Temperature and calcium ions affect aggregation of mesenchymal stem cells in phosphate buffered saline

Bone marrow-derived mesenchymal stem cells (MSC) are being extensively studied as potential therapeutic agents for various diseases and have demonstrated tremendous promise to date. To reduce immunological and inflammatory reaction upon delivery of MSC in situ, the cells are often suspended in protein-free and nutrient-poor buffered saline solution at high titers and kept on ice (0 °C) until completion of the transplantation procedure. This study investigated the effects of suspending MSC (5 × 10⁶ cells/mL) in phosphate buffered saline (PBS) with and without calcium, over a time course of 90 and 180 min, at temperatures of 0 and 37 °C. The results at 0 °C showed a small but significant decrease in cell viability within calcium-free PBS after 180 min, whereas no significant changes in cell viability were observed with PBS containing calcium. Additionally, it was observed that significant aggregation of MSC into cellular clumps occurred when incubated in PBS at 0 °C, with a higher degree of aggregation occurring under calcium-free conditions. By contrast at 37 °C, there was a more pronounced decrease in cell viability after 90 and 180 min, but lesser aggregation of MSC both in the presence and absence of calcium. The aggregation of MSC into cellular clumps could pose an embolic hazard if delivered into the arterial vasculature in cardiac applications, can clog-up injection or infusion catheters utilized for cell delivery during surgery, and can also possibly reduce the overall efficacy of transplantation therapy.