The Humoral Immune Response - Immunology Lecture Notes PDF

The Humoral Immune Response Literature: Chapter 10, Janeway’s Immunobiology ETH Zurich Lecture on “Pharmaceutical Immunology I” Prof. Dr. Cornelia Halin Winter 535-0830-00L HS 2024  Revision from Chapter 1 Antibodies pa...

The Humoral Immune Response Literature: Chapter 10, Janeway’s Immunobiology ETH Zurich Lecture on “Pharmaceutical Immunology I” Prof. Dr. Cornelia Halin Winter 535-0830-00L HS 2024  Revision from Chapter 1 Antibodies participate in humoral immunity in multiple ways Antibodies are found in plasma - the fluid component of blood - and in extracellular fluids. Because body fluids, were once known as humors, immunity mediated by antibodies is know as humoral immunity. Neutralisation Opsonization Complement activation Antibody-dependent cellular cytotoxicity (ADCC) Inducers of mast cell degranulation (IgE) Þ In this Chapter we will learn how T cells induce B cells to secrete antibodies and study in detail the effector functions of the different antibody isotypes 1 Content 1) B cell activation by helper T cells 2) The distributions and functions of immunoglobulin classes 3) The destruction of antibody-coated pathogens via Fc-receptors 2 1. B cell activation by antigen and helper T cells The B cell receptor (BCR) serves to internalize antigen and to induce signaling However, typically a second signal is required for B cell activation: Thymus-dependent antigens: Thymus-independent antigens: Protein antigens alone Mostly microbial constituents (e.g. tetanus vaccine) Come together with a TLR ligand Require help from T helper cells: Typically arrayed antigens that CD40 – CD40L engage (cluster) many BCRs B-cell helping T helper cells are Only induce IgM responses; knows as follicular helper cells => rapid first protection 3  T follicular helper cells (TFH ) provide several signals that activate B cells and control their subsequent differentiation Cell-surface bound: CD40L interacting with B-cell expressed CD40 (“handshake”) Secreted: IL-21 and various other cytokines required to induce proliferation, differentiation and class switching (depending on the cytokines!) of B cells 4  T cells and B cells must recognize antigens contained within the same molecular complex in order to interact The antigen that elicited the T cell response (in the example shown an internal viral protein – red – presented on MHC of a dendritic cell) does not have to be identical with the antigen recognized by the B cell via its BCR (in the example shown, a structure on the viral surface). As long as the T cell antigen was also taken up by the B cell and gets presented on the B cell’s MHC, the T cell can give help to activate the B cell and induce antibody production. This phenomenon is known as “linked recognition” 5 Activation of B cells in lymph nodes B cells that have encountered antigen move to the border of the T cell area. Upon interaction with an antigen-specific T cell, the B cells start to proliferate and to differentiate Form a germinal center. Germinal center: sites of sustained B cell proliferation and differentiation (somatic hypermutation leading to antibody affinity maturation and class switching take place here) Plasma cells: Terminally differentiated antibody-producing B cells. Remain in lymph node or migrate to the bone marrow. 6  The primary antibody repertoire is diversified by somatic hypermutation and by class switching V(D)J joining => determines the specificity for the antigen: takes place in the bone marrow Somatic hypermutation => improves the affinity for the antigen: takes place in germinal centers Class switching => determines the isotype: takes place in germinal centers 7  Activated B cells differentiate into antibody-secreting plasmablasts and plasma cells Plasma cells are terminally differentiated B cells. Typically, they have undergone class switching (changing of isotype) and now secrete antibody at a high rate. However, they can no longer proliferate or respond respond to antigen. Plasmablasts are at an intermediary stage. 8  The germinal center proliferating cells, FDCs, CD4+ T cells Germinal centers are sites of intense cell proliferation and cell death Represent a specialized microenvironment where B cell proliferation, somatic hypermutation and selection for strength of antigen binding (affinity maturation) occur The germinal center is divided into a closely packed ”dark zone” and a less densely packed “light zone”. B cells cycle between these two zones and cell states Their migration is controlled by differential expression of chemokines (CXCL13 & CXCL12) and up/downregulation of the corresponding chemokine receptors on the B cells  Germinal center reaction: Selection of high-affinity mutants In the dark zone: B cells proliferate at high rate and somatic hypermutation occurs. After exiting the dark zone the B cells enter the light zone: Here, they capture antigen trapped on follicular dendritic cells. B cells with a higher affinity for the antigen will have an advantage in presenting antigen to TFH and receiving survival signals (stimulation via CD40 and cytokines). => High affinity clones will