Life-supporting pig-to-primate renal xenotransplantation using genetically modified donors

Low molecular weight dextran sulfate prevents complement activation and delays hyperacute rejection in pig-to-human xenotransplantation models

Dextran sulfate of 5000 molecular weight (DXS 5000) is known to block complement activation as well as the intrinsic coagulation cascade by potentiation of C inhibitor. The effect of DXS 5000 on hyperacute rejection (HAR) was tested in pig-to-human xenotransplantation models. For in vitro testing, a cytotoxicity assay was used with the pig kidney cell line PK15 as target cells and fresh, undiluted human serum as antibody and complement source. Ex vivo pig lung perfusion was chosen to assess DXS 5000 in a physiologic model. Pig lungs were perfused with fresh, citrate-anticoagulated whole human blood to which 1 or 2 mg/ml DXS 5000 were added; the lungs were ventilated and the blood de-oxygenated. Pulmonary vascular resistance (PVR) and blood oxygenation (deltapO2) were monitored throughout the experiment. Autologous pig blood and human blood without DXS 5000 served as controls. In the PK 15 assay DXS 5000 led to a complete, dose-dependent inhibition of human serum cytotoxicity with an average IC50 of 43 +/- 18 microg/ml (n=8). Pig lungs perfused with untreated human blood (n=2) underwent HAR within 105 +/- 64 min, characterized by increased PVR, decrease of deltapO2, and generalized edema. Microscopically, capillary bleeding as well as deposition of human antibodies, complement and fibrin could be observed. Addition of DXS 5000 (n=4) prolonged lung survival to 170 +/- 14 min for 1 mg/ml and 250 +/- 42 min for 2 mg/ml. and PVR values as well as edema formation were comparable to control lungs that were perfused with autologous pig blood (n=2). Activation of complement (activation products in serum, deposition on lung tissue) and the coagulation system (fibrin monomers) were significantly diminished as compared to human blood without DXS 5000. Binding of anti-Gal antibodies was not influenced, and in vitro experiments showed no evidence of complement depletion by DXS 5000. In conclusion, DXS 5000 is an efficient complement inhibitor in pig-to-human xenotransplantation models and therefore a candidate for complement-inhibitory/anti-inflammatory therapy either alone or in combination with other substances and warrants further investigation.

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.

A human CD46 transgenic pig model system for the study of discordant xenotransplantation

BACKGROUND

The chronic shortage in the supply of human organs available for allotransplantation has turned attention toward the use of animals as potential donors, with pigs as the most likely species under consideration. Hyperacute rejection, the initial and immediate barrier to a pig-to-primate xenograft, has been addressed by generation of transgenic pigs that express the human membrane-bound complement-regulatory proteins CD59 and/or CD55. Difficulty has been encountered in generation of transgenic animals that express a third membrane-bound complement-regulatory protein, CD46.

METHODS

We have generated transgenic animals by using a large genomic construct that encompasses the entire human CD46 gene.

RESULTS

We report the first description of transgenic mice and pigs that express high levels of human CD46 in a cell and tissue type-specific manner, resembling patterns of endogenous CD46 expression observed in human tissues. Furthermore, when human CD46 transgenic porcine hearts were transplanted into baboons, the grafts did not succumb to hyperacute rejection, and survival extended for up to 23 days. Under the same conditions, nontransgenic grafts underwent hyperacute rejection within 90 min.

CONCLUSIONS

This is the first report to describe generation of transgenic pigs that express human CD46, and the first in vivo demonstration of the ability of human CD46 expressed on pig organs to regulate complement activation and overcome hyperacute rejection upon transplantation of a vascularized organ into nonhuman primates.

Physiologic and immunologic hurdles to xenotransplantation

The major problem in the field of renal transplantation is currently the shortage of available kidneys. However, the use of animals as a source of kidneys, i.e., xenotransplantation, is increasingly being viewed as a potential solution to this problem. One preeminent hurdle to xenotransplantation is the immune response of the recipient against the graft; other hurdles include the physiologic limitations of the transplant, infection, and ethical considerations. This review summarizes what is currently known regarding the obstacles to xenotransplantation and some potential solutions to those problems.

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.

Report of the Xenotransplantation Advisory Committee of the International Society for Heart and Lung Transplantation: the present status of xenotransplantation and its potential role in the treatment of end-stage cardiac and pulmonary diseases

