5.5 en Immune Response-2014

8/19/2019 5.5 en Immune Response-2014

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IMMUNE RESPONSE

BiochemistryLecture 5.5

Prof. Dalė Vieželienė

Department of Biochemistry

E-mail: [email protected]


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• Uses two basic strategies

Humoral Immunity

- works to eliminate antigens that are extracellular

Cellular Immunity

- deals with antigens within host cell

OVERVIEW OF ADAPTIVE IMMUNITY


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Humoral

Immunity


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IMMUNOGLOBULINS Ig

(ANTIBODIES)

Glycoprotein molecules that are produced by plasma cells in

response to an immunogen and which function as antibodies

Receptors for antigen on B-cells When secreted – immunoglobulin (antibodies)

One B-cell makes only one kind of antibody

Immunoglobulins comprise approx. 20 % proteins of blood

plasma


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• Antigen - any structure which el ici ts an immuneresponse

• Recognition is a central feature of the immuneresponse, required for detection and elimination ofdangerous organisms (pathogens)

• This recognition is mediated by specific antigenreceptors which bind to structures (antigens) on, orderived from, pathogens.

• Antigen receptors can be secreted or cell-associated

Antigen recognition is mediated

by specific antigen receptors


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Antigenic determinant (epitope)


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Structure of Immunoglobulins

•2 identical Light chainsκ or λ – each about 220

amino acids

•2 identical Heavy chains -a d e g

or m -

each about

450 amino acids

2 λ + 2γ

•Disulfide bonds:• Inter-chain

• Intra-chain

•

Oligosaccharides

CH1

VL

CL

VH

CH2 CH3

Hinge Region

Carbohydrate

Disulfide bond

•Variable & Constant Regions

VL & CL

VH & CH

• Hinge Region

•Domains (Domains are folded,compact, protease resistant

structures)

VL & CLVH & CH1 - CH3 (or CH4)


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A typical antibody molecule

CHO CHO

•Fab - fragment

antigen binding

•Fc - fragment

crystallisable

Fc – effector

functions. Fc

interacts with

innate effector

mechanisms and

greatly amplifiesthem

Carbo-

hydrate

Fab

Fc

Immunoglobulins are

Bifunctional Proteins


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Pepsin cleavage sites - 1 x (Fab)2 & 1 x Fc

Papain cleavage sites - 2 x Fab 1 x Fc


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Antigens can bind in

pockets or grooves or on

extended surfaces in the

binding site of antibodies

by non-covalent forces


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Why do antibodies need an Fc region?

• Detect antigen

• Precipitate antigen

• Block the active sites of toxins or pathogen-associated

molecules

• Block interactions between host and pathogen-associated

molecules

The (Fab)2

fragment can:

• Inflammatory and effector functions associated with cells

• Inflammatory and effector functions of complement

• The trafficking of antigens into the antigen processing

pathways

The Fc fragment can activate:


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Consequences of antibody-antigen

binding

A. Viral Inhibition: virus preventing it from attaching to cellB. Neutralization: make toxins unable to bind to cellsC. Opsonization: antibodies bind to antigen and facilitate

attachment of phagocytic cells

D. and E. Agglutination and Precipitation: antibodies bind toantigen and get them into clumps, then one big “mouthful”for phagocyte

F. Phagocytosis: Fc portion of antibody encouragesphagocytosis

Complement Activation: binding of antigen to antibody cantrigger one pathway of complement cascade


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The complement system

Interacting set of enzymes (proteases) that upon activation giverise to cascade of reactions culminating in the destruction of

pathogens and infected cells


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Classes of immunoglobulins

(depend on H chain)


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• Classes: IgM, IgG, IgD, IgA, IgE

• Subclasses - IgG1, IgG2, IgG3, IgG4

ANTIBODY CLASSES AND SUBCLASSES

(ISOTYPES)

MONOMER PENTAMER MONOMER DIMER MONOMER

MONOMER WHEN

ACTING AS B-CELL

RECEPTOR FOR ANTIGEN


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IgG

(Monomeric)

– Major serum Ig

– Major Ig in extravascular spaces – 70-80% of total serum

antibody

– Half-life in serum – 23 days

– Placental transfer (IgG2) – Fixes complement (IgG4)

– Binds to Fc receptors (IgG2, IgG4)

• Phagocytes - opsonization

IgG1, IgG2 and IgG4 IgG3


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The pentameric IgM molecule

irst line of defence

Extra domain (CH4)J chain

First Ig made by fetus and B cells

5-10% of total serum antibody

Half-life in serum – 5 days

Fixes complement

Agglutinating Ig

Binds to Fc receptors

B cell surface Ig


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Dimeric IgA- protection of the mucosa

•Serum – monomer

•Secretions (sIgA) - major secretory Ig (Mucosal or Local Immunity)

•10-15 % of total serum antibody (if mucous membranes and body secretions

included, percentage is much higher)•Half-life in serum – 6 days

Tears, saliva, gastric and pulmonary secretions

Dimer: J chain; Secretory component

Does not fix complement (unless aggregated)

Binds to Fc receptors on some cells


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IgD (Monomer)

– 0,2% of total serum antibody


– B cell surface Ig, blood, lymph

– Does not bind complement, serum function

not known

I l b li St t


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•Secreted antibody

Neutralisation

Arming/recruiting effector cells

Complement fixation

• Cell surface antigen receptor on B cells

Allows B cells to sense their antigenic

environment

Connects extracellular space with intracellular

signalling machinery

Immunoglobulin Structure-

Function Relationship


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IgE (Monomer)

