Citation: Mohiuddin MM (2007) Clinical Xenotransplantation of Organs: Why Aren't We There Yet? PLoS Med 4(3): e75. https://doi.org/10.1371/journal.pmed.0040075
Published: March 27, 2007
This is an open-access article distributed under the terms of the Creative Commons Public Domain declaration which stipulates that, once placed in the public domain, this work may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose.
Funding: The author received no specific funding for this article.
Competing interests: The author has declared that no competing interests exist.
Abbreviations: HAR, hyperacute rejection; KO, knockout; NHP, nonhuman primate; pCMV, porcine cytomegalovirus; PERV, porcine endogenous retrovirus; XNA, xenogeneic natural antibody
Organ transplantation across a species barrier—xenotransplantation—has been attempted for over a century. Given the rapidly increasing gap between the organs required and those available for transplantation, over the last 20 years xenotransplantation has been aggressively pursued as a promising supplement to allotransplantation (see Glossary).
Progress in this field from the late eighties to the late nineties had been steady, but shrinking funding, ethical and regulatory issues, threats of transmission of infection, and diminished interest by industry have resulted in a significant decline of enthusiasm in this field. But the recent development of genetically modified pigs that are more compatible with humans has reinstated hope for the success of xenotransplantation of organs. However, whether such genetic modifications are necessary to prevent xenograft rejection of porcine cells is questionable.
Early Experiments Using Small Animal Models
A significant amount of information about the mechanism of solid organ xenograft rejection was gained from earlier experiments using small animal models. Experimental protocols were successfully generated to induce graft accommodation and donor-specific tolerance, the latter, for example, through the generation of microchimerism [1–6]. In accommodation studies, production of antibodies in transplanted animals was delayed, and when the antibodies were later allowed to return, the transplanted organ had developed a means of protection from these antibodies, thus preventing antibody-mediated rejection [5,7]. In tolerance studies, the immune system of the recipient was manipulated so that it learned to recognize the foreign graft as self [1,3,4,8]. Costimulatory blockade and suppression of T and B cells were also successful in achieving long-term graft survival in small animal models [9–14].
Allogeneic: Two or more strains are stated to be allogeneic to each other when the genes at one or more loci are not identical in sequence in each organism.
Allotransplantation: Transplantation of an allograft.
Autologous: Derived from the same organism.
Heterotopic: Occurring in an abnormal position.
Microchimerism: The presence of two genetically distinct and separately derived populations of cells, one population being in low concentration, in the same individual or organ ( e.g., bone marrow).
Thus, work in small animal models of solid organ xenografts clearly showed that xenotransplantation initiates a variety of inflammatory, immune, and coagulation responses, and the successful suppression of these responses encouraged researchers to move to larger animal models. Unfortunately, the task of extending graft survival in large animal models such as pig-to-nonhuman primate (NHP) has proven to be a tall order. The mechanism of rejection is found to be more complex and experiments using large animals have resulted in identification of new pathways responsible for substantial anti-donor xenogeneic responses [15,16].
In this article, I discuss the lessons learned from large animal xenograft models and why the immunological barrier is still the most important hurdle preventing clinical xenotransplantation of organs. I also briefly consider other barriers, such as ethical concerns and concerns about viral disease transmission.
Alternatives for Overcoming End-Stage Organ Failure
Patients requiring organ transplantation have limited options. For example, total artificial hearts or mechanical devices have great potential for replacing or improving the function of a diseased heart. However, while ventricular devices have helped patients with cardiac failure , implantation devices have suffered from thrombotic complications and are not yet proven suitable for replacing transplantation .
Autologous adult stem cell transplantation has garnered significant interest over the past few years. This procedure has the potential to repair damage due to myocardial infarction and local defects [19–21]. Allogeneic stem cell transplantation may play a role in delaying the need for transplantation. However, neither of these methods have the potential to replace entire organs.
The idea of growing organs in culture dishes has fascinated scientists for years. Attempts to grow organs (e.g., kidneys) in vitro have yielded small sized organs that lack vascularization . Attempts to grow organs in vivo, in which fetal tissue has been shown to grow into functional organs, have shown some promise. The progress in this field is gradual but I believe that attempts to grow organs are further away from clinical practice than xenotransplantation. Considering all these options, xenotransplantation seems to be one of the most viable and complete options for replacing organs to treat end-stage diseases.
Mechanisms of Xenograft Rejection in Animal Models
In experimental xenotransplantation between discordant species, i.e., species that are phylogenetically distant, the graft undergoes hyperacute rejection (HAR) within minutes. In the pig-to-NHP combination, an example of discordant species combination, HAR is primarily mediated by preformed xenogeneic natural antibody (XNA, predominantly IgM) against a galactose residue (Galactose alpha 1,3-Galactose [Gal]) expressed on pig vascular endothelium [23,24]. Gal is expressed by pigs and most other mammals [25,26]. Binding of XNA to Gal leads to activation of the complement cascade, which causes endothelial damage, thrombus formation, and ultimately a very rapid graft rejection within minutes [23,27,28].
If this antibody- and complement-mediated rejection is averted by measures described below, the transplanted organ undergoes delayed xenograft rejection or acute vascular rejection . In these cases, elicited ( IgM and IgG) antibodies recognize Gal and other non-Gal antigens on the vascular endothelium leading to its activation and rejection of xenografts .
Inefficient regulation of homeostasis leading to intravascular coagulation
Intravascular coagulation is triggered by either antibody/cell-mediated damage of the endothelium or by coagulation factor incompatibilities between two species, and plays a significant role in xenograft rejection. On activation by either antibody binding, or directly by T cells, NK cells, or macrophages, endothelium changes from its anticoagulant state to a procoagulant state by up regulation of von Willebrand factor and production of tissue factor leading to thrombus formation, hemorrhage, and rejection of the graft.
The molecular incompatibilities of coagulation and complement systems further contribute to the rejection process in many porcine-to-NHP systems. For example, porcine thrombomodulin cannot bind to NHP thrombin, and therefore cannot activate protein C and prevent thrombosis .