An urgent and steadily increasing need exists world-wide for a greater supply of donor thoracic organs. Xenotransplantation offers the possibility of an unlimited supply of hearts and lungs that could be available electively when required. However, anti-body- mediated mechanisms cause the rejection of pig organs transplanted into non-human primates, and these mechanisms provide major immunologic barriers that have not yet been overcome. Having reviewed the literature on xenotransplantation, we present a number of conclusions on its present status with regard to thoracic organs, and we make a number of recommendations relating to eventual clinical trials. Although pig hearts have functioned in heterotopic sites in non-human primates for periods of several weeks, median survival of orthotopically transplanted hearts is currently ,1 month. No transplanted pig lung has functioned for even 24 hours. Current experimental results indicate that a clinical trial would be premature. A potential risk exists, hitherto undetermined, of transferring infectious organisms along with the donor pig organ to the recipient, and possibly to other members of the community. A clinical trial of xeno-transplantation should not be undertaken until experts in microbiology and the relevant regulatory authorities consider this risk to be minimal. A clinical trial should be considered when approximately 60% survival of life-supporting pig organs in non-human primates has been achieved for a minimum of 3 months, with at least 10 animals surviving for this minimum period. Furthermore, evidence should suggest that longer survival (.6 months) can be achieved. These results should be achieved in the absence of life-threatening complications caused by the immunosuppressive regimen used. The relationship between the presence of anti-HLA antibody and anti-pig antibody and their cross-reactivity, and the outcome of pig-organ xenotransplantation in recipients previously sensitized to HLA antigens require further investigation. We recommend that the patients who initially enter into a clinical trial of cardiac xenotransplantation be unacceptable for allotransplantation, or acceptable for allotransplantation but unlikely to survive until a human cadaveric organ becomes available, and in whom mechanical assist-device bridging is not possible. National bodies that have wide-reaching government-backed control over all aspects of the trials should regulate the initial clinical trial and all subsequent clinical xenotransplantation procedures for the foreseeable future. We recommend coordination and monitoring of these trials through an international body, such as the International Society for Heart and Lung Transplantation, and setting up a registry to record and widely disperse the results of these trials. Xenotransplantation has the potential to solve the problem of donor-organ supply, and therefore research in this field should be actively encouraged and supported.

Coagulation and thrombotic disorders associated with pig organ and hematopoietic cell transplantation in nonhuman primates

BACKGROUND

Efforts to achieve tolerance to transplanted pig organs in nonhuman primates by the induction of a state of mixed hematopoietic chimerism have been associated with disorders of coagulation and thrombosis. Activation of recipient vascular endothelium and platelets by porcine hematopoietic cells and/or activation of donor organ vascular endothelium and/or molecular differences between the species may play roles. Irradiation or drug therapy could possibly potentiate endothelial cell activation and/or injury.

METHODS

We have investigated parameters of coagulation and platelet activation in nonhuman primates after (1) a regimen aimed at inducing mixed hematopoietic chimerism and tolerance (TIR that included total body irradiation, T cell depletion, and splenectomy; (2) pig bone marrow or pig peripheral blood mobilized progenitor cell transplantation (PCTx); and/or (3) pig organ transplantation (POTx). Five experimental groups were studied. Baboons were the recipient subjects in all groups except Group 1. Gp 1 Cynomolgus monkeys (n=6) underwent TIR + allotransplantation of hematopoietic cells and a kidney or heart or TIR + concordant xenotransplantation (using baboons as donors) of cells and a kidney; Gp 2 Baboons (n=4) underwent TIR with or without (+/-) autologous hematopoietic cell infusion; Gp 3 (n=12) PCTx+/-TIR; Gp 4 (n=5) POTx+/-TIR; Gp 5 (n=4) TIR + PCTx + POTx. Platelet counts, with plasma prothrombin time, partial thromboplastin time, fibrinogen levels, fibrin split products and/or D-dimer were measured.

RESULTS

In the absence of a discordant (porcine) cellular or organ transplant (Groups 1 and 2), TIR resulted in transient thrombocytopenia only, in keeping with bone marrow depression from irradiation. PCTx alone (Group 3) was associated with the rapid development of a thrombotic thrombocytopenic (TTP)-like microangiopathic state, that persisted longer when PCTx was combined with TIR. POTx (+/-TIR) (Group 4) was associated with a gradual fall (over several days) in platelet counts and fibrinogen with disseminated intravascular coagulation (DIC); after graft excision, the DIC generally resolved. When TIR, PCTx and POTx were combined (Group 5), an initial TTP-like state was superseded by a consumptive picture of DIC within the first week, necessitating graft removal.

CONCLUSIONS

Both PCTx and POTx lead to profound alterations in hemostasis and coagulation parameters that must be overcome if discordant xenotransplantation of hematopoietic cells and organs is to be fully successful. Disordered thromboregulation could exacerbate vascular damage and potentiate activation of coagulation pathways after exposure to xenogeneic cells or a vascularized xenograft.

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

BACKGROUND

It is not known whether the pig liver is capable of functioning efficiently when transplanted into a primate, neither is there experience in transplanting a liver from a transgenic pigs expressing the human complement regulator human complement regulator decay accelerating factor (h-DAF) into a baboon. The objective of this study was to determine whether the porcine liver would support the metabolic functions of non-human primates and to establish the effect of hDAF expression in the prevention of hyperacute rejection of porcine livers transplanted into primates.

METHODS

Five orthotopic liver xenotransplants from pig to baboon were carried out: three from unmodified pigs and two using livers from h-DAF transgenic pigs.

