Immunology-Immune Regulation And Tolerance

Front 1

Q: What is the purpose of immune regulation?

Back 1

-Immune regulation controls immune responses, maintaining Ab levels, keeping T and B memory cells, preventing excess proliferation of lymphocytes and preventing weak anti-self responses from becoming strong anti-self responses. thus immune regulation controls both responses to foreign antigens and to “self”

Front 2

Q: How do T and B cells develop to tolerate self?

Back 2

-In the normal course of both T and B cell development, tolerance is induced to "self' before the lymphocytes encounter foreign antigens. Strong anti-self reactive lymphocytes are centrally deleted in the primary lymphoid organs (thymus for T, bone marrow for B cells). In the periphery signal 1 (recognition of antigen) without signal 2 (co-stimulation, that usually accompanies inflammation) results in either B or T cell death or anergy.

Front 3

Q: Define tolerance.

Back 3

-Tolerance is a state of un responsiveness in an animal or person to a specific potential immunogen. Tolerance naturally occurs to many "self' antigens (preventing immune responses against "self'). B and T lymphocytes with receptors that are able to recognize self are continually generated in the body. There are several mechanisms that prevent them from mediating autoimmunity. Tolerance is specific for the tolerogen. Tolerance maybe induced centrally (in the primary lymphoid organs when T and B cells are generated) or peripherally (after the mature T and B cells are released).

Front 4

Q: Define tolerogen.

Back 4

-A tolerogen is a substance capable of inducing tolerance. In general, a toleragen in one situation can become an immunogen in another situation. Normally, many self antigens are tolerogens. Other tolerogens can be experimental antigens or antigens induced therapeutically.

Front 5

Q: Define central tolerance.

Back 5

-Central tolerance occurs during lymphocyte development in primary lymphoid organs, e.g., the thymus for T cells. Central T cell tolerance is dependent upon negative thymic selection which eliminates of highly reactive anti-self T cells on the thymus

Front 6

Q: Define peripheral tolerance.

Back 6

-Peripheral tolerance occurs outside the primary lymphoid organs, after mature T and B cells are released into circulation. T or B cell peripheral tolerance is a reflection of cell deletion, anergy or ignorance.

Front 7

Q: Define clonal deletion.

Back 7

-Clonal deletion is the elimination of a f or B cell capable of responding to an antigen, usually to a self or auto-antigen. (The cell dies.)

Front 8

Q: Define clonal anergy.

Back 8

-Clonal anergy is unresponsiveness of a lymphocyte that remains alive. The unresponsive cell still has its antigen specific receptors but just doesn't respond when given a particular antigen.
-applies to a whole person, rather than to a cell. Clinical anergy is the inability of a patient to respond to antigens. This anergy usually causes a patient to have debilitating microbial infections. Clinical anergy lacks antigen specificity and the person is unable to respond to new immunogens. The person with clinical anergy is also unable to mount secondary responses to antigens to which he was immune and was previously responsive. Clinical anergy frequently occurs in patients with disseminated mycobacterial tuberculosis and disseminated mycobacterial tuberculosis and disseminated coccidioidomycosis (infection with the fungus Coccidiodes immitus).

Front 9

Q: Define clonal ignorance.

Back 9

-Clonal ignorance means that a lymphocyte actually ignores a self antigen even though the lymphocyte survives in circulation. The self antigens may be hidden from the lymphocyte or the lymphocytes may have such low-affinity receptors for these self antigens that the antigens are normally unrecognized.

Front 10

Q: What role do natural T regulatory CD4+ cells have in tolerance?

Back 10

-'Natural' T regulatory CD4+ cells recognize auto-antigens and then actively suppress other lymphocytes' responses to these antigens. ('Induced' T regulatory CD4+ cells may recognize foreign antigens and are outside the scope of this course.)

Front 11

Q: What role does CD8+ T cell-mediated suppression have on tolerance?

Back 11

-CD8+ T cell-mediated suppression affects proliferation of any nearby Immune T or B cells. When the cognate antigen for the T suppressor cells activates the cells, the suppressive responses apply to nearby cells responding to other antigens.

Front 12

Q: When do lymphoproliferative disease occur?

Back 12

-Lymphoproliferative diseases occur when immune responses to foreign antigens are improperly controlled

Front 13

Q: Describe the induction of tolerance in animals.

