Circulating Dendritic Cells, Farm Exposure and Asthma at Early Age

Farm environment has been shown to protect from childhood asthma. Underlying immunological mechanisms are not clear yet, including the role of dendritic cells (DCs). The aim was to explore whether asthma and farm exposures are associated with the proportions and functional properties of DCs from 4.5‐year‐old children in a subgroup of the Finnish PASTURE birth cohort study. Myeloid DCs (mDCs), plasmacytoid DCs (pDCs) and CD86 expression on mDCs ex vivo (n = 100) identified from peripheral blood mononuclear cells (PBMCs) were analysed using flow cytometry. MDCs and production of interleukin (IL)‐6 and tumour necrosis factor alpha (TNF‐α) by mDCs were analysed after 5 h in vitro stimulation with lipopolysaccharide (LPS) (n = 88). Prenatal and current farm exposures (farming, stables, hay barn and farm milk) were assessed from questionnaires. Asthma at age 6 years was defined as a doctor′s diagnosis and symptoms; atopic sensitization was defined by antigen‐specific IgE measurements. Asthma was positively associated with CD86 expression on mDCs ex vivo [adjusted odds ratio (aOR) 4.83, 95% confidence interval (CI) 1.51–15.4] and inversely with IL‐6 production in mDCs after in vitro stimulation with LPS (aOR 0.19, 95% CI 0.04–0.82). In vitro stimulation with LPS resulted in lower percentage of mDCs in the farm PBMC cultures as compared to non‐farm PBMC cultures. Our results suggest an association between childhood asthma and functional properties of DCs. Farm exposure may have immunomodulatory effects by decreasing mDC proportions.