Carbohydrates in xenotransplantation

Human ST3Gal II and ST6GalNAc IV genes increase human serum-mediated cytotoxicity to xenogeneic cells

Carbohydrate-antigens widely existed on glycoproteins and glycosphingolipids of all mammalian cells play a crucial role in self-defense and immunity. Xeno-reactive antibodies included in natural human sera play a protecting role in an acute phase-rejection of xenotransplantation. In this study, we investigated the effect of an alteration of glycosylation-pattern, caused by human sialyltransferases such as hST3Gal II or hST6GalNAc IV, on human serum mediated cytotoxicity in pig kidney PK15 cells. From LDH cytotoxicity assay, cytotoxicity to human serum was significantly increased in hST3Gal II and hST6GalNAc IV-transfected PK15 cells, as compared to the control. In the hST6Gal I-carrying cells, the cytotoxicity to human serum was rather decreased. Moreover, flow cytometry analysis revealed that an alteration of pig glycosylation-pattern by hST3Gal II or hST6GalNAc IV influences on a binding of human IgM or IgG, respectively, in pig kidney cells, regardless of Gal antigen alteration. Finally, we found that hST6GalNAc IV contributed to increase of terminal disialylated tetrasaccharide structure, disialyl T antigen, as evidenced by increase of the MAL II lectin binding capacity in the hST6GalNAc IV-transfected PK15 cells, compared with control. Therefore, our results suggest that carbohydrate antigens, such as disialyl T antigen, newly synthesized by the ST3Gal II- and ST6GalNAc IV are potentially believed to be new xeno-reactive elements.

Innate Immune Response Analysis in Meniscus Xenotransplantation Using Normal and Triple Knockout Jeju Native Pigs

Although allogenic meniscus grafting can be immunologically safe, it causes immune rejection due to an imbalanced tissue supply between donor and recipient. Pigs are anatomically and physiologically similar to adult humans and are, therefore, considered to be advantageous xenotransplantation models. However, immune rejection caused by genetic difference damages the donor tissue and can sometimes cause sudden death. Immune rejection is caused by genes; porcine GGTA1, CMAH, and B4GLANT2 are the most common. In this study, we evaluated immune cells infiltrating the pig meniscus transplanted subcutaneously into BALB/c mice bred for three weeks. We compared the biocompatibility of normal Jeju native black pig (JNP) meniscus with that of triple knockout (TKO) JNP meniscus (α-gal epitope, N-glycolylneuraminic acid (Neu5Gc), and Sd (a) epitope knockout using CRISPR-Cas 9). Mast cells, eosinophils, neutrophils, and macrophages were found to have infiltrated the transplant boundary in the sham (without transplantation), normal (normal JNP), and test (TKO JNP) samples after immunohistochemical analysis. When compared to normal and sham groups, TKO was lower. Cytokine levels did not differ significantly between normal and test groups. Because chronic rejection can occur after meniscus transplantation associated with immune cell infiltration, we propose studies with multiple genetic editing to prevent immune rejection.

An Overview of the Importance and Value of Porcine Species in Sialic Acid Research

Simple Summary

Humans frequently interact with pigs and porcine meat is the most consumed red meat in the world. In addition, due to the many physiological and anatomical similarities shared between pigs and humans, in contrast to most mammalian species, pigs are a suitable model organism and pig organs can be used for xenotransplantation. However, one major challenge of porcine meat consumption and xenotransplantation is the xenoreactivity between red meat Neu5Gc sialic acid and human anti-Neu5Gc antibodies, which are associated with certain diseases and disorders. Furthermore, pigs express both α2-3 and α2-6 Sia linkages that could serve as viable receptors for viral infections, reassortments, and cross-species transmission of viruses. Therefore, pigs play a significant role in sialic acid research and, in general, in human health.

Abstract

Humans frequently interact with pigs, whose meat is also one of the primary sources of animal protein. They are one of the main species at the center of sialic acid (Sia) research. Sias are sugars at terminals of glycoconjugates, are expressed at the cell surfaces of mammals, and are important in cellular interactions. N-glycolylneuraminic acid (Neu5Gc) and N-acetylneuraminic acid (Neu5Ac) are notable Sias in mammals. Cytidine monophospho-N-acetylneuraminic acid hydroxylase (CMAH) encodes the CMAH enzyme that biosynthesizes Neu5Gc. Although humans cannot endogenously synthesize Neu5Gc due to the inactivation of this gene by a mutation, Neu5Gc can be metabolically incorporated into human tissues from red meat consumption. Interactions between Neu5Gc and human anti-Neu5Gc antibodies have been associated with certain diseases and disorders. In this review, we summarized the sialic acid metabolic pathway, its regulation and link to viral infections, as well as the importance of the pig as a model organism in Sia research, making it a possible source of Neu5Gc antigens affecting human health. Future research in solving the structures of crucial enzymes involved in Sia metabolism, as well as their regulation and interactions with other enzymes, especially CMAH, could help to understand their function and reduce the amount of Neu5Gc.

Fluorescent Sensor Array for Quantitative Determination of Saccharides

Accurate monitoring of sugar levels is essential for many fields from food industry to human health. Here, we developed FRET-based dual chromophore sensors for saccharides that form oxazolidine boronate and may be employed as a noninvasive method for monitoring of sugar levels in biological fluids, namely, urine. The saccharide-binding properties of the sensors were studied using fluorescence spectroscopy and utilized in the determination of saccharides in a high-throughput manner. Here, two fluorescent sensors were successful in the classification of nine different monosaccharides and disaccharides with 100% correct classification. Furthermore, the dual chromophore self-assembled sensors were successfully utilized for the quantitative determination of important carbohydrates such as glucose in the presence of competitive saccharides (fructose) and in complex media (urine) without sample pretreatment. The present fluorescent sensors allow for quantification of glucose in a concentration range of 0-60 mM, which matches the concentration range of frequently used urinalysis test strips.

Human immune reactivity of GGTA1/CMAH/A3GALT2 triple knockout Yucatan miniature pigs

In this study, we investigated the effect of a triple knockout of the genes alpha-1,3-galactosyltransferase (GGTA1), cytidine monophosphate-N-acetylneuraminic acid hydroxylase (CMAH), and alpha 1,3-galactosyltransferase 2 (A3GALT2) in Yucatan miniature pigs on human immune reactivity. We used the CRISPR/Cas9 system to create pigs lacking GGTA1 (GTKO) and GGTA1/CMAH/A3GALT2 triple gene knockout (TKO). The expression of all three xenoantigens was absent in TKO pigs, but there was no additional reduction in the level of Galα1,3Gal (αGal) epitopes expression in the A3GALT2 gene KO. Peripheral blood mononuclear cells (PBMCs), aorta endothelial cells (AECs), and cornea endothelial cells (CECs) were isolated from these pigs, and their ability to bind human IgM/IgG and their cytotoxicity in human sera were evaluated. Compared to wild type (WT) pigs, the level of human antibody binding of the PBMCs, AECs, and CECs of the transgenic pigs (GTKO and TKO) was significantly reduced. However, there were significant differences in human antibody binding between GTKO and TKO depending on the cell type. Human antibody binding of TKO pigs was less than that of GTKO on PBMCs but was similar between GTKO and TKO pigs for AECs and CECs. Cytotoxicity of transgenic pig (GTKO and TKO) PBMCs and AECs was significantly reduced compared to that of WT pigs. However, TKO pigs showed a reduction in cytotoxicity compared to GTKO pigs on PBMCs, whereas in AECs from both TKO and GTKO pigs, there was no difference. The cytotoxicity of transgenic pig CECs was significantly decreased from that of WT at 300 min, but there was no significant reduction in TKO pigs from GTKO. Our results indicate that genetic modification of donor pigs for xenotransplantation should be tailored to the target organ and silencing of additional genes such as CMAH or A3GALT2 based on GTKO might not be essential in Yucatan miniature pigs.