FINDINGS

The three control animals transplanted with livers from unmodified pigs survived for less than 12 hr. Baboons transplanted with livers from h-DAF transgenic pigs survived for 4 and 8 days. Hyperacute rejection was not detected in the baboons transplanted with hDAF transgenic pig livers; however, it was demonstrated in the three transplants from unmodified pigs. Baboons transplanted with livers from h-DAF transgenic pigs were extubated at postoperative day 1 and were awake and able to eat and drink. In the recipients of hDAF transgenic pig livers the clotting parameters reached nearly normal levels at day 2 after transplantation and remained normal up to the end of the experiments. In these hDAF liver recipients, porcine fibrinogen was first detected in the baboon plasma 2 hr postreperfusion, and was present up to the end of the experiments. One animal was euthanized at day 8 after development of sepsis and coagulopathy, the other animal arrested at day 4, after an episode of vomiting and aspiration. The postmortem examination of the hDAF transgenic liver xenografts did not demonstrate rejection.

INTERPRETATION

The livers from h-DAF transgenic pigs did not undergo hyperacute rejection after orthotopic xenotransplantation in baboons. When HAR is abrogated, the porcine liver maintains sufficient coagulation and protein levels in the baboon up to 8 days after OLT.

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.

Animal models in xenotransplantation

The severe shortage of donor organs has provided a strong impetus to push the investigation into the use of animal organs for humans. Xenotransplantation will not only benefit patients, but also represents a unique and potentially profitable business opportunity. However, there are many barriers to successful clinical xenotransplantation, including immunological barriers, physiological incompatibility, zoonosis and ethical concerns. This overview will focus on currently available animal models used in attempts to break through the immunological barriers to xenotransplantation. There are many advantages to using small animal, namely rodent, models in xenotransplantation research. For example, the use of the mouse model allows the use of knockout mice and careful dissection of rejection mechanisms at the molecular level. The following models can be used to study hyperacute rejection (HAR): guinea-pig-to-rat, mouse-to-rabbit, guinea-pig-to-mouse, rat-to-presensitised mouse and rat-to-alpha-Gal knockout mouse. The hamster-to-rat, mouse-to-rat and rat-to-mouse models are commonly used to study acute vascular rejection. Large animal models are complex and expensive, but they are more relevant to clinical xenotransplantation. Based on experiments using transgenic pig-to-primate models, HAR can be overcome. However, acute vascular rejection remains a major barrier at the present time. A pig cartilage-to-monkey model has been developed to study chronic rejection. Other novel models such as pig venous segment-to-monkey model and rat-to-primate model may represent viable options to study immunological barriers following xenotransplantation. Like many other medical breakthroughs, animal research will continue to make enormous contributions towards the eventual success of xenotransplantation.

Ontogeny of antipig xenoantibody and hyperacute rejection

BACKGROUND

Neonatal primates have been reported to receive pig hearts without hyperacute rejection (HAR). We examined the ontogeny of the anti-pig xenoantibody (XenoAb) and HAR in the neonatal and infant monkeys.

METHODS

Twenty-six serum samples from 15 monkeys ages 14-192 days were subjected to hemagglutination titration against pig erythrocytes. Ten pig hearts were heterotopically transplanted into the monkeys.

RESULTS

Six monkeys, ages 52-114 days, received pig hearts without HAR, and those ages 129-191 days hyperacutely rejected them. XenoAb titers were increased according to the age (Spearman's rank correlation value=0.909 (P<0.01)). XenoAb titers in 16 monkeys <4 months were significantly (P<0.01) lower than those in 10 monkeys >4 months.

CONCLUSIONS

Anti-pig XenoAb titers increased with the age of the monkeys. XenoAb levels in monkeys >4 months are high enough to reject pig hearts hyperacutely.

Costimulatory blockade by the induction of an endogenous xenospecific antibody response

Xenogeneic tissues induce vigorous T cell immunity, reflecting the ability of costimulatory molecules to function across species barriers. We describe a strategy to inhibit costimulation that exploits species differences using the model of porcine pancreatic islet transplantation into mice. Mice were immunized with chimeric peptides that contained a known T cell epitope and selected sequences of the porcine costimulatory molecule CD86. This resulted in anti-peptide antibody responses that recognized intact porcine CD86, blocked costimulation by porcine CD86 but not murine CD86 in vitro, and prolonged the survival of porcine islet grafts in vivo. This strategy of inducing endogenous donor-specific costimulatory blockade has potential clinical applicability.

Acute vascular rejection is associated with systemic complement activation in a pig-to-primate kidney xenograft model

The introduction of h-DAF transgenic porcine organs into pre-clinical pig-to-primate discordant xenotransplantation has led to complete and reliable abrogation of hyperacute xenograft rejection (HAR). Despite additional heavy immunosuppression however, most xenografts are still lost due to acute vascular rejection (AVR), with current treatment protocols being of only limited value. In a life-supporting model of pig-to-primate kidney transplantation, unmodified (n=8) or h-DAF-transgenic (n=9) porcine kidneys were transplanted into cynomolgus monkeys under cyclophosphamide (CyP), cyclosporine and low-dose steroid immunosuppression. Longest recipient survival was 11 days in the control group and 68 days in the h-DAF transgenic group. Stable initial graft function with recipient survival >4 days was generated in eight animals (two controls and six transgenics). In these animals, plasma complement levels were analyzed during ongoing AVR. Compared with baseline levels, a two-fold increase in C3a levels and a four-fold increase in sC5b-9 levels were measured. In parallel to systemic complement activation, increased deposition of C3 and C5b-9 along with massive staining for recipient IgM immunoglobulins was detected in the xenografts on immunohistochemistry. We conclude that acute vascular xenograft rejection of porcine kidneys in cynomolgus monkeys is associated with classical pathway complement activation following binding of induced recipient anti-porcine antibodies. This complement activation can be observed despite membrane bound expression of human complement regulators in the porcine xenografts. Therefore, additional short-term fluid phase complement inhibition seems necessary for the future development of protocols designed for treatment of AVR in the pig-to-primate combination.

