Persistent repression of a functional allele can be responsible for galactosyltransferase deficiency in Tn syndrome

CAR T cells redirected against tumor-specific antigen glycoforms: can low-sugar antigens guarantee a sweet success?

Immune-based therapies have experienced a pronounced breakthrough in the past decades as they acquired multiple US Food and Drug Administration (FDA) approvals for various indications. To date, six chimeric antigen receptor T cell (CAR-T) therapies have been permitted for the treatment of certain patients with relapsed/refractory hematologic malignancies. However, several clinical trials of solid tumor CAR-T therapies were prematurely terminated, or they reported life-threatening treatment-related damages to healthy tissues. The simultaneous expression of target antigens by healthy organs and tumor cells is partly responsible for such toxicities. Alongside targeting tumor-specific antigens, targeting the aberrantly glycosylated glycoforms of tumor-associated antigens can also minimize the off-tumor effects of CAR-T therapies. Tn, T, and sialyl-Tn antigens have been reported to be involved in tumor progression and metastasis, and their expression results from the dysregulation of a series of glycosyltransferases and the endoplasmic reticulum protein chaperone, Cosmc. Moreover, these glycoforms have been associated with various types of cancers, including prostate, breast, colon, gastric, and lung cancers. Here, we discuss how underglycosylated antigens emerge and then detail the latest advances in the development of CAR-T-based immunotherapies that target some of such antigens.

Epitopes of MUC1 Tandem Repeats in Cancer as Revealed by Antibody Crystallography: Toward Glycopeptide Signature-Guided Therapy

Abnormally O-glycosylated MUC1 tandem repeat glycopeptide epitopes expressed by multiple types of cancer have long been attractive targets for therapy in the race against genetic mutations of tumor cells. Glycopeptide signature-guided therapy might be a more promising avenue than mutation signature-guided therapy. Three O-glycosylated peptide motifs, PDTR, GSTA, and GVTS, exist in a tandem repeat HGVTSAPDTRPAPGSTAPPA, containing five O-glycosylation sites. The exact peptide and sugar residues involved in antibody binding are poorly defined. Co-crystal structures of glycopeptides and respective monoclonal antibodies are very few. Here we review 3 groups of monoclonal antibodies: antibodies which only bind to peptide portion, antibodies which only bind to sugar portion, and antibodies which bind to both peptide and sugar portions. The antigenicity of peptide and sugar portions of glyco-MUC1 tandem repeat were analyzed according to available biochemical and structural data, especially the GSTA and GVTS motifs independent from the most studied PDTR. Tn is focused as a peptide-modifying residue in vaccine design, to induce glycopeptide-binding antibodies with cross reactivity to Tn-related tumor glycans, but not glycans of healthy cells. The unique requirement for the designs of antibody in antibody-drug conjugate, bi-specific antibodies, and chimeric antigen receptors are also discussed.

The cytokine-cosmc signaling axis upregulates the tumor-associated carbohydrate antigen Tn

Tn antigen (GalNAc-α-O-Ser/Thr), a mucin-type O-linked glycan, is a well-established cell surface marker for tumors and its elevated levels have been correlated with cancer progression and prognosis. There are also reports that Tn is elevated in inflammatory tissues. However, the molecular mechanism for its elevated levels in cancer and inflammation is unclear. In the current studies, we have explored the possibility that cytokines may be one of the common regulatory molecules for elevated Tn levels in both cancer and inflammation. We showed that the Tn level is elevated by the conditioned media of HrasG12V-transformed-BEAS-2B cells. Similarly, the conditioned media obtained from LPS-stimulated monocytes also elevated Tn levels in primary human gingival fibroblasts, suggesting the involvement of cytokines and/or other soluble factors. Indeed, purified inflammatory cytokines such as TNF-α and IL-6 up-regulated Tn levels in gingival fibroblasts. Furthermore, TNF-α was shown to down-regulate the COSMC gene as evidenced by reduced levels of the COSMC mRNA and protein, as well as hypermethylation of the CpG islands of the COSMC gene promoter. Since Cosmc, a chaperone for T-synthase, is known to negatively regulate Tn levels, our results suggest elevated Tn levels in cancer and inflammation may be commonly regulated by the cytokine-Cosmc signaling axis.

The Cosmc connection to the Tn antigen in cancer

The Tn antigen is a tumor-associated carbohydrate antigen that is not normally expressed in peripheral tissues or blood cells. Expression of this antigen, which is found in a majority of human carcinomas of all types, arises from a blockage in the normal O-glycosylation pathway in which glycans are extended from the common precursor GalNAcα1-O-Ser/Thr (Tn antigen). This precursor is generated in the Golgi apparatus on newly synthesized glycoproteins by a family of polypeptide α-N-acetylgalactosaminyltransferases (ppGalNAcTs) and then extended to the common core 1 O-glycan Galβ1-3GalNAcα1-O-Ser/Thr (T antigen) by a single enzyme termed the T-synthase (core 1 β3-galactosyltransferase or C1GalT). Formation of the active form of the T-synthase requires a unique molecular chaperone termed Cosmc, encoded by Cosmc on the X-chromosome (Xq24 in humans, Xc3 in mice). Cosmc resides in the endoplasmic reticulum (ER) and prevents misfolding, aggregation, and proteasome-dependent degradation of newly synthesized T-synthase. Loss of expression of active T-synthase or Cosmc can lead to expression of the Tn antigen, along with its sialylated version Sialyl Tn antigen as observed in several cancers. Both genetic and epigenetic pathways, in addition to potential metabolic regulation, can result in abnormal expression of the Tn antigen. Engineered expression of the Tn antigen by disruption of either C1GalT (T-syn) or Cosmc in mice is associated with a tremendous range of pathologies and engineered expression of the Tn antigen in mouse embryos leads to embryonic death. Studies indicate that many membrane glycoproteins expressing the Tn antigen and/or truncated O-glycans may be dysfunctional, due to degradation and/or misfolding. Thus, expression of normal O-glycans is associated with health and homeostasis whereas truncation of O-glycans, e.g. the Tn and/or Sialyl Tn antigens is associated with cancer and other pathologies.

