Chapter 10 Immunologic Tolerance General Features and Mechanisms T Lymphocyte Tolerance B Lymphocyte Tolerance Tolerance induced by Foreign Protein Antigens Homeostasis in the Immune System

What is Immunologic Tolerance? Immunologic Tolerance—— an antigen induced specific unresponsiveness Tolerogens  Immunogens Self-tolerance  Autoimmunity Tolerance induced by foreign antigens Failure Introduction: Tolerance refers to the specific immunological non-reactivity to an antigen resulting from a previous exposure to the same antigen. While the most important form of tolerance is non-reactivity to self antigens, it is possible to induce tolerance to non-self antigens. When an antigen induces tolerance, it is termed tolerogen.

General Features and Mechanisms Immunologically specific A result of antigen recognition by specific lymphocytes

_ + + Studies of graft rejection in inbred mice Strain A Strain A Strain A Neonate Adult _ Skin graft rejection + + 移植以另一品系小鼠的骨髓，至小鼠长至6周，再移植以该骨髓来源品系小鼠的皮肤 During lymphocyte maturation in the generative lymphoid organs, all lymphocytes pass through a stage in which encounter with Ag leads to tolerance Immunologically specific a result of the recognition of antigens by specific lymphocytes

General Features and Mechanisms Immunologically specific Central tolerance: induced in generative lymphoid organs immature self-reactive lymphocyte The repertoire of mature lymphocytes cannot recognize ubiquitous or widely disseminated self antigens The repertoire of mature lymphocytes cannot recognize ubiquitous or widely disseminated self antigens

Burnet: Clonal selection hypothesis

General Features and Mechanisms Immunologically specific Central tolerance: generative lymphoid organs immature self-reactive lymphocyte Peripheral tolerance: peripheral sites mature self-reactive lymphocytes The repertoire of mature lymphocytes cannot recognize ubiquitous or widely disseminated self antigens The repertoire of mature lymphocytes cannot recognize ubiquitous or widely disseminated self antigens Important for maitaining unresponsiveness to self antigens that are not expressed in the generative lymphoid organs.

Peripheral tolerance is induced when mature lymphocytes recognize antigens without adequate levels of the costimulators.

The principal mechanisms of lymphocyte tolerance Central tolerance Deletion: apoptotic cell death Anergy: functional inactivation without cell death Treg Peripheral tolerance

Apoptosis Apoptosis. Light micrographs of (a) normal thymocytes (developing T cells in the thymus) and (b) apoptotic thymocytes. Scanning electron micrographs of (c) normal and (d) apoptotic thymocytes. [From B. A. Osborne and S. Smith, 1997, Journal of NIH Research 9:35; courtesy B. A. Osborne, University of Massachusetts at Amherst.]

Immunologic Tolerance General Features and Mechanisms T Lymphocyte Tolerance B Lymphocyte Tolerance Tolerance induced by Foreign Protein Antigens Homeostasis in the Immune System

T Lymphocyte Tolerance Central T Cell Tolerance Peripheral T cell Tolerance

Maturation of T cells in the thymus Diagrammatic cross section of a portion of the thymus, showing several lobules separated by connective tissue strands (trabeculae). The densely populated outer cortex is thought to contain many immature thymocytes (blue), which undergo rapid proliferation coupled with an enormous rate of cell death. Also present in the outer cortex are thymic nurse cells (gray), which are specialized epithelial cells with long membrane extensions that surround as many as 50 thymocytes. The medulla [me”dʌlə] is sparsely populated and is thought to contain thymocytes that are more mature. During their stay within the thymus, thymocytes interact with various stromal cells, including cortical epithelial cells (light red), medullary epithelial cells (tan), interdigitating dendritic cells (purple), and macrophages (yellow). These cells produce thymic hormones and express high levels of class I and class II MHC molecules. Hassalls corpuscles, found in the medulla, contain concentric layers of degenerating epithelial cells. [Adapted, with permission, from W. van Ewijk, 1991,

Negative selection: Development of central tolerance In immature t cells, the consequence of high avidity antigen recognition is the triggering of apoptosis, leading to death or deletion of the cells High affinity High concentration

Self antigens expressed in the thymus ubiquitous self-antigen: widely expressed in the body tissue-specific antigen autoimmune regulator gene, AIRE

