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Herpes simplex virus type 1 (HSV-1), a member of the herpes virus family, is characterized by a short replication cycle, high cytopathogenicity and distinct neurotropism. Primary infection and reactivation may cause severe diseases in immunocompetent and immunosuppressed individuals. This study investigated the role of human plasmacytoid dendritic cells (pDC) in the activation of natural killer (NK) cells for the control of herpesviral infections. Within peripheral blood mononuclear cells, UV-inactivated HSV-1 and CpG-A induced CD69 up-regulation on NK cells, whereas infectious HSV-1 was particularly active in inducing NK cell effector functions interferon-γ (IFN-γ) secretion and degranulation. The pDC-derived IFN-α significantly contributed to NK cell activation, as evident from neutralization and cell depletion experiments. In addition, monocyte-derived tumour necrosis factor-α (TNF-α) induced after exposure to infectious HSV-1 was found to stimulate IFN-γ secretion. A minority of monocytes was shown to be non-productively infected in experiments using fluorescently labelled viruses and quantitative PCR analyses. HSV-1-exposed monocytes up-regulated classical HLA-ABC and non-classical HLA-E molecules at the cell surface in an IFN-α-dependent manner, whereas stress molecules MICA/B were not induced. Notably, depletion of monocytes reduced NK cell effector functions induced by infectious HSV-1 (P < 0·05). Altogether, our data suggest a model in which HSV-1-stimulated pDC and monocytes activate NK cells via secretion of IFN-α and TNF-α. In addition, infection of monocytes induces NK cell effector functions via TNF-α-dependent and TNF-α-independent mechanisms. Hence, pDC and monocytes, which are among the first cells infiltrating herpetic lesions, appear to have important bystander functions for NK cells to control these viral infections.

This study tested the hypothesis that mtDNA fragments carry immunostimulatory motifs that naturally induce immune activation by PDC. Genomic and mtDNA induced similar IFN-α production after transfection into PBMCs using the liposomal transfection reagent DOTAP. Shortening of mtDNA to CpG islands enhanced the immunostimulatory activity, based on the presence of unmethylated CpG DNA. Further fragmentation into mtODN, which exhibited similarities to published CpG ODN, resulted in a strong immunostimulatory activity in addition to PDC maturation and migration. The addition of the human cathelicidin LL-37 to CpG islands induced spontaneous PDC IFN-α production. Notably, one phosphodiester mtODN with a double-palindromic structure induced PDC IFN-α production in the absence of DOTAP. Flow cytometry, life-cell, and confocal imaging revealed attachment and spontaneous uptake into PDC, colocalizing, in part, with TLR9 in early endosomal vesicles. This process was accompanied by a moderate but significant PDC maturation in addition to B cell and NK cell activation (P