Week 7 S - Generation of Diversity

Flashcards on Immunogenetics and Generation of Antibody/T-cell receptor diversity

Number of antibody specificities

Estimate of at least 2.5 x 10^7

Estimate of the number of genes in the human genome

3 x 10^4

Number of immunoglobulin loci

3

Partial Answer to Generation of Diversity (GOD)

Random combination of light and heavy chains would demand only 5 x 10^3 chains of each class

Immunoglobulin diversity

Vary in isotype, allotype, and idiotype; allelic exclusion limits diversity

During the antibody response

Heavy chain produced first, specificities switch isotype, affinity and specificity increase, hypervariability is focussed

Allotypic determinants

Subtle differences caused by allelic variation

Idiotypic determinants

Actual binding site

Germ Line Theory

Genome contains a huge number of immunoglobulin genes.

Somatic Variation Theory

Small number of genes that either mutate or recombine to give diversity.

Structure of immunoglobulin loci

3 loci (k, l light chain; heavy chain)

Structure of immunoglobulin loci

Each locus contains repeated gene segments – V, (D), J, C

Structure of immunoglobulin loci

Structure differs between germline and B cells; in B cells DNA is rearranged and some lost

Functional consequences of Immunoglobulin Diversity

Variation is largely combinatorial; diversity comes at a price (many lymphocytes die, changes can be pathogenic); success prevents further change; success leads to gene expression

Allelic exclusion

Only ONE chromosome per cell is expressed

Transcription in germ line

Enhancer is too far from promoter to get transcription, re-arrangement get increase of 10^4 times rate transcription

T-cell receptor diversity

Repertoire is similar = 2.5 x 10^7; number of chains and loci = 2; each locus contains repeated gene segments – V, (D), J, C; structure differs between germline and T cells; variation is largely combinatorial

V(D)J recombination

The process by which T-cell receptors and immunoglobulins increase their diversity.

Enzyme(s) responsible for V(D)J recombination

RAG-1 and RAG-2; only expressed in lymphocytes.

TdT (terminal deoxynucleotidyl transferase)

Enzyme that randomly inserts N nucleotides into the junctions between gene segments.

RSS (recombination signal sequences)

Sequences that flank each V, D, and J gene segment; RSSs are signals for the RAG complex.

12/23 rule

The rule that dictates that a gene segment flanked by a 12 bp spacer RSS can only join to a gene segment flanked by a 23 bp spacer RSS.

Junctional diversity

Diversity in the hypervariable CDR3 regions of Ig and TCR variable regions, brought about by the addition or removal of nucleotides at the junctions between V, D, and J gene segments.

P nucleotides

Palindromic nucleotides created by asymmetrical cleavage of the hairpin structures formed after RAG1/2 cleavage.

N nucleotides

Non-templated nucleotides added by TdT.