Immunological Tolerance and Autoimmunity

Question 1

What is the significance of immunological tolerance?

Answer 1

• IT is specific UNRESPONSIVENESS to an Ag.
• SELF-TOLERANCE – All individuals are tolerant to self-Ags.
• AUTOIMMUNITY results from breakdown of self-tolerance.
• NEGATIVE SELECTION of self-reactive T lymphocytes occurring in the thymus is NOT perfect.
• There is low level of physiological AUTO-REACTIVITY that is crucial to normal immune function.
• The CHALLENGE is to understand how it becomes a pathologic process and how T cells and B cells recognize self-Ags and contribute to tissue injury.

Question 2

Describe Central and Peripheral Tolerance.

Answer 2

• Unlike “nonspecific” immunosuppression that can be mediated by antiinflammatory IL-10, TOLERANCE is Ag specific.
• CENTRAL TOLERANCE is induced in immature self-reactive lymphocytes in the primary lymphoid organs.
• Central tolerance ensures that mature LYMPHOCYTES are NOT reactive to self-Ags.
• PERIPHERAL TOLERANCE is induced in mature self-reactive lymphocytes in the lymph nodes or peripheral sites such as submucosal tissues.
• Peripheral tolerance is needed to prevent activation of these potentially dangerous lymphocyte clones in the PERIPHERY.

Question 3

Describe central tolerance.

Answer 3

Central Tolerance

• Immature lymphocytes specific for self-Ags may encounter these Ags in the generative (central) lymphoid organs that results in either:

1. deletion by apoptosis
2. change of BCR specificity
3. development of Treg cells.

Question 4

Describe peripheral tolerance.

Answer 4

Peripheral tolerance

• Mature self-reactive lymphocytes in peripheral tissues may be either:

1. inactivated (anergy)
2. deleted (apoptosis)
3. suppressed by the Treg cells

Question 5

Describe central T cell tolerance.

Answer 5

• The thymus has a special mechanism for expressing many protein Ags that are present only in certain peripheral tissues.
• Recognition of self Ags by immature T cells in the thymus leads to:

 death of the cells by negative selection or

 development of Treg cells

• Takes place in THE THYMUS.
• T cells undergo maturation and selection process in which cells with STRONG BINDING of self-Ags are deleted by apoptosis.
• Nonfunctional T cells showing NO AFFINITY at all also undergo apoptosis.
• Only T cells that bind self-Ags BELOW A CERTAIN THRESHOLD of binding affinity are positively selected and allowed to migrate into the blood as mature T cells.
• Most of these thymic emigrants develop into effector CD4+ and CD8+ T cells, and mediate both cell-mediated (CD4 and CD8 T cells) and humoral (CD4 T cells) immune responses.
• A SMALL PERCENTAGE OF T CELLS that emigrate from the thymus and develop into natural CD4+CD25+FOXP3+ Treg cells.

Question 6

Describe CD4+ FoxP3+ CD25high in Treg cells

Answer 6

Natural Treg cells develop in THE THYMUS. • Treg cells are POSITIVELY SELECTED in the thymus via strong TCR interactions with self-Ags.

• After recognition of self-Ags they are NOT ELIMINATED by apoptosis.
• Treg cells are able to produce ANTI-APOPTOTIC MOLECULES which protect them from negative selection in the thymus.
• Treg cells express FOXP3 transcriptional factor and are CD4+CD25+ positive.
• Treg cells typically express high levels of CTLA-4.
• CYTOKINE IL-2 is critical factor for survival and functional competence of Treg cells.
• Treg cells represent a LONG-LIVED population of self-Ag-specific T cells.
• Treg cells serve to prevent potentially AUTOIMMUNE REACTIONS in various tissues.

Question 7

Explain How Induced Treg Cells Differentiate in the Periphery.

Answer 7

In addition to the natural Treg cells which differentiate in the thymus, mature Th0 cells OUTSIDE THE THYMUS can also acquire Treg phenotype and function.

