Exposure to a diversity of microbes has been implicated in playing a major role in susceptibility to the development of allergic lung-type diseases. The hygiene hypothesis suggests that those exposed to a broad diversity of microbes are more likely to be protected against developing allergic-type diseases. However, changes in exposure to microbial diversity can occur in both younger individuals, as well as in adults, and the effects are not always understood. We investigated the effect of exposure to broad microbial diversity on the airway T cell response in house dust mite-induced allergic airway disease (a model of allergic asthma) in mice. We increased exposure to broad microbial diversity by cohousing specific pathogen-free adult or newborn mice with pet store mice (pet store exposure or born in pet store exposed, respectively). Mice were then exposed to house dust mite to induce allergic airway disease. Our findings suggest an effect of increased microbial exposure on the development of allergic airway inflammation that differs by age. Increasing exposure to diverse microbes as adults likely exacerbates the development of allergic airway inflammation, whereas this was not observed when exposure occurred at birth. We suggest that experimental evaluation of the hygiene hypothesis in inflammation, particularly those using mouse models, may need to consider age of the host and time of microbial exposure.
Elmore et al. (Sun,) studied this question.