Background: Exposure to bacterial products in early life could protect against development of atopy. We examined the effect of bacterial lipopolysaccharide on allergic inflammation and expression of cytokines and lipopolysaccharide receptor (toll-like receptor 4 TLR4) in nasal mucosa of 15 atopic children and ten atopic adults.

Methods: Explanted mucosa was cultured with allergen with or without lipopolysaccharide (0.1 mg/L) for 24 h. Immunocytochemistry and in-situ hybridisation were used to phenotype the cells and cytokines.

Findings: In explants from atopic children, lipopolysaccharide prevented allergen-induced T-helper type 2 (Th2) inflammation and upregulated Th1 cytokine reactivity and expression. These effects were blocked by antibody to interleukin 10. In children but not in adults, lipopolysaccharide caused increases of three times in T-cell reactivity, five times in T-cell proliferation, and four times in expression of interleukin 10 compared with mucosa stimulated with allergen alone. This difference in response was mirrored by lipopolysaccharide-induced increases in TLR4 reactivity in children but not adults. TLR4 receptor was expressed by CD3-positive T cells, and TLR4-positive cells contained interleukin 10. Lipopolysaccharide increased expression of cells positive for both CD3 and TLR4; both TLR4 and interleukin 10; and both CD4 and CD25.

Interpretation: Lipopolysaccharide inhibits allergic inflammation in nasal mucosa of atopic children by skewing local immune responses from Th2 to Th1 and upregulating production of interleukin 10. These effects are mediated by TLR4. Our results emphasise an important difference between adults and children in their ability to respond to bacterial products. These differences could have a role in normal maturation of the immune system.