Inhibition of complement, evoked antibody, and cellular response prevents rejection of pig-to-primate cardiac xenografts

Cardiac Xenotransplantation: Progress in Preclinical Models and Prospects for Clinical Translation

Survival of pig cardiac xenografts in a non-human primate (NHP) model has improved significantly over the last 4 years with the introduction of costimulation blockade based immunosuppression (IS) and genetically engineered (GE) pig donors. The longest survival of a cardiac xenograft in the heterotopic (HHTx) position was almost 3 years and only rejected when IS was stopped. Recent reports of cardiac xenograft survival in a life-sustaining orthotopic (OHTx) position for 6 months is a significant step forward. Despite these achievements, there are still several barriers to the clinical success of xenotransplantation (XTx). This includes the possible transmission of porcine pathogens with pig donors and continued xenograft growth after XTx. Both these concerns, and issues with additional incompatibilities, have been addressed recently with the genetic modification of pigs. This review discusses the spectrum of issues related to cardiac xenotransplantation, recent progress in preclinical models, and its feasibility for clinical translation.

Porcine to Human Heart Transplantation: Is Clinical Application Now Appropriate?

Cardiac xenotransplantation (CXTx) is a promising solution to the chronic shortage of donor hearts. Recent advancements in immune suppression have greatly improved the survival of heterotopic CXTx, now extended beyond 2 years, and life-supporting kidney XTx. Advances in donor genetic modification (B4GALNT2 and CMAH mutations) with proven Gal-deficient donors expressing human complement regulatory protein(s) have also accelerated, reducing donor pig organ antigenicity. These advances can now be combined and tested in life-supporting orthotopic preclinical studies in nonhuman primates and immunologically appropriate models confirming their efficacy and safety for a clinical CXTx program. Preclinical studies should also allow for organ rejection to develop xenospecific assays and therapies to reverse rejection. The complexity of future clinical CXTx presents a substantial and unique set of regulatory challenges which must be addressed to avoid delay; however, dependent on these prospective life-supporting preclinical studies in NHPs, it appears that the scientific path forward is well defined and the era of clinical CXTx is approaching.

Emergence of Oseltamivir-Resistant H7N9 Influenza Viruses in Immunosuppressed Cynomolgus Macaques

Antiviral compounds (eg, the neuraminidase inhibitor oseltamivir) are invaluable for the treatment of individuals infected with influenza A viruses of the H7N9 subtype (A[H7N9]), which have infected and killed hundreds of persons. However, oseltamivir treatment often leads to the emergence of resistant viruses in immunocompromised individuals. To better understand the emergence and properties of oseltamivir-resistant A(H7N9) viruses in immunosuppressed individuals, we infected immunosuppressed cynomolgus macaques with an A(H7N9) virus and treated them with oseltamivir. Disease severity and mortality were higher in immunosuppressed than in immunocompetent animals. Oseltamivir treatment at 2 different doses reduced A(H7N9) viral titers in infected animals, but even high-dose oseltamivir did not block viral replication sufficiently to suppress the emergence of resistant variants. Some resistant variants were not appreciably attenuated in cultured cells, but an oseltamivir-resistant A(H7N9) virus did not transmit among ferrets. These findings are useful for the control of A(H7N9) virus infections in clinical settings.

Study of the microcirculation in hDAF transgenic rat livers xenoperfused with human blood

BACKGROUND

The microcirculation was assessed in the livers of human decay accelerating factors (hDAF) and wild-type transgenic rats by fluorescent intravital microscopy, histology and histomorphology to determine the benefits of hDAF expression for the microcirculation of a rat liver xenograft perfused with human blood.

METHODS

Male hDAF transgenic rats (group A; n = 20) and wild-type Sprague-Dawley rats (group B; n = 20) were xenoperfused with human blood, while other male wild-type Sprague-Dawley rats (group C; n = 10) were perfused with allogeneic blood. Following plasma and leukocyte staining with fluorescein sodium, and platelet staining with rhodamine, the right lobe of the liver was assessed by intravital microscopy, counting the numbers of perfused sinusoids and leukocytes adhering to the endothelium per mm(2), and calculating the acinar perfusion index (Pi). The liver underwent histological assessment at the end of each experiment. Mean +/- SEM values were calculated and the Mann-Whitney U-test was used for statistical analysis.

RESULTS

The number of perfused sinusoids was higher in the group of hDAF rat livers (group A) and controls (group C) than in the group of non-transgenic rat livers perfused with human blood (group B) (P < 0.05), although only group C still had a significantly more perfused sinusoids than the other groups after 90 min of perfusion (P < 0.05). The acinar perfusion index was higher in groups A and C than in group B (P < 0.05); here again, only group C still had a significantly higher Pi than group B after 90 min of perfusion (P < 0.05). There was a massive accumulation of leukocytes that peaked after 5 min and persisted throughout the perfusion in all three groups. Histology showed portal and subendothelial hepatic vein hemorrhage, necrosis and inflammatory reaction, which were particularly evident in group B.

CONCLUSION

In our study, rat livers transgenic for hDAF were better protected against early tissue damage by perfusion with human blood, but this did not result in a longer xenograft survival.

Anatomical particularities of the porcine immune system--a physician's view

In this article the anatomical structure of the porcine immune organs is described. The focus is on their particularities that are related to the use of pigs as an animal model. Key issues of the intrauterine development of the lymphoid organs are presented, such as the specific epithelio-chorial placenta, the appearance of the thymic tissue and the initial development of B cells. The role of the thymus for the development of alpha/beta and gamma/delta T cells and the location of tonsillar tissue in the naso-pharynx, in the oral cavity and at the basis of the tongue are described. The porcine spleen is of interest for surgical techniques to treat splenic trauma adequately. The observation of the inverted lymph node structure of pigs is puzzling and it remains unclear why only few species have this distinct morphological organisation. Based on the functional differences in lymphocyte recirculation observed in pigs, specific lymph cannulation experiments are possible in the porcine immune system. The porcine intestinal lymphoid tissue and the lymphocytes in the mucosal epithelium and lamina propria are of interest for studying the gut immune responses. For use as a model the fact that the pig is a monogastric omnivorous animal represents an advantage, although the porcine ileal Peyer's patch has no obvious anatomical equivalent in man. Based on the detailed knowledge of porcine immune morphology the pig is suitable as model animal for immunology--in addition to the various experimental approaches in physiology, pharmacology, surgery, etc. that are applicable to human medicine.

