Hepatitis C virus fails to activate NF-kappaB signaling in plasmacytoid dendritic cells

Plasmacytoid dendritic cells (pDCs) respond to viral infection by production of alpha interferon (IFN-alpha), proinflammatory cytokines, and cell differentiation. The elimination of hepatitis C virus (HCV) in more than 50% of chronically infected patients by treatment with IFN-alpha suggests that pDCs can play an important role in the control of HCV infection. pDCs exposed to HCV-infected hepatoma cells, in contrast to cell-free HCV virions, produce large amounts of IFN-alpha. To further investigate the molecular mechanism of HCV sensing, we studied whether exposure of pDCs to HCV-infected hepatoma cells activates, in parallel to interferon regulatory factor 7 (IRF7)-mediated production of IFN-alpha, nuclear factor kappa B (NF-kappaB)-dependent pDC responses, such as expression of the differentiation markers CD40, CCR7, CD86, and tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) and secretion of the proinflammatory cytokines TNF-alpha and interleukin 6 (IL-6). We demonstrate that exposure of pDCs to HCV-infected hepatoma cells surprisingly did not induce phosphorylation of NF-kappaB or cell surface expression of CD40, CCR7, CD86, or TRAIL or secretion of TNF-alpha and IL-6. In contrast, CpG-A and CpG-B induced production of TNF-alpha and IL-6 in pDCs exposed to the HCV-infected hepatoma cells, showing that cell-associated virus did not actively inhibit Toll-like receptor (TLR)-mediated NF-kappaB phosphorylation. Our results suggest that cell-associated HCV signals in pDCs via an endocytosis-dependent mechanism and IRF7 but not via the NF-kappaB pathway. In spite of IFN-alpha induction, cell-associated HCV does not induce a full functional response of pDCs. These findings contribute to the understanding of evasion of immune responses by HCV.