Infection with Oncolytic Herpes Simplex Virus-1 Induces Apoptosis in Neighboring Human Cancer Cells

Abstract

Purpose:The antitumor efficacy of a herpes simplex virus (HSV)-1 oncolytic virus depends on the cytotoxic effect of the virus, but also on viral replication and spread within the tumor. Apoptosis is considered a defense mechanism of infected cells that minimizes the spread of viral progeny by limiting cellular production of virus. We sought to determine whether oncolytic HSV-1 infection induces apoptosis in neighboring, uninfected cells and whether manipulation of apoptosis can increase viral replication and cytotoxicity.Experimental Design: NV1066 is an oncolytic HSV-1 mutant that contains the marker gene for enhanced green fluorescent protein. OCUM human gastric cancer cells were infected with NV1066 in vitro and inspected for apoptosis by Hoechst and terminal deoxynucleotidyltransferase-mediated nick end labeling staining and for infection by expression of green fluorescence.Results: A significant increase in apoptosis was seen in cells infected by NV1066. More interestingly, a significant percentage (10%) of uninfected cells also proceeded to apoptosis. After NV1066 infection, cells were also treated with N-acetylcysteine (NAC), an inhibitor of apoptosis. By day 4 after infection, 2.7؋ more NV1066 was produced in cells exposed to NAC than in those not exposed to NV1066 (P ‫؍‬ 0.04). NAC also increased tumor kill when administered with virus.Conclusions: These data suggest that NV1066 induces apoptosis in uninfected cocultured cells, potentially hindering propagation of viral progeny and concomitant tumor kill. Inhibition of apoptosis may improve the efficacy of oncolytic HSV-1 therapy.

Widespread intratumoral virus distribution with fractionated injection enables local control of large human rhabdomyosarcoma xenografts by oncolytic herpes simplex viruses

“…Based on data in cell culture, it has been proposed that apoptosis of adjacent, uninfected cells might limit virus spread within a tumor. 26 While we found nuclear changes consistent with apoptosis in early infected cells on tumor sections, cells surrounding infected cells or the debris of infected cells (identified by eGFP expression) appeared healthy. It stands to reason that to improve the antitumor effects of oncolytic viruses efforts should be focused not only on being able to inject higher virus doses but also on achieving better initial virus distribution.…”

“…Infection with replication-defective HSV recombinants that do not express early proteins causes high levels of cellular apoptosis in a cell-type-specific manner. 25 Uninfected neighboring cells may also undergo apoptosis following injection of some cell lines with NV1066, 26 and it has been proposed that such apoptosis might limit virus spread. Tumors injected with oncolytic HSV mutants often have regions of necrosis associated with HSV-infected cells, [27][28][29][30][31] and in some cases there is a concomitant increase in apoptotic cells.…”

Oncolytic recombinant herpes simplex virus for treatment of orthotopic liver tumors in nude mice

“…The antitumor effect of rHSV is reported to be much greater than its evident distribution throughout tumors (20)(21)(22)(23)(24)(25). HSV infection induces apoptosis in adjacent, uninfected cells in vitro and in vivo (26). The route of administration will be a critical determining factor in the efficacy of viral therapy.…”

Effect of a caspase inhibitor, zVADfmk, on the inhibition of breast cancer cells by herpes simplex virus type 1

“…21,22,24,25 Others have therefore proposed that anti-apoptosis drugs might allow infected tumor cells to resist apoptosis, at least temporarily, thus allowing greater replication of the virus and a stronger anti-tumor effect in the long term. 26 Apoptosis is a complex process, offering many target sites at which it could be inhibited. These include the various caspases, and it has previously been demonstrated that the pan-caspase inhibitor N-benzyloxycarbonylVal-Ala-Asp-fluoromethylketone (zVADfmk) can prevent HSV-1-induced apoptosis.…”