Around half of hospitalised patients and nearly all outpatients tested positive for virus, with all those who tested positive having similar viral loads. Significantly higher levels of IL-13 and IL-17 were found in hospitalised patients with undetectable virus. IL6 was found to show a significant inverse association with arterial blood oxygen pressure in hospitalised patients and a similar inverse relationship was found for IL-8 in the critically ill patients.
Th1 and Th17 cells form an important part of host defence against pathogens but TH17 cells have also been linked to the pathogenesis of autoimmune and inflammatory diseases. It is presently unclear whether the increase in Th1 and Th17 responses reflects a vigorous antiviral defence necessary to clear lower respiratory infection or whether the inflammatory response contributes to disease severity. Although the ability of influenza viruses to evoke an inflammatory response is well known, this is the first study to link a Th17 response to severe influenza disease in humans. The authors suggest that immunomodulatory drugs which down-modulate Th1 and Th17 responses could be used to clarify the role of these pathways in the pathogenesis of the acute respiratory symptoms shown by patients with severe H1N1 disease. “Hypercytokinemia” of specific chemokines and cytokines has previously been shown to be associated with severe and often fatal cases of human H5N1 infections.
The study is published in the journal Critical Care.