Antigen Presentation/Cell cooperation in Antibody response Pin Ling ( 凌 斌 ), Ph.D. ext 5632; References: 1. Male D., J. Brostoff, D. B Roth, and I. Roitt Immunology (7th ed., 2006), Chapters 7 & 8

Questions What is the advantage of MHC Polymorphism? Is that good if MHC is as diverse as Ig or TCR?  MHC polymorphism may provide a broader protective immunity in a population during pathogen infection.

Outline Antigen presentation to T cells T-cell activation Cell cooperation in Ab response (B-cell activation) B cell affinity maturation Summary & Question

Key Concepts in Ag presentation between APCs & T cells 1. Most T cells recognize only peptides, whereas B cells can recognize peptides, lipids, nucleic acids,….etc. NK-T cells can recognize lipids. 2. T cells only recognize peptides displayed by MHC molecules on Ag-presenting cells (APCs). 3. APCs are responsible for capturing and displaying different Ags to T cells. 4. APCs serve two key functions for T cell activation: 1 st function => process protein Ags to small peptides => form & present the peptide-MHC complex to T cells 2nd function => provide 2nd co-stimulatory signals, e.g. Cytokines & Surface Molecules

T cells require APCs to respond to a specific Ag

Features of different APCs

Localization of APCs in lymph nodes

Functions of different APCs

Overview of Dendritic cells in Ag capture & presentation

The Class I MHC pathway for Ag (intracellular) Presentation

The Class II MHC pathway of Ag (extracellular) Presentation

Cross presentation of exogenous peptides by MHC-I on DCs

Pathogen presentation by MHCs

Ag Presentation to different T cell subsets

Outline Antigen presentation to T cells T-cell activation Cell cooperation in Ab response (B-cell activation) B cell affinity maturation Summary & Question

Key Concepts in lymphocyte development 1. Lymphocyte development-A process of differentiation of lymphoid progenitor cells into mature lymphocytes (T & B). 2. Rearrangement and expression of Ag receptor genes are associated with lymphocyte development. 3. Selection events are involved in preserving cells w/ correct Ag receptors and eliminating dangerous cells w/ self-recognition Ag receptor. 4. Proliferation in the early lymphocyte development is stimulated by IL-7.

Checkpoints in Lymphocyte Development

Stages of T lymphocyte development-I

Maturation of T lymphocytes in Thymus

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 2 o 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 (e. g. CD28) interact with ligands from APCs. 4. T cell responses decline after Ag is eliminated.

Key Concepts in T cell Activation-II 5. 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 6. Differentiation of naïve CD8 T cells requires Signals: - Ag presentation, Cytokines (eg. IL-12), and Effector CD4 T cells in some cases. 7. The Best characterized costimulatory pathway: CD28 (T cell)-B7 (APC) interaction.

Key Molecules in APC-T cell Interaction (Immunological Synapse)

T-cell Activation Signaling

T-cell Activation, Anergy, & Down- regulation

T cell Activation by Superantigens

Outline Antigen presentation to T cells T-cell activation Cell cooperation in Ab response (B-cell activation) B cell affinity maturation Summary & Question

Features of B lymphocyte development 1. Maturation of B Lymphocytes development - Rearrangement & expression of Ig gene in a precise order - Selection & proliferation of pre-B cells via pre-Ag receptor 2. Selection of the mature B cell repertoire - Self Ag => Affect the strength of the BCR signal - Immature B cells => self Ag/high avidity => Receptor editing => Additional L chain recombination => Not Self-reactive Fail to receptor editing => Apoptosis 3. During this maturation, B cell lineage cells go through distinct stages => A specific Ig gene expression => Distinct surface markers 4. At Pre-B cell stage, H chain recombination occurs first and associates w/ Surrogate light chains (  & VpreB). - 5 & VpreB are similar to  & light chains but invariant - form pre-B cell receptor => Development

Stages of B lymphocyte development-I

Stages of B lymphocyte development

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 & 2 nd Ab responses differ qualitatively & quantitatively.

Activation of Lymphocytes in Lymph Nodes

The Phases of B cell Responses

Cell cooperation in B cell Responses 1.APCs w/Ag  T cells 2.B cell w/Ag  Th cells 3.B cells activation => Plasma cells => memory

B-cell Activation Signaling

Interface between T and B-cells

Outline Antigen presentation to T cells T-cell activation Cell cooperation in Ab response (B-cell activation) B cell affinity maturation (in Germinal Center) Summary & Question

Key Concepts in B cell Activation-II 6. Features of antibody responses in vivo include: the enhanced secondary response; isotype switching; affinity maturation; the development of memory.

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

B-cell development in germinal centers

Cytokines help B-cell isotype switching & affinity maturation

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

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

SUMMARY 1. Two classes of MHC molecules. - Class-I MHC => peptides from cytosolic (intracellular) proteins => CD8 T cells - Class-II MHC => peptides from extracellular (exogenous) proteins from phagocytosis => CD4 T cells 2. APCs serve two key functions for T cell activation: 1 st function => process & present Ag peptides w/MHC to T cells 2nd function => provide 2nd co-stimulatory signals, ex. cytokines & surface molecules 3. B cell activation mediates “Humoral Immune Responses”. Isotype switching & Hypermutation are key features in T cell-dep humoral immune responses.

Question What happens to the immune system if RAG1 or RAG2 is mutated?