Fernando J. Irazoqui

The Antineoplastic Lectin of the Common Edible Mushroom (Agaricus bisporus) Has Two Binding Sites, Each Specific for a Different Configuration at a Single Epimeric Hydroxyl

The lectin from the common mushroom Agaricus bisporus, the most popular edible species in Western countries, has potent antiproliferative effects on human epithelial cancer cells, without any apparent cytotoxicity. This property confers to it an important therapeutic potential as an antineoplastic agent. The three-dimensional structure of the lectin was determined by x-ray diffraction. The protein is a tetramer with 222 symmetry, and each monomer presents a novel fold with two β sheets connected by a helix-loop-helix motif. Selectivity was studied by examining the binding of four monosaccharides and seven disaccharides in two different crystal forms. The T-antigen disaccharide, Galβ1–3GalNAc, mediator of the antiproliferative effects of the protein, binds at a shallow depression on the surface of the molecule. The binding of N-acetylgalactosamine overlaps with that moiety of the T antigen, but surprisingly, N-acetylglucosamine, which differs from N-acetylgalactosamine only in the configuration of epimeric hydroxyl 4, binds at a totally different site on the opposite side of the helix-loop-helix motif. The lectin thus has two distinct binding sites per monomer that recognize the different configuration of a single epimeric hydroxyl. The structure of the protein and its two carbohydrate-binding sites are described in detail in this study.

Agaricus bisporus lectin binds mainly O-glycans but also N-glycans of human IgA subclasses.

The primary interaction between purified Agaricus bisporus lectin (ABL) and human IgA subclasses was studied by ABL-affinity chromatography, dot blot assay and competitive enzyme-lectin assay, considering that ABL could be an alternative tool for detection of IgA1 O-glycans. Both secretory IgA subclasses bound to ABL-Sepharose and the IgA2 subclass (which contains only N-glycans) was recovered with a high degree of purity when NH4OH was used as eluent. ABL-Ig interaction was also observed by dot blot assays using ABL-peroxidase against monoclonal IgA1 k Pan, IgA2m(1)k Gir, IgA2m(2)k Bel, secretory IgA2 and normal IgG (also contains only N-glycans). When these immunoglobulins were enzymatically treated with peptide N-glycosidase F (N-glycan hydrolysis), the ABL-IgA2 and -IgG interaction did not occur while IgA1 maintained a high degree of interaction with ABL. Also, the ABL-IgA interaction was observed by competitive enzyme-lectin assay, and when IgA1 subclass was treated with endo-α-N-acetylgalactosaminidase for O-glycans hydrolysis, the reactivity with ABL was very low. We conclude that the complementary use of ABL and peptide N-glycosidase F could be a useful tool to assess the O-glycosylation state of human IgA1 subclass, which is of relevant importance in the effector functions of immunoglobulins. Abbreviations: ABL, Agaricus bisporus lectin; α1 and α2, heavy chains from human IgA1 and IgA2; C1-3α, constant domains (1–3) of heavy chains from human IgA; ECL, Erythrina cristagalli lectin; EEO, electroendoosmosis; EIA, enzyme immunoassay; ELA, enzyme lectin assay; Ig, immunoglobulin; HRP, horseradish peroxidase; ID50, 50% inhibitory dose; Ka, affinity constant; O.D., optical density; PBS, phosphate buffered saline; PBS-t, phosphate buffered saline with Tween 20; PNGase F, peptide N-glycosidase F; RID, radial immunodiffusion; SD, standard deviation; TBS, Tris-HCL buffered saline; TBS-t, Tris-HCI buffered saline with Tween 20; T-disaccharide, Thomsen-Friedenreich disaccharide

Normal human plasma contains antibodies that specifically block neuropathy-associated human anti-GM1 IgG-antibodies

Intravenous immunoglobulin (IVIg) is used in the treatment of a variety of autoimmune diseases. The blocking of disease-associated antibodies by anti-idiotype antibodies present in IVIg has been proposed as an action mechanism. Anti-GM1 antibodies have been implicated in motor neuropathies. Although IVIg is frequently applied for these diseases, the presence in IVIg or in human plasma of anti-idiotype antibodies that recognize anti-GM1 antibodies has not been clearly demonstrated. Here we present evidence that normal human plasma contains antibodies that inhibit the binding of anti-GM1 IgG-antibodies from neuropathy patients but do not inhibit anti-GM1 IgG-antibodies of rabbit origin with the same fine specificity. The significance of these findings in the course of acute and chronic neuropathies is discussed.

