W4L7 - Autoimmune Diseases

Question 1

Criteria for Autoimmune Response and Disease

Answer 1

1. Auto-antibodies or auto-reactive T cells found in disease
2. Auto-antibodies/T cells found at site of damage (organ)
3. Level of auto-antibody or T cell response reflect disease activity
4. Reduction of antibody or T cell response leads to reduction of disease
5. Transfer of autoantibody or T cell to another host leads to disease in recipient
6. Immunisation with autoantigen causes disease

Question 2

MHC

Answer 2

Strongest association with autoimmune diseases
HLA contributes approx 50% of genetic susceptibility in most autoimmune diseases
- HLA is the proteins encoded for, by the MHC
The strongest association is between HLA-B27 and ankylosing spondylitis
- inflammatory disease of vertebral joints, presumed to be autoimmune
- almost all people with ankylosing spondylitis are HLA-B27 positive (>95%), but very few people that are positive for HLAB27 have ankylosing spondylitis
- HLA-B27 testing used for exclusion

Question 3

Features of MHC

Answer 3

HLA allele associated may not be involved in disease
- the HLA may be inherited in linkage with another gene that is involved with the disease
Nucleotide differences within the polymorphic binding site is important
- hence association with different HLA types
- binding site binds the antigen for presentation
Disease associated sequences in the MHC are found in health people
- so MHC is not the cause of disease
- one of several contributing factors

Question 4

Non-MHC Genetic Associations

Answer 4

Many of these are genes that are involved in the immune system
Low contribution to disease risk
Some polymorphisms may protect and others increase incidence of disease
Many polymorphisms occur in non-coding regions
- effects expression of genes that encode a protein
- linkage with another gene

Question 5

Triggers of Autoimmunity

Answer 5

Cross reactivity with microbial antigen
- T cell or antibody directed against a microbe antigen also reacts against self antigen
- epitopes highly homologous (similar)
- known as molecular mimicry (important)
Sequestered antigen
- antigens normally hidden to immune system (e.g. cornea, cardiac antigens, cartilage)
- accident or infection exposes antigen
Polyclonal activation
- microbial activation of many clones of T or B cells
- non-specific so auto-reactive clones may be activated
Non-infectious triggers
- drugs, chemicals, hormones
- alter T cells directly or inhibit T suppressor cells

Question 6

Tolerance

Answer 6

Tolerance - unresponsiveness to an antigen that is induced by previous exposure to that antigen
- self tolerance when antigen is self antigen
- can have tolerance to foreign antigen
- antigen that induce tolerance called tolerogens or tolerogenic antigens
Only lymphocytes since they have receptors for antigen (TCR and BCR)
Two types:
1. Central Tolerance - during lymphocyte maturation
- bone marrow for B cells, thymus for T cells
2. Peripheral Tolerance - mature lymphocytes
- tissue specific self tolerance

Question 7

Mechanisms of Self Tolerance - Central

Answer 7

Receptor editing
- genetic rearrangement of the variable region of BCR and TCR
- BCR and TCR no longer specific for antigen
Clonal Deletion
- auto reactive cells eliminated by apoptosis
- B and T cells with strong affinity for self antigens

Question 8

Mechanisms of Self Tolerance - Peripheral

Answer 8

Anergy
- non-responsiveness of cells upon contact with antigen
- lack of second co-stimulatory signal (CD40/CD40L for B cells, and B7/CD28 for T cells)
Clonal Ignorance
- cells that remain inactivated due to low affinity with self antigen
- low concentration of antigen, low signal
T regulator cell
- T reg suppress T and B cells as well as NK cells
Fas – Fas L mediated apoptosis
- removal of mature auto-reactive B and T cells

Question 9

Central Tolerance - Receptor Editing

Answer 9

If the receptor binds self antigen - change it!
TCR and BCR are made from fragments of DNA to generate the diversity of receptor binding regions (somatic recombination)
Receptor editing occurs during development of the receptor when receptor binds to self antigen
Alpha chain (TCR) or light chain (BCR) is removed and a second somatic recombination occurs to produce new chain
New chain moves to join receptor and so the binding site is different and no longer binds self antigen
Occurs early in development of lymphocyte

