Anticarbohydrate Antibody Repertoires in Patients Transplanted with Fetal Pig Islets Revealed by Glycan Arrays

Immunoprotection Strategies in β-Cell Replacement Therapy: A Closer Look at Porcine Islet Xenotransplantation

Type 1 diabetes mellitus (T1DM) is characterized by absolute insulin deficiency primarily due to autoimmune destruction of pancreatic β-cells. The prevailing treatment for T1DM involves daily subcutaneous insulin injections, but a substantial proportion of patients face challenges such as severe hypoglycemic episodes and poorly controlled hyperglycemia. For T1DM patients, a more effective therapeutic option involves the replacement of β-cells through allogeneic transplantation of either the entire pancreas or isolated pancreatic islets. Unfortunately, the scarcity of transplantable human organs has led to a growing list of patients waiting for an islet transplant. One potential alternative is xenotransplantation of porcine pancreatic islets. However, due to inter-species molecular incompatibilities, porcine tissues trigger a robust immune response in humans, leading to xenograft rejection. Several promising strategies aim to overcome this challenge and enhance the long-term survival and functionality of xenogeneic islet grafts. These strategies include the use of islets derived from genetically modified pigs, immunoisolation of islets by encapsulation in biocompatible materials, and the creation of an immunomodulatory microenvironment by co-transplanting islets with accessory cells or utilizing immunomodulatory biomaterials. This review concentrates on delineating the primary obstacles in islet xenotransplantation and elucidates the fundamental principles and recent breakthroughs aimed at addressing these challenges.

A systematic review reveals conflicting evidence for the prevalence of antibodies against the sialic acid 'xenoautoantigen' Neu5Gc in humans and the need for a standardised approach to quantification

Despite an array of hypothesised implications for health, disease, and therapeutic development, antibodies against the non-human sialic acid N-glycolylneuraminic acid (Neu5Gc) remain a subject of much debate. This systematic review of 114 publications aimed to generate a comprehensive overview of published studies in this field, addressing both the reported prevalence of anti-Neu5Gc antibodies in the human population and whether experimental variation accounts for the conflicting reports about the extent of this response. Absolute titres of anti-Neu5Gc antibodies, the reported prevalence of these antibodies, and the individual variation observed within experiments were analysed and grouped according to biological context ('inflammation', 'xenotransplantation', 'biotherapeutic use', 'cancer', and 'healthy populations'), detection method, target epitope selection, and choice of blocking agent. These analyses revealed that the experimental method had a notable impact on both the reported prevalence and absolute titres of anti-Neu5Gc antibodies in the general population, thereby limiting the ability to ascribe reported trends to genuine biological differences or the consequence of experimental design. Overall, this review highlights important knowledge gaps in the study of antibodies against this important xenoautoantigen and the need to establish a standardised method for their quantification if the extent of the importance of Neu5Gc in human health is to be fully understood.

Serum antibody screening using glycan arrays

Humans and other animals produce a diverse collection of antibodies, many of which bind to carbohydrate chains, referred to as glycans. These anti-glycan antibodies are a critical part of our immune systems' defenses. Whether induced by vaccination or natural exposure to a pathogen, anti-glycan antibodies can provide protection against infections and cancers. Alternatively, when an immune response goes awry, antibodies that recognize self-glycans can mediate autoimmune diseases. In any case, serum anti-glycan antibodies provide a rich source of information about a patient's overall health, vaccination history, and disease status. Glycan microarrays provide a high-throughput platform to rapidly interrogate serum anti-glycan antibodies and identify new biomarkers for a variety of conditions. In addition, glycan microarrays enable detailed analysis of the immune system's response to vaccines and other treatments. Herein we review applications of glycan microarray technology for serum anti-glycan antibody profiling.

