Lecture 2 - Tolerance, B cells Flashcards

1
Q

Define Immunological Tolerance

A

The non-responsiveness to an antigen which is induced through previous exposure to that antigen

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2
Q

Define Self Tolerance

A

The non-responsiveness to self-antigens (those found in one’s own body)

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3
Q

Why do we need immunological tolerance?

A

Tolerance to:
• Self antigens
• Some foreign antigens (eg. food etc)

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4
Q

Describe the contributions to the understanding of tolerance by Peter Medawar

A

Tolerance to antigens is set up early on

Experiment:
Newborn mice injected with histo-incompatible cells and tolerated

As adults, the mice receive grafts of these histo-incompatible cells, and they are not rejected

i.e. Tolerance has been induced

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5
Q

Describe the contributions to the understanding of tolerance by Macfarlane Burnet

A

Clonal selection theory

Antigen specific clones are sensitive to toleration during their development

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6
Q

List some of the mechanisms of immunological tolerance

A

Remove:
• Deletion
• Receptor editing

Switch off:
• Anergy (cell intrinsic)
• Regulation (cell extrinsic)

Limit access:
• Immunoprivilege
• Ignorance (site or dose)

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7
Q

List some immunoprivileged sites in the body

A
  • The eye
  • The brain
  • Testes
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8
Q

What are the divisions of immunological tolerance?

A

Central
• Thymus (T cells)
• Bone marrow (B cells)

Peripheral
• Lymphoid tissue
• Organs

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9
Q

What are transgenic mice?

How are they used with respect to immunological tolerance?

A

Mice that has been bred with human genes

Process:
• Desired genes on a plasmid are injected into a single cell mouse embryo

They are very useful as an animal model for studying tolerance

Genes:
• Model antigens (e.g. BM or pancreas tissue)
• Clonal antigen receptors

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10
Q

Describe the HEL model

A

HEL: Hen egg lysozyme model

Two transgenic mice:
• Anti-HEL mouse
• HEL mouse

These two mice are crossed
By looking at the lymphocyte populations, we can learn about tolerance

HEL mouse:
• Membrane bound HEL (strongly cross linking) → Deletion of B cells
• Soluble HEL (weakly cross linking) → Anergic B cells

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11
Q

What are the main mechanisms of central B cell tolerance?

When does each occur?
What does each involve?

A
1. Deletion
 • Immature B cell apoptosis
Occurs when:
 • High avidity ligand present
 • Strong cross-linking of BCR to antigen
Involves:
 • Pro-apoptotic molecule 'bim'
 • Reduced expression of BAFF receptor
2. Anergy
 • Cell cannot be activated
Occurs when:
 • Self antigen is soluble
 • Less receptor cross linking
Involves:
 • Cell survives to migrate into the periphery
 • Reduced surface IgM → poorly responsive
 • Most likely eventually dies
 • Unable to up regulate CD86
  1. Receptor editing
    • Can occur when strongly cross linking Ag is present in BM
    • Immature B cell undergoes more light chain rearrangement
    • If new receptor is not self-reactive, the cell lives, if not, deletion or anergy occurs
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12
Q

What sort of antigen will trigger the various mechanisms of central B cell tolerance?

A

Anergy: soluble antigen

Clonal deletion:
• Membrane bound antigen; i.e. multivalent

Receptor editing:
• Membrane bound antigen, i.e. multivalent

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13
Q

At what stage does central B cell tolerance occur?

A

Occurs at the immature B cell stage

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14
Q

Outline the stage of development of B cells

Briefly give the features of the cells at each stage

A
  1. MPP
  2. CLP
  3. Early pro-B cell
    • D-J rearranging
  4. Late pro-B cell
    • V-DJ rearranging
  5. Large pre-B cell
    • VDJ rearranged
    • Pre-BCR checkpoint
  6. Small pre-B cell
    • V-J rearranging of light chain
  7. Immature B cell
    • Heavy and light chains rearranged
    • IgM expressed on surface
  8. Mature B cell
    • IgM and IgD expressed on the surface
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15
Q

Describe the features of self-reactive B cells that make it to the periphery

A
They will (hopefully) be: 
 • Anergic, i.e. the Ag that they are specific for is soluble

• Clonally ignorant: the Ag is soluble and present, but not cross linking

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16
Q

In which instance will B cells never reach the periphery?

A

If the Ag the B cell is specific for is strongly cross-linking and multivalent (i.e. membrane bound)

In this instance, the B cell will undergo clonal deletion and never make it to the periphery

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17
Q

Strongly cross linking Ag in BM → ?

Weakly cross linking Ag in BM → ?

A

Strongly cross linking Ag in BM → Apoptosis in BM (clonal deletion)

Weakly cross linking Ag in BM → Anergy

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18
Q

Compare IgM expression on mature B cells and anergic B cells in the periphery

A

Anergic B cells in periphery have lower expression of IgM

19
Q

What is notable about the expression of molecules on the surface of anergic B cells?

A
  • ↓ Expression of IgM
  • Fail to up regulate CD86 when they encounter Ag
  • ↓ CXCR5 expression
  • ↓ BAFF receptor expression
20
Q

What is the outcome of anergic B cells’ poor expression of CXCR5?

