The specificity of the various STAT SH2 domains for different tyrosine-containing peptides enables cytokines to activate different signaling pathways and to induce distinct patterns of gene expression. We show that STAT4 has a unique peptide specificity and binds to the peptide sequence pYLPSNID (where pY represents phosphotyrosine). This motif is found at tyrosine residue 800 in the ␤2 subunit of the interleukin-12 receptor and is required for DNA binding and transcriptional activity of STAT4. Our data demonstrate that transfection of interleukin-12 receptor ␤1 and ␤2 subunits is sufficient for STAT4 activation but not for STAT1 or STAT3 activation.IL-12 1 is a heterodimeric cytokine secreted from antigen presenting cells in response to bacteria and intracellular parasites and thus plays an important role in host defense against bacterial pathogens (1). IL-12 promotes the proliferation of T cells and NK cells and is required for the differentiation of T cells into the Th1 subset of T helper cells. Th1 cells are critical for cell-mediated immune responses, because they secrete IFN-␥, which enhances the activity of cytotoxic T cells and NK cells (1-3). In addition to these essential functions, Th1-dominated responses are associated with pathologic autoimmune and inflammatory conditions such as rheumatoid arthritis and inflammatory bowel disease (4). Given this potential for immunopathology, it is important to understand the mechanisms that control the Th1 response to develop possible therapeutic interventions.IL-12 signals through the IL-12 receptor, which is composed of at least two subunits designated ␤1 and ␤2 (5, 6). Both IL-12 receptor subunits are members of the hemapoietin receptor superfamily and have strong homology to the gp130 receptor (5). The ␤1 receptor, although a low affinity binder of IL-12, is not capable of transducing an IL-12-mediated signal (6). A second subunit of the IL-12 receptor was subsequently identified that when coexpressed with the ␤1 subunit forms a high affinity receptor for IL-12 and confers IL-12 signaling (6). IL-12R ␤2 expression is differentially regulated in Th1 versus Th2 cells (7). Th1 cells but not Th2 cells express the ␤2 subunit of the IL-12 receptor (8). Following T cell activation, IL-12 and IFN-␥ treatment induces ␤2 expression, whereas IL-4, a cytokine produced by Th2 cells, inhibits ␤2 expression resulting in loss of IL-12 signaling (7,8). Therefore, expression of the ␤2 subunit of the IL-12 receptor is a crucial determinant of Th1 versus Th2 development.Cytokine binding to its receptor leads to activation of JAK kinases that phosphorylate the receptor on tyrosines located in the intracellular domain. The phosphorylated regions are binding sites for signal transduction molecules called STATs that are rapidly recruited to the receptor and tyrosine phosphorylated by JAK kinases. Tyrosine phosphorylation of STAT proteins induces their dimerization and translocation to the nucleus where they bind to specific DNA sequences and regulate transcription. IL-12 stimulation ...