Tolerance and autoimmunity

Question 1

Define autoimmunity

Answer 1

Autoimmunity: adaptive immune responses with
specificity for self “antigens” (autoantigens)

However, innate immunity also involved in pathogenesis

Question 2

What can lead to the development of autoimmune disease

Answer 2

“normal
autoimmunity”
(healthy)

breakdown of self tolerance- due to environmental and self tolerance

this can lead to autoimmune disease

Question 3

What is important to remember about the normal immune system

Answer 3

Due to how TCRs are synthesised- random combination of gene segments- we can make TCRs reactive against self-antigens- need to develop tolerance against autoimmunity.

Question 4

Describe the different criteria for a disease to be autoimmune

Answer 4

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
Elimination of the autoimmune response modifies disease
History of autoimmune disease (personal or family), and/or MHC associations

Question 5

Describe the different genetic and environmental risk factors for autoimmune disease

Answer 5

Genes: twin and family studies, GWAS (e.g. 40 key loci in SLE)- identical twins- 40% increased risk of T1DM.
Sex: women more susceptible (e.g. 9:1 in SLE)
Infections: inflammatory environment
Diet: obesity, high fat, effects on gut microbiome: diet modification may relieve autoimmune symptoms
Stress: physical and psychological, stress-related hormones- cortisol
Microbiome: gut/oral microbiome helps shape immunity, perturbation (dysbiosis) may help trigger autoimmune disease (sex differences?)

Question 6

Describe the gradient of sex tropism in autoimmune disease

Answer 6

 There is a gradient of AI disease sex tropism though; DM affects more men whilst SLE and thyroid disease affects much more women.

Question 7

Where is the microbiome located

Answer 7

Intestines- especially large
Stomach
Mouth, pharynx and respiratory system
skin
urogenital tract

Question 8

Outline the mechanisms of autoimmune disease

Answer 8

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

Autoimmune diseases involve breaking T-cell tolerance- they are IgG mediated so need T cells to get this class-switch

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

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

Question 9

Describe the impact of autoimmune disease

Answer 9

Approx. 100 chronic disorders have been identified which relate to aberrant immune responses causing the body to attack it’s own tissues

Approx. 8% of individuals are affected by autoimmune disease

Approx. 80% of affected individuals are women

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

Question 10

Summarise the hygiene hypothesis

Answer 10

The environment when our immune system evolved in is not the same as the environment now, therefore the normal regulation of the immune system is different.

Question 11

Describe the role of pregnancy in autoimmune disease

Answer 11

Don’t want inflammation during pregnancy.
therefore we get more Th2 responses- leading to more production of antibodies- worsening diseases mediated by antibodies such as SLE

Due to less inflammation, we get less Th1 activation- therefore Rheumatoid Arthritis gets better during pregnancy.

Question 12

Describe the epidemiology of some common autoimmune diseases

Answer 12

Rheumatoid Arthritis: 1 in 100: 2.1 million cases, 30-50,000 children
Type I diabetes: 1 in 800: 300-500,000 cases, (123,000< 20yrs old)
Multiple Sclerosis: 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 13

How do we describe autoimmune disease in humans

Answer 13

Organs affected

Involvement of specific autoantigens

Types of immune responses

Question 14

Describe the graded increase of autoimmune diseases from organ-specific to multi-system

Answer 14

Graves’ disease			Thyroid
Hashimoto’s thyroiditis		Thyroid
Type I diabetes			Pancreas
Goodpasture’s syndrome		Kidney
Pernicious anaemia		Stomach
Primary biliary cirrhosis		Liver, Bile 
Myasthenia gravis		Muscles
Dermatomyositis/Polymyositis	Skin/ Muscles
Vasculitis			Blood vessels
Rheumatoid Arthritis		Joints
SLE				Multiple targets

Question 15

Describe the pathophysiology of autoimmune haemolytic anaemia

Answer 15

There are autoantibodies against red blood cells, which bind to red blood cells and activate complement
This results in clearance and complement-mediated lysis of the autologous erythrocytes - leading to anaemia

Question 16

Describe how we know haemolytic anaemia is autoimmune

Answer 16

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

IgG antibodies- so can cross placenta

A woman with Graves’s disease during pregnancy may pass the antibodies onto the fetus- making them hyperthyroid until the IgG is turned over.

