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.

Cancer Research Center of Marseille CNRS UMR7258 INSERM U1068 Institut Paoli Calmettes Aix Marseille Université UM105 Marseille France

Department of Genetics and Microbiology Faculty of Sciences Biocev Charles University Prague Czechia

Etablissement Français du Sang Rhône Alpes Grenoble France

INSERM U 1209 CNRS UMR 5309 Institute for Advanced Biosciences Université Grenoble Alpes Grenoble France

Institute of Molecular Genetics of the Czech Academy of Sciences Prague Czechia

Institute of Molecular Genetics of the Czech Academy of Sciences Prague Czechia Department of Genetics and Microbiology Faculty of Sciences Biocev Charles University Prague Czechia Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences Gilead Sciences and IOCB Research Centre Prague Czechia

Institute of Molecular Genetics of the Czech Academy of Sciences Prague Czechia Department of Genetics and Microbiology Faculty of Sciences Biocev Charles University Prague Czechia Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences Gilead Sciences and IOCB Research Centre Prague Czechia Cancer Research Center of Marseille CNRS UMR7258 INSERM U1068 Institut Paoli Calmettes Aix Marseille Université UM105 Marseille France

Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences Gilead Sciences and IOCB Research Centre Prague Czechia

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