Which are the environmental factors thought to be responsible for allergy and asthma in early life? A review
A review of the subject tries to answer this question
As many other disease, allergy pathogenesis relies on genetic and environmental factors. However, the significant increase of allergy in the last decades suggests a change in the environmental factors. This review in JACI focuses on the environmental exposures in early life that may cause immune dysregulation and influence the development of allergic disease. These environmental factors are (Fig. 1):
- Microbial exposure: the host microbiome consists of intestinal and lung microbiome. In early life atopic children show a reduced gut colonization of the species Lactobacillus, Bifidobacterium and Bacteroides and increased concentration of Clostridium species and yeasts. In the airways the presence of Streptococcus pneumoniae or Haemophilus influenzae in the first years of life is associated to a greater risk of wheezing and asthma. Mode of delivery, pre- and post-natal antibiotic therapy and diet (breastfeeding vs formula feeding) may influence the colonization of different microbes. Considering external microbial exposure, the exposure during childhood to a rich environmental microbiome as in the natural green, in a farm or in contact with animals promotes immune tolerance and protects from allergy risk (this concept has been demonstrated in USA in the Amish and Hutterites communities - Stein MM 2016 NEJM).
- Respiratory infections: the role of Rhinovirus and RSV in the first 2 years of life has been extensively studied. Both have been associated to a greater risk of wheezing and asthma.
- Indoor allergen exposure: the mechanism underlying the development of allergic rhinitis and asthma following the exposure to indoor allergens is still not clear but many studies show that exposure to HDM, mold, cockroach and mice allergens increases the risk of these conditions. The exposure to pet allergens is not associated to increased risk of allergic disease.
- Ambient air pollution exposure: if the exposure to traffic-related air pollution (especially benzene, PM2.5 or NO2) occurs in the last trimesters of pregnancy there is an increased risk of impaired foetal lung development resulting in reduced lung function and increased risk of asthma in school age. A similar risk not only for asthma but also for aeroallergens sensitization is demonstrated for children exposed to air pollution during infancy and childhood. The exposure to tobacco smoke has a profound influence on lung development if this happens during pregnancy and in the postnatal period with an associated risk of asthma, eczema and allergic sensitization likely due to a reduced umber of T reg cells.
The review concludes saying that allergy or asthma onset depends on the combination of all these exposures (plus the genetic factors) and that the real goal is understanding the epigenetic effects of these exposures. This is the aim of the –omics studies on exposomes such as EXPOsOMICS (funded by the European Commission and coordinated by Imperial College London) but so far the most important data come from large birth cohort studies.
Fig.1.Environmental factors involved in allergic disease pathogenesis. A, determinants that influence subject’s microbiome and external microbial environment. B, allergens linked to atopy development. C, air pollution exposure. In the center a question mark: how do these factors interact in the onset and severity of allergic disease?
Burbank AJ, Sood AK, Kesic MJ, Peden DB, Hernandez ML. Environmental determinants of allergy and asthma in early life. J Allergy Clin Immunol. 2017 Jul;140(1):1-12