['Bruton tyrosine kinase (BTK) plays an essential role in various biologic functions of different cell types. Mutations in BTK lead to X-linked agammaglobulinemia (XLA) in humans. BTK was recently linked to the innate immune system, in particular, the Toll-like receptor (TLR) pathway; however, the TLR9 pathway has never been studied in dendritic cells (DCs) of patients with XLA.', 'The aim of this study was to investigate the role of BTK in human DC activation upon TLR9 stimulation.', 'DCs of patients with XLA and healthy donors were stimulated via TLR4/9 and evaluated for cell activation and cytokine production.', 'We showed that BTK plays an essential role in DC responses to unmethylated CpG oligodeoxynucleotide: although responses to lipopolysaccaride/TLR4 induce normal DC activation in terms of upregulation of specific markers (CD86, CD83, CD80, HLA-DR), the CpG/TLR9 pathway is completely impaired in patients with XLA. Furthermore, cytokine production upon TLR9 activation in patients with XLA is radically impaired in terms of IL-6, IL-12, TNF-α, and IL-10 production. Interestingly, BTK mediated STAT1/3 upregulation in a TLR9-dependent manner. The important role of BTK in human DC activation was confirmed after incubation of healthy DCs with ibrutinib, the specific BTK inhibitor, which resulted in impairment of TLR9 responses as seen in patients with XLA.'] Does bruton tyrosine kinase mediate TLR9-dependent human dendritic cell activation?