Carbohydrate antigen microarray analysis of serum IgG and IgM antibodies before and after adult porcine islet xenotransplantation in cynomolgus macaques

Understanding the anti-carbohydrate antibody response toward epitopes expressed on porcine cells, tissues, and organs is critical to advancing xenotransplantation toward clinical application. In this study, we determined IgM and IgG antibody specificities and relative concentrations in five cynomolgus monkeys at baseline and at intervals following intraportal xenotransplantation of adult porcine islets. This study utilized a carbohydrate antigen microarray that comprised more than 400 glycoconjugates, including historically reported α-Gal and non-α-Gal carbohydrate antigens with various modifications. The elicited anti-carbohydrate antibody responses were predominantly IgM compared to IgG in 4 out of 5 monkeys. Patterns of elicited antibody responses greater than 1.5 difference (log2 base units; 2.8-fold on a linear scale) from pre-serum to post-serum sampling specific for carbohydrate antigens were heterogeneous and recipient-specific. Increases in the elicited antibody response to α-Gal, Sda, GM2 antigens, or Lexis X antigen were found in individual monkeys. The novel carbohydrate structures Galβ1-4GlcNAcβ1-3Galβ1 and N-linked glycans with Manα1-6(GlcNAcβ1-2Manα1-3)Manβ1-4GlcNAcβ structure were common targets of elicited IgM antibodies. These results provide important insights into the carbohydrate epitopes that elicit antibodies following pig-to-monkey islet xenotransplantation and reveal possible targets for gene editing.

Unique repertoire of anti-carbohydrate antibodies in individual human serum

Humoral immunity to pathogens and other environmental challenges is paramount to maintain normal health, and individuals lacking or unable to make antibodies are at risk. Recent studies indicate that many human protective antibodies are against carbohydrate antigens; however, little is known about repertoires and individual variation of anti-carbohydrate antibodies in healthy individuals. Here we analyzed anti-carbohydrate antibody repertoires (ACARs) of 105 healthy individual adult donors, aged 20-60+ from different ethnic backgrounds to explore variations in antibodies, as defined by binding to glycan microarrays and by affinity purification. Using microarrays that contained > 1,000 glycans, including antigens from animal cells and microbes, we profiled the IgG and IgM ACARs from all donors. Each donor expressed many ACAs, but had a relatively unique ACAR, which included unanticipated antibodies to carbohydrate antigens not well studied, such as chitin oligosaccharides, Forssman-related antigens, globo-type antigens, and bacterial glycans. We also saw some expected antibodies to ABO(H) blood group and α-Gal-type antigens, although these also varied among individuals. Analysis suggests differences in ACARs are associated with ethnicity and age. Thus, each individual ACAR is relatively unique, suggesting that individualized information could be useful in precision medicine for predicting and monitoring immune health and resistance to disease.

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.

Profiling natural serum antibodies of non-human primates with a carbohydrate antigen microarray

BACKGROUND

Engineering of α-Galactosyltransferase gene-knockout pigs circumvented hyperacute rejection of pig organs after xenotransplantation in non-human primates. Overcoming this hurdle revealed the importance of non-α-Gal carbohydrate antigens in the immunobiology of acute humoral xenograft rejection.

METHODS

This study analyzed serum from seven naïve cynomolgus monkeys (blood type O/B/AB = 3/2/2) for the intensity of natural IgM and IgG signals using carbohydrate antigen microarray, which included historically reported α-Gal and non-α-Gal carbohydrate antigens with various modifications.

RESULTS

The median (range) of IgM and IgG signals were 12.71 (7.23-16.38) and 9.05 (7.23-15.90), respectively. The highest IgM and IgG signals with narrowest distribution were from mono- and disaccharides, followed by modified structures. Natural anti-α-Gal antibody signals were medium to high in IgM (11.2-15.9) and medium in IgG (8.5-11.6) spectra, and was highest with Lac core structure (Galα1-3Galβ1-4Glc, iGb3) and lowest with LacNAc core structure (Galα1-3Galβ1-4GlcNAc). Similar signal intensities (up to 15.8 in IgM and up to 11.8 in IgG) were observed for historically detected natural non-α-Gal antigens, which included Tn antigen, T antigen, GM2 glycolipid, and Sd^a antigen. The hierarchical clustering analysis revealed the presence of clusters of anti-A antibodies and was capable of distinguishing between the blood group B and AB non-human primates.

