antibody affinity maturation and class switch

these cards are missing the details of how class switch recombination works

what do B cells go through in the germinal center?

they undergo V-region somatic hypermutations and antibody class switch

what does V-region somatic hypermutation lead to?

leads to enhanced antibody affinity for the B cell

what is affinity maturation?

process where repeated exposure to the same antigen results antibodies of successively greater affinities.

what recombination events occur for class switch?

Class switch involves recombination events that bring the C-region gene adjacent to the rearranged V-region so that a different class of immunoglobulins is produced by a B cell.

what is the inducer of mutations in the Ig gene?

Activation-induced cytidine deaminase (AID)

where is AID selectively expressed?

it is selectively expressed in the germinal centre B cells.

how does AID induce mutations?

AID acts by deamination of cytidine (C) to uracil (U) in DNA of the immunoglobulin locus.

The presence of U in DNA can trigger several types of DNA repair – including the mismatch repair and the base-excision repair – which further alter the DNA sequence.

what can AID deaminate?

can only deaminate C residues in single-stranded DNA molecules (when DNA double helix is unwound for transcription)

where can targeting of immunoglobulin genes occur?

only occur in the germinal centre B cells where AID is expressed and in the actively transcribed rearranged V regions.

how is somatic hypermutation initiated?

When C residues in the immunoglobulin V regions are deaminated, somatic hypermutation is initiated.

how is class switch recombination initiated?

When C residues in the switch regions are deaminated, class switch recombination is initiated

what happens in mismatch repair? (somatic hypermutation)

the U residue is detected by proteins MSH2 and MSH6 to recruit nucleases that remove the U residue along with several adjacent nucleotides. This is followed by a fill-in ‘patch repair’ by the error-prone ‘translesion’ DNA polymerase (polη), which introduces mutations at A:T base pairs.

what happens in base-excision repair? (somatic hypermutation)

uracil-DNA glycosylase (UNG) removes the uracil base from the U residue to create an abasic site in the DNA. During the next round of DNA replication, random insertion of nucleotides opposite the abasic site leads to mutations at the site of the original C:G base pair.

which B cells are selected and expanded in germinal centers?

Only B cells with mutations that improve BCR affinity are selected and expanded in the germinal centres. T_FH cells aid in the selection of high-affinity B cell mutants

where do positively selected mutations tend to cluster?

in the CDRs of the immunoglobulin V-region genes.

how is a single stranded DNA nick repaired?

using homologous sequences flanking the immunoglobulin gene as a template results in gene conversion.

what happens if the single-stranded DNA nicks are made in the switch (S) regions upstream of the constant (C) regions of the Ig gene?

repair commences though generation of double-stranded DNA breaks and recombination events bringing the C-region gene adjacent to the recombined V-region, resulting in class switch.

how is the selection of a particular C region regulated?

it is regulated by the cytokine produced by T_FH cells in the germinal centre.

how do cytokines induce class switch?

by inducing the production of RNA transcript through the S-region that lie immediately upstream of each H-chain C gene segment.

how many class switch rcombinations can occur?

In any given germinal center B cell, only one class switch recombination can occur