Kinetics of thymic regeneration in female mice following short-term rapamycin administration

Thymus is the core organ that reflects the degree of aging, and the anti-aging effect of rapamycin(RAPA) has been confirmed by multiple studies, but the dynamic changes of the thymus after the administration of RAPA have been lacking systematic research. Here, we observed the involution of thymus induced by RAPA (1 mg/kg/day, i.p. for 3 days) and its regeneration kinetics after withdrawal of RAPA, to explore the possible mechanism of RAPA's anti-aging effects based on changes in thymus function. The results show that the thymus weight decreased by 46.15% post-RAPA, then regenerated from 20.45 mg to 57.97 mg, reaching 1.3-fold baseline levels in age-matched controls. RAPA can cause acute involution in thymic structure and function, such as loss of medullary thymic epithelial cells (mTECs) and subcapsular thymic epithelial cells (sTECs), abnormal differentiation of thymocytes, reduction of peripheral T cell proportion, and up-regulation of Foxn1 and Klf6 mRNA levels. Regeneration featured expanded double positive (DP) T cells, CD3⁺TCRβ⁺ T cells, CD8 single-positive (SP) T cells, medullary dilatation, and thymocyte telomere elongation. IL-7 and MET have pro-regeneration effects, but neither of them could accelerate RAPA-mediated thymic regeneration. IL-7 can prolong telomere length of thymocytes and restore peripheral T cell homeostasis, while MET can promote T cell positive selection and increase Sirt3 and Gimap4 mRNA levels. These results demonstrate that RAPA triggers transient thymic suppression followed by thymus volume and functional recovery. It seems that RAPA-mediated immune reconstitution may be an important reason for the potential anti-aging effect of RAPA.