HLA-specific antibodies in highly sensitized patients can cause a positive crossmatch against pig lymphocytes

The respective relevance of sensitization to alloantigens and xenoantigens in pig organ xenotransplantation

BACKGROUND

Antibody-mediated rejection is a major cause of graft injury and contributes to failure of pig xenografts in nonhuman primates (NHPs). Most 'natural' or elicited antibodies found in humans and NHPs are directed against pig glycan antigens, but antibodies binding to swine leukocyte antigens (SLA) have also been detected. Of clinical importance is (i) whether the presence of high levels of antibodies directed towards human leukocyte antigens (HLA) (i.e., high panel-reactive antibodies) would be detrimental to the outcome of a pig organ xenograft; and (ii) whether, in the event of sensitization to pig antigens, a subsequent allotransplant would be at increased risk of graft failure due to elicited anti-pig antibodies that cross-react with human HLA or other antigens.

SUMMARY

A literature review of pig-to-primate studies indicates that relatively few highly-HLA-sensitized humans have antibodies that cross-react with pigs, predicting that most would not be at increased risk of rejecting an organ xenograft. Furthermore, the existing evidence indicates that sensitization to pig antigens will probably not elicit increased alloantibody titers; if so, 'bridging' with a pig organ could be carried out without increased risk of subsequent antibody-mediated allograft failure.

KEY MESSAGE

These issues have important implications for the design and conduct of clinical xenotransplantation trials.

First clinical-grade porcine kidney xenotransplant using a human decedent model

A radical solution is needed for the organ supply crisis, and the domestic pig is a promising organ source. In preparation for a clinical trial of xenotransplantation, we developed an in vivo pre-clinical human model to test safety and feasibility tenets established in animal models. After performance of a novel, prospective compatible crossmatch, we performed bilateral native nephrectomies in a human brain-dead decedent and subsequently transplanted two kidneys from a pig genetically engineered for human xenotransplantation. The decedent was hemodynamically stable through reperfusion, and vascular integrity was maintained despite the exposure of the xenografts to human blood pressure. No hyperacute rejection was observed, and the kidneys remained viable until termination 74 h later. No chimerism or transmission of porcine retroviruses was detected. Longitudinal biopsies revealed thrombotic microangiopathy that did not progress in severity, without evidence of cellular rejection or deposition of antibody or complement proteins. Although the xenografts produced variable amounts of urine, creatinine clearance did not recover. Whether renal recovery was impacted by the milieu of brain death and/or microvascular injury remains unknown. In summary, our study suggests that major barriers to human xenotransplantation have been surmounted and identifies where new knowledge is needed to optimize xenotransplantation outcomes in humans.

Aspects of histocompatibility testing in xenotransplantation

Xenotransplantation, using genetically-modified pigs for clinical organ transplantation, is a solution to the organ shortage. The biggest barrier to clinical implementation is the antigenicity of pig cells. Humans possess preformed antibody to pig cells that initiate antibody-mediated rejection of pig organs in primates. Advances in genetic engineering have led to the development of a pig lacking the three known glycan xenoantigens (triple-knockout [TKO] pigs). A significant number of human sera demonstrate no antibody binding to TKO pig cells. As a result of the TKO pig's low antigen expression, survival of life-supporting pig organs in immunosuppressed nonhuman primates has significantly increased, and hope has been renewed for clinical trials of xenotransplantation. It is important to understand the context in which xenotransplantation's predecessor, allotransplantation, has been successful, and the steps needed for the success of xenotransplantation. Successful allotransplantation has been based on two main immunological approaches - (i) adequate immunosuppressive therapy, and (ii) careful histocompatibility matching. In vivo studies suggest that the available immunosuppressive regimens are adequate to suppress the human anti-pig cellular response. Methods to evaluate and screen patients for the first clinical xenotransplantation trial are the next challenge. The goal of this review is to summarize the history of histocompatibility testing, and the available tools that can be utilized to determine xenograft histocompatibility.

The Role of SLAs in Xenotransplantation

Advances in genetic engineering, particularly CRISPR/Cas9, have resulted in the development of a triple glycan-knockout (TKO) pig. There is minimal human antipig antibody binding to TKO pig cells. The TKO background has decreased antibody binding to a sufficiently low level that any additional xenoantigens expressed on the cells can now be more easily detected. One of these xenoantigens is the swine major histocompatibility complex, termed swine leukocyte antigens (SLA). SLA are the homolog to HLAs, a protein complex expressed on human tissue capable of stimulating the development of new antibodies in allotransplantation. These antibodies can result in graft failure through hyperacute, acute, or chronic rejection. Our knowledge of SLA, particularly in the last 5 years, has grown considerably. The presence, cause, and methods to detect anti-SLA antibodies will need to be carefully considered for the first clinical trial of xenotransplantation. The focus of this review is to summarize the role of SLA in xenotransplantation and consider whether it will prove to be a major barrier. Techniques are now available to mutate target SLA amino acids to ensure that cross-reactive anti-HLA antibodies no longer bind to SLA on the cells of the organ-source pigs. While deletion of SLA expression is possible, it would render the pig at risk for infectious complications. The ideal organ-source pig for HLA highly sensitized recipients may therefore be 1 with site-specific mutations to eliminate cross-reactive binding.

HLA Class I-sensitized Renal Transplant Patients Have Antibody Binding to SLA Class I Epitopes

BACKGROUND

Highly sensitized patients are difficult to match with suitable renal allograft donors and may benefit from xenotransplant trials. We evaluate antibody binding from sensitized patients to pig cells and engineered single allele cells to identify anti-human leukocyte antigen (HLA) antibody cross-species reactivity with swine leukocyte antigen (SLA). These novel testing strategies assess HLA/SLA epitopes and antibody-binding patterns and introduce genetic engineering of SLA epitopes.

METHODS

Sensitized patient sera were grouped by calculated panel reactive antibody and luminex single antigen reactivity profile and were tested with cloned GGTA1/CMAH/B4GalNT2 glycan knockout porcine cells. Pig reactivity was assessed by direct flow cytometric crossmatch and studied following elution from pig cells. To study the antigenicity of individual class I HLA and SLA alleles in cells, irrelevant sera binding to lymphoblastoid cells were minimized by CRISPR/Cas9 elimination of endogenous class I and class II HLA, B-cell receptor, and Fc receptor genes. Native HLA, SLA, and mutants of these proteins after mutating 144K to Q were assessed for antibody binding.

