Influenza A viruses cause recurrent outbreaks of local or global scale with potentially severe consequences for human health and the global economy. Recently, a new strain of influenza A virus was detected that causes disease in and transmits among humans, probably owing to little or no pre-existing immunity to the new strain. On June 11, 2009, the WHO declared that the infections caused by the new strain had reached pandemic proportion. Characterized as an influenza A virus of the H1N1 subtype, the genomic segments of the new strain were most closely related to swine viruses1. Most human infections with swine-origin H1N1 influenza viruses (S-OIVs) appear to be mild; however, more than 50% of hospitalized individuals do not have underlying health issues, attesting to the pathogenic potential of S-OIVs. To better assess the risk posed by the new virus, we characterized one of the first US S-OIV isolates, A/California/04/09 (H1N1; CA04), as well as several other S-OIV isolates, in vitro and in vivo. In mice and ferrets, CA04 and other S-OIV isolates tested replicate more efficiently than a currently circulating human H1N1 virus. In addition, CA04 replicates efficiently in nonhuman primates, causes more severe pathologic lesions in the lungs of infected mice, ferrets, and nonhuman primates than a currently circulating human H1N1 virus, and transmits among ferrets. In specific-pathogen free miniature pigs, CA04 replicates without clinical symptoms. The assessment of human sera from different age groups suggests that infection with human H1N1 viruses antigenically closely related to viruses circulating in 1918 confers neutralizing antibody activity to CA04. Finally, we show that CA04 is sensitive to approved and experimental antiviral drugs, suggesting these compounds as a first line of defence against the recently declared S-OIV pandemic.

The 1918 influenza pandemic was unusually severe, resulting in about 50 million deaths worldwide. The 1918 virus is also highly pathogenic in mice, and studies have identified a multigenic origin of this virulent phenotype in mice. However, these initial characterizations of the 1918 virus did not address the question of its pathogenic potential in primates. Here we demonstrate that the 1918 virus caused a highly pathogenic respiratory infection in a cynomolgus macaque model that culminated in acute respiratory distress and a fatal outcome. Furthermore, infected animals mounted an immune response, characterized by dysregulation of the antiviral response, that was insufficient for protection, indicating that atypical host innate immune responses may contribute to lethality. The ability of influenza viruses to modulate host immune responses, such as that demonstrated for the avian H5N1 influenza viruses, may be a feature shared by the virulent influenza viruses.

Highly pathogenic avian H5N1 influenza A viruses have spread throughout Asia, Europe, and Africa, raising serious worldwide concern about their pandemic potential. Although more than 250 people have been infected with these viruses, with a consequent high rate of mortality, the molecular mechanisms responsible for the efficient transmission of H5N1 viruses among humans remain elusive. We used a mouse model to examine the role of the amino acid at position 627 of the PB2 viral protein in efficient replication of H5N1 viruses in the mammalian respiratory tract. Viruses possessing Lys at position 627 of PB2 replicated efficiently in lungs and nasal turbinates, as well as in cells, even at the lower temperature of 33 °C. Those viruses possessing Glu at this position replicated less well in nasal turbinates than in lungs, and less well in cells at the lower temperature. These results suggest that Lys at PB2–627 confers to avian H5N1 viruses the advantage of efficient growth in the upper and lower respiratory tracts of mammals. Therefore, efficient viral growth in the upper respiratory tract may provide a platform for the adaptation of avian H5N1 influenza viruses to humans and for efficient person-to-person virus transmission, in the context of changes in other viral properties including specificity for human (sialic acid α-2,6-galactose containing) receptors.

Pulmonary tissue damage resulting from influenza virus infection is caused by both the cytolytic activity of the virus and the host immune response. Immune-mediated injury results from T cellmediated destruction of virus-infected cells and by release of cytokines and chemokines that attract polymorphonuclear leukocytes (PML) and macrophages to the infected site. The cytokines/ chemokines potentiate dendritic cell (DC) activation and T cell expansion, which further enhances local damage. Here we report that immune modulation by local administration to the respiratory tract of sphingosine analog AAL-R significantly dampens the release of cytokines and chemokines while maintaining protective neutralizing antibody and cytotoxic T cell responses. As a result there was a marked reduction of infiltrating PML and macrophages into the lung and resultant pulmonary tissue injury. DC maturation was suppressed, which limited proliferation of specific antiviral T cells in the lung and draining lymph nodes. Further, AAL-R was effective in controlling CD8 ؉ T cell accumulation in the lungs even when given 4 days after initiation of influenza virus infection. These data indicate that sphingosine analogs display useful potential for controlling the immunopathology caused by influenza virus.immunopathology ͉ dendritic cells ͉ cytokine storm ͉ T cells T he antiviral host response evolved to limit the spread of infection at the cost of causing tissue injury. There is a balance between the protective and injurious responses that leads either to the purging of infectious virus and host recovery or to severe disease and even death. Thus, strategies to balance the antiviral immune response in favor of host outcome need to be developed.T cell response elicited early in the course of infection recognizes, attacks and lyses virus-infected cells to eliminate potential factories of progeny viruses (1-5). The protective influenza antibody response is elicited later and plays a role in controlling re-infection (6, 7). The innate and adaptive immune systems release wide varieties of cytokines and chemokines that activate and attract inflammatory cells to the site of infection (2,8,9). However, these molecules can also cause the host harm by a phenomenon known as cytokine storm. Cytokine storm has been convincingly documented both in experimental animals infected with the 1918/1919 and H5N1 influenza viruses (2, 10-13) as well as in humans (14-18) succumbing to H5N1 infection. Although antiviral drugs can be used to treat the virus, a strategy to balance the resultant cytokine release and lung injury while maintaining benefits of the antiviral protective immune response is needed (19). Influenza virus replication is most often limited primarily to the respiratory tract but the systemic signs and symptoms of disease, e.g., fever, muscle pain and shakes, intestinal tract involvement, are related to cytokine effects (20, 21).We reported that intratracheal (i.t.) delivery of the sphingosine analog AAL-R or its phosphate ester inhibited virus-specifi...

During the last decade, more than half of humans infected with highly pathogenic avian influenza (HPAI) H5N1 viruses have died, yet virus-induced host signaling has yet to be clearly elucidated. Airway epithelia are known to produce inflammatory mediators that contribute to HPAI H5N1-mediated pathogenicity, but a comprehensive analysis of the host response in this cell type is lacking. Here, we leveraged a system approach to identify and statistically validate signaling subnetworks that define the dynamic transcriptional response of human bronchial epithelial cells after infection with influenza A/Vietnam/1203/2004 (H5N1, VN1203). Importantly, we validated a subset of transcripts from one subnetwork in both Calu-3 cells and mice. A more detailed examination of two subnetworks involved in the immune response and keratinization processes revealed potential novel mediators of HPAI H5N1 pathogenesis and host response signaling. Finally, we show how these results compare to those for a less virulent strain of influenza virus. Using emergent network properties, we provide fresh insight into the host response to HPAI H5N1 virus infection and identify novel avenues for perturbation studies and potential therapeutic interventions for fatal HPAI H5N1 disease.