Characterization of the rat alpha(1,3)galactosyltransferase: evidence for two independent genes encoding glycosyltransferases that synthesize Galalpha(1,3)Gal by two separate glycosylation pathways

Mechanisms of Formation of Antibodies against Blood Group Antigens That Do Not Exist in the Body

The system of the four different human blood groups is based on the oligosaccharide antigens A or B, which are located on the surface of blood cells and other cells including endothelial cells, attached to the membrane proteins or lipids. After transfusion, the presence of these antigens on the apical surface of endothelial cells could induce an immunological reaction against the host. The final oligosaccharide sequence of AgA consists of Gal-GlcNAc-Gal (GalNAc)-Fuc. AgB contains Gal-GlcNAc-Gal (Gal)-Fuc. These antigens are synthesised in the Golgi complex (GC) using unique Golgi glycosylation enzymes (GGEs). People with AgA also synthesise antibodies against AgB (group A [II]). People with AgB synthesise antibodies against AgA (group B [III]). People expressing AgA together with AgB (group AB [IV]) do not have these antibodies, while people who do not express these antigens (group O [0; I]) synthesise antibodies against both antigens. Consequently, the antibodies are synthesised against antigens that apparently do not exist in the body. Here, we compared the prediction power of the main hypotheses explaining the formation of these antibodies, namely, the concept of natural antibodies, the gut bacteria-derived antibody hypothesis, and the antibodies formed as a result of glycosylation mistakes or de-sialylation of polysaccharide chains. We assume that when the GC is overloaded with lipids, other less specialised GGEs could make mistakes and synthesise the antigens of these blood groups. Alternatively, under these conditions, the chylomicrons formed in the enterocytes may, under this overload, linger in the post-Golgi compartment, which is temporarily connected to the endosomes. These compartments contain neuraminidases that can cleave off sialic acid, unmasking these blood antigens located below the acid and inducing the production of antibodies.

α-Gal antigen-deficient rabbits with GGTA1 gene disruption via CRISPR/Cas9

Background

Previous studies have identified the carbohydrate epitope Galα1–3Galβ1–4GlcNAc-R (termed the α-galactosyl epitope), known as the α-Gal antigen as the primary xenoantigen recognized by the human immune system. The α-Gal antigen is regulated by galactosyltransferase (GGTA1), and α-Gal antigen-deficient mice have been widely used in xenoimmunological studies, as well as for the immunogenic risk evaluation of animal-derived medical devices. The objective of this study was to develop α-Gal antigen-deficient rabbits by GGTA1 gene editing with the CRISPR/Cas9 system.

Results

The mutation efficiency of GGTA1 gene-editing in rabbits was as high as 92.3% in F0 pups. Phenotype analysis showed that the α-Gal antigen expression in the major organs of F0 rabbits was decreased by more than 99.96% compared with that in wild-type (WT) rabbits, and the specific anti-Gal IgG and IgM antibody levels in F1 rabbits increased with increasing age, peaking at approximately 5 or 6 months. Further study showed that GGTA1 gene expression in F2-edited rabbits was dramatically reduced compared to that in WT rabbits.

Conclusions

α-Gal antigen-deficient rabbits were successfully generated by GGTA1 gene editing via the CRISPR/Cas9 system in this study. The feasibility of using these α-Gal antigen-deficient rabbits for the in situ implantation and residual immunogenic risk evaluation of animal tissue-derived medical devices was also preliminarily confirmed.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12863-022-01068-4.

Advances in Innate Immunity to Overcome Immune Rejection during Xenotransplantation

Transplantation is an effective approach for treating end-stage organ failure. There has been a long-standing interest in xenotransplantation as a means of increasing the number of available organs. In the past decade, there has been tremendous progress in xenotransplantation accelerated by the development of rapid gene-editing tools and immunosuppressive therapy. Recently, the heart and kidney from pigs were transplanted into the recipients, which suggests that xenotransplantation has entered a new era. The genetic discrepancy and molecular incompatibility between pigs and primates results in barriers to xenotransplantation. An increasing body of evidence suggests that innate immune responses play an important role in all aspects of the xenogeneic rejection. Simultaneously, the role of important cellular components like macrophages, natural killer (NK) cells, and neutrophils, suggests that the innate immune response in the xenogeneic rejection should not be underestimated. Here, we summarize the current knowledge about the innate immune system in xenotransplantation and highlight the key issues for future investigations. A better understanding of the innate immune responses in xenotransplantation may help to control the xenograft rejection and design optimal combination therapies.

Aspects of the Complement System in New Era of Xenotransplantation

After producing triple (Gal, H-D and Sd^a)-KO pigs, hyperacute rejection appeared to no longer be a problem. However, the origin of xeno-rejection continues to be a controversial topic, including small amounts of antibodies and subsequent activation of the graft endothelium, the complement recognition system and the coagulation systems. The complement is activated via the classical pathway by non-Gal/H-D/Sda antigens and by ischemia-reperfusion injury (IRI), via the alternative pathway, especially on islets, and via the lectin pathway. The complement system therefore is still an important recognition and effector mechanism in xeno-rejection. All complement regulatory proteins (CRPs) regulate complement activation in different manners. Therefore, to effectively protect xenografts against xeno-rejection, it would appear reasonable to employ not only one but several CRPs including anti-complement drugs. The further assessment of antigens continues to be an important issue in the area of clinical xenotransplantation. The above conclusions suggest that the expression of sufficient levels of human CRPs on Triple-KO grafts is necessary. Moreover, multilateral inhibition on local complement activation in the graft, together with the control of signals between macrophages and lymphocytes is required.

Pathological features in 'humanized' neonatal pig

Cytidine monophosphate-Nacetylneuraminic acid (Neu5Ac) hydroxylase (CMAH) and glycoprotein, alpha1, 3-galactosyltransferase (GGTA1) double knockout (DKO) pig models were produced to reduce immune reaction for xenotransplantation. However, the role of Neu5Gc and α-Gal in pigs has not been fully elucidated and it is necessary to consider the after-effect of inactivation of GGTA1 and CMAH in pigs. Hematological profiles of DKO pigs were analyzed through complete blood count (CBC). Histology of liver and spleen of DKO were investigated, and lectin blotting and mass spectrometry (MS) were performed to explore glycosylation changes in red blood cell (RBC) membranes of DKO pigs. DKO pigs showed common clinical signs such as weakness (100%), dyspnea (90%) and constipation (65%). DKO pigs revealed a significant decrease in RBC, hemoglobin (HGB) and hematocrit (HGB), and an increase in white blood cell (WBC), lymphocyte (LYM), monocyte (MON), and erythrocyte mean corpuscular volume (MCV). DKO piglets showed swollen liver and spleen, and exhibited raised deposition of hemosiderin and severe bleeding. Lectin assay and MS proved variations in glycosylation on RBC membranes. GGTA1/CMAH DKO pigs developed pathological features which are similar to anemic symptoms, and the variations in glycosylation on RBC membranes of DKO pigs may be attributed to the pathologies observed.