Natural Tregs arise in the thymus Natural Tregs arise in the thymus as thymocytes interact with mTEC expressing tissue-specific antigens driven by the transcription factor AIRE. An increased affinity interaction between the pMHC complex and the TCR along with the secondary signals stimulates developing thymocytes to express Foxp3 and activates a gene expression profile driving natural Treg development 天然调节T细胞在维持自身耐受中其主要作用

天然调节T细胞在维持自身耐受中其主要作用

T Lymphocyte Tolerance Central T Cell Tolerance Peripheral T cell Tolerance

Peripheral T cell Tolerance Antigen recognition without adequate costimulation Use CTLA-4 to recognize costimulators on APCs Activation induced cell death (AICD) Regulatory T Lymphocytes Factors that determine the tolerogenicity of self antigens

Peripheral T cell Tolerance Antigen recognition without adequate costimulation Use CTLA-4 to recognize costimulators on APCs AICD Treg Factors that determine the tolerogenicity of self antigens

Two Signal model Sig2 (co-stimulation) 1975 Lafferty & Cunningham T helper cells die when they see antigen unless rescued by co-stimulation(signal two) from APCs. 21

co-stimulatory signal For a T cell to be activated by a specific antigen, the T cell receptor must recognize complexes of MHCI with the antigen on the surface of an antigen-presenting cell. T cells and the T cell receptor complex do not respond to antigen in solution, but even for the specific antigen they only respond to antigen-MHC-1 complexes on the cell surface. This interaction is necessary for T cell activation, but it is not sufficient. T cell activation also requires a co-stimulatory signal involving interaction of CD28 on the T cell with CD80 or CD86 (B7 family genes) on the antigen-presenting cell. CD28 activates a signal transduction pathway acting through PI-3K, Lck and Grb-2/ITK to provide its co-stimulatory signal for T cell activation. Another means to control T cell activation is by expressing factors that down-regulate T cell activation. Signaling by activated T cell receptors induces expression of CTLA-4, a receptor that opposes T cell activation. CTLA-4 has a higher affinity than CD28 for B7 proteins, terminating T cell activation. ICOS is a protein related to CD28 that is only expressed on activated T cells, and that provides another important co-stimulatory signal. The requirement for co-stimulatory signals provides additional control mechanisms that prevent inappropriate and hazardous T cell activation 22

Figure 8-20 Gowth factor: IL-2

Anergy is induced when mature lymphocytes recognize antigens without adequate levels of the costimulators.

Peripheral T cell Tolerance Antigen recognition without adequate costimulation Use CTLA-4 to recognize costimulators on APCs AICD Treg Factors that determine the tolerogenicity of self antigens

Figure 8-12 CTLA-4/B7 initiate the inhibitory signal

Anergy may be induced if T cells use CTLA-4 to recognize costimulators on APCs

Question Factors ? ? T cells T cells T cells recognize B7 molecules with CD28 (active receptor) ? ? ? ? ? ? Recognize the same B7 with CTLA-4 (inhibitory receptor) Immune response Raise the questions Tolerance

Making and breaking tolerance 感染致树突状细胞成熟和活化 The nature of tissue APCs is an important determinant of whether self-tolerance or autoimmunity develops.

Peripheral T cell Tolerance Antigen recognition without adequate costimulation Use CTLA-4 to recognize costimulators on APCs AICD Treg Factors that determine the tolerogenicity of self antigens

Activation induced cell death Repeated stimulation of T cells by persistent antigen results in death of the activated cells by a process of apoptosis

Fas-mediated activation-induced cell death AICD is a form of apoptosis induced by signals from membrane death receptors

activation activation The net effect is that the population of mature lymphocytes is depleted of antigen specific lymphocytes by repeated stimulation.

Peripheral T cell Tolerance Antigen recognition without adequate costimulation Use CTLA-4 to recognize costimulators on APCs AICD Regulatory T Lymphocytes (Treg) Factors that determine the tolerogenicity of self antigens

