The Effect of Clean and Microbiota-Free Environment on Host Immune Response

Accompanied with improved living conditions and hygiene practices, there is a steady and steep increase of occurrence of allergic diseases in developed countries. Hygiene Hypothesis, which states that lack of microbes at the early childhood attributes the improper development of a healthy immune system in adulthood, has been formulated to explain this clinical observation. The mechanisms underlying Hygiene Hypothesis remain obscure largely due to the complex interface between environmental factors and genetic composition in clinical studies and animal model systems. Here, we explored the potential of deploying a simple plant system to examine the effect of clean environment on host immune system. We compared the growth phenotype and the immune response of the model plant Arabidopsis thaliana grown on autoclaved soil (clean environment) and non-autoclaved soil (dirty environment). We observed that plants grown on autoclaved soil exhibited a weaker immune response than plants grown on non-autoclaved soil at the adulthood stage. Plants grown on autoclaved soil showed severer infestation by fungal gnats, the common Arabidopsis pests, and less uniformity in size than those grown on non-autoclaved soil. These data suggest that individuals living in a clean environment may develop a dysfunctional immune system at the adulthood compared to those grown in a dirty environment. The autoclave process kills soil microbiota, which might be a reason for altered growth and immunity. Thus, our studies suggest that host immune response and microbiota should be considered when studying the underlying mechanisms of Hygiene Hypothesis.