GGTA1/iGb3S Double Knockout Mice: Immunological Properties and Immunogenicity Response to Xenogeneic Bone Matrix

Background

Animal tissues and tissue-derived biomaterials are widely used in the field of xenotransplantation and regenerative medicine. A potential immunogenic risk that affects the safety and effectiveness of xenografts is the presence of remnant α-Gal antigen (synthesized by GGTA1 or/and iGb3S). GGTA1 knockout mice have been developed as a suitable model for the analysis of anti-Gal antibody-mediated immunogenicity. However, we are yet to establish whether GGTA1/iGb3S double knockout (G/i DKO) mice are sensitive to Gal antigen-positive xenoimplants.

Methods

α-Gal antigen expression in the main organs of G/i DKO mice or bovine bone substitutes was detected via a standardized ELISA inhibition assay. Serum anti-α-Gal antibody titers of G/i DKO mice after immunization with rabbit red blood cells (RRBC) and implantation of raw lyophilized bone substitutes (Gal antigen content was 8.14 ± 3.17 × 10¹²/mg) or Guanhao Biotech bone substitutes (50% decrease in Gal antigen relative to the raw material) were assessed. The evaluation of total serum antibody, inflammatory cytokine, and splenic lymphocyte subtype populations and the histological analysis of implants and thymus were performed to systematically assess the immune response caused by bovine bone substitutes and bone substitute grafts in G/i DKO mice.

Results

α-Gal epitope expression was reduced by 100% in the main organs of G/i DKO mice, compared with their wild-type counterparts. Following immunization with RRBC, serum anti-Gal antibody titers of G/i DKO mice increased from 80- to 180-fold. After subcutaneous implantation of raw lyophilized bone substitutes and Guanhao Biotech bone substitutes into G/i DKO mice, specific anti-α-Gal IgG, anti-α-Gal IgM, and related inflammatory factors (IFN-γ and IL-6) were significantly increased in the raw lyophilized bone substitute group but showed limited changes in the Guanhao Biotech bone substitute group, compared with the control.

Conclusion

G/i DKO mice are sensitive to Gal antigen-positive xenogeneic grafts and can be effectively utilized for evaluating the α-Gal-mediated immunogenic risk of xenogeneic grafts.

A New Humanized Mouse Model Mimics Humans in Lacking α-Gal Epitopes and Secreting Anti-Gal Antibodies

Mice have been used as accepted tools for investigating complex human diseases and new drug therapies because of their shared genetics and anatomical characteristics with humans. However, the tissues in mice are different from humans in that human cells have a natural mutation in the α1,3 galactosyltransferase (α1,3GT) gene and lack α-Gal epitopes on glycosylated proteins, whereas mice and other nonprimate mammals express this epitope. The lack of α-Gal epitopes in humans results in the loss of immune tolerance to this epitope and production of abundant natural anti-Gal Abs. These natural anti-Gal Abs can be used as an adjuvant to enhance processing of vaccine epitopes to APCs. However, wild-type mice and all existing humanized mouse models cannot be used to test the efficacy of vaccines expressing α-Gal epitopes because they express α-Gal epitopes and lack anti-Gal Abs. Therefore, in an effort to bridge the gap between the mouse models and humans, we developed a new humanized mouse model that mimics humans in that it lacks α-Gal epitopes and secretes human anti-Gal Abs. The new humanized mouse model (Hu-NSG/α-Gal^null) is designed to be used for preclinical evaluations of viral and tumor vaccines based on α-Gal epitopes, human-specific immune responses, xenotransplantation studies, and in vivo biomaterials evaluation. To our knowledge, our new Hu-NSG/α-Gal^null is the first available humanized mouse model with such features.

Gal epitope expression and immunological properties in iGb3S deficient mice

The Gal antigen is synthesized by glycoprotein galactosyltransferase alpha 1, 3 (GGTA1) or (and) isoglobotrihexosylceramide 3 synthase (iGb3S). However, whether iGb3S deletion changes Gal epitope expression and immunological properties in animals is still not clear. The objective of this study was to develop iGb3S deficient mice, and characterize their Gal epitope expression and Gal epitope-related immunological properties. iGb3S gene knockout mice were generated on the C57BL/6 background using the bacterial artificial chromosome homology region recombination technique. Gal epitope expression in the iGb3S deficient mice was determined by using a monoclonal anti-Gal antibody. Immunological properties were analyzed by enzyme linked immune sorbent assay. It was found that Gal epitope expression was decreased from 5.19% to 21.74% in the main organs of iGb3S deficient mice, compared with that of C57BL/6 wild type mice, suggesting that the iGb3S gene participated to Gal epitope expression. However, iGb3S deletion alone did not cause significant changes in the immunological properties of iGb3S deficient mice with or without exogenous Gal antigen (Rabbit Red Blood Cell) stimulation. The data from this study suggest that the iGb3S gene likely contributes to Gal epitope expression, but may have a very weak effect on immunological properties of the iGb3S deficient mice.

Tick galactosyltransferases are involved in α-Gal synthesis and play a role during Anaplasma phagocytophilum infection and Ixodes scapularis tick vector development

The carbohydrate Galα1-3Galβ1-(3)4GlcNAc-R (α-Gal) is produced in all mammals except for humans, apes and old world monkeys that lost the ability to synthetize this carbohydrate. Therefore, humans can produce high antibody titers against α-Gal. Anti-α-Gal IgE antibodies have been associated with tick-induced allergy (i.e. α-Gal syndrome) and anti-α-Gal IgG/IgM antibodies may be involved in protection against malaria, leishmaniasis and Chagas disease. The α-Gal on tick salivary proteins plays an important role in the etiology of the α-Gal syndrome. However, whether ticks are able to produce endogenous α-Gal remains currently unknown. In this study, the Ixodes scapularis genome was searched for galactosyltransferases and three genes were identified as potentially involved in the synthesis of α-Gal. Heterologous gene expression in α-Gal-negative cells and gene knockdown in ticks confirmed that these genes were involved in α-Gal synthesis and are essential for tick feeding. Furthermore, these genes were shown to play an important role in tick-pathogen interactions. Results suggested that tick cells increased α-Gal levels in response to Anaplasma phagocytophilum infection to control bacterial infection. These results provided the molecular basis of endogenous α-Gal production in ticks and suggested that tick galactosyltransferases are involved in vector development, tick-pathogen interactions and possibly the etiology of α-Gal syndrome in humans.