Treg cell development Immunity 2009; 30: p626

T cell-mediated suppression

Mechanisms of action of regulatory T cells Figure 1 | Two classes of regulatory T cells can be envisioned. a | In this hypothetical model, the natural regulatory T (TReg) cells (blue) suppress immune responses in a contact-dependent manner and function in general homeostasis to block the actions of autoimmune T cells (red) in non-inflammatory settings. b | The adaptive TReg-cell subset enhances the robust nature of suppression in an inflammatory milieu. Importantly, adaptive TReg cells can develop either from CD4+CD25+ natural TReg cells (blue striped) or by altering the activity of T helper (TH) cells (red striped). APC, antigen-presenting cell; IL-10, interleukin-10; TGF-β, transforming growth factor-β. Physiological roles of TReg-cell subsets The two TReg-cell subsets might function in different immunological settings, depending on the context of antigen exposure, the nature of the inflammatory response and the TCR repertoires of the individual cells. We argue that natural TReg cells would be most effective at suppressing autoreactive T-cell responses locally, in non-inflammatory settings circumstances in which antigen-specific, self limiting reactions are required to achieve a fine homeostatic balance. By contrast, during self-damaging inflammatory reactions to microbes or transplanted tissue, or in settings of inflammatory autoimmune disease that are more similar to the infectious setting (for example, inflammatory bowel disease), adaptive TReg cells might be induced to suppress the pathological immune responses. Although this functional activity might also require cell–cell contact, the resulting suppression is amplified by the secretion of inhibitory cytokines.

Peripheral T cell Tolerance Antigen recognition without adequate costimulation Use CTLA-4 to recognize costimulators on APCs AICD Regulatory T Lymphocytes (Treg) Factors that determine the tolerogenicity of self antigens

Factors That Determine the Immunogenicity and Tolerogenicity of Protein Factors that favor stimulation of immune response Factors that favor tolerance Amount Optimal doses that vary for different antigens High doses Persistence Short-lived (eliminated by immune response) Prolonged (AICD) Portal of entry; location Subcutaneous, intradermal; absence from generative organs Intravenous, oral; presence in generative organs Presence of adjuvants Antigens with adjuvants: stimulate helper T cells Antigens without adjuvants: Properties of APCs High levels of costimulators Low levels of costimulators and cytokines

Immunologic Tolerance General Features and Mechanisms T Lymphocyte Tolerance B Lymphocyte Tolerance Tolerance induced by Foreign Protein Antigens Homeostasis in the Immune System

B Lymphocyte Tolerance Central B Cell Tolerance Peripheral B cell Tolerance Checkpoints during B cell maturation and activation at which encounter self Ags may abort these process

B cell development in bone marrow The maturation of B and T lymphocytes consists of sequential stages-lineage commitment and proliferation, expression of antigen receptor genes, and selection of mature repertoires. The earliest bone marrow cells committed to the B cell lineage is called a pro-B cell. The pre-B cell represents the next stage of development

淋巴样干细胞、未接触抗原的B细胞、受抗原驱动成为记忆细胞或浆细胞

Central tolerance in B cells Immature B cells that recognize self antigens in the bone marrow with high affinity are deleted or change their specificity. multivalent self antigens: cell membrane molecules、polymeric molecules

Down-regulation of antigen receptor expression What is the result of self antigen recognition in generative lymphoid organ? HEL transgenic Down-regulation of antigen receptor expression Change in receptor specificity (receptor editing)

B Lymphocyte Tolerance Central B Cell Tolerance Peripheral B cell Tolerance

B7 CD28 T-B Collaboration

Peripheral tolerance in B cells Mature B cells that recognize self antigens in peripheral tissues in the absence of specific helper T cells may be rendered functionally unresponsive. If anergic B cells do encounter any antigen-specific helper T cells, what happened? The B cells maybe killed by FasL on the T cells engaging Fas on the B cells.

systemic lupus erythematosus 脑 Brain 癫痫 seizures 精神病 psychosis 头痛 headache 头发 Hair 脱发 alopecia 脸 Face 颊部红斑 malar rash 盘状红斑 discoid rash 肺 Lungs 胸膜炎 pleurisy 间质性肺炎 interstitial pneumonia 心 Heart 心包炎 pericarditis 心肌炎 myocarditis 心内膜炎 endocarditis 肺动脉高压 pulmonary hypertension 血液、血管 Blood & Vessels 血管葱皮样改变 onion-skin like artery 贫血 anemia 血栓 thrombosis 小肠 Intestines 血管炎 vasculitis 手 Hands 雷诺氏现象 Raynauds phenomenon Jaccoud关节病 肾 Kidneys 蛋白尿 proteinuria 管型 casts 关节 Joints 关节炎 arthritis 足 Feet 血管炎 vasculitis systemic lupus erythematosus 

蝶型红斑 （Butterfly rash）

Homogenous pattern Peripheral pattern Speckled pattern

Normal individals do not produce autoantibodies against self protein antigens, and this may be due to deletion or tolerance of helper T lymphocytes even if functional B cells are present. Defects in the maintenance of T cell tolerance may result in autoantibody production

