Th17 inflammation in the lungs is sustained by multiple exposures to house dust and is dependent on TLR4 and conventional dendritic cells

Microbial products can prime antigen-specific T helper 17 cell responses to inhaled allergens. However, once established, it is unclear how pathogenic lung Th17 responses are sustained. To study this, we sensitized mice to inhaled ovalbumin (OVA)together with various environmental adjuvants, including house dust extract(HDE). Animals were then chronically challenged with OVA alone, or OVA plus the adjuvant previously used during sensitization. Challenge with OVA plus HDE elicited greater airway neutrophilia, lymphocytosis and IL-17A than did challenge with OVA alone. HDE exposure with OVA also promoted greater expansion of adoptively transferred, OVA-specific, IL-17A-fate mapping Th17 cells. These effects were independent of enzymatic activity in HDE, but required TLR4 in recipient mice. Analysis of Tlr4 bone marrow chimeric mice and conditional TLR4 knockout mice indicated that Th17 proliferation is sustained by HDE via TLR4 signaling predominantly in hematopoietic cells, including CD11c+ cells, rather than in airway epithelial cells. IL-23 contributes to the maintenance of these lung Th17 cells. Use of genetically modified mice lacking distinct lung myeloid cell subsets, and co-culture of various lung antigen-presenting cell types with mature Th17 cells, revealed that conventional dendritic cells (either CD103+ or CD11b+), rather than lung macrophages or B cells, support the revival of Th17 clonal expansion and activation. Thus, maintenance of Th17-dependent inflammation in the lung requires persistent innate immune responses, TLR4, and the presence of conventional dendritic cells.