L3 - Cellular basis of autoimmunity (I) Flashcards
- to review evidence for the role of CD4+ T cells in the pathogenesis of organ-specific autoimmunity - To understand current hypotheses for the initiation of autoimmunity at the level of immune recognition - To learn about cellular mechanisms underlying common patterns of disease
Overview of autoimmunity
A. You get initiation of disease
B. this results in the expansion and trafficking of cells
C. there is localisation to a target organ e.g. eye, joint or brain where there is activation and damage
D. There is amplification, regulation and establishment of immunological memory (+ tissue-resident memory)
E. In the “wild” if left untreated, these conditions often remit but then there can be reactivation and the cycle starts again.
what is a classic presentation of autoimmunity
the cycle of people getting ill and then getting better ( why it is often difficult to diagnose)
When looking at autoimmunity and how the cells are responding what do we use
animal modules which can have spontaneous or induced autoimmunity
what are some spontaneous autoimmune animal models
- type I diabetes = Non obese diabetic (NOD) mouse
- TCR transgenic animals (have had a susceptibility element introduced e.g a T cell receptor introduced as a transgene)
what are some induced autoimmune animal models
Involves immunising the animals to initiate an immune response
- Multiple Sclerosis: Experimental Autoimmune Encephalomyelitis (EAE).
- Uveitis: Experimental Autoimmune Uveoretinitis (EAU).
- Other models: TCR transgenic animals to mimic specific autoimmune conditions (mimics TCR which we know are dangerous)
what is the advantage of inducing a model to have autoimmunity
allows scientists to time the disease in a way which is impossible in a stochastic situation
When did autoimmunity become recognised
in the late 1800s (1885) through work from Pasteur’s lab while developing a treatment for rabies. They prepared extracts from rabbit spinal cords to immunise people, which triggered a protective response against rabies but also provided early insights into the immune system’s ability to attack self-tissues.
How potent is rabies disease
Very potent and is almost always fatal once symptoms appear
What was the side efffect rate for the rabies treatment
1:1000 with patients deeloping acute paralytic illness ( although is high, the risk is better than the fatal outcomes of rabies)
What was happening in people who develop the paralytic disease after rabies treatment
The immune system in the human was confusing rabbit brain and human brain tissue. While they generated an immune response that protected them from the rabies, they were getting a cross reaction that targeted brain tissue causing paralytic effects. This showed how the immune system could also damage the body.
When did autoimmunity become a governing / mainstream idea
1956
Which organ was one that established an important medical concept for autoimmunity
the thyroid ( can see slides where e.g. lymphocytes have infiltrated the organ demonstrating it is a disease of immunity / antibodies rather than the gland itself)
What causes Grave’s disease
Autoantiboes (predominantly IgG1 which are T-helper dependent) which stimulate the TSHR. thyroid grows and secretes too much thyroid hormones
What are Grave’s disease symtoms
increased metabolic rate with rapid or irregular heartbeat, bulging eyes, goiter ( enlarged thyroid gland) and skin disease
What is Grave’s Disease a subset of
Hasmimotis thyroiditis
What does genetics demonstrate about assocation with increased susceptibility to autoimmune diseases
MHC ( Class II in specific over class I)
what implies that T cell help is associated with the pathogenicity of thyroid autoimmunity
the fact that pathogenic immunoglobulins in the thyroid are high affinity IgGs and that Class II expression is upregulated in inflamed tissues (CD4 T cells)
History of CD4+ T Cells as Key Mediators in Specificity **
1960: Mixed lymphocyte populations were transferred between animal models to test for pathogenicity. This revealed that autoimmune encephalomyelitis (EAE) could be transferred with cells but not with antibodies.
1969: Thoracic duct lymphocytes were shown to transfer EAE. These lymphocytes are highly concentrated as they return through the lymphatic drainage system.
1981: Antigen-specific T cell lines were developed to transfer EAE from immunised animals. Researchers expanded these lines to demonstrate that they were responsible for disease transfer.
1984: T cell receptors (TCRs) were cloned molecularly for the first time.
1985: MHC class II-restricted, antigen-specific T cell clones were confirmed to transfer EAE. Uniform cell lines with a single T cell receptor were found to be MHC class II restricted.
