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HYGIENE Report Summary

Project ID: 250290
Funded under: FP7-IDEAS-ERC
Country: France


The central objective of our “Hygiene” project was to revisit the Hygiene Hypothesis applied to allergic and autoimmune diseases on robust epidemiological and experimental bases. The Hygiene Hypothesis proposes that the decrease of infections observed in developed countries is the cause of the increase in allergic and autoimmune diseases in these countries. The goal was not only to confirm the reality of the Hygiene Hypothesis in allergy using modern tools available but also to propose novel therapeutic strategies that could replace the protective effect of infectious diseases. For the epidemiological studies we have chosen to focus on a particular allergic disease, atopic dermatitis.

From the experimental point of view, we could address the mechanisms of the protective effect of infections by conducting our studies in parallel on allergic diseases, especially using the model of induced experimental asthma, and on autoimmunity in a model of spontaneous insulin-dependent type 1 diabetes, the non-obese diabetic (NOD) mouse. Indeed, we could confirm and extend preliminary data showing that the NOD mouse, on which we have worked for many years, is one of the most responsive mouse strains to the development of allergic responses including anaphylaxis. It became important therefore to understand the immunological mechanisms predisposing NOD mice to the development of both allergic asthma and autoimmune diabetes. We have undertaken studies that have led us to characterize sub-strains of NOD mice expressing a low or high susceptibility to develop allergy and autoimmunity, which opened the prospect of genetic and molecular studies. Taken together our present data strongly indicate that varying predisposition to allergy and autoimmune diabetes onset in genetically close individuals not only depends on differential exposure to commensals or pathogens but also on potential genetic variants, epigenetic marks and microbiota diversity which are presently under analysis.

The aim of epidemiological section was to investigate the association of various factors, especially those implicated by the Hygiene Hypothesis (i.e., direct and indirect markers of infection), with the occurrence of atopic dermatitis in early childhood. This included a case-control study of 500 cases of incident atopic dermatitis and 500 controls matched by sex, age and period of interview from 10 Italian centers. Atopic dermatitis was defined by the participating dermatologist according to standardized diagnostic criteria. A validated questionnaire was used to investigate potential risk factors for atopic dermatitis in young children according to strict time related criteria. With reference to main factors considered in the Hygiene Hypothesis, we found a strong inverse relation with family size, with a significant odds ratio (OR) of 0.55, for children with two or more siblings compared with those without, and with pet exposure (to dogs only; OR= 0.72). This result was consistent with a comprehensive meta-analysis on pet exposure conducted within the project. Early weaning (i.e. the introduction of solid foods at 4-5 months of age), was inversely related to atopic dermatitis occurrence as compared to children exclusively breastfed (OR=0.42 and 0.38 at 4 and 5 months of age respectively). Results for direct markers of infections were largely non-significant. A meta-analysis of probiotic supplementation during pregnancy and infancy, showed a decreased incidence of atopic dermatitis (summary relative risk=0.79).
Our results confirm the protective effect of family size and daily contact with dogs on atopic dermatitis occurrence. The underlying immunological mechanisms may be related to sub-threshold but continuous exposure to infectious agents rather than to the exposure to infections agents resulting in symptomatic episodes of infection.

An additional pilot study concerning the potential interaction between intestinal microbiota composition and direct and/or indirect makers of infections related to atopic dermatitis occurrence was also implemented. In fact, a major event occurred in the year following the acceptance of our project, which involved widening the definition of the microbial environment in the context of the Hygiene Hypothesis. Until then the microbial environment that mainly included pathogens has expanded to include commensal and, in a first step, especially those composing the gut microbiota. Therefore, it proved essential to incorporate this new element in our work objectives, both in experimental and epidemiological parts of the project, to clarify the role of changes in the diversity of intestinal microbiota in the progression of dysimmune diseases. It also became important to analyze the effect of different treatments including prebiotics, probiotics or antibiotics, which modify more or less the diversity of the intestinal microbiota, on the progression of the pathologies of interest.

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