Question 10

Peripheral Tolerance

Answer 10

Central tolerance does not remove all self reactive cells
- not all the self reactive cells in bone marrow/thymus
- some antigens expressed only when older
Therefore peripheral tolerance is important
- regulatory T cells (Treg) suppress T-help and B cells
- deals with sequestered self antigens that are hidden from immune system
- low affinity for self-antigen (clonal ignorance)
- anergy by absence of co-stimulatory signals
- removal of self reactive lymphocytes in periphery (Fas expression removal by FasL expressing T cells)

Question 11

Peripheral Tolerance

Answer 11

Peripheral Tolerance is important

• regulatory T cells (Treg) suppress T-help and B cells
• sequestered self antigen are hidden from immune system
• low affinity for self-antigen (clonal ignorance)
• anergy by absence of co-stimulatory signals
• removal of self reactive lymphocytes in periphery (Fas expression removal by FasL expressing T cells)

Question 12

Anergy

Answer 12

Functional unresponsiveness
Due to:
1. Lack of co-stimulation
- lymphocyte binds antigen but also requires costimulatory molecules to bind for activation
- lack of second co-stimulatory surface molecules (CD40/CD40L for B cells, and B7/CD28 for T cells)
- leads to a lack of innate response (no stimulating cytokines)
2. Self antigen binding sends stop signal
- without co-stimulation self antigen binding activates ubiquitin ligase causing proteolytic degradation of signalling proteins
3. Inhibitory receptors
- T cell binds self antigen, instead of co-stimulatory engagement, there is inhibitory receptor engagement (CTLA-4)
- terminate T cell response

Question 13

T-reg Cells

Answer 13

Originally call suppressor T cells
CD4+ T cells that suppress T-help, B cells and NK cells
Maintain self-tolerance
Express high levels of CD25 (IL-2 receptor α chain)
- IL-2 stimulation critical for T-reg
- expressed on activated T-help and B cells (but lower levels)
Typically express CTLA-4
Transcription factor FoxP3 is critical for development of T-reg
- TGF-β stimulates expression of FoxP3
- FoxP3 may exerts influence by binding DNA promoting regulatory genes

Question 14

Natural and Adaptive T-reg Cells

Answer 14

Natural - regulatory T cells development in thymus
Adaptive - strong innate immune responses can form Treg from naïve CD4+ T cells in the periphery
- can be specific for self or foreign antigens
- TGF-β stimulates expression of FoxP3

Question 15

How do T-reg Cells Suppress Immune Response?

Answer 15

Production of immunosuppressive cytokines TGF-β, IL-35 and IL-10
CTLA4 binding to B7 on APC
Binding high amounts of IL-2, reducing IL-2 levels for other cells
Defects in T-reg cells or reduced resistance of effector cells to T-reg suppression contributes to autoimmune disease

Question 16

Apoptosis

Answer 16

Self antigen reacting T cells may bind antigen with strong affinity and without co-stimulatory signals => induces apoptosis
Intrinsic:
- TCR binding activate pro-apoptotic molecules (Bim, Bax, Bak)
- co-stimulation activates anti-apoptotic molecules (Bcl-2, Bcl-XL which act by suppressing pro-apoptotic molecules)
- binding TCR without co-stimulation leads to apoptosis
Extrinsic:
- when T cells are repeatedly stimulated the express the death receptor FasL (Fas ligand)
- Fas (CD95) from the same or another T cell binds and activates caspases that lead to apoptosis

Question 17

Dendritic Cells

Answer 17

Involved in antigen presentation to T cells (CD4)
- initiate adaptive immune response
- phagocytose foreign antigen in periphery, as move to LN they mature and express co-stim molecules
DCs may be involved in tolerance
- resting DC do not express co-stimulatory molecules
- when uptake self antigens they may not mature
- these immature DC may be presenting self antigen to T cells without co-stimulation
- anergy of T cells
- differentiation into T-reg

Question 18

Graves Disease

Answer 18

Antibodies (IgG) directed against receptors for TSH
- thyroid stimulating antibodies
Stimulates thyroid follicle to produce thyroid hormones (T3/T4)
Hyperactive thyroid (hyperthyroidism)
- thyroid infiltrated with CD4 and CD8 cells
Termed “thyrotoxicosis”
5 to 10 times more common in women
Antibodies can cross placenta and result in “Neonatal Graves Disease”

