Immunochemical studies of conjugates of isomaltosyl oligosaccharides to lipid. I. Antigenicity of the glycolipids and the production of specific antibodies in rabbits

Polyclonal human antibodies against glycans bearing red meat-derived non-human sialic acid N-glycolylneuraminic acid are stable, reproducible, complex and vary between individuals: Total antibody levels are associated with colorectal cancer risk

BACKGROUND

N-glycolylneuraminic acid (Neu5Gc) is a non-human red-meat-derived sialic acid immunogenic to humans. Neu5Gc can be metabolically incorporated into glycan chains on human endothelial and epithelial surfaces. This represents the first example of a "xeno-autoantigen", against which circulating human "xeno-autoantibodies" can react. The resulting inflammation ("xenosialitis") has been demonstrated in human-like Neu5Gc-deficient mice and contributed to carcinoma progression via antibody-mediated inflammation. Anti-Neu5Gc antibodies have potential as biomarkers for diseases associated with red meat consumption such as carcinomas, atherosclerosis, and type 2 diabetes.

METHODS

ELISA assays measured antibodies against Neu5Gc or Neu5Gc-glycans in plasma or serum samples from the Nurses' Health Studies, the Health Professionals Follow-up Study, and the European Prospective Investigation into Cancer and Nutrition, including inter-assay reproducibility, stability with delayed sample processing, and within-person reproducibility over 1-3 years in archived samples. We also assessed associations between antibody levels and coronary artery disease risk (CAD) or red meat intake. A glycan microarray was used to detected antibodies against multiple Neu5Gc-glycan epitopes. A nested case-control study design assessed the association between total anti-Neu5Gc antibodies detected in the glycan array assay and the risk of colorectal cancer (CRC).

RESULTS

ELISA assays showed a wide range of anti-Neu5Gc responses and good inter-assay reproducibility, stability with delayed sample processing, and within-person reproducibility over time, but these antibody levels did not correlate with CAD risk or red meat intake. Antibodies against Neu5Gc alone or against individual Neu5Gc-bearing epitopes were also not associated with colorectal cancer (CRC) risk. However, a sialoglycan microarray study demonstrated positive association with CRC risk when the total antibody responses against all Neu5Gc-glycans were combined. Individuals in the top quartile of total anti-Neu5Gc IgG antibody concentrations had nearly three times the risk compared to those in the bottom quartile (Multivariate Odds Ratio comparing top to bottom quartile: 2.98, 95% CI: 0.80, 11.1; P for trend = 0.02).

CONCLUSIONS

Further work harnessing the utility of these anti-Neu5Gc antibodies as biomarkers in red meat-associated diseases must consider diversity in individual antibody profiles against different Neu5Gc-bearing glycans. Traditional ELISA assays for antibodies directed against Neu5Gc alone, or against specific Neu5Gc-glycans may not be adequate to define risk associations. Our finding of a positive association of total anti-Neu5Gc antibodies with CRC risk also warrants confirmation in larger prospective studies.

Carbohydrate Microarrays

Carbohydrates, like nucleic acids and proteins, are essential biological molecules. Owing to their intrinsic physicochemical properties, carbohydrates are capable of generating structural diversity in a multitude of ways and are prominently displayed on the surfaces of cell membranes or on the exposed regions of macromolecules. Recent studies highlight that carbohydrate moieties are critical for molecular recognition, cell-cell interactions, and cell signaling in many physiological and pathological processes, and for biocommunication between microbes and host species. Modern carbohydrate microarrays emerged in 2002 and brought in new high-throughput tools for “glyco code” exploration. In this section, some basic concepts of sugar chain diversity, glyco-epitope recognition, and the evolving area of glyco-epitomics and biomarker discovery are discussed. Two complementary technologies, carbohydrate antigen arrays and photogenerated glyco-chips, serve as models to illustrate how to apply carbohydrate microarrays to address biomedical questions.

New synthetic amphiphilic polymers for steric protection of liposomes in vivo

Carboxy group-terminated synthetic polymers--branched poly(ethylene glycol), poly(acryloylmorpholine), and poly(vinylpyrrolidone)--were made amphiphilic by derivatization with phosphatidyl ethanolamine via the terminal carboxy group and then incorporated into lecithin-cholesterol liposomes prepared by the detergent dialysis method. Following the biodistribution of liposomes in mice, all three polymers were shown to be effective steric protectors for liposomes and were able to sharply increase liposome circulation times in a concentration-dependent manner. The accumulation of liposomes in the liver decreases. The effects observed are similar to those found for liposomes modified with linear poly(ethylene glycol). At low polymer concentration, amphiphilic branched poly(ethylene glycol) seems to be the most effective liposome protector, most probably, because at the same molar content of anchoring groups, each attachment point carries two polymeric chains and doubles the quantity of liposome-grafted polymer comparing to linear poly(ethylene glycol).

