Bruton’s tyrosine kinase (Btk), defective in X-linked agammaglobulinemia (XLA), has been shown to play a role in the signaling pathways of multiple toll-like receptors (TLRs) in a variety of cell types. Mouse models of XLA have suggested cellular dysfunction in response to TLR ligands in vivo, and increased TLR-induction of pro-inflammatory cytokines by Btk deficient cells in vitro. Human studies have been highly contradictory, possibly due to the use of in vitro differentiated cells and the use of non-specific Btk inhibitors to model Btk-deficiency.

To clarify the effect of Btk deficiency on TLR-mediated inflammation, non-differentiated human primary cells lacking Btk from patients with XLA were cultured with ligands for TLRs 4, 7 and 8, and inflammatory signaling and cytokine production was assessed. Btk-deficient mouse neutrophils had previously demonstrated dysfunctional responsiveness to LPS, however, in this study both LPS and CL097, a synthetic TLR7/8 ligand, activated XLA neutrophil shedding of surface CD62L, and led to phosphorylation of MAP kinases p38, JNK and ERK, similar to control cells. TLR activation also induced respiratory burst and retarded apoptosis of XLA neutrophils comparable to normal controls.

In XLA peripheral blood mononuclear cells, TLR activation of the NF-κB and MAP kinase signaling pathways was also intact. Btk-deficient plasmacytoid dendritic cells (pDCs) produced normal levels of IFN-α in response to TLR7 stimulation, while production of the pro-inflammatory cytokines IL-6 and TNF-α by Btk-deficient monocytes and classical dendritic cells (cDCs) was increased compared to control cells. In contrast to previously presented murine data, regulation of degradation of the TLR adaptor TIRAP, which terminates TLR4 signaling, was found intact in XLA patient cells, suggesting that this regulatory control is not dependent on Btk in humans. These data demonstrate that XLA neutrophils demonstrate intact TLR responsiveness, and XLA monocytes and dendritic cells have intact TLR signaling and even augmented cytokine responses to selected TLR ligands.