Question 19

Graves Disease - Diagnosis and Treatment

Answer 19

Signs and symptoms
- family history (found in ~50% of cases)
- raised T3 and T4
- agglutination assays for antibodies to thyroid peroxidase
Treatment with anti thyroid drugs (carbimazole), radioactive iodine, surgery

Question 20

Hashimoto’s Thyroiditis

Answer 20

Both humoral and cellular response to thyroid tissue
Direct killing of the thyroid cells
First is T cell response
- infiltration of thyroid with lymphocytes (CD4), macrophages and plasma cells
Antibody production later and contributes to damage
It is a Hypothyroidism
Potentially due to antigens similar to thyroid antigens (molecular mimicry?)
Antibodies produced:
1. Anti-thyroglobulin antibodies
- precursor of thyroglobulin
- over 90% of patients with Hashimoto’s
2. Anti-thyroid peroxidase (anti-TPO) antibodies
- > 80% of patients with Hashimoto’s

Question 21

Clinical Presentation of Hashimoto’s Thyroiditis

Answer 21

Early in disease there may be extra release of T3/T4 due to damage and compensation
- appears like hyperthyroidism
Onset 30 -60 years, females (7 X more than Males)
HLA-DR3, 4, 5 associated, while DQ1 protective
Goiter - enlarged thyroid (compensatory)
Fatigue, dry coarse cold skin, weight gain, mild depression, slow speech and movement
Myxoedema (puffy face, pasty skin, large tongue)

Question 22

Hashimoto’s Thyroiditis - Diagnosis and Treatment

Answer 22

Low T4 and elevated TSH
- (T3 sometimes low)
ELISA (or other) for anti-thyroglobulin and/or anti-TPO
Treated with Thyroglobulin (T3 and T4) replacement therapy

Question 23

Systemic Lupus Erythematosus (SLE)

Answer 23

Systemic (multi-systemic disorder), Lupus (wolf), Erythematosus (reddening of skin)
Production of anti nuclear antibodies (ANA)
- negative ANA does not exclude SLE
- positive ANA strongly supports SLE but is not confirmatory
- other diseases are associated with ANA
- ANA is a screening test
Patients can develop arthritis, skin lesions, kidney disease, pulmonary and neurological damage
SLE 10 times more common in women (high oestrogen levels accelerate disease)

Question 24

SLE - Pathology

Answer 24

Antigen is DNA (associated with proteins)
- primary antigen may be the nucleosome (particle of DNA + histone as a result of cell break down)
- cells break down continually
- DNA-proteins exposed to immune system
SLE mediated by auto-antibody with type II and type III hypersensitivity
Tissue damage due to deposition of dsDNA-Ab complexes (Kidney)
- complement consumed and depleted
- C3 and C4 measured to determine active disease

Question 25

SLE - Further Tests

Answer 25

ANA is a broad screening test
Antibodies to a variety of nuclear antigens
- nucleic acid, nuclear proteins, ribonuclear proteins
- extractable nuclear antigens (ENA)
Detection of antibodies to individual nuclear antigens used to help diagnosis of disease
Sometimes a combination of antibodies to more than one antigen
Antibodies to dsDNA is most common for SLE (>90% of SLE patients have antibodies to dsDNA)

Question 26

SLE Diagnosis

Answer 26

Often SLE symptoms are similar to RA so differential diagnosis is important:
- SLE usually younger women (<40)
- SLE may have facial rash, neurological symptoms, pericarditis, raynaud’s, venous thrombosis, no joint erosion, vasculitis
- RA have joint erosion and deformity, rheumatoid nodules on elbows and knees, women >40, vasculitis
Blood tests:
- RF in 70% of RA patients but <30% in SLE
- ANA (IgG) in 99% of SLE but <30% ANA (IgM in low titre) in RA
- dsDNA antibody in SLE, very rare in RA
- complement low in SLE, raised in RA
- CRP raised in RA, normal levels in SLE

Question 27

SLE Treatment

Answer 27

No cure for SLE, disease is managed
Disease may be very mild in which very little is required:
- avoid stimuli (UV light)
- small dose non-steroid anti-inflammatory drugs
- monitor
Severe disease or “flare-up”:
- immunosuppression (corticosteroids, azathioprine)
- rarely anti-CD20 (anti-B cell) rituximab
- monitor ANA, anti-dsDNA, C3, C4 and CRP