Autoimmunity

Question 1

What defines autoimmunity?

Answer 1

• Generation of adaptive immune cell receptor diversity comes with the danger of producing cells that are able to strongly bind SELF
• If these SELF-specific T or B cells which recognize autoantigens are not checked, then the result is autoimmunity
• Leads to serious tissue/organ damage and possibly death if not treated 

  *Autoimmune disorders are an adaptive response, not innate

Question 2

What are 3 examples of autoimmune disorders?

Answer 2

1. Rheumatoid arthritis → autoreactive lymphocytes reacting to Ag to the joints
2. Systemic lupus erythematosus → auto-ab generated to DNA are deposited in the kidney
3. Graves’ disease → Auto-Ab against thyroid hormone receptor, too much thyroid hormone

Question 3

Which of the following is an autoimmune disorder?
1. Antibodies generated against cellular DNA leading to kidney failure
2. Chronic production of IL1-beta leading to intermittent fever
3. Macrophages located in plaques contributing to tissue damage in heart disease (atherosclerosis)

Answer 3

Answer: 1. Antibodies generated against cellular DNA leading to kidney failure

Question 4

What experiment first showed autoimmunity?

Answer 4

Auto-ab bind to RBC upon cold exposure → activate complement → RBC get lysed

Question 5

What defines immunological tolerance vs self-tolerance? What defines breaking tolerance?

Answer 5

A state of functional unresponsiveness for a particular antigen
- Cells are not there or cells are there but not responding

Breaking Tolerance → making the immune system responsive to an antigen
Breaking Tolerance to self-antigen → Autoimmunity

Tolerance is an active process and tolerance to self is an essential feature of the immune system
Self-tolerance protects an individual from potentially self- reactive lymphocytes 


Question 6

What defines central tolerance?

Answer 6

Deletion of the T- and B-cell clones before the cells are allowed to mature if they possess receptors that recognize self Ags with greater than a threshold affinity.
- Clonal deletion during lymphocyte development

Question 7

What defines peripheral tolerance? What are 3 mechanisms?

Answer 7

Deletion or rendering anergic lymphocytes that possess receptors reacting with self Ags.

Self is detected:
1. Suppression by regulatory T cells
2. Anergy
3. Activation induced cell death

Self not detected or not accessible:
1. Ignorance

Question 8

What are different levels of tolerance?

Answer 8

1. Individual choice → at level of individual cells
2. Committee governance → interaction between other cells (with DCs)
3. Governance by police → Specific reg cells that shut down autoreactive processes (Tregs)

Question 9

What antigens are presented in the thymus to educate T cells? How is tolerance to peripheral tissue-specific antigens (TSA) achieved?

Answer 9

1. Self-proteins are presented → membrane channels, house keeping proteins involved in cell division/maintenance (this is limited to cells and genes that can be expressed in the thymus)

Achieving tolerance to peripheral tissue-specific antigens (TSAs) → TSA expression in mTECs:
2. AIRE: autoimmune regulator

Question 10

What is AIRE? What is the effect of lack of AIRE?

Answer 10

Autoimmune Regulator expressed in mTECs
- A crucial transcriptional regulator which can promote the expression of thousands of TSAs

Lack of AIRE causes autoimmune polyglandular syndrome type 1 (APS-1) → autosomal recessive disease in humans

Taking self-reactive T cell from mouse that has APS-1 → you would transfer autoimmunity if you transfer that T cell
Autoimmune component = cell intrinsic
*Expression of Aire is specific to medullary region (where they undergo negative selection as developping, can generate lots of self-proteins in the thymus that would not normally be made

Question 11

What are mimetic cells?

Answer 11

Mimetic cells are specialized mTECs that present tissue-restricted antigens
Mimicking peripheral cell counterparts of other epithelium

Question 12

What is T cell fate defined by?

Answer 12

T cell fate in the thymus is determined by the strength of the signal transmitted by the T cell receptor (avidity/affinity)

Big number of T cells generated are fltered out because they don’t recognize peptide:MHC (death by neglect = majority!!) → After selection ~ 5% of T cells you make a making it out of the thymus

Question 13

Does central tolerance get rid of all self-reactive T cells? Descrive the experiment.

