The magnetic cell separation technique: Executing cell separation by magnetic activation has been a trusted technique by scientists for decades. Cell sorting is ubiquitously used in research and clinical settings where a target cell of interest needs to be isolated from a heterogenous mixture such as serum or plasma. It is used in several scientific disciplines such as immunology, where it helps identify cells present during immune responses, or in cancer research elucidating tissue environment of tumors.
There are now many options to do cell separation beyond centrifugation, which separates cells based on their density, but has difficulty achieving good purity and specificity of your sample.
The magnetic cell separation technique makes use of monoclonal antibodies to dramatically increase the ability to bind target cells and magnetic beads allow for their extraction at high yields. The magnetic cell separation is done by a magnet, and is a crucial part of the process. Modern magnets are made to create a magnetic force that can carefully and efficiently separate magnetic beads from a solution.
Overview of the Magnetic Cell Separator
A magnetic cell separator is an immunomagnetic separation system. The process of separating cells utilizes antibody-antigen binding specificity to label cells for separation as well as a powerful but stable magnet to bind the molecules of interest out of solution. Monoclonal antibodies are conjugated to magnetic beads and subsequently mixed with a heterogeneous solution of cells. The monoclonal antibodies that are introduced into the mixture will specifically bind the surface proteins of your target cells of interest. A magnetic cell separator utilizes a magnet to pull those pre-bound target cells away from a mixture of cells. Such a process can be used for positive and negative selection. The magnetic cell separator can come in many sizes, from eppendorf size to a magnet capable of separating biological materials in several liters of solution. It is very important, especially as the volume gets large, that this magnet produces a force that can efficiently but steadily separate molecules. Companies that produce magnetic cell separators, such as Sepmag are producing magnets with constant force that is held constant to keep the separation useful.
Typically for positive selection, the separation method begins by placing your container of pre-bound mixture into the magnetic cell separator, which attracts the magnetic bead-conjugated antibodies to the edges of the container towards the magnetic chamber. While your cells of interest are bound to their container by the attraction of the magnetic beads to the magnetic force of the chamber, the solution within the container can be disposed, removing all non-labeled cells. A new cell-free solution can be placed into the container in the magnetic chamber. Removing the tube from the magnetic chamber will release your target cells back into solution.
Similar to positive selection, in a negative selection experiment, while cells are magnetized by the magnetic cell separator to the edges of the container you can remove the solution of non-labeled cells. The solution you removed from the container in the magnetic chamber is now selected for all cells except the cell type you bead-conjugated.
Other methods have been developed such as the column separation. These approaches flow the pre-bound mixture of cells through a column of ferromagnetic beads, which become magnetized in the presence of a magnetic field created by an adjacent magnet. Magnetic bead bound cells of interest are bound by ferromagnetic beads in the column while the rest of the solution of cells flows through. To elute target cells, the magnet is removed from the vicinity of the column and beads can flow out.
Column based Magnetic Cell Separation
Magnetic cell separation can be run both with and without a column. Column based magnetic cell separation involves passing a previously magnetically labeled sample through a column matrix within a magnetic field. The column contained ferromagnetic spheres that can capture magnetic particles within the sample. This, however, requires the additional costs of a column and provides additional time to run the sample. Some columns are prone to clogging and this risks loss of sample. However, this generally results in a higher overall purity of the final sample.
Alternatively, column-free magnetic separation simply places a tube with magnetically labeled samples in a magnetic field. The target cells will then migrate towards the magnet in a magnetic field. This allows for a simpler filtration method, but may not result in as high purity.
Overall, the addition of a column for filtration can help enhance purity, but will result in a more time consuming process that is more expensive.
Benefits of the Magnetic Cell Separator
Magnetic cell separation is regarded as the ideal approach when balancing cost and specificity needs for cell sorting. The process is faster than fluorescence activated cell sorting (FACS) because while FACS sorts cells one at a time, the magnetic cell separator can separate in bulk. Immunomagnetic sorting is also cheaper because it does not require expensive machinery and its required reagents. Centrifugation and filtration are commonly used for separation but have become favored as preliminary separation techniques as they produce low purity and yield. Overall, the magnetic cell separator is favored for its simplicity and the purity of the samples that it can produce.