Recent reports of mild to severe influenza-like illness in humans caused by a novel swine-origin 2009 A(H1N1) influenza virus underscore the need to better understand the pathogenesis and transmission of these viruses in mammals. In this study, selected 2009 A(H1N1) influenza isolates were assessed for their ability to cause disease in mice and ferrets and compared with a contemporary seasonal H1N1 virus for their ability to transmit to naïve ferrets through respiratory droplets. In contrast to seasonal influenza H1N1 virus, 2009 A(H1N1) influenza viruses caused increased morbidity, replicated to higher titers in lung tissue, and were recovered from the intestinal tract of intranasally inoculated ferrets. The 2009 A(H1N1) influenza viruses exhibited less efficient respiratory droplet transmission in ferrets in comparison with the highly transmissible phenotype of a seasonal H1N1 virus. Transmission of the 2009 A(H1N1) influenza viruses was further corroborated by characterizing the binding specificity of the viral hemagglutinin to the sialylated glycan receptors (in the human host) by use of dose-dependent direct receptor-binding and human lung tissue-binding assays.

Rapid antigenic evolution in the influenza A virus hemagglutinin precludes effective vaccination with existing vaccines. To understand this phenomenon, we passaged virus in mice immunized with influenza. Neutralizing antibodies selected mutants with single amino acid hemagglutinin substitutions that increased virus binding to cell surface glycan receptors. Passaging these high avidity-binding mutants in naïve mice, but not immune mice, selected for additional hemagglutinin substitutions that decreased cellular receptor binding avidity. Analyzing a panel of monoclonal antibody hemagglutinin escape mutants revealed a positive correlation between receptor binding avidity and escape from polyclonal antibodies. We propose that in response to variation in neutralizing antibody pressure between individuals, influenza A virus evolves by adjusting receptor binding avidity via amino acid substitutions throughout the hemagglutinin globular domain, many of which simultaneously alter antigenicity.Influenza A virus remains an important human pathogen due largely to its ability to evade antibodies specific for its attachment protein, the hemagglutinin (HA). This "antigenic drift" is due to accumulation of amino acid substitutions in HA epitopes recognized by antibodies that neutralize viral infectivity by blocking interaction of HA with sialic acid residues on hostcell membranes (1-3). The H1 subtype HA has four antigenic sites recognized by monoclonal antibodies with high neutralizing activity, designated Sa, Sb, Ca, and Cb (4). How can HA escape polyclonal antibodies given that the frequency of variants with simultaneous multiple point mutations is exceedingly low (5)? A popular model posits sequential selection by different individuals whose antibody responses focus on different individual antigenic sites (6,7).To better understand how antigenic drift occurs in human populations, we revisited classical experiments modeling drift in outbred Swiss mice (8). We generated three separate infectious stocks of the mouse-adapted strain A/Puerto Rico/8/34 (H1N1) (PR8) in MDCK cells using

A switch in specificity of avian influenza A viruses' hemagglutinin (HA) from avian-like (alpha2-3 sialylated glycans) to human-like (alpha2-6 sialylated glycans) receptors is believed to be associated with their adaptation to infect humans. We show that a characteristic structural topology--and not the alpha2-6 linkage itself--enables specific binding of HA to alpha2-6 sialylated glycans and that recognition of this topology may be critical for adaptation of HA to bind glycans in the upper respiratory tract of humans. An integrated biochemical, analytical and data mining approach demonstrates that HAs from the human-adapted H1N1 and H3N2 viruses, but not H5N1 (bird flu) viruses, specifically bind to long alpha2-6 sialylated glycans with this topology. This could explain why H5N1 viruses have not yet gained a foothold in the human population. Our findings will enable the development of additional strategies for effective surveillance and potential therapeutic interventions for H5N1 and possibly other influenza A viruses.

The extracellular environment is largely comprised of complex polysaccharides, which were historically considered inert materials that hydrated the cells and contributed to the structural scaffolds. Recent advances in development of sophisticated analytical techniques have brought about a dramatic transformation in understanding the numerous biological roles of these complex polysaccharides. Glycosaminoglycans (GAGs) are a class of these polysaccharides, which bind to a wide variety of proteins and signaling molecules in the cellular environment and modulate their activity, thus impinging on fundamental biological processes. Despite the importance of GAGs modulating biological functions, there are relatively few examples that demonstrate specificity of GAG-protein interactions, which in turn define the structure-function relationships of these polysaccharides. Focusing on heparin/heparan (HSGAGs) and chondroitin/dermatan sulfate (CSGAGs), this review provides structural insights into the oligosaccharide-protein interactions and discusses some key and challenging aspects of understanding GAG structure-function relationships.