Antigens & Antigen Receptors Lecture A PDF

Antigens & Antigen Receptors Lecture A Dr Natalie Riddell [email protected] 12AY02 Theme 3 Discussion Board Theme 3 Introduction The Aims of this Theme are: To provide an understanding of the composition and properties of antigens, and how they elicit an immune response To develop a knowledge o...

Antigens & Antigen Receptors Lecture A Dr Natalie Riddell [email protected] 12AY02 Theme 3 Discussion Board Theme 3 Introduction The Aims of this Theme are: To provide an understanding of the composition and properties of antigens, and how they elicit an immune response To develop a knowledge of the structure and function of B cell receptor, T cell receptor and the major histocompatibility complexes Lecture content Lecture A (today): Part 1: General Principals, Antigens and immunogens Part 2: The B cell receptor and Antibodies Part 3: The T cell receptor, Polyclonal responses Lecture B (Thursday): Part 1: MHC class I & II, Antigen processing and presentation Part 2: Genetics of antigen recognition Lecture A, Part 1 Introduction, refresher Antigens/immunogens Learning profile items: Define the term antigen and distinguish between an antigen and an immunogen Explain the different factors that influence an antigen’s immunogenicity Describe the differences between T and B cell antigens Refresher The Immune system The body’s ability to recognize and defend itself against distinct invaders and their products Is a ‘smart’ system whose ‘memory’ allows it to respond rapidly to a second encounter with a pathogen Antigens (Ag) trigger specific immune responses Various cells are part of specific immunity including antigen presenting cells, as well as B and T lymphocytes that have specific antigen receptors (AgR) This is the central principle of vaccines How do vaccines work? Immune Response is antigen driven Phases of adaptive immune responses Function of secondary lymphoid tissue Increases the chances of the antigen encountering the right antigen receptor that can recognise it Fig. 2-13 Much like trying to find a friend in a “Starbucks”, as opposed to anywhere in Guildford. Antigens are presented in different tissues depending on the compartment they appear in Antigens/Immunogens Some terminology Antigen receptors B cells - B cell receptor (BCR) T cells - T cell receptor (TCR) Paratopes (antigen binding sites) Epitopes (antigenic determinants) Antigens Immunogens What is the difference between an immunogen and antigen? ANTIGEN (Ag): any substance that binds to specific receptors on lymphocytes i.e. B cell receptor (BCR) or T cell receptor (TCR) ANTIGENIC DETERMINANT or EPITOPE: that part of Ag that binds to these receptors IMMUNOGEN: any molecule or group of molecules that induce an immune response All immunogens are antigens Not all antigens are immunogens HAPTEN: Small molecule that can act as an epitope but not elicit an immune response Antigens An antigen may be protein, lipid, carbohydrate or any combination of these May be foreign or altered self molecule May be soluble or particulate, simple or complex with many different antigenic determinants each of which comprises a small number of amino acids, carbohydrate or lipid residues Many are proteins or large polysaccharides from foreign organisms: – components of bacterial cell walls, capsules, pili, and flagella, proteins of viruses, fungi, and protozoa Non microbe derived antigens might include pollen, food and dust or ‘self’ antigens from dead/senescent cells or altered self molecules Antigens enter the body by various methods: – – – – breaks in the skin and mucous membranes, direct injection, as with a bite or needle or through organ transplants and skin grafts M cells in the mucosal surfaces Examples Simpler Antigens Benzene Ovalbumin Pollen Examples Complex Antigens Complex antigens can have many different antigenic determinants, each of which comprises a small portion of the protein, carbohydrate or lipid Other nomenclature Adjuvants Adjuvants enhance the immune response to an antigen, making it more immunogenic Frequently used in vaccination They increase the: persistence (‘depot’) effective size (for uptake by APCs) activation of dendritic cells, macrophages → inflammatory cytokine production Some examples include: Complete Freund’s