Question 17

Describe the different types of immune reaction that play a role in the pathology of autoimmune disease

Answer 17

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)

Question 18

Summarise the type 2 hypersensitivity reactions

Answer 18

Autoimmune haemolytic anaemia- autoantigens against Rh blood group antigens or I antigens- leading to destruction of red cells by complement and phagocytes leading to anaemia

Myasthenia Gravis- auto antigens against Ach receptor- leading to progressive muscle weakness

Autoimmune thrombocytopenia purpura- auto antigens against platelet integrin gp2b:3a leading to abnormal bleeding

Graves’s disease. – auto antigens against TSH receptor, leading to hyperthyroidism

Pemphigus vulgaris- auto antigens against edpidermal cadherin leading to blistering of skin

acute rheumatic fever- auto antigens against streptococcal cell wall antigens, antibodies cross-react with cardiac muscle– leading to arthritis, myocarditis, and late scarring of heart valves

good pasture’s syndrome– auto antigens against non-collagenous domain of BM collagen IV, leading to glomerulonephritis and pulmonary haemorrhage.

Question 19

Describe the diagnosis of good pasture’s syndrome

Answer 19

fluorescent
anti-IgG
stain

neutrophil
infiltration – -will see lots of purple indicating proliferation and infiltration.

Question 20

What is good pasture’s syndrome

Answer 20

Type 2 hypersensitivity reaction in which there are IgG antibodies against a type IV collagen found on the basement membrane in the glomerulus
This results in deposition of autoantibodies in the renal corpuscle and activation of complement leading to infiltration of inflammatory cells and kidney damage
NOTE: the inflammatory cells (e.g. neutrophils) bind to the Fc portion of antibodies via their own Fc receptors

Question 21

How do type 2 and type 3 immune reactions recruit inflammatory cells

Answer 21

Inflammatory cells are recruited via the binding of inflammatory cells to the Fc portion of antibodies via their Fc receptors

Question 22

Describe the type 3 hypersensitivity reactions involved in autoimmune diseases

Answer 22

o Type 3 hypersensitivity – immune complex (formed by antigen against soluble antigens).
 SLE – immune complex deposition in glomerulus.
• Autoantigen – DNA, histones, ribosomes, snRNP, scRNP.
• Consequence – glomerulonephritis, vasculitis, arthritis

Destruction of tissues via neutrophil granule enzymes, ROS

Question 23

Describe the type 4 hypersensitivity reactions involved in autoimmune disease

Answer 23

o Type 4 hypersensitivity – T-cell mediated (delayed type hypersensitivity) – CD8+(cytotoxic) and CD4+ (T-cell) responses may become involved AS WELL AS B-cell responses.
 Diabetes mellitus:
• Autoantigen – pancreatic beta cell antigen.
• Consequence – beta-cell destruction.
 Rheumatoid arthritis:
• Autoantigen – synovial joint antigen.
• Consequence – join inflammation & destruction.
 Multiple sclerosis:
• Autoantigen – myelin basic protein, proteolipid protein.
• Consequence – brain degeneration (demyelination), weakness/paralysis.

Can have antibodies against insulin or cyclin citrulinated proteins in RA

Question 24

Summarise the normal T-cell response

Answer 24

 Antigen is presented to T-cells by MHC.
o MHC II (DP, DQ, DR)  CD4+ TCR.
o MHC I (A, B, C)  CD8+ TCR.
 MHC II is the dominant genetic factor affecting susceptibility to autoimmune disease – T-cells may initiate AI disease…

Activation leads to Response:
Proliferation
Function

Co-stimulation also important in activation of naive T cells

Question 25

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

Answer 25

Lots of mutations in HLA-DR (class 2) alleles associated with an increased risk of autoimmune diseases such as T1DM, graves's and multiple sclerosis 
also SLE and RA.

Question 26

Summarise human autoimmune diseases

Answer 26

Mechanisms in autoimmunity are the same as in normal responses against foreign antigens
Immune responses to autoantigens (self) have a direct role in the pathology of autoimmune diseases
Both B cells (antibody) and T cells can be involved
HLA associations strongly imply a role for T cells in initiating autoimmune disease

Question 27

Describe the freeman cattle experiment

Answer 27

Freemartin cattle have fused placentas
and exchange cells and antigen in utero.

Non-identical twins have different sets of blood group antigens.

As these twins are non-identical, as adult cattle they would normally be expected to react to each others cells and tissues

However:
Adult cattle tolerate blood transfusions from a non-identical twin
They also accept skin grafts from each other

Question 28

What did the freeman cattle experiment demonstrate

Answer 28

It showed that early exposure to foreign antigens allows the development of tolerance to those antigens

Question 29

Describe how the mouse models showed that the timing of the exposure is important

Answer 29

 Mouse models – these models show the TIMING of tolerance is important:
o If the donor supplied spleen and BM cells to a NEONATAL mouse, then the same adult mouse can accept a skin graft.
o If the donor supplied to an adult mouse, that same adult could not then accept a skin graft – cells had to be received in neonatal phase.

