TECHNIQUES IN PLANT VIROLOGY

CIP Training Manual

2.3 DETECTION/Serology

Section 2.3.1
An Introduction to
Serology

Introduction
The morphological characteristics of plants and animals are used as
classification criteria by the descriptive natural sciences. However, the
development of serology as an independent science has made it possible
to clarify the differences between living beings based on chemical
considerations by determining their structures. These considerations
were arrived at indirectly and not as the consequence of simple
observation.

The concept of specificity originates in the knowledge that after recovery

from an infectious illness there is a certain resistance to that disease
(immunity). Jenner used this fact to develop the first practical vaccine.
 The search for an explanation to this phenomenon led to the discovery of
a peculiar group of substances in blood called antibodies. Some
antibodies protect the organism against infectious agents (bacteria and
viruses) or neutralize toxins.

These substances, which have now been identified as modified globulins,

are formed not only as the result of an infection, but also as the
consequence of the administration of certain dangerous substances of
high molecular weight—toxins of bacteria, animals, or plants—or of dead
bacteria.

Apart from the initial interest in resistance (immunity) to diseases, other

research in immunology found that immunity caused by bacteria and
toxins is only a part of a more general principle: the same mechanism is
triggered when animals are inoculated with materials, such as cells or
proteins derived from foreign species. In this case, antibodies appear in
the blood causing agglomeration, cell destruction, or precipitation of
those proteins.

All immuno-antibodies are specific. This means, literally, that they react
only to one antigen (the one used for immunization). Antigens may be
proteins, cells from the blood of a different species, bacteria, or viruses.

Modern biological sciences have developed efficient and precise

biochemical and immunological techniques for the diagnosis of various
important diseases, thus replacing more conventional methods.

History

Edward Jenner (1749–1823) pioneered the use of vaccines for the

prevention of infectious diseases.

Louis Pasteur (1822–1895) studied various methods for attenuating

pathogen virulence, leading to the development of vaccines.

H. Bence-Jones (1847) discovered an abnormal protein in the urine of

multiple myeloma patients. This protein, produced in excess, is a dimer
of light (L) chains.

Paul Ehrlich (1854–1915) discovered that antibodies are synthesized as a

response to antigens.

George Nuttall (1888) discovered the bactericidal property of blood.

Gruber and Durham (1896) produced the first serological agglutination

reaction of vibrium cholera, and the corresponding antiserum.

Albert Coons (1942) developed histochemical methods for detecting

pathogens by utilizing fluorescent antibodies.

O. Ouchterlony, J. Ondin, and S. Elek (1946–1948) described the gel

diffusion tests for virus study and detection.

P. Grabar and C. Williams (1953) developed the immuno-electrophoresis

technique.

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 Alick Isaacs (1957) produced interferon.

Solomon Berson and Rosalyn Yallw (1959–1960) developed

radioimmunology.

Rodney Porter (1962) proposed a basic four-chain model for the structure
of immunoglobulin molecules.

G. Edelman et al. (1968) established the complete amino acid sequence

of an IgG antibody from the blood of a multiple myeloma patient.

A. Völler et al. (1976) first used the enzyme linked immunosorbent assay
(ELISA) for detecting three plant viruses.

George Köhler and Cesar Miltstein (1981) developed the technique for
producing monoclonal antibodies.

Immunochemistry

Leukocytes are blood cells that protect organisms. There are several
kinds of leukocytes, among which the most important are:

Monocytes: 15 to 18 m long; are up to 8% of all leukocytes. (They

are macrophages and along with the phagocytosis are responsible for
consuming large molecules).

Lymphocytes: 8–9 m long; constitute 30–35% of all leukocytes and

take part in antigen–antibody reactions.

Eosinophiles: 12–13 m long; are up to 8% of blood components.

They take part in antigen–antibody reactions, particularly in allergies.
The immune system reacts in either of two ways, through cell
immunity or humoral immunity. Lymphocytes are the primary cells
involved in these responses. Two different lymphocyte populations
have been identified: T cells (T lymphocytes) and B cells (B
lymphocytes).

T lymphocytes are responsible for cell immunity, including cutaneous

reactions, graft rejection, anti-tumor immunity, and cellular defense
against fungi and intracellular pathogens.

B lymphocytes develop in the Fabrician sack of birds, but are thought

to derive from the bone marrow of mammals. They are in charge of
humoral immunity expressed in the production of specific circulating
plasma proteins called antibodies or immunoglobulins.