adjuvant (oil, water, dead:lysed mycobacteria) Alum (aluminium potassium sulphate) Introduction to Antigen Receptors Different types of antigen receptors Innate (non-specific): Pattern Recognition Receptors Adaptive (specific): BCR – B cell receptor (Immunoglobulin/antibody) TCR – T cell receptor In between: NK cells killer activatory immunoreceptors (KARs such as NKG2D) Antigen recognition by lymphocytes B cells bind ‘native’ antigen in solution or suspension & recognise 3D conformation of the antigen via the B cell Receptor (BCR) (The BCR is sometimes called surface immunoglobulin or sIg) Fig. 2-7 Antigen recognition by lymphocytes T cells bind linear arrays of approximately 9 amino acids via the TCR i.e. NOT native Ag; it is broken down to 1o structure (PROCESSED) and PRESENTED to T cell by molecules on surface of Ag presenting cells (e.g. macrophages & dendritic cells) Fig. 2-7 Summary Antigens Antigens are defined by their ability to interact with antigen receptors Usually non-self (in healthy people) Many antigens are proteins Can also be carbohydrate, lipids, nucleic acid etc etc Immunogens are antigens that elicit an immune response Antigen receptors Antigens receptors recognise antigen Differences between the mechanisms for BCR (native antigen) and TCR (presented antigen) Lecture A, Part 2 Immunoglobulins: The B cell receptor (BCR) and Antibodies + Affinity vs Avidity Learning profile items: Describe and draw the typical structure of an antibody molecule Name the 5 classes of antibody, and compare their structures, abundance and main functions Explain the difference between affinity and avidity Illustrate the structure of the B cell receptor (BCR) complex Refresher Antigen receptors B cells - B cell receptor (BCR) T cells - T cell receptor (TCR) Paratopes (antigen binding sites) Epitopes (antigenic determinants) Antigens Immunogens Refresher Antigen receptors B cells -B cell receptor (BCR) T cells - T cell receptor (TCR) Paratopes (antigen binding sites) Epitopes (antigenic determinants) Antigens Immunogens Refresher B cells bind ‘native’ antigen in solution or suspension & recognise 3D conformation of the antigen via the B cell Receptor (Sometimes called surface immunoglobulin or sIg) Fig. 2-7 The B Cell Receptor Each B lymphocyte has multiple copies of an immunoglobulin with single antigen specificity This is due to the ‘randomly’ generated antibody variable regions that occur genetically during B cell 4 E M development E TH However, the BCRs on all of an individual’s B cells combined are capable of recognizing millions of different antigenic determinants How do vaccines work? Immunoglobulins (Antibodies and the BCR) Immunoglobulins Immunoglobulins are proteins that recognize and bind to a particular antigen with very high specificity They come in two forms Bound to the surface of B cells: The B cell Receptor Made in response to exposure to the antigen : Antibodies Both molecules have the same structure but are either bound to the membrane or secreted by fully activated B cells (plasma cells) The structure of the Ig or antibody molecule allows it to both recognise and bind to antigen as well as carry out different effector mechanisms Antibodies vs the B Cell Receptor Fig. 3-9 Apart from the presence of a transmembrane domain, the structure of the BCR and an antibody is identical – The BCR and antibody made by the same cell will recognize and bind to ONE particular antigen with very high specificity – They will also be of the same class of immunoglobulin (more of this later) Immunoglobulin structure Monomer: A flexible Y-shaped molecule with four protein chains: 2 identical light chains 2 identical heavy chains The heavy and light chains are joined by disulphide bonds Different regions of antibodies have different names: The ‘Fab’ contains the variable antigen binding site The rest of the molecule has a relatively constant structure, and is referred to as ‘Fc’ Fig. 2 p81 Immunoglobulin structure Fig. 3-10 The amino-terminal of each chain is characterised by sequence variability (V) in both the heavy and light chains (VH and VL regions) which form the antigen binding site Note that the antigen binding site consists of 2 variable regions – one from the heavy chain and one from the light chain Immunoglobulin