Replication-Dependent Potent IFN-α Induction in Human Plasmacytoid Dendritic Cells by a Single-Stranded RNA Virus

Abstract

Plasmacytoid dendritic cells sense viral ssRNA or its degradation products via TLR7/8 and CpG motifs within viral DNA via TLR9. Although these two endosomal pathways operate independently of viral replication, little is known about the detection of actively replicating viruses in plasmacytoid dendritic cell (PDC). Replication and transcription of the viral genome of ssRNA viruses as well as many DNA viruses lead to the formation of cytosolic dsRNA absent in noninfected cells. In this study, we used human respiratory syncytial virus (HRSV) encoding a fusion (F) protein for direct cytosolic entry. Both HRSV infection and cytosolic delivery of a 65-nt dsRNA led to potent IFN-α induction in PDC, but not in myeloid dendritic cells. Inactivation of HRSV by UV irradiation abrogated IFN-α induction in PDC. The comparison of two respiratory syncytial virus (RSV) constructs carrying either the HRSV or the bovine RSV F protein revealed that F-mediated cytosolic entry of RSV was absolutely required for IFN-α induction in PDC. HRSV-induced IFN-α production was independent of endosomal acidification and of protein kinase R (PKR) kinase activity, as demonstrated with chloroquine and the PKR inhibitor 2-aminopurine, respectively. In contrast, the induction of IFN-α by the TLR7/8 ligand R848, by the TLR9 ligand CpG-A ODN 2216, and by inactivated influenza virus (TLR7/8 dependent) was completely blocked by 2-aminopurine. IFN-α induction by mouse pathogenic Sendai virus was not affected in PKR- and MyD88-deficient mice, confirming that a ssRNA virus, which is able to directly enter host cells via fusion at the plasma membrane, can be detected by PDC independently of PKR, TLR7/8, and TLR9.

The histopathology of fatal untreated human respiratory syncytial virus infection

“…Macrophages can be infected in cell culture, 17 so this may represent actual infection, but it is also possible that this represents phagocytosed antigen. Although DCs can be infected with RSV in vitro 18 and CD1a þ cells were detected in peribronchiolar tissue, no RSV-positive cells were identified that appeared to have DC morphology. DCs are highly mobile and, if infected, may have already moved to areas of organized lymphoid tissue for induction of adaptive immune responses.…”

IFN-α production by human mononuclear cells infected with varicella-zoster virus through TLR9-dependent and -independent pathways

“…[18][19][20] It has been reported that replication and transcription of the viral genome of single-stranded viral RNA viruses and many DNA viruses often lead to the formation of cytosolic dsRNA. 45,46 Thus, dsRNA could induce PKR activation. Recently, 2AP was suggested to have suppressive effect on other kinases; 47 therefore, further studies to explore the roles of PKR and other kinases in VZV infection are needed.…”

TLR signaling

“…It is reported that RSV infection triggers type I IFN induction in pDCs in a MyD88-independent manner. 75 This induction is entirely dependent on virus entry into cytosol and replication. Furthermore, while pDCs recognize HSV-1 genomic DNA via TLR9, there is no increase in HSV-1 replication in TLR9-deficient mice after local infection.…”