Lecture 11 - Immunological Tolerance

Question 1

Define Immunological Tolerance.

What are the clinical implications to loss of tolerance to self-antigens?

What are some examples of areas that we need tolerance to maintain homeostasis of the immune system?

Answer 1

Immunological tolerance means non-responsiveness to specific antigens. (different from immune suppression)

The clinical implications of loss of tolerance to self-antigens are manifest in a wide spectrum of autoimmune diseases.
D.

Tolerance against antigens we are exposed commonly (food, commensal bacterium) is important to maintain homeostasis of the immune system and prevent autoimmune disorders such as IBD or hyper-immune disorders (allergy).

E. Tumor environment is often tolerogenic and induce tolerance against tumor antigen, inhibiting anti-tumor immunity.

Question 2

What subset of T cells are required for tolerance against self antigens?

Answer 2

A subset of T cells (called regulatory T cells) are required for tolerance against self-antigens

Question 3

Define Central Tolerance.

Where does this usually take place?

Answer 3

This is based on elimination of T cells that are reactive to antigens present in the thymus—self antigens.

T cell differentiation in the thymus (takes place in ONE place, hence CENTRAL)
they undergo 2 types of selection

POSITIVE selection = select T cells that work only with own MHC molecules, and eliminate clones that are highly reactive against self-antigens

NEGATIVE selection = select against auto- reactive T cells (AIRE needed for this?)

Question 4

What are 3 examples of PERIPHERAL tolerance?

Answer 4

Peripheral tolerance:

1. Regulatory T cells—professional T cells that are designed to impose suppression to other immune cells.
2. Myeloid Derived Suppressor cells (MDSCs): A group of myeloid cells become potent immunoregulatory cells when exposed to inflammatory cytokines (e.g.IFN-g) and blocks T cell responses. Often tumor associated.
3. Clonal anergy: When T cells are stimulated in a manner that are not “complete”, cells become non-responsive to further stimulation.

Question 5

What is a main player involved in Negative Selection (Central Tolerance)?

What is negative selection?

Answer 5

AIRE

Negative Selection:
- intrathymic cloncal deletion during the maturation process of thymocytes eliminate cells that have high affinity to antigens present in the thymus

Question 6

How are non-thymic self-antigen reactive cells eliminate?

Answer 6

AIRE gene enables thymic stromal cells to express non-thymic genes and present self-antigens to developing thymocytes

AIRE is a transcription factor that is expressed mainly in the medulla of the thymus.

This leads to negative selection of thymocytes reactive to these self antigens.

Question 7

What disease is linked to the Failure of Central tolerance and loss of function of AIRE gene?

What are the results of this disease?

Answer 7

Loss of functions of AIRE gene leads to severe auto-recessive genetic autoimmune disorder :

Autoimmune polyendocrine syndrome (APS).

Endocrine organs are destructed by antibodies and lymphocytes.

Question 8

What 2 main functions are associated with Peripheral Tolerance?

Answer 8

1. Dominant Suppression
   - imposed by professional regulators, stops the response of other T cells
2. Cell Intrinsic Inactivation
   - change of the state of T cells
   - become unresponsive to antigen stimulation

Question 9

What are the 3 types of regulatory T cells?

Where are they derived?

Are they FOXP3+?

Answer 9

nTregs:
thymus derived Foxp3+ cells.
- mainly against self antigens

iTregs:
peripheral derived Foxp3+ cells.
Mainly against foreign antigens.
- antigens from food, commensal bacteria, etc…

Tr1:
peripheral derived Foxp3- cells.

Express high levels of IL-10, an immunoregulatory cytokine.

Question 10

Where are nTregs generated?

What do they express?

What do MATURE entrees express?

What is the essential factor for the maintenance of nTregs?

Answer 10

1. Naturally arising Tregs (nTregs) are those generated in the THYMUS due to their reactivity against self antigens.
2. They express a transcription factor called Foxp3.
   Foxp3 is essential for generation and/or maintenance of Tregs survival and functions.
3. Mature nTregs also express CD25, IL-2R alpha chain.
4. IL-2 is an essential factor for the maintenance of nTregs.

Question 11

How was Fox P3 found?

Answer 11

Patient with Immunodysregulation, Polyendocrinopathy, and Enteropathy, X-linked (IPEX) suffer from multiple tissue damages caused by self-reactive T cells.

The cause of this disease is mutations in a gene called foxp3.

Foxp3 was found being expressed by a small fraction of CD4 T cells and lack of this gene caused loss of this fraction of T cells, suggesting the significance of Foxp3+ T cells for immune regulation.

Question 12

What is the purpose of Hassall’s Corpuscles?

What group of thymocytes lives here?

Where is Hassall’s Corpuscle?

What do they express?

Answer 12

A group of thymocytes that have reactivity against self-antigens are converted into nTregs.

This takes place in the structure called Hassall’s corpuscles.

Hassall’s corpuscles are groups of epithelial cells within the thymic medulla.

Human Hassall’s corpuscles express thymic stromal lymphopoietin (TSLP) and help differentiation of FOXP3+ regulatory T cells via activation of a subset of dendritic cells.

Question 13

State the following for Inducible Tregs:

1. What induces them?
2. What does the cytokine milieu drive?
   - specifically what 2 cytokines play CRITICAL roles
3. What inflammatory cytokine is INHIBITORY for iTregs?
4. What induces iTregs? Where?