Rejection of Gal knockout pig xenografts
Recently pigs have been generated in which the enzyme 1,3-galactosyltransferase (GT) gene, which encodes the enzyme responsible for adding Gal residues to many cell surface molecules, has been disrupted. When organs from these Gal-deficient pigs are transplanted into baboons, there is no activation of complement due to the binding of preformed anti-Gal antibody and hyperacute rejection is prevented. Nevertheless, antibodies to non-Gal antigens, which can also directly activate endothelium, and persistence of coagulation incompatibilities leads to graft rejection . These non-Gal antigens have not been fully characterized yet and their potential role in xenograft rejection is under investigation. In Gal knockout (KO) pigs, some investigators have shown an alternate mechanism of surface expression of Gal, suggesting that elimination of Gal is not complete .
In vitro studies analyzing the human response to porcine antigens indicates that human T cells can directly recognize porcine major histocompatibility complex, swine leukocyte antigen I & II, and can also recognize porcine antigens indirectly in the context of self major histocompatibility complex, human leukocyte antigen . A recent paper by Davila et al. shows the induction of cytotoxic pig-specific CD4+CD28- lymphocytes capable of direct tissue destruction . Multiple NK cell-mediated xenograft rejection pathways also exist and complicate efforts to neutralize the potent anti-xenograft activity of NK cells. Recent studies using Gal-/- porcine endothelial cells showed resistance to NK-mediated antibody-dependent cellular cytotoxicity, but susceptibility to direct NK cell lysis [35,36]. Macrophages have also been shown to target and directly destroy islet xenografts .
Large Animal Models of Xenotransplantation: Recent Progress and Limitations
Cellular (islet) transplantation
The most promising reports have come from transplanting wild type porcine islets in NHP for the treatment of diabetes, where complete reversal of diabetes was shown consistently for over 100 days [38,39]. Much evidence suggests that adult porcine islets, unlike endothelial cells and many other cell types, do not express the Gal epitope, and are not susceptible to XNA-mediated hyperacute rejection.
But there were a few drawbacks in both of these studies [38,39]. First, a much larger number of islets compared to the number used in clinical transplantation of human islets were infused to control hyperglycemic states. Second, the level of immunosuppression used in the NHP studies would be unacceptable in humans. For the effective use of this method in clinics, a more acceptable immunosuppressive regimen will be needed and if a larger dose of xeno islets is required to overcome hyperglycemia, an alternate site, such as the omental pouch, could be considered to avoid compromising liver function. To avoid immune rejection, porcine islets have also been transplanted successfully using alginate encapsulation; while this method offers promise, additional work is needed .
Solid organ transplantation
Mixed results have emerged from experiments using genetically modified pig-to-baboon organ transplantation. To date, only two groups have reported long-term survival (over three months) of heterotopic porcine cardiac xenografts for in baboons. McGregor et al. have shown long-term survival of human CD46 transgenic pig hearts in baboons by using synthetic Gal conjugate (TPC) and strong immunosuppression [41,42]. Kuwaki et al. transplanted Gal KO pig hearts heterotopically into baboon recipients and demonstrated graft survival of over six months with costimulation blockade and immune suppression [43,44]. McGregor et al. have recently reported (unpublished data; World Transplant Congress, Boston, MA) survival of orthotropic cardiac xenografts with the longest graft survival of over 60 days, indicating the ability of pig heart to sustain life of recipient baboon.
Recently developed techniques of vascular thymic transplantation are also a major step towards tolerance induction to xenografted kidneys [45,46]. Successful prolongation of pig kidney graft survival by simultaneous transplantation of pig thymus has also been reported using Gal KO pigs as source animals . However, the extensive immunosuppression used in these experiments significantly increased the mortality rate due to infections.
Several other manipulations targeting specific causes of rejection have been developed to overcome xenograft rejection in large animal models. Some of these approaches and their resultant effects are described below (see also Figure 1).
Targeting antibody-mediated rejection
Investigators have successfully delayed HAR by immunoadsorption of antibody on columns or by blocking the epitope binding with GAS 914 or TPC (Gal-Polyethylene glycol conjugates) [41,48–50]. But reemergence of antibodies had not been properly controlled and still posed a credible problem.
In experiments using Gal-deficient pigs as sources, and in recipients in which anti-Gal antibodies were neutralized, antibodies against non-Gal antigens have induced acute xenograft rejection [15,51]. Kidneys transplanted into baboons from either Gal KO pigs or wild type pigs with Gal neutralized by synthetic Gal conjugates are subject to acute rejection by non-Gal antibodies . The results of this study  and studies performed by other groups suggests that when Gal is present, anti-Gal is the primary antibody involved in rejection, but in its absence other non-Gal antibodies play a prominent role in xenograft rejection. Anti-CD20 antibody has been effectively used to eliminate antibody-producing B cells in allotransplantation and in therapies for several leukemias. When this antibody was used for an extended period of time to avoid xenograft rejection, it resulted in serious infections, thus limiting its use to induction therapy. CD20 is not expressed on memory B cells, therefore the antibody against this molecule is unable to suppress anti-pig antibody production by these cells.
Suppressing complement and thromboembolism
Complement is a major contributor to both antibody-mediated rejection and coagulopathies responsible for xenograft rejection. Therefore, measures to prevent complement activation included use of soluble complement receptor 1 (sCR1) , cobra venom factor (CVF) , and transgenic expression of human complement regulatory proteins such as decay accelerating factor (CD55) , CD59 , and membrane cofactor protein (CD46) . Expression of decay accelerating factor prevented HAR to some extent , but was not sufficient to prevent microangiopathic coagulopathy . Transgenic expression of human CD46 along with the use of strong immunosuppressive agents could partially inhibit graft rejection . Triple expression of human CD55/CD59/alpha 1,2-fucosyltransferase (HT) (alpha 1,2-fucosyltransferase competes with GT for substrate to reduce Gal expression) also averted HAR, but was also not successful in significantly prolonging the graft survival .
Drug strategies to prevent thromboembolism included warfarin or low molecular weight heparin , aspirin , and anti-platelet therapy with aspirin and clopidogrel , but none of them were able to significantly prolong graft survival. Genetic modifications to prevent thrombosis include transgenic expression of human tissue factor pathway inhibitor, CD39, or thrombomodulin. Any reports using these transgenic factors in large animal models are yet to be published. Coagulopathy has also been associated with latent porcine cytomegalovirus (pCMV) infections, but early weaning of pigs has prevented activation of pCMV infections, and consumptive coagulopathy was thereby averted .