Disordered regulation of coagulation and platelet activation in xenotransplantation

Rejection of xenografts is associated with vascular-based inflammation, thrombocytopenia and the consumption of coagulation factors that may evolve into disseminated intravascular coagulation (DIC). Similarly, bone marrow-derived cellular xenotransplantation procedures are associated with endothelial cell activation and thrombotic microangiopathic injury. These complications generally develop despite the best available measures for depletion of xenoreactive natural antibody, inhibition of complement activation and suppression of T- and B-cell mediated immune responses. The mechanisms underlying the DIC and thrombotic microangiopathy associated with xenotransplantation are unclear. A proposed primary biological dysfunction of xenografts with respect to regulation of clotting could amplify vascular injury, promote immunological responses and independently contribute to graft failure. Disordered thromboregulation could have deleterious effects, comparable to unregulated complement activation, in the pathogenesis of xenograft rejection and may therefore represent a substantive barrier to xenotransplantation.

Cyclosporine inhibits class II major histocompatibility antigen presentation by xenogeneic endothelial cells to human T lymphocytes by altering expression of the class II transcriptional activator gene

BACKGROUND

Cyclosporine (CsA) is currently given to recipients of vascularized xenografts as part of the immunosuppressive regimen required to prevent the hyperacute rejection phase. The effects of CsA on non-lymphoid immune cells, such as endothelial cells (ECs), have not been well characterized and sometimes seem contradictory, because both protective and adverse effects have been reported. In the present study, we investigated in vitro whether CsA could alter the antigenicity of activated porcine aortic endothelial cells (PAECs) by reducing class I and class II MHC antigen expression.

METHODS

The effect of CsA on MHC antigen expression during tumor necrosis factor (TNF)-alpha- or lymphocyte-mediated PAEC activation was evaluated in vitro by flow cytometry and correlated to the ability of porcine ECs to promote human T lymphocyte proliferation. The effect of CsA on class II MHC antigen mRNA expression was also analyzed and related to class II transcriptional activator (CIITA) mRNA expression.

RESULTS

Flow cytometry analysis showed that TNF-alpha-mediated induction of class II MHC antigen expression on PAECs was completely inhibited by CsA, whereas expression of class I MHC was reduced by 50%. The inhibition was dose dependent (at drug concentrations ranging from 2.5 microg/ml to 20.0 microg/ml) and was consistently observed at all time points (24-72 hr) during the activation period. Decreased MHC antigen expression dramatically reduced the ability of PAECs to further promote human T-cell proliferation. Similar levels of inhibition were achieved using an anti-porcine class II MHC blocking monoclonal antibody. Pretreatment of PAECs with CsA for 4 hr before coculture with human peripheral blood leukocytes efficiently blocked the induction on PAECs of E-selectin and class II MHC antigens and inhibited overexpression of class I antigens. Semiquantitative reverse transcriptase-polymerase chain reaction experiments showed that CsA markedly reduced the steady-state level of porcine class II (SLA-DRA and SLA-DQA) mRNA at 16 hr, compared with PAECs stimulated with TNF-alpha alone. The reduced level of class II MHC mRNA was associated with a lack of CIITA expression at this time point, suggesting that CsA could alter transcription or promote the rapid decay of CIITA mRNA.

CONCLUSION

Our study indicates that CsA could play a role in preventing porcine MHC antigens being directly presented to human T lymphocytes by xenogeneic ECs.

Renal xenografts from triple-transgenic pigs are not hyperacutely rejected but cause coagulopathy in non-immunosuppressed baboons

BACKGROUND

The genetic modification of pigs is a powerful strategy that may ultimately enable successful xenotransplantation of porcine organs into humans.

METHODS

Transgenic pigs were produced by microinjection of gene constructs for human complement regulatory proteins CD55 and CD59 and the enzyme alpha1,2-fucosyltransferase (H-transferase, HT), which reduces expression of the major xenoepitope galactose-alpha1,3-galactose (alphaGal). Kidneys from CD55/HT and CD55/CD59/HT transgenic pigs were transplanted into nephrectomised, nonimmunosuppressed adult baboons.

RESULTS

In several lines of transgenic pigs, CD55 and CD59 were expressed strongly in all tissues examined, whereas HT expression was relatively weak and did not significantly reduce alphaGal. Control nontransgenic kidneys (n=4) grafted into baboons were hyperacutely rejected within 1 hr. In contrast, kidneys from CD55/HT pigs (n=2) were rejected after 30 hr, although kidneys from CD55/CD59/HT pigs (n=6) maintained function for up to 5 days. In the latter grafts, infiltration by macrophages, T cells, and B cells was observed at days 3 and 5 posttransplantation. The recipients developed thrombocytopenia and abnormalities in coagulation, manifested in increased clotting times and an elevation in the plasma level of the fibrin degradation product D-dimer, within 2 days of transplantation. Treatment with low molecular weight heparin prevented profound thrombocytopenia but not the other aspects of coagulopathy.

