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

Besma Aouar,Katerina Trejbalova,Daniel Olive,David Durantel,Ivan Hirsch,Jan Weber,Laurence Chaperot,Jonathan Florentin,Jacques A Nunès,Patrice Dubreuil,Ruzena Stranska,Sebastien Letard,Jiri Hejnar,Albert Font-Haro,Joel Plumas,Denisa Kovarova

doi:10.1371/journal.pone.0156063

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.

Highlights

Plasmacytoid dendritic cells are a highly specialized subset of dendritic cells that plays a central role at the interface of innate and adaptive immunity
Our results show that triggering of TLR9 and RR signaling differentially induced kinetics of spleen tyrosine kinase (Syk) phosphorylation (Fig 1A)
Our results demonstrate that Syk is involved in Plasmacytoid dendritic cells (pDCs) in both RR and TLR7/9 signaling

Summary

Introduction

Plasmacytoid dendritic cells (pDCs) are a highly specialized subset of dendritic cells that plays a central role at the interface of innate and adaptive immunity They are important actors in antiviral and antitumor immunity and potent inducers of autoimmune diseases [1,2,3,4,5]. In addition to TLR7/9, pDC express multiple specific receptors that regulate pDC function and prevent aberrant immune responses These include Fc (FcR) and C-type lectin (CLRs) receptors [9, 10], which signal through the B cell receptor (BCR)-like pathway involving Syk, Mek-Erk1/2, and BLNK [6, 10]. Signaling via pDC regulatory receptors (RR) attenuates TLR7/9-induced production of IFN and proinflammatory cytokines [6, 7, 10]. We hypothesized that inhibition of BCR-like pathway could restore TLR7/9 signaling in pDCs exposed simultaneously to TLR7/ 9 and RR agonists [7]

Methods

Results

Conclusion