RESULTS

Those with predominately anti-HLA-B&C antibodies, including Bw6 and Bw4 sensitization, frequently have low pig reactivity. Conversely, antibodies eluted from porcine cells are more commonly anti-HLA-A. Single HLA/SLA expressing engineered cells shows variable antigenicity and mutation of 144K to Q reduces antibody binding for some sensitized patients.

CONCLUSIONS

Anti-HLA antibodies cross-react with SLA class I in predictable patterns, which can be identified with histocompatibility strategies, and SLA class I is a possible target of genetic engineering.

Examining epitope mutagenesis as a strategy to reduce and eliminate human antibody binding to class II swine leukocyte antigens

Xenotransplantation of pig organs into people may help alleviate the critical shortage of donors which faces organ transplantation. Unfortunately, human antibodies vigorously attack pig tissues preventing the clinical application of xenotransplantation. The swine leukocyte antigens (SLA), homologs of human HLA molecules, can be xenoantigens. SLA molecules, encoded by genes in the pig major histocompatibility complex, contribute to protective immune responses in pig. Therefore, simply inactivating them through genome engineering could reduce the ability of the human immune system to surveil transplanted pig organs for infectious disease or the development of neoplasms. A potential solution to this problem is to identify and modify epitopes in SLA proteins to eliminate their contribution to humoral xenoantigenicity while retaining their biosynthetic competence and ability to contribute to protective immunity. We previously showed that class II SLA proteins were recognized as xenoantigens and mutating arginine at position 55 to proline, in an SLA-DQ beta chain, could reduce human antibody binding. Here, we extend these observations by creating several additional point mutants at position 55. Using a panel of monoclonal antibodies specific for class II SLA proteins, we show that these mutants remain biosynthetically competent. Examining antibody binding to these variants shows that point mutagenesis can reduce, eliminate, or increase antibody binding to class II SLA proteins. Individual mutations can have opposite effects on antibody binding when comparing samples from different people. We also performed a preliminary analysis of creating point mutants near to position 55 to demonstrate that manipulating additional residues also affects antibody reactivity.

Immune Responses of HLA Highly Sensitized and Nonsensitized Patients to Genetically Engineered Pig Cells

BACKGROUND

We investigated in vitro whether HLA highly sensitized patients with end-stage renal disease will be disadvantaged immunologically after a genetically engineered pig kidney transplant.

METHODS

Blood was drawn from patients with a calculated panel-reactive antibody (cPRA) 99% to 100% (Gp1, n = 10) or cPRA 0% (Gp2, n = 12), and from healthy volunteers (Gp3, n = 10). Serum IgM and IgG binding was measured (i) to galactose-α1-3 galactose and N-glycolylneuraminic acid glycans by enzyme-linked immunosorbent assay, and (ii) to pig red blood cell, pig aortic endothelial cells, and pig peripheral blood mononuclear cell from α1,3-galactosyltransferase gene-knockout (GTKO)/CD46 and GTKO/CD46/cytidine monophosphate-N-acetylneuraminic acid hydroxylase-knockout (CMAHKO) pigs by flow cytometry. (iii) T-cell and B-cell phenotypes were determined by flow cytometry, and (iv) proliferation of T-cell and B-cell carboxyfluorescein diacetate succinimidyl ester-mixed lymphocyte reaction.

RESULTS

(i) By enzyme-linked immunosorbent assay, there was no difference in IgM or IgG binding to galactose-α1-3 galactose or N-glycolylneuraminic acid between Gps1 and 2, but binding was significantly reduced in both groups compared to Gp3. (ii) IgM and IgG binding in Gps1 and 2 was also significantly lower to GTKO/CD46 pig cells than in healthy controls, but there were no differences between the 3 groups in binding to GTKO/CD46/CMAHKO cells. (iii and iv) Gp1 patients had more memory T cells than Gp2, but there was no difference in T or B cell proliferation when stimulated by any pig cells. The proliferative responses in all 3 groups were weakest when stimulated by GTKO/CD46/CMAHKO pig peripheral blood mononuclear cell.

CONCLUSIONS

(i) End-stage renal disease was associated with low antipig antibody levels. (ii) Xenoreactivity decreased with increased genetic engineering of pig cells. (iii) High cPRA status had no significant effect on antibody binding or T-cell and B-cell response.

Does human leukocyte antigens sensitization matter for xenotransplantation?

The major histocompatibility complex class I and class II human leukocyte antigens (HLA) play a central role in adaptive immunity but are also the dominant polymorphic proteins targeted in allograft rejection. Sensitized patients with high levels of panel-reactive anti-HLA antibody (PRA) are at risk of early allograft injury, rejection, reduced allograft survival and often experience prolonged waiting times prior to transplantation. Xenotransplantation, using genetically modified porcine organs, offers a unique source of donor organs for these highly sensitized patients if the anti-HLA antibody, which places the allograft at risk, does not also enhance anti-pig antibody reactivity responsible for xenograft rejection. Recent improvements in xenotransplantation efficacy have occurred due to improved immune suppression, identification of additional xenogeneic glycans, and continued improvements in donor pig genetic modification. Genetically engineered pig cells, devoid of the known xenogeneic glycans, minimize human antibody reactivity in 90% of human serum samples. For waitlisted patients, early comparisons of patient PRA and anti-pig antibody reactivity found no correlation suggesting that patients with high PRA levels were not at increased risk of xenograft rejection. Subsequent studies have found that some, but not all, highly sensitized patients express anti-HLA class I antibody which cross-reacts with swine leukocyte antigen (SLA) class I proteins. Recent detailed antigen-specific analysis suggests that porcine-specific anti-SLA antibody from sensitized patients binds cross-reactive groups present in a limited subset of HLA antigens. This suggests that using modern genetic methods, a program to eliminate specific SLA alleles through donor genetic engineering or stringent donor selection is possible to minimize recipient antibody reactivity even for highly sensitized individuals.

Swine Leukocyte Antigen Class II Is a Xenoantigen

BACKGROUND

Over 130 000 patients in the United States alone need a lifesaving organ transplant. Genetically modified porcine organs could resolve the donor organ shortage, but human xenoreactive antibodies destroy pig cells and are the major barrier to clinical application of xenotransplantation. The objective of this study was to determine whether waitlisted patients possess preformed antibodies to swine leukocyte antigen (SLA) class II, homologs of the class II HLA.