A

They are unable to access the B cell follicle

21
Q

What are ‘ignorant B cells’?

A

Mature B cells in the periphery that are specific for self-Ag, but do not respond because:
• Self-Ag is non-cross linking, or
• Only expressed outside the BM

They are unaware of their self-antigen at the time of development

22
Q

Describe how breaks down in tolerance can lead to kidney damage

A

Molecular mimicry:

Ignorant naïve T cells in the periphery

Infection with a microbe that shares epitopes with a tissue specific self Ag

B cell response launched against microbe epitope

Now B cells are activated, they can move into the periphery, where they encounter the self-Ag for which they are specific

Formation of immune complexes, which are deposited in the organs that filter blood, e.g. kidney, joints

Activation of the C’ cascade

23
Q

List some immune-mediated disease with a proposed role of molecular mimicry

A
  • Acute rheumatic fever
  • Chagas’ disease
  • Guillain-Barré syndrome
  • Herpetic keratitis
  • Insulin-dependent DM (T1DM)
  • Lyme arthritis
  • Multiple sclerosis
24
Q

What is the microbe or Ag responsible for triggering molecular mimicry in the following diseases:
• Acute rheumatic fever
• Guillain-Barré syndrome
• Glomerulonephritis?

A

ACF: Group A strep

G-B syndrome: Campylobacter

Glomerulonephritis: Group A strep

25
What are the target host-Ag in the following diseases caused by molecular mimicry: • ARF • G-B syndrome?
ARF: multiple cardiac glycoproteins G-B syndrome: Peripheral nerve gangliosides
26
Describe the mechanism of peripheral B cell tolerance Where does it occur?
Occurs in 2° lymphoid tissues (i.e. B cell encounters Ag once mature) Mechanism: • Self-reactive B cells migrate from blood into LNs • B cells don't experience T cell help • B cells die Also: • Anergic B cells struggle to get into follicle When B cells recognise Ag, but do not receive T cell help, they die
27
Describe the signals required by B cells to respond to antigen
Need two signals: • Antigen • T cell help (CD40L, cytokines)
28
What happens to B cells in the absence of T cell help?
They die In the sole presence of signal 1, B cells eventually die
29
Describe the structure of a LN
* Afferent & efferent lymphatics * Blood vessels * 1° lymphoid follicle (B cell zone) * 2° lymphoid follicle (containing germinal centres) * Paracortex (T cell zone) * HEV (in paracortex)
30
Describe how naïve lymphocytes enter LNs
Through HEVs
31
Describe how the structure of LNs is set up and maintained
1°LF: • FDCs produce CXCL13 • Mature B cells express CXCR5 • B cells migrate into the follicle Paracortex: • FRC make CCL19/21 • Naïve T cells express CCR7 • T cells migrate to paracortex
32
Where are HEV located?
In paracortex
33
Describe the events that occur when B cells experience Ag
1. Naïve (mature) B cell experiences Ag 2. B cell upregulated CCR7 and migrates to the boundary between the follicle and the paracortex 3. B cell and TFH interact and form the primary focus 4. B cell migrates back into the follicle to form a GC 5. SHM, affinity maturation and CSR occur in GC
34
What are the fates of auto-reactive B cells in the periphery?
* Deletion * Anergy * Survival
35
If mature naïve lymphocytes are anergised or deleted when they experience their antigen in the periphery, how can lymphocytes ever be activated against foreign antigen?
1. Infection with a microbe triggers inflammation 2. Inflammation results in the up-regulation of co-stimulatory molecules on the APCs such as DCs, and the production of cytokines which activate the lymphocytes
36
What happens when DCs present Ag to naive lymphocytes in the absence of infection or inflammation?
The interaction with the lymphocyte will be tolerogenic: i.e. tolerance inducing
37
Describe the important receptors / molecules in B cells that are clonally deleted in central tolerance
Increased bim | Decreased BAFF receptor
38
What is bim?
Pro-apoptotic molecule Removes Bcl-2 from Bax/Bak, resulting in the triggering of apoptosis
39
What is BAFF receptor?
Detects the survival signal in B cells
40
What is happening in receptor editing?
They light chain is rearranged again NB the heavy chain cannot be edited at this point
41
Where does CCLR19/21 come from?
FRCs (Fibro-reticular cells) in the paracortex
42
Describe the apoptotic pathway in B cells
1. Bcl-2 normally keeping Bax and Bak separate 2. Bim binds Bcl-2 3. Bcl-2 removed from Bax and Bak 4. Bax and Bak unite to form a pore in the membrane of the mitochondria 5. Cytochrome c moves out of the mitochondria through this pore 6. Formation of apoptosome, activation of caspases 7. Apoptosis
43
Compare cell surface molecule expression of anergic and ignorant B cells
Ignorant B cells express the same surface molecules are normal naïve B cells (i.e. they are fully able to respond to Ag) Anergic B cells: ↓ IgM, ↓ BAFF, ↓CXCR5, ↓CD86
44
In which situations will B cells in the periphery die?
If they fail to get into the B cell follicle If they do not receive T cell help after becoming activated