structure Fig. 3-10 m g a IgM IgG IgA d e IgD IgE The constant portion of the light chain is termed the CL region Can be κ or λ chains The constant portion of the heavy chain is further divided into structurally discrete regions (immunoglobulin-like domains): CH1, CH2, CH3 and stabilised by intrachain disulfide bonds Can be µ, γ, α, δ, or ε chains depending on the class The chains present are determined genetically by ‘class-switching’ T 4 E M E H Immunoglobulin structure Fig. 3-10 m g a IgM IgG IgA d e IgD IgE The constant portion of the light chain is termed the CL region Can be κ or λ chains The constant portion of the heavy chain is further divided into structurally discrete regions (immunoglobulin-like domains): CH1, CH2, CH3 and stabilised by intrachain disulfide bonds Can be µ, γ, α, δ, or ε chains depending on the class The chains present are determined genetically by ‘class-switching’ T 4 E M E H Variable regions – antigen binding Variable regions are made of immunoglobulin-like domains that contain 3 hypervariable loops between the strands of the β-pleated sheets Called CDRs (complementaritydetermining regions), which vary in length and amino acid composition in the BCRs found on different B cells This variability is what gives the immune system diversity, and it’s ability to bind antigens that have a wide range of different shapes Constant regions – Effector functions Many of the effector functions of secreted immunoglobulins (antibodies) are endowed by the constant domains These are mediated through the binding to cellular Fc receptors and complement activation 4 E M E TH Classes of Immunoglobulin Classes of Immunglobulins Immunoglobulins come in 5 types “classes”: – IgM – Found on the surface of naïve B cells – IgA, IgE and IgG – Found on the surface of B cells that have been activated and have undergone “class-switching” – IgD – Found on all B cells (function unknown) Because the 5 classes are determined by the genome of the B cell, the B cell receptor and the antibody made by a certain B cell clone will be the SAME Therefore the body can make 5 different classes of antibody: Different antibody types are induced by different types of antigen, the entry route and antibody function needed to mould the immune response to the type of invader encountered Figure 16.7 Classes of Antibodies Monomers Dimer Pentamer Fig. 3-12 16.7 Note that when in the BCR (membrane bound) form, all classesFigure are monomeric Characteristics of the five classes of antibodies THEME 4 Table 16.1 The B Cell Receptor Complex B cell antigen receptor (BCR) complex Comprises membrane-bound Ig (mIg) AND 2 Igα and 2 Igβ chains (CD79 α and CD79β) The mIg itself has very little cytoplasmic domain and cannot signal Igα and Igβ have signalling motifs in their cytoplasmic chains (Immunoreceptor Tyrosine kinase Activatory Motif: ITAM) The majority of resting or naïve B cells express membrane mIgM (and mIgD) Fig. 3-14 44 B cell antigen receptor (BCR) complex The ITAMs of Igα and Igβ transmit the signal if the mIgM binds antigen, leading to development of either plasma or memory cells Once developed in to plasma cells, they (probably) no longer express the BCR on their surface, instead they secrete antibodies (same variable regions and Ig subclass as the surface BCR) Memory B cells continue to express the BCR (but as IgG, IgA or IgE) Fig. 3-14 45 Epitopes, affinity and avidity Epitopes Antibodies recognise epitopes on antigen “The parts of antigen that contact the antigen binding sites of an antibody or T cell receptor” So, a discreet portion of the antigen (epitope in red) Immunoglobulin-antigen interactions - definitions The molecular ‘shape’ of the epitope and of the variable regions of the antibody (paratope) must ‘fit’ The epitope can be linear or discontinuous Immunoglobulin-antigen interactions - definitions Each antigen binding site is made up of the Complementarity Determining Regions (CDRs) of two different Immunoglobulin folds These are at the furthest extreme of the heavy (VH) and light chains (VL) These are so close together in the 3D structure of the antibody that they make one continuous paratope Immunoglobulin-antigen interactions - definitions Antibodies bind to antigens based on the ‘complementarity’ of the epitope on the antigen and