Adaptive Immunity - B Cells PDF (November 2024)

November 2024 Adaptive Immunity - B cells Dr Patrick Walsh Class Year 1 Module BMF Title Adaptive Immunity – B cells  LECTURE LEARNING OUTCOMES By the end o...

November 2024 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 T H E W O R L D T O B E T T E R H E A LT H 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 RCSI LEADING antibodies List diseases associated with B cell deficiencies Dr Chiara De Santi, Adaptive Immunity – B cells  KEY ROLES OF B CELLS T H E W O R L D T O B E T T E R H E A LT H B cells are cells of the adaptive immune system (antigen-specific RCSI LEADING response) They produce antibodies  crucial in the fight against pathogens Dr (mainly extracellular pathogens) Chiara De Santi, Adaptive Immunity – B cells  Foreign KEY ROLES OF B CELLS antigen BCR Y Each B cell has a unique B cell receptor B Cell (BCR) They recognise antigen in any biological T H E W O R L D T O B E T T E R H E A LT H form (protein, carbohydrate, lipid, Y Y Y polysaccharide) Plasma Plasma Plasma cell Cell Cell Antigen recognition causes B cell activation where they proliferate and Y Y differentiate into plasma cells Y Y Each plasma cell produce trillions of Y Y Y Y YY Y Y antibodies (immunoglobulin = Ig) Y antibodies Y Antibodies are secreted into circulation RCSI LEADING Y and mucus membranes – trigger effector responses Dr Chiara De Santi, Adaptive Immunity – B cells  Pertinent Questions Where do B cells come from? What are the key stages of B cell development? T H E W O R L D T O B E T T E R H E A LT H How do they obtain unique B cell receptors? How do antibodies work and what do they do? RCSI LEADING Dr Chiara De Santi, Adaptive Immunity – B cells  B CELL DEVELOPMENT Primary Secondary Lymph organs Lymph organs 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  MEMBRANE IGM RECEPTOR (BCR) A membrane bound IgM is also called the Variable BCR region It is composed of four polypeptide chains T H E W O R L D T O B E T T E R H E A LT H – 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 Constant via disulphide bonds region Each domain folds into a characteristic 3D shape called the ‘Ig domain’ RCSI LEADING The V region is the variable ‘antigen- binding site’ The C region is the constant region Dr Chiara De Santi, Adaptive Immunity – B cells How do we create B cells able to respond to all possible antigens? 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 T H E W O R L D T O B E T T E R H E A LT H with unique specificities are generated! VDJ recombination – randomly combine V(D)J genes to form VDJ or VJ segments  1011 different combinations!!!!! RCSI LEADING Dr Chiara De Santi, Adaptive Immunity B cells  B CELL DEVELOPMENT T H E W O R L D T O B E T T E R H E A LT H 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 RCSI LEADING 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 1. Do you have a BCR T H E W O R L D T O B E T T E R H E A LT H (positive selection)? Yes! Developing B cells are positively selected when they express a complete BCR 2. Do you recognise self- antigen (negative selection)? Only weakly! B cells are removed by negative RCSI LEADING selection if they bind to self-peptide too tightly Dr Chiara De Santi, Adaptive Immunity – B cells  Naïve B cells leave the bone marrow with a functioning BCR T H E W O R L D T O B E T T E R H E A LT H RCSI LEADING PRIOR TO ANTIGEN EXPOSURE ANTIGEN EXPOSURE 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 ACTIVATION RCSI LEADING Dr Chiara De Santi, Adaptive Immunity – B cells  B cell activation – 2 signals Signal 1 – Antigen binding to BCR B cell antigen receptor will recognise antigen T H E W O R L D T O B E T T E R H E A LT H (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 RCSI LEADING 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 In the lymph node, B cells and T cells specific for a particular antigen come together T H E W O R L D T O B E T T E R H E A LT H RCSI LEADING 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 Dr Chiara De Santi, Adaptive Immunity – B cells MHC class II  B CELL ACTIVATION Signal 2 – T cell help T H E W O R L D T O B E T T E R H E A LT H ACTIVATED T helper cells will recognise antigen presented by B cells RCSI LEADING The T cell upregulates costimulatory molecules (CD40L) which binds to CD40 on B cells The production of cytokines drive B cell aid proliferation and Dr Chiara De Santi, Adaptive Immunity – B cells differentiation  Exercise – quote quiz challenge! ‘My maturation (mainly) happens in the bone marrow’ ‘I only like peptide antigens when they T H E W O R L D T O B E T T E R H E A LT H are ‘presented’ by other cells’ ‘My activation requires 3 steps’ ‘I express CD40 on my surface which interacts with CD40L on the cell I am interacting with’ RCSI LEADING ‘I can generate memory cells’ Dr Chiara De Santi, Adaptive Immunity – B cells  B Cell activation results in…. 