Hepatitis C Virus Fails To Activate NF-κB Signaling in Plasmacytoid Dendritic Cells

Abstract

Plasmacytoid dendritic cells (pDCs) respond to viral infection by production of alpha interferon (IFN-␣P lasmacytoid dendritic cells (pDCs) are a highly specialized subset of dendritic cells that function as sentinels for viral infection and are responsible for production of type I interferons (IFN), proinflammatory cytokines, and antigen presentation during viral infection (15,19,32). pDCs are able to detect the genetic material of viruses with a subset of Toll-like receptors (TLR) localized to the endosomal compartment (10). These nucleotidesensing TLRs include TLR7 and TLR8, which recognize singlestranded RNA, and TLR9, which recognizes DNA. TLR7 also recognizes synthetic imidazoquinoline components, for example R848 (resiquimod), whereas TLR9 recognizes synthetic CpG oligonucleotides, for example CpG-A or CpG-B. Ligation of TLR7 and TLR9 with their agonists triggers a signaling cascade, which starts with recruitment of the MyD88 adaptor molecule to the cytoplasmic domain of nucleotide-sensing TLR. This activates the assembly of a multiprotein signal-transducing complex in the cytoplasm that includes interferon-regulatory factor 7 (IRF7) (10). Activated IRF7, which is constitutively expressed in pDCs, translocates to the nucleus and initiates the transcription of type I IFN.The elimination of hepatitis C virus (HCV) in more than 50% of chronically infected patients by treatment with alpha interferon (IFN-␣) (9, 20) suggests that pDCs can play an important role in the control of HCV infection. Several reports have shown that exposure of pDCs from healthy donors to HCV particles results in no or only weak production of type I IFN and cell differentiation (4,7,11,13,31). A recent report has shown that pDCs exposed in direct cell-to-cell contact with HCV-infected hepatoma cells, unlike those exposed to cell-free HCV virions, produce large amounts of type I IFN via TLR7 signaling (35). This suggests that pDCs could be responsible for production of intrahepatic type I IFN (17, 35). Importantly, these events require viral RNA replication but not virion formation in the stimulator cells. In parallel to IRF7-mediated production of IFN-␣, MyD88 signaling also leads to activation of nuclear factor kappa B (NF-B) and mitogenactivated protein kinases (MAPKs). Both NF-B and MAPKs stimulate secretion of the proinflammatory cytokines interleukin 6 (IL-6) and tumor necrosis factor ␣ (TNF-␣) and stimulate expression of costimulatory molecules such as CD80 and CD86. Recent reports have identified a new signaling pathway induced by TLR7 and dependent on PI3K-p38MAPK, which stimulates the early IFN-inducible genes MxA and CXCL10 and the TNF-related apoptosis-inducing ligand (TRAIL) in the absence of type I IFN (6, 27).To better understand the molecular mechanism of HCV sensing, we investigated whether exposure of pDCs to HCV-infected hepatoma cells induces not only IRF7 signaling but also NF-B signaling pathways necessary for pDC functions. We demonstrate that in comparison to influenza virus or synthetic agonists of TLR7 and TLR...

Cross Talk between Inhibitory Immunoreceptor Tyrosine-Based Activation Motif-Signaling and Toll-Like Receptor Pathways in Macrophages and Dendritic Cells

“…Mature pDCs are characterized by the upregulation of CD40 and co-stimulatory molecules, including CD80 and CD86. Recent results have shown that BDCA-2 mAb cross-linking inhibits CpG-A and CpG-B-induced upregulation of co-stimulatory molecules CD40 and CD86 (62, 77–79). In contrast, CD40L-stimulated upregulation of CD86 in pDCs is unaffected by BDCA-2 cross-linking.…”

The Plasmacytoid Dendritic Cell as the Swiss Army Knife of the Immune System: Molecular Regulation of Its Multifaceted Functions

“…For example, the hepatitis C virus (HCV) envelope glycoprotein E2 binds to BDCA2 and DCIR, which inhibits IFNα production in pDC when exposed to HCV-infected hepatocytes(23). Moreover, exposure of pDC to HCV-infected hepatoma cells prevents NFκB phosphorylation via an endocytosis-dependent mechanism resulting in a lack of cell surface expression of CD40, CCR7, CD86 and TRAIL, and of TNFα and IL6 secretion(24). Another example is HIV, that induces production of IFNα via TLR7 signaling to elicit antiviral activity in acute infection(25).…”

HCV glycoprotein E2 is a novel BDCA-2 ligand and acts as an inhibitor of IFN production by plasmacytoid dendritic cells

“…pDCs purified from PBMCs as described previously were from 75% to 95% pure, with a contamination of Ͻ 5% myeloid dendritic cells. [11][12][13]16 Isolated pDCs were cultured in RPMI 1640 supplemented with 10% FCS and antibiotics. To optimize viability in overnight experiments, recombinant IL-3 (R&D Systems Europe) was added to a final concentration of 10 ng/mL.…”

“…30 Thus, our demonstration that HCV-infected hepatoma cells do not stimulate phosphorylation of NF-B can explain lower levels of type I and III IFNs induced by cell-associated HCV than those induced by synthetic agonists of TLR7/9. 16 Activation of IFN-stimulated genes in infected liver, one of the hallmarks of HCV pathogenesis and chronic HCV infection, remains elusive with respect to the cellular source of type I and III IFNs. In infected liver, pDCs exposed to hepatocyte-associated HCV are good candidates for production of intrahepatic type I IFN, 14,31,32 which not only activates expression of IFN-stimulated genes but also promotes greater activation of both pDCs and myeloid dendritic cells during virus infections.…”