Transcript 05-Humoral_Immunity__Ig_structure_and_func_2008

Humoral Immunity &
Immunoglobulin Structure and
Function
Dr. Adel Almogren
Adaptive Immunity:
@ Humoral Immunity
(Antibody mediated response)
@ Cellular Mediated Immunity
(CMI)
Humoral (Antibody-Mediated) Immunity
+ Involves production of antibodies against foreign
antigens.
+ Antibodies are produced by a subset of lymphocytes
called B cells.
+ B cells that are stimulated will actively secrete
antibodies and are called plasma cells.
+ Antibodies are found in extracellular fluids (blood
plasma, lymph, mucus, etc.) and the surface of B cells.
+ Defense against bacteria, bacterial toxins, and
viruses that circulate freely in body fluids, before they
enter cells.
+ Also cause certain reactions against transplanted
tissue.
How Do B Cells Produce Antibodies?
 B cells develop from stem cells in the bone marrow
of adults (liver of fetuses).
 After maturation B cells migrate to lymphoid organs
(lymph node or spleen).
 Clonal Selection:
When a B cell encounters an
antigen it recognizes, it is stimulated and divides into
many clones called plasma cells, which actively
secrete antibodies, and memory B cells
 Each B cell produces antibodies that will recognize
only one antigenic determinant.
Clonal Selection of B Cells is Caused by
Antigenic Stimulation
Humoral Immunity (Continued)
Clonal Selection
 Clonal Selection:
B cells that encounter
stimulating antigen will proliferate into a large
group of cells (also apply to T cells).
 Why don’t we produce antibodies against our
own antigens? We have developed tolerance to
them.
 Clonal Deletion: B (and T) cells that react against
self antigens appear to be destroyed during fetal
development. Process is poorly understood.
Antibody Production
T-Dependent Antigens:
 Antibody production requires assistance from T helper cells.
 A macrophage cells ingest antigen and presents it to TH cell.
 TH cell stimulates B cells specific for antigen to become
plasma cells.
 Antigens are mainly proteins on viruses, bacteria, foreign red
blood cells, and hapten-carrier molecules.
T-Independent Antigens:
 Antibody production does not require assistance from T cells.
 Antigens are mainly polysaccharides or lipopolysaccharides
with repeating subunits (bacterial capsules).
 Weaker immune response than for T-dependent antigens.
B cell activation
Some responses require T
help whereas other do not
Thymus-independent because
T cells are not needed
Thymus-dependent because T
cells are required
T-independent antibody response
generally have
1. no memory
2. no isotype switching
3. no somatic mutations
Immunoglobulin (Antibody)
Structure and Function
Immunoglobulin Structure-Function Relationship
• Cell surface antigen receptor on B cells
• Secreted antibody
Immunoglobulins are Bi-functional
proteins
Ag binding
Fc receptor
Complement protein binding
Domain Structure of Immunoglobulins
Domains are folded, compact, protease resistant structures
Fab
Fc
Light chain C
domains
k or l
S
S
S S
S S
S
Heavy chain C
domains
a, d, e, g, or m
Pepsin cleavage sites
Papain cleavage sites
S
F(ab)2
- 1 x (Fab)2 & 1 x Fc
- 2 x Fab 1 x Fc
CH3
CH2
CH3
CH1
CH2
CH3
VH1
CH1
CH2
CH3
VH1
CH1
CL
CH2
CH3
VH1
CH1
VL
CL
CH2
CH3
VH1
CH1
VL
CL
CH2
CH3
VH
CH1
CL
VL
CH2
Elbow
Hinge
CH3
Flexibility and
motion of
immunoglobulins
Elbow
Hinge
Hypervariable regions
•
Most hypervariable regions coincided with antigen contact points the COMPLEMENTARITY DETERMINING REGIONS (CDRs)
FR1
CDR1 FR2 CDR2
FR3
CDR3
FR4
100
Variability
80
60
40
20
20
40
60
80
100
120
Amino acid No.
Hypervariable CDRs are located
on loops at the end of the Fv regions
Hypervariable loops and framework: Summary
• The sequences of the hypervariable loops are highly variable
amongst antibodies of different specificities
• Variable amino acid sequence in the
hypervariable loops accounts for the diversity of
antigens that can be recognised by a repertoire of
antibodies
Antigens vary in size and complexity
Protein:
Influenza haemagglutinin
Hapten:
5-(para-nitrophenyl
phosphonate)-pentanoic acid.
Concept: Epitopes can bind in pockets or
grooves or on extended surfaces in the
binding site of antibodies.
Electron micrographs of the effect of antibodies and
complement upon bacteria
Healthy E. coli
Antibody + complement- mediated
damage to E. coli
Why do antibodies need an Fc region?
