Adaptive Immunity Central objective: Protect against foreign invaders Create memory of invasion to prevent recurrent infection

Adaptive Immunity Central problems: Distinguishing Self vs. Non-self Generating Diversity

Adaptive Immunity Unique Features: Specific receptors recognize foreign invaders: –B cell receptor (BCR) –T cell receptor (TCR)

Adaptive Immunity There 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 Proteins 2 H chains and 2 L chains Bilateral symmetry Globular domains -each domain is 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), IMMUNOGEN, HAPTEN Antigen: A substance capable of interacting with an Ag receptor. Epitope

Hapten/Carrier hapten carrier A hapten 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 carrier is immunogenic.

Haptens/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-Hapten 2) anti-protein-carrier 3) anti-hapten+carrier

IMMUNOGEN 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 OF 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 (K a and K d ).

CHARACTERISTIC FEATURES OF 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 understanding immune system function

Figure 1-27

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

The Ag Binding Conundrum The immune system must accommodate at least 10e6 Ag binding specificities How does it do it?

Diversity of Ag Binding Resides in the V Regions The N-terminal portion of the H and L chains fold to form the Ag binding pocket.

Substructure of V regions in Ig Diversity in V regions is non-random and primarily located in the hyper-variable regions (or complementarity determining regions [CDRs]).

Diversity of Ig Binding CDRs form the Ag binding domain of the V region.

Generation of Ab Diversity Where does diversity of Ag binding come from?

One Source of Diversity: Combinatorial Association of 500 L chains with 1000 H chain equals 5x10e5

Combinatorial Association of H and L chains Assume: 500 L chains H chains= 5x10e5 antigen binding sites However, there are only ~30,000 genes in the human genome

Another Source of Diversity: Hypothesis L and H chain V genes are composed of multiple gene segments i) L  and 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 Experiment: circa 1976 B cell Embryo C V+C V Bam HI

The Tonegawa Experiment C V+C Probes

Generation of Diversity 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  /4J = 30x4=150) (50 VH/30 D/ 4 J= 50x30=1500x4=6000) ii) H chain V genes are generated from V, D and J segments (50 VH/30 D/ 4 J= 50x30=1500x4=6000)

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

Mechanism of V(D)J Joining DNA rearrangement: via deletion looping out mechanism

Deletion and looping out Inversion

Other Sources of 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 Joining 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 Joining: VDJ joining conforms to the 12/23 Rule

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

Mechanism of V(D)J Joining: Hairpins are opened with the help of Artemis

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 Mutations

Generation of TCR Diversity: TCRs are generated by V(D)J joining of mini- gene segments

Primary Immunodeficiency: Severe combined immunodeficient: scid Phenotype: absence of B cells and T cells, thus no humoral or cell mediated immunity Underlying cause?

BCR and TCR Break the Rules of Mendelian Genetics Monospecific Ag receptors: Only one H chain and one L chain is expressed per B/T cell whereas each cell has 2 alleles for H and L chains

Adaptive Immunity Unique Features: Antigen specific receptors: allelic exclusion –Exhibit allelic exclusion monospecific –Are monospecific with respect to Ag recognition