The freed Bcl-2 presumably exerts a prosurvival function, which would enhance the efficacy of the IL-15-induced Bcl-2 increment. Being resident in the intestine epithelium, it may be beneficial for CD8αα+ iIELs to control Bim activity by phosphorylation and dephosphorylation rather than synthesis and degradation, as the former can be achieved in a timely manner in response to the complex environment of the intestinal mucosa. Further studies are needed to test these possibilities. Activation of the Jak3-Jak1-PI3K-Akt-ERK signaling pathway is essential for IL-15-mediated CD8αα+ iIEL survival (Fig. 1). Although ERK activation is downstream of PI3K-Akt,

it was obviously delayed

compared to the activation of PI3K-Akt (Fig. 1C, D and Supporting Information Fig. 6, left panel). Consistently, the reduction of Bcl-2 level by MEK inhibition ABT-737 occurred later than that induced by Jak3 or PI3K inhibitor (Fig. 2A). It is possible that ERK1/2 activation was secondary to IL-15 selleck inhibitor stimulation. However, our preliminary experiments using supernatant from 40 h IL-15-treated CD8αα+ iIELs did not support the possibility that IL-15-induced secretory factor(s) activated ERK1/2 in CD8αα+ iIELs (Supporting Information Fig. 6, right panel). Other possible causes for the delayed and sustained ERK1/2 activation includes prolonged activation of upstream kinase and diminished activation of phosphatase. In view of these findings, we propose a stepwise model for IL-15-mediated CD8αα+ iIEL survival (Supporting Information Fig. 7). IL-15 first upregulates prosurvival Bcl-2 and Mcl-1 via activation of the Jak3-Jak1-PI3K-Akt pathway. With elevated Bcl-2, IL-15 induces ERK1/2-mediated phosphorylation of Bim at Ser65 to release Bcl-2 from the Bcl-2-Bim complex and to keep Bim in a phosphorylated SPTLC1 state. Activated ERK1/2 also participates in the maintenance of Bcl-2 level. The increase of Bcl-2 abundance and freed Bcl-2 shift the balance of Bcl-2 and Bim function toward promoting CD8αα+ iIEL survival. C57BL/6J (B6) and B6 human (hu) BCL-2

transgenic (B6-Tg (BCL2) 36Wehi/J) mice were purchased from the Jackson Laboratories. Il15ra−/− mice were generated in our lab [43] and backcrossed to B6 for 24 generations. RNA polymerase II-driven huMCL-1 transgenic mice in the B6 background were generated in Dr. S.-F. Yang-Yen’s lab [44]; Bim−/− mice were kindly provided by Dr. Jeffery C. Y. Yen (Institute of Biomedical Science, Academia Sinica, Taiwan). All mice were raised in a specific pathogen-free facility at the Institute of Molecular Biology, Academia Sinica. The mice were used at 8–22 weeks of age. All mice experiments were approved by the Institutional Animal Care and Use Committee at Academia Sinica and conformed to the relevant regulations.