Transcript Antigenicity - immunology.unideb.hu

4th SEMINAR
THE ADAPTIVE IMMUNE RESPONSE:
ANTIGENS AND ANTIGEN-SPECIFIC RECEPTORS
DEFENSE SYSTEMS
ADAPTIVE IMMUNITY
INNATE IMMUNITY
SENSING
Cells
SENSING
RECOGNITION
Receptors
RECOGNITION
SIGNALING
Signaling
pathways
SIGNALING
Cell-Cell
collaboration
RESPONSE
Effector
functions
RESPONSE
RECOGNITION BY CELLS OF THE
ADAPTIVE IMMUNE SYSTEM
Antigen-specific receptors:
B cell receptor (BCR) and T cell
receptor (TCR)
• The basic structure (90%) of the
receptors (BCR or TCR) is common
• Each cell expresses a receptor that is
unique in specificity (the 10% difference
means different specificity)
• These differences in antigen-specificity
are achieved during maturation in the
central lymphoid organs (bone marrow
and thymus)
ANTIGEN
Any structure that can be recognized by
the adaptive immune system (BCR, TCR).
Antigenicity: ability of a chemical structure to bind specifically to a
TCR or a BCR/antibody
According to the results of the specific binding an antigen may be
either:
• Immunogenic: recognition induces an immune response
• Tolerogenic: recognition induces tolerance (specific immune nonresponsiveness)
FACTORS INFLUENCING IMMUNOGENICITY
• Size (the bigger the better)
• haptens: antigens that can not provoke an
immune response because of their small
size unless they are attached to a carrier
molecule (e.g. a self peptide)
• Genetics
• Species (evolutionary the farther the better)
• Individual (e.g. transplantation antigens)
• Age (young: immature, old: decreasing number of
lymphocytes)
• Dose
• Route (vaccination)
subcutaneous > intravenous > oral /
intranasal
Not true for live vaccines (e.g. oral polio vaccine)
• Adjuvant (vaccination)
• substances that enhance the immune
response to an antigen (aluminum salts,
LPS, Freund’s adjuvant, TLR ligands)
• depot effect – slower biodegradation,
prolonged antigen intake by antigen
presenting cells
• activation of innate immunity
• Physical status
•
•
corpuscle (cell, colloid) or soluble
denatured or native
• Degradability
antigen presentation by APCs
ANTIGENIC DETERMINANT (EPITOPE)
Part of the antigen that
directly interacts with the
antigen binding site of the
TCR or BCR/antibody.
B CELL EPITOPE
T CELL EPITOPE
Recognized by B cells
Recognized by T cells
proteins
polysaccharides
lipids
DNA
steroids
etc. (many artificial molecules)
proteins mainly (8-23 amino
acids)
cell- or matrix-associated or
soluble
requires processing and
presentation by APCs
ANTIGEN RECOGNITION ≠ CELL ACTIVATION
BCR AND ANTIBODY
Antigen binding
L
H
H
H
Membrane-bound Ig
Antigen-specific
Receptor (BCR)
RECOGNIZING
MOLECULE
L
ab
H
L
L
Secreted Ig
Antigen-specific
soluble protein
EFFECTOR MOLECULE
signaling
B CELL
PLASMA CELL
IMMUNOGLOBULINS
Definition: Glycoprotein molecules that are present on
B cells as part of the BCR or produced by plasma cells
as antibodies in response to an immunogenic antigen.
Membrane-bound immunoglobulin (mIg) - BCR
Secreted immunoglobulin (sIg) – antibody
serum antibodies = gamma globulin fraction
STRUCTURE
• 2x Heavy chain (light blue)
disulfide
bond
• 2x light chain (dark blue)
carbohydrate
• Variable regions  antigen
binding
CL
VL
• Constant regions
CH1
VH
CH2
hinge region
CH3
HYPERVARIABLE REGIONS
CDR1
CDR2
CDR3
Light
chain
Epitope
CDR1
CDR2
CDR3
Heavy
chain
CDR3
CDR = Complementarity Determining Region – those amino acids of
the variable regions that directly interact with the epitope
variability index
150
100
CDR1
50
FR1
0
FR = frame – those amino acids of the variable regions that do not
interact directly with the epitope (stabilizer function)
CDR2
FR2
FR3
25
75
50
aminoacid sequence N – C terminal
FR4
100
DIFFERENT VARIABLE REGIONS 
DIFFERENT ANTIGEN-BINDING SITE 
DIFFERENT SPECIFICITY
isotype
allotype
idiotype
(Classes/subclasses)
Sequence variability of H/Lchain constant regions
Allelic variants
Sequence variability of H and
L-chain variable regions
(individual, clone- specific)
HUMAN IMMUNOGLOBULIN CLASSES
encoded by different structural gene segments (isotypes)
• IgG - gamma (γ) heavy chains
• IgM - mu (μ) heavy chains
• IgA - alpha (α) heavy chains
• IgD - delta (δ) heavy chains
• IgE - epsilon (ε) heavy chains
light chain types
• kappa (κ)
• lambda (λ)
Ig isotype
Serum
concentration
Characteristics, functions
Trace
amounts
 Major isotype of secondary
(memory) immune response
 Complexed with antigen activates
effector functions (Fc-receptor
binding, complement activation
 The first isotype in B-lymphocyte
membrane
 Function in serum is not known
Trace
amounts
 Major isotype in protection against
parasites
 Mediator of allergic reactions (binds
to basophils and mast cells)
3-3,5 mg/ml
 Major isotype of secretions (saliva,
tear, milk)
 Protection of mucosal surfaces
12-14 mg/ml
1-2 mg/ml
 Major isotype of primary immune
responses
 Complexed with antigen activates
complement
 Agglutinates microbes
 The monomeric form is expressed in
B-lymphocyte membrane as antigen
binding receptor
IMMUNOGLOBULIN FRAGMENTS
STRUCTURE/FUNCTION RELATIONSHIPS
• Fab
papain
• antigen binding
• valence = 1
• specificity determined
by VH and VL
• Fc
VH
VL
• effector functions
Fc
Fab
IMMUNOGLOBULIN FRAGMENTS
STRUCTURE/FUNCTION RELATIONSHIPS
pepsin
• F(ab’)2
- Bivalent!
