Lesson 3: Immunoglobulins: Structure and Function PDF

Lesson 3: Immunoglobulins: structure and function Generation of B cells repertoire  Immunoglobulins: molecular structure The “pattern” immunoglobulin is a...

Lesson 3: Immunoglobulins: structure and function Generation of B cells repertoire  Immunoglobulins: molecular structure The “pattern” immunoglobulin is a glycoprotein formed by 4 polipeptidic chains: two heavy (blue) and two light chains (green) A light chain and a heavy chain are linked by a disulfide bond. Heavy chains are linked together by one (or several) disulfide bond. Aproximate image of an IgG molecule structure.  A) Variable regions (Antigen binding) HVR (high variable regions) form the antigen binding surface. Among them there are framework regions with more conserved sequences. The antigen binding site is formed by three heavy chain loops and three light chain loops. HVR = CDR Immunoglobulins: diversity and specificity Specificity: each Ig binds selectively its specific antigen Diversity: there are around 109 different specificities B) Constant regions (with effector functions) ANTIBODY CLASSES (ISOTYPES) depend on C domains of Ig heavy chain HEAVY CHAINS: Gamma (g): IgG Delta (d): IgD Epsilon (e): IgE Alfa (a): IgA Mu (m): IgM LIGHT CHAINS: Kappa (k) Lambda (l) Both k or l in every Ig molecule IgE and IgM have an extra constant domain in their heavy chains (4). IgA and Ig M tend to form dimers (IgA) or pentamers (IgM), using the so called J (Joining) chain Immunoglobulins: effector functions Opsonization / phagocytosis Antibody-mediated cytotoxicity Complement activation Ig M 5-10% serum Igs (pentamers) Primary response Ig Great agregation capacity and complement activation, but it does not have direct opsonization capacity. It does not cross placenta Membrane IgM: BcR, monomer.  Ig G Four subclasses: IgG1-IgG4. More abundant serum isotype (80%) Main secondary response Ig Cross placenta. Good opsonization capacity and complement activation. Ig A Main mucosal Ig, always a dimer. Both IgA monomers are joined together by the so called J chain. In secretions, IgA is bound to another chain called “secretory component” which protects it from proteases of gut lumen In blood: monomer or dimer It is the secretions Ig: Salive, tears, nasal, bronquial, genitourinary, digestive fluids, breast milk. Ig E Very small amounts in serum, its concentration increases in parasite infestations and allergy. Special Ig in defense against parasites. It does not activates complement. Its Fc regions binds membrane receptors is basophils and masts cells (FcRI). Role in pathology: allergies. Ig D Biological function: BcR (together with IgM). The serum free form is very scarce and without known function. GAMDE ANTIGEN-ANTIBODY REACTION  ANTIGENS AND IMMUNOGENS Immunogenicity is a molecule´s capacity to induce a detectable immune response (to activate lymphocytes). These substances are called immunogens. Proteins and glycoproteins are the most potent ones. Antigenicity. A molecule´s capacity to specifically combine to antibodies or T cell receptors. In general most of them are immunogens. Antigen and immunogen: all immunogens are ALSO antigens, but not all antigens are able to induce an immune response. Haptens are antigens not immunogens by themselves but they become immunogens if they associate to another molecule called carrier. They normaly have low molecular weight and many of them are drugs or antibiotics. Epitope is the precise part of an antigen that combines with a specific antibody or T cell receptor. In the past they were called antigenic determinants.  Main features of antigen-antibody reactions 1. Ag/Ab binding is reversible 2. Antibodies can fix big macromolecules. 3. Antigen-antibody reactions are defined by affinity and avidity  Affinity: Strength of union between antibody and antigen It is defined by the disociation constant (Kd): Ag concentration needed to occupy the biding sites of half the antibodies in a solution.  AVIDITY GLOBAL STRENGTH OF INTERACTION BETWEEN TWO MOLECULES It depens on: Affinity Valency (number of binding sites) Relationship between antibody structure and function a) Regarding the antigen recognition: 1.- Specificity: Antibodies are extremely specific (they recognize very small differences) Exception: Cross-reactivity phenomenon. High affinity 2.- Diversity: each individual is able to synthesize 109 different antibody specificities = antibody repertoire 3.- Affinity maturation: During the humoral response the V regions slightly change its structure, due to a somatic mutation Low affinity process in B lymphocytes stimulated by the Ag. This generates Abs of higher affinity. b) Regarding the effector functions: 1.- Functions such as phagocytosis, opsonization, complement fixation, depend on Fc structure. 2.- Effector functions take place only when the Ab is bound to the During the immune response the Ag and do not occur with free Ab. antibodies affinity increases and 3.- The activation of a B lymphocyte by an antigen produces isotype switching: immunoglobulin class switching occurs ❖ Virgin or naïve B cells--->Membrane IgD and IgM ❖ B cell activated by an Ag-→IgG, IgE of the same specificity (V fragments-dependent) and higher capacity to eliminate antigen (C- fragments dependent) 4.- Fc dictates the tissular distribution of Abs. B cells and its antigen receptor.  1. B cell: Phenotype, function and differentiation ▪ Most B cells express membrane IgM and IgD: They are mature lymphocytes that do not have found yet their specific Ag. ▪ CD19, CD21, CD45 and CD40 are accessory BcR molecules that help in B cell activation. B lymphocytes express in its membrane a protein group called B cell antigen receptor BcR complex  Events after recognition of antigen by the B lymphocyte 1.1 ▪ Ag recognition begins an intracelular activation signal that includes: ❖ phosphorylation and dephosphorylation events ❖ membrane phospholipids hydrolysis and 2nd mesengers generation ❖ cytosolic Ca2+ increase ▪ The purpose of this activation is to modify the B cell gene expression profile: mitosis activation (proliferation), Igs synthesis and differentiation.  T-dependent and T-independent antigens Proteic antigens need always the T-B cooperation for the production of antibodies against them. – High affinity: affinity maturation – Immune memory – Isotype switching in secondary response Non-protein antigens, they do not need T-B cooperation – Low affinity (affinity maturation does not take place) – Almost never immunological memory – Almost never isotype switching: always IgM or limited to IgG2 at most 1.2 ▪ B cell recognizes Ag in native form, procceses and presents it to T cell, which cooperates (helps) in activating B cell and begining the humoral response. ▪B cell internalizes Ag and presents it to CD4+ T cell, using a HLA class II molecule, to achieve T cell collaboration. Consequences: ▪ Increasing proliferation and differentiation. ▪ Igs isotype switching ▪ Affinity maturation  Where do these events happen?  Genesis of Immunoglobulin repertoire Somatic recombination: Mechanism that generates diversity: from a relatively limited number of gene sequences, an almost infinite number of different proteins are produced. Together with the somatic recombination, another two mechanisms amplify Igs diversity: a) Junctional diversity: VDJ recombinations are unprecise, several nucleotides are included in each of them (P nucleotide). On the other hand, terminal deoxinucleotidil transferase (TdT) enzyme randomly adds nucleotides in the same regions (N nucleotides). Both mechanisms occur during the Ig recombination. b) Somatic hypermutation: new mutations are introduced after the antigen recognition. They affect the total V domain. They are responsable for affinity maturation.  The end https://www.youtube.com/watch?v=C_GRI3fxUWw https://www.youtube.com/watch?v=v9jl9QpVz10