Enforced expression of Bcl-2 partially restores cell numbers but not functions of TCRγδ intestinal intraepithelial T lymphocytes in IL-15-deficient mice

IL-15 knockout (KO) mice have severely reduced numbers of TCR gamma delta intestinal intraepithelial T lymphocytes (i-IEL), suggesting requirements of IL-15 signaling in the development or maintenance of i-IEL. To determine an involvement of survival signals via Bcl-2 in IL-15-mediated homeostasis of TCR-gamma delta i-IEL, we introduced a bcl-2 transgene into IL-15 KO mice. In situ apoptosis of TCR gamma delta i-IEL was decreased in Bcl-2 transgenic (Tg) x IL-15 KO mice compared with IL-15 KO mice. The enforced expression of Bcl-2 partially restored the numbers of TCR gamma delta i-IEL in 11-15 KO mice. However, effector functions of TCR gamma delta i-IEL., including cytokine production and cytotoxic activity, were not recovered in Bcl-2 Tg x IL-15 KO mice. Importantly, TCR gamma delta i-IEL in Bcl-2 Tg x IL-15 KO mice expressed a reduced level of eomesodermin, a transcription factor critical for effector functions of NK cells and CD8(+) T cells. Similar to the case of TCR gamma delta i-IEL, enforced expression of Bcl-2 restored the numbers but not the functions of NK cells in IL-15 KO mice. These results suggest that Bcl-2-mediated survival signal is involved in the IL-15-mediated homeostasis of TCR gamma delta i-IEL and NK cells, but other signals from 11-15 are critical for inducing transcription factors, such as eomesodermin for their effector functions.