Intercellular cytosolic transfer correlates with mesenchymal stromal cell rescue of umbilical cord blood cell viability during ex vivo expansion.

BACKGROUND AIMS: Mesenchymal stromal cells (MSC) have been observed to participate in tissue repair and to have growth-promoting effects on ex vivo co-culture with other stem cells. METHODS: In order to evaluate the mechanism of MSC support on ex vivo cultures, we performed co-culture of MSC with umbilical cord blood (UCB) mononuclear cells (MNC) (UCB-MNC). RESULTS: Significant enhancement in cell growth correlating with cell viability was noted with MSC co-culture (defined by double-negative staining for Annexin-V and 7-AAD; P < 0.01). This was associated with significant enhancement of mitochondrial membrane potential (P < 0.01). We postulated that intercellular transfer of cytosolic substances between MSC and UCB-MNC could be one mechanism mediating the support. Using MSC endogenously expressing green fluorescent protein (GFP) or labeled with quantum dots (QD), we performed co-culture of UCB-MNC with these MSC. Transfer of these GFP and QD was observed from MSC to UCB-MNC as early as 24 h post co-culture. Transwell experiments revealed that direct contact between MSC and UCB-MNC was necessary for both transfer and viability support. UCB-MNC tightly adherent to the MSC layer exhibited the most optimal transfer and rescue of cell viability. DNA analysis of the viable, GFP transfer-positive UCB-MNC ruled out MSC transdifferentiation or MSC-UCB fusion. In addition, there was statistical correlation between higher levels of cytosolic transfer and enhanced UCB-MNC viability (P < 0.0001). CONCLUSIONS: Collectively, the data suggest that intercellular transfer of cytosolic materials could be one novel mechanism for preventing UCB cell death in MSC co-culture.