Epigenetic silencing of the chaperone Cosmc in human leukocytes expressing tn antigen

Cosmc is the specific molecular chaperone in the endoplasmic reticulum for T-synthase, a Golgi β3-galactosyltransferase that generates the core 1 O-glycan, Galβ1-3GalNAcα-Ser/Thr, in glycoproteins. Dysfunctional Cosmc results in the formation of inactive T-synthase and consequent expression of the Tn antigen (GalNAcα1-Ser/Thr), which is associated with several human diseases. However, the molecular regulation of expression of Cosmc, which is encoded by a single gene on Xq24, is poorly understood. Here we show that epigenetic silencing of Cosmc through hypermethylation of its promoter leads to loss of Cosmc transcripts in Tn4 cells, an immortalized B cell line from a male patient with a Tn-syndrome-like phenotype. These cells lack T-synthase activity and express the Tn antigen. Treatment of cells with 5-aza-2'-deoxycytidine causes restoration of Cosmc transcripts, restores T-synthase activity, and reduces Tn antigen expression. Bisulfite sequencing shows that CG dinucleotides in the Cosmc core promoter are hypermethylated. Interestingly, several other X-linked genes associated with glycosylation are not silenced in Tn4 cells, and we observed no correlation of a particular DNA methyltransferase to aberrant methylation of Cosmc in these cells. Thus, hypermethylation of the Cosmc promoter in Tn4 cells is relatively specific. Epigenetic silencing of Cosmc provides another mechanism underlying the abnormal expression of the Tn antigen, which may be important in understanding aberrant Tn antigen expression in human diseases, including IgA nephropathy and cancer.

The entirely carbohydrate immunogen Tn-PS A1 induces a cancer cell selective immune response and cytokine IL-17

The tumor-associated carbohydrate antigen/hapten Thomsen-nouveau (Tn; a-D-GalpNAc-ONH2) was conjugated to a zwitterionic capsular polysaccharide, PS A1, from commensal anaerobe Bacteroides fragilis ATCC 25285/NCTC 9343 for the development of an entirely carbohydrate cancer vaccine construct and probed for immunogenicity. This communication discloses that murine anti-Tn IgG3 antibodies both bind to and recognize human tumor cells that display the Tn hapten. Furthermore, the sera from immunization of mice with Tn-PS A1 contain cytokine interleukin 17 (IL-17A), which is known to possess anti-tumor function and represents a striking difference to an IL-2, and IL-6 profile obtained with anti-PS A1 sera.

Glycosylation of IgA1 and pathogenesis of IgA nephropathy

IgA nephropathy, described in 1968 as IgA-IgG immune-complex disease, is an autoimmune disease. Galactose-deficient IgA1 is recognized by unique autoantibodies, resulting in the formation of pathogenic immune complexes that ultimately induce glomerular injury. Thus, formation of the galactose-deficient IgA1-containing immune complexes is a critical factor in the pathogenesis of IgA nephropathy. Studies of molecular defects of IgA1 can define new biomarkers specific for IgA nephropathy that can be developed into clinical assays to aid in the diagnosis, assessment of prognosis, and monitoring of disease progression.

The Tn antigen-structural simplicity and biological complexity

Glycoproteins in animal cells contain a variety of glycan structures that are added co- and/or posttranslationally to proteins. Of over 20 different types of sugar-amino acid linkages known, the two major types are N-glycans (Asn-linked) and O-glycans (Ser/Thr-linked). An abnormal mucin-type O-glycan whose expression is associated with cancer and several human disorders is the Tn antigen. It has a relatively simple structure composed of N-acetyl-D-galactosamine with a glycosidic α linkage to serine/threonine residues in glycoproteins (GalNAcα1-O-Ser/Thr), and was one of the first glycoconjugates to be chemically synthesized. The Tn antigen is normally modified by a specific galactosyltransferase (T-synthase) in the Golgi apparatus of cells. Expression of active T-synthase is uniquely dependent on the molecular chaperone Cosmc, which is encoded by a gene on the X chromosome. Expression of the Tn antigen can arise as a consequence of mutations in the genes for T-synthase or Cosmc, or genes affecting other steps of O-glycosylation pathways. Because of the association of the Tn antigen with disease, there is much interest in the development of Tn-based vaccines and other therapeutic approaches based on Tn expression.

IgA1-secreting cell lines from patients with IgA nephropathy produce aberrantly glycosylated IgA1

Aberrant glycosylation of IgA1 plays an essential role in the pathogenesis of IgA nephropathy. This abnormality is manifested by a deficiency of galactose in the hinge-region O-linked glycans of IgA1. Biosynthesis of these glycans occurs in a stepwise fashion beginning with the addition of N-acetylgalactosamine by the enzyme N-acetylgalactosaminyltransferase 2 and continuing with the addition of either galactose by beta1,3-galactosyltransferase or a terminal sialic acid by a N-acetylgalactosamine-specific alpha2,6-sialyltransferase. To identify the molecular basis for the aberrant IgA glycosylation, we established EBV-immortalized IgA1-producing cells from peripheral blood cells of patients with IgA nephropathy. The secreted IgA1 was mostly polymeric and had galactose-deficient O-linked glycans, characterized by a terminal or sialylated N-acetylgalactosamine. As controls, we showed that EBV-immortalized cells from patients with lupus nephritis and healthy individuals did not produce IgA with the defective galactosylation pattern. Analysis of the biosynthetic pathways in cloned EBV-immortalized cells from patients with IgA nephropathy indicated a decrease in beta1,3-galactosyltransferase activity and an increase in N-acetylgalactosamine-specific alpha2,6-sialyltransferase activity. Also, expression of beta1,3-galactosyltransferase was significantly lower, and that of N-acetylgalactosamine-specific alpha2,6-sialyltransferase was significantly higher than the expression of these genes in the control cells. Thus, our data suggest that premature sialylation likely contributes to the aberrant IgA1 glycosylation in IgA nephropathy and may represent a new therapeutic target.

