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

Intact reducing glycan promotes the specific immune response to lacto-N-neotetraose-BSA neoglycoconjugates

The mammalian immune system responds to eukaryotic glycan antigens during infections, cancer, and autoimmune disorders, but the immunological bases for such responses are unclear. Conjugate vaccines containing bacterial polysaccharides linked to carrier proteins (neoglycoconjugates) have proven successful, but these often contain repeating epitopes and the reducing end of the glycan is less important, unlike typical glycan determinants in eukaryotes, which are shorter in length and may include the reducing end. Here, we have compared the effects of two linkage methods, one that opens the ring at the reducing end of the glycan, and one that leaves the reducing end closed, on the glycan specificity of the vaccine response in rabbits and mice. We immunized rabbits and mice with bovine serum albumin (BSA) conjugates of synthetic open- and closed-ring forms (OR versus CR) of a simple tetrasaccharide lacto-N-neotetraose (LNnT, Galβ1-4GlcNAcβ1-3Galβ1-4Glc), and tested reactivity to the immunogens and several related glycans in both OR and CR versions on glycan microarrays. We found that in rabbits the immune response to the CR conjugate was directed toward the glycan, whereas the OR conjugate elicited antibodies to the reducing end of the glycan and linker region but not specifically to the glycan itself. Unexpectedly, mice did not generate a glycan-specific response to the CR conjugate. Our findings indicate that the reducing end of the sugar is crucial for generation of a glycan-specific response to some eukaryotic vaccine epitopes, and that there are species-specific differences in the ability to make a glycan-specific response to some glycoconjugates. These findings warrant further investigation with regard to rational design of glycoconjugate vaccines.

On the dark side of therapies with immunoglobulin concentrates: the adverse events

Therapy by human immunoglobulin G (IgG) concentrates is a success story ongoing for decades with an ever increasing demand for this plasma product. The success of IgG concentrates on a clinical level is documented by the slowly increasing number of registered indication and the more rapid increase of the off-label uses, a topic dealt with in another contribution to this special issue of Frontiers in Immunology. A part of the success is the adverse event (AE) profile of IgG concentrates which is, even at life-long need for therapy, excellent. Transmission of pathogens in the last decade could be entirely controlled through the antecedent introduction by authorities of a regulatory network and installing quality standards by the plasma fractionation industry. The cornerstone of the regulatory network is current good manufacturing practice. Non-infectious AEs occur rarely and mainly are mild to moderate. However, in recent times, the increase in frequency of hemolytic and thrombotic AEs raised worrying questions on the possible background for these AEs. Below, we review elements of non-infectious AEs, and particularly focus on hemolysis and thrombosis. We discuss how the introduction of plasma fractionation by ion-exchange chromatography and polishing by immunoaffinity chromatographic steps might alter repertoire of specificities and influence AE profiles and efficacy of IgG concentrates.

The glycosphingolipid P₁ is an ovarian cancer-associated carbohydrate antigen involved in migration

Background:

The level of plasma-derived naturally circulating anti-glycan antibodies (AGA) to P₁ trisaccharide has previously been shown to significantly discriminate between ovarian cancer patients and healthy women. Here we aim to identify the Ig class that causes this discrimination, to identify on cancer cells the corresponding P₁ antigen recognised by circulating anti-P₁ antibodies and to shed light into the possible function of this glycosphingolipid.

Methods:

An independent Australian cohort was assessed for the presence of anti-P₁ IgG and IgM class antibodies using suspension array. Monoclonal and human derived anti-glycan antibodies were verified using three independent glycan-based immunoassays and flow cytometry-based inhibition assay. The P₁ antigen was detected by LC-MS/MS and flow cytometry. FACS-sorted cell lines were studied on the cellular migration by colorimetric assay and real-time measurement using xCELLigence system.

Results:

Here we show in a second independent cohort (n=155) that the discrimination of cancer patients is mediated by the IgM class of anti-P₁ antibodies (P=0.0002). The presence of corresponding antigen P₁ and structurally related epitopes in fresh tissue specimens and cultured cancer cells is demonstrated. We further link the antibody and antigen (P₁) by showing that human naturally circulating and affinity-purified anti-P₁ IgM isolated from patients ascites can bind to naturally expressed P₁ on the cell surface of ovarian cancer cells. Cell-sorted IGROV1 was used to obtain two study subpopulations (P₁-high, 66.1% and P₁-low, 33.3%) and observed that cells expressing high P₁-levels migrate significantly faster than those with low P₁-levels.

Conclusions:

This is the first report showing that P₁ antigen, known to be expressed on erythrocytes only, is also present on ovarian cancer cells. This suggests that P₁ is a novel tumour-associated carbohydrate antigen recognised by the immune system in patients and may have a role in cell migration. The clinical value of our data may be both diagnostic and prognostic; patients with low anti-P₁ IgM antibodies present with a more aggressive phenotype and earlier relapse.

