The MEK1/2-ERK Pathway Inhibits Type I IFN Production in Plasmacytoid Dendritic Cells

Abstract

Recent studies have reported that the crosslinking of regulatory receptors (RRs), such as blood dendritic cell antigen 2 (BDCA-2) (CD303) or ILT7 (CD85g), of plasmacytoid dendritic cells (pDCs) efficiently suppresses the production of type I interferons (IFN-I, α/β/ω) and other cytokines in response to toll-like receptor 7 and 9 (TLR7/9) ligands. The exact mechanism of how this B cell receptor (BCR)-like signaling blocks TLR7/9-mediated IFN-I production is unknown. Here, we stimulated BCR-like signaling by ligation of RRs with BDCA-2 and ILT7 mAbs, hepatitis C virus particles, or BST2 expressing cells. We compared BCR-like signaling in proliferating pDC cell line GEN2.2 and in primary pDCs from healthy donors, and addressed the question of whether pharmacological targeting of BCR-like signaling can antagonize RR-induced pDC inhibition. To this end, we tested the TLR9-mediated production of IFN-I and proinflammatory cytokines in pDCs exposed to a panel of inhibitors of signaling molecules involved in BCR-like, MAPK, NF-ĸB, and calcium signaling pathways. We found that MEK1/2 inhibitors, PD0325901 and U0126 potentiated TLR9-mediated production of IFN-I in GEN2.2 cells. More importantly, MEK1/2 inhibitors significantly increased the TLR9-mediated IFN-I production blocked in both GEN2.2 cells and primary pDCs upon stimulation of BCR-like or phorbol 12-myristate 13-acetate-induced protein kinase C (PKC) signaling. Triggering of BCR-like and PKC signaling in pDCs resulted in an upregulation of the expression and phoshorylation of c-FOS, a downstream gene product of the MEK1/2-ERK pathway. We found that the total level of c-FOS was higher in proliferating GEN2.2 cells than in the resting primary pDCs. The PD0325901-facilitated restoration of the TLR9-mediated IFN-I production correlated with the abrogation of MEK1/2-ERK-c-FOS signaling. These results indicate that the MEK1/2-ERK pathway inhibits TLR9-mediated type I IFN production in pDCs and that pharmacological targeting of MEK1/2-ERK signaling could be a strategy to overcome immunotolerance of pDCs and re-establish their immunogenic activity.

TLR7 and TLR8 activate distinct pathways in monocytes during RNA virus infection

“…We observed that the TLR7-dependent increase in FOSL1 expression was dependent on both (99) and FOSL1 (72). Moreover, the absence of type I IFN expression upon TLR7 stimulation had previously been observed in human macrophages upon HCV infection (14) and in human dendritic cells upon Influenza infection or small molecule activation(100).…”

Regulatory role of SphK1 in TLR7/9‐dependent type I interferon response and autoimmunity

“…Mitogen activated protein kinases, including ERK1/2 and JNK1/2 signalling pathways, have also been implicated in IFN production in response to TLR activation . Inhibition of SphK1 attenuated activation of these signalling pathways during TLR stimulation (Figures E and S3C).…”

Human Blood and Tonsil Plasmacytoid Dendritic Cells Display Similar Gene Expression Profiles but Exhibit Differential Type I IFN Responses to Influenza A Virus Infection

“…Cross-linking of the Ig-like transcript ILT7 and the transmembrane adapter FcεRIg on primary human PDCs inhibited IFN-a production (92). CD303 (BDCA-2) can also directly bind FcεRIg and activated a novel BCR-like signaling pathway suppressing IFN-a production (93,94). Either of the inhibitory pathways may be responsible for dampened IFN-a production by tonsil PDCs via mechanisms currently unknown.…”