Release of pig leukocytes during pig kidney perfusion and characterization of pig lymphocyte carbohydrate 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.

Structure and activity of an anti-epidermal growth factor receptor antibody without galactose-α-1,3-galactose residues

Epidermal growth factor receptor (EGFR) is a transmembrane glycoprotein, which has been considered as one of the key targets for cancer therapy. However, currently approved therapeutic anti-EGFR antibody may cause the hypersensitivity reaction induced by galactose-α-1,3-galactose (α-Gal) structure, which is inevitable in insect cell expression system. In this study, the Chinese hamster ovary cell line was used to produce a monoclonal antibody containing simplified glycosylation patterns (code: AB01). And cetuximab was used as a control. The two antibodies were highly similar in molecular weight, secondary structure, binding affinity and endocytosis behavior, whereas the glycotypes are extremely distinct. The flow cytometry assay suggested that AB01 induced cell cycle arrest in G1, thus inhibit cell proliferation. Moreover, both cetuximab and AB01 showed similar sensitivity for all tested cell lines in this research. In conclusion, AB01 could be a potential anti-EGFR drug candidate for cancer therapy.

Are there specific antibodies against Neu5Gc epitopes in the blood of healthy individuals?

Strong discrepancies in published data on the levels and epitope specificities of antibodies against the xenogenic N-glycolyl forms of sialoglycans (Hanganutziu-Deicher Neu5Gcɑ2-3Galβ1-4Glc and related antigens) in healthy donors prompted us to carry out a systematic study in this area using the printed glycan array and other methods. This article summarizes and discusses our published and previously unpublished data, as well as publicly available data from the Consortium for Functional Glycomics. As a result, we conclude that (1) the level of antibodies referred to as anti-Neu5Gc in healthy individuals is low; (2) there are antibodies that seem to interact with Neu5Gc-containing epitopes, but in fact they recognize internal fragments of Neu5Gc-containing glycans (without sialic acids), which served as antigens in the assays used and; (3) a population capable of interacting specifically with Neu5Gc (it does not bind the corresponding NAc analogs) does exist, but it binds the monosaccharide Neu5Gc better than the entire glycans containing it. In other words, in healthy donors, there are populations of antibodies capable of binding the Neu5Gc monosaccharide or the inner core -Galβ1-4Glc, but very few true anti-Neu5Gcɑ2-3Galβ1-4Glc antibodies, i.e., antibodies capable of specifically recognizing the entire trisaccharide.

The Possible Role of Anti-Neu5Gc as an Obstacle in Xenotransplantation

Seventy to ninety percentage of preformed xenoreactive antibodies in human serum bind to the galactose-α(1,3)-galactose Gal epitope, and the creation of Gal knockout (KO) pigs has eliminated hyperacute rejection as a barrier to xenotransplantation. Now other glycan antigens are barriers to move ahead with xenotransplantation, and the N-glycolyl neuraminic acid, Neu5Gc (or Hanganutziu-Deicher antigen), is also a major pig xenoantigen. Humans have anti-Neu5Gc antibodies. Several data indicate a strong immunogenicity of Neu5Gc in humans that may contribute to an important part in antibody-dependent injury to pig xenografts. Pig islets express Neu5Gc, which reacted with diet-derived human antibodies and mice deleted for Neu5Gc reject pancreatic islets from wild-type counterpart. However, Neu5Gc positive heart were not rejected in Neu5Gc KO mice indicating that the role of Neu5Gc-specific antibodies has to be nuanced and depend of the graft situation parameters (organ/tissue, recipient, implication of other glycan antigens). Recently generated Gal/Neu5Gc KO pigs eliminate the expression of Gal and Neu5Gc, and improve the crossmatch of humans with the pig. This review summarizes the current and recent experimental and (pre)clinical data on the Neu5Gc immunogenicity and emphasize of the potential impact of anti-Neu5Gc antibodies in limiting xenotransplantation in humans.

