Molecular mechanisms underlying IFN-γ-mediated tumor growth inhibition induced during tumor immunotherapy with rlL-12

Abstract

The present study investigates the molecular mechanisms by which IFN-γ produced as a result of in vivo IL-12 administration exerts its anti-tumor effects. rIL-12 was administered three or five times into mice bearing CSA1M fibrosarcoma, OV-HM ovarian carcinoma or MCH-1-A1 fibrosarcoma. This regimen induced complete regression of CSA1M and OV-HM tumors but only transient growth inhibition of MCH-1-A1 tumors. The anti-tumor effects of IL-12 were associated with enhanced induction of IFN-γ because these effects were abrogated by pretreatment of hosts with anti-IFN-γ antibody. Exposure in vitro of the three types of tumor cells to rIFN-γ resulted in moderate to potent inhibition of tumor cell growth. IFN-γ stimulated the expression of mRNAs for an inducible type of NO synthase (iNOS) in CSA1M cells and indoleamine 2,3-dioxygenase (IDO), an enzyme capable of degrading tryptophan, in OV-HM cells, but induced only marginal levels of these mRNAs in MCH-I-AI cells. In association with iNOS gene expression, IFN-γ-stimulated CSA1M cells produced a large amount of NO which functioned to inhibit their own growth in vitro. Although OV-HM and MCH-1-A1 cells did not produce NO, they also exhibited NO susceptibility. Whereas the tumor masses from IL-12-treated CSA1M-bearing or OV-HM-bearing mice induced higher levels of iNOS (for CSA1M) or IDO and iNOS (for OV-HM) mRNAs, the MCH-1-A1 tumor mass expressed lower levels of iNOS mRNA alone. Moreover, massive infiltration of CD4+ and CD8+ T cells and Mac-1+ cells was seen only in the CSA1M and OV-HM tumors. Thus, these results indicate that IFN-γ produced after IL-12 treatment induces the expression of various genes with potential to modulate tumor cell growth by acting directly on tumor cells or stimulating tumor-infiltrating lymphoid cells and that the effectiveness of IL-12 therapy is associated with the operation of these mechanisms.