T-cell-specific Stat3-deficient mice displayed impaired IL-6-induced and IL-2-induced T-cell proliferation.[16, 17] Also, Stat3 plays a crucial role in promoting T-cell survival in response to various stimuli.[18] Furthermore, Johnston et al.[19] suggested that the T-cell growth factors IL-2, IL-7 and IL-15 all activate Stat3 and Stat5. Therefore, transcription complexes

that include Stat3 and Stat5 may be of general importance to promote cell proliferation in T cells. Also, Durant et al.[11] examined the CD4+ T cells in the spleen and found that the majority of control (Stat3fl/fl) T cells underwent multiple cell divisions after 5 days. In contrast, fewer than half of Stat3−/− T cells had

divided, Sorafenib order as indicated by CFSE dilution. By 7 days, essentially all of the control T cells had divided, whereas Cytoskeletal Signaling inhibitor 18% of Stat3−/− T cells remained quiescent. In spite of current knowledge about the link between Stat3 and T-cell survival, little is known about how Stat3 regulates T-cell homeostasis in peripheral lymphoid tissues. Using mice with targeted deletion of Stat3 in T cells, we showed that Stat3 maintains the CD4 or CD8 single-positive (SP) thymocytes and naive T-cell pool in the resting condition by promoting the expression of Bcl-2 family genes. This discovery magnifies the significance of Stat3 as a master regulator of homeostatic signals for the maintenance and functional adjustment of the naive T-cell population. Mice homozygous for the loxP-flanked (floxed) Stat3 gene (Stat3fl/fl) were a kind gift from Dr S. Akira.

Mice carrying a Cre transgene under the control of the distal Lck promoter (Lck-CRE+/+)were purchased from The Jackson Laboratory (Bar Harbor, ME). Mice with a Stat3 deletion in T cells were generated by crossing mice with the floxed Cyclic nucleotide phosphodiesterase Stat3 allele with mice expressing Cre under the control of the Lck promoter.[17] Genomic DNA was isolated from tail tips using a NucleoSpin genomic DNA purification kit (Macherey-Nagel GmbH & Co., Duren, North Rhine-Westphalia, Germany). Genotyping was performed with the primers CCTGAAGACCAAGTTCATCTGTGTGAC and CACACAAGCCATCAAACTCTGGTCTCC, which are specific for exons 22 and 23 of Stat3, respectively.[20] Mice carrying Cre were identified by genotyping with the primers GCGGTCTGGCAGTAAAAACTATC and GTGAAACAGATT-GCTGTCACTT, which are specific for the Cre transgene, according to the manufacturer’s instructions. All animals were maintained under specific pathogen-free conditions and all experimental procedures were reviewed and approved by the Institutional Animal Care and Use Committee (IACUC) of the College of Medicine, Seoul National University.