Anti-CD154 was used by some groups to prolong graft survival [38,65–67], but thromboembolic complications limited the successful use of this agent [68,69]. Other methods of costimulation blockade that work well in inducing tolerance in small xenograft models have not proven successful in prolonging pig-to-baboon cardiac xenograft .
Most of the above techniques currently used to over come xenograft rejection are summarized in Figure 1.
Beyond Immunological Barriers: Concerns about Ethics and Viruses
All the current regimens used to prolong xenograft survival involve vigorous immune suppression leading to an immunocompromised recipient. This immune suppression could result in infection by pathogens not normally associated with human disease or by newly emerging infectious agents. Categories of potential pathogens are discussed elsewhere . The demonstration that porcine endogenous retroviruses (PERVs) infect human cells in vitro has raised concerns about disease transmission by retroviruses through xenotransplantation .
But selective breeding techniques may eliminate a large number of potential pathogens, including pCMV, and possibly reduce infectious endogenous retroviruses. Swine that do not express PERVs that are infectious for human cells have been identified . Long-term retrospective studies of patients treated with pig tissues have not found any evidence of PERV infection in any of the patients tested [73,74]. Antibodies against the highly conserved epitopes encoded by the retroviral genome have been shown to neutralize PERV infectivity, suggesting that there may be a basis for producing a PERV vaccine .
However, the risk of infection via xenotransplantation could be perceived differently by patients with end-stage heart disease or a patient with kidney failure inadequately controlled by dialysis. New viruses and other pathogens continue to infect human beings. Whether the risk of transmission of these pathogens will increase with xenotransplantation is not yet known. But the risk can be anticipated, and thus prepared for. A long-term careful follow-up of transplanted patients will be required to monitor for infection by latent viruses and other pathogens. A timely intervention would be important to treat the infection and control its spread to other individuals.
Other barriers to clinical xenotransplantation include ethical concerns, including the objection of animal rights proponents to the use and genetic modification of pigs. Commercial considerations (e.g., high cost) and regulatory requirements to ensure safety and the potential for efficacy may also play a significant role in delaying the transition of xenotransplantation from bench to bedside.
The Next Steps
It is evident from the progress to date that there are several mechanisms of xenograft rejection which still must be overcome and which will require extensive investigations to make xenotransplantation a clinical reality. There are fewer hurdles to worry about in xenogeneic cellular/islet transplantation, which therefore has greater potential for reaching clinics than solid organ xenotransplantation.
For organ xenotransplantation, just replacing one immunosuppressive agent with another may not serve the purpose. Serious attempts should be made to induce tolerance to xenografts, especially for B lymphocyte mediated immunity, or to further modify the genetic makeup of the Gal KO pig to make it less immunogenic in both NHP and humans. More experiments are needed with life supporting organ transplantation to determine the physiologic restrictions of this procedure. Further, cross species transmission of pathogens should be studied in greater depth as this issue will also limit the transition of xenotransplantation to clinics.
With diminishing industry support and limited funding from granting agencies, it has become more evident that finding a solution to xenograft rejection is not within the scope of one investigator or laboratory. Therefore, it is imperative that major groups working on xenotransplantation share their information and expertise to formalize a joint approach to make this unique field a clinical reality.
I believe that the research in this field is progressing in the right direction and, in the course of seeking to solve xenograft rejection, has significantly advanced our understanding of several important immunological mechanisms, including the role of anti-carbohydrate antibodies, memory B cells, coagulation cascades, and cancer therapy. There is a fair amount of optimism that with careful planning and a coordinated effort, the dream of clinical xenotransplantation can be achieved.
The author greatly acknowledges the reviewing help of Drs. Keith Horvath, Eda Bloom, Cynthia Porter, and Robert Zhong. Also, the author is thankful to Drs. Robert Zhong and Bernard Hering for sharing their recent experimental results. Manuscript editing help was kindly provided by Patricia Jackson and Brenda Sonneveldt.
- 1. Yang YG, deGoma E, Ohdan H, Bracy JL, Xu Y, et al. (1998) Tolerization of anti-Galalpha1-3Gal natural antibody-forming B cells by induction of mixed chimerism. J Exp Med 187: 1335–1342.YG YangE. deGomaH. OhdanJL BracyY. Xu1998Tolerization of anti-Galalpha1-3Gal natural antibody-forming B cells by induction of mixed chimerism.J Exp Med18713351342
- 2. Ohdan H, Swenson KG, Kitamura H, Yang YG, Sykes M (2001) Tolerization of Gal alpha 1,3Gal-reactive B cells in pre-sensitized alpha 1,3-galactosyltransferase-deficient mice by nonmyeloablative induction of mixed chimerism. Xenotransplantation 8: 227–238.H. OhdanKG SwensonH. KitamuraYG YangM. Sykes2001Tolerization of Gal alpha 1,3Gal-reactive B cells in pre-sensitized alpha 1,3-galactosyltransferase-deficient mice by nonmyeloablative induction of mixed chimerism.Xenotransplantation8227238
