Imunoglobulins – structure and function Production of immunoglobulins Genetic determination of immunoglobulin production Clonal selection theory Antigen and epitope Molecule of IgG Hypervaribale region of immunoglobulin molecule binds epitope of the antigen Variable region of immunoglobulin molecule Clonal selection theory F.M. Burnet, 1957 • During (mainly fetal) development immunocompetent cells of the immune system develop. Each sell is characterized by its own antigen specific receptor. Each cell reacts only with one concrete specific antigen. • After exposure to autoantigen during fetal life autoreactive clones are eliminated ( „forbidden clones“). • If a concrete cell recognizes its specific antigen, it is stimulated, proliferates and forms a clone = clonal selection. • After repeated divisions the cell becomes a terminally differentiated cell, that does not proliferate and after some time dies. • The cells of the clone that do not differentiate into the terminal stage become a memory cells which will quickly react after the second exposure to the antigen. Clonal selection theory Effector cells antigen Memory cells Elimination of autoreactive clones Blood and periperal lymphatic organs expansion death death VDJ Recombination Germline configuration D to J recombination V to DJ recombination transcription, splicing V segments D segments J segments Constant region exons AAA Issembly Adapted from Janeway 2001 Downloaded from: StudentConsult (on 18 July 2006 11:18 AM) © 2005 Elsevier VDJ genes for BCR, and TCR Somatic hypermutations • The process occurs in activated B-lymphocytes, takes place in germinal centers of secondary lymphoid organs. • Key enzyme is AID (activation-induced deaminase). • Mutation frequency is approx. 106 times higher than in other parts of human genome. • Antigen presentation by lymphoid dendritic cells to B-cells leads to selection of clones with higher affinity – the process is called affinity maturation. Isotype switching Isotype switching Downloaded from: StudentConsult (on 18 July 2006 11:29 AM) © 2005 Elsevier Activation and differentiation of B-lymphocytes (clonal selection theory in B-lymphocyte development) Primary phase of the antibody response • Naive or opsonised antigen captured by follicular dendritic cells. • Primary stimulation of B-cells in lymphoid folicles. • The antigen also stimulates T cells (after adequate presentation) in T-cell zones. T-cells migrate toward the lymphoid folicles. • Newly formed plasma cells produce mainly IgM(mainly in bone marrow). Secondary phase of the antibody response • Occurs in newly formed germinal centers of lymphoid folicles • Th lympocytes stimulate B-lymphocytes to somatic hypermutations and isotype switching. • This leads to selection of B- cells producing high-affinity antibodies (affinity maturation). • Majority of B-cells producing low-affinty antibodies die. Development of B-cells in the bone marrow • Stem cells: no B-cell surface markers, no rearrangement of Ig genes • Pro-B lymphocyte – rearrangement of heavy chain , expression of several B-cell surface markers (e.g. CD19). • Pre-B-lymphocytes VDJ has of heavy chain has been completed, m chain can be detected in cytoplasm. Pre-B receptor – composed of m chain and surrogate chains V-preB and l5 is expressed on the surface of the cell. Signal transduction though this receptor is essential for B- cell development. • Imature B-cell – light chain rearrangement (V-J) completed B-cell receptor is composed of monomeric IgM • Mature B-lymphocyte has IgM and IgD B-cell receptors Downloaded from: StudentConsult (on 18 July 2006 11:18 AM) © 2005 Elsevier Development of B-cells in the bone marrow B-cell receptorHeavy chain μ/δ Light chain κ/λ Igα Igβ Differentiation Proliferation Survival CD19 Pre B-cell receptor Bruton´s tyrosine kinase (BTK) • BTK binds fosfatidylinoisitoltriphosphate (3,4,5)-(PIP3). • Activates phospholipase C, leading to hydrolysis of phapsphatidylinositol (PIP2) into inositoltriphophate (IP3) and diacyglycerol(DAG). • This is key process in activation, differentian and development of B-cells. • Mutations of BTK lead to X-linked (Bruton´s) agammaglobulinemia. • BTK blockers (e.g. ibrutimib) are used for the treatment of B- cell malignancies. Downloaded from: StudentConsult (on 18 July 2006 11:29 AM) © 2005 Elsevier Isotype • The class or subclass of an immunoglobulin. • Antigenic determinats are on constant part of immunoglobulin molecule. Idiotype • An antigenic determinant on the variable region of a specific antibody. Interaction idiotype-antiidiotype antigen Antiidiotyp idiotyp Antibody Ab 1 Antibody Ab 2 Antibody Ab 3 Paratop Epitop www.immunology.klimov.tom.ru/1-1.php. Downloaded from: StudentConsult (on 18 July 2006 11:18 AM) © 2005 Elsevier Characteristics of immunoglobulin classes IgG Antibody response after primary and secondary antigen exposure Secondary antibody responsePrimary antibody response IgM IgG Firstexporsure Serumantibodytiters Weeks Secondexposuretoantigen Downloaded from: StudentConsult (on 18 July 2006 11:29 AM) © 2005 Elsevier IgM on B-cell membrane Expression of surface immunoglobulins on B-cells Downloaded from: StudentConsult (on 20 July 2006 11:29 AM) © 2005 Elsevier Formation of Secretory IgA • Affinity: The strength of the binding between a single site of an antibody (one variable region) and an epitope. • Avidity: The overall strength of interaction between and antibody and antigen. The avidity depends on affinity and the valency of interactions. Antibody affinity http://pathmicro.med.sc.edu/mayer/rx-2.jpg Affinity and avidity IgA IgG IgM Biological functions of immunoglobulin molecules • Activation of complement system (IgG, IgM) • Opsonization (particularly IgG) • Neutralization of antigens (IgG, IgA, IgM) • Adherence interference (IgA, IgG) • Antibody dependent cellular cytotoxicity (ADCC) • Agglutation, precipitation (IgG, IgM) • Mast cells degranulation (IgE) • Transport through placenta (IgG) • Imunoregulation (mainly IgG) Downloaded from: StudentConsult (on 20 July 2006 11:29 AM) © 2005 Elsevier Antibody dependent cellular cytotoxicity (ADCC) Fc receptor Virus-infected Cell NK Cell perforin granzyne Ig GFa b Fc Epitope