Anti-Gal(alpha)1-3Gal antibody response to porcine bone marrow in unmodified baboons and baboons conditioned for tolerance induction

Modifying the sugar icing on the transplantation cake

As a transplant surgeon, my interest in glycobiology began through my research into ABO-incompatible allotransplantation, and grew when my goal became overcoming the shortage of organs from deceased human donors by the transplantation of pig organs into patients with terminal organ failure (xenotransplantation/cross-species transplantation). The major target for human "natural" (preformed) anti-pig antibodies is galactose-α(1,3)-galactose (the "Gal" epitope), which is expressed on many pig cells, including the vascular endothelium. The binding of human IgM and IgG antibodies to Gal antigens initiates the process of hyperacute rejection, resulting in destruction of the pig graft within minutes or hours. This major barrier has been overcome by the production of pigs in which the gene for the enzyme α(1,3)-galactosyltransferase (GT) has been deleted by genetic engineering, resulting in GT knockout (GTKO) pigs. The two other known carbohydrate antigenic targets on pig cells for human anti-pig antibodies are (i) the product of the cytidine monophosphate-N-acetylneuraminic acid hydroxylase (CMAH) gene, i.e., N-glycolylneuraminic acid, and (ii) the product of the β1,4 N-acetylgalactosaminyltransferase gene, i.e., the Sd(a) antigen. Expression of these two has also been deleted in pigs. These genetic manipulations, together with others directed to overcoming primate complement and coagulation activation (the latter of which also relates to glycobiology) have contributed to the prolongation of pig graft survival in nonhuman primate recipients to many months rather than a few minutes. Clinical trials of the transplantation of pig cells are already underway and transplantation of pig organs may be expected within the relatively near future.

Changes of the Structural and Biomechanical Properties of the Bovine Pericardium after the Removal of α-Gal Epitopes by Decellularization and α-Galactosidase Treatment

Background

Bovine pericardium is one of the most widely used materials in bioprosthetic heart valves. Immunologic responses have been implicated as potential causes of limited durability of xenogenic valves. This study aimed to determine the effectiveness of decellularization and α-galactosidase (α-gal) to remove major xenoreactive antigens from xenogenic tissues.

Materials and Methods

Recombinant Bacteroides thetaiotaomicron (B. thetaiotaomicron) α-gal or decellularization, or both were used to remove α-gal from bovine pericardium. It was confirmed by α-gal-bovine serum albumin-based enzyme-linked immunosorbent assay (ELISA), high-performance anion exchange chromatography, flow cytometry, 3,3'-diaminobenzidine-staining, and lectin-based ELISA. The mechanical properties of bovine pericardium after decellularization or α-gal treatment were investigated by tests of tensile-strength, permeability, and compliance. Collagen fiber rearrangement was also evaluated by a 20,000× transmission electron microscope (TEM).

Results

Recombinant B. thetaiotaomicron α-gal could effectively remove α-gal from bovine pericardium B. thetaiotaomicron (0.1 U/mL, pH 7.2) while recombinant human α-gal removed it recombinant human α-gal (10 U/mL, pH 5.0). There was no difference in the mechanical properties of fresh and recombinant α-gal-treated bovine pericardium. Furthermore, the TEM findings demonstrated that recombinant α-gal made no difference in the arrangement of collagen fiber bundles with decellularization.

Conclusion

Recombinant B. thetaiotaomicron α-gal effectively removed α-gal from bovine pericardium with a small amount under physiological conditions compared to human recombinant α-gal, which may alleviate the harmful xenoreactive immunologic responses of α-gal. Recombinant α-gal treatment had no adverse effects on the mechanical properties of bovine pericardium.

Current progress in xenogeneic tolerance

PURPOSE OF REVIEW

The present review updates the current status of research regarding the immunologic responses of the recipient following xenotransplantation. Additionally, we present the recent progress with attempts to induce xenogeneic tolerance induction.

RECENT FINDINGS

There continues to be great interest in xenotransplantation. Recently, descriptions of the mechanisms responsible for attempted T-cell xenogeneic tolerance in both large and small animal models have improved xenogeneic graft survivals. Additionally, the cellular signaling mechanisms, such as those involving CD39, CD44, and CD47, are proving to be highly important. Using the mixed chimerism approach to tolerance in xenogeneic model may be encouraging, especially given the recent clarification of the role for macrophage-induced phagocytosis of xenogeneic donor cells.

SUMMARY

Induction of tolerance to xenogeneic antigens has been accomplished only in small animals; however, graft survivals in large animal models continue to improve. Further clarification of both the adaptive and innate immune responses to xenogeneic antigens is required for success to continue.

Occurrence of specific humoral non-responsiveness to swine antigens following administration of GalT-KO bone marrow to baboons

BACKGROUND

Hematopoietic chimerism induces transplantation tolerance across allogeneic and xenogeneic barriers, but has been difficult to achieve in the pig-to-primate model. We have now utilized swine with knockout of the gene coding for alpha-1,3-galactosyltransferase (GalT-KO pigs) as bone marrow donors in an attempt to achieve chimerism and tolerance by avoiding the effects of natural antibodies to Gal determinants on pig hematopoietic cells.

METHODS

Baboons (n = 4; Baboons 1 to 4 = B156, B158, B167, and B175, respectively) were splenectomized and conditioned with TBI (150 cGy), thymic irradiation (700 cGy), T cell depletion with rabbit anti-thymocyte globulin (rATG) and rat anti-primate CD2 (LoCD2b), and received FK506 and supportive therapy for 28 days. All animals received GalT-KO bone marrow (1 to 2 x 10(9) cells/kg) in two fractions on days 0 and 2, and were thereafter monitored for the presence of pig cells by flow cytometry, for porcine progenitor cells by PCR of BM colony-forming units, and for cellular reactivity to pig cells by mixed lymphocyte reaction (MLR). In vitro antibody formation to LoCD2b and rATG was tested by ELISA; antibody reactivity to GalT-KO pig cells was tested by flow cytometry and cytotoxicity assays. Additionally, Baboons 3 and 4 received orthotopic kidney transplants on days 17 and 2, respectively, to test the potential impact of the protocol on renal transplantation.

RESULTS

None of the animals showed detectable pig cells by flow cytometry for more than 12 h post-BM infusion. However, porcine progenitor cell engraftment, as evidenced by pig-derived colony forming units in the BM, as well as peripheral microchimerism in the thymus, lymph node, and peripheral blood was detected by PCR in baboons 1 and 2 for at least 28 days post-transplant. ELISA results confirmed humoral immunocompetence at time of transplantation as antibody titers to rat (LoCD2b) and rabbit (ATG) increased within 2 weeks. However, no induced antibodies to GalT-KO pig cells or increased donor specific cytotoxicity was detectable by flow cytometry. In contrast, baboons 3 and 4 developed serum antibodies to pig cells as well as to rat and rabbit immunoglobulin by day 14. Retrospective analysis revealed that although all four baboons possessed low levels of antibody-mediated cytotoxicity to GalT-KO cells prior to transplantation, the two baboons (3 and 4) that became sensitized to pig cells (and rejected pig kidneys) had relatively high pre-transplantation titers of anti-non-Gal IgG detectable by flow cytometry, whereas baboons 1 and 2 had undetectable titers.

CONCLUSIONS

Engraftment and specific non-responsiveness to pig cells has been achieved in two of four baboons following GalT-KO pig-to-baboon BMT. Engraftment correlated with absence of preformed anti-non-Gal IgG serum antibodies. These results are encouraging with regard to the possibility of achieving transplantation tolerance across this xenogeneic barrier.

Renal and cardiac endothelial heterogeneity impact acute vascular rejection in pig-to-baboon xenotransplantation

Xenograft outcomes are dictated by xenoantigen expression, for example, Gal alpha1, 3Gal (Gal), but might also depend on differing vascular responses. We investigated whether differential vascular gene expression in kidney and cardiac xenografts correlate with development of thrombotic microangiopathy (TM) and consumptive coagulation (CC). Immunosuppressed baboons underwent miniswine or hDAF pig kidney (n = 6) or heart (n = 7), or Gal-transferase gene-knockout (GalT-KO) (thymo)kidney transplantation (n = 14). Porcine cDNA miniarrays determined donor proinflammatory, apoptosis-related and vascular coagulant/fibrinolytic gene expression at defined time points; validated by mRNA, protein levels and immunopathology. hDAF-transgenic and GalT-KO xenografts, (particularly thymokidneys) exhibited prolonged survival. CC was seen with Gal-expressing porcine kidneys (3 of 6), only 1 of 7 baboons postcardiac xenotransplantation and was infrequent following GalT-KO grafts (1 of 14). Protective-type genes (heme oxygenase-I, superoxide dismutases and CD39) together with von Willebrand factor and P-selectin were upregulated in all renal grafts. Transcriptional responses in Gal-expressing xenografts were comparable to those seen in the infrequent GalT-KO rejection. In cardiac xenografts, fibrin deposition was associated with increased plasminogen activator inhibitor-1 expression establishing that gene expression profiles in renal and cardiac xenografts differ in a quantitative manner. These findings suggest that therapeutic targets may differ for renal and cardiac xenotransplants.