B4GALNT2 and xenotransplantation: A newly appreciated xenogeneic antigen

Analysis of non‐Gal antibody induced after pig‐to‐baboon cardiac xenotransplantation identified the glycan produced by porcine beta‐1,4‐N‐acetyl‐galactosaminyltransferase 2 (B4GALNT2) as an immunogenic xenotransplantation antigen. The porcine B4GALNT2 enzyme is homologous to the human enzyme, which synthesizes the human SDa blood group antigen. Most humans produce low levels of anti‐SDa IgM which polyagglutinates red blood cells from rare individuals with high levels of SDa expression. The SDa glycan is also present on GM2 gangliosides. Clinical GM2 vaccination studies for melanoma patients suggest that a human antibody response to SDa can be induced. Expression of porcine B4GALNT2 in human HEK293 cells results in increased binding of anti‐SDa antibody and increased binding of Dolichos biflorus agglutinin (DBA), a lectin commonly used to detect SDa. In pigs, B4GALNT2 is expressed by vascular endothelial cells and endothelial cells from a wide variety of pig backgrounds stain with DBA, suggesting that porcine vascular expression of B4GALNT2 is not polymorphic. Mutations in B4GALNT2 have been engineered in mice and pigs. In both species, the B4GALNT2‐KO animals are apparently normal and no longer show evidence of SDa antigen expression. Pig tissues with a mutation in B4GALNT2, added to a background of alpha‐1,3‐galactosyltransferase deficient (GGTA1‐KO) and cytidine monophosphate‐N‐acetylneuraminic acid hydroxylase deficient (CMAH‐KO), show reduced antibody binding, confirming the presence of B4GALNT2‐dependent antibodies in both humans and non‐human primates. Preclinical xenotransplantation using B4GALNT2‐deficient donors has recently been reported. Elimination of this source of immunogenic pig antigen should minimize acute injury by preformed anti‐pig antibody and eliminate an induced clinical immune response to this newly appreciated xenotransplantation antigen.

Characteristics of α-Gal epitope, anti-Gal antibody, α1,3 galactosyltransferase and its clinical exploitation (Review)

The α-Gal epitope (Galα1,3Galα1,4GlcNAc-R) is ubiquitously presented in non-primate mammals, marsupials and New World Monkeys, but it is absent in humans, apes and Old World monkeys. However, the anti-Gal antibody (~1% of immunoglobulins) is naturally generated in human, and is found as the immunoglobulin G (IgG), IgM and IgA isotypes. Owing to the specific binding of the anti-Gal antibody with the α-Gal epitope, humans have a distinct anti-α-gal reactivity, which is responsible for hyperacute rejection of organs transplanted from α-gal donors. In addition, the α1,3 galactosyltransferases (α1,3GT) can catalyze the synthesis of the α-Gal epitope. Therefore, the α1,3GT gene, which encodes the α1,3GT, is developed profoundly. The distributions of the α-Gal epitope and anti-Gal antibody, and the activation of α1,3GT, reveal that the enzyme of α1,3GT in ancestral primates is ineffective. Comparison of the nucleotide sequence of the human α1,3-GT pseudogene to the corresponding different species sequence, and according to the evolutionary tree of different species, the results of evolutionary inactivation of the α1,3GT gene in ancestral primates attribute to the mutations under a stronger selective pressure. However, on the basis of the structure, the mechanism and the specificity of the α-Gal epitope and anti-Gal antibody, they can be applied to clinical exploitation. Knocking out the α1,3GT gene will eliminate the xenoantigen, Gal(α1,3)Gal, so that the transplantation of α1,3GT gene knockout pig organ into human becomes a potential clinically acceptable treatment for solving the problem of organ shortage. By contrast, the α-Gal epitope expressed through the application of chemical, biochemical and genetic engineering can be exploited for the clinical use. Targeting anti-Gal-mediated autologous tumor vaccines, which express α-Gal epitope to antigen-presenting cells, would increase their immunogenicity and elicit an immune response, which will be potent enough to eradicate the residual tumor cells. For tumor vaccines, the way of increasing immunogenicity of certain viral vaccines, including flu vaccines and human immunodeficiency virus vaccines, can also be used in the elderly. Recently, α-Gal epitope nanoparticles have been applied to accelerate wound healing and further directions on regeneration of internally injured tissues.

α1,3Galactosyltransferase knockout pigs produce the natural anti-Gal antibody and simulate the evolutionary appearance of this antibody in primates

BACKGROUND

Anti-Gal is the most abundant natural antibody in humans and Old World primates (apes and Old World monkeys). Its ligand, the α-gal epitope (Galα1-3Galβ1-4GlcNAc-R), is abundant in nonprimate mammals, prosimians and New World monkeys whereas it is absent in humans and Old World primates as a result of inactivation of the α1,3galactosyltransferase (α1,3GT) gene in ancestral Old World primates, as recent as 20-28 million years ago. Since anti-Gal has been a "forbidden" autoantibody for >140 million years of evolution in mammals producing α-gal epitopes it was of interest to determine whether ancestral Old World primates could produce anti-Gal once α-gal epitopes were eliminated, i.e. did they carry anti-Gal encoding immunoglobulin genes, or did evolutionary selection eliminate these genes that may be detrimental in mammals synthesizing α-gal epitopes. This question was studied by evaluating anti-Gal prodution in α1,3GT knockout (GT-KO) pigs recently generated from wild-type pigs in which the α-gal epitope is a major self-antigen.

METHODS

Anti-Gal antibody activity in pig sera was assessed by ELISA, flow cytometry and complement mediated cytolysis and compared to that in human sera.

RESULTS

The study demonstrates abundant production of the natural anti-Gal antibody in GT-KO pigs at titers even higher than in humans. The fine specificity of GT-KO pig anti-Gal is identical to that of human anti-Gal.

CONCLUSIONS

Pigs and probably other mammals producing α-gal epitopes carry immunoglobulin genes encoding anti-Gal as an autoantibody. Once the α-gal epitope is eliminated in GT-KO pigs, they produce anti-Gal. These findings strongly suggest that similar to GT-KO pigs, inactivation of the α1,3GT gene in ancestral Old World primates enabled the immediate production of anti-Gal, possibly as a protective antibody against detrimental microbial agents carrying α-gal epitopes.

Possible role of a cell surface carbohydrate in evolution of resistance to viral infections in old world primates

Due to inactivation of the α1,3-galactosyltransferase gene (GGTA1, or the α1,3GT gene) approximately 28 million years ago, the carbohydrate αGal (Galα1,3Galβ1,4GlcNAc) is not expressed on the cells of Old World monkeys and apes (including humans) but is expressed in all other mammals. The proposed selective advantage of this mutation for these primates is the ability to produce anti-Gal antibodies, which may be an effective immune component in neutralizing αGal-expressing pathogens. However, loss of α1,3GT expression may have been advantageous by providing natural resistance against viral pathogens that exploited the α1,3GT pathway or cell surface αGal for infection. Infections of paired cell lines with differential expression of α1,3GT showed that Sindbis viruses (SINV) preferentially replicate in α1,3GT-positive cells, whereas herpes simplex viruses type 1 and type 2 (HSV-1 and HSV-2) preferentially grow in cells lacking α1,3GT. Viral growth and spread correlated with the ability of the different viruses to successfully initiate infection in the presence or absence of α1,3GT expression. GT knockout (KO) suckling mice infected with SINV strains (AR339 and S.A.AR86) experienced significant delay in onset of disease symptoms and mortality compared to wild-type (WT) B6 suckling mice. In contrast, HSV-2-infected GT KO mice had higher viral titers in spleen and liver and exhibited significantly more focal hepatic necrosis than WT B6 mice. This study demonstrates that α1,3GT activity plays a role in the course of infections for certain viruses. Furthermore, this study has implications for the evolution of resistance to viral infections in primates.

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.

