Lecture 8: autoimmunity

Question 1

what is the tolerance?

Answer 1

• Failure of immune system to respond to antigen
• Self-tolerance
• Failure of self-tolerance results in autoimmunity

Question 2

what is the autoimmunity

Answer 2

Autoimmunity refers to an immune reaction against the body’s own cells that occurs as a result of a loss of immunological tolerance.

Question 3

how is T cell tolerance maintained?

Answer 3

Central T cell tolerance
•	Thymic “education”
PeripheralT cell tolerance
•	Absence of Signal 2/danger signal
•	Active regulation

Question 4

how is B cell tolerance maintained?

Answer 4

•	Failure of T cell help
•	Bone marrow (Immature B cells)
–	Deletion (multi-valent self-antigen)
–	Anergy (soluble self-antigen)
•	Periphery (mature B cells)
–	Deletion (multi-valent self-antigen)
–	Anergy/ deletion (soluble self-antigen)

Question 5

what is the anergy?

Answer 5

A lack of a normal immune response to particular antigens, cytokines, and allergens. Can be due to a lack of costimulatory signals. Anergy testing is a diagnostic procedure used to obtain information on the competence of the cellular immune system (e.g., PPD tuberculin skin test).

Question 6

what is the negative T cell selection?

thymic education

Answer 6

A process of T cell selection that takes place in the thymic medulla. Tests if T cells can bind to self-antigens presented on MHC. T cells that do not bind receive a survival signal; dysfunctional T cells undergo apoptosis. Ensures that the thymus does not produce self-reacting T-cells.
Mediated by the autoimmune regulator protein (AIRE)

Question 7

what is the positive T cell selection?

thymic education

Answer 7

A process by which developing T cells that can bind appropriately to major histocompatibility complex receptors on the thymic cortical cells are allowed to survive. This process ensures that the thymus produces functional T cells.

Question 8

what are the outcomes of T cell stimulation?

Answer 8

anergy
apoptosis
proliferation

Question 9

what is the molecular mimicry?

Answer 9

The antigenic resemblance between molecules on some pathogens and those of normal cells in the body. Can cause autoimmune disease if the immune response against pathogens also attacks normal cells (e.g., in acute rheumatic fever, GBS).

Question 10

what is the superantigen?

Answer 10

Viral or bacterial antigens that cause nonspecific activation of multiple clones of T cells, resulting in massive cytokine release. Unlike normal antigens, superantigens do not have to be processed and presented by macrophages for recognition by specific T cells. Superantigens bind directly to the MHC II complexes of T cells, resulting in widespread polyclonal T-cell activation. Exotoxins produced by certain bacteria are important examples of superantigens (e.g., TSST-1 toxin produced by S. aureus can result in toxic shock syndrome).

Question 11

what is the pathophysiology of autoimmunity?

Answer 11

Autoreactive B lymphocytes are physiologically eliminated in the bone marrow, spleen, or lymph nodes. T lymphocytes that attack the body’s own cells are either sorted out in the thymus or undergo apoptosis in peripheral lymphoid tissues (e.g, lymph nodes, adenoids, Peyer’s patches) due to a lack of stimulation. If the selection mechanisms fail, this results in the immune cells attacking the body’s own cells, which leads to autoimmune inflammation.

Question 12

what is rheumatic fever?

Answer 12

• An acute systemic inflammatory illness
• Occurs 2-4 weeks after infection with a group A
beta-hemolytic Streptococcal pharyngitis
• Molecular mimicry - heart muscle, valves, articular structures & neurons

Question 13

what are the clinical features of rheumatic fever?

Answer 13

•	Fever
•	Migrating arthritis
•	Destructive inflammatory lesions
-myocardium
-endocardium & heart valves 
-pericardium
-Periarticular structures 
-Subcutaneous tissues
•	Chorea

Question 14

what s the chorea?

Answer 14

A type of involuntary movement characteristic of a group of neurological disorders called dyskinesias. Chorea movements are continuous, involuntary, rapid, abrupt, irregular, non-stereotyped, and are not urge- or compulsion-driven. Most commonly involve the shoulders, hips, and face.

Question 15

what are the Jones criteria of rheumatic fever?

Answer 15

two major OR one major plus two minor criteria are required for diagnosis.