Controlled delivery of Gd-containing liposomes to lymph nodes: surface modification may enhance MRI contrast properties

Surface modification of liposomes containing Gd-diethylenetriaminepentaacetyl-phosphatidylethanolamine (Gd-DTPA-PE) as an amphiphilic paramagnetic label influences their contrast properties as MRI agents. Liposomes with the mean diameter ca. 220 nm and 10% mol content of Gd-DTPA-PE were modified with dextran (molecular weight 6 kDa) or polyethyleneglycol (PEG, molecular weight 5 kDa). The preparations obtained were used for MR visualization of axillary and subscapular lymph nodes after SC injection into rabbit's paw. Dextran-modified liposomes were found to have limited or no effect on lymph node/muscle MR signal intensity ratio during the first 2 h after the administration when compared to nonmodified plain liposomes, whereas the liposomes modified with PEG demonstrated a 3-3.5-fold enhancement of lymph node signal. However, the partial biodistribution studies with the 111In-labeled liposomes revealed that PEG-modified vesicles accumulated in the axillary lymph node at half the amount comparing with nonmodified and dextran-modified liposomes. The phenomenon found might be explained by noticeably increased relaxivity of PEG-modified Gd-liposomes.

Amphiphilic vinyl polymers effectively prolong liposome circulation time in vivo

Newly synthesized amphiphilic polyacrylamide and poly(vinyl pyrrolidone), single terminus-modified with long-chain fatty acyl groups, are able to incorporate into the liposomal membrane, and similar to poly(ethylene glycol) prolong liposome circulation in vivo and decrease liposome accumulation in the liver. Protective efficacy of modified polymers increases with the increase in the length of acyl moiety and decreases for higher molecular weight polymers. The data on amphiphilic polymer-modified liposome biodistribution are presented.

Poly(ethylene glycol) on the liposome surface: on the mechanism of polymer-coated liposome longevity

The hypothetical model is built explaining the molecular mechanism of protective action of poly(ethylene glycol) on liposomes in vivo. The protective layer of the polymer on the liposome surface is considered as a statistical 'cloud' of polymer possible conformations in solution. Computer simulation was used to demonstrate that relatively a small number of liposome-grafted molecules of hydrophilic and flexible polymer can create a dense protective conformational cloud over the liposome surface preventing opsonizing protein molecules from contacting liposome. A more rigid polymer fails to form this dense protective cloud, even when hydrophilic. Computer simulation was also used to reveal possible heterogeneity of reactive sites on a polymer-coated liposome surface, and to estimate the optimal polymer-to-lipid ratio for efficient liposome protection. Experiments have been performed with the quenching of liposome-associated fluorescent label (nitrobenzoxadiazole or fluorescein) with protein (rhodamine-ovalbumin or anti-fluorescein antibody) from solution. It was shown that poly(ethylene glycol) grafting to liposomes hinders protein interaction with the liposome surface, whereas liposome-grafted dextran (more rigid polymer) in similar quantities does not affect protein-liposome interaction. Highly-reactive and low-reactive populations of chemically identical reactive sites have been found on polymer-coated liposomes. Experimental data satisfactory confirm the suggested mechanism for the longevity of polymer-modified liposome.

Molecular requirements for hapten binding to antibodies against glutamate and aspartate

Molecular requirements for hapten recognition by antibodies raised in rabbits against glutaraldehyde conjugates of L-glutamate and L-aspartate were determined in enzyme immunoassays by measuring the displacement of binding of glutamate and aspartate, respectively, by a large number of selected haptens to two anti-glutamate and two anti-aspartate sera. The results indicate that N-terminal modifications of the amino acids, such as the presence of an N-acetyl or N-carbamyl group or the addition of a second amino acid to form dipeptides with C-terminal glutamate or aspartate, are tolerated to variable degrees, more so by the aspartate than the glutamate antisera. The antibodies possess point-to-point recognition sites for the two carboxyl groups present in both amino acids. Strong shape complementarity between the amino acids and their respective binding sites is suggested by the lack of recognition of the appropriate D stereoisomers by any of the antibodies. Changes in the distance between the two carboxyl groups, or modification, replacement or loss of either or both carboxyl groups, strongly reduce or eliminate binding. Based on these results, we suggest that other antibodies raised to similar conjugates of these amino acids are likely to share similar recognition characteristics. In addition, the results provide a rational background for the evaluation of antibody specificity and the interpretation of results in immunocytochemical studies using antisera to glutamate and aspartate.