METHODS

Sera from people currently awaiting solid organ transplant were tested for IgG binding to class II SLA proteins when expressed on mammalian cells. Pig fibroblasts were made positive by transfection with the class II transactivator. As a second expression system, transgenes encoding the alpha and beta chains of class II SLA were transfected into human embryonic kidney cells.

RESULTS

Human sera containing IgG specific for class II HLA molecules exhibited greater binding to class II SLA positive cells than to SLA negative cells. Sera lacking antibodies against class II HLA showed no change in binding regardless of the presence of class II SLA. These antibodies could recognize either SLA-DR or SLA-DQ complexes.

CONCLUSIONS

Class II SLA proteins may behave as xenoantigens for people with humoral immunity toward class II HLA molecules.

Is sensitization to pig antigens detrimental to subsequent allotransplantation?

An important question in xenotransplantation is whether an allotransplant can safely be carried out in a patient who has become sensitized to a pig xenograft. To answer this question, we have searched the literature. We primarily limited our review to the clinically relevant pig-to-non-human primate (NHP) model and found five studies that explored this topic. No NHP that had received a pig graft developed antibodies to alloantigens, and in vitro studies indicated no increased humoral and/or cellular alloreactivity. We carried out a small in vitro study ourselves that confirmed this conclusion. There have been three experiments in which patients undergoing dialysis were exposed to wild-type pig kidneys and three clinical studies related to bridging a patient in hepatic failure to liver allotransplantation. Despite the development of anti-pig antibodies, all subsequent organ (kidney or liver) allografts were successful (except possibly in one case). In addition, pig fetal islets were transplanted into patients with kidney allografts; there was no increase in panel-reactive alloantibodies and the kidney grafts continued to function satisfactorily. In conclusion, the limited data suggest that, after sensitization to pig antigens, there is no evidence of antibody-mediated or accelerated cellular rejection of a subsequent allograft.

Humoral Reactivity of Renal Transplant-Waitlisted Patients to Cells From GGTA1/CMAH/B4GalNT2, and SLA Class I Knockout Pigs

BACKGROUND

Antipig antibodies are a barrier to clinical xenotransplantation. We evaluated antibody binding of waitlisted renal transplant patients to 3 glycan knockout (KO) pig cells and class I swine leukocyte antigens (SLA).

METHODS

Peripheral blood mononuclear cells from SLA identical wild type (WT), α1, 3-galactosyltransferase (GGTA1) KO, GGTA1/ cytidine monophosphate-N-acetylneuraminic acid hydroxylase (CMAH) KO, and GGTA1/ CMAH /b1,4 N-acetylgalactosaminyl transferase (B4GalNT2) KO pigs were screened for human antibody binding using flow cytometric crossmatch (FCXM). Sera from 820 patients were screened on GGTA1/CMAH/B4GalNT2 KO cells and a subset with elevated binding was evaluated further. FCXM was performed on SLA intact cells and GGTA1/SLA class I KO cells after depletion with WT pig RBCs to remove cell surface reactive antibodies, but leave SLA antibodies. Lastly, human and pig reactive antibodies were eluted and tested for cross-species binding and reactivity to single-antigen HLA beads.

RESULTS

Sequential glycan KO modifications significantly reduce antibody binding of waitlisted patients. Sera exhibiting elevated binding without reduction after depletion with WT RBCs demonstrate reduced binding to SLA class I KO cells. Human IgG, eluted from human and pig peripheral blood mononuclear cells, interacted across species and bound single-antigen HLA beads in common epitope-restricted patterns.

CONCLUSIONS

Many waitlisted patients have minimal xenoreactive antibody binding to the triple KO pig, but some HLA antibodies in sensitized patients cross-react with class I SLA. SLA class I is a target for genome editing in xenotransplantation.

Immunological challenges and therapies in xenotransplantation

Xenotransplantation, or the transplantation of cells, tissues, or organs between different species, was proposed a long time ago as a possible solution to the worldwide shortage of human organs and tissues for transplantation. In this setting, the pig is currently seen as the most likely candidate species. In the last decade, progress in this field has been remarkable and includes a better insight into the immunological mechanisms underlying the rejection process. Several immunological hurdles nonetheless remain, such as the strong antibody-mediated and innate or adaptive cellular immune responses linked to coagulation derangements, precluding indefinite xenograft survival. This article reviews our current understanding of the immunological mechanisms involved in xenograft rejection and the potential strategies that may enable xenotransplantation to become a clinical reality in the not-too-distant future.

Allogeneic Adipose-Derived Stem Cells with Low Immunogenicity Constructing Tissue-Engineered Bone for Repairing Bone Defects in Pigs

The ideal cells for tissue engineering should have the following characteristics: easy obtainment, safety, immune privilege, the capability of self-renewal, and multipotency. Adipose-derived stem cells (ADSCs) are a promising candidate. However, the immunogenicity of allogeneic mesenchymal stem cells limits their long-term benefits. In this study, we introduced human cytomegalovirus US2/US3 gene into the ADSCs to decrease the expression of MHC I protein of ADSCs and reduce the activation of T-cells of the recipient animals. Moreover, the biosafety and biological characteristics of ADSCs transfected with the US2/US3 genes (ADSCs-US2/US3) were similar to normal ADSCs. Then we took ADSCs-US2/US3 to construct a tissue-engineered bone for repairing bone defects in pigs and found that there were no great differences in repair effects or healing time between the allogeneic ADSCs-US2/US3 group and the autologous ADSC group. These results suggest that allogeneic ADSCs-US2/US3 have the advantages of biological safety, low immunogenicity, and effective osteogenesis. Such barely immunogenic ADSCs will be crucial for the success of future tissue-regenerative approaches.