- 3. Ohdan H, Yang YG, Swenson KG, Kitamura H, Sykes M (2001) T cell and B cell tolerance to GALalpha1,3GAL-expressing heart xenografts is achieved in alpha1,3-galactosyltransferase-deficient mice by nonmyeloablative induction of mixed chimerism. Transplantation 71: 1532–1542.H. OhdanYG YangKG SwensonH. KitamuraM. Sykes2001T cell and B cell tolerance to GALalpha1,3GAL-expressing heart xenografts is achieved in alpha1,3-galactosyltransferase-deficient mice by nonmyeloablative induction of mixed chimerism.Transplantation7115321542
- 4. Mohiuddin MM, Ogawa H, Yin DP, Galili U (2003) Tolerance induction to a mammalian blood group-like carbohydrate antigen by syngeneic lymphocytes expressing the antigen, II: tolerance induction on memory B cells. Blood 102: 229–236.MM MohiuddinH. OgawaDP YinU. Galili2003Tolerance induction to a mammalian blood group-like carbohydrate antigen by syngeneic lymphocytes expressing the antigen, II: tolerance induction on memory B cells.Blood102229236
- 5. Mohiuddin MM, Ogawa H, Yin DP, Shen J, Galili U (2003) Antibody-mediated accommodation of heart grafts expressing an incompatible carbohydrate antigen. Transplantation 75: 258–262.MM MohiuddinH. OgawaDP YinJ. ShenU. Galili2003Antibody-mediated accommodation of heart grafts expressing an incompatible carbohydrate antigen.Transplantation75258262
- 6. Ogawa H, Mohiuddin MM, Yin DP, Shen J, Chong AS, et al. (2004) Mouse-heart grafts expressing an incompatible carbohydrate antigen. II. Transition from accommodation to tolerance. Transplantation 77: 366–373.H. OgawaMM MohiuddinDP YinJ. ShenAS Chong2004Mouse-heart grafts expressing an incompatible carbohydrate antigen. II. Transition from accommodation to tolerance.Transplantation77366373
- 7. Lin SS, Hanaway MJ, Gonzalez-Stawinski GV, Lau CL, Parker W, et al. (2000) The role of anti-Galalpha1-3Gal antibodies in acute vascular rejection and accommodation of xenografts. Transplantation 70: 1667–1674.SS LinMJ HanawayGV Gonzalez-StawinskiCL LauW. Parker2000The role of anti-Galalpha1-3Gal antibodies in acute vascular rejection and accommodation of xenografts.Transplantation7016671674
- 8. Galili U (2004) Immune response, accommodation, and tolerance to transplantation carbohydrate antigens. Transplantation 78: 1093–1098.U. Galili2004Immune response, accommodation, and tolerance to transplantation carbohydrate antigens.Transplantation7810931098
- 9. Hua N, Yamashita K, Hashimoto T, Masunaga T, Fujita M, et al. (2004) Gene therapy-mediated CD40L and CD28 co-stimulatory signaling blockade plus transient anti-xenograft antibody suppression induces long-term acceptance of cardiac xenografts. Transplantation 78: 1463–1470.N. HuaK. YamashitaT. HashimotoT. MasunagaM. Fujita2004Gene therapy-mediated CD40L and CD28 co-stimulatory signaling blockade plus transient anti-xenograft antibody suppression induces long-term acceptance of cardiac xenografts.Transplantation7814631470
- 10. Wang GM, Yang Y, Jin YZ, Li AL, Hao J, et al. (2005) Blockade of both CD28/B7 and OX40/OX40L co-stimulatory signal pathways prolongs the survival of islet xenografts. Transplant Proc 37: 4449–4451.GM WangY. YangYZ JinAL LiJ. Hao2005Blockade of both CD28/B7 and OX40/OX40L co-stimulatory signal pathways prolongs the survival of islet xenografts.Transplant Proc3744494451
- 11. Benda B, Ljunggren HG, Peach R, Sandberg JO, Korsgren O (2002) Co-stimulatory molecules in islet xenotransplantation: CTLA4Ig treatment in CD40 ligand-deficient mice. Cell Transplant 11: 715–720.B. BendaHG LjunggrenR. PeachJO SandbergO. Korsgren2002Co-stimulatory molecules in islet xenotransplantation: CTLA4Ig treatment in CD40 ligand-deficient mice.Cell Transplant11715720
- 12. Yin D, Ma L, Shen J, Byrne GW, Logan JS, et al. (2002) CTLA-41g in combination with anti-CD40L prolongs xenograft survival and inhibits anti-gal ab production in GT-Ko mice. Am J Transplant 2: 41–47.D. YinL. MaJ. ShenGW ByrneJS Logan2002CTLA-41g in combination with anti-CD40L prolongs xenograft survival and inhibits anti-gal ab production in GT-Ko mice.Am J Transplant24147
- 13. Singh NP, Guo L, Que X, Shirwan H (2004) Blockade of indirect recognition mediated by CD4+ T cells leads to prolonged cardiac xenograft survival. Xenotransplantation 11: 33–42.NP SinghL. GuoX. QueH. Shirwan2004Blockade of indirect recognition mediated by CD4+ T cells leads to prolonged cardiac xenograft survival.Xenotransplantation113342
- 14. Sutherland RM, McKenzie BS, Zhan Y, Corbett AJ, Fox-Marsh A, et al. (2002) Anti-CD45RB antibody deters xenograft rejection by modulating T cell priming and homing. Int Immunol 14: 953–962.RM SutherlandBS McKenzieY. ZhanAJ CorbettA. Fox-Marsh2002Anti-CD45RB antibody deters xenograft rejection by modulating T cell priming and homing.Int Immunol14953962
- 15. Chen G, Qian H, Starzl T, Sun H, Garcia B, et al. (2005) Acute rejection is associated with antibodies to non-Gal antigens in baboons using Gal-knockout pig kidneys. Nat Med 11: 1295–1298.G. ChenH. QianT. StarzlH. SunB. Garcia2005Acute rejection is associated with antibodies to non-Gal antigens in baboons using Gal-knockout pig kidneys.Nat Med1112951298
- 16. Buhler L, Xu Y, Li W, Zhu A, Cooper DK (2003) An investigation of the specificity of induced anti-pig antibodies in baboons. Xenotransplantation 10: 88–93.L. BuhlerY. XuW. LiA. ZhuDK Cooper2003An investigation of the specificity of induced anti-pig antibodies in baboons.Xenotransplantation108893
