EXPRESSION OF COSTIMULATORY MOLECULES - B7 AND ICAM UP-REGULATION AFTER TREATMENT WITH A SUICIDE GENE

The herpes simplex virus thymidine kinase gene (HSV-TK) in combination with ganciclovir (GCV), is currently being used in gene therapy-based clinical trials for cancer treatment. its therapeutic effect is based on a ''bystander effect'' whereby HSV-TK gene-modified tumor cells ar e toxic to nearby unmodified tumor cells when exposed to the antiviral drug GCV. We have recently hypothesized that the in vive mechanism of this bystander effect is due to alterations in the tumor microenviron ment in response to release of cytokines and an infiltration of leukoc ytes after treatment with HSV-TK gene-modified tumor cells and GCV, wh ich results in tumor regression. Expression of B7, a recently identifi ed costimulatory molecule that is important for T-cell stimulation, ha s been shown to be modulated by stimulatory cytokines interferon-gamma , tumor necrosis factor-alpha, and inhibited by interleukin-10. in the present study, we investigated whether the cytokines released after H SV-TK and GCV treatment could induce the expression of the costimulato ry molecules B7-1 and B7-2 and the adhesion molecule (ICAM)-1 in the t umor. Furthermore, we investigated whether this altered environment af fected the antitumor properties of host lymphocytes. An in vitro model was developed to establish the effects of HSV-TK gene-modified tumor ce[ts and GCV on tumor infiltrating cells. The murine macrophage cell line (IC21) was exposed to either supernatants or cell lysates collect ed from a mixture of HSV-TK-transduced (KBALB-STK) and non-transduced (KBALB) murine fibrosarcoma tumor cells previously exposed to GCV (exp erimental). Immunohistochemical analysis showed a significant expressi on (P <.0001) of B7-1 and B7-2 post exposure of IC21 cells to either s upernatant or lysate. In contrast, the level of expression in IC21 cel ls exposed to the control lysate or supernatant remained unchanged for B7-1 and B7-2. In vivo analysis for B7-1 and B7-2 expression by immun ohistochemistry in tumor tissues from experimental mice receiving HSV- TK gene-modified tumor cells and GCV treatment showed a significant ex pression of B7.1 (35%, P <.0001) and B7.2 (38.2%, P <.0001) on tumor-i nfiltrating mononuclear cells. In contrast, tumor-bearing control anim als showed low levels of B7-2 expression (5.8%), whereas B7-1 was unde tectable, as confirmed by reverse-transcriptase polymerase chain react ion. In addition, a significant up-regulation of ICAM expression (50%) on tumor tissues was observed in the experimental group (P=.0317) as compared with the control group (25%). Furthermore, T cells isolated f rom experimental mice showed a significant in vitro proliferative resp onse (p=.0202) when exposed to syngeneic tumor cells as compared with the control group. These data demonstrated that the use of HSV-TK gene -modified tumor cells and GCV as a suicide gene in the treatment of an intraperitoneal tumor resulted in the expression of the B7 costimulat ory molecules and ICAM-1 adhesion molecule and enhanced proliferative response of host T cells. These findings help to understand the mechan ism of tumor cell killing in vivo using HSV-TK gene-modified tumor cel ls.