinne et al., 1993). Recipient cynomolgus monkeys were treated with either anti-T cell monoclonal antibodies or ATG to remove mature T cell subsets and natural killer (NK) cells. They received similar sublethal irradiation to that utilized in our previous r a t ~ m o u s e studies, and also received bone marrow from the pig donor. At the time of operation, the recipient's blood was perfused from the aorta, through a freshly isolated pig liver, and back to the recipient vena cava, using Silastic catheters. A l h perfusion was found to be sufficient to remove the vast majority of natural antibodies. Following this procedure, a pig renal allograft was transplanted as a test organ for the induction of transplant tolerance. Our results to date have shown elimination of hyperacute rejection by this absorption, and we have achieved kidney graft survival times of up to 13 days. During this time, the kidney functioned normally, and maintained a normal creatinine and acid-base balance. However, we have not yet achieved persistence of mixed xenogeneic chimerism in this model, nor has long-term tolerance yet been established. Clearly, a variety of parameters will need to be refined to achieve this goal. Among the approaches being tested are the use of columns bearing specific sugars to absorb natural antibodies, rather than the use of a whole liver, which is a major stress to the cardiovascular system of the host. Production and use of specific monoclonal
190
D.H. Sachs/Veterinarylmmunologyandlmmunopathology43
(1994) 185-191
antibodies rather than the use of ATG to eliminate host immune cells is also in progress. We are encouraged by the survivals achieved without additional immunosuppression, and by the fact that the recipients have maintained normal renal function during this time with only a functioning pig xenograft kidney.
5. Conclusions The pig has numerous advantages as a potential xenograft donor. Miniature swine, in particular, are of exactly the right size to provide organs to human beings of all ages and weights. In addition, the breeding characteristics of swine have made it possible to produce genetically defined lines, and should make it possible in the future to produce appropriate genetic alterations to favor xenograft acceptance. These animals can be produced under conditions of well-controlled flora, which will hopefully make it possible to avoid introduction of dangerous pathogens along with the xenogeneic tissues and organs. The major obstacle to discordant xenotransplantation, that of preformed natural antibodies, can probably be overcome by absorption techniques. It will remain to be seen whether or not the use of standard immunosuppressive agents will be sufficient to prevent cellular rejection, the return of natural antibodies, or the production of immune antibodies. It seems likely, however, that the amount of immunosuppression which will be required to prevent rejection of xenografls will be higher than that needed for mismatched allografts, the complications of which already provide limitations to the field of transplantation. In this case, the induction of specific tolerance may be of even greater importance to the future of this field.
Acknowledgments This work was supported by a Sponsored Research Agreement from BioTransplant Inc.
References G.P.J. Alexandre et al. ( 1989 ), in Xenografl 25, M.A. Hardy, Ed. (Excerpta Medica, New York), pp. 259-266. H. Auchincloss, Jr. ( 1988 ), Transplantation 46, 1. J.F. Burdick, P.S. Russell, H.J. Winn (1979), J. Immunol. 123, 1732. R.Y. Calne (1970), Transplant. Proc. 2, 550. D.K.C. Cooper, Y. Ye, L.L Rolf, N. Zuhdi ( 1991 ), in Xenotransplantation: The Transplantation of Organs and Tissues Between Species, D.K.C. Cooper, E. Kemp, K. Reemtsma, D.J.G. White, Eds. (Springer, Berlin), pp. 481-500. S.T. Ildstad and D.H. Sachs (1984), Nature 307 (5947), 168. R.L. Kirkman (1989), in Xenograft 25, M.A. Hardy, Ed. (Excerpta Medica, Amsterdam), pp. 125132.
D.H. Sachs / Veterinary lmmunology and lmmunopathology 43 (1994) 185-191
191
E.M. Lance, R.H. Levey, P.B. Medawar, M. Ruszkiewicz (1969), Proc. Natl. Acad. Sci. USA 64, 1356. D. Latinne et al. (1993), Transplant. Proc. 25,336. W. Muller-Rucholtz, H.K. Muller-Hermelink, H.U. Wottge (1979), Transplant. Proc. 11, 517. D.H. Sachs and F.H. Bach (1990), Hum. Immunol. 28, 245. D.H. Sachs, in M.M. Swindle, D.C. Moody, L.D. Phillips (1992), Eds. (Iowa State University Press, Ames), pp. 3-15. D.H. Sachs et al. (1993), Exp. Nephrol. 1, 128. Y. Sharabi and D.H. Sachs (1989), J. Exp. Med. 169, 493. Y. Sharabi, I. Aksentijevich, T.M. Sundt III, D.H. Sachs, M. Sykes (1990), J. Exp. Med. 172, 195. Y. Sharabi, V.S. Abraham, M. Sykes, D.H. Sachs (1992), Bone Marrow Transplant. 9, 91. M.E. Tumbleson ( 1986 ), Swine in Biomedical Research (Plenum Press, New York). O.B. Zoalberg, O. Vos, D.W. van Bekkum (1957), Nature 180, 238.