High-mannose type N-glycans with core fucosylation and complex-type N-glycans with terminal neuraminic acid residues are unique to porcine islets

OBJECTIVES

Islet transplantation is an emerging treatment option for type 1 diabetes but its application is limited by the shortage of human pancreas donors. Characterization of the N- and O-glycan surface antigens that vary between human and genetically engineered porcine islet donors could shed light on targets of antibody mediated rejection.

METHODS

N- and O-glycans were isolated from human and adult porcine islets and analyzed using matrix-assisted laser-desorption time-of-flight mass spectrometry (MALDI-TOF-MS) and electrospray ionization mass spectrometry (ESI-MS/MS).

RESULTS

A total of 57 porcine and 34 human N-glycans and 21 porcine and 14 human O-glycans were detected from cultured islets. Twenty-eight of which were detected only from porcine islets, which include novel xenoantigens such as high-mannose type N-glycans with core fucosylation and complex-type N-glycans with terminal neuraminic acid residues. Porcine islets have terminal N-glycolylneuraminic acid (NeuGc) residue in bi-antennary N-glycans and sialyl-Tn O-glycans. No galactose-α-1,3-galactose (α-Gal) or Sda epitope were detected on any of the islets.

CONCLUSIONS

These results provide important insights into the potential antigenic differences of N- and O-glycan profiles between human and porcine islets. Glycan differences may identify novel gene targets for genetic engineering to generate superior porcine islet donors.

Possible Link Between the ABO Blood Group of Bioprosthesis Recipients and Specific Types of Structural Degeneration

Background Pigs/bovines share common antigens with humans: α-Gal, present in all pigs/bovines close to the human B-antigen; and AH-histo-blood-group antigen, identical to human AH-antigen and present only in some animals. We investigate the possible impact of patients' ABO blood group on bioprosthesis structural valve degeneration (SVD) through calcification/pannus/tears/perforations for patients ≤60 years at implantation. Methods and Results This was a single-center study (Paris, France) that included all degenerative bioprostheses explanted between 1985 and 1998, mostly porcine bioprostheses (Carpentier-Edwards second/third porcine bioprostheses) and some bovine bioprostheses. For the period 1998 to 2014, only porcine bioprostheses with longevity ≥13 years were included (total follow-up ≥29 years). Except for blood groups, important predictive factors for SVD were prospectively collected (age at implantation/longevity/number/site/sex/SVD types) and analyzed using logistic regression. All variables were available for 500 explanted porcine bioprostheses. By multivariate analyses, the A group was associated with an increased risk of: tears (odds ratio[OR], 1.61; P=0.026); pannus (OR, 1.5; P=0.054), pannus with tears (OR, 1.73; P=0.037), and tendency for lower risk of: calcifications (OR, 0.63; P=0.087) or isolated calcification (OR, 0.67; P=0.17). A-antigen was associated with lower risk of perforations (OR 0.56; P=0.087). B-group patients had an increased risk of: perforations (OR, 1.73; P=0.043); having a pannus that was calcified (OR, 3.0, P=0.025). B-antigen was associated with a propensity for calcifications in general (OR, 1.34; P=0.25). Conclusions Patient's ABO blood group is associated with specific SVD types. We hypothesize that carbohydrate antigens, which may or may not be common to patient and animal bioprosthetic tissue, will determine a patient's specific immunoreactivity with respect to xenograft tissue and thus bioprosthesis outcome in terms of SVD.

Identification by mass spectrometry and immunoblotting of xenogeneic antigens in the N- and O-glycomes of porcine, bovine and equine heart tissues

Animal bioprosthetic heart valves (BHV) are used to replace defective valves in patients with valvular heart disease. Especially young BHV recipients may experience a structural valve deterioration caused by an immune reaction in which α-Gal and Neu5Gc are potential target antigens. The expression of these and other carbohydrate antigens in animal tissues used for production of BHV was explored. Protein lysates of porcine aortic and pulmonary valves, and porcine, bovine and equine pericardia were analyzed by Western blotting using anti-carbohydrate antibodies and lectins. N-glycans were released by PNGase F digestion and O-glycans by β-elimination. Released oligosaccharides were analyzed by liquid chromatography – tandem mass spectrometry. In total, 102 N-glycans and 40 O-glycans were identified in animal heart tissue lysates. The N- and O-glycan patterns were different between species. α-Gal and Neu5Gc were identified on both N- and O-linked glycans, N,N´-diacetyllactosamine (LacdiNAc) on N-glycans only and sulfated O-glycans. The relative amounts of α-Gal-containing N-glycans were higher in bovine compared to equine and porcine pericardia. In contrast to the restricted number of proteins carrying α-Gal and LacdiNAc, the distribution of proteins carrying Neu5Gc-determinants varied between species and between different tissues of the same species. Porcine pericardium carried the highest level of Neu5Gc-sialylated O-glycans, and bovine pericardium the highest level of Neu5Gc-sialylated N-glycans. The identified N- and O-linked glycans, some of which may be immunogenic and remain in BHVs manufactured for clinical use, could direct future genetic engineering to prevent glycan expression rendering the donor tissues less immunogenic in humans.

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

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

Bioprosthetic Heart Valves: Upgrading a 50-Year Old Technology

Prosthetic heart valves have been commonly used to address the increasing prevalence of valvular heart disease. The ideal prosthetic heart valve substitute should closely mimic the characteristics of a normal native heart valve. Despite the development of various interventions, an exemplary valve replacement does not exist. This review provides an overview of the novel engineering valve designs and explores emergent immunologic insights into age-dependent structural valve degeneration (SVD).

Recipients with blood group A associated with longer survival rates in cardiac valvular bioprostheses

Background

Pigs/bovines share with humans some of the antigens present on cardiac valves. Two such antigens are: the major xenogenic Ag, “Gal” present in all pig/bovine very close to human B-antigen of ABO-blood-group system; the minor Ag, pig histo-blood-group AH-antigen identical to human AH-antigen and present by some animals. We hypothesize that these antigens may modify the immunogenicity of the bioprosthesis and also its longevity. ABO distribution may vary between patients with low (<6 years) and high (≥15 years) bioprostheses longevity.

