HUMAN GENE THERAPY 10:2741–2755 (November 20, 1999) Mary Ann Liebert, Inc. Systemic Antitumor Immunity in Experimental Brain Tumor Therapy Using a Multimutated, Replication-Competent Herpes Simplex Virus TOMOKI TODO, 1 SAMUEL D. RABKIN, 1,2 PERIASAMY SUNDARESAN, 1 AIGUO WU, 3 KENNETH R. MEEHAN, 3 HERBERT B. HERSCOWITZ, 2 and ROBERT L. MARTUZA 1 ABSTRACT Replication-competent, attenuated herpes simplex virus (HSV) vectors have been developed for viral oncolytic therapy of primary and metastatic malignant brain tumors. However, the role of the host immune responses in the brain has not been elucidated. N18 neuroblastoma cells were used as a tumor model in syngeneic A/J mice to test the therapeutic efficacy of G207, a conditionally replicating HSV vector, in an immunocompetent condition. G207 inoculated intraneoplastically exhibited a prominent oncolytic antitumor effect in mice har- boring N18 tumors in the brain or subcutaneously, and, in addition, elicited a systemic antitumor immune re- sponse. Subcutaneous tumor therapy with G207 caused regression of a remote, established tumor in the brain or in the periphery, which was potentially mediated by the systemic antitumor immune response, and pro- vided persistent tumor-specific protection against N18 tumor rechallenge in the brain as well as in the pe- riphery. Antitumor immunity was associated with an elevation of specific CTL activity against N18 tumor cells that persisted for at least 13 months. The results suggest that the oncolytic antitumor action of replica- tion-competent HSV may be augmented by induction of specific and systemic antitumor immunity effective both in the periphery and in the brain. 2741 OVERVIEW SUMMARY G207 is a multimutated, conditionally replicating herpes simplex virus (HSV) vector developed for viral oncolytic therapy of malignant brain tumors. A/J mice harboring tu- mors of syngeneic N18 neuroblastoma were used to eluci- date the role of the host immune responses in G207 tumor therapy. G207 inoculated intraneoplastically exhibited a prominent oncolytic antitumor effect against N18 tumors in the brain or skin. Subcutaneous tumor therapy with G207 elicited a systemic antitumor immune response that led to regression of a remote, established tumor in the brain or skin, and provided persistent tumor-specific protection against N18 tumor rechallenge in the brain and in the pe- riphery. Antitumor immunity was associated with the ele- vation of specific and persistent CTL activity against N18 tumor cells. The oncolytic antitumor action of replication- competent HSV may be augmented by induction of specific and systemic antitumor immunity effective both in the pe- riphery and in the brain. INTRODUCTION B RAIN TUMORS are the second leading cause of death from neurological disease, and the annual age-adjusted incidence of primary brain tumors in the United States has been reported to be 5.0 to 14.1 per 100,000 population (Radhakrishnan et al ., 1994). Gliomas account for 40–67% of the primary tumors, and about three-fourths of gliomas are considered malignant. De- spite advances in microsurgical techniques and adjuvant ther- apy such as chemotherapy or radiotherapy, these modalities have caused little improvement in the survival rate of patients with malignant brain tumors. In addition, treatment of systemic tumors often fails owing to central nervous system (CNS) metastases. Genetically engineered, conditionally replicating viruses have been used to kill tumor cells by direct oncolysis (Martuza et al ., 1991; Markert et al., 1993; Jia et al ., 1994; Kaplitt et al., 1994; Mineta et al., 1994, 1995; Chambers et al ., 1995; Kesari et al ., 1995; Bischoff et al., 1996; Kramm et al ., 1997; Miyatake et al., 1997; Pyles et al ., 1997; Rodriguez et al ., 1997). In this regard, herpes simplex virus (HSV) has ad- 1 Molecular Neurosurgery Laboratory, Department of Neurosurgery, 2 Department of Microbiology and Immunology, and 3 Bone Marrow Trans- plant Program, Georgetown University Medical Center, Washington, D.C. 20007.