L15 - Tolerance and immunoregulation

Question 1

Innate Immunity: what receptors are involved with its activation, what does it recognise, and how is it encoded?

Answer 1

Pattern Recognition Receptors

Recognise MAMPs/PAMPS & DAMPS

Encoded in the germline

Question 2

Adaptive Immunity: what receptors are involved with its activation, how is it generated, and how diverse is the repertoire of the antibodies produced?

Answer 2

BCRs (Antibodies) & TCRs

Somatically generated after TCR activation - Rearrangement of multiple gene segments in developing B and T cells

Highly diverse random repertoire (polyclonal)

Question 3

Does self-reactivity always lead to an autoimmune disease?

Answer 3

Autoimmunity is normal within a healthy immune system - restrained by mechanisms of tolerance

Question 4

Self-tolerance: what is it and how is it obtained?

Answer 4

Failure to respond to intrinsic self-antigens and response to harmless non-self antigens (ie healthy bacteria)

Acquired not inherited - Non-identical twin calves sharing a common placenta become tolerant of each other

Question 5

Is tolerance always useful?

Answer 5

Can be useless in some instances:
* Self-tolerance - failure to respond to intrinsic self-antigens
* Harmless non-self antigen - commensal bacteria, etc
* therapeutically relevant antigens - allergens, transplantation antigens, etc

Question 6

Autoimmunity: what mechanisms prevent it?

Answer 6

Immature or developing lymphocytes whose antigen receptor binds self-antigen are mechanised to die

Question 7

Mechanisms contributing to tolerance

Answer 7

• Inactivation of potentially self-reactive clones - Death, Editing or Anergy
• Immune regulation - Suppression, Functional Deviation
• Self-epitopes not available for recognition - Ignorance, Sequestration, Immune Privilege

Question 8

Tolerance checkpoints

Answer 8

1 - Central Tolerance:
* Primary lymphoid organs (bone marrow, thymus) - removal of highly self-reactive clones during lymphocyte development (deletion, editing, anergy; functional skewing to tTreg)

2 - Peripheral Tolerance:
* Peripheral organs & tissues, secondary lymphoid organs (LNs, spleen) - Multiple mechanisms limit reactivity against self and harmless antigens in the periphery (eg lack of T cell help for B cells; T cells - ignorance, anergy, deletion, functional skewing, regulation….)

Question 9

B-cell tolerance process

Answer 9

B Cell Fate is Determined by Interaction with Self-Antigen in the Bone Marrow

The strength of interaction with self-antigen is influenced by:
* The concentration of antigen
* It’s ability to cross-link surface receptors

Question 10

If B-cells pass the central tolerance check what happens?

Answer 10

They move to the periphery

RAG genes stay on - further recombination at the light chain locus
Self-reactive clones are deleted or rescued by editing

Question 11

Self-reactive clones: what do they do?

Answer 11

Self-reactive clones are ANERGIC (unresponsive) – die rapidly

Question 12

T-cells: where do they develop and what forms are there?

Answer 12

Thymus

• Alpha - most abundant along with β T-cell (~95%)
• Beta - most abundant along with α T-cell (~95%)
• Gamma - roles in epithelial surface immune responses (innate?)
• Delta - roles in epithelial surface immune responses (innate?)

Question 13

T-cells: what are their stages of development and what are they named by?

Answer 13

• Early thymocyte
• Double negative (DN) thymocyte
• Double positive (DP) thymocyte
• Single positive (SP) thymocytes (CD4/CD8 cells)

Stages named according to the expression of CD4 and/or CD8

Question 14

T-cells: what antigens do they react to?

Answer 14

MHC class I - 8-10 peptide, presents intracellular antigens (ie viral antigens) to CD8+ T cells

MHC class II - 12-20+ peptide, presents extracellular, engulfed antigens to CD4+ T cells

T-cell receptors bind to both the MHC and the antigen peptide

Question 15

T-cell tolerance process

Answer 15

TCR gene rearrangements occur and give rise to various forms
* TCRs that fail to make a surface receptor
* TCRs that fail to recognise self-antigens
* TCRs that recognise self-antigens too strongly
* TCRs that recognise self-antigens but don’t bind them too strongly

The latter is the ideal and only ~2% fit that objective and enter the periphery from the Thymus

Question 16

Checkpoints during TCR development

Answer 16

1 - Quality Check - Failure to receive a survival signal -> USELESS cell deleted by apoptosis (subcapsular region)
2 - Positive selection - Recognition of self-antigen (cortex)
3 - Negative selection - Strength of recognition tested (medulla)

Question 17

Positive selection of T-cells during development: what is the point, where does it occur, what cells do they interact with, what happens if positive selection fails, and what happens to cells if positive selection succeeds?