Antigens

Substances capable of inducing an immunological response (humoral,

cellular, or mixed T and B cells), when introduced in animals are called
antigens or immunogens. Most antigens are macromolecular proteins but
also may be immunogenes, polysaccharides, synthetic polypeptides, as
well as other synthetic polymers.

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 Immunogenic molecules have the following characteristics:

• The molecules should be foreign to the host.

• Molecules with a molecular weight higher than 10,000 are weak

immunogens; proteins with a molecular weight higher than
100,000 are stronger immunogens.

• The molecule must have a certain degree of complexity to be

antigenic. Immunogenicity increases with structural complexity.
Moreover, aromatic amino acids contribute to a larger degree than
residues of non-aromatic amino acids in immunogenicity.

• The capacity to respond to an antigen varies with the animal

species, and even with its genetic constitution.

Antigenic Determinants

The production of immunoglobulins requires the linkage of the antigen to

the surface of the lymphocyte.

Combination sites on the surface of the lymphocyte that consist of

molecules similar to antibodies are called antigen receptors. Only
restricted portions of the antigenic molecules are related to antibody
combination sites. These areas are called antigenic determinants, and
they determine antigen–antibody reactions. An antigenic determinant can
comprise only 6 to 7 amino acids of a protein's total number. In other
words, a virus, for example, induces the production of a mixture of
antibodies that react specifically to various antigenic determinants
present in the particle. Thus, we can define the antisera containing a
mixture of antibodies as "polyclonal antisera." The number of antigenic
determinants in a single molecule varies with the size of the molecule and
its complexity.

Hapten

This is a small, chemically defined molecule that cannot induce the

production of specific antibodies against itself. However, when it is
covalently linked to a larger molecule, it can act as an antigenic determinant
and induce antibody synthesis.

Adjuvants

Adjuvants are chemical substances used to enhance immunological

response. They not only stimulate the formation of antibodies, but also
localize them at the site of injection as deposits from which they are slowly
released during the period of antibody synthesis, either through adsorption to
solid particles or through their incorporation in to an oily emulsion. The most
frequently used adjuvants are calcium phosphate, inorganic gels, aluminum
hydroxide, bentonite, methiolated serum albumin, and incomplete Freund
adjuvant. This is the most commonly used adjuvant with viral antigens and is
made of 9 parts mineral oil (lanolin) and one part detergent; it allows the
formation of a stable emulsion. If the mixture also contains dead
Mycobacterium sp. cells, it is called a complete Freund adjuvant.

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 Antibodies

Antibodies are immunoglobulins produced by an organism as a response to

the invasion of foreign compounds such as proteins, glucosides, or nucleic
acid polymers. The antibody molecule associates non-covalently to the
foreign substance starting a process for eliminating the foreign substance
from the organism.

The antibody molecule is basically formed by four polypeptide chains

forming the basic unit: two identical heavy (H) chains (MW 53,000 to
75,000), and two light (L) chains (MW 23,000) united by disulfur bonds.
There are two regions in the antibody molecule: the common fraction (Fc)
and the variable fraction (Fab):

• Fc: constituted of a portion of heavy chain.

• Fab: this fraction chain (MW 50,000) determines the specificity of

the antibodies and is constituted by the other portion of the heavy
chains (MW 100,000).

Immunoglobulins made up of more than one basic monomeric unit are

called polymers. Electrophoretic and ultracentrifugation studies have
allowed the identification of 5 groups of immunoglobulins.

1. IgG. This is the main fraction of antibodies and comprises 80% of

immunoglobulins having molecular weights of between 150,000 and
160,000. It contains 2–4% carbohydrates and shows the lowest
electrophoretic mobility among all immunoglobulins. It is the only
immunoglobulin capable of crossing the placenta.

2. IgA. This immunoglobulin has a molecular weight of between

180,000 and 400,000. It has a higher carbohydrate content (5–10%)
than IgG. IgA is found in high concentrations in blood, in secretions
such as colostrum, saliva, tears, and bronchial and digestive tube
secretions. IgA cannot cross the placenta.

3. IgM. This immunoglobulin has the most proteins and its amino acid
sequence has not yet been determined. It contains 576 amino acids
and has a molecular weight of 950,000. It is the first antibody
synthesized by a newborn animal or human being. The cells
producing IgM are divided into two daughter cells, which produce
IgG. The IgMs can promote phagocytosis of microorganisms by
macrophages and polymorphonuclear leukocytes. IgMs do not cross
the placenta.

4. IgD. No antibody activity is attributed to IgDs. Its role in the general

scheme of immunoglobulins is still unknown.

5. IgE. This appears in serum at very low concentrations. It has a

molecular weight of 190,000. Approximately half of the patients with
allergic diseases show high IgE concentrations.