1990: The immunodominant class II-restricted T cell epitope of myelin basic protein (MBP) was identified in humans.
1993: TCR transgenic mice were observed to develop spontaneous EAE.
2000: CD4-activating peptide analogues of MBP were shown to precipitate relapses in multiple sclerosis.
How can one measure (T) cell growth experimentally
by measuring how much radiolabeled thymidine is incorporated into the DNA of dividing T cells - a measure of DNA replication and cell growth.
What happens to T-cell proliferation in encephalitogenic clones when an anti-MHC class II antibody is applied?
T-cell proliferation is significantly inhibited, with levels reduced by 1 to 2 orders of magnitude, demonstrating the crucial role of MHC class II in T-cell activation.
How did blocking antibodies help identify the role of specific MHC molecules in T-cell activation? ( looking at EAE in mice)
Blocking antibodies targeting specific MHC molecules (e.g., I-A𝑢 and I-E𝑠/𝑢) revealed that T-cell activation is MHC-restricted and depends on a single MHC molecule, confirming its role in antigen-specific responses.
Why do encephalitogenic T-cell clones cause auto-inflammation in the brain?
These T-cells recognise an antigen from the brain and are activated in the brain. In other organs like the liver or spleen, they don’t encounter this antigen and remain inactive.
What is the purpose of using different antibodies in this experiment?
Different antibodies were used to determine which MHC molecule (e.g., I-A𝑢u or I-E𝑠/𝑢s /u) mediates the antigen-specific activation of T-cells in this mouse strain.
What does the table show about T-cell proliferation in response to anti-I-A𝑢u and anti-I-E𝑠/𝑢s/u?
Proliferation is reduced significantly with anti-I-A 𝑢 u , suggesting dependency on this MHC molecule for activation, whereas anti-I-E𝑠/𝑢s /u does not block activation as effectively.
How does the use of blocking antibodies help in this study?
Blocking antibodies help identify which MHC molecules are involved in the activation of specific T-cell clones by preventing their interaction with the antigen.
What conclusion can be drawn about MHC specificity from this study?
T-cell activation is antigen-specific and restricted to a single MHC molecule, which explains the tissue-specific auto-inflammatory response.
What is EAE / experimental autoimmune encephalomyelitis
an animal model of the autoimmune disease multiple sclerosis (MS) used in research to study the mechanisms and potential treatments for MS
What did the transgenic mice have when investigating spontaneous EAE in 1994 and what was shown
the mice had a T cell receptor expressed as a transgene, so some mice only had antigen specific transgenic T cells that recognise auto antigens. over time it was found that all these mice developed spontaneous EAE. - shows that all you have is a TCR that is sufficient to for every animal to develop autoimmune disease.
What drives the proliferation of T cells
when the TCR binds to the correct peptide-MHC complex. This activates signalling pathways, promoting T cell division and cytokine release. The peptide presented by MHC differs across tissues, guiding the immune response / autoimmunity specificity
identifying autoantigens
- Choose a candidate protein e.g. a brain protein like MBP
- Analyse immune response to whole proteins when experimented on animals
- Screen overlapping peptides to find epitopes
- proof of principle where they take peptides / proteins that they have identified to see if they induce disease
- Identify and characterise the inciting cells
- Correlate with responses in humans
- CAUTION - the presence of autoreactive cells does not equal disease
What was surprising about autoreactive cells
that most people have autoreactive T cells (or T cells that have the potential to recognise autoantigens) circulating in their periphery without having autoimmune diseases.
What are examples of autoantigens in T1D
- Insulin
- Glutamic acid decarboxylase (GAD)
- Zinc transporter 9 (ZnT8)
- Islet-specific glucose 6 phosphatase catalytic subunit-related protein (IGRP)
what are examples of autoantigens for Multiple sclerosis
- Myelin Basic Protein (MBP)
- Myelin Proteolipid Protein (PLP)
- Myelin oligodendrocyte protein (MOG)
- aBcrystallin
What are the characteristics of autoantigens that are recognised by the immune system in the context of autoimmunity
- epitopes that bind to MHC
- Naturally processed
- Induce disease
What is a key characteristic of autoantigens related to MHC binding?
Autoantigens have epitopes that bind to MHC, allowing them to be presented and activate autoreactive immune cells.
How are autoantigens processed in the context of autoimmunity?
Autoantigens are naturally processed inside cells and presented by MHC molecules. However, in autoimmunity, this process can trigger inappropriate immune responses. Processing rules in vitro differ from those in cell culture.
How do autoantigens contribute to autoimmune diseases?
Autoantigens can induce disease by provoking immune responses that cause tissue damage, inflammation, and the development of autoimmune conditions.
What is the test used to determinine if an autoantigen induces disease?
the acid test
What happens in an acid test
Transfer immune cells (e.g., T cells or B cells) from a diseased individual to a healthy recipient.
If the recipient develops the disease, it confirms the immune response induces the disease.
Example: Transferring T cells in Experimental Autoimmune Encephalomyelitis (EAE) causes MS-like symptoms.
How many autoantigens are there in a single protein and give an example
several autoantigens (peptide epitopes)
What is an example of a protein with multiple autoantigens that can cause EAE
Proteolipid protein (PLP) has three peptide epitopes that can cause EAE when presented by the I-A^s MHC class II molecule in mice.
What do professional antigen-presenting cells (APCs) present in the absence of additional triggers / infection in healthy individuals?
MHC class II molecules on APCs are constantly loaded with self-derived antigens, which are presented to T cells in secondary lymphoid organs, maintaining homeostasis without causing disease
Why do T cells need some level of cross-reactivity in secondary lymphoid organs?
as part of their homeostasis to maintain responsiveness, but under healthy conditions, this does not lead to disease or autoimmunity.
How do specific T cells get activated
Through molecular mimicry and bystander activation
What is the critical question in understanding autoimmunity?
understanding how the immune system transitions from being balanced and protective to becoming mis-activated, leading to autoimmunity and damage
What is molecular mimicry, and who introduced the concept?
credited to Fujinami and Oldstone (1985), refers to the idea that T cell receptors (TCRs) can recognise similar peptides from autoantigens and pathogens, leading to cross-reactivity that may cause autoimmunity.
Why are TCRs more likely to cross-react with different peptides?
Because they are parsimonious, focusing on a few key residues within a peptide rather than every amino acid, increases the potential for cross-reactivity with similar antigens.
How does molecular mimicry contribute to autoimmunity?
Pathogen-derived peptides can share key recognition elements (e.g., KWLG in a bacterial protein vs self) with autoantigens, leading to T cell activation and potential modulation of autoimmune responses, which can either enhance or reduce autoimmunity.
How was molecular mimicry validated?
Databases were used to identify naturally occurring peptides resembling autoantigens. These mimics were shown to modulate autoimmune responses, demonstrating their role in autoimmunity.
How sensitive are TCRs to amino acid changes? ( based on amino acid substitution studies)
highly sensitive to single amino acid changes; one substitution can eliminate reactivity entirely.
Can TCRs respond to peptides with multiple substitutions or completely different sequences? ( based on amino acid substitution studies)
Yes, TCRs can accept multiple substitutions without losing the ability to respond, and they can be activated by peptides with completely different sequences.
Why is predicting TCR recognition bioinformatically challenging?
TCRs recognise a range of peptides rather than a single one, making it difficult to establish structural rules for prediction.
a single TCR in a mouse has the potential to be stimulated by how many self peptides ( based on aa substitution studies)
~13 self peptides
How common is TCR cross-reactivity, and what does this mean for immune system education?
more common than previously appreciated. A single TCR can be stimulated by ~13 self-peptides, requiring the immune system to balance self and foreign recognition.
How does thymic selection handle TCR auto-reactivity?
eliminates strongly self-reactive cells but retains TCRs with low-level auto-reactivity, allowing some auto-reactive T cells to remain.
What is the difference between low- and high-avidity TCR interactions?
Low avidity: Weaker interactions dominate and are less likely to trigger auto-reactivity.
High avidity: Stronger interactions are more selective but can still permit some auto-reactivity.
What is molecular mimicry in the context of TCRs and autoimmunity
occurs when TCRs react to both pathogen-derived and self-antigens, potentially leading to autoimmune responses.
What is experimental evidence for molecular mimicry
In an experiment using HSV keratitis (jertitis) mice, the eye was scratched and infected with a virus. After 5 days, the virus was undetectable, yet inflammation and disease continued to progress in susceptible strains. This persistence of disease despite the absence of the virus demonstrates molecular mimicry, where T cells react to self-antigens resembling the viral epitopes.
What was the purpose of the HSV keratitis experiment in mice?
The experiment aimed to investigate molecular mimicry by studying how TCRs react to viral and self-antigens in the context of autoimmune keratitis.
What happens in wild-type (WT) mice infected with the wild-type HSV virus?
Wild-type HSV virus causes autoimmune keratitis in WT mice at a viral dose of 4×10⁴ PFU.
What happens when WT mice are infected with the mutated virus (S309L)?
WT mice require a much higher dose of the mutated virus (4×10⁷ PFU, three orders of magnitude higher) to cause disease.
Why does the mutated virus (S309L) require a higher dose to cause disease?
The mutation alters the viral epitope recognised by TCRs, reducing the TCR’s ability to respond effectively. As a result, a higher viral concentration is needed to trigger the autoimmune response.
How do transgenic mice with dangerous TCRs respond to the wild-type virus?
Transgenic mice are much more sensitive, developing autoimmune keratitis at a lower viral dose (4×10³ PFU), compared to WT mice.
How do transgenic mice respond to the mutated virus (S309L)?
The mutated virus did not cause disease in transgenic mice, even at the highest tested concentrations.
What does the inability of the S309L virus to cause disease in transgenic mice suggest?
that the S309L mutation disrupts the molecular mimicry process by altering the viral epitope, preventing TCR activation and autoimmune inflammation.
How does this experiment demonstrate molecular mimicry?
It shows that specific TCRs can react to both viral and self-antigens, leading to autoimmune disease (e.g., keratitis). The need for higher doses of the mutated virus highlights the importance of precise epitope recognition in driving molecular mimicry.
How does this experiment demonstrate molecular mimicry?
The S309L mutation cripples the viral epitope, reducing its ability to mimic self-antigens and activate TCRs, thereby failing to induce significant autoimmune inflammation
When does bystander activation occur in the context of T cells and autoimmunity?
occurs when T cells recognising self-antigens are activated due to a highly inflammatory environment with strong innate immune signals, even though they were not specifically responding to a pathogen.
What role do DAMPs and PAMPs play in bystander activation?
In a “hot” inflammatory environment with many DAMPs and PAMPs, dendritic cells present antigens aggressively, reducing the activation threshold for T cells, including self-reactive ones.
What are DAMPs and PAMPs
DAMPs (Damage-Associated Molecular Patterns) are molecules released from damaged or dying host cells and signal the presence of tissue damage triggering an inflammatory response whilst PAMPs (Pathogen-Associated Molecular Patterns) are molecules found on the surface of pathogens that act as a red flag for the immune system
How does thymic selection influence the likelihood of autoimmunity?
Thymic selection eliminates T cells with high affinity for self-antigens, leaving behind T cells that only respond weakly to self. However, under extreme inflammatory conditions, these weakly self-reactive T cells can still become activated, contributing to autoimmunity.
How does T cell activation relate to signal strength and frequency patter
Foreign Antigens:
A small number of T cells produce high TCR signal strength, crossing the activation threshold to trigger an immune response.
Most T cells produce low signals and remain unactivated.
Self-Antigens:
Thymic selection removes T cells with strong affinity for self-antigens, leaving only weakly self-reactive T cells.
These weak signals are typically below the activation threshold, preventing autoimmunity.
Inflammatory Environment:
In a “hot” environment with many DAMPs and PAMPs, the activation threshold lowers.
This can activate self-reactive T cells, increasing the risk of autoimmunity.
Signal strength and frequency help distinguish between immune responses to foreign and self-antigens.
What is sympathetic ophthalmia, and how does it relate to autoimmunity?
an autoimmune condition where trauma to one eye causes inflammation and damage in the uninjured eye. This occurs when self-proteins from the injured eye are presented in an inflammatory context, triggering an autoimmune response. T cells that weakly recognise these self-antigens can become activated due to the strong innate immune signals associated with the trauma. This case underscores how self-proteins, when presented alongside intense inflammatory signals, can provoke autoimmunity.
