IRF4 and graft versus host disease: To examine whether there was a difference in these signaling pathways between WT and Itk-/- donor T cells in the GVHD and GVL model, we examined expression of IRF4, JAK1, JAK2 and STAT3 by purified splenic T cells that had been stimulated overnight with CD3 and CD28 followed by lysis for analysis of protein.