Structures of complexes of a metal-independent glycosyltransferase GT6 from Bacteroides ovatus with UDP-N-acetylgalactosamine (UDP-GalNAc) and its hydrolysis products

Mammalian members of glycosyltransferase family 6 (GT6) of the CAZy database have a GT-A fold containing a conserved Asp-X-Asp (DXD) sequence that binds an essential metal cofactor. Bacteroides ovatus GT6a represents a GT6 clade found in more than 30 Gram-negative bacteria that is similar in sequence to the catalytic domains of mammalian GT6, but has an Asn(95)-Ala-Asn(97) (NXN) sequence substituted for the DXD motif and metal-independent catalytic activity. Co-crystals of a low activity mutant of BoGT6a (E192Q) with UDP-GalNAc contained protein complexes with intact UDP-GalNAc and two forms with hydrolysis products (UDP plus GalNAc) representing an initial closed complex and later open form primed for product release. Two cationic residues near the C terminus of BoGT6a, Lys(231) and Arg(243), interact with the diphosphate moiety of UDP-GalNAc, but only Lys(231) interacts with the UDP product and may function in leaving group stabilization. The amide group of Asn(95), the first Asn of the NXN motif, interacts with the ribose moiety of the substrate. This metal-independent GT6 resembles its metal-dependent homologs in undergoing conformational changes on binding UDP-GalNAc that arise from structuring the C terminus to cover this substrate. It appears that in the GT6 family, the metal cofactor functions specifically in binding the UDP moiety in the donor substrate and transition state, actions that can be efficiently performed by components of the polypeptide chain.

Natural antibodies to glycans

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

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.

High-throughput profiling of anti-glycan humoral responses to SIV vaccination and challenge

Recent progress toward an HIV vaccine highlights both the potential of vaccines to end the AIDS pandemic and the need to boost efficacy by incorporating additional vaccine strategies. Although many aspects of the immune response can contribute to vaccine efficacy, the key factors have not been defined fully yet. A particular area that may yield new insights is anti-glycan immune responses, such as those against the glycan shield that HIV uses to evade the immune system. In this study, we used glycan microarray technology to evaluate anti-glycan antibody responses induced by SIV vaccination and infection in a non-human primate model of HIV infection. This comprehensive profiling of circulating anti-glycan antibodies found changes in anti-glycan antibody levels after both vaccination with the Ad5hr-SIV vaccine and SIV infection. Notably, SIV infection produced generalized declines in anti-glycan IgM antibodies in a number of animals. Additionally, some infected animals generated antibodies to the Tn antigen, which is a cryptic tumor-associated antigen exposed by premature termination of O-linked glycans; however, the Ad5hr-SIV vaccine did not induce anti-Tn IgG antibodies. Overall, this study demonstrates the potential contributions that glycan microarrays can make for HIV vaccine development.

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

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

Carbohydrate microarrays

In the last decade, carbohydrate microarrays have been core technologies for analyzing carbohydrate-mediated recognition events in a high-throughput fashion. A number of methods have been exploited for immobilizing glycans on the solid surface in a microarray format. This microarray-based technology has been widely employed for rapid analysis of the glycan binding properties of lectins and antibodies, the quantitative measurements of glycan-protein interactions, detection of cells and pathogens, identification of disease-related anti-glycan antibodies for diagnosis, and fast assessment of substrate specificities of glycosyltransferases. This review covers the construction of carbohydrate microarrays, detection methods of carbohydrate microarrays and their applications in biological and biomedical research.

Development of a convenient competitive ELISA for the detection of the free and protein-bound nonhuman galactosyl-α-(1,3)-galactose epitope based on highly specific chicken single-chain antibody variable-region fragments

The presence of the nonhuman galactosyl-α-(1,3)-galactose (Gal-α-(1,3)-Gal) carbohydrate epitope on a number of recombinant therapeutic proteins has recently been reported, renewing interest in this immunogenic carbohydrate epitope. It is well-known that this motif is the primary contributing factor in hyperacute rejection of porcine organ xenograft, due to the existence of natural antibodies against this epitope in human serum. Though the number of epitopes on recombinant glycoproteins may be low when compared directly to whole tissue, circulating anti-Gal-α-R immunoglobulins can still induce anaphylaxis. Therefore, there is a need for rapid and convenient methods for detection and monitoring of this epitope in biopharmaceuticals produced in recombinant mammalian systems. To this end, we have generated immune-challenged chicken single-chain antibody variable-region fragment (scFv) libraries targeting the Gal-α-(1,3)-Gal motif and have selected a panel of scFv's that bind the target. We have used one of these antibodies to develop a competitive ELISA for both free and protein-bound Gal-α-(1,3)-Gal and have demonstrated that the ELISA is specific for the target and can be used to determine the loading of the target on glycoproteins. This competitive ELISA will provide a convenient method of detecting and quantifying Gal-α-(1,3)-Gal on therapeutic glycoproteins.

Tumor-associated glycans and their role in gynecological cancers: accelerating translational research by novel high-throughput approaches

Glycans are important partners in many biological processes, including carcinogenesis. The rapidly developing field of functional glycomics becomes one of the frontiers of biology and biomedicine. Aberrant glycosylation of proteins and lipids occurs commonly during malignant transformation and leads to the expression of specific tumor-associated glycans. The appearance of aberrant glycans on carcinoma cells is typically associated with grade, invasion, metastasis and overall poor prognosis. Cancer-associated carbohydrates are mostly located on the surface of cancer cells and are therefore potential diagnostic biomarkers. Currently, there is increasing interest in cancer-associated aberrant glycosylation, with growing numbers of characteristic cancer targets being detected every day. Breast and ovarian cancer are the most common and lethal malignancies in women, respectively, and potential glycan biomarkers hold promise for early detection and targeted therapies. However, the acceleration of research and comprehensive multi-target investigation of cancer-specific glycans could only be successfully achieved with the help of a combination of novel high-throughput glycomic approaches.