Identification of functional type I and type II interferon receptors.

Although the interferons were the first cytokines purified and cloned, their receptors have remained an enigma as fully functional receptors have not been defined and characterized. We previously discovered that the IFN-gamma receptor (IFN-gamma R) was able to bind IFN-gamma, but required one or more accessory factors to exhibit functional activity. We have now identified a gene and sequence of one of these accessory factors. Both genomic and cDNA clones were used to reconstitute a functional receptor. The presence of the Hu-IFN-gamma R and the human accessory factor-1 (designated AF-1) are both necessary and sufficient to induce MHC class I antigens in response to Hu-IFN-gamma in hamster cells. AF-1 is a transmembrane protein with no significant amino acid homology to other sequences in nucleic acid data bases, but has overall structural homology to the class 2 cytokine receptor family. Whereas total human DNA was used to first isolate a Type I interferon receptor, the only genomic or cDNA clone reported exhibited activity essentially only in response to Hu-IFN-alpha B2 (Hu-IFN-alpha 8). Yet hamster or mouse cells containing human Chromosome 21 seemed to have the full complement of Hu-IFN-alpha activities and responded to all Hu-IFN-alpha species as well as Hu-IFN-beta. To investigate this dilemma, we extended our earlier studies with total human DNA to employ defined YAC clones. We have now isolated a genomic segment of human Chromosome 21 that exhibits the properties expected of the Type I interferon receptor: response to multiple species of Hu-IFN-alpha; and excellent binding of Hu-IFN-alpha A and Hu-IFN-alpha B2 species. Reconstitution of functional Type I and Type II interferon receptors now provides the first steps to link the receptors to the signal transduction mechanisms; and will help to integrate our understanding of the ligand receptor interactions to the molecular signals that activate specific transcription factors that in turn induce specific genes carrying the interferon regulatory elements.