G.L.C.-M.S. of N-(1-deoxyalditol-1-yl)octadecylamine derivatives in the analysis of methanolysates of neoglycolipids obtained by reductive amination

Hydrophobic conjugates of a series of aldoses have been prepared by reductive amination with octadecylamine and sodium cyanoborohydride, as model compounds for the analysis of reductively aminated oligosaccharides derived from capsular polysaccharides of Streptococcus pneumoniae. In the context of the methanolysis procedure for sugar analysis, g.l.c. and g.l.c.-m.s. (e.i.-mode) studies were carried out on the N-(1-deoxyalditol-1-yl)octadecylamine derivatives obtained after treatment with methanolic HCl, and subsequent N-acetylation and trimethylsilylation.

Improved procedure for the construction of neoglycolipids having antigenic and lectin-binding activities, from reducing oligosaccharides

Conditions have been established for the rapid and efficient conjugation of reducing oligosaccharides (di- to deca-saccharides) to dipalmitoyl phosphatidylethanolamine. The resulting neoglycolipids derived from several naturally occurring oligosaccharides and a series of N-linked high-mannose-type oligosaccharides released by hydrazinolysis from RNAase B showed specific and potent reactivities, as appropriate, with monoclonal antibodies to blood group Lewis(b), blood group A or a stage-specific embryonic (SSEA-1) antigen, or the lectin concanavalin A.

Immunological studies with liposome-bound corynetoxins

Attempts were made to raise antibodies against corynetoxins, a family of toxins responsible for annual ryegrass toxicity. The glycolipid nature of corynetoxins made them ideally suited for incorporation into the structure of small unilamellar liposomes. Sheep were injected with corynetoxin liposomes with and without adjuvants such as lipid A and muramyl dipeptide, and the sera tested for anti-corynetoxin antibody. Similarly, rabbits were injected with hydrolysed corynetoxin coupled to human IgG and keyhole limpet haemocyanin and with corynetoxin coupled to bovine serum albumin. These preparations were administered with complete Freund's adjuvant. The failure of any of these preparations to elicit an anti-corynetoxin antibody response in either sheep or rabbits is discussed.

Immunochemical studies on monoclonal antibodies to stearyl-isomaltotetraose from C58/J and a C57BL/10 nude mouse

Six hybridomas, five from C58/J and one from C57BL/10 nu/nu, immunized with stearyl-isomaltotetraose (S-IM4) were established. One produced IgG3, one IgM and four IgA. The specificities and sizes of the antibody combining sites were determined by quantitative precipitin and ELISA quantitative inhibition assays. All cross-react with alpha(1----6)dextran B512. Their combining sites were complementary to five to seven glucose residues. Association constants of hybridoma antibodies for dextran B512 ranged from 10(3) to 10(5) ml/g, and for isomaltoheptaose (IM7) from 10(3) to 10(4)M-1. Two hybridoma antibodies, one IgA and another IgM derived from different fusions have identical inhibition curves and combining sites, and very close association constants. It will be important to establish whether they have very similar or identical nucleotide sequences in their variable regions. These studies provide further insight into the specificity and repertoire of antidextran combining sites.

Novel approach to the study of the antigenicities and receptor functions of carbohydrate chains of glycoproteins

This report describes the construction of neoglycolipids as a novel approach to determining the antigenicities and receptor functions of minute amounts of oligosaccharides derived from glycoproteins. Reduced oligosaccharides are converted into oligosaccharide alditols by controlled selective periodate oxidation and conjugated to phosphatidyl ethanolamine dipalmitoyl by reductive amination. The resulting neoglycolipids can be rendered multivalent by binding to polyvinylchloride or silica plates or they can be incorporated into liposomes and their antigenicities and receptor activities determined in low concentrations by direct binding or inhibition of binding assays. This approach, which has been successfully used with two monoclonal antibodies and a plant lectin, should be widely applicable to the direct analysis of O- and N-glycosidically linked carbohydrate chains of glycoproteins and proteoglycans both as antigens and recognition structures of diverse receptor systems.