CONCLUSIONS

The results presented here provide the most comprehensive evaluation of natural antibodies present in cynomolgus monkeys.

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.

Carbohydrate antigen expression and anti-pig antibodies in New World capuchin monkeys: Relevance to studies of xenotransplantation

BACKGROUND

Old World non-human primates (OWNHPs) are used for preclinical pig-to-NHP studies. However, like pigs, OWNHPs express Neu5Gc, and therefore do not develop natural anti-Neu5Gc antibodies. New World NHPs (NWNHPs) have been reported not to express Neu5Gc. We investigated the potential of NWNHPs in xenotransplantation research.

METHODS

We investigated expression of Gal, Neu5Gc, and Sd^a antigens on RBCs and PBMCs from humans, selected OWNHPs, and capuchin monkeys (a NWNHP). Serum anti-Gal and anti-Neu5Gc IgM and IgG levels were measured by ELISA. Binding of primate serum IgM and IgG to pig RBCs was measured by flow cytometry.

RESULTS

(a) Neither humans, OWNHPs, or capuchin monkeys expressed Gal on their RBCs, but capuchins expressed Gal on PBMCs. Humans and capuchins did not express Neu5Gc on either RBCs or PBMCs, but OWNHPs expressed Neu5Gc on both cells. Sd^a was not expressed on any RBCs or PBMCs. (b) By ELISA, human and OWNHP, but not capuchin, sera showed IgM and IgG binding to Gal. Human and capuchin, but not OWNHP, sera demonstrated some binding to Neu5Gc. (c) Anti-Sd^a IgM/IgG antibodies were detected in OWNHP sera. Knockout of Sd^a on pig RBCs did not significantly reduce human and capuchin antibody binding.

CONCLUSION

Capuchin monkeys could be surrogates for humans in experiments using RBCs, islets, neuronal cells, etc, from triple-knockout pigs (but may be too small to be used as recipients of pig organ grafts).

Anti-Neu5Gc and anti-non-Neu5Gc antibodies in healthy humans

Our group previously investigated the levels of anti-Gal and anti-nonGal IgM and IgG in a cohort of 75 healthy humans of various backgrounds, and found some significant differences related to factors such as age, gender, ABO blood group, diet, vaccination history, and geographic location during childhood. We have now expanded our cohort (n = 84) to investigate the levels of anti-Neu5Gc and anti-nonGal/nonNeu5Gc antibodies in healthy humans. Anti-nonGal and anti-nonGal/nonNeu5Gc human IgM and IgG binding to pRBCs and pAECs from GTKO/CD46 and GTKO/CD46/Neu5GcKO pigs were measured by flow cytometry. Anti-Gal and anti-Neu5Gc IgM and IgG levels were measured by ELISA. In summary, (i) the great majority (almost 100%) of humans had anti-Neu5Gc IgM and IgG antibodies that bound to pAECs and approximately 50% had anti-Neu5Gc antibodies that bound to pRBCs, (ii) there was significantly less human antibody binding to pig cells that did not express either Gal or Neu5Gc compared with those that did not express Gal alone, (iii) the levels of both IgM and IgG binding to GTKO/CD46/Neu5GcKO pRBCs and pAECs were low, (iv) the level of anti-Neu5Gc IgG was higher in men than women, (v) the level did not change with age or diet, and there was some variability associated with (vi) previous vaccination history and (vii) the geographic region in which the individual spent his or her childhood. Our study confirms that human antibody binding to RBCs and AECs from GTKO/CD46/Neu5GcKO pigs is greatly reduced compared to binding to GTKO/CD46 cells. However, all humans appear to have a low level of antibody that binds to pAECs that is not directed to either Gal or Neu5Gc. Our findings require consideration in planning clinical trials of xenotransplantation.