Xenotransplantation: role of natural immunity

Hyperacute rejection, mediated by natural anti-Galalpha1,3Galbeta1,4GlcNAc (alphaGal) antibodies and the classically activated complement pathway, was identified as the first major barrier to the survival of porcine organs in humans. Subsequently, discordant pig-to-nonhuman primate and concordant rodent models revealed key roles for T and B lymphocytes in the second form of rejection, acute vascular rejection (AVR) or delayed xenograft rejection (DXR). As significant progress was made in strategies to circumvent or suppress xenoreactivity of the adaptive immune system, it became clear that, apart from natural antibodies, other innate immune system elements actively participate in AVR/DXR and represent a barrier to xenograft acceptance that may be particularly difficult to overcome. Observations in pig-to-primate and semi-discordant and concordant rodent models indicate that Natural Killer (NK) cells play a more prominent role in xenograft than in allograft rejection. Several mechanisms through which human NK cells recognize porcine endothelial cells have been elucidated and these appear to be more diverse than those involved in NK cell alloreactivity. Further, it has been demonstrated that human macrophages and neutrophils can directly recognize pig derived cells and can mediate direct xenograft damage. Here, we review the recent progress in the understanding of the xenoreactivity of the natural immune system, focussing on preclinical pig-to-(non)human primate systems, and discuss the proposed strategies to overcome these barriers.

Human complement-activating immunoglobulin (Ig)G3 antibodies are essential for porcine endothelial cell activation

BACKGROUND

Complement-activating naturally occurring anti-porcine endothelial cell antibodies (Abs) are responsible for hyperacute rejection in porcine-to-primate transplantation, whereas the role of complement in acute vascular rejection, characterized by type II endothelial cell activation, is less well understood. We previously demonstrated a correlation between porcine type II endothelial cell activation, as detected by E-selectin expression, and human immunoglobulin (Ig)G3 anti-Gal alpha1-3Gal (Gal) Abs, which was not seen for IgG1, IgG2 or IgG4. The present study was undertaken to investigate whether there is a causal relationship between human anti-porcine IgG3 Abs and porcine endothelial cell activation.

METHODS

IgG3 was isolated employing a Protein A column to 98.3% purity. Porcine endothelial cells were incubated with isolated human IgG3 or the combination of IgG1, IgG2 and IgG4. E-selectin expression and complement activation were investigated by flow cytometry and Western blotting, respectively.

RESULTS

Purified IgG3, in contrast to the other IgG subclasses, induced a substantial increase in E-selectin expression. This activation was accompanied by complement activation as detected by C3 cleavage, and was abolished by heat inactivation or by adding the complement inhibitor FUT-175. Depletion of anti-Gal Abs reduced E-selectin expression by 60%, consistent with the presence of complement-activating anti-porcine non-Gal Abs of the IgG3 subclass.

CONCLUSIONS

Collectively, these data strengthen the hypothesis that human anti-porcine endothelial cell Abs of the IgG3 subclass are essential for endothelial cell activation in porcine-to-human species grafts and demonstrate such activation to be partly independent of Gal epitopes.

Endothelial cell protection in xenotransplantation: looking after a key player in rejection

The endothelium, as an organ at the interface between the intra- and extravascular space, actively participates in maintaining an anti-inflammatory and anti-coagulant environment under physiological conditions. Severe humoral as well as cellular rejection responses, which accompany cross-species transplantation of vascularized organs as well as ischemia/reperfusion injury, primarily target the endothelium and disrupt this delicate balance. Activation of pro-inflammatory and pro-coagulant pathways often lead to irreversible injury not only of the endothelial layer but also of the entire graft, with ensuing rejection. This review focuses on strategies targeted at protecting the endothelium from such damaging effects, ranging from genetic manipulation of the donor organ to soluble, as well as membrane-targeted, protective strategies.

Oral feeding with pig peripheral lymphocytes decreases the xenogeneic delayed type hypersensitivity reaction in galactosyltransferase knockout mice

PURPOSE

Oral tolerance induction has shown promising results in experimental allotransplantation models but is not well investigated in xenotransplantation. We investigated the possibility to induce tolerance against pig peripheral lymphocytes (pPBL) in galactosyltransferase knockout mice (gal -/-), which produce antibodies against Galalpha1-3Gal.

MATERIAL AND METHODS

Female (gal -/-) mice 6 to 8 weeks old weighing 35 to 40 g (n = 10) were fed orally every third day five times with 2 x 10(7) isolated, viable pPBL, or with phosphate-buffered saline (PBS) only (n = 7). They were then immunized subcutaneously on day 0 with a subcellular lysate from 4 x 10(7) isolated, viable pPBL. On day 13, 25 microL of a subcellular lysate corresponding to 1 x 10(7) isolated, viable pPBL was injected in the right dorsal foot pad, and the delayed type hypersensitivity (DTH) reaction was calculated after 24 hours by subtracting the swelling response from 25 microL PBS in the left footpad. Anti-Galalpha1-3Gal immunoglobulin IgG and IgM antibody titers were measured in the serum before oral feeding and at day 14.

RESULTS

The DTH reaction of the pPBL fed mice was 0.07 +/- 0.05 mm vs 0.57 +/- 0.23 mm for the controls (P < .001). No significant differences in anti Gal alpha1-3 Gal IgG and IgM antibody titers were seen.

CONCLUSIONS

This study demonstrates for the first time that oral delivery of pPBL can counteract the indirect T-cell reaction against xenogeneic subcellular antigens from pPBL. These observations warrant further investigation in immunologically modified mice and perhaps in primate models of xenotransplantation.