Methods

Single-centre registry study (Paris, France) including all degenerative porcine bioprostheses (mostly Carpentier-Edwards 2nd/3rd generation heart valves) explanted between 1985 and 1998 and some bovine bioprostheses. For period 1998–2014, all porcine bioprostheses with longevity ≥13 years (follow-up ≥29 years). Important predictive factors for bioprosthesis longevity: number, site of implantation, age were collected. Blood group and other variables were entered into an ordinal logistic regression analysis model predicting valve longevity, categorized as low (<6 years), medium (6–14.9 years), and high (≥15 years).

Findings

Longevity and ABO-blood group were obtained for 483 explanted porcine bioprostheses. Mean longevity was 10.2 ± 3.9 years [0–28] and significantly higher for A-patients than others (P = 0.009). Using multivariate analysis, group A was a strong predictive factor of longevity (OR 2.09; P < 0.001). For the 64 explanted bovine bioprosthesis with low/medium longevity, the association, with A-group was even more significant.

Interpretation

Patients of A-group but not B have a higher longevity of their bioprostheses. Future graft-host phenotyping and matching may give rise to a new generation of long-lasting bioprosthesis for implantation in humans, especially for the younger population.

Fund

None.

Different approaches to heart valve decellularization: A comprehensive overview of the past 30 years

Xenogeneic decellularized heart valve scaffolds have the potential to overcome the limitations of existing bioprosthetic heart valves that have limited duration due to calcification and tissue degeneration phenomena. This article presents a review of 30 years of decellularization approaches adopted in cardiovascular tissue engineering, with a focus on the use, either individually or in combination, of different detergents. The safety and efficacy of cell-removal procedures are specifically reported and discussed, as well as the structure and biomechanics of the treated extracellular matrix (ECM). Detergent residues within the ECM, production of hyaluronan fragments, safe removal of cellular debris, and the persistence of the alpha-Gal epitope after the decellularization treatments are of particular interest as parameters for the identification of the best tissue for the manufacture of bioprostheses. Special attention has also been given to key factors that should be considered in the manufacture of the next generation of xenogeneic bioprostheses, where tissues must retain the ability to be remodeled and to grow in weight along with body reshaping.

Molecular Evolution of the Glycosyltransferase 6 Gene Family in Primates

Glycosyltransferase 6 gene family includes ABO, Ggta1, iGb3S, and GBGT1 genes and by three putative genes restricted to mammals, GT6m6, GTm6, and GT6m7, only the latter is found in primates. GT6 genes may encode functional and nonfunctional proteins. Ggta1 and GBGT1 genes, for instance, are pseudogenes in catarrhine primates, while iGb3S gene is only inactive in human, bonobo, and chimpanzee. Even inactivated, these genes tend to be conversed in primates. As some of the GT6 genes are related to the susceptibility or resistance to parasites, we investigated (i) the selective pressure on the GT6 paralogs genes in primates; (ii) the basis of the conservation of iGb3S in human, chimpanzee, and bonobo; and (iii) the functional potential of the GBGT1 and GT6m7 in catarrhines. We observed that the purifying selection is prevalent and these genes have a low diversity, though ABO and Ggta1 genes have some sites under positive selection. GT6m7, a putative gene associated with aggressive periodontitis, may have regulatory function, but experimental studies are needed to assess its function. The evolutionary conservation of iGb3S in humans, chimpanzee, and bonobo seems to be the result of proximity to genes with important biological functions.

Human antibody reactivity against xenogeneic N-glycolylneuraminic acid and galactose-α-1,3-galactose antigen

BACKGROUND

Despite the development of α1,3-galactosyl transferase-knockout (GTKO) pigs, acute humoral xenograft rejection caused by antibodies against non-Gal antigens, along with complement activation, are hurdles that need to be overcome. Among non-Gal antigens, N-glycolylneuraminic acid (Neu5Gc) is considered to play an important role in xenograft rejection in human.

METHODS

We generated human embryonic kidney 293 (HEK293) cells that expressed xenogeneic Neu5Gc (HEK293-pCMAH) or α1,3Gal (HEK293-pGT) antigen and investigated the degree of human antibody binding and complement-dependent cytotoxicity (CDC) against these antigens using 100 individual human sera.

RESULTS

Both IgM and IgG bound to α1,3Gal, while only IgG bound to Neu5Gc. Of the ABO blood groups, the degree of IgG binding to α1,3Gal was highest for blood group A. The degree of CDC against HEK293-pCMAH cells was significantly lower than that against HEK293-pGT cells. However, CDC against HEK293-pCMAH cells was significantly higher than that against control HEK293 cells. In addition, the severity of CDC against HEK293-pCMAH cells positively correlated with that against GTKO pig aortic endothelial cells (PAECs), suggesting that Neu5Gc is the main antigen in GTKO PAECs. Similar to antibody-binding activity, only IgG binding correlated with CDC against HEK293-pCMAH cells. The most common subclass of IgGs against Neu5Gc was IgG1, which typically induces strong complement activation.

CONCLUSIONS

We showed that IgG-mediated CDC was detected in Neu5Gc-overexpressed HEK293 cells incubated with human sera; however, this antibody reactivity to Neu5Gc was highly variable among individuals. Our results suggest that additional modifications to the CMAH gene should be considered for widespread use of pig organs for human transplants.

Inclusion of homologous DNA in nuclease-mediated gene targeting facilitates a higher incidence of bi-allelically modified cells

BACKGROUND

Recent advancements in gene editing techniques have increased in number and utility. These techniques are an attractive alternative to conventional gene targeting methods via homologous recombination due to the ease of use and the high efficiency of gene editing. We have previously produced cytidine monophosphate-N-acetylneuraminic acid hydroxylase (CMAH) knockout (KO) pigs in a Minnesota miniature pig genetic background. These pigs were generated using zinc-finger nucleases (ZFNs) in combination with donor DNA containing a total homology length of 1600 bp (800-bp homology on each arm). Our next aim was to introduce the targeted disruption of alpha-1,3-galactosyltransferase (GGTA1) in the CMAH KO genetic background and evaluate the effect of donor DNA homology length on meganuclease-mediated gene targeting.

METHODS

Zinc-finger nucleases from a previous CMAH KO experiment were used as a proof of concept to identify a correlation between the length of donor DNA homology and targeting efficiency. Based on those results, experiments were designed to use transcription activator-like effector nucleases (TALENs) to generate bi-allelically modified GGTA1 cells using donor DNAs carrying various lengths of homology. Donor DNA was designed to symmetrically flank the predicted cleavage sites in CMAH and GGTA1 for both ZFN and TALEN cleavage sites, respectively. For both genes, the length of total homology ranged from 60 to 1799 bp. Sialyltransferase gene expression profiles were evaluated in CMAH and GGTA1 double KO pig cells and were compared to wild-type and CMAH KO cells.

RESULTS

Introduction of donor DNA with ZFNs demonstrated that small amounts of homology (60 bp) could facilitate homology-directed repair during ZFN-mediated targeting of CMAH; however, donor DNA with longer amounts of homology resulted in a higher frequency of homology-directed repair. For the GGTA1 KO experiments that used TALENs and donor DNA, donor DNA alone did not result in detectable bi-allelic conversion of GGTA1. As the length of donor DNA increased, the bi-allelic disruption of GGTA1 increased from 0.5% (TALENs alone, no donor DNA present) to a maximum of 3% (TALENs and donor DNA with total homology of 1799 bp). Inclusion of homologous donor DNA in TALEN-mediated gene targeting facilitated a higher incidence of bi-allelically modified cells. Using the generated cells, we were able to demonstrate the lack of GGTA1 expression and the decrease in gene expression sialyltransferase-related genes.

