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Individual interferon (IFN)-producing cells were identified by hybridization in situ followed by autoradiography. cDNAs corresponding to murine IFN-a and murine IFN-P labeled by nick-translation to high specific activity (24 x 10 dpm/,ug) with a-35S-labeled dATP were used as probes for hybridization with IFN mRNA in mouse C-243 cells induced with Newcastle disease virus. Control experiments with non-induced cells or with non-IFN-related labeled DNA monitored the specificity of the autoradiographic signal. Under optimal conditions of IFN induction, between 15% and 40% of the cells gave a hybridization signal with a mixture of IFN-a and -P probes. Differential hybridization with either the IFNa or -13 probe or a mixture of both, at three different time intervals after induction, revealed that only a small fraction of cells had detectable amounts of IFN-a mRNA, whereas in the majority of the positive cells IFN-(3 mRNA was present.Studies on the genetics and on the cellular origin of interferon (IFN) production have suffered from the limitation that the results represented average values obtained from cultures or from cell populations in vivo, because there was no method to identify and characterize individual IFN-producing cells at the same time. To be able to study IFN production at the single cell level is of obvious importance, because it can provide answers to several questions-for example, the number of IFN-producing cells in a given culture system or the nature and the number of IFN-producing cells in vivo as a function of the inducing agent. In the present study, we have used cDNA probes of murine IFN-a and -,B to develop a method for detecting the presence of specific mRNA by in situ hybridization. This method makes it possible to distinguish individual IFN-producing cells that can, at the same time, be stained for morphological identification. MATERIALS AND METHODSLabeling of cDNA Probes. The recombinant plasmids pMIF 1204 (carrying a partial cDNA for murine IFN-a2) and pM l3-3 (carrying the complete nucleotide sequence for murine IFN-f3) have been described (1, 2). After digestion with Pst I, the plasmids were electrophoresed through 1% agarose slab gels and the inserts [820 base pairs (bp) for IFN-a and 680 bp for IFN-,B1 were recovered from the gel slices by electroelution and chromatography on DE-52 columns. The inserts were labeled to high specific activity (2-4 x 108 dpm/,ug) with a-35S-labeled dATP (>1000 Ci/mmol; 1 Ci = 37 GBq; New England Nuclear) in a nick-translation reaction essentially as described by Haase et al. (3). Briefly, a 25-,ul reaction mixture contained 400 ng of DNA template/50 mM Tris-HCl, pH 7.4/10 mM MgCl/1 mM dithiothreitol/50,ug of nuclease-free bovine serum albumin per ml (Bethesda Research Laboratories)/30 ,uM of each dCTP, dGTP, and dTTP/250 ,JCi of a-35S-labeled dATP/5 units of DNA polymerase 1/100 pg of DNase I. The mixture was incubated at 15TC for 2 or 4 hr and incorporation of labeled nucleotides was determined by trichloroacetic acid precipitation in the presen...

Type I interferons (IFNs) constitute a family of structurally related proteins that are all derived from the same ancestral gene and act on a common cell-surface receptor. Contrary to many other cytokines, the production of type I IFNs is not a specialized function, and all cells in the organism can produce them, usually as a result of induction by viruses, via the formation of double-stranded RNA. Type I IFNs are indeed responsible for the first line of defense during virus infection and act through the induction of a great number of proteins. Of these, at least thirty have been characterized, and there are probably many more. In addition to their direct antiviral effect, type I IFNs exert a wide variety of other activities, such as for example the induction of various cytokines and the stimulation of different effector cells of the immune system. Due to these pleiotropic effects, recombinant interferons are used in the clinic to treat a variety of diseases, among which cancer, viral hepatitis and multiple sclerosis.