The Structural Complexity and Animal Tissue Distribution of N-Glycolylneuraminic Acid (Neu5Gc)-Terminated Glycans. Implications for Their Immunogenicity in Clinical Xenografting

N-Glycolylneuraminic acid (Neu5Gc)-terminated glycans are present in all animal cells/tissues that are already used in the clinic such as bioprosthetic heart valves (BHV) as well as in those that potentially will be xenografted in the future to overcome end stage cell/organ failure. Humans, as a species lack this antigen determinant and can react with an immune response after exposure to Neu5Gc present in these products/cells/tissues. Genetically engineered source animals lacking Neu5Gc has been generated and so has animals that in addition lack the major αGal xenoantigen. The use of cells/tissues/organs from such animals may improve the long-term performance of BHV and allow future xenografting. This review summarizes the present knowledge regarding structural complexity and tissue distribution of Neu5Gc on glycans of cells/tissue/organs already used in the clinic or intended for treatment of end stage organ failure by xenografting. In addition, we briefly discuss the role of anti-Neu5Gc antibodies in the xenorejection process and how knowledge about Neu5Gc structural complexity can be used to design novel diagnostics for anti-Neu5Gc antibody detection.

Glycosphingolipids of porcine, bovine, and equine pericardia as potential immune targets in bioprosthetic heart valve grafts

BACKGROUND

Pericardial tissue from various animal species is utilized for the production of the bioprosthetic heart valves (BHV) used clinically. Experimental data show that the eventual breakdown of BHV is partly due to immunological interactions with carbohydrate tissue antigens. To understand these processes, we have examined the glycolipid-based carbohydrate antigens in naïve porcine, bovine, and equine pericardia.

EXPERIMENTAL

Total non-acid and acid glycosphingolipid fractions were isolated from porcine, bovine, and equine pericardia, and individual glycolipid compounds were characterized by thin-layer chromatography, mass spectrometry, and binding of monoclonal antibodies, lectins and bacteria in chromatogram binding assays.

RESULTS

The non-acid glycolipid fractions from all species contained glycosphingolipids based on the globo- and neolacto-series, including pentaglycosylceramides with terminal Galα3 determinants. Terminal blood group A and H (O) structures based on type 2 core chains were present in porcine pericardium, while the Forssman pentaosylceramide was found in equine pericardium. All acid glycolipid fractions contained sulfatide and several gangliosides with both N-acetyl- and N-glycolyl-neuraminic acid as terminal saccharide chain determinants.

CONCLUSION

Several carbohydrate antigens which are potential targets for the human immune system have been identified in the animal pericardial tissues used for the production of BHV. Which of these antigens are left in the tissues after industrial BHV production processes, as well as their potential role in eventual BHV degradation, remains to be elucidated.

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.

Release of pig leukocytes and reduced human NK cell recruitment during ex vivo perfusion of HLA-E/human CD46 double-transgenic pig limbs with human blood

BACKGROUND

In pig-to-human xenotransplantation, interactions between human natural killer (NK) cells and porcine endothelial cells (pEC) are characterized by recruitment and cytotoxicity. Protection from xenogeneic NK cytotoxicity can be achieved in vitro by the expression of the non-classical human leukocyte antigen-E (HLA-E) on pEC. Thus, the aim of this study was to analyze NK cell responses to vascularized xenografts using an ex vivo perfusion system of pig limbs with human blood.

METHODS

Six pig forelimbs per group, respectively, stemming from either wild-type (wt) or HLA-E/hCD46 double-transgenic (tg) animals, were perfused ex vivo with heparinized human blood for 12 hours. Blood samples were collected at defined time intervals, cell numbers counted, and peripheral blood mononuclear cells analyzed for phenotype by flow cytometry. Muscle biopsies were analyzed for NK cell infiltration. In vitro NK cytotoxicity assays were performed using pEC derived from wt and tg animals as target cells.

RESULTS

Ex vivo, a strong reduction in circulating human CD45 leukocytes was observed after 60 minutes of xenoperfusion in both wt and tg limb groups. NK cell numbers dropped significantly. Within the first 10 minutes, the decrease in NK cells was more significant in the wt limb perfusions as compared to tg limbs. Immunohistology of biopsies taken after 12 hours showed less NK cell tissue infiltration in the tg limbs. In vitro, NK cytotoxicity against hCD46 single tg pEC and wt pEC was similar, while lysis of double tg HLA-E/hCD46 pEC was significantly reduced. Finally, circulating cells of pig origin were observed during the ex vivo xenoperfusions. These cells expressed phenotypes mainly of monocytes, B and T lymphocytes, NK cells, as well as some activated endothelial cells.