CONCLUSIONS

Strong expression of CD55 and CD59 completely protected porcine kidneys from hyperacute rejection and allowed a detailed analysis of xenograft rejection in the absence of immunosuppression. Coagulopathy appears to be a common feature of pig-to-baboon renal transplantation and represents yet another major barrier to its clinical application.

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.

Anti-galactose-alpha(1,3) galactose antibody production in alpha1,3-galactosyltransferase gene knockout mice after xeno and allo transplantation

Antibodies (Abs) that mediate the hyperacute rejection and acute vascular rejection/delayed xenograft rejection of pig organs in humans and Old World primates are predominantly directed at a single carbohydrate epitope, galactose-alpha1,3-galactose (alpha1,3Gal). The T-cell dependence of elicited anti-alpha1,3Gal Ab responses in humans and Old World primates is controversial. In this study we have characterized anti-alpha1,3Gal Ab production in mice with disrupted alpha1,3-galactosyltransferase genes (GT-Ko mice) and determined the T-cell dependence of anti-alpha1,3Gal Ab responses, following xenograft and allograft transplantation. GT-Ko mice produce natural anti-alpha1,3Gal IgM and IgG in an age-dependent manner, however, these Abs could not elicit hyperacute rejection nor affect the rate of cardiac xenograft (3-5 days) or allograft rejection (7-9 days). Transplantation of xenogeneic Lewis rats hearts elicited modest anti-alpha1,3Gal Ab, but vigorous xenoAb responses. The anti-alpha1,3Gal Ab response was restricted to the IgM and IgG3 subclass while the xenoAb response comprised IgM and all four IgG subclasses. Transplantation of allogeneic C3H hearts elicited weak anti-alpha1,3Gal Ab responses that were primarily IgM, but vigorous alloAb responses. Despite the restriction of elicited anti-alpha 1,3Gal Ab responses to the IgM and IgG3 isotypes, these responses are T-cell dependent. The ability of allografts to elicit weak anti-alpha1,3Gal but strong allo-Ab responses, can be explained by the dependence of alpha1,3Gal-specific B cells on cognate help from T cells.

Growth of porcine kidneys in their native and xenograft environment

The increased survival of hDAF pig-to-primate renal xenografts for up to two months has afforded the opportunity to study physiological aspects such as organ growth. Experimental evidence exists of species restriction of the activity of growth hormone, although growth itself is also controlled by a number of other endocrine, paracrine and autocrine substances. This study consisted of four parts: (1) measurement of pig kidney size according to pig body weight; (2) measurement of pig kidney size according to pig age; (3) serial length measurement of pig-to-primate renal xenografts; (4) correlation of terminal weight of renal xenograft with age and histology. The xenografted pig kidneys in a primate recipient grow as they would in the pig for the first two weeks after transplantation. After this time there is a reduction in the rate of increase in the length of the xenograft. Over the same period, changes in weight are greatly increased by the presence of rejection. This observational study supports the notion that regulation of growth of a xenotransplanted porcine kidney occurs.

Comparative study of theefficacy of removal of anti-ABO and anti-gal antibodies by double filtration plasmapheresis

Successful clinical ABO-incompatible renal transplantation has been achieved by the removal of anti-A or anti-B antibodies using double filtration plasmapheresis (DFPP). We have compared changes in the levels of anti-donor antibodies and the histopathology of the renal grafts following human ABO-incompatible allotransplantation and pig-to-baboon xenotransplantation using pretransplant DFPP. DFPP was performed on days 6, -4, -2 and -1 before the ABO-incompatible transplants (n=25) and on days -2 and 0 (immediately before reperfusion) in the xenotransplants (n=4). In two baboons (XenoTx Group I) the extent of antibody removal was comparable to that in the ABO-incompatible patients, and an even greater level of removal was achieved in another two baboons (XenoTx Group II). Anti-A and anti-B and anti-pig IgM and IgG antibodies were measured by flow cytometry. All clinical ABO-incompatible renal grafts are functioning, except two which were lost from recurrence of the original disease or from chronic rejection. Three other grafts underwent humoral rejection episodes, which were successfully treated. DFPP reduced the mean anti-A/B IgM and IgG antibody levels to 8% and 13% of pretreatment levels, respectively. After kidney transplantation, they were maintained at 37% and 46% of pre-DFPP level. No antibody binding to the transplanted kidney was detected at any time (1 h to 2 yr) after ABO-incompatible allotransplantation. In contrast, in XenoTx Group I, the same extent of antibody removal (90%) prevented hyperacute rejection, but the two grafts were rejected on days 6 and 7, respectively, from acute vascular rejection. In XenoTx Group II, the additional DFPP that was required to deplete the remaining 10% of anti-pig antibody was poorly tolerated and the two baboons died 4 h and 2 days, respectively, after renal transplantation. Although anti-pig IgM antibodies were reduced to 2% of pre-treatment level, IgM and C3 binding were detected in the graft as early as 1 h posttransplantation. These data suggest that the concentration of xeno-antigen epitopes expressed on pig organs may need to be reduced by genetic engineering to the much lower level of blood group A/B antigens on human kidneys if discordant xenotransplantation is to be successful.

