2 - Tolerance and Autoimmunity

Question 1

What is autoimmunity?

Answer 1

AUTOIMMUNITY = adaptive immune responses with specificity for self “antigens” (autoantigens)

• B Cells
• T Cells

Everyone has receptors for self-antigens, but they do not cause disease in most people. In normal people, they are formed in recombination.

Question 2

What are the criteria for disease to be ‘autoimmune’?

Answer 2

Evidence of disease-specific adaptive immune response in the affected target tissue, organ or blood

Passive transfer of autoreactive cells or antibodies replicates the disease

• can occur sometimes naturally
• antibody-mediated disease is often IgG
• IgG can cross from mother to fetus via umbilical cord
• therefore, if mother has Grave’s Disease, fetus may have symtpoms of the disease initially when born

Elimination of the autoimmune response modifies disease

History of autoimmune disease (personal or family), and/or MHC associations

• doesn’t have to be the same autoimmune disease

Question 3

What genetic and environmental factors can predispose to autoimmunity?

Answer 3

Genes:

• twin and family studies
• GWAS (e.g. 40 key loci in SLE)

Sex:

• women more susceptible (e.g. 9:1 in SLE)
• women have stronger immune responses

Infections:

• inflammatory environment
• more activation of immune cells

Diet:

• obesity
• high fat
• effects on gut microbiome
• diet modification may relieve autoimmune symptoms

Stress:

• physical and psychological
• stress-related hormones

Microbiome:

• gut/oral microbiome helps shape immunity
• perturbation (dysbiosis) may help trigger autoimmune disease
• potential sex differences in microbiome between men and women

Question 4

Outline the importance of the human microbiome

Answer 4

Gut microbiome helps regulate our immune responses (homeostasis)

Question 5

What are the mechanisms of autoimmunity?

Answer 5

Adaptive immune reactions against self use the same mechanisms as immune reactions against pathogens (and environmental antigens)

Autoimmune diseases involve breaking T-cell tolerance

• T cells help B cells with class switching, despite the disease being mediated by antibodies

Because self tissue is always present, autoimmune diseases are chronic conditions (often relapsing)

• autoimmunity can be treated but will still require life-long management
• symptoms can be well controlled but relapse is common
• very difficult to cure

Effector mechanisms resemble those of hypersensitivity reactions, types II, III, and IV

Question 6

Describe the impact of autoimmune diseases

Answer 6

Approx:

• 100 chronic disorders have been identified which relate to aberrant immune responses causing the body to attack it’s own tissues
• 8% of individuals are affected by autoimmune disease
• 80% of affected individuals are women

The incidence of autoimmune disease (and hypersensitivity) is increasing (hygiene hypothesis)

Question 7

Describe the sex differences in autoimmune disease incidence

Answer 7

Different autoimmune diseases have different levels of prevalence in males and females

e. g.
* SLE: Much more prevalent in females

However, some diseases have less of a difference between the sexes than others

Question 8

List examples of important autoimmune diseases

Answer 8

Rheumatoid Arthritis

• 1 in 100
• 2.1 million cases
• 30-50,000 children (not just older people susceptible)

Type I diabetes

• 1 in 800
• 300-500,000 cases
• 123,000< 20yrs old

Multiple Sclerosis (MS)

• 1 in 700
• 250-300,000 cases
• 25,000 hospitalisations per year

Systemic Lupus Erythematosus (SLE)

• 240,000 cases

Autoimmune Thyroid Disease (ATD)

• including Hashimoto’s and Graves’ disease
• 5 cases/ 1000 women
• 0.8 cases per 1000 men

Question 9

How do you describe autoimmune reactions in humans?

Answer 9

Used to be described based on organs affected

Now:

• Involvement of specific autoantigens
• Types of immune responses

Question 10

List organ-specific autoimmune diseases and multi-systemic autoimmune disease, as well as their targets

Answer 10

Question 11

What type of antigens have been identified as playing a direct role in immune diseases?

Answer 11

Autoantigens have been identified in various autoimmune diseases which play a direct role in the immunopathogenesis

Early experiments showed that autoantibodies against red blood cells were responsible for autoimmune haemolytic anaemia in humans

Result in the clearance or complement-mediated lysis of autologous erythrocytes

• Direct link between autoantibodies and disease (also antibody transfer experiments)

Question 12

What immune reactions are known to play a direct role in the pathology of human autoimmune disease?

Answer 12

Antibody response to cellular or extracellular matrix antigen (Type II)

Immune complex formed by antibody against soluble antigen (Type III)

T-cell mediated disease (Delayed type hypersensitivity reaction, Type IV)

In most conditions, it is not one type of reaction but a mixture of the different types

Question 13

Name autoimmune diseases that are type II reactions

Answer 13

Question 14

What is Goodpasture’s syndrome?

Answer 14

It is also known as anti-glomerular basement membrane disease

It is a rare autoimmune disease in which antibodies attack the basement membrane in lungs and kidneys, leading to bleeding from the lungs and kidney failure.

Question 15

What is Graves’ Disease and outline its autoimmune nature?

Answer 15

Antibody stimulates TSH receptor

Normal negative feedback doesn’t work because antibody continues binding to receptor

Hyperthryoidism

Question 16

Name an autoimmune disease that involves Type III reactions

Answer 16

Systemic Lupus Erythematosus (SLE)

• antibodies against nuclear components (histones, double-stranded DNA)
• antibodies bind to antigens
• circulate as immune complexes
• deposited in organs and joints
• causes problems particularly in the kidneys
  
  • leading to glomeruler nephritis

Question 17

Why do some autoimmune diseases get better during pregnancy?

Answer 17

During pregnancy, there is a shift in immune responses away from inflammatory responses in order to protect the foetus

• e.g. rheumatoid arthritis symptoms may get better during pregnancy

However, for an antibody-mediated autoimmune disease, symptoms often get worse

• e.g. SLE

Question 18

What occurs in Type II and Type III hypersensitivity reactions?

Answer 18

TYPE II

• Insoluble antigen
• Antibody binds
• Localised tissue injury and recruitment
• Activation of complement
• Recruitment of inflammatory cells

TYPE III

• Same mechanism as Type II, just the location differs
• Soluble immune complexes in the circulation
• These complexes get deposited once they bind to a surface
• Then they can activate complement and inflammatory cells
• Inflammation at the site of deposition of the immune complexes

Question 19

Give some examples of Type IV (T-Cell mediated diseases) hypersensitivity

Answer 19

These reactions involve predominantly T-cells instead of antibodies

Question 20

Outline the normal T-cell response to antigens

Answer 20

Antigen is presented to T-cells by MHC molecules expressed on the surface of antigen-presenting cells

• CD4 T-cells restricted by class 2 molecules
• CD8 T-cells restricted by class 1 molecules

Question 21

What is MHC called in humans?

Answer 21

HLA - Human Leukocyte Antigen

Question 22

What MHC class is the dominant genetic factor affecting susceptibility to autoimmune disease?

Answer 22

Human MHC (HLA) class II is the dominant genetic factor affecting susceptibility to autoimmune disease

• strongest association with autoimmune disease
• relative risk goes up if you have HLA-DR molecules

Question 23

What is unique about Freemartin cattle?

Answer 23

They have fused placentas, and so can exchange cells and antigen in utero

Non-identical twins have different sets of blood group antigens. As they are non-identical, as adult cattle they would normally be expected to react to each others cells and tissues.

However, the adult freemartin cattle can tolerate blood transfusions froma non-identical twin. They can also accept skin grafts from each other.

THIS IS EVIDENCE THAT, IF YOU ARE EXPOSED TO SOMETHING IN UTERO, AS AN ADULT YOU DON’T FORM AN IMMUNE REACTION AGAINST IT

Question 24

What does early exposure to foreign antigens in neonatal mice allow for in adult mice?

Answer 24

If an adult mouse is given a skin graft from another mouse, the graft will be rejected.

However, if you give a neonatal mice some of the foreign antigens from another mouse, then attempt a skin graft when the mouse is older, the graft won’t be rejected.

THIS SHOWS THAT TIMING OF ANTIGEN EXPOSURE IS CRITICAL TO BUILD IMMUNE TOLERANCE.

Question 25

Give evidence to show that tolerance has specificity

Answer 25

Even if cells are given to a neonate mouse to introduce them to foreign antigens, if they are given a skin graft as an adult from a different strain, they will still reject it

Question 26

Define immunological tolerance

Answer 26

**IMMUNOLOGICAL TOLERANCE** = The acquire inability to respond to an antigenic stimulus

Question 27

What are 'The 3 As' involved in immunological tolerance?

Answer 27

**_THE 3 As_** **ACQUIRED** - involves cells of the acquired immune system and is 'learned' **ANTIGEN SPECIFIC** **ACTIVE** - active process in neonates, the effects of which are maintained throughout life

Question 28

How does self tolerance work and how does it fail?

Answer 28

**CENTRAL TOLERANCE** * deletion of auto-reactive cells in the primary lymphoid organs **PERIPHERAL TOLERANCE** * anergy * active suppression (regulatory T cells) * immune privilege, ignorance of antigen - never see the antigen Failure in one or more of these mechanisms may result in autoimmune disease.

Question 29

Outline the mechanism behind central tolerance

Answer 29

**CENTRAL TOLERANCE** All lymphocytes come from stem cells in the bone marrow T-cell precursors go to the thymus where a selection process occurs after generation of the T-cell receptor based on the peptides being presented on the thymic epithelial cells and dendritic cells in the thymus CD4+ T-Lymphocytes are selected on MHC II plus self peptide CD8+ T-Lymphocytes are selected on MHC I plus self peptide

Question 30

What happens to T-Lymphocytes during selection in the thymus?

Answer 30

**USELESS - can't see MHC** * die by apoptosis **USEFUL - see MHC weekly** * receive signal to survive * 'positive selection' **DANGEROUS - see self strongly** * receive signal to die by apoptosis * don't want these in the periphery as they will react strongly and cause disease * 'negative selection' ONLY 5% OF THYMOCYES SURVIVE SELECTION 95% ARE DELETED IN THE THYMUS

Question 31

What happens to B-Cells and how do they develop tolerance?

Answer 31

**IMMATURE B-CELLS = BONE MARROW** NO SELF-REACTION * become a mature B-cell * express IgG and IgM REACT STRONGLY WITH MULTIVALENT SELF MOLECULE * get deleted * or receptor editing occurs to make a receptor that isn't strongly self-reactive REACT AGAINST SOLUBLE SELF-MOLECULES WITH INTERMEDIATE AFFINITY * escape to periphery * become anergic * can't receive T-cell help * don't become fully activated to produce antibodies * reside in secondary lymphoid organs * slowly turned-over and removed REACT VERY WEAKLY WITH SELF-ANTIGEN BUT CAN REACT IF SELF-ANTIGEN IS IN HIGH AMOUNTS * have receptors to bind to self-antigen * "ignorant" * if they escape to the periphery, they can cause autoimmune disease * not deleted because they haven't seen their antigen in the bone marrow

Question 32

What is APECED?

Answer 32

**A**utoimmune **P**oly**E**ndocrinopathy **C**andidiasis **E**ctodermal **D**ystrophy = **APECED** It is an autoimmune polyglandular disease, or APD It is a rare disease that affects the endocrine glands

Question 33

What are the affected endocrine glands and symptoms in APECED?

Answer 33

Thyroid Kidneys Chronic mucocutaneouscandidiasis Gonadalfailure Diabetes mellitus Pernicious anaemia

Question 34

What causes APECED?

Answer 34

**APECED result froma failure to delete T-cells in the thymus** * caused by mutations in the transcription factor AIRE (autoimmune regulator) gene * AIRE is important for the expression of 'tissue specific' genes in the thymus * involved in the negative selection of self-reactive T-cells in the thymus * if the transcription factor doesn't work, the auto-reactive T-cells escape to the periphery and cause multi-organ disease

Question 35

What causes most autoimmune diseases?

Answer 35

Most autoimmune diseases are associated with multiple defects and genetic traits **SLE:** * Genes affecting multiple biological pathways * May lead to a failure of tolerance * (40-50 genes have been implicated in genetic susceptibility) * Some of these genes are associated with: * Induction of tolerance (B lymphocyte activation: CD22, SHP-1): autoantibody production * Apoptosis (Fas, Fas-ligand): failurein cell death * Clearance of antigen (Complement proteins C1q, C1r and C1s): abundance/persistence of autoantigen

Question 36

How does induction and maintenance of tolerance occur in the periphery?

Answer 36

Some antigens may not be expressed in the thymus or bone marrow, and may be expressed only after the immune system has matured Mechanisms are required to prevent mature lymphocytes becoming auto-reactive and causing disease * Anergy * Suppression by regulatory T cells * (Ignorance of antigen)

Question 37

Explain anergy

Answer 37

**ANERGY - absence of costimulation** Naïve T-cells require costimulation for full activation: CD80, CD86 and CD40 are examples of costimulatory molecules expressed on APC These are absent on most cells of the body Without costimulation then cell proliferation and/or factor production does not proceed Subsequent stimulation leads to a refractory state termed ‘ANERGY’

Question 38

Explain immunological ignorance

Answer 38

Occurs: * when antigen concentration is too low in the periphery * when relevant antigen presenting molecule is absent: most cells in the periphery are MHC class II negative * at immunologically privileged sites where immune cells cannot normally penetrate: for example in the eye, central and peripheral nervous system and testes. In this case, cells have never been tolerisedagainst the auto-antigens

Question 39

Outline an example of a failure of immunological ignorance

Answer 39

**FAILURE OF IGNORANCE - ex: Sympathetic Opthalmia** Normally, immune cells do not enter the eye. Therefore, the proteins in the eye are not seen by immune cells. Physical damage to eye causes a release of antigens that are only expressed in the eye via the lymphatics to the draining lymph node If there's some AP in the lymph node, the T-cells can get activated and cause immune response in both eyes

Question 40

Outline suppression/regulation by T-cells

Answer 40

**SUPPRESSION/REGULATION** Autoreactive T-cells may be present but do not respond to autoantigen Controlled by other cell types: * Regulatory T-cells…..CD4+, CD25+, CTLA-4+, FOXP3+ * control response to other cells * CD25 is the Interleukin-2 Receptor * CTLA-4 binds to B7 and sends a negative signal * FOX P3 is a transcription factor required for regulatory T-cell development * expressed in a high level

Question 41

What is IPEX?

Answer 41

**I**mmune dysregulation, **P**olyendocrinopathy, **E**nteropathy and **X**-linked inheritance syndrome = **IPEX**

Question 42

What causes IPEX?

Answer 42

A failure in the regulation of peripheral tolerance Fatal recessive disorder presenting early in childhood Mutation in the FOXP3 gene which encodes a transcription factor critical for the development of regulatory T-cells ---\> leads to an accumulation of autoreactive T-cells

Question 43

What are the symptoms of IPEX?

Answer 43

**SYMPTOMS INCLUDE:** * early onset insulin dependent diabetes mellitus * severe enteropathy * eczema * variable autoimmune phenomena * severe infections

Question 44

How are infections linked to autoimmune disease?

Answer 44

Question 45

How could infections affect the tolerant state?

Answer 45

Molecular mimicry of self molecules Induce changes in the expression and recognition of self proteins Induction of co-stimulatory molecules or inappropriate MHC class II expression: pro-inflammatory environment Failure in regulation : effects on regulatory T-cells Immune deviation: shift in type of immune response e.g. Th1-Th2 Tissue damage at immunologically privileged sites