Lecture 14: Class switching and generation of diversity and memory Flashcards
11/11/2024
What are the light and heavy chains of antibodies composed of?
Light and heavy chains are composed of repeating Ig domains.
What are the N-terminal domains of light and heavy chains called?
variable’ (V) domains. (The remaining are ‘constant’ (C) domains.)
What is the antigen binding site made from?
The antigen binding site is made from VL and VH domain interactions
Does the structure of heavy chains differ considerably?
Yes
Describe the heavy chain structure of IgM
IgM: H chains have a CH4 domain
(VH-CH1-h-CH2-CH3-CH4)5+J.
Extra terminal domain (CH4)- longer heavy chain
J chain aids in the formation of the whole complex
There are multiple disulfide bonds, and the H chain is heavily N-glycosylated.
N-glycans are complex carbohydrates that are added to asparagine residues during folding prior to secretion: they are large and so they hold domains apart, allowing exposure of functional motifs (e.g. complement binding sites).
N glycans near hinge impact the flexibiltiy of the arm
Describe the heavy chain structure of IgG
Different domains have different functions
Cγ1, Cγ2: bind complement components
Cγ2, Cγ3: bind Fc receptors on neutrophils
Cγ3: binds Fc receptor on macrophages and NK
- Doesn’t have many N-glycans. Pushes heavy chains apart, exposing functional domains
Describe the heavy chain structure of IgE
The multiple N-glycans make this a stiff rigid molecule: good for targeting large pathogens, but it cannot cross-link small targets
Additional constant domains
Describe the heavy chain structure of IgA
V flexible, good cross-linker, valency =4
Secreted antibody that goes around as a dimer
Similar to IgG
Not man y N-glycans
Flexibility is useful when it comes to secretion
Class switching summary
Pre B cells in the bone marrow express IgM, membrane-bound.
During maturation, they express IgM and IgD, membrane bound, in lymphoid tissue.
The IgM+ IgD+ B cells are then selected by antigen and undergo clonal selection.
Mature B cells can switch classes from IgM to other Ig classes, whilst maintaining the same specificity for antigen.
To do that requires the same VH domain on a different heavy chain.
There is no protease/protein ligase that can swap domains.
Why do we class switch?
Different antibody classes are required for different immune responses.
Class switching allows B cells to change antibody type without changing antigen specificity.
How does class switching happen?
Heavy chain gene contains different segments (μ, γ, ε, α).
Switch regions (red stars in diagrams) enable DNA recombination.
Looping out of DNA deletes unwanted heavy chain segments.
Somatic recombination moves V region next to a new C region. (the heavy domain hasn’t changed, so same specificty as the orignial IgM)
Expression then relies on the removal of introns from mRNA
Translation can then occur
Allows transition from IgM to IgG, IgE, or IgA.
Describe the clonal selection theory
An antigen activates only those lymphocytes that are already committed to respond to it.
A lymphocyte committed to an antigen displays cell surface receptors that specifically recognise the antigen, even if that antigen has never been encountered before.
Receptors: TCR and BCR. BCR are membrane-bound antibodies.
There are millions of different clones of lymphocytes in the human immune system: before encountering immunogen/antigen, there would be only a few cells in each clone.
Upon encountering antigen to which they are committed, lymphocytes undergo CLONAL EXPANSION and differentiation.
Briefly describe clonal selection
Following infection, individual clones are selected by antigen, based on how well the antigen and the receptor fit together: the result is pathogen-specific lymphocytes are selected from pools of B and T cells.
Briefly describe clonal expansion
The selected clones undergo mitosis, proliferate and differentiate into effector/ plasma cells
What is clonal deletion
Those lymphocytes that react inappropriately with ‘self’ antigens are destroyed.
Pre existing diversity in the adaptive immune system
An antigen activates only those lymphocytes (T and B cells) that are already committed to respond to it.
The antigen-specific receptors (TCR and membrane-bound antibodies for T and B cells respectively) are encoded by unusual segmented genes.
These genes are assembled from a series of gene segments by an unusual form of recombination called SOMATIC GENE RECOMBINATION.
Somatic gene recombination is used for class switching … but a slight modification leads to massive pre-existing diversity
How can there be so many antibodies capable of recognising antigens that have never before been encountered?
It’s all to do with the variable domains that encode antigen specificity.
① There are only 3 antibody genes. There are two classes of light chains, which increases diversity.
Thus the same VH domain can be partnered with variable domains from two classes of light chain, increasing the repertoire of possible binding sites, allowing different antigens to be recognised.
② There are multiple gene segments encoding V domains that can be combined with C domains by somatic recombination. (There are about 4,500 different VH: VL combinations
).
③ The somatic recombination (called V(D)J recombination) used to select a V gene segment is particularly complex and sophisticated and involves selection also of small pieces of ‘diversity’ and ‘joining’ DNA that ultimately link the V domain with the constant domains at the protein level.
This added layer of sophistication means that 3 genes (1H, 2L) can generate
> 1014 proteins with unique potential antigen binding sites
(100,000,000,000,000 different sites!)
What is affinity maturation?
The process by which antibodies change over time to improve specificity and affinity
What is the cause of affinity maturation in antibodies?
accumulation of point mutations in the Variable domains long after the coding sequences have been assembled from the segmented genes.
Where does affinity maturation occur?
Affinity maturation occurs in the lymph nodes. Some of the activated B cells proliferate in the lymphoid follicles and form structures called germinal centres.
Describe the mutation in the germinal centres
The mutation rate in the germinal centres is about 1 million times greater than the spontaneous mutation rate in other genes.
It is also confined to the gene segments that encode V domains.
It is therefore often referred to as somatic hypermutation.
Describe the steps of affinity maturation
① Antigen stimulation causes activation and clonal expansion of B cells with pre-existing fits to the antigen, from a pool of 1012 B cells that are potentially capable of generating 1014 unique V domain combinations.
Class switching allows this selected pre-existing specificity to be transferred to other Ig, providing extra immune functions.
② Some B cells of the expanded clonal population proliferate in germinal centres and undergo somatic hypermutation, generating antibodies with altered V domain specificity.
③ Most of these hypermutated clones are worse than the original, and will not be stimulated by the original antigen: most will die ….
but the rare B cells with mutated BCR versions that have higher affinity for the original antigen will proliferate.
Repeated cycles result in rapid EVOLUTION of a HIGH AFFINITY antibody response. It’s a Darwinian process: survival by selective advantage.
The antibody secondary response
Exposure to antigen (e.g. to pathogen or by immunisation) results in a primary response (antibody, T-cell or both) that appears after a few days, rises rapidly and exponentially and then declines gradually.
Second exposure (up to years later) results in a greater and more efficient secondary response with a short lag period. Thus the immune system has a memory.
And the secondary response is greater and more specific than the primary response, because it is dominated by class-switched antibodies that have undergone somatic hypermutation.
What is immunological memory generated by?
Generated by the primary response
