Abstract
Bruton's tyrosine kinase (Btk) is a signaling molecule involved in development and activation of B cells through B-cell receptor (BCR) and Toll-like receptor (TLR) signaling. We have previously shown that transgenic mice that overexpress human Btk under the control of the CD19 promoter (CD19-hBtk) display spontaneous germinal center formation, increased cytokine production, anti-nuclear autoantibodies (ANAs), and systemic autoimsmune disease upon aging. As TLR and BCR signaling are both implicated in autoimmunity, we studied their impact on splenic B cells. Using phosphoflow cytometry, we observed that phosphorylation of ribosomal protein S6, a downstream Akt target, was increased in CD19-hBtk B cells following BCR stimulation or combined BCR/TLR stimulation, when compared with wild-type (WT) B cells. The CD19-hBtk transgene enhanced BCR-induced B cell survival and proliferation, but had an opposite effect following TLR9 or combined BCR/TLR9 stimulation. Although the expression of TLR9 was reduced in CD19-hBtk B cells compared to WT B cells, a synergistic effect of TLR9 and BCR stimulation on the induction of CD25 and CD80 was observed in CD19-hBtk B cells. In splenic follicular (Fol) and marginal zone (MZ) B cells from aging CD19-hBtk mice BCR signaling stimulated in vitro IL-10 production in synergy with TLR4 and particularly TLR9 stimulation, but not with TLR3 and TLR7. The enhanced capacity of CD19-hBtk Fol B cells to produce the pro-inflammatory cytokines IFNγ and IL-6 compared with WT B cells was however not further increased following in vitro BCR or TLR9 stimulation. Finally, we used crosses with mice deficient for the TLR-associated molecule myeloid differentiation primary response 88 (MyD88) to show that TLR signaling was crucial for spontaneous formation of germinal centers, increased IFNγ, and IL-6 production by B cells and anti-nuclear autoantibody induction in CD19-hBtk mice. Taken together, we conclude that high Btk expression does not only increase B cell survival following BCR stimulation, but also renders B cells more sensitive to TLR stimulation, resulting in increased expression of CD80, and IL-10 in activated B cells. Although BCR-TLR interplay is complex, our findings show that both signaling pathways are crucial for the development of pathology in a Btk-dependent model for systemic autoimmune disease.
Highlights
B cells are crucial players in autoimmunity, as B cell depletion therapy was proven effective in patients with several systemic autoimmune diseases including Sjögren’s syndrome (SjS) and rheumatoid arthritis (RA) [1, 2]
Bruton’s tyrosine kinase (Btk) is a critical kinase in the B cell receptor (BCR) signaling pathway and is known to interact with various proteins that are downstream of Toll-like receptor (TLR)
Our findings provide evidence that TLR signaling is critical in systemic autoimmunity driven by overexpression of Btk in transgenic mice, which is characterized by the induction of TLR7/9-associated auto-antibody specificities, including dsDNA, histone, RNP-A, RNP-C, and SmB
Summary
B cells are crucial players in autoimmunity, as B cell depletion therapy was proven effective in patients with several systemic autoimmune diseases including Sjögren’s syndrome (SjS) and rheumatoid arthritis (RA) [1, 2]. These diseases are marked by altered B cell selection leading to the production of autoreactive antibodies, an important hallmark in the pathology of systemic autoimmune diseases. BCR signaling is crucial for the selection of B cells. Evidence is accumulating that additional signals derived from CD40, Toll-like receptors (TLRs) and BAFFR affect selection of B cells [4]
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have