Immunochemical studies of conjugates of isomaltosyl oligosaccharides to lipid: production and characterization of mouse hybridoma antibodies specific for stearyl-isomaltosyl oligosaccharides

Twelve C57BL/6J hybridoma clones, 9, 2 and 1 from mice immunized with stearyl-isomaltotetraose, stearyl-isomaltopentaose and stearyl-isomaltohexaose respectively were characterized. Seven produced IgA and 5 IgM. The specificities and sizes of their combining sites were determined by quantitative precipitin and precipitin inhibition assays. All 12 hybridoma antibodies precipitated with alpha 1----6 dextran B512 and linear dextran LD7, indicating that they recognize an internal -Glc alpha 1----6Glc alpha 1----6Glc- determinant. This in contrast with the results with rabbit antisera obtained in response to the same immunogen which recognize the non-reducing terminal determinant Glc alpha 1----6Glc alpha 1----6Glc-. Of the 12 hybridoma antibodies, 1 has an antibody combining site complementary to 4 alpha 1----6-linked glucoses while others have combining sites complementary to isomaltohexaose or isomaltoheptaose. The large combining-site sizes found in C57BL/6 hybridoma clones may be related to the pre-existing clonal repertoire in this strain. Binding constants of monomers of these antibodies for dextran B512 and isomaltoheptaose determined by affinity electrophoresis range from 1.4 X 10(3) to 4.6 X 10(5) ml/g and from 1.2 X 10(3) to 3.5 X 10(4) M-1 respectively, which is consistent with previous studies in the anti-dextran B512 system. The use of synthetic glycolipids as antigens enables us to study the gene control of antibody responses to glycolipids and to investigate the combining-site specificities of antibodies to a single antigenic determinant. Results so far show that all 12 hybridoma proteins are different despite the simplicity of the antigens. The findings provide further insight into the specificity of antibody combining sites.

Genetic and nongenetic control of the immune response of mice to a synthetic glycolipid, stearylisomaltotetraose

Evidence for genetic control of antibody response to stearylisomaltotetraose (ST-IM4), a chemically defined synthetic glycolipid, was studied in various mouse strains. Anti-glycolipid antibodies were induced by repeated injections of ST-IM4 in complete Freund's adjuvant. C57BL and CBA/J mice were found to be good responders, while A/J, SJL/J, and AKR/J mice were poor responders. Responsiveness is independent of the H-2 genotype since AKR/J and CBA/J mice share the same H-2 locus. In addition, B10.A and B10.PL mice developed antibody levels similar to those of C57BL. The immunoglobulin heavy-chain locus (IgCH) was also found not to control antibody levels to ST-IM4 since C57BL and SJL/J mice share the same IgCH allotype. In C58/J mice, wide variation in response was observed among individual mice. Study of the response to ST-IM4 in 12 breeder pairs from different family lines of the C58/J colony also indicated that the regulation of the immune response to ST-IM4 is apparently more complex than can be explained by single gene control. C58 mice, unlike many other strains, had very low preimmune titers.

Characterization of anti-N-acetyl-D-glucosamine antibodies elicited through haptenated liposomes

Antiserum was raised in rabbits against p-aminophenyl 2-acetamido-2-deoxy-beta-D-glucoside covalently coupled to phosphatidylaminoethanol-containing lecithin liposomes. The affinity-purified antibodies were found to be IgM. The properties of the antibodies were studied by ligand inhibition of quantitative precipitation, fluorescence titration with 2-acetamido-2-deoxy-D-glucose, and complement lysis of haptenated liposomes.

Immunogenicity of galactosylated liposomes

The kinetics of the immune response to galactosylated liposomes in saline was studied in rabbits. The antibody level in the secondary immunization was found to be higher compared to that in the primary response. The antibodies were purified on an immunoadsorbent column of galactosylated bovine serum albumin. Anti-galactosyl antibodies belonged to both IgM and IgG classes, the former being the major fraction. Both failed to agglutinate normal or neuraminidase treated human or rabbit erythrocytes but they agglutinated trypsinized cells.

Immunogenicity of a synthetic HBsAg peptide: enhancement by conjugation to a fatty acid carrier

Effective immunization with short polypeptide antigens has typically only been possible when the peptide is conjugated to a large carrier substance, usually a protein. Such immunizations suffer from difficulties in producing conjugates of reliable composition, and from unwanted anti-carrier immune responses. When a chemically synthesized peptide, bearing hepatitis B virus a-determinant specificity, was conjugated to a dipalmityl-lysine moiety, a significant improvement in anti-hepatitis B surface antigen response was obtained, in comparison to the corresponding peptide-keyhole limpet hemocyanin conjugate. Dipalmityl lysyl peptide conjugates are readily made by standard Merrifield synthesis procedures, and are relatively free of byproducts that might cause unwanted immune responses. Gel filtration experiments suggest that the conjugates form large aggregates, possibly micelles, which may play a significant role in the enhancement of the anti-peptide response. These properties suggest that fatty acid conjugation may be a useful procedure for producing chemically synthesized peptide vaccines.