L24 Tolerance and Autoimmunity PDF

Tolerance & Development of Autoimmunity Suggested reading for Lecture 24: Basic Immunology, Chapter 9 MICRG 1553 Immunology Kathryn Leyva, Ph.D. Learning Objectives 1. Define tolerance 2. Understand how central tolerance in T cells & B cells develops a. Describe negativ...

Tolerance & Development of Autoimmunity Suggested reading for Lecture 24: Basic Immunology, Chapter 9 MICRG 1553 Immunology Kathryn Leyva, Ph.D. Learning Objectives 1. Define tolerance 2. Understand how central tolerance in T cells & B cells develops a. Describe negative selection of T & B cells during development of these cells b. Understand the function of the transcription factor AIRE in development of T cell central tolerance c. Understand how Treg cells can develop in the thymus 3. Understand how peripheral tolerance in T cells & B cells develops a. Describe how anergy and apoptosis (deletion) of self-reactive T & B cells occurs b. Understand the inhibitory role of CD22 in B cell peripheral tolerance c. Identify FOXP3 as the transcription factor for Treg development d. Understand the role of Treg cells e. Evaluate the 4 mechanisms used by regulatory T cells to inhibit self-reactive T cells 4. Explain how genetic susceptibility may play a role in the development of autoimmunity 5. Evaluate the 5 proposed mechanisms for how infections/inflammation can result in the loss of tolerance 2 Tolerance and Autoimmunity  Autoimmunity = an inappropriate response of the immune system against autologous (self) antigens that results in the activation of autoreactive lymphocytes – both T and B cells. ◼ Autoimmune diseases are classified as hypersensitivities.  Autoimmunity involves a loss of self tolerance in B cells, T cells, or both. ◼ Tolerance = a state of unresponsiveness by lymphocytes to a particular antigen as a result of their interaction with that antigen.  Tolerance to our own self antigens develops during lymphocyte (B and T cell) maturation as well as occurs in the tissues (in the periphery). Optional video on tolerance (the big picture): 3 https://www.youtube.com/watch?v=rHx30H3dUKQ Overview: Tolerance  Central Tolerance: ◼ Develops during selection of immature lymphocytes within the primary lymphatic tissues  Peripheral Tolerance: = Treg cell ◼ Deletion (apoptosis) or anergy of lymphocytes that recognize self-antigens in the periphery ◼ Suppression of responses by regulatory CD4+CD25+ T lymphocytes = Treg cells = Treg cell Optional video on tolerance (focus is on peripheral tolerance): https://www.youtube.com/watch?v=jpL3wCxS-BM 4 T Cell Central Tolerance  We covered (in the T cell lectures) the process of negative selection, which resulted in deletion of immature T cells that expressed a TCR specific for a self antigen. This means that immature T cells need to be exposed to many different self antigens! ◼ Thymic epithelial cells express an active MEC = medullary transcription factor called AIRE (autoimmune (thymic) epithelial cells regulator). ◼ AIRE allows thymic medullary epithelial cells to express many types of peripheral tissue antigens to immature T cells.  Goal: expose immature T cells to as many self antigens as possible during development to: ▪ Delete self-reactive T cells ▪ Develop Treg cells (see next slide too) (Treg) Fig modified from https://www.nature.com/articles/nri2136 5 T Cell Central Tolerance  While most self-reactive CD4+ T cells recognizing self antigens in the thymus are deleted, some are not! These self-reactive CD4+ T cells differentiate into regulatory CD4+CD25+ T cells = Treg cells. How??  The strength of TCR-MHC binding dictates the outcome (see fig): ◼ Strong or “no” binding → Deletion ◼ Weak binding → Helper T cell ◼ Intermediate binding → Treg cell  Treg cells leave the thymus and inhibit responses against self antigens in the periphery. ◼ We’ll look at the specific functions of Treg cells in the next section (slides 12-13). Modified figure from: https://www.nature.com/ articles/s41571-019-0175-7 6 Clinical Application: Mutation in AIRE  If a mutation in the AIRE gene results in a non-functional AIRE protein, autoimmunity can develop due to: ◼ Defective negative selection  Self-reactive T cells are not deleted, so these cells are released into circulation.  Activation of self-reactive T cells can lead to the development of autoimmunity. ◼ Defective Treg development  Treg cells are important in inhibiting self-reactive T cells from responding to self antigens.  A reduction in Treg cells can lead to the development of autoimmunity. FYI: Patients with null mutations in AIRE are diagnosed with Autoimmune Polyendocrinopathy Syndrome (APS1 or APECED), a https://maureensulab.wixsite.com/maureensugroup/research multi-organ autoimmune disease. 