SummaryHerpes 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-c (IFN-c) secretion and degranulation. The pDC-derived IFN-a significantly contributed to NK cell activation, as evident from neutralization and cell depletion experiments. In addition, monocyte-derived tumour necrosis factor-a (TNF-a) induced after exposure to infectious HSV-1 was found to stimulate IFN-c 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-a-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 < 0Á05). Altogether, our data suggest a model in which HSV-1-stimulated pDC and monocytes activate NK cells via secretion of IFN-a and TNF-a. In addition, infection of monocytes induces NK cell effector functions via TNF-a-dependent and TNF-a-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.

SummaryMalignant melanoma is an aggressive tumour of the skin with increasing incidence, frequent metastasis and poor prognosis. At the same time, it is an immunogenic type of cancer with spontaneous regressions. Most recently, the tumoricidal effect of plasmacytoid dendritic cells (pDC) and their capacity to overcome the immunosuppressive tumour microenvironment are being investigated. In this respect, we studied the effect of the infectious, but replication-deficient, herpes simplex virus 1 (HSV-1) d106S vaccine strain, which lacks essential immediate early genes, in pDC co-cultures with 11 melanoma cell lines. We observed a strong cytotoxic activity, inducing apoptotic and necrotic cell death in most melanoma cell lines. The cytotoxic activity of HSV-1 d106S plus pDC was comparable to the levels of cytotoxicity induced by natural killer cells, but required only a fraction of cells with effector : target ratios of 1 : 20 (P < 0Á05). The suppressive activity of cell-free supernatants derived from virus-stimulated pDC was significantly neutralized using antibodies against the interferona receptor (P < 0Á05). In addition to type I interferons, TRAIL and granzyme B contributed to the inhibitory effect of HSV-1 d106S plus pDC to a minor extent. UV-irradiated viral stocks were significantly less active than infectious particles, both in the absence and presence of pDC (P < 0Á05), indicating that residual activity of HSV-1 d106S is a major component and sensitizes the tumour cells to interferon-producing pDC. Three leukaemic cell lines were also susceptible to this treatment, suggesting a general anti-tumour effect. In conclusion, the potential of HSV-1 d106S for therapeutic vaccination should be further evaluated in patients suffering from different malignancies.