Role of aberrant glycosylation of IgA1 molecules in the pathogenesis of IgA nephropathy

Studies of the properties of immune complexes (IC) in the circulation, urine, and mesangium of IgA nephropathy (IgAN) patients have provided data relevant to the pathogenesis of this disease. IC contain predominantly polymeric IgA1 molecules which are deficient in galactose (Gal) residues on O-linked glycan chains in the hinge region (HR) of their heavy (H) chains. As a result of this aberrancy, a novel antigenic determinant(s) involving N-acetylgalactosamine (GalNAc) and perhaps sialic acid (SA) of O-linked glycans is generated and recognized by naturally occurring GalNAc-specific antibodies. Thus, IC in IgAN consist of Gal-deficient IgA1 molecules as an antigen, and GalNAc-specific IgG and/or IgA1 as an antibody. IgG antibodies to Gal-deficient IgA1 are probably induced by cross-reactive microbial antigens; they are present at variable levels not only in humans with or without IgAN but also in many phylogenetically diverse vertebrate species. Incubation of human mesangial cells with IC from sera of IgAN patients indicated that stimulation of cellular proliferative activity was restricted to the large (>800 kDa) complexes. These findings suggest that experimental approaches that prevent the formation of large Gal-deficient IgA1-IgG IC may be applied ultimately in an immunologically mediated therapy.

Peripheral B lymphocyte beta1,3-galactosyltransferase and chaperone expression in immunoglobulin A nephropathy

PURPOSE

Aberrant O-glycosylation of serum IgA(1) is presumed to be one of the main pathogenesis of immunoglobulin A nephropathy (IgAN). beta1,3-galactosyltransferase (beta1,3GT), whose activity requires coexistence of a specific chaperone, is the main enzyme which participate in the glycosylation process. The current study was carried out to elucidate the expression level of beta1,3GT (C1GALT1) and its chaperone (Cosmc) in IgAN, and their relationships with clinical features as well as IgA glycosylation level.

DESIGN, SETTING AND SUBJECTS

Forty-one patients with IgAN, 21 patients with non-IgAN glomerulonephritis and 26 normal controls were included in the present study. Peripheral B lymphocytes were isolated, and then expression level of C1GALT1 and Cosmc were quantitatively measured by real-time reverse transcriptase polymerase chain reaction (RT-PCR). Serum IgA level and glycosylation level were determined by enzyme-linked immunosorbent assay (ELISA) and VV lectin-binding method. Correlation analysis was performed between C1GALT1/Cosmc expression levels and clinical manifestations (severe proteinuria, renal dysfunction, gross haematuria).

RESULTS

B-lymphocyte Cosmc gene expression level was significantly lower in IgAN patients than that of normal control and non-IgAN patients (P<0.05), whilst no apparent disparity was observed in C1GALT1 expression level. Cosmc expression showed a negative correlation with IgA O-glycosylation level indicated by VV lectin-binding assay. Statistical analysis also indicated that the level of Cosmc expression was negatively correlated with severe proteinuria (P<0.05) instead of gross haematuria (P>0.05).

CONCLUSION

These data suggested that the aberrant IgA O-glycosylation in IgAN was resulted from a downregulation of beta1,3GT chaperone (Cosmc) expression in B lymphocyte, which is closely associated with clinical characteristics of the disease. This downregulation might be one of the fundamental pathogenic abnormalities in IgAN.

A Fully Synthetic Therapeutic Vaccine Candidate Targeting Carcinoma-Associated Tn Carbohydrate Antigen Induces Tumor-Specific Antibodies in Nonhuman Primates

We recently developed an efficient strategy based on a fully synthetic dendrimeric carbohydrate display (multiple antigenic glycopeptide; MAG) to induce anticarbohydrate antibody responses for therapeutic vaccination against cancer. Here, we show the superior efficacy of the MAG strategy over the traditional keyhole limpet hemocyanin glycoconjugate to elicit an anticarbohydrate IgG response against the tumor-associated Tn antigen. We highlight the influence of the aglyconic carrier elements of such a tumor antigen for their recognition by the immune system. Finally, we additionally developed the MAG system by introducing promiscuous HLA-restricted T-helper epitopes and performed its immunological evaluation in nonhuman primates. MAG:Tn vaccines induced in all of the animals strong tumor-specific anti-Tn antibodies that can mediate antibody-dependent cell cytotoxicity against human tumor. Therefore, the preclinical evaluation of the MAG:Tn vaccine demonstrates that it represents a safe and highly promising immunotherapeutic molecularly defined tool for targeting breast, colon, and prostate cancers that express the carbohydrate Tn antigen.

Mucin-type O-glycosylation in helminth parasites from major taxonomic groups: evidence for widespread distribution of the Tn antigen (GalNAc-Ser/Thr) and identification of UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase activity

This article focuses on the initiation pathway of mucin-type O-glycosylation in helminth parasites. The presence of the GalNAc-O-Ser/Thr structure, also known as Tn antigen, a truncated determinant related to aberrant glycosylation in mammal cells, and the activity of the UDP-GalNAc:polypeptide N-acetyl-galactosaminyltransferase (ppGaNTase), the enzyme responsible for its synthesis, were studied in species from major taxonomic groups. Tn reactivity was determined in extracts from Taenia hydatigena, Mesocestoides corti, Fasciola hepatica, Nippostrongylus brasiliensis, and Toxocara canis using the monoclonal antibody 83D4. The Tn determinant was revealed in all preparations, and multiple patterns of Tn-bearing glycoproteins were observed by immunoblotting. Additionally, the first evidence that helminth parasites express ppGaNTase activity was obtained. This enzyme was studied in extracts from Echinococcus granulosus, F. hepatica, and T. canis by measuring the incorporation of UDP-(3H)GalNAc to both deglycosylated ovine syalomucin (dOSM) and synthetic peptide sequences derived from tandem repeats of human mucins. Whereas significant levels of ppGaNTase activity were detected in all the extracts when dOSM was used as a multisite acceptor, it was only observed in F. hepatica and E. granulosus extracts when mucin-derived peptides were used, suggesting that T. canis ppGaNTase enzyme(s) may represent a member of the gene family with a more restricted specificity for worm O-glycosylation motifs. The widespread expression of Tn antigen, capable of evoking both humoral and cellular immunity, strongly suggests that simple mucin-type O-glycosylation does not constitute an aberrant phenomenon in helminth parasites.