CONCLUSIONS

The approach of using donor DNA in conjunction with a meganuclease can be used to increase the efficiency of gene targeting. The gene editing methods can be applied to other genes as well as other mammalian systems. Additionally, gene expression analysis further confirms that the CMAH/GGTA1 double KO pigs can be a valuable source for the study of pig-to-human xenotransplantation.

Modifying the sugar icing on the transplantation cake

As a transplant surgeon, my interest in glycobiology began through my research into ABO-incompatible allotransplantation, and grew when my goal became overcoming the shortage of organs from deceased human donors by the transplantation of pig organs into patients with terminal organ failure (xenotransplantation/cross-species transplantation). The major target for human "natural" (preformed) anti-pig antibodies is galactose-α(1,3)-galactose (the "Gal" epitope), which is expressed on many pig cells, including the vascular endothelium. The binding of human IgM and IgG antibodies to Gal antigens initiates the process of hyperacute rejection, resulting in destruction of the pig graft within minutes or hours. This major barrier has been overcome by the production of pigs in which the gene for the enzyme α(1,3)-galactosyltransferase (GT) has been deleted by genetic engineering, resulting in GT knockout (GTKO) pigs. The two other known carbohydrate antigenic targets on pig cells for human anti-pig antibodies are (i) the product of the cytidine monophosphate-N-acetylneuraminic acid hydroxylase (CMAH) gene, i.e., N-glycolylneuraminic acid, and (ii) the product of the β1,4 N-acetylgalactosaminyltransferase gene, i.e., the Sd(a) antigen. Expression of these two has also been deleted in pigs. These genetic manipulations, together with others directed to overcoming primate complement and coagulation activation (the latter of which also relates to glycobiology) have contributed to the prolongation of pig graft survival in nonhuman primate recipients to many months rather than a few minutes. Clinical trials of the transplantation of pig cells are already underway and transplantation of pig organs may be expected within the relatively near future.

The pathobiology of pig-to-primate xenotransplantation: a historical review

The immunologic barriers to successful xenotransplantation are related to the presence of natural anti-pig antibodies in humans and non-human primates that bind to antigens expressed on the transplanted pig organ (the most important of which is galactose-α1,3-galactose [Gal]), and activate the complement cascade, which results in rapid destruction of the graft, a process known as hyperacute rejection. High levels of elicited anti-pig IgG may develop if the adaptive immune response is not prevented by adequate immunosuppressive therapy, resulting in activation and injury of the vascular endothelium. The transplantation of organs and cells from pigs that do not express the important Gal antigen (α1,3-galactosyltransferase gene-knockout [GTKO] pigs) and express one or more human complement-regulatory proteins (hCRP, e.g., CD46, CD55), when combined with an effective costimulation blockade-based immunosuppressive regimen, prevents early antibody-mediated and cellular rejection. However, low levels of anti-non-Gal antibody and innate immune cells and/or platelets may initiate the development of a thrombotic microangiopathy in the graft that may be associated with a consumptive coagulopathy in the recipient. This pathogenic process is accentuated by the dysregulation of the coagulation-anticoagulation systems between pigs and primates. The expression in GTKO/hCRP pigs of a human coagulation-regulatory protein, for example, thrombomodulin, is increasingly being associated with prolonged pig graft survival in non-human primates. Initial clinical trials of islet and corneal xenotransplantation are already underway, and trials of pig kidney or heart transplantation are anticipated within the next few years.

Gene expression and pathway analysis of effects of the CMAH deactivation on mouse lung, kidney and heart

Background

N-glycolylneuraminic acid (Neu5Gc) is generated by hydroxylation of CMP-Neu5Ac to CMP-Neu5Gc, catalyzed by CMP-Neu5Ac hydroxylase (CMAH). However, humans lack this common mammalian cell surface molecule, Neu5Gc, due to inactivation of the CMAH gene during evolution. CMAH is one of several human-specific genes whose function has been lost by disruption or deletion of the coding frame. It has been suggested that CMAH inactivation has resulted in biochemical or physiological characteristics that have resulted in human-specific diseases.

Methodology/Principal Findings

To identify differential gene expression profiles associated with the loss of Neu5Gc expression, we performed microarray analysis using Illumina MouseRef-8 v2 Expression BeadChip, using the main tissues (lung, kidney, and heart) from control mice and CMP-Neu5Ac hydroxylase (Cmah) gene knock-out mice, respectively. Out of a total of 25,697 genes, 204, 162, and 147 genes were found to be significantly modulated in the lung, kidney, and heart tissues of the Cmah null mouse, respectively. In this study, we examined the gene expression profiles, using three commercial pathway analysis software packages: Ingenuity Pathways Analysis, Kyoto Encyclopedia of Genes and Genomes analysis, and Pathway Studio. The gene ontology analysis revealed that the top 6 biological processes of these genes included protein metabolism and modification, signal transduction, lipid, fatty acid, and steroid metabolism, nucleoside, nucleotide and nucleic acid metabolism, immunity and defense, and carbohydrate metabolism. Gene interaction network analysis showed a common network that was common to the different tissues of the Cmah null mouse. However, the expression of most sialytransferase mRNAs of Hanganutziu-Deicher antigen, sialy-Tn antigen, Forssman antigen, and Tn antigen was significantly down-regulated in the liver tissue of Cmah null mice.

Conclusions/Significance

Mice bearing a human-like deletion of the Cmah gene serve as an important model for the study of abnormal pathogenesis and/or metabolism caused by the evolutionary loss of Neu5Gc synthesis in humans.

Development of a consensus protocol to quantify primate anti-non-Gal xenoreactive antibodies using pig aortic endothelial cells

BACKGROUND

Scientists working in the field of xenotransplantation do not employ a uniform method to measure and report natural and induced antibody responses to non-Galα(1,3)Gal (non-Gal) epitopes. Such humoral responses are thought to be particularly pathogenic after transplantation of vascularized GalTKO pig organs and having a more uniform assay and reporting format would greatly facilitate comparisons between laboratories.

METHODS

Flow cytometry allows examination of antibody reactivity to intact antigens in their natural location and conformation on cell membranes. We have established a simple and reproducible flow cytometric assay to detect antibodies specific for non-Gal pig antigens using primary porcine aortic endothelial cells (pAECs) and cell culture-adapted pAEC cell lines generated from wild type and α1,3galactosyl transferase knockout (GalTKO) swine.

RESULTS

The consensus protocol we propose here is based on procedures routinely used in four xenotransplantation centers and was independently evaluated at three sites using shared cells and serum samples. Our observation support use of the cell culture-adapted GalTKO pAEC KO:15502 cells as a routine method to determine the reactivity of anti-non-Gal antibodies in human and baboon serum.