Gal/non-Gal antigens in pig tissues and human non-Gal antibodies in the GalT-KO era

Our knowledge regarding Gal and non-Gal antigens in GalT-KO pig tissues can be summarized as α3Galactosyl-tranferase gene knock out eliminates the Galα3Galβ4GlcNAc-R antigen expression in pig tissues as well as anti-Gal antibody binding. Other Galα-terminating saccharides (e.g. iGb3 glycolipids and Galα2 determinants) may be present but have not been documented. α3Galactosyl-tranferase gene knock out slightly changes the carbohydrate antigen expression but no "new" antigens recognized by the human immune system have been found. Non-Gal antigens are both of protein and carbohydrate nature but their exact chemical structures are poorly defined. Regarding human non-Gal antibodies our knowledge is as Non-Gal antibodies exist naturally and increase in humans/non-human primate (NHP) receiving WT or GalT-KO pig grafts. Non-Gal antibodies with new antigen epitope recognition can be induced in humans/NHP after challenge by WT or GalT-KO pig grafts. Non-Gal antibodies react with both carbohydrates and proteins. Part of the protein reactivity is directed to glycoprotein carbohydrates chains. Non-Gal antibodies reacting with neuraminic acid terminated saccharides (both N-Acetyl and N-Glycoloyl variants) are present in humans/NHP. Anti-neuraminic acid antibodies are increased, as well as induced, after grafting pig organs into humans/NHP. Non-Gal antibodies does not cause hyperacute xenorejection but can be cytotoxic and cause xenoorgan damage. If humans sensitized to HLA antigens are at a higher risk of rejecting pig xenograft compared with non-sensitized individuals is not fully clarified. Clinical trials are needed to evaluate the relevance of non-Gal antigens/antibodies and for the xenofield to advance.

Human monoclonal HLA antibodies reveal interspecies crossreactive swine MHC class I epitopes relevant for xenotransplantation

Crossreactivity of anti-HLA antibodies with SLA alleles may limit the use of pig xenografts in some highly sensitized patients. An understanding of the molecular basis for this crossreactivity may allow better selection of xenograft donors. We have tested 68 human monoclonal HLA class I antibodies (mAbs) for reactivity with pig lymphocytes from SLA defined pigs and found nine to be crossreactive. Eight of nine were broadly HLA reactive IgM-mAbs. The putative HLA epitopes for seven mAbs. were conserved in the aminoacid sequence of the SLA alleles studied. The lack of reactivity of a large number of mAbs largely correlated with the absence of the putative epitopes in the SLA alleles studied. We conclude that most patients with anti-HLA class I antibodies should be able to find pig donors lacking SLA antigens that cross react with their antibodies and that many of the crossreacting epitopes can be defined by analysis of shared epitopes in the aminoacid sequence of human and pig MHC antigens.

Acute humoral rejection of renal transplants in alloimmunized pigs

BACKGROUND

Despite progress in immunosuppression, acute humoral rejection (AHR) remains an unsolved issue in transplantation, with a possible reversibility in only about 50% of cases. AHR is characterized by antidonor antibodies in the recipient circulation and characteristic histological lesions within the graft itself, as well as complement degradation products and immunoglobulins (IgM and IgG) deposition in vascular zones. The lack of a large animal models of AHR in experimental allotransplantation led us to establish such a model in the pig.

MATERIALS AND METHODS

Pigs were immunized with peripheral blood mononuclear cells from allogeneic donors and subsequently received a kidney from the same donor, once antidonor antibodies (Ab) had reached a plateau. The efficiency of the alloimmunization, the nature of the induced antibodies and rejection were studied.

RESULTS

Six out of seven recipients developed specific antidonor Ab. Three days post-transplantation, characteristic lesions of AHR were observed together with intragraft IgG, IgM, and complement deposition. The AlloAb were found to be cytotoxic and directed against donor MHC class I molecules.

CONCLUSIONS

We described, for the first time, a large animal model of AHR, which will enable us to more extensively study phenomena implicated in AHR and to test new strategies aimed at its prevention or cure, as well as improve transplant protocols in the case of presensitized or hyperimmunized patients.

Allosensitization does not increase the risk of xenoreactivity to alpha1,3-galactosyltransferase gene-knockout miniature swine in patients on transplantation waiting lists

BACKGROUND

The recent availability of alpha1,3-galactosyltransferase knockout (GalT-KO) miniature swine has eliminated anti-Gal antibodies as the major barrier to xenotransplantation, potentially bringing this modality closer to clinical application. Highly-allosensitized patients, who have poor prospects of receiving a suitable cross-match negative human organ, might be the first patients to benefit from xenotransplantation of porcine organs. However, concerns exist regarding cross-reactivity of alloreactive anti-human leukocyte antigen (HLA) antibodies against xenogeneic swine leukocyte antigen (SLA) antigens. We have investigated this question using sera from such patients on GalT-KO target cells.

METHODS

Using flow cytometry and complement-dependent cytotoxicity (CDC) assays, we have tested a panel of 88 human serum samples from patients awaiting cadaveric renal allotransplantation for reactivity against: 1) human; 2) standard miniature swine; and 3) GalT-KO peripheral blood lymphocytes (PBL) and cultured endothelial cells.

RESULTS

Anti-swine IgM and IgG antibody binding, as well as CDC, were significantly attenuated on GalT-KO versus standard swine. No correlation was found between the degree of anti-human panel reactive antibodies (PRA) and xenoreactivity against either standard or GalT-KO miniature swine. Treatment of sera with dithiothreitol (DTT) showed that the majority of remaining lymphocytotoxicity against GalT-KO swine was mediated by preformed IgM antibodies. Patients with high alloreactivity but low anti-GalT-KO xenoreactivity were readily identified.

CONCLUSIONS

Highly allosensitized patients awaiting renal transplants appear to be at no increased risk of xenosensitization over their non-sensitized cohorts, and could therefore be candidates for xenotransplantation using GalT-KO swine donors.

Human DAF on pig cells protects against human and non-human primate sera cytotoxicity mediated by exogenous or endogenous complement, as determined by flow cytometry

Expression of human complement regulatory proteins (CRP) in pig cells through transgenesis was proposed to prevent complement activation and the ensuing rejection of pig tissues and organs following pig-to-primate transplantation. Transplantation in non-human primates of organs from transgenic pigs for human decay accelerating factor (hDAF) did not undergo hyperacute rejection, but hDAF could not prevent humoral xenograft rejection (AHXR). A possible explanation for the lack of efficacy of the expression of human complement regulatory proteins in pig cells to prevent AHXR may be interspecies differences between human and non-human complement regulatory system. We assayed the efficacy of transgenic hDAF expressed on porcine cells to inhibit the in vitro complement activity of primate sera. The individual cytotoxicity of sera from seven untreated baboons and of pools of normal human and baboon sera was assayed with endogenous and exogenous complement using a flow-cytometry complement-mediated cytotoxicity assay (FCCA) against peripheral blood lymphocytes (PBL) from hDAF and non-transgenic pigs. We also analyzed the anti-Galalpha1-3Gal (alphaGal) antibody titre of the baboon sera by ELISA and the expression of hDAF on the PBL surface by immunofluorescence. Transgenic hDAF expression was capable of protecting pig cells against injury produced by both baboon and human serum. Cellular expression of hDAF reduced cytotoxicity mediated by endogenous and exogenous complement, although the former was slightly higher. Humoral cytotoxicity was not related to a particular antibody but was inversely related to hDAF expression. The presence of hDAF protected pig cells against lysis by NHS more effectively than against NBS. These results confirm in vitro the protective role of hDAF in pig cells to heterologous complement mediated damage, but they also suggest that the extent of hDAF protection decreases, however, if cells express low levels of hDAF.

