LOW-LEVEL EXPOSURE TO HOUSE-DUST MITES STIMULATES T-CELL RESPONSES DURING EARLY-CHILDHOOD INDEPENDENT OF ATOPY

Background The immune responses which underlie the expression of aller gic symptoms in childhood are believed to be initiated in infancy and early childhood. The kinetics of this response have hardly been resear ched. Objective To analyse, in an environment with low house dust mite (HDM) exposure levels, the relationship between house dust mite (HDM) -specific T-cell reactivity as expressed by in vitro proliferation of blood mononuclear cells. Methods The study comprised a prospective ana lysis of patterns of allergen-specific T-cell reactivity in a cohort o f 19 children, from whom blood samples were obtained in the spring dur ing their second and third years of life. Blood mononuclear cell cultu res were established in 200 mu L AIM-V serum free medium. Crude house dust mite (HDM) and purified Der p 1 and Der p 2 extracts were used at optimal concentrations, i.e. 100 mu g/mL for HDM and 30 mu g/mL for t he purified allergens. Tetanus toroid (0.5 mu g/mL) and ovalbumin (10 mu g/mL) Served as positive controls. A clinical diagnosis of allergy was verified with skin-prick tests. Dust samples were collected from a mattress and/or carpet or sofa in homes, day care centres and day car e homes. Major mite allergen levels (Der p 1/Der f 1) in dust were ana lysed by an enzyme linked immunosorbent assay (ELISA). Results Specifi c T-cell responses were seen in the majority of the children against h ouse dust mite (crude HDM extract, Der p 1 and Der p 2). The levels of the house dust mite allergens Der p 1 and Der f 1 were low, i.e. < 0. 68 mu g/g fine dust in the homes of the children and the day care cent res that they were attending. This indicates that doses of mite antige n well below the suggested sensitization threshold level of 2 mu g/g d ust can induce mite-specific T-cell responses in young children. None of them showed clinical reactivity to house dust mites as indicated by negative skin-prick tests. Conclusions The findings suggest that acti ve immunological recognition of environmental allergens and the ensuin g initiation of allergen-specific T-cell responses, is a normal part o f the 'education' of the immune system in early childhood and can occu r even at very low exposure levels. Priming per se does not imply clin ically significant sensitivity, however.