Stable antiviral expression in BALB/c 3T3 cells carrying a beta interferon sequence behind a major histocompatibility complex promoter fragment.

We are exploring the use of interferon (IFN) genes for somatic cell gene therapy and have investigated the possibility of transforming cells into low constitutive IFN producers over prolonged periods of time without impeding cell survival or replication. Here we report the possibility of conferring a permanent antiviral state to cells by introducing an IFN-beta gene with a modified transcriptional control. This was achieved by placing the murine IFN-beta-coding sequence behind the IFN-inducible 0.6-kb XhoII-NruI promoter region of the H-2Kb major histocompatibility complex gene. BALB/c 3T3 cells that are normally permissive for virus infection were transformed with this construction, and 14 of the 21 clonal cell lines obtained displayed different levels of enhanced resistance to the replication of vesicular stomatitis virus, encephalomyocarditis virus, and Semliki Forest virus. The permanent antiviral state was dependent on the presence of new IFN-beta fragments in Southern blots and was accompanied by very low constitutive synthesis of IFN-beta, and the presence of construct-derived IFN mRNA could be demonstrated only after polymerase chain reaction amplification of cDNA. The antiviral state was stable over a 9-month period in culture, corresponding to about 250 cell generations, and did not affect cell replication, since the average population doubling time of these cells was not significantly different from that of the control clone. Twenty-nine control clonal cell lines, stably transformed with either the neo gene alone (22 lines) or the neo gene together with a mutated murine IFN-beta gene coding for an inactive protein (7 lines), did not develop stable antiviral expression. We conclude that low constitutive synthesis of autocrine IFN-beta is sufficient to induce a permanent antiviral state, is compatible with cell survival and replication, and therefore merits further exploration for use in somatic cell gene therapy.