Novel complement inhibitors

The complement system provides natural immunity against microbes and is an effector arm of antibody-mediated humoral immunity. It promotes the inflammatory process by activating cells and facilitates microbial destruction through opsonisation and lysis. Given this tissue damaging potential, it is not surprising that nearly half of the proteins of the complement system are regulators. The complement system can mediate undesirable cellular damage in autoantibody-mediated conditions, for example myasthenia gravis, immune-complex excess syndromes, such as systemic lupus erythaematosus, ischaemia-reperfusion states, hyperacute rejection of transplants, organ failure conditions (e.g., adult respiratory distress syndrome [ARDS]), Alzheimer's disease (AD) and related neurodegenerative disorders. A complement inhibitor has been lacking in the therapeutic arsenal. However, there are now several such agents being assessed in clinical trials and others under development. Current approaches include soluble versions of membrane regulatory proteins, humanised antibodies to components, small molecule inhibitors at various stages of the pathway and transgenic animals expressing human complement regulators for xenotransplantation. These and other strategies should lead to an effective means with which to inhibit complement activation in clinical medicine.

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 effect of soluble complement receptor type 1 on acute humoral xenograft rejection in hDAF-transgenic pig-to-primate life-supporting kidney xenografts

BACKGROUND

In pig-to-nonhuman primate solid organ xenotransplantation using organs from donors transgenic for human decay-accelerating factor (hDAF), the main type of rejection is antibody-mediated (acute humoral xenograft rejection, AHXR). This occurs despite the complement-regulatory function of the transgene, neutralization of natural antibodies to Galalpha1-3Gal (Gal) using soluble glycoconjugates, and chronic immunosuppression. As complement components play a major role in graft destruction after antibody binding, we evaluated the efficacy of chronic complement inhibition by soluble complement receptor type 1 (TP10).

METHODS

Life-supporting hDAF-transgenic kidney transplantation was performed in cynomolgus monkeys, using cyclophosphamide induction, and maintenance immunosuppression with cyclosporin A, mycophenolate sodium, and tapering steroids. Rejection was treated with bolus steroid injections: if not successful animals were terminated. Three groups were studied: in group 1 (n=4) GAS914 (a soluble glycoconjugate comprising Gal on a poly-L-lysine backbone) was added before and after transplantation; group 2 (n=2) received GAS914 as in group 1 and in addition TP10 before and after transplantation; in group 3 (n=4) GAS914 was only given before transplantation and TP10 as in group 2. Monitoring included the regular assessment of anti-porcine antibodies, complement activity (soluble C5b-9), therapeutic drug monitoring, and graft histology.

RESULTS

Survival in group 1 was 6, 12, 31 and 37 days, respectively, and in all four cases graft histology showed AHXR. The two animals in groups 2 survived 3 and 15 days, respectively, and similarly showed AHXR in graft histology. In group 3 two animals showed AHXR (10 and 37 days survival, respectively), and two others did not show AHXR (20 and 32 days survival, respectively). The diagnosis AHXR included the deposition of complement activation products in the graft, which were present at lower intensity in animals treated with TP10. In all animals GAS914 effectively neutralized circulating anti-Gal antibody. Antibodies were detectable in the circulation of all animals using porcine erythrocytes in a hemolytic assay, although at lower levels than before transplantation. Soluble C5b-9 was not detectable in the circulation of animals receiving TP10, and circulating TP10 concentrations in these animals were in a presumed pharmacologically active range.

CONCLUSIONS

The inclusion of TP10 in the immunosuppressive protocol does not clearly lead to improved xenograft survival. Despite effective neutralization of anti-Gal antibodies and effective inhibition of systemic complement activity, AHXR was apparent in four of six animals under chronic TP10 treatment, including deposits of complement activation products in the graft. Apparently, effective systemic complement inhibition by TP10 in combination with local complement regulation by the hDAF transgene product does not necessarily result in effective inhibition of complement activation at locations in the xenograft upon binding of anti-porcine antibodies to the grafted endothelium.

Improved suppression of circulating complement does not block acute vascular rejection of pig-to-rhesus monkey cardiac transplants

At present, acute vascular rejection (AVR) remains a primary obstacle inhibiting long-term graft survival in the pig-to-non-human primate transplant model. The present study was undertaken to determine whether repetitive injection of low dose Yunnan-cobra venom factor (Y-CVF), a potent complement inhibitor derived from the venom of Naja kaouthia can completely abrogate hemolytic complement activity and subsequently improve the results in a pig-to-rhesus monkey heterotopic heart transplant model. Nine adult rhesus monkeys received a heterotopic heart transplant from wild-type pigs and the recipients were allocated into two groups: group 1 (n = 4) received repetitive injection of low dose Y-CVF until the end of the study and group 2 (n = 5) did not receive Y-CVF. All recipients were treated with cyclosporine A (CsA), cyclophosphamide (CyP) and steroids. Repetitive Y-CVF treatment led to very dramatic fall in CH50 and serum C3 levels (CH50 < 3 units/C3 remained undetectable throughout the experiment) and successfully prevented hyperacute rejection (HAR), while three of five animals in group 2 underwent HAR. However, the continuous suppression of circulating complement did not prevent AVR and the grafts in group 1 survived from 8 to 13 days. Despite undetectable C3 in circulating blood, C3 deposition was present in these grafts. The venular thrombosis was the predominant histopathologic feature of AVR. We conclude that repetitive injection of low dose Y-CVF can be used to continuously suppress circulating complement in a very potent manner and successfully prevent HAR. However, this therapy did not inhibit complement deposition in the graft and failed to prevent AVR. These data suggest that using alternative pig donors [i.e. human decay accelerating factor (hDAF)-transgenic] in combination with the systemic use of complement inhibitors may be necessary to further control complement activation and improve survival in pig-to-non-human primate xenotransplant model.

