Adaptive ImmunityAdaptive Immunity

•Central objective:

•Protect against foreign invaders

•Create memory of invasion to prevent recurrent infection

Adaptive ImmunityAdaptive Immunity

Central problems:

•Distinguishing Self vs. Non-self

•Generating Diversity

Adaptive ImmunityAdaptive Immunity

Unique Features:Specific receptors recognize foreign invaders:– B cell receptor (BCR)

– T cell receptor (TCR)

Adaptive ImmunityAdaptive ImmunityThere are two forms of immunoglobulin (also called antibodies) which function as Ag receptors.

• Form I is located on the B cell surface (also called the BCR) and

• Form II is secreted from the B cell.

• The TCR is only expressed in the cell surface.

Basic Features of Ig Basic Features of Ig ProteinsProteins

•2 H chains and 2 L chains•Bilateral symmetry•Globular domains-each domain is 100-110 aa residues•L chain has 2 domains•H chain has 4-5 domains including a hinge

-the hinge is a flexible stretch of polypeptide chain

ANTIGENS (Ag), ANTIGENS (Ag), IMMUNOGEN, HAPTENIMMUNOGEN, HAPTEN

Antigen: A substance capable of interacting with an Ag receptor.

Epitope

Hapten/CarrierHapten/CarrierA haptenhapten is a substance which can physically interact with antibodies (Ab) or with TCR but does NOT elicit an immune response from the B cells or T cells, respectively. The carriercarrier is immunogenic.

Haptens/CarrierHaptens/Carrier•Small organic molecule of simple structure.

•Does not induce an antibody response by themselves.

•However, can induce Ab response when coupled to a •protein carrier.

•Three types of Abs produced:1) anti-Hapten2) anti-protein-carrier3) anti-hapten+carrier

IMMUNOGENIMMUNOGEN

An immunogen is a substance which interacts with the B cell receptor (BCR) or the T cell receptor (TCR) and elicits a response from these cells. Thus, an immunogen is an Ag which causes either a B cell mediated humoral response or a T cell mediated response or both.

CHARACTERISTIC FEATURES CHARACTERISTIC FEATURES OF AGOF AG

•Ags interact with the ligand binding sites in Ab and TCR.

•Ags can be large molecules which contain several epitopes which are substructures (also called antigenic determinants) capable of interacting with the Ab receptor or the TCR.

•The physical parameters of the Ab (TCR) binding site define the Ag

binding constants (Ka and Kd).

CHARACTERISTIC FEATURES CHARACTERISTIC FEATURES OF AGOF AG

Ag valence is the number of epitopes or reactive sites on the Ag molecules. A single Ag might have multiple epitopes.

Epitopes

BCR and TCR are key to BCR and TCR are key to understanding immune understanding immune

system functionsystem function

Figure 1-27

CHARACTERISTIC FEATURES CHARACTERISTIC FEATURES OF AbsOF Abs

In the body, Ig is a pool of proteins with a diverse set of binding capabilities.

The Ag Binding The Ag Binding ConundrumConundrum

The immune system must accommodate at least 10e6 Ag binding specificities

How does it do it?How does it do it?

Diversity of Ag Diversity of Ag Binding Resides in Binding Resides in

the V Regionsthe V RegionsThe N-terminal portion of the H and L chains fold to form the Agbinding pocket.

Substructure of V Substructure of V regionsregionsin Igin Ig

Diversity in V regions is non-random and primarilylocated in the hyper-variableregions (or complementaritydetermining regions [CDRs]).

Diversity of Ig Diversity of Ig BindingBinding

CDRs form the Ag binding domain of the V region.

Generation of Ab Generation of Ab DiversityDiversity

Where does diversity of Ag Where does diversity of Ag binding come from?binding come from?

One Source of One Source of Diversity:Diversity:

Combinatorial Association of 500 L chains with 1000 H chain equals 5x10e5

Combinatorial Combinatorial Association of H Association of H

and L chainsand L chains•Assume: 500 L chains + 1000 H chains= 5x10e5 antigen binding sites

• However, there are only ~30,000 genes in the human genome

Another Source of Another Source of Diversity: Diversity: HypothesisHypothesis

L and H chain V genes are L and H chain V genes are composed of multiple gene composed of multiple gene segmentssegments

i) Land L chain V genes are generated from V and J segments

ii) H chain V genes are generated from V, D and J segments

The Tonegawa The Tonegawa Experiment:Experiment:

circa circa 11976976

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The Tonegawa The Tonegawa ExperimentExperiment

1 2 3 4 5 67

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20

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80

100

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Probes

Generation of Generation of Diversity of BCRDiversity of BCR

L and H chain V genes are composed of multiple gene segments

i) L chain V genes are generated from V and J segments

( 30 V( 30 V/4J = 30x4=150)/4J = 30x4=150)

ii) H chain V genes are generated from V, D and J segments (50 VH/30 D/ 4 J= (50 VH/30 D/ 4 J= 50x30=1500x4=6000)50x30=1500x4=6000)

L and H chain V genes are composed L and H chain V genes are composed of multiple gene segmentsof multiple gene segments

Mechanism of V(D)J Mechanism of V(D)J JoiningJoining

DNA rearrangement:DNA rearrangement:

via deletion looping out mechanism

Deletion andlooping out

Inversion

Other Sources of Other Sources of Diversity...Diversity...

Note:Assembly of V genes from mini-gene segments and combinatorial assoc. is insufficient to account for Ab diversity

Mechanism of V(D)J Mechanism of V(D)J JoiningJoining

The VDJ joining process is sloppy and generates additional diversity at the junctions of the joined DNA segments. This raises the potential diversity to 2x10e7 Ag binding sites.

Mechanism of V(D)J Mechanism of V(D)J Joining:Joining:

VDJ joining conforms to the 12/23 Rule

V(D)J V(D)J Joining is Joining is Catalyzed Catalyzed by the RAG by the RAG Enzymes:Enzymes:

Mechanism of V(D)J Mechanism of V(D)J Joining:Joining:

Hairpins Hairpins are opened are opened with the with the help of help of ArtemisArtemis

Hairpins cont…Hairpins cont…

•Hypomorphic mutations give rise to a partial loss of function

•Ommens syndrome arises from a hypomorphic mutations in the Rag genes

•A leaky form of SCID arises from a hypomorphic mutation in DNA-PK

Hypomorphic MutationsHypomorphic Mutations

Generation of TCR Generation of TCR Diversity:Diversity:

TCRs are generated by V(D)J joining of mini-gene segments

Primary Primary Immunodeficiency:Immunodeficiency:

Severe combined Severe combined immunodeficient: scidimmunodeficient: scid

Phenotype: absence of B cells and T cells, thus no humoral or cell mediated immunity

Underlying cause?

BCR and TCR Break the BCR and TCR Break the Rules of Mendelian Rules of Mendelian

GeneticsGeneticsMonospecific Ag Monospecific Ag receptors:receptors:

Only one H chain and one L chain is expressed per B/T cellwhereas each cell has 2 alleles for H and L chains

Adaptive ImmunityAdaptive Immunity

•Unique Features:

•Antigen specific receptors:–Exhibit allelic exclusionallelic exclusion–Are monospecificmonospecific with respect to Ag recognition