Magnetic-activated cell sorting (MACS) is a fast, scalable method for purifying cells based on surface marker expression. It is widely used for T cell isolation, stem cell enrichment, and sample preparation before downstream analysis.

Principle

Cells are labeled with superparamagnetic nanoparticles (typically 50–100 nm) conjugated to antibodies against a specific surface antigen. The labeled suspension is passed through a column packed with ferromagnetic steel wool placed in a strong magnetic field. Labeled cells (expressing the target antigen) are retained in the column, while unlabeled cells pass through. After removing the column from the magnet, the retained cells are eluted as the positive fraction.

Magnetic Beads

• MicroBeads (Miltenyi): 50 nm superparamagnetic particles. They do not activate cells or alter function and can remain on cells during culture or injection. The bead–antibody complex is typically not crosslinked, allowing the beads to dissociate from the cell surface over time or to be removed by a detachment reagent.
• EasySep (STEMCELL): 200 nm dextran-coated magnetic particles that are incubated with the cell suspension and then separated in a tube placed in a magnet without a column (the positive fraction sticks to the tube wall).

Positive vs. Negative Selection

Positive selection: target cells are directly labeled with magnetic beads against a marker of interest (e.g., CD4⁺ T cells using anti-CD4 beads). The positive fraction contains the target cells, which are bead-bound (but the beads are small and non-activating).

Negative selection (depletion): unwanted cells are labeled with a cocktail of antibodies against multiple markers, and the desired cells are untouched in the flow-through. For example, CD4⁺ T cells can be isolated by depleting CD8⁺, CD19⁺, CD14⁺, CD16⁺, and CD56⁺ cells. Negative selection avoids any antibody binding to the target cells, preserving their native state.

Protocol

1. Prepare a single-cell suspension. Remove clumps by passing through a 30–70 µm strainer.
2. Count cells and centrifuge at 300 × g for 10 minutes.
3. Resuspend in degassed buffer (PBS with 0.5% BSA and 2 mM EDTA). EDTA chelates Ca²⁺ and prevents integrin-mediated clumping.
4. Incubate with magnetic beads (typically 10–20 µL per 10⁷ cells) at 4 °C for 15–30 minutes.
5. Wash to remove excess beads.
6. Apply the cell suspension to a magnetic column (LS column for up to 10⁸ cells, MS for up to 10⁷, or autoMACS for automated processing).
7. Wash the column 3 times with buffer while in the magnetic field.
8. Remove the column from the magnet and elute the retained cells by plunging buffer through the column.

Purity and Recovery

Purity depends on the specificity of the antibody, the stringency of washes, and the flow rate. Typical purity for positive selection is 90–99%. Recovery is 50–90%, with some loss in the column and wash steps. Multiple passes over a fresh column can improve purity at the cost of recovery.

Comparison with FACS

MACS is faster (30 minutes vs. hours for FACS), gentler (lower shear stress), and scalable (up to 10¹¹ cells). It does not require a specialized operator or instrument. FACS offers higher purity (99%+), can sort based on multiple parameters simultaneously, and can isolate rare subsets with high precision. MACS and FACS are often combined: MACS for bulk enrichment, FACS for fine sorting.