TLR4 sensing of IsdB of Staphylococcus aureus induces a proinflammatory cytokine response via the NLRP3-caspase-1 inflammasome cascade

The iron-regulated surface determinant protein B (IsdB) of Staphylococcus aureus is involved in the acquisition of iron from hemoglobin. Moreover, IsdB elicits an adaptive immune response in mice and humans. Here, we show that IsdB also has impact on innate immunity. IsdB induces the release of proinflammatory cytokines, including IL-6 and IL-1 beta, in innate immune cells of humans and mice. In silico analysis and thermophoresis show that IsdB directly binds to TLR4 with high affinity. TLR4 sensing was essential for the IsdB-mediated production of IL-6, IL-1 beta, and other cytokines as it was abolished by blocking of TLR4-MyD88-IRAK1/4-NF-kappa B signaling. The release of IL-1 beta additionally required activation of the NLRP3 inflammasome. In human monocytes infected with live S. aureus, IsdB was necessary for maximal IL-1 beta release. Our studies identify S. aureus IsdB as a novel pathogen-associated molecular pattern that triggers innate immune defense mechanisms.