The innate immune response and activation of coagulation in alpha1,3-galactosyltransferase gene-knockout xenograft recipients

BACKGROUND

The role of the innate immune system in the development of thrombotic microangiopathy (TM) after alpha1,3-galactosyltransferase gene-knockout (GTKO) pig organ transplantation in primates is uncertain.

METHODS

Twelve organs (nine hearts, three kidneys) from GTKO pigs were transplanted into baboons that received no immunosuppressive therapy, partial regimens, or a full regimen based on costimulation blockade. After graft failure, histologic and immunohistologic examinations were carried out.

RESULTS

Graft survival of less than 1 day was prolonged to 2 to 12 days with partial regimens (acute humoral xenograft rejection) and to 5 and 8 weeks with the full regimen (TM). Clinical or laboratory features of consumptive coagulopathy occurred in 7 of 12 baboons. Immunohistochemistry demonstrated IgM, IgG, and complement deposition in most cases. Histopathology demonstrated neutrophil and macrophage infiltrates, intravascular fibrin deposition, and platelet aggregation (TM). Grafts showed expression of primate tissue factor (TF), with increased mRNA levels, and TF was also expressed on baboon macrophages/monocytes infiltrating the graft.

CONCLUSIONS

Our data suggest that (1) irrespective of the presence or absence of the adaptive immune response, early or late xenograft rejection is associated with activation of the innate immune system; and (2) porcine endothelial cell activation and primate TF expression by recipient innate immune cells may both contribute to the development of TM.

Achieving tolerance in pig-to-primate xenotransplantation: reality or fantasy

Because the immunologic differences between species are far greater than those within species, it is likely that the amount of immunosuppression that would be required for successful xenografting would be so much greater than that now used for allografting, that the side-effects and complications would be unacceptable. Tolerance approaches to xenotransplantation would overcome this concern. Studies in humanized mouse models have demonstrated that human T cells can be tolerized to porcine xenografts, providing important proofs of principle of the potential feasibility of pig-to-primate xenograft tolerance. The results available from studies of pig-to-primate xenotransplantation to date have demonstrated that while chronic immunosuppressive drugs have not completely avoided either T cell responses or humoral rejection, approaches directed toward tolerance induction have been encouraging with regard to avoiding immunization at both of these levels.

Antibodies directed to pig non-Gal antigens in naïve and sensitized baboons

BACKGROUND

As pigs homozygous for alpha1,3-galactosyltransferase gene-knockout (GT-KO) are available, primate antibodies to pig non-Gal antigens can be studied.

METHODS

Sera from 56 baboons were tested for binding of IgM and IgG to peripheral blood mononuclear cells (PBMC) from both wild-type (WT) and GT-KO pigs by flow cytometry. Complement-dependent cytotoxicity was measured in 39 sera. Antibody and cytotoxicity responses were measured in two baboons exposed to a GT-KO pig heart, one not immunosuppressed and one that received only cobra venom factor.

RESULTS

IgM and IgG bound to 95% and 79% of WT PBMC, and 32% and 9% GT-KO PBMC, respectively (WT vs. GT-KO, P<0.01). Whereas 97% of sera were cytotoxic to WT PBMC, only 64% were cytotoxic to GT-KO PBMC, and the level of cytotoxicity was less (mean 60% vs. 25% lysis, P<0.05). In the two baboons exposed to GT-KO hearts, anti-non-Gal antibodies increased markedly, peaking after 2 (IgM) and 3 (IgG) weeks, associated with an increase in lysis of GT-KO PBMC.

CONCLUSIONS

Two-thirds of baboon sera demonstrated cytotoxicity to GT-KO PBMC. After GT-KO organ transplantation, if an elicited antibody response develops, it is likely to cause rapid graft rejection.

alpha1,3-Galactosyltransferase gene-knockout pig heart transplantation in baboons with survival approaching 6 months

BACKGROUND

The recent generation of alpha1,3-galactosyltransferase gene-knockout (GalT-KO) pigs has allowed investigation of the survival of GalT-KO pig organs in nonhuman primates.

METHODS

Heterotopic heart transplantation from GalT-KO pigs was carried out in baboons (n=8) using a human antihuman CD154 monoclonal antibody-based immunosuppressive regimen.

RESULTS

In six of the eight cases, graft survival extended to between approximately 2 and 6 months. All grafts developed thrombotic microangiopathy (TM). In particular, the clinical course of one baboon in which the graft functioned for 179 days is summarized. This baboon received aspirin (40 mg on alternate days) from day 4 in addition to heparin, which may have been a factor in the delay of onset and progression of TM and in prolonged graft survival. Maintenance therapy with anti-CD154 mAb, mycophenolate mofetil, and methylprednisolone was associated with persistently low numbers of CD3CD4 and CD3CD8 cells. Despite persisting depletion of these cells, no infectious complications occurred.

CONCLUSIONS

It remains to be established whether TM is related to a very low level of natural preformed or T-cell-induced antibody deposition on the graft, inducing endothelial activation and injury, or to molecular incompatibilities in the coagulation mechanisms between pig and baboon, or to both. However, function of a pig organ in a baboon for a period approaching six months, which has not been reported previously, lends encouragement that the barriers to xenotransplantation will eventually be overcome.

Thrombotic Microangiopathic Glomerulopathy in Human Decay Accelerating Factor–Transgenic Swine-to-Baboon Kidney Xenografts

Models of pig-to-baboon xenografting were examined to identify the mechanisms and pathologic characteristics of acute humoral xenograft rejection (AHXR). Thymus and kidney (composite thymokidney) from human decay accelerating factor-transgenic swine were transplanted into baboons (n = 16) that were treated with an immunosuppressive regimen that included extracorporeal immunoadsorption of anti-alphaGal antibody and inhibition of complement activation. Morphologic and immunohistochemical studies were performed on protocol biopsies and graftectomy samples. All renal xenografts avoided hyperacute rejection. However, graft rejection coincided with the increase of anti-alphaGal antibody in the recipient's circulation. The 16 xenografts studied were divided into two groups dependent on the rapid return (group 1) or gradual return (group 2) of anti-alphaGal antibody after immunoadsorption. In group 1 (n = 6), grafts were rejected to day 27 with development of typical AHXR, characterized by marked interstitial hemorrhage and thrombotic microangiopathy in the renal vasculature. In group 2 (n = 10), grafts also developed thrombotic microangiopathy affecting mainly the glomeruli by day 30 but also showed minimal evidence of interstitial injury and hemorrhage. In the injured glomeruli, IgM and C4d deposition, subsequent endothelial cell death and activation with upregulation of von Willebrand factor and tissue factor, and a decrease of CD39 expression developed with the formation of fibrin-platelet multiple microthrombi. In this model, the kidney xenografts, from human decay accelerating factor-transgenic swine, in baboons undergo AHXR. In slowly evolving AHXR, graft loss is associated with the development of thrombotic microangiopathic glomerulopathy. Also, anti-alphaGal IgM deposition and subsequent complement activation play an important role in the mechanism of glomerular endothelial injury and activation and the formation of multiple microthrombi.

Progress in xenotransplantation

Organ transplantation is considered the most effective treatment for end-stage organ failure; currently it is limited by a severe worldwide shortage of human donor organs. This has led to investigation of the potential use of animals as organ donors. For a number of reasons, the pig represents the most likely organ donor candidate. Transplantation of a vascularised porcine organ into a human or non-human primate results in an immediate and dramatic rejection process, known as hyperacute rejection, which is mediated by the binding of pre-existing antibody to the porcine graft and the subsequent activation of host complement. Strategies aimed at preventing this initial rejection have been largely successful in experimental models. This has allowed attention to turn towards an understanding of the immunological barriers comprising the next phase of xenograft rejection, termed acute vascular rejection. This delayed rejection process appears to be a humoral event, and it is likely that the control of antibody synthesis will play a pivotal role in overcoming the current barrier to successful xenotransplantation.

The active role played by xenogeneic endothelial cells in the indirect presentation pathway is not lymphocyte trans-co-stimulation

The human CD4+ T lymphocyte response to major histocompatibility complex (MHC) class II-negative porcine endothelial cells is dependent on the presence of human monocytes through a human leukocyte antigen (HLA) class II-restricted indirect presentation pathway. Because the role of porcine endothelial cells had been previously shown to do more than simply supply xenopeptides, co-stimulatory signals were analysed. Endothelial cells were shown to express the CD54, CD58, CD59 and CD86 transcripts; however, no membrane B7 molecule could be detected. Blocking experiments in a direct pathway model confirmed that porcine endothelial cells could provide co-stimulatory signals to human T cells through the CD2 and LFA-1 pathways. Nevertheless, the proliferation achieved in the indirect presentation model required co-stimulation by LFA-1, CD2 and CD28, engaged by co-stimulation molecules expressed in the cis-form by the human monocytes. These results clearly show that the active role played by the endothelial cells in the indirect pathway is not lymphocyte trans-co-stimulation and suggest that cis-co-stimulation dominates trans-co-stimulation when both are present.

Bone marrow transplantation from alpha1,3-galactosyltransferase gene-knockout pigs in baboons

BACKGROUND

Successful hematopoietic cell allotransplantation results in donor-specific tolerance, but this approach has been unsuccessful in the wild-type pig-to-baboon xenotransplantation model, as pig cells were lost from the circulation within 5 days. However, after cessation of immunosuppressive therapy on day 28, all baboons demonstrated non-specific unresponsiveness on mixed leukocyte reaction (MLR) for at least 30 days. We have now investigated the transplantation of bone marrow (BM) cells from miniature swine homozygous for alpha1,3-galactosyltransferase gene-knockout (GalT-KO).