Does exposure to swine leukocyte antigens after pig-to-nonhuman primate xenotransplantation provoke antibodies that cross-react with human leukocyte antigens?

BACKGROUND

A potential concern of using pig kidney xenografts for human transplantation is that antibodies produced to swine leukocyte antigens (SLA) may cross-react with human leukocyte antigens (HLA) and thereby limit the scope for a subsequent human organ donor transplant. We therefore investigated whether exposure to SLA after pig-to-nonhuman primate kidney xenotransplantation gives rise to HLA cross-reactive antibodies.

METHODS

Serum samples were obtained from 52 cynomolgus monkeys that received kidney transplants from human decay-accelerating factor (hDAF) transgenic pigs. Samples were collected pre-transplant and at time of autopsy (mean 20 days post-transplantation, range 1 to 53 days) and analyzed for IgG HLA class I and HLA class II specific antibodies by enzyme-linked immunosorbent assay (ELISA) against pooled purified HLA antigens. To ensure the ability of the HLA ELISA to detect cynomolgus monkey IgG binding, parallel experiments were performed to detect IgG Gal-alpha-1,3-Gal-specific antibodies known to be present in cynomolgus monkey serum.

RESULTS

Analysis of both pre- and post-transplantation serum samples by ELISA demonstrated no detectable IgG antibody binding to HLA class I or class II antigens. Using the same ELISA antibody detection reagents, IgG Gal-alpha-1,3-Gal-specific antibodies were identified in 13 of 38 (34%) sera obtained before transplantation and 21 of 52 (40%) sera collected post-transplantation, confirming that the negativeHLA ELISA results were not due to a technical aspect of the assay.

CONCLUSION

This study suggests that exposure to SLA following transplantation of porcine kidneys in nonhuman primates does not give rise to antibodies that cross-react with HLA.

Flow cytometry complement-mediated cytotoxicity assay detects baboon xenoantibodies directed to porcine epitopes undetected by hemolytic assay

The pig-to-primate model is increasingly being utilized as the final preclinical means of assessing therapeutic strategies aimed at allowing discordant xenotransplantation. To obtain information about the nature of cytotoxic response in pig-to-baboon xenotransplants, we sought to determine if serum cytotoxicity in this model was assay dependent. Sera from nine kidney or heart xenotransplanted baboons were obtained before transplantation and at the time of acute humoral xenograft rejection (AHXR). Cytotoxicity was measured by an anti-pig haemolytic assay (APHA) and by a flow cytometry complement-dependent assay (FCCA), using pig blood lymphocytes (PBLs). Serum samples showing inter-assay differences were absorbed with pig erythrocytes and assayed by APHA and FCCA, as well as by measuring anti-alphaGal and total anti-pig xenoantibodies. The results showed that in four AHXR samples, FCCA cytotoxicity was higher than APHA cytotoxicity. Absorption with pig erythrocytes diminished FCCA and removed APHA cytotoxicity. Residual FCCA activity was due to total anti-pig and IgM anti-alphaGal and non-Gal antibodies. Our results indicate that some cytotoxic antibodies present in the sera of xenotransplanted baboons at time of AHXR are IgM antibodies directed against pig PBL antigens not detected by APHA.

Hyperacute rejection of hDAF-transgenic pig organ xenografts in cynomolgus monkeys: influence of pre-existing anti-pig antibodies and prevention by the alphaGAL glycoconjugate GAS914

BACKGROUND

Our introductory pig-to-cynomolgus monkey heart or kidney transplantation using organs from pigs transgenic for human decay-accelerating factor (hDAF), showed a high incidence of hyperacute rejection (HAR), which was ascribed to extraordinary high levels of anti-pig antibodies. We evaluated the efficacy of GAS914, a Gal alpha 1-3Gal trisaccharide linked to a poly-l-lysine backbone, in inhibition of HAR.

METHODS

hDAF transgenic heterotopic heart (n = 15) or life-supporting kidney (n = 8) transplantation included induction with cyclophosphamide or anti-thymocyte globulin, and maintenance with cyclosporine or tacrolimus, steroids and mycophenolate sodium/mofetil. Four doses of GAS914 were given before transplantation. Rejection was confirmed by graft histology, and anti-pig antibody levels were determined in various assays.

RESULTS

Four of six heart transplants without GAS914 treatment showed HAR. Nine subsequent transplants with GAS914 pre-treatment, did not show HAR (chi-square, P < 0.05). Two of four kidney transplants without GAS914 treatment ended with HAR. Four subsequent transplants with GAS914 did not show HAR. Animals with HAR showed extremely high antibody levels. Samples just before transplantation showed significantly higher antibody levels in recipients presenting with HAR. In all assays antibody levels were significantly lowered by GAS914 pre-treatment.

CONCLUSIONS

HAR of hDAF solid organs could be ascribed to high levels of anti-pig antibodies. It is hypothesized that the hDAF transgene shows a threshold in efficacy, above which an overwhelming attack by antibodies and complement activation cannot be modulated to prevent HAR. HAR does not occur when animals with lower levels are used, or when antibodies are effectively depleted from the circulation by GAS914 treatment.