Question 30

Describe how the mouse models show that the specificity of tolerance is important

Answer 30

 Mouse models – these models show the SPECIFICITY of tolerance:
o If donor supplies cells to neonate then the same adult couldn’t accept a graft from a random other mouse.

Question 31

Define what is meant by tolerance

Answer 31

Defined as the acquired inability to respond to an antigenic stimulus

Question 32

What are the key features of tolerance

Answer 32

The 3 As’

Acquired -involves cells of the acquired immune system and is ‘learned’

Antigen specific

Active process in neonates, the effects of which are maintained throughout life

Question 33

What are the two types of immunological tolerance

Answer 33

Central Tolerance = happens during lymphocyte development

Peripheral Tolerance = once we’ve developed mature lymphocytes, there are mechanisms to develop tolerance

Question 34

What are the mechanisms of peripheral tolerance

Answer 34

Peripheral tolerance:
- anergy
- active suppression (regulatory T cells)
(- immune privilege, ignorance of antigen)

Question 35

What can failure in tolerance result in

Answer 35

Failure in one or more of these mechanisms may result

in autoimmune disease

Question 36

Summarise lymphocyte development

Answer 36

Immature B-cells ---- Immunoglobulin-secreting 
plasma cells (takes place in the bone marrow)

pre T-cells migrate from bone marrow to thymus — then mature in thymus

Question 37

What do T cells recognise in the thymus

Answer 37

 T-cells mature in the thymus.
o T-cells recognise peptides presented on MHC in the thymus – Thymic epithelial cells (TEC) or DC:
 MHC II (DP, DQ, DR)  CD4+ TCR.
 MHC I (A, B, C)  CD8+ TCR.

Question 38

Describe the selection of T-cells in the thymus as a result of central tolerance

Answer 38

Useless (can’t see MHC): die by apoptosis

Useful (see MHC weakly): receive signal to survive. “Positive selection”

Dangerous (see self strongly): receive signal to die by apoptosis. “Negative selection”

only 5% of thymocytes survive selection

Question 39

Describe central tolerance of B cells in the bone marrow

Answer 39

B-cells mature in the bone marrow:
 B-cell selection:
o No self-reaction  migration to periphery  mature b-cell.
o Multi-valent self-molecule  clonal deletion or receptor editing  apoptosis or mature b-cell.
o Soluble self-antigen  migrate to periphery  anergic b-cell (only expresses IgM, no IgD- not capable of mounting immune response).
o Low-affinity, non-crosslinking self-molecule  migrates  mature b-cell that is clonally ignorant.
 This last one has potential to become autoreactive.
 B-cell selection occurs by x-linking of surface IG by polyvalent antigens expressed on BM stromal cells to facilitate deletion.

Question 40

When do B cells express IgD

Answer 40

IgD is expressed after alternative splicing of heavy chain transcripts (Chapter 6). After B cells progress through this stage, they become mature B cells that express surface IgM and surface IgD. Once the B cell completes the early maturation stages in the bone marrow, it begins to migrate to peripheral lymphoid organs to complete its development. B cells that have fully rearranged Ig genes but have not yet encountered nonself antigen are known as naive B cells.

Question 41

Describe how central tolerance fails in autoimmune disease

Answer 41

APECED
Autoimmune
PolyEndocrinopathy-
Candidiasis-
Ectodermal 
Dystrophy
(autoimmune polyglandular disease, or APD)

Rare autoimmune disease which affects the endocrine glands
 Thyroid
 Kidneys
 Chronic mucocutaneous candidiasis
 Gonadal failure
 Diabetes mellitus
 Pernicious anaemia

Question 42

What does APECED result from

Answer 42

APECED results from a 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 (i.e expression of insulin in thymus)
Involved in the negative selection of self reactive T-cells in the thymus

So, if this is mutated - the medullary thymic epithelial cell (mTHEC) can’t express all peptides from around the body- so you can get persistence of auto-reactive T cells ( no clonal deletion or tolerance).

Question 43

Describe the genetic defects in SLE

Answer 43

SLE: Genes affecting multiple biological pathways
may lead to a failure of tolerance
(40-50 genes have been implicated in genetic susceptibility)

induction of tolerance (B lymphocyte activation: CD22, SHP-1): autoantibody production

apoptosis (Fas, Fas-ligand): failure in cell death

clearance of antigen (Complement proteins C1q, C1r and C1s): abundance/persistence of autoantigen- I.E CLEARANCE OF NUCLEAR ANTIGENS FROM APOPTOSING CELLS BEFORE THEY CAN BE RECOGNISED BU AUTO-REACTIVE T-CELLS

Question 44

Summarise T cell selection in the thymus

Answer 44

• Dependent on MHC: peptide: T-cell receptor (TCR) interaction
• Most cells die by neglect: no or very weak recognition
• Negative selection occurs for cells with high affinity TCRs, which die by apoptosis
• Surviving cells are MHC-restricted, with low/intermediate affinity for self-peptide

Question 45

Summarise B cell selection in the bone marrow

Answer 45

Crosslinking of surface immunoglobulin by polyvalent antigens expressed on bone marrow stromal cells facilitates deletion

Failure in central tolerance can lead to autoimmunity. In most diseases, a complex interaction of multiple factors is usually involved.