CONCLUSIONS

Ex vivo perfusion of pig forelimbs using whole human blood represents a powerful tool to study humoral and early cell-mediated rejection mechanisms of vascularized pig-to-human xenotransplantation, although there are several limitations of the model. Here, we show that (i) transgenic expression of HLA-E/hCD46 in pig limbs provides partial protection from human NK cell-mediated xeno responses and (ii) the emergence of a pig cell population during xenoperfusions with implications for the immunogenicity of xenografts.

Production of α1,3-galactosyltransferase and cytidine monophosphate-N-acetylneuraminic acid hydroxylase gene double-deficient pigs by CRISPR/Cas9 and handmade cloning

Gene-knockout pigs hold great promise as a solution to the shortage of organs from donor animals for xenotransplantation. Several groups have generated gene-knockout pigs via clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated 9 (Cas9) and somatic cell nuclear transfer (SCNT). Herein, we adopted a simple and micromanipulator-free method, handmade cloning (HMC) instead of SCNT, to generate double gene-knockout pigs. First, we applied the CRISPR/Cas9 system to target α1,3-galactosyltransferase (GGTA1) and cytidine monophosphate-N-acetylneuraminic acid hydroxylase (CMAH) genes simultaneously in porcine fetal fibroblast cells (PFFs), which were derived from wild-type Chinese domestic miniature Wuzhishan pigs. Cell colonies were obtained by screening and were identified by Surveyor assay and sequencing. Next, we chose the GGTA1/CMAH double-knockout (DKO) cells for HMC to produce piglets. As a result, we obtained 11 live bi-allelic GGTA1/CMAH DKO piglets with the identical phenotype. Compared to cells from GGTA1-knockout pigs, human antibody binding and antibody-mediated complement-dependent cytotoxicity were significantly reduced in cells from GGTA1/CMAH DKO pigs, which demonstrated that our pigs would exhibit reduced humoral rejection in xenotransplantation. These data suggested that the combination of CRISPR/Cas9 and HMC technology provided an efficient and new strategy for producing pigs with multiple genetic modifications.

Initial in vitro studies on tissues and cells from GTKO/CD46/NeuGcKO pigs

BACKGROUND

The impact that the absence of expression of NeuGc in pigs might have on pig organ or cell transplantation in humans has been studied in vitro, but only using red blood cells (pRBCs) and peripheral blood mononuclear cells (pPBMCs) as the target cells for immune assays. We have extended this work in various in vitro models and now report our initial results.

METHODS

The models we have used involve GTKO/hCD46 and GTKO/hCD46/NeuGcKO pig aortas and corneas, and pRBCs, pPBMCs, aortic endothelial cells (pAECs), corneal endothelial cells (pCECs), and isolated pancreatic islets. We have investigated the effect of the absence of NeuGc expression on (i) human IgM and IgG binding, (ii) the T-cell proliferative response, (iii) human platelet aggregation, and (iv) in an in vitro assay of the instant blood-mediated inflammatory reaction (IBMIR) following exposure of pig islets to human blood/serum.

RESULTS

The lack of expression of NeuGc on some pig tissues (aortas, corneas) and cells (RBCs, PBMCs, AECs) significantly reduces the extent of human antibody binding. In contrast, the absence of NeuGc expression on some pig tissues (CECs, isolated islet cells) does not reduce human antibody binding, possibly due to their relatively low NeuGc expression level. The strength of the human T-cell proliferative response may also be marginally reduced, but is already weak to GTKO/hCD46 pAECs and islet cells. We also demonstrate that the absence of NeuGc expression on GTKO/hCD46 pAECs does not reduce human platelet aggregation, and nor does it significantly modify the IBMIR to pig islets.

CONCLUSION

The absence of NeuGc on some solid organs from GTKO/hCD46/NeuGcKO pigs should reduce the human antibody response after clinical transplantation when compared to GTKO/hCD46 pig organs. However, the clinical benefit of using certain tissue (e.g., cornea, islets) from GTKO/hCD46/NeuGcKO pigs is questionable.

