Enhancement of tumor killing using a combination of tumor immunization andHSV-tk suicide gene therapy

Tumor cells genetically modified with the herpes simplex virus thymidine ki nase (HSV-tk) gene in combination with ganciclovir (GCV) demonstrate a "bys tander effect", Previous attempts to enhance the bystander tumor killing by combining cytokine genes with HSV-tk/GCV have met with varying results. Th e present study was designed to determine the effects of tumor immunization in combination with HSV-tk gene-modified tumor cells and GCV on tumor kill ing and to determine if the bystander tumor killing could be enhanced. Tumo r-bearing mice immunized with syngeneic tumor (KBALB) prior to treatment wi th an i.p. injection of xenogeneic HSV-tk gene-modified tumor cells (PA-IST K) had prolonged animal survival (group 4, 56.4 days). In contrast, unimmun ized tumor-bearing mice (group 2) or tumor bearing mice immunized to the xe nogeneic PA-ISTK tumor cells (group 5) showed a mean survival of about: 27 days after receiving an i.p, injection of PA-ISTK cells and GCV, Control gr oups, which were either not immunized and did not receive HSV-tk cells (gro up I) or immunized but treated only with GCV (group 3) showed short surviva l (16-18 days), Analysis of tumors for cytokine mRNA expression revealed in creased TNF-a! and Il-la! mRNA expression in group 4 mice. Furthermore, IL- 2 mRNA expression was detectable on days 2 and 4 only in group 4 mice. Immu nophenotypic analysis for tumorinfiltrating lymphocytes demonstrated an inc rease in macrophage (4%, p = 0.0001)and T cells (1.8%, p < 0.001)in group 4 mice with an enhanced T-cell response as compared with mice from groups 1, 2 and 3, Our results demonstrate that tumor immunization combined with HSV -tk/GCV treatment results in increased animal survival with enhanced immune response. Furthermore, the cytokine milieu observed in the present study c an modulate the tumor micra-environment in vivo from one that is immunosupp ressive to one that is immune-stimulatory, (C) 1999 Wiley-Liss, Inc.