Utility of xenografts: Lack of correlation between PRA and natural antibodies to swine

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.

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.

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.

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.

Increased levels of anti-non-Gal IgG following pig-to-baboon bone marrow transplantation correlate with failure of engraftment

BACKGROUND

The development of genetically modified pigs, which lack the expression of alpha 1-3 galactosyl transferase, (GalT-KO pigs) has facilitated the xenogeneic transplantation of porcine organs and tissues into primates by avoiding hyperacute rejection due to pre-existing antibodies against the Gal epitope. However, antibodies against other antigens (anti-non-Gal antibodies), are found at varying levels in the pre-transplant sera of most primates. We have previously found that baboons with high levels of pre-transplant anti-non-Gal IgG, conditioned with a non-myeloablative conditioning regimen, failed to engraft following pig-to-baboon bone marrow transplantation (Xenotransplantation, 17, 2010 and 300). Two baboons with low levels of pre-transplant anti-non-Gal IgG, conditioned with the same regimen, showed porcine bone marrow progenitors at 28 days following transplantation, suggesting engraftment. These baboons also showed evidence of donor-specific hyporesponsiveness. This observation led us to investigate the hypothesis that selecting for baboon recipients with low pre-transplant anti-non-Gal IgG levels might improve engraftment levels following GalT-KO pig-to-baboon bone marrow transplantation.

METHODS

Five baboons, with low pre-transplant anti-non-Gal IgG levels, received transplantation of bone marrow cells (1-5 × 10(9) /kg of recipient weight) from GalT-KO pigs. They received a non-myeloablative conditioning regimen consisting of low-dose total body irradiation (TBI) (150 cGy), thymic irradiation (700 cGy), anti-thymocyte globulin (ATG), and tacrolimus. In addition, two baboons received Rituximab and Bortezomib (Velcade) treatment as well as extra-corporeal immunoadsorption using GalT-KO pig livers. Bone marrow engraftment was assessed by porcine-specific PCR on colony forming units (CFU) of day 28 bone marrow aspirates. Anti-non-Gal antibody levels were assessed by serum binding toward GalT-KO PBMC using flow cytometry (FACS). Peripheral macro-chimerism was measured by FACS using pig and baboon-specific antibodies and baboon anti-pig cellular responses were assessed by mixed lymphocyte reactions (MLR).

RESULTS

As previously reported, two of five baboons demonstrated detectable bone marrow engraftment at 4 weeks after transplantation. Engraftment was associated with lack of an increase in anti-non-Gal IgG levels as well as cellular hyporesponsiveness toward pig. Three subsequent baboons with similarly low levels of pre-existing anti-non-Gal IgG showed no engraftment and an increase in anti-non-Gal IgG antibody levels following transplantation. Peripheral macrochimerism was only seen for a few days following transplantation regardless of antibody development.

CONCLUSIONS

Selecting for baboon recipients with low levels of pre-transplant anti-non-Gal IgG did not ensure bone marrow engraftment. Failure to engraft was associated with an increase in anti-non-Gal IgG levels following transplantation. These results suggest that anti-non-Gal-IgG is likely involved in early bone marrow rejection and that successful strategies for combating anti-non-Gal IgG development may allow better engraftment. Since engraftment was only low and transient regardless of antibody development, innate immune, or species compatibility mechanisms will likely also need to be addressed to achieve long term engraftment.

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.

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.

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.

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.

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.

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.

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.

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

BACKGROUND

The presence of IgG HLA-specific antibodies in the serum of patients awaiting transplantation indicates T- and B-cell priming and would result in acute rejection of a poorly matched human allograft. Recent advances in xenotransplantation, with the amelioration of hyperacute rejection using transgenic pig kidneys, may benefit such patients. However, accelerated cellular rejection might result from the primed T-cell recognition of antigenic epitopes shared between pig and human MHC molecules.

METHODS

We have compared the reactivity of IgG antibodies from 8 nonsensitized (NS) and 13 highly sensitized (HS) patients with human and pig lymphocytes by flow cytometry. Xenoreactive natural antibodies (XNA) were absorbed with pig red blood cells, and HLA class I-specific antibodies were further absorbed with pooled human platelets.

RESULTS

Before XNA absorption, 20 of the 21 patients had a positive IgG crossmatch with pig lymphocytes, and there was no difference between NS and HS patients. In contrast, after XNA absorption, none of the 8 NS patients were positive, compared with 9 of the 13 HS patients (mean of the median channel fluorescence values of 7.7 and 86.5, respectively; P=<0.001). For XNA-absorbed HS patient sera, 20 of 30 (67%) pig lymphocyte crossmatch combinations were positive, with a mean median channel fluorescence value of 125 (range 31 to 294) compared with 9.5 (range 7 to 13) for the 10 crossmatch-negative combinations. Platelet absorption resulted in a concomitant reduction in antibody binding to pig lymphocytes in three of six HS patient sera, indicating that HLA class I-specific antibodies are responsible, at least in part, for the positive crossmatch.

CONCLUSION

These results suggest that some IgG HLA-specific antibodies can bind to pig lymphocytes, analogous to a positive crossmatch with allogeneic donors.