Week 7 S - Immunogenetics

Flashcards on Immunogenetics and Antibody Diversity

Immunogenetics

The mechanisms by which the body generates antibody (Ab) and T-cell receptor (TCR) diversity.

Structure of immunoglobulin loci

Three loci (k, l light chain; heavy chain). Each locus contains repeated gene segments – V, (D - heavy chain), J, C. Structure differs between germline and B cells. In B cells DNA is rearranged and some lost

DNA loss during rearrangement

V(D)J joining involves loss of DNA; V(D)J joining is recombinational within a DNA strand; loss of loops of DNA

RSS (recombination signal sequences)

DNA sequences next to V, D, and J segments; each consists of a nonamer and a heptamer separated by 12 or 23 bp

Mechanism of V(D)J rejoining

Recognition of RSS sequences. Two RSS and adjacent gene segments brought into proximity (synapsis). Recombination is initiated by the lymphocyte-specific RAG1 and RAG2 double strand nucleases. The ds break on the coding side folds back to give a hairpin.

Recombination variation sources

Rejoining is not precise; on rejoining, some arrangements are nonproductive; further variation by the addition of either P or N nucleotide addition

P-nucleotide addition

Random cleavage of hairpin by endonuclease generates sites for the addition of P-nucleotides

N-nucleotide addition

Optional addition to H-chain segments of N-nucleotides by TdT

N-nucleotide addition

N-nucleotide addition occurs only in heavy-chain DNA.

Further changes during antibody maturation

Isotype switching in the heavy chain and affinity of the antibody increases. Both effects are dependent on transcription and can involve AID (activation-induced cytidine deaminase)

Activation induced cytidine deaminase (AID)

De-aminates selected cytosine changing them to uracil both in RNA and also DNA (this is repaired to either AT or GC or completely excised!).

Isotype switching

Initial heavy chain transcript contains both IgM and IgD constant regions. Selection of polyadenylation site selects between IgM and IgD. Membrane-bound and secreted forms are produced by the same mechanism.

Alternative polyA sites

Use of alternative polyA sites results in either secreted or membrane bound form

Isotype switching

Switch regions exist between the different C regions of the heavy chain locus. DNA loops out and recombination occurs between switch regions. Switch sequences are GC-rich with C on the template strand and G on the coding strand.

Possible mechanism for Isotype Switching

During transcription, the displaced coding strand is affected by AID, and the U residues produced are repaired by the mismatch repair system – this causes ds breaks which allow recombination. Similar effects in the hypervariable regions lead to mutations.

Increasing affinity

Frequency of mutation increases during the course of the primary response and is higher following subsequent immunisations. Note that most of the mutation is in CDR1 and 2

Gene Conversion

Gene conversion with pseudogenes upstream of functional V genes can diversify the repertoire; seen in birds and rabbits, but not humans.

Allelic Exclusion

Allelic exclusion ensures that each B cell expresses only one heavy chain and one light chain allele, preventing multiple specificities.

Receptor Editing

Receptor editing modifies autoreactive receptors by replacing the light chain, ensuring self-tolerance.

Central Tolerance

The process by which B cells that bind to self-antigens in the bone marrow undergo receptor editing or apoptosis.

T-Cell Dependent B Cell Activation

Mediated by interactions between CD40 on B cells and CD40L on T cells; essential for isotype switching and affinity maturation.

Affinity Maturation

Occurs in germinal centers of secondary lymphoid organs; B cells undergo somatic hypermutation and selection for higher affinity to antigen.