Maintenance triple immunosuppression with cyclosporin A, mycophenolate sodium and steroids allows prolonged survival of primate recipients of hDAF porcine renal xenografts

To date, the best results in life-supporting pig-to-primate renal xenotransplantation have been obtained in recipients exposed to long-term immunosuppression with cyclophosphamide. As this agent is frequently associated with side-effects, we have explored the potential of a mycophenolate sodium-based maintenance immunosuppression in this model. Human decay-accelerating factor (hDAF) transgenic kidneys were transplanted into splenectomized and bilaterally nephrectomized cynomolgus monkeys immunosuppressed with mycophenolate sodium, cyclosporin A and steroids, and exposed to a brief induction course with cyclophosphamide (up to four doses). After transplantation, the primates were monitored daily for biochemical and haematological evaluations and for the measurements of haemolytic anti-pig antibodies (APA). A detailed histological analysis of each explanted graft was also performed. All the animals showed very poor initial graft function but survived for up to 51 days. In contrast to our previous studies in xenograft recipients on long-term immunosuppression with cyclophosphamide, minimal or no circulating xeno-directed antibodies, as measured by the evaluation of APA titres, were detected in this series although some degree of acute humoral rejection was observed in all the explanted grafts and was the primary cause of graft failure. Furthermore, in addition to areas of humorally mediated graft damage, we have observed for the first time areas with exclusive and prominent infiltration by CD2+ and CD8+ mononuclear cells presenting patterns compatible with tubulitis, glomerulitis and arteritis, which we have called acute cellular xenograft rejection (ACXR). In addition, CD68+ infiltrating macrophages and CD20+ B-cells were also present. This study demonstrates that a triple maintenance immunosuppression with mycophenolate sodium, cyclosporin A and steroids is a viable alternative to a cyclophosphamide-based immunosuppression to obtain prolonged survival of porcine organs transplanted into primates. However, a more stringent control of antibody forming cells remains essential to further extend the survival of xenografts in this model. In addition, the use of the immunosuppressive regimen reported here in the primate is associated with the occurrence of a new category of cell-mediated xenograft injury (ACXR) whose significance has yet to be clarified.

Role of complement in xenotransplantation

The xenotransplantation research is driven by the increasing gap between the number of patients with end-stage organ failure on waiting lists for transplantation and the supply of allografts. The lack of success in developing suitable artificial organs for permanent treatment of organ failure has further strengthened the need for xenotransplantation research. Pigs are now generally accepted to be the source animal of choice. Transplantation of pig organs to humans faces several barriers which have to be overcome before it comes to clinical application: (1) anatomical and physiological conditions; (2) immunological rejection mechanisms; (3) molecular compatibility between signal molecules of the two species; (4) risk of transmission of microorganisms, particularly pig endogenous retroviruses; and (5) legal and ethical aspects both with respect to the animal and the recipient. Here we will focus on the role of the complement system in the rejection of immediately vascularized pig-to-primate xenografts. The hyperacute rejection occurring within minutes after transplantation is mediated by binding of natural antibodies to the Galalpha(l-3)Gal epitope on the endothelial cells with subsequent complement activation. Whereas inhibition of complement activation protects against hyperacute rejection, the role of complement in the later rejection phases is less clarified.

Human serum-induced porcine endothelial cell E-selectin expression is associated with IgG3 and IgM anti-Gal antibodies

Naturally occurring anti-Galalpha1-3Gal (anti-Gal) antibodies and complement induce hyperacute rejection (HAR) of porcine organs transplanted to primates. If the hyperacute reaction is prevented, an acute vascular rejection (AVR) occurs within hours to few days. Antibodies are important for the development of AVR, whereas the role of complement is still not clarified. AVR is characterized by protein synthesis-dependent endothelial cell (EC) activation. In the present study we investigated the relation between EC activation as measured by E-selectin expression, and the concentrations of anti-Gal antibodies of IgM, IgG and IgG subclasses in sera from 80 healthy blood donors selected on the basis of sex and age. There was a significant correlation between E-selectin expression and the concentration of IgG3 anti-Gal (r=0.39; P=0.019), which was not seen for the other IgG subclasses or for total IgG anti-Gal. A modest, but significant correlation was found between the concentration of IgM anti-Gal and E-selectin expression (r=0.38; P=0.040), but not between IgM and IgG3 anti-Gal. There was a large interindividual variation in anti-Gal antibodies, 50-fold for IgM and 70-fold for IgG. Females had significantly higher concentrations of IgM anti-Gal than males (P=0.0006), which was explained by a substantial increase in IgM anti-Gal concentration in younger women. The concentration of IgG anti-Gal and the degree of E-selectin expression did not differ between sex or age groups. In conclusion, the close correlation between anti-Gal antibodies of the potent complement activating IgG3 subclass and porcine EC activation, may imply that these antibodies play a role in EC activation characteristic of AVR.

C1-inhibitor for prophylaxis of xenograft rejection after pig to cynomolgus monkey kidney transplantation

BACKGROUND

Early rejection of discordant porcine xenografts in primate recipients is initiated by the intragraft binding of either preformed (hyperacute xenograft rejection) or induced (acute vascular rejection) antiporcine recipient antibodies with subsequent complement activation via the classical pathway. We have investigated the efficacy of the supplemental administration of C1-inhibitor (C1-INH), a specific inhibitor of the classical complement activation pathway, for prophylaxis of xenograft rejection in a pig to primate kidney xenotransplantation setting.

METHODS

Based on the results of pharmacokinetic studies performed in two nontransplanted monkeys, supplemental C1-INH therapy was administered daily to three Cynomolgus monkeys receiving a life-supporting porcine kidney transplant together with cyclophosphamide-induction/cyclosporine A/mycophenolat-mofetil/steroid immunosuppressive therapy.

RESULTS

In the three monkeys receiving porcine kidney xenografts and continuous C1-INH treatment none of the grafts underwent hyperacute rejection; all xenografts showed initial function. Recipient survival was 13, 15, and 5 days. No graft was lost due to acute vascular rejection. All animals died with a functioning graft (latest creatinine 96, 112, and 96 micromol/liter) due to bacterial septicemia.

CONCLUSION

We conclude that, in our model, supplemental C1-INH therapy together with a standard immunosuppressive regimen can be helpful for prevention of xenograft rejection in a pig to primate kidney xenotransplantation setting. The optimal dose and duration of C1-INH treatment, however, has yet to be determined.

Human serum-induced expression of E-selectin on porcine aortic endothelial cells in vitro is totally complement mediated

BACKGROUND

Whereas complement is a key mediator of hyperacute xenograft rejection, its role in acute vascular rejection (AVR) is a matter of controversy. AVR is associated with de novo synthesis of endothelial cell-derived inflammatory mediators, including the leukocyte-recruiting adhesion molecule E-selectin. Here we investigate the role and mechanism of complement in human serum-induced porcine endothelial cell activation.