Purification, characterization, and subunit structure of rat core 1 Beta1,3-galactosyltransferase

The O-linked oligosaccharides (O-glycans) in mammalian glycoproteins are classified according to their core structures. Among the most common is the core 1 disaccharide structure consisting of Galbeta1-->3GalNAcalpha1-->Ser/Thr, which is also the precursor for many extended O-glycan structures. The key enzyme for biosynthesis of core 1 O-glycan from the precursor GalNAc-alpha-Ser/Thr is UDP-Gal:GalNAc-alpha-Ser/Thr beta3-galactosyltransferase (core1 beta3-Gal-T). Core 1 beta3-Gal-T activity, which requires Mn2+, was solubilized from rat liver membranes and purified 71,034-fold to apparent homogeneity (>90% purity) in 5.7% yield by ion exchange chromatography on SP-Sepharose, affinity chromatography on immobilized asialo-bovine submaxillary mucin, and gel filtration chromatography on Superose 12. The purified enzyme is free of contaminating glycosyltransferases. Two peaks of core 1 beta3-Gal-T activity were identified in the final step on Superose 12. One peak of activity contained protein bands on non-reducing SDS-PAGE of approximately 84- and approximately 86-kDa disulfide-linked dimers, whereas the second peak of activity contained monomers of approximately 43 kDa. Reducing SDS-PAGE of these proteins gave approximately 42- and approximately 43-kDa monomers. Both the 84/86-kDa dimers and the 42/43-kDa monomers have the same novel N-terminal sequence. The purified enzyme, which is remarkably stable, has an apparent Km for UDP-Gal of 630 microm and an apparent Vmax of 206 micromol/mg/h protein using GalNAcalpha1-O-phenyl as the acceptor. The reaction product was generated using asialo-bovine submaxillary mucin as an acceptor; treatment with O-glycosidase generated the expected disaccharide Galbeta1-->3GalNAc. These studies demonstrate that activity of the core 1 beta1,3-Gal-T from rat liver is contained within a single, novel, disulfide-bonded, dimeric enzyme.

Cloning and expression of human core 1 beta1,3-galactosyltransferase

The common core 1 O-glycan structure Galbeta1--> 3GalNAc-R is the precursor for many extended mucin-type O-glycan structures in animal cell surface and secreted glycoproteins. Core 1 is synthesized by the transfer of Gal from UDP-Gal to GalNAcalpha1-R by core 1 beta3-galactosyltransferase (core 1 beta3-Gal-T). Amino acid sequences from purified rat core 1 beta3-Gal-T (Ju, T., Cummings, R. D., and Canfield, W. M. (2002) J. Biol. Chem. 277, 169-177) were used to identify the core 1 beta3-Gal-T sequences in the human expressed sequence tag data bases. A 1794-bp human core 1 beta3-Gal-T cDNA sequence was determined by sequencing the expressed sequence tag and performing 5'-rapid amplification of cDNA ends. The core 1 beta3-Gal-T predicts a 363-amino acid type II transmembrane protein. Expression of both the full-length and epitope-tagged soluble forms of the putative enzyme in human 293T cells generated core 1 beta3-Gal-T activity that transferred galactose from UDP-Gal to GalNAcalpha1-O-phenyl, and a synthetic glycopeptide with Thr-linked GalNAc and the product was shown to have the core 1 structure. Northern analysis demonstrated widespread expression of core 1 beta3-Gal-T in tissues with a predominance in kidney, heart, placenta, and liver. Highly homologous cDNAs were identified and cloned from rat, mouse, Drosophila melanogaster, and Caenorhabditis elegans, suggesting that the enzyme is widely distributed in metazoans. The core 1 beta3-Gal-T sequence has minimal homology with conserved sequences found in previously described beta3-galactosyltransferases, suggesting this enzyme is only distantly related to the known beta3-galactosyltransferase family.

Progress in molecular and genetic studies of IgA nephropathy

Several new findings emerged recently from biochemical, genetic, and molecular studies of patients with IgA nephropathy. It appears that immunoglobulin A1-secreting cells of IgA nephropathy patients produce increased amounts of aberrantly glycosylated IgA1 in which the O-linked glycans in the hinge region are deficient in the content of galactose. The galactose-deficient IgA1 in the circulation is recognized by naturally occurring antibodies with anti-glycan specificity, and immune complexes are formed. These circulating immune complexes escape hepatic degradation and eventually are deposited in the kidney mesangium. Resident mesangial cells bind the IgA-containing immune complexes with the involvement of a novel IgA receptor and become activated. A familial form of IgA nephropathy has been linked to chromosome 6q22-23. Recent progress in molecular analyses of IgA nephropathy thus defines this disease as an autoimmune process with a novel IgA mesangial receptor and certain genetically determined traits.

O-glycosylation in Echinococcus granulosus: identification and characterization of the carcinoma-associated Tn antigen

In the present work we demonstrate that the cancer-associated O-glycosylated Tn antigen (GalNAc-O-Ser/Thr) is expressed by the cestode Echinococcus granulosus. This antigen was detected in both larval and adult worm extracts, with the highest specific activity observed in the adult excretion/secretion preparation. Histochemical analysis showed that Tn is preferentially expressed in the parenchyma in both parasite stages and the external part of tegument in adult worms. A similar pattern was observed for sialyl-Tn, a related O-linked antigen. Tn glycoproteins from protoscoleces were resolved by SDS-PAGE in two main components of 43 and 49 kDa. After purification, this material was reactive with lectins which bind GlcNAc/sialic acid, GalNAc, and T antigen. In a preliminary evaluation, high levels of Tn antigen were detected in serum samples from patients with hydatid cyst, suggesting that the measure of Tn in serum could be a biomarker of this disease, although extensive work is necessary in order to determine the clinical usefulness of this assay. The results reported here, the first evidence of O-glycosylation pathways in E. granulosus and the presence of Tn antigen in cestodes, suggest that the evaluation of O-glycosylated antigens might give new insights in the host-parasite relationship.

Heterogeneity of O-glycosylation in the hinge region of human IgA1

Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry was applied to studies of the molecular heterogeneity of desialylated human IgA1 hinge region glycopeptides released with two IgA1 proteases. Typically, the hinge region of an alpha1 chain contains three to five O-linked glycan chains. Variants of the hinge region peptides released from IgA1(Kni) myeloma protein carrying 0, 1, 2, or 3 GalNAc residues were observed in the mass spectra as well as the nonglycosylated peptide. Variable numbers of Gal residues indicated additional heterogeneity in O-glycosylation of IgA1. In the hinge region preparation from normal human serum IgA1, glycopeptides carrying 2, 3, 4, or 5 GalNAc residues with variable numbers of Gal residues were detected. In conclusion, our new approach using the site-specific cleavage with two IgA1 proteases allowed precise and sensitive MALDI-TOF mass spectrometric analysis of O-glycosylation heterogeneity in IgA1 hinge region.

