Common gamma receptor–dependent cytokines and their JAK/STAT pathways play pivotal roles in T cell immunity. Abnormal activation of this system was pervasive in diverse T cell malignancies assessed by pSTAT3/pSTAT5 phosphorylation. Activating mutations were described in some but not all cases. JAK1 and STAT3 were required for proliferation and survival of these T cell lines whether or not JAKs or STATs were mutated. Activating JAK and STAT mutations were not sufficient to initiate leukemic cell proliferation but rather only augmented signals from upstream in the cytokine pathway. Activation required the full pathway, including cytokine receptors acting as scaffolds and docking sites for required downstream JAK/STAT proteins. JAK kinase inhibitors have depressed leukemic T cell line proliferation. The insight that JAK/STAT system activation is pervasive in T cell malignancies suggests novel therapeutic approaches that include antibodies to common gamma cytokines, inhibitors of cytokine-receptor interactions, and JAK kinase inhibitors that may revolutionize therapy for T cell malignancies.

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