Magnetic-activated cell sorting

Magnetic-activated cell sorting (MACS) is a trademark name for a method for separation of various cell populations depending on their surface antigens (CD molecules). The term MACS is a registered trademark of Miltenyi Biotec.

Magnetic cell separation using antibodies is also possible using monosized magnetic beads (Dynabeads) from Invitrogen (now part of Life Technologies). The Dynabeads technology is not column based - instead these magnetic beads with attached cells enjoy liquid phase kinetics in a sample tube, and the cells are isolated by placing the tube on a magnetic rack.

Procedure
The MACS method allows cells to be separated by incubating with magnetic nanoparticles coated with antibodies against a particular surface antigen. This causes the cells expressing this antigen to attach to the magnetic nanoparticles. Afterwards the cell solution is transferred on a column placed in a strong magnetic field. In this step, the cells attached to the nanoparticles(expressing the antigen) stay on the column, while other cells (not expressing the antigen) flow through. With this method, the cells can be separated positively or negatively with respect to the particular antigen(s).

Positive selection
In positive selection the cells expressing the antigen(s) of interest, which attached to the magnetic column, are washed out to a separate vessel, after removing the column from the magnetic field. This method is useful for isolation of a particular cell type, for instance CD4 lymphocytes.

Negative selection
In negative selection the antibody used is against surface antigen(s) which are known to be present on cells that are not of interest. After administration of the cells/magnetic nanoparticles solution onto the column the cells expressing these antigens bind to the column and fraction that goes through is collected, as it contains almost no cells with undesired antigens.

Modifications
Magnetic nanoparticles conjugated to an antibody against an antigen of interest are not always available, but there is a way to circumvent it. Since fluorophore-conjugated antibodies are much more prevalent, it is possible to use magnetic nanoparticles coated with anti-fluorochrome antibodies. They are incubated with the fluorescent-labelled antibodies against the antigen of interest and may thus serve for cell separation with respect to the antigen.