Back 13

-Animals are naturally tolerant or unresponsive to "self' antigens. Therapeutic induction of tolerance to specific antigens would be desirable to promote organ grafts and to "re-train" autoimmune people to tolerate the self antigens to which they have become responsive. In experimental (and clinical) practice, tolerance is very difficult to induce. Experiments tell us that induced tolerance is possible but usually only to a few epitopes. When tolerance of self is broken, the autoimmune responses are generally to many epitopes. So far, therapeutic tolerance in adults is possible only with specially prepared tolerogens (usually peptides and soluble proteins). In contrast, in neonates, tolerance can be induced to foreign cells as well as to soluble foreign proteins. In most cases of experimentally induced tolerance, the tolerogen must remain present in the body to maintain tolerance to if. Tolerance can be broken when the tolerogen is absent and can sometimes be broken under other conditions such as high cytokine production.

Front 14

Q: What does toerlance depend on?

Back 14

-Tolerance depends on one of three scenarios listed below:
1. Reactive lylnphocytes can be deleted.
2. Lymphocytes can be made unresponsive/anergic if they encounter antigen (signal 1) in the absence of co-stimulation (signal 2). Under healthy conditions, auto-antigens naturally occur, but do so in the absence of co-stimulatory signals. Anergy must be actively maintained by the presence of the auto-antigens or tolerogens.
3. Auto immune cells may be competent, but regulated or suppressed by other cells

Front 15

Q: What are the different types of tolerance?

Back 15

-There can be tolerance of either T and B cells, induced either centrally (in the primary lymphoid organs) or peripherally.

Front 16

Q: Describe T cell tolerance.

Back 16

-T cell tolerance is induced centrally in the thymus and depends on negative selection (which results in T cell clonal deletion of T cells with TCRs that bind too tightly to self peptides.)

Front 17

Q: Where does programming to limit recognition of self occur?

Back 17

-Programming to limit recognition of self occurs in the thymus. New T cells are continuously made throughout life. T cell are selected in the thymus so that only cell that weakly recognize MHC class I or II proteins live.

Front 18

Q: Define positive selection in terms of tolerance.

Back 18

-Positive selection is the process whereby newly generated T cells whose T cell antigen receptors bind weakly to (self) MHC proteins are selected for survival. This happens at the 'double positive' CD4+/CD8+ stage.
-MHC I and MHC II proteins in the thymus positively select CD8 and CD4 cells, respectively.
-T cells that don't recognize self MHC proteins at all die.

Front 19

Q: Describe negative selection in terms of tolerance.

Back 19

-Negative selection in the thymus. Epithelial cells of the thymus, under the control of a unique transcription factor, AIRE, express proteins of other tissues. The proteins are processed into peptides and presented in MHC I or II that these potential auto-antigens can "delete" the T cells that recognize the self-peptide-MHC complexes.

Front 20

Q: What happens with T cells that strongly recognize MHC proteins containing a variety of self peptides?

Back 20

-T cells that strongly recognize MHC proteins containing a variety of self peptides die in the thymus. The T cells with TCRs that bind tightly to self peptide-MHC complexes are deleted. The selects at both the MHC class I and MHC class II level, eliminating CD8+ and CD4+ anti-self. About 98% of all T cells die in the thymus.
-Thus, only T cells that bind weakly to MHC-self peptides are allowed to live and leave the thymus. They have T cell receptors that are very low affinity for anti-self MHC I or II. The survival means that the MHCs bearing foreign peptides can later be recognized with greater affinity than just MHC self peptide, and thereby trigger the T cell receptor

Front 21

Q: What are the two ways that T cell tolerance is peripherally induced?

Back 21

1. T cell anergy or death is caused by T cell encounter with (self) antigens (signal 1) without co-stimulation by CD28-B7 interactions (lack of signal 2). In the case of anergy, the T cell lives but is unable to respond to the tolerogen. The result of the interaction is an anergic T cell which does not divide when it is given both antigen and co-stimulation much later.
2. Also illustrated are the consequences of CTLA-4-B7 interactions, when B7 binds CTLA-4 instead of CD28. CTLA-4 is induced quite a few days after initial encounter with antigen and T cell activation. The CTLA-4 signal is an "off' signal that instructs the T cells to not respond to antigen any longer, and can counterbalance any remaining "go" signals coming from CD28.

Front 22

Q: Can T cell autoimmune responses be suppressed?

Back 22

-T cell autoimmune responses can occur in normal individuals but be actively suppressed. Some "natural" CD4+ CD25+ T regulatory cells that recognize self antigens in MHC class II protein may directly bind to and affect potentially auto-responsive T cells (if the T cells have induced MHC II!). CD4+ T regulatory cells may also secrete suppressive cytokines, IL-10 and TGF-beta. What the TCRs of most T regulatory cells recognize is still unknown.