1) Major criteria
- Arthritis (migratory polyarthritis involving primarily the large joints)
- Carditis (pancarditis, including valvulitis)
- Sydenham chorea (CNS involvement)
- Subcutaneous nodules
- Erythema marginatum
2) Minor criteria
- Arthralgia
- Fever
- ↑ Acute phase reactants (ESR, CRP)
- Prolonged PR interval on electrocardiogram

Question 16

what are the long term sequelae of rheumatic fever

Answer 16

• Valvular Heart Disease
• Increased risk of endocarditis
• Chorea may be persistent

Question 17

what is the Kawasaki disease?

Answer 17

Acute, necrotizing vasculitis of unknown etiology, primarily affecting children under the age of five (more common among those of Asian descent). Characterized by a high fever, desquamative rash, conjunctivitis, mucositis (e.g., “strawberry tongue”), cervical lymphadenopathy, as well as erythema and edema of the distal extremities. Coronary artery aneurysms are the most concerning complication as they can lead to myocardial infarction or arrhythmias.

Question 18

what are the antigen-presenting cells?

Answer 18

Antigens are processed by antigen-presenting cells (i.e., macrophages, monocytes, B cells, and dendritic cells). These cells present antigens (peptide fragments) via MHC molecules.

Question 19

what are the signal 1 and signal 2 for antigen presentation?

Answer 19

1) Antigen-presentation by a dendritic cell
- -Exogenous antigens are presented via MHC II to TCR/CD4.
- -Endogenous antigens, cross-presentation of antigens are presented via MHC I to TCR/CD8.
2) Co-stimulatory signal: Interaction of a second set of molecules mediates survival and proliferation of T cell.
- -On the dendritic cell: B7 protein (CD80 or CD86)
- -On the T cell: CD28
- -Antigen presentation without this co-stimulatory signal will lead to T-cell anergy.
3) Effect:
- -T-helper cell: via activation of NFAT produces cytokines that activate other cells, e.g., B cells, macrophages, and cytotoxic T cells
- -Cytotoxic T cell: destroys cells that possess antigens (signals malignancy or viral infection)

Question 20

what is the role of infection of antigen-presenting cells in autoimmunity?

Answer 20

• Signal 1 – self-antigen & MHC class I
• Signal 2 – costimulation
– Usually absent
– Infection may activate DC & produce signal 2
• Insulin-dependent DM
• ? Role of Coxsackie viral infection

Question 21

what virus is proposed as a cause of type 1 DM

Answer 21

coxsackie

Question 22

what is the autoimmune lymphoproliferative syndrome?

Answer 22

an autosomal dominant disorder in which defective Fas-FasL interaction results in failure of the extrinsic (death receptor) apoptosis pathway. Leads to proliferation of antigen-specific lymphocyte lineages. Clinical manifestations include generalized adenopathy, hepatosplenomegaly, and autoimmunity (typically Evans syndrome).
• Mutations in Fas/Fas ligand
• Prevents lymphocyte death
• Immune responses cannot be switched off
• Equivalent to lupus-prone mouse

Question 23

what are the mechanisms of tissue injury in autom=immune diseases?

Answer 23

•	Type II Hypersensitivity
Humoral immunity to tissue components
•	Type III Hypersensitivity 
Immune complex deposition
•	Type IV Hypersensitivity 
Cellular immunity to tissue
Indirect antibody effects

Question 24

how autoantibodies are detected?

Answer 24

• Agglutination
• indirect immunofluorescence
• ELISA

Question 25

what is the ELISA?

Answer 25

A diagnostic test that uses an antibody coupled with an enzyme as a marker to detect a specific antigen (e.g., tumor markers, viruses, drug, other antibodies). The coupled enzyme catalyzes a reaction involving a chromogenic or fluorogenic substrate. The antigen level in the test sample can then be quantified based on the degree of color change or fluorescence.

Question 26

what is the difference between direct and indirect immunofluorescence?

Answer 26

Direct IF uses a single antibody directed against the target of interest. The primary antibody is directly conjugated to a fluorophore.
Indirect IF uses two antibodies. The primary antibody is unconjugated and a fluorophore-conjugated secondary antibody directed against the primary antibody is used for detection.

Question 27

what are the agglutination assays used in diagnosis of autoimmune diseases?

Answer 27

• Antigen coated beads + serum
• No antibody – remain in suspension
• Antibody – agglutinate to form button

Question 28

what is the tissue-restricted self-antigen?

Answer 28

n antigen that is expressed in a specific type of tissue or organ. The epithelial cells of the medullary thymus can express all the different tissue-restricted antigens found in the body by a process called promiscuous gene expression.

