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Malignant mesothelioma remains an incurable disease for which immune-modulatory therapies, such as exogenous cytokines, have shown some promise. One such cytokine, IFN-beta, has potent antiproliferative and immunostimulatory activity in vitro, but its in vivo use has been limited by toxicity. We thus conducted studies evaluating intracavitary delivery of a replication-deficient adenoviral (Ad) vector encoding for the murine IFN-beta gene (Ad.muIFN-beta) in mouse models of malignant mesothelioma. In contrast to multiple injections of recombinant protein, a single i.p. injection of Ad.muIFN-beta into animals with established tumors elicited remarkable antitumor activity leading to long-term survival in >90% of animals bearing either AB12 or AC29 i.p. mesotheliomas. A control adenovirus vector had minimal antitumor effect in vivo. Significant therapeutic effects were also seen in animals treated with large tumor burdens. Importantly, treatment of i.p. tumor also led to reduction of growth in tumors established at a distant site (flank). A number of experiments suggested that these effects were attributable to an acquired CD8(+) T-cell-mediated response including: (a) the induction of long-lasting antitumor immunity; (b) loss of efficacy of Ad.muIFN-beta in tumor-bearing, immune-deficient (SCID, SCID/beige) mice; (c) detection of high levels of specific antitumor cytolytic activity from unstimulated splenocytes harvested from Ad.muIFN-beta-treated animals that was abolished by CD8(+) T-cell depletion; and (d) abrogation of antitumor effects of Ad.muIFN-beta in tumor-bearing CD8(+) T-cell-depleted animals. These data show that intracavitary IFN-beta gene therapy using an adenoviral vector provides strong CD8(+) T-cell-mediated antitumor effects in murine models of mesothelioma and suggest that this may be a promising strategy for the treatment of localized tumors such as mesothelioma or ovarian cancer in humans.

Previous studies with a mycobacterial heat shock protein (hsp-65) have demonstrated some efficacy using cationic liposome-mediated gene transfer in murine i.p. sarcoma models. To further analyze the efficacy of hsp-65 immunotherapy in clinically relevant models of localized cancer, immunocompetent mice bearing i.p. murine mesothelioma were treated with four i.p. doses of a cationic lipid complexed with plasmid DNA (pDNA) containing hsp65, LacZ, or a null plasmid. We observed >90% long-term survival (median survival, 150 days versus approximately 25 days, treated versus saline control, respectively) in a syngeneic, i.p. murine mesothelioma model (AC29). Long-term survivors were observed in all groups treated with lipid complexed with any pDNA. Lipid alone or DNA alone provided no demonstrable survival advantage. In a more aggressive i.p. model of mesothelioma (AB12), we observed >40% long-term survival in groups treated with lipid:pDNA complexes, again irrespective of the transgene. To ask whether these antitumor effects had led to an adaptive immune response against the tumor cell, we rechallenged long-term survivors in both murine models s.c. with the parental tumor cell line. Specific, long-lasting systemic immunity against the tumor was readily demonstrated in both models (AB12 and AC29). Consistent with these results, splenocytes from long-term survivors specifically lysed the parental tumor cell lines. Depleting the CD8+ T-cells from the splenocyte pool eliminated this lytic activity. Lipid:pDNA treatment of athymic, SCID, and SCID/Beige mice bearing a murine i.p. mesothelioma (AC29) resulted in only a slight survival advantage, but there were no long-term survivors. Treatment of immunocompetent mice depleted of specific immune effector cells demonstrated roles for CD8+ and natural killer cells. Although the exact mechanism(s) responsible for these antitumor effects is unclear, the results are consistent with roles for both innate and adaptive immune responses. An initial tumor cell killing stimulated by cationic lipid:pDNA complexes appears to be translated into long-term, systemic immunity against the tumor cell. These results are the first to demonstrate that adaptive immunity against a tumor cell can be induced by the administration of lipid:pDNA complexes. Multiple administrations of cationic lipid complexed with pDNA lacking an expressed transgene could provide a promising generalized immune-mediated modality for treating cancer.

Modified, nonneurovirulent herpes simplex viruses (HSVs) have shown promise in the treatment of brain tumors. However, HSV-1 can infect and lyse a wide range of cell types. In this report, we show that HSV-1716, a mutant lacking both copies of the gene coding ICP-34.5, can effectively treat a localized i.p. malignancy. Human malignant mesothelioma cells supported the growth of HSV-1716 and were efficiently lysed in vitro. i.p. injection of HSV-1716 into animals with established tumor nodules reduced tumor burden and significantly prolonged survival in an animal model of non-central nervous system-localized human malignancy without dissemination or persistence after i.p. injection into SCID mice bearing human tumors. These findings suggest that this virus may be efficacious and safe for use in localized human malignancies of nonneuronal origin such as malignant mesothelioma.

Transfer of the herpes simplex virus thymidine kinase (HSVtk) gene into tumor cells using retroviral vectors followed by administration of ganciclovir provides a potential strategy for the treatment of malignancy. Because of the limitations of using retroviral vectors for clinical application, the feasibility of using a recombinant adenovirus containing HSVtk was examined. Cell lines derived from human malignant mesotheliomas and non-small cell lung cancers infected with a recombinant adenovirus containing HSVtk showed strong expression of HSVtk protein as determined by immunohistochemical staining. Infection with a recombinant adenovirus containing HSVtk rendered cells sensitive to doses of ganciclovir that were 2-3 logs lower than uninfected cells or those infected with a control virus. A strong "bystander effect" was noted in mesothelioma lines; there was no diminution in the efficacy of ganciclovir treatment until the ratio of infected:uninfected cells fell below 1:10. This study thus demonstrates in vitro efficacy of an adenovirus-transduced HSVtk drug sensitization gene therapy system in thoracic malignancies. Recombinant adenovirus transfer of the HSVtk gene followed by ganciclovir may have promise as an in situ treatment for tumors.