Lack of iGb3 and Isoglobo-Series Glycosphingolipids in Pig Organs Used for Xenotransplantation: Implications for Natural Killer T-Cell Biology

α-1,3-Terminated galactose residues on glycoproteins and glycosphingolipids are recognized by natural anti-α-1,3-galactose antibodies in human serum and cause hyperacute rejection in pig-to-human xenotransplantation. Genetic depletion of α-1,3-galactosyltransferase-1 in pigs abolishes the hyperacute rejection reaction. However, the isoglobotriosylceramide (iGb3) synthase in pigs may produce additional α-1,3-terminated galactose residues on glycosphingolipids. In both α-1,3-galactosyltranserase-1 knockout mice and pigs, cytotoxic anti-α-1,3-galactose antibodies could be induced; thus, a paradox exists that anti-α-1,3-galactose antibodies are present in animals with functional iGb3 synthases. Furthermore, iGb3 has been found to be an endogenous antigen for natural killer T (NKT) cells, an innate type of lymphocyte that may initiate the adaptive immune responses. It has been reasoned that iGb3 may trigger the activation of NKT cells and cause the rejection of α-1,3-galactosyltransferase-1-deficient organs through the potent stimulatory effects of NKT cells on adaptive immune cells (see ref.([20])). In this study, we examined the expression of iGb3 and the isoglobo-series glycosphingolipids in pig organs, including the heart, liver, pancreas, and kidney, by ion-trap mass spectrometry, which has a sensitivity of measuring 1% iGb3 among Gb3 isomers, when 5 μg/mL of the total iGb3/Gb3 mixture is present (see ref.([35])). We did not detect iGb3 or other isoglobo-series glycosphingolipids in any of these organs, although they were readily detected in mouse and human thymus and dendritic cells. The lack of iGb3 and isoglobo-series glycosphingolipids in pig organs indicates that iGb3 is unlikely to be a relevant immune epitope in xenotransplantation.

Complete absence of the αGal xenoantigen and isoglobotrihexosylceramide in α1,3galactosyltransferase knock-out pigs

BACKGROUND

Anti-Galα1,3Galβ-R natural antibodies are responsible for hyperacute rejection in pig-to-primate xenotransplantation. Although the generation of pigs lacking the α1,3galactosyltransferase (GalT) has overcome hyperacute rejection, antibody-mediated rejection is still a problem. It is possible that other enzymes synthesize antigens similar to Galα1,3Gal epitopes that are recognized by xenoreactive antibodies. The glycosphingolipid isoglobotrihexosylceramide (iGb₃) represents such a candidate expressing an alternative Galα1,3Gal epitope. The present work determined whether the terminal Galα1,3Gal disaccharide is completely absent in Immerge pigs lacking the GalT using several different highly sensitive methods.

METHODS

The expression of Galα1,3Gal was evaluated using a panel of antibodies and lectins by flow cytometry and fluorescent microscopy; GalT activity was detected by an enzymatic assay; and ion trap mass spectroscopy of neutral cellular membranes extracted from aortic endothelial was used for the detection of sugar structures. Finally, the presence of iGb₃ synthase mRNA was tested by RT-PCR in pig thymus, spleen, lymph node, kidney, lung, and liver tissue samples.

RESULTS

Aortic endothelial cells derived from GalT knockout pigs expressed neither Galα1,3Gal nor iGb₃ on their surface, and GalT enzymatic activity was also absent. Lectin staining showed an increase in the blood group H-type sugar structures present in GalT knockout cells as compared to wild-type pig aortic endothelial cells (PAEC). Mass spectroscopic analysis did not reveal Galα1,3Gal in membranes of GalT knockout PAEC; iGb₃ was also totally absent, whereas a fucosylated form of iGb₃ was detected at low levels in both pig aortic endothelial cell extracts. Isoglobotrihexosylceramide 3 synthase mRNA was expressed in all pig tissues tested whether derived from wild-type or GalT knockout animals.

CONCLUSIONS

These results confirm unequivocally the absence of terminal Galα1,3Gal disaccharides in GalT knockout endothelial cells. Future work will have to focus on other mechanisms responsible for xenograft rejection, in particular non-Galα1,3Gal antibodies and cellular responses.

Structural characterization of neutral and acidic oligosaccharides in the milks of strepsirrhine primates: greater galago, aye-aye, Coquerel's sifaka and mongoose lemur

The structures of milk oligosaccharides were characterized for four strepsirrhine primates to examine the extent to which they resemble milk oligosaccharides in other primates. Neutral and acidic oligosaccharides were isolated from milk of the greater galago (Galagidae: Otolemur crassicaudatus), aye-aye (Daubentoniidae: Daubentonia madagascariensis), Coquerel's sifaka (Indriidae: Propithecus coquereli) and mongoose lemur (Lemuridae: Eulemur mongoz), and their chemical structures were characterized by (1)H-NMR spectroscopy. The oligosaccharide patterns observed among strepsirrhines did not appear to correlate to phylogeny, sociality or pattern of infant care. Both type I and type II neutral oligosaccharides were found in the milk of the aye-aye, but type II predominate over type I. Only type II oligosaccharides were identified in other strepsirrhine milks. α3'-GL (isoglobotriose, Gal(α1-3)Gal(β1-4)Glc) was found in the milks of Coquerel's sifaka and mongoose lemur, which is the first report of this oligosaccharide in the milk of any primate species. 2'-FL (Fuc(α1-2)Gal(β1-4)Glc) was found in the milk of an aye-aye with an ill infant. Oligosaccharides containing the Lewis x epitope were found in aye-aye and mongoose lemur milk. Among acidic oligosaccharides, 3'-N-acetylneuraminyllactose (3'-SL-NAc, Neu5Ac(α2-3)Gal(β1-4)Glc) was found in all studied species, whereas 6'-N-acetylneuraminyllactose (6'-SL-NAc, Neu5Ac(α2-6)Gal(β1-4)Glc) was found in all species except greater galago. Greater galago milk also contained 3'-N-glycolylneuraminyllactose (3'-SL-NGc, Neu5Gc(α2-3)Gal(β1-4)Glc). The finding of a variety of neutral and acidic oligosaccharides in the milks of strepsirrhines, as previously reported for haplorhines, suggests that such constituents are ancient rather than derived features, and are as characteristic of primate lactation is the classic disaccharide, lactose.

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.

Investigation of potential carbohydrate antigen targets for human and baboon antibodies

BACKGROUND

The continued presence of a primate antibody-mediated response to cells and organs from alpha1,3-galactosyltransferase gene-knockout (GTKO) pigs indicates that there may be antigens other than Gal alpha 1,3Gal (alpha Gal) against which primates have xenoreactive antibodies. Human and baboon sera were tested for reactivity against a panel of saccharides that might be potential antigen targets for natural anti-non-alpha Gal antibodies.

METHODS

Human sera (n = 16) and baboon sera (n = 15) of all ABO blood types were tested using an enzyme-linked immunoadsorbent assay for binding of IgM and IgG to a panel of synthetic polyacrylamide-linked saccharides (n = 15). Human sera were also tested after adsorption on alpha Gal immunoaffinity beads. Sera from healthy wild-type (WT, n = 6) and GTKO (n = 6) pigs and from baboons (n = 4) sensitized to GTKO pig organ or artery transplants (of blood type O) were also tested. Forssman antigen expression on baboon and pig tissues was investigated by immunohistochemistry.

