Lecture 2 - B Cell Pathology Flashcards
How long after vaccination do serum IgG levels stay elevated?
A very long time. Tens of years
Another name for isotype switching
Class switch recombination
Features of secondary antibody memory response
1)
2)
3)
1) Faster kinetics
2) Greater magnitide
3) Higher affinity
Symptoms of CD40 and CD40L deficiency
Normal IgM levels
Very low IgG, IgA levels
No B cell memory response
Normal T cell levels
Features of cytophotometry of hyper IgM individuals
1)
2)
1) CD27 absent (top left)
2) Higher than average IgM levels (bottom)
Surface marker expressed by memory B cells
CD27
On which cells is CD27 expressed?
Memory B cells
When do clinical symptoms of hyper IgM normally present?
Between one and two years of age
Clinical presentation of hyper IgM syndromes 1) 2) 3) 4)
1) Recurrent URT and LRT bacterial infections
2) Lung infections by cytomegalovirus or cryptococcus fungi
3) GIT problems (malabsorption, diarrhoea) reported in some patients
4) Often enlarged spleen, tonsils, lymph nodes
Which part of the spleen is lymphocyte-rich?
White pulp
Which cell types are common in white pulp?
Lymphocytes
Do lymph nodes contain much red pulp?
No
Structure of white pulp 1) 2) 3) 4) 5)
1) Central arteriole
2) Periarteriolar lymphoid sheath
3) Follicle
4) Marginal zone
5) Red pulp (surrounding)
Function of the central arteriole in spleen
To bring blood through the white pulp
Function of periarteriolar lymphoid sheath in white pulp
Rich in CD4 and CD8 lymphocytes
Function of follicle in spleen
Rich in mature B cells and follicular dendritic cells
Function of the marginal zone in spleen
Rich in macrophages and B cells
Separated from follicle by marginal sinus
Early stages of T and B cell activation
1) Antigen enters into lymphoid organ. Enters intact into B region, is processed and presented at T
2) Antigen-specific T or B cells contact antigen or are presented antigen
3) Chemokine receptor expression is changed. B cells express CCR7. T cells express CXCR5
4) Activated T and B cells move towards boundary between periarteriolar lymphoid sheath and follicle.
Outcomes of encounter of activated T and B cells in a lymphoid organ
1) Plasma cell proliferation
2) Germinal centre formation
How do B cells become plasma cells or form germinal centres?
Need correct T cell signalling
Transcription factor for B cells to become plasma cells
Blimp1
Transcription factor for B cells to form germinal centre
Bcl6
What does Blimp1 do?
Transcription repressor.
Shuts off B cell program, allows plasma cell program.
What does bcl6 do?
Transcription repressor.
Promotes cell cycle
Inhibits response to DNA damage
What is a germinal centre?
Where mature B cells proliferate, produce antibodies, undergo somatic hypermutaiton
Composition of germinal centres
95% B cells
5% CD4 T cells
1% Follicular dendritic cells
Functions within a germinal centre 1) 2) 3) 4) 5)
1) Clonal expansion
2) Isotype switching
3) Somatic hypermutation
4) Affinity maturaiton
5) Memory formation
Difference between somatic hypermutation and affinity maturation
Somatic hypermutation is mutations in V region
Affinity maturation is selection of somatically hypermutated V regions with the greatest affinity for antigen
Why is IgM the first antibody produced?
Pentameric, so 10 binding sites
High avidity for epitope, despite low affinity
Difference between IgM and IgG in terms of affinity and avidity
IgM has greater avidity
IgG has fewer binging sites, but has higher affinity for epitope
Which antibodies exist in mucosa?
IgA1, IgA1, IgM (to a lesser extent)
Which antibodies can cross the placenta?
IgG1, IgG2, IgG3, IgG4 (kind of)
Which antibodies result in complement activation?
IgG1, IgG2, IgG3, IgM
Which antibodies can sensitise mast cells?
IgE
Most common antibody in the blood
IgG1
Least common antibody in the blood
IgE
IgG in order of most common to least common
IgG1
IgG2
IgG3
IgG4
IgA in order of most common to least common
IgA1
IgA2
Does class switch recombination affect binding affinity?
