Human mitochondrial transfer modeling reveals biased delivery from mesenchymal-to-hematopoietic stem cells

Within the bone marrow (BM), the intercellular communication between hematopoietic stem and progenitor cells (HSPCs) and mesenchymal stem/stromal cells (MSCs) is critical for the life-long maintenance of functional hematopoiesis. In recent years, the transfer of mitochondria between MSCs and HSPCs has emerged as a key aspect of this communication, occurring both in stress and homeostatic conditions. In human, the mesenchymal-to-hematopoietic transfer process and functional impact remain cryptic, primarily due to a lack of robust models. To this end, we here describe the development and exploitation of iMSOD-mito, an immortalized human MSCs line bearing an inducible mCherry mitochondrial tag. Co-culture with primary healthy HSPCs or a leukemic cell line revealed a high mitochondrial transfer rate (>15%), exclusively relying on cell-to-cell contact. While all CD34+ blood cells received mitochondria, a preferential transfer towards phenotypic hematopoietic stem cells was identified. Similarly, using primary MSCs with genetically labelled mitochondria we confirmed a transfer to all CD34+ populations, albeit occurring at a lower frequency than with the iMSOD-mito (3.38%). By engineering 3D bone marrow niches in perfusion bioreactor, this transfer rate could be significantly increased, while the biased towards HSC as receiver was maintained. Functionally, mitochondria-receiving cells exhibited an increased mitochondria membrane potential and reactive oxygen species (ROS) production, which in HSPCs was associated with retained quiescence in single cell divisional assay. In summary, we propose the iMSOD-mito as a standardized tool to model human mesenchymal-to-hematopoietic mitochondria transfer in 2D or 3D culture systems. Our work prompts the study of mitochondria transfer in both healthy or disease conditions, towards the design of regenerative therapies or identification of new targets in a malignant context.