Stat5b Is Essential for Natural Killer Cell–mediated Proliferation and Cytolytic Activity By Kazunori Imada,* Eda T. Bloom,‡ Hiroshi Nakajima,* Judith A. Horvath-Arcidiacono,‡ Garry B. Udy,§ Helen W. Davey,¶ and Warren J. Leonard* From the *Laboratory of Molecular Immunology, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892-1674; the ‡Division of Cell and Gene Therapy, Center for Biologics Evaluation and Research, Food and Drug Administration, Bethesda, Maryland 20892; the §Ministry of Agriculture and Forestry (Veriﬁcation Agency),Te Kuiti, New Zealand; and the ¶Dairy Science Group, AgResearch, Ruakura, Hamilton, New Zealand Summary We have analyzed the immune system in Stat5-deficient mice. Although Stat5a2/2 splenocytes have a partial defect in anti-CD3-induced proliferation that can be overcome by high dose in- terleukin (IL)-2, we now demonstrate that defective proliferation in Stat5b2/2 splenocytes can- not be corrected by this treatment. Interestingly, this finding may be at least partially explained by diminished expression of the IL-2 receptor b chain (IL-2Rb), which is a component of the receptors for both IL-2 and IL-15, although other defects may also exist. Similar to the defect in proliferation in activated splenocytes, freshly isolated splenocytes from Stat5b2/2 mice ex- hibited greatly diminished proliferation in response to IL-2 and IL-15. This results from both a decrease in the number and responsiveness of natural killer (NK) cells. Corresponding to the diminished proliferation, basal as well as IL-2– and IL-15–mediated boosting of NK cytolytic activity was also greatly diminished. These data indicate an essential nonredundant role for Stat5b for potent NK cell–mediated proliferation and cytolytic activity. Key words: natural killer cells • Stat5b • Stat5a • interleukin 2 • interleukin 15 S(STAT)1 proteins are cytosolic latent transcription fac- ignal transducers and activators of transcription tors that are rapidly activated after cellular exposure to in- terferons, cytokines, or growth factors (1–3). A consider- able amount of investigation has centered on the specificity of the signals transduced by each of the seven mammalian STAT proteins. For three of these STATs (Stat1, Stat4, and Stat6), knockout mice reveal specific defects limited to the immune system consistent with defective signaling in re- sponse to interferons (4, 5), IL-12 (6, 7), or IL-4/IL-13 (8– 10), respectively. In contrast, Stat3 knockout mice exhibit fetal lethality (11), a finding consistent with the activation of Stat3 by many important cytokines, including but not limited to the IL-6 family of cytokines (IL-6, IL-11, ciliary neurotrophic factor, leukemia inhibitory factor, onco- statin M, and cardiotrophin 1; reference 3). Stat5 was discovered as a mammary gland factor induced by prolactin (12). Subsequently, a number of studies re- vealed the existence of two closely related Stat5-like pro- 1Abbreviation used in this paper: STAT, signal transducers and activators of transcription. teins with .90% amino acid identity, now denoted as Stat5a and Stat5b (13–17). The genes encoding Stat5a and Stat5b are closely linked on human chromosome 17 (17) and mouse chromosome 11 (18). Both Stat5a and Stat5b are activated not only by prolactin but also by a very wide range of other cytokines, including growth hormone, erythropoietin, thrombopoietin, hematopoietic cytokines (IL-3, GM-CSF, and IL-5), and cytokines more specific for lymphocytes (IL-2, IL-7, IL-9, and IL-15; reference 3). Despite their similarities, Stat5a and Stat5b differ in their COOH-terminal transactivation domains (19), and rela- tively specific actions have been attributed to Stat5a and Stat5b. For example, mice lacking Stat5a exhibit defective lactation with impaired lobuloalveolar proliferation of the mammary epithelium during pregnancy, and reduced ex- pression of whey acidic protein (20), indicative of defective prolactin signaling. Mice lacking Stat5b exhibit a loss of sexually dimorphic growth, which results from sex differ- ences in the pattern of growth hormone secretion from the pituitary (21). A recent study analyzing mice in which the two Stat5 genes were deleted individually or together (by retargeting Stat5a1/2 embryonic stem cells with a Stat5b targeting construct) confirmed these findings and addition- 2067 The Journal of Experimental Medicine • Volume 188, Number 11, December 7, 1998 2067–2074 http://www.jem.org