Lecture 6: Antigen Recognition by Lymphocytes PDF

Lecture 6 Antigen Recognition by Lymphocytes 09/24/2024 Learning objectives Antigen recognition by B cells Structural The interaction of The structure of a Antigen...

Lecture 6 Antigen Recognition by Lymphocytes 09/24/2024 Learning objectives Antigen recognition by B cells Structural The interaction of The structure of a Antigen variation in the antibody typical antibody recognition by T immunoglobulin molecule with molecule cells constant regions specific antigen How does adaptive immunity differ from innate immunity? Innate Immunity Adaptive Immunity Complement proteins – promote pathogen Based on specificity and diversity recognition & destruction Innate immune cells bind to complement attached to pathogen surface recognize PAMPs via PRRs Limit an infection Lack of specific Recognize very specific foreign molecules receptors Not capable of recognizing diverse types of pathogens Limitless in the diversity of molecules recognized Not sufficient to halt an infection Combat new and previously encountered pathogens  The cells of the adaptive immune system Develop in bone marrow Lymphocytes Develop and mature in bone marrow Mature in thymus T Cells B Cells +ag CD4 +ag Helps cytotoxic T cells and B cells in their immune functions Helper T cells Plasma cells Produce antibodies Kills virus-infected and damaged cells Humoral Immunity Cytotoxic T cells CD8 Cell-mediated Immunity Antigen (ag): any molecule or part of a molecule that is specifically recognized by BCRs/TCRs Antigen and Epitope An antigen is a macromolecule (protein, polysaccharide, glycoproteins) that is recognized by the body as foreign. Stimulates an immune response Can be foreign or self Foreign antigen – can be microbial (e.g., cell wall, nucleic acid, bacterial toxins) or non-microbial (pollen, egg white, transplanted tissue, transfused blood) Self antigen – own cells and molecules An epitope (aka antigenic determinants) is the region or sites of antigen that are recognized by the immune system They bind to specific Igs or TCRs B-cell receptor (BCR) and Immunoglobulins (aka Antibodies) Highly specialized recognition proteins of B cells Plasma cells are specialized Known as immunoglobulins (Igs) effector B cells that produce produced by B cells in a vast range of antigen soluble antibodies (Igs) specificities each B cell produces Igs of a single-specificity Membrane-bound form on the B-cell surface – BCR – functions as the cell’s receptor for antigens Secreted form – antibody – produced by terminally differentiated B cells o binds pathogens or their toxic products in the extracellular spaces – main effector function of B cells in adaptive immunity o Recruits other cells and molecules to destroy the pathogen it has bound Effector functions  1. The structure of a typical antibody Structure of an antibody (or Ig) molecule molecule NH2 terminal Roughly Y-shaped Two hinged heavy or H chains (green) - 50kDa Two light or L chains (yellow) - 25 kDa Joined by disulphide bonds – each heavy chain is λ or κ linked to a light chain and the two heavy chains are linked together Two identical H and L chains – two identical antigen- binding sites μ, γ, α, δ or ε Avidity: the total strength of the interaction between an antibody and antigen COOH terminal Affinity: the strength of the interaction between A typical IgG molecule Fig. 4.2 a single antigen-binding site and antigen Ig heavy and light chains are composed of constant and variable regions Antigen-binding 1. Each H and L chain consists of a series of domains – sites Ig domains Each domain is about 110 aa in length L chain has 2 Ig domains H chain has 4 or 5 Ig domains (depending on the class of antibody) 2. The amino (N)-terminus has variable Ig domains (V domains) of the heavy (VH) and light (VL) chains effector molecules Interacts with makeup the variable region of the antibody and cells Determine the antigen-binding specificity 3. The constant Ig domains (C domains) of the heavy (CH) and light (CL) chains Makes the constant regions of the antibody Distinguishes diff. classes – IgM, IgG, IgD, IgA, IgE Fig. 4.1 The antibody molecule can be readily cleaved