Page 386 - Veterinary Immunology, 10th Edition

VetBooks.ir Lymphocyte Mitogens

In addition to their surface proteins, lymphocytes can be
characterized by the stimulants that make them divide. The most

important of these are the lectins that bind to cell surface
glycoproteins and trigger cell division (Box 13.2). These lectins are
commonly obtained from plants. Examples include
phytohemagglutinin (PHA) obtained from the red kidney bean
(Phaseolus vulgaris), concanavalin A (Con A) obtained from the jack

bean (Canavalia ensiformis), and pokeweed mitogen (PWM) obtained
from the pokeweed plant (Phytolacca americana). Lectins bind sugar
residues on glycoprotein side chains. For example, PHA binds N-

acetylgalactosamine, and Con A binds α-mannose and α-glucose.
Not all lymphocytes respond equally well to all lectins. Thus PHA
primarily stimulates T cells, although it has a slight effect on B cells.
Con A is also a T cell mitogen, whereas PWM acts on both T and B
cells.

Box 13.2

How to Measure Mitogenicity

To measure the effect of mitogens, lymphocytes are grown in tissue

culture. Lymphocytes can be obtained directly from blood. The
lymphocytes are cultured for at least 24 hours before the mitogen is
added. Once this is done they begin to divide, synthesize new
DNA, and take up any available nucleotides from the medium. It is

usual to incorporate a small quantity of thymidine labeled with the
3
radioactive isotope of hydrogen, tritium ( H), in the tissue culture
fluid. The thymidine is only incorporated into the DNA of cells that
are dividing. After about 24 hours, the cultured cells are separated

from the tissue culture fluid, either by centrifugation or filtration,
and their radioactivity is counted. The amount of radioactivity in
the mitogen-treated cells may be compared with that in an
untreated lymphocyte culture. This ratio is called the stimulation

index (see Fig. 33.8). As an alternative to the use of tritiated
14
thymidine a radiolabeled amino acid such as C-leucine can be
used. Uptake of this compound indicates increased protein

386