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 Properties of human inmunoglobulins (Harper, 1980)

LgG lgA lgM lgD lgE

Molecular weight 150,000 160,000 900,000 180,000 190,000

Sedimentation coefficient 6–7 7 19 7–8 8
Concentration in serum (mg %) 1,000 200 120 3 0.05
Placenta transfer yes no no no no
Bacterial lysis + +++ + ? ?
Antiviral activity + +++ + ? ?

Monoclonal Antibodies

Monoclonal antibodies are specific antibodies to only one antigenic

determinant. Monoclonal antibodies are produced by cloned cells
(hybridomes) resulting from the union of a B lymphocyte with a
carcinogenic myeloma cell. The lymphocyte confers the monoclonal
antibody the capability to produce antibodies, and the myeloma cell gives
it the ability to reproduce itself indefinitely. Because all cell clones from a
hybridome come from a single B lymphocyte, they produce specific
antibodies to only one antigenic determinant.

Antigen–Antibody Reactions

The antigen–antibody union is non-covalent and irreversible under normal

laboratory conditions, and it includes hydrogen bonds, Van der Waals
forces, and hydrophobic and coulombic interactions. The resulting chemical
complementarity is similar to that between a key and lock.

The union area between the antigen and the antibody is only 1% of the total
globulin surface and its specificity depends on the sequence of amino acids
present in that region. The degree of association between the antigen and
the antibody depends on the characteristics of each molecule and is
determined by the combined effects of the interactions between them. If
affinity and avidity are high, the molecules will unite more rapidly and
dissociation will be very slow. Antisera with strong precipitation reactions
show more avidity than those reacting weakly at a similar degree of dilution.

Antiserum affinity depends on:

• The temperature at which the reaction takes place. Higher
temperatures cause stronger reactions. However, temperatures over
40OC result in protein denaturation.
• Salt concentration. Low salt content favors the union of reacting
substances.
• The pH level can cause changes in antigen–antibody affinity.
• The concentration of reacting substances for the formation of
precipitates.

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 Preparation of Viral Antigens

1. Animal-infecting viruses

These may be propagated in laboratory animals, embryonic eggs, or in

vitro cultures. The animal species used for the production of antiserum
must be as phylogenetically distant as possible from the usual species to
be infected by the pathogen. The viral preparation must contain a high
virus concentration, and the viruses must be free of host contaminants.
For example, in the case of rabies, the inactivated virus must be used in
the first immunizations in a series.

2. Plant-infecting viruses

These can be propagated to obtain high particle-concentration in vivo in

host plants allowing virus propagation in their tissues. Difficulty of
purification depends on the virus and plant host. For viruses whose
multiplication is restricted to the phloem, the concentration obtained in
plant tissue will be low and purification difficult.

Immunization and Production of Antibodies

a) Immunization

Animals commonly used in the production of antisera for research

purposes include rabbits, guinea pigs, mice, goats, sheep, monkeys,
horses, and hens. Selection of species depends on availability and
volume of antiserum required. The mechanism regulating antibody
synthesis and the reactions produced in the cells of the immune system
in the presence of viral particles are extremely complex, and some
characteristics are still unknown.

The immunization process can be summarized as follows (see Figure):

Some of the viruses entering the organism (1) are ingested by the
macrophage cells (2).

Some of the many millions of the helper T lymphocytes (3) normally

flowing in the blood system are activated to identify the new enemy, in
this case a viral particle. The T cell is activated by its union with the
macrophage that captures the particle. The union takes place when the T
lymphocyte recognizes in the macrophage one of its own markers and
another corresponding to the antigen.

The activated helper T lymphocyte multiplies itself and thus stimulates

the multiplication of killer T lymphocytes (4) and of B lymphocytes (5).

Once the B lymphocytes have multiplied (approximately a million different

lines), the T lymphocytes signal them to start the production of a great
number of antibodies (6) that will join the blood stream.

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 Figure 1. Production of antibodies by organisms.

When a virus infects an organism, some particles penetrate and start

multiplying in the infected cells. Killer T lymphocytes cause chemical
damage to the membranes of the infected cells, thus interrupting virus
replication.

Some of the synthesized antibodies neutralize the remaining virus

particles.

When the infection stops, the suppressor T lymphocyte (7) stops all
activity of the immune system, thus preventing uncontrolled reactions.

