Pathogenesis of avian flu H5N1 and SARS

Avian influenza A (H5N1) and severe acute respiratory syndrome (SARS) coronavirus are infections that cause a severe viral pneumonia leading to acute respiratory dysfunction syndrome and carry a high case-fatality rate. We have investigated innate immune responses to both viruses using primary human macrophages and respiratory epithelial cells as in vitro models. In contrast to human influenza A H1NI viruses, the H5N1 viruses hyper-induce cytokines (tumour necrosis factor [TNF]alpha, interferon beta) and chemokines (IP10, MIP1alpha, MCP) in in vitro cultures of primary human macrophages. A similar differential effect is observed in primary human bronchial epithelial cells and in type 2 pneumocytes although TNFalpha is not induced in respiratory epithelial cells. The cell signalling pathways responsible for this differential effect remain to be explored. Preliminary data suggest that such differential signalling involves p38 MAP kinase rather than NF-kappaB. SARS coronavirus infection of primary human macrophages is associated with a strong induction of chemokines without an associated type 1 interferon response. These observations may be relevant in disease pathogenesis.