Molecular mimicry of carbohydrate and protein structures by hybridoma antibodies

The requirement of ammonium or other cations linked with p-cresol sulfate for cross-reactivity with a peptide of myelin basic protein

Urinary myelin basic protein-like material (MBPLM), so designated because of its immunoreactivity with a polyclonal antibody directed against a cryptic epitope located in residues 83-89 of myelin basic protein (MBP), exists in humans normally but increases in concentration in patients with multiple sclerosis who have progressive disease. Given its possible role in reflecting events of neural tissue destruction occurring in multiple sclerosis, urinary MBPLM is a candidate surrogate marker for this phase of the disease. Previously, it has been demonstrated that p-cresol sulfate (PCS) is the dominant component of MBPLM; however, another component(s) was essential in enabling p-cresol sulfate to have molecular mimicry with MBP peptide 83-89 detected by immunoreactivity. In the present investigation, this remaining component(s) was characterized by a combination of high performance size exclusion chromatography followed by nuclear magnetic resonance spectroscopy and shown to be ammonium. The monovalent cation ammonium could be substituted in vitro by several different monovalent and divalent cations, most notably zinc, in restoring to deprotonated p-cresol sulfate its immunoreactivity as MBPLM. These findings indicate the basis for the unexpected molecular mimicry between an epitope of an encephalitogenic protein and a complex containing a small organic molecule, p-cresol sulfate. Furthermore, the reaction of either ammonium or other cations with p-cresol sulfate may represent an in vivo process directly related to damage of axonal membranes.

Inhibition of Escherichia coli heat-labile enterotoxin by neoglycoprotein and anti-lectin antibodies which mimic GM1 receptor

Escherichia coli producing heat-labile enterotoxin is responsible for numerous cases of diarrhea worldwide, leading to considerable morbidity and mortality. The B subunits of this toxin are responsible for the binding to the receptor, the complex ganglioside GM1 which has galactose as its terminal sugar. In this study we showed that analogs of galactose (gal) and N-acetylgalactosamine (GalNAc) interfere with the binding of heat-labile toxin to GM1. Antibodies to lectins which mimic sugar structures and neoglycoprotein were employed. These compounds were able to inhibit heat-labile toxin activity efficiently in Vero cells: 37 microg of IgG-enriched fraction from an antiserum inhibited up to 70% of this activity, and 50% of the binding of heat-labile toxin to GM1. Neoglycoprotein was more efficient than antibodies, since 2.5 microg of this ligand completely abolished the activity of heat-labile toxin on Vero cells. These data suggest that these molecules could be developed for prophylaxis and diagnosis of diarrhea caused by heat-labile toxin.

Towards the development of peptide mimotopes of carbohydrate antigens as cancer vaccines

Tumor-associated carbohydrate antigens are considered important targets in efforts to develop cancer vaccines. To further enhance vaccine efforts, we are developing peptide mimotopes of tumor-associated carbohydrate antigens that can elicit functional immune responses. Mapping peptide epitopes with anticarbohydrate antibodies can lend to defining structural relationships that can go undetected by screening of carbohydrate antigens alone. Here we contrast reactivity patterns for peptides using monoclonal antibodies (MAbs) directed to the neolactoseries related Lewis Y (LeY) and sialyl-Lewis X (sLeX) antigen and the GD3/GD2 ganglioside antigen. We observe that representative MAbs cross-react with a WRY-containing peptide and that this motif type is isolated by the respective monoclonal in peptide phage display screening. Primary immunization with multiple antigen peptide preparations with QS-21 adjuvant efficiently elicited cytotoxic IgM antibodies for a murine Meth A fibrosarcoma line expressing sLeX. The cytotoxicity of IgG polyclonal response was found to be as effective as IgM in mediating complement-dependent cytotoxicity against the Meth A line. These experiments suggest that peptide mimotopes of the LeY and sLeX tumor-associated carbohydrate antigen and QS-21 adjuvant could be considered as an immunogenic therapeutic vaccine in carcinoma and melanoma patients in the minimal residual disease setting.

Antigenic and immunological mimicry of peptide mimotopes of Lewis carbohydrate antigens

Peptides may substitute for carbohydrates in reactions with carbohydrate-specific molecules. Recently, we found that peptides containing aromatic residues mimic mucin and histo-blood group related carbohydrate epitopes, eliciting polyclonal responses cross-reactive with bacterial and viral antigens that express these carbohydrate forms. These results demonstrate that peptides can function in in vivo and in vitro models as carbohydrate surrogate antigens. To further explore the nature of the antigenic and immunogenic properties of such mimotopes, synthetic peptides with aromatic amino acids were tested to delineate reactivity patterns with several anti-neolactoseries monoclonal antibodies (MAbs). These MAbs recognize biologically important conformations of the histo-blood group related Lewis antigens expressed on the surface of a variety of human cancers. Results by ELISA demonstrate that the MAbs can distinguish particular peptide motifs that include the sequences GGIYYPYDIYYPYDIYYPYD, GGIYWRYDIYWRYDIYWRYD and GGIYYRYDIYYRYDIYYRYD. Substitution of Arg by Pro diminished the reactivity of the anti-Lewis Y (LeY) MAb BR55-2. Binding of LeY to BR55-2 was inhibitable by the Arg containing peptides. Serum against all three peptides displayed reactivity with synthetic histo-blood group related antigen probes. Immunologic presentation of the peptides as multiple antigen peptides (MAPs) improved peptide ability to induce LeY specific immune responses. Serum bound to human tumor cells that preferentially expressed neolactoseries antigens, but not to normal tissues. Immunoprecipitation of human breast tumor cell lysates before and after treatment with tunicamycin confirmed serum carbohydrate binding. The anti-peptide sera mediated tumor cell killing by complement mediated cytotoxicity. These results indicate that mapping peptide epitopes with anti-carbohydrate antibodies can lend to defining antibody fine specificities that can go undetected by screening of carbohydrate antigens alone. In addition, these results confirm that peptides and carbohydrates can bind to the same antibody binding site and that peptides can structurally mimic salient features of carbohydrate epitopes.

Parasite glycoconjugates: towards the exploitation of their structure

The parasitic protozoa express many unusual complex carbohydrates at the cell surface in the form of glycoproteins and glycophospholipids. In several cases, such molecules have been shown to be involved in parasite survival, infectivity and host-cell recognition. The carbohydrate chains of these glycoconjugates are often highly immunogenic, and can in some cases elicit protective immunity. The immunogenicity of some parasite glycans is a function of their unusual chemical structure as compared with mammalian glycans. This suggests differences in the glycosylation pathways between host and parasite due to their evolutionary distance. This article describes how a combination of biophysical and biochemical techniques can be used to determine the primary and three-dimensional structures of parasite carbohydrate and how this information might be exploited towards the development of new selective chemotherapeutic agents and synthetic vaccines.