Scientists might have identified one of the reasons why the bird flu virus H5N1 is so deadly to humans. A study published today in the open access journal Respiratory Research reveals that, in human cells, the virus can trigger levels of inflammatory proteins more than 10 times higher than the common human flu virus H1N1. This might contribute to the unusual severity of the disease caused by H5N1 in humans, which can escalate into life-threatening pneumonia and acute respiratory distress.
Michael Chan and colleagues from the University of Hong Kong and collaborators in Vietnam, studied the levels of a subset of the pro-inflammatory proteins called 'cytokines' and 'chemokines', induced by the virus H5N1 in human lung cells, in vitro. The authors compared protein levels induced by strains of the H5N1 virus that had appeared in Hong Kong in 1997 (H5N1/97) and Vietnam in 2004 (H5N1/04), with levels induced by the human flu virus H1N1.
Their results show that H5N1 is a much more potent inducer of pro-inflammatory proteins than H1N1. Twenty-four hours after infection with H5N1/04, the levels of the chemokine IP-10 in bronchial epithelial cells reach 2200 pg/ml, whereas in cells infected with H1N1 they only reach 200pg/ml. In H5N1/97-infected cells, IP-10 levels reach 1750 pg/ml. Similar results were found for other chemokines and cytokines.
Chemokines and cytokines are the "messengers of the immune system" and are critical in coordinating and regulating the immune response. Altering this balance is likely to lead to an uncontrolled inflammatory response in the lung and probably explains, at least in part, the severe lung inflammation associated with avian flu virus H5N1.
Proinflammatory cytokine responses induced by influenza A (H5N1) viruses in primary human alveolar and bronchial epithelial cells.
MCW Chan, CY Cheung, WH Chui, SW Tsao, JM Nicholls, YO Chan, RWY chan, HT Long, LLM Poon, Y Guan and JSM Peiris
Respiratory Research, 2005, in press
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