Activation of B Cells notes PDF

3/23/22 The Immune System Dr. Dalia Zakaria 1 Adaptive Immunity VII Mechanisms of Lymphocytes Activation Activation of Naïve B Cells Textbook Reading: Chapter 7 2 1 3/23/22 What Do You Need to Learn? Components of the immune system (an overview) Components of the innate immune system Mechanisms of i...

3/23/22 The Immune System Dr. Dalia Zakaria 1 Adaptive Immunity VII Mechanisms of Lymphocytes Activation Activation of Naïve B Cells Textbook Reading: Chapter 7 2 1 3/23/22 What Do You Need to Learn? Components of the immune system (an overview) Components of the innate immune system Mechanisms of innate immune responses Transition from innate to adaptive immune response Components of the adaptive immune system (lymphocytes and lymphatic system) How do lymphocytes recognize their specific antigens (development) and how do they distinguish between self and non-self molecules (maturation) Mechanisms of lymphocytes activation Effector mechanisms of the adaptive immune responses 3 Objectives After completing this module, you need to understand the following: The difference between the T dependent and independent activation of B cells in terms of the mechanism, type of B cells and the outcome The mechanism of T dependent activation of B cells and the role of helper T cells and costimulation and the consequences of the lack of costimulation The mechanism of isotype switching and affinity maturation The difference between plasma cells and memory cells The mechanism of T independent activation of B cells 4 2 3/23/22 Recognition of and Binding to Specific Antigen The membrane Ig of B cells is a high-affinity receptor that enables a B cell to specifically bind a particular antigen, even when the extracellular concentration of the antigen is very low In B cells, antigen receptor–mediated signal transduction requires cross-linking of two or more membrane Ig molecules This response is greatest when the antigen is multivalent, cross-links many antigen receptors, and activates complement and innate immune receptors strongly All these features are typically seen with polysaccharides and other T-independent microbial antigens Most soluble protein antigens do not contain multiple identical epitopes, are not capable of cross-linking many receptors on B cells, and by themselves, typically do not stimulate high levels of B cell activation 5 Activation of Naive B Cells Naive B lymphocytes express two classes of membrane-bound antibodies (receptors), IgM and IgD The antibodies secreted in response to an antigen have the same specificity as the surface receptors on naive B cells One activated B cell may generate a few thousand plasma cells, each of which can produce several thousands antibody molecules per hour Some B cells may begin to produce antibodies of different heavy-chain isotypes (isotype or class switching), which mediate different effector functions and are specialized to combat different types of microbes Repeated exposure to a protein antigen leads to the production of antibodies with improved capacity to bind to and neutralize microbes and their toxins (affinity maturation) 6 3 3/23/22 B Cell Receptor 7 Activation of Naive B Lymphocytes (Clonal Expansion and Differentiation) T cell dependent (B-2 cells) T cell independent (B-1 and B-Mz cell)s 8 4 3/23/22 Activation of T-Dependent B Lymphocytes Naturally occurring protein antigens usually are not multivalent, possibly explaining why they do not induce full B cell responses In the absence of T cell help, protein antigens elicit weak or no antibody responses Therefore, protein antigens and the antibody responses to these antigens are called Tdependent 9 T-Dependent B Lymphocytes In which Compartments of the Lymph Node B Cells are Activated? 10 5 3/23/22 In which compartments of the lymph node are B cells activated? 11 Activation of T Dependent B Cells Same antigen but different epitopes Start here B cell binds to and process an antigen and displays it on MHCII Germinal Center Proliferation, class switching, affinity maturation B Cell Zone (Primary Follicle) Interaction CD4+ T cell previously activated by an antigen Extrafollicular Focus Short lived plasma cells Some short lived plasma cells and helper T cells migrate back to the follicle T Cells Zone Some short lived plasma cells produce IgM antibodies which migrate back to the site of infection 12 6 3/23/22 Activation of T Dependent Naive B Lymphocytes (Clonal Expansion and Differentiation) B cells recognize and bind to their specific antigens (in the follicle) B cells Endocytose and process protein antigens and display them on MHCII (in the follicle) Antigen presentation to CD4+ helper T cells (extrafollicular) Proliferation and differentiation of activated B cells (extrafollicular focus) Isotype class switching (germinal center) Affinity maturation (germinal center) Generation of plasma cells and memory cells 13 Role of Innate Immunity in B Cell Activation B lymphocytes, similar to dendritic cells and other leukocytes, express numerous Toll-like receptors TLR engagement on the B cells by microbial products triggers activating signals that work in concert with signals from the antigen receptor (BCR) This combination of signals stimulates B cell proliferation, differentiation, and Ig secretion, thus promoting antibody responses against microbes 14 7 3/23/22 Endocytosis, Processing and Antigen Display In the follicle B cell may bind an epitope of a native