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  What are antibodies and what do they do? Antibodies are secreted proteins which recognise ‘epitopes’ on the surface of pathogens using their antigen binding Variable site within the variable regions region T H E W O R L D T O B E T T E R H E A LT H Epitopes can be any biological molecule – protein, carbohydrate, lipid, polysaccharide The constant region activates different effectors (complement proteins or Constant innate immune cells) that eliminates region these microbes and toxins The ‘type’ of constant region defines RCSI LEADING the isotype ofThe an “epitope” antibodyis the specific part of the antigen that is recognised by T and B cell receptors Dr Chiara De Santi, Adaptive Immunity – B cells  Antibodies structural features Fab (Fragment antigen-binding) portion 1. Antigen binding site – recognises antigen and 1 binds incredibly tightly – Incredibly specific – can T H E W O R L D T O B E T T E R H E A LT H distinguish between very similar epitopes 2. Complement binding site – site where C1q 2 protein binds to start the classical complement cascade 3 3. Binding site for immune cells – RCSI LEADING Macrophages, mast cells, Fc (Fragment, crystallizable) portion basophils, NK cells, neutrophils and Dr Chiara De Santi, Adaptive Immunity – B cells eosinophils all have Fc  Isotype switching T H E W O R L D T O B E T T E R H E A LT H 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 RCSI LEADING 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 ligand 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  AFFINITY MATURATION Affinity maturation is the process by which the affinity of antibodies produced in response to an antigen increases with prolonged or repeated exposure T H E W O R L D T O B E T T E R H E A LT H Random mutation produces B cell clones of variable affinity RCSI LEADING Limiting amounts of antibody results in selection of Dr high affinity clones Chiara De Santi, Adaptive Immunity – B cells  Antibody effector T H E W O R L D T O B E T T E R H E A LT H functions - how do they protect us??? RCSI LEADING Dr Chiara De Santi, Adaptive Immunity – B cells  ANTIBODY EFFECTOR FUNCTIONS IgG, IgA IgG T H E W O R L D T O B E T T E R H E A LT H IgG, IgE RCSI LEADING IgG, IgM Dr Chiara De Santi, Adaptive Immunity – B cells  PATHOGEN NEUTRALISATION Antibody neutralization of microbe and toxins T H E W O R L D T O B E T T E R H E A LT H Binds microbes/toxins and makes them too bulky to enter cells Neutralization function of antibody requires only the antigen-binding regions of antibodies, not the Fc region RCSI LEADING Very efficient at preventing extracellular infections Dr Chiara De Santi, Adaptive Immunity – B cells OPSONISATION  PHAGOCYTOSIS 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 T H E W O R L D T O B E T T E R H E A LT H RCSI LEADING 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 COMPLEMENT ACTIVATION Antigen binding site and complement recognition sites needed Antibody binds to pathogen surface (typically IgG/IgM) T H E W O R L D T O B E T T E R H E A LT H 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 RCSI LEADING bacteria Dr Chiara De Santi, Adaptive Immunity – B cells ANTIBODY-DEPENDENT CELLULAR CYTOTOXICITY T H E W O R L D T O B E T T E R H E A LT H Natural Killer (NK) cells and other leukocytes bind to antibody- coated cells and destroy these cells NK cells express FcγRIII receptor which binds to IgG attached to the RCSI LEADING surface of a cell NK cells discharges granules to kill opsonised cell Dr Chiara De 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  EOSINOPHIL/MAST CELL- MEDIATED REACTIONS Most helminths are too large to be phagocytosed IgE antibodies are generated against the worms T H E W O R L D T O B E T T E R H E A LT H 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”, RCSI LEADING releasing their contents, which can kill the worms Mast Dr Chiaracells De – histamine Santi, and Adaptive Immunity – B cells ANTIBODY CLASSES 5 types of heavy chains (gamma, alpha, mu, delta, epsilon) IgG, IgA, IgM, IgD, IgE Ab Form Description most abundant Ig in the body; only antibody class that can IgG Monomer Y cross placenta; involved in all antibody functions IgA Dimer Y Secreted at mucosal surfaces; blocks pathogen entry; Y involved in neutralisation; IgA immunity acquired via colostrum and milk IgM Pentamer Y Y Y Y Y On naïve B cells; first Ig to be secreted; involved in complement activation IgD Monomer Y Part of antigen receptor on naïve B cell surface; unclear role (binds mast cells and basophils and drive the production of anti-microbial peptides) IgE Monomer Y Attaches to mast cells and eosinophils; implicated in worm  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? T H E W O R L D T O B E T T E R H E A LT H A. IgA B. IgD C. IgE D. IgG E. IgM What region of an antibody is important for the neutralization function? A. Fc RCSI LEADING B. Fab (antigen-binding) C. Disulphide bonds Dr Chiara De Santi, Adaptive Immunity – B cells  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 T H E W O R L D T O B E T T E R H E A LT H 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 RCSI LEADING 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 Dr Chiara De Santi, Adaptive OVERVIEW Immunity – B cells