7 T Cell Peripheral Tolerance  Peripheral tolerance of T cells is the mechanisms by which mature T cells recognizing self antigens in peripheral tissues are made incapable of subsequently responding to these antigens. Three main mechanisms: ◼ Anergy ◼ Deletion ◼ Suppression  Tolerance in T cells (especially CD4+ T cells) is an effective way of preventing immune responses to protein antigens! Optional video on peripheral tolerance: https://www.youtube.com/watch?v=jpL3wCxS-BM 8 1. T Cell Peripheral Tolerance: Anergy  Anergy of T cells develops due to lack of costimulation or by inhibitory signals. ◼ T cells binding to APCs without B7-CD28 costimulation become anergic (refer to T cell lectures).  Normally, APCs expressing self-antigens do not express B7. APCs upregulate B7 expression in response to infections or inflammatory signals – when PAMPs bind to PRRs! ◼ T cells binding to APCs without costimulation will begin to express inhibitory receptors:  CTLA-4  PD-1 (not shown) 9 2. T Cell Peripheral Tolerance: Deletion  T cells that recognize self antigens without inflammation or that are repeatedly stimulated by antigens die by apoptosis. Two ways this can happen: A. T cells that recognize self antigens in the absence of costimulation may activate the intrinsic apoptotic pathway. Details in this figure are FYI https://www.sinobiological.com/research/signal- transduction/intrinsic-apoptosis 10 2. T Cell Peripheral Tolerance: Deletion Optional video on Fas signaling: https://app.jove.com/science- education/v/12428/the-extrinsic-apoptotic-pathway B. Repeated stimulation of self-reactive T cells results in coexpression of Fas & FasL. Fas-FasL binding activates the FasL = Fas extrinsic apoptotic pathway in the T cells, resulting in apoptosis. ligand  This is another mechanism T cells can use to induce apoptosis! FasR = Fas receptor (Fas) FADD = Fas associated death domain Details in this figure Clinical Application are FYI Mutations in Fas or FasL results in failure of activation- induced cell death.  FYI: This autoimmune disease is known as autoimmune lymphoproliferative https://commons.wikimedia.org/wiki syndrome (ALPS). /File:Extrinsic_apoptosis.jpg 11 3. T Cell Peripheral Tolerance: Treg Cells  Development of Treg cells is controlled primarily by the transcription factor FOXP3. ◼ Most Tregs develop in the thymus (see fig). FYI: some can develop in the periphery, too. ◼ Treg cells are important in the inhibition of immune responses against self-antigens and maintain self-tolerance. Clinical Application FOXP3 mutations cause a rare but severe, systemic multiorgan autoimmune disease due to failure to induce peripheral tolerance.  FYI: Disease is known as IPEX = immune dysregulation, polyendocrinopathy, enteropathy, X- linked syndrome. 12 Function of Treg Cells Modified from https://www.jacionline.org/article/S0091-6749(16)30499-7/pdf  Four main ways that Treg cells function to suppress responses to self-antigens: 1 1. Direct cytotoxic effect: ◼ Release of perforins/granzymes 2 2. Inhibit T cell proliferation: ◼ CD25 binds and sequesters IL-2 3. Secrete inhibitory cytokines: 4 ◼ IL-10, TGF-β, & IL-35 3 4. Down-modulation of APCs: B7 = ◼ CTLA-4 binding to B7 CD80/ CD86 Take home message: Treg cells inhibit the activation, activity, and survival of self-reactive lymphocytes! 13 B Cell Central Tolerance  Central tolerance develops during maturation: ◼ Fig A: Immature B cells that bind with high avidity to a self antigen can undergo receptor editing  If successful, the B cell receptor will not recognize self and complete maturation.  If unsuccessful, the immature B cell remains self-reactive and will undergo apoptosis. ◼ Fig B: Immature B cells that bind with low avidity, to self antigens (i.e., have weak signaling through the BCRs) are released from the bone marrow in an anergic state = unresponsive state.  These anergic cells have downregulated their BCRs and/or have reduced signaling capacity. 14 B Cell Peripheral Tolerance  Peripheral tolerance is not well understood. Proposed mechanisms: 1. B cells that are repeatedly stimulated by self-antigens without T cell help often become anergic or are deleted. 2. Regulation by the inhibitory receptor CD22 - sets a threshold for activation B cells responding to self antigens get signals through only the BCR, allowing sialic acid (glycoproteins) on host cells to bind CD22 CD22 inhibits B cell activation against self antigens = auto-antigens 3. Treg cells also can inhibit https://link.springer.com/referenceworkentry/ self-reactive B (see slide 12) 10.1007/978-4-431-54841-6_131 15 Loss of Self-Tolerance…  Results in autoimmunity (AI disease)  There are a variety of hypotheses explaining how tolerance mechanisms can fail. ◼ Genetic influences are known or suspected to play a significant role. ◼ Previous infections &/or inflammatory responses have been implicated in the development of several autoimmune diseases. 