INDUCTION OF GENE-EXPRESSION BY INTRACELLULAR INTERFERON-GAMMA - ABROGATION OF THE SPECIES-SPECIFICITY BARRIER

We reported previously that murine L-929 cells expressing a human inte rferon (IFN)-gamma cDNA lacking a signal peptide sequence synthesize b ut fail to secrete human IFN-gamma and support viral replication at a reduced level. These cells also had elevated levels of IFN-inducible g ene products. We show here that a similar response is seen in human ce lls expressing a mutated murine IFN-gamma cDNA. The ability of human I FN-gamma to induce gene expression in murine cells is shown to be due to the intracellular IFN-gamma rather than to clonal variation, induct ion of endogenous murine IFN, or alternative mediators of antiviral ac tivity. 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-gamma cDNA lacking a signal sequence support virus replication at the same level as control cells. However, unlike transfectants containing only the neo(R) selec tion gene, clones expressing the mutated human lFN-gamma gene show str ong protection against viral infection and elevated levels of 2,5 A sy nthetase mRNA and MHC class I protein after treatment with IFN-beta al one. Reverse transcriptase-PCR rules out the induction of endogenous m urine IFN expression as a mediator of these effects. Thus, expression of intracellular human IFN-gamma mimics treatment with extracellular m urine IFN-gamma in permitting a synergistic response to IFN-beta. Give n the inability of human IFN-gamma to bind to the murine cell-surface receptor our results show that intracellular IFN-gamma can activate ce rtain responses independent of cell-surface binding. (C) 1995 Academic Press, Inc.