Environmental Biodiversity, Human Microbiota, and Allergy are Interrelated
Rapidly declining biodiversity may be a contributing factor to another global megatrend—the rapidly increasing prevalence of allergies and other chronic inflammatory diseases among urban populations worldwide.
By 2050, some predict that two-thirds of the global human population will live in urban areas with little green space and limited contact with nature and biodiversity. At the same time, an increasing fraction of the urban population will suffer from chronic inflammatory disorders, of which allergic and autoimmune diseases are prime examples. Building on the hygiene hypothesis, the notion that growing up in a farming environment protects children from allergic sensitization, and the emerging understanding of the role of microbes in the development and maintenance of epithelial cell integrity and tolerance, the “biodiversity hypothesis” proposes that reduced contact of people with natural environmental features and biodiversity, including environmental microbiota, leads to inadequate stimulation of immunoregulatory circuits. Importantly, interactions with the natural environment may influence the composition of the human commensal microbiota, the members of which are not equal in their ability to stimulate the regulatory circuits via Toll-like and other antigen-recognizing receptors to prevent or terminate inappropriate inflammatory responses.
To test the biodiversity hypothesis of inflammatory disorders, we studied a random sample of 118 adolescents inhabiting a small town, villages of different sizes, and isolated houses within a 100 × 150 km region in eastern Finland. The inflammatory disorder that we examined is atopic sensitization, which involves the propensity to develop IgE antibodies in response to allergen exposure. Here we address four questions. First, we examine whether the environmental biodiversity influences the composition of the commensal microbiota of the study subjects. Environmental biodiversity was characterized at two spatial scales, the vegetation cover of the yards and the major land use types within 3 km of the homes of the study subjects. Commensal microbiota sampling evaluated the skin bacterial flora, identified to the genus level from DNA samples obtained from the volar surface of the forearm. Second, we investigate whether atopy is related to environmental biodiversity in the surroundings of the study subjects’ homes. Third, we examine whether atopy is related to the composition of the skin microbial community. Finally, we characterize the immune function of the study subjects by in vitro measurement of IL-10 expression in peripheral blood mononuclear cells (PBMCs) and relate it to the composition of the skin microbiota. IL-10 is one of the key anti-inflammatory cytokines in immunologic tolerance.