protein antigen, internalize and process the protein, and display multiple peptides from that protein on MHCII One of the displayed epitopes must be the same epitope that previously activated CD4+ T cell B cells and T cells may recognize different epitopes of the same protein antigen 15 Antigen Presentation by B Cells to Helper T Cells Any one B cell may bind a conformational epitope of a native protein antigen, internalize and process the protein, and display multiple peptides from that protein for T cell recognition Therefore, B cells and T cells recognize different epitopes of the same protein antigen Because B cells efficiently present the antigen for which they have specific receptors, and helper T cells recognize peptides derived from the same antigen, the interaction remains antigen specific B cells are capable of activating previously differentiated effector T cells but are inefficient at initiating the responses of naive T cells 16 8 3/23/22 Mechanism of Activation of B Cells by Helper T Cells Activated helper T lymphocytes that recognize antigen presented by B cells use CD40 ligand (CD40L) and secreted cytokines to activate the antigen-specific B cells CD40L expressed on activated helper T cells binds to CD40 on B lymphocytes Which delivers signals to stimulate proliferation and the synthesis and secretion of antibodies Meanwhile, cytokines produced by the helper T cells bind to cytokine receptors on B lymphocytes and stimulate more B cell proliferation and Ig production 17 Mechanism of Activation of B Cells by Helper T Cells 18 9 3/23/22 CD40L-CD40 binding is essential for T-B cell interaction Naive B cells are activated in the follicles by an exposed epitope on the same protein that previously activated the helper T cells The antigen-activated helper T cells and B cells migrate toward one another and interact at the edges of the follicles, where the initial antibody response develops Therefore, B lymphocytes are efficient APCs for the antigens they specifically recognize 19 Back to the follicle 20 10 3/23/22 Proliferation and Differentiation of Activated B Cells (Germinal Center Reactions) Plasma cells that are generated in the extra-follicular foci are short-lived and few memory B cells are generated Some of the activated B cells migrate back into follicles to form germinal centers and divide in response to signals from Tfh cells (one cell may produce several thousand progeny within a week) Some of the activated helper T cells express high levels of CXCR5, which draws these cells into the adjacent follicles The CD4+ T cells that migrate into the follicles interact with B cells B cells undergo extensive isotype switching and somatic mutation of Ig genes in the germinal center 21 Heavy Chain Isotype (Class) Switching Heavy-chain isotype switching is induced by a combination of T Tfh CD40L-mediated signals and cytokines In the absence of CD40 or CD40L, B cells secrete only IgM and fail to switch to other isotypes A disease called the X-linked hyper-IgM syndrome is caused by mutations in the CD40L gene, which is located on the X chromosome, leading to production of nonfunctional forms of CD40L In this disease, much of the serum antibody is IgM, because of defective heavy-chain isotype switching 22 11 3/23/22 X-Linked Hyper IgM Syndrome X-linked hyper-IgM syndrome is caused by mutations in the CD40L gene, which is located on the X chromosome, leading to production of nonfunctional forms of CD40L In this disease, much of the serum antibody is IgM, because of defective heavy-chain isotype switching 23 Heavy Chain Isotype (Class) Switching Production of IgG antibodies is stimulated by IL-10 and other cytokines produced by Tfh cells Switching to the IgE class is stimulated by IL-4 produced by Tfh cells IgE functions to eliminate helminths, acting in concert with eosinophils, which are activated by another Th2 cytokine, IL-5. Predictably, helminths induce strong Th2 and related Tfh cell responses IgA antibody is the major isotype in mucosal lymphoid tissues, probably because TGF-β that promote switching to IgA are made in these tissues B cells activated in these lymphoid tissues are also induced to express chemokine receptors and adhesion molecules that favor migration into the sites just below mucosal epithelial barriers 24 12 3/23/22 Mechanism of Isotype Switching Switch recombination, takes the previously formed VDJ exon encoding the V domain of an Ig µ heavy chain and moves it adjacent to a downstream C region The heavy-chain mRNA is produced by splicing a VDJ exon to Cμ exons in the initially transcribed RNA which combines with a light chain to give rise to an IgM antibody Thus, the first antibody produced by B cells is IgM Signals from CD40 and cytokine receptors stimulate transcription through one of the constant regions that is downstream of Cμ An enzyme called activation-induced deaminase (AID), which is induced by CD40 signals, plays a key role in this process 25 Functions of the Different Isotypes of Antibodies Microbes opsonization is best mediated by IgG1 and IgG3 (in humans), that bind to highaffinity phagocyte Fc receptors specific for the γ heavy chain Helminths, in contrast, are too large to be phagocytosed, and they are best eliminated by eosinophils. Eosinophils have high-affinity receptors for the Fc portion of the ε heavy chain (IgE) IgG antibodies are able to bind to the neonatal Fc receptor (FcRn) expressed in the placenta and mediate the transfer