- 17. Rose EA, Gelijns AC, Moskowitz AJ, Heitjan DF, Stevenson LW, et al. (2001) Long-term mechanical left ventricular assistance for end-stage heart failure. N Engl J Med 345: 1435–1443.EA RoseAC GelijnsAJ MoskowitzDF HeitjanLW Stevenson2001Long-term mechanical left ventricular assistance for end-stage heart failure.N Engl J Med34514351443
- 18. Frazier OH, Dowling RD, Gray LA Jr, Shah NA, Pool T, et al. (2004) The total artificial heart: Where we stand. Cardiology 101: 117–121.OH FrazierRD DowlingLA Gray JrNA ShahT. Pool2004The total artificial heart: Where we stand.Cardiology101117121
- 19. Taylor DA, Atkins BZ, Hungspreugs P, Jones TR, Reedy MC, et al. (1998) Regenerating functional myocardium: Improved performance after skeletal myoblast transplantation. Nat Med 4: 929–933.DA TaylorBZ AtkinsP. HungspreugsTR JonesMC Reedy1998Regenerating functional myocardium: Improved performance after skeletal myoblast transplantation.Nat Med4929933
- 20. Hutcheson KA, Atkins BZ, Hueman MT, Hopkins MB, Glower DD, et al. (2000) Comparison of benefits on myocardial performance of cellular cardiomyoplasty with skeletal myoblasts and fibroblasts. Cell Transplant 9: 359–368.KA HutchesonBZ AtkinsMT HuemanMB HopkinsDD Glower2000Comparison of benefits on myocardial performance of cellular cardiomyoplasty with skeletal myoblasts and fibroblasts.Cell Transplant9359368
- 21. Menasche P, Desnos M, Hagege AA (2006) Routine delivery of myoblasts during coronary artery bypass surgery: Why not? Nat Clin Pract Cardiovasc Med 3(Suppl 1): S90–S93.P. MenascheM. DesnosAA Hagege2006Routine delivery of myoblasts during coronary artery bypass surgery: Why not?Nat Clin Pract Cardiovasc Med3Suppl 1S90S93
- 22. Ekblom P (1981) Formation of basement membranes in the embryonic kidney: An immunohistological study. J Cell Biol 91: 1–10.P. Ekblom1981Formation of basement membranes in the embryonic kidney: An immunohistological study.J Cell Biol91110
- 23. Dalmasso AP, Vercellotti GM, Fischel RJ, Bolman RM, Bach FH, et al. (1992) Mechanism of complement activation in the hyperacute rejection of porcine organs transplanted into primate recipients. Am J Pathol 140: 1157–1166.AP DalmassoGM VercellottiRJ FischelRM BolmanFH Bach1992Mechanism of complement activation in the hyperacute rejection of porcine organs transplanted into primate recipients.Am J Pathol14011571166
- 24. Lin SS, Kooyman DL, Daniels LJ, Daggett CW, Parker W, et al. (1997) The role of natural anti-Gal alpha 1-3Gal antibodies in hyperacute rejection of pig-to-baboon cardiac xenotransplants. Transpl Immunol 5: 212–218.SS LinDL KooymanLJ DanielsCW DaggettW. Parker1997The role of natural anti-Gal alpha 1-3Gal antibodies in hyperacute rejection of pig-to-baboon cardiac xenotransplants.Transpl Immunol5212218
- 25. Good AH, Cooper DK, Malcolm AJ, Ippolito RM, Koren E, et al. (1992) Identification of carbohydrate structures that bind human antiporcine antibodies: Implications for discordant xenografting in humans. Transplant Proc 24: 559–562.AH GoodDK CooperAJ MalcolmRM IppolitoE. Koren1992Identification of carbohydrate structures that bind human antiporcine antibodies: Implications for discordant xenografting in humans.Transplant Proc24559562
- 26. Galili U, Clark MR, Shohet SB, Buehler J, Macher BA (1987) Evolutionary relationship between the natural anti-Gal antibody and the Gal alpha 1—3Gal epitope in primates. Proc Natl Acad Sci U S A 84: 1369–1373.U. GaliliMR ClarkSB ShohetJ. BuehlerBA Macher1987Evolutionary relationship between the natural anti-Gal antibody and the Gal alpha 1—3Gal epitope in primates.Proc Natl Acad Sci U S A8413691373
- 27. Geller RL, Bach FH, Vercellotti GM, Nistler RS, Bolman RM III, et al. (1992) Activation of endothelial cells in hyperacute xenograft rejection. Transplant Proc 24: 592.RL GellerFH BachGM VercellottiRS NistlerRM Bolman III1992Activation of endothelial cells in hyperacute xenograft rejection.Transplant Proc24592
- 28. Auchincloss H Jr, Sachs DH (1998) Xenogeneic transplantation. Annu Rev Immunol 16: 433–470.H. Auchincloss JrDH Sachs1998Xenogeneic transplantation.Annu Rev Immunol16433470
- 29. Bach FH, Winkler H, Ferran C, Hancock WW, Robson SC (1996) Delayed xenograft rejection. Immunol Today 17: 379–384.FH BachH. WinklerC. FerranWW HancockSC Robson1996Delayed xenograft rejection.Immunol Today17379384
- 30. Blakely ML, Van der Werf WJ, Berndt MC, Dalmasso AP, Bach FH, et al. (1994) Activation of intragraft endothelial and mononuclear cells during discordant xenograft rejection. Transplantation 58: 1059–1066.ML BlakelyWJ Van der WerfMC BerndtAP DalmassoFH Bach1994Activation of intragraft endothelial and mononuclear cells during discordant xenograft rejection.Transplantation5810591066
- 31. Schulte am Esch J 2nd, Rogiers X, Robson SC (2001) Molecular incompatibilities in hemostasis between swine and men—Impact on xenografting. Ann Transplant 6: 12–16.J. Schulte am Esch 2ndX. RogiersSC Robson2001Molecular incompatibilities in hemostasis between swine and men—Impact on xenografting.Ann Transplant61216
- 32. Milland J, Christiansen D, Lazarus BD, Taylor SG, Xing PX, et al. (2006) The molecular basis for galalpha(1,3)gal expression in animals with a deletion of the alpha1,3galactosyltransferase gene. J Immunol 176: 2448–2254.J. MillandD. ChristiansenBD LazarusSG TaylorPX Xing2006The molecular basis for galalpha(1,3)gal expression in animals with a deletion of the alpha1,3galactosyltransferase gene.J Immunol17624482254
- 33. Yamada K, Sachs DH, DerSimonian H (1995) Human anti-porcine xenogeneic T cell response. Evidence for allelic specificity of mixed leukocyte reaction and for both direct and indirect pathways of recognition. J Immunol 155: 5249–5256.K. YamadaDH SachsH. DerSimonian1995Human anti-porcine xenogeneic T cell response. Evidence for allelic specificity of mixed leukocyte reaction and for both direct and indirect pathways of recognition.J Immunol15552495256