Recent investigations into pig antigen and anti-pig antibody expression

Genetic engineering of donor pigs to eliminate expression of the dominant xenogeneic antigen galactose α1,3 galactose (Gal) has created a sea change in the immunobiology of xenograft rejection. Antibody mediated xenograft rejection of GGTA-1 α-galactosyltransferase (GTKO) deficient organs is now directed to a combination of non-Gal pig protein and carbohydrate antigens. Glycan analysis of GTKO tissues identified no new neo-antigens but detected high levels of N-acetylneuraminic acid (Neu5Gc) modified glycoproteins and glycolipids. Humans produce anti-Neu5Gc antibody and in very limited clinical studies sometimes show an induced anti-Neu5Gc antibody response after challenge with pig tissue. The pathogenicity of anti-Neu5Gc antibody in xenotransplantation is not clear however as non-human transplant models, critical for modelling anti-Gal immunity, do not produce anti-Neu5Gc antibody. Antibody induced after xenotransplantation in non-human primates is directed to an array of pig endothelial cells proteins and to a glycan produced by the pig B4GALNT2 gene. We anticipate that immune suppression will significantly affect the T-cell dependent and independent specificity of an induced antibody response and that donor pigs deficient in synthesis of multiple xenogeneic glycans will be important to future studies.

Generation of α1,3-galactosyltransferase and cytidine monophospho-N-acetylneuraminic acid hydroxylase gene double-knockout pigs

Zinc-finger nucleases (ZFNs) and transcription activator-like effector nucleases (TALENs) are new tools for producing gene knockout (KO) animals. The current study reports produced genetically modified pigs, in which two endogenous genes were knocked out. Porcine fibroblast cell lines were derived from homozygous α1,3-galactosyltransferase (GalT) KO pigs. These cells were subjected to an additional KO for the cytidine monophospho-N-acetylneuraminic acid hydroxylase (CMAH) gene. A pair of ZFN-encoding mRNAs targeting exon 8 of the CMAH gene was used to generate the heterozygous CMAH KO cells, from which cloned pigs were produced by somatic cell nuclear transfer (SCNT). One of the cloned pigs obtained was re-cloned after additional KO of the remaining CMAH allele using the same ZFN-encoding mRNAs to generate GalT/CMAH-double homozygous KO pigs. On the other hand, the use of TALEN-encoding mRNAs targeting exon 7 of the CMAH gene resulted in efficient generation of homozygous CMAH KO cells. These cells were used for SCNT to produce cloned pigs homozygous for a double GalT/CMAH KO. These results demonstrate that the combination of TALEN-encoding mRNA, in vitro selection of the nuclear donor cells and SCNT provides a robust method for generating KO pigs.

Potential deleterious role of anti-Neu5Gc antibodies in xenotransplantation

Human beings do not synthesize the glycolyl form of the sialic acid (Neu5Gc) and only express the acetylated form of the sugar, whereas a diet-based intake of Neu5Gc provokes a natural immunization and production of anti-Neu5Gc antibodies in human serum. However, Neu5Gc is expressed on mammal glycoproteins and glycolipids in most organs and cells. We review here the relevance of Neu5Gc and anti-Neu5Gc antibodies in the context of xenotransplantation and the use of animal-derived molecules and products, as well as the possible consequences of a long-term exposure to anti-Neu5Gc antibodies in recipients of xenografts. In addition, the importance of an accurate estimation of the anti-Neu5Gc response following xenotransplantation and the future contribution of knockout animals mimicking the human situation are also assessed.

Carbohydrate antigens

PURPOSE OF REVIEW

To summarize the current knowledge of carbohydrate antigens as related to xenotransplantation. The emphasis is on non-Gal carbohydrate antigens identified in many institutes. In addition, several topics such as glycosyltransferase-transgenic pigs, innate cell receptors and porcine endogenous retrovirus (PERV) will be discussed.