Variable patterns of anti-GM1 IgM-antibody populations defined by affinity and fine specificity in patients with motor syndromes: evidence for their random origin

Elevated titers of serum antibodies against GM(1)-ganglioside are associated with a variety of autoimmune neuropathies. Although much evidence indicates that these autoantibodies play a primary role in the disease processes, the mechanism of their appearance is unclear. Low-affinity anti-GM(1) antibodies of the IgM isotype are part of the normal human immunological repertoire. In patients with motor syndromes, we found that in addition to the usual anti-GM(1) antibodies, the sera contain IgM-antibodies that recognize GM(1) with higher affinity and/or different specificity. This latter type of antibodies was not detected in other autoimmune diseases. We studied the fine specificity of both normal and motor disease-associated antibodies using HPTLC-immunostaining of GM(1) and structurally related glycolipids, soluble antigen binding inhibition, and GM(1) affinity columns. Normal low-affinity anti-GM(1) antibodies cross-react with GA(1) and/or GD(1b). In the motor syndrome patients, different populations of antibodies characterized by their affinity and cross-reactivity were detected. Although one population is relatively common (low affinity, not cross-reacting with GA(1) and GD(1b)), there are remarkably few sera having the same set of populations. These results suggest that the appearance of the new antibody populations is a random process. When the different antibody populations were analyzed in relation to the three-dimensional structure of GM(1), a restricted area of the GM(1) oligosaccharide (the terminal Galbeta1-3GalNAc) was found to be involved in binding of normal anti-GM(1) antibodies. Patient antibodies recognize slightly different areas, including additional regions of the GM(1) molecule such as the NeuNAc residue. We hypothesize that disease-associated antibodies may originate by spontaneous mutation of normal occurring antibodies.

Anti-GM1 IgG Antibodies In Guillain-BarrÉ Syndrome: Fine Specificity is Associated with Disease Severity

Background Clinical severity of Guillain–Barré syndrome (GBS) is highly variable, but the immunopathological reason is unknown. Objective The study was designed to show which antibody parameters are associated with disease severity in GBS patients with serum anti-GM1 IgG antibodies. Methods Thirty-four GBS patients with anti-GM1 IgG antibodies were grouped into two categories according to disease severity at nadir: mild (grades 1–3 by Hughes functional scale, n=13) and severe (grades 4 and 5, n=21). Titre, affinity, fine specificity and cell binding of anti-GM1 antibodies were obtained and compared between the two groups. Results No differences in antibody titre (GM1-ELISA) or affinity were found between the two patient groups. In contrast, the severe group showed a significantly higher frequency (95%, vs 46% in the mild group, p=0.002) of specific (not cross-reacting with GD1b) anti-GM1 antibodies. In addition, the severe group also exhibited a higher antibody binding titre to cellular GM1. Conclusions Differences in fine specificity of antibodies are strong indications that different regions of the GM1-oligosaccharide are involved in antibody binding. High titres of specific anti-GM1 antibody binding to cellular GM1 can be explained by antigen exposure, that is, GM1 exposes or forms mainly epitopes recognised by specific antibodies, and ‘hides’ those involved in binding of cross-reacting antibodies. Thus, the fine specificity of anti-GM1 antibodies may influence disease severity by affecting antibody binding to cellular targets. Additionally, since antibody specificity studies are relatively easy to implement, fine specificity could be considered a useful predictor of disease severity.