Answer 13

What was thought was that was that negative selection efficiently deletes self-reactive T cells and peripheral tolerance mechanisms regulate only a small number of T cells

Actually, negative selection is very incomplete and many T cells escape deletion
Experiment: CD8 T cells recognizing a Y chromosome specific antigen (SMCY, self for males, foreign for women) is only 3x greater in women than in men.
- A large fraction (1/3) of SMCY-specific CD8 T cells are not deleted in men 


Question 14

How does central tolerance involve in the case of B cells?

Answer 14

Immature B cells expressing an autoreactive receptor recognizing a multivalent self antigen can undergo receptor editing

IgM cross-linking results in IgM downregulation, RAG re-expression and light-chain gene rearrangement

B cells remaining autoreactive undergo clonal deletion

Question 15

What conditions induce anergy in T cells?

Answer 15

T cell receptor binding to peptide-MHC on a cell that does not express the co-stimulatory factor B7, results in a long-lasting non-responsive state called anergy

Once anergic, no cell division upon antigen stimulation + costim (even when full signal)

This is dependent on the way DCs were activated (if fully activated or not)

Question 16

What experiment showed the existence of Tregs?

Answer 16

1. Take CD25+ and CD25- T cells from a WT mouse
   *Nude mice don’t have a thymus

Inject in group 1 of nude mice → CD25- T cells → autoimmunity
Inject in group 2 of nude mice → CD25+ and CD25- T cells → healthy

Showed that there was a subset of CD25+ T cells that where required for self-tolerance

Question 17

What transcription factor defines Tregs?
What do these cells develop from?

Answer 17

FoxP3
FoxP3+ Tregs develop from CD4 T cells that have a higher reactivity for self antigen than conventional T cells

Question 18

What is the effect of a mutation in FoxP3?
Give an example of syndrome.

Answer 18

Mutations in FoxP3 cause a human disease with aggressive and lethal lymphoproliferative syndrome characterised by multisystem autoimmunity

IPEX: Immunodysregulation, Polyendocrinopathy, Enteropathy, X-linked syndrome

Question 19

What are the different mechanisms of action of T regs to suppress effector T cells responses?

Answer 19

1. Secretion of immuno-suppressive cytokines
   ex: TGFbeta
2. Metabolic disruption
   - CD25 receptor on Tregs have high affinity with IL-2 → Mop-up IL-2 which is critical for survival of effector T cells, expansion and division → T cells undergo cell death without iL-2

• CD39 and CD73 are exonuclease → generate adenosine → act on receptor to dampen effector T cells phenotype (immunosuppressive effect)

3. Inhibition of dendritic cell maturation and function
   - Through CTLA-4-4:CD80/86 → DC produce IDO → immunosuppressive cytokine
   *CTLA-4 on Tregs
4. Cytolysis
   - Release of Granzyme A or B to lyse and kill effector T cells

Question 20

Where are Tregs found in the lymph nodes?

Answer 20

In the deep t cell zone, colocalized with other T cells

Question 21

What is involved in activation-induced cell death as a mechanism of peripheral tolerance?

Answer 21

Following clonal expansion of effector T cells when pathogen is cleared, effector cells undergo apoptosis and only a small pool of memory T cells remains

Activation induced Fas/FasL expression (both expressed by T cells) → Autocrine and paracrine apoptosis

Question 22

What is ALPS?
(Autoimmune Lymphoproliferative Syndrome)

Answer 22

Autosomal dominant disorder where defect in T cell receptor induced apoptosis leads to non-malignant T cell division
*problem in peripheral activation induced cell death

Unchecked cell division can result from stimulation by self or foreign antigens

Question 23

What does ignorance imply as a mechanism of peripheral tolerance?

Answer 23

Antigens that are not accessible or presented to T cells induce neither tolerance nor activation = T cells are ignorant

Antigens to which this applies in particular are those that are in immunologically privileged sites**

If the ignorant T cells are activated elsewhere, then the sequestered autoantigens could become targets of autoimmune attack – eg. multiple sclerosis

Question 24

What are different immunologically privileged sites?

