Hygiene hypothesis and innate immunity. evaluating the role of environmental factors and genetic polymorphisms on the expression of receptors of the innate immune system

Background
In ‘westernised’ countries, the prevalence of childhood asthma and allergy has risen throughout the last few decades. A series of epidemiological studies showed reduced prevalence of asthma and allergy among farmers’ children compared to non-farmers’ children. These findings were in favour of the so-called ‘hygiene hypothesis’: Limited exposure to bacterial and viral pathogens during early childhood results in a higher risk of developing allergic diseases.
Pattern-recognition receptors (PRR) of the innate immune system, such as toll-like receptors (TLR) or CD14, recognize microbial compounds. Activation of PRR signalling pathways initiates regulatory mechanisms which might be the biological basis for the protective effect of the farming environment. Gene-environment interactions have been found for several genetic polymorphisms in PRR genes, therefore differing environmental exposures between study populations might explain the conflicting results of genetic studies.
Objectives
To assess factors of the farming environment that influence the innate immune system with respect to gene expression of crucial proteins of innate immunity pathways and test if this association is modulated by unknown genetic factors.
Methods
The PARSIFAL study was a multi-center cross-sectional survey in rural areas across Europe. 14893 children aged 5-13 years (farmer’s children, children attending Steiner schools, and their respective control groups) completed a standardised questionnaire. Specific IgE to common inhalant and food allergens were obtained from selected children. Indoor dust samples were collected. Furthermore, gene expression measurements and SNPs (single nucleotide polymorphisms) of PRR were available.
Results
Gene expression of CD14, TLR2, and TLR4 was higher in farmers’ children compared to non-farmers’ children. Mainly prenatal factors accounted for these differences. Expression of PRR was higher in children when the mother worked on the farm during pregnancy. In addition, we observed a dose-dependent increase in gene expression of CD14, TLR2 and TLR4 with the number of farm animal species the mother had contact with during pregnancy, which probably serves as proxy for an increasing variation in microbial exposure. Prenatal factors also exert their effect on the child’s health later in life. Children of mothers who worked on the farm during pregnancy were less sensitised at school age to common inhalant and food allergens than children of unexposed mothers. However, the development of clinical symptoms of atopic diseases seemed to depend on exposures that occurred postnatally.
Farm milk consumption was significantly associated with lower prevalence of atopic sensitisation and atopic health outcomes. Of particular importance is the consistency of the findings across the four study groups.
Neither of the genotyped SNPs of innate immunity genes was directly associated with differential gene expression. However, a polymorphism in the CD14 gene (CD14/A-1721G) significantly modified the effect of farm milk consumption on CD14 gene expression. Farm milk drinking children homozygous for the A allele expressed significantly more CD14 than non-exposed children. No association between farm milk drinking was observed in children with the GG genotype, heterozygous children showed an intermediate difference in CD14 gene expression between exposed and non-exposed children. We observed the same interaction between genotypes of CD14/A-1721G and farm milk exposure on asthma and related atopic health outcomes. Moreover, we were able to replicate the gene-environment interaction on atopic health outcomes in two independent populations.
Conclusions and Outlook
Whereas the manifestation of atopic diseases such as asthma and allergies depends on postnatal exposures, protection against atopic sensitisation is conferred by prenatal exposure to a farming environment. Immune modulation by activation of innate immune mechanisms might underlay these observations. However, these results need confirmation by longitudinal studies.
Despite limited power we could show a interaction between farm milk consumption and a CD14 polymorphism. Genome wide association studies study might give further insight into the interplay between genetics, environment and atopic diseases.
Current scientific evidence has not developed strongly enough to provide a reliable course of action for primary prevention or therapy. Infectious diseases resulting from exposure to pathogens continue to be a serious public health problem. However, further investigation and characterisation of environmental compounds conferring protection against atopic diseases is promising and will hopefully result in future efficient preventive measures.