Abstract: BACKGROUND: Aging of human body may be due to the senescence and depletion of various stem cells in the body. Bone marrow mesenchymal stromal cells have important physiological functions and have shown certain therapeutic effects on various diseases. It is of great significance to study the senescence and mechanism of bone marrow mesenchymal stromal cells. 
OBJECTIVE: To investigate whether human bone marrow mesenchymal stromal cells exhibit senescence phenotypes with increasing donor age, and further determine whether endogenous retrovirus drives the senescence of bone marrow mesenchymal stromal cells, offering a novel reference for the investigation of stem cell senescence mechanism.
METHODS: The senescence of bone marrow mesenchymal stromal cells at different ages was characterized by flow cytometry, β-galactosidase staining, qPCR, western blotting, and EdU fluorescence imaging. Bone marrow mesenchymal stromal cells and cell culture supernatant were collected from donors of different ages. The content of human endogenous retrovirus was detected by qPCR. Furthermore, highly sensitive droplet digital PCR was used to detect the expression of endogenous retrovirus-like particles in the cell culture supernatant. The content of endogenous retrovirus in bone marrow plasma samples of different ages was detected by ELISA.
RESULTS AND CONCLUSION: Bone marrow mesenchymal stromal cells exhibited obvious senescence with increasing age, including significant morphological changes, increased proportion of β-galactosidase positive cells, increased expression of senescence markers P16 and P21 protein, decreased expression of LMNB1 protein, and reduced cell proliferation ability. There was no significant difference in the content of endogenous retrovirus in bone marrow mesenchymal stromal cells at different ages, almost no endogenous retrovirus-like particles in the cell culture supernatant. There was no significant difference in endogenous retrovirus-like particles detected in bone marrow plasma samples at different ages. These findings indicate that human bone marrow mesenchymal stromal cells have normal physiological senescence with increasing age, but the mechanism of senescence is not mediated by abnormal activation of endogenous retroviruses, which may have a more complex driving mechanism. 

Key words: bone marrow mesenchymal stromal cell, aging mechanism, cellular senescence, bone marrow plasma, endogenous retrovirus, endogenous retrovirus-like particle, droplet digital PCR