cDNA sequencing revealed that chick Mx protein consists of 705 amino acids. Its 84 N-terminal amino acids show no significant sequence homology to other Mx proteins. They are followed by 514 residues that include a tripartite GTP binding consensus motif. This region shows 50-70% sequence identity to mammalian and duck Mx proteins. Sequences near the C terminus, including a leucine zipper motif, are also conserved, whereas the intervening 19 amino acids lack sequence similarity. This unique sequence corresponds to a highly variable region in mammalian Mx proteins, suggesting that it serves as a spacer between functional domains. Chick and mouse cells transiently transfected with cDNA expression constructs synthesized chick Mx protein at a level that could easily be detected with specific antibodies. Chick Mx protein in such cells was mainly cytoplasmic and had a granular appearance. Permanently transfected cell lines expressing high levels of chick Mx protein could not be established, suggesting low metabolic stability of chick Mx protein or incompatibility with cell proliferation. The antiviral activity of chick Mx protein was tested at the single-cell level using immunofluorescence techniques. Transfected cells expressing chick Mx protein showed no enhanced resistance to influenza A virus, vesicular stomatitis virus, Thogoto virus, or Sendai virus. Thus, chick Mx joins the list of Mx proteins without recognized antiviral activity, supporting the concept that Mx proteins serve unrelated functions.

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Influenza A viruses continue to represent a severe threat worldwide, causing large epidemics and pandemics responsible for thousands of deaths every year. Excessive inflammation due to overabundant production of proinflammatory cytokines by airway epithelial cells is considered an important factor in disease pathogenesis. Here we report that influenza A virus induced IB kinase (IKK) activity in human airway epithelial A549 cells, resulting in persistent activation of nuclear factor-B (NF-B), a critical regulator of the inflammatory response. Although lung epithelial cells are highly sensitive to stimulation of the IKK/NF-B pathway by influenza virus infection, NF-B was not activated in several non-pulmonary cells permissive to the virus, indicating a cell-specific response. Moreover, NF-B was not essential for virus replication but triggered the expression of proinflammatory cytokines in infected lung cells and was directly responsible for production of high levels of interleukin-8, a chemokine associated with influenzainduced inflammation and airway pathology. We also report that 9-deoxy-⌬ 9 ,⌬ 12 -13,14-dihydro-prostaglandin D 2 , a cyclopentenone prostanoid with therapeutic efficacy against influenza in preclinical studies, was a powerful inhibitor of influenza virus-induced IKK activity and interleukin-8 production by human pulmonary cells. The results identify IKK as an important factor in triggering influenza virus-induced inflammatory reactions in pulmonary epithelium, suggesting novel therapeutic approaches in the treatment of influenza.