The immunological barriers to xenotransplantation

The availability of cells, tissues and organs from a non-human species such as the pig could, at least in theory, meet the demand of organs necessary for clinical transplantation. At this stage, the important goal of getting over the first year of survival has been reported for both cellular and solid organ xenotransplantation in relevant preclinical primate models. In addition, xenotransplantation is already in the clinic as shown by the broad use of animal-derived medical devices, such as bioprosthetic heart valves and biological materials used for surgical tissue repair. At this stage, however, prior to starting a wide-scale clinical application of xenotransplantation of viable cells and organs, the important obstacle represented by the humoral immune response will need to be overcome. Likewise, the barriers posed by the activation of the innate immune system and coagulative pathway will have to be controlled. As far as xenogeneic nonviable xenografts, increasing evidence suggests that considerable immune reactions, mediated by both innate and adaptive immunity, take place and influence the long-term outcome of xenogeneic materials in patients, possibly precluding the use of bioprosthetic heart valves in young individuals. In this context, the present article provides an overview of current knowledge on the immune processes following xenotransplantation and on the possible therapeutic interventions to overcome the immunological drawbacks involved in xenotransplantation.

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.

Competition between serum IgG, IgM, and IgA anti-glycan antibodies

Anti-glycan antibodies are an abundant subpopulation of serum antibodies with critical functions in many immune processes. Changes in the levels of these antibodies can occur with the onset of disease, exposure to pathogens, or vaccination. As a result, there has been significant interest in exploiting anti-glycan antibodies as biomarkers for many diseases. Serum contains a mixture of anti-glycan antibodies that can recognize the same antigen, and competition for binding can potentially influence the detection of antibody subpopulations that are more relevant to disease processes. The most abundant antibody isotypes in serum are IgG, IgM, and IgA, but little is known regarding how these different isotypes compete for the same glycan antigen. In this study, we developed a multiplexed glycan microarray assay and applied it to evaluate how different isotypes of anti-glycan antibodies (IgA, IgG, and IgM) compete for printed glycan antigens. While IgG and IgA antibodies typically outcompete IgM for peptide or protein antigens, we found that IgM outcompete IgG and IgA for many glycan antigens. To illustrate the importance of this effect, we provide evidence that IgM competition can account for the unexpected observation that IgG of certain antigen specificities appear to be preferentially transported from mothers to fetuses. We demonstrate that IgM in maternal sera compete with IgG resulting in lower than expected IgG signals. Since cord blood contains very low levels of IgM, competition only affects maternal IgG signals, making it appear as though certain IgG antibodies are higher in cord blood than matched maternal blood. Taken together, the results highlight the importance of competition for studies involving anti-glycan antibodies.

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.

Cloning and expression of porcine β1,4 N-acetylgalactosaminyl transferase encoding a new xenoreactive antigen

Background

Xenograft rejection of pigs organs with an engineered mutation in the GGTA-1 gene (GTKO) remains a predominantly antibody mediated process which is directed to a variety of non-Gal protein and carbohydrate antigens. We previously used an expression library screening strategy to identify six porcine endothelial cell cDNAs which encode pig antigens that bind to IgG induced after pig-to-primate cardiac xenotransplantation. One of these gene products was a glycosyltransferase with homology to the bovine β1,4 N-acetylgalactosaminyltransferase (B4GALNT2). We now characterize the porcine B4GALNT2 gene sequence, genomic organization, expression, and functional significance.

Methods

The porcine B4GALNT2 cDNA was recovered from the original library isolate, subcloned, sequenced, and used to identify a bacterial artificial chromosome (BAC) containing the entire B4GALNT2 locus from the Children's Hospital Oakland Research Institute BACPAC Resource Centre (#AC173453). PCR primers were designed to map the intron/exon genomic organization in the BAC clone. A stable human embryonic kidney (HEK) cell line expressing porcine B4GALNT2 (HEK-B4T) was produced. Expression of porcine B4GALNT2 in HEK-B4T cells was characterized by immune staining and siRNA transfection. The effects of B4GALNT2 expression in HEK-B4T cells was measured by flow cytometry and complement mediated lysis. Antibody binding to HEK and HEK-B4T cells was used to detect an induced antibody response to the B4GALNT2 produced glycan and the results were compared to GTKO PAEC specific non-Gal antibody induction. Expression of porcine B4GALNT2 in pig cells and tissues was measured by qualitative and quantitative real time reverse transcriptase PCR and by Dolichos biflorus agglutinin (DBA) tissue staining.

