Antigen
ANTIGENS
Compiled by: Zenisha Acharya
Sadhana
Khanal
Christina
Khadka
Nirmala Joshi
Sandesh
Dhakal
Central Department of
Microbiology
Kirtipur,
Kathmandu
Antigenicity and Antigens
•
Antigenicity is the ability to
combine specifically with the final products of the adaptive immune responses (antibodies and/or
TCRs)
•
Substances that can be recognized by
the immunoglobulin receptor of B cells
or by the T cell receptor when complexed
with MHC, are called antigens
Immunogenicity and Immunogens
• Immunogenicity
is the ability to induce a humoral and/or cell mediated immune response
• Immunogens are the substances that elicit/ trigger/induce adaptive immune response
•
Haptens are the small organic molecules that interact but do not activate the immune response (non-immunogenic antigens)
•
Chemical coupling of a hapten to a
large protein, called a carrier, yields
an immunogenic hapten-carrier conjugate
•
Examples: penicillin, aspirin,
formaldehyde, sulfonamide,
picrylchloride
Hapten-carrier conjugate
• Animals immunized with a conjugate produce antibodies specific for
1. The hapten determinant
2. Unaltered epitopes on the carrier protein
3. New epitopes formed by combined parts of both the
hapten and carrier
•
The immune system actually
recognizes particular macromolecules of
an infectious agent, generally either proteins
or polysaccharides
•
Proteins are the most potent immunogens,
with polysaccharides ranking second
•
Lipids and nucleic acids of an
infectious agent generally do not serve
as immunogens unless they are complexed
with proteins or polysaccharides
•
Proteins must first be processed into
small peptides and then presented
together with MHC molecules on the
membrane of a cell before they can be recognized as immunogens
•
Lipids and glycolipids that can elicit cell-mediated immunity must also be combined with MHC-like membrane molecules called CD1
•
Immunogenicity is not an intrinsic
property of an antigen
•
Also depends on a number of properties of the particular biological system that the antigen encounters
Contribution of the immunogen
- Foreignness:
• A
molecule must be recognized as non-self by the
biological system in order to elicit an immune response
• The
greater the phylogenetic distance between two
species, the greater the structural disparity between them
• Examples
Bovine
serum albumin (BSA) is not immunogenic when
injected into a cow but is strongly immunogenic when injected into a rabbit
Egg
albumin is not immunogenic when injected into fowls but is strongly immunogenic when injected
into a rabbit
Exception
•
Collagen and cytochrome c have been
highly conserved throughout evolution
and therefore display very little
immunogenicity across diverse species
•
Some self-components (e.g., corneal
tissue and sperm) are effectively
sequestered from the immune system, so
that if these tissues are injected even
into the animal from which they
originated, they will function as immunogens
- Molecular
size:
•
Generally, substances with a molecular
mass less than 5,000-10,000 Da are poor
immunogens
•
Very large proteins, e.g. keyhole limpet hemocyanin (>2,000 kDa) is a very
powerful immunogen
•
The best immunogens tend to have a
molecular mass of 100 kDa
•
Polysaccharides vary in immunogenicity
(dextran with MW of 600 kDa is a good immunogen whereas dextran with MW of 100 kDa is not)
- Chemical
composition and heterogeneity
•
Synthetic homopolymers (polymers
composed of a single amino acid or sugar) eg. polystyrene, nylon, polyacrylamide tend to lack immunogenicity
regardless of their size
•
Copolymers composed of different amino
acids or sugars are usually more
immunogenic than homopolymers
•
It is notable that all four levels of
protein organization (primary, secondary,
tertiary, and quaternary) contribute to the structural complexity of a
protein and hence affect its
immunogenicity
- Susceptibility
to Ag processing and presentation
• Large,
insoluble macromolecules generally are
more immunogenic than small, soluble ones because the larger molecules are more
readily phagocytosed and processed
• Macromolecules
that cannot be degraded and presented
with MHC molecules are poor immunogens
• Example
o
polymers of D-amino acids
Contribution of the
biological system
- Genotype
of the recipient:
• The
gene controlling immune responsiveness
is mapped to regions of MHC
• The
response of an animal to an antigen is
also influenced by
o
The genes that encode B-cell and T-cell
receptors
o
The genes that encode various proteins
involved in immune regulatory mechanisms
- Dosage
and route of antigen administration:
•
An insufficient dose will not stimulate
an immune response because it fails to
activate enough lymphocytes
•
A single dose of Ag is less
effective than repeated administration over a period of
weeks
• Repeated
administrations, or boosters, increase
the clonal proliferation of antigen-
specific T cells or B cells
Administration routes
of Ag
1. Intravenous (iv):
into a vein
2. Intradermal (id):
into the skin
3. Subcutaneous (sc): beneath the skin
4. Intramuscular (im): into a muscle
5. Intraperitoneal (ip): into the peritoneal
cavity
•
The immune response is high if soluble
Ag is administered subcutaneously or
intramuscularly and cellular Ag is
administered intravenously or intraperitoneally
•
Antigen administered intravenously is
carried first to the spleen, whereas
antigen administered subcutaneously moves first to local lymph nodes
- Adjuvants
•
Substances that enhance the
immunogenicity of that antigen when
mixed and injected with it
•
Adjuvants are used when an antigen has
low immunogenicity or when only small
amounts of an antigen are available
Effects of adjuvants on
immunogenicity
- Antigen
persistence is prolonged
- Co-stimulatory
signals are enhanced
- Local
inflammation is increased
- The
nonspecific proliferation of lymphocytes
is stimulated
Aluminum potassium
sulfate (alum)