into functionally distinct fragments Hinge region Lies within the C region VH VH Allows flexibility in binding to VL VL hinge multiple antigens CH 1 CH 1 CL CL Differs between isotypes CH 2 CH 2 VL Fragment antigen binding (Fab) CH 3 CH 3 Antigen-binding activity When fully assembled, an Fragment crystallizable (Fc) antibody molecule comprises Does not interact with the antigen three equal-sized globular portions Biological activity The two arms are joined to the Differs between H chain isotypes trunk by a polypeptide chain – the hinge region Fig. 4.4 Fig. 4.2 Spot the difference! No hinge region Additional domain The Ig (antibodies) isotypes 2. Structural variation in immunoglobulin constant regions The class or isotype of an antibody is determined by the structure of its CH regions Different isotypes have different effector functions Five main heavy-chain isotypes – IgM (μ), IgG (γ), IgA (α), IgD (δ) and IgE (ε) Distinct characteristics in the C region: 1) the number and location of disulphide bonds, 2) the number of attached carbohydrate groups, 3) the number of C domains, and 4) the length of the hinge regions Fig. 4.6 The Ig (antibodies) isotypes The 5 main classes of Ig occur as transmembrane antigen receptors (___) or secreted ____ IgG – 4 subclasses: IgG1 > IgG2 > IgG3 > IgG4 IgA – 2 subclasses: IgA1 & IgA2 IgG is the most abundant Ig in serum IgM is the first Ig produced after B-cell activation o IgM is secreted as a pentamer – normally present in the bloodstream but not in tissues o Pentamer increases the avidity of IgM for antigens  The physical and functional properties of the human Ig isotypes IgM and most IgG isotypes interact with C1. IgA and IgE do not activate complement IgA acts at mucosal surfaces, secreted into the gut and respiratory tract, and breast milk IgE in parasite immunity as well as allergic reactions IgD: role unknown. Present on the surface of MATURE B cell – marker! Fig. 4.7 The IgM and IgA molecules can form polymers by interacting with the J chain J chain – polypeptide chain IgM molecules found as pentamers in plasma. 10 antigen-binding site IgA molecules found as dimers in mucous secretions but as monomers in plasma Fig. 4.8 IgM antibodies frequently recognize repetitive structures such as bacterial cell wall polysaccharides, but individual binding sites are often of low affinity because IgM is made early in immune responses, before somatic hypermutation and affinity maturation. Multisite binding makes up for this, marked Lee improving the overall functional binding strength Localized regions of hypervariable 3. The interaction of the antibody molecule with sequence form the antigen-binding site specific antigen Recap: Constant region – aa sequence in the C-terminal regions of H and L chains is the same Variable regions – aa sequence in the N-terminal regions of H and L chains is different. This region provides antibodies with unique specificity CDRs Hypervariable regions – regions within the variable regions of both H and L chains (greater specificities) These regions of the variable domains actually contact the antigen They, therefore, makeup the antigen-binding site These regions are also called the complementarity- determining regions (CDRs) The hypervariable regions Fig. 4.9 HV: Hypervariable regions – three: HV1, HV2, HV3 The most variable part of the domain is in the HV3 FR: Framework regions – regions between HV regions. Four FRs in each V domain: FR1, FR2, FR3, FR4. Provide structural framework of the Ig domain Antibodies and Combinatorial Diversity The immune system generates antibodies of different specificities. This is achieved by creating various combinations of H-chain and L-chain V regions. This process is known as combinatorial diversity. It involves the formation of VH and VL region genes from smaller DNA segments. This occurs during the development of B cells in the bone marrow. More next week! (Chapter 5) Recap on BCR/Igs Antibodies are comprised of repeating 110 aa units referred to as domains or Ig folds The C-terminal domains are constant from antibody to antibody (within a class) The constant region domains are responsible for all functions of an