Results

The porcine B4GALNT2 gene shares a conserved genomic organization and encodes an open reading frame with 76 and 70% amino acid identity to the human and murine B4GALNT2 genes, respectively. The B4GALNT2 gene is expressed in porcine endothelial cells and shows a broadly distributed expression pattern. Expression of porcine B4GALNT2 in human HEK cells (HEK-B4T) results in increased binding of antibody to the B4GALNT2 enzyme, and increased reactivity with anti-Sd^a and DBA. HEK-B4T cells show increased sensitivity to complement mediated lysis when challenged with serum from primates after pig to primate cardiac xenotransplantation. In GTKO and GTKO:CD55 cardiac xenotransplantation recipients there is a significant correlation between the induction of a non-Gal antibody, measured using GTKO PAECs, and the induction of antibodies which preferentially bind to HEK-B4T cells.

Conclusion

The functional isolation of the porcine B4GALNT2 gene from a PAEC expression library, the pattern of B4GALNT2 gene expression and its sensitization of HEK-B4T cells to antibody binding and complement mediated lysis indicates that the enzymatic activity of porcine B4GALNT2 produces a new immunogenic non-Gal glycan which contributes in part to the non-Gal immune response detected after pig-to-baboon cardiac xenotransplantation.

Natural antibodies to glycans

A wide variety of so-called natural antibodies (nAbs), i.e. immunoglobulins generated by B-1 cells, are directed to glycans. nAbs to glycans can be divided in three groups: 1) conservative nAbs, i.e. practically the same in all healthy donors with respect to their epitope specificity and level in blood; 2) allo-antibodies to blood group antigens; 3) plastic antibodies related to the first or the second group but discussed separately because their level changes considerably during diseases and some temporary conditions, in particular inflammation and pregnancy. Antibodies from the third group proved to be prospective markers of a number of diseases, whereas their unusual level (below or above the norm) is not necessarily the consequence of disease/state. Modern microarrays allowed the determination of the human repertoire, which proved to be unexpectedly broad. It was observed that the content of some nAbs reaches about 0.1% of total immunoglobulins. Immunoglobulins of M class dominate for most nAbs, constituting up to 80-90%. Their affinity (to a monovalent glycan, in KD terms) were found to be within the range 10(-4)-10(-6) M. Antibodies to Galβ1-3GlcNAc (Le(C)), 4-HSO3Galβ1-4GalNAc (4'-O-SuLN), Fucα1-3GlcNAc, Fucα1-4GlcNAc, GalNAcα1-3Gal (Adi), Galα1-4Galβ1-4Glc (P(k)), Galα1-4Galβ1-4GlcNAc (P1), GlcNAcα-terminated glycans, and hyaluronic acid should be noted among the nAbs revealed and studied during the last decade. At the same time, a kind of "taboo" is observed for a number of glycans: antibodies to Le(X) and Le(Y), and almost all gangliosides have not been observed in healthy persons. Many of the revealed nAbs were directed to constrained inner (core) part of glycan, directly adjoined to lipid of cell membrane or protein. The biological function of these nAbs remains unclear; for anti-core antibodies, a role of surveillance on appearance of aberrant, especially cancer, antigens is supposed. The first data related to oncodiagnostics based on quantitation of anti-glycan nAbs are reported.

