october 5, 2004. immunity adaptiveinnate cell mediatedhumoral
TRANSCRIPT
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October 5, 2004
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IMMUNITY
ADAPTIVE INNATE
CELL MEDIATED HUMORAL
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IMMUNITY
ADAPTIVE INNATE
CELL MEDIATED HUMORAL
ANTIBODIESStructureSequence diversityNature of reaction with AgDifferent functional properties
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If bacteria are injected into blood specific substances are found that exhibit different biologic properties
Agglutinin - specifically clump only the bacteria used for immunization
Opsonin - facilitate engulfment
Antitoxin - neutralize toxin associated with the immunizing bacteriaCytolysis - lyse bacteria
Precipitins - form flocculate precipitates when added to supernatant from the bacteria
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If bacteria are injected into blood specific substances are found that exhibit different biologic properties
Agglutinin - specifically clump only the bacteria used for immunization
Opsonin - facilitate engulfment
Antitoxin - neutralize toxin associated with the immunizing bacteriaCytolysis - lyse bacteria
Precipitins - form flocculate precipitates when added to supernatant from the bacteria
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A n t i b o d y ( A b )
A m o l e c u l e p r o d u c e d b y a n i m a l i n r e s p o n s e t o
f o r e i g n s u b s t a n c e ( A g ) w h i c h h a s t h e p r o p e r t y o f
b i n d i n g t h e e l i c i t i n g a g e n t
I m m u n o g l o b u l i n ( I g )
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A n t i b o d y ( A b )
A m o l e c u l e p r o d u c e d b y a n i m a l i n r e s p o n s e t o
f o r e i g n s u b s t a n c e ( A g ) w h i c h h a s t h e p r o p e r t y o f
b i n d i n g t h e e l i c i t i n g a g e n t
I m m u n o g l o b u l i n ( I g )
Antibodies are found both as cell-associated receptors (BCR) and as circulating effector
molecules
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A n t i g e n ( A g )
A n y m o l e c u l e c a p a b l e o f b e i n g b o u n d b y t h e
c o m b i n i n g s i t e o f a n A b o r t h e T - c e l l r e c e p t o r
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I m m u n o g e n
A s u b s t a n c e c a p a b l e o f e l i c i t i n g a n i m m u n e
r e s p o n s e ; a l l i m m u n o g e n s a r e a n t i g e n s b u t n o t a l l
a n t i g e n s a r e i m m u n o g e n s .
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We will now consider the experiments that were done to determine antibody atructure
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albumin
αβ
γ
Electrophoretic migration
Absorbance
Immune serum from rabbits before and after the antibodies were removed by precipitation with Ag(ovalbumin)
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albumin
αβ
γ
Electrophoretic migration
Absorbance
The antibodies migrated as a γ-globulinShowed they w
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g a m m a g l o b u l i n
When purified antibodies (produced by dissolving immunoprecipiates) were analyzed in an ultracentrifuge they ran with a 7S sedimentation co-efficient indicating a m.w. of approx. 150,000
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gamma globulin
7S = 150,000 molecular weight
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gamma globulin
7S = 150,000 molecular weight
Molecular analysis of
precipitates showed
the valence = 2
Two molecules of Ag were precipitated by each Ab
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gamma globulin
7S = 150,000 molecular weight
valence = 2
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Digestion of antibodies with the enzyme papain yielded two fragments, the Fab (fragment antigen binding), and the Fc (fragment crystallizable)
Ratio: 2 Fab and 1 Fc
gamma globulin
7S = 150,000 molecular weight
valence = 2
papain --> 2Fab + 1Fc
Fab binds Ag but no precipitate
Fc forms crystals
MW of Fab and Fc = 50,000
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1 ( F a b ' )
2
o f 1 0 0 , 0 0 0
b i n d s 2 m o l e c u l e s o f A g