antibody other than antigen binding – biological function The N-terminal domains are variable from antibody to antibody are referred to as variable domains The variable domains contain three hypervariable regions – the CDRs The CDRs of the variable domain in both H and L chains make up the antigen-binding site  4. Antigen T-Cell-Receptor (TCR) recognition by T cells Transmembrane protein with an almost entirely extracellular structure Consists of two protein chains α chain and β chain (most typical) some TCRs have γ and δ chains (γδT cells) Each chain has a C and V region. The V region of α and β chains make the antigen-binding site Differences in V region allow TCR recognize different antigens (diversity) Each TCR can recognize one specific antigen (specificity) Each T cell expresses one type of TCR – each T cell offers protection against only one pathogen Fig. 4.18 TCRs and MHC/HLA molecules Most TCRs can only recognize small peptides These peptides MUST be presented via cell-surface protein receptors of the major histocompatibility complex (MHC) family in mice, or the human leukocyte antigen (HLA) family in humans A major function of TCR is to recognize a specific MHC-peptide complex APC Peptide is processed through proteolysis in the cytosol or lysosomal digestion and presented on the surface of the cell Two classes of MHC molecules MHC class I MHC class II differ in structure and expression pattern Fig. 4.21 and 4.22 MHC Class I o Two subunits 1. α chain – anchors MHC to plasma membrane Three subunits – α1, α2, α3 α1 and α2 form the peptide-binding groove 2. β2 macroglobulin soluble Provides structural support (along with α3) o Binds small peptides of 8-10 aa in length. Peptides are generated from intracellular proteins MHC Class II o Two subunits 1. α chain – anchors MHC to plasma membrane Two subunits – α1, α2 2. β chain Two subunits – β1, β2 o Both the chains are transmembrane o α1 and β1 form the peptide-binding groove o Binds larger peptides of 13-25 aa in length. Peptides are generated from extracellular proteins Co-receptors and MHC molecules The MHC molecule presenting the antigen binds to both the TCR and a co-receptor expressed on the T-cell surface T cells have two co-receptors: 1. CD4 – binds to MHC-II targets extracellular pathogens 2. CD8 – binds to MHC-I – targets intracellular pathogens Fig. 4.30 Fig. 4.31  T-cell receptor complex TCR MHC Co-receptor Recognition CD3 complex – 6 subunits TCR is not expressed without CD3. It is required to bring TCR to surface transduction CD3 complex recruits signaling CD3g Signal molecules that are activated upon TCR engagement – drives T cell activation Recap on TCRs The receptor for antigen on most T cells is the αβ T-cell receptor. It consists of two protein chains: TCRα and TCRβ. αβ T-cell receptors are always membrane-bound. They recognize a composite ligand formed by a peptide antigen bound to an MHC molecule. The peptide antigen is generated intracellularly and bound stably in the MHC's peptide-binding cleft. There are two classes of MHC molecules: MHC Class I: Interacts with CD8 molecules MHC Class II: Interacts with CD4 molecules T-cell receptors have dual specificity: They interact with both the antigenic peptide and the polymorphic features of the MHC molecule. This dual specificity is the basis for MHC restriction of T-cell responses. Checkpoint questions for Lecture #6 1. Describe the structure and composition of an antibody and the functions associated with its primary components. 2. Define the Fab and Fc regions of an antibody? 3. What is the importance of an MHC molecule in the adaptive immune system? 4. What are the five major functions of soluble Igs? 5. What types of antigens do MHC class I and II typically present? 6. What subunits are required for a properly functioning TCR complex? 7. List the five major Ig isotypes. 8. Draw similarities and differences between TCRs and BCRs/Igs 9. Why is the CD3 complex required in a functional TCR complex? 10. Compare and contrast the structures of MHC I and II. Assigned Readings Chapter 4 4-1, 4-2, 4-4, 4-5, 4-6, 4-7, 4-8, 4-9, 4-14, 4-15, 4-16, 4-18, 4-21 InQuizitive 20240924 Readings Due Oct 1, noon