Lecture 2 - B cells - Immune Deficiencies 2 Flashcards by Eleanor Nicklason

What is the phenotype of mice lacking CD40L?

Unable to undergo isotope switching
No memory
Low affinity

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What is a combined immune deficiency?

Spans both humoral and cellular immunity

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What is the name for humoral deficiencies (as opposed to combined)?

Antibody deficiencies

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Describe persistence of B cell immunity

Even 60 years after vaccination, there can be protective levels of antibody in the serum

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Describe the antibody response after vaccination

→ Vaccination
• IgM response
• Later, isotope switching to IgG

→ Booster
• Increased levels of IgG

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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

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What are the symptoms of Hyper-IgM syndromes?

Recurrent infections (upper and lower respiratory)
GIT dysfunction (malabsorption)
Autoimmune disorders
Enlarged 2° lymphoid organs

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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

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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

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Describe the cell distribution within white pulp

Mature B lymphocytes: follicle
T lymphocytes: periarteriolar sheath (central)
Dendritic cells: PALS
MZ B lymphocytes: marginal zone

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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

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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%

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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

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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

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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

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List the processes occurring in the germinal centre

Clonal expansion
Isotope switching
Somatic hypermutation
Affinity maturation
Memory formation

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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

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Describe affinity maturation

Cells with mutations in Ig with increased affinity are selected, others are discarded

(Evolution on a microscale)

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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

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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

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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

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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)

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Compare the location of the following:
• IgG
• IgM
• IgA
• IgE

IgG
• Serum

IgM
• Serum

IgA
• Secretions

IgE
• Bound to mast cells

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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

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Describe the process of CSR Which enzyme is required?

1. 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 2. Formation of loop, cleaved out, deletion intervening sequences 3. Ligation of ends IgM → IgG → IgE → IgA

Describe the likely events after helminth infection

1. DC recognition and migration to lymph node 2. DC presentation to Th cell; 3 signals 3. Th cell releases cytokines and presents antigen to B cell 4. 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

1. Interaction between TCR on Tfh and MHC-peptide complex on B cell 2. Interaction between CD40 and CD40L on B cell and T cell respectively 3. 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?

1. CD40L • B-T cell interaction at MZ • No germinal centre • HIGM 2. AID • Maturation events in GC • GC still forms • HIGM 3. ICOS • B-T cell interaction • No germinal centre • CVID 4. UNG • Maturation events in GC • GC still forms • HIGM 5. CD40 • B-T cell interaction • No germinal centre • HIGM 6. CD19 • B-T cell interaction • No GC • CVID 7. TACI • Maturation events in GC • GC still forms • CVID 8. 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