Answer 17

Check if TCRs recognise self-antigen

Cortex

Cortical epithelial cells

Receptor editing of the alpha chain occurs, attempting to recover the TCR but if this fails, death by apoptosis occurs after 3-4 days

~10-30 cells pass positive selection and move into the medulla as SP CD4/8+ cells

Question 18

Negative selection of T-cells during development: what is the point, where does it occur, what cells do they interact with, ????

Answer 18

Detect if TCRs recognise self-antigen too strongly

Medulla

DCs & medullary epithelial cells (?)

MEC/AIRE - what on earth is this watch leccy

Question 19

Thymic emigrants: what are they and what further differentiated forms do they make up?

Answer 19

• CD8+ (CDLₙₐᵢᵥₑ)
• CD4+ (Thₙₐᵢᵥₑ) - Th1, Th2, Th17, ThFH, Foxp3⁺iTreg, Foxp3⁻iTreg (Tr1,Th3)
• CD4+ (Foxp3⁺tTreg)

Question 20

T-cells: what three signallings are required for activation to occur?

Answer 20

• Antigen-specific activation signal (TCR/MHC:pep)
• Co-stimulation
• Cytokines

Question 21

Peripheral Tolerance in B cells

Answer 21

Mainly due to Lack of T Cell Help

Question 22

Is dendritic signalling always activating?

Answer 22

Dendritic cells signal to TCRs but this may not always be activating - presentation in the absence of danger (no co-stimualation) is tolerogenic and leads to anergy

Question 23

Tolerogenic

Answer 23

Anergy - incapable of producing immunological tolerance

When T-cells are tolerogenic, they are unreactive and unresponsive

Question 24

Clonal Anergy: Turning T Cells Off

Answer 24

When only signal 1 is presented, T-cells are inactivated

Question 25

Regulation: peripheral induced Treg (pTreg)

Answer 25

Antigen along with immunosuppressive signals may turn T cells off

Question 26

Thymic derived Treg (tTreg) and induced peripheral Treg (pTreg)

Answer 26

Antigen along with immunosuppressive signals may turn T cells off

Thymic Tregs regulate immunity against self-antigen
Peripheral Tregs regulate immunity against self-antigen and harmless antigens

Question 27

Tregs: what are they, what do they do, and how do they do their function?

Answer 27

T-regulatory cells

Inhibit harmful T-cells (still not completely understood)

• Production of anti-inflammatory, pro-regulatory cytokines; TGFβ, IL10, IL35 (dependent on signals driving differentiation of the Treg)
• Modulation of dendritic cells
• Outcompete effector T cells for resources; IL2 (Treg commonly express IL2R)
• Direct killing of T cells
• Exosome transfer of miRNAs

Question 28

Are all tissues are immunologically equivalent?

Answer 28

No - ie gut

Question 29

Immune privileged sites: what are they, why do they exist, and how does the body work with them?

Answer 29

Antigens in immune privilege sites do not induce immune attack

Enclosed by a physical barrier - limited lymphatic drainage; selective entry

• Low MHC I
• Rich in suppressive cytokines, eg TGFβ
• Express FasL

Question 30

Breaking Immune Privilege - Sympathetic Ophthalmia

Answer 30

Eye infection may cause both eyes to be damaged as T-cells ‘see’ eye antigens and destroy them

They were unseen due to the physical barrier of the eye to the rest of the body causing a lack of tolerance checks due to no reference point

Question 31

Recruitment of immune suppression can allow tumors to attain immune privilege

Answer 31

ITC type stuff

Cancerous cells can secrete immune suppressive cytokines and facilitate T-cell destruction

Question 32

Examples of Self-Tolerance: whar are they and what are their mechanisms of tolerance?

Answer 32

• Central Tolerance - Deletion/Editing
• Antigen segregation - Physical barrier to self-antigen access to lymphoid system
• Peripheral anergy - Cellular inactivation by weak signalling without co-stimulus
• Tregs - Suppression by cytokines, intercellular signals
• Functional deviation - Differentiation of Tregs that limit inflammatory cytokines
• Activation-induced cell death - Apoptosis

Question 33

Healing peripheral tolerance

Answer 33

Can slowly desensitise yourself

Ie small amounts of peanuts may lead to reducing allergies