Question 46

Why is peripheral tolerance important

Answer 46

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 47

Describe anergy

Answer 47

Absence of cositmulation

Naïve T-cells require costimulation for full activation: CD80, CD86 and CD40 are examples of costimulatory molecules expressed on APC- bind to CD28 on T-cell
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

Particular important for naive cd4 t-CELLS

Question 48

Describe immunological ignorance

Answer 48

Occurs when antigen concentration is too low in the periphery

Occurs when relevant antigen presenting molecule is absent: most cells in the periphery are MHC class II negative- however, in inflammation, IFN-y can lead to normal cells expressing MHC and thus present self peptides to CD4

Occurs 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 tolerised against the auto-antigens

Question 49

Describe sympathetic ophthalmia as a failure of ignorance

Answer 49

Sympathetic ophthalmia
Damage to the eye can release eye antigens into the lymphatics and lymph nodes (intra-ocular antigens)
These antigens are recognised by T cells, which become activated against the eye antigens (immune privilege- so have never been tolerated to these antigens- so they are recognised as foreign).
The T cells then go back to both eyes and cause damage

Question 50

Why does ignorance exist

Answer 50

 Ignorance occurs when:
o Occurs when antigen concentration is too low.
o Occurs when relevant APC is absent – most cells in periphery are MHC II –ve.
o Occurs at immunologically privileged sites – immune cells cannot penetrate as an immune reaction could do more harm than good – i.e. the brain

Question 51

Summarise the suppression/regulaiton of auto-reactive T cells in the periphery

Answer 51

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+
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

Question 52

What is IPEX

Answer 52

IPEX: a failure in the regulation of peripheral tolerance

Immune dysregulation, Polyendocrinopathy, Enteropathy and X-linked inheritance syndrome

Question 53

Describe IPEX

Answer 53

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

	Symptoms include: 
early onset insulin dependent diabetes mellitus
severe enteropathy
eczema
variable autoimmune phenomena
severe infections

Accumulation of autoreactive T cells

Question 54

Give some examples of infections that can break peripheral tolerance

Answer 54

Multiple sclerosis
Epstein-Barr virus (EBV), measles virus

Lyme arthritis
Borrelia burgdorferi

Type I diabetes
Coxsackie virus B4, rubella virus, cytomegalovirus (CMV), mumps virus

Rheumatoid arthritis
Escherichia coli, mycobacteria, EBV, hepatitis C virus (HCV)

Lupus erythematosus
EBV

Myocarditis
Coxsackie virus B3, CMV, chlamydia

Rheumatic fever/myocarditis
Streptococci

Chagas’ disease/myocarditis
Trypanosoma cruzi

Myasthenia gravis
Herpes simplex virus, HCV

Guillain-Barré syndrome
CMV, EBV, Campylobacter spp.

Question 55

Describe how infection can break peripheral tolerance

Answer 55

 This can be done via:
o Molecular mimicry of self-molecules – i.e. Grave’s disease.
o Induce changes in expression and recognition of self-proteins.
o Induction of co-stimulatory molecules or inappropriate MHC II expression.
o Failure of regulation – effects in T-reg cells.
o Immune deviation – shift in type of immune response – e.g. Th1  Th2.
o Tissue damage at immunologically privileged sites such as the eye.

APC may become more effective at presenting self peptides- lots of co-stumulation- trigger response against self-antigens

Question 56

Summarise peripheral tolerance

Answer 56

Induction and maintenance of peripheral tolerance will depend on:
Site of antigen expression (MHC expression, immune privilege)
Timing of antigen expression
Amount of antigen expression
Costimulation
T cell help for B cell responses
Regulation

Infections may help break tolerance by a variety of mechanisms

Question 57

Describe links between the immune system and mental health

Answer 57

Inflamed mind

antibodies against structures in the brain – psychoses

Question 58

What is important to remember about autoimmunity

Answer 58

Results from failures in central or peripheral tolerance
Failure in central tolerance can occur:

When a particular self protein is absent or mutated. Subsequent exposure to the native form of this protein will promote an auto-immune response.
When the mechanisms involved in the removal of autoreactive cells during ​
T- or B-cell selection are faulty.
Failure in peripheral tolerance can occur

When mechanisms of peripheral tolerance are faulty.
As a result of infection.