Malignant mesothelioma ( MM ) is a solid tumor largely unresponsive to conventional therapies. Immunological gene therapy shows promise in murine models and human clinical trials; however, the role of surgery in combination with gene therapy has not been widely studied. The aim of this study was to determine if debulking surgery improved the effectiveness of gene therapy in a murine MM model. Mice were subcutaneously inoculated with the MM cell line, AC29, at two different sites, 4 days apart, to allow a surgical and distal site tumor to develop. Once tumors were established, the surgical site tumor was debulked and vaccination of syngeneic tumor transfectants encoding genes for IL -4, IL -2, GM -CSF, B7 -1 or allogeneic MHC molecules commenced at a site away from both tumors, and tumor growth was measured. Neither debulking surgery nor gene therapy alone delayed tumor growth. However, there was a clear delay of tumor growth when debulking surgery was combined with vaccination of tumor transfectants expressing B7 -1 or high levels of GM -CSF. Combinations of these two transfectants did not lead to a synergistic effect. This study demonstrates that debulking surgery can augment the immunostimulatory effects of immunological gene therapy and can delay tumor growth. This has implications for the future design of human gene therapy trials for solid tumors such as MM.

Summary Stable IL-2 transfectanl clones have been derived frotn two non-immunogenic murine malignant tnesothelioma (MM) cell lines to investigate the induction of protective antitumour immunity to MM. AC29-IL-2 transfectant clones grew at a slower rate in vivo than the parental cell line or a transfectant control clone but all inoculated mice developed tumours despite the continued ability of the tumour cells to express IL-2. Tumour development after inoculation of ABl-IL 2 transfectants varied, the degree of in vivo inhibition (40-100%) being directly related to the rate of IL-2 secretion of the transfectants. When mice which had rejected the ABl-lL-2 transfectants were challenged with parental ABl cells, a proportion (16-709'f) of mice from each group remained tumour free at least 45 days after challenge (naive mice developed tumours within 26 days). The inhtbttion of growth ofthe initial inoculum of ABl-IL-2 transfectants was independent of CD4-' and CD8+ cells, consistent with the demonstration of non-specific cytotoxic activity by splenocytes from mice inoculated with the IL-2 transfectants. These data suggest that IL-2 expression by MM cells is capable of generating in vivo immunity to the tumour. This immunity may be relatively weak or may be subject to downregulation so that consistent rejection of unmodified tumour cells is not achieved. Genetic modification with combinations of genes, including lL-2 and B7-1. will be necessary for reliable generation of protective immunity to MM.

Many cancers express an array of chemokines which have the capacity to modulate the nature and function of intratumoural leukocyte infiltrates. In malignant mesothelioma (MM) neither the chemokine signalling networks nor their regulation have been investigated despite the prominence of leucocytic infiltrates in both clinical and experimental tumours. In this study, we examined constitutive and cytokine-regulated expression of CC and CXC chemokine genes in mesothelioma and mesothelial cell cultures derived from two different mouse strains (BALB/C and CBA/CaH). In mouse MM and mesothelial cells MCP-1/JE, GRO-α/KC and RANTES were expressed whereas MIP-1α and MIP-2 were infrequently expressed. Comparison of basal chemokine expression showed that GRO-α/KC mRNA was overexpressed in the malignant cells whereas MCP-1 gene expression and release was downregulated. Treatment of mesothelioma cells with IL-4, IFN-γ or TNF-α revealed that chemokine genes could be more responsive to cytokines in the malignant compared to their mesothelial cells. TNF-α was consistently the most potent positive regulator of both CC and CXC chemokine expression and MCP-1 release. The present study for the first time provides a mechanistic insight into the differential regulation of chemokine expression in malignant mesothelioma cells and has implications for mesothelial chemokine signalling in mouse models.

Transfection of the genes encoding the co-stimulatory molecules B7-1 and B7-2 has enhanced the development of immunity to a variety of experimental tumors, although most of these were inherently immunogenic. We have determined the effect of expression of these genes on the induction of immunity to 2 non-immunogenic murine malignant mesothelioma (MM) cell lines (AC29 and AB1). We had previously shown that B7-1 transfection into AC29 delayed but did not prevent tumor development by certain of the transfectant clones. Here we demonstrate that over-expression of B7-1 can inhibit tumor development by certain AB1-B7-1 clones, that inhibition of transfectant growth is dependent on CD4 1 and CD8 1 T cells and that mice that reject some of these transfectant clones are capable of rejecting subsequent inocula of the parental cell line, AB1. The transfectant clones can generate tumor-specific cytotoxic T cells. By contrast, expression of B7-2 in several clones derived from either AB1 or AC29 had no significant effect on the development of tumors in vivo. Our data are consistent with data from other systems that show differences in the effect of modification by B7-1 or B7-2 on the modulation of anti-tumor immune responses. They demonstrate that such modifications can induce protective immunity against an MM cell line but confirm the intra-and inter-tumoral heterogeneity in the effect of genetic modification on the induction of immunity. Our observations are relevant to human MM because these cell lines have been derived from asbestos-induced tumors and share many properties with human cell lines of the same histological type. Int.

No abstract