Signaling by Toll-like Receptors 8 and 9 Requires Bruton's Tyrosine Kinase

Toll-like receptors (TLRs) are a primary surveillance system for the detection of pathogens and are crucial to the activation of host defense. TLR7 and TLR8 sense single-stranded RNA from viruses or host ribonucleoproteins and synthetic imidazoquinolines such as R848, whereas TLR9 senses unmethylated CpG motifs in viral and bacterial DNA and in host DNA. Here we report the endogenous interaction between Brutons's tyrosine kinase (Btk) and human TLR8 and TLR9 in the monocytic cell line THP1. We also show that R848, single-stranded RNA, and CpGB-DNA activate Btk in THP1 cells as shown by phosphorylation of the tyrosine 223 residue of Btk and also by increased autokinase activity. We demonstrate that Btk is required for NFkappaB activation, participating in the pathway to increased phosphorylation of p65 on serine 536 activated by TLR8 and TLR9. Finally we demonstrate that peripheral blood mononuclear cells from patients with X-linked agammaglobulinaemia (XLA) that have dysfunctional Btk are impaired in the induction of interleukin-6 by CpGB-DNA. This study therefore establishes Btk as a key signaling molecule that interacts with and acts downstream of TLR8 and TLR9. Lack of functioning Btk in XLA patients downstream of TLR8 and TLR9 might explain the susceptibility of XLA patients to viral infections.