therefore be selected (survive) and can subsequently return to the dark zone for another round of mutations! EXTRA SLIDE (not relevant for exam) Antigens are trapped in immune complexes that bind to the surface of follicular dendritic cells (FDCs) FDCs are specialized stromal cells in B cell follicles. They are not of hematopoietic origin. FDCs express abundantly Fc receptors and complement receptors. These allow them to bind antigen on their surface. The antibodies that bind the antigen to the FDCs are the ones formed early during the adaptive immune response (or during a previous immune response to the same antigen) FDCs can retain antigen on their surface for long time periods (days – weeks) Left panel: Intense dark staining shows the germinal center localization of of radiolabeled antigen that had been injected 3 days previously. 11 AID initiates DNA lesions. Their repair leads to somatic hypermutation (and class switch recombination and gene conversion) The presence of uridine can trigger either a mismatch repair or a base-excision repair Together, these pathways generate point mutations AID: activation-induced cytidine deaminase UNG: uracil-DNA-glycosylase APE-1: apurinic/apyrimidinc endonuclease-1 (APE-1) AID is also the initiator of class switching (discussed later) and gene conversion 12 EXTRA SLIDE (not relevant for exam) Activation-induced cytidine deaminase (AID): Expressed in germinal center B cells (which are proliferating) Deamidates cytidine to uridine Only active on single-stranded DNA in loci that are actively being transcribed (e.g. immunoglobulin locus) Uridine is foreign to DNA => induces mismatch repair and excision programs Different from all other cells in the body, the DNA polymerase involved is error- prone => => mutations 13 Somatic hypermutation: introduces mutations into the rearranged immunoglobulin variable (V) regions that improve antigen binding Mutations that change anywhere from one to a few amino acids in the immunoglobulin Produces closely related B cell clones that subtly vary in affinity Mutations induced by enzyme AID (activation-induced cytidine deaminase) Most mutation have a negative impact on the BCR to bind antigen (e.g. prevent folding, block antigen binding). Such cells will die by apoptosis. => germinal centers contain specialized macrophages to remove all the apoptotic cells Less frequently, mutations will improve the antibody affinity for the antigen => these cells will have a survival advantage and therefore will be expanded/selected 14 Somatic hypermutation: introduces mutations into the rearranged immunoglobulin variable (V) regions that improve antigen binding Mutations that change anywhere from one to a few amino acids in the immunoglobulin Have a look at Figure 10.14: What might be the reason Produceswhyclosely related mutations B cell clones mainly cluster that subtly varyininthe CDRs ? affinity Mutations induced by enzyme AID (activation-induced cytidine deaminase) Most mutation have a negative impact on the BCR to bind antigen (e.g. prevent folding, block antigen binding). Such cells will die by apoptosis. => germinal centers contain specialized macrophages to remove all the apoptotic cells Less frequently, mutations will improve the antibody affinity for the antigen => these cells will have a survival advantage and therefore will be expanded/selected 15 AID initiates DNA lesions. Their repair leads to class switch recombination (and somatic hypermutation and gene conversion) AID induces uridine that can trigger either a mismatch repair or a base-excision repair these may eventually lead to the generation of double-stand breaks, which are required for class switching AID: activation-induced cytidine deaminase UNG: uracil-DNA-glycosylase APE-1: apurinic/apyrimidinc endonuclease-1 (APE-1) 16 Class switching (e.g. to IgE) Also known as isotype switching constant-region portion of the antibody heavy chain is exchanged Remember: The first isotypes expressed in a humoral Sµ: switch region in fron of constant heavy chain sequence of IgM immune response are IgM and IgD. Se: switch region in fron of constant heavy chain sequence of IgM Expression of IgM / IgD is determined by RNA DSBR: Double-Strand Break Repair splicing. Switching to other isotypes requires irreversible DNA recombination Happens in activated B cells and requires T cell help Involves the enzymes AID, UNG & APE1, which induced nicks in the switch regions upstream of the Cµ and the target constant region portion (Ce in Figure 10.21) Upon class switching, the intervening DNA region gets deleted  Class switching: The choice of the isotype depends on the cytokines produced by the T follicular helper (TFH) cells Interactions between germinal center B cells and TFH are essential for class switching. They consist of CD40-CD40L and IL-21 and other cytokine signaling Different cytokines preferentially induce switching to a particular isotype (see Table 10.23) Germinal center B cells eventually differentiate into antibody- secreting plasma cells and memory B cells Memory B cells are long lived, hardly divide and express some surface antibodies. They are rapidly activated during antigen re-challenge (secondary exposure) 18 Different cytokines induce switching to different antibody classes Class switching: Determines the type of immune response => different antibody isotypes have different effector functions 19 Exercise: Activation-induced cytidine deaminase (AID) deficiency 1. Which immunologic phenotype would you expect in patients with mutations in the AID gene? 2. How immunosuppressed would such individuals be? 3. Could you think of other mutations giving rise to similar immunodeficiencies? 