BMF_B cells_2023x PW.pptx

19th November 2023 Adaptive Immunity - B cells Dr Patrick Walsh Class Year 1 Module BMF Title Adaptive Immunity – B cells LECTURE LEARNING OUTCOMES By the end of this lecture you should be able to: Outline the key roles of B cells in the adaptive immune response Describe the stages of B cell d...

19th November 2023 Adaptive Immunity - B cells Dr Patrick Walsh Class Year 1 Module BMF Title Adaptive Immunity – B cells LECTURE LEARNING OUTCOMES By the end of this lecture you should be able to: Outline the key roles of B cells in the adaptive immune response Describe the stages of B cell development including B cell receptor rearrangement Define the steps in B cell activation Explain isotype class switching and affinity maturation process Compare and contrast the different classes of antibodies Describe the structure and effector functions of antibodies List diseases associated with B cell deficiencies T H E W O R L D T O B E T T E R H E A LT H • • • • RCSI LEADING • • • Dr Chiara De Santi, Adaptive Immunity – B cells RCSI LEADING T H E W O R L D T O B E T T E R H E A LT H KEY ROLES OF B CELLS • B cells are cells of the adaptive immune system (antigen-specific response) • They produce antibodies  crucial in the fight against pathogens (mainly extracellular pathogens) Dr Chiara De Santi, Adaptive Immunity – B cells Foreign antigen KEY ROLES OF B CELLS Y Chiara De Santi, Adaptive Immunity – B cells Y Y Plasma cell Plasma Cell Plasma Cell Y Y Y Y Y Y Y Y antibodies Y RCSI LEADING Y Y Dr B Cell Y Y YY • Each B cell has a unique B cell receptor (BCR) • They recognise antigen in any biological form (protein, carbohydrate, lipid, polysaccharide) • Antigen recognition causes B cell activation where they proliferate and differentiate into plasma cells • Each plasma cell produce trillions of antibodies (immunoglobulin = Ig) • Antibodies are secreted into circulation and mucus membranes – trigger effector responses Y T H E W O R L D T O B E T T E R H E A LT H BCR Pertinent Questions T H E W O R L D T O B E T T E R H E A LT H • Where do B cells come from? • What are the key stages of B cell development? • How do they obtain unique B cell receptors? RCSI LEADING • How do antibodies work and what do they do? Dr Chiara De Santi, Adaptive Immunity – B cells B CELL DEVELOPMENT RCSI LEADING T H E W O R L D T O B E T T E R H E A LT H Primary Lymph organs Dr Chiara De Santi, Adaptive Immunity – B cells Secondary Lymph organs MEMBRANE IGM RECEPTOR (BCR) RCSI LEADING T H E W O R L D T O B E T T E R H E A LT H • A membrane bound IgM is also called the BCR • It is composed of four polypeptide chains – two heavy (H) chains - two light (L) chains • They are assembled to form a ‘Y’ Ig domains structure • The heavy and light chains are attached via disulphide bonds • Each domain folds into a characteristic 3D shape called the ‘Ig domain’ • The V region is the variable ‘antigenbinding site’ • The C region is the constant region Dr Chiara De Santi, Adaptive Immunity – B cells Variable region Constant region How do we create B cells able to respond to all possible antigens? RCSI LEADING T H E W O R L D T O B E T T E R H E A LT H • At any one time we have millions of B cells in our body, each having a TCR with a different specificity for antigen • How? During B cell development gene rearrangement of BCR genes (VDJ recombination) occurs and variable BCRs with unique specificities are generated! • VDJ recombination – randomly combine V(D)J genes to form VDJ or VJ segments  1011 different combinations!!!!! Dr Chiara De Santi, Adaptive Immunity B cells T H E W O R L D T O B E T T E R H E A LT H B CELL DEVELOPMENT RCSI LEADING • It is a stepwise process which happens mainly in the bone marrow • Immature B cell has a membrane bound IgM antigen receptor (otherwise known as BCR) • Mature B cell occurs when a IgD receptor is coexpressed with the IgM receptor (IgM+IgD+ B cell) – this steps happens in the spleen Mature B cells circulate in the spleen, lymph and blood waiting antigen Dr Chiara De Santi, Adaptive Immunity – B cells B CELL DEVELOPMENT CHECKPOINTS Checkpoints are required to ensure the selection of appropriate B cells T H E W O R L D T O B E T T E R H E A LT H 1. Do you have a BCR (positive selection)? Yes! Developing B cells are positively selected when they express a complete BCR 2. Do you recognise selfantigen (negative selection)? Only weakly! RCSI LEADING B cells are removed by negative selection if they bind to self-peptide too tightly Dr Chiara De Santi, Adaptive Immunity – B cells RCSI LEADING T H E W O R L D T O B E T T E R H E A LT H Naïve B cells leave the bone marrow with a functioning BCR PRIOR TO ANTIGEN EXPOSURE Dr Chiara De Santi, Adaptive