Histopathologic insights into the mechanism of anti-non-Gal antibody-mediated pig cardiac xenograft rejection

The histopathology of cardiac xenograft rejection has evolved over the last 20 yr with the development of new modalities for limiting antibody-mediated injury, advancing regimens for immune suppression, and an ever-widening variety of new donor genetics. These new technologies have helped us progress from what was once an overwhelming anti-Gal-mediated hyperacute rejection to a more protracted anti-Gal-mediated vascular rejection to what is now a more complex manifestation of non-Gal humoral rejection and coagulation dysregulation. This review summarizes the changing histopathology of Gal- and non-Gal-mediated cardiac xenograft rejection and discusses the contributions of immune-mediated injury, species-specific immune-independent factors, transplant and therapeutic procedures, and donor genetics to the overall mechanism(s) of cardiac xenograft rejection.

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.

Lectin array analysis for wild-type and α-Gal-knockout pig islets versus healthy human islets

PURPOSE

We performed lectin microarray analyses of islets from wild-type (WT) pigs and α1-3galactosyltransferase gene knockout (GKO) pigs and compared the results with the corresponding values for islets from healthy humans.

METHODS

Islets were isolated from the pancreas. After sonication and centrifugation, the proteins in the supernatant from each islet were labeled with Cy3 and applied to a lectin array.

RESULTS

Despite negligible expression of the Gal antigen on the adult pig islets (APIs), GKO-islets showed weaker signals, not only for GS-I-B4 but also for PNA, WFA, PTL-I, and GS-I-A4, than the WT islets, indicating reduced contents of α-linked GalNAc and Galβ1-3GalNAc. In comparing the islets of pigs vs. humans, human islets showed stronger signals for UEA-I, AAL, TJA-II, EEL, WFA, HPA, DBA, SBA and PTL-I, indicating that besides ABO blood type antigens, high levels of fucose and α-linked GalNAc are present. On the other hand, the high mannose form was very rich in the APIs.

CONCLUSION

GKO reduced alpha-linked GalNAc, despite negligible expression of the Gal antigen on WT-API. On the other hand, the high-mannose form was richer in both APIs than in healthy human islets. These results provide useful information for future studies.

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.

Studies on carbohydrate xenoantigens

Naturally occurring and elicited anti-carbohydrate antibodies play a major role in immune responses to xenografts. The original obstacles associated with the Gal antigen have been largely resolved by the generation of knockout pigs. In contrast, much less is known about the nature and role of non-Gal carbohydrate antigens and the antibodies recognizing these. These antibodies can be identified and characterized by enzyme-linked immunosorbent assay. Furthermore, the biological significance of the non-Gal antigen(s) can be determined by expression of the relevant glycosyltransferase(s) by transfection and analyzed by antibody and/or lectin binding.

Toward automated oligosaccharide synthesis

Carbohydrates have been shown to play important roles in biological processes. The pace of development in carbohydrate research is, however, relatively slow due to the problems associated with the complexity of carbohydrate structures and the lack of general synthetic methods and tools available for the study of this class of biomolecules. Recent advances in synthesis have demonstrated that many of these problems can be circumvented. In this Review, we describe the methods developed to tackle the problems of carbohydrate-mediated biological processes, with particular focus on the issue related to the development of the automated synthesis of oligosaccharides. Further applications of carbohydrate microarrays and vaccines to human diseases are also highlighted.

Identification of new carbohydrate and membrane protein antigens in cardiac xenotransplantation

BACKGROUND

α1,3-Galactosyltransferase gene knockout (GTKO) pigs reduced the significance of antibody to galactose alpha 1,3-galactose (Gal) antigens but did not eliminate delayed xenograft rejection (DXR). We hypothesize that DXR of GTKO organs results from an antibody response to a limited number of non-Gal endothelial cell (EC) membrane antigens. In this study, we screened a retrovirus expression library to identify EC membrane antigens detected after cardiac xenotransplantation.

METHODS

Expression libraries were made from GT:CD46 and GTKO porcine aortic ECs. Viral stocks were used to infect human embryonic kidney cells (HEK) that were selected by flow cytometry for IgG binding from sensitized cardiac heterotopic xenograft recipients. After three to seven rounds of selection, individual clones were assessed for non-Gal IgG binding. The porcine complementary DNA was recovered by polymerase chain reaction amplification, sequenced, and identified by homology comparisons.

