Apoptosis and proinflammatory cytokine responses of primary mouse microglia and astrocytes induced by human H1N1and avian H5N1 influenza viruses

Patients with an influenza virus infection can be complicated by acute encephalopathy and encephalitis. To investigate the immune reactions involved in the neurocomplication, mouse microglia and astrocytes were isolated, infected with human H1N1 and avian H5N1 influenza viruses, and examined for their immune responses. We observed homogeneously distributed viral receptors, sialic acid (SA)-alpha 2,3-Galactose (Gal) and SA-alpha 2,6-Gal, on microglia and astrocytes. Both viruses were replicative and productive in microglia and astrocytes. Virus-induced apoptosis and cytopathy in infected cells were observed at 24 h post-infection (p.i.). Expression of IL-1 beta, IL-6 and TNF-alpha mRNA examined at 6 h and 24 h p.i. was up-regulated, and their expression levels were considerably higher in H5N1 infection. The amounts of secreted proinflammatory IL-1 beta, IL-6 and TNF-alpha at 6 h and 24 h p.i. were also induced, with greater induction by H5N1 infection. This study is the first demonstration that both human H1N1 and avian H5N1 influenza viruses can infect mouse microglia and astrocytes and induce apoptosis, cytopathy, and proinflammatory cytokine production in them in vitro. Our results suggest that the direct cellular damage and the consequences of immunopathological injury in the CNS contribute to the influenza viral pathogenesis.