Date Published: August 13, 2012
Publisher: Hindawi Publishing Corporation
Author(s): Bonnie Reinhart, Lucia Mazzacurati, Adriana Forero, Chang-Sook Hong, Junichi Eguchi, Hideho Okada, Wendy Fellows, Ajay Niranjan, Justus B. Cohen, Joseph C. Glorioso, Paola Grandi.
Successful oncolytic virus treatment of malignant glioblastoma multiforme depends on widespread tumor-specific lytic virus replication and escape from mitigating innate immune responses to infection. Here we characterize a new HSV vector, JD0G, that is deleted for ICP0 and the joint sequences separating the unique long and short elements of the viral genome. We observed that JD0G replication was enhanced in certain glioblastoma cell lines compared to HEL cells, suggesting that a vector backbone deleted for ICP0 may be useful for treatment of glioblastoma. The innate immune response to virus infection can potentially impede oncolytic vector replication in human tumors. Indoleamine-2,3-dioxygenase (IDO) is expressed in response to interferon γ (IFNγ) and has been linked to both antiviral functions and to the immune escape of tumor cells. We observed that IFNγ treatment of human glioblastoma cells induced the expression of IDO and that this expression was quelled by infection with both wild-type and JD0G viruses. The role of IDO in inhibiting virus replication and the connection of this protein to the escape of tumor cells from immune surveillance suggest that IDO downregulation by HSV infection may enhance the oncolytic activity of vectors such as JD0G.
Glioblastoma multiforme (GBM) is the most common type of primary brain tumor with an incidence rate of approximately 3 cases per 100,000 people per year in the United States (2004-2005, http://www.cbtrus.org/). The current treatment modality for GBM typically involves surgery to remove the tumor, followed by radiotherapy and adjuvant chemotherapy with temozolomide . The infiltrative nature of the tumor makes complete surgical removal difficult, and tumor recurrence is common at the tumor margin. The median survival for patients with GBM is generally less than two years despite treatment. Clearly, new and effective therapies are needed.
The development of attenuated viruses that are adapted for preferential replication in solid tumors is an attractive approach to treatment of malignancies where more standard therapies are either ineffective or difficult to apply. HSV-1-oncolytic vectors used in early-phase clinical trials for the treatment of GBM have shown some success without serious side effects [33, 34]. Tumor specificity can be achieved by deleting viral genes that permit mutant virus replication in tumor cells while profoundly impairing virus replication in normal host cells . The vector prototype is G207, that is, deleted for γ34.5 and produces a nonfunctional ICP6-LacZ fusion protein . ICP6 encodes the large subunit of the viral ribonucleotide reductase, a protein that permits virus growth in nondividing cells by maintaining the nucleotide pool, while γ34.5 counteracts the virus-induced activation of the PKR pathway. The more advanced oncolytic vector examined in this study, MGH2, was derived from G207 by replacing lacZ with eGFP at the ICP6 locus and inserting two antitumor genes, CYP2B1 (encoding cytochrome p450) and shiCE (encoding secreted human intestinal carboxylesterase) . These enzymes activate the anticancer drugs cyclophosphamide and irinotecan, respectively, both of which are potent tumor toxic products. In one study, MGH2 showed oncolytic activity in vivo only with the addition of cyclophosphamide and irinotecan , indicating that the MGH2 vector backbone alone does not function as an effective OV. Some evidence suggests that vector replication in certain tumor cells may require γ34.5 activity and that oHSV is susceptible to innate immune responses, potentially limiting the effectiveness of this and other oHSV vectors . We therefore sought to examine other mutant backbones that may overcome these limitations.