Counter‐regulation mechanism of IL‐4 and IFN‐α signal transduction through cytosolic retention of the pY‐STAT6:pY‐STAT2:p48 complex

IFN‐α and IL‐4 induce Th1 and Th2 responses, respectively, and often display antagonistic actions against each other. To elucidate the molecular mechanism of counter‐regulation, we have investigated the signal interception by IFN‐α and IL‐4, employing a human B‐cell line Ramos, sensitive to both cytokines. In these cells, IFN‐α effectively inhibited IL‐4‐induced Fc epsilon receptor II (CD23) expression, whereas IL‐4 suppressed IFN‐α‐mediated IRF7 expression. The counter‐regulatory action by IL‐4 and IFN‐α proceeded with a delayed kinetics requiring 4 h. Notably, IFN‐α did not affect the IL‐4‐induced tyrosine phosphorylation of STAT6, but induced a time‐dependent cytoplasmic accumulation of phosphotyrosine(pY)‐STAT6 and a corresponding decrease in nuclear pY‐STAT6. By confocal analysis and co‐immunoprecipitation assays, we demonstrated the colocalization and molecular interaction of IL‐4‐induced pY‐STAT6 with IFN‐α‐induced pY‐STAT2:p48 in the cytosol. In addition, the over‐expression of STAT2 or STAT6 induced the concomitant cytosolic accumulation of pY‐STAT6 or pY‐STAT2, leading to the suppression of IL‐4‐induced CD23 or IFN‐α‐induced IRF7 gene expression, respectively. Our data suggest that the signals ensued by IFN‐α and IL‐4 induce cytoplasmic sequestration of IL‐4‐activated STAT6 and IFN‐α‐activated STAT2:p48 in B cells through the formation of pY‐STAT6:pY‐STAT2:p48 complex, which provides a novel mechanism by which IFN‐α and IL‐4 cross‐regulate their signaling into the nucleus.