RESULTS

Both human and baboon sera showed high IgM and IgG binding to alpha Gal saccharides, alpha-lactosamine, and Forssman disaccharide. Human sera also demonstrated modest binding to N-glycolylneuraminic acid (Neu5Gc). When human sera were adsorbed on alpha Gal oligosaccharides, there was a reduction in binding to alpha Gal and alpha-lactosamine, but not to Forssman. WT and GTKO pig sera showed high binding to Forssman, and GTKO pig sera showed high binding to alpha Gal saccharides. Baboon sera sensitized to GTKO pigs showed no significant increased binding to any specific saccharide. Staining for Forssman was negative on baboon and pig tissues.

CONCLUSIONS

We were unable to identify definitively any saccharides from the selected panel that may be targets for primate anti-non-alpha Gal antibodies. The high level of anti-Forssman antibodies in humans, baboons, and pigs, and the absence of Forssman expression on pig tissues, suggest that the Forssman antigen does not play a role in the primate immune response to pigs.

Structural biology of carbohydrate xenoantigens

Transplantation of organs across species (xenotransplantation) is being considered to overcome the shortage of human donor organs. However, unmodified pig organs undergo an antibody-mediated hyperacute rejection that is brought about by the presence of natural antibodies to Galalpha(1,3)Gal, which is the major carbohydrate xenoantigen. Genetic modification of pig organs to remove most of the Galalpha(1,3)Gal epitopes has been achieved, but the human immune system may still recognize residual lipid-linked Galalpha(1,3)Gal carbohydrates, new (cryptic) carbohydrates or additional non-Galalpha(1,3)Gal carbohydrate xenoantigens. The structural basis for lectin and antibody recognition of Galalpha(1,3)Gal carbohydrates is starting to be understood and is discussed in this review. Antibody binding to Galalpha(1,3)Gal carbohydrates is predicted to primarily involve end-on insertion of the terminal alphaGal residue, but it is possible that groove-type binding can occur, as for some lectins. It is likely that similar antibody and lectin recognition will occur with other non-Galalpha(1,3)Gal xenoantigens, which potentially represent new barriers for pig-to-human xenotransplantation.

Immune responses to alpha1,3 galactosyltransferase knockout pigs

PURPOSE OF REVIEW

To summarize the current knowledge of the immune response generated against xenografts stemming from alpha1,3-galactosyltransferase knockout (GalT-KO) pigs. In particular, we will address the nature of potentially remaining Gal epitopes, the role of non-Gal xenoantigens, and the cellular response directed against GalT-KO tissues.

RECENT FINDINGS

New findings support the view that porcine cells do not express isoglobotrihexosylceramide 3, and GalT-KO pigs, if at all, express negligible levels of Gal. The anti-non-Gal antibody response to GalT-KO cells allowed the identification of several potentially relevant porcine xenoantigens, mainly carbohydrates. Coculture of wildtype pig aortic endothelial cells but not of GalT-KO pig aortic endothelial cells with whole human blood induces the secretion of porcine and human cytokines and the upregulation of E-selectin; in contrast, the transmigration of human leukocytes across porcine endothelium is not regulated by Gal.

SUMMARY

New immunological problems are arising after the elimination of Gal by the generation of GalT-KO pigs; these include non-Gal antibodies and the identification of their elusive antigens, as well as cellular components of the immune system, including neutrophils, macrophages, natural killer cells, and T cells.

Xenotransplantation: role of natural immunity

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

CD1d presentation of glycolipids

The CD1 family of antigen-presenting molecules consists of five members, CD1a to e. Of these molecules CD1d has been the subject of much interest over the past 10 years following the discovery that this molecule presents antigens to a group of T cells known as invariant natural killer T cells (iNKT). iNKT cells carry an invariant T cell receptor which contains homologous gene segments in mouse and man. iNKT cells are positively selected in the thymus in the same manner as major histocompatibility complex restricted T cells, except iNKT cells require CD1d to be presented by thymocytes rather than epithelial cells. Once in peripheral organs, iNKT cells appear to play multiple roles in host defence against pathogens and cancer. If the numbers of iNKT cells are not correctly regulated it can result in autoimmune disorders, such as diabetes. The ligands for iNKT cells have been the subject of much research but identifying physiologically relevant candidate ligands for positive selection or activation has proved technically very challenging. This is largely due to the fact that the ligands for iNKT cells are lipids. The lipid ligands for thymic selection and some of those involved in peripheral activation are self-derived. Glycosphingolipids are suggested to be the class of lipid for iNKT cell thymic development. For peripheral activation it appears multiple classes of self-derived lipids may play a role, in addition to pathogen-derived lipids. This review will cover essential background to iNKT cell and CD1d biology with emphasis on the candidate iNKT cell ligands proposed to date.

Distribution of the alphaGal- and the non-alphaGal T-antigens in the pig kidney: potential targets for rejection in pig-to-man xenotransplantation

Carbohydrate antigens, present on pig vascular endothelial cells, seem to be the prime agents responsible for graft rejection, and although genetically modified animals that express less amounts of carbohydrate antigen are available, it is still useful to decide the localization of the reactive xenoantigens in organs contemplated for xenotransplantation. Here we compare the distribution in pig kidney of antigens important in xenograft destruction, namely the Galalpha1-3Gal (alphaGal) glycans, with the localization of the T-antigen (Galbeta1-3GalNAc). The alpha-galactose-specific lectin Griffonia simplicifolia isolectin 1B4 was used to detect the Galalpha1-3Gal glycans, whereas Arachis hypogaea (PNA) lectin and a monoclonal antibody (3C9) detected T-antigen. In addition, two vascular markers (anti-caveolin-1 and anti-von Willebrand factor) served to identify vascular structures of the kidney. Both conventional fluorescence and confocal microscopy were used to distinguish lectin and immunohistochemical staining. On the basis of fluorescence signals, the results indicate that the carbohydrate antigens are heterogeneously distributed in the pig kidney. alphaGal epitopes were sparse in the capillary loops forming the glomeruli and in the capillaries surrounding the convoluted tubules, but showed stronger staining in capillaries surrounding the limbs of Henle. In addition, the brush border and basement membranes of the convoluted tubules strongly reacted with the GS1-B4-lectin. Finally, the Galalpha1-3Gal glycans were also present on epithelial cells of the large collecting tubules. Regarding the T-antigen, PNA and 3C9 reacted with different glomerular cells, whereas both reacted strongly with the endothelial cells lining the large kidney vessels. Human serum incubation of pig kidney sections, in which the alphaGal epitopes were blocked by unconjugated GS1-B4, showed staining of the same vascular structures as were obtained after incubation with the T-antigen-detecting agents. The study thus proves a complex spatial distribution of carbohydrate antigens relevant for xenotransplantation of pig kidney.

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.