Front 23

Q: Describe B cell tolerance.

Back 23

-New B cells are generated all the time. Thus, B cell tolerance must be continuously maintained by the presence of the tolerogen.

Front 24

Q: Describe central B cell tolerance.

Back 24

-B cell tolerance of self-antigens can occur centrally by either clonal deletion of developing B cells in the bone marrow that have receptors that bind strongly to self antigens or by "receptor editing" to make new light chains so that the, anti-self reaction is lost.

Front 25

Q: Define receptor editing.

Back 25

-Receptor editing occurs in the bone marrow during B cell development. A B cell that binds to too well to self may not die. Instead, it gets a second chance. The B cell rearranges a second light chain and substitutes it for the light chain that previously made a high affinity anti-self receptor. As many as 25% of all mature B cells may be made this way.

Front 26

Q: Describe peripheral B cell tolerance.

Back 26

-B cell tolerance can occur peripherally if B cells receive signal 1 (antigen) without a signal 2 for B cell co-stimulation (binding to CD40 of the B cell, etc.). These B cells may die in. the periphery.

Front 27

Q: Can tolerance be therapeutically induced?

Back 27

-Therapeutic induction of tolerance to foreign antigens is extremely difficult, more of a hope than a reality.

Front 28

Q: Which antigens make the best tolerogens?

Back 28

-The antigens that make the best tolerogens are either free peptides or proteins that are similar in amino acid sequence to self and very soluble. Soluble (and peptide) tolerogens are effective in both neonate and adult mice. (We know that peptides can be tolerogens in people by exchanging into the MHCs and providing a signal 1 in a noninflammatory environment where signal 2 is missing, Co-stimulation [signal 2] increases with inflammation.). Without repeated re-introduction of the tolerogen, tolerance stops. Tolerance, like immunity, is specific

Front 29

Q: Can adults or neonates be tolerized to cellular tolerogens?

Back 29

-Adults cannot be tolerized to cellular tolerogens but neonates can. This observation is very disappointing for clinical induction of tolerance to organ grafts. Note: the neonatal tolerance is immunospecific to the tolerogen. The animal that was neonatally tolerized can still respond to other cellular antigens when it now an adult. In the figure below, the animal was tolerized as a neonate and is now an adult. The adult is thought to maintain some strain B lymphocytes that persisted from the original inoculation. Therefore, the cellular tolerogen persists in this animal's body. The tolerogen persists in a state without inflammatory cytokine production.

Front 30

Q: Is it possible to break natural tolerance?

Back 30

-Breaking natural tolerance is possible with imunogens that are similar in amino acid sequence to self proteins or with immunogens made from modified or denatured self proteins, when these immunogens are introduced with strong adjuvants to produce inflammatory cytokines.

Front 31

Q: What protein is the antigen that breaks tolerance in experimental allergic encephalomyelitis in mice?

Back 31

-Myelin basic protein, from a closely related species (such as rat), is the antigen that breaks tolerance in experimental allergic encephalomyelitis (EAE) in mice, a disease that d resembles multiple sclerosis. The disease can be transferred between inbred mice by CD4+ T cells, not by antibody. T cell tolerance was broken autoimmune T helper cells promote EAE.

Front 32

Q: What are the different ways to regulate the immune response?

Back 32

-end of proliferation, negative feedback, and CTLA-4 down regulation
-The proliferation of immune responses must be stopped (to prevent tumor formation and to avoid lympho proliferative diseases that crowd other immune responses in secondary lymphoid organs). Antibody levels and memory T and B cells must be maintained in the absence of antigen.

Front 33

Q: What effect does termination of proliferation have on the immune response?

Back 33

-Termination of proliferation limits immune responses. The extent of proliferation determines the size of the clones, which affects the amounts of T helper cytokines produced, the amounts of antibody produced, the cytotoxic T cell activity to different viruses and the number of memory cells afterwards

Front 34

Q: What does the clearance of immunogen do during termination of proliferation?

Back 34

-The clearance of immunogen removes signal 1 and stops proliferative responses. Only quiescent memory cells remain. Some cytotoxic T cells become resting memory cells, without granules or FasL and are unable to kill targets, unless re-stimulated Memory helper T cells no longer secrete cytokines.

Front 35

Q: What does consumption of ILs do to the immune proliferative response?

