Sialic acid-dependent binding specificities of Helicobacter pylori and influenza virus. Importance of different parts of the binding sequences

Sammanfattning: Many viruses and bacteria adhere to host cell glycoconjugates in a specific manner, utilizing selected carbohydrate sequences. The aim of this work was to investigate at a molecular level the interactions of two important human pathogens, influenza virus and Helicobacter pylori, with sialylated carbohydrate structures. Influenza virus causes yearly epidemics and H. pylori is associated with common gastric diseases such as gastritis and duodenal ulcer and may contribute to development of gastric cancer. Detailed knowledge of the binding sequences may be of importance for development of new drugs against these microbes.In the present study, binding of human and avian influenza A viruses to sialylated epitopes was investigated, utilizing mixtures of gangliosides prepared from human leukocytes. The methods used were overlay of TLC plates with viruses and various lectins and antibodies, chemical modifications and mass spectrometry. The results showed a difference in binding between human and avian strains. The human virus recognized complex structures whereas the avian (duck) virus bound to both simple and complex species with a preference for less extended molecules. It was also shown that the binding of human virus was not dependent on the presence of sialyl-Lewis x or VIM-2 structures. In accordance with work from other laboratories the human virus recognized gangliosides with Ñ6-linked sialic acid, whereas the avian virus interacted with Ñ3-linked sialic acid. Recently it has been reported that chicken-specific influenza viruses (but not duck influenza viruses) also prefer complex carbohydrate structures as receptors. This is of interest because chicken influenza may be transferred to humans resulting in serious influenza symptoms and high mortality among patients.The interaction of H. pylori with sialylated carbohydrates was tested using a combination of various techniques including epitope dissection, i.e. chemical and enzymatic modifications of a known Neu5Ac-containing binding sequence followed by binding studies. As a model substance in these experiments sialyl-3-paragloboside (Neu5Ac Ñ3Gal Ò4GlcNAc Ò3Gal Ò4Glc Ò1Cer, S-3-PG) was used. The modifications included: oxidation of the sialic acid glycerol chain followed by reduction or coupling with methylamine and ethanolamine, methylation followed by reduction or amidation of the carboxyl group, lactonization, formation of lyso S-3-PG and N-acylation, de-N-acetylation and N-acylation. To evaluate the binding activity of the S-3-PG derivatives to H. pylori TLC overlay assay was used. The binding to a panel of natural carbohydrate structures and neoglycolipids was also studied using TLC overlay assay and hemagglutination inhibition experiments. Some of the carbohydrates were, in addition, tested in a novel binding assay using labeled neoglycoproteins. The results showed that in the terminal trisaccharide, Neu5Ac Ñ3Gal Ò4GlcNAc, Neu5Ac and its three side chains are critical for the interaction, which indicates the participation of this sugar residue in a direct binding to the bacterial adhesin. Parts of Gal also seem to be involved in this process since Neu5Ac Ñ £Gal but not free Neu5Ac turned out to be active as inhibitor. However, the third monosaccharide (GlcNAc) seems to have an auxiliary role, probably as a guiding carrier for the binding area. This sugar residue may be modified at C2 and C3 without loss of the activity. Other sugars of the core chain, especially additional N-acetyllactosamine units, may also be of importance for the definition of the extended epitope and its presentation on cell membranes and in vivo recognition by H. pylori. The current treatment of H. pylori is based on the use of antibiotics, which may induce antibiotic-resistance. Therefore an alternative treatment is required.