the antigen-binding site of the antibody Complementarity can be thought of as ‘goodness of fit’ The better the fit, the stronger the binding. The strength of the reaction is referred to as the affinity of the antibody Generally speaking, a high affinity antibody is more protective than a low affinity antibody because it will bind antigens at lower concentrations Antibody-antigen interactions - definitions The Antibody ‘avidity’ is the total strength of binding of the Fab regions of the population of antibodies raised against an antigen, and involves the reaction with all antigenic determinants Thus it is the total strength of the binding of antibodies to antigens Affinity One antigen binding site One epitope Avidity Multiple antigen binding sites Repeated epitopes Affinity vs. Avidity IgM antibodies are often low affinity (1° immune response) However, due to the 10 variable (Fab) regions, the have high avidity, and are still very effective against whole pathogens and are very effective activators of complement Summary Antibodies/BCR Antibodies are the secreted form of the B cell receptor Each B cell makes an antibody with a different variable region and therefore can bind different antigens Each B cell can only make an antibody of one particular subclass There are 5 sub-classes of antibody. IgM is made in the primary immune response, other subclasses IgA, IgE and IgG depend on the type of immune response Lecture A, Part 3 Immunoglobulin folds The T cell Receptor (TCR) Non-classical T cell activation Learning profile items: Illustrate the structure of the T cell receptor (TCR) complex Describe the differences between T and B cell antigens Define superantigens Refresher Antigen receptors B cells - B cell receptor (BCR) T cells - T cell receptor (TCR) Paratopes (antigen binding sites) Epitopes (antigenic determinants) Antigens Immunogens Refresher Antigen receptors B cells - B cell receptor (BCR) T cells -T cell receptor (TCR) Paratopes (antigen binding sites) Epitopes (antigenic determinants) Antigens Immunogens Antigen recognition by lymphocytes T cells bind linear arrays of approximately 9 amino acids via the TCR i.e. NOT native Ag; it is broken down to 1o structure (PROCESSED) and PRESENTED to T cell by molecules on surface of Ag presenting cells (e.g. macrophages & dendritic cells) Fig. 2-7 Antigen receptors B cells T cells T cell receptor Paratopes (antigen binding sites) Epitopes Mechanisms of antigen presentation MHC class I MHC class II Antigen presenting molecules T cell receptor complex The T cell Receptor (TCR) Similarities between TCRs and BCRs TCRs are structurally very similar to Fab fragments of B cell receptors Both receptor types are composed of two different peptide chains and have variable regions for binding antigen, constant regions, and hinge regions Structure of the T cell receptor Very similar to Fab fragments of B cell receptors Two types of TCR – αβ and γδ - with distinct functions Variable Region: Made up of the V domains (Vα/Vβ or Vγ/Vδ). Antigen binding Constant Region: Made up of the C domains (Cα/Cβ or Cγ/Cδ) TCRs remain membrane-bound and contain only a single antigen-binding site. Fig. 3-15 The T cell receptor complex – CD3 Like the BCR, the TCR cannot signal – this is provided by CD3 CD3 consists of 6 polypeptides; ζζ, γε and εδ dimers ζ chains have 3 ITAMs γ, ε and δ have extracellular Ig folds and one ITAM apiece ITAMS TCRs also use “coreceptors” (CD4 or CD8) to carry out their function Fig. 3-17 The T cell receptor complex The TCR/CD3 complex forms a compact structure in the T cell surface TCR and CD3 are held together by electrostatic interactions Each T cell has multiple copies of a single variant of the TCR, randomly generated in the thymus Features of antigens recognised by T cells Most T cells recognise only peptides T cells are specific for amino acid sequences of peptides T cells recognise and respond to foreign peptide antigens only when the antigens are presented by Major Histocompatibility Complex Molecules (MHC) CD8 T cells recognise MHC class I molecules and CD4 T cells class II. Whilst the TCR recognises and binds the peptide/MHC molecule complex, the CD4 and CD8 molecules also interact with and bind the MHC molecule ensuring stability of the ‘immunological synapse’ Antigen recognition by the TCR Paratope Epitope = parts of peptide and parts of MHC A defining feature of antigen recognition by the TCR is that the antigen must be presented in the MHC The epitope recognised includes some amino acids from the presented peptide, and also parts of the MHC T cell activation Fig. 3-17 First, the variable regions of the TCR αβ chains interact with the peptide in the MHC. The epitope of the TCR must ‘fit’ the combined paratope of the peptide/MHC together After binding of MHC-peptide on the APC with the TCR, either CD4 or CD8 can join the complex Cellular kinases and the ITAMs of CD3 initiate T cell activation Non-classical T cell activation Superantigens Fig. 10-7 Some bacteria and viruses make superantigens These are proteins that bypass normal antigen recognition and bind: – non-variable sequences of TCR variable regions – non-polymorphic sequences of MHCII α-chain They can therefore activated ~ 5-20% of T-helper cell population They are effective at very low concentrations ~ 10-9M Superantigens are not processed by antigen presenting cells and bind in their native state 68 ab vs. gd T cells 95-99% of circulating T cells are αβ T cells 1-5% are γδ T cells – instead, these are found mostly in epithelial rich tissues Major sites are: – FETUS (precede αβ T cells) – MUCOSA (up to 50% of the T cell population in these sites) Fig. 8-4 69 Antigen recognition by gd T cells The structure of the TCR is the same, but uses different proteins: g and d instead of a and b The g and d TCR genes are less diverse than those of a and b They still recognise presented antigen, but it is not normally processed peptides Antigen recognition by gd T cells  gd T cells recognise antigen differently to αβ T cells  gd T cells recognise antigen presented in CD1 rather than MHC Function still not clear, but they can recognise lipids, pathogens (such as M. tuberculosis) and pathogenic toxins, cellular stress markers Fig. 7-19 71 Summary Antigen recognition by the TCR Each T cell makes a TCR with a different variable region and therefore can bind different antigens The TCR complex also includes CD4 or CD8 The αβ TCR on T cells can ONLY recognise antigen when presented by the MHC This means that T cell activation requires accessory cells Superantigens made by pathogens can activate T cell independently of antigen specificity The gd TCR recognises antigens (often lipids) presented in CD1 Polyclonal responses Pathogens and antigens When a pathogen invades, it will be carrying many different antigens – proteins, carbohydrates, lipids, metabolites Each of these will have several epitopes, so in an infection, you get a polyclonal response – i.e. many clones of B and T cells will expand as they recognise the different antigens and/or different epitopes within them Pathogens and antigens Can you spot many antigens on this virus and bacterium? Which ones could be processed, presented and recognised by the TCR? Which could be recognised by the BCR in their native form? Polyclonal responses The activation of different T and/or B cell clones in the adaptive immune response is the root of the term “polyclonal”, especially with regard to antibodies Monoclonal Polyclonal Antibodies from a single B cell clone Antibodies from multiple B cell clones All the antibodies recognise a single (the same) epitope The mixture an antibodies can recognise range of epitopes Typically only produced in a lab when Typical in the natural response to an producing commercial antibodies infection Summary The structure of the BCR and TCR have similarities: Both involve repeating units of structural domains that are known as immunoglobulin folds Both have constant and variable regions, with the antigen recognition residing in the variable regions. Neither are able to signal by themselves The structure of the BCR and TCR have differences: The number of both constant and variable regions differ The number of antigen binding sites can differ TCR is always cell-associated, whereas the BCR can be soluble (antibody) The accessory proteins required for signalling are different Complex antigens (such as pathogens) have many epitopes and therefore give a polyclonal response Thank you for your attention Next Lecture: Antigen Presentation MHC I & II Genetics of antigen recognition