- 34. Davila E, Byrne GW, Labreche PT, McGregor HC, Schwab AK, et al. (2006) T-cell responses during pig-to-primate xenotransplantation. Xenotransplantation 13: 31–40.E. DavilaGW ByrnePT LabrecheHC McGregorAK Schwab2006T-cell responses during pig-to-primate xenotransplantation.Xenotransplantation133140
- 35. Baumann BC, Forte P, Hawley RJ, Rieben R, Schneider MK, et al. (2004) Lack of galactose-alpha-1,3-galactose expression on porcine endothelial cells prevents complement-induced lysis but not direct xenogeneic NK cytotoxicity. J Immunol 172: 6460–6467.BC BaumannP. ForteRJ HawleyR. RiebenMK Schneider2004Lack of galactose-alpha-1,3-galactose expression on porcine endothelial cells prevents complement-induced lysis but not direct xenogeneic NK cytotoxicity.J Immunol17264606467
- 36. Baumann BC, Schneider MK, Lilienfeld BG, Antsiferova MA, Rhyner DM, et al. (2005) Endothelial cells derived from pigs lacking Gal alpha(1,3)Gal: No reduction of human leukocyte adhesion and natural killer cell cytotoxicity. Transplantation 79: 1067–1072.BC BaumannMK SchneiderBG LilienfeldMA AntsiferovaDM Rhyner2005Endothelial cells derived from pigs lacking Gal alpha(1,3)Gal: No reduction of human leukocyte adhesion and natural killer cell cytotoxicity.Transplantation7910671072
- 37. Yi S, Hawthorne WJ, Lehnert AM, Ha H, Wong JK, et al. (2003) T cell-activated macrophages are capable of both recognition and rejection of pancreatic islet xenografts. J Immunol 170: 2750–2758.S. YiWJ HawthorneAM LehnertH. HaJK Wong2003T cell-activated macrophages are capable of both recognition and rejection of pancreatic islet xenografts.J Immunol17027502758
- 38. Hering BJ, Wijkstrom M, Graham ML, Hardstedt M, Aasheim TC, et al. (2006) Prolonged diabetes reversal after intraportal xenotransplantation of wild-type porcine islets in immunosuppressed nonhuman primates. Nat Med 12: 301–303.BJ HeringM. WijkstromML GrahamM. HardstedtTC Aasheim2006Prolonged diabetes reversal after intraportal xenotransplantation of wild-type porcine islets in immunosuppressed nonhuman primates.Nat Med12301303
- 39. Cardona K, Korbutt GS, Milas Z, Lyon J, Cano J, et al. (2006) Long-term survival of neonatal porcine islets in nonhuman primates by targeting costimulation pathways. Nat Med 12: 304–306.K. CardonaGS KorbuttZ. MilasJ. LyonJ. Cano2006Long-term survival of neonatal porcine islets in nonhuman primates by targeting costimulation pathways.Nat Med12304306
- 40. Dufrane D, Goebbels RM, Saliez A, Guiot Y, Gianello P (2006) Six-month survival of microencapsulated pig islets and alginate biocompatibility in primates: Proof of concept. Transplantation 81: 1345–1353.D. DufraneRM GoebbelsA. SaliezY. GuiotP. Gianello2006Six-month survival of microencapsulated pig islets and alginate biocompatibility in primates: Proof of concept.Transplantation8113451353
- 41. McGregor CG, Teotia SS, Byrne GW, Michaels MG, Risdahl JM, et al. (2004) Cardiac xenotransplantation: Progress toward the clinic. Transplantation 78: 1569–1575.CG McGregorSS TeotiaGW ByrneMG MichaelsJM Risdahl2004Cardiac xenotransplantation: Progress toward the clinic.Transplantation7815691575
- 42. McGregor CG, Davies WR, Oi K, Teotia SS, Schirmer JM, et al. (2005) Cardiac xenotransplantation: Recent preclinical progress with 3-month median survival. J Thorac Cardiovasc Surg 130: 844–851.CG McGregorWR DaviesK. OiSS TeotiaJM Schirmer2005Cardiac xenotransplantation: Recent preclinical progress with 3-month median survival.J Thorac Cardiovasc Surg130844851
- 43. Tseng YL, Kuwaki K, Dor FJ, Shimizu A, Houser S, et al. (2005) alpha1,3-Galactosyltransferase gene-knockout pig heart transplantation in baboons with survival approaching 6 months. Transplantation 80: 1493–1500.YL TsengK. KuwakiFJ DorA. ShimizuS. Houser2005alpha1,3-Galactosyltransferase gene-knockout pig heart transplantation in baboons with survival approaching 6 months.Transplantation8014931500
- 44. Kuwaki K, Tseng YL, Dor FJ, Shimizu A, Houser SL, et al. (2005) Heart transplantation in baboons using alpha1,3-galactosyltransferase gene-knockout pigs as donors: initial experience. Nat Med 11: 29–31.K. KuwakiYL TsengFJ DorA. ShimizuSL Houser2005Heart transplantation in baboons using alpha1,3-galactosyltransferase gene-knockout pigs as donors: initial experience.Nat Med112931
- 45. Yamamoto S, Lavelle JM, Vagefi PA, Arakawa H, Samelson-Jones E, et al. (2005) Vascularized thymic lobe transplantation in a pig-to-baboon model: A novel strategy for xenogeneic tolerance induction and T-cell reconstitution. Transplantation 80: 1783–1790.S. YamamotoJM LavellePA VagefiH. ArakawaE. Samelson-Jones2005Vascularized thymic lobe transplantation in a pig-to-baboon model: A novel strategy for xenogeneic tolerance induction and T-cell reconstitution.Transplantation8017831790
- 46. Johnston DR, Muniappan A, Hoerbelt R, Guenther DA, Shoji T, et al. (2005) Heart and en-bloc thymus transplantation in miniature swine. J Thorac Cardiovasc Surg 130: 554–559.DR JohnstonA. MuniappanR. HoerbeltDA GuentherT. Shoji2005Heart and en-bloc thymus transplantation in miniature swine.J Thorac Cardiovasc Surg130554559
- 47. Yamada K, Yazawa K, Shimizu A, Iwanaga T, Hisashi Y, et al. (2005) Marked prolongation of porcine renal xenograft survival in baboons through the use of alpha1,3-galactosyltransferase gene-knockout donors and the cotransplantation of vascularized thymic tissue. Nat Med 11: 32–34.K. YamadaK. YazawaA. ShimizuT. IwanagaY. Hisashi2005Marked prolongation of porcine renal xenograft survival in baboons through the use of alpha1,3-galactosyltransferase gene-knockout donors and the cotransplantation of vascularized thymic tissue.Nat Med113234