Thomsen-Friedenreich Disaccharide Immunogenicity

Mucin-type O-glycans are upregulated and aberrantly glycosylated in many carcinomas. O-glycan Core 1 (Galbeta1-3GalNAcalpha-O-), also called Thomsen-Friedenreich disaccharide, is a cryptic structure overexpressed in cancer cells through modification of its glycosyltransferase profile. This molecule is a useful model for study of carbohydrate immunogenicity as well as a candidate for active specific immunotherapy of cancer patients. Several strategies are discussed for enhancing immune response to a particular region of carbohydrate: carbohydrate-protein conjugation, linkers, synthetic clustered sugars, chemical modifications, peptide/protein mimetics, and molecular rotation.

Differential reactivity of Agaricus bisporus lectin with human IgA subclasses in gel precipitation

The interaction between purified Agaricus bisporus lectin and several human proteins was studied using the Ouchterlony double diffusion and immunoelectrophoresis techniques. Only one precipitation line was observed with normal human serum, normal human colostrum, IgA1 myeloma serum, both serum monoclonal and secretory IgA1 and monoclonal IgD. No reaction was observed with monoclonal and secretory IgA2, IgG, IgM, alpha 2 macroglobulin or pregnancy-associated alpha 2 glycoprotein. These results were confirmed by hemagglutination inhibition assays when IgA1, IgA2 and IgD were tested. On the basis of this reactivity, ABL could be a useful tool for distinguishing and isolating human IgA subclasses.

The origin of anti-GM1 antibodies in neuropathies: the "binding site drift" hypothesis.

Elevated titers of serum antibodies against GM1-ganglioside are associated with a variety of autoimmune neuropathies. The origin of these autoantibodies is still unknown, although there is evidence that they are produced by CD5+ B-lymphocytes and that antigen mimicry is involved. Anti-GM1 IgM-antibodies in the normal human immunological repertoire are low affinity antibodies that cross-react with other glycoconjugates carrying Galβ1-3GalNAc and probably do not have GM1-mediated biological activity. Other anti-GM1 IgM-antibodies with higher affinity and/or different fine specificity are present in patients with motor syndromes. Based on our studies of structural requirement for binding, we hypothesize that disease-associated anti-GM1 antibodies originate at random by mutations affecting the binding site of naturally-occurring ones. The hypothesis is conceptually similar to the established phenomenon of “genetic drift” in species evolutionary biology and is therefore termed “binding site drift”.

Validation of a rabbit model of neuropathy induced by immunization with gangliosides

The induction of neurological signs by immunization of rabbits with gangliosides has been a controversial topic for many years. Recently, Yuki et al. [N. Yuki, M. Yamada, M. Koga, M. Odaka, K. Susuki, Y. Tagawa, et al. Animal model of axonal Guillain-Barré syndrome induced by sensitization with GM1 ganglioside. Ann Neurol 2001;49:712-720.] described an immunization protocol, including keyhole lympet hemocyanin in addition to ganglioside that induced a neurological disease resembling human Guillain-Barré syndrome. We employed this protocol in our laboratory and succeeded in reproducing the disease. Five different experiments were performed during a period of two years by different operators, using different batches of drugs, in a total of 26 rabbits. Despite minor variations in onset time and severity of the induced disease, the model proved to be reproducible. Both gangliosides and keyhole limpet hemocyanin are required for induction of disease.

Immune response to Thomsen-Friedenreich disaccharide and glycan engineering.

Cancer‐associated mucins show frequent alterations of their oligosaccharide chain profile, with a switch to unmask normally cryptic O‐glycan backbone and core regions. Epithelial tumour cells typically show overexpression of the uncovered Galβ1‐3GalNAcα‐O‐Ser/Thr (Core 1) structure, known as the T antigen or the Thomsen–Friedenreich antigen, the oligosaccharide chain of which is called the Thomsen–Friedenreich disaccharide (TFD). T antigen expression has been associated with immunosuppression, metastasis dissemination, and the proliferation of cancer cells. Several different strategies have been used to trigger a specific immune response to TFD. Natural T antigen and synthetic TFD residues have low immunodominance. In the T antigen, flexibility of the glycosidic bond reduces the immunogenicity of the sugar residue. Enhanced rigidity should favour certain glycan conformations and thereby improve TFD immunotargeting. We propose the term ‘glycan engineering’ for this approach. Such engineering of TFD should reduce the flexibility of its glycan moiety and thereby enhance its stability, rigidity and immunogenicity.