Lecture 2 - Tolerance, B cells Flashcards
Define Immunological Tolerance
The non-responsiveness to an antigen which is induced through previous exposure to that antigen
Define Self Tolerance
The non-responsiveness to self-antigens (those found in one’s own body)
Why do we need immunological tolerance?
Tolerance to:
• Self antigens
• Some foreign antigens (eg. food etc)
Describe the contributions to the understanding of tolerance by Peter Medawar
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
Describe the contributions to the understanding of tolerance by Macfarlane Burnet
Clonal selection theory
Antigen specific clones are sensitive to toleration during their development
List some of the mechanisms of immunological tolerance
Remove:
• Deletion
• Receptor editing
Switch off:
• Anergy (cell intrinsic)
• Regulation (cell extrinsic)
Limit access:
• Immunoprivilege
• Ignorance (site or dose)
List some immunoprivileged sites in the body
- The eye
- The brain
- Testes
What are the divisions of immunological tolerance?
Central
• Thymus (T cells)
• Bone marrow (B cells)
Peripheral
• Lymphoid tissue
• Organs
What are transgenic mice?
How are they used with respect to immunological tolerance?
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
Describe the HEL model
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
What are the main mechanisms of central B cell tolerance?
When does each occur?
What does each involve?
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
- 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
What sort of antigen will trigger the various mechanisms of central B cell tolerance?
Anergy: soluble antigen
Clonal deletion:
• Membrane bound antigen; i.e. multivalent
Receptor editing:
• Membrane bound antigen, i.e. multivalent
At what stage does central B cell tolerance occur?
Occurs at the immature B cell stage
Outline the stage of development of B cells
Briefly give the features of the cells at each stage
- MPP
- CLP
- Early pro-B cell
• D-J rearranging - Late pro-B cell
• V-DJ rearranging - Large pre-B cell
• VDJ rearranged
• Pre-BCR checkpoint - Small pre-B cell
• V-J rearranging of light chain - Immature B cell
• Heavy and light chains rearranged
• IgM expressed on surface - Mature B cell
• IgM and IgD expressed on the surface
Describe the features of self-reactive B cells that make it to the periphery
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
In which instance will B cells never reach the periphery?
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
Strongly cross linking Ag in BM → ?
Weakly cross linking Ag in BM → ?
Strongly cross linking Ag in BM → Apoptosis in BM (clonal deletion)
Weakly cross linking Ag in BM → Anergy
Compare IgM expression on mature B cells and anergic B cells in the periphery
Anergic B cells in periphery have lower expression of IgM
What is notable about the expression of molecules on the surface of anergic B cells?
- ↓ Expression of IgM
- Fail to up regulate CD86 when they encounter Ag
- ↓ CXCR5 expression
- ↓ BAFF receptor expression
What is the outcome of anergic B cells’ poor expression of CXCR5?
They are unable to access the B cell follicle
What are ‘ignorant B cells’?
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
Describe how breaks down in tolerance can lead to kidney damage
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
List some immune-mediated disease with a proposed role of molecular mimicry
- Acute rheumatic fever
- Chagas’ disease
- Guillain-Barré syndrome
- Herpetic keratitis
- Insulin-dependent DM (T1DM)
- Lyme arthritis
- Multiple sclerosis
What is the microbe or Ag responsible for triggering molecular mimicry in the following diseases:
• Acute rheumatic fever
• Guillain-Barré syndrome
• Glomerulonephritis?
ACF: Group A strep
G-B syndrome: Campylobacter
Glomerulonephritis: Group A strep
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
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
Describe the signals required by B cells to respond to antigen
Need two signals:
• Antigen
• T cell help (CD40L, cytokines)
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
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)
Describe how naïve lymphocytes enter LNs
Through HEVs
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
Where are HEV located?
In paracortex
Describe the events that occur when B cells experience Ag
- Naïve (mature) B cell experiences Ag
- B cell upregulated CCR7 and migrates to the boundary between the follicle and the paracortex
- B cell and TFH interact and form the primary focus
- B cell migrates back into the follicle to form a GC
- SHM, affinity maturation and CSR occur in GC
What are the fates of auto-reactive B cells in the periphery?
- Deletion
- Anergy
- Survival
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?
- Infection with a microbe triggers inflammation
- 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
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
Describe the important receptors / molecules in B cells that are clonally deleted in central tolerance
Increased bim
Decreased BAFF receptor
What is bim?
Pro-apoptotic molecule
Removes Bcl-2 from Bax/Bak, resulting in the triggering of apoptosis
What is BAFF receptor?
Detects the survival signal in B cells
What is happening in receptor editing?
They light chain is rearranged again
NB the heavy chain cannot be edited at this point
Where does CCLR19/21 come from?
FRCs (Fibro-reticular cells) in the paracortex
Describe the apoptotic pathway in B cells
- Bcl-2 normally keeping Bax and Bak separate
- Bim binds Bcl-2
- Bcl-2 removed from Bax and Bak
- Bax and Bak unite to form a pore in the membrane of the mitochondria
- Cytochrome c moves out of the mitochondria through this pore
- Formation of apoptosome, activation of caspases
- Apoptosis
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
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