METHODS

Baboons (n = 3) were pre-treated with whole body and thymic irradiation, anti-thymocyte globulin, and splenectomy, and received immunosuppressive and supportive therapy for 28 days. BM was harvested from GalT-KO swine (n = 3). The baboons were monitored for the presence of pig cells by flow cytometry and colony-forming units (CFUs), and for cellular reactivity by MLR.

RESULTS

A mean of 11 x 10(8) BM cells/kg was infused into each baboon. The mean absolute numbers and percentages of pig cells detected in the blood at 2 h and on days 1, 2 and 4, respectively, were 641/microl (9.5%), 132/microl (3.4%), 242/microl (3.9%), and 156/microl (2.9%). One baboon died (from accidental hemorrhage) on day 6, at which time chimerism was present in the blood (2.0%) and BM (6.4%); pig cell engraftment in the BM was confirmed by polymerase chain reaction (PCR) of CFUs. In the two other baboons, blood chimerism was lost after day 5 but returned at low levels (<1%) between days 9 to 16 and 7 to 17, respectively, indicating transient BM engraftment. Both surviving baboons showed non-specific unresponsiveness on MLR until they were euthanized on days 85 and 110, respectively.

CONCLUSIONS

By using BM cells from GalT-KO pigs, chimerism was detected at levels comparable with previous studies when 30-fold more growth factor-mobilized peripheral blood progenitor cells had been transplanted. In addition, cellular hyporesponsiveness was prolonged. However, long-term engraftment and chimerism were not achieved.

Porcine Hematopoietic Progenitor Cell Transplantation in Nonhuman Primates: A Review of Progress

The critical shortage of human donor organs for transplantation would be overcome if a suitable animal, e.g., the pig, could be used as an organ source. There are, however, several immune barriers that have to date resulted in limited function of pig organs transplanted into nonhuman primates. It would be beneficial, and indeed may be essential, to induce a state of tolerance in the primate recipient to the pig organ. In allotransplantation, the successful transplantation of hematopoietic progenitor cells with the development of mixed chimerism is associated with the induction of tolerance toward a donor-specific organ. For some years, this approach has been explored in the pig-to-nonhuman primate model. This experience is briefly reviewed. The problems of natural and elicited anti-pig antibodies, recipient platelet adhesion to pig hematopietic progenitor cells, and the rapid removal of these cells by the host macrophage-phagocytic system are highlighted. Recent experience with the use of hematopoietic cells from pigs homozygous for alpha1,3-galactosyltransferase gene-knockout is reported.

Initial experience with the human anti-human CD154 monoclonal antibody, ABI793, in pig-to-baboon xenotransplantation

BACKGROUND

ABI793 (ABI) is a human monoclonal antibody (mAb) specific for human CD154. To assess the suitability of ABI for baboon transplantation studies, we carried out in vitro studies to determine ABI's reactivity with baboon cells expressing CD154, performed in vivo pharmacokinetic studies in two baboons, and tested the effect of ABI administration on elicited antibody production in two baboons undergoing either pig hematopoietic progenitor cell (PBPC) or heterotopic heart transplantation.

METHODS

In vitro: Baboon peripheral blood mononuclear cells were activated in vitro to upregulate CD154, and binding of ABI to CD154 was measured by flow cytometry. In vivo: Serum levels of ABI were measured immediately before and 15 min after the intravenous administration of ABI (20 mg/kg) to two baboons over 28 days. Subsequently, ABI (25 mg/kg on days 0, 1, 4 and 7, and then 20 mg/kg every 5 days) was included in the immunosuppressive regimen in two pig-to-baboon transplants (PBPC or heart transplantation).

RESULTS

In vitro: ABI was almost non-reactive to baboon T cells before stimulation, but bound to activated T cells. In vivo: In the pharmacokinetic study, trough levels of ABI (before the next dose) ranged between 190 and 580 microg/ml, and the estimated half-life was 10-15 days. There was no apparent toxicity. Following pig PBPC or heart transplantation, no elicited antibody was detected while ABI was being administered or during several weeks of follow-up.

CONCLUSIONS

ABI functions in baboons, is well-tolerated, and prevents an elicited antibody response to pig antigens.

Initial investigation of the potential of modified porcine erythrocytes for transfusion in primates

There is a shortage of human blood for transfusion. The possibility of using alpha-galactosidase-treated pig red blood cells (pRBCs) for transfusion into humans has been investigated. pRBCs were treated in vitro with alpha-galactosidase. In vitro binding of antibodies (Abs) in baboon or human sera to untreated/treated pRBCs was assessed by flow cytometry and serum cytotoxicity. In vivo clearance rates of (1) autologous baboon red blood cells (RBCs), (2) unmodified pRBCs, and (3) alpha-galactosidase-treated pRBCs were measured after transfusion into baboons receiving either no treatment or depletion of complement +/- depletion of anti-Gal alpha 1-3Gal (Gal) Ab or of macrophage phagocytes. In vitro binding of baboon or human Abs to treated pRBCs was absent or minimal compared with untreated pRBCs, and serum cytotoxicity was completely inhibited. In vivo autologous baboon RBCs survived for >16 days and unmodified pRBCs for <15 min in an untreated baboon. Treated pRBCs survived for 2 h in an untreated baboon, for 24 h in a complement-depleted baboon, and for 72 h when the baboon was depleted of both complement and anti-Gal Ab, or of complement and macrophage phagocytes. All baboons, however, became sensitized to Gal antigens. Failure to prolong the in vivo survival of treated pRBCs could be due to inadequate removal of Gal epitopes because sensitization to Gal developed, or could imply other, as yet unidentified, causes for RBC destruction. To fully assess the potential of pRBC transfusion in humans, more complete alpha-galactosidase treatment of pRBCs will be required.

Correlation of biochemical and hematological changes with graft failure following pig heart and kidney transplantation in baboons

We have explored biochemical and hematologic parameters that might indicate acute humoral xenograft rejection (AHXR) following pig organ transplantation in baboons. Baboons (n = 15) received an immunosuppressive regimen, and underwent a miniature swine or hDAF kidney (Group 1, n = 6) or heart (Group 2, n = 7) transplantation. Control baboons (Group 3, n = 2) received the immunosuppressive regimen without organ transplantation. Blood chemistry and hematologic parameters were measured daily. Baboon and porcine cytomegalovirus were monitored. In Groups 1 and 2, organ grafts survived for up to 29 days. A plasma fibrinogen of <80 mg/dL on 2 consecutive days, and a serum lactate dehydrogenase of >600 U/L and aspartate transaminase of >300 U/L, were associated with the development of AHXR in both heart and kidney grafts. In Group 1, a decrease in platelet count of >150,000/microL within 3 days, or a count of <50,000/microL, were associated with AHXR. In Group 2, a creatine phosphokinase of >500 U/L was associated with graft failure. In Group 3, no abnormalities were observed. The possibility that porcine CMV may play a role in graft injury could not be excluded. Noninvasive parameters were identified that have predictive potential for AHXR. Monitoring of these might enable therapeutic intervention to reverse rejection.

Release of pig leukocytes during pig kidney perfusion and characterization of pig lymphocyte carbohydrate xenoantigens

The Galalpha1-3Gal (alphaGal) antigen is considered the main xenoantigen in the pig to human species combination but other porcine antigens have to be considered such as the swine lymphocyte antigen (SLA), the blood group A/O and the Hanganutziu-Deicher (H-D) antigens. The H-D antigens are N-glycolyl-neuraminic acid (NeuGc) terminated gangliosides that are widely distributed in mammalian species but absent in humans. Upon exposure to a vascularized pig organ, the human recipient can be immunized by direct interaction with the pig tissue or/and by transfer of tissue/cells from the organ into the recipient. In the present work, we describe the release of cells from porcine kidneys upon perfusion and the expression of glycolipid based alphaGal, blood group A/O and H-D antigens in pig lymphocytes. Pig kidneys were flushed with 20 ml of NaCl or Lidocain containing 5000 U heparin, and thereafter perfused with 3000-ml perfusion solution and the cells released were counted and examined microscopically. Neutral glycolipid and ganglioside fractions were extracted from purified pig lymphocytes. The extracted components were characterized by thin layer chromatography, degradation and mass spectrometry. The expression of alphaGal and H-D epitopes on cells released from pig kidneys and purified pig lymphocytes were studied by immune electron microscopy. A total amount of about 300 x 106 leukocytes, mainly lymphocytes were released in the perfusate from the kidneys, of which about 100 x 106 cells were eluated in the 600 to 2400 ml perfusate fraction. Immunelectron microscopical analysis with Griffonia simplicifolia isolectin B4 showed staining of pig leukocytes and other cells, morphologically similar to endothelial cells, released in the perfusate. The purified porcine lymphocytes contained 930 microg neutral glycolipid (4.2 microg/mg cell protein) of which 95% was glycolipids with one to four sugar residues. Immunostaining of the neutral glycolipid fractions revealed alphaGal terminated compounds migrating in the five and 10 to 12 sugar regions and blood group A compounds in the six and eight sugar regions. Two major gangliosides NeuGc-GM3 and NeuGc-GD3 were found in the pig lymphocytes. In a patient extracorporeally xenoperfused with a pig kidney, an increased staining of both alphaGal terminated structures as well as the H-D reactive gangliosides were found in the post-perfusion serum samples. In summary, leukocytes, mainly lymphocytes are released from pig kidneys during perfusion which may contribute to immunization of human xenograft recipients.

