Lecture 2 - B cells - Immune Deficiencies 2 Flashcards
What is the phenotype of mice lacking CD40L?
- Unable to undergo isotope switching
- No memory
- Low affinity
What is a combined immune deficiency?
Spans both humoral and cellular immunity
What is the name for humoral deficiencies (as opposed to combined)?
Antibody deficiencies
Describe persistence of B cell immunity
Even 60 years after vaccination, there can be protective levels of antibody in the serum
Describe the antibody response after vaccination
→ Vaccination
• IgM response
• Later, isotope switching to IgG
→ Booster
• Increased levels of IgG
Compare B memory cells in the healthy people and hyper IgM patients
Normal: Many memory cells in serum
HIGM: no memory B cells in their serum
What are the symptoms of Hyper-IgM syndromes?
- Recurrent infections (upper and lower respiratory)
- GIT dysfunction (malabsorption)
- Autoimmune disorders
- Enlarged 2° lymphoid organs
What happens on a cellular level to a B cell when differentiating into a plasma cell?
What brings about this change?
Which other cell mediated this?
Why?
Major organelle reorganisation
- Expression of BLIMP1
- Switching off of B cell program
- Change to plasma cell program
Tightly regulation:
• By CD4+ cells (Tfh)
• This is because making Ab is dangerous: once it’s made, it can’t be unmade
• Need to be sure that the Ab being made is not going to cause problems
What are the various areas in the spleen?
Describe the structure
- Red pulp:
• Erythrocytes
2. White pulp: • Lymphocytes • Surrounding blood vessels a. Follicle: B cells b. PALS: T cells c. Marginal zone: around follicle
Describe the cell distribution within white pulp
- Mature B lymphocytes: follicle
- T lymphocytes: periarteriolar sheath (central)
- Dendritic cells: PALS
- MZ B lymphocytes: marginal zone
Describe the early and late stages of B and T cell activation in lymphoid organs
- B cell clonally selected; Ag on follicular DC; T cell stimulated by Ag presented on DC
- B cell and T cell migrate to boundary of B cell follicle and paracortex
a. B cell upregulation of CCR7
b. T cell upregulation of CXCR5 - TFH cell and B cell interaction:
a. MHC II:peptide – TCR
b. CD40 – CD40L - B cell forms a germinal centre under the action of Bcl-6 and undergoes maturation events:
a. CSR
b. SHM
c. Affinity maturation - Alternatively, some B cells differentiate immediately into plasma cells under the action of BLIMP1:
a. BLIMP1: transcription repressor that turns of B cell program
b. Immediate production of low affinity IgM
What is a germinal centre?
Which cells are present, and in what proportions?
An area within a secondary lymphoid organ in which B cells mature and proliferate
Composition:
• B cells: 90%
• T cells: 5%
• Follicular dendritic cells: 1%
What are the outcomes for B cells after interaction with T cells?
- Initial plasma cells
• Low affinity IgM
• Short lived
• Initial, rapid response - Germinal centre formation
• Maturation events (SHM, CSR, affinity maturation)
• Production of memory cells and ‘better’ plasma cells
Which signals are required for Plasma cell development?
Describe the function
BLIMP1
• Transcription factor (repressor)
• Expressed in B cells
Brings about:
• Switching off of B cell program
• Switching on of plasma cell program
• Massive intracellular reorganisation
What is the transcription factor vital for B cells to form Germinal centres?
Describe its function
Why is it so vital?
Bcl6
Function:
• Transcription repressor
• Promotes cell cycling
Vital:
• Inhibition of the DNA damage response to SHM and CSR
List the processes occurring in the germinal centre
- Clonal expansion
- Isotope switching
- Somatic hypermutation
- Affinity maturation
- Memory formation
Describe the mechanism of somatic hypermutation
- AID converts Cystosine to Uracil in variable region
- Induction of error prone DNA repair
- Random mutation in the V region of the heavy and light chains
Describe affinity maturation
Cells with mutations in Ig with increased affinity are selected, others are discarded
(Evolution on a microscale)
Describe memory formation
High affinity GC B cells differentiate into either:
• Plasma cells
• Memory cells
Memory cells persist after primary infection in secondary lymphoid organs, as well as the BM
What is the difference between the plasma cells derived from the GC and those originally derived from the B cell that was yet to undergo maturation events?
GC derived plasma cells have much higher affinity, and are class switched
Which class of immunoglobulin is made first?
What would be the benefit of this?
IgM
Initially, the immunoglobulins have low affinity, so the pentamer IgM, which confers higher avidity, compensates for this fact
Compare the effector functions of the following: • IgG • IgM • IgA • IgE
IgG
• Complement activation
• Placental transfer
IgM
• Complement activation
• Little bit of mucosal protection
IgA
• Mucosal protection
IgE
• Sensitising of Mast cells
IgD
• Present on mature B cells (naïve)
Compare the location of the following: • IgG • IgM • IgA • IgE
IgG
• Serum
IgM
• Serum
IgA
• Secretions
IgE
• Bound to mast cells
Describe the regions on the Ig molecule that are changed through CSR
What is the significance of this?
Constant region: changes
Variable region: no change
This ensures the antibody still has the same affinity after isotype switching
Describe the process of CSR
Which enzyme is required?