Answer 13

1. These Tregs can be induced by the environment (food), commensal organisms, pathogens and tumors
2. The cytokine milieu drives the differentiation of a naïve T cell in the presence of its antigen.
3. For iTregs, the presence of TGF- and IL-2 plays critical roles.
4. Presence of inflammatory cytokines such as IL-6 is inhibitory for induction of iTregs. (under these conditions, pro-inflammatory Th17 cells are generated).
5. iTregs are induced by antigen presenting cells that are present in the mucosal environment such as intestine.

Vitamin A and/or D functions as a co-factor to induce iTregs.

3. Some chronic infectious agents actively use this mechanism to establish their state of infections.

Question 14

What cytokine do Tr1 cells produce?

What 2 cytokines INDUCE their development?

Are they FOXP3+ or -?

Answer 14

1. IL-10
2. TGF- Beta & IL-27

FOXP3-

Question 15

What vitamin is linked to the up regulation of IL-10 and generation of Tr1 cells?

Answer 15

Vitamin D upregulates IL-10 production and is linked to generation of Tr1 cells

Question 16

How do Tres suppress the immune response?

What plays a significant role?

What enhances their function?

What is another effector molecule generated by Tregs to suppress other lymphocytes?

Answer 16

Direct cell-cell contact is essential for Tregs to suppress target cells.

CTLA-4 plays a significant role in the suppression by Tregs.

Foxp3+ Tregs use soluble factors (TGF-beta, IL-10) to enhance their functions. Tr1 mainly uses IL-10.

Adenosine is another effector molecule generated by Tregs to suppress other lymphocytes.

Question 17

What is clonal anergy?

Answer 17

Clonal anergy is a state of T cells that are unresponsive to antigenic stimulation.

Anergy is induced when antigen is presented in the absence of co-stimulatory signal (CD28).

Ligand of CD28 are B7.1 (CD80) and B7.2 (CD86).*

• These co-stimulators expressed only by a limited group of cells (so called professional APCs). ***

This limits the risk of false stimulations.

Question 18

Clonal Anergy relies ont he absence of what?

Answer 18

Co-stimulatory signal

Question 19

What does CTLA-4 compete with for binding?

When does CTLA-4 Increase?

What does CTLA-4 mediated suppression play a critical role in?

Answer 19

1. CTLA-4 is expressed by T cells after activation and competes with CD28 for B7 binding and will ultimately win the contest because it has higher affinity for B7 than does CD28.
2. CTLA4 also recruits signaling molecules that suppress TCR signaling and blocks antigen-activation.
3. In the natural sequence of antigen activation of T cells, there is progressive increasing production of CTLA-4 as a regulatory mechanism to prevent runaway T cell proliferation.
4. . CTLA4 mediated suppression plays a critical role in immune homeostasis evidenced by lethal autoimmune disorders observed in CTLA4 gene knock out mice.

Question 20

What is the application of CTLA4 in terms of clinical procedures?

Answer 20

Inhibition of co-stimulation significantly improves the frequency of graft versus host disease (GVH) after bone marrow transplantations.

This can be done by using CTLA-4 in a “trans” manner to block T cell activation form the outside.

This is accomplished by the use of the man-made biologic modifier CTLA4-Ig, which is now also tested in clinical trial to treat autoimmune disorders.

Question 21

What are Myeloid Derived Suppressor Cells?

What is their function?

What cytokines do they activate?

Answer 21

1. heterogeneous population of cells that are defined by their myeloid origin and ability to potently suppress T cell responses.
   - initially recognized as cells that infiltrate solid tumors and have potent immune-suppressive functions.

2.They suppress in part by response to pre-existing inflammations and activated by cytokines
such as IFN-g. ***

Then in turn produce immuno-inhibitory factors.

Question 22

State the following for mutations in: AIRE

1. What is destroyed?
2. What mechanism of failure occurs?
3. What human disease is associated with this?

Answer 22

1. What is destroyed?
   - destruction of endocrine organs by antibodies, lymphocytes
2. What mechanism of failure occurs?
   - failure of central tolerance
3. What human disease is associated with this?
   - Autoimmune Potyendocrine Syndrome (APS)

Question 23

State the following for mutations in: FoxP3

1. What is destroyed?
2. What mechanism of failure occurs?
3. What human disease is associated with this?

Answer 23

1. What is destroyed?
   - multi organ lymphocytic infiltrates, wasting
2. What mechanism of failure occurs?
   - deficiency of regulatory T cells
3. What human disease is associated with this?
   - IPEX

Question 24

State the following for mutations in: 1L2: IL2R

1. What is destroyed?
2. What mechanism of failure occurs?
3. What human disease is associated with this?

Answer 24

1. What is destroyed?
   - inflammatory bowel disease (IBS)
   - anti - erythrocyte and anti-DNA antibodies
2. What mechanism of failure occurs?
   - defective development, survival, or function of regulatory T cells
3. What human disease is associated with this?
   - NONE

Question 25

State the following for mutations in: CTLA-4

1. What is destroyed?
2. What mechanism of failure occurs?
3. What human disease is associated with this?

Answer 25

1. What is destroyed?
   - lymphoproliferation; T cell infiltrates in multiple organs
2. What mechanism of failure occurs?
   - failure of anergy in CD4+ cells
3. What human disease is associated with this?
   CTLA-4 Polymorphisms associated with several autoimmune diseases

Question 26

State the following for mutations in: IL-10

1. What is destroyed?
2. What mechanism of failure occurs?
3. What human disease is associated with this?

Answer 26

1. What is destroyed?
   - causes INFLAMMATORY BOWEL DISEASE
2. What mechanism of failure occurs?
   - failure of T cell and accessory cell suppression against commensal bacterium
3. What human disease is associated with this?
   - Polymorphism link to IBD, various cancers