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Warren J. Leonard is an American immunologist and scientist at the National Institutes of Health, where he is Chief of the Laboratory of Molecular Immunology and Director of the Immunology Center in the National Heart, Lung, and Blood Institute. He received his A.B. in mathematics from Princeton University and M.D. from Stanford University.

Leonard is an expert in cytokine biology, molecular biology, and immunodeficiency. Leonard cloned the interleukin-2 receptor (IL-2R) α chain, discovered the IL-2R β chain, and demonstrated that the IL-2R γ chain is mutated in humans with X-linked severe combined immunodeficiency disease (XSCID, also known as SCIDX1 and as the "Bubble Boy disease"). XSCID is characterized by profoundly diminished numbers of T cells and natural killer (NK) cells; B cells are present but are non-functional. Thus, XSCID is a form of T-B+NK-SCID. He demonstrated that IL-2Rγ is shared by the multiple cytokine receptors and renamed it as the common cytokine receptor γ chain, γc. γc is now known to be shared by the receptors for IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21. Leonard also found mutations in γc caused a moderate form of X-linked combined immunodeficiency (XCID). He showed that the kinase, JAK3, associates with γc, and his group and that of Notarangelo independently reported the first cases of JAK3-deficient human SCID, as a second form of T-B+NK- SCID. Leonard also discovered IL7R-deficient human SCID, which is characterized by a selective lack of T cells.He also predicted that agents that inhibit the γc-JAK3 interaction or the catalytic activity of JAK3 would be immunosuppressive, providing the foundation for the subsequent development of Jak3 inhibitors as immunosuppressive agents.

Leonard also cloned the IL-21 receptor and showed it is critical for terminal B cell differentiation to plasma cells via its induction of BLIMP1 and that IL-21 also promotes CD8 T cell expansion. He also showed with collaborators that IL-21 has anti-cancer actions and that IL-2 is required for the development of autoimmune disease, including type 1 diabetes, systemic lupus erythematosus, and and experimental autoimmune uveitis. He also demonstrated the the IL-7-related cytokine, thymic stromal lymphopoietin (TSLP) is required for the development of allergic lung inflammation in a model of asthma. Dr. Leonard has also elucidated the mechanism of signaling of γc family cytokines via genome wide analysis of signal transducer and activator of transcription (STAT) proteins.

Awards and activities: Leonard has won numerous awards including the American Federation for Clinical Research Foundation Outstanding Investigator Award and the American Association of Immunologists-Huang Foundation Meritorious Career Award. Leonard is past-President of the International Cytokine Society, past-co-Editor of Immunity, and has organized major conferences in the cytokine field.