CONCLUSIONS

We have developed an assay that allows the detection of natural and induced non-Gal xenoreactive antibodies present in human or baboon serum in a reliable and consistent manner. This consensus assay and format for reporting the data should be accessible to laboratories and will be useful for assessing experimental results between multiple research centers. Adopting this assay and format for reporting the data should facilitate the detection, monitoring, and detailed characterization of non-Gal antibody responses.

Distribution of non-gal antigens in pig cornea: relevance to corneal xenotransplantation

PURPOSE

The aim of this study was to investigate the distribution of antigens other than galactose-α-1,3-galactose (Gal) (non-Gal) recognized by human and rhesus monkey serum antibodies in the α-1,3-galactosyltransferase gene-knockout (GTKO) pig cornea.

METHODS

The distribution of non-Gal, specifically N-glycolylneuraminic acid (NeuGc), in the corneas from wild-type (WT) and GTKO pigs was identified. Corneal sections from WT and GTKO pigs were incubated with human or rhesus monkey serum to determine immunoglobulin (Ig)M and IgG binding to corneal tissue by means of fluorescent microscopy.

RESULTS

Strong expression of NeuGc was found in all layers of both WT and GTKO pig corneas. In both humans and monkeys, antibody binding (IgG > IgM) to GTKO was found to be weaker than that to entire WT pig corneas, but in both, most antibody binding, especially IgG, was to the epithelium. There was weak diffuse antibody binding, especially of IgG, to the corneal stroma, suggesting binding to antigens expressed on collagen. There was no or minimal binding of IgM/IgG to the corneal endothelium.

CONCLUSIONS

Although the cornea is avascular, antibodies in primate serum can bind to pig antigens, especially on epithelial cells and stromal collagen. Although the binding to entire GTKO corneas was weaker than that to WT corneas, deletion of the expression of NeuGc and expression of human complement-regulatory proteins in the pig cornea will be important if prolonged clinical corneal xenograft survival is to be achieved.

Production of biallelic CMP-Neu5Ac hydroxylase knock-out pigs

After the knock-out (KO) of α1,3 galactosyltransfease (Gal-T), the Hanganutziu-Deicher antigen became a major antigen of the "non-Gal antigen" that is implicated in subsequent xenograft rejection. For deletion of non-Gal antigen, we successfully produced zinc finger nuclease (ZFN)-mediated monoallelic/biallelic male and female CMP-N-acetylneuraminic acid hydroxylase (CMAH) KO miniature pigs: the efficiency of the gene targeting (41.7%) was higher when donor DNA was used with the ZFN than those of ZFN alone (9.1%). Monoallelic KO pigs had no integration of exogenous DNA into their genome, indicating that this technique would provide a new avenue to reduce the risk of antibiotics resistance when organs from genetically modified pigs are transplanted into patients. Until now, both monoallelic and biallelic CMAH KO pigs are healthy and show no sign of abnormality and off-target mutations. Therefore, these CMAH null pigs on the Gal-T KO background could serve as an important model for the xenotransplantation.

Immunogenicity in xenogeneic scaffold generation: antigen removal vs. decellularization

Decades of research have been undertaken towards the goal of tissue engineering using xenogeneic scaffolds. The primary advantages associated with use of xenogeneic tissue-derived scaffolds for in vitro development of replacement tissues and organs stem from the inherent extracellular matrix (ECM) composition and architecture. Native ECM possesses appropriate mechanical properties for physiological function of the biomaterial and signals for cell binding, growth and differentiation. Additionally, xenogeneic tissue is readily available. However, translation of xenogeneic scaffold-derived engineered tissues or organs into clinical therapies requires xenoantigenicity of the material to be adequately addressed prior to implantation. Failure to achieve this goal will result in a graft-specific host immune rejection response, jeopardizing in vivo survival of the resultant scaffold, tissue or organ. This review explores (i) the appropriateness of scaffold acellularity as an outcome measure for assessing reduction of the immunological barriers to the use of xenogeneic scaffolds for tissue engineering applications and (ii) the need for tissue engineers to strive for antigen removal during xenogeneic scaffold generation.

The role of platelets in coagulation dysfunction in xenotransplantation, and therapeutic options

Xenotransplantation could resolve the increasing discrepancy between the availability of deceased human donor organs and the demand for transplantation. Most advances in this field have resulted from the introduction of genetically engineered pigs, e.g., α1,3-galactosyltransferase gene-knockout (GTKO) pigs transgenic for one or more human complement-regulatory proteins (e.g., CD55, CD46, CD59). Failure of these grafts has not been associated with the classical features of acute humoral xenograft rejection, but with the development of thrombotic microangiopathy in the graft and/or consumptive coagulopathy in the recipient. Although the precise mechanisms of coagulation dysregulation remain unclear, molecular incompatibilities between primate coagulation factors and pig natural anticoagulants exacerbate the thrombotic state within the xenograft vasculature. Platelets play a crucial role in thrombosis and contribute to the coagulation disorder in xenotransplantation. They are therefore important targets if this barrier is to be overcome. Further genetic manipulation of the organ-source pigs, such as pigs that express one or more coagulation-regulatory genes (e.g., thrombomodulin, endothelial protein C receptor, tissue factor pathway inhibitor, CD39), is anticipated to inhibit platelet activation and the generation of thrombus. In addition, adjunctive pharmacologic anti-platelet therapy may be required. The genetic manipulations that are currently being tested are reviewed, as are the potential pharmacologic agents that may prove beneficial.

The Xeno-glycomics database (XDB): a relational database of qualitative and quantitative pig glycome repertoire

SUMMARY

In recent years, the improvement of mass spectrometry-based glycomics techniques (i.e. highly sensitive, quantitative and high-throughput analytical tools) has enabled us to obtain a large dataset of glycans. Here we present a database named Xeno-glycomics database (XDB) that contains cell- or tissue-specific pig glycomes analyzed with mass spectrometry-based techniques, including a comprehensive pig glycan information on chemical structures, mass values, types and relative quantities. It was designed as a user-friendly web-based interface that allows users to query the database according to pig tissue/cell types or glycan masses. This database will contribute in providing qualitative and quantitative information on glycomes characterized from various pig cells/organs in xenotransplantation and might eventually provide new targets in the α1,3-galactosyltransferase gene-knock out pigs era.

AVAILABILITY

The database can be accessed on the web at http://bioinformatics.snu.ac.kr/xdb.

Is there a correlation between anti-pig antibody levels in humans and geographic location during childhood?

BACKGROUND

An initial observation suggested high levels of anti-pig antibodies in healthy humans who had spent their childhood in the Middle East. We tested larger cohorts to determine whether anti-pig antibody levels correlated with the geographic location in which the subject spent his/her childhood, because this might have implications for clinical trials of xenotransplantation.

METHODS

Anti-pig IgM and IgG levels (by flow cytometry using peripheral blood mononuclear cells from wild-type and α1,3-galactosyltransferase gene-knockout pigs) and anti-Gal IgM and IgG levels (by enzyme-linked immunosorbent assay) were measured in 75 volunteers. Comparisons of antibody levels were also made based on subject age, gender, ABO blood group, diet, and history of vaccination.