Depletion of anti-Gal antibodies by the intravenous infusion of Gal type 2 and 6 glycoconjugates in baboons

BACKGROUND

Natural anti-Gal antibodies (NAb) to Gal epitopes play a key role in the rejection of pig cells or organs transplanted into primates. We have investigated the effect on NAb return after extracorporeal immunoadsorption (EIA) of the continuous intravenous (i.v.) infusion of (i) bovine serum albumin conjugated to Gal type 6 oligosaccharides (BSA-Gal) or (ii) a poly l-lysine backbone conjugated to Gal type 2 or 6 oligosaccharides (PLL-Gal).

METHODS

Porcine mobilized peripheral blood progenitor cells (PBPC) obtained by leukapheresis from MHC-inbred miniature swine (n = 9) were infused intravenously (i.v.) into baboons: Group 1 baboons (n = 4) received whole body and thymic irradiation, splenectomy, antithymocyte globulin, cobra venom factor, cyclosporine, mycophenolate mofetil, anti-CD154mAb, porcine hematopoietic growth factors, and EIA before transplantation of high doses (2 to 4 x 1010 cells/kg) of PBPC; Group 2 baboons (n = 3) received the Group 1 regimen plus a continuous i.v. infusion of BSA-Gal for up to 30 days; Group 3 baboons (n = 5) received the Group 1 regimen plus a continuous i.v. infusion of PLL-Gal type 2 (n = 2) or both PLL-Gal types 2 and 6 (n = 3) for up to 30 days.

RESULTS

Group 1: NAb returned to pre-PBPC levels within 20-30 days, but there was no induction of antibody to Gal or non-Gal determinants; Group 2: NAb was undetectable or at very low level during BSA-Gal therapy. In one baboon, however, IgG to Gal type 2, but not to type 6, returned during BSA-Gal therapy; Group 3: NAb was undetectable or at very low level during PLL-Gal therapy. In two baboons that received PLL-Gal type 2, NAb to Gal type 6, but not to type 2, returned during PLL-Gal treatment. Two of five baboons, however, developed systemic infection. Four of five baboons died within 14 days; autopsy revealed focal hemorrhagic injury to their hearts, lungs, and small intestines, with histologic abnormalities that varied between animals from hemorrhage and/or thrombosis in some organs (heart, lungs, or intestine) to signs of infections (bacteria in intestine, cytomegalovirus in liver).

CONCLUSIONS

(i) BSA-Gal and PLL-Gal therapy maintained depletion of NAb. (ii) Some heterogeneity in specificity of NAb was identified, indicating that the infusion of a combination of Gal type 2 and 6 glycoconjugates may be required. (iii) The addition of PLL-Gal to the immunosuppressive regimen was associated with a high incidence of morbidity and mortality without a clear histopathologic entity underlying the cause of death.

Xenogeneic thymokidney and thymic tissue transplantation in a pig-to-baboon model: I. Evidence for pig-specific T-cell unresponsiveness

BACKGROUND

The potential of xenotransplantation for clinical application will require overcoming barriers of humoral and cellular rejection, through strategies using immune suppression or tolerance induction. This laboratory has previously reported the induction of tolerance in the discordant xenogeneic model of pig-to-rodent thymic transplantation. We also have described a miniature swine model of fully mismatched allogeneic composite vascularized thymokidney transplantation that induced transplantation tolerance. We tested a combination of these approaches in a clinically relevant pig-to-primate model of xenotransplantation.

METHODS

Composite thymokidney grafts were prepared 40 to 80 days before transplantation by the autologous implantation of thymic tissue under the renal capsule of human decay-accelerating factor transgenic swine. Baboons received xenotransplants of both human decay-accelerating factor composite thymokidneys and omental implants of thymic tissue. Recipients were treated with an immunosuppressive-conditioning regimen including thymectomy or thymic irradiation, extracorporeal immunoadsorption of anti-alphaGal antibodies and T-cell depletion. Recipients were followed for indicators of xenograft rejection, T-cell depletion and reconstitution, anti-alphaGal antibody levels, and mixed lymphocyte responses. Immunologic responses were studied in those animals that survived for more than 3 weeks.

RESULTS

Thymokidney xenografts survived for up to 30 days, with evidence of viable thymic epithelium and Hassall's corpuscles under the renal capsule and in the omental implants, and with evidence of few host lymphocytes. Three animals demonstrated donor-specific unresponsiveness, while maintaining normal alloresponses, in mixed-lymphocyte-response assays performed after immunosuppression had been stopped. Rejected grafts demonstrated humoral damage without evidence of cellular infiltrates. After graftectomy, one animal maintained donor-specific cellular unresponsiveness and stable anti-alphaGal antibody levels for more than 2 months.

CONCLUSIONS

We concluded that composite thymokidney and thymic-tissue xenotransplantation from swine to baboons can induce donor-specific cellular unresponsiveness and stable anti-alphaGal antibody levels, suggesting avoidance of sensitization after xenotransplantation. The presence of viable donor-swine thymic epithelium could have a role in the development of donor-specific T-cell tolerance. Further strategies to address humoral rejection could prolong graft survival and result in long-term tolerance to xenografts.

Xenogeneic thymus transplantation in a pig-to-baboon model

BACKGROUND

We have tested whether fetal porcine thymic tissue transplantation can lead to tolerance across a discordant (pig-to-baboon) xenogeneic barrier.

METHODS

Six baboons underwent a conditioning regimen with thymectomy, splenectomy, and anti-monkey CD3 antibody conjugated to a diphtheria toxin binding site mutant (FN18-CRM9). Porcine fetal or neonatal thymic tissue was transplanted into three baboons. Three control baboons received either no transplanted pig tissue (n=1) or adult pig lymph node (n=2). Cellular responses and skin xenografts were used to test for tolerance.

RESULTS

After T-cell depletion and thymic transplantation, recovery of thymus-dependent naïve-type CD4 cells (CD4/CD45RA ) and in vitro xenogeneic hyporesponsiveness were observed. No sensitization of alpha-galactosyl antibody responses was observed. The thymic grafts survived up to 48 days. Porcine skin xenografts were performed in two of these animals with survival of 22 and 24 days. Only two of these animals were completely T-cell depleted, and both failed to recover thymus-dependent T cells (CD4/CD45RA ). In one animal, general in vitro hyporesponsiveness was observed, with subsequent death from infection. The second animal demonstrated delayed recovery of T cells and prolonged general hyporesponsiveness in vitro. Neither animal demonstrated prolongation of porcine skin grafts compared with allografts (both rejected by day 13).

CONCLUSIONS

Porcine thymic tissue is able to induce xenogeneic hyporesponsiveness. More efficient thymic engraftment may allow this approach to induce xenograft tolerance.

The in vitro and in vivo effects of anti-galactose antibodies on endothelial cell activation and xenograft rejection

We have previously produced a series of antigalactose (anti-Gal) hybridomas and characterized their heavy chain gene usage. Here we have quantified the affinity of these Abs for the alpha-Gal epitope and characterized their in vitro effects on endothelial cell activation and apoptosis. We report that anti-Gal mAbs derived from Gal(-/-) mice show a range of affinity for the alpha-Gal epitope, and that affinity was generally increased as the V(H) gene usage transitioned from germline sequences to sequences exhibiting somatic maturation. Despite an 85-fold range in affinity, all the anti-Gal mAbs examined induced alpha-Gal-specific endothelial cell activation, and after prolonged exposure induced endothelial cell apoptosis in a complement-independent manner. Only murine anti-Gal mAbs of the IgM or IgG3 subclass, but not IgG1, were effective at initiating complement-dependent cell lysis. Using a novel rat to mouse xenograft model, we examined the in vivo ability of these mAbs to induce xenograft rejection and characterized the rejection using histology and immunohistochemistry. Infusion of complement-fixing IgG3 mAbs resulted in either hyperacute rejection or acute vascular rejection of the xenograft. Surprisingly, infusion of an equal amount of a high affinity anti-Gal IgG1 mAb, that fixed complement poorly also induced a rapid xenograft rejection, which we have labeled very acute rejection. These studies emphasize the importance of in vivo assays, in addition to in vitro assays, in understanding the role of anti-Gal IgG-mediated tissue injury and xenograft rejection.

An investigation of the specificity of induced anti-pig antibodies in baboons

AIM

To provide information on the specificity of induced anti-pig antibodies (Abs) in baboons after exposure and sensitization to pig antigens.

