Skeletal muscle interleukin 15 promotes CD8(+) T-cell function and autoimmune myositis.

BACKGROUND: Interleukin 15 (IL-15) is thought to be abundant in the skeletal muscle under steady state conditions based on RNA expression; however, the IL-15 RNA level may not reflect the protein level due to post-transcriptional regulation. Although exogenous protein treatment and overexpression studies indicated IL-15 functions in the skeletal muscle, how the skeletal muscle cell uses IL-15 remains unclear. In myositis patients, IL-15 protein is up-regulated in the skeletal muscle. Given the supporting role of IL-15 in CD8(+) T-cell survival and activation and the pathogenic role of cytotoxic CD8(+) T cells in polymyositis and inclusion-body myositis, we hypothesize that IL-15 produced by the inflamed skeletal muscle promotes myositis via CD8(+) T cells. METHODS: Expression of IL-15 and IL-15 receptors at the protein level by skeletal muscle cells were examined under steady state and cytokine stimulation conditions. The functions of IL-15 in the skeletal muscle were investigated using Il15 knockout (Il15 (-/-) ) mice. The immune regulatory role of skeletal muscle IL-15 was determined by co-culturing cytokine-stimulated muscle cells and memory-like CD8(+) T cells in vitro and by inducing autoimmune myositis in skeletal-muscle-specific Il15 (-/-) mice. RESULTS: We found that the IL-15 protein was not expressed by skeletal muscle cells under steady state condition but induced by tumor necrosis factor alpha (TNF-α) and interferon gamma (IFN-γ) stimulation and expressed as IL-15/IL-15 receptor alpha (IL-15Rα) complex. Skeletal muscle cells expressed a scanty amount of IL-15 receptor beta (IL-15Rß) under either conditions and only responded to a high concentration of IL-15 hyperagonist, but not IL-15. Consistently, deficiency of endogenous IL-15 affected neither skeletal muscle growth nor its responses to TNF-α and IFN-γ. On the other hand, the cytokine-stimulated skeletal muscle cells presented antigen and provided IL-15 to promote the effector function of memory-like CD8(+) T cells. Genetic ablation of Il15 in skeletal muscle cells greatly ameliorated autoimmune myositis in mice. CONCLUSIONS: These findings together indicate that skeletal muscle IL-15 directly regulates immune effector cells but not muscle cells and thus presents a potential therapeutic target for myositis.

Adipocyte IL-15 regulates local and systemic NK cell development.

NK cell development and homeostasis require IL-15 produced by both hematopoietic and parenchymal cells. Certain hematopoietic IL-15 sources, such as macrophages and dendritic cells, are known, whereas the source of parenchymal IL-15 remains elusive. Using two types of adipocyte-specific Il15(-/-) mice, we identified adipocytes as a parenchymal IL-15 source that supported NK cell development nonredundantly. Both adipocyte-specific Il15(-/-) mice showed reduced IL-15 production specifically in the adipose tissue but impaired NK cell development in the spleen and liver in addition to the adipose tissue. We also found that the adipose tissue harbored NK progenitors as other niches (e.g. spleen) for NK cell development, and that NK cells derived from transplanted adipose tissue populated the recipient's spleen and liver. These findings suggest that adipocyte IL-15 contributes to systemic NK cell development by supporting NK cell development in the adipose tissue, which serves as a source of NK cells for other organs.

IL-15Rα of radiation-resistant cells is necessary and sufficient for thymic invariant NKT cell survival and functional maturation.

The development of invariant NKT (iNKT) cells depends on the thymus. After positive selection by CD4(+)CD8(+)CD1d(+) cortical thymocytes, iNKT cells proceed from CD44(low)NK1.1(-) (stage 1) to CD44(high)NK1.1(-) (stage 2), and then to CD44(high)NK1.1(+) (stage 3) cells. The programming of cytokine production occurs along the three differentiation stages, whereas the acquisition of NK receptors occurs at stage 3. Stage 3 thymic iNKT cells are specifically reduced in Il15ra(-/-) mice. The mechanism underlying this homeostatic deficiency and whether the IL-15 system affects other thymic iNKT cell developmental events remain elusive. In this study, we demonstrate that increased cell death contributed to the reduction of stage 3 cells in Il15ra(-/-) mice, as knockout of Bim restored this population. IL-15-dependent upregulation of Bcl-2 in stage 3 cells affected cell survival, as overexpression of hBcl-2 partially restored stage 3 cells in Il15ra(-/-) mice. Moreover, thymic iNKT cells in Il15ra(-/-) mice were impaired in functional maturation, including the acquisition of Ly49 and NKG2 receptors and the programming of cytokine production. Finally, IL-15Rα expressed by radiation-resistant cells is necessary and sufficient to support the survival as well as the examined maturation events of thymic iNKT cells.

IL-15 does not affect IEL development in the thymus but regulates homeostasis of putative precursors and mature CD8 alpha alpha+ IELs in the intestine.

Mice devoid of the IL-15 system lose over 90% of CD8alphaalpha(+) TCRalphabeta and TCRgammadelta intestinal intraepithelial lymphocytes (iIELs). Previous work revealed that IL-15Ralpha and IL-15 expressed by parenchymal cells, but not by bone marrow-derived cells, are required for normal CD8alphaalpha(+) iIEL homeostasis. However, it remains unclear when and how the IL-15 system affects CD8alphaalpha(+) iIELs through their development. This study found that IL-15Ralpha is dispensable for the thymic stage of CD8alphaalpha(+) TCRalphabeta and TCRgammadelta iIEL development but is required for the maintenance and/or differentiation of the putative lineage marker negative precursors in the intestinal epithelium, especially for the most mature CD8 single positive subset. Moreover, the IL-15 system directly supports the survival of mature CD8alphaalpha(+) iIEL in vivo. Taken together, this study suggests that regulation of CD8alphaalpha(+) iIEL homeostasis by the IL-15 system does not occur in the thymus but involves mature cells and putative precursors in the intestine.