Back 35

-Consumption of interleukins limits the immune proliferative responses. Growth factors (such as IL-2) are destroyed after they bind to their receptors, thus IL-2 levels drop. CD4+ T helper cells produce interleukin 2 for only a few days, unless they are re-stimulated with antigen. Ultimately, all the growth factors are used up and the cells stop dividing.

Front 36

Q: What happens to the IL-2 receptors from T cells if the receptors are not engaged by IL-2?

Back 36

-Also, the induced, high affinity IL-2 receptors (CD2S) disappear from cells within a few days if the receptors are not engaged by IL-2. The induced mRNA for the high affinity IL-2 alpha chain CD25 in T cells has a finite halflife. When the mRNAs disappear, these cells will lack receptors for the critical growth factor and stop growing.

Front 37

Q: Describe the role of T suppressor cells in regulation of the immune response.

Back 37

-T Suppressor cells are CD8+ T cells and differ from Treg's which are CD4+ T cells. T suppressors are one of the least understood of immune phenomena. They prevent unlimited proliferation of lymphocytes.

Front 38

Q: Describe the phenomena that is T suppressor cells.

Back 38

-These phenomena are easier to document when the CD8+ suppressor cells are generated in vivo and evaluated in vitro.
-Some of the best examples are human. In vitro, antigens of tubercle bacilli elicit suppressive factors from lymphocytes of clinically anergic patients with fulminant, disseminated TB. When the TB antigens are cleared after drug therapy, the suppression is lifted and the patients' previous immune responses restored. The patients can now also respond to new immunogens.
-Elicitation of the suppressive cells from the CD8+ T cells is antigen specific, but the effects are antigen irrelevant. The Ts cells recognize their cognate specific antigens and secrete soluble suppressive mediators that will block any lymphoproliferation.

Front 39

Q: What are the soluble mediators that mediate suppression?

Back 39

-Suppression is mediated by one or several soluble mediators, including transforming growth factor beta and IL-10
-Transforniing growth factor beta (TGF-beta) stops and T and B cell division (proliferation).
-IL-10 stops macrophage and dendritic cell production of monokines such as TNFbeta and IL-l.

Front 40

Q: How are B cells able to down regulate the immune response?

Back 40

-via antibody feedback inhibition
-Natural immunogens are usually multivalent. Normally, their antigens crosslink the Ig antigen receptors on B cells to transduce 'signal l' to allow B cells to proceed in their immune responses to antigen.
-However, when antigen-antibody complexes (instead of pure antigen) bind to the B cell, another signal counteracts 'signal 1' and the cell fails to respond to antigen. (See right hand side of the figure below). B cells have Fcgamma receptors for IgG as well as membrane Ig receptors for antigen. When the Fc receptors on the B cell are crosslinked, negative signals are transduced. When negative signals are sent simultaneously with positive signals from the antigen receptors on a single B cell, the B cell's response is negated. Antigen In Immune complexes will bind to the Ig antigen receptors while the antibody, of the immune complexes binds to the Fc receptors. Two counteracting signals (phosphorylation and dephosphorylation) are generated concurrently.

Front 41

Q: How do T cells help in the immune response?

Back 41

-T cells responding to foreign antigens (as well as self antigens above) can be down regulated by interactions with antigen-presenting cells when the T cells acquire the protein CTLA-4 cells express CTLA-4 at late stages of their responses to antigen
-CTLA-4 mediates a different form of 'second signal' that down regulates T cell responses
-Normally, T cells get a 'signal l' of peptide antigen in MHC, recognized by the TCR. The second signal generated when B7.1 or B7.2 on macrophages (or B cells) binds to CD28 of the T cells. However, T cells express CTLA-4 later, after expressing CD28. When CTLA-4 is engaged by B7.1 or B7.2, a signal is transduced that down regulates T cell responsiveness, including IL-2 production

Front 42

Q: Describe the practical medical information about immune regulation and tolerance.

Back 42

-Most clinical efforts to induce antigen specific unresponsiveness have had limited success. Next week, you will learn about one success, anti-Rho gamma globulin used to prevent sensitization of Rh- women to the Rh+ antigens of their babies.
-At present, tolerance cannot be induced to antigens of solid tissues such as transplanted organs. Thus, for organ transplants, clinicians resort to continuous, generalized immunosuppression of the immune response.
-Peripheral re-tolerance to islet antigens is being attempted in type I diabetic patients who are responding to these auto antigens. Peptides from the islet cell antigens appear to produce antigen specific anergy (after first displacing self-peptides in MHCs so that islet peptide-MHC complexes are formed). Thus the antigens would be presented without IL-1 and TNF-alpha and without signal 2.