MATERIALS AND METHODS

Baboons (n=7) received either porcine mobilized peripheral blood progenitor cells (n=3), kidney (n=3) or heart (n=1) transplants. After rejection of these cells or organs, pre- and post-rejection sera were analyzed by enzyme-linked immunosorbent assay (ELISA) and flow cytometry to detect and measure anti-Galactosealpha1,3Galactose (Gal) and anti-non-Gal Abs. To study the anti-non-Gal carbohydrate response, the sera were incubated with pig red blood cells pretreated with alpha-galactosidase (to remove Gal) and three other exoglycosidases to remove other potential oligosaccharide epitopes, and studied by flow cytometry. To study the anti-swine leukocyte antigen (SLA) response, non-Gal Abs from two baboons sensitized with kidneys from inbred miniature swine of dd or aa haplotype, respectively, were adsorbed on cells of aa, cc, or dd haplotypes, and binding to aa, cc or dd cells was measured by flow cytometry. Cytotoxicity of anti-non-Gal Abs was tested in vitro by a complement-mediated cytotoxicity assay, using pig cells as targets.

RESULTS

In pre-transplant and pre-rejection sera, anti-Gal Abs were detected, but anti-non-Gal Abs were either absent or at minimal levels. After exposure to pig antigens, baboons developed induced anti-Gal and anti-non-Gal Abs. No anti-non-Gal Abs directed to the tested carbohydrate epitopes could be detected. Anti-non-Gal Abs showed minor evidence of specific SLA haplotype reactivity, suggesting that the major Ab response was to pan-pig determinants. Anti-non-Gal Abs showed a low level of complement-mediated lysis of pig cells in vitro.

CONCLUSIONS

In this limited study, no Ab response to non-Gal carbohydrates was observed, and anti-SLA specificity was minor, indicating that most induced anti-non-Gal Ab was directed against non-specific pig proteins, including SLA-epitopes.

Induction of xenograft accommodation by modulation of elicited antibody responses1 2

BACKGROUND

We have established that the timing of splenectomy influences the magnitude of the xenoreactive antibody (XAb) response and thus hamster heart survival in cyclosporine (CyA)-treated rats. This model has been used to test our hypothesis that modulation of XAb responses without perturbation of complement may influence the development of graft accommodation.

METHODS

Pretransplantation splenectomy (day -1/day 0) fully abrogated anti-graft IgM response, whereas a delayed procedure (day 1/day 2) caused significantly delayed (3-4 days) and decreased levels (two- to threefold) of XAb. Both interventions resulted in long-term graft survival. After surviving for 7 or more days, xenografts in CyA-treated rats with post-, but not pre-, transplantation splenectomy were also resistant to exogenous anti-graft XAb. Such grafts meet the criteria for accommodation. Accommodating hearts displayed progressive and increasing expression of protective genes, such as heme oxygense (HO)-1 and A20, in endothelial cells and smooth muscle cells.

RESULTS

Our results suggest that XAb responses may influence the kinetics of accommodation development possibly by promoting protective gene expression. This hypothesis was directly tested in vitro. Pretreatment of porcine aortic endothelial cells with sublytic amounts of baboon anti-pig serum for 24 hr induced HO-1 expression; this was associated with cell resistance to lytic amounts of such serum. Overexpression of HO-1 by adenoviral-mediated gene transfer in porcine aortic endothelial cells resulted in similar protective effects.

CONCLUSIONS

Delayed and relatively low levels of XAb IgM promote expression of protective genes in the graft and thereby aid in the progress of accommodation. Expression of HO-1 protects xenoserum-mediated endothelial cell destruction.

Analysis of the control of the anti-gal immune response in a non-human primate by galactose alpha1-3 galactose trisaccharide-polyethylene glycol conjugate

BACKGROUND

The current limitation to the clinical application of xenotransplantation using pig organs is a rejection process that has been termed delayed xenograft rejection or acute vascular rejection. It is thought that acute vascular rejection may be mediated at least in part by both the continued synthesis, of preexisting, and the induction, posttransplantation, of antibodies against the carbohydrate moiety galalpha1-3gal that is present on glycoproteins and glycolipids of the pig endothelium. The synthesis of these antibodies has proven difficult to control with currently available immunosuppressive agents.

METHODS

We have synthesized galalpha1-3gal conjugated polyethylene glycol polymers that can bind to anti-galalpha1-3gal antibodies and tested their activity in non-human primates.

RESULTS

These conjugates when administered to non-human primates can substantially reduce the levels of preexisting and control the induction of anti-galalpha1-3gal antibodies. The level of circulating antibody-secreting cells that make anti-galalpha1-3gal antibodies is also reduced.

CONCLUSION

These alpha-gal polyethylene glycol conjugates may have the potential to control the anti-gal antibody response in a pig to primate organ transplant setting and may be a useful therapeutic agent in prolonging graft survival.

Anti-Gal alpha 1-3Gal IgM and IgG antibody levels in sera of humans and old world non-human primates

Organs transplanted from pig to primate are rejected within minutes or hours by an antibody-dependent, complement-mediated mechanism [hyperacute rejection (HAR)]. Even after depletion of anti-Gal alpha 1-3Gal (Gal) antibody (Ab), for example by extracorporeal immunoadsorption, return of natural Ab is believed to be a major factor in the initiation of acute humoral xenograft rejection. Various non-human primates are used as recipients of pig organs in experimental discordant xenotransplantation (XTx) models. However, anti-Gal IgM and IgG levels in non-human primates may differ from those in humans. Serum levels of anti-Gal IgM and IgG were measured by enzyme-linked immunosorbent assay (ELISA) in humans (n=14), chimpanzees (n=8), baboons (n=214), cynomolgus monkeys (n=29), rhesus monkeys (n=23) and Japanese monkeys (n=6). The mean level of anti-Gal IgM was significantly higher in chimpanzees than in other groups, while in rhesus monkeys it was significantly lower than in other groups, except baboons and Japanese monkeys. The mean human anti-Gal IgG level was higher than in other groups and this difference reached statistical significance except with regard to chimpanzees. The mean anti-Gal IgG level in baboons was significantly lower than that in humans, chimpanzees and cynomolgus monkeys. The measured differences in anti-Gal IgM and IgG levels may affect the kinetics of Ab removal and rate of return in different species, and thus may have relevance for translating work in non-human primate models to the clinical setting.

Pig hematopoietic cell chimerism in baboons conditioned with a nonmyeloablative regimen and CD154 blockade

BACKGROUND

In an attempt to induce mixed hematopoietic chimerism and transplantation tolerance in the pig-to-primate model, we have infused high-dose porcine peripheral blood progenitor cells (PBPC) into baboons pretreated with a nonmyeloablative regimen and anti-CD154 monoclonal antibody (mAb).

METHODS

Group 1 baboons (n=2) received a nonmyeloablative regimen including whole body irradiation, pharmacological immunosuppression, porcine hematopoietic growth factors, and immunoadsorption of anti-Galalpha1,3Gal (Gal) antibody before infusion of high doses of PBPC (2.7-4.6x10(10) cells/kg). In group 2 (n=5), cyclosporine was replaced by anti-CD154 mAb. Group 3 (n=3) received the group 1 regimen plus anti-CD154 mAb.

RESULTS

In group 1, pig chimerism was detected in the blood by flow cytometry (FACS) for 5 days (with a maximum of 14%), and continuously up to 13 days by polymerase chain reaction (PCR). In group 2, pig chimerism was detectable for 5 days by FACS (maximum 33%) and continuously up to 28 days by PCR. In group 3, initial pig chimerism was detectable for 5 days by FACS (maximum 73%). Two of three baboons showed reappearance of pig cells on days 11 and 16, respectively. In one, in which no anti-Gal IgG could be detected for 30 days, pig cells were documented in the blood by FACS on days 16-22 (maximum 6% on day 19) and pig colony-forming cells were present in the blood on days 19-33, which we interpreted as evidence of engraftment. Microchimerism was continuous by PCR up to 33 days.

CONCLUSIONS

These results suggest that there is no absolute barrier to pig hematopoietic cell engraftment in primates, and that this may be facilitated if the return of anti-Gal IgG can be prevented.

Depletion of anti-gal antibodies in baboons by intravenous therapy with bovine serum albumin conjugated to gal oligosaccharides

BACKGROUND

Anti-Galalpha 1-3Gal (Gal) antibodies (Ab) play a key role in the rejection of pig cells or organs transplanted into primates. A course of extracorporeal immunoadsorption (EIA) of anti-Gal Ab using an immunoaffinity column of a Gal type 6 oligosaccharide depletes Ab successfully, but Ab returns during the next few days. Although therapy with an anti-CD154 monoclonal antibody (mAb) prevents an induced Ab response to Gal or non-Gal epitopes, T cell-independent natural anti-Gal IgM and IgG return to baseline (pretransplant) levels. We have investigated the capacity of continuous i.v. infusion of bovine serum albumin conjugated to Gal type 6 oligosaccharide (BSA-Gal) to deplete or maintain depletion of circulating anti-Gal Ab.

METHODS

Porcine peripheral blood mobilized progenitor cells (PBPC) obtained by leukapheresis from MHC-inbred miniature swine (n=6) were transplanted into baboons. Group 1 baboons (n=4) underwent whole body (300 cGy) and thymic (700 cGy) irradiation, T cell depletion with antithymocyte globulin, complement depletion with cobra venom factor, short courses of anti-CD154 mAb therapy (20 mg/kg i.v. on alternate days), cyclosporine (CyA) (in two baboons only), mycophenolate mofetil, and porcine hematopoietic growth factors. Anti-Gal Ab depletion by EIA was carried out before transplantation of high doses (2-4x 1010 cells/kg) of PBPC. Group 2 baboons (n=3) received the group 1 regimen (including CyA) plus a continuous i.v. infusion of BSA-Gal. To prevent sensitization to BSA, anti-CD154 mAb therapy was continued until BSA-Gal administration was discontinued.