T lymphocytes and memory B lymphocytes (8) store information on the

viral particles and remain in the blood and lymphatic systems. Thus, they
are ready to mobilize and start a reaction if the same viral particles
invade the organism again.

b) Immunization methods

Immunization can be intravenous, intramuscular, subcutaneous,

intraperitoneal, intradermal, intra-articular, or intranodular. This increases
the stimulating effect upon the immune response.

c) Antisera production

Antibody production increases during the first days following the first
immunization to reach a maximum concentration that can be maintained
for a few days. If a new dose of antigen is injected, the synthesis of
antibodies increases rapidly to a higher concentration than initially

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 produced by the first dose of antigen. Antibodies are found in the blood
serum of immunized animals. This serum is called serum.

To produce antiserum, allow a blood sample to stand to separate serum

from other blood components. The serum is centrifuged (7840 g x 15
min) to remove debris that have not separated. Serum may be stored
frozen (–20 OC to –70 OC) and lyophylized, or as liquid at 4OC mixed with
an antiseptic.

Adequately stored, the antisera can remain active for many years.

Immunodiagnostic Techniques

A. Precipitation tests

Antigen and antibody precipitation occurs when the reaction between

these substances forms a grid-like structure preventing the passage of
water molecules (hydrophobic reaction). The formation of the grid
structure requires a proportional concentration of reacting substances.
For every antigen molecule, a given number of antibodies are needed.

Antibodies usually act as bivalent molecules and the antigens as

multivalent molecules. The presence of too many antibodies will make
the antibodies act as monovalent molecules only to one particle of the
antigen, thus preventing the formation of the grid structure. Too many
antigens also prevent the formation of the grid, because the antigens will
then act only as monovalent molecules. The antigen–antibody reaction
cannot be observed in either case.

1. Interface ring tests

In this test, a solution containing both antigens and antibodies is used. A

proportional amount of the sample containing the antigen is placed on a
given volume of antibodies. Precipitation occurs in the interface and the
precipitation plane creates the illusion of a ring.

2. Tube liquid precipitation test

This quantitative test is mainly used to determine the titer of antisera, and
the antigen concentration. In this test, several (double) dilutions of the
antiserum and the antigen are mixed and incubated in two small test
tubes for the formation of precipitates. Varying degrees of precipitation
allow determination of the concentration of the reacting substances.

3. Microprecipitation test

In this test, individual drops of each of the reacting substances are placed
on a petri dish. The plaque is stirred to mix the antibody and the antigen.
After incubation, the precipitate forms. A stereoscopic microscope is
used to observe the reaction.

B. Gel diffusion tests

These precipitation tests are usually done using agar or agarose gels.
Their advantage is that the mix of the antigen and its corresponding

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 antibody can be physically separated by the difference in the diffusion
coefficients of the components of their gels. Thus, these tests can
provide information on the homogeneity and purity of the reacting
substances, as well as on their size and relations.

1. Simple immunodiffusion (Oudin 1946)

This is based on the use of antibodies immobilized in an agar or agarose

matrix melted in a water bath at 50OC. It is pre-cooled for use and put into
10 x 75 ml or smaller tubes. The antigen can diffuse through the agar,
thus provoking the appearance of a zone or band migrating along the
tube until the concentration of both reacting substances reaches an
optimal stage. Precipitation occurs at this point.

Some factors affecting band migration are antigen concentration and

diffusion coefficient. The latter depends on molecular weight and on the
size of the antigen molecule. Lower molecular weight and size mean
higher diffusion coefficients. The distance migrated by the diffusion band
also depends on time, as well as on the kind, quality, and concentration
of the antibody.

2. Double immunodiffusion (Ouchterlony 1948)

• Simple dimension system. A neutral agar layer is placed between

the antigen and antibody solutions. The same principles apply as in
the Oudin simple diffusion test. However, both reacting substances
are diffusable and a precipitation line appears when they meet.
The position and width of the band allow determination of the
concentration of the reacting substances.

! Double diffusion system. Plaques or slab holders are covered with

neutral agar at a concentration of 0.7–1.5%. A well pattern is
designed that suits the purpose of the test, and the antigen and the
antibody are placed in separate wells.

Types of Reactions

Identity reaction: The fusion of precipitation lines occurs when both

antigens are identical. A compact barrier, the immunospecific barrier, is
thus formed.

Non-identity reaction: In this case, diffusion varies and the precipitation

arches do not act as barriers to the antigen, which are not related and
therefore intercross.

Partial double identity reaction: A partial double identity occurs between

antigens. (Almost identical antigens differ in only one antigenic
determinant.)

Partial identity reaction: The formation of a "spur" is observed, indicating

partial identity between antigens (e.g., as in the case of different strains
of the same virus).