Carbohydrate residues downstream of the terminal Galalpha(1,3)Gal epitope modulate the specificity of xenoreactive antibodies

Carbohydrates are involved in many immunological responses including the rejection of incompatible blood, tissues and organs. Carbohydrate antigens with Galalpha(1,3)Gal epitopes are recognized by natural antibodies in humans and pose a major barrier for pig-to-human xenotransplantation. Genetically modified pigs have been established that have no functional alpha1,3-galactosyltransferase (alpha1,3GT), which transfers alphaGal to N-acetyllactosamine (LacNAc) type oligosaccharides. However, a low level of Galalpha(1,3)Gal is still expressed in alpha1,3GT knockout animals in the form of a lipid, isoglobotrihexosylceramide (iGb3), which is produced by iGb3 synthase on lactose (Lac) type core structures. Here, we define the reactivity of a series of monoclonal antibodies (mAb) generated in alpha1,3GT-/- mice immunized with rabbit red blood cells (RbRBC), as a rich source of lipid-linked antigens. Interestingly, one mAb (15.101) binds weakly to synthetic and cell surface-expressed Galalpha(1,3)Gal on LacNAc, but strongly to versions of the antigen on Lac cores, including iGb3. Three-dimensional models suggest that the terminal alpha-linked Gal binds tightly into the antibody-binding cavity. Furthermore, antibody interactions were predicted with the second and third monosaccharide units. Collectively, our findings suggest that although the terminal carbohydrate residues confer most of the binding affinity, the fine specificity is determined by subsequent residues in the oligosaccharide.

[Clinical xenotransplantation]

The growing numerical gap between the number of patients and available human donor organs have led to a revival interest in xenotransplantation. This review will mainly focus on the clinical affairs of xenotransplantation and the project of producing the gene modified pigs. Trials, designed to overcome xenogenic rejection by the expression of human complement regulatory protein (CRP), such as DAF (CD55), on the pig organ and knocking out the alpha-Gal epitope(Galalpha1-3Galbeta1-4GlcNAc-R), which is biosynthesized by the action of alpha1,3 galactosyltransferase (alpha1,3GT), were accomplished in several institutes, such as Harvard University, Pittsburgh University, Mayo Clinic, and BresaGen. We have also produced the [DAF(CD55)+GnT-III+alpha-Gal KO] pigs in last year. On the other hand, the clinical pig islets transplantation was done in many countries, such as Russia, Sweden, Mexico and China, until 2005. In addition, the new clinical trials of pig islets transplantation will be started in USA within three years. In addition, as the current studies in the xenotransplantation field, the strategies for the downregulation of the glycoantigen, complement activation, NK cell, and other immuno responces on the xenografts, are reviewed. The studies for the infectivity of porcine endogenous retrovirus (PERV) to human cells are also introduced.

Enzymes that synthesize the IB4 epitope are not sufficient to impart IB4 binding in dorsal root ganglia of rat

The isolectin B4 (IB4) stains a subset of small and medium-sized dorsal root ganglion (DRG) neurons by binding to terminal alpha-galactose on glycoproteins and glycolipids. The enzymes alpha(1,3)galactosyltransferase (1,3GT) and isoglobotriaosylceramide synthase (iGb3S) synthesize the galactose-alpha(1,3)-galactose group, which is the most common carbohydrate containing terminal alpha-galactose. 1,3GT preferentially glycosylates proteins whereas iGb3S glycosylates lipids. We generated antibodies against rat 1,3GT and iGb3S that were used for immunohistochemical staining of DRG cells. Virtually all neurons that bound IB4 expressed both enzymes, suggesting that IB4 binds to both glycoproteins and glycolipids in IB4-positive neurons. 1,3GT immunoreactivity was observed in small and medium-sized neurons and satellite cells. iGb3S immunoreactivity was observed in neurons of varying sizes. Many neurons that expressed these enzymes did not bind IB4. Additionally, the majority of neurons that expressed substance P expressed both enzymes but did not bind IB4. Ultrastructual studies revealed that 1,3GT was predominantly associated with the Golgi apparatus, whereas iGb3S was found near the Golgi apparatus and in large, clear vesicles throughout the soma. These data suggest that, although expression of 1,3GT and/or iGb3S appears to be necessary for IB4 binding, expression of these enzymes is not sufficient to impart IB4 binding.

Normal development and function of invariant natural killer T cells in mice with isoglobotrihexosylceramide (iGb3) deficiency

CD1d-restricted natural killer T (NKT) cells, expressing the invariant T cell antigen receptor (TCR) chain encoded by Valpha14-Jalpha18 gene segments in mice and Valpha24-Jalpha18 in humans [invariant NKT (iNKT) cells], contribute to immunoregulatory processes, such as tolerance, host defense, and tumor surveillance. iNKT cells are positively selected in the thymus by CD1d molecules expressed by CD4(+)/CD8(+) cortical thymocytes. However, the identity of the endogenous lipid(s) responsible for positive selection of iNKT cells remains unclear. One candidate lipid proposed to play a role in positive selection is isoglobotrihexosylceramide (iGb3). However, no direct evidence for its physiological role has been provided. Therefore, to directly investigate the role of iGb3 in iNKT cell selection, we have generated mice deficient in iGb3 synthase [iGb3S, also known as alpha1-3galactosyltransferase 2 (A3galt2)]. These mice developed, grew, and reproduced normally and exhibited no overt behavioral abnormalities. Consistent with the notion that iGb3 is synthesized only by iGb3S, lack of iGb3 in the dorsal root ganglia of iGb3S-deficient mice (iGb3S(-/-)), as compared with iGb3S(+/-) mice, was confirmed. iGb3S(-/-) mice showed normal numbers of iNKT cells in the thymus, spleen, and liver with selected TCR Vbeta chains identical to controls. Upon administration of alpha-galactosylceramide, activation of iNKT and dendritic cells was similar in iGb3S(-/-) and iGb3S(+/-) mice, as measured by up-regulation of CD69 as well as intracellular IL-4 and IFN-gamma in iNKT cells, up-regulation of CD86 on dendritic cells, and rise in serum concentrations of IL-4, IL-6, IL-10, IL-12p70, IFN-gamma, TNF-alpha, and Ccl2/MCP-1. Our results strongly suggest that iGb3 is unlikely to be an endogenous CD1d lipid ligand determining thymic iNKT selection.

Implications for invariant natural killer T cell ligands due to the restricted presence of isoglobotrihexosylceramide in mammals

Development of invariant natural killer T (iNKT) cells requires the presentation of lipid ligand(s) by CD1d molecules in the thymus. The glycosphingolipid (GSL) isoglobotrihexosylceramide (iGb3) has been proposed as the natural iNKT cell-selecting ligand in the thymus and to be involved in peripheral activation of iNKT cells by dendritic cells (DCs). However, there is no direct biochemical evidence for the presence of iGb3 in mouse or human thymus or DCs. Using a highly sensitive HPLC assay, the only tissue where iGb3 could be detected in mouse was the dorsal root ganglion (DRG). iGb3 was not detected in other mouse or any human tissues analyzed, including thymus and DCs. Even in mutant mice that store isoglobo-series GSLs in the DRG, we were still unable to detect these GSLs in the thymus. iGb3 is therefore unlikely to be a physiologically relevant iNKT cell-selecting ligand in mouse and humans. A detailed study is now warranted to better understand the nature of iNKT cell-selecting ligand(s) in vivo.

