T cell receptor & MHC complexes-Antigen presentation 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 5-7 2. Male D., J. Brostoff, D. B Roth, and I. Roitt Immunology (7th ed., 2006), Chapter 5 & 7

Question What mechanisms to achieve the generation of Ab diversity? Ans: 1. VDJ gene recombination => V regions => Ag binding 2. Somatic hypermutation 3. Isotype switching => Fc portion => Effector function

Outline Structures & Features of T-cell antigen receptor (TCR) Structures & Features of Major Histocompatibility Complex (MHC) Antigen presentation to T cells Summary & Question

Antibody, TCR & MHC

Key Concepts in T-cell Receptor (TCR) 1. T-cell antigen receptor (TCR) is similar to the F(ab) of Ab but only located on the surface of T cells => No secreted form => Two major types: ab TCR and gd TCR 2. TCR functions to recognize Ag peptide and then to activate T cells => Adaptive immunity 3. Ag recognition by ab TCR requires Ag presented by Major Histocompatibility Complex (MHC). => consider both Ag peptide & MHC => Cell-Cell interaction 4. The Ag-binding site region of the TCR is formed by the Va and Vb regions.

Similarities & Differences between T-cell Receptor (TCR) and Ab

The Structure of T-cell Receptor (TCR)

T-cell Receptor (TCR) vs. Ab

Binding of a TCR to a peptide-MHC complex Side Front

TCR & Accessory Molecules -Help T cells in response to a specific Ag 1. CD3 (e, g, d & z chains): associates w/ TCR => intracellular signaling transduction 2. CD4/CD8: CD4  MHC-II CD8  MHC-I 3. CD28: a co-stimulatory receptor 4. Integrin: Adhesion & co-stimulation

The TCR/CD3 Complex TCR/CD3 Complex: 1. TCRab dimer + multiple CD3 dimers - CD3eg dimer - CD3ed dimer - CD3zz dimer 2. The ITAM motif on CD3 => Signaling Transduction

TCR/CD3 Complex vs BCR/Igab Complex

Interactions of Accessory molecules between T cells & APCs

Outline Structures & Features of T-cell antigen receptor (TCR) Structures & Features of Major Histocompatibility Complex (MHC) Antigen presentation to T cells Summary & Question

Key Concepts in Major Histocompatibility Complex (MHC) 1. Ag presentation to TCR is mediated by Two classes of MHC molecules. - Class-I MHC => peptides from cytosolic (intracellular) proteins => CD8 T cells - Class-II MHC => peptides from intracellular (exogenous) proteins from phagocytosis => CD4 T cells 2. In humans, the MHC is also called as the HLA (Human Leukocyte Antigen). 3. MHC genes are the most polymorphic genes present in the genome and co-dominantly expressed in each individual. 4. MHC molecules express on the cellular surfaces of only in presence of Ag-peptides. Class-I => all nucleated cells Class-II => APCs (DC, Macrophages & B cells)

TCR-Ag w/ Histocompatibility Complex (MHC)

The Discovery of Major Histocompatibility Complex (MHC) Histocompatibility genes: George Snell in 1940s => tumors or tissues => same strain, OK => foreign strains, No

For their work leading to the discoveries of MHC (mouse ) and HLA (human). - Immune recognition => The foundation of adaptive immunity - Transplantation

Schematic maps of human & mouse MHC loci

Class-I vs. Class-II MHC molecule

Structure of Class-I MHC molecule a1a2 domains are polymorphic and form the peptide-binding cleft. a3 domain is conserved among all MHC class-I, and folds into an Ig domain for CD8 binding.

Structure of Class-II MHC molecule a1b1 domains are polymorphic and form the peptide-binding cleft. b2 domain is conserved among all MHC class-II, and folds into an Ig domain for CD4 binding.

Polymorphic residues of MHC molecules In Class-I, polymorphic residues => the peptide-binding cleft formed by a1a2 domains In Class-II, polymorphic formed by a1b1 domains In this case HLA-DR, polymorphism is on b1 domain

MHC genes are highly Polymorphic

Expression of MHC alleles is co-dominant

Polymorphism & Polygeny both contribute to the MHC diversity

Gene conversion creates new alleles in MHC genes => Copying sequences from one MHC gene to another

Gene recombination creates new alleles in MHC genes

MHC expression on cells-I

MHC expression on cells-II Expression of MHC molecules is increased by cytokines produced during innate & adaptive immune cells, ex. IFN

Features of Peptide-MHC interactions 1. MHC molecules show a broad spectrum for peptide binding, in contrast to the fine specificity of Ag recognition by Ab. 2. Peptide-MHC interactions are non-covalent and mediated by residues both in the peptides and in the clefts of MHC molecules. 3. Each MHC molecule binds one peptide at a time but can bind many different peptides. 4. MHC molecules do not discriminate between foreign peptides & self peptides.

Outline Antigen presentation to T cells Structures & Features of T-cell antigen receptor (TCR) Structures & Features of Major Histocompatibility Complex (MHC) Antigen presentation to T cells 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: 1st function => process protein Ags to small peptides => form & present the peptide-MHC complex to T cells 2nd function => provide 2nd co-stimulatory signals, ex. Cytokines & surface molecules

Features of Ag recognition by T cells

T cells require APCs to respond to a specific Ag

MHC Restriction of cytotoxic T cells

Functions of different APCs

Features of different APCs

Dendritic cells – The Most effective APCs 1. Located at common sites of entry of microbes 2. Express receptors to capture microbes. 3. Migrate preferentially to T- cell zones in LNs. 4. Mature DCs express high levels of costimulators => T-cell activation.

Overview of Dendritic cells in Ag capture & presentation

Class I vs Class II MHC pathway

The Class II MHC pathway of Ag Presentation

The Class I MHC pathway of Ag Presentation

Ag Presentation to different T cell subsets

SUMMARY 1. TCR functions to recognize Ag peptides presented the MHC complexes => Ag peptide specificity => MHC restriction 2. Two classes of MHC molecules. - Class-I MHC => peptides from cytosolic (intracellular) proteins => CD8 T cells - Class-II MHC => peptides from intracellular (exogenous) proteins from phagocytosis => CD4 T cells 3. APCs serve two key functions for T cell activation: 1st function => process & present Ag peptides w/MHC to T cells 2nd function => provide 2nd co-stimulatory signals, ex. cytokines & surface molecules

Questions What is the Bare Lymphocyte Syndrome? What is the advantage of MHC Polymorphism? Is that good if MHC is as diverse as Ig or TCR?