Immunologic tolerance, autoimmunity, and chemokines

Question 1

Central tolerance

Answer 1

• Happens before exogenous Ag exposure
• Developing lymphocytes are exposed to self-Ags in the thymus (T cells) and BM (B cells)
• Positive selection: cells useful against pathogens are kept (binds either MHC1 or 2 not too strong but not too weak)
• Negative selection: removes self-reactive cells
• Central tolerance dependent on AIRE locus expression

Question 2

Thymus maturation of T cells

Answer 2

• Thymocytes differentiate while moving from cortex->medulla
• Positive selection occurs in cortex as T cells differentiate (>95% die from +selection)
• In the medulla negative selection takes place
• Cells in the medulla are kept only if they pass negative selection and their receptors/co-receptors match their MHC class
• The AIRE locus promotes expression of self-Ags in medullary epithelial cells, ensuring sufficient purging of self-reactive T cells
• The cells that pass leave the thymus as mature naive T cells, which circulate through secondary lymph tissue to encounter an Ag

Question 3

Role of AIRE locus

Answer 3

• AIRE found in thymus epithelium to ensure negative selection (expression of self Ag)
• AIRE expressing cells also found in secondary lymphoid tissue, which may act to eliminate autoimmune cells
• Mutations in AIRE gene can result in severe multi-organ autoimmune disease

Question 4

B cell maturation

Answer 4

• Occurs in BM, immature B cells that interact strongly w/ self-Ags in the BM are clonal deleted
• B cells can also alter their receptor specificity by expressing a new light chain, so the receptor no longer self-reacts (receptor editing)

Question 5

Differences btwn B and T cell lifespan

Answer 5

• T cells are long-lived, and by the age of 20-30 the thymus involutes and ceases to be functional
• B cells are constantly turning over and being made, so the BM remains active for most of your life

Question 6

Peripheral tolerance: anergy 1

Answer 6

• Anergy is induced when mature naive lymphocytes that recognize self-Ags in peripheral tissue are inactivated or killed by regulatory T cells
• 2 ways to get anergy for T cells: binding of MHC presenting self Ag w/o co-stimulation, or binding of MHC presenting self-Ag w/ recognition of B7 (APC costimulatory ligand) by inhibitor receptor (CTLA4) on T cell
• Anergic T cells cannot express IL2 and therefore cannot stimulate their own proliferation and differentiation
• Peripheral tolerance in part dependent on Treg expression of FoxP3

Question 7

Peripheral tolerance: anergy 2

Answer 7

• Mature B cells that encounter high concentrations of self-Ags in peripheral lymphoid tissues also become anergic
• Anergic B cells express IgD but little surface IgM and develop a partial block in signal transduction
• They migrate to periphery and are rapidly lost

Question 8

Activation-induced cell death

Answer 8

• T cells that are activated by self Ags are destroyed by apoptosis
• T cell divides, both of the clones then express Fas and FasL proteins
• The two proteins interact w/ their ligands on both cells and the cells undergo apoptosis

Question 9

Tregs

Answer 9

• Tregs produce IL10 (inhibits APC function) and TGF-b (inhibits T cel proliferation)
• Powerful Tregs express FoxP3 TF and CD4/CD25
• Tregs are produced (in thymus and peripheral lymphoid organs) in response to strong recognition of self-Ags
• They inhibit activation and differentiation of naive T cells by contact-dependent and cytokine mechanisms
• Mutations in FoxP3 can result in severe, acute autoimmune diseases

Question 10

Immunologically privileged sites

Answer 10

• These organs have tightly regulated immune access, and anti-inflammatory profiles
• The brain and eyes are examples
• They have limited capacity for regeneration, and immune mediated inflammation can be devastating
• Fetus is another example (father’s MHC will be different from mother’s), and the placenta appears to sequester the fetus from the mothers T cells

Question 11

Classification of autoimmune diseases

Answer 11

• Abs directed against self (surface Ags or extracellular matrix Ags); autoAbs
• Soluble immune complexes that are deposited in tissues
• Effector T cells (autoimmune T cells)

Question 12

Tissue involvement 1

Answer 12

• Endocrine glands are particularly prone to autoimmune targeting, b/c they express tissue-specific protein hormones
• They are well-vascularized, and often lead to organ-specific autoimmunity
• Common endocrine glands targeted:
• Thyroid: Abs block TSH receptor leading to elevated TSH in blood due to inability to bind and destruction of negative feedback
• Islets of langerhans in pancreas: results in diabetes (type 1)
• Adrenal gland: addison’s disease

Question 13

Tissue involvement 2

Answer 13

• Multiple sclerosis: inflammation of brain leads to T cell infiltration and attack of myelin
• Systemic lupus: Ab/Ag complexes (Abs directed to a number of systemic Ags) precipitate in kidney and cause renal failure

