MTOR and Osteopenia: To further evaluate whether inhibition of mTOR signaling rescues the functional decline of MSCs in TNFα−/− mice in vivo, we infused rapamycin into TNFα−/− mice intraperitoneally at a dosage of 1.5 mg·kg−1 per day (Fig. 4a); this approach was based on our previous study using rapamycin to ameliorate osteopenia.30 After 2 weeks of rapamycin administration, MSCs demonstrated remarkable recovery despite TNFα deficiency, as indicated by the formation of more fibroblastic colonies ex vivo (Fig. 4b).