- AID introduces nicks into ‘S’ region (upstream from each constant region segment) of heavy chain
• By converting C to U, and then U is removed - Formation of loop, cleaved out, deletion intervening sequences
- Ligation of ends
IgM → IgG → IgE → IgA
Describe the likely events after helminth infection
- DC recognition and migration to lymph node
- DC presentation to Th cell; 3 signals
- Th cell releases cytokines and presents antigen to B cell
- B cell undergoes CSR differentiation and starts producing antibody against helminth
Describe DC and Th cell interaction
Signal 1:
• TCR binds to MHC-peptide complex
Signal 2:
Costimulation: CD28 and CD80/CD86
Signal 3:
• Release of cytokines from DC that act on IL-R on T cell
• Drives differentiation into various Th cell subtypes
Describe T cell help for B cells
- Interaction between TCR on Tfh and MHC-peptide complex on B cell
- Interaction between CD40 and CD40L on B cell and T cell respectively
- Release of cytokines from Tfh cell (IL-21) onto B cell
4. B cell forms a GC Undergoes: • CSR • SHM • Affinity maturation • Memory formation
What is AID?
Where is it expressed?
Describe its function and mechanism of this function
Activation induced deaminase
Expressed in the germinal centre
Role in:
• CSR
• SHM
Mechanism
• Converts Cytosines to urasils
• Error prone DNA repair is then induced → SHM
By which process do the variable portions of Ig’s change?
SHM: Somatic hyper mutation
Where does B and T cell interaction occur?
At Marginal zone; the border of the B cell follicle
What are some of the molecular bases of HIGM?
- CD40 mutation
- CD40L mutation (X linked)
- AID mutation
- UNG mutation
Which processes is AID important for?
- CSR (class switch recombination)
* SHM (somatic hypermutation)
What happens on AID deficiency?
GC form, but there is no:
• CSR
• High affinity Ig
What happens in CD40L deficiency?
• Defective B cell proliferation
No GC formation:
• No memory
• No CSR
• No SHM and Affinity maturation
What are some therapies for HIGM?
Intragam (intravenous immunoglobulin)
Bone marrow transplantation
What is CVID?
What are the manifestations?
Common variable immune deficiency
Individuals unable to produce one or two of the isotypes
More common than XLA and HIGM (1 / 25 000)
Less severe than those as well
What are symptoms of CVID?
Recurrent pyogenic infections from encapsulated bacteria
List some mutations that can cause CVID
- TACI (most common)
- ICOS
- CD19
- STAT3
How many binding sites on the IgM pentamer?
Compare this with IgG and IgA
IgM: 10
IgG: 2
IgA: 4
Which classes of Ig can be transferred to the placenta?
IgG only
NB IgA present in milk
How do B cells ‘know’ how to produce the appropriate Immunoglobulin?
The helper T cell releases a certain program of cytokines that bring the CSR about.
Which PID is most important?
(Primary immune deficiency)
CVID (common variable immunodeficiency)
It has the highest incidence
Which antibody deficiency is the most severe?
Why?
Order of severity: • Agammaglobulinaemia: no B cells or Ig • HIGM: only IgM • CVID: missing only one or two isotopes • SAD: missing Ab against certain polysaccharide antigens
What is the treatment for HIGM?
IVIg
Bone marrow transplantation (in more serious cases)
In which types of HIGM will a GC form?
When won’t a GC form?
GC: • UNG mutation • AID mutation • TACI • STAT3
No GC: • CD40L mutation • CD40 mutation • ICOS • CD19
What is the therapy for CVID?
IVIg
Where are the CD40L and CD40 molecules located?
CD40L: Th cell
CD40: B cell
Where is white pulp found?
In the spleen
Which sort of infections is one more susceptible to in XLA?
Why?
Extracellular bacteria
Can’t make Ab, which is most important in protection against bacterial infections
T cell compartment, which mediated viral immunity, is still intact
Which type of B cells are seen in HIGM patients?
Which are lacking?
Why is so?
- Naïve cells present
- Lacking memory B cells
This is because there is a mutation (CD40L, CD40, AID, UNG) that means that CSR and affinity maturation can not occur
Compare the location of erythrocytes in the lymph nodes and the spleen
Spleen: red pulp
Lymph nodes: not found here
Compare location of the genes for CD40L and CD40
CD40L: X-chromosome
CD40: Autosomal chromosome
What is the driver of clonal expansion of B cells in GC?
T cells stimulate the clonal population of B cells to proliferate to produce a robust response through IL-21 signalling
Describe the germ line configuration of constant regions at the heavy chain locus
Downstream of VDJ mini gene segments: • Cμ • Cδ • Cγ3 • Cγ1 • Cγ2b • Cγ2a • Cε • Cα
What happens to B cells in the absence of Blimp1?
The B cell can never be a plasma cell
Compare the effect of the following on B cells:
• Bcl6
• Blimp1
B cell expression of
Blimp1 → plasma cell
Bcl6 → Maturation events in GC
What is the function of UNG?
In which cells is it found?
Enzyme that removes uracils that AID inserted during SHM or CSR
Ubiquitous throughout the body
What determines which class of Ig is made through CSR?
Depending on the infection, the DC will induce different Th cell subsets
These Th cell subsets will release specific cytokines that induce CSR to a specific isotype
When do the following mutations affect B cells: • CD40L • AID • ICOS • UNG • CD40 • CD19 • TACI • STAT3
In which cases would a germinal centre form?
Which disorder results?
- CD40L
• B-T cell interaction at MZ
• No germinal centre
• HIGM - AID
• Maturation events in GC
• GC still forms
• HIGM - ICOS
• B-T cell interaction
• No germinal centre
• CVID - UNG
• Maturation events in GC
• GC still forms
• HIGM - CD40
• B-T cell interaction
• No germinal centre
• HIGM - CD19
• B-T cell interaction
• No GC
• CVID - TACI
• Maturation events in GC
• GC still forms
• CVID - STAT3
• Defective B cell response to cytokines
• GC still forms
• Hyper-IgE
Compare cellular expression of the following:
• AID
• UNG
AID: only in B cells
UNG: all cells
Which PID (of the ones spoken about) is less severe?
CVID