Chapter 12 B Cell Activation and Antibody Production

• Humoral immunity is mediated by secreted antibodies, which are produced by cells of the B lymphocyte lineage • Two types of microbial antigens can induce robust antibody responses: – First, multivalent antigens of microbial origin can activate B cells through the B cell receptor (BCR), often accompanied by signals provided by engagement of pattern recognition receptors (PRR) on B cells by microbial products, but without T cell help (T independent antigen) – Second, microbial protein antigens can be presented by B cells to helper T cells, resulting in T-dependent responses in which helper T cells drive B cell activation (T dependent antigen)

Phases of the humoral immune response

General Feature of Humoral Immune Responses • The type and amount of antibodies produced vary according to the: type of antigen driving the immune response, the involvement of T cells, a prior history of antigen exposure, and the anatomic site at which activation occurs • Antibody responses to protein antigens require that the antigen be specifically recognized and internalized by B cells and that a peptide fragment of the internalized protein be presented to CD 4+ helper T lymphocytes that then activate these B cells • Antibody responses to multivalent non-protein antigens with repeating determinants, such as polysaccharides, some lipids, and nucleic acids, do not require antigen specific helper T lymphocytes (thymus independent or Tindependent antigens)

• Some of the progeny of activated B cells are long-lived antibody-secreting plasma cells, which continue to produce antibodies for months or years, and others are long-lived memory cells • Heavy chain isotype switching and affinity maturation are typically seen in helper T cell–dependent humoral immune responses to protein antigens (CD 40 ligand (CD 40 L) on the surface of activated helper T cells and cytokines Secreted and CD 40 on B cells

• Primary and secondary antibody responses to protein antigens differ qualitatively and quantitatively

• Distinct subsets of B cells respond preferentially to different types of antigens

• ANTIGEN RECOGNITION AND ANTIGEN-INDUCED B CELL ACTIVATION

Antigen Recognition and Antigen-induced B Cell Activation • Step 1. Antigen Capture and Delivery to B Cells: • Mature B lymphocytes (Naive B cells) migrate from one secondary lymphoid organ to the next in search of antigen • Entry into the follicles is guided by the chemokine CXCL 13 (binds to the CXCR 5 on naïve B cell) secreted by follicular dendritic cells (FDC) and other stromal cells in the follicle • Antigen may be delivered to naive B cells in lymphoid organs in different forms and by multiple routes

Pathways of antigen delivery to follicular B cells • Most antigens from tissue sites are transported to lymph nodes by afferent lymphatic vessels that drain into the subcapsular sinus of the nodes. Soluble antigens, generally smaller than 70 k. D, may reach the B cell zone through conduits that extend between the subcapsular sinus and the follicle and interact directly with specific B cells • Subcapsular sinus macrophages capture large microbes and antigen-antibody complexes and deliver these to follicles, which lie under the sinus • Many relatively large antigens that enter the node through afferent lymphatic vessels are not captured by subcapsular sinus macrophages but are too large to enter the conduits

• Antigens in immune complexes may bind to complement receptors (in particular the complement receptor type 2 or CR 2) on marginal zone B cells, and these cells can transfer the immune complex–containing antigens to follicular B cells • Antigen in immune complexes may also bind to CR 2 on the surface of follicular dendritic cells (FDC) and be presented to antigen-specific B cells • Blood-borne pathogens may be captured by plasmacytoid dendritic cells in the blood and transported to the spleen, where they may be delivered to marginal zone B cells • Polysaccharide antigens can be captured by macrophages in the marginal zone of splenic lymphoid follicles and displayed or transferred to B cells in this area

Step 2. Activation of B Cells by Antigens and Other Signals • The activation of antigen-specific B lymphocytes is initiated by the binding of antigen to membrane Ig molecules, which, in conjunction with the associated Igα and Igβ proteins, make up the antigen receptor complex of mature B cells • B cell activation is facilitated by the CR 2/CD 21 coreceptor on B cells, which recognizes complement fragments covalently attached to the antigen or that are part of immune complexes containing the antigen • Microbial products engage Toll-like receptors (TLR 5, 7) on B cells, which also enhances B cell activation

