Interferon α (IFNα) signaling in cells expressing the variant form of the type I IFN receptor

Two different Type I interferon receptors (IFN-R) have been described: the normal and the variant receptors. The α subunit of the Type I IFN-R has a molecular mass of 110 kDa in cells expressing normal and variant receptors. The β subunit has a molecular mass of approximately 100 kDa in cells that express normal receptors and 55 kDa in cells expressing the variant form of the receptor. The IFNα-resistant U-937 cell line expresses variant receptors and fails to down-regulate and phosphorylate the α subunit on tyrosine residues. We report that two other myelomonocytic cell lines, YK-M2 and ML- 2, also expressing the variant form of the receptor, fail to down-regulate and phosphorylate the α subunit on tyrosine residues. However, YK-M2 and ML- 2 cells are sensitive to the antiproliferative and antiviral effects of IFNα2, indicating that phosphorylation of the α subunit is not necessary to elicit an IFNα response and that expression of variant receptors is not a source of IFNα resistance. We also determined if other proteins involved in the IFNα signal transduction pathway had a different phosphorylation pattern. Treatment of cells expressing variant receptors induced tyrosine phosphorylation of the p135(tyk2) tyrosine kinase, and the three interferon- stimulated gene factor 3α (ISGF3α) polypeptides (p113, p91, and p84), albeit at lower levels. These results indicate that cells expressing either form of the Type I IFN-R phosphorylate a similar set of proteins, with the exception of the α subunit.