c a n p r e c i p i t a t e
( F a b ' )
2 p e p s i n
b r e a k S S - - > 2 F a b
gamma globulin
7S = 150,000 molecular weight
valence = 2
papain --> 2Fab + 1Fc
pepsin--> 1 (Fab')
2
of 100,000
binds 2 molecules of Ag
can precipitate
(Fab')
2 break SS-->2Fab
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If break disulfide bonds and separate on the basis of size you get two products
gamma globulin
7S = 150,000 molecular weight
valence = 2
papain --> 2Fab + 1Fc
Ab--> cleave disulfides-->
2H(50000) + 2L(25000)
Anti-H reacts with Fab and Fc
Anti-L reacts only with Fab
Anti-Fab reacts with H and L
Antiserum
pepsin--> 1 (Fab')
2
of 100,000
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gamma globulin
7S = 150,000 molecular weight
valence = 2
papain --> 2Fab + 1Fc
Ab--> cleave disulfides-->
2H(50000) + 2L(25000)
Anti-H reacts with Fab and Fc
Anti-L reacts only with Fab
Anti-Fab reacts with H and L
Antiserum
pepsin--> 1 (Fab')
2
of 100,000
gamma globulin
7S = 150,000 molecular weight
valence = 2
papain --> 2Fab + 1Fc
Ab--> cleave disulfides-->
2H(50000) + 2L(25000)
Anti-H reacts with Fab and Fc
Anti-L reacts only with Fab
Anti-Fab reacts with H and L
Antiserum
pepsin--> 1 (Fab')
2
of 100,000
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gamma globulin
7S = 150,000 molecular weight
valence = 2
papain --> 2Fab + 1Fc
Ab--> cleave disulfides-->
2H(50000) + 2L(25000)
Anti-H reacts with Fab and Fc
Anti-L reacts only with Fab
Anti-Fab reacts with H and L
Antiserum
pepsin--> 1 (Fab')
2
of 100,000
Papain
DigestionPepsin
Disulfide
Reduction
L
H
Fc
Fab Fab F(ab')
2
+
Small Fragments
L chain
H chain
NH
3
+
COO
-
NH
3
+
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gamma globulin
7S = 150,000 molecular weight
valence = 2
papain --> 2Fab + 1Fc
Ab--> cleave disulfides-->
2H(50000) + 2L(25000)
Anti-H reacts with Fab and Fc
Anti-L reacts only with Fab
Anti-Fab reacts with H and L
Antiserum
pepsin--> 1 (Fab')
2
of 100,000
Papain
DigestionPepsin
Disulfide
Reduction
L
H
Fc
Fab Fab F(ab')
2
+
Small Fragments
L chain
H chain
NH
3
+
COO
-
NH
3
+
This is the structure of the fundamental building block of an Ab molecule. Some antibodies (such as IgG) are made of one of these building blocks. Others (IgA and IgM) are made of multiple copies of this basic building block.
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albumin
αβ
γ
Electrophoretic migration
Absorbance
One problem with further analysis was the heterogeneity of antibodies: note the narrow peak of the homogenous protein albumin compared to the broad peak of the Ig
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It was a major breakthrough when it was realized that multiple myeloma is a tumor of plasma cells generally synthesizing a single species of Ig and that the myeloma proteins are homogeneous Abs produced by the plasma cells
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Bence-Jones proteins are monoclonal (homogeneous) L chains in patients with multiple myeloma
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How are antibodies capable of
binding many different
antigens?
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Light chains are of two types:
κ λ
anti - κ
anti - λ
++++
++++
-
-
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Light chains are of two types:
κ λ
anti - κ
anti - λ
++++
++++
-
-
Take two kappa (or lambda) lights chains from two different patients with multiple myeloma and produce proteolytic fragments
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Light chains are of two types:
κ λ
anti - κ
anti - λ
++++
++++
-
-
Take two kappa (or lambda) lights chains from two different patients with multiple myeloma, produce proteolytic fragments.
Analyze the products by 2-D peptide map (chromatography in one dimension, electrophoresis in the other).
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You find that about half of the cleavage products are identical between the two chains while the other half differ.
electrophoresis-->
chromatography-->
The shaded spots indicate peptides shared by the two κ or λ chains. Note the difference between κ and λ.The conclusion is that the lights chains have a portion that is variable and a portion that is constant.