RESULTS

Antibody binding to α1,3-galactosyltransferase gene-knockout pig cells was less than to wild-type cells. There was a reduction in anti-pig IgM and anti-Gal IgM, but a slight increase in anti-nonGal IgG, with age. Women had higher levels of anti-Gal IgM than men. Blood group A subjects had higher levels of anti-pig IgM and IgG than those of group AB. Diet had no influence on antibody levels. Typhoid or measles-mumps-rubella vaccination was associated with lower anti-nonGal IgG or anti-Gal IgG, respectively, whereas influenza vaccination was associated with higher anti-nonGal IgG. There were some significant variations in antibody levels associated with location during childhood, with subjects from the Middle East demonstrating higher anti-nonGal IgG and anti-Gal IgG.

CONCLUSION

Clinical trials of xenotransplantation may be influenced by various factors, including the geographic location of the recipient during childhood, possibly associated with exposure to different microorganisms.

Exploration of sialic acid diversity and biology using sialoglycan microarrays

Sialic acids (Sias) are a group of α-keto acids with a nine-carbon backbone, which display many types of modifications in nature. The diversity of natural Sia presentations is magnified by a variety of glycosidic linkages to underlying glycans, the sequences and classes of such glycans, as well as the spatial organization of Sias with their surroundings. This diversity is closely linked to the numerous and varied biological functions of Sias. Relatively large libraries of natural and unnatural Sias have recently been chemically/chemoenzymatically synthesized and/or isolated from natural sources. The resulting sialoglycan microarrays have proved to be valuable tools for the exploration of diversity and biology of Sias. Here we provide an overview of Sia diversity in nature, the approaches used to generate sialoglycan microarrays, and the achievements and challenges arising.

Long-term IgG response to porcine Neu5Gc antigens without transmission of PERV in burn patients treated with porcine skin xenografts

Acellular materials of xenogenic origin are used worldwide as xenografts, and phase I trials of viable pig pancreatic islets are currently being performed. However, limited information is available on transmission of porcine endogenous retrovirus (PERV) after xenotransplantation and on the long-term immune response of recipients to xenoantigens. We analyzed the blood of burn patients who had received living pig-skin dressings for up to 8 wk for the presence of PERV as well as for the level and nature of their long term (maximum, 34 y) immune response against pig Ags. Although no evidence of PERV genomic material or anti-PERV Ab response was found, we observed a moderate increase in anti-αGal Abs and a high and sustained anti-non-αGal IgG response in those patients. Abs against the nonhuman sialic acid Neu5Gc constituted the anti-non-αGal response with the recognition pattern on a sialoglycan array differing from that of burn patients treated without pig skin. These data suggest that anti-Neu5Gc Abs represent a barrier for long-term acceptance of porcine xenografts. Because anti-Neu5Gc Abs can promote chronic inflammation, the long-term safety of living and acellular pig tissue implants in recipients warrants further evaluation.

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.

The sweets standing at the borderline between allo- and xenotransplantation

Animal cells are densely covered with glycoconjugates, such as N-glycan, O-glycan, and glycosphingolipids, which are important for various biological and immunological events at the cell surface and in the extracellular matrix. Endothelial α-Gal carbohydrate epitopes (Galα3Gal-R) expressed on porcine tissue or cell surfaces are such glycoconjugates and directly mediate hyperacute immunological rejection in pig-to-human xenotransplantation. Although researchers have been able to develop α1,3-galactosyltransferase (GalT) gene knockout (KO) pigs, there remain unclarified non-Gal antigens that prevent xenotransplantation. Based on our expertise in the structural analysis of xenoantigenic carbohydrates, we describe the immunologically significant non-human carbohydrate antigens, including α-Gal antigens, analyzed as part of efforts to assess the antigens responsible for hyperacute immunological rejection in pig-to-human xenotransplantation. The importance of studying human, pig, and GalT-KO pig glycoprofiles, and of developing adequate pig-to-human glycan databases, is also discussed.

Natural Antibodies Against Sialoglycans

Natural antibodies, part of the innate immunity system, are produced at strictly regulated levels in normal sera without immunization and thus are part of the innate immune system. The best studied natural antibodies are those directed against blood group antigens A and B and xeno-antigens including glycolylneuraminic acid containing Hanganutziu-Deicher (HD) glycolipid. Abnormal levels of anti-glycan antibodies were found in a number of pathologies. In many cases pathological antibodies are known to bind gangliosides. The genesis of anti-glycan antibodies in healthy humans and the reasons for their changes in pathologies are poorly understood. With a growing interest in their diagnostic applications, it is important to determine the carbohydrate structures that are recognized by antibodies present in the circulation of healthy individuals. We tested a large number of healthy donors using a printed glycan array (PGA) in a microchip format. The PGA contained ~300 glycans, representing mostly normal mammalian structures of glycoproteins and glycolipids, and many of the structures presented are biologically relevant sialylated motifs. As revealed by PGA, the sera interacted with at least 70 normal human glycans. With only few exceptions, antibodies recognizing sialosides have not been identified. Moderate levels of antibodies and moderate variability were observed in the case of SiaT n and its glycolyl variant. Unexpectedly, we found minimal antibody titer directed against Neu5Gcα and the trisaccharide Neu5Gcα2-6Galβ1-4GlcNAc, although this form of neuraminic acid does not occur naturally in humans. Antibodies recognizing sialosides in unnatural β-configuration have been detected and confirmed Springer's paradigm that circulating antibodies represent a reaction against bacteria. Gram-negative bacteria contain LPS with βKDN and/or βKDO which are very close analogs of Neu5Ac that are found in β-connected form. Antibodies against the biantennary N-glycan chain, (Neu5Acα2-6Galβ1-4GlcNAcβ1-2Manα)2-3,6-Manβ1-4GlcNAcβ1-4GlcNAc were never observed and similarly we never saw antibodies directed against the SiaLe(a)/SiaLe (x) motifs. Anti-sialoglycan antibodies can be masked with gangliosides: for example, we observe about a five times higher level of anti-GD3 in purified total IgG compared to the same concentration of total Ig in the composition of native serum. For several antibodies we observed anomalous binding in diluted sera, namely, the signals towards sialylated glycans were increased in the PGA if diluted sera were used.

Porcine sialoadhesin: a newly identified xenogeneic innate immune receptor

Extracorporeal porcine liver perfusion is being developed as a bridge to liver allotransplantation for patients with fulminant hepatic failure. This strategy is limited by porcine Kupffer cell destruction of human erythrocytes, mediated by lectin binding of a sialic acid motif in the absence of antibody and complement. Sialoadhesin, a macrophage restricted lectin that binds sialic acid, was originally described as a sheep erythrocyte binding receptor. Given similarities between sialoadhesin and the unidentified macrophage lectin in our model, we hypothesized porcine sialoadhesin contributed to recognition of human erythrocytes. Two additional types of macrophages were identified to bind human erythrocytes-spleen and alveolar. Expression of sialoadhesin was confirmed by immunofluorescence in porcine tissues and by flow cytometry on primary macrophages. A stable transgenic cell line expressing porcine sialoadhesin (pSn CHO) bound human erythrocytes, while a sialoadhesin mutant cell line did not. Porcine macrophage and pSn CHO recognition of human erythrocytes was inhibited approximately 90% by an antiporcine sialoadhesin monoclonal antibody and by human erythrocyte glycoproteins. Furthermore, this binding was substantially reduced by sialidase treatment of erythrocytes. These data support the hypothesis that porcine sialoadhesin is a xenogeneic receptor that mediates porcine macrophage binding of human erythrocytes in a sialic acid-dependent manner.