Pharmacotherapeutic agents in xenotransplantation

The ability to transplant pig organs into humans would resolve the current crisis in the supply of cadaveric human organs for the treatment of end stage disease. Several immunologic barriers need to be overcome if pig-to-primate transplantation is to be successful. The presence of preformed antibodies in humans, apes and Old World monkeys directed against galactose epitopes on pig vascular endothelium provides the major barrier, as binding of antibody to antigen leads to graft destruction by complement activation and other mechanisms. Hyperacute rejection can result from the action of complement. If this is prevented, delayed antibody-mediated rejection develops, which can be associated with a state of consumptive coagulopathy (disseminated intravascular coagulation, DIC). Efforts being made to overcome antibody-mediated rejection include depletion of antibody by extracorporeal immunoadsorption, prevention of an induced antibody response by co-stimulatory blockade, B-cell and/or plasma cell depletion, depletion or inhibition of complement, or the use of organs from pigs transgenic for a human complement regulatory protein, such as hDAF. The ultimate solution would be the induction of both B- and T-cell tolerance to the transplanted pig organ, which is being explored by attempting to induce haematopoietic cell chimerism. One complication of this is a thrombotic microangiopathy, similar to thrombotic thrombocytopenic purpura. The many and diverse roles in which pharmacotherapy is involved in attempts to overcome the barriers of xenotransplantation are reviewed and current progress, particularly in our own laboratory, is discussed.

The axis of interleukin 12 and gamma interferon regulates acute vascular xenogeneic rejection

Recent advances using transgenic animals or exogenous complement inhibitors have demonstrated prevention of hyperacute rejection of vascularized organs, but not graft loss due to acute vascular rejection. Using various wild-type and cytokine-deficient mice strains, we have examined the mechanisms of acute vascular rejection. C57BL/6 mice deficient in interleukin12 or gamma interferon showed faster acute vascular rejection than did wild-type mice. Furthermore, mice defective in B-cell development showed no acute vascular rejection. These results demonstrate that the axis of interleukin 12 and gamma interferon provides a survival advantage in vascularized xenografts by delaying or preventing acute vascular rejection caused by a B cell-dependent mechanism.

Influence of cold ischemia time, pretransplant anti-porcine antibodies, and donor/recipient size matching on hyperacute graft rejection after discordant porcine to cynomolgus kidney transplantation

Organs transplanted between phylogenetically disparate species, such as from the pig into the primate, are subject to hyperacute rejection (HAR). This form of xenograft rejection is mediated by preformed natural antibodies and is believed to occur invariably in discordant xenografts thus leading to rapid destruction and complete thrombosis of the graft. Recent data, however, have shown that in the porcine to cynomolgus monkey setting, HAR is not inevitably seen after porcine kidney transplantation. The influence of preoperative antiporcine antibody levels in the recipient, cold ischemia time, and donor organ weight on the onset of HAR was investigated by using unmodified large white pigs (aged 3-12 weeks) as organ donors and adult cynomolgus monkeys (aged 1.5-3.5 years) as recipients. Porcine kidney xenotransplantation was performed in either a non-life-supporting model (n=7) or in a life-supporting model (n=8). In both models, no correlation was found between cold ischemia time and HAR. When preoperative anti-porcine antibody levels were investigated, a significant increase in incidence of HAR was observed in animals with elevated anti-porcine IgM (P<0.05) but not IgG levels (P=NS). Interestingly, although 5 of 12 grafts with an organ weight of less than 50 g underwent HAR, none of three grafts with a donor organ weight of more than 70 g showed signs of HAR. In addition, all three larger grafts showed intraoperative and postoperative urine production, although only in 1 (48 g) of the 12 grafts weighing less than 50 g primary graft function was observed. In one animal, a second porcine kidney (23 g) was successfully transplanted (without HAR) immediately after HAR and subsequent removal of a first porcine kidney (20 g). These results indicate that in the porcine to cynomolgus monkey setting anti-porcine IgM rather than IgG anti-porcine antibody levels seem to be of predominant importance for the induction of HAR. By increasing the donor organ size and weight the frequency of the onset of HAR can be at least reduced. This is most likely due to immunoabsorption of the recipients preformed antibodies in the porcine kidney without lethal damage for the graft.

High sequence homology between human and pig CD40 with conserved binding to human CD154

BACKGROUND

Understanding the molecular interactions between pig tissues and human immune cells is fundamental to achieving long-term pig to human xenograft survival. CD40 has been shown to be central in the interaction of T cells with many antigen-presenting cells including B cells, and dendritic cells. It has been clearly shown in vitro that human T cells can effectively recognize pig major histocompatibility complex proteins, and that various accessory molecule interactions are compatible between these species, including human CD28 with pig B7 family members (CD80/CD86). The importance of CD40 in transplantation has been established using blocking antibodies to its ligand, CD154, which prolong allograft survival in mouse and primate models.

