Dual Role of the Tyrosine Kinase Syk in Regulation of Toll-Like Receptor Signaling in Plasmacytoid Dendritic Cells

Abstract

Crosslinking of regulatory immunoreceptors (RR), such as BDCA-2 (CD303) or ILT7 (CD85g), of plasmacytoid dendritic cells (pDCs) efficiently suppresses production of type-I interferon (IFN)-α/β and other cytokines in response to Toll-like receptor (TLR) 7/9 ligands. This cytokine-inhibitory pathway is mediated by spleen tyrosine kinase (Syk) associated with the ITAM-containing adapter of RR. Here we demonstrate by pharmacological targeting of Syk that in addition to the negative regulation of TLR7/9 signaling via RR, Syk also positively regulates the TLR7/9 pathway in human pDCs. Novel highly specific Syk inhibitor AB8779 suppressed IFN-α, TNF-α and IL-6 production induced by TLR7/9 agonists in primary pDCs and in the pDC cell line GEN2.2. Triggering of TLR9 or RR signaling induced a differential kinetics of phosphorylation at Y352 and Y525/526 of Syk and a differential sensitivity to AB8779. Consistent with the different roles of Syk in TLR7/9 and RR signaling, a concentration of AB8779 insufficient to block TLR7/9 signaling still released the block of IFN-α production triggered via the RR pathway, including that induced by hepatitis B and C viruses. Thus, pharmacological targeting of Syk partially restored the main pDC function—IFN-α production. Opposing roles of Syk in TLR7/9 and RR pathways may regulate the innate immune response to weaken inflammation reaction.

A human tissue-based functional assay platform to evaluate the immune function impact of small molecule inhibitors that target the immune system

“…This data is consistent with previous reports demonstrating that Syk kinase is essential for mediating NK cell killing function [31]. While a majority of the TLR signaling pathways are mediated by IRAK4 kinase [32], we observed that inhibitors of other kinases such as PI3Kδ, SYK/ZAP-70 and JAK also have an impact on functional TLR responses, consistent with previous reports [33–35]. Among the TLR responses evaluated, cyclosporine, PI3Kδ and SYK/ZAP70 inhibitors inhibited TLR3 and TLR9, but not TLR7 induced responses.…”

Tyrosine kinase SYK is a potential therapeutic target for liver fibrosis

“…We confirmed that GS‐9973 (1.0 μM) effectively inhibited SYK phosphorylation at Y525/526 (Fig. B), the key activation site at the kinase domain of SYK . We also found that GS‐9973 down‐regulated protein expression of α‐SMA, PDGFRβ and PAI‐1 (Fig.…”

FcγRIIIa Signaling Modulates Endosomal TLR Responses in Human CD4+ T Cells

“…Another report suggested a role for the C330b-DAP12-TLR4-Syk-PI3K signaling complex in amplifying inflammatory responses in macrophages (53). A role for Syk in the regulation of TLR signaling in pDCs is also observed (54). These studies and our results show that the FcγRIIIa-pSyk signaling induced NA-TLRs are critical for CD4 + T-cell responses.…”