Magnetic Activated Cell Sorting (MACS) is a technique to enrich a specific cell type from a mixed population. Scientists or companies sort or isolate cells so they can study or grow colonies of a single type of cell. They may use that type of cell for a specific type of functional assay crucial to that cell type or they might be interested in stem cells. MACS technique emerged as a cheap alternative to cell sorting. MACS uses the highly specific antibody-antigen interaction to probe cells by their surface antigens by their specific antibody. Magnetic bead cell isolation has been implemented with MACS to offer a more precise cell sorting.
Fundamentals of Magnetic Activated Cell Sorting
Basically, there are two types of magnetic cell separation, positive and negative separation: Positive separation techniques recognize and conjugate the target cell. Those targets are isolated and retained at the edges of the container by a magnetic separator while the remaining unwanted solution of cells is removed and exchanged with a fresh buffer.
While, in negative separation techniques the magnetic beads are functionalized to recognize the unwanted cells. These are then removed from solution by magnetic separation and the remaining targets are decanted and retained. One benefit of negative selection is that the target cells are not conjugated to the magnetic beads at any point in the process. However, negative selection may lack the specificity of positive selection and may require multiple separation steps. Some recent studies have used this lack of specificity to their advantage however, by studying heterogeneous populations of cells with a defined characteristic.
Within the context of magnetic cell separation technologies, the tube-based method offers distinct advantages to its alternative, which involves passing cells through a dense column-matrix. The entire tubular cell separation is completed in a single vessel. Magnetic beads are added to a cell-sample and given time to incubate. Targeted cells are pulled toward the magnet when its power is applied, effectively separating cells that have bound to beads. Properly implemented, this process eliminates undue cell stress that can be generated by column-based separation methods or from exposure to iron, significantly diminishing the risk of experimental procedures negatively impacting cell function and phenotype.
There are two mechanisms of labeling cells in MACS namely as direct labeling and Indirect labeling. Direct labeling is more specific with microbeads containing the antibody that correlates directly to an antigen on the surface of a cell. Indirect labeling, on the other hand, uses a combination of monoclonal and polyclonal antibodies. In this case, some antibody is initially attached to the cell, and then a secondary antibody with a magnetic bead then binds to the primary antibody. This method is less specific, but is often less expensive. Some cells have weak antigen expression, where a direct method is impossible.
Application of Magnetic Activated Cell Sorting
Magnetic beads are composed of a ferrous iron-oxide core surrounded by a polymer shell, or a magnetic ‘pigment’ embedded in a polymer matrix. Magnetic beads range from a few nanometers to a few micrometers in size and size of the beads affects their behavior in a magnetic field. If the beads are small enough, they will demonstrate paramagnetic behavior, i.e. no permanent magnetism in a zero magnetic field. The tiny paramagnetic beads will rapidly become magnetized and demagnetized when an external magnetic field is applied and removed, respectively. This feature offers the advantage of magnetic bead cell sorting and isolation to be completed in a single vessel without the need for a centrifuge or disposable columns. Essentially, there are two types of magnetic beads based on the size:
- Magnetic nanoparticles are often irregularly shaped, and have slower magnetic separation kinetics due to a lack of chain formation, and in some cases have been found to enter into the cell through the cell membrane.
- Magnetic microbeads, on the other hand, are spherical, have a narrower size distribution and form cooperative chains during magnetic isolation. As a result, microbeads have a more predictable behavior during magnetic bead cell isolation and significantly decrease separation time. They also are too large to enter the cell and therefore will not disrupt cellular growth.
Magnetic Bead Sorting (MBS) generates a simple method for cell isolation. It just separates unlabeled cells without attached beads from those treated by beads. When appropriately implemented, such as a pre-enrichment procedure prior to flow cytometry sorting, MBS can significantly diminish the time required to complete the sorting process simultaneously reducing the total quantity of cells that need to be sorted. However, if a higher quantity of cells is required for analytical purposes, cell populations can be raised more readily than with other cytometric methodologies.
MBS offers higher selectivity of separation without complex protocols or relying on costly and disposable lab-equipment. The process and outcomes are more stable than electric-field separation technologies or other methods of separation. MBS generates extended sample enrichment potential if additional analyses – chromatographic/electromigratory – are required. Liquid-phase kinetics are enabled, leading to enhanced isolation of targeted cells. Repeated washing processes are unnecessary, yet cell samples are generally pure, unaltered and viable.
Magnetic Activated Cell Sorting technique is compatible with the majority of contemporary life science/biomedical techniques and applications. Tubular cell sorting methods are gentler than alternative techniques, offering less threat of contamination from the host cell population or stress to cells caused by more complicated separation regimens. It is very adaptable, effective with small or large cell populations.
Remember to download The basic guide to magnetic bead cell separation in order to learn much more about magnetic activated cell sorting: