Tolerance Flashcards
Define tolerance
A state of antigen-specific immunological unresponsiveness.
What are the different types of tolerance?
• Central → mechanisms to limit production of self reactive T and B cell clones
• Peripheral → mechanism to prevent unwanted destructive responses by any self-reactive clones that are produced.
• Self → failure to respond to intrinsic self-antigens
• Acquired → failure to respond to an external antigen:
− Harmless non-self eg) commensal bacteria, fetomaternal antigens
− Induction of therapeutic tolerance in situations of allergy and transplantation
What is the mechanism of central tolerance in B cells?
No self reaction:
• Immature B cells with no strong reactivity to self antigens are allowed to mature
Clonal Deletion/Receptor Editing
• Clonal deletion (removal of particular antigen specificity from the repertoire) seems to predominate when the interacting self antigen is multi-valent and strongly cross-linking.
• Not all lymphocytes undergo clonal deletion, some undergo receptor editing
• Interval before cell death during which the B cell can be resuced by further gene rearrangement
• Strong cross-linking of the IgM ensures RAG gene expression continues
• light chain rearrangement can continue → these secondary rearrangements can resuce immature self-reactive B cells by deleting the self reactive light chain and replacing it with a new one.
• If the new light chain is non-autoreactive, B cell continues normal development
• If it is autoreactive, rearrangement continues until a non-autoreactive one is produced, or segments are exhausted and the B cell undergoes apoptosis
• Not clear whether receptor editing occurs at the heavy chain
• No available D segments at a rearranged heavy chain locus, so a new rearrangement cant occur by the normal mechanism
• Instead, a process of Vh replacement may occur
• It has always been thought that RAG gene turn-off results in allelic exclusion. The ability of cells to keep them turned on to undergo receptor editing suggests that it may not be the sole method.
Anergy
• Immature B cells that encounter more weakly cross-linking antigen of low valence respond differently
• They tend to be inactivated, entering a state of permanent un-respinsiveness (anergy) but do not immediately die
• Anergic B cells cannot be active by their specific antigen.
• Migration of anergic B cells to the LN is impaired
• They will eventually die as they cannot get survival signals from T cells
Clonal Ignorance
• Remain in as state of immunological tolerance
• Have affinity for self-antigen, but do not sense and respond to it
• Antigen may not be accessible to the B cell, it may be in low concentration, or it may bind too weakly
• They are not however, inert → may be activated in times of inflammation or if the self antigen reaches an unsually high concentration.
What is the mechanism of central tolerance in T cells?
Checkpoints during T Cell Development
- T cell that fails to make a functional TCR → useless
- TCR that cannot recognize self MHC → useless
- TCR that recognizes self MHC and self antigen too strongly → dangerous
- TCR that recognizes self MHC and does not bind self antigen too strongly → GOOD.
Checkpoint 1 → Quality check (Cortex)
• Has TCR gene rearrangement resulted in a functional pre-TCR expression?
• Occurs at the DN3 stage → CD44lo CD25+
• Here we have the beta chain V-J recombination, then V-DJ, forming the pre-TCR
• If it hasn’t resulted in a functional pre-TCR → no survival signal → apoptosis
Checkpoint 2 → Positive Selection (Moving towards the medulla)
• Can the rearranged ab TCR recognize self MHC?
• DP thymocytes encounter antigen on MHC-I or II on the surface of cTECs
• Thymocytes whos TCR bind to the self MHC receive a survival signal → only around 10-30% survive. These move to the next stage as SP.
• Cells that cannot recognize either attemot to be recovered by receptor editing of the alpha chainor they die by apoptosis
Checkpoint 3 → Negative Selection (Medulla)
• Does the abTCR bind self-MHC and self peptide too strongly?
• The thymus at the cortico-medullary junction has DCs expressing MHC-I and II which present self antigen
• T cells are deleted if they bind too strongly.
How does the thymus present antigen for central tolerance?
Cortex:
cTECs:
• Constitutive MHC-I and II expression
• MHC-I peptides processed by thymo-proteasome
• MHC-II peptides processed by :
− Cathepsin L in the endosome
− thymus specific serine proteases in the endosome or lysosome
− constitutive macroautophagy to delivery self-antigen to the MHC
Medulla:
mTECs and DCS:
• Constitutive MHC-I and II expression
• MHC I → processed by 26S proteasome and immunoproteasome
• MHC-II → processed by cathepsin S and macroautophagy
What are the sources of antigen for T cell central tolerance?