- 48. Kroshus TJ, Dalmasso AP, Leventhal JR, John R, Matas AJ, et al. (1995) Antibody removal by column immunoabsorption prevents tissue injury in an ex vivo model of pig-to-human xenograft hyperacute rejection. J Surg Res 59: 43–50.TJ KroshusAP DalmassoJR LeventhalR. JohnAJ Matas1995Antibody removal by column immunoabsorption prevents tissue injury in an ex vivo model of pig-to-human xenograft hyperacute rejection.J Surg Res594350
- 49. Leventhal JR, John R, Fryer JP, Witson JC, Derlich JM, et al. (1995) Removal of baboon and human antiporcine IgG and IgM natural antibodies by immunoadsorption. Results of in vitro and in vivo studies. Transplantation 59: 294–300.JR LeventhalR. JohnJP FryerJC WitsonJM Derlich1995Removal of baboon and human antiporcine IgG and IgM natural antibodies by immunoadsorption. Results of in vitro and in vivo studies.Transplantation59294300
- 50. Domenech N, Diaz T, Moscoso I, Lopez-Pelaez E, Centeno A, et al. (2003) Elicited non-anti-alphaGAL antibodies may cause acute humoral rejection of hDAF pig organs transplanted in baboons. Transplant Proc 35: 2049–2050.N. DomenechT. DiazI. MoscosoE. Lopez-PelaezA. Centeno2003Elicited non-anti-alphaGAL antibodies may cause acute humoral rejection of hDAF pig organs transplanted in baboons.Transplant Proc3520492050
- 51. Rood PP, Hara H, Busch JL, Ezzelarab M, Zhu X, et al. (2006) Incidence and cytotoxicity of antibodies in cynomolgus monkeys directed to nonGal antigens, and their relevance for experimental models. Transpl Int 19: 158–165.PP RoodH. HaraJL BuschM. EzzelarabX. Zhu2006Incidence and cytotoxicity of antibodies in cynomolgus monkeys directed to nonGal antigens, and their relevance for experimental models.Transpl Int19158165
- 52. Lechler RI, Sykes M, Thomson AW, Turka LA (2005) Organ transplantation—How much of the promise has been realized? Nat Med 11: 605–613.RI LechlerM. SykesAW ThomsonLA Turka2005Organ transplantation—How much of the promise has been realized?Nat Med11605613
- 53. Lam TT, Hausen B, Hook L, Lau M, Higgins J, et al. (2005) The effect of soluble complement receptor type 1 on acute humoral xenograft rejection in hDAF-transgenic pig-to-primate life-supporting kidney xenografts. Xenotransplantation 12: 20–29.TT LamB. HausenL. HookM. LauJ. Higgins2005The effect of soluble complement receptor type 1 on acute humoral xenograft rejection in hDAF-transgenic pig-to-primate life-supporting kidney xenografts.Xenotransplantation122029
- 54. Kobayashi T, Taniguchi S, Neethling FA, Rose AG, Hancock WW, et al. (1997) Delayed xenograft rejection of pig-to-baboon cardiac transplants after cobra venom factor therapy. Transplantation 64: 1255–1261.T. KobayashiS. TaniguchiFA NeethlingAG RoseWW Hancock1997Delayed xenograft rejection of pig-to-baboon cardiac transplants after cobra venom factor therapy.Transplantation6412551261
- 55. Ramirez P, Chavez R, Majado M, Munitiz V, Munoz A, et al. (2000) Life-supporting human complement regulator decay accelerating factor transgenic pig liver xenograft maintains the metabolic function and coagulation in the nonhuman primate for up to 8 days. Transplantation 70: 989–998.P. RamirezR. ChavezM. MajadoV. MunitizA. Munoz2000Life-supporting human complement regulator decay accelerating factor transgenic pig liver xenograft maintains the metabolic function and coagulation in the nonhuman primate for up to 8 days.Transplantation70989998
- 56. Menoret S, Plat M, Blancho G, Martinat-Botte F, Bernard P, et al. (2004) Characterization of human CD55 and CD59 transgenic pigs and kidney xenotransplantation in the pig-to-baboon combination. Transplantation 77: 1468–1471.S. MenoretM. PlatG. BlanchoF. Martinat-BotteP. Bernard2004Characterization of human CD55 and CD59 transgenic pigs and kidney xenotransplantation in the pig-to-baboon combination.Transplantation7714681471
- 57. Adams DH, Kadner A, Chen RH, Farivar RS (2001) Human membrane cofactor protein (MCP, CD 46) protects transgenic pig hearts from hyperacute rejection in primates. Xenotransplantation 8: 36–40.DH AdamsA. KadnerRH ChenRS Farivar2001Human membrane cofactor protein (MCP, CD 46) protects transgenic pig hearts from hyperacute rejection in primates.Xenotransplantation83640
- 58. Cozzi E, Vial C, Ostlie D, Farah B, Chavez G, et al. (2003) Maintenance triple immunosuppression with cyclosporin A, mycophenolate sodium and steroids allows prolonged survival of primate recipients of hDAF porcine renal xenografts. Xenotransplantation 10: 300–310.E. CozziC. VialD. OstlieB. FarahG. Chavez2003Maintenance triple immunosuppression with cyclosporin A, mycophenolate sodium and steroids allows prolonged survival of primate recipients of hDAF porcine renal xenografts.Xenotransplantation10300310
- 59. Shimizu A, Yamada K, Yamamoto S, Lavelle JM, Barth RN, et al. (2005) Thrombotic microangiopathic glomerulopathy in human decay accelerating factor-transgenic swine-to-baboon kidney xenografts. J Am Soc Nephrol 16: 2732–2745.A. ShimizuK. YamadaS. YamamotoJM LavelleRN Barth2005Thrombotic microangiopathic glomerulopathy in human decay accelerating factor-transgenic swine-to-baboon kidney xenografts.J Am Soc Nephrol1627322745
- 60. Cowan PJ, Aminian A, Barlow H, Brown AA, Chen CG, et al. (2000) Renal xenografts from triple-transgenic pigs are not hyperacutely rejected but cause coagulopathy in non-immunosuppressed baboons. Transplantation 69: 2504–2515.PJ CowanA. AminianH. BarlowAA BrownCG Chen2000Renal xenografts from triple-transgenic pigs are not hyperacutely rejected but cause coagulopathy in non-immunosuppressed baboons.Transplantation6925042515