METHODS

Pig CD40 was cloned from a porcine spleen cDNA library and subsequently sequenced. Expression of pig CD40 was detected by flow cytometry using soluble human CD154 (hCD154-Ig). Results. Comparison of the derived amino acid sequence of pig with human shows 74% identity. Significantly, there is conservation between pig and human at 5 residues shown by mutagenesis studies to be essential for binding of human CD40 to CD154. hCD154Ckappa was shown to bind pig B cell lines and a proportion of human and pig lymphocytes and further confirmed by staining of COS cells transfected with pig CD40. Conclusions. Recipient human cells bearing CD154 will, therefore, be able to bind donor pig CD40, and these interactions might modulate effector functions and hence influence xenograft survival. Further investigation is necessary to ascertain the exact nature of these interactions and their implications for xenograft survival.

A surface-bound form of human C1 esterase inhibitor improves xenograft rejection

BACKGROUND

The purpose of the present study was to investigate the effect of the C1 esterase inhibitor (C1-INH) molecule against human complement attack on a swine endothelial cell (SEC) membrane. Human C1-INH functions as an inhibitor for complement reaction in the first step of the classical pathway in the fluid phase.

METHODS

A surface-bound form of human C1-INH (C1-INH-PI) consisting of a full-length coding sequence of C1-INH and a glycosylphosphatidylinositol (GPI) anchor of the decay-accelerating factor (CD55) was constructed, and stable Chinese hamster ovarian tumor (CHO) cell lines and SEC lines expressing C1-INH-PI were then prepared by transfection of the constructed cDNA. The basic function of the transfected molecules on the xenosurface was investigated using CHO transfectants for the sake of convenience. The efficacy of C1-INH-mediated protection of SEC from human complement was then assessed as an in vitro hyperacute rejection model of a swine-to-human discordant xenograft.

RESULTS

Flowcytometric profiles of the stable CHO and SEC transfectants with C1-INH-PI showed a medium level of expression of these molecules. The C1-INH levels were significantly reduced as a result of phosphatidylinositol-specific phospholipase C (PI-PLC) treatment, suggesting that the molecules were present as the PI-anchor form. Approximately 51.3 x 10(4) and 13.3 x 10(4) molecules of C1-INH-PI blocked human complement-mediated cell lysis by approximately 75% on the CHO cell and by 60-65% on the SEC cell, respectively. In addition, the complement-inhibiting activity of human C1-INH molecules is not homologously restricted.

CONCLUSIONS

The results suggest that the surface-bound form of C1-INH represents a good candidate as a safeguard against hyperacute rejection of xenografts.

Anti-Galalpha1-3Gal antibody levels in organ transplant recipients receiving immunosuppressive therapy

The effect of long-term pharmacologic immunosuppression (PI) on anti-Galalpha1-3Gal (alphaGal) antibody (Ab) levels has not been determined previously in humans. In this study, we measured alpha Gal Ab levels by ELISA in 14 healthy volunteers (controls) and in 70 patients with grafts (kidney, heart, liver) who had received different combinations of PI (including cyclosporine, tacrolimus, azathioprine, mycophenolate mofetil, and steroids) for >3 months. There was great variation in Gal IgM (<80-fold) and IgG (<160-fold). There was no difference in Gal IgM or Gal IgG between any one group and any other. In kidney patients with either high (mean 68%) or low (mean 6%) panel-reactive alloantibodies, there was no difference in alpha Gal Ab level or serum cytotoxicity to pig cells. In vitro immunoadsorption of alphaGal Ab from the serum did not change panel-reactive alloantibody positivity. Therapy with OKT3, a mouse product that might stimulate alphaGal Ab production, led to no significant change in patient Ab levels. We conclude that long-term (>3 months) PI does not reduce Gal Ab levels sufficiently to be of clinical value in xenotransplantation.

Immunosuppressive therapies after heart transplantation: best, better, and beyond

Despite the significant advances in transplantation immunology and immunosuppressive therapies over the past 30 years, current immunosuppressive regimens are still inadequate in the majority of cardiac transplant recipients. Although short- and long-term survival rates have improved significantly, only 50% will survive 10 years and very few will survive 20 years. Complications of overimmunosuppression and underimmunosuppression account for the majority of these deaths. Only true "immunologic" tolerance can provide the outcome we pursue, namely, prolonged allograft function and otherwise normal immune function without chronic immunosuppressive therapy and its risks. Until a successful tolerance-inducing protocol is developed, we must use the current and upcoming immunosuppressive agents and techniques.

Xenotransplantation

BACKGROUND

The success of clinical transplantation has led to a large discrepancy between donor organ availability and demand; considerable pressure exists to develop an alternative source of organs. The use of animal organs for donation is a possible solution that is not yet clinically applicable.

METHODS AND RESULTS

A literature review was performed based on a Medline search to find articles on xenotransplantation. Keywords included hyperacute, acute vascular, xenograft rejection combined with concordant and discordant. Additional references cited in these articles from journals not included in Medline were obtained from the British Library. Limited information on unpublished, preliminary work has been included from sources known to the authors, based on their research work in the field. One hundred and forty-six references and four personal communications have been included in this review article.

