Asthma in children is increasingly common in Westernized countries. Ironically, the greater prevalence of allergic disease is thought to be due to life-style improvements that bring with them better hygiene. The less exposure you have during childhood to harmful pathogens, the less you seem to be protected against allergy. However, in the March issue of Nature Immunology scientists report that this is not necessarily the case, at least for influenza infection; prior exposure to virus enhanced, rather than suppressed, allergy.
The 'hygiene hypothesis' suggests that the 'type 1' (TH1) cytokines produced after childhood exposure to many pathogens are what protected previous generations from allergic asthma, a disease mediated by 'type 2' (TH2) cytokines. David Lewis and colleagues tested this hypothesis directly in a mouse model. They analyzed the impact of influenza infection, which induces a TH1 response, on subsequent allergen responses. Surprisingly, their data did not confirm the hygiene hypothesis. Having the flu actually enhanced subsequent allergen-induced asthma symptoms by promoting both TH1 and TH2 allergen-specific responses. The culprits were the dendritic cells and the immune modulator interferon-gamma, which remain in the lung after the infection is cleared. If other common respiratory viruses are also found to have similar effects, a favored hypothesis will have to be rethought.
David B. Lewis
Stanford University Medical School
Tel: +1 650 498 4189
Additional contact for comment on paper:
Steven D. Shapiro
Brigham and Women's Hospital
Tel: +1 617 732 7599; E-mail: firstname.lastname@example.org
Abstract available online.
(C) Nature Immunology press release.
Message posted by: Trevor M. D'Souza