METHODS

An in vitro xenotransplantation method was designed using porcine aortic endothelial cells stimulated with human serum in microculture wells. E-selectin expression was measured by cell-enzyme immunoassay. Complement inhibitors acting at different levels in the cascade were investigated for their effect on E-selectin expression.

RESULTS

E-selectin was strongly induced by normal human serum but not by heat-inactivated serum. Compstatin, a synthetic C3 inhibitor, markedly reduced human serum-induced E-selectin expression. Purified C1-inhibitor suppressed E-selectin induction completely, indicating activation through the classical or lectin pathway. Furthermore, a monoclonal antibody (mAb) that inhibits cleavage of C5 or another mAb that blocks the function of C7, completely inhibited the expression of serum-induced E-selectin, consistent with the terminal C5b-9 complement complex being the mediator of the endothelial cell activation. Inhibition of the alternative pathway using a novel antifactor D mAb did not reduce E-selectin expression.

CONCLUSION

Human serum-induced expression of porcine E-selectin is totally complement dependent, induced by a C1-inhibitor regulated pathway and mediated through the terminal complement complex. The data may have implications for therapeutic strategies, particularly of C1-inhibitor and anti-C5 mAb, to protect against endothelial cell activation and subsequent AVR of porcine xenografts.

The effect of immunosuppressive protocols on spontaneous CNS remyelination following toxin-induced demyelination

Glial cell transplantation is a potential therapy for human demyelinating disease, though obtaining large numbers of human myelinating cells for transplantation remains a major stumbling block. Autologous transplantation is currently not possible, since the adult human CNS is not a good source of oligodendrocyte precursors, and long-term immunosuppression of engrafted allogeneic or xenogeneic cells is therefore likely to be necessary. Immunosuppressive drugs may need to be used in situations where more recent, active areas of demyelination are undergoing endogenous remyelination. It is therefore pertinent to establish the extent to which immunosuppressive protocols will suppress spontaneous remyelination. In order to investigate this issue, we created demyelinating lesions in the spinal cord of adult rats and compared the extent of remyelination in animals receiving different immunosuppressive treatments. In animals given only cyclosporin A, there was no difference in the extent of either Schwann cell or oligodendrocyte remyelination of ethidium bromide-induced demyelinating lesions. However, in animals given cyclophosphamide, either alone or in combination with cyclosporin, there was a significant reduction in the extent of oligodendrocyte-mediated remyelination. These results demonstrate that cyclophosphamide is deleterious to oligodendrocyte remyelination and for this reason should be used with caution in patients with demyelinating disease.

Analysis of potential porcine endogenous retrovirus (PERV) transmission in a whole-organ xenotransplantation model without interfering microchimerism

The question whether porcine xenografts can lead to porcine endogenous retrovirus (PERV) infection of recipients is critical for the evaluation of the safety of pig-to-man xenotransplantation. Unfortunately, polymerase chain reaction (PCR)-based analysis of potential PERV infections in nonhuman-primate whole-organ xenotransplantation models is hampered by false positive results due to chimeric porcine cells. To avoid the inherent analytical problem of xenomicrochimerism, we developed a non-life-supporting pig-to-primate kidney xenotransplantation model: porcine kidneys were transplanted, whereas the functioning recipient kidneys remained in situ. Subsequent to rejection (after 2 hours to 15 days), xenografts were removed, and recipients remained alive for up to 287 days. Immunosuppressive therapy based on cyclophosphamide, cyclosporine, and steroids was maintained for 28 days after transplantation. Using appropriate PCR assays, xenochimerism was found in tissue samples and partly even in peripheral blood leukocytes (PBLs) while the porcine kidneys were in situ. After graft removal, xenochimerism was no longer detectable, thus allowing analysis for possible PERV transmission.

Xenogeneic transplantation

This review summarizes the clinical history and rationale for xenotransplantation; recent progress in understanding the physiologic, immunologic, and infectious obstacles to the procedure's success; and some of the strategies being pursued to overcome these obstacles. The problems of xenotransplantation are complex, and a combination of approaches is required. The earliest and most striking immunologic obstacle, that of hyperacute rejection, appears to be the closest to being solved. This phenomenon depends on the binding of natural antibody to the vascular endothelium, fixation of complement by that antibody, and finally, activation of the endothelium and initiation of coagulation. Therefore, these three pathways have been targeted as sites for intervention in the process. The mechanisms responsible for the next immunologic barrier, that of delayed xenograft/acute vascular rejection, remain to be fully elucidated. They probably also involve multiple pathways, including antibody and/or immune cell binding and endothelial cell activation. The final immunologic barrier, that of the cellular immune response, involves mechanisms that are similar to those involved in allograft rejection. However, the strength of the cellular immune response to xenografts is so great that it is unlikely to be controlled by the types of nonspecific immunosuppression used routinely to prevent allograft rejection. For this reason, it may be essential to induce specific immunologic unresponsiveness to at least some of the most antigenic xenogeneic molecules.

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.

Cardiac xenotransplantation: clinical experience and future direction

The shortage of human organs has focused research on finding an animal source of replacement organs. The immunological barriers to xenotransplantation are now more clearly defined, allowing retrospective interpretation of past clinical experience in humans. Due to physiological compatibilities as well as ethical and infectious considerations, pigs have now emerged as the most likely source of future xenografts. The introduction of transgenic pigs expressing human complement regulatory proteins and new immunosuppressive regimens have shown early promise in the laboratory, although further advancements are needed to advance to clinical trials. Additional clarification of infectious risks and patient strategies are remaining obstacles to application in the clinical arena.

