Interaction of anti-HLA antibodies with pig xenoantigens

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.

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.

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.

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.

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.

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.

The study of mitoxantrone as a potential immunosuppressor in transgenic pig renal xenotransplantation in baboons: comparison with cyclophosphamide

Mounting evidence suggests that delayed xenograft rejection (DXR) of discordant xenografts has a strong humoral component. To explore the possibility of targeting this humoral response more efficiently, we performed a preliminary study in baboons immunized against pig blood cells using the immunosuppressor mitoxantrone (Mx). The results from this study showed that, in comparison with cyclophosphamide (CyP), Mx induced a long-lasting depletion of circulating B cells within 6 days of its administration and delayed secondary anti-Gal antibody (Ab) responses to pig blood cell immunizations. Given these results, we next evaluated Mx in an in vivo model of pig to baboon renal xenotransplantation. We performed a series of renal xenotransplantations in baboons using human CD55-CD59 transgenic donor pigs. In the first group of baboons (Mx group; n = 4) Mx was administered 6 days prior to the day of transplantation, the objective being to perform the xenotransplantation in a context where the recipient would have few remaining circulating B cells and thus have an impaired capacity to mount an Ab response to the xenograft. We compared this group to a second group of baboons treated with CyP starting 1 day prior to transplantation (CyP group; n = 2). All baboons receiving Mx or CyP received an additional immunosuppression of cyclosporin A, mycophenolate mofetil and steroids. No hyperacute rejection was observed in either group but all xenografts underwent DXR. Mx did not show superiority to CyP in terms of graft survival with a mean survival time of 8 +/- 2 days compared with 9 days for both CyP-treated baboons. Neither CyP nor Mx decreased serum levels of pre-existing anti-Gal Abs but levels of these Abs decreased dramatically within 1 day of transplantation, likely reflecting their immediate trapping within the xenograft. Interestingly however, in contrast to CyP, Mx inhibited the return of anti-Gal immunoglobulin M (IgM) to the circulation, even at the time of rejection. Nevertheless, strong intragraft deposits of IgM, IgG and the activated complement complex C5b-9 were observed in biopsies at rejection. Furthermore, despite the expected profound depletion of circulating B cells by Mx within 6 days of its administration, biopsies from both groups at rejection displayed a mild B cell infiltrate accompanied by a strong macrophage and intermediate T-cell infiltration, the latter tending to be more abundant in Mx-treated animals. Our data show that in this particular model of pig to baboon xenotransplantation and at the dose used, Mx was not superior to CyP in conferring protection against rejection, despite its capacity to profoundly deplete circulating B cells and to inhibit anti-Gal Ab responses to xenografts. DXR was thus possible without the return of anti-Gal Abs and may have been mediated by the early fixation of pre-existing Abs with secondary complement activation. However, although Mx was not more efficient than CyP in controlling DXR, its capacity to deplete B cells and delay Ab recovery may be beneficial in the context of Gal knockout organ transplantation where the induced Ab response is likely to take precedence over the preformed response.

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.

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.

The effect of immunoglobulin immunoadsorptions on delayed xenograft rejection of human CD55 transgenic pig kidneys in baboons

Delayed xenograft rejection (DXR) remains a major obstacle in discordant xenotransplantation. As strategies of complement inhibition and xenogeneic natural antibody (Ab) removal have been shown to give prolonged xenograft survival, we endeavored to determine whether combining these two strategies would lead to an additive effect in terms of graft survival. The study was initiated with two groups, A and B, where group A received normal kidneys and group B received hCD55 transgenic kidneys. Both groups underwent pre-transplant (day-1) total immunoglobulin (Ig) immunoadsorption (IA) and received an immunosuppression of cyclophosphamide, cyclosporine A, mycophenolate mofetil and corticosteroids. Two subsequent groups (C and D) receiving hCD55 transgenic pig kidneys were then performed with an 'optimized' immunosuppression (Cyclophosphamide starting 1 day earlier) but only group D recipients were immunoadsorbed. Biopsies taken during the post-transplantation period were analyzed for Ab deposition, compliment activation and cellular infiltration. No hyperacute rejection was observed. In the initial immunoadsorbed groups A and B, all baboons underwent DXR, which started surprisingly early (day 5 in most cases. In the subsequent two groups, the immunoadsorbed group D baboons also underwent DXR, again as early as day 5. In contrast, group C baboons did not show any signs of DXR on their day 6 biopsy or at their time of death. Analysis of graft biopsies from the kidneys undergoing rejection or with stable function showed strong deposition of anti-Gal IgM in all cases whereas strong complement C5b-9 deposits were only observed in biopsies at rejection. Cellular infiltration consisted mostly of monocytes/macrophages, was more pronounced in biopsies taken at rejection and was associated with a pro-inflammatory environment involving interleukins 1alpha, 6 and 8. Our findings suggest non-specific Ig (anti-Gal and non-Gal Ig of all isotypes) IA or even incomplete IA in immunosuppressed baboon recipients of transgenic pig kidneys is detrimental to graft survival by being associated with an Ab and compliment driven rejection. We speculate that the IA were insufficient in terms of Ig depletion or frequency inducing an Ab rebound or that this total Ig depletion also removed components facilitating graft survival.