α1,3-galactosyltransferase deficiency in germ-free miniature pigs increases N-glycolylneuraminic acids as the xenoantigenic determinant in pig-human xenotransplantation

In this study, we examined whether Hanganutziu-Deicher (H-D) antigens are important as an immunogenic non-α1,3-galactose (Gal) epitope in pigs with a disrupted α1,3-galactosyltransferase gene. The targeting efficiency of the AO blood genotype was achieved (2.2%) in pig fibroblast cells. A total of 1800 somatic cell nuclear transfer (SCNT) embryos were transferred to 10 recipients. One recipient developed to term and naturally delivered two piglets. The α1,3-galactosyltransferase activity in lung, liver, spleen, and testis of heterozygote α1,3-galactosyltransferase gene knockout (GalT-KO) pigs was significantly decreased, whereas brain and heart showed very low decreasing levels of α1,3-galactosyltransferase activity when compared to those of control. Enzyme-linked lectinosorbent assay showed that the heterozygote GalT-KO pig had more sialylα2,6- and sialylα2,3-linked glycan than the control. Furthermore, the heart, liver, and kidney of the heterozygote GalT-KO pig had a higher N-glycolylneuraminic acid (Neu5Gc) content than the control, whereas the lung of the heterozygote GalT-KO pig had Neu5Gc content similar to the control. Collectively, the data strongly indicated that Neu5Gc is a more critical xenoantigen to overcoming the next acute immune rejection in pig to human xenotransplantation.

The role of complement in the early immune response to transplantation

The complement system is a key element of the innate immune system, and the production of complement components can be divided into central (hepatic) and peripheral compartments. Essential complement components such as C3 are produced in both of these compartments, but until recently the functional relevance of the peripheral synthesis of complement was unclear. Here, we review recent findings showing that local peripheral synthesis of complement in a transplanted organ is required for the immediate response of the donor organ to tissue stress and for priming alloreactive T cells that can mediate transplant rejection. We also discuss recent insights into the role of complement in antibody-mediated rejection, and we examine how new treatment strategies that take into account the separation of central and peripheral production of complement are expected to make a difference to transplant outcome.

Clinical lung xenotransplantation--what donor genetic modifications may be necessary?

Barriers to successful lung xenotransplantation appear to be even greater than for other organs. This difficulty may be related to several macro anatomic factors, such as the uniquely fragile lung parenchyma and associated blood supply that results in heightened vulnerability of graft function to segmental or lobar airway flooding caused by loss of vascular integrity (also applicable to allotransplants). There are also micro-anatomic considerations, such as the presence of large numbers of resident inflammatory cells, such as pulmonary intravascular macrophages and natural killer (NK) T cells, and the high levels of von Willebrand factor (vWF) associated with the microvasculature. We have considered what developments would be necessary to allow successful clinical lung xenotransplantation. We suggest this will only be achieved by multiple genetic modifications of the organ-source pig, in particular to render the vasculature resistant to thrombosis. The major problems that require to be overcome are multiple and include (i) the innate immune response (antibody, complement, donor pulmonary and recipient macrophages, monocytes, neutrophils, and NK cells), (ii) the adaptive immune response (T and B cells), (iii) coagulation dysregulation, and (iv) an inflammatory response (e.g., TNF-α, IL-6, HMGB1, C-reactive protein). We propose that the genetic manipulation required to provide normal thromboregulation alone may include the introduction of genes for human thrombomodulin/endothelial protein C-receptor, and/or tissue factor pathway inhibitor, and/or CD39/CD73; the problem of pig vWF may also need to be addressed. It would appear that exploration of every available therapeutic path will be required if lung xenotransplantation is to be successful. To initiate a clinical trial of lung xenotransplantation, even as a bridge to allotransplantation (with a realistic possibility of survival long enough for a human lung allograft to be obtained), significant advances and much experimental work will be required. Nevertheless, with the steadily increasing developments in techniques of genetic engineering of pigs, we are optimistic that the goal of successful clinical lung xenotransplantation can be achieved within the foreseeable future. The optimistic view would be that if experimental pig lung xenotransplantation could be successfully managed, it is likely that clinical application of this and all other forms of xenotransplantation would become more feasible.

Repertoire of human natural anti-glycan immunoglobulins. Do we have auto-antibodies?

BACKGROUND

Profiling of donor's antibodies using glycan arrays demonstrated presence of antibodies capable of binding to >100 mammalian glycans or their fragments. For example, relatively high binding to Galα1-4Galβ1-4GlcNAc (P(1)), Galα1-4Galβ1-4Glc (P(k)), Galβ1-3GlcNAc (Le(c)), 4-O-SuGalβ1-4GlcNAc, and GalNAcα1-3GalNAc (Fs) was found in all tested individuals. Affinity isolation using hapten-specific chromatography in combination with epitope mapping revealed their glycotopes. Notably, a significant part of the antibodies was capable of recognizing a fragment of larger glycans, for example, -Galβ1-4Glc of glycolipids, or Fucα1-3GlcNAc motif of Le(X)/Le(Y) antigens. Their epitope specificity did not vary between different healthy individuals. Nominally, all the mentioned immunoglobulins could be classified as auto-antibodies.

METHODS

In this work we re-evaluated results published earlier and analyzed new data to address the question why autologous antibodies found in healthy individuals do not cause severe auto-immune reactions.

RESULTS

In all cases the presumably "auto" antibodies were found to bind short fragments "subtracted" from larger glycans whereas recognition of the same fragment in the context of the whole natural chain was completely abolished. Thus, in spite of numerous formally positive signals observed on the printed glycan array, we are yet unable to identify in blood serum of healthy individuals true auto-antibodies capable of binding carbohydrate chains in their naturally occurring form.

GENERAL SIGNIFICANCE

The identified natural anti-glycan antibodies were found to be specific, high-titer and population conservative immunoglobulins - all of this suggesting as yet unknown biological role(s) of the studied proteins. This article is part of a Special Issue entitled Glycoproteomics.

Alpha 1,3-galactosyltransferase deficiency in pigs increases sialyltransferase activities that potentially raise non-gal xenoantigenicity

We examined whether deficiency of the GGTA1 gene in pigs altered the expression of several glycosyltransferase genes. Real-time RT-PCR and glycosyltransferase activity showed that 2 sialyltransferases [α2,3-sialyltransferase (α2,3ST) and α2,6-sialyltransferase (α2,6ST)] in the heterozygote GalT KO liver have higher expression levels and activities compared to controls. Enzyme-linked lectin assays indicated that there were also more sialic acid-containing glycoconjugate epitopes in GalT KO livers than in controls. The elevated level of sialic-acid-containing glycoconjugate epitopes was due to the low level of α-Gal in heterozygote GalT KO livers. Furthermore, proteomics analysis showed that heterozygote GalT KO pigs had a higher expression of NAD⁺-isocitrate dehydrogenase (IDH), which is related to the CMP-N-acetylneuraminic acid hydroxylase (CMAH) enzyme reaction. These findings suggest the deficiency of GGTA1 gene in pigs results in increased production of N-glycolylneuraminic acid (Neu5Gc) due to an increase of α2,6-sialyltransferase and a CMAH cofactor, NAD⁺-IDH. This indicates that Neu5Gc may be a critical xenoantigen. The deletion of the CMAH gene in the GalT KO background is expected to further prolong xenograft survival.

