2. The cellular basis of autoimmunity 1 - T cells Flashcards
What is the functional unit of the immune system?
The immune cells
What is the overview of autoimmunity?
- Initiation or reactivation.
- Expansion and trafficking.
- Target organ localisation, activation and damage.
- Amplification, Regulation and systemic and tissue-specific memory generation.
- Remission (or reactivation)
Can autoimmune disease enter remission?
- Yes
- even untreated it can enter remission.
- This is a classic presentation of autoimmunity and makes it difficult to diagnose.
What are spontaneous models of autoimmunity?
- They introduce susceptibility, which leads to the random development of autoimmunity.
- NOD mice
- TCR transgenic animals.
What are induced models of autoimmunity?
- These are models where the disease can be induced by treating them with something that initiates the immune response.
- These can be used to time the disease that you can’t do in spontaneous models.
- Also TCR transgenic animals.
What is the induced autoimmune model for multiple sclerosis?
Experimental autoimmune encephalomyelitis (EAE)
What is the induced autoimmune model for uveitis?
Experimental autoimmune Uveoretinitis (EAU)
When was the concept of autoimmunity thought of?
around 1880s
How was the concept of autoimmunity discovered?
- The Pasteur lab was trying to find a rabies treatment.
- It is a deadly disease so a vaccine or treatment was desperately needed.
- They invented a process for drying and extracting serum from rabbit spinal cords.
- This provoked a protective response. It was a risky vaccine but much better then the disease.
- The immune system confused the rabbit brain with the human brain. This generated an immune response that combats rabies and attacks the brain causing an acute paralytic illness.
- Most people thought the immune system couldn’t do autoimmunity as it’s too dangerous.
When did autoimmunity become a mainstream idea?
1956
What help cemented autoimmunity as a mainstream idea?
- A thyroid disease showed 1000s of lymphocytes present in the tissue.
- 2 scientists came up with the idea this thyroid disease was caused by the immune system and not an infection.
- This was investigating in animal models and human tissues.
- Antibodies were proven to be the cause of this inflammation and disease. They targeted the thyroid specifically.
- This was fortunate, as in the 1950s, only antibodies were understood. T cells, lymphocytes, and other immune cells were unknown.
What can autoantibodies be?
Blocking or stimulating
What do blocking autoantibodies cause?
Under activity, e.g. Hashimoto’s thyroiditis
What do stimulating autoantibodies cause?
Overactivity, e.g. Graves disease
What type of antibodies were present in the thyroid?
IgG
What drives the tissue specificity in autoimmunity?
- Genetics determine MHC (specifically MHC2) association with autoimmune susceptibility.
- Pathogenic antibodies in thyroid autoimmunity are high-affinity IgG.
- These antibodies have been class-switched so T cells are involved.
- MHC2 expression is upregulated in inflamed tissues.
- This shows that CD4 T cells are involved in the disease process.
What was the 1st step in looking for pathogenic lymphocytes?
- in 1960 Mixed lymphocyte populations could transfer EAE.
- this showed that you can transfer disease with cells but not with antibodies.
- This is a complex mix of cells so it is unclear which cells are responsible for disease.
- Cells are required for disease induction.
What was the 2nd step in finding the lymphocytes responsible for autoimmunity?
- in 1969 thoracic duct cells were shown to transfer EAE.
- Thoracic duct cells have a high concentration of lymphocytes from lymphatic drainage.
- This is almost 100% lymphocytes, so it is a more specific test.
What was the 3rd step in finding the lymphocytes responsible for autoimmunity?
- in 1981 antigen-specific T cell lines were isolated and shown to transfer EAE.
- These are T cells that are immunised from animals.
- There is still a mix of different cells
- Something in the cloning process could cause the pathogenicity in the cells.
What was the final step of finding the lymphocytes responsible for mediating autoimmune disease?
- in 1984 TCR were cloned for the 1st time.
- In 1985 Transgenic T cells were cloned for specific receptors.
- This creates uniform cell lines baring a single TCR.
- It was shown that these cells were MHC2 restricted therefore CD4 T cells.
What does encephalitogenic mean?
Cells that cause encephalitis
How were the T-cell clones made in the 1985 experiment to prove MHC2 restriction?
- All cells together and diluted down to a single cell.
- These were then grown up.
- Then, transferred back to the mice to create a mouse with around 90% of the T cells to specific for a single antigen
How was MHC2 restriction proved with blocking antibodies?
- with no antigen expressed, the T cell clone numbers are low.
- With antigen present, there is lots of T cell proliferation and, therefore, disease.
- With antigen present and anti-MHC antibody, T cell numbers reduce again. Not back to 0 but a significant reduction.
- then, specific antibodies for the different MHC2 present in the mice showed the specific for the type of MHC2 that was presenting the antigen.
- A control set of clones was used that were not blockable, showing they were responding to something outside the antigen presentation pathway.
What did the 1985 MHC restriction experiment prove?
To cause autoimmunity, there was MHC-restricted antigen-specific activation of CD4 T cells
How is autoimmunity tissue restricted?
- The autoreactive T cells recognise specific antigens.
- Some of these antigens are tissue restricted.
- If there is no antigen expressed there is no autoimmunity.
- So, an autoreactive T cell specific for a brain antigen couldn’t cause disease in the liver.
What is the 1 thing you need to cause autoimmune disease?
specific autoreactive T cells
What did the long process of discovering CD4+ T cells were the mediators of autoimmunity teach us?
- specific autoantigen recognising T cells are enough to develop spontaneous autoimmunity.
- These theories discovered in animals models (mostly mice) apply to humans.
- Cross-reactivity can cause relapse in autoimmune disease.
What do CD4+ T cells recognise?
- A peptide presented on an MHC molecule.
- This peptide is an epitope and is present in a particular way on an MHC.
What MHC interactions are needed for T cell activation?
- The peptide must be present and recognised by the TCR.
- The MHC must be recognised by the Co receptor and TCR.
How does peptide presentation change?
- The peptides presented change depending on the part of the body.
- An APC from a lymph node in the foot will display different peptides to one from the lungs.
How are autoantigens identified?
- A candidate protein is chosen
- The immune response to this whole protein is analysed using animal models.
- The overlapping peptides are screened to find epitopes.
- The disease is induced in a model.
- The inciting cells are identified and characterised.
- This correlates with the human response.
What is important to remember about autoreactive cells?
- The presence of autoreactive cells does not equal disease.
- Other factors are needed to activate autoimmune disease.
- Everyone has autoreactive cells in the body.
What are lots of autoantigens?
Organ specific antigens
What are some potential autoantigens for type 1 diabetes?
- Insulin
- Glutamic acid decarboxylase
- zinc transporter 8
What are some potential autoantigens for multiple sclerosis?
- Myelin basic protein (MBP)
- Myelin proteolipid protein
- Myelin oligodendrocyte protein
What are some potential autoantigens in the retina?
- S-antigen
- Retinol binding protein 3
- Rhodopsin
- Recoverin
- Phosducin
What creates the specificity in organ specific autoimmunity?
- Tissue restricted antigens
- These are the antigens being recognised by the autoreactive T cells.
- There are lots of different potential autoantigens in the tissues.
- Knowing what is being specifically recognised is hard and mostly still unknown.
What are the characteristics of an autoantigen?
- The epitope needs to be able to bind MHC and be presented.
- The antigen needs to be naturally processed in the cell.
- The antigen needs to induce disease.
Why can identifying autoantigens in vitro be difficult?
Antigen processing occurs differently in cell cultures and in vitro then it does in vivo.
What makes the candidate pool for autoantigens large?
Many different epitopes can be generated from the same protein.
Does autoantigen recognition trigger autoimmune disease?
- MHC2 on all APCs are constantly expressing self peptides.
- The T cells interact with these in secondary lymphoid organs and nothing happens.
- So another factor needs to be present to activate autoreactive T cells
What are the two theories of activation of autoreactive T cells?
- Molecular mimicry
- Bystander activation
What parts of the epitope is the TCR recognising?
- T cells focus on a few residues with the epitope.
- They have a key recognition element that is normally 1 or 2 amino acids.
How big are T cell epitopes?
around a dozen residues
What does the small scope of TCR antigen recognition cause?
An increase in cross-reactivity in the T cells as there are less unique combinations when you are looking at only a few residues.
How does molecular mimicry work?
- An autoantigen epitope has a specific sequence.
- Other peptides from the body or pathogens look similar
- These other peptides mimic the autoantigen and can modulate autoimmunity. They can increase or decrease risk.
- Both self and foreign antigen can bind the same TCR to mount a protective response to infection or cause autoimmunity.
What do different amino acid substitution studies in TCR epitope tell us?
- TCRs can be very sensitive to single amino acid changes and this can remove all reactivity.
- TCRs can also accept multiple substitutions and still be able to respond.
- TCRs can recognise a range of peptide that look similar.
- TCRs can be activated by 2 peptides with completely different sequences.
- TCR cross-reactivity is common.
How would controlling autoreactive T cells work if a TCR had a single specificity?
- It would be very easy
- You could just eliminate all autoreactive T cells easily.
- However human T cells have multiple specificities.
How does controlling autoreactive T cells work as a TCR has multiple specificities?
- Most autoreactive T cells recognise self peptides with low avidity and are fine.
- T cells that recognise self peptides with high avidity are deleted in thymic selection.
- However, no matter the threshold between low and high avidity, the body will always retain some self-reactive T cells.
How does cross reactivity with TCRs work?
- Any TCR will have some cross reactivity with self antigens.
- Most of these T cells will recognise pathogenic antigens at low concentrations.
- They could also recognise self-antigens but at very high concentrations.
- The difference in avidity and response the T cell makes to the antigen is used by the immune system to determine recognition.
- The level of activation caused by the antigen feeds into the process of distinguishing between self and non-self.
What is keratitis?
Inflammation of the cornea
how can keratitis be induced in an animal model
- HSV infection was induced in an animal eye.
- The infection cleared but the inflammation remain.
- So what caused the continued inflammation? A self-antigen.
How was autoimmune keratitis used as experimental evidence for molecular mimicry?
- A wild-type mouse infected with the wild-type virus developed keratitis with moderate viral load.
- A wild-type mouse infected with mutant virus required a much higher viral load to cause keratitis. Alter epitope meant recognition was less efficient.
- a low affinity TCR transgenic mouse for the WT epitope needed a lower concentration of virus to cause autoimmunity as there were lots of TCRs present to recognise the antigen.
- the transgenic mouse treated with the mutant virus developed no disease.
- The control could not recognise the viral antigen and no disease was induced.
- This provides evidence for molecular mimicry. Infection can produce epitopes that autoreactive T cells can recognise and then cause disease.
What is sympathetic Ophthalmia?
A condition where an injury in 1 eye causes that other eye to develop inflammation.
What is needed to activate autoimmunity through bystander activation?
- Normal cell proteins
- Innate immune activation.
How are T cells normally activated in terms of signal strength?
- Some T cells recognise foreign antigens with high avidity. These are the ones that are activated.
- Most T cells ignore most peptides.
- Due to thymic selection there are few T cells with TCRs that respond to self antigens with high avidity.
- There is an activation threshold which is sufficient to activate foreign peptide recognising cells but high enough that it doesn’t activate self peptides recognising cells.
What happens to activate autoreactive T cells during bystander activation?
- If there are lots of innate activation signals like DAMPs and PAMPs, the threshold of activation could be reduced.
- This strong immune environment can provide the co-stimulation and cytokines an autoreactive T cell wouldn’t otherwise get.
- A really strong immune response that activates a lot of different T cells including some that wouldn’t normally be activated to expand the range of the immune response and increase the likelihood of activating a T cell that can deal with the underlying cause.
How was autoimmune keratitis use as evidence for bystander activation?
- Transgenic mice for the low avidity TCR C1-6.
- These mice developed autoimmunity with innate stimulus alone.
- The T cells need to have the potential for autoimmunity.
Why do autoimmune diseases go through cycles of relapse and remission?
- Elimination of autoantigens is difficult due to their essential function like myelin.
- ‘new’ immune responses to previously untargeted antigens via epitope spreading.
- Changes in the activation threshold of the environment.
- Changes in the target tissue that make reactivation more likely.
What changes to the tissue can make autoimmune reactivation more likely?
- Inflammation can cause increased antigen presentation.
- Tissues that undergo autoimmune disease have increased immunosurveillance.
- Tissue-specific memory is present in autoimmune tissues.
What autoimmune disease is associated with relapse due to activation threshold changes?
Infections causing relapse in MS
What is epitope spreading?
- A theoretical concept with some experimental evidence.
- Ongoing inflammation causes other peptide epitopes from the same protein to be presented to autoreactive T cells.
- This makes more autoantigens available to be recognised by and activate autoreactive T cells.
- This can also happen with peptides from other antigens in the tissue.
- It can drive disease progression.
What is the experimental evidence for epitope spreading?
- An immune response was generated in mice 1 with an epitope from the self antigen MBP and one from the foreign hen egg lysozyme.
- After a week both mice had high T cell proliferation and a strong immune response. They don’t have responses to other epitopes in the protein they have the potential to respond to.
- After a month the hen egg lysozyme mice were no longer sick, behaved normally and had a reduced antigen specific response to the hen egg.
- After a month the MBP mice still had inflammation, developed experimental autoimmune encephalomyelitis and had generated antigen specific responses to other peptides in the same protein.
What drives organ specific autoimmunity?
Tissue antigens
What initiates autoimmunity?
- Infection
- Danger
- Chance
Why is autoimmunity a relapsing and remitting process?
- Reactivity can spread through epitope spreading.
- Activation threshold can vary on a day by day basis based on health, environmental stimuli and infection.
Why can’t autoreactive T cells be eliminated from the repertoire?
- The only thing worse than autoimmunity is no immunity.
- Elimination means getting rid of all randomly generated antigen specific cells which is very dangerous.