Question 14

Genetic factors contributing to autoimmunity (AI)

Answer 14

• Both genetic and environmental factors
• Certain HLA alleles are associated w/ AI
• Ex: ankylosing spondylitis (B27 HLA allele) has an 87% correspondence rate
• Females more susceptible to AI than males
• HLA correlations for the most part are weak, and predisposing HLAs do not usually result in AI
• Non-HLA genes also play a role: complement protein defects (C2, C4) can result in lupus-like diseases
• Fas/FasL mutations lead to inhibition of AICD and prevent elimination of self-reactive T cells
• IL2 mutations may induce diabetes
• FoxP3 mutations leads to reduction in Treg activity and greatly increases chance of AI developing
• Mutations in AIRE
• Complement gene mutations
• GWAS used to find

Question 15

Environmental factors contributing to AI

Answer 15

• Trauma, chemical exposure, infection
• Infections (such as strep pyogenes) can initiate Abs that cross react w/ self-Ags
• One possible mechanism is molecular mimicry: microbe Ags may appear similar to a self-Ag, causing cross-rxn
• If a T cell that is activated by a pathogen then reacts to self Ags, it will continue to react w/ self Ag even after infection has cleared (the recurrence of AI caused by the infection may be years after the infection)
• Trauma (specifically to immune privileged areas) can cause AI by exposing immune cells to tissues they are infrequently exposed to, and thus may look foreign
• Drugs/toxins/chemicals: may change the epitopes on self-Ags to look more like pathogen Ags

Question 16

Animal models

Answer 16

• Needed to elucidate the mechanisms of AI, and for discovering AI Rx
• Non-obese diabetic (NOD) mouse model used in diabetes research
• Experimental autoimmune encephalomyelitis (EAE) mouse model mimics multiple sclerosis
• In EAE we transfer in stimulated, autoreactive T cells (stimulated w/ self and pathogenic Ags) to induce AI

Question 17

Chemokines

Answer 17

• Large family of chemotactic cytokines that are secreted
• Involved in leukocyte migration
• Proteins of 60-140 AAs, classified according to location of cysteine bridges: C, CC, CXC, CXXXC
• Receptors are denoted w/ “R” ligand is denoted w/ “L”
• Some guide the homing of lymphocytes to different regions in lymphoid organs
• Some recruit immune cells to sites of inflammation
• Some are involved in tissue maintenance and development
• Thus they play a role in cancer, infection, and autoimmune

Question 18

Chemokine receptors

Answer 18

• GPCRs (7 transmembrane domains) that are activated by chemokine binding and intracellular GTP binding
• Activation of the receptor leads to signals to change patterns of gene expression (via second messenger)
• Control many cellular functions like motility, metabolism, and cell division (significant redundancy btwn the chemokines and their receptors)
• The response to a particular chemokine can vary based on the receptor it binds to and the cell/tissue type
• Chemokine effects regulated by D6 receptor (decoy chemokine receptor)

Question 19

Chemokines and leukocyte migration 1

Answer 19

• WBCs (primarily PMNs) leave the blood and migrate to site of infection
• Dependent on adhesive interactions that are regulated by chemokines and cytokines
• Begins w/ rolling on the vascular endothelium through the interactions btwn selectins expressed on the endothelium (induced by chemo/cytokines) and carbohydrate lectin ligands on WBC
• Tight binding then occurs btwn integrins on WBCs (expression induced by chemokines such as IL8) and their ligands (ICAM, VCAM) on endothelium, the expression of which are induced by inflammatory cytokines
• The tight binding btwn integrins and I/VCAM leads to arrest of rolling and squeezing, or extravasation, of the WBC btwn the endothelium into the tissue

Question 20

Chemokines and leukocyte migration 2

Answer 20

• The binding of integrins to their ligand cause the cytoplasmic portion of the protein to bind to the cytoskeleton, allowing for cellular shape change
• WBCs only exit from veins, never from arteries
• The WBCs then penetrate the basement membrane w/ the aid of nzs to break down the ECM
• Movement through the basement membrane and tissues is diapedesis
• The WBCs then migrate through the tissue to the site of inflammation under the influence of a chemokine gradient produced by macrophages (mostly IL8)
• Defects in this system are implicated in cancer (APCs w/ cancer Ag cannot migrate to lymph nodes due to lack of chemokine receptor- induced by tumor cell)

Question 21

Chemokine mimics

Answer 21

• One way that viral pathogens can evade/infect the immune system
• They can also inhibit the interaction btwn chemokines and their receptors
• HIV requires a chemokine receptor for entry into CD4 cells (either CCR5 or CXCR4, and always requires CD4)
• CCR5 deletion (∆32) makes one resistant to HIV infection, w/o any immunologic complications
• HIV infects via mucosal transmission using CCR5+CD4
• Later on in infection HIV switches to utilize CXCR4 and CD4