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electrophoresis-->
chromatography-->
V
L
C
L
V
H
C
H
( κ or λ )
( )many different options
Amino acid sequence analysis shows that the variable portions is at one end of the light (and heavy) chains
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V
L
C
L
V
H
C
H
( κ or λ )
( )many different options
More extensive analysis of the sequences of variable regions shows that they in fact contain relatively conserved regions and other regionswhich are hypervariable
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More extensive analysis of the sequences of variable regions shows that they in fact contain relatively conserved regions and other regionswhich are hypervariable.
There are 20 different amino acids.If all are found with equal frequencyvariability is 20/.05 = 400.If only one amino acid is found thevariability is 1/1 =1/
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There are 20 different amino acids.If all are found with equal frequencyvariability is 20/.05 = 400.If only one amino acid is found thevariability is 1/1 =1/
Hypervariable regions are also called complementarity determining regions (CDRs). The prediction is that the
CDRs contact antigen.
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Hypervariable regions are also called complementarity determining regions (CDRs). The prediction is that the
CDRs contact antigen.
X-crystallography shows that the Ab molecule folds into compact domains. The CDRs are loops extending from the variable regions so that they are easily accessible for interaction with Ag. The other amino acids in the variable region are the “framework” amino acids and providea scaffold to maintain the CDRs in the proper orientation.
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Light chains have two domains: one variable and one constantHeavy chains have four or five domains, one of which is variable with the remains constantAll heavy chains have at least one carbohydrate moiety attached
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The forces are weak and operate at short distances
Therefore the closer the approach (the better the fit) the stronger the interaction
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A complex of an Ab and a
protein Ag shows:
the reaction occurs between two
interacting surfaces
the better the fit, the stronger the interaction
most but not all of the interaction with Ag
occurs through the CDRs
on a protein Ag the amino acids compri s ing
the e pi topes are adjacent in 3D space but not
necessarily in linear sequence
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Abs make contact with a protein Ag across a large planar face
usually 15-22 amino acids on Ag contact a similar number on the Ab giving a complementary surface of 650-900 A2
epitopes are exposed on the surfacemost Abs are elicited by proteins in their native statenote that Abs are designed to react with Ags in solution; the TCR reacts with Ags bound to cell surfaces
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Another characteristic of the antibody molecule is a flexible hinge
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D N P l i g a n d
When a divalent hapten reacts with Abs, trimers, tetramers and other large complexes form. Because of the flexibility of the hinge region the angle between the arms of the Abs vary.
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D N P l i g a n d
Digestion with papain removes the Fc
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Classes Heavy Chain Subclasses (human)
IgM μ
IgE ε
IgD δ
IgG γ 1, 2, 3, 4IgG IgG IgG IgG
IgA α 1, 2IgA IgA
+ = Classes subclasses isotypes
: Light chains have two isotypes κ and λ
Each isotype is encoded by a separate gene
There are several different possible structure of the constant regions of the heavy chains
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IgMH chain has four constant region domains and no hinge
In addition to H and L also contains J chain
(H2L2)5J or (H2L2)6
Usually the first Ig made in response to antigen
Increased valence leads to increased avidity
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IgG
IgGIgG
Most abundant serum immunoglobulin
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J
SC
Monomer Dimeric IgA Secretory IgA
IgA
Present in secretions where it provides protection at epithelial surfaces
Frequently found as a polymer with J chain
In secretions it also has a fourth chain, secretory component, a product of the epithelial cells
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Submucosa
Plasma
cell
Dimeric
IgA
IgA
receptor
Lumen
Epithelial
cells
Vesicle Enzymatic
cleavage
Secretory
component
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IgE
Present at low concentrations
Responsible for allergy
Binds to receptors on mast cells
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IgD
Principle role seems to be as a membrane Ig
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All isotypes can exist as both secreted
and membrane bound antibodies.
The two forms differ in their carboxy
terminal sequence.
Secreted antibodies have a hydrophilic
terminus.
Membrane antibodies have a hydrophobic
sequence which inserts into the plasma
membrane and a short cytoplasmic tail.
All isotypes can exist as both secreted and membrane bound antibodies