Cross-reactivity between swine leukocyte antigen and human anti-HLA-specific antibodies in sensitized patients awaiting renal transplantation

Xenotransplantation is increasingly viewed as a promising way to alleviate the problem of patients who have alloreactive lymphocytotoxic antibodies and therefore tend to accumulate on the waiting list for renal transplantation. One barrier to xenotransplantation in these patients could be the hyperacute or acute vascular rejection as a result of preexisting anti-HLA antibodies that recognize swine leukocyte antigens. The cross-reactivity of sera from 98 patients with pig lymphocytes was studied by flow cytometry. After absorption of xenoreactive natural antibodies (XNA), isotype, class, and antibody specificity causing a positive cross-match (XM) were determined. For nonsensitized patients, all of the antibody binding to pig lymphocytes was due to XNA, which were removed by pig red blood cells absorption. In contrast, in sensitized patients, after removal of XNA, pig lymphocyte XM remained positive. There was no correlation between antibody binding to pig lymphocytes and Ig isotype (IgG or IgM) or HLA class-specific antibodies. For testing evidence that class II-specific antibodies were responsible for antibody binding to pig lymphocytes, HLA class I-specific antibodies were absorbed with pooled human platelets. It was confirmed that HLA class II-specific antibodies were responsible for the positive pig XM, but the strength of the positive XM was weaker than the strength caused by HLA class I-specific antibodies. Sera with multiple specificities (plurispecific sera) displayed a greater frequency of cross-reactivity with swine leukocyte antigens (P < 0.05). Seven of 11 highly immunized patients without cross-reactivity IgG with porcine lymphocytes showed positive XM before an IgM was used. The results demonstrate the cross-reactive nature of HLA antibodies and therefore point out the need to perform a prospective XM after absorption of XNA in presensitized individuals.

Anti-pig antibody levels in non-human primates of various origin

BACKGROUND

Natural anti-porcine antibodies play a major role in hyperacute solid organ xenograft rejection in the pig-to-non-human primate model. Work from other groups and our experience in transplantation experiments has shown that antibody levels are highly variable between non-human primate species, and that extremely high levels can mediate hyperacute rejection even if organs from animals transgenic for human decay-accelerating factor are used.

METHODS

Sera were obtained from cynomolgus monkeys wild-caught in Mauritius, captive-bred in the Philippines, captive-bred in Indonesia (Indonesia-Ind), and originating from Indonesia but colony-bred in USA (Indonesia-USA), from baboons wild-caught in Kenya, and from rhesus monkeys originating from India but colony-bred in USA (10 animals in each group). Antibody levels were determined using assays for haemolytic antibody (APA), IgM and IgG class anti-Galalpha1-3Gal antibody, and IgM and IgG class anti-endothelial cell antibody.

RESULTS

Cynomolgus monkeys from the Philippines and Indonesia-USA and rhesus monkeys showed median APA and IgM antibody levels in the same range as a pooled human serum standard, and median IgG levels well below the level in this standard. Cynomolgus monkeys from Mauritius and Indonesia-Ind showed extremely high APA levels (median seven to 10 times the human serum standard): IgM class antibodies were also higher, while IgG class antibodies were in the range of the level in the human serum standard. Antibody levels in baboons were in between these two categories. The results of the APA assay showed a highly statistically significant correlation with the assays of IgM antibody, and this was also the case for the IgM antibody assays, indicative of the assessment of the same antibodies in these assays. The same was observed for the assays for IgG antibody. Taking body weight as an indicator for age, there was no relationship between body weight and levels of antibodies.

CONCLUSIONS

Natural antibody levels show a significant variation between various groups of non-human primates, with levels in some groups well above those in a human serum standard.

Expression of CTLA-4 in nonhuman primate lymphocytes and its use as a potential target for specific immunotoxin-mediated apoptosis: results of in vitro studies

T-cell-mediated immunoregulation is one of the main mechanisms implicated in induction and maintenance of transplantation tolerance. In this regard, deletion or modulation of xeno/alloantigen-specific T cells, as well as blocking of their interactions with other cell populations, are currently being pursued for tolerance induction in humans as well as nonhuman primates. In order to investigate whether cytotoxic T-lymphocyte antigen-4 (CTLA-4) may represent a suitable target for a T cell depletion approach in nonhuman primate models, we analysed CTLA-4 expression in peripheral blood mononuclear cells (PBMCs) from nonhuman primates and the potential role of two anti-CTLA-4 saporin-conjugated immunotoxins. The analysis was performed in PBMCs from 8 cynomolgus monkeys from Philippines and from Mauritius both at protein level by flow cytometry and at transcriptional level by RT-PCR. In addition, the apoptotic role of the immunotoxins was investigated. The results showed that CTLA-4 was expressed at variable levels depending on the origin of the cynomolgus monkeys and the resting or activated cell condition. CTLA-4 was not expressed on resting Mauritius PBMCs and showed a lower up-regulation upon PMA/PHA activation compared to the Philippines PBMCs that expressed CTLA-4 also before activation. Two CTLA-4 RNA transcripts (672 and 550 bp) were detected with levels variations after cell stimulation. Two anti-CTLA-4 immunotoxins induced in vitro apoptosis of activated PBMCs from both sources of cynomolgus monkeys. This is the first report that documents CTLA-4 expression both at protein and transcriptional level by nonhuman primate PBMCs and provides novel perspectives of xeno/allograft rejection immunotherapy based on CTLA-4 targeting.

Alloantibody and xenoantibody cross-reactivity in transplantation

The recent availability of pigs homozygous for alpha1,3-galactosyltransferase gene knockout, and improved immunosuppressive regimens that prevent an elicited antibody response, are expected to contribute to significantly increased survival of pig organs transplanted into primates, bringing clinical trials of xenotransplantation closer. Patients highly sensitized to human leukocyte antigens, who may be precluded from obtaining a human donor organ, would be one group that might benefit from xenotransplantation. However, there have been few studies on whether there is cross-reactivity of anti-human leukocyte antigen antibodies with pig antigens. What data there are suggest that such cross-reactivity exists and that this may be detrimental to the outcome after transplantation of a pig organ. Neither is it known whether sensitization after a pig xenograft would preclude subsequent allotransplantation, although the data available suggest that this will not be the case. Further investigation on allo- and xenoantibody cross-reactivity is required.

