Mesenchymal stem cells (MSCs) closely interact with the immune system, and they are known to secrete inflammatory cytokines in response to stress stimuli. The biological function of MSC-derived inflammatory cytokines remains elusive. Here, we reveal that even under physiological conditions, MSCs produce and release a low level of tumor necrosis factor alpha (TNFα), which is unexpectedly required for preserving the self-renewal and differentiation of MSCs via autocrine/paracrine signaling. Furthermore, TNFα critically maintains MSC function in vivo during bone homeostasis. Mechanistically, we unexpectedly discovered that physiological levels of TNFα safeguard MSC homeostasis in a receptor-independent manner through mechanical force-driven endocytosis and that endocytosed TNFα binds to mammalian target of rapamycin (mTOR) complex 2 and restricts mTOR signaling. Importantly, inhibition of mTOR signaling by rapamycin serves as an effective osteoanabolic therapeutic strategy to protect against TNFα deficiency and mechanical unloading. Collectively, these findings unravel the physiological framework of the dynamic TNFα shuttle-based mTOR equilibrium that governs MSC and bone homeostasis.

间充质干细胞 (MSCs) 与免疫系统密切相互作用，已知它们会分泌炎性细胞因子以响应压力刺激。MSC 衍生的炎性细胞因子的生物学功能仍然难以捉摸。在这里，我们揭示即使在生理条件下，MSCs 也会产生和释放低水平的肿瘤坏死因子 α (TNFα)，这是通过自分泌/旁分泌信号传导来保持 MSCs 的自我更新和分化所出乎意料的要求。此外，TNFα 在骨稳态过程中关键地维持体内 MSC 功能。机械地，我们意外地发现，生理水平的 TNFα 通过机械力驱动的内吞作用以不依赖受体的方式保护 MSC 稳态，并且内吞的 TNFα 与哺乳动物雷帕霉素靶标 (mTOR) 复合物 2 结合并限制 mTOR 信号传导。重要的是，雷帕霉素抑制 mTOR 信号传导是一种有效的骨合成代谢治疗策略，可防止 TNFα 缺乏和机械卸载。总的来说，这些发现揭示了控制 MSC 和骨稳态的动态 TNFα 穿梭基 mTOR 平衡的生理框架。