RESULTS

In group 1, Gal-reactive Ab returned to pre-PBPC transplant levels within 15-21 days, but no induced Ab to Gal or non-Gal determinants developed while anti-CD154 mAb therapy was being administered. In group 2, anti-Gal Ab was either not measurable or minimally measurable while BSA-Gal was being administered. After discontinuation of BSA-Gal, Ab did not return to pre-PBPC transplant level for more than 40-60 days, and no sensitization developed even when all therapy was discontinued. In one baboon, however, Ab to Gal type 2, but not type 6, returned during BSA-Gal therapy.

CONCLUSIONS

Prevention of the induced humoral response to Gal and non-Gal epitopes by anti-CD154 mAb therapy has been reported previously by our group, but our studies are the first to demonstrate a therapy that resulted in an absence of natural anti-Gal Ab for a prolonged period. The combination of BSA-Gal and T cell costimulatory blockade may facilitate survival of pig cells and organs transplanted into primates. The return in one baboon of Ab reactive with the Gal type 2 oligosaccharide, but not type 6, indicates some polymorphism of anti-Gal Ab and suggests that, to be effective in all cases, the infusion of a combination of type 6 and type 2 BSA-Gal may be required.

Transgenic pigs designed to express human CD59 and H-transferase to avoid humoral xenograft rejection

Research in pig-to-primate xenotransplantation aims to solve the increasing shortage of organs for human allotransplantation and develop new cell- and tissue-based therapies. Progress towards its clinical application has been hampered by the presence of xenoreactive natural antibodies that bind to the foreign cell surface and activate complement, causing humoral graft rejection. Genetic engineering of donor cells and animals to express human complement inhibitors such as hCD59 significantly prolonged graft survival. Strategies to decrease the deposition of natural antibodies were also developed. Expression of human alpha1,2-fucosyltransferase (H transferase, HT) in pigs modifies the cell-surface carbohydrate phenotype resulting in reduced Galalpha1,3-Gal expression and decreased antibody binding. We have developed transgenic pigs that coexpress hCD59 and HT in various cells and tissues to address both natural antibody binding and complement activation. Functional studies with peripheral blood mononuclear cells and aortic endothelial cells isolated from the double transgenic pigs showed that coexpression of hCD59 and HT markedly increased their resistance to human serum-mediated lysis. This resistance was greater than with cells transgenic for either hCD59 or HT alone. Moreover, transgene expression was enhanced and protection maintained in pig endothelial cells that were exposed for 24 h to pro-inflammatory cytokines. These studies suggest that engineering donor pigs to express multiple molecules that address different humoral components of xenograft rejection represents an important step toward enhancing xenograft survival and improving the prospect of clinical xenotransplantation.

Adult porcine islet transplantation in baboons treated with conventional immunosuppression or a non-myeloablative regimen and CD154 blockade

The aim of the present study was to assess the survival of adult porcine islets transplanted into baboons receiving either (I) conventional triple drug immunosuppressive therapy or (2) a non-myeloablative regimen and an anti-CD154 monoclonal antibody (mAb) aimed at tolerance-induction. Group 1 baboons (n = 3) were pancreatectomized prior to intraportal injection of 10,000 porcine islet equivalents (IE)/kg and immunosuppressed with anti-thymocyte globulin (ATG), cyclosporine and azathioprine. In Group 2 (n = 2), non-pancreatectomized baboons underwent induction therapy with whole body and thymic irradiation, and ATG. Extracorporeal immunoadsorption (EIA) of anti-Galalpha1,3Gal (Gal) antibody was carried out. Maintenance therapy was with cobra venom factor, cyclosporine. mycophenolate mofetil, methylprednisolone and anti-CD154 mAb. Porcine islets were injected intraportally (14,000 and 32,000 IE/kg, respectively) and high-dose pig mobilized peripheral blood progenitor cells (3 x 10(10) cells/kg) were infused into a systemic vein. Porcine islets were also implanted in the sternomastoid muscle to facilitate subsequent biopsies. In both groups. porcine C-peptide was measured, and histological examination of liver or sternomastoid muscle biopsies was performed at regular intervals. In Group 1, total pancreatectomy reduccd human C-peptide to < 0.1 ng/ml and induced insulin-requiring diabetes. The transplantation of porcine islets was followed by normalization of glycemia for 15-24 h. Porcine C-peptide was detected only transiently immediately after porcine islet injection (maximum 0.12 ng/ml). Histological examination of liver biopsies taken between days 2 and 19 did not reveal viable islets, but necrotic cell structures with mononuclear cell infiltrates were identified in portal venules. In Group 2, injection of porcine islets into non-pancreatectomized recipients induced a transient hypoglycemia (2-4 h) requiring concentrated intravenous dextrose administration. Porcine C-peptide was detectable for 5 and 3 days (maximum 2.8 and 1.0 ng/ml), respectively. Baboon #4 died on day 12 from small bowel intussusception. Liver and sternomastoid muscle biopsies showed well-preserved porcine islets, staining positive for insulin and glucacon, without signs of rejection. In baboon #5, viable islets were detected in the sternomastoid muscle biopsy on day 14, but not on day 28 or thereafter. A progressive mononuclear cell and macrophage infiltration was seen in the biopsies. In conclusion, conventional immunosuppression allowed survival of porcine islets in baboons for < 24 h. The non-myeloablative regimen prolonged survival of porcine islets for > 14 days. However, despite depletion of T cells, anti-Gal antibody and complement, and CD154-hlockade, porcine islets were rejected by day 28. These results suggest that powerful innate immune responses are involved in rejection of discordant xenogencic islets.

CD40-CD154 pathway blockade requires host macrophages to induce humoral unresponsiveness to pig hematopoietic cells in baboons

The effect of CD154 blockade and macrophage depletion or inhibition on baboon humoral and cellular immune responses to pig antigens was studied in a pig-to-baboon peripheral blood mobilized progenitor cell (PBPC) transplantation model aimed at inducing tolerance. We infused pig PBPCs in baboons pretreated with a nonmyeloablative regimen along with murine anti-human CD154 monoclonal antibody (mAb) and macrophage-depleting or -inhibiting agents. Group 1 baboons (n=2) underwent a nonmyeloablative regimen and immunoadsorption of anti-Gal(alpha)1,3Gal (Gal) antibody (Ab) before intravenous infusion of high doses (1.3-4.6 x 10(10)cells/kg) of PBPCs. In group 2 (n=5), cyclosporine was replaced by 8 doses of anti-CD154 mAb over 14 days. Group 3 (n=3) received the group 2 regimen plus medronate liposomes (n=2) or commercially available human intravenous immunoglobulin G depleted of anti-Gal Ab (n=1) to deplete/inhibit recipient macrophages. Group 1 developed sensitization to Gal and also developed new Ab to non-Gal porcine antigens within 10 to 20 days. In group 2, no sensitization to Gal or non-Gal determinants was seen, but Gal-reactive antibodies did return to their preleukocyte transplantation levels. CD154 blockade, therefore, induced humoral unresponsiveness to pig cells. In group 3, sensitization to Gal was seen in all three baboons at 20 days, and Abs against new porcine determinants developed in one baboon. The depletion or inhibition of host macrophages, therefore, prevented the induction of humoral unresponsiveness by CD154 blockade. These results suggest that CD154 blockade induces humoral unresponsiveness by a mechanism that involves the indirect pathway of antigen presentation. In vitro investigation of baboon anti-pig mixed lymphocyte reaction confirmed that only the indirect pathway is efficiently blocked by anti-CD154 mAb. The mechanism in which blockade of the CD40-CD154 pathway induces its effect remains to be determined, but it could involve the generation of regulatory cells capable of suppressing the direct pathway.

Pig kidney transplantation in baboons: anti-Gal(alpha)1-3Gal IgM alone is associated with acute humoral xenograft rejection and disseminated intravascular coagulation

BACKGROUND

Kidneys harvested from miniature swine or pigs transgenic for human decay-accelerating factor (hDAF) were transplanted into baboons receiving an anti-CD154 monoclonal antibody (mAb) and either a whole body irradiation (WBI)- or cyclophosphamide (CPP)-based immunosuppressive regimen.

METHODS

Group 1 baboons (n=3) underwent induction therapy with WBI and thymic irradiation, pretransplantation antithymocyte globulin, and immunoadsorption of anti-Gal(alpha)1-3Gal (Gal) antibody (Ab). After transplantation of a miniature swine kidney, maintenance therapy comprised cobra venom factor, mycophenolate mofetil, and an anti-CD154 mAb (for 14-28 days). In group 2 (n=2), WBI was replaced by CPP in the induction protocol. Group 3 (n=3) animals received the group 2 regimen, but underwent transplantation with hDAF pig kidneys.

