B-Cell Biology and Development

Introduction  Humoral immunity is mediated by antibodies  Arm of the adaptive immune response  Functions to neutralize and eliminate: I. Extracellular microbes II. Microbes with capsules (rich in polysaccharides and lipids) III. Microbial toxins Introduction  B cells the mediators of humoral immunity respond to, and produce antibodies specific for, many types of extracellular and cell surface molecules, including: A. B. C. D. E. Polysaccharides Lipids Proteins Nucleic acids Small chemicals  T cells, the mediators of cellular immunity, recognize and respond only to protein antigens that are internalized into or synthesized in cells Introduction  Naive B lymphocytes recognize antigens but do not secrete antibodies, and activation of these cells stimulates their differentiation into antibody-secreting plasma cells.  This class focusing on the following questions: I. How are receptor-expressing naive B lymphocytes activated and converted to antibody-secreting cells? II. How is the process of B cell activation regulated so that the most useful types of antibodies are produced in response to different types of microbes? Phase and types of humoral immune responses  Naive B lymphocytes express two classes of membranebound antibodies: IgM and IgD, that function as receptors for antigens  Activation of naïve B cells: antigens binding to BCR and by other signals  Results of B lymphocytes activation: I. II. Proliferation of antigen-specific cells (clonal expansion) Differentiation into plasma cells (effector cells of humoral immunity) B cell activation and differentiation  Clonal expansion: One activated B cell may generate up to 4000 plasma cells, which can produce up to 1012 antibody molecules per day  Heavy-chain isotype or class switching: During differentiation of B cells, some B cells may begin to produce antibodies of different heavy-chain isotypes (or classes), which mediate different effector functions  Affinity maturation: Repeated exposure to a protein antigen results in the production of antibodies with increasing affinity for the antigen Phases of humoral immune responses  Antibody responses to different antigens: I.T dependent II.T independent  Protein antigens are processed in antigen-presenting cells (APCs) and recognized by helper T lymphocytes a. b. c. B cell activation Induction of heavy-chain isotype switching Affinity maturation  Polysaccharides, lipids, and other nonprotein antigens stimulate antibody production without the involvement of helper T cells. a. b. Little heavy-chain isotype switching Little affinity maturation Subsets of B cells 1. follicular B cells:      2. The majority of B cells they reside in and circulate through the follicles of lymphoid organs T-dependent Class-switched High-affinity antibody responses to protein antigens Long-lived plasma cells Marginal-zone B cells:     3. Located in the peripheral region of the splenic white pulp Respond to blood-borne polysaccharide antigens Antigen receptors of limited diversity Predominantly IgM responses B-1 cells:    Respond to non-protein antigens in the mucosal tissues and peritoneum Antigen receptors of limited diversity Predominantly IgM responses Subsets of B cells Features of primary and secondary antibody responses Features of primary and secondary antibody responses STIMULATION OF B LYMPHOCYTES BY ANTIGEN  Humoral immune responses are initiated when antigenspecific B lymphocytes in the spleen, lymph nodes, and mucosal lymphoid tissues recognize antigens  The recognition of antigen triggers signaling pathways that initiate B cell activation Antigen-Induced Signaling in B Cells  The process of B lymphocyte activation is, in principle, similar to the activation of T cells  Ig receptor–mediated signal transduction: • Cross-linking of two or more receptor molecules of B cells Antigen-Induced Signaling in B Cells  Polysaccharides, lipids, and other nonprotein antigens often contain multiple identical epitopes in each molecule  Membrane IgM and IgD receptors have short cytoplasmic tails, so although they recognize antigens, they do not themselves transduce signals  Igα and Igβ are noncovalently associated with Membrane IgM and IgD receptors (B cell receptor (BCR) complex) Antigen receptor–mediated signal transduction in B lymphocytes Syk tyrosine kinase (equivalent to ZAP-70 in T cells) The net result of receptor-induced signaling in B cells is the activation of transcription factors that switch on the expression of genes whose protein products are involved in B cell proliferation and differentiation. Role of Complement Proteins and Other Innate Immune Signals in B Cell Activation  B lymphocytes express a receptor for a protein of the complement system that provides signals for the activation of these cells  The complement system is a collection of plasma proteins that are activated by:  Microbes  Antibodies attached to microbes  The complement system acts as effector mechanisms of host defense  B lymphocytes express complement receptor type 2 (CR2, or CD21), which binds C3d Role of the complement protein C3d in B cell activation  Microbes or innate immune responses to microbes provide signals in addition to antigen that are necessary for lymphocyte activation.  