20  3) Immunoglobulin classes 2. The distributions and functions of immunoglobulin classes => Revise what we learnt in Chapter 5 21 Major effector functions and distribution of immunoglobulin classes The isotype determines: the type of immune response (effector functions) the distribution of the antibody Receptors for the Fc region and their expression patterns 23 Summary of antibody distributions IgM: mainly intravasular (too big to extravasate) IgA: mucosal surfaces, e.g. gastrointestinal (GI) tract, nasal cavity, saliva, tear fluids IgE: epithelial surfaces, e.g. GI tract, skin, lung, nasal cavity IgG: all tissues within the body (exception; uninflamed central nervous system) 24 IgA: Most abundant antibody present in the lumen of the gut IgA mainly works by neutralization (poor in inducing opsonisation or complement activation). It needs to transported in a dimeric form across mucosal epithelia. Polymeric immunoglobulin receptor: binds to the Fc regions of IgA and transports it across epithelial barriers 25  FcRN Receptor (neonatal Fc reeptor): transport of IgG across the placenta Recycling of IgG in blood => responsible for half-life of 2-3 weeks! FcRN expression: - endothelial cells - monocytes - podoytes - proximal tubular epithelial cells 26  4. Effector mechanisms Antibodies participate in humoral immunity in multiple ways I. Neutralisation II. Opsonization III. Complement activation IV. Antibody-dependent cellular cytotoxicity (ADCC) V. Inducers of mast cell degranulation (IgE) 27 I. Neutralization of toxins => prevent cellular damage Example: diphtheria, pertussis, tetanus vaccination 28 I. Neutralization of viruses: => prevent infection Example: influenza, polio vaccination, SARS-CoV-2 29 I. Neutralization of adhesion: Þ prevent bacterial colonization 30 II. Opsonization: Coating of pathogens with antibodies and complement C3b => initiates phagocytosis 31 II. Opsonization Aggregation /clustering induces cellular activation Of note: Binding of one antibody to an Fc Receptor alone does not induce cellular activation: crosslinking by simultaneous binding of several antibodies is needed III. Complement Activation IgM are particularly efficient at activating complement!! Complement activation on a cell / microbial surface can lead to formation of the membrane-attack complex and induce cell lysis 33 IV. Antibody-dependent cell cytotoxicity (ADCC) Mediated by Natural Killer (NK) cells expressing FcgRIII Mechanism of action of several therapeutic antibodies, e.g. Rituxan (anti-CD20) 34 Rituxan / MabThera (Rituximab; anti-CD20) antibody directed against CD20; a surface protein on B cells induces the destruction of B cells by NK cells approved for the treatment of malignant B-cell tumors (including non-Hodgkin lymphoma) developed by Genentech (Roche) – one of the bestselling biologicals (7.9 billion USD in 2017) Positron emission tomography (PET) image of a patient with non-Hodgkin before (left) and after (right) treatment with Rituxan plus chemotherapy Nature Reviews Clinical Oncology 9, 671 (2012) 35 V. IgE-Response Important for fighting parasites Eosinophils attacking a schistosome larva in the presence of serum from an infected patient 36  V. IgE-Response IgE-crosslinking on the surface of mast cells leads to a rapid release of granules containing histamine and other inflammatory mediators and secretion of lipid mediators Þ Allergy Þ Protection against helminths 37 Major effector functions and distribution of immunoglobulin classes The isotype determines: the type of immune response (effector functions) the distribution of the antibody Summary exercise: I) How do antibodies contribute to host defense? 39 Take-home messages Chapter 10 Humoral immune response to infection involves the production of antibodies by plasma cells derived from B cells. Production of antibodies (other than IgM) usually requires the help of CD4 T cells (=> thymus-dependent antigens) The T cell is specific for a peptide fragment derived from the antigen that the B cell initially took up via its B cell receptor (=> linked recognition) Initially activated B cells undergo somatic hypermutation and clonal selection in germinal centers, leading to antibody affinity maturation Cytokines produced by T follicular helper cells in germinal centers additional affect the class switch process (i.e. change of antibody isotype) The various antibodies produced have different affinity (for both the antigen and Fc receptors!), distribution in the body, and effector functions. Once the infection is cleared, memory B cells remain in the body to provide rapid protection in case of a second challenge. 40