- 61. Byrne GW, Schirmer JM, Fass DN, Teotia SS, Kremers WK, et al. (2005) Warfarin or low-molecular-weight heparin therapy does not prolong pig-to-primate cardiac xenograft function. Am J Transplant 5: 1011–1020.GW ByrneJM SchirmerDN FassSS TeotiaWK Kremers2005Warfarin or low-molecular-weight heparin therapy does not prolong pig-to-primate cardiac xenograft function.Am J Transplant510111020
- 62. Dor FJ, Kuwaki K, Tseng YL, Shimizu A, Houser SL, et al. (2005) Potential of aspirin to inhibit thrombotic microangiopathy in alpha1,3-galactosyltransferase gene-knockout pig hearts after transplantation in baboons. Transplant Proc 37: 489–490.FJ DorK. KuwakiYL TsengA. ShimizuSL Houser2005Potential of aspirin to inhibit thrombotic microangiopathy in alpha1,3-galactosyltransferase gene-knockout pig hearts after transplantation in baboons.Transplant Proc37489490
- 63. Schirmer JM, Fass DN, Byrne GW, Tazelaar HD, Logan JS, et al. (2004) Effective antiplatelet therapy does not prolong transgenic pig to baboon cardiac xenograft survival. Xenotransplantation 11: 436–443.JM SchirmerDN FassGW ByrneHD TazelaarJS Logan2004Effective antiplatelet therapy does not prolong transgenic pig to baboon cardiac xenograft survival.Xenotransplantation11436443
- 64. Mueller NJ, Kuwaki K, Dor FJ, Knosalla C, Gollackner B, et al. (2004) Reduction of consumptive coagulopathy using porcine cytomegalovirus-free cardiac porcine grafts in pig-to-primate xenotransplantation. Transplantation 78: 1449–1453.NJ MuellerK. KuwakiFJ DorC. KnosallaB. Gollackner2004Reduction of consumptive coagulopathy using porcine cytomegalovirus-free cardiac porcine grafts in pig-to-primate xenotransplantation.Transplantation7814491453
- 65. Buhler L, Yamada K, Alwayn I, Kitamura H, Basker M, et al. (2001) Miniature swine and hDAF pig kidney transplantation in baboons treated with a nonmyeloablative regimen and CD154 blockade. Transplant Proc 33: 716.L. BuhlerK. YamadaI. AlwaynH. KitamuraM. Basker2001Miniature swine and hDAF pig kidney transplantation in baboons treated with a nonmyeloablative regimen and CD154 blockade.Transplant Proc33716
- 66. Knosalla C, Ryan DJ, Moran K, Gollackner B, Schuler W, et al. (2004) Initial experience with the human anti-human CD154 monoclonal antibody, ABI793, in pig-to-baboon xenotransplantation. Xenotransplantation 11: 353–360.C. KnosallaDJ RyanK. MoranB. GollacknerW. Schuler2004Initial experience with the human anti-human CD154 monoclonal antibody, ABI793, in pig-to-baboon xenotransplantation.Xenotransplantation11353360
- 67. Wu G, Pfeiffer S, Schroder C, Zhang T, Nguyen BN, et al. (2005) Co-stimulation blockade targeting CD154 and CD28/B7 modulates the induced antibody response after a pig-to-baboon cardiac xenograft. Xenotransplantation 12: 197–208.G. WuS. PfeifferC. SchroderT. ZhangBN Nguyen2005Co-stimulation blockade targeting CD154 and CD28/B7 modulates the induced antibody response after a pig-to-baboon cardiac xenograft.Xenotransplantation12197208
- 68. Koyama I, Kawai T, Andrews D, Boskovic S, Nadazdin O, et al. (2004) Thrombophilia associated with anti-CD154 monoclonal antibody treatment and its prophylaxis in nonhuman primates. Transplantation 77: 460–462.I. KoyamaT. KawaiD. AndrewsS. BoskovicO. Nadazdin2004Thrombophilia associated with anti-CD154 monoclonal antibody treatment and its prophylaxis in nonhuman primates.Transplantation77460462
- 69. Kawai T, Andrews D, Colvin RB, Sachs DH, Cosimi AB (2000) Thromboembolic complications after treatment with monoclonal antibody against CD40 ligand. Nat Med 6: 114.T. KawaiD. AndrewsRB ColvinDH SachsAB Cosimi2000Thromboembolic complications after treatment with monoclonal antibody against CD40 ligand.Nat Med6114
- 70. Fishman JA, Patience C (2004) Xenotransplantation: Infectious risk revisited. Am J Transplant 4: 1383–1390.JA FishmanC. Patience2004Xenotransplantation: Infectious risk revisited.Am J Transplant413831390
- 71. Chapman LE, Bloom ET (2001) Clinical xenotransplantation. JAMA 285: 2304–2306.LE ChapmanET Bloom2001Clinical xenotransplantation.JAMA28523042306
- 72. Scobie L, Taylor S, Wood JC, Suling KM, Quinn G, et al. (2004) Absence of replication-competent human-tropic porcine endogenous retroviruses in the germ line DNA of inbred miniature Swine. J Virol 78: 2502–2509.L. ScobieS. TaylorJC WoodKM SulingG. Quinn2004Absence of replication-competent human-tropic porcine endogenous retroviruses in the germ line DNA of inbred miniature Swine.J Virol7825022509
- 73. Paradis K, Langford G, Long Z, Heneine W, Sandstrom P, et al. (1999) Search for cross-species transmission of porcine endogenous retrovirus in patients treated with living pig tissue. The XEN 111 Study Group. Science 285: 1236–1241.K. ParadisG. LangfordZ. LongW. HeneineP. Sandstrom1999Search for cross-species transmission of porcine endogenous retrovirus in patients treated with living pig tissue. The XEN 111 Study Group.Science28512361241
- 74. Cunningham DA, Herring C, Fernandez-Suarez XM, Whittam AJ, Paradis K, et al. (2001) Analysis of patients treated with living pig tissue for evidence of infection by porcine endogenous retroviruses. Trends Cardiovasc Med 11: 190–196.DA CunninghamC. HerringXM Fernandez-SuarezAJ WhittamK. Paradis2001Analysis of patients treated with living pig tissue for evidence of infection by porcine endogenous retroviruses.Trends Cardiovasc Med11190196
- 75. Fiebig U, Stephan O, Kurth R, Denner J (2003) Neutralizing antibodies against conserved domains of p15E of porcine endogenous retroviruses: basis for a vaccine for xenotransplantation? Virology 307: 406–413.U. FiebigO. StephanR. KurthJ. Denner2003Neutralizing antibodies against conserved domains of p15E of porcine endogenous retroviruses: basis for a vaccine for xenotransplantation?Virology307406413