The effect of mitoxantrone on anti-pig immunization in baboons

Besides virological and physiological concerns, the success of xenotransplantation (Xt) is still dependent on the prevention of delayed xenograft rejection (DXR). Although multifactorial, DXR is mainly due to xenonatural antibody (Ab) recognizing their xenogenic antigen (Ag) followed by complement activation. Despite the use of intensive treatments capable of inhibiting the humoral response, DXR can still not be avoided and always occurs within weeks following transplantation. Moreover, these latter treatments currently used in Xt could not be used clinically in humans because of their high risk of over-immunosuppressing the patients. Mitoxantrone (Mx) is a drug well known for its antiproliferative properties and is used clinically in oncology and in the treatment of relapsing multiple sclerosis. In models of arthritis in rats, it has been shown to be 10 to 20 times more powerful than cyclophosphamide (CyP) at blocking both inflammatory and B-cell responses. Because of its B-cell inhibitory capacity and considering the implication of the humoral response in xenograft rejection, we have compared Mx with CyP for its ability to block in vivo anti-pig immunization induced via subcutaneous injections of pig red blood cells into baboons. Neither drug was able to inhibit the anti-pig responses following the first and second immunizations, emphasizing the particularity of preformed Ab responses. However, the rise in Ab in the Mx treated animals was significantly delayed as compared with the non-treated as well as the CyP treated animals and was mainly because of a profound depletion of circulating B-cells. Mx displays an interesting antihumoral effect that we now intend to test in a pig kidney to baboon Xt model, with anticipated administration of the drug allowing an early B-cell depletion.

Cellular participation in delayed xenograft rejection of hCD55 transgenic pig hearts by baboons

Delayed xenograft rejection (DXR) of pig organs by baboons currently represents the major obstacle to successful xenotransplantation. Although antibodies (Abs) are believed to play a fundamental role in this form of rejection, so far little is known concerning the potential cellular component. Biopsies taken during DXR of human CD55 transgenic pig hearts by non-treated (n = 2), alpha-Gal immunoadsorbed (n = 2), or immunosuppressed (n = 9) baboons were studied. The cellular element was explored by determining not only its phenotype by classical immunohistochemical techniques but also its activity in terms of cytokines, cytolytic enzymes and other mediators using quantitative reverse transcription polymerase chain reaction. All porcine xenografts underwent DXR; within 5 days in non-treated and immunoadsorbed animals but significantly delayed (6 to 29 days) in immunosuppressed animals. Cellular infiltration in non-immunosuppressed grafts consisted predominantly of monocytes/macrophages, CD8 cells and a few CD4 T-cells. The predominant baboon transcripts detectable were the proinflammatory cytokines interleukin1-alpha and tumor necrosis factor-alpha, the lymphokine interferon-gamma and the cytotoxic enzyme granzyme B. However, these cellular components were lacking in the immunosuppressed animals. Despite these differences, strong immunoglobulin M (IgM) and C5b-9 complement deposition was observed in all animals at rejection. Together our findings suggest that although the humoral response plays a predominant role in DXR through IgM Abs and complement activation, there is a clear cellular infiltrate in DXR in this model that is likely to contribute to rejection through a strong pro-inflammatory and cytotoxic environment, necessitating substantial immunosuppression for a prolonged graft survival.

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.

Rapid failure of pig islet transplantation in non human primates

We have previously demonstrated that adult pig islets of Langerhans are not destroyed in vitro by primate sera. Whether these islets can function when placed into the liver of non-human primates is not known. We now report on the outcome of pig islet xenotransplantation into five non diabetic primates (four baboons and one macacus fascicularis) receiving intraportally purified adult pig islets. The average number of islet-equivalent per graft was 110,000 (60-180,000). All animals received associations of ATG, cyclosporine or LF 195 (a deoxyspergualin analog), mycophenolate mofetil and corticosteroids. A specific porcine C-peptide (C-pep) RIA test was used to monitor insulin secretion. Two hours after grafting, porcine C-peptide was positive (from 0.37 to 4.25 ng/ml) in all monkeys except one. Primate C-pep was normal in all cases. Only two monkeys had detectable levels of porcine C-pep on day 1 or 2 with undetectable levels thereafter, even after glucagon challenge between days 6 and 10. Several normal islets with moderate inflammatory infiltration were observed in one animal liver on day 2 (the time of necropsy) as well as islets with IgM and complement deposition. Among animals sacrificed on days 14, 16 and 38, some residual islet cells could be identified only in livers collected on day 14. Partial glycaemic control was achieved in some rats receiving islets from the same preparations. In conclusion, adult pig islets are not able to maintain insulin secretion for more than 24 h when injected intraportally into non diabetic immunosuppressed monkeys. suggesting immediate islet xenograft destruction.