Fig. 1: Magnetic-activated cell sorting (MACS) represents one potential method for preparation of transplantation-friendly stem cells. ESCs (left) can be reprogrammed to yield mature cells such as muscle or skin, but a subpopulation of cells remains undifferentiated (middle). MACS using antibodies against ESC surface markers could enable efficient separation of these cells (top right) from the desired, mature cells (bottom right).
© 2010 K. Schriebl
The capacity of embryonic stem cells (ESCs) to differentiate into virtually any adult cell type makes them a promising clinical resource. However, it also poses a challenge for researchers hoping to derive transplant-ready cells: every population of mature cells derived from ESCs contains a small minority that remains undifferentiated, and these ultimately have the potential to cause serious problems for patients by developing into tumors known as teratomas.
Kornelia Schriebl and co-workers at the A*STAR Bioprocessing Technology Institute (BTI) in Singapore have been working to develop strategies that eliminate this risk by separating out undifferentiated ESCs. Her team has been particularly interested in magnetic-activated cell sorting (MACS), a popular technique that employs magnetic beads coated with antibodies that recognize and bind specific cell-surface proteins, enabling scientists to efficiently and specifically pluck undesirable cells from a mixture (Fig. 1).
Schriebl’s co-worker Andre Choo, also from the BTI, has identified several ESC-specific antibodies that might prove useful in this technique, but Schriebl and Choo started by simply assessing whether MACS could realistically deliver cell preparations with the 99.99999999% purity necessary for truly safe transplantation. “The intention was to evaluate the MACS technique in combination with an established cell surface marker, SSEA-1, based on its suitability for future cell therapy applications,” says Schriebl.
SSEA-1 is known to be highly expressed in undifferentiated ESCs and the antibodies used by Schriebl and co-workers exhibited strong affinity for their target, but the separation efficiency dropped as the number of stem cells within a sample decreased. As a result, each subsequent round of MACS was relatively less successful at eliminating the small number of contaminating ESCs, and the researchers estimated that ultimately 31 rounds of separation would be needed to attain the necessary purity.
Besides being time-consuming, this many rounds of sorting is likely to result in a marked loss of target cells, a problem that even higher-affinity antibodies are highly unlikely to solve. “It is possible to improve the system by targeting alternative antigens,” says Schriebl, “but you will not be able to reduce it to a 2- or 3-step process.”
Nevertheless, MACS still offers an effective first step for bulk cell sorting, and her team is now pairing this method with other, more aggressive approaches. “There is an antibody called mAb 84 that selectively kills undifferentiated human ESCs,” she says. “Using mAb 84 as a second step makes selective killing of residual traces of undifferentiated cells possible, thus leading to safer cell therapy.”
The A*STAR-affiliated researchers mentioned in this highlight are from the Bioprocessing Technology Institute.
