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

Abstract

 Interferon γ (IFNγ) 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 IFNγ gene into EMT6 tumor cells to assess the effect of IFNγ gene expression on tumor immunogenicity. Transfectants were selected in G418-containing tissue-culture medium and were determined to express the inserted IFNγ 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 IFNγ stimulation, whereas class II MHC was induced in IFNγ-transfected cells. The induction of class II MHC in IFNγ-transfected cells correlated with the expression of a mouse class II transactivator gene that was dormant in unmodified or mock-transfected cells. In addition, IFNγ-gene-transfected tumor cells were found to secrete up to 17 ng IFN (equivalent to 75 units/10⁶ 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 IFNγ-transfected EMT6 clone (EMT6-B17) with parental EMT6 cells resulted in tumor rejection, suggesting that IFNγ-transfected EMT6 cells were able to induce long-term immunity. Mixing experiments using gene-transfected and unmodified tumor cells demonstrated that 10% of IFNγ-transfected cells in the population was sufficient to protect mice against subsequent challenge with tumorigenic EMT6 cells. These studies demonstrate that the immunogenicity of tumor cells that are poorly responsive to exogenous IFNγ can be enhanced by inserting and expressing the IFNγ transgene. These findings also suggest a role for class II MHC in reducing tumorigenicity of the EMT6 tumor and inducing long-term tumor immunity.