No
How does class switch recombination take place?
1) Each C region has a Switch region upstream of it. All S regions are homologous
2) AID enzyme recognises Switch regions, makes a nick in dsDNA
3) Double strand breaks are brought together. DNA between S-S forms a loop
4) Loop is excised
What is class switch recombination?
Deletional recombination mediated by S-S recognition
Where does class switch recombination take place?
Only at the heavy chain locus
What is AID?
Activation Induced cytidine Deaminase
How does activation induced cytidine deaminase work?
1) Recognises Switch region
2) Deaminates cytosine to urasil
3) Urasil is removed from DNA, leaving a staggered double stranded break
4) Staggered double stranded break is homologous with all other S regions cut by AID
How is DNA repaired after cutting by AID?
Using similar DNA repair enzymes to V(D)J recombination
Role of AID in somatic hypermutation
1) Replaces C with U
2) Error-prone DNA repair enzymes recruited
3) Mistakes that improve affinity are selected for
Stages of affinity maturation
1)
2)
3)
1) In early germinal centre. No somatic hypermutation taking place. Isotype switching
2) SHM gene activated, leads to somatic hypermutation
3) Selective proliferation of B cells with greater affinity. Rest of B cells die
Surface proteins On B and T cells involved in isotype switching and somatic hypermutaiton
1) CD40L (T cell), CD40 (B cell)
2) TCR (T cell), MHCII (B cell)
3) ICOS (T cell), ICOSL (B cell)
Effect of CD40-CD40L interaction
1) Within B cell, NEMO activates NFkappaB
2) NFkappaB activates AID
Is CD40L deficiency autosomal or sex-linked?
X-linked
Is CD40 deficiency autosomal or sex-linked?
Autosomal recessive
What causes X-linked hyper IgM syndrome with anhydrotic ectodermal dysplasia?
Issues with either NEMO or NFkappaB
What do issues with NEMO or NFkappaB result in?
X-linked hyper IgM syndrome with anhydroptic ectodermal dysplasia
What causes autosomal recessive AID deficiency?
Lack of functional AID
Effect of CD40L deficiency 1) 2) 3) 4)
1) Defective B cell production
2) No germinal centres
3) No memory
4) Defective dendritic cell activation
Effect of CD40 deficiency
1)
2)
3)
1) Defective humoral immunity
2) No germinal centres
3) No memory
Effect of AID deficiency
1)
2)
3)
1) Germinal centre formation
2) No isotype switching
3) No switched memory
Effect of defective NFkappaB signalling
1) Abrogates signals from CD40
2) No germinal centres
3) No somatic hypermutation
4) No isotype switching
Mode of NFkappaB inheritance
X-linked
Autosomal dominant
Therapy for hyper IgM syndrome
1) Intravenous Ig infusion
2) For X-linked HIGM, bone marrow transplant
Common variable immune deficiencies
1)
2)
3)
1) Characterised by hypogammaglobulinaemia
2) Recurrent pyogenic infections by capsulated bacteria
3) Normally manifests in early adulthood
Definition of hypogammaglobulinaemia
IgG under 3g/L
IgA under 0.05g/L
Frequency of common variable immune deficiencies
1/25,000
Number of common variable immune deficiencies that have an identified genetic basis
~20%
What occurs in the germinal centre?
Isotype switching, affinity maturation
How much can affinity maturation increase the affinity of an antibody for an antigen?
10,000 times
How does somatic hypermutation occur? 1) 2) a) b) c)
1) AID replaces C with U in variable region
2) One of three things can happen
a) U is converted to T. Adjacent G is converted to A
b) Excision repair
c) Mismatch repair
There is a chance in excision and mismatch repair that a mistake will be made. This is a mutation
Where do somatic hypermutation mutations cluster?
At CDR1, CDR2 or CDR3
Which T cells express ICOS and CD40L?
Activated T cells
Which B cells express ICOSL and CD40?
All B cells
What are two proteins involved in DNA repair?
PMS2
UNG
What does UNG do?
Removes urasil form DNA
Which enzyme removes urasil from DNA?
UNG