Functional Responses of B Cells to Antigens

HELPER T CELL–DEPENDENT ANTIBODY RESPONSES TO PROTEIN ANTIGENS • Antibody responses to protein antigens require recognition and processing of the antigen by B cells, followed by presentation of a peptide fragment of the antigen to helper T cells, leading to cooperation between the antigen specific B and T lymphocytes

Initial Activation and Migration of Helper T Cells and B Cells

Role of CD 40 L: CD 40 Interaction in T-Dependent B Cell Activation

Extrafollicular B Cell Activation

The Germinal Center B Cell Reaction and the Function of Follicular Helper T Cells

Molecular events in follicular helper T cell generation

T follicular helper cell

Follicular Dendritic Cells (FDCs) • FDCs are found only in lymphoid follicles and express complement receptors (CR 1, CR 2, and CR 3) and Fc receptors, are involved in displaying antigens for the selection of germinal center B cells • FDCs do not express class II MHC molecules and are not derived from progenitors in the bone marrow • The rim of naive B cells in the follicle, surrounding the germinal center, is called the mantle zone

Heavy Chain Isotype (Class) Switching

• The key enzyme required for isotype switching (and somatic mutation) is activationinduced deaminase (AID). Humans and knockout mice lacking this enzyme have profound defects in isotype switching and affinity maturation. AID expression is activated mainly by CD 40 signals. The enzyme deaminates cytosines in single-stranded DNA templates, converting cytosine (C) residues to uracil (U) residues

Affinity Maturation: Somatic Mutation of Ig Genes and Selection of High-Affinity B Cells • Helper T cells and CD 40: CD 40 L interactions are required for somatic mutation to be initiated • In proliferating germinal center B cells in the dark zone, Ig V genes undergo point mutations at an extremely high rate. This rate is estimated to be 1 in 103 V gene base pairs per cell division, which is about a thousand times higher than the spontaneous rate of mutation in other mammalian genes. (For this reason, mutation in Ig V genes is also called hypermutation)

B Cell Differentiation into Antibody-Secreting Plasma Cells

Generation of Memory B Cells and Secondary Humoral Immune Responses • Such vaccines, which are called conjugate vaccines, more readily induce highaffinity antibodies and memory than do polysaccharide vaccines without linked proteins

Antigen Presentation by B Cells and the Hapten-Carrier Effect

ANTIBODY RESPONSES TO T CELL–INDEPENDENT ANTIGENS

Nature of B Cells That Respond to T-Independent Antigens • The marginal zone and B-1 subsets of B cells are especially important for antibody responses to TI antigens • Marginal zone B cells are a distinct population of B cells that mainly respond to polysaccharides. After activation, these cells differentiate into short-lived plasma cells that produce mainly Ig. M. In humans these cells are also called Ig. M memory cells • B-1 cells represent another lineage of B cells that responds readily to TI antigens mainly in the peritoneum and in mucosal sites

Functions of T-Independent Antibody Responses • The practical significance of TI antigens is that many bacterial cell wall polysaccharides belong to this category, and humoral immunity is the major mechanism of host defense against infections by such encapsulated bacteria • TI antigens contribute to the generation of natural antibodies, which are present in the circulation of normal individuals and are apparently produced without overt exposure to pathogens. Most natural antibodies are low-affinity anticarbohydrate antibodies, postulated to be produced by B-1 peritoneal B cells stimulated by bacteria that colonize the gastrointestinal tract and by marginal zone B cells in the spleen. Antibodies to the A and B glycolipid blood group antigens are examples of these natural antibodies

ANTIBODY FEEDBACK: REGULATION OF HUMORAL IMMUNE RESPONSES BY Fc RECEPTORS