Long-term survival of hamster hearts in presensitized rats

We transplanted hamster hearts into rats that had been sensitized to hamster cardiac grafts 5 days earlier as a model for discordant xenotransplantation. Sensitized rats had high serum levels of elicited anti-donor IgM and IgG that caused hyperacute rejection. Transient complement inhibition with cobra venom factor (CVF) plus daily and continuing cyclosporin A (CyA) prevented hyperacute rejection. However, grafts underwent delayed xenograft rejection (DXR). DXR involved IgG and associated Ab-dependent cell-mediated rejection, because depletion of IgG or Ab-dependent cell-mediated rejection-associated effector cells prolonged graft survival and the serum-mediated Ab-dependent cell-mediated cytotoxicity in vitro. Blood exchange in combination with CVF/CyA treatment dramatically decreased the level of preexisting Abs, but DXR still occurred in association with the return of Abs. Splenectomy and cyclophosphamide acted synergistically to delay Ab return, and when combined with blood exchange/CVF/CyA facilitated long-term survival of grafts. These grafts survived in the presence of anti-donor IgM, IgG, and complement that precipitated rejection of naive hearts, indicating that accommodation (survival in the presence of anti-graft Abs and complement) had occurred. We attribute the long-term survival to the removal of preexisting anti-donor Abs and therapy that attenuated the rate of Ab return. Under such conditions, the surviving hearts showed expression in endothelial cells and smooth muscle cells of protective genes and an intragraft Th2 immune response. Th2 responses and protective genes are associated with resistance to IgM- and IgG-mediated, complement-dependent and -independent forms of rejection.

Mixed chimerism as an approach to transplantation tolerance

The induction of tolerance to transplanted organs could make transplantation safer and more uniformly successful. One of the most promising approaches currently being investigated involves the induction of deletional tolerance through the establishment of "mixed chimerism." In this laboratory, we first studied mixed chimerism as an approach to transplantation tolerance in mice, using a nonmyeloablative preparative regimen consisting of 300 R whole-body irradiation, 700 R thymic irradiation, and treatment with monoclonal antibodies to CD4 and CD8. This approach has subsequently been extended successfully to the induction of tolerance to renal transplants in fully mismatched cynomolgus monkeys. In addition, the same approach, with minor modifications, has been found effective in producing mixed chimerism and transplantation tolerance in the concordant xenogeneic baboon to cynomolgus monkey species combination. Because pigs have many advantages as a potential xenograft donor for humans, we are also trying to extend our nonmyeloablative regimen for production of mixed chimerism to the discordant pig --> primate combination. We have used absorption of natural antibodies to prevent hyperacute rejection and then proceeded with a mixed chimerism approach. Administration of pig hematopoietic stem cells along with pig recombinant cytokines (SCF and IL-3) to primates has enabled the pig bone marrow to survive in these xenogeneic hosts for over 6 months. This chimerism has apparently been sufficient to markedly diminish T cell immunity and the induction of new T-cell-dependent responses. However, to date we have not succeeded in preventing the return of natural antibodies, which appear to be the cause of eventual loss of organ transplants and are the subject of further intensive investigations.

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.

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.

Role of complement in xenotransplantation

1. Xenotransplantation, or transplantation across species, leads to rejection, which destroys the xenograft within hours to days of transplantation. 2. Complement is a major barrier to xenotransplantation of vascularized organs and is believed to play an important role in the rejection process. 3. The present paper reviews three aspects of complement in xenotransplantation. These include the mechanisms and regulation of complement activation as well as tissue injury mediated by complement activation.

Expression of the human alpha1,2-fucosyltransferase in transgenic pigs modifies the cell surface carbohydrate phenotype and confers resistance to human serum-mediated cytolysis

Hyperacute rejection (HAR) is the first critical immunological hurdle that must be addressed in order to develop xenogeneic organs for human transplantation. In the area of cell-based xenotransplant therapies, natural antibodies (XNA) and complement have also been considered barriers to successful engraftment. Transgenic expression of human complement inhibitors in donor cells and organs has significantly prolonged the survival of xenografts. However, expression of complement inhibitors without eliminating xenogeneic natural antibody (XNA) reactivity may provide insufficient protection for clinical application. An approach designed to prevent XNA reactivity during HAR is the expression of human alpha1, 2-fucosyltransferase (H-transferase, HT). H-transferase expression modifies the cell surface carbohydrate phenotype of the xenogeneic cell, resulting in the expression of the universal donor O antigen and a concomitant reduction in the expression of the antigenic Galalpha1,3-Gal epitope. We have engineered various transgenic pig lines that express HT in different cells and tissues, including the vascular endothelium. We demonstrate that in two different HT transgenic lines containing two different HT promoter constructs, expression can be differentially regulated in a constitutive and cytokine-inducible manner. The transgenic expression of HT results in a significant reduction in the expression of the Galalpha1,3-Gal epitope, reduced XNA reactivity, and an increased resistance to human serum-mediated cytolysis. Transgenic pigs that express H-transferase promise to become key components for the development of xenogeneic cells and organs for human transplantation.

Accommodated Xenografts Survive in the Presence of Anti-Donor Antibodies and Complement That Precipitate Rejection of Naive Xenografts

Hamster hearts transplanted into transiently complement-depleted and continuously cyclosporin A (CyA)-immunosuppressed rats survive long-term despite deposition of anti-donor IgM Abs and complement on the graft vascular endothelium. This phenomenon is referred to as “accommodation.” The hypothesis tested here is that accommodated xenografts are resistant to IgM Abs and complement that could result in rejection of naive xenografts. After first hamster hearts had been surviving in cobra venom factor (CVF) + CyA-treated rats for 10 days, a time when the anti-donor IgM Ab level was maximal and complement activity had returned to approximately 50% of pretreatment levels, naive hamster hearts or hamster hearts that had been accommodating in another rat for 14 days were transplanted into those rats carrying the surviving first graft. The naive hearts were all hyperacutely rejected. In contrast, a majority of regrafted accommodating hearts survived long-term. There was widespread Ab and activated complement deposition on the vascular endothelium of accommodating first hearts, second accommodating hearts, and rejected second naive hearts. However, only the rejected naive hearts showed extensive endothelial cell damage, myocardial necrosis, fibrin deposition, and other signs of inflammation. Accommodating first and second hearts but not rejected second naive hearts expressed high levels of the protective genes A20, heme oxygenase-1 (HO-1), bcl-2, and bcl-xL. These data demonstrate that accommodated xenografts become resistant to effects of anti-donor IgM Abs and complement that normally mediate rejection of xenografts. We hypothesize that this resistance involves expression by accommodated xenografts of protective genes.

