The role of the immune response to oncolytic Herpes Simplex viral (oHSV) therapy for glioblastoma is controversial. Within hours of oHSV infection of human or syngeneic glioblastoma in mice, activated natural killer (NK) cells are recruited to the site of infection. This response significantly diminished the efficacy of glioblastoma virotherapy. oHSV-activated NK cells coordinated macrophage and microglia activation within tumors. In vitro, human NK cells preferentially lysed oHSV-infected human glioblastoma cell lines. This enhanced killing depended on NK cell natural cytotoxicity receptors (NCR) NKp30 and NKp46, whose ligands were up-regulated in oHSV-infected glioblastoma cells. HSV titers and oHSV efficacy were increased in Ncr1−/− mice and in a Ncr1−/− NK cell adoptive transfer model of glioma, respectively. These in vitro and in vivo (mouse) results demonstrate that glioblastoma virotherapy is partly limited by an antiviral NK cell response involving specific NCRs, uncovering novel potential targets to enhance cancer virotherapy.

MiR-128 is an important suppressor of PRC activity, and its absence is an early event in gliomagenesis.

Tumor virotherapy has been and continues to be used in clinical trials. One barrier to effective viral oncolysis, consisting of the interferon (IFN) response induced by viral infection, is inhibited by valproic acid (VPA) and other histone deacetylase inhibitors (HDACi). Innate immune cell recruitment and activation have been shown to be deleterious to the efficacy of oncolytic herpes simplex virus (oHSV) infection, and in this report we demonstrate that VPA limits this deleterious response. VPA, administered prior to oHSV inoculation in an orthotopic glioblastoma mouse model, resulted in a decline in NK and macrophage recruitment into tumor-bearing brains at 6 and 24 h post-oHSV infection. Interestingly, there was a robust rebound of recruitment of these cells at 72 h post-oHSV infection. The observed initial decline in immune cell recruitment was accompanied by a reduction in their activation status. VPA was also found to have a profound immunosuppressive effect on human NK cells in vitro. NK cytotoxicity was abrogated following exposure to VPA, consistent with downmodulation of cytotoxic gene expression of granzyme B and perforin at the mRNA and protein levels. In addition, suppression of gamma IFN (IFN-␥) production by VPA was associated with decreased STAT5 phosphorylation and dampened T-BET expression. Despite VPA-mediated immune suppression, mice were not at significantly increased risk for HSV encephalitis. These findings indicate that one of the avenues by which VPA enhances oHSV efficacy is through initial suppression of immune cell recruitment and inhibition of inflammatory cell pathways within NK cells. Despite intense investigations to improve the standard of therapy for glioblastoma (GBM), current regimens result in approximately 15 months of median survival following initial diagnosis, emphasizing the need for new therapies. Oncolytic viruses (OV) are promising biological agents, intensely investigated for nearly 2 decades. These naturally occurring and biologically engineered viruses, which are designed to replicate in a relatively selective manner within tumors and culminate in the destruction of the host's cancer cells (1, 10), have demonstrated effectiveness in preclinical models. Five different clinical trials have tested oncolytic herpes simplex virus (oHSV) (22,35,36,47,50), and a maximum tolerated dose was not achieved and toxicity was not demonstrated. Additionally, oncolytic adenovirus (11), Newcastle disease virus (16), and reovirus (14) have been shown to be safe in dose escalation trials in humans with malignant glioma; moreover, there are ongoing clinical trials with measles virus (24), retrovirus (45), parvovirus H-1, poliovirus, and Seneca Valley virus (see http://www.clinicaltrials.gov/ct2 /results?termϭglioblastomaϩANDϩvirus). However, therapeutic efficacy has been elusive to demonstrate. It is evident that efficacy should depend on the ability of the initially injected oHSV to replicate and distribute within the GBM mass. Identification of both barriers in the host that could li...