Fc
peptides
F(ab’)2
ANTIBODY FUNCTION
• Role of the Fab part:
• Binds the antigen
• May form crosslinks between
antigens (precipitation /
agglutination – see later)
• Neutralization: binding can block
the enzyme or toxin or other
virulence factors of pathogens and
can avoid damage to host cells
• Role of the Fc part:
• Activate cells carrying Fcreceptors on their surfaces:
• Phagocytic cells – opsonized
phagocytosis
• NK cells – antibody-dependent
cellular cytotoxicity (ADCC)
• Activates the complement
system via the classical pathway
NEUTRALIZATION
OPSONIZATION
ADCC
(A) High-affinity FcRs on the surface of the cell bind monomeric
Ig before it binds to antigen. (mast cell)
(B) Low-affinity FcRs bind multiple Igs that have already bound
to a multivalent antigen. (macrophage, NK cell)
COMPLEMENT ACTIVATION
PRODUCTION OF IMMUNOGLOBULINS
BEFORE BIRTH
AFTER BIRTH
breast milk
IgA
100%
(adult)
maternal IgG
IgM
IgG
IgA
0
3
month
6
9
1 2 3 4 5 adult
year
SECRETORY IgA AND TRANSCYTOSIS
SS
SS
ss
J
SS
S
S
ss
S
S
SS
SS
SS
SS
ss
ss
SS
S
S
S
S
ss
SS
SS
J
J
S
S
S
S
S
S
S
S
S
S
J
S
S
ss
ss
SS
SS
J
S
S
J
J
B
SS
S
S
S
S
B cells located in the
submucosa produce
dimeric IgA
S
S
IgA and pIgR are
transported to
the apical
surface in
vesicles
SS
‘Stalk’ of the pIgR is degraded to release IgA
containing part of the pIgR (the secretory component)
Epithelial
cell
pIgR and IgA are
internalised
Polymeric Ig receptors are
expressed on the basolateral
surface of epithelial cells to
capture IgA produced in the
mucosa
M
U
C
U
S
TCR
• Alpha and beta chains instead of light and heavy (innate
subgroup of T cells express gamma-delta chains as TCR)
• Both chains are membrane-bound
• Monovalent interaction with the antigen
• Antigen recognition requires presentation by antigen
presenting cells via MHC molecules
• Recognize peptide antigens (mainly)
• No secreted form
ANTIGEN PRESENTATION
•
T cells that express CD8 as co-receptor (cytotoxic T cells) recognize peptides presented via MHC class I molecules
•
T cells that express CD4 as co-receptor (helper T cells) recognize peptides presented via MHC class II molecules
•
For activation both cell types require the help of APCs
•
Antigen presentations that lead to T cell activation take place in the secondary lymphoid tissues (e.g. lymph nodes)
•
MHC I is expressed by every nucleated cell  RBCs don’t express them
•
Professional antigen presenting cells express MHC class I and class II molecules:
» macrophage (innate)
» DC (innate)
» B cell (adaptive)
MHC RESTRICTION OF T CELL RECOGNITION
TCR
TCR
TCR
1. A given TCR recognizes a
defined MHC – peptide
complex
2. The same peptide presented
by another MHC is not
recognized by the same TCR
M HC
MHC
MHC
APC
APC
APC
1.
2.
3.
3. Another peptide bound to the
same MHC is not recognized
by the same TCR
ACTIVATION OF TCR AND BCR
APC
MHC
Antigen
Antigen
TCR
BCR
αβ
s
V s
s
sV
s
C s
s
sC
ss
ITAM
Immunoreceptor Tyrosine-based
Activation Motif
CD3
CD3
εδ
εγ
s s
s s
s s
s s
ζζ
s s
ACTIVATION
SUPERANTIGENS
Microbial proteins that bind to and
activate all the T cells that express
a particular set or family of TCR
molecules.
The activation is independent from
the presented antigen.
Leads to polyclonal T cell activation
that causes life threatening
inflammatory responses.
SUPERANTIGENS
conventional antigen
superantigen
monoclonal/oligoclonal
polyclonal
T cell response
T cell response
1:104 - 1:105
1:4 - 1:10
107 – 108 / 1011
activated T cells
1010 / 1011