of maternal IgG to the fetus, providing protection to the newborn Thus, effective host defense requires that the immune system to make different antibody isotypes in response to different types of microbes 26 13 3/23/22 Cytokines and Isotype Switching Various Cytokines (IL-10) IgG IL-4 TGF-β and BAFF IgE IgA IgA is the major isotype produced in the mucosal lymphoid tissues where TGF-β is produced 27 Functions of the Different Isotypes Center Reactions Extrafollicular and Germinal of Antibodies IL-10 28 14 3/23/22 Affinity Maturation (Somatic Hypermutation) Affinity maturation increases the affinity of antibodies produced in response to a protein antigen with prolonged or repeated exposure to that antigen This is caused by point mutations in the V regions, and particularly in the antigenbinding hypervariable regions, of the genes encoding the antibodies produced It takes place only in responses to helper T cell–dependent protein antigens, indicating that helper cells are critical in the process 29 Affinity Maturation (Somatic Hypermutation) The enzyme AID also plays a critical role in somatic mutation 30 15 3/23/22 Affinity Maturation (Somatic Hypermutation) Affinity maturation occurs in the germinal centers of lymphoid follicles due to somatic hypermutation of Ig genes in dividing B cells, followed by the selection of high-affinity B cells by antigen The enzyme AID also plays a critical role in somatic mutation Frequency of mutations is estimated to be 1 in 103 base pairs per cell per division (much greater than the mutation rate in other genes) This extensive mutation results in the generation of different B cell clones whose Ig molecules may bind with widely varying affinities to the antigen that initiated the response The next step in the process is the selection of B cells with the most useful antigen receptors (highest affinity) 31 Affinity Maturation Somatic hypermutation occurs in the rapidly dividing B cells in the germinal center The rearranged variable region genes in B cells are somehow targeted for the introduction of random somatic point mutations B cells interact with the antigens retained on the surface of the follicular dendritic cells (FDCs) B cells with the best antigen binding receptors are selected for survival Germinal centers have macrophages which engulf apoptotic cells Affinity maturation occurs in the primary response as well as in secondary and subsequent responses Therefore, with repeated exposures to the same antigen, a host will produce antibodies of successively greater affinities 32 16 3/23/22 Generation of Plasma Cells and Memory Cells Plasma Cells Memory Cells The antibody-secreting cells enter circulation then migrate to the bone marrow or mucosal tissues, and may continue to produce high-affinity antibodies for years, even after the antigen is eliminated A fraction of the progeny of isotypeswitched high-affinity B cells become memory cells They circulate in blood and reside in mucosal and other tissues It is estimated that more than half of the antibodies in blood of a normal adult are produced by these long-lived plasma cells They are ready to respond rapidly if the antigen is reintroduced Memory from a T-dependent antibody response can last for a lifetime Circulating antibodies reflect each individual’s history of antigen exposure 33 Activation of Naive B Lymphocytes (Clonal Expansion and Differentiation) T cell dependent T cell independent 34 17 3/23/22 T-Independent B Lymphocytes B cells can recognize native (unprocessed) antigens, so the secreted antibodies are also able to bind to the native microbe Polysaccharides, lipids, and other nonprotein antigens contain multivalent arrays of the same epitope and may be able to cross-link many antigen receptors on a specific B cell which could be enough to stimulate proliferation and differentiation without a T cell help Defense against encapsulated bacteria is mediated primarily by antibodies that bind to capsular polysaccharides and target bacteria for phagocytosis 35 T-Independent B Lymphocytes Unlike soluble proteins, polysaccharides are multivalent and can cross-link many B cell receptors which is required for B cell activation 36 18 3/23/22 B Cells with Limited Diversity B-1 cells are a population of B lymphocytes found in the peritoneal cavity and mucosal tissues Produce most of blood circulating IgM antibodies (natural antibodies) May also produce IgM in response to nonprotein antigens An important sources of IgA antibody in mucosal tissues especially against nonprotein antigens Marginal-zone B cells, are present at the edges of lymphoid follicles in the spleen and other organs They are involved in rapid antibody responses to bloodborne polysaccharide-rich microbes They are not fully formed until 1 to 2 years in humans 37 T-Independent B Lymphocytes B-1 cells also appear to be important sources of IgA antibody in mucosal tissues especially against nonprotein antigens IgM antibodies may be produced spontaneously by B-1 cells, without overt immunization. These antibodies, called natural antibodies, help to clear apoptotic cells and may also provide protection against some bacterial pathogens Marginal-zone B cells in the spleen are the major contributors to T-independent antibody responses to blood-borne antigens, and B-1 cells make T-independent responses to antigens in mucosal tissues and in the peritoneum 38 19 3/23/22 Antibody Response to T-Dependent and Independent Antigens 39 20