Question 29

what is the difference between direct and indirect ELISA?

Answer 29

1)Direct ELISA
–The patient’s sample supposedly containing the protein of interest (i.e. the antigen) is added to a well of microtiter plates with a buffered solution.
–The specific antibody-enzyme conjugate is added to the solution.
–A substrate for the enzyme is added
–Spectrometry is used to detect the generated chromophore
–Higher concentration of antibodies binding to the antigen: stronger signal
–Lower concentration of antibodies binding to the antigen: weaker signal
2)Indirect ELISA
–Same procedure as direct ELISA but
the antibody specific for the antigen of interest is not labeled itself and is called primary antibody.
–the primary antibody is detected by a secondary, labeled antibody.

Question 30

what is the test sensitivity?

Answer 30

The proportion of people with a condition who test positive. A test with high sensitivity (i.e., few false negatives) can be used for screening purposes. The formula for calculating sensitivity is: True Positives/(True Positives + False Negatives). TP/(TP+FN)

Question 31

why highly sensitive tests are used in screening diseases?

Answer 31

A high sensitivity is usually achieved at the expense of specificity. If a highly sensitive test yields a negative result, the disease can most definitely be ruled out because individuals with disease will very rarely have a false negative result. Therefore, tests with a high sensitivity are used as screening tests (e.g., ELISA for HIV antibodies). A screening test does not have 100% specificity, which means a positive result could be falsely positive and a confirmatory test is necessary.

Question 32

what is the test specificity?

Answer 32

he proportion of people without a condition who test negative. A test with high specificity (i.e., few false positives) can be used for confirmatory purposes. The formula for calculating specificity is True Negatives/(True Negatives + False Positives). TN/(TN+FP)

Question 33

what is the positive predictive value?

Answer 33

The likelihood that an individual with a positive test result has the condition that is being tested. PPV has a direct relationship with the condition’s prevalence and the pretest probability. The formula for calculating PPV is: True Positives/(True Positives + False Positives).
• % of people with a positive test who have the disease being tested for
• Depends on the cut-off chosen for the test
• Also dependent on the prevalence of the disease in the population being tested

Question 34

what is the negative predictive value

Answer 34

he likelihood that an individual with a negative test result does not have the condition that is being tested. NPV has an inverse relationship with the condition’s prevalence and the pretest probability. The formula for calculating NPV is: True Negatives/(True Negatives + False Negatives).
• % of people with a negative test who do not have the disease being tested for
• Also dependent on cut-off chosen
• Affected by prevalence of the disease in the population tested; but less than PPV

Question 35

How sure are you that a patient with a positive test has the disease?

Answer 35

PPV

TP/(TP+FP)

Question 36

How sure are you that patient with a negative test does NOT have the disease?

Answer 36

NPV

TN/(TN+FN)

Question 37

what are the cutoff values?

Answer 37

Every diagnostic test involves a trade-off between sensitivity and specificity.
Sensitivity, specificity, positive predictive values, and negative predictive values vary according to the criterion or cutoff values of data

Question 38

What happens when a cutoff value is raised or lowered?

Answer 38

• -depends on whether a diagnostic test requires a high value (e.g., tumor marker for cancer, lipase for pancreatitis) or a low value (e.g., hyponatremia, agranulocytosis)
  1) Lowering or raising a cutoff value for a high value test
• -↓ cutoff value (i.e., broadening the inclusion criteria): lower specificity, higher sensitivity, lower positive predictive value, higher negative predictive value
• -↑ cutoff value (i.e., narrowing the inclusion criteria): higher specificity, lower sensitivity, higher positive predictive value, lower negative predictive value
  2) Lowering or raising a cutoff value for a low value test (causes opposite results)
• -↓ cutoff value (i.e., narrowing the inclusion criteria): higher specificity, lower sensitivity, higher positive predictive value (decrease in false positive > decrease in true positives), lower negative predictive value (increase in false negatives > increase in true negatives)
• -↑ cutoff value (i.e., broadening the inclusion criteria): lower specificity, higher sensitivity, lower positive predictive value (increase in true positives > increase in false positives), higher negative predictive value (decrease in false negatives > decrease in true negatives)

Question 39

what are the impacts of indiscriminate tests?

Answer 39

False positive results create anxiety & risk to patient
- unnecessary tests
- may get inappropriate treatment
- delay getting correct diagnosis
Cost
Delay test results for patients who need them