Humoral immunity and antibody-mediated rejection in solid organ transplantation

The humoral arm of the immune system provides robust protection against extracellular pathogens via the production of antibody molecules that neutralize or facilitate the destruction of microorganisms. However, the humoral immune system also provides a significant barrier to solid organ transplantation due to the antibody-mediated recognition of non-self proteins and carbohydrates expressed on transplanted organs. Historically, the presence of donor-specific antibodies (DSA) that recognize donor HLA molecules, incompatible ABO blood group antigens and other endothelial or xenogeneic antigens was considered a contraindication to transplantation. However, recent advances in antibody testing and immunosuppressive therapies have made it possible to cross certain antibody barriers successfully. In this article, we review our current understanding of antibody-mediated processes in solid organ transplantation and discuss the clinically available treatment options for preventing and treating antibody-mediated rejection.

Absence of Gal epitope prolongs survival of swine lungs in an ex vivo model of hyperacute rejection

BACKGROUND

Galactosyl transferase gene knock-out (GalTKO) swine offer a unique tool to evaluate the role of the Gal antigen in xenogenic lung hyperacute rejection.

METHODS

We perfused GalTKO miniature swine lungs with human blood. Results were compared with those from previous studies using wild-type and human decay-accelerating factor-transgenic (hDAF(+/+) ) pig lungs.

RESULTS

GalTKO lungs survived 132 ± 52 min compared to 10 ± 9 min for wild-type lungs (P = 0.001) and 45 ± 60 min for hDAF(+/+) lungs (P = 0.18). GalTKO lungs displayed stable physiologic flow and pulmonary vascular resistance (PVR) until shortly before graft demise, similar to autologous perfusion, and unlike wild-type or hDAF(+/+) lungs. Early (15 and 60 min) complement (C3a) and platelet activation and intrapulmonary platelet deposition were significantly diminished in GalTKO lungs relative to wild-type or hDAF(+/+) lungs. However, GalTKO lungs adsorbed cytotoxic anti-non-Gal antibody and elaborated high levels of thrombin; their demise was associated with increased PVR, capillary congestion, intravascular thrombi and strong CD41 deposition not seen at earlier time points.

CONCLUSIONS

In summary, GalTKO lungs are substantially protected from injury but, in addition to anti-non-Gal antibody and complement, platelet adhesion and non-physiologic intravascular coagulation contribute to Gal-independent lung injury mechanisms.

Peptide inhibitors of xenoreactive antibodies mimic the interaction profile of the native carbohydrate antigens

Carbohydrate-antibody interactions mediate many cellular processes and immune responses. Carbohydrates expressed on the surface of cells serve as recognition elements for particular cell types, for example, in the ABO(H) blood group system. Antibodies that recognize host-incompatible ABO(H) system antigens exist in the bloodstream of all individuals (except AB individuals), preventing blood transfusion and organ transplantation between incompatible donors and recipients. A similar barrier exists for cross-species transplantation (xenotransplantation), in particular for pig-to-human transplantation. All humans express antibodies against the major carbohydrate xenoantigen, Galalpha (1,3)Gal (alphaGal), preventing successful xenotransplantation. Although antibody binding sites are precisely organized so as to selectively bind a specific antigen, many antibodies recognize molecules other than their native antigen. A range of peptides have been identified that can mimic carbohydrates and inhibit anti-alphaGal antibodies. However, the structural basis of how the peptides achieved this was not known. Previously, we developed an in silico method which we used to investigate carbohydrate recognition by a panel of anti-alphaGal antibodies. The method involves molecular docking of carbohydrates to antibodies and uses the docked carbohydrate poses to generate maps of th antibody binding sites in terms of prevalent hydrogen bonding and van der Waals interactions. We have applied this method to investigate peptide recognition by the anti-alphaGal antibodies. It was found that the site maps of the peptides and the carbohydrates were similar, indicating that the peptides interact with the same residues as those involved in carbohydrate recognition. This study demonstrates the potential for "design by mapping" of anti-carbohydrate antibody inhibitors.

Initial in vitro investigation of the human immune response to corneal cells from genetically engineered pigs

PURPOSE

To compare the in vitro human humoral and cellular immune responses to wild-type (WT) pig corneal endothelial cells (pCECs) with those to pig aortic endothelial cells (pAECs). These responses were further compared with CECs from genetically engineered pigs (α1,3-galactosyltransferase gene-knockout [GTKO] pigs and pigs expressing a human complement-regulatory protein [CD46]) and human donors.

METHODS

The expression of Galα1,3Gal (Gal), swine leukocyte antigen (SLA) class I and class II on pCECs and pAECs, with or without activation by porcine IFN-γ, was tested by flow cytometry. Pooled human serum was used to measure IgM/IgG binding to and complement-dependent cytotoxicity (CDC) to cells from WT, GTKO, and GTKO/CD46 pigs. The human CD4(+) T-cell response to cells from WT, GTKO, GTKO/CD46 pigs and human was tested by mixed lymphocyte reaction (MLR).

RESULTS

There was a lower level of expression of the Gal antigen and of SLA class I and II on the WT pCECs than on the WT pAECs, resulting in less antibody binding and reduced human CD4(+) T-cell proliferation. However, lysis of the WT pCECs was equivalent to that of the pAECs, suggesting more susceptibility to injury. There were significantly weaker humoral and cellular responses to the pCECs from GTKO/CD46 pigs compared with the WT pCECs, although the cellular response to the GTKO/CD46 pCECs was greater than to the human CECs.

CONCLUSIONS

These data provide the first report of in vitro investigations of CECs from genetically engineered pigs and suggest that pig corneas may provide an acceptable alternative to human corneas for clinical transplantation.

Xenotransplantation--the future of corneal transplantation?

Although corneal transplantation (Tx) is readily available in the United States and certain other regions of the developed world, the need for human donor corneas worldwide far exceeds supply. There is currently renewed interest in the possibility of using corneas from other species, especially pigs, for Tx into humans (xeno-Tx). The biomechanical properties of human and pig corneas are similar. Studies in animal models of corneal xeno-Tx have documented both humoral and cellular immune responses that play roles in xenograft rejection. The results obtained from the Tx of corneas from wild-type (ie, genetically unmodified) pigs into nonhuman primates have been surprisingly good and encouraging. Recent progress in the genetic manipulation of pigs has led to the prospect that the remaining immunological barriers will be overcome. There is every reason for optimism that corneal xeno-Tx will become a clinical reality within the next few years.