• These cells are called induced Treg cells (iTreg cells).
• FoxP3 EXPRESSION can be induced in naive CD4 + cells in vitro upon Ag recognition in the presence of TGF-β.
• There is a close DEVELOPMENTAL RELATIONSHIP between iTregs and Th17 cells.
• Ag RECOGNITION in the presence of TGF-β induces FoxP3 expression, if IL-6 is NOT present.
• Ag recognition in the presence of TGF-β + IL-6 prevents FoxP3 expression leading to Th17 cell DIFFERENTIATION.

RETINOIC ACID produced by DCs facilitates the generation of Foxp3+-positive induced Treg cells from naive CD4+CD25− T cells.

Question 8

What are Natural and Inducible Treg Cells?

Answer 8

The development and survival of T regulatory cells require IL-2 and FoxP3.

• In peripheral tissues, Treg cells suppress the activation of self-reactive lymphocytes.

Natural Treg cells are generated in the thymus

Inducible Treg cells are produced in LNs and GI tract.

Question 9

Describe the role of T cell Anergy and Suppression in peripheral T cell tolerance.

Answer 9

• Ag recognition without adequate CD80:CD28 costimulation induces ANERGY.
• ANERGIC CELLS survive but are incapable of responding to the Ag.
• T cell may engage inhibitory receptors CTLA-4 or PD-1 that causes SUPPRESSION of T cell response.
• Both CTLA-4 and PD-1 are expressed on CD4 + and CD8 + T cells after Ag stimulation.

Question 10

Describe Central B Cell Tolerance.

Answer 10

• Immature B cells that RECOGNIZE SELF AgS in the bone marrow with high avidity die by apoptosis or undergo receptor editing and change the specificity of their BCRs.
• Receptor editing — further rearrangement and replacement of the IgL-chain genes that occurs until nonself-recognizing receptors are produced or the cell dies.
• WEAK RECOGNITION of self Ags in the bone marrow may lead to anergy (functional inactivation) of the B cells.

Question 11

Describe The BCR Editing.

Answer 11

• Precursor (pre)-B cells, which already expresses rearranged heavy (IgH) chains first recombines the locus that encodes κ light (IgL) chain yielding a lymphocyte with an autoreactive BCR.
• BCR signaling promotes developmental arrest and continued recombination.
• Receptor editing of the IgL chain leads to expression of a distinct IgL chain, generating cell-surface immunoglobulin that lacks self-reactivity.
• All B cells with BCR containing the λ light chain are underwent BCR editing.

Question 12

Describe Peripheral B Cell Tolerance.

Answer 12

Mature B cells that recognize self Ag in peripheral tissues in the absence of specific Th cells may be rendered functionally UNRESPONSIVE or die by APOPTOSIS.

• The CD22 inhibitory receptor is phosphorylated by Lyn and then recruits SHP-1 tyrosine phosphatase attenuating BCR signaling.

Question 13

Why Are Treg Cells Are Central in the Peripheral Tolerance?

Answer 13

• Treg cells are key mediators of PERIPHERAL TOLERANCE.
• Treg cells may inhibit CD4+ T cell activation by APCs and inhibit T-cell differentiation into CD8+ CTLs.
• Treg cells may prevent T cells from providing help to B cells in the production of Abs.

Question 14

Describe Mechanisms of Autoimmunity.

Answer 14

It is postulated that SUSCEPTIBILITY GENES interfere with pathways of self-tolerance and lead to the persistence of self-reactive T and B cells.

• ENVIRONMENTAL TRIGGERS, such as infections and other inflammatory stimuli, promote the influx of lymphocytes into tissues and the activation of APCs.
• APCs may activate these SELF- REACTIVE LYMPHOCYTES, resulting in the generation of effector T cells and autoantibodies that are responsible for the autoimmune disease.

Question 15

Describe AIRE in Central Tolerance.

Answer 15

The NEGATIVE SELECTION of T cells in the thymus is necessary for the maintenance of self-tolerance.

• Medullary THYMIC EPITHELIAL CELLS have a key function as APCs.
• They EXPRESS a large number of peripheral tissues-restricted SELF-Ags that are presented to the developing T cells.
• MUTATIONS in AIRE (autoimmune regulator) protein cause a breakdown of the central tolerance.
• AIRE has been proposed to function as a TRANSCRIPTION FACTOR.
• Mutation in AIRE is associated with DECREASED EXPRESSION of the peripheral tissues self-Ags in the thymus.