Tn antigen is a pre-cancerous biomarker in breast tissue and serum in n-nitrosomethylurea-induced rat mammary carcinogenesis

The Tn determinant (GalNAcalpha-O-Ser/Thr), normally a cryptic structure in mucin-type O-glycans, is a tumor-associated marker which has attracted particular interest in cancer biology. We herein report the characterization of N-nitrosomethylurea (NMU)-induced breast cancer in rats as a new model for the study of aberrant O-glycosylation products. Tn-antigen expression is detectable not only in mammary carcinoma induced by NMU but also in carcinogen-initiated mammary epithelium, indicating that Tn could be a pre-cancerous biomarker in rats treated with NMU. Serum Tn levels were followed up longitudinally in 30 rats from the time of the first injection of NMU to the development of advanced breast cancer. Tn antigen increased in serum several weeks before tumor development, and became highly positive after 56 days of carcinogenesis (prior to breast-cancer occurrence), and the levels correlated with Tn expression in mammary tissues. However, during the follow-up after detection of mammary cancer, all animals displayed a significant decrease of serum Tn antigen, and low levels were observed in animals with advanced breast cancer. We have shown that the humoral immune response to cancer, with the production of anti-Tn antibodies, could hamper the detection of Tn antigen in animals with advanced breast cancer. These results suggest that NMU-induced rat mammary carcinogenesis is a useful experimental model to study the regulation of O-glycosylation at the cellular level during malignant transformation.

Pathways of O-glycan biosynthesis in cancer cells

Glycoproteins with O-glycosidically linked carbohydrate chains of complex structures and functions are found in secretions and on the cell surfaces of cancer cells. The structures of O-glycans are often unusual or abnormal in cancer, and greatly contribute to the phenotype and biology of cancer cells. Some of the mechanisms of changes in O-glycosylation pathways have been determined in cancer model systems. However, O-glycan biosynthesis is a complex process that is still poorly understood. The glycosyltransferases and sulfotransferases that synthesize O-glycans appear to exist as families of related enzymes of which individual members are expressed in a tissue- and growth-specific fashion. Studies of their regulation in cancer may reveal the connection between cancerous transformation and glycosylation which may help to understand and control the abnormal biology of tumor cells. Cancer diagnosis may be based on the appearance of certain glycosylated epitopes, and therapeutic avenues have been designed to attack cancer cells via their glycans.

Tn-syndrome

The idiopathic Tn-syndrome, formerly called 'permanent mixed-field polyagglutinability', is a rare hematological disorder characterized by the expression of the Tn-antigen on all blood cell lineages. The immunodominant epitope of the Tn-antigen is terminal alpha-N-acetylgalactosamine, O-glycosidically linked to protein. Normally this residue is 3'-substituted by 5-galactose thereby forming the core 1 structure known as the Thomsen-Friedenreich (TF) antigen (Galbeta1 ==> 3GalNAcalpha1 ==> Thr/Ser). The cause of the exposure of the Tn-antigen appears to be due to the silencing of the gene expression of beta1,3galactosyltransferase, since treatment of deficient Tn(+) lymphocyte T clones with 5'azacytidine or Na butyrate leads to reexpression of enzyme activity and the sialylated TF-antigen. The Tn-syndrome is acquired and permanent and affects both sexes at any age. Its origin is unknown. Pluripotent stem cells are affected since all lineages are involved but each one to a variable extent. Therefore, normal cells co-exist with Tn-transformed cells. Clinically, patients suffering from the Tn-syndrome appear healthy. Laboratory findings usually reveal moderate thrombocyto- and leukopenia and some signs of hemolytic anemia not warranting any treatment.

Circulating immune complexes in IgA nephropathy consist of IgA1 with galactose-deficient hinge region and antiglycan antibodies

Circulating immune complexes (CICs) isolated from sera of patients with IgA nephropathy (IgAN) consist of undergalactosylated, mostly polymeric, and J chain-containing IgA1 and IgG antibodies specific for N-acetylgalactosamine (GalNAc) residues in O-linked glycans of the hinge region of IgA1 heavy chains. Antibodies with such specificity occur in sera of IgAN patients, and in smaller quantities in patients with non-IgA proliferative glomerulonephritis and in healthy controls; they are present mainly in the IgG (predominantly IgG2 subclass), and less frequently in the IgA1 isotype. Their specificity for GalNAc was determined by reactivity with IgA1 myeloma proteins with enzymatically removed N-acetylneuraminic acid (NeuNAc) and galactose (Gal); removal of the O-linked glycans of IgA1 resulted in significantly decreased reactivity. Furthermore, IgA2 proteins that lack the hinge region with O-linked glycans but are otherwise structurally similar to IgA1 did not react with IgG or IgA1 antibodies. The re-formation of isolated and acid-dissociated CICs was inhibited more effectively by IgA1 lacking NeuNAc and Gal than by intact IgA1. Immobilized GalNAc and asialo-ovine submaxillary mucin (rich in O-linked glycans) were also effective inhibitors. Our results suggest that the deficiency of Gal in the hinge region of IgA1 molecules results in the generation of antigenic determinants containing GalNAc residues that are recognized by naturally occurring IgG and IgA1 antibodies.

Protein glycosylation in development and disease

N- and O-linked glycan structures of cell surface and secreted glycoproteins serve a variety of functions related to cell-cell communication in systems affecting development and disease. The more sophisticated N-glycan biosynthesis pathway of metazoans diverges from that of yeast with the appearance of the medial-Golgi beta-N-acetylglucosaminyltransferases (GlcNAc-Ts). Tissue-specific regulation of medial- and trans-Golgi glycosyltransferases contribute structural diversity to glycoproteins in metazoans, and this can affect their molecular properties including localization, half-life, and biological activity. Null mutations in glycosyltransferase genes positioned later in the biosynthetic pathway disrupt expression of smaller subsets of glycan structures and are progressively milder in phenotype. In this review, we examine data on targeted mutations affecting glycosylation in mice and congenital mutations in man, with a view to understanding the molecular functions of glycan structures as modulators of glycoprotein activity. Finally, pathology associated with the expression of GlcNAc-Ts in cancer and diabetes-induced cardiac hypertrophy suggest that inhibitors of these enzymes may have therapeutic value.

