Antibody, Lymphocytes & Diversity (King's College London) PDF

Lymphocyte receptors, B cells and antibodies Jo Spencer Department of Immunobiology Faculty of Life Sciences & Medicine Immunology Roadmap Innate immunity Adaptive immunity Microbes B lymphocytes Antibodies...

Lymphocyte receptors, B cells and antibodies Jo Spencer Department of Immunobiology Faculty of Life Sciences & Medicine Immunology Roadmap Innate immunity Adaptive immunity Microbes B lymphocytes Antibodies Epithelial barriers T lymphocytes Phagocytes C3a C3b Effector T cells NK cells Complement  Topics Adaptive immunity Diversity of lymphocyte receptors B cell receptor and antibody Functions of different antibody isotypes Refinement of antibody specificity after meeting antigen Antibodies; friend and foe Adaptive immunity Not as fast as innate immunity, but more specific and with memory! Innate Immunity works to a point………. …but sometimes you need something better Adaptive Immunity Without adaptive immunity, we die ‘ The ‘Boy in the Bubble’ David Phillip Vetter was born in 1971 with no lymphocytes Severe Combined Immune Deficiency or SCID. Died 1984 of an infection following a bone marrow transplant to try to generate lymphocytes. Adaptive immunity requires lymphocytes T cell B cell T cell receptor - heterodimer one binding site for antigen Alpha chain Beta chain T cell B cell receptor is an antibody Two antigen binding sites for antigen Two identical Two identical heavy chains light chains B cell Adaptive Immunity – depends on cells that are individual and recognise their own unique antigenic shapes Constant migration of lymphocytes with diverse receptors through blood and lymphoid tissues increases the probability that they will encounter specific antigens  Memory Memory Memory Memory Memory Memory Memory Memory  Memory Same principle for B cells and T cells higher frequency of specific cells respond more rapidly Only in adaptive immunity Memory Memory Memory Memory Memory Memory Memory Memory Innate Immunity Adaptive Immunity Minutes to hours First exposure (primary response) 12 days Booster (secondary response) 5-7 days Diversity of lymphocyte receptors  Variable Constant T cell B cell Total number of human genes in the genome = approximately 25,000 But the microbe diversity exceeds millions and they can mutate and adapt. Gene rearrangement Gene rearrangement The variable region of the heavy chain is composed of three parts Variable, Diversity and Joining Variable magnified Diversity Joining  Antibody heavy chain Variable Diversity Joining Human DNA encodes approximately 48 different options for V 27 different options for D 6 different options for J There are 48 x 27 x 6 =7776 different combinations of V, D and J for immunoglobulin heavy chain variable region  Variable regions are made up from Antibody light chain Variable and Joining segments Variable Joining Using the same rearrangement process of variants of V and J there are 340 alternatives for light chains. From previous slide: there are 48 x 27 x 6 =7776 different combinations of V, D and J for antibody heavy chain variable region So, for one heavy and light chain together there would be 7776 x 340 = 2,643,840 variants = Combinatorial diversity In addition… Heavy Variable Diversity Joining …joining is not precise. Nucleotides are added and removed from junctions during rearrangement. This adds further diversity termed junctional diversity Light Variable Joining Gene rearrangement creates the unique specificity of B and T cell receptors B cell development with gene rearrangement happens in bone marrow T cell development with gene rearrangement happens in thymus B cell receptor and antibody Binding or ‘ligating’ the T cell or B cells receptors have very different consequences T cell B cell  Differentiate to None of these pathways secrete cytotoxic involve the T cell receptor granules beyond recognition of Activated antigen Cytotoxic T cell Differentiate to produce Activated different sets of cytokines Th1 Th2 Th17 Treg Helper T cell Activated B cell Plasma cell Antibody = Immunoglobulin Antibody = Immunoglobulin Two identical Variable regions heavy chains V determine specificity D Kappa Two identical light V J or chains J Lambda Constant regions Constant region of the heavy chain … and therefore determines function determines if the antibody is IgM, IgG, IgA, IgE or IgD… Some terminology IgM, IgD, IgG, IgA and IgE are refered to as classes or isotypes defined by the constant regions of their heavy chains. IgG1, IgG2, IgG3 and IgG4 and IgA1 and IgA2 are subclasses. The subclasses are coded by different constant region gene segments. Some terminology IgM, IgD, IgG, IgA and IgE are refered to as classes or isotypes defined by the constant regions of their heavy chains. IgG1, IgG2, IgG3 and IgG4 and IgA1 and IgA2 are subclasses. The subclasses are coded by different constant region gene segments. Some terminology IgM, IgD, IgG, IgA and IgE are refered to as classes or isotypes defined by the constant regions of their heavy chains. IgG1, IgG2, IgG3 and IgG4 and IgA1 and IgA2 are subclasses. Fc = the Some cells such paired The subclasses are coded by as macrophages have Fc constant different constant region gene receptors that can region segments. bind antibodies segments General