Absence of functional STAT4 activation despite detectable tyrosine phosphorylation induced by murine IFN-α

We previously reported that IL-12, but not IFN-alphaA/D, induces T helper type (Th) 1 development and STAT4 phosphorylation in murine CD4(+) T cells. However, a recent study reported that IFN-(alpha)A/D and recombinant murine IFN-alphaA can induce STAT4 phosphorylation, although more weakly than IL-12, largely in CD8(+) rather than CD4+ T cells. That report did not examine Th1 development or directly demonstrate induction of IFN-gamma by IFN-alpha. To address these differences, we compared IFN-alphaA/D, murine IFN-alphaA, and IL-12 for STAT4 phosphorylation, formation of active nuclear DNA binding complexes, induction of Thl development, and production of IFN-gamma in murine CD4(+) T cells. IFN-alphaA induced detectable STAT4 phosphorylation, although at significantly lower levels than induced by IL-12. Furthermore, in contrast to IL-12, IFN-alphaA failed to induce Thl development or the formation of DNA binding complexes or to synergize with IL-18 for induction of IFN-gamma production. STAT1-deficient CD4(+) T cells showed increased IFN-alphaA-induced STAT4 phosphorylation but still exhibited significantly lower amounts of cytokine-induced IFN-gamma compared to IL-12. In summary, these results suggest that in contrast to IL-12, IFN-alphaA does not play a functionally significant role in meditating the STAT4-dependent induction of Th1 development or IFN-gamma production in CD4(+) T cells.