Interferon gamma(IFN gamma) gene transfer of an EMT6 tumor that is poorly responsive to IFN gamma stimulation: Increase in tumor immunogenicity is accompanied by induction of a mouse class II transactivator and class II MHC

Interferon gamma (IFN gamma) is an important cytokine with immunomodulatory properties that include activation of immune cells and induction of class I and class II major histocompatibility complex antigens. In this study a retroviral vector was used to introduce the IFN gamma gene into EMT6 tumor cells to assess the effect of IFN gamma gene expression on tumor immunogenicity. Transfectants were selected in G418-containing tissue-culture medium and were determined to express the inserted IFN gamma gene by reverse transcriptase/polymerase chain reaction. Flow-cytometric analysis revealed that parental unmodified EMT6 cells constitutively expressed only class I MHC and were poorly responsive to exogenous IFN gamma stimulation, whereas class II MHC was induced in IFN gamma-transfected cells. The induction of class II MHC in IFN gamma-transfected cells correlated with the expression of a mouse class II transactivator gene that was dormant in unmodified or mock-transfected cells. In addition, IFN gamma-gene-transfected tumor cells were found to secrete up to 17 ng IFN (equivalent to 75 units/10(6) cells) by enzyme-linked immunosorbent assay (ELISA). Whereas parental EMT6 cells grew unchecked, the growth of genetically modified tumor cells was significantly inhibited in immunocompetent mice. Rechallenge of animals that rejected an IFN gamma-transfected EMT6 clone (EMT6-B17) with parental EMT6 cells resulted in tumor rejection, suggesting that IFN gamma-transfected EMT6 cells were able to induce long-term immunity. Mixing experiments using gene-transfected and unmodified tumor cells demonstrated that 10% of IFN gamma-transfected cells in the population was sufficient to protect mice against subsequent challenge with tumorigenic EMT6 cells. These studies demonstrate that the immunegenicity of tumor cells that are poorly responsive to exogenous IFN gamma can be enhanced by inserting and expressing the IFN gamma transgene. These findings also suggest a role for class II MHC in reducing tumorigenicity of the EMT6 tumor and inducing long-term tumor immunity.