– Extra domain CH4

– Least common serum Ig

– 0,002% of total serum antibody


• Binds to basophils and mast cells

– Allergic reactions – Parasitic infections (Helminths)

• Binds to Fc receptor on eosinophils

– Does not fix complement


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Diversity: Antibody diversity is

generated by:

Multiple genes encoding both VL and VH

Segmental joining of additional DNA segments to form the

mature VL and VH genes

Addition of nucleotide bases during the joining event

Random association of Heavy and Light chains (facilitated by

the VC, CH1 disulfide bond)

Somatic (body) mutation of mature heavy and light chain

genes after antigen stimulation generate higher affinity

antibodies


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Variable domains in immunoglobulins are made

by combinatorial joining of gene segments


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Immunoglobulins and Development:

Fetal synthesis of IgM and IgA begin during the 5th month.

Immature B-cells express IgM on surface. IgM is monomeric on

surface.

Mature B-cells express IgM and IgD on surface.

Plasma cells can secrete IgM, IgG, IgA, IgE (all but IgD)

Memory cells display IgG, IgA, IgE, alone or combined with

IgM. These are usually higher affinity than the orginal B-cell

clone because of affinity maturation via somatic cell mutation

Th i l b li f il f


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The immunoglobulin superfamily of

proteins

• Variety of other proteinswhich exhibit amino acid

sequence homology with Ig

also contain Ig-like domains

and are considered as

members of the

immunoglobulin superfamily

• The genes that encode these

proteins have evolved from a

common ancestor gene coding

for a single domain.

• The structural hallmark of

immunoglobulin structure is the

existence of globular „domains with

β-pleated sheet folding pattern.


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The immunoglobulin superfamily of proteins


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B Cell Antigen Receptor (BCR)

Ig-β Ig-α Ig-α Ig-β


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B-cells and Antibody Response

• B-cell receptor binds to antigen

• One of two things happen:

– B-cell needs confirmation by T-cell to begin

responding

– B-cell does not need confirmation by T-cell to

begin responding


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When B-cell does not need

confirmation from T-cell• B-cell receptors bind epitopes

• B-cells respond by proliferating, producing

antibody and differentiating into memory B-cells


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When B-cell needs confirmation from T-cell


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CELLULAR IMMUNITY

Types of T cells (cellular immune


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Types of T cells (cellular immune

response)

• Also called CD8 T cells

• Once activated, induce apoptosis in

“self” cells infected with virus; destroy

cancerous host cells

• Distinguish infected “self” cells, because

these cells present peptides on surface in

MHC class I molecule

• Also called CD4 T-cells

• Antigen presenting cells, present

antigen to T-helper cells in MHCclass II

• If recognize antigen presented as

foreign, activate macrophages,

release cytokines that recruit other

cells of immune system, stimulate NK cells activate B cells

T Cell Receptor (TCR)


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TcR molecule resembles Fab and is monovalent

T Cell Receptor (TCR)

The TCRs are

transmembrane,

insoluble proteins.

Approximately 105

TcR molecules arepresent on the surface

of a T cell.

•Consists of an α and

β chain, or a γ and δ

chain.

• In human majority of T cells express the αβ heterodimer;

the remaining T cells (small percentage) express the γδ

heterodimer (no αδ, or γβ T cells exist).

• Each chain has a variable (V) and a constant domain (C).

TCR Structure


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TCR Structure

•The variable domains in both chains contain threehypervariable regions which are equivalent to CDRs present in

Ab light and heavy chains.

• Transmembrane domains of both chains contain positively

charged amino acid residues

• The most αβ TCRs interact with peptide antigens presented by

MHC molecules.

• Certain αβ T cells react with nonpeptide antigens

(carbohydrate and lipids)

• In contrast to αβ TCR, γδ TCR exhibits limited diversity • The γδ T cells react with antigen (e.g. phospholipid antigen of

Mycobacter ium tuberculosis ) that is neither processed or

presented in the context of a MHC molecule.


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TCR αβ chain receptor gene

rearrange similar to antibody genes


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C i f TCR d BCR


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Comparison of TCR and BCR

(antibody)

BCR TCR

Ligand Any structure peptide & lipid

Bind native ligand Yes No

Ag processing No Yes

MHC restriction No Yes

Somatic mutation Yes No

Affinity for ligand Low to very high Low

Co-receptors No Yes

T ll i th ti l h it i


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• T cells recognize the antigen only when it is

presented by self-MHC molecule.

•This phenomenon, called self -MHC

restriction, differentiates recognition of

antigen by T cells and B cells.The MHC is highly polymorphic from individual

to individual

Single receptorrecognizes

an alteration in self-

MHC molecules induced

because of association

with foreign antigen.


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Class I molecules are expressed on the surfaces of virtually all

nucleated cells at varying densities

Class II molecules are more restricted to cells of the immune

system, primarily B lymphocytes and monocytes.

Class I molecules function at the effector phase of immunity by

presenting antigens to CD8+ T cells, which generally have cytotoxic

or suppressor function Class II molecules present antigenic fragments to the CD4+ inducer

(or helper) T cells on antigen presenting cells such as macrophages

MHC class I molecules present peptides derived from cytosolic

proteins; the pathway is called the cytosolic or endogenous pathway

MHC class II molecules present peptides derived from extracellular

proteins; the pathway is called the endocytic or exogenous pathway.


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Epitopes recognized by TCR does not

need to lie on the surface of the protein


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Interplay between T-cells and B-cells

5.5 en Immune Response-2014

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