Lymphocyte Activation & Immune Tolerance Pin Ling (凌 斌), Ph.D. ext 5632; lingpin@mail.ncku.edu.tw References: 1. Abbas, A, K. et.al, Cellular and Molecular Immunology (6th ed., 2007), Chapter 9-11

Question What is happened to the immune system if RAG1 & RAG2 are mutated? Severe Combined Immunodeficiency Disease (SCID) Lacking functional T & B cells

Outline Activation of T Lymphocytes B cell Activation & Ab Production Immune Tolerance Summary & Question

Key points in T cell activation Activation of CD4 & CD8 T cells 2. Role of Costimulators in T cell activation 3. Signaling Transduction by the TCR/CD3 complex 4. T cell signal “Attenuation”: Roles of inhibitory receptors & Protein degradation

Key Concepts in T cell Activation-I 1. Adaptive immune responses are initiated and mediated by T-cell activation. 2. Naïve T lymphocytes migrate from Thymus to 2o Lymphoid organs, whereby encounter Ag presented by APCs (eg. DCs) and then become activated. 3. T-cell activation requires Two Signals: - Primary Signal-TCR/CD3 –Ag/MHC complex - Second Signal (Costimulatory)- Other T cell surface molecules (ex. CD28) interact with ligands from APCs. 4. T cell responses decline after the Ag is eliminated.

Activation of Lymphocytes in Lymph Nodes

Phases of T cell Responses IL2 –A T cell autocrine factor

Cross presentation (priming) of Ag to CD8 T cells DC has an unique feature: Allow protein traffic from endosomal vesicles to the cytosol Ag-MHC-I presentation Ingest virus-infected or tumor cells => CD8 T cells

Role of CD4 T helper cells in the differentiation of CD8 T cells

Functions of co-stimulation in T cell activation

T-cell Receptor Signaling

The T-cell Immunological Synapse The interface between T cells and APC cells => Prolong interaction => Enhance signaling transduction

IL-2 Expression during TCR Signaling Cyclosporin A

Signal Attenuation by the Inhibitory Receptor (CTLA-4)

TCR Signal Termination by Protein Degradation

Key Concepts in T cell Activation-II 1. Activated CD4 T helper type -1 (Th1) cells regulate the cell-mediated immunity (CMI) => Macrophages & CD8 T cells. Th2 cells help Humoral immunity => B cell activation 2. Differentiation of naïve CD8 T cells requires Signals: - Ag presentation, Cytokines (eg. IL-12), and Effector CD4 T cells in some cases. The Best characterized costimulatory pathway: CD28 (T cell)-B7 (APC) interaction.

Outline B cell Activation & Ab Production Immune Tolerance Activation of T Lymphocytes B cell Activation & Ab Production Immune Tolerance Summary & Question

Key points in B cell activation & Ab production Overview of B cell activation 2. Signaling Transduction by BCR & its Co-receptor 3. Mechanisms of CD4 T helper cell-mediated B cell activation 4. The Germinal Center Reaction: -Isotype switching -Affinity maturation (Somatic Hypermutation)

Key Concepts in B cell Activation-I 1. B cell activation mediates Humoral Immune Responses. 2. Activating B cells => Ab responses => protein Ag => Require CD4 T helper cells => These protein Ag => T-dependent (TD) Ag 3. Activating B cells => Ab responses to multivalent Ags w/ repeating epitopes (lipids, polysaccharides) => No CD4 T helper cells => These lipid & polysaccharide Ags => T- independent (TI) Ag 4. Heavy chain isotype switching & Hypermutation are often seen in T cell-dependent humoral immune responses. 5. Primary & 2nd Ab responses differ qualitatively & quantitatively.

Activation of Lymphocytes in Lymph Nodes

Phases of B cell Responses

Primary vs. Secondary Ab responses

B-cell Receptor Signaling

CD21/CR2 complement receptor as a Co-receptor for B cells

B cell Responses by cross-linking BCR

Events of T-B cell Interaction

T-B collaboration & the Hapten-Carrier effect

Helper T cell-mediated B cell activation

Germinal center reactions in T cell-mediated Ab responses Germinal centers (2nd follicle) 1. T-dep B cell activation 2. Somatic hypermutation 3. Isotype switching 4. Memory B cells

Ig Heavy chain isotype switching

Ig Heavy chain isotype switching-II 1. AID (Activation- Induced Deaminase) , ApeI endonuclease & UNG (Uracil N Glycosylase) but not RAG. 2. Other DNA repair proteins 3. Recognize Ig exon or Switch region

Ig Heavy chain isotype switching-III

Somatic Hypermutation in Ig V region AID is required in this process.

B cell selection in Germinal Centers Only B cells w/ a high affinity for Ag on follicular DCs (FDCs) => Survive

Outline Immune Tolerance Summary & Question Activation of T Lymphocytes B cell Activation & Ab Production Immune Tolerance Summary & Question

Key Concepts in Immune Tolerance 1. Self-tolerance may be induced in immature self-reactive lymphocytes in primary lymphoid organs (Central tolerance) or mature lymphocytes in peripheral sites (Peripheral tolerance). 2. Central Tolerance => Cell Death (T & B cells) (Negative selection) => Receptor Editing (B cells) => A change in functional abilities (Regulatory T cells) 3. Peripheral Tolerance Mature lymphocytes  self Ags => No response (Anergy) => Cell Death 4. Foreign Ags may be administered in ways that inhibit immune responses by inducing tolerance.

Fates of lymphocytes after encounter with antigens

Central vs. Peripheral Tolerance

Central T cell Tolerance ?

T cell tolerance-A transgenic mouse model

Mechanisms of T cell Anergy

Regulatory T cell-mediated Suppression

Self Ag-induced death of peripheral T cells

Central & Peripheral B cell Tolerance

Features of T cell vs.B cell Tolerance

Outline Summary & Question Activation of T Lymphocytes B cell Activation & Ab Production Immune Tolerance Summary & Question

SUMMARY 1. T-cell activation requires “Two Signals”: - Primary Signal-TCR/CD3 –Ag/MHC complex - Second Signal (Costimulatory)- Other T cell surface molecules (eg. CD28) interact with ligands from APCs. 2. B cell activation mediates “Humoral Immune Responses”. Isotype switching & Hypermutation are key features in T cell-dep humoral immune responses. 3. Self-tolerance may be induced in immature self-reactive lymphocytes in primary lymphoid organs (Central tolerance) or mature lymphocytes in peripheral sites (Peripheral tolerance). 4. The failure of self tolerance leads to autoimmune diseases.

Question Have you learned something from this series of Immunology Lectures? Yes More or less Not Sure Definitely Not