Magnetic-activated cell sorting
Magnetic-activated cell sorting (MACS) is a method for separation of various cell populations depending on their surface antigens (CD molecules) invented by Miltenyi Biotec. The name MACS is a registered trademark of the company.
The method was developed with Miltenyi Biotec's MACS system, which uses superparamagnetic nanoparticles and columns. The superparamagnetic nanoparticles are of the order of 100 nm. They are used to tag the targeted cells in order to capture them inside the column. The column is placed between permanent magnets so that when the magnetic particle-cell complex passes through it, the tagged cells can be captured. The column consists of steel wool which increases the magnetic field gradient to maximize separation efficiency when the column is placed between the permanent magnets.
Magnetic-activated cell sorting is a commonly used method in areas like immunology, cancer research, neuroscience, and stem cell research. Miltenyi sells microbeads which are magnetic nanoparticles conjugated to antibodies which can be used to target specific cells.
Procedure
The MACS method allows cells to be separated by using magnetic nanoparticles coated with antibodies against a particular surface antigen. This causes the cells expressing this antigen to attach to the magnetic nanoparticles. After incubating the beads and cells, the solution is transferred to a column 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 and negative selection
With 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.
With 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 the fraction that goes through is collected, as it contains almost no cells with these 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.
Use in Sperm Selection for fertilization
It is a technique that allows us to select the spermatozoa with the best qualities to use them in assisted reproduction treatments and eliminate the apoptotic ones. This is a way to increase the chance of pregnancy. The MACS technique involves the use of tiny biodegradable magnetic particles attached to an antibody (adnexin V protein). This protein recognizes the sperm that will die before fertilizing the egg, binding them and allowing them to be separated from healthy sperm. Once the particles with adnexin V protein are added to the semen sample, it passes through columns in the presence of a magnetic field, so that the apoptotic sperm will be retained by the column and the healthy sperm pass through the column by gravity. In this way, we can obtain healthy sperm with the best physiological qualities to be used in assisted reproduction treatments.
A Cochrane review from 2019 could not verify that magnetic-activated cell sorting (MACS) could influence on the number of pregnancies, live births or miscarriages.[1]
In which cases it is indicated
This method of sperm selection can greatly benefit couples where the cause of their infertility is the male factor or of unknown origin. Doctors indicate the use of this technique in the following cases: Patients with a pathological sperm DNA fragmentation analysis. Patients who have completed more than two cycles of treatment without resulting in pregnancy. Patients with repeated abortions without apparent cause. Bad embrionary quality. For patients being treated with chemotherapy or radiotherapy. Sterility of an unknown origin.
Frequent questions
How much does the MACS technique cost?
The price of the MACS technique is usually around 300-350 euros. Usually the Health System currently does not finance the MACS technique in assisted reproduction treatments.
From how many failed cycles is it recommended to use the MACS technique?
MACS should be used whenever a higher than normal level of apoptosis is detected in a semen sample, in order to avoid spermatozoa with cell damage. Therefore, it is indicated in cases of: severe male factor, implantation failure (usually we talk about two transfers with high quality embryos, which have not generated pregnancy) and those previous IVF (in vitro fertilisation) cycles in which fertilization or embryo development has not been as expected, despite the fact that the semen sample was normal or almost normal.
Can MACS be performed with sperm obtained by testicular biopsy?
It is not possible, since the spermatozoa obtained by this technique are rare. Therefore, when they are passed through the columns of adnexin V, it is very likely that there will not be enough to carry out the treatment.
See also
References
- Lepine S, McDowell S, Searle LM, Kroon B, Glujovsky D, Yazdani A (30 July 2019). "Advanced sperm selection techniques for assisted reproduction". Cochrane Database of Systematic Reviews. doi:10.1002/14651858.CD010461.pub3. PMID 31425620.
- S. Miltenyi, W. Muller, W. Weichel, and A. Radbruch, “High Gradient Magnetic Cell Separation With MACS,” Cytometry, vol. 11, no. 2, pp. 231–238, 1990.