Reactivity of human natural antibodies to endothelial cells from Galalpha(1,3)Gal-deficient pigs

BACKGROUND

Xenoreactive human natural antibodies (NAb) are predominantly directed against galactose-alpha(1,3)galactose (Gal). Binding of immunoglobulin (Ig) G and IgM NAb activates porcine endothelial cells (pEC) and triggers complement lysis responsible for hyperacute xenograft rejection. In vitro, IgG NAb induce human natural killer (NK) cell-mediated lysis of pEC by antibody-dependent cell-mediated cytotoxicity (ADCC). The present study examined the levels of anti-porcine NAb in a large number of individuals and addressed the functional role of non-Gal anti-porcine NAb.

METHODS

Sera from 120 healthy human blood donors were analyzed for the presence of anti-porcine NAb by flow cytometry using porcine red blood cells (pRBC), lymphoblastoid cells (pLCL), and pEC derived from control or Gal-deficient pigs. Xenogeneic complement lysis was measured by flow cytometry using human serum and rabbit complement. ADCC was analyzed by chromium-release assays using human serum and freshly isolated NK cells.

RESULTS

Human IgM binding to pRBC was found in 93% and IgG binding in 86% of all samples. Non-Gal NAb comprised 13% of total IgM and 36% of total IgG binding to pEC. NAb/complement-induced lysis and ADCC of Gal-deficient compared to Gal-positive pEC were 21% and 29%, respectively. The majority of anti-Gal and non-Gal IgG NAb were of the IgG2 subclass.

CONCLUSIONS

The generation of Gal-deficient pigs has overcome hyperacute anti-Gal-mediated xenograft rejection in nonhuman primates. Non-Gal anti-porcine NAb represent a potentially relevant immunological hurdle in a subgroup of individuals by inducing endothelial damage in xenografts.

Long-term evolution of the CAZY glycosyltransferase 6 (ABO) gene family from fishes to mammals—a birth-and-death evolution model

Functional glycosyltransferase 6 (GT6) family members catalyze the transfer of galactose or N-acetylgalactosamine in alpha1,3 linkage to various substrates and synthesize structures related to the A and B histo-blood group antigens, the Forssman antigen, alphaGal epitope, and iGb3 glycolipid. In rat, mouse, dog, and cow genomes, we have identified three new mammalian genes (GT6m5, GT6m6, and GT6m7) encoding putative proteins belonging to the GT6 family. Among these, GT6m6 protein does not display major alterations of the GT6 motifs involved in binding of the divalent cation and the substrate. Based on protein sequence comparison, gene structure, and synteny, GT6 homologous sequences were also identified in bird, fish, and amphibian genomes. Strikingly, the number and type of GT6 genes varied widely from species to species, even within phylogenetically related groups. In human, except ABO functional alleles, all other GT6 genes are either absent or nonfunctional. Human, mouse, and cow have only one ABO gene, whereas rat and dog have several. In the chicken, the Forssman synthase-like is the single GT6 family member. Five Forssman synthase-like genes were found in zebrafish, but are absent from three other fishes (fugu, puffer fish, and medaka). Two iGb3 synthase-like genes were found in medaka, which are absent from zebrafish. Fugu, puffer fish, and medaka have an additional GT6 gene that we termed GT6m8, which is absent from all other species analyzed here. These observations indicate that individual GT6 genes have expanded and contracted by recurrent duplications and deletions during vertebrate evolution, following a birth-and-death evolution type.

Progress in xenotransplantation following the introduction of gene-knockout technology

The production of alpha1,3-galactosyltransferase gene-knockout (GT-KO) pigs has overcome the barrier of preformed anti-Galalpha1,3Gal (Gal) antibodies that has inhibited progress in pig-to-primate organ xenotransplantation for many years. Survival of GT-KO pig organs in nonhuman primates is currently limited by the development of a thrombotic microangiopathy that results in increasing ischemic injury of the transplanted organ over weeks or months. Potential causative factors include vascular endothelial activation from preformed anti-nonGal antibodies or cells of the innate immune system that recognize nonGal pig antigens directly, and coagulation dysregulation associated with molecular incompatibilities between pig and primate. Carefully isolated pancreatic islets from wild-type (genetically unmodified) adult pigs express minimal Gal epitopes, allowing survival sometimes for weeks or months after transplantation into nonhuman primates receiving immunosuppression directed only at T-cell function. However, there is a considerable immediate loss of islets, probably related to activation of coagulation and complement cascades. Further genetic manipulation of organ-source pigs is therefore required to overcome these problems. GT-KO pigs expressing a human complement-regulatory protein, e.g. decay-accelerating factor, and/or an 'anti-coagulant' gene, e.g. human tissue factor pathway inhibitor, might prevent the change in vascular endothelium from an anti-coagulant to a procoagulant phenotype, and protect the islets from early loss.

Antibodies directed to pig non-Gal antigens in naïve and sensitized baboons

BACKGROUND

As pigs homozygous for alpha1,3-galactosyltransferase gene-knockout (GT-KO) are available, primate antibodies to pig non-Gal antigens can be studied.

METHODS

Sera from 56 baboons were tested for binding of IgM and IgG to peripheral blood mononuclear cells (PBMC) from both wild-type (WT) and GT-KO pigs by flow cytometry. Complement-dependent cytotoxicity was measured in 39 sera. Antibody and cytotoxicity responses were measured in two baboons exposed to a GT-KO pig heart, one not immunosuppressed and one that received only cobra venom factor.

RESULTS

IgM and IgG bound to 95% and 79% of WT PBMC, and 32% and 9% GT-KO PBMC, respectively (WT vs. GT-KO, P<0.01). Whereas 97% of sera were cytotoxic to WT PBMC, only 64% were cytotoxic to GT-KO PBMC, and the level of cytotoxicity was less (mean 60% vs. 25% lysis, P<0.05). In the two baboons exposed to GT-KO hearts, anti-non-Gal antibodies increased markedly, peaking after 2 (IgM) and 3 (IgG) weeks, associated with an increase in lysis of GT-KO PBMC.

CONCLUSIONS

Two-thirds of baboon sera demonstrated cytotoxicity to GT-KO PBMC. After GT-KO organ transplantation, if an elicited antibody response develops, it is likely to cause rapid graft rejection.

The Molecular Basis for Galα(1,3)Gal Expression in Animals with a Deletion of the α1,3Galactosyltransferase Gene

The production of homozygous pigs with a disruption in the GGTA1 gene, which encodes alpha1,3galactosyltransferase (alpha1,3GT), represented a critical step toward the clinical reality of xenotransplantation. Unexpectedly, the predicted complete elimination of the immunogenic Galalpha(1,3)Gal carbohydrate epitope was not observed as Galalpha(1,3)Gal staining was still present in tissues from GGTA1(-/-) animals. This shows that, contrary to previous dogma, alpha1,3GT is not the only enzyme able to synthesize Galalpha(1,3)Gal. As iGb3 synthase (iGb3S) is a candidate glycosyltransferase, we cloned iGb3S cDNA from GGTA1(-/-) mouse thymus and confirmed mRNA expression in both mouse and pig tissues. The mouse iGb3S gene exhibits alternative splicing of exons that results in a markedly different cytoplasmic tail compared with the rat gene. Transfection of iGb3S cDNA resulted in high levels of cell surface Galalpha(1,3)Gal synthesized via the isoglobo series pathway, thus demonstrating that mouse iGb3S is an additional enzyme capable of synthesizing the xenoreactive Galalpha(1,3)Gal epitope. Galalpha(1,3)Gal synthesized by iGb3S, in contrast to alpha1,3GT, was resistant to down-regulation by competition with alpha1,2fucosyltransferase. Moreover, Galalpha(1,3)Gal synthesized by iGb3S was immunogenic and elicited Abs in GGTA1 (-/-) mice. Galalpha(1,3)Gal synthesized by iGb3S may affect survival of pig transplants in humans, and deletion of this gene, or modification of its product, warrants consideration.

