One of the major lines of defense against foreign substances entering
the body of mammals is called the humoral, or body fluid immune
response. Humoral immunity is based on the production of special cell markers
called immunoglobulins (antibodies) which detach from the cells
that produce them and circulate in the bloodstream, lymph, and other body
fluids. Each immunoglobulin is specific in its ability to bind to
a foreign substance called an antigen at key portions of the antigen
molecule called antigenic determinants or epitopes. Once
immunoglobulins have become bound to antigens, they can facilitate several
reactions, including precipitation of the antigen out of the fluid,
or clumping of antigens together to facilitate their removal from the body,
detoxification of poison substances, and delivery of nonspecific
immune chemicals such as complement which lyses cells and enhances phagocytosis
by wandering macrophages and neutrophils. The study of antibody-antigen
reactions is called serology, and knowledge of serological reactions
has led to the development of many techniques for the identification of
antigen-carrying cells, ranging from the well-known blood type agglutination
reactions to complement fixation tests used to diagnose the presence of
endotoxins released by gram-negative bacteria, and fluorescent antibody
techniques used for the identification of human pathogens such as Treponema
pallidum, the etiologic agent of syphilis.
In this lab, we will utilize a technique for the identification of a
plant pathogen which using two antibodies, a primary antibody specific
for antigens expressed on the surface of the tobacco mosaic virus (TMV),
and a secondary antibody produced against the primary. The secondary is
called a conjugated antibody, since it carries an enzyme called
phosphatase which has been chemically attached to it. The primary antibody
is produced by injecting laboratory rabbits with TMV, which, while not
pathogenic to the animals, elicits the production of antibodies which circulate
in the serum. Blood is removed from the rabbits and the serum fraction
containing the antibodies purified. Some of the primary antibodies are
then injected into the bloodstream of goats, which stimulates the production
of the secondary immunoglobulin. Secondary antibodies are then purified
and chemically bound to alkaline phosphatase enzyme.
Fluid taken from a plant suspected to be infected with TMV is placed
on a nitrocellulose membrane and allowed to dry. The TMV antigen, if present,
will adhere to the membrane. The primary antibody will then bind to the
antigen. Excess unbound antigen and antibody are washed away, then the
membrane is exposed to the conjugated secondary antibody. The secondary
will bind to the primary, exposing the alkaline phosphatase enzyme. Excess
secondary antibody is then washed from the membrane. Finally, a substrate
for the enzyme called nitroblue tetrazolium (NBT) is added. If the
antigen-primary-secondary complex is present on the membrane, the enzyme
will react with NBT and cause a blue color to appear.
Whatman filter paper anti-rabbit IgG AP conjugate (Sigma # A0418)
1X TBS (Tris-Borate) sterile diH20
2% nonfat dry milk sterile plastic petri dishes
Sigma FastTM BCIP/NBT tablets 10 X 75 mm disposable culture tubes
PBS-Tween (PBS, 0.05% Tween-20) vinyl or latex gloves
1000 ml and 10 ml pipettors
0.2 mm nitrocellulose sheets cut into 2 X 80 mm strips
TMV-specific polyclonal antibodies (ATCC # PVAS-135 or freshly prepared rabbit anti-TMV)
Remember to always wear gloves when preparing and handling samples.
1. Label two culture tubes C (control) and T (test).
2. Place approximately 0.1 g fresh cigarette
tobacco in a 1.5 ml eppindorf microcentrifuge
tube. Add 1000 ml sterile 1 X TBS and grind with a minipestle or clean glass rod until
the tobacco is finely macerated.
3. Place approximately 0.1 g fresh tobacco leaf in a separate tube and repeat step 2.
4. Place the two samples in a microcentrifuge and spin for 1minute to pellet the leaf matter.
5. Soak two nitrocellulose strips in diH2O
in the bottom of a plastic petri dish until they are
completely wetted. Remove and place on a paper towel. Gently blot with a chemwipe to
remove excess moisture.
6. Remove the strips from water and place
them on a sheet of filter paper. Dot
2 ml anti-rabbit 2o antibody on one strip near the bottom (positive control) and
2 ml sterile diH2O approximately 1 mm above the first dot (negative control). Allow
the membrane to air dry (see below).
7. On the second strip, dot 2 ml
cigarette supernatant, then 2 ml of
the ground tobacco leaf
supernatant approximately 1 mm above the cigarette dot. Allow the strip to air dry.
Note: Air dried samples can be sealed in
a dry plastic ziplock bag and stored at room
temperature for several days prior to use.
8. Soak dried nitrocellulose strips in
1X TBS for 20 seconds, then transfer them to the
9. Add 1000 ml
of the blocking buffer to each tube and incubate for 20 minutes at room
10. Prepare 2000 ml 1:5000 dilution of the primary antibody.
11. Discard blocking buffer, rinse once
with diH2O, then add 1000 ml
of the primary
antibodies to each tube. Incubate for one-half hour at room temperature.
12. Prepare 2000 ml 1:1000 dilution of the secondary (anti-rabbit goat) antibody.
13. Discard primary antibody, rinse once
with diH2O, then add 1000 ml
of the secondary
antibody to each tube. Incubate at room temperature for one-half hour.
14. Discard secondary antibody, then rinse three times 10 minutes each, with PBS-Tween.
15. Prepare NBT-BCIP solution by adding
one Sigma FastTM BCIP/NBT tablet
to 10 ml
diH2O and mixing with a vortex until the tablet is dissolved.
16. Discard the wash, then add 400 ml
of the BCIP/NBT solution to each tube. Immediately
place the tubes into the dark and incubate until a blue color appears. Stop the reaction
by immersing the nitrocellulose strips in diH2O. Allow the samples to air dry.
Questions for Review
1. Did the cigarette tobacco contain tobacco mosaic virus?
2. If the answer to question #1 was yes, do you think that this virus
could be transmissable to
other tobacco plants? If so, how?
3. Describe some other potential uses for an immunoblot technique as described above.