CONCLUSION

A greater understanding of the pathogenesis of xenograft rejection is developing rapidly. Strategies to abrogate hyperacute rejection have proved successful, but control of antibody-driven acute vascular rejection has not yet been achieved. The safety and viability of xenotransplantation as a therapeutic modality are still unproven.

Interaction of anti-HLA antibodies with pig xenoantigens

BACKGROUND

Many patients with renal failure are condemned to long-term dialysis with little prospect of transplantation because they are highly sensitized with immunoglobulin G (IgG) directed against class I human leukocyte antigens (HLA) of virtually all donors. Xenotransplantation could represent an attractive solution providing their alloantibodies (alloAb) do not recognize porcine motifs. Hitherto there has been no in vivo demonstration of any cross-reactivity and the objective of this work was to investigate this problem using a technique of extracorporeal pig kidney perfusion as a model of clinical xenografting.

METHODS

Pig kidneys were perfused ex vivo with plasma from both a group of highly sensitized patients and healthy individuals. Sequential plasma samples were analyzed for the titer of anti-Galalpha1-3Gal antibody (Ab) (major natural xenoreactive Ab) by enzyme-linked immunosorbent assay and anti-HLA class I Ab against a cell panel. At the end of perfusion, kidneys were perfused with a citric acid buffer to elute bound Ab.

RESULTS

Galalpha1-3Gal Ab were shown to decrease rapidly in the plasma (in less than 10 min) and then reached a plateau. A fractional decrease in anti-HLA Ab was also found in some of the perfused plasma samples. Anti-Gal Ab were readily detected in all citric acid perfusates and anti-HLA Ab in 8 of 10. The HLA specificities of eluted Ab were mainly concordant with the originally designated specificities for each patient.

CONCLUSION

Anti-HLA class I Ab presumably cross-react with pig class I homologues. However, some plasma samples did not cross-react, suggesting that negatively cross-matched pig kidneys could be identified in the pig population for xenotransplantation in these patients. Further studies are required to precisely describe these cross-reactivities and to understand their functional significance in xenotransplantation.

Function of human factor H and I on xenosurface

The cell membrane-bound forms of mini-factor H with 1-4 short consensus repeats (fH-PI) and factor I (fI-PI) were constructed. Swine endothelial cell (SEC) lines and Chinese hamster ovary (CHO) cell expressing fH-PI or fI-PI were established and confirmed by flow cytometry. The cell lysate of the SEC line expressing fH-PI showed strong cofactor activity for the cleavage of C3b, and fI-PI demonstrated the protease activity for C4b and C3b not only in the fluid phase but also on the cell membrane. In addition, fH-PI blocked human complement-mediated cell lysis by approximately 30-40%. An SEC line with a low expression of fI-PI showed a weak inhibition of cell lysis in human serum, whereas a CHO cell transfectant with a high expression of fI-PI showed over a 60% inhibition of cell lysis. The results suggest that fH-PI and fI-PI have potential for use in clinical xenotransplantation.

Hyperacute rejection in the guinea pig-to-rat model without formation of the membrane attack complex

The guinea pig (GP)-to-rat transplantation model has been widely used to study hyperacute rejection (HAR) of xenografts. In this model heart graft survival beyond 8 days has never been reported. In contrast, survival times of kidney and heart grafts up to 62 days have been reported in the discordant pig-to-primate model. It is not clear why it is so much more difficult to obtain long-term graft survival in the GP-to-rat model as compared to the pig-to-primate model. We hypothesized that mechanisms other than activation of complement may be involved in the rejection of guinea pig grafts by rat recipients. Therefore, we have studied in detail the rejection of GP aortic grafts by rat recipients, either PVG/c+ (complement competent, group 1), or PVG/c- (complement C6 deficient, group 2). PVG/c- rats are not able to form the membrane attack complex (MAC) of complement. Forty-four GP-to-rat aortic transplantations were performed successfully. Recipient rats were sacrificed at various intervals after transplantation (4, 24 and 48 h, and 7 and 28 days, three to six animals per time point per group). Twenty-four hours after transplantation the number of cells in the media was significantly decreased from 11.1 +/- 0.9 cells/mm2 to 3.1 +/- 2.8 cells/mm2 in group 1, whereas the number of medial cells in group 2 remained the same. The number of medial cells was significantly decreased in both groups at 48 h post-transplantation (group 1: 1.8 +/- 2.2 cells/mm2; group 2: 5.5 +/- 3.0 cells/mm2). At that time no infiltrating cells were apparent in the grafts of either two groups. Seven days after transplantation, the number of medial cells remained low in group 1 (1.8 +/- 2.9 cells/mm2) but was increased in group 2 (10.7 +/- 2.6 cells/mm2) as a consequence of infiltrating immune cells. These infiltrating cells consisted mainly of macrophages, but also T cells and NK cells. At 28 days after transplantation the grafts in both groups were completely reorganized and no distinction could be made between media and adventitia. These results show that rejection of GP grafts by rat recipients can occur in the absence of both MAC of complement and immune competent cells. This MAC and immune cells independent type of rejection has not been described before and may explain the difficulty in obtaining long-term graft survival in the GP-to-rat xenotransplantation model.