Magnetic Decoration and Labeling of Prokaryotic and Eukaryotic Cells - Cell Surface Engineering: Fabrication of Functional Nanoshells - page 195

If magnetic particles are used throughout the crosslinking process, magnetic
cells or cell walls derivatives can be prepared. 23

10.2.5 Specific Interactions with Immunomagnetic
Nano- and Microparticles
Immunomagnetic detection and modification of cells implies the use of
magnetic microbeads or magnetic nanoparticles-antibody system causing
the particles to be selectively attached to target cells when added to a cell
suspension. After incubation, target cells with attached magnetic particles
(and also excess particles) are isolated with the help of an appropriate
magnetic separator. Both monoclonal and polyclonal antibodies (Abs) can
be used in the course of magnetic modification. In the direct method, the
appropriate antibodies are coupled to the magnetic particles, which are then
added directly to the cells containing sample. Ideally, the antibody should be
oriented with its F c (fragment crystallizable) region towards the magnetic
particle so that the F ab (fragment antigen-binding) region is pointing out-
wards from the particle. 3,17
The indirect method can also be used. In the first step, the cell suspension
is incubated with primary antibodies that bind to the target cells. Prior
sensitization of the target cells will ensure a proper orientation of the anti-
bodies and an optimal number of interaction possibilities between magnetic
particles and cells. Not only purified primary antibodies have to be used;
crude antibody preparations or serum can be used, too. After incubation, the
unbound antibodies are usually removed by washing. Thereafter, the mag-
netic particles with immobilized secondary antibodies are added, permitting
the beads to bind rapidly and firmly to the primary antibodies on the target
cells. Target cells-primary antibody complexes can be also captured by
protein A or protein G immobilized on magnetic carriers. Alternatively,
primary antibodies can be biotinylated or labeled with fluorescein and
magnetic particles with immobilized streptavidin or antifluorescein anti-
bodies are used for capturing the target cells. 3,17
This binding mediated by a specific antigen-antibody reaction is pref-
erably used in immunomagnetic separation of both eukaryotic and pro-
karyotic cells (e.g. microbial pathogens, stem and cancer cells, etc.). Both
magnetic nanoparticles and microparticles can be used as antibodies
carriers. After target-cell labeling the modified cells are usually separated
from the sample. Target diamagnetic cells can be separated using two basic
strategies, namely positive selection or depletion. The optimal separation
strategy depends on the frequency of target cells in the cell sample, their
phenotype compared with the other cells in the sample, the availability of
reagents and a full consideration of how the target cells are to be used.
Positive selection means that the desired target cells are magnetically la-
beled and isolated directly as the positive cell fraction (Figure 10.7). It is the
most direct and specific way to isolate the target cells from a heterogeneous
cell suspension.

d n 8 y 4 n g | 5

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