How are antigens not normally expressed in the thymus presented for test binding? Because otherwise you would just be tolerance to your thymus?
mTECs
• Inefficient at uptake and presentation of exogenous antigen
• AIRE allows for promiscuous expression of tissue-specific genes not normally expressed in the thymus (tissue restricted antigens)
• Genetic deficiency of AIRE results in autoimmune polyendocrinopathy.
• mTECS also perform macroautophagy
DCs
• DCs bring in antigen from the blood
• DCs take part in uptake of tissue restricted antigens shed from mTECs
• DCs cross-present ingested antigens
Describe peripheral tolerance in B cells?
• Mainly due to lack of T cell help
- DCs see antigen in the periphery. Takes it up, presents it, and migrates to the LN
- In the LN, a T cell will be activated by this DC
- Some of these activated T cells will move toward the B cell region
- Some of the same antigen will have drained into the lymph node, and taken up and presented by B cells
So the same antigen is presented on the DC and the B cell
- The T cell has seen that the B cell has recognized the same antigen as the DC, and it gives the B cell the extra co-stimulation it needs to become a mature antibody secreting plasma cell
- If however, a self-antigen had arrived in the lymph node, the B cell would have taken it up and presented it, but there would be no T cell help, because the DC wouldn’t have recognized any antigen, and wouldn’t have activated the T cell.
There is also selection in the germinal centre:
• Auto-reactive B cells arising from somatic hypermutations are deleted in the GC
1. Competition for limiting antigen and Tfh drives selection of the highest affinity binders
2. Failure to bind and present antigen → failure to receive survival signals
3. Binding to soluble self-antigen → apoptosis
4. Role of T follicular regulators → have an antigen specific suppression of B cell responses
➢ Foxp3+ Blimp-1+ CD4+
➢ Have the same phenotypic characteristics of Tfh cells and conventional Foxp3+ T regs, yet are distinct from both
➢ Derived from thymic-derived Foxp3+ precursors, not naïve T cells or Tfh cells
➢ They limit Tfh cell and germinal centre B cell numbers
➢ In the absence of these Tfr cells, an out-growth of non-antigen specific B cells leads to fewer antigen-specific B cells.
- Not all potential self antigens are expressed in the lymphoid organs → some are highly tissue specific, or compartmentalized
- Newly emigrated self-reacitve lymphocytes that encounter their specifc autoantigen in the periphery must be eliminated or inactivated
- In the absence of infection, newly mature B cells that encounter a strongly cross-linking antigen in the periphery will undergo clonal deletion
- There is no receptor editing – they are mature and cant rearrange gene loci any more
- As with immature B cells, mature B cells that encounter and bind abundant soluble antigen become anergised
- The question arises: if the encounter of a mature naïve lymphocyte with a self antigen leads to death or anergy, why does this not happen to a lymphocyte that recognizes a pathogen antigen?
- Infection sets up inflammation, which induces expression of co-stimulatory molecules and cytokine production that promote lymphocyte activation
Describe peripheral tolerance in T cells
- Antigen must be presented on a DC in an activation context to initiate naïve T cell activation
- Presentation in the absence of danger (signal 2 and/or signal 3) is tolerogenic → leads to anergy or regulation.
Clonal Anergy
• Presence of signal 1 alone without signal 2 or 3 renders the T cell anergic
Peripheral Induced T regs
• Presence of antigen with an immunosuppressive signal skews the T cell to become a regulatory T cell
Different Tregs depending on signal 3:
• TGFb → iTreg (Foxp3+)
− Most prominent in the gut, induced via specialsed DCs → CD103+
− avb8 expressed on DCs is critically important for maintaining immune homeostatsis → mice lacking avb8 on DCs develop autoimmune colitis
− Lack of avb8 on DCs also characterized by a reduced population of T regs in the intestine (Travis et al, 2007)
− Lack of avb8 is associated with reduced TGF-b activity → highly suggests activation of TGF-B by avb8 on DCs is crucial for inducing Tregs in the intestine
• IL-10 → Tr1 (Foxp3-)
• suppress the NLRP3 inflammasome
There are some differences between nTregs and iTregs:
• nTregs recognize self, this is why they were selected in the thymus
• iTregs can see any antigen in the context of the immunosuppressive signal 3, and have been skewed off to become a regulator
How do T regs suppress?
Cytokine Dependent
• Initial in vitro assays suggested not important, but later in vivo assays suggested TGF-B and IL-10 are key in Treg mediated suppression
Induction of T cell Death
• Tregs secrete perforin and granzyme
Disruption of T cell Effector Mechanisms
• Tregs mop up IL-2 so it cant be used by effector T cells for proliferation and differentiation
• CD25 is part of the IL-2 receptor → gets upregulated on Tregs upon activation
• Tregs also produce adenosine that inactivates lymphocytes
• Tregs appropriate partial ‘aborted’ forms of the transcriptional programmes of their respective target Th cells by expressing their master transcription factors and co-opting their function
− eg) Treg cells upregelate T-bet, which induces expression of some Th1 related genes but not others
− Allows the Treg cells to migrate to sites of Th1 cell mediated inflammation, while restaining their differentiation into Th1 cells.
