Activation of Eosinophils via Toll-like Receptor (tlr)3, Tlr7 and Tlr9: Link between Viral Infection and Asthma?

WINNING ABSTRACT: Asthma is disease characterized by a massive accumulation of eosinophils that release an array of tissue-damaging mediators. Respiratory viral infections are thought to be a leading cause of exacerbations of asthma. One possible explanation might be a direct activation of viral components through Toll-like receptors (TLRs), a receptor family comprising 10 different pathogen-recognizing members (TLR1-TLR10). The virus-recognizing TLRs are TLR3, TLR7/8 and TLR9, which respond to viral dsRNA, ssRNA and CpG-DNA. The present study aimed to investigate the expression of these TLRs and their functions in human eosinophils. Eosinophils were isolated from peripheral blood using magnetic beads (purity .97%). Cells were incubated with or without poly(I:C), R-837 or CpG alone, the synthetic ligands of TLR3, TLR7 and TLR9, respectively, or combined with IL-4 or histamine. Flow cytometry, and ELISA were used to analyze expression of TLRs and various surface markers, viability and secretion of inflammatory mediators. Eosinophils expressed proteins for TLR3, TLR7 and TLR9. Poly(I:C), R-837 and CpG prolonged survival, up-regulated expression of the adhesion molecule CD11b and increased secretion of IL-8 compared to unstimulated controls. These effects were affected by the presence of IL-4 and histamine. This study shows that several viral products directly activate eosinophils through their TLRs. Since eosinophils are central in asthma, the TLR system may be an important mechanism of eosinophil activation linking viral infections with exacerbations. Consequently, this system represents a future clinical target for the resolution of asthmatic disease.