The role of complement in transplantation

The complement system contributes critically to the barrier to transplantation of cells and organs. In the case of tissues and organs transplanted between individuals of the same species, that is in allotransplantation, the barrier posed by complement is seemingly eclipsed by the barrier posed by cellular immune responses. In the case of cells and organs transplanted between individuals of disparate species, that is xenotransplantation, the complement system has been thought to pose a nearly insurmountable barrier. With our understanding on how the complement system contributes to rejection, it is now clear that the complement system is more important in allotransplantation and more forgiving in xenotransplantation than was previously thought.

Factors in xenograft rejection

Important mechanisms underlying immediate xenograft loss by hyperacute rejection (HAR), in the pig-to-primate combination, have been recently delineated. There are now several proposed therapies that deal with the problem of complement activation and xenoreactive natural antibody (XNA) binding to the vasculature that have been shown to prevent HAR. However, vascularized xenografts are still lost, typically within days, by delayed xenograft rejection (DXR), alternatively known as acute vascular rejection (AVR). This process is characterized by endothelial cell (EC) perturbation, localization of XNA within the graft vasculature, host NK cell and monocyte activation with platelet sequestration and vascular thrombosis. Alternative immunosuppressive strategies, additive anti-complement therapies with the control of any resulting EC activation processes and induction of protective responses have been proposed to ameliorate this pathological process. In addition, several potentially important molecular incompatibilities between activated human coagulation factors and the natural anticoagulants expressed on porcine EC have been noted. Such incompatibilities may be analogous to cross-species alterations in the function of complement regulatory proteins important in HAR. Disordered thromboregulation is potentially relevant to the progression of inflammatory events in DXR and the disseminated intravascular coagulation seen in primate recipients of porcine renal xenografts. We have recently demonstrated the inability of porcine tissue factor pathway inhibitor (TFPI) to adequately neutralize human factor Xa (FXa), the aberrant activation of both human prothrombin and FXa by porcine EC and the failure of the porcine natural anticoagulant, thrombomodulin to bind human thrombin and hence activate human protein C. The enhanced potential of porcine von Willebrand factor to associate with human platelet GPIb has been demonstrated to be dependent upon the isolated A1 domain of von Willebrand factor. In addition, the loss of TFPI and vascular ATPDase/CD39 activity following EC activation responses would potentiate any procoagulant changes within the xenograft. These developments could exacerbate vascular damage from whatever cause and enhance the activation of platelets and coagulation pathways within xenografts resulting in graft infarction and loss. Analysis of these and the other putative factors underlying DXR should lead to the development and testing of genetic approaches that, in conjunction with selected pharmacological means, may further prolong xenograft survival to a clinically relevant extent.

Disseminated intravascular coagulation in association with the delayed rejection of pig-to-baboon renal xenografts

BACKGROUND

Intravascular fibrin deposition and platelet sequestration occur with porcine xenograft rejection by baboons. Disseminated intravascular coagulopathy may arise either as a direct consequence of the failure to fully deplete xenoreactive natural antibodies and block complement, or because of putative cross-species molecular incompatibilities in this discordant species combination.

METHODS

Three baboons were conditioned with retrovirally transduced autologous bone marrow to induce tolerance to swine antigens. Xenoreactive natural antibodies and complement were depleted by plasmapheresis and the use of Gal alpha1-3Gal column adsorptions; baboons were then splenectomized and underwent renal xenografting from inbred, miniature pigs. Soluble complement receptor type-1 with protocol immunosuppression (mycophenolate mofetil, 15-deoxyspergualin, steroids, and cyclosporine) was administered.

RESULTS

A bleeding diathesis was clinically evident from days 5 to 12 after transplantation in two baboons. Low levels of circulating C3a, C3d, and iC3b were measured despite the absence of functional circulating complement components. Profound thrombocytopenia with abnormalities in keeping with disseminated intravascular coagulopathy were observed. Prolongation of prothrombin and partial thromboplastin times was accompanied by evidence for tissue factor-mediated coagulation pathways, high levels of thrombin generation (prothrombin fragment F(1+2) production and thrombin-antithrombin complex formation), fibrinogen depletion, and production of high levels of the fibrin degradation product D-dimer. Importantly, these disturbances resolved rapidly after the excision of the rejected xenografts in two surviving animals. Histopathological examination of the rejected xenografts confirmed vascular injury, fibrin deposition, platelet deposition, and localized complement activation.

CONCLUSIONS

Systemic coagulation disturbances are associated with delayed xenograft rejection.

Discordant organ xenotransplantation in primates: world experience and current status

The pig-to-primate model is increasingly being utilized as the final preclinical means of assessing therapeutic strategies aimed at allowing discordant xenotransplantation. We review here the world experience of both pig-to-human and pig-to-nonhuman primate organ transplantation. Eight whole organ transplants using discordant mammalian donors have been carried out in human recipients; only one patient was reported (in 1923) to have survived for longer than 72 hr. Therapeutic approaches in the experimental laboratory setting have included pharmacologic immunosuppression, antibody and/or complement depletion or inhibition, the use of pig organs transgenic for human complement regulatory proteins, and conditioning regimens aimed at inducing a state of tolerance or specific immunologic hyporesponsiveness. The greatest success to date has been obtained with methods that inhibit complement-mediated injury, either by the administration of cobra venom factor or soluble complement receptor I to the recipient (with organ survival up to 6 weeks) or by the use of donor organs transgenic for human decay-accelerating factor (with organ survival up to 2 months). The future of xenotransplantation may lie in the judicious combination of current approaches.