Absence of humoral and cellular alloreactivity in baboons sensitized to pig antigens

AIM

to study whether sensitization to pig antigens results in humoral and/or cellular sensitization to alloantigens in baboons, and thus increases the risks of organ allotransplantation after xenotransplantation. Serum from baboons that were naive (n = 4), sensitized to Gal alpha 1,3Gal (Gal) antigens (n = 2), or sensitized to Gal + non-Gal pig antigens (n = 2) were tested by flow cytometry for the presence of immunoglobulin G (IgG) and IgM antibodies that bind to pig or baboon peripheral blood mononuclear cells (PBMC). Two allosensitized baboons were used as positive controls. The same 10 sera were tested in a complement-mediated cytotoxicity assay to detect cytotoxic antibodies against pig, allo and self-PBMC. The T-cell responses of the same baboons to allogeneic and pig PBMC stimulators in mixed lymphocyte reaction (MLR) were studied. All baboon sera contained cytotoxic antibodies that bound to pig PBMC. Binding and cytotoxicity were higher in xenosensitized baboons, particularly in those sensitized to Gal + non-Gal antigens (P < 0.001). None of the naive or xenosensitized baboon sera bound to baboon PBMC. Serum from allosensitized baboons showed anti-baboon IgG and IgM binding, but there was no increase in binding to pig PBMC or in cytotoxicity to pig cells. The MLR response to pig stimulators in baboons sensitized to non-Gal pig antigens was greater than that of naive or Gal-sensitized baboons (P < 0.001), but there was no increase in the response to baboon cells. In baboons, no in vitro evidence that a previous pig xenograft might endanger the outcome of a subsequent allograft was documented.

Comparison of allogeneic and xenogeneic in vitro T-cell proliferative responses in sensitized patients awaiting kidney transplantation

Highly sensitized patients awaiting kidney transplantation may be potential candidates for future clinical trials using pig organ donors. Because of crossreactivity between human leucocyte antigens (HLA) and swine leucocyte antigens (SLA), such patients might have heightened T-cell responses to porcine xenoantigens. We determined whether lymphocytes from allo-sensitized patients displayed secondary (cyclosporine resistant) T-cell proliferative responses against porcine xenoantigens. Lymphocytes from six non-sensitized, seven sensitized [immunoglobulin G (IgG) panel reactive antibodies (PRA) 11 to 84%], 14 highly sensitized patients (IgG PRA > 84%) and 12 healthy individuals were tested [in the presence and absence of Cyclosporin A (CsA)] to determine their proliferative response to human (allogeneic) and to porcine (xenogeneic) stimulator cells. Lymphocytes from all study groups showed a strong proliferative response to allogeneic and xenogeneic stimulator cells with no significant difference between non-sensitized and sensitized individuals. Addition of CsA (100 and 500 ng/ml) inhibited (>90%) proliferation of lymphocytes from all non-sensitized patients to both allogeneic and xenogeneic stimulators. CsA was less effective at inhibiting proliferation of lymphocytes from sensitized patients and highly sensitized patients to allogeneic stimulators [29% (n=21) and 50% (n=42) respectively were resistant to CsA inhibition (100 ng/ml)]. In contrast, cyclosporine inhibited proliferation of lymphocytes from the majority of sensitized and highly sensitized patients to xenogeneic stimulator cells [14% (n=21) and 14% (n=42) respectively were resistant to CsA inhibition (100 ng/ml)]. HLA sensitized patients awaiting renal transplantation display cyclosporine resistant proliferative T-cell responses to allogeneic stimulators but proliferative responses to xenogeneic stimulators are more amenable to suppression by CsA. This finding suggests that humoral sensitisation to HLA antigens is not necessarily indicative of a heightened in vitro T-cell response to SLA antigens.

Potential implications of ABO blood group for vascular rejection in pig to human kidney xenotransplantation

BACKGROUND

A substantial hurdle for successful xenotransplantation is to negate the effect of xenoreactive natural antibodies [mainly Galalpha1-3Galbeta1-4GlcNAc (alpha-Gal) specific] that cause hyperacute xenograft rejection. Galalpha1-3Gal molecules (alpha-Gal) have close structural homology with human ABO blood groups and therefore an individual's blood group might influence the formation of alpha-Gal specific antibodies. Genetic heterogeneity controlling alpha-Gal specific antibody formation could have important implications for future pig to human xenotransplantation clinical trials. We have investigated the relationship between ABO blood group and immunoglobulin M (IgM) and immunoglobulin G (IgG) alpha-Gal specific antibody titres in sera obtained from renal dialysis patients and healthy blood donors.

METHODS

Serially diluted sera (n = 166) obtained from renal dialysis patients awaiting kidney transplantation (n = 116) and healthy blood donors (n = 50) were tested for IgM and IgG alpha-Gal antibodies using an enzyme-linked immunosorbent assay (ELISA) specific for alpha-Gal. The study cohort comprised 62, 48, 36 and 20 sera obtained from blood group O, A, B and AB individuals, respectively. Reciprocal alpha-Gal specific antibody titres were calculated from ELISA titration curves and stratified by individual blood group.

RESULTS

No significant heterogeneity was found in IgM alpha-Gal specific antibody titres across ABO blood groups. In contrast, marked heterogeneity was observed in IgG alpha-Gal specific antibody titres when stratified by blood group. IgG alpha-Gal specific antibody titres were higher in sera obtained from blood group O renal dialysis patients [median titre 40, interquartile range (IQR) 14 to 72], compared with blood group A (median titre 18, IQR 7 to 54, P = 0.05), blood group B (median titre 6, IQR 0 to 15, P < 0.001) and blood group AB patients (median titre 3.5, IQR 0 to 16, P = 0.002). A similar correlation was found for IgG alpha-Gal specific antibody titres in sera obtained from healthy blood donors with median titres of 20 (IQR 12 to 34), 37 (10 to 91), 9 (0 to 20), and 5.5 (0 to 12) in blood groups O, A, B and AB individuals, respectively. There was a strong interrelationship between alpha-Gal specific antibody class and blood group, with both IgM and IgG alpha-Gal specific antibodies found in 84% of the blood group O sera, 73% of blood group A sera, 50% of blood group B sera and 40% of blood group AB sera (P < 0.001). In a subgroup of 39 renal dialysis patients, IgM and IgG alpha-Gal specific antibody titres were measured in two serum samples obtained at different time-points (median time interval 581 days, range 42 to 4414), and showed a high degree of stability (correlation coefficient 0.88 and 0.90 for IgM and IgG, respectively).

CONCLUSION

IgG alpha-Gal specific antibody titres are significantly higher in the sera of blood group O and A renal dialysis patients and healthy individuals compared with blood groups B and AB. These data indicate that future clinical trials of pig to human xenotransplantation may be more problematic for non-blood group B patients who are likely to have high levels of IgG alpha-Gal specific antibodies that are associated with acute vascular rejection.