3. Immunoelectrophoresis (Grabar and Williams, 1953)

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 This is one of the most important analytical tools used in the solution of
complex antigen mixes. It is based on electrophoretic mobility and
antigenic specificity. The antigen mix is first separated in its components
through electrophoresis in an agar gel. Then the antiserum is placed in a
parallel channel or electrophoretic migration pathway to allow the
formation of precipitation lines. Alkaline buffers between pH 7.5 and 8.6
are generally used because this provides conditions under which the
proteins are negatively charged and move toward the anode. This
method is used for virus characterization and strain differentiation.

C. Agglutination Test

The main difference between precipitation and agglutination tests is that

in the latter the antigen is very often larger than the antibody. For this
reason, few antibody molecules are necessary to form a visible particle
grouping. The principles governing reactions are similar to those of the
precipitation tests.

1. Passive agglutination (indirect or reverse)

This test is based on the use of inert substances that carry antigens or
antibodies. These substances (latex spheres, bentonite) are several
times larger than the reacting substances, thus making it possible to use
soluble antigens (viral particles) in agglutination tests.

2. Latex test

This test is based on the use of polysterene spheres (800 nm diameter)

covered by immunoglobulin molecules. This technique is 10–100 times
more sensitive than the traditional microprecipitation tests for detecting
plant viruses. A reaction may be seen with the naked eye.

3. Neutralization tests

In some cases, the interaction of biologically active antigens with

homolog antibodies results in loss of the antigen's biological activity. This
reaction is called neutralization. Since the sensitivity of these tests
fundamentally depends on the activity of the antiserum, the antigen's
activity must be biologically detectable.

D. Immunological tests with markers

These tests use antibodies and antigens marked with independently

acting substances called markers that increase test power and sensitivity.
The higher the marker's level of activity, the faster the antigen–antibody
reaction can be detected.

1. Immunofluorescence (IFA)

This technique uses substances transforming light in the ultraviolet range

(200 to 400 nm) into longer wavelength radiation. A modified microscope
(a fluorescence microscope) allows you to see the light emitted by the
fluorescing substance (fluoresceine isocyanate: FITC).

2. Radioimmunology

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 The immunoglobulins are marked with radioactive substances (P32, I128).
Their presence is determined by a reaction against photographic
material.

3. Immunologic assays with enzymatic conjugates

[Enzyme Linked Immunosorbent Assay (ELISA)]

These tests are based on the property of certain antigens and antibodies
to be absorbed into a solid medium allowing them to construct an ordered
sequence of biological material (antibody, antigen, antibody conjugated
with an enzyme), and which can be seen thanks to the color reaction
resulting from the addition of the enzyme-specific substrate conjugated to
the antibody, thus allowing adequate quantifying of the antigen.

Depending on research needs, the following assays of this kind are

frequently used.

DAS-ELISA

(Double Antibody Sandwich)

NCM-ELISA

(Nitro Cellulose Membrane)

E. Immunosorbent Electron Microscopy (ISEM)

This is used for detecting antigens, or in ultrathin sections of virus-

infected tissues.

When the virus is suspended, the positive reaction can be identified with
the help of an electron microscope.

1. Aggregation of viral particles

An adequate dilution of specific antiserum is prepared for detecting the

suspected virus in the tissue suspension. As a result of particle addition,
complexes appear that can be seen under the electron microscope.

This technique is particularly useful when low virus concentration

prevents direct observation. An excess of antibodies will result in reaction
inhibition, as in precipitation tests.

2. Antibody coating

This is based on the immobilization of viral particles in a grid and their

posterior covering by specific antibodies.

3. Antigen capture

Conversely, the grid is previously covered by specific antibodies.

Recommended Literature
Ball, E.M. 1974. Serological test of identification of plant viruses.
Published by American Phytopathological Society Inc. USA.

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 Devlin, T. 1985. Textbook of biochemistry with clinical correlation. John
Wiley & Sons Inc. New York, USA.
Harlow, E. and D. Lane. 1988. Antibodies: a laboratory manual. Cold
Spring Harbor Laboratory. New York, USA.
Harper, H.A. 1980. Manual de Química Fisiológica. 7ma. Edición. El
Manual Moderno, Mexico.
Landsteiner, K. 1969. The specifics of serological reactions. Dover
Publications Inc. New York, USA.
Lehninger, A. 1978. Biochemistry. Second Edition. Worth Publisher Inc.
New York, USA.
Nowotny A. 1969. Basic exercises in immuno-chemistry. Laboratory
manual. Springer-Verlag. Berlin, Germany.

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