Induction of Species-Specific Host Accommodation in the Hamster-to-Rat Xenotransplantation Model

The combination of two immunosuppressants, leflunomide and cyclosporin A (CsA), completely inhibits immune xenoreactions in the hamster-to-Lewis rat xenotransplantation model. In addition, the control of acute xenograft rejection with this combination of immunosuppressants subdues early T-independent xenoreactivity and uncovers a late immune response that can be controlled by CsA alone. We attribute this acquired responsiveness to CsA to a modification in the recipient’s humoral response to the xenograft, and refer to this change as host accommodation. Host accommodation can be induced in Lewis rats receiving hamster hearts by the combination of leflunomide and CsA. A 7-day treatment with leflunomide and CsA was able to convert xenoreactivity from one that was resistant to CsA treatment into one that was controlled by CsA. The presence of the hamster xenograft was critical for the induction of host accommodation since the immunosuppressive regimen, either alone or in combination with a transfusion with donor-specific spleen cells, was unable to modify the anti-hamster reactivity in Lewis rats. When accommodation was induced in the presence of hamster hearts, these accommodated rats were able to acutely reject third-party mouse hearts while under CsA therapy, thus indicating that the host accommodation is species specific. Finally, we demonstrate that host accommodation is associated with a loss in the ability to produce species-specific, T-independent xenoantibodies. These novel observations suggest that xenoreactive T-independent humoral responses can be deleted selectively without significant loss of other innate, Ag-specific T-independent humoral responses.

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

BACKGROUND

In order to circumvent the complement-mediated hyperacute rejection of discordant xenografts, a colony of pigs transgenic for the human regulator of complement activity, human decay-accelerating factor (hDAF), has been produced.

METHODS

Seven kidneys from hDAF transgenic pigs and six kidneys from nontransgenic control pigs were transplanted into cynomolgus monkeys; both native kidneys were removed during the same operation. The recipient animals were immunosuppressed with cyclosporine, steroids, and cyclophosphamide.

RESULTS

In the transgenic group, the median survival time was 13 days (range, 6-35 days); the median survival time in the control group was 6.5 days (range, 0.3-30 days). There were no cases of hyperacute rejection in the transgenic group, and the two longest-surviving kidneys in this group showed no evidence of rejection on histological examination. In contrast, all control kidneys underwent antibody-mediated rejection, one demonstrating hyperacute rejection and the others acute vascular rejection.

CONCLUSION

This study demonstrates that (i) a kidney from an hDAF transgenic pig can support the life of a primate for up to 35 days (and also shows the basic physiological compatibility between the pig and nonhuman primate); (ii) nontransgenic kidneys are not routinely hyperacutely rejected; and (iii) the presence of hDAF on the kidney confers some protection against acute vascular rejection. Improved immunosuppression and immunological monitoring may enable extended survival.

Intravenous infusion of Galalpha1-3Gal oligosaccharides in baboons delays hyperacute rejection of porcine heart xenografts

BACKGROUND

Hyperacute rejection (HAR) of pig-to-primate discordant xenografts is caused by the deposition of preexisting natural antibodies that recognize Galalpha1-3Gal (alphaGal)-terminating oligosaccharides on glycoproteins and glycolipids, followed by complement-mediated lysis of the graft's endothelium. In vitro, these natural xenoantibodies can be blocked by alphaGal-containing oligosaccharides. We undertook in vivo pig-to-baboon cardiac xenotransplantation experiments to evaluate free oligosaccharides as inhibitors of HAR.

METHODS

Initial 15-min intravenous infusions of alphaGal-oligosaccharides into baboons were used to measure pharmacokinetic parameters, and to assess the extent of neutralization of anti-alphaGal antibody activity. AlphaGal trisaccharide (Galalpha1-3Galbeta1-4GlcNAc) or pentasaccharide (Galalpha1-3Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glc ) was administered at 0.5 mmol/kg into baboons. Next, two baboons that received porcine heterotopic heart xenografts were continuously infused with alphaGal pentasaccharide for 4-5 hr, maintaining the serum oligosaccharide concentration in the millimolar range.

RESULTS

Pharmacokinetic analysis indicated that the oligosaccharides were rapidly cleared from the blood, with a serum half-life of 50 min. In the period during which blood oligosaccharide concentration was above 1 mM, as determined by high-pressure liquid chromatography, the serum cytotoxic activity against porcine cells was completely abolished. HAR of the xenograft was inhibited during the infusions, although there was some histological and immunohistological evidence of antibody-mediated injury on biopsies taken at the end of this period.

CONCLUSIONS

Intravenous alphaGal oligosaccharides, by inhibiting anti-alphaGal antibody activity, delay but do not abolish the onset of HAR.

Delayed xenograft rejection of pig-to-baboon cardiac transplants after cobra venom factor therapy

BACKGROUND

This study sought to (i) investigate the efficacy of cobra venom factor (CVF) in preventing hyperacute rejection (HAR) after pig-to-baboon heart transplantation, (ii) examine the effect of additional splenectomy (Spx) and pharmacologic immunosuppression (IS), and (iii) study delayed graft rejection when HAR is avoided by complement depletion.

METHODS

Eleven recipient baboons received heterotopic pig heart transplants. Three received either no therapy or IS (cyclosporine + methylprednisolone +/- cyclophosphamide +/- methotrexate) at clinically well-tolerated doses, with graft survival for only 40, 32, and 15 min, respectively. Two received CVF+/-Spx, which extended survival to 5 and 6 days, respectively. Six underwent Spx + CVF therapy + IS; graft survival was 3 hr (technical complication), 6 days (death from sepsis), 10, 12, and 22 days (vascular rejection), and <25 days (euthanized for viral pneumonia with a functioning graft that showed histopathologic features of vascular rejection).

RESULTS

Dense deposition of IgM and, to a lesser extent, IgG and IgA were seen on the endothelial cells within 1 hr of transplantation, but only trace levels of complement deposition were present in CVF-treated recipients. Within approximately 5-12 days, cardiac xenografts showed progressive infiltration by mononuclear cells, consisting primarily of activated macrophages producing tumor necrosis factor-alpha and small numbers of natural killer cells; T and B cells were absent.

CONCLUSIONS

We conclude that (i) CVF prevents HAR, (ii) the addition of Spx + IS delays rejection, but (iii) the early deposition of antibody leads to progressive graft injury, resulting in (iv) delayed vascular rejection. Our findings indicate that the features of delayed xenograft rejection described in small animal models also occur in the pig-to-baboon model, and that rejection may occur in a complement-independent manner from the effects of antibody and/or host macrophages.