Immunity – B cells ANTIGEN EXPOSURE T H E W O R L D T O B E T T E R H E A LT H RCSI LEADING B CELL ACTIVATION Dr Chiara De Santi, Adaptive Immunity – B cells B cell activation – 2 signals RCSI LEADING T H E W O R L D T O B E T T E R H E A LT H Signal 1 – Antigen binding to BCR •B cell antigen receptor will recognise antigen (protein, carbohydrate, lipid, polysaccharide) •Triggers phagocytosis of antigen •B cells excellent phagocytes and APCs •Peptides now presented on MHC class II •Get up-regulation of CD40 a co-stimulatory receptor •Get up-regulation of cytokine receptors But to get full activation requires T helper cell Dr Chiara De Santi, Adaptive Immunity – B cells B CELLS AND T CELLS MUST COME TOGETHER RCSI LEADING T H E W O R L D T O B E T T E R H E A LT H In the lymph node, B cells and T cells specific for a particular antigen come together Dr Chiara • The CD4 T cell must be fully activated by DC presenting peptides on MHC class II • Naïve B cells are primed by BCR recognition and presentation on De Santi, Adaptive Immunity – B cells MHC class II B CELL ACTIVATION RCSI LEADING T H E W O R L D T O B E T T E R H E A LT H Signal 2 – T cell help Dr Chiara De • ACTIVATED T helper cells will recognise antigen presented by B cells • The T cell upregulates costimulatory molecules (CD40L) which binds to CD40 on B cells • The production of cytokines drive B cell aid proliferation and Santi, Adaptive Immunity – B cells differentiation Exercise – quote quiz challenge! ‘My maturation (mainly) happens in the bone marrow’ ‘My activation requires 3 steps’ ‘I express CD40 on my surface which interacts with CD40L on the cell I am interacting with’ RCSI LEADING THE WORLD TO BETTER H E A LT H ‘I only like peptide antigens when they are ‘presented’ by other cells’ ‘I can generate memory cells’ Dr Chiara De Santi, Adaptive Immunity – B cells RCSI LEADING THE WORLD TO BETTER H E A LT H B Cell activation results in…. Dr Chiara De Santi, Adaptive Immunity – B cells What are antibodies and what do they do? RCSI LEADING T H E W O R L D T O B E T T E R H E A LT H • Antibodies are secreted proteins which recognise ‘epitopes’ on the surface of pathogens using their antigen binding site within the variable regions • Epitopes can be any biological molecule – protein, carbohydrate, lipid, polysaccharide • The constant region activates different effectors (complement proteins or innate immune cells) that eliminates these microbes and toxins • The ‘type’ of constant region defines the isotype ofThe an “epitope” antibody is the specific part of the antigen that is recognised by T and B cell receptors Dr Chiara De Santi, Adaptive Immunity – B cells Variable region Constant region Antibodies structural features Fab (Fragment antigen-binding) portion T H E W O R L D T O B E T T E R H E A LT H 1 2 RCSI LEADING 3 Fc (Fragment, crystallizable) portion Dr Chiara De Santi, Adaptive Immunity – B cells 1. Antigen binding site – recognises antigen and binds incredibly tightly – Incredibly specific – can distinguish between very similar epitopes 2. Complement binding site – site where C1q protein binds to start the classical complement cascade 3. Binding site for immune cells – Macrophages, mast cells, basophils, NK cells, neutrophils and eosinophils all have Fc T H E W O R L D T O B E T T E R H E A LT H Isotype switching RCSI LEADING • IgM is the first antibody secreted upon infection • But B cells can isotype switch into making IgG, IgA, IgE, IgD isoforms as required • During isotype switching the antigen binding site remains the same but the constant region of the heavy chain is replaced • The process of isotype switching is induced by CD40-CD40L interaction and cytokines released by T helper cells • Each isotype has a slightly different function as discussed later Dr Chiara De Santi, Adaptive Immunity – B cells ISOTYPE SWITCHING RCSI LEADING T H E W O R L D T O B E T T E R H E A LT H ligand Dr Chiara De Santi, Adaptive Immunity – B cells AFFINITY MATURATION T H E W O R L D T O B E T T E R H E A LT H Affinity maturation is the process by which the affinity of antibodies produced in response to an antigen increases with prolonged or repeated exposure RCSI LEADING Random mutation produces B cell clones of variable affinity Dr Limiting amounts of antibody results in selection of high affinity clones Chiara De Santi, Adaptive Immunity – B cells T H E W O R L D T O B E T T E R H E A LT H Antibody effector functions - RCSI LEADING how do they protect us??? Dr Chiara De Santi, Adaptive Immunity – B cells ANTIBODY EFFECTOR FUNCTIONS IgG, IgA T H E W O R L D T O B E T T E R H E A LT H IgG RCSI LEADING IgG, IgE IgG, IgM Dr Chiara De Santi, Adaptive Immunity – B cells PATHOGEN NEUTRALISATION T H E W O R L