We constructed chimeric receptors wherein the extracellular domain of the erythropoietin receptor (EpoR) was fused to the transmembrane and intracellular domains of the interferon (IFN) type I receptor subunits, IFNaR1 or IFNaR2-2. Transfection into 2fTGH and Tyk2-deficient 11,1 cells showed that EpoR/IFNaR2-2 alone was able to transduce a signal upon stimulation with erythropoietin (Epo), as judged by induction of the interferon type I-inducible 6-16 promoter. In contrast, protection against infection with encephalomyocarditis virus or vesicular stomatitis virus was reduced or absent, respectively. To further investigate the role of IFNaR1 in the induction of an antiviral state, we analyzed the Epo-versus IFN␣-induced transcription of a set of genes, involved in antiviral protection. Up to 24 h after stimulation with Epo or IFN␣, comparable transcription of the p56, dsRNA-dependent protein kinase, 2-5A synthetase, and MxA genes was seen. However, at later time points, only in the case of Epo induction, a sharp decrease of mRNA levels was observed. Western blotting analysis of dsRNA-dependent protein kinase showed a similar pattern at the protein level. Taken together, our results imply a role for IFNaR1 in the induction of sustained mRNA and protein levels that are likely required for optimal antiviral activity.It is generally accepted that activation of a cell by a cytokine is initiated by ligand-induced clustering of receptor subunits, which can occur as di-, tri-, or higher order oligomers, involving identical or related subunits. With the exception of the heptamembrane-spanning receptors, members from the hematopoietin/IFN, 1 tumor necrosis factor/nerve growth factor, and tyrosine kinase receptor families are all activated by this mechanism of cytokine-driven multimerization. Ligand binding to the first receptor component induces the association with additional receptor subunits eventually resulting in an increase of affinity for the ligand (1, 2). Alternatively, preformed receptor complexes may also exist on the cell membrane (3). Interferons belong to the class I cytokine family and are divided into type I interferons (at least 14 IFN␣ subtypes, IFN, IFN, and the bovine embryonic IFN) that have antiviral, cytostatic, and hematopoietic activities on many cell types, and the type II interferon or IFN␥ that is also involved in many immune functions. Both types of interferons bind to distinct receptors that belong to the class I cytokine receptors. The interferon type I receptor (IFNaR) is composed of two subunits, IFNaR1 and IFNaR2-2. As a result of alternative splicing, subtypes of the latter subunit do also exist: a cytoplasmic truncated transmembrane form (IFNaR2-1) and a soluble form (IFNaR2-3) (4 -8).A large body of evidence has shown that the class I cytokine receptors make use of associated kinases (JAKs) to start intracellular tyrosine phosphorylation, resulting in the activation of the so-called Stat proteins. This JAK/Stat pathway is essential for transcription of many of the cytokine-inducible ge...