RESULTS

Group 1 and 2 animals developed features of disseminated intravascular coagulation (DIC), with reductions of fibrinogen and platelets and increases of prothrombin time, partial thromboplastin time, and fibrin split products. Graft survival was for 6-13 days. Histology showed mild acute humoral xenograft rejection (AHXR) of the kidneys, but severe rejection of the ureters. Group 3 animals developed features of DIC in two of three cases during the fourth week, with AHXR in the third case. Graft survival was for 28 (n=1) or 29 (n=2) days. Histology of day 15 biopsy specimens showed minimal focal mononuclear cellular infiltrates, with predominantly CD3+ cells. By days 28 and 29, kidneys showed mild-to-moderate features of AHXR. In all groups, the humoral response was manifest by reappearance of anti-Gal IgM below baseline level, with no or low return of anti-Gal IgG. All excised kidneys showed IgM deposition, but no complement and no or minimal IgG deposition. No baboon showed a rebound of anti-Gal Ab immediately after excision of the graft, and anti-Gal Ab increased over pretransplantation levels only when anti-CD154 mAb was discontinued.

CONCLUSIONS

DIC was observed with WBI- or CPP-based therapy, and after miniature swine or hDAF kidney transplantation. AHXR+/-DIC was observed in all recipients even in the absence of complement and no or low levels of anti-Gal IgG, but was significantly delayed in the hDAF recipients. These results confirm our earlier observation that CD154 blockade prevents T cell-dependent sensitization in baboons to pig antigens, but that baseline natural anti-Gal Ab production is not inhibited. We suggest that IgM deposition, even in the absence of IgG and complement, leads to endothelial cell activation with the development of DIC, even when there are only minimal histologic changes of AHXR.

Effects of specific anti-B and/or anti-plasma cell immunotherapy on antibody production in baboons: depletion of CD20- and CD22-positive B cells does not result in significantly decreased production of anti-alphaGal antibody

Anti-Galalpha1-3Gal antibodies (antialphaGal Ab) are a major barrier to clinical xenotransplantation as they are believed to initiate both hyperacute and acute humoral rejection. Extracorporeal immunoadsorption (EIA) with alphaGal oligosaccharide columns temporarily depletes antialphaGal Ab, but their return is ultimately associated with graft destruction. We therefore assessed the ability of two immunotoxins (IT) and two monoclonal antibodies (mAb) to deplete B and/or plasma cells both in vitro and in vivo in baboons, and to observe the rate of return of antialphaGal Ab following EIA. The effects of the mouse anti-human IT anti-CD22-ricin A (proportional to CD22-IT, directed against a B cell determinant) and anti-CD38-ricin A (proportional to CD38-IT, B and plasma cell determinant) and the mouse anti-human anti-CD38 mAb (proportional to CD38 mAb) and mouse/human chimeric anti-human anti-CD20 mAb (proportional to CD20 mAb, Rituximab, B cell determinant) on B and plasma cell depletion and antialphaGal Ab production were assessed both in vitro and in vivo in baboons (n = 9) that had previously undergone splenectomy. For comparison, two baboons received nonmyeloablative whole body irradiation (WBI) (300 cGy), and one received myeloablative WBI (900 cGy). Depletion of B cells was monitored by flow cytometry of blood, bone marrow (BM) and lymph nodes (LN), staining with anti-CD20 and/or anti-CD22 mAbs, and by histology of LN. EIA was carried out after the therapy and antialphaGal Ab levels were measured daily. In vitro proportional to CD22-IT inhibited protein synthesis in the human Daudi B cell line more effectively than proportional to CD38-IT. Upon differentiation of B cells into plasma cells, however, less inhibition of protein synthesis after proportional to CD22-IT treatment was observed. Depleting CD20-positive cells in vitro from a baboon spleen cell population already depleted of granulocytes, monocytes, and T cells led to a relative enrichment of CD20-negative cells, that is plasma cells, and consequently resulted in a significant increase in antialphaGal Ab production by the remaining cells, whereas depleting CD38-positive cells resulted in a significant decrease in antialphaGal Ab production. In vivo, WBI (300 or 900 cGy) resulted in 100% B cell depletion in blood and BM, > 80% depletion in LN, with substantial recovery of B cells after 21 days and only transient reduction in antialphaGal Ab after EIA. Proportional to CD22-IT depleted B cells by > 97% in blood and BM, and by 60% in LN, but a rebound of B cells was observed after 14 and 62 days in LN and blood, respectively. At 7 days, serum antialphaGal IgG and IgM Ab levels were reduced by a maximum of 40-45% followed by a rebound to levels up to 12-fold that of baseline antialphaGal Ab by day 83 in one baboon. The results obtained with proportional to CD38-IT were inconclusive. This may have been, in part, due to inadequate conjugation of the toxin. Cell coating was 100% with proportional to CD38 mAb, but no changes in antialphaGal Ab production were observed. Proportional to CD20 mAb resulted in 100% depletion of B cells in blood and BM, and 80% in LN, with recovery of B cells starting at day 42. Adding 150cGy WBI at this time led to 100% depletion of B cells in the BM and LN. Although B cell depletion in blood and BM persisted for > 3 months, the reduction of serum antialphaGal IgG or IgM Ab levels was not sustained beyond 2 days. Proportional to CD20 mAb + WBI totally and efficiently depleted CD20- and CD22-positive B cells in blood, BM, and LN for > 3 months in vivo, but there was no sustained clinically significant reduction in serum antialphaGal Ab. The majority of antibody secretors are CD38-positive cells, but targeting these cells in vitro or in vivo with proportional to CD38-IT was not very effective. These observations suggest that CD20-and CD22-positive B cells are not the major source of antialphaGal Ab production. Future efforts will be directed towards suppression of plasma cell function.

Mechanisms of thrombotic microangiopathy following xenogeneic hematopoietic progenitor cell transplantation

INTRODUCTION

Attempts to induce tolerance though mixed hematopoietic chimerism in the discordant pig-to-baboon xenotransplantation model are sometimes complicated by a potentially fatal thrombotic microangiopathy in the recipient baboons. This state develops immediately after the infusion of porcine mobilized peripheral blood leukocytes, containing progenitor cells (PBPC). In our study, we examined the interaction of infused porcine PBPC with recipient platelets in vivo in baboons and investigated the underlying mechanisms using an in vitro model.

METHODS

Two naïve baboons and six baboons preconditioned with irradiation and immunosuppression that received porcine PBPC were evaluated in vivo. The interaction of porcine and baboon PBPC with baboon platelets was investigated by an in vitro platelet aggregation assay. Fresh and cryopreserved PBPC were evaluated as well as PBPC obtained from growth-factor mobilized and unmobilized pigs. Furthermore, cellular subsets of PBPC were assessed for potential to induce platelet aggregation. Immunohistochemical staining was performed on platelet-leukocyte aggregates and potential inhibition of aggregation with anti-P-selectin and anti-CD154 mAbs, or eptifibatide (a GPIIb/IIIa receptor antagonist), was tested.

RESULTS

All baboons that received porcine PBPC rapidly developed marked thrombocytopenia (<20,000/microl), elevated serum lactate dehydrogenase (>1,500U/liter), schistocytosis, and platelet aggregates on blood smear. Three baboons died (two untreated and one preconditioned), and substantive platelet aggregates containing porcine leukocytes were observed in the microvasculature of lungs and kidneys. In vitro, porcine, but not baboon, PBPC induced aggregation of baboon platelets in a dose-dependent manner. Immunohistological examination of these aggregates confirmed the incorporation of porcine leukocytes. Cryopreserved PBPC caused less aggregation than fresh PBPC, and growth-factor-mobilized PBPC induced less aggregation than unmobilized PBPC. Aggregation was fully abrogated by the addition of eptifibatide, and modulated by anti-P-selectin and anti-CD154 monoclonal antibodies that recognize adhesion receptors on activated platelets. Purified fractions (granulocytes, CD2+, and CD- cells) of porcine PBPC did not initiate aggregation, whereas addition of exogenous porcine PBPC membranes (erythrocytes, dead cells, and/or platelets) to the purified fractions exacerbated the aggregation response.

CONCLUSIONS

These data indicate that porcine PBPC mediate aggregation of baboon platelets. This process likely contributes to the thrombotic microangiopathy observed after PBPC transplantation in the pig-to-baboon model. Eptifibatide can fully abrogate platelet aggregation induced by porcine PBPC in vitro. Purification of the progenitor cells from porcine PBPC and/or treatment of baboons with eptifibatide may be beneficial.

Porcine hematopoietic cell xenotransplantation in nonhuman primates is complicated by thrombotic microangiopathy

Thrombotic microangiopathy (TM) is a serious complication of bone marrow transplantation (BMT) that resembles thrombotic thrombocytopenic purpura (TTP). In attempting to achieve hematopoietic cell chimerism in the pig-to-baboon model, we have observed TM following infusion of high doses (>10(10) cells/kg) of porcine peripheral blood mobilized progenitor cells (PBPC) into baboons. We performed investigations to analyze the pathobiology of this TM and to test therapeutic interventions to ameliorate it. PBPC were obtained by leukapheresis of cytokine-stimulated swine. The initial observations were made in two baboons that underwent a non-myeloablative regimen (NMR) prior to PBPC transplantation (TX) (group 1). We then studied three experimental groups. Group 2 (n = 2) received NMR without PBPC TX. Group 3 (n = 2) received PBPC TX alone. Group 4 (n = 6) received NMR + PBPC TX combined with prostacyclin, low-dose heparin, methylprednisolone, and cyclosporine was replaced by anti-CD40L mAb in five cases. Baboons in groups 1 and 3 developed severe thrombocytopenia (<10,000/mm3), intravascular hemolysis with schistocytosis (>10/high powered field (hpf)), increase in plasma lactate dehydrogenase (LDH) (2500-9000 U/l), transient neurologic changes, renal insufficiency, and purpura. Autopsy on two baboons confirmed extensive platelet thrombi in the microcirculation, and, similar to clinical BMT-associated TM/TTP, no unusually large vWF multimers or changes in vWF protease activity were observed in the plasma of baboons with TM. In group 2, self-limited thrombocytopenia occurred for 10-15 days following NMR. Group 4 baboons developed thrombocytopenia (<20,000/mm3) rarely requiring platelet transfusion, minimal schistocytosis (<3/hpf), minor increase in LDH (<1000 U/l), with no clinical sequelae. We conclude that high-dose porcine PBPC infusion into baboons induces a microangiopathic state with vWF biochemical parameters resembling clinical BMT-associated TM/TTP and that administration of antithrombotic and anti-inflammatory agents can ameliorate this complication.

