Recruitment of Stat4 to the human interferon-α/β receptor requires activated Stat2

Stat4 activation is involved in differentiation of type 1 helper (Th1) T cells. Although State is activated by interleukin (IL)-12 in both human and murine T cells, State is activated by interferon (IFN)-alpha only in human, but not murine, CD4+ T cells. This species-specific difference in cytokine activation of State underlies critical differences in Th1 development in response to cytokines and is important to the interpretation of murine models of immunopathogenesis. Here, we sought to determine the mechanism of Stat4 recruitment and activation by the human IFN-alpha receptor. Analysis of phosphopeptide binding analysis suggests that State does not interact directly with tyrosine-phosphorylated amino acid residues within the cytoplasmic domains of either of the subunits of the IFN-alpha receptor complex. Expression of murine State in the Stat1-deficient U3A and the Stat2-deficient U6A cell lines shows that IFN-alpha-induced State phosphorylation requires the presence of activated Stat2 but not Stat1. Thus, in contrast to the direct recruitment of Stat4 by the IL-12 receptor, Stat4 activation by the human IFN-alpha receptor occurs through indirect recruitment by intermediates involving Stat2.