Anti-xenograft immune responses in alpha 1,3-galactosyltransferase knock-out mice

Although originally generated to test the effect of eliminating the alpha-Gal epitope on HAR, it is becoming increasingly clear that GalT KO mice offer a convenient and inexpensive model to investigate many aspects of the anti-xenorgraft immune response. Clearly, not all aspects of anti-xenograft rejection responses are identical in mice and primates, which should be kept in mind when interpreting results of GalT KO mouse studies. However, with this and other mouse models it is possible to test a large number of variables, which is impractical for both logistical and financial reasons with primates. Furthermore the short gestation time and large litter size of mice means that genetic strategies targeting different aspects of the anti-xenograft immune response can be combined and subsequently tested to identify the optimal combination of genetic and therapeutic approaches to achieve long term xenograft survival. In this regard the GalT KO mouse has been and will continue to be a valuable small animal model for the study of all facets of xenograft rejection involving anti-Gal antibodies.

Repressed beta-1,3-galactosyltransferase in the Tn syndrome

The human hematopoietic disorder named Tn syndrome has been ascribed to an acquired stem cell mutation resulting in loss of beta-1,3-galactosyltransferase activity in affected Tn+ cells of the hematopoietic lineages. Recently, we could demonstrate that this deficiency is due to a repression of a functional allele of the beta-1,3-Gal-T gene since treatment of Tn+ T-lymphocytes from a patient (R.R.) afflicted with the Tn-syndrome with 5-azacytidine or Na n-butyrate resulted in re-expression of the Thomsen-Friedenreich (TF) antigen, the product of beta-1,3-Gal-T activity [M. Thurnher, S. Rusconi, E.G. Berger. Persistent repression of functional allele can be responsible for galactosyltransferase deficiency in Tn syndrome. J. Clin. Invest. 91 (1993) 2103-2110]. To reduce these observations to a common pathogenetic mechanism responsible for the Tn-syndrome, more Tn patients need to be investigated. Here, we describe similar Tn+ T-lymphocytes cultured ex vivo from patient M.Z. whose Tn+ syndrome was newly recognized. Tn+ and TF+ T-lymphocyte cultures were characterized by flow cytometry and measurement of beta-1,3-Gal-T and shown to be deficient in Tn+ cells. Furthermore, Tn+ cells were treated with 5-azacytidine and Na n-butyrate as described before. Reoccurrence of beta-1,3-Gal-T activity dependent epitopes on the cell surface of Tn+ cells was shown by flow cytometry. These support the notion of beta-1,3-Gal-T gene repression as a common pathogenetic mechanism underlying the Tn-syndrome.

Xenoantigens and xenoantibodies

Major efforts are being directed towards determining and modifying the glycosylated epitopes on pig vascular endothelial cells, against which human natural antibodies are directed. Genetic engineering techniques are being used in an effort to knock out or replace the major alpha galactosyl (alpha Gal) epitopes in mice, but to-date these have been only modestly successful in prolonging functional survival of such modified organs. Competitive glycosylation involving insertion of the gene for alpha 1,2 fucosyltransferase results in reduction of alpha Gal expression but also of presentation of hitherto cryptic antigens against which natural human antibodies are directed or could develop. The introduction of the gene for N-acetylglucosaminyltransferase III has been demonstrated to significantly reduce alpha Gal expression, and the intracellular expression of single chain Fv antibodies against alpha 1,3 galactosyltransferase also represses this enzyme activity. Several other carbohydrate antigens have been identified that could act as targets for human natural antibodies, and these include Gal alpha 1-3Le(x), Hanganutziu-Deicher, Tn, and Forssman antigens. The alternative approach, namely, the depletion of the recipient's natural antibodies, is proving difficult, but techniques for inducing B cell tolerance are being explored. The induction of a state of mixed hematopoietic chimerism in alpha Gal knockout mice has resulted in tolerance to the alpha Gal antigen. Tolerance to the SLA antigens of miniature swine is also being attempted in baboons by the transfer of SLA Class II genes into baboon bone marrow cells. It is hoped that one or a combination of these approaches may overcome the problem created by the presence of pig antigens against which humans have xenoreactive antibodies.

Galactose-deficient IgA1 in sera of IgA nephropathy patients is present in complexes with IgG

IgA1 proteins from sera of patients with IgA nephropathy (IgAN) are galactosylated to a lesser degree than those from healthy controls. The increased reactivity of intact or de-sialylated serum IgA1 with N-acetylgalactosamine (GalNAc)-specific lectins, Helix aspersa (HAA) and Caragana arborescens (CAA) and de-sialylated IgA1 with Helix pomatia (HPA) and Bauhinia purpurea (BPA) indicated that the Gal deficiency is in glycans located in the hinge region of IgA1 molecules. De-sialylated IgA from sera of 81 IgAN patients bound biotin-labeled lectin HAA more effectively than did de-sialylated IgA from 56 healthy controls (P < 0.0001). Similar results were observed for 67 IgAN patients and 52 controls with second lectin, CAA (P < 0.001). The binding patterns for 9 patients with mesangial proliferative glomerulonephritis of non-IgA origin were similar to those for controls. Incompletely galactosylated IgA1 capable of binding GalNAc-specific lectins was detected in complexes with IgG as demonstrated by ELISA, size-exclusion chromatography and sucrose gradient ultracentrifugation. The formation of IgA1-IgG complexes may affect the serum level of IgA1 by reducing the rate of its elimination and catabolic degradation by the liver.

Changes in cell surface glycosylation in alpha1,3-galactosyltransferase knockout and alpha1,2-fucosyltransferase transgenic mice

BACKGROUND

Inactivation of the alpha1,3-galactosyltransferase (GalT) gene by homologous recombination (knockout [KO] mice) and competition for the enzyme's N-acetyllactosamine substrate by transgenically expressed alpha1,2-fucosyltransferase (H-transferase) are two genetic approaches to elimination of the Gal alpha1,3Gal (alphaGal) epitope, which is the major xenoantigen in pigs against which humans have preformed antibodies. Such genetic manipulations often have unpredictable results.

METHODS

A panel of 19 selected lectins was used to characterize the changes in cell surface glycosylation in GalT KO and H-transferase transgenic mice, compared with nontransgenic littermate controls.