D T O B E T T E R H E A LT H • Antibody neutralization of microbe and toxins • Binds microbes/toxins and makes them too bulky to enter cells RCSI LEADING • Neutralization function of antibody requires only the antigen-binding regions of antibodies, not the Fc region • Very efficient at preventing extracellular infections Dr Chiara De Santi, Adaptive Immunity – B cells OPSONISATION  PHAGOCYTOSIS RCSI LEADING T H E W O R L D T O B E T T E R H E A LT H • Antibodies can “coat” microbes and make it easier for phagocytes to recognise them • The opsonised microbe will bind to antibody-specific receptors on the phagocytes • This leads to efficient phagocytosis and killing of the internalised microbe Neutrophils, Macrophages and dendritic cells all have Fcg receptors for IgG and phagocytose IgG coated pathogens Dr Chiara De Opsonisation – marking a pathogen for ingestion and destruction Santi, Adaptive Immunity – B cells RCSI LEADING T H E W O R L D T O B E T T E R H E A LT H COMPLEMENT ACTIVATION • Antigen binding site and complement recognition sites needed • Antibody binds to pathogen surface (typically IgG/IgM) • Complement binding site exposed • Get activation of the complement cascade, generating products that can: - Attract leukocytes  inflammation - Opsonise microbes  promote phagocytosis - Cause assembly of the MAC  lyse bacteria Dr Chiara De Santi, Adaptive Immunity – B cells RCSI LEADING T H E W O R L D T O B E T T E R H E A LT H ANTIBODY-DEPENDENT CELLULAR CYTOTOXICITY Dr Chiara De • Natural Killer (NK) cells and other leukocytes bind to antibodycoated cells and destroy these cells • NK cells express FcγRIII receptor which binds to IgG attached to the surface of a cell • NK cells discharges granules to kill opsonised cell S•a n tSome i , A d a p t itherapeutic v e I m m u n i t y – B antibodies cells use this mechanism to kill cancer cells RCSI LEADING T H E W O R L D T O B E T T E R H E A LT H EOSINOPHIL/MAST CELLMEDIATED REACTIONS • Most helminths are too large to be phagocytosed • IgE antibodies are generated against the worms • IgEs bind to (“prime”) the surface of either mast cells or eosinophils (via Fcε receptors) • When the worm antigens bind to their specific IgE antibodies, this perturbs the membrane of the mast cells/eosinophils • These cells “degranulate”, releasing their contents, which can kill the worms • Mast cells – histamine and Dr Chiara De Santi, Adaptive Immunity – B cells ANTIBODY CLASSES Ab 5 types of heavy chains (gamma, alpha, mu, delta, epsilon) IgG, IgA, IgM, IgD, IgE Form Description IgG Monomer IgA Dimer IgM Pentamer Y Y Monomer Y Monomer Y Y IgE Y Y IgD Y Y Y • most abundant Ig in the body; only antibody class that can cross placenta; involved in all antibody functions • Secreted at mucosal surfaces; blocks pathogen entry; involved in neutralisation; IgA immunity acquired via colostrum and milk • On naïve B cells; first Ig to be secreted; involved in complement activation • Part of antigen receptor on naïve B cell surface; unclear role (binds mast cells and basophils and drive the production of anti-microbial peptides) • Attaches to mast cells and eosinophils; implicated in worm THE WORLD TO BETTER H E A LT H Luke suffers from recurring bacterial and viral infections due to a deficiency in one antibody isotype. What isotype is most abundant in serum and can carry out all known antibody functions including opsonisation, ADCC, neutralization and complement activation? A. IgA B. IgD C. IgE D. IgG E. IgM RCSI LEADING What region of an antibody is important for the neutralization function? A. Fc B. Fab (antigen-binding) C. Disulphide bonds Dr Chiara De Santi, Adaptive Immunity – B cells RCSI LEADING T H E W O R L D T O B E T T E R H E A LT H B cell/Antibody Deficiencies Common Variable Immune Deficiency (CVID) • Characterised by low antibody levels, specificially IgG, IgM and IgA • Symptoms vary but recurrent infections of the lungs and gastrointestinal tract are more common Selective IgA Deficiency • Most common immunodeficiency • Failure of B cells to differentiate into IgA producing plasma cells • At least 1/3 patients are asymptomatic (as IgM can compensate) X-linked agammaglobulinemia (XLA) • Caused by a mutation in a protein involved in B cell development  Absent B cells • Recurrent bacterial infections Dr Chiara De Santi, Adaptive Immunity – B cells RCSI LEADING T H E W O R L D T O B E T T E R H E A LT H OVERVIEW Dr Chiara De Santi, Adaptive Immunity – B cells Immune System Questions for Review https://vle.rcsi.com/mod/folder/view.php?id =491567 Tu e s d a y , 1 1 t h O c t 2 0 2 2 Thank you Dr Patrick Walsh Class Year 1 Module Foundations for Practice 1 Click to edit master title style Title Adaptive Immunity – B cells