Question 16

Why Does Aberrant Expression of AIRE Leads to Autoimmunity?

Answer 16

The AIRE regulates expression of tissue-restricted Ags (TRAs).

• Peptides derived from these Ags are displayed within HLA molecules on Medullary Thymic Epithelial Cells.
• Peptide-HLA complexes are recognized by immature T cells, leading to the deletion of SELFREACTIVE T CELLS.
• In the ABSENCE OF FUNCTIONAL AIRE, these self-reactive T cells are not eliminated and they can enter tissues and cause TISSUE INJURY where the Ags continue to be produced.

Question 17

What is Regulation of T Cell Responses by Inhibitory Receptors?

Answer 17

When T cells recognize self Ags, they may engage inhibitory receptors of the CD28 family, whose function is to terminate T cell responses. The best established inhibitory receptors are CTLA-4.

Question 18

Describe the Role of CTLA-4 in Peripheral Tolerance (CTLA4- Cytotoxic T-Lymphocyte Antigen 4)

Answer 18

Upon Ag ENCOUNTER, individual populations of T cells undergo expansion and later contraction after Ag is eliminated.

• T cell activation is regulated by members of the B7(CD80/CD86)-CD28 family of COSTIMULATORY MOLECULES.
• CTLA4 (Cytotoxic T-Lymphocyte Antigen 4) is a homolog of CD28.
• CTLA4 is an INHIBITORY RECEPTOR.
• CTLA4 provides signals that terminate immune responses and maintain self-tolerance

Question 19

What Are The Functions of CTLA-4?

Answer 19

UNCONTROLLED LYMPHOCYTE ACTIVATION with massively enlarged LNs and spleen and fatal multi-organ lymphocytic infiltrates is seen in CTLA-4 KO mice.

• BLOCKING of CTLA-4 with Abs also enhances autoimmune diseases in animal models.
• POLYMORPHISMS in the CTLA4 are associated with several autoimmune diseases in humans, including type 1 diabetes and Graves’ disease.
• CTLA-4 has two important properties: – CTLA-4 expression is low on resting T cells until the cells are activated by Ag. – Once expressed CTLA-4, terminates continuing activation of these responding T cells.
• CTLA-4 is expressed on REGULATORY T cells and mediates the suppressive function of these cells by inhibiting the activation of naive T cells.

Question 20

What Are The Modes of CTLA-4 Action?

Answer 20

CELL-INTRINSIC FUNCTION OF CTLA-4: Engagement of CTLA-4 on a T cell may deliver INHIBITORY SIGNALS that terminate further activation of that cell.

CELL-EXTRINSIC ACTION OF CTLA-4: CTLA-4 on Treg cells or responding T cells binds to B7 molecules on APCs or makes unavailable to CD28 and blocking T cell activation.

Question 21

What is the significance of autoimmunity?

Answer 21

• About 5% or ~ 12-15 million people in the US alone suffer have AUTOIMMUNE DISEASES.
• There are 60-70 diverse autoimmune diseases which affect various tissues of human body.
• There is NO known CURE or clear UNDERSTANDING the cause for any autoimmune disorder.
• Most autoimmune diseases are treated symptomatically.
• The autoimmune diseases bring the PARADOX PROPOSITION that “the body both is and is not itself”
• There is NO FUNDAMENTAL DIFFERENCE between the structure of selfAgs and non-self Ags because Ags are all proteins composed by the same amino acids.
• A PATHOLOGIC immune RESPONSE against self Ags is clinically manifested as an “immune-mediated inflammatory disease”.
• AUTOIMMUNITY is caused by the activation of T cells and/or B cells in the absence of an ongoing infection.
• Autoimmunity results in activation of HYPERSENSITIVE REACTIONS which cause one’s own immune system to attack the self.

Question 22

How Autoimmunity Is Prevented:

Answer 22

T cells that are physically separated from their specific Ag (the BBB) cannot become activated, a process termed immunologic ignorance.

• T cells that express the Fas (CD95) can receive their signals from cells that express FasL and undergo apoptosis, a process known as deletion.
• CTLA4 (CD152) can bind CD80 on APC and inhibit costimulatory signal that inhibits T cells activation.
• Regulatory T cells can inhibit through the production of inhibitory cytokines such as IL-10 and TGFb.