Thymus (cortex); peripheral Bone marrow; peripheral Feature T lymphocyte B lymphocyte Principal sites Thymus (cortex); peripheral Bone marrow; peripheral Tolerance-sensitive stage CD4+CD8+ thymocyte Immature B lymphocytes Stimuli for tolerance induction Central: high-avidity recognition of Ag in thymus Central: high-avidity recognition of multivalent Ag in bone marrow Peripheral: Ag presentation by APCs lacking costimulators; repeated stimulation by self Ag Peripheral: Ag recognition without T cell helper Principal mechanisms of tolerance Central tolerance: clonal deletion (apoptosis) Central tolerance: clonal deletion (apoptosis), receptor editing Peripheral tolerance: anergy, AICD, suppression Peripheral tolerance: block in signal transduction (anergy); failure to enter FC

Immunologic Tolerance General Features and Mechanisms T Lymphocyte Tolerance B Lymphocyte Tolerance Tolerance induced by Foreign Protein Antigens Homeostasis in the Immune System

Factors That Determine the Immunogenicity and Tolerogenicity of Protein Factors that favor stimulation of immune response Factors that favor tolerance Amount Optimal doses that vary for different antigens High doses Persistence Short-lived (eliminated by immune response) Prolonged (AICD) Portal of entry; location Subcutaneous, intradermal; absence from generative organs Intravenous, oral; presence in generative organs Presence of adjuvants Antigens with adjuvants: stimulate helper T cells Antigens without adjuvants: Properties of APCs High levels of costimulators Low levels of costimulators and cytokines

Immunologic Tolerance General Features and Mechanisms T Lymphocyte Tolerance B Lymphocyte Tolerance Tolerance induced by Foreign Protein Antigens Homeostasis in the Immune System Termination of normal immune response

Mechanisms of the decline of normal immune response (homeostasis) CTLA-4、Fas、FasL

T cell B7 CD28 Ag TCR CTLA-4 Active signal I T I M I TAM 24 h B7 CD28 Ag TCR CTLA-4 Active signal I T I M I TAM Inhibitory signal T cell

Antibody feedback

The repertoire for TCR and BCR (Ab) Niels Jerne: Network hypothesis Ab1 Ab2 Ag Ab3 threshold 阈值之下的各种显示独特型的淋巴细胞，就是由数量极大的感知元件组成的一种洞察抗原刺激并具有引发特异性应答潜在能力的网络。 The repertoire for TCR and BCR (Ab) Idiotype and anti-idiotypic Reach a steady state at which the immune system is at homeostasis

Summary What is immunologic tolerance? Central and peripheral tolerance Tolerance in T and B cells Homeostasis

What is Immunologic Tolerance? Tolerance—— an antigen induced specific unresponsiveness Tolerogens  Immunogens Self-tolerance  Autoimmunity Tolerance induced by foreign antigens Introduction: Tolerance refers to the specific immunological non-reactivity to an antigen resulting from a previous exposure to the same antigen. While the most important form of tolerance is non-reactivity to self antigens, it is possible to induce tolerance to non-self antigens. When an antigen induces tolerance, it is termed tolerogen.

Thymus (cortex); peripheral Bone marrow; peripheral Feature T lymphocyte B lymphocyte Principal sites Thymus (cortex); peripheral Bone marrow; peripheral Tolerance-sensitive stage CD4+CD8+ thymocyte Immature B lymphocytes Stimuli for tolerance induction Central: high-avidity recognition of Ag in thymus Central: high-avidity recognition of multivalent Ag in bone marrow Peripheral: Ag presentation by APCs lacking costimulators; repeated stimulation by self Ag Peripheral: Ag recognition without T cell helper Principal mechanisms of tolerance Central tolerance: clonal deletion (apoptosis) Central tolerance: clonal deletion (apoptosis), receptor editing Peripheral tolerance: anergy, AICD, suppression Peripheral tolerance: block in signal transduction (anergy); failure to enter FC

THANK YOU

Innate and adaptive Immunity Naïve lymphocytes Mature lymphocytes that have not previously encountered antigen; function -- antigen recognition Preferential migration to peripheral lymphoid organs (lymph nodes), the sites where immune responses start Effector lymphocytes Activated lymphocytes capable of performing the functions required to eliminate microbes (‘effector functions”) Effector T lymphocytes: cytokine secretion (helper cells), killing of infected cells (CTLs) B lymphocytes: antibody-secreting cells (e.g. plasma cells)