Huang Po-Lin, Hou Mau-Sheng, Wang Szu-Wen, Chang Chin-Ling, Liou Yae-Huei, Liao Nan-Shih
Molecular Cell Biology, Taiwan International Graduate Program, Institute of Molecular Biology, Academia Sinica, and Graduate Institute of Life Sciences, National Defense Medical Center, Taipei, Taiwan ; Institute of Molecular Biology, Academia Sinica, Taipei, 11529 Taiwan.
Institute of Molecular Biology, Academia Sinica, Taipei, 11529 Taiwan.
Skelet Muscle. 2015 Sep 28;5:33. doi: 10.1186/s13395-015-0058-2. eCollection 2015.
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.
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.
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-15Rβ) 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.
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.
基于RNA表达，白细胞介素15（IL-15）在稳态条件下被认为在骨骼肌中含量丰富；然而，由于转录后调控，IL-15 RNA水平可能无法反映蛋白质水平。尽管外源性蛋白质处理和过表达研究表明IL-15在骨骼肌中发挥作用，但骨骼肌细胞如何利用IL-15仍不清楚。在肌炎患者中，骨骼肌中的IL-15蛋白上调。鉴于IL-15在CD8(+) T细胞存活和激活中的支持作用以及细胞毒性CD8(+) T细胞在多发性肌炎和包涵体肌炎中的致病作用，我们假设炎症骨骼肌产生的IL-15通过CD8(+) T细胞促进肌炎。
在稳态和细胞因子刺激条件下，检测骨骼肌细胞中IL-15和IL-15受体的蛋白水平表达。使用Il15基因敲除（Il15 (-/-) ）小鼠研究IL-15在骨骼肌中的功能。通过体外共培养细胞因子刺激的肌肉细胞和记忆样CD8(+) T细胞以及在骨骼肌特异性Il15 (-/-) 小鼠中诱导自身免疫性肌炎，确定骨骼肌IL-15的免疫调节作用。
我们发现，在稳态条件下骨骼肌细胞不表达IL-15蛋白，但在肿瘤坏死因子α（TNF-α）和干扰素γ（IFN-γ）刺激下可诱导表达，并以IL-15/IL-15受体α（IL-15Rα）复合物形式存在。在两种条件下，骨骼肌细胞均表达少量的IL-15受体β（IL-15Rβ），且仅对高浓度的IL-15超激动剂有反应，对IL-15无反应。一致地，内源性IL-15的缺乏既不影响骨骼肌生长，也不影响其对TNF-α和IFN-γ的反应。另一方面，细胞因子刺激的骨骼肌细胞呈递抗原并提供IL-15以促进记忆样CD8(+) T细胞的效应功能。骨骼肌细胞中Il15的基因缺失极大地改善了小鼠的自身免疫性肌炎。
这些发现共同表明，骨骼肌IL-15直接调节免疫效应细胞而非肌肉细胞，因此是肌炎的一个潜在治疗靶点。