Question 23

What are the IMMUNE PRIVILEGED SITES?

Answer 23

Eyes: cornea, anterior chamber, vitreous cavity, subretinal space

Brain

Pregnant uterus

Ovary

Testic

Adrenal cortex

Hair follicles

FROM ANIMAL STUDIES: • Donor skin allografts placed at the recipient’s skin are rejected. • The same grafts placed in the eye and brain of the recipient survive for prolonged, sometimes indefinite, periods of time. • Mechanical breaking of the barriers due to a trauma results in generation of an autoimmune response against normal tissues in the immune privileged sites.

Question 24

Describe General Features of Autoimmune Disorders.

Answer 24

• Autoimmune diseases may be either SYSTEMIC or ORGAN SPECIFIC depending on the distribution of the auto-Ag involved.
• VARIOUS EFFECTOR MECHANISMS are responsible for tissue injury in autoimmune diseases.
• Autoimmune diseases are chronic, progressive, and self-perpetuating.
• FAILURE of T or B cells self-tolerance is the cause of all autoimmune diseases.
• Most autoimmune diseases are COMPLEX POLYGENIC TRAITS .
• Affected individuals inherit multiple GENETIC POLYMORPHISMS that contribute to susceptibility to autoimmunity.
• Among the genes that are associated with autoimmunity, the strongest associations are with MHC genes.
• Polymorphisms in NON-HLA genes (such as CTLA-4) are also associated with autoimmunity.
• SUSCEPTIBILITY GENES interact with ENVIRONMENTAL FACTORS to cause the diseases.

Question 25

What is the Role of HLA in Autoimmunity?

Answer 25

The ASSOCIATION between HLA alleles and autoimmune diseases in humans was recognized for many years ago.

• Most of autoimmune diseases are associated with class II HLA alleles (HLA-DR and HLA-DQ) because class II HLA control the action of CD4 + T cells.
• Of importance, although an HLA allele may increase the risk of developing a particular autoimmune disease, the HLA allele itself do not cause autoimmunity.

Question 26

What is the Role of Environment Triggers in Autoimmunity?

Answer 26

• Microbial Ags can initiate autoimmune disorder through:

– Molecular mimicry: • Rheumatic fever is triggered by streptococcal infection and mediated by crossreactivity between streptococcal Ags and cardiac myosin. • Multiple sclerosis – T cells react with myelin basic protein and peptides from Epstein–Barr virus, influenza virus type A, and human papillomavirus.

– Polyclonal (bystander) activation: • Microbial infection that results in a ROBUST INFLAMMATORY RESPONSE can cause a polyclonal activation of autoreactive lymphocytes in the cytokine field.

– Release of previously sequestered Ags: • Microbes that kill cells and cause the release of sequestered Ags (DAMPs) leading to the development of autoimmunity.

Question 27

What is Systemic Lupus Erythematosus?

Answer 27

• SLE is the prototypic immune complex– mediated disease (type III hypersensitivity).
• The principal CLINICAL MANIFESTATIONS are rashes, arthritis, and glomerulonephritis.
• Many different auto-Abs are found. The most frequent are anti-DNA Abs.
• IMMUNE COMPLEXES formed from these auto-Abs and their specific Ags are responsible for glomerulonephritis, arthritis, and vasculitis involving small arteries.
• The principal DIAGNOSTIC TEST for the disease is the presence of anti-nuclear Abs.

Question 28

What is Rheumatoid Arthritis (RA)?

Answer 28

RA is an INFLAMMATORY DISEASE involving small and large joints.

• INFLAMMATION of the SYNOVIUM associated with destruction of the joint cartilage and bone.
• Mediated by type IV hypersensitivity reactions in which Th17 cells play a key role.
• CELLS INVOLVED – Th1 cells, Th17 cells, activated B cells and plasma cells, and Mφ.
• Patients frequently have circulating IgM or IgG called RHEUMATOID FACTOR (RF) that reacts with the Fc portion circulating IgG.
• These auto-Abs are called RHEUMATOID FACTOR, and their presence is used as a diagnostic test for RA.