MyD88-dependent signaling for IL-15 production plays an important role in maintenance of CD8 alpha alpha TCR alpha beta and TCR gamma delta intestinal intraepithelial lymphocytes.

Interaction between commensal bacteria and intestinal epithelial cells (i-ECs) via TLRs is important for intestinal homeostasis. In this study, we found that the numbers of CD8alphaalpha TCRalphabeta and TCRgammadelta intestinal intraepithelial lymphocytes (i-IELs) were significantly decreased in MyD88-deficient (-/-) mice. The expression of IL-15 by i-ECs was severely reduced in MyD88(-/-) mice. Introduction of IL-15 transgene into MyD88(-/-) mice (MyD88(-/-) IL-15 transgenic mice) partly restored the numbers of CD8alphaalpha TCRalphabeta and TCRgammadelta i-IELs. The i-IEL in irradiated wild-type (WT) mice transferred with MyD88(-/-) bone marrow (BM) cells had the same proportions of i-IEL as WT mice, whereas those in irradiated MyD88(-/-) mice transferred with WT BM cells showed significantly reduced proportions of CD8alphaalpha TCRalphabeta and TCRgammadelta i-IELs, as was similar to the proportions found in MyD88(-/-) mice. However, irradiated MyD88(-/-) IL-15 transgenic mice transferred with WT BM cells had increased numbers of CD8alphaalpha TCRalphabeta and TCRgammadelta subsets in the i-IEL. These results suggest that parenchymal cells such as i-ECs contribute to the maintenance of CD8alphaalpha TCRalphabeta and gammadelta i-IELs at least partly via MyD88-dependent IL-15 production.