Prospect of Plasmacytoid Dendritic Cells in Enhancing Anti-Tumor Immunity of Oncolytic Herpes Viruses.

The major type I interferon-producing plasmacytoid dendritic cells (pDC) surround and infiltrate certain tumors like malignant melanoma, head and neck cancer, and ovarian and breast cancer. The presence of pDC in these tumors is associated with an unfavorable prognosis for the patients as long as these cells are unstimulated. Upon activation by synthetic Toll-like receptor agonists or viruses, however, pDC develop cytotoxic activities. Viruses have the additional advantage to augment cytotoxic activities of pDC via lytic replication in malignant lesions. These effects turn cold tumors into hotspots, recruiting further immune cells to the site of inflammation. Activated pDC contribute to cross-presentation of tumor-associated antigens by classical dendritic cells, which induce cytotoxic T-cells in particular in the presence of checkpoint inhibitors. The modification of oncolytic herpes viruses via genetic engineering favorably affects this process through the enhanced production of pro-inflammatory cytokines, curbing of tumor blood supply, and removal of extracellular barriers for efficient viral spread. Importantly, viral vectors may contribute to stimulation of memory-type adaptive immune responses through presentation of tumor-related neo- and/or self-antigens. Eventually, both replication-competent and replication-deficient herpes simplex virus 1 (HSV-1) may serve as vaccine vectors, which contribute to tumor regression by the stimulation of pDC and other dendritic cells in adjuvant and neo-adjuvant situations.

Generation of an Oncolytic Herpes Simplex Virus 1 Expressing Human MelanA.

Robust anti-tumor immunity requires innate as well as adaptive immune responses. We have shown that plasmacytoid dendritic cells develop killer cell-like activity in melanoma cell cocultures after exposure to the infectious but replication-deficient herpes simplex virus 1 (HSV-1) d106S. To combine this innate effect with an enhanced adaptive immune response, the gene encoding human MelanA/MART-1 was inserted into HSV-1 d106S via homologous recombination to increase direct expression of this tumor antigen. Infection of Vero cells using this recombinant virus confirmed MelanA expression by Western blotting, flow cytometry, and immunofluorescence. HSV-1 d106S-MelanA induced expression of the transgene in fibroblast and melanoma cell lines not naturally expressing MelanA. Infection of a melanoma cell line with CRISPR-Cas9-mediated knockout of MelanA confirmed de novo expression of the transgene in the viral context. Dependent on MelanA expression, infected fibroblast and melanoma cell lines induced degranulation of HLA-matched MelanA-specific CD8+ T cells, followed by killing of infected cells. To study infection of immune cells, we exposed peripheral blood mononuclear cells and in vitro-differentiated macrophages to the parental HSV-1 d106S, resulting in expression of the transgene GFP in CD11c+ cells and macrophages. These data provide evidence that the application of MelanA-encoding HSV-1 d106S could enhance adaptive immune responses and re-direct MelanA-specific CD8+ T cells to tumor lesions, which have escaped adaptive immune responses via downregulation of their tumor antigen. Hence, HSV-1 d106S-MelanA harbors the potential to induce innate immune responses in conjunction with adaptive anti-tumor responses by CD8+ T cells, which should be evaluated in further studies.

Combined cytotoxic activity of an infectious, but non-replicative herpes simplex virus type 1 and plasmacytoid dendritic cells against tumour cells.

Malignant 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 < 0·05). The suppressive activity of cell-free supernatants derived from virus-stimulated pDC was significantly neutralized using antibodies against the interferon-α receptor (P < 0·05). 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 < 0·05), 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.

A subset of human plasmacytoid dendritic cells expresses CD8α upon exposure to herpes simplex virus type 1.

Classical and plasmacytoid dendritic cells (DC) play important roles in the defense against murine and human infections with herpes simplex virus (HSV). So far, CD8α expression has only been reported for murine DC. CD8α(+) DC have prominent cross-presenting activities, which are enhanced by murine CD8α(+) PDC. The human orthologue of murine CD8α(+) DC, the CD141 (BDCA3)(+) DC, mainly cross-present after TLR3 ligation. We report here the serendipitous finding that a subset of human PDC upregulates CD8α upon HSV-1 stimulation, as shown by gene array and flow cytometry analyses. CD8α, not CD8ß, was expressed upon exposure. Markers of activation, migration, and costimulation were upregulated on CD8α-expressing human PDC. In these cells, increased cytokine and chemokine levels were detected that enhance development and function of T, B, and NK cells, and recruit immature DC, monocytes, and Th1 cells, respectively. Altogether, human CD8α(+) PDC exhibit a highly activated phenotype and appear to recruit other immune cells to the site of inflammation. Further studies will show whether CD8α-expressing PDC contribute to antigen cross-presentation, which may be important for immune defenses against HSV infections in vitro and in vivo.

Both plasmacytoid dendritic cells and monocytes stimulate natural killer cells early during human herpes simplex virus type 1 infections.

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.

Identification of novel oligonucleotides from mitochondrial DNA that spontaneously induce plasmacytoid dendritic cell activation.

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<0.05). Altogether, our data indicate that fragmented mtDNA, which may be released as a consequence of apoptotic, necrotic, and necroptotic cell death, can act as a DAMP. For the first time, our study provides a mechanism how longer and shorter mtDNA fragments can be taken up naturally by the PDC and thus, may contribute to acute and chronic immune activation.