Activation of T cells with PHA + IFN-γ augments whereas activation with PHA + IL-4 inhibits the IL-12–induced tyrosine phosphorylation of Jak2 and Tyk2. PBMC were cultured with either PHA, PHA + IFN-γ (A and B), or PHA + IL-4 (C and D), and T cells were then stimulated with medium alone (lanes 1 and 3) or IL-12 (lanes 2 and 4). Immunoprecipitations were performed on whole cell lysates with an antibody to Jak2 (A and C) or Tyk2 (B and D) followed by Western blots using an antiphosphotyrosine antibody (upper panels) or antibodies to either Jak2 or Tyk2 (lower panels). Each result is representative of two separate experiments.