What does the case study of the man with an eye injury teach about autoimmunity?
involves a 56-year-old man who suffered retinal detachment and blindness after a squash court accident. Doctors considered removing the injured eye due to the risk of developing sympathetic ophthalmia,
What experimental evidence supports bystander activation?
In HSV keratitis mice with a transgenic TCR, it was shown that strong innate immune activation alone (without foreign antigens) could activate T cells, demonstrating how bystander activation contributes to autoimmunity.
How do foreign antigen-specific T cells differ from self-antigen-specific T cells in their activation profiles?
Foreign antigen-specific T cells typically generate a stronger TCR signal and are more likely to meet the activation threshold.
Self-antigen-specific T cells usually generate weaker signals, but strong costimulation in inflammatory environments can still activate them.
How does bystander activation differ from molecular mimicry?
Molecular mimicry involves T cells mistaking self-antigens for foreign antigens due to structural similarity.
Bystander activation happens when a strong inflammatory response lowers the activation threshold, activating self-reactive T cells independently of mimicry.
Why is there relapses and remissions in autoimmune diseases
a number of reasons e.g.
- it is difficult to eliminate all the autoantigens e.g. myelin in the brain safely
- “new” immune responses end up developing to previously untargeted antigens
- changes in activation threshold of the environment ( infection)
- The target tissue is changed by the inflammatory process 9the immune system doesn’t reset back to a naive set but back to a stage that is at higher risk of developing further autoimmune disease
What is epitope spreading
Process in an autoimmune response, where peptides from the same autoantigens or from other proteins are progressively recognised leading to the activation of autoreactive T cells as the disease advances
What role does inflammation play in epitope spreading?
Ongoing inflammation causes the presentation of additional peptides from the same autoantigen, and possibly from other proteins, which leads to the activation of autoreactive T cells.
How does epitope spreading progress over time
As the disease progresses, inflammation leads to the presentation of more peptides, including ones from different proteins, further activating autoreactive T cells and amplifying the autoimmune response.
What happens when peptides from other proteins are presented during epitope spreading?
they can also be recognised by autoreactive T cells, potentially expanding the autoimmune response beyond the initial autoantigen.
What was the focus of the 1992 paper regarding epitope spreading?
focused on intramolecular epitope spreading, demonstrating how immune responses to specific peptides from antigens can expand over time to involve other peptides from the same antigen or related proteins.
What two antigens were used in the 1992 study?
Myelin Basic Protein (MBP) and henoglycosine, a completely foreign antigen.
How were the immune responses to the peptides studied in the mice?
were measured by the proliferation of T cells in response to the peptides from MBP and henoglycosine. Mice were monitored after a week of exposure and then again after a month to see how they reacted to different peptides and compare them
What was observed in the mice one week after immune response induction?
One week after exposure, mice that had been given MBP showed a strong immune response, but only to AC1-11 and no other epitope. In contrast, the mice that received henoglycosine responded to 30-53 ( no abnormal response)
What happened after a month in the study, specifically with the MBP and henoglycosine groups?
After a month, mice that received henoglycosine behaved normally, with no signs of disease. Mice that had been given MBP, however, developed Experimental Autoimmune Encephalomyelitis (EAE), showing signs of ongoing brain inflammation and clinical symptoms.
What were the differences in immune responses after a month?
the mice exposed to MBP developed immune responses to a wide range of other antigens (epitope spreading) and more of a response to Ac1-11 , while those exposed to henoglycosine maintained a specific immune response.
What does the difference in response between the MBP and henoglycosine groups suggest about epitope spreading?
that intramolecular epitope spreading can lead to the activation of autoreactive T cells to additional peptides, potentially amplifying the autoimmune response, as seen in MBP-treated mice developing EAE.
What drives organ specific autoimmunity
tissue antigens
What initiates autoimmunity
Infection, danger and change
Why is it a relapsing remitting process
reactivation can spread and activation thresholds can vary
why cant autopathogenic T cells be eliminated from the repertoire
Because the only thing worse than autoimmunity is no immunity at all
