Induction of Gene Expression by Intracellular Interferon-γ: Abrogation of the Species Specificity Barrier

AbstractWe reported previously that murine L-929 cells expressing a human interferon (IFN)-γ cDNA lacking a signal peptide sequence synthesize but fail to secrete human IFN-γ and support vital replication at a reduced level. These cells also had elevated levels of IFN-inducible gene products. We show here that a similar response is seen in human cells expressing a mutated murine IFN-γ cDNA. The ability of human IFN-γ to induce gene expression in murine cells is shown to be due to the intracellular IFN-γ rather than to clonal variation, induction of endogenous murine IFN, or alternative mediators of antiviral activity. We have used a murine cell line, Ltk-aprt-, which is resistant to both type I and II IFNs but responsive to combined treatment with both, Ltk-aprt- cells transfected with human IFN-γ cDNA lacking a signal sequence support virus replication at the same level as control cells. However, unlike transfectants containing only the neoR selection gene, clones expressing the mutated human IFN-γ gene show strong protection against viral infection and elevated levels of 2,5 A synthetase mRNA and MHC class I protein after treatment with IFN-β alone. Reverse transcriptase-PCR rules out the induction of endogenous murine IFN expression as a mediator of these effects. Thus, expression of intracellular human IFN-γ mimics treatment with extracellular murine IFN-γ in permitting a synergistic response to IFN-β. Given the inability of human IFN-γ to bind to the murine cell-surface receptor our results show that intracellular IFN-γ can activate certain responses independent of cell-surface binding