G-CSF Mobilized Human Mesenchymal Stem Cells Are Found in the Peripheral Blood and Have Telomere Limited Growth Potential.

Mesenchymal stem cells (MSC) are multipotent cells found lining the bone marrow cavity. Their primary function is to support the growth and differentiation of hematologic progenitors. MSC have been shown to differentiate into a variety of cell types including: bone, adipocytes, cartilage, neuron-like, and muscle-like cells. There is mounting evidence that these cells can, under the right circumstances, enter the peripheral circulation. However, MSC have not been routinely isolated from peripheral blood. Granulocyte colony stimulating factor (G-CSF) is commonly used to mobilize hematopoietic stem cells from the bone marrow into the peripheral circulation. We show that G-CSF mobilized peripheral blood also contains a small percentage of MSC although lower than that of bone marrow derived MSC (BMMSC): 0.012% vs 0.04%. Isolates were morphologically similar to BMMSC and were successfully expanded and shown to differentiate into osteogenic and adipogenic lineages in the appropriate differentiation conditions. FACS analysis showed that the cells reliably expressed cell surfaces markers commonly found on MSC including CD105, CD29, CD166, and CD13. They were negative for CD14, CD34, CD133, and CD45. Mobilized peripheral blood derived MSC (MPB-MSC) had limited expansion potential when compared with bone marrow isolated MSC. Most cells appeared to cease cell division 20–25 days after isolation. MPB-MSC did not have any detectable telomerase activity (as determined by TRAP assay) and consequently were found to have undergone significant telomere shortening (shown by Southern analysis.) The rarity of this cell in G-CSF-mobilized peripheral blood and the subsequent tremendous pressure to divide in cell culture are likely contributing factors leading to the telomere loss seen in MPB-MSC. This phenomenon probably also accounts for the observed senescence observed in vitro. While we have conclusively shown that MSC can be found in G-CSF mobilized peripheral blood, the use of such cells for transplant or gene therapy may be of limited potential due to the telomere-restricted capability of expansion.