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.

α1,3‐Galactosyltransferase knockout pigs are available for xenotransplantation: Are glycosyltransferases still relevant?

In the early 1990s, the Galalpha(1,3)Gal carbohydrate linkage was found to be the major xenoepitope causing hyperacute rejection. This carbohydrate, the antibodies that bind to it, and the enzyme that produces it (alpha1,3-galactosyltransferase) were the foci of research by many groups. Nearly a decade later, alpha1,3-galactosyltransferase knockout pigs were finally produced; hyperacute rejection could be avoided in these pigs. Having achieved this goal, enthusiasm declined for the study of glycosyltransferases and their carbohydrate products. To examine whether this decline was premature, we evaluate whether gene deletion has indeed solved the initial rejection problem or, in fact, created new problems. This review addresses this by examining the impact of the gene deletion on cell surface carbohydrate. Surprisingly, Galalpha(1,3)Gal is still present in alpha1,3-galactosyltransferase knockout animals: it is possibly synthesized on lipid by iGb3 synthase. Furthermore, removal of alphaGal resulted in the exposure of the N-acetyllactosamine epitope. This exposed epitope can bind natural antibodies and perhaps should be capped by transgenic expression of another transferase. We believe the continued study of glycosyltransferases is essential to examine the new issues raised by the deletion of alpha1,3-galactosyltransferase.

The possible contribution of anti-Gal to Graves' disease

Anti-Gal is a natural antibody specific for the alpha-galactosyl epitope. Previous studies suggested that Graves' disease (GD) patients had elevated anti-Gal titers compared to normal controls, but titers returned to normal after treatment. We developed an anti-Gal enzyme-linked immunosorbent assay (ELISA) using the property of anti-Gal to bind tightly to mouse laminin. We found no significant correlations between anti-Gal and thyroidstimulating immunoglobulin (TSI) or free thyroxine (T(4)) in untreated hyperthyroid GD patients (n = 15) without clinical ophthalmopathy or euthyroid, previously treated GD patients with ophthalmopathy. There was a significant regression between TSI and free T(4) in the hyperthyroid patients (p < 0.01). Addition of total anti- Gal antibody to the regression showed a trend toward improved correlation (p = 0.15 for improved correlation relative to TSI and free T(4) alone), suggesting it may stimulate GD thyroid tissue. However, in contrast to previous studies we found hyperthyroid patients (n = 20) had lower levels of anti-Gal immunoglobulin G (IgG) (18.4 +/- 4.0 vs. 41.8 +/- 8.9) than normals (n = 36 p < 0.05). Interestingly, hyperthyroid patients without clinical ophthalmopathy tended to have lower IgG anti-Gal levels than euthyroid patients with ophthalmopathy (p = 0.1). Hyperthyroidism significantly lowers anti-Gal, but the possible increase of anti-Gal in patients with ophthalmopathy suggests anti-Gal may play a role in ophthalmopathy, or may reflect the euthryoid status of these patients. This trend needs further study.

Specificity of a soluble UDP-galactose: fucoside alpha1,3-galactosyltransferase that modifies the cytoplasmic glycoprotein Skp1 in Dictyostelium

Skp1 is an adaptor-like protein in E3(SCF)-ubiquitin ligases and other multiprotein complexes of the cytoplasm and nucleus. In Dictyostelium, Skp1 is modified by an unusual pentasaccharide containing a Galalpha1-Fuc linkage, whose formation is examined here. A cytosolic extract from Dictyostelium was found to yield, after 2400-fold purification, an activity that could transfer Gal from UDP-Gal to both a Fuc-terminated glycoform of Skp1 and synthetic Fuc conjugates in the presence of Mn(2+) and dithiothreitol. The microsomal fraction was devoid of activity. The linkage formed was Galalpha1,3Fuc based on co-chromatography with only this synthetic isomer conjugate, and sensitivity to alpha1,3/6-galactosidase. Skp1 exhibited an almost 1000-fold lower K(m) and 35-fold higher V(max) compared with a simple alpha-fucoside, but this advantage was abolished by denaturation or alkylation of Cys residues. A comparison of a complete series of synthetic glycosides representing the non-reducing terminal mono-, di-, and trisaccharides of Skp1 revealed, surprisingly, that the disaccharide is most active owing primarily to a V(max) advantage, but still much less active than Skp1 itself because of a K(m) difference. These findings indicate that alpha-GalT1 is a cytoplasmic enzyme whose modification of Skp1 requires proper presentation of the terminal acceptor disaccharide by a folded Skp1 polypeptide, which correlates with previous evidence that the Galalpha1,3Fuc linkage is deficient in expressed mutant Skp1 proteins.

Lack of Galactose-α-1,3-Galactose Expression on Porcine Endothelial Cells Prevents Complement-Induced Lysis but Not Direct Xenogeneic NK Cytotoxicity

The galactose-alpha-1,3-galactose (alphaGal) carbohydrate epitope is expressed on porcine, but not human cells, and therefore represents a major target for preformed human anti-pig natural Abs (NAb). Based on results from pig-to-primate animal models, NAb binding to porcine endothelial cells will likely induce complement activation, lysis, and hyperacute rejection in pig-to-human xenotransplantation. Human NK cells may also contribute to innate immune responses against xenografts, either by direct recognition of activating molecules on target cells or by FcgammaRIII-mediated xenogeneic Ab-dependent cellular cytotoxicity (ADCC). The present study addressed the question as to whether the lack of alphaGal protects porcine endothelial cells from NAb/complement-induced lysis, direct xenogeneic NK lysis, NAb-dependent ADCC, and adhesion of human NK cells under shear stress. Homologous recombination, panning, and limiting dilution cloning were used to generate an alphaGal-negative porcine endothelial cell line, PED2*3.51. NAb/complement-induced xenogeneic lysis of PED2*3.51 was reduced by an average of 86% compared with the alphaGal-positive phenotype. PED2*3.51 resisted NK cell-mediated ADCC with a reduction of lysis ranging from 30 to 70%. However, direct xenogeneic lysis of PED2*3.51, mediated either by freshly isolated or IL-2-activated human NK cells or the NK cell line NK92, was not reduced. Furthermore, adhesion of IL-2-activated human NK cells did not rely on alphaGal expression. In conclusion, removal of alphaGal leads to a clear reduction in complement-induced lysis and ADCC, but does not resolve adhesion of NK cells and direct anti-porcine NK cytotoxicity, indicating that alphaGal is not a dominant target for direct human NK cytotoxicity against porcine cells.