TLR engagement on the B cells by microbial products triggers activating signals that work in concert with signals from the antigen receptor  Consequences:     Optimal B cell proliferation Differentiation Ig secretion Prepares B cells to interact with Th Functional consequences of immunoglobulinmediated B cell activation  Early phase of the humoral immune response is greatest when: I. Antigen is multivalent II. Cross-links many antigen receptors III. Activation of complement system  Polysaccharides and other T-independent antigens  Most soluble protein antigens do not contain multiple identical epitopes  Protein antigens can induce signals in B lymphocytes enhance their ability to interact with helper T lymphocytes Problems of B and T cells intraction  How do B cells and T cells specific for epitopes of the same antigen find one another? (both types of lymphocytes specific for any one antigen are rare, probably less than 1 in 100,000 of all the lymphocytes in the body)  How do helper T cells specific for an antigen interact with B cells specific for an epitope of the same antigen and not with irrelevant B cells?  What signals are delivered by helper T cells that stimulate not only the secretion of antibody but also the special features of the antibody response to proteins, namely, heavy-chain isotype switching and affinity maturation? Sequence of events in helper T cell–dependent antibody responses The process of T-B cell interaction and T cell– dependent antibody responses I. Independent activation of CD4+ helper T cells and B cells by a Ag II. Interaction of T and B cells outside the follicles and migration toward each other. III. B cells process and present antigen to the T cells IV. Helper T cells express CD40L and secrete cytokines (B cells proliferation and differentiation to plasma cells) V. Antibody secretion, some degree of isotype switching (in extrafollicular) The process of T-B cell interaction and T cell– dependent antibody responses VI. Some activated B cells migrate back into the follicle, accompanied by helper T cells (follicular helper T cells; TFH) VII. B cells begin to proliferate, forming an organized structure called a germinal center VIII. B cells undergo extensive somatic mutation of antibody gene variable regions and Ig heavy chain isotype switching IX. High-affinity B cells are selected in the germinal center (production of high-affinity antibodies) X. Generation of long-lived plasma cells (many of which Activation and Migration of Helper T Cells  T cell activation is induced best by: I. Microbial antigens II. Protein antigens that are administered with adjuvants  Ag presentation by dendritic cells to CD4+ T cells: I. Differentiation of CD4+ T cells II. Production of various cytokines Activation and Migration of Helper T Cells  The directed migration of activated B and T cells toward one another depends on changes in the expression of certain chemokine receptors on the activated lymphocytes  T cells reduction of CCR7 (recognized chemokines produced in T cell zones), increasement of CXCR5 (recognized chemokines produced in follicles)  B cells undergo precisely the opposite Antigen presentation by B lymphocytes to helper T cells  Any one B cell may bind a conformational epitope of a protein antigen  Helper T cells specifically recognize peptides derived from the same antigen Hapten-carrier conjugates  A hapten is a small chemical that is recognized by B cells but stimulates strong antibody responses only if it is attached to a carrier protein  In this situation the B cell binds the hapten portion, ingests the conjugate, and displays peptides derived from the carrier to helper T cells.  Polysaccharides themselves stimulate weak (T-independent) antibody  If the polysaccharide is coupled to a carrier protein, however, effective responses against the polysaccharide (helper T cells are engaged in the response)  Conjugate vaccines (Haemophilus influenzae) Mechanisms of helper T cell–mediated activation of B lymphocytes heavy-chain isotype switching and affinity maturation Extrafollicular and Germinal Center Reactions Follicular helper T (TFH) cells: CXCR5 The generation and function of TFH cells are dependent on a costimulator of the CD28 family called ICOS (inducible costimulator) Mutations in the ICOS gene are the cause of some antibody deficiencies TFH cells may develop from: Uncommitted T cells Other subsets, including TH1, TH2, and TH17 TFH secrete cytokines, such as IFN-γ, IL-4, or IL-17, which are characteristic of these subsets Most TFH cells secrete the cytokine IL-21 (role in antibody production) Extrafollicular and Germinal Center Reactions  B cells doubling time: 6 hours (5000 progeny within a week) in germinal center  Germinal center B cells:  Extensive isotype switching  Somatic mutation of Ig genes  The highest affinity B cells are the ones that are selected at the end of the germinal center reaction to differentiate into memory B cells and long-lived plasma cells. Immunoglobulin heavy-chain isotype (class) switching Different antibody isotypes perform different functions I. The nature of the helper T cell response (cytokine) to a microbe guides the subsequent Ab response II. Site of immune responses • Neonatal Fc receptor (FcRn) expressed in the placenta. • FcRn expressed on endothelial cells and phagocytes plays a special role in protecting IgG antibodies from intracellular catabolism, thereby prolonging its halflife in the blood X-linked hyper-IgM syndrome:  Mutations in the CD40L gene, which is located on the X chromosome (nonfunctional forms of CD40L)  Much of the serum antibody is IgM  Defective cell-mediated immunity against intracellular microbes (CD40L is important for T cell–mediated activation of macrophages and for the amplification of T cell responses by dendritic cells) Mechanism of immunoglobulin heavy-chain isotype switching S: Switch region (except Cδ) An enzyme called activation-induced deaminase (AID), which is induced by CD40 signals, plays a key role in this process AID converts cytosines (C) in DNA to uracil (U). switch recombination Affinity Maturation  Affinity maturation is the process by which the affinity of antibodies produced in response to a protein antigen increases with prolonged or repeated exposure to that antigen  Point mutations in the V regions (particularly in the antigenbinding hypervariable)  Affinity maturation is seen only in responses to helper T cell– dependent protein antigens  Affinity maturation occurs in the germinal centers of lymphoid follicles  Ig genes of rapidly dividing B cells undergo numerous point mutations Affinity maturation  The enzyme AID required for isotype switching also plays a critical role in somatic mutation.  The U’s that are produced by AID in Ig V-region DNA are frequently converted to T’s during DNA replication, or they are removed and repaired by error-prone mechanisms that often lead to mutations  The frequency of Ig gene mutations one in 1000 base pairs per cell per division, about 100,000-fold to 1 million–fold greater than the mutation rate in most other genes (somatic hypermutation) Affinity maturation in antibody responses Selection of high-affinity B cells in germinal centers ANTIBODY RESPONSES TO T-INDEPENDENT ANTIGENS  Polysaccharides, lipids, and other nonprotein antigens elicit antibody responses without the participation of helper T cells  These nonprotein antigens cannot bind to MHC molecules  Bacterial Polysaccharide rich capsules defense against such bacteria is mediated primarily by antibodies ANTIBODY RESPONSES TO T-INDEPENDENT ANTIGENS  Polysaccharides and lipids contain multivalent arrays of the same epitop cross-link many B cell antigen receptors stimulate their proliferation and differentiation without a requirement for T cell help  Marginal-zone B cells in the spleen are the major contributors to T-independent antibody responses to bloodborne antigens  B-1 cells make T-independent responses to antigens in mucosal tissues and in the peritoneum. Features of antibody responses to T-dependent and Tindependent antigens REGULATION OF HUMORAL IMMUNE RESPONSES: ANTIBODY FEEDBACK  B lymphocytes antibody secreting cells memory cells  Most of the activated B cells die by programmed cell death physiologic decline of the humoral immune response Mechanism of antibody feedback FcγRIIB This process, in which antibody bound to antigen inhibits further antibody production, is called antibody feedback. SUMMARY  Humoral immunity is mediated by antibodies that bind to extracellular microbes and their toxins  Neutralized  Targeted for destruction by phagocytes  Complement system  Humoral immune responses to nonprotein antigens:  Recognition of the antigens by specific BCR  Binding of multivalent antigen cross-links specific B cell Ig receptors  Signaling  B cell clonal expansion  IgM secretion SUMMARY  Humoral immune responses to a protein antigen:  Binding of the protein to specific Ig receptors of naive B cells (lymphoid follicles)  Signaling  Interaction with helper T cells  Internalization and processing of Ag by B cells and presentation to class II MHC activating helper T cells  CD40L expression by helper T cells and cytokines secretion  Stimulate of high levels of B cell proliferation and differentiation  Follicular helper T cells (TFH) effective at stimulating isotype switching and affinity maturation SUMMARY Heavy-chain isotype switching depends on the combination of CD40L and cytokines, both expressed by helper T cells  Different cytokines induce switching to different antibody isotypes Affinity maturation  Initiated by signals from TFH cells  B cells in germinal centers proliferate rapidly, and their Ig V genes undergo extensive somatic mutation  Antigen complexed with secreted antibody is displayed by FDS in the germinal centers  B cells that recognize the antigen with high affinity are selected to survive, giving rise to affinity maturation of the antibody response SUMMARY  The early T-dependent humoral response occurs in extrafollicular foci and generates low levels of antibodies, with little isotype switching, that are produced by short-lived plasma cells.  The later response develops in germinal centers  Isotype switching  Affinity maturation  Generation of long-lived plasma cells  Long-lived memory B cells  T-independent antigens contain multiple identical epitopes that are able to cross-link many Ig receptors on a B cell  shutting off B cell activation