functions of Antibodies Neutralise toxins and viruses by binding to them and blocking their interaction with other cells Opsonise pathogens by binding to them to promote phagocytosis and killing activity by other cells by recognition of Fc receptors Activate the complement cascade which helps kill pathogens Agglutinates particles (pathogen debris, viruses etc) Functions of different antibody isotypes Main serum antibody IgG Measurement of antibody titre in serum in response to vaccine is generally measurement of specific IgG  Good at opsonisation – coating pathogens so IgG that phagocytic cells can recognise them. Pathogens coated in IgG also become targets for killing by Natural killer cells – antibody- dependent cellular cytotoxicity There are 4 subclasses of IgG (IgG1,IgG2,IgG3,IgG4 named by increasing concentration in serum ) IgA monomer IgA IgA dimer= 2 monomers joined by a joining ‘J’ chain  Dimeric IgA Retention of antigen by adherence to mucus due to Agglutination, neutralisation charge CHO rich SC  Pentameric IgM First antibody made in immune responses. Pentameric Pentameric IgM Pentamer has 10 possible binding sites – high Avidity AGGLUTINATION  IgE aTISHoo MAST CELL has receptors for IgE Individuals with allergies have higher concentrations of IgE in serum IgM and IgD with the same specificity are expressed on the surface of newly formed B cells IgM IgD Very little IgD in serum B cell Refinement of antibody specificity after meeting antigen In B cells ONLY B cells can improve their specificity for antigen by affinity maturation. Lymphocytes constantly recirculate through blood and lymphoid tissues. Secondary lymphoid tissues contain zones of dividing B cells called germinal centres (dotted circle below). Brown cells are B cells migrating through. M+D+ up to approximately 1mm  If B cells encounter specific antigen and they have help from T cells they can enter germinal centres of dividing B cells. Blood up to approximately 1mm B cells in the germinal centre start to divide very rapidly As B cell divide they start to MUTATE their immunoglobulin variable region genes by somatic hypermutation. High affinity variants are SELECTED in the germinal centre Somatic hypermutation then selection = AFFINITY MATURATION. B cells can also class switch in germinal centres, so they express isotypes other than IgM and IgD. A switch is represented as a change to a pink constant region in the animation below Two populations of cells leave the germinal centre Memory Quiescent circulating Plasma cells locate to Bone memory B cells Marrow or the intestine. Plasma cells can be very long lived. Affinity matured Bone marrow plasma cells can Possibly class secrete protective antibodies for switched a lifetime Antibodies; friend and foe See also – ‘When the immune system goes wrong’ Monoclonal antibodies Monoclonal antibodies are powerful laboratory reagents and also biological therapeutics used to treat diseases from autoimmune diseases such as rheumatoid arthritis, to inflammatory diseases such as Crohn’s disease, to cancer. Monoclonal antibodies Normal B cell populations in blood are polyclonal – even when some cells have formed memory clones. Also individual cells comprising the memory clones will most likely be different due to somatic hypermutation. Monoclonal antibodies Monoclonal antibodies IgG First monoclonal antibodies were made using mouse B cells and mouse monoclonal antibodies are powerful reagents for research and diagnosis. However, mouse monoclonal antibodies to not make good therapeutics because the mouse protein can been seen as foreign antigen and immune responses against the antibody generated Monoclonal antibodies A chimeric antibody is one where the mouse IgG monoclonal is engineered so that the mouse variable region of heavy and light chains are Mouse associated with human constant regions Chimeric antibodies have the ending –ximab An example is Rituximab that is an antibody to CD20. CD20 is expressed by B cells. Rituximab targets B cells for killing and removes them from Human circulation and lymphoid tissues. Rituximab is effective against B cell lymphomas but also some autoimmune diseases including rheumatoid arthritis. Monoclonal antibodies IgG A monoclonal antibody can be ‘humanised’ so that only the parts of the variable region that contact antigen ( )are from the mouse. Humanised antibodies have the ending –zumab An example is antibody omalizumab Omalizumab is specific for IgE and can be used to treat moderate to severe allergic asthma. Monoclonal antibodies IgG Monoclonal antibodies can be fully human Human monoclonal antibodies have the ending –umab An example is antibody adalimumab adalimumab is specific for TNFa and is used to treat inflammatory diseases. Antibodies to red blood cell antigens ABO- function unknown Rh system: NH4 transporter? CO2 NH2 diffusion? Kell : endothelin- NH2 converting enzyme Cromer: Complement inhibition ABO Universal donor A B AB Universal recipient  Cross placental transfer of IgG Rhesus targeting rhesus antigen Mother who is Rh- has from mother to fetus causes IgG antibodies to red blood cell lysis in the Rh following exposure to fetus and haemolytic disease Rh antigen from a first of the newborn. pregnancy Plasmapheresis Summary IgM IgD B cell Plasma cell Antibody