The (Fab)2 fragment can •
Detect antigen
•
Precipitate antigen
•
Block the active sites of toxins or pathogen-associated
molecules
•
Block interactions between host and pathogen-associated
molecules
but can not activate
•
Inflammatory and effector functions associated with cells
•
Inflammatory and effector functions of complement
•
The trafficking of antigens into the antigen processing
pathways
Structure and function of the Fc region
IgA IgD IgG
IgE IgM
CH2
The hinge region is replaced
by an additional Ig domain
Fc structure is common to all specificities of antibody within an ISOTYPE
(although there are allotypes)
The structure acts as a receptor for complement proteins and a ligand
for cellular binding sites
Monomeric IgM
IgM only exists as a monomer on the surface of B cells
Monomeric IgM has a very low affinity for antigen
Cm2
N.B. Only constant
heavy chain
domains are shown
IgM forms pentamers and hexamers
Cm4
ss
C
C
Cm2
Multimerisation of IgM
Cm4
IgM facts and figures
Heavy chain:
m - Mu
Half-life:
5 to 10 days
% of Ig in serum:
10
Serum level (mgml-1):
0.25 - 3.1
Complement activation:
++++ by classical pathway
Interactions with cells:
Phagocytes via C3b receptors
Epithelial cells via polymeric Ig receptor
Transplacental transfer:
No
Affinity for antigen:
Monomeric IgM - low affinity - valency of 2
Pentameric IgM - high avidity - valency of 10
IgD facts and figures
Heavy chain:
d - Delta
Half-life:
2 to 8 days
% of Ig in serum:
0.2
Serum level (mgml-1):
0.03 - 0.4
Complement activation: No
Interactions with cells:
T cells via lectin like IgD receptor
Transplacental transfer: No
??IgD & IgM ??
IgA dimerisation and secretion
IgA is the major isotype of antibody secreted at mucosal surfaces
Exists in serum as a monomer, but more usually as a J chainlinked dimer, that is formed in a similar manner to IgM pentamers.
S
S
S
J
S
ss
S
S
S
S
IgA exists in two subclasses
IgA1 is mostly found in serum and made by bone marrow B cells
IgA2 is found in higher concentration in mucosal secretions, colostrum and
milk and is made by (??? )
Secretory IgA and transcytosis
S
S
SS
SS
SS
SS
ss
ss
S
S
J
S
S
S
S
S
S
J
ss
S
S
S
S
SS
S
S
B
J
J
Epithelial
cell
pIgR & IgA are
internalised
ss
SS
S
S
SS
J
SS
S
S
ss
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
SS
B cells located in the submucosa
produce dimeric IgA
Polymeric Ig receptors
are expressed on the
basolateral surface of
epithelial cells to
capture IgA produced
in the mucosa
IgA facts and figures
Heavy chains:
a1 or a2 - Alpha 1 or 2
Half-life:
IgA1 5 - 7 days
IgA2 4 - 6 days
Serum levels (mgml-1):
IgA1 1.4 - 4.2
IgA2 0.2 - 0.5
% of Ig in serum:
IgA1 11 - 14
IgA2 1 - 4
Complement activation: IgA1 - by alternative and lectin pathway
IgA2 - No
Interactions with cells:
Epithelial cells by pIgR
Phagocytes by IgA receptor
Transplacental transfer: No
IgE facts and figures
Heavy chain:
e - Epsilon
Half-life:
1 - 5 days
Serum level (mgml-1):
0.0001 - 0.0002
% of Ig in serum:
0.004
Complement activation: No
Interactions with cells:
Via high affinity IgE receptors expressed
by mast cells, eosinophils, basophils
and Langerhans cells
Via low affinity IgE receptor on B cells
and monocytes
Transplacental transfer: No
its role in protecting against parasitic infections
IgE is also closely linked with allergic diseases
IgG facts and figures
Heavy chains:
g 1 g 2 g3 g4 - Gamma 1 - 4
Half-life:
IgG1
IgG3
21 - 24 days
7 - 8 days
IgG2
IgG4
21 - 24 days
21 - 24 days
Serum level (mgml-1):
IgG1
IgG3
5 - 12
0.5 - 1
IgG2
IgG4
2-6
0.2 - 1
% of Ig in serum:
IgG1
IgG3
45 - 53
3-6
IgG2
IgG4
11 - 15
1-4
+++
++++
IgG2
IgG4
+
No
Complement activation: IgG1
IgG3
Interactions with cells:
All subclasses via IgG receptors on macrophages
and phagocytes
Transplacental transfer: IgG1
IgG3
++
++
IgG2
IgG4
+
++
The neonatal Fcg receptor may be responsible!
C1q binding motif is
located on the Cg2
domain
Carbohydrate is essential for
complement activation
Subltly different hinge regions
between subclasses accounts
for differing abilities to activate
complement
Fcg receptors
Receptor
FcgRI
FcgRIIA
FcgRIIB1
FcgRIIB2
FcgRIII
Cell type
Effect of ligation
Macrophages Neutrophils,
Eosinophils, Dendritic cells
Uptake, Respiratory burst
Macrophages Neutrophils,
Eosinophils, Platelets
Langerhans cells
Uptake, Granule release
B cells, Mast Cells
No Uptake, Inhibition of stimulation
Macrophages Neutrophils,
Eosinophils
Uptake, Inhibition of stimulation
NK cells, Eosinophils,
Macrophages, Neutrophils
Mast cells
Induction of killing (NK cells)
Fv
VH1
CH1
Fb
VL
CL
Fab
CH2
Elbow
Hinge
Fc
Carbohydrate
CH3
Antibody Dependent Cell Mediated Cytotoxicity
(ADCC)
Target cell is covered with antibodies,
leaving Fc portion sticking outwards.
Natural killer and other nonspecific cells
that have receptors for Fc region are
stimulated to kill targeted cells.
Target organism is lysed by substances
secreted by attacking cells.
Used to destroy large organisms that
cannot be phagocytosed.
Destruction of Large Parasites by ADCC