Oncolytic viruses induce tumor cell death and antitumor immune memory in the GL261 orthotopic mouse glioma model

Abstract

Introduction and Aim: Glioblastoma Multiforme is a WHO grade IV neoplasm and is the most frequent primary brain tumor in adults. In spite of current multimodal treatment the prognosis remains poor, with a median overall survival of 14.6 months and a mortality rate of 88% within 3 years. At time of relapse (which is universal), the prognosis is particularly dismal with reports of 100% mortality within 1.5 years. In view of these discouraging results, (pre)clinical research is focused on long-term and tumor-specific treatments, such as oncolytic virotherapy. Not only do oncolytic viruses have the potential to lower tumor volume and viability considerably, they might also alter the local microenvironment within the tumor, to subsequently induce a better immune control through immunotherapeutic approaches. Here we investigate the efficacy of the oncolytic Reovirus Type 3 Dearing, Parvovirus H-1 and Newcastle disease virus (NDV) Hitchner B1 strains in the orthotopic murine GL261 glioma model. Materials and Methods: monolayers of GL261 cells and healthy astrocytes were inoculated with Reovirus, Parvovirus or NDV at different multiplicities of infection, ranging from 0,05 to 10 plague forming units/cells. 72 hours later, cell death was measured via MTT assays. C57/BL6 and Rag2-/- mice were challenged with GL261 cells orthotopically and treated intratumorally with NDV on day 3 or day 7 after tumor induction. Animals were followed for symptoms and survival. Long-term surviving animals were rechallenged with GL261 cells after 100 days. Results: We demonstrated in vitro that Reovirus, Parvovirus and NDV exhibit strong cytotoxicity towards the GL261 cell line. In vivo we showed that local injection of NDV was feasible and significantly prolonged survival of treated animals versus untreated controls. Furthermore, 45% of treated animals survived long-term versus none in the control group. Similar survival data were obtained whether NDV was injected at time of minimal disease or at time of bulky tumor. Tumor rechallenge in long-term surviving animals resulted in 80% survival, suggesting immunization in most mice after NDV therapy. In immunodeficient Rag2-/- animals, NDV failed to prolong survival after glioma challenge, further substantiating the notion that the anticancer effects of NDV can be tracked not only to direct viral oncolysis, but also to an additional immune component. Conclusion: For the first time, we describe that NDV has therapeutic activity against GL261 tumor cells, evidenced in an orthotopic mouse model. NDV therapy significantly improves median survival and induces an antitumor memory response in treated animals. This is a highly innovative finding and ongoing work is focused on further elucidating the immune component in successful oncolytic virotherapy. Furthermore, this model will allow us to study the potential synergism between NDV therapy and dendritic cell mediated immunotherapy, which thus far has not been studied in the field of glioma research