Diesel exhaust enhances virus- and poly(I:C)-induced Toll-like receptor 3 expression and signaling in respiratory epithelial cells

Prior exposure of respiratory epithelial cells to an aqueous-trapped solution of diesel exhaust (DEas) enhances the susceptibility to influenza infections. Here, we examined the effect of DEas on the Toll-like receptor 3 (TLR3) pathway, which is responsible for the recognition of and response to viruses and double-stranded RNA. Flow cytometric and confocal microscopy analyses showed that TLR3 is predominantly expressed in the cytoplasm of respiratory epithelial cells. To examine the effect of DE on TLR3 expression and function, differentiated human bronchial or nasal epithelial cells as well as A549 cells were exposed to DEas and then infected with influenza A or treated with polyriboinosinic acid-polyribocytidylic acid [poly(I:C)], a synthetic form of double-stranded RNA. Exposure to DEas before infection with influenza or stimulation with poly(I:C) significantly upregulated the expression of TLR3. Additionally, preexposure to DEas significantly increased the poly(I:C)-induced expression of IL-6. Overexpression of a dominant-negative mutant form of TNF receptor-associated factor 6 reversed the effects of DEas on poly(I:C)-induced IL-6 expression, suggesting that the response was TLR3 dependent. Similarly, preexposure to DEas significantly increased nuclear levels of interferon regulatory factor 3 and the expression of IFN-beta in response to poly(I:C). Pretreatment with wortmannin, a specific inhibitor of phosphatidylinositol 3-kinase, was able to abate the effect of DEas on poly(I:C)-induced IFN-beta expression. Together, these results indicate that exposure of respiratory epithelial cells to DEas could potentially alter the response to viral infections by increasing the expression and function of TLR3.