RESULTS

GalT KO mice showed complete elimination of the alphaGal epitope, as reported previously. Surprisingly, however, this was associated with only a modest increase in N-acetyllactosamine residues and had little other effect on the pattern of lectin binding. In contrast, the pattern of lectin binding to H-transferase transgenic mouse cells was more profoundly disturbed and indicated, in addition to the expected expression of H substance and suppression of the alphaGal epitope, that there was a marked reduction in alpha2,3-sialylation and exposure of the normally cryptic antigens, sialylated Tn and Forssman antigens. Similar changes in lectin reactivity with porcine aortic endothelial cells were induced by neuraminidase treatment.

CONCLUSIONS

Lectins were able to bind underlying carbohydrate structures (sialylated Tn and Forssman antigens) that are normally cryptic antigens on H-transferase transgenic mouse spleen and cardiac endothelial cells, probably as a consequence of the reduction in the electronegativity of the cell surface due to reduced sialylation. As humans have preformed anti-Tn and anti-Forssman antibodies, it is possible that these structures may become targets of the xenograft rejection process, including hyperacute rejection.

Immunoreactivities of polyclonal and monoclonal anti-T and anti-Tn antibodies with human carcinoma cells, grown in vitro and in a xenograft model

Human polyclonal, monospecific anti-T and -Tn antibodies were found to be reactive in ELISA tests with human ovarian (IGROV-1, OVCAR-3 and SKOV-3), breast (SKBr-3 and T47D)- and oral (KB)-carcinoma cell lines, but less so or non-reactive with normal epithelia and fibroblasts. The direct binding radioimmunoassay, using 125I-labeled human antibodies, to the IGROV-1 cancer cells was inhibited by homologous unlabeled antibodies of the same concentration, but not by the respective immunodominant haptenic monosaccharides (Gal for T and GalNAc for Tn). Rodent ascitic monoclonal anti-T (Ca3114 and Ca3741) and anti-Tn (Ca3250, Ca3268 and Ca3638) antibodies were also reactive with the ovarian- and breast-cancer cells, as measured by FACS and ELISA tests, but to a lower extent than the polyclonal human antibodies. Both the monoclonal anti-T (Ca3741) and anti-Tn (Ca3250 and Ca3638) antibody-binding reactivities were significantly inhibited by the haptenic free monosaccharides. Addition of the above MAbs to IGROV-1 ovarian-cancer or T47D breast-cancer cells cultured in vitro resulted in significant cytological change and inhibition of the viability of the tumor cells, but not of normal epithelial breast cells. This effect on viability was shown to be complement-independent, yet it was profoundly influenced by the concentration of the serum added to the assay medium. In vivo biodistribution of the anti-T (Ca3114) and anti-Tn (Ca3638) MAbs administered i.p. to athymic IGROV-1 tumor-bearing CD1 female nude mice revealed higher 125I-labeled antibody accumulation in the tumor xenografts and in their lung tissues, as compared with other organs of the same mice tested. The above results thus suggest the feasibility of utilizing these antibodies in immunotherapy and drug targeting.

Characterization of a CD43/leukosialin-mediated pathway for inducing apoptosis in human T-lymphoblastoid cells

The monoclonal antibody (mAb) J393 induces apoptosis in Jurkat T-cells. NH2-terminal amino acid sequence analysis identified the 140-kDa surface antigen for mAb J393 as CD43/leukosialin, the major sialoglycoprotein of leukocytes. While Jurkat cells co-expressed two discrete cell-surface isoforms of CD43, recognized by mAb J393 and mAb G10-2, respectively, only J393/CD43 signaled apoptosis. J393/CD43 was found to be hyposialylated, bearing predominantly O-linked monosaccharide glycans, whereas G10-2/CD43 bore complex sialylated tetra- and hexasaccharide chains. Treatment with soluble, bivalent mAb J393 killed 25-50% of the cell population, while concomitant engagement of either the CD3.TcR complex or the integrins CD18 and CD29 significantly potentiated this effect. Treatment of Jurkat cells with mAb J393 induced tyrosine phosphorylation of specific protein substrates that underwent hyperphosphorylation upon antigen receptor costimulation. Tyrosine kinase inhibition by herbimycin A diminished J393/CD43-mediated apoptosis, whereas inhibition of phosphotyrosine phosphatase activity by bis(maltolato)oxovanadium-IV enhanced cell death. Signal transduction through tyrosine kinase activation may lead to altered gene expression, as J393/CD43 ligation prompted decreases in the nuclear localization of the transcriptional regulatory protein NF-kappaB and proteins binding the interferon-inducible regulatory element. Since peripheral blood T-lymphocytes express cryptic epitopes for mAb J393, these findings demonstrate the existence of a tightly regulated CD43-mediated pathway for inducing apoptosis in human T-cell lineages.

Deficiency in beta1,3-galactosyltransferase of a Leishmania major lipophosphoglycan mutant adversely influences the Leishmania-sand fly interaction

To study the function of side chain oligosaccharides of the cell-surface lipophosphoglycan (LPG), mutagenized Leishmania major defective in side chain biosynthesis were negatively selected by agglutination with the monoclonal antibody WIC79.3, which recognizes the galactose-containing side chains of L. major LPG. One such mutant, called Spock, lacked the ability to bind significantly to midguts of the natural L. major vector, Phlebotomus papatasi, and to maintain infection in the sand fly after excretion of the digested bloodmeal. Biochemical characterization of Spock LPG revealed its structural similarity to the LPG of Leishmania donovani, a species whose inability to bind to and maintain infections in P. papatasi midguts has been strongly correlated with the expression of a surface LPG lacking galactose-terminated oligosaccharide side chains. An in vitro galactosyltransferase assay using wild-type or Spock membranes was used to determine that the defect in Spock LPG biosynthesis is a result of defective beta1,3-galactosyltransferase activity as opposed to a modification of LPG, which would prevent it from serving as a competent substrate for galactose addition. The results of these experiments show that Spock lacks the beta1, 3-galactosyltransferase for side chain addition and that the LPG side chains are required for L. major to bind to and to produce transmissible infection in P. papatasi.