RECENT FINDINGS

Studies related to iGb3 and neoantigens after knocking out GalT (GGTA1) were reviewed. Available data do not support the conclusion that GalT-KO remains iGb3 and/or that neoantigens are produced in the pigs. Concerning non-Gal antigen, in addition to the Hanganutziu-Deicher (H-D) antigen (NeuGc), Forrsman antigen, Galα1-3Lew(x), α-linked or β-linked GalNAc, β3 linked Gal, NeuAc, such as Neu5Acα2-3Galβ1-3GlcNAc, and Sid blood group (Sd(a))-like antigens are candidates. However, to date some of these remain controversial and others need further study to completely identify them. Regarding the H-D antigen, different from the α-Gal, it has a complicated expression system, but has cytotoxic effects toward pig cells. To modify other carbohydrate antigen apart from α-Gal, only the overexpression of GnT-III appears to have an effect on the suppression of the N-linked sugar of non-Gal antigen. Concerning innate cell receptors related to carbohydrates (ligands), the focus turned from natural killer (NK) receptor to others, such as monocytes. Finally, PERV contains a ligand with an N-linked sugar. Modification of the glycosylation pattern appears to be associated with regulating PERV infectivity.

SUMMARY

A considerable amount of data related to carbohydrate antigens is now available. At the same time, however, discrepancies between studies complicate this issue. Further studies will be needed to completely understand this complicated area of interest.

Trial using pig cells with the H-D antigen knocked down

PURPOSE

This report describes an attempt to reduce the expression level of Hanganutziu-Deicher (H-D) antigens by small interfering RNA (siRNA) for pig cytidine monophospho-N-acetylneuraminic acid hydroxylase (pCMAH).

METHODS

A pig endothelial cell (PEC) line, and PEC and fibroblasts from an α1,3galactosyltransferase knockout (GalT-KO) piglet were used. Real-time PCR was used to evaluate the degradation of mRNA by siRNA. The H-D antigen was stained, and then the cells were incubated with human serum for the FACS analysis. The extent of lysis in human serum was next calculated using an LDH assay.

RESULTS

Suppression of the mRNA of pCMAH by each siRNA was first determined. The mixture of siRNAs for pCMAH reduced the expressions of the H-D antigen on the PEC and fibroblasts to a considerable extent. The further reduction in the xenoantigenicity for human serum of the GalT-KO cells was then confirmed. In addition, the PEC line showed a significant downregulation in complement-dependent cytotoxicity by the siRNAs, thus indicating that the anti-H-D antigen in human serum is capable of causing lysis of the pig cells.

CONCLUSION

pCMAH silencing by siRNA reduced the expression of the H-D antigen and its antigenicity, thus confirming that the H-D antigen is one of the major non-Gal antigens in this situation.

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.

Structural characterization of alpha1,3-galactosyltransferase knockout pig heart and kidney glycolipids and their reactivity with human and baboon antibodies

BACKGROUND

alpha1,3-galactosyltranferase knockout (GalT-KO) pigs have been established to avoid hyperacute rejection in GalT-KO pig-to-human xenotransplantation. GalT-KO pig heart and kidney glycolipids were studied focusing on elimination of Gal-antigens and whether novel antigens would appear. Non-human primates are used as pre-clinical transplantation experimental models. Therefore, sera from baboons transplanted with GalT-KO hearts were compared with human serum regarding reactivity with pig glycolipids.

METHODS

Neutral and acidic glycolipids were isolated from GalT-KO and WT pig hearts and kidneys. Glycolipid immune reactivity was tested on TLC plates using human affinity-purified anti-Gal Ig, anti-blood group monoclonal antibodies, lectins, and human serum as well as baboon serum collected before and after GalT-KO pig heart transplantations. Selected glycolipid fractions, isolated by HPLC, were structurally characterized by mass spectrometry and proton NMR spectroscopy.

RESULTS

GalT-KO heart and kidney lacked alpha3Gal-terminated glycolipids completely. Levels of uncapped N-acetyllactosamine precursor compounds, blood group H type 2 core chain compounds, the P1 antigen and the x(2) antigen were increased. Human serum antibodies reacted with Gal-antigens and N-glycolylneuraminic acid (NeuGc) in WT organs of which only the NeuGc reactivity remained in the GalT-KO tissues. A clear difference in reactivity between baboon and human antibodies with pig glycolipids was found. This was most pronounced for acidic, not yet identified, compounds in GalT-KO organs which were less abundant or lacking in the corresponding WT tissues.

