Toll‐like receptor (TLR) 2 and TLR4 are essential for Aspergillus‐induced activation of murine macrophages

Aspergillus fumigatius is a ubiquitous saprophytic fungus that has become the most prevalent airborne fungal pathogen for immunocompromised patients during the last two decades. In this report we have analysed how macrophages recognize this microorganism. Using transfected human HEK 293 cells we demonstrate that NF‐κB‐dependent promoter activation triggered by A. fumigatus is mediated by Toll‐like receptors TLR2 and TLR4, whereas no activation was observed in cells overexpressing other distinct TLR proteins (TLR1, TLR3, TLR5–10). Using macrophages derived from mice lacking TLR2 expression, expressing defective TLR4 or both we found that A. fumigatus conidia and hyphae induce NF‐κB translocation, release of pro‐inflammatory molecules, like TNFα, and the chemoattractant MIP‐2 in a TLR2‐ and TLR4‐dependent manner. Recognition of A. niger and A. fumigatus , was similar in terms of the parameters analysed, suggesting that pathogenic and non‐pathogenic aspergilli are sensed by macrophages in a similar fashion. Finally, we found that recruitment of neutrophils is severely impaired in mice lacking both functional TLR2 and TLR4, but is less impaired in single TLR2‐ or TLR4‐deficient mice, providing evidence that both receptors are required for an optimal immune response to Aspergillus in vivo.