4th SEMINARTHE ADAPTIVE IMMUNE RESPONSE:

ANTIGENS AND ANTIGEN-SPECIFIC RECEPTORS

SENSING

RECOGNITION

SIGNALING

RESPONSE

INNATE IMMUNITY

Cells

Receptors

Signaling pathways

Cell-Cell collaboration

Effector functions

DEFENSE SYSTEMS

ADAPTIVE IMMUNITY

SENSING

RECOGNITION

SIGNALING

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 non-

responsiveness)

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

• Degradabilityantigen 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

proteinspolysaccharideslipidsDNAsteroidsetc. (many artificial molecules)

cell- or matrix-associated or soluble

Recognized by T cells

proteins mainly (8-23 amino acids)

requires processing and presentation by APCs

ANTIGEN RECOGNITION ≠ CELL ACTIVATION

H HL L

H H

L L

Secreted IgAntigen-specificsoluble protein

EFFECTOR MOLECULE

Membrane-bound IgAntigen-specificReceptor (BCR)RECOGNIZING

MOLECULE ba

signaling

B CELL PLASMA CELL

Antigen binding

BCR AND ANTIBODY

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)

• 2x light chain (dark blue)

• Variable regions antigen binding

• Constant regionshinge region

carbohydrate

disulfide bond

CH1

VL

CL

VH

CH2 CH3

Epitope

CDR1 CDR2CDR3

CDR1CDR2

CDR3

Light chain

Heavy chain

CDR3

FR1 FR2 FR3 FR4

CDR1CDR2

varia

bili

ty in

de

x

25 7550 100aminoacid sequence N – C terminal

150

100

50

0

HYPERVARIABLE REGIONS

CDR = Complementarity Determining Region – those amino acids of the variable regions that directly interact with the epitope

FR = frame – those amino acids of the variable regions that do not interact directly with the epitope (stabilizer function)

DIFFERENT VARIABLE REGIONS DIFFERENT ANTIGEN-BINDING SITE

DIFFERENT SPECIFICITY

(Classes/subclasses)

Sequence variability of H/L-chain constant regions

Sequence variability of H and L-chain variable regions

(individual, clone- specific)

Allelic variants

isotype idiotypeallotype

• 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)

HUMAN IMMUNOGLOBULIN CLASSESencoded by different structural gene segments (isotypes)

Ig isotype Serum concentration

Characteristics, functions

12-14 mg/ml

Major isotype of secondary (memory) immune response

Complexed with antigen activates effector functions (Fc-receptor binding, complement activation

Trace

amounts

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

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

• Fab• antigen binding• valence = 1• specificity determined

by VH and VL

• Fc• effector functions

IMMUNOGLOBULIN FRAGMENTS STRUCTURE/FUNCTION RELATIONSHIPS

papain

Fc

Fab

VH

VL

• F(ab’)2

- Bivalent!

IMMUNOGLOBULIN FRAGMENTS STRUCTURE/FUNCTION RELATIONSHIPS

pepsin

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 Fc-receptors 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

IgG

IgM

IgA

A F T E R B IR T H

breas t milkIgA

0

1 0 0 %( a d u l t )

3 3y e a r

2 546 a d u l t9 1m o n t h

maternal IgG

B E F O R E B IR T H

PRODUCTION OF IMMUNOGLOBULINS

Epithelialcell

JC C

SS

SS

C

C

SS

SS

CC

ss

JC C

SS

SS

CC

SS

SS

CCss

JC C

SS

SS

C

C

SS

SS

CC

ss

pIgR and IgA areinternalised

‘Stalk’ of the pIgR is degraded to release IgA containing part of the pIgR (the secretory component)

JC C

SS

SS

C

C

SS

SS

CC

ss

IgA and pIgR are transported to the apical surface in vesicles

B cells located in the submucosa produce dimeric IgA

B

Polymeric Ig receptors are expressed on the basolateral surface of epithelial cells to capture IgA produced in the mucosa

SECRETORY IgA AND TRANSCYTOSIS

MUCUS

JC C

SS

SS

C

C

SS

SS

CC

ss

JC C

SS

SS

C

C

SS

SS

CC

ss

JC C

SS

SS

C

C

SS

SS

CC

ss

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)

TCR

APC

MHC

TCR TCR

APC

MHC

APC

MHC

MHC RESTRICTION OF T CELL RECOGNITION

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

3. Another peptide bound to the same MHC is not recognized by the same TCR

1. 2. 3.

MHC

TCR

CD3

APC

s s

ss

ss

s

ss

V V

C C

s

α β

ss

ss

ss

ss

CD3

s s

ε δ ε γ

ζ ζ

ITAMImmunoreceptor Tyrosine-based

Activation Motif

ACTIVATION

ACTIVATION OF TCR AND BCR

Antigen

BCR

Antigen

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.

conventional antigen

monoclonal/oligoclonal

T cell response

1:104 - 1:105

superantigen

polyclonal

T cell response

1:4 - 1:10

107 – 108 / 1011 1010 / 1011activated T cells

SUPERANTIGENS