CONCLUSIONS

GalT-KO pig heart and kidney completely lacked Gal glycolipid antigens whilst glycolipids synthesized by competing pathways were increased. Baboon and human serum antibodies showed a different reactivity pattern to pig glycolipid antigens indicating that non-human primates have limitations as a human pre-clinical model for immune rejection studies.

Glycosylation of Recombinant Antibody Therapeutics

The adaptive immune system has the capacity to produce antibodies with a virtually infinite repertoire of specificities. Recombinant antibodies specific for human targets are established in the clinic as therapeutics and represent a major new class of drug. Therapeutic efficacy depends on the formation of complexes with target molecules and subsequent activation of downstream biologic effector mechanisms that result in elimination of the target. The activation of effector mechanisms is dependent on structural characteristics of the antibody molecule that result from posttranslational modifications, in particular, glycosylation. The production of therapeutic antibody with a consistent human glycoform profile has been and remains a considerable challenge to the biopharmaceutical industry. Recent research has shown that individual glycoforms of antibody may provide optimal efficacy for selected outcomes. Thus a further challenge will be the production of a second generation of antibody therapeutics customized for their clinical indication.

Studies on glycolipid antigens in small intestine and pancreas from alpha1,3-galactosyltransferase knockout miniature swine

BACKGROUND

To avoid hyperacute rejection of xeno-organs, alpha1,3-galactosyltransferase knockout (GalT-KO) pigs have been produced. Galalpha1,3Gal determinant elimination may expose cryptic carbohydrate antigens and/or generate new antigens. This is the first biochemical study of carbohydrate antigens in GalT-KO pig organs.

METHODS

Neutral and acidic glycolipids were isolated from small intestine and pancreas of two GalT-KO and one wild-type (WT) pig. Glycolipid immune reactivity was tested on thin-layer chromatograms. Small intestine neutral glycolipids were separated by high-performance liquid chromatography and selected fractions were analyzed by proton nuclear magnetic resonance spectroscopy. Total gangliosides were quantified on thin-layer chromatograms and in microtiter wells.

RESULTS

Using Galalpha1,3nLc4 glycolipid reference, total Galalpha1,3Gal glycolipid antigens in the WT animal was estimated at about 30 microg (small intestine) and 3 microg (pancreas) per gram of dry tissue. Galalpha1,3Gal determinants were not detected in GalT-KO tissues at a detection limit of less than 0.25% (small intestine) and 0.5% (pancreas) of the WT tissues. Isoglobotriaosylceramide (iGb3) was absent but trace amounts of Fuc-iGb3 was found in both GalT-KO and WT pig small intestine. Blood group H type 2 core saccharide compounds were increased in GalT-KO pancreas. Total amount of gangliosides was decreased in GalT-KO tissues. The alpha1,3-galactosyltransferase acceptor, N-acetyllactosamine determinant, was not increased in GalT-KO tissues. Human serum antibodies reacted with WT organ Galalpha1,3Gal antigens and gangliosides, of which the ganglioside reactivity remained in GalT-KO tissues.

CONCLUSIONS

Knockout of porcine alpha1,3-galactosyltransferase gene results in elimination of Galalpha1,3Gal-terminated glycolipid compounds. GalT-KO genetic modification did not produce new compensatory glycolipid compounds reactive with human serum antibodies.

Characteristics of protein-carbohydrate interactions as a basis for developing novel carbohydrate-based antirejection therapies

The relative shortage of human organs for transplantation is today the major barrier to a broader use of transplantation as a means of treating patients with end-stage organ failure. This barrier could be partly overcome by an increased use of blood group ABO-incompatible live donors, and such trials are currently underway at several transplant centres. If xenotransplantation can be used clinically in the future, the human organ shortage will, in principle, be eradicated. In both these cases, carbohydrate antigens and the corresponding anti-carbohydrate antibodies are the major primary immunological barriers to overcome. Refined carbohydrate-based therapeutics may permit an increased number of ABO-incompatible transplantations to be carried out, and may remove the initial barriers to clinical xenotransplantation. Here, we will discuss the chemical characteristics of protein-carbohydrate interactions and outline carbohydrate-based antirejection therapies as used today in experimental as well as in clinical settings. Novel mucin-based adsorbers of natural anti-carbohydrate antibodies will also be described.