Answer 24

• Brain
• Eye
• Testis
• Uterus (fetus)

*These organs doo not have antigen cells in them and peptides from these organs are not presented in the thymus so they would be recognized as non-self

Question 25

What is sypathetic ophthalmia?

Answer 25

It is an example where ignorance is maintained only until there is damage to an immunologically privileged site 1. Trauma to 1 eye result in the release of sequestered intraocular protein antigens 2. Released intraoccular antigen is carried to lymph nodes and activates T cells 3. Effector T cells return via bloodstream and encounter antigen in both eyes *If not treated, both eyes will be damaged → complete loss of vision

Question 26

What is autoimmunity the result of? What factors are involved?

Answer 26

It is the result of a combination of multiple factors coming together. 1. Genetic susceptibility 2. Environmental triggers such as infections 3. Breakdown in neural tolerance mechanisms *Does not occur as the result of only 1 factor

Question 27

Where is multiple sclerosis mostly present world-wide?

Answer 27

High prevalence in Canada and North of the US + Northern Europe

Question 28

What are 2 ways in which Autoimmunity is multifactorial?

Answer 28

1. Takes multiple factors to trigger it (genetic + environmental) 2. Although some autoimmune diseases have traditionally been thought to be mediated by B cells or T cells, it is important to consider that all aspects of the immune system have a role to play.

Question 29

How is the full immune system involved in autoimmunity although it is only caused by an imbalance in the adaptative immune response? Give an example.

Answer 29

Ex: Systemic Lupus Erythematosus Type 1 IFN plays a critical role... Genetic susceptibility: 1. Antigen presentation (HLA-DR) 2. IFN/TLR signaling 3. TNF/NF-kB signaling 4. T cell signaling 5. B cell signaling 6. Immune complex clearance etc. Envrionmental/endogenous triggers lead to DCs releasing more IFN-a → monocytes, autoreactive Cd4 T cells, autoreactive CD8 T cells, B cells, etc. - TLR receptors recognize it - Plaque formation through Ab-Ag complexes etc.

Question 30

What are the 2 main classifications of autoimmune diseases? *For clinical purposes

Answer 30

1. Organ-specific autoimmune diseases - Type 1 diabetes mellitus - Goodpasture's syndrome - Multiple sclerosis - Graves' disease 2. Systemic autoimmune diseases - Rheumatoid arthritis - Scleroderma - SLE In systemicautoimmune diseases multiple organs are affected and have a tendency to become chronic, because the autoantigens cannot be cleared from the body.

Question 31

What characterizes type 1 diabetes as an autoimmune disease?

Answer 31

Specific destruction of the β-cell producing insulin in the pancreatic islets of Langerhans. - Other cells in the islets are not destroyed Studies in the NOD mouse model of type I diabetes have shown that peptides from insulin itself are recognized by the CD8 T cells (Tc). - These studies confirm that insulin is the autoantigen. *Glucagon and somatostatin are still produced by alpha and delta cells, but no insulin can be made

Question 32

What characterizes multiple sclerosis as an autoimmune disease?

Answer 32

Multiple sclerosis is an example of a T-cell mediated chronic neurological disease that is caused by the destructive immune response against several brain Ags like: - myelin basic protein - proteolipid protein - myelin oligodendrocyte glycoprotein 1. Unkown trugger sets up initial focus of inflammation in brain and blood-brain barrier becomes locally permeable to leukocytes and blood proteins 2. T cells specific for CNS antigen and activated is peripheral lymphoid tissues reencounter antigen presented on microglia or DC in brain 3. Inflammatory reaction in brain due to mast-cell activation, complement activation, Abs and cytokines 4. Demyelination of neurons

Question 33

Which cells are specifically involved in MS response?

Answer 33

Th1/Th17 CD4+ T cells

Question 34

What characterizes rheumatoid arthritis as an autoimmune disease?

Answer 34

Rheumatoid arthritis is a chronic disease characterized by inflammation of the synovium (thin lining of a joint). - As the disease progresses, the inflamed synovium invades and damages the cartilage followed by erosion of the bone. - Patients suffer from chronic pain, loss of function and disability. - Matrix metalloproteinases (MMPs) → attacks tissues which activates bone-destroying osteoclasts resulting in joint destruction