16 Genetic Susceptibility to AI Diseases  Among the genes that are associated with autoimmunity, the strongest associations are with MHC genes.  Expression of particular MHC alleles increases an individual’s relative risk for AI disease. ◼ Relative risk = the probability of developing of a disease No need to memorize the alleles or numbers…the concept is what is important!  Many other genes have been shown to influence the maintenance of tolerance to self antigens by altering the normal regulation of lymphocyte development and/or activation. ◼ Ones we’ve covered: FoxP3, CTLA-4, & AIRE. There are others! 17 Loss of Self-Tolerance…  Results in autoimmunity (AI disease)  There are a variety of hypotheses explaining how tolerance mechanisms can fail. ◼ Genetic influences are known or suspected to play a significant role. ◼ Previous infections &/or inflammatory responses have been implicated in the development of several autoimmune diseases.  Five major mechanisms that promote the loss of tolerance (see next 5 slides for details) 18 1. Bystander T Cell Activation  Infections in the tissues promote the influx and activation of lymphocytes. Some of the lymphocytes that extravasate into the tissues may be self-reactive. ◼ Fig. A: In the absence of infection, APCs present self antigens, but not B7, so tolerance is maintained. ◼ Fig. B: APCs express B7 when induced by a microbe. Recall, APCs present both foreign & self-antigens in MHC molecules. ◼ These activated APCs provide B7 co- stimulation to both microbe-specific (not shown) & self-reactive T cells in the tissues. Activation of self-reactive T cells can result in generation of an AI response. 19 2. Molecular Mimicry  Microbial antigens that are similar to self- antigens can activate T and B cells, producing cross-reacting antibodies and/or CTLs. https://www.immunopaedia.org.za/breaking- news/perfect-match-propels-pathologies/ Examples of how similar self and foreign peptides can be! (details are FYI). 20 CTLs = cytotoxic T lymphocytes 3. Altered Self FYI: PAD = peptidylarginine deiminase  Self antigens can be altered, and thus recognized as foreign in two main ways: a) Haptens binding to self- b) Enzymatic modification of antigens, resulting in a antigens, such as new antigen that can citrullination, stimulate an autoimmune cause changes response. to the original antigen that can be recognized as foreign. 21 4. Release of Sequestered Antigens  Self antigens within immune-privileged sites (e.g., brain, eye, ovary, testis) are sequestered from the immune system. ◼ These sequestered antigens may not have been exposed to developing lymphocytes to induce central tolerance.  If these antigens become exposed to the immune system through tissue damage (via injury or infection), then these newly exposed antigens may be recognized as foreign. 22 5. Cryptic Self Epitopes  Some molecules may shelter (hide) epitopes in the interior from contact with the immune system.  If the molecule is denatured or has a conformational change, epitopes may become exposed and can be recognized as foreign. From Lippincott’s Illustrated Reviews: Immunology, Figure 16.9 https://blog.addgene.org/antibodies-101-epitope-availability 23 Review: Immune Responses Against Self Antigens  Both cell-mediated & humoral responses can occur against self antigens when tolerance mechanisms fail. ◼ Helper T cells can activate macrophages and cytotoxic T cells that can result in inflammation and tissue damage. The same immune mechanisms as for pathogens! ◼ Helper T cells assist B cell activation; B cells produce antibodies that can result in tissue damage. 24 Summary  Tolerance = unresponsiveness to self antigens ◼ Central tolerance = negative selection in primary lymphatics  Immature B cells that bind to self-Ag during development are rendered anergic (reduced BCR expression), deleted via apoptosis, or undergo receptor editing  Immature T cells that bind to MHC+self-Ag during development are deleted via apoptosis or differentiate into Treg cells that participate in peripheral tolerance ◼ Peripheral tolerance mechanisms in secondary lymphatics  B & T cells: no costimulation leads to anergy; self-antigen recognition can lead to apoptosis  B cells: may express inhibitory receptors in response to self-antigens  Treg cells: secrete immunosuppressive cytokines (e.g., IL-10 & TGF-β), perforin/granzymes, CD25 sequesters IL-2, and express inhibitory receptors (e.g., CTLA-4)  The breakdown of tolerance leads to AI: ◼ Genetic susceptibility to AI development: some MHC alleles, others ◼ 5 proposed mechanisms of breaking tolerance: bystander T cell activation, molecular mimicry, altered self, release of sequestered antigens, & cryptic self epitopes  Autoreactive B and T cell immune responses against self antigens occur, causing tissue 25 damage