• Transfer of Treg cell exosomal miRNA to conventional T cells abrogated the capacity of Treg cells to prevent disease in model of colitis → miRNA mediated non-autonomous gene silencing. Redirects transcriptional circuitry in favour of a tolerogenic profile.
Targeting of DCs
• CTLA-4 is expressed on Tregs and activated T cells
• Binds with high affinity to CD80 and 86
• Downregulates their expression
• Tregs also block T cell acess to DCs → if Foxp3+ Tregs bind with a higher affinity to the antigen, T cells cant bind
− Suggested that Tregs are more enriched to recognize antigen than normal effector T cells
Describe oral tolerance
Oral Tolerance
Challenges of the Gut
• Thin, permeable barrier
• Continuously exposed to pathogens to which it must react
• Also continuously exposed to food and commensal organism (to which it must not react, because not only are they harmless, they are beneficial)
− Coeliac disease is immune response to wheat protein
− Chrohns is an immune response to commensals
It has a specialized immune environment • MALT • Th1, Th17 activity balanced by the regulatory enviromnet • Anti-inflammatory mediates • Immune deviation • IgA
Also has specialized DCs, the CD103+ DCs
• Low response to TLR
• IL-10 production
• Migrates to MLN and imprints gut homing
• Induces Foxp3+ Tregs via RA and TGFb (via avb8 integrin)
→ Oral tolerance demonsrated by American Indians with posion ivy digestion – immune system recognizes it in a TGFb environment, and tolerises against it.
- Various mechanisms are likely to account for oral tolerance such as anergy, deletion of antigen specific T cells and generation of Tregs
- Although Tregs can be generated in Peyers Patches, the MLNs play the domiantn role
- Treg production of TGF-b has been particularly associated with oral tolerance → immunosuppressive and stimulates class switch to IgA (non-inflammatory)
Describe immune privilege
• Certain sites of the body have immune prilvelage → they can tolerance the introduction of antigens without eliciting an inflammatory response
• Thought to be an evolutionary advantage to protect vital structures from the potentially damaging effects of an inflammatory response
− Brain, eye, testis, uterus
• Generally, immunologically privileged sites are:
− Enclosed by a physical barrier
− Low MHC-I
− Rich in suppressive cytokines, eg, TGFb
− Express FasL – can stimulate apoptosis
Fetomaternal tolerance:
• Fetus is a repeatedly tolerated foreign graft
• Carries both major and minor paternal antigens
• Trophoblast: maternal/fetus interface
− Physical barrier
− Complement control proteins
− No MHCI or MHCII
− HLA-G binds inhibitor receptors on NK cells
− Nutrient starvation
− TGFb and IL-10
− Tregs
Breaking Immune Privilege: Sympathetic Opthalmua
• Normally, the eye is immune privileged – the antigens are sequestered
• Trauma or infection to one eye breaks the barrier → results in the release of sequestered intraocular protein antigens. They have not been tolerised against as they have never been seen before
• The released antigen is carried to LNs and activated effector T cells
• T cells return via the blood stream and enter both eyes – not just the injured one
• This tells you it isn’t that T cells cant enter the eye – it is just because they never saw the antigen before
Describe tolerance and autoimmunity
Autoimmune disease:
• Affects 5-8% population
• Develops as a consequence of failure of self-tolerance
• Normal effector mechanisms triggered
• Failure to clear self-antigen → chronic
Some rare gene defects give rise to autoimmune syndromes:
• Foxp3+ → immune dysregulation polyendocrinopathy X-linked syndrome
• AIRE → autoimmune polyendocrine syndrome
• FAS → autoimmune lymphoproliferative syndrome
There are also problems if we get too much tolerance!
Recruitment of immune suppression can allow tumours to attain immune privilege:
• Low immunogenicity → no peptide:MHC ligand, no adhesion molecules, no co-stimulatory molecules
• Tumour treated as self → tumour antigens taken up and presented in the absence of co-stimulation tolerises T cells
• Antigenic modulation → antibodies against tumour cell-surface antigens can induce endocytosis degradation of the antigen.
• Tumour-induced immune suppression → secrete TGFB, IL-10, induce Tregs
• Tumour-induced privileged sites → tumours secrete factors creating a physical barrier
