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 allergic symptoms in childhood are believed to be initiated in infancy and early childhood. The kinetics of this response have hardly been researched.

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 analysis of patterns of allergen-specific T-cell reactivity in a cohort of 19 children, from whom blood samples were obtained in the spring during their second and third years of life. Blood mononuclear cell cultures were established in 200 microL 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 micrograms/mL for HDM and 30 micrograms/mL for the purified allergens. Tetanus toxoid (0.5 microgram/mL) and ovalbumin (10 micrograms/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 care homes. Major mite allergen levels (Der p 1/Der f 1) in dust were analysed by an enzyme linked immunosorbent assay (ELISA).

RESULTS: Specific T-cell responses were seen in the majority of the children against house 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 microgram/g fine dust in the homes of the children and the day care centres that they were attending. This indicates that doses of mite antigen well below the suggested sensitization threshold level of 2 micrograms/g dust 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 active immunological recognition of environmental allergens and the ensuing initiation of allergen-specific T-cell responses, is a normal part of the 'education' of the immune system in early childhood and can occur even at very low exposure levels. Priming per se does not imply clinically significant sensitivity, however.