Histo-blood group p: biosynthesis of globoseries glycolipids in EBV-transformed B cell lines

The genetic and biosynthetic basis of the histo-blood group P-system is not fully understood. Individuals with the rare p phenotype do not express the three glycolipid antigens (Pk, P and P1) of this system, probably because of deficiencies in glycosyltransferases involved in their biosynthesis. Iiuka et al. [Iiuka S, Chen SH, Yoshida A (1986) Biochem Biophys Res Commun 137: 1187-95], however, previously reported that detergent extracts from an EBV-transformed B cell line derived from a p individual did express the glycosyltransferase activity (Pk transferase) assumed to be missing in this blood group status. Here, we have reinvestigated the antigen expression and glycosyltransferase activities in two p individuals by analysing EBV-transformed cell lines as well as erythrocytes to confirm the blood group P status. The thin layer chromatography glycolipid profile of extracts from erythrocytes and EBV-transformed B cell lines showed characteristic accumulation of lactosylceramide and absence of Pk and P antigens. Glycosyltransferase activities of the B cell lines were analysed using glycolipid substrates and both extracts were found to contain lactosylceramide synthetase and P transferase activities but to be completely devoid of Pk transferase activity. The presented data indicate that p individuals, in contrast to previous reports, do not express a functional Pk glycosyltransferase.

Constitutively hyposialylated human T-lymphocyte clones in the Tn-syndrome: binding characteristics of plant and animal lectins

Previously, beta 1,3-galactosyltransferase-deficient (Tn+) and normal (TF+) T-lymphocyte clones have been established from a patient suffering from Tn-syndrome [Thurnher et al. (1992) Eur J Immunol 22: 1835-42]. Tn+ T lymphocytes express only Tn antigen GalNAc alpha 1-O-R) while other O-glycan structures such as sialosyl-Tn (Neu5Ac alpha 2,6GalNAc alpha 1-O-R) or TF (Gal beta 1-3GalNAc alpha 1-O-R) antigens are absent from these cells as shown by flow cytometry using specific mABs for TF and sialosyl-Tn antigen, respectively. Normal T lymphocytes express the TF antigen and derivatives thereof. The surface glycans of Tn+ and TF+ cells were then analysed by flow cytometry using the following sialic acid-binding lectins: Amaranthus caudatus (ACA), Maackia amurensis (MAA), Limax flavus (LFA), Sambucus nigra (SNA) and Triticum vulgare (WGA). Equal and weak binding of MAA and SNA to both TF+ and Tn+ cells was found. WGA, LFA and ACA bound more strongly to TF+ cells than to Tn+ cells. Binding of ACA to TF+ cells was enhanced after sialidase treatment. To investigate the possible biological consequences of hyposialylation, binding of three sialic acid-dependent adhesion molecules to Tn+ and TF+ cells was estimated using radiolabelled Fc-chimeras of sialoadhesin (Sn), myelin-associated glycoprotein (MAG) and CD22. Equal and strong binding of human CD22 to both TF+ and Tn+ cells was found. Whereas binding of Sn and MAG to TF+ cells was strong (100%), binding to Tn+ cells amounted only to 33% (Sn) and 19% (MAG). These results indicate that the in vivo interactions of T lymphocytes in the Tn syndrome with CD22 are not likely to be affected, whereas adhesion mediated by Sn or MAG could be strongly reduced.

Flow cytometric analysis of T and Tn epitopes on chronic lymphocytic leukemia cells

Immunophenotyping of peripheral blood lymphocytes from six patients with B-cell chronic lymphocytic leukemia (B-CLL) and five normal volunteers was done and their T and Tn epitopes analyzed using specific monoclonal antibodies and flow cytometry. Lymphocytes from all patients showed strong Tn expression as compared to normal control lymphocytes. By contrast, T antigen was not expressed, The TN expression may be a useful diagnostic and prognostic marker for B-CLL.

Tumor-infiltrating T lymphocytes from renal-cell carcinoma express B7-1 (CD80): T-cell expansion by T-T cell co-stimulation

B7-1 (CD80) provides co-stimulation for T-cell activation by interacting with CD28 or CTLA4. Here we demonstrate the expression of B7-1 in freshly isolated and cultured lymphocytes from renal-cell carcinoma. In fresh preparations of lymphocytes infiltrating renal-cell-carcinoma tissue, B7-1 mRNA could readily be detected by reverse transcription PCR, and 2-color flow-cytometry analysis revealed substantial B7-1 expression on T cells from these isolates. As expected, tumor-derived T cells also expressed CD28, the B7 receptor. While B7-1 expression of tumor-derived T cells was maintained during culture in interleukin-2-supplemented medium, CD28 expression was further enhanced. We also show that B7-1 is functionally involved in T-cell expansion: anti-B7-1 MAb inhibited the PHA-induced proliferation of tumor-derived B7-1+ T cells (35%) in the absence of exogenous antigen-presenting cells, indicating that B7-1 mediates T-T cell co-stimulation (self-co-stimulation). Our data demonstrate that T cells infiltrating renal-cell carcinoma express B7-1, and that mutual co-stimulation via the B7-1/CD28 pathway contributes to the interleukin-2-driven expansion of tumor-derived T cells in vitro. The frequency of B7-1+ T cells in tumor lesions and the level of B7-1 on these cells may determine the time course of T-cell expansion in vivo. Self-co-stimulation, however, might also represent one mechanism leading to the state of suppression or anergy characteristic of tumor-infiltrating lymphocytes.

Expression of the T antigen on a T-lymphoid cell line, supT1

We have measured glycosyltransferase activities of SupT1 cells, a T-lymphoid cell line shown to react with autoantibodies in the sera of many HIV patients. Since considerable alpha-N-acetylgalactosaminyl-transferase and beta 1, 3 galactosyltransferase activities were found in SupT1 cells, at least the O-glycan core 1 structure can probably be synthesized. FACS analysis using an anti-T monoclonal antibody showed expression of the T antigen (Gal beta 1-3 GalNAc). Glycoproteins with the T antigen were isolated by immunoprecipitation with the anti-T antibody from a SupT1 cell lysate labelled metabolically with 3H-glucosamine and then analysed by SDS-PAGE. It was revealed that the precipitate contained a glycoprotein with a molecular weight corresponding to that of leukosialin. O-glycans were prepared from the immunoprecipitate by alkaline-borohydride treatment and then fractionated on Bio-Gel P-2, GalNAcOH and Gal-GalNAcOH being identified inter alia. These results suggest that an anti-T antibody may be included in the autoantibodies found in HIV-1 infected individuals.