•
It prolongs the persistence of antigen
•
When an antigen is mixed with alum, the
salt precipitates the antigen
•
Injection of this alum precipitate
results in a slower release of antigen
from the injection site
•
The effective time of exposure to the
antigen increases from a few days
without adjuvant to several weeks with
the adjuvant
•
The alum precipitate also increases the
size of the antigen, thus increasing the likelihood of phagocytosis
Freund’s incomplete adjuvant
•
It contains antigen in aqueous solution,
mineral oil and an emulsifying agent
such as mannide monooleate, which
disperses the oil into small droplets
surrounding the antigen
•
The antigen is then released very slowly
from the site of injection
•
It contains heat-killed mycobacterial
extract, muramyl dipeptide, a component
of the mycobacterial cell wall that
activates macrophages
•
Activated macrophages are more
phagocytic than inactivated macrophages
and express higher levels of class II
MHC molecules and the membrane molecules
of the B7 family
•
The increased expression of class II MHC
increases the ability of the APC to
present antigen to TH cells
•
B7 molecules on the APC bind to CD28, a
cell-‐surface protein on TH cells, triggering co-stimulation, an enhancement of the T- cell immune response
•
Alum and Freund’s adjuvants also
stimulate a local, chronic inflammatory
response that attracts both phagocytes
and lymphocytes
•
The infiltration of cells at the site of
the adjuvant injection ofen results in
formation of a dense, macrophage-rich
mass of cells called a granuloma
•
Macrophages in a granuloma are
activated, so it also enhances the
activation of TH cells
•
Other adjuvants (synthetic
polyribonucleotides and bacterial LPS)
stimulate the nonspecific proliferation
of lymphocytes
Epitopes
•
Immune cells do not interact with,
or recognize, an entire immunogen
molecule
•
Lymphocytes recognize discrete sites on
the macromolecule called epitopes, or
antigenic determinants
•
Epitopes are the immunologically active
regions of an immunogen that bind
to antigen-‐specific membrane receptors
on lymphocytes or to secreted antibodies
•
B cells recognize soluble antigen when
it binds to their membrane-‐bound
antibody and the epitopes they
recognize tend to be highly accessible sites on the exposed surface of
the immunogen
•
T cells recognize only peptides combined
with MHC molecules on the surface of
antigen‐ presenting cells and altered
self-cells and epitope cannot be
considered apart from their associated
MHC molecules
Types of antigens
- On
the basis of immunogenicity
Ø Pure
immunogen
Ø Hapten
- On
the basis of biological function
Ø Thymus-dependent
(TD) Ags:
•
TD
Ags are those that require the help of T-
cells to stimulate the production of Ab by B- cells
•
Proteins are TD Ags
•
Ag processing is required
•
They initiate memory
Ø Thymus-independent
(TI) Ags:
•
They are the Ags, which can directly
stimulate B‐cells to produce Ab without
the requirement of T-cell help
•
Polysaccharides are TI Ags
•
No memory
•
IgM predominant
•
Low level of class switching
•
Classified into type 1 and type 2
Type 1 TI Ags
•
TI-1 antigens are polyclonal B-cell
activators (mitogens)
•
They are able to activate B cells
regardless of their antigenic specificity
•
The mechanism by which TI-1 antigens
activate B cells is not well understood
•
When B cells are exposed to lower
concentrations of TI-1 antigens, only
those B-cells specific for epitopes of
the antigen will be activated
•
At high
concentrations, some TI-1 antigens will
stimulate proliferation and antibody secretion by as many as one third of all B cells
•
Example: LPS
Type 2 TI Ags
•
Highly repetitious molecules
such as polymeric proteins
•
They activate B cells by extensively crosslinking the mIg receptor
•
Examples
Ø Bacterial
flagellin
Ø Bacterial
cell wall polysaccharides with repeating
polysaccharide units
•
TI-2 antigens differ from TI-1 antigens
in three important respects:
Ø They
are not B-cell mitogens and so do not act as
polyclonal activators
Ø TI-1
antigens will activate both mature and immature
B cells, but TI-2 antigens activate mature B cells only
Ø Although
the B-cell response to TI-2 antigens does not
require direct involvement of TH cells, cytokines
derived from TH cells are required for efficient B-cell proliferation and for class switching to
isotypes other than IgM
- Heterophiles
• Two
different Ags share an identical epitope
•
Consequence: cross-reactivity
Ab specific for one epitope also binds to an unrelated epitope possessing similar chemical properties
- Superantigens
•
Superantigens are viral or bacterial
proteins that bind simultaneously to the
Vβ domain of TCR and to the α
chain of a class II MHC molecule
•
Crosslinkage of a T-cell receptor and
class II MHC molecule by superantigen
produces an activating signal that
induces T-cell activation and
proliferation
•
Conventional T-dependent Ags activate
only a small fraction (1 in 104-105)
of T-cell population i.e. monoclonal or
oligoclonal response
•
The activation by superantigens is polyclonal
and can affect a significant
percentage (5-25%) of the total TH population
Exogenous superantigens:
•
They are soluble proteins secreted by
bacteria
•
A variety of exotoxins secreted
by gram- positive bacteria, such
as
Ø Staphylococcal
enterotoxins
Ø Toxic-shock-syndrome
toxin
Ø Exfoliative-dermatitis
toxin
•
Each of the exogenous superantigens
binds particular Vβ sequences
in TCR and crosslinks the TCR to a class
II MHC molecule
Endogenous
superantigens
•
They are cell-membrane proteins encoded
by certain viruses that infect mammalian
cells
•
Viral proteins are expressed on the
membrane of the infected cells
•
The viral proteins, called minor
lymphocyte stimulating (Mls) determinants,
bind particular Vβ sequences
in TCR and crosslink the TCR to a class
II MHC molecule
Superantigens and Diseases
Comments
Post a Comment