RESULTS

A total of 199 and 317 clones were analyzed from GT:CD46 and GTKO porcine aortic EC complementary DNA libraries, respectively. Sequence analysis identified porcine CD9, CD46, CD59, and the EC protein C receptor. We also identified porcine annexin A2 and a glycosyltransferase with homology to the human β1,4 N-acetylgalactosaminyl transferase 2 gene.

CONCLUSION

The identified proteins include key EC functions and suggest that non-Gal antibody responses may compromise EC functions and thereby contribute to DXR. Recovery of the porcine β1,4 N-acetylgalactosaminyl transferase 2 suggests that an antibody response to a SD-like carbohydrate may represent a new carbohydrate moiety involved in xenotransplantation. The identification of these porcine gene products may lead to further donor modification to enhance resistance to DXR and further reduce the level of xenograft antigenicity.

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.

A high-throughput O-glycopeptide discovery platform for seromic profiling

Biomarker microarrays are becoming valuable tools for serological screening of disease-associated autoantibodies. Post-translational modifications (PTMs) such as glycosylation extend the range of protein function, and a variety of glycosylated proteins are known to be altered in disease progression. Here, we have developed a synthetic screening microarray platform for facile display of O-glycosylated peptides (O-PTMs). By introduction of a capping step during chemical solid-phase glycopeptide synthesis, selective enrichment of N-terminal glycopeptide end products was achieved on an amine-reactive hydrogel-coated microarray glass surface, allowing high-throughput display of large numbers of glycopeptides. Utilizing a repertoire of recombinant glycosyltransferases enabled further diversification of the array libraries in situ and display of a new level of potential biomarker candidates for serological screening. As proof-of-concept, we have demonstrated that MUC1 glycopeptides could be assembled and used to detect autoantibodies in vaccine-induced disease-free breast cancer patients and in patients with confirmed disease at time of diagnosis.

Glycan and lectin microarrays for glycomics and medicinal applications

Three different array formats to study a challenging field of glycomics are presented here, based on the use of a panel of immobilized glycan or lectins, and on in silico computational approach. Glycan and lectin arrays are routinely used in combination with other analytical tools to decipher a complex nature of glycan-mediated recognition responsible for signal transduction of a broad range of biological processes. Fundamental aspects of the glycan and lectin array technology are discussed, with the focus on the choice and availability of the biorecognition elements, fabrication protocols, and detection platforms involved. Moreover, practical applications of both technologies especially in the field of clinical diagnostics are provided. The future potential of a complementary in silico array technology to reveal details of the protein-glycan-binding profiles is discussed here.

Glycan array: a powerful tool for glycomics studies

Glycan arrays have become a powerful tool for the high-throughput elucidation of interactions of different carbohydrate structures with a wide variety of biological targets, including antibodies, proteins, viruses and cells. This technique is especially suitable for glycomics studies, because arrays present carbohydrate ligands in a manner that mimics interactions at cell-cell interfaces. This review assesses the recent advances involving glycan arrays, including new methods for glycan-array fabrication, new platforms for novel biological information, and new perceptions of glycomics for improving the understanding of disease-related glycobiology. Furthermore, this review attempts to forecast trends in the development of glycan arrays and possible solutions for some remaining challenges to improve this new technology.

Survey of glycoantigens in cells from alpha1-3galactosyltransferase knockout pig using a lectin microarray

BACKGROUND

Glycoantigens represent major obstacles to successful xenotransplantation. Even after the alpha1-3galactosyltransferase (GalT) gene knockout (GalT-KO) pigs were produced, non-Gal antigens continue to be present. This study reports on lectin blot analyses for endothelial cells (EC) and fibroblasts from GalT-KO pigs.