Mixed chimerism and transplantation tolerance

Achieving transplantation tolerance is an important goal in the effort to reduce long-term morbidity and mortality in organ transplant recipients. Robust, lifelong, donor-specific tolerance can be reliably achieved by induction of mixed chimerism in various animal models. To date, the clinical application of these proto-cols has been impeded partly by the potential toxicity of the required host conditioning regimens and the lack of successful studies in large animals. This article reviews the progress achieved in recent years in developing considerably milder conditioning protocols in rodents, and in extending some of these models to achieve permanent mixed chimerism and tolerance in large animals. Advances in the induction of xenogeneic tolerance through mixed chimerism are also discussed.

Antigen expression in xenotransplantation: how low must it go?

BACKGROUND

Acute vascular rejection (AVR) is an important immunological barrier to xenotransplantation. Thought to be initiated by xenoreactive antibodies, acute vascular rejection might, in principle, be avoided by engineering animals to express low levels of antigen. The extent to which antigen expression would have to be decreased to achieve such a goal is unknown.

METHODS

We estimated the decrease in expression of a xenogeneic antigen, Galalpha1-3Gal, which might be needed to avert acute vascular rejection of xenotransplants based on the decrease in antibody binding to endothelium that would prevent tissue damage.

RESULTS

The level of decrease needed in Galalpha1-3Gal expression needed to avoid acute vascular rejection was estimated to exceed 96% of baseline. The extent of the decrease needed reflected, in part, a substantial "excess" of Galalpha1-3Gal on porcine endothelial cell surfaces.

CONCLUSIONS

Although the change in antigen expression required to avoid acute vascular rejection might be conditioned by various factors, the very large magnitude of this change necessitates application of highly efficient approaches to antigen modification.

The role of anti-Galalpha1-3Gal antibodies in acute vascular rejection and accommodation of xenografts

BACKGROUND

A major impediment to the transplanting of porcine organs into humans is the susceptibility of porcine organs to acute vascular rejection, which can destroy a vascularized xenograft over a period of hours to days. Acute vascular rejection of porcine-to-primate xenografts is thought to be triggered by binding of xenoreactive antibodies to the graft. We tested whether antibodies, binding to Galalpha1-3Gal epitopes in porcine tissue, initiate this phenomenon.

METHODS AND RESULTS

Specific depletion of anti-Galalpha1-3Gal antibodies from the blood of baboons, using extracorporeal perfusion of separated plasma through columns of Sepharose beads covalently linked to the antigenic trisaccharide, Galalpha1-3Galbeta1-4GlcAc, averted the development of acute vascular rejection in porcine organs transgenic for human decay-accelerating factor and CD59. More importantly, after immunodepletion was stopped and Gala1-3Gal antibodies were allowed to return, these same organs continued to function and remained pathologically normal and thus seemed to achieve a state of accommodation.

CONCLUSION

These results demonstrate that anti-Galalpha1-3Gal antibodies cause acute vascular rejection and suggest that depletion of these antibodies leads to accommodation of the donor cardiac xenograft and could supply an important model for additional study.

Prospects for xenotransplantation

Pig-to-primate organ survival has been extended from a few minutes to weeks and occasionally months, following the development of transgenic pigs that express human complement-regulatory proteins, efficient antibody removal technologies and immunosuppressive strategies. The current limitation to the clinical application of this technology is acute vascular rejection, and an understanding of the mechanisms of this process and the development of modalities to overcome it are key to making significant progress at solving the critical shortage of organs for transplantation. Approaches that address this issue are underway in a number of laboratories.

Plasma perfusion by apheresis through a Gal immunoaffinity column successfully depletes anti-Gal antibody: experience with 320 aphereses in baboons

BACKGROUND

Anti-Galalpha1-3Gal (Gal) antibodies (Gal Ab) contribute to the rejection of porcine organs transplanted into primates. Extracorporeal immunoadsorption (EIA) has been developed to eliminate Gal Ab from the circulation.

METHODS

Between 1995 and 1999 we performed 320 EIAs in baboons using a COBE-Spectra apheresis unit incorporating a synthetic Gal immunoaffinity column. Three plasma volumes were immunoadsorbed on each occasion. The 221 consecutive EIAs performed in 41 immunosuppressed baboons between January 1997 and April 1999 form the basis of this review. Of these 41 baboons, 29 underwent a series of three or four EIAs at daily intervals, seven had multiple series of three EIAs, and the remainder underwent single or double EIAs. Serum Gal Ab levels were monitored by ELISA before and at intervals after the course of EIA.

RESULTS

There were two fatal complications, one from a respiratory mishap (unrelated to the EIA) and one from persistent hypotension unresponsive to therapeutic interventions. Seven procedures (3%) were terminated early owing to technical difficulties and/or persistent hypotension. Mean pre-EIA Gal Ab levels in naive baboons were 33.1 microg/ml (IgM) and 14.5 microg/ml (IgG). Immediately after three consecutive EIAs, IgM was depleted by a mean of 97.3% and IgG by 99.4%. By 18 to 24 h later, Gal Ab was returning but depletion remained at 80.1% (IgM) and 84.7% (IgG). The subsequent rate of return of Gal Ab depended on the immunomodulatory protocol used.

CONCLUSIONS

(1) With appropriate monitoring, EIA is an acceptably safe procedure, even in small (<10 kg) baboons. (2) Three consecutive EIAs are effective in removing >97% of Gal Ab. (3) In the majority of cases, return of Gal Ab begins within 24 h, irrespective of the immunomodulatory protocol.

High-dose porcine hematopoietic cell transplantation combined with CD40 ligand blockade in baboons prevents an induced anti-pig humoral response

BACKGROUND

In pig-to-primate organ transplantation, hyperacute rejection can be prevented, but the organ is rejected within days by acute vascular rejection, in which induced high-affinity anti-Gal alpha1-3Gal (alphaGal) IgG and possibly antibodies directed against new porcine (non-alphaGal) antigenic determinants are considered to play a major role. We have explored the role of an anti-CD40L monoclonal antibody in modifying the humoral response to porcine hematopoietic cells in baboons pretreated with a nonmyeloablative regimen.

METHODS

Porcine peripheral blood mobilized progenitor cells obtained by leukapheresis from both major histocompatibility complex-inbred miniature swine (n=7) and human decay-accelerating factor pigs (n=3) were transplanted into baboons. Group 1 baboons (n=3) underwent whole body (300 cGy) and thymic (700 cGy) irradiation, T cell depletion with ATG, complement depletion with cobra venom factor, short courses of cyclosporine, mycophenolate mofetil, porcine hematopoietic growth factors, and anti-alphaGal antibody depletion by immunoadsorption before transplantation of high doses (2-4 x 10(10)/cells/kg) of peripheral blood mobilized progenitor cells. In group 2 (n=5), cyclosporine was replaced by eight doses of anti-CD40L monoclonal antibodies over 14 days. The group 3 baboons (n=2) received the group 1 regimen plus 2 doses of anti-CD40L monoclonal antibodies (on days 0 and 2).

RESULTS

In group 1, sensitization to alphaGal (with increases in IgM and IgG of 3- to 6-fold and 100-fold, respectively) and the development of antibodies to new non-alphaGal porcine antigens occurred within 20 days. In group 2, no sensitization to alphaGal or non-alphaGal determinants was seen, but alphaGal-reactive antibodies did return to their pre- peripheral blood mobilized progenitor cells transplant levels. In group 3, attenuated sensitization to alphaGal antigens was seen after cessation of cyclosporine and mycophenolate mofetil therapy at 30 days (IgM 4-fold, IgG 8-30-fold), but no antibodies developed against new porcine determinants. In no baboon did anti-CD40L monoclonal antibodies prevent sensitization to its own murine antigens.

CONCLUSIONS

We believe these studies are the first to consistently demonstrate prevention of a secondary humoral response after cell or organ transplantation in a pig-to-primate model. The development of sensitization to the murine elements of the anti-CD40L monoclonal antibodies suggests that nonresponsiveness to cell membrane-bound antigen (e.g., alphaGal) is a specific phenomenon and not a general manifestation of immunological unresponsiveness. T cell costimulatory blockade may facilitate induction of mixed hematopoietic chimerism and, consequently, of tolerance to pig organs and tissues.