Serum anti-pig antibodies as potential indicators of acute humoral xenograft rejection in pig-to-cynomolgus monkey kidney transplantation

BACKGROUND

Hyperacute rejection of solid organ pig xenografts in nonhuman primates has been overcome by using donors transgenic for human complement regulatory proteins, but grafts are still susceptible to humoral (antibody-mediated) rejection. We investigated whether circulating xenoreactive antibodies are a useful indicator of this xenograft rejection.

METHODS

Five assays were employed in a retrospective analysis on 20 selected cynomolgus monkey recipients of renal xenografts transgenic for human decay-accelerating factor, with survival between 4 and 60 days. The assays included hemolytic and hemagglutination assays and the measurement of immunoglobulin (Ig)G and IgM binding to porcine endothelial cells and leukocytes, and to the Gal alpha 1-3Gal trisaccharide (Gal) antigen. To assess non-Gal-directed antibodies, sera were absorbed with a Gal-coated resin. A predictive value was defined as an increase in antibody levels before a decline in graft failure (>20% increase in creatinine levels) and humoral rejection in graft pathology.

RESULTS

Data on hemolytic anti-pig antibody correlated with those on IgM antibody to endothelial cells, leukocytes, and Gal. In absorbed sera IgM and IgG antibody to endothelial cells and leukocytes correlated with each other, indicative for an elicited antibody response to non-Gal antigens. Sixteen animals showed humoral rejection, and in all but two animals one or more assays was considered of predictive value. On the other hand, increased antibody levels were noted in two animals without signs of rejection in graft pathology and in two cases with cellular xenograft rejection.

CONCLUSIONS

It is recommended to use multiple assays (preferably hemolytic, anti-Gal, and anti-endothelial cell) to be able to fully monitor the peripheral antibody responses in pig-to-primate xenograft recipients.

Anti-pig antibody levels in naïve baboons and cynomolgus monkeys

Anti-pig antibodies (APA) were analysed in serum from 28 naïve wild-caught baboons (originating from Kenya) and 31 naïve captive-bred cynomolgus monkeys (13 from the Philippines and 18 from Mauritius), using a haemolytic assay with pig erythrocytes (APA), flow cytometry on the porcine lymphoma T-cell cell line L35, and enzyme linked immunosorbent assay (ELISA) using alpha-Gal type II and type VI antigen. This was extended in baboon samples by the evaluation in two laboratories (Imutran, Cambridge, UK and Immerge, Boston, USA), and by antibody absorption using either immobilized alpha-Gal type II or alpha-Gal type VI. Anti-porcine antibodies were demonstrated in all assays with substantial variability within and between the three non-human primate groups. Immunoglobulin (Ig)M antibody levels tended to be similar to or higher than those in a pooled normal human standard serum while IgG levels tended to be lower. Highest antibody levels were recorded in Mauritius cynomolgus monkeys. There were statistically significant correlations between assays for IgM or IgG class anti-Gal antibodies using either alpha-Gal type II or alpha-Gal type VI as antigen, both for different assays and two laboratories involved. Also, significant correlations were observed between the anti-Gal and L35 binding assays. Baboon sera before and after absorption to immobilized alpha-Gal type II or type VI were analysed for anti-Gal type VI or type II antibody: levels were almost undetectable indicating that most anti-Gal antibodies react to epitopes shared between alpha-Gal type II and type VI oligosaccharides. Finally, the relation between APA and outcome of porcine heart xenotransplantation in cynomolgus monkeys and baboons showed no apparent relation between pre-transplant APA levels and the occurrence of hyperacute rejection (HAR) when compared with non-immunological cause of organ/recipient dysfunction or acute humoral xenograft rejection during the first 4 days post-transplantation or survival exceeding 4 days post-transplantation.

Sensitisation to swine leukocyte antigens in patients with broadly reactive HLA specific antibodies

We have previously shown that IgG HLA specific antibodies in the sera of highly sensitised patients awaiting renal transplantation can cross-react with swine leukocyte antigens (SLA). In this study we determined the frequency of patient serum IgG HLA specific antibody binding to a porcine lymphocyte panel and the likelihood of locating a cross-match negative pig donor for sensitised patients. Serum samples (n = 82) were obtained from 35 sensitised [current IgG panel reactive antibodies (PRA) > 10%] and seven nonsensitised patients awaiting renal transplantation at Addenbrooke's Hospital, Cambridge, UK. Fifty sera had IgG HLA specific PRA of 11-84%, 20 had IgG PRA of >84% and 12 had 0% PRA (negative controls). Sera were absorbed with porcine erythrocytes to remove xenoreactive natural antibodies and tested for cross-reactive IgG HLA specific antibody binding by flow cytometry against a panel of porcine lymphocytes obtained from 23 human decay accelerating factor (hDAF) transgenic pigs. A total of 1,884 cross-match combinations were tested and 369 (20%) gave a positive porcine lymphocyte cross-match. For sera from sensitised patients with IgG PRA (11-64%), only 6 of 805 (0.75%) cross-match tests were positive. In contrast, for sera from patients with high IgG PRA (>64%), 363 of 805 (45%) cross-match tests were positive (p < 0.0001). There was no difference in the frequency of positive cross-matches between patient sera with IgG PRA 65-84% and highly sensitised patient sera with IgG PRA 85-100% [156/345 (45%) vs. 207/460 (45%)]. This study demonstrates that only patient sera with broadly reactive IgG HLA specific PRA (>64%) cross-react with porcine lymphocytes. If future clinical trials of xenotransplantation are undertaken, it may be of value to select a cross-match-negative pig organ donor for such patients.

The potential role of xenotransplantation in treating endstage cardiac disease: a summary of the report of the Xenotransplantation Advisory Committee of the International Society for Heart and Lung Transplantation

There is an increasing shortage of human donor hearts for transplantation. One potential solution is using hearts from a suitable animal source, such as the pig. A committee of the International Society for Heart and Lung Transplantation has reviewed the current status of research into xenotransplantation. Furthermore, the committee considered what results in the pig-to-nonhuman primate experimental model would justify a clinical trial of xenotransplantation, and the criteria for selecting patients to be entered in the trial. This review emphasizes initial patient selection issues, although the committee's overall conclusions and recommendations are summarized. Although the current experimental results do not presently justify initiating a clinical trial, the committee concluded that xenotransplantation theoretically has immense potential, and that research in this field should be encouraged and supported. Human cadaveric organ donation will still be of the highest priority for the foreseeable future.

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.

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.

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.