METHODS

Differences in glycoantigens that are produced on cell surfaces in humans and pigs were surveyed. Differences between ECs and fibroblasts from wild-type and GalT-KO pigs were also examined. EC and fibroblasts from GalT-KO pigs (heterozygous and homozygous) with N-acetylglucosaminyltransferase-III (GnT-III), a wild-type EC from the sibling, human EC lines, HUVEC (human EC from umbilical veins), & HAOEC (human EC from aortas), and human fibroblast line were used. EC and fibroblasts were cultured in gelatin-coated dishes for several days. After sonication and centrifugation, the supernatant protein from each cell was labeled with Cy3, applied to a lectin array and scanned with an SC Profiler, and analyzed using an Array Pro Analyzer.

RESULTS

The pig EC showed higher signals in Euonymus Europaeus (EEL) & Griffonia simplicifolia I-B(4) (GSI-B4), binds alpha-Gal, and in Wisteria Floribunda (WFA), Helix pomatia (HPA), Glycine max (SBA), & Griffonia simplicifolia I-A(4) (GSI-A4), binds GalNAc including the Thomsen-Friedenreich precursor (Tn)-antigen, while the human EC showed strong signals in Ulex europaeus I (UEA-I), Maackia amurensis (MAL), Erythrina cristagalli (ECA), & Trichosanthes japonica I (TJA-I) instead. The EC from the GalT-KO pig signals for EEL & GSI-B4 disappeared and those for Bauhinia purpurea alba (BPL), HPA, SBA, & GSI-A4 were greatly diminished as well, while it up-regulated signals for Sambucus Nigra (SNA), Sambucus sieboldiana (SSA), & TJA-I, bind alpha2-6 sialic acid, compared to the wild-type pig EC. Concerning fibroblasts, the signals for HPA, SBA, & GSI-A4 were the most intense in the wild-type, and the intensities for homozygous-KO were less, approaching those of humans. In addition, the order of the intensities, as detected by Arachis hypogaea (PNA) & Maclura pomifera (MPA), binding Galbeta1-2GalNAc, indicates that the Thomsen-Friedenreich (T)-antigen is likely present on pig fibroblasts.

CONCLUSION

It is possible that the T-antigen and Tn-antigen related to GalNAc are non-Gal antigens, but, fortunately, not only alpha-Gal but also GalNAc were found to be decreased in the KO-pig.

Profiling human serum antibodies with a carbohydrate antigen microarray

Carbohydrate antigen arrays (glycan arrays) have been recently developed for the high-throughput analysis of carbohydrate macromolecule interactions. When profiling serum, information about experimental variability, interindividual biological variability, and intraindividual temporal variability is critical. In this report, we describe the characterization of a carbohydrate antigen array and assay for profiling human serum. Through optimization of assay conditions and development of a normalization strategy, we obtain highly reproducible results with a within-experiment coefficient of variation (CV) of 10.8% and an overall CV (across multiple batches of slides and days) of 28.5%. We also report antibody profiles for 48 human subjects and evaluate for the first time the effects of age, race, sex, geographic location, and blood type on antibody profiles for a large set of carbohydrate antigens. We found significant dependence on age and blood type of antibody levels for a variety of carbohydrates. Finally, we conducted a longitudinal study with a separate group of 7 serum donors to evaluate the variation in anti-carbohydrate antibody levels within an individual over a period ranging from 3 to 13 weeks and found that, for nearly all antigens on our array, antibody levels are generally stable over this period. The results presented here provide the most comprehensive evaluation of experimental and biological variation reported to date for a glycan array and have significant implications for studies involving human serum profiling.

Glycan arrays: recent advances and future challenges

Carbohydrate arrays, also referred to as glycan arrays, are composed of various oligosaccharides and/or polysaccharides immobilized on a solid support in a spatially defined arrangement. This technology provides a powerful, high-throughput approach to examining carbohydrate-macromolecule interactions, and glycan arrays have had a significant impact on the field of glycobiology. This review focuses on recent advances in glycan array technology, limitations, and opportunities for improvement. In particular, new methods for the production of natural glycan arrays and chemoenzymatic approaches are greatly expanding the diversity of structures on arrays. Since multivalent complex formation is generally required to achieve tight binding, methods to evaluate and modulate presentation are vital for enhancing the capabilities of this technology.