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      Soluble Host Defense Lectins in Innate Immunity to Influenza Virus

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          Abstract

          Host defenses against viral infections depend on a complex interplay of innate (nonspecific) and adaptive (specific) components. In the early stages of infection, innate mechanisms represent the main line of host defense, acting to limit the spread of virus in host tissues prior to the induction of the adaptive immune response. Serum and lung fluids contain a range of lectins capable of recognizing and destroying influenza A viruses (IAV). Herein, we review the mechanisms by which soluble endogenous lectins mediate anti-IAV activity, including their role in modulating IAV-induced inflammation and disease and their potential as prophylactic and/or therapeutic treatments during severe IAV-induced disease.

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          Receptor binding and membrane fusion in virus entry: the influenza hemagglutinin.

          Hemagglutinin (HA) is the receptor-binding and membrane fusion glycoprotein of influenza virus and the target for infectivity-neutralizing antibodies. The structures of three conformations of the ectodomain of the 1968 Hong Kong influenza virus HA have been determined by X-ray crystallography: the single-chain precursor, HA0; the metastable neutral-pH conformation found on virus, and the fusion pH-induced conformation. These structures provide a framework for designing and interpreting the results of experiments on the activity of HA in receptor binding, the generation of emerging and reemerging epidemics, and membrane fusion during viral entry. Structures of HA in complex with sialic acid receptor analogs, together with binding experiments, provide details of these low-affinity interactions in terms of the sialic acid substituents recognized and the HA residues involved in recognition. Neutralizing antibody-binding sites surround the receptor-binding pocket on the membrane-distal surface of HA, and the structures of the complexes between neutralizing monoclonal Fabs and HA indicate possible neutralization mechanisms. Cleavage of the biosynthetic precursor HA0 at a prominent loop in its structure primes HA for subsequent activation of membrane fusion at endosomal pH (Figure 1). Priming involves insertion of the fusion peptide into a charged pocket in the precursor; activation requires its extrusion towards the fusion target membrane, as the N terminus of a newly formed trimeric coiled coil, and repositioning of the C-terminal membrane anchor near the fusion peptide at the same end of a rod-shaped molecule. Comparison of this new HA conformation, which has been formed for membrane fusion, with the structures determined for other virus fusion glycoproteins suggests that these molecules are all in the fusion-activated conformation and that the juxtaposition of the membrane anchor and fusion peptide, a recurring feature, is involved in the fusion mechanism. Extension of these comparisons to the soluble N-ethyl-maleimide-sensitive factor attachment protein receptor (SNARE) protein complex of vesicle fusion allows a similar conclusion.
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            Assembly of asparagine-linked oligosaccharides.

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              Characterization of a novel influenza A virus hemagglutinin subtype (H16) obtained from black-headed gulls.

              In wild aquatic birds and poultry around the world, influenza A viruses carrying 15 antigenic subtypes of hemagglutinin (HA) and 9 antigenic subtypes of neuraminidase (NA) have been described. Here we describe a previously unidentified antigenic subtype of HA (H16), detected in viruses circulating in black-headed gulls in Sweden. In agreement with established criteria for the definition of antigenic subtypes, hemagglutination inhibition assays and immunodiffusion assays failed to detect specific reactivity between H16 and the previously described subtypes H1 to H15. Genetically, H16 HA was found to be distantly related to H13 HA, a subtype also detected exclusively in shorebirds, and the amino acid composition of the putative receptor-binding site of H13 and H16 HAs was found to be distinct from that in HA subtypes circulating in ducks and geese. The H16 viruses contained NA genes that were similar to those of other Eurasian shorebirds but genetically distinct from N3 genes detected in other birds and geographical locations. The European gull viruses were further distinguishable from other influenza A viruses based on their PB2, NP, and NS genes. Gaining information on the full spectrum of avian influenza A viruses and creating reagents for their detection and identification will remain an important task for influenza surveillance, outbreak control, and animal and public health. We propose that sequence analyses of HA and NA genes of influenza A viruses be used for the rapid identification of existing and novel HA and NA subtypes.
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                Author and article information

                Journal
                J Biomed Biotechnol
                J. Biomed. Biotechnol
                JBB
                Journal of Biomedicine and Biotechnology
                Hindawi Publishing Corporation
                1110-7243
                1110-7251
                2012
                16 May 2012
                : 2012
                : 732191
                Affiliations
                1Department of Microbiology and Immunology, The University of Melbourne, Parkville, VIC 3010, Australia
                2Centre for Innate Immunity and Infectious Disease, Monash Institute of Medical Research, Monash University, Clayton, VIC 3168, Australia
                3WHO Collaborating Centre for Reference and Research on Influenza, Victorian Infectious Diseases Reference Laboratory, North Melbourne, VIC 3051, Australia
                Author notes
                *Patrick C. Reading: preading@ 123456unimelb.edu.au

                Academic Editor: Nobutaka Wakamiya

                Article
                10.1155/2012/732191
                3362216
                22665991
                45c12faa-69b3-4f8e-bdde-f68e4d2c7db5
                Copyright © 2012 Wy Ching Ng et al.

                This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

                History
                : 20 December 2011
                : 21 February 2012
                Categories
                Review Article

                Molecular medicine
                Molecular medicine

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