Carbohydrates in xenotransplantation

The success of allotransplantation has led to an increasing shortage of human organs from deceased donors. This crisis could be resolved by the use of organs from an anatomically suitable animal, such as the pig. The pig and human have, however, been evolving differently for approximately 80 million years, and numerous immunological and physiological barriers have developed that need to be overcome. Differences in carbohydrate epitopes on pig and human cells have been found to play a major role in some of the immunological barriers that have been identified to date. The rejection caused by the presence of galactose-alpha1,3-galactose (Gal) on the pig vascular endothelium and of natural anti-Gal antibodies in humans has recently been prevented by the breeding of pigs that do not express Gal, achieved by knocking out the gene for the enzyme alpha1,3-galactosyltransferase, which was made possible by the introduction of nuclear transfer/embryo transfer techniques. N-glycolylneuraminic acid (the so-called Hanganutziu-Deicher antigen) has been identified as another carbohydrate antigen present in pigs that may need to be deleted if xenotransplantation is to be successful, although some doubt remains regarding its importance. There remain other antipig antibodies against hitherto unidentified antigenic targets that may well be involved in graft destruction; their possible carbohydrate target epitopes are discussed.

Are N-glycolylneuraminic acid (Hanganutziu-Deicher) antigens important in pig-to-human xenotransplantation?

BACKGROUND

N-glycolylneuraminic acid (NeuGc) epitopes, so called Hanganutziu-Deicher (HD) antigens, which are widely expressed on endothelial cells of all mammals except humans, are considered to be potential targets for natural and elicited anti-nonGalalpha1-3 Gal (Gal) antibodies in humans. We previously reported that anti-NeuGc antibodies were not detected in healthy humans by enzyme-linked immunosorbent assay (ELISA) using NeuGc-GM3-coated plates, and no antibody production was observed in patients with a history of exposure to pig cells. However, a recent paper has revealed that (i) anti-NeuGc antibodies to porcine red blood cells (PRBC) are detectable in most healthy humans, and (ii) the majority of anti-nonGal antibodies are specific for NeuGc epitopes. The purpose of this study was to reassess whether NeuGc is important as an immunogenic nonGal epitope.

METHODS

The binding of antibodies to PRBC and porcine aortic endothelial cells (PAEC) was compared. Cells were treated with (i) alpha-galactosidase, and then (ii) neuraminidase, which digests sialic acids, including NeuGc epitopes. Cells were incubated with human pooled sera, and applied to flow cytometric analysis. After enzyme digestion, almost complete reduction of Gal and NeuGc expression was confirmed by GS-IB4 and HU/Ch2-7 (a chicken monoclonal antibody against HD antigens), respectively. Trypsin, which removes membrane glycoproteins, and endoglycoceramidase which cleaves glycolipids, were used for differentiating between NeuGc-containing glycoproteins and glycolipids.

RESULTS

Neuraminidase-treatment reduced the binding of immunoglobulin G (IgG) antibodies to PRBC; about half of the anti-nonGal IgG antibodies to PRBC were directed to NeuGc. In contrast, anti-nonGal antibodies to PAEC were not directed to NeuGc. Trypsin-treatment markedly reduced the expression of NeuGc only on PRBC. Endoglycoceramidase-treatment was followed by a greater reduction in NeuGc epitopes on PAEC than on PRBC. Most NeuGc on PRBC appeared to be linked to proteins, but most NeuGc on PAEC was expressed on glycolipids.

CONCLUSIONS

Carbohydrate structures on PRBC are different from those on PAEC. Healthy human sera contain anti-nonGal IgG antibodies to NeuGc expressed on PRBC, but not on PAEC. We speculate that anti-nonGal IgG antibodies to PRBC can recognize both NeuGc and protein, and this may be the reason why such antibodies have not been detected by ELISA. A definite conclusion about the immunogenicity of NeuGc has not been obtained. More sera from patients (not from non-human primates) sensitized with porcine cells or organs need to be studied.