Humoral immunity is mediated by antibodies and is the arm of the adaptive immune response that functions to neutralize and eliminate extracellular microbes and microbial toxins. Humoral immunity is the principal defense mechanism against microbes with capsules rich in polysaccharides and lipids, because antibodies can be produced against polysaccharides and lipids but T cells cannot respond to nonprotein antigens. Antibodies are produced by B lymphocytes and their progeny. Naive B lymphocytes recognize antigens but do not secrete antibodies, and activation of these cells stimulates their differentiation into antibody-secreting plasma cells.
This chapter describes the process and mechanisms of B cell activation and antibody production, focusing on the following questions:
How are antigen receptor–expressing naive B lymphocytes activated and converted to antibody secreting cells?
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?
Chapter 8 describes how the antibodies that are produced during humoral immune responses function to defend individuals against microbes and toxins.
Phases and Types of Humoral Immune Responses
The activation of B lymphocytes results in their proliferation, leading to expansion of antigen-specific clones, and their differentiation into plasma cells, which secrete antibodies ( Fig. 7.1 ). Naive B lymphocytes express two classes of membrane-bound antibodies, immunoglobulins M and D (IgM and IgD), that function as receptors for antigens. These naive B cells are activated by antigen binding to membrane immunoglobulin (Ig) and by other signals discussed later in the chapter. The antibodies secreted in response to an antigen have the same specificity as the surface receptors on naive B cells that recognize that antigen in order to initiate the response. One activated B cell may generate a few thousand plasma cells, each of which can produce copious amounts of antibody, in the range of several thousand molecules per hour. In this way, humoral immunity can keep pace with rapidly proliferating microbes. During their differentiation, some B cells may begin to produce antibodies of different heavy-chain isotypes (or classes) that mediate different effector functions and are specialized to combat different types of microbes. This process is called heavy-chain isotype (or class) switching. During the course of a B cell response to an infection, the affinity of antibodies specific for microbial proteins increases over time. This process is called affinity maturation, and it leads to the production of antibodies with improved capacity to bind to and neutralize microbes and their toxins.
Antibody responses to different antigens are classified as T-dependent or T-independent, based on the requirement for T cell help ( Fig. 7.2 ). B lymphocytes recognize and are activated by a wide variety of chemically distinct antigens, including proteins, polysaccharides, lipids, nucleic acids, and small chemicals. Helper T lymphocytes play an important role in B cell activation by protein antigens. (The designation helper came from the discovery that some T cells stimulate, or help, B lymphocytes to produce antibodies.) T cells help B cells respond to only protein antigens because T cells can only recognize peptides derived from proteins presented as peptide–major histocompatibility complex (MHC) complexes. In the absence of T cell help, most protein antigens elicit weak or no antibody responses. Therefore, protein antigens and the antibody responses to these antigens are called T-dependent. Polysaccharides, nucleic acids, lipids, and other multivalent antigens (which contain the same structural unit repeated multiple times in tandem) can stimulate antibody production without the involvement of helper T cells. Therefore, these multivalent nonprotein antigens and the antibody responses to them are called T-independent. The antibodies produced in response to proteins exhibit more isotype switching and affinity maturation than antibodies against T-independent antigens because helper T cells stimulate these processes. Furthermore, T-dependent antigens stimulate the generation of long-lived plasma cells and memory B cells. Thus, the most specialized and long-lived antibody responses involve protein antigens and are generated under the influence of helper T cells, whereas T-independent responses are relatively simple and transient, and involve only the direct activation of B cells by antigens.
Different subsets of B cells respond preferentially to T-dependent and T-independent antigens (see Fig. 7.2 ). The majority of B cells are called follicular B cells because they reside in and circulate through the follicles of lymphoid organs (see Chapter 1 ). These follicular B cells make the bulk of T-dependent, class-switched, and high-affinity antibody responses to protein antigens and give rise to long-lived plasma cells. Marginal-zone B cells , which are located in the peripheral region of the splenic white pulp and also in the outer rim of follicles in lymph nodes, respond largely to blood-borne polysaccharide and lipid antigens; B-1 cells respond to multivalent antigens in the mucosal tissues and peritoneum. Marginal-zone B cells and B-1 cells express antigen receptors of limited diversity and make predominantly T-independent IgM responses. IgM antibodies may be produced spontaneously by B-1 cells, without overt immunization. These antibodies, called natural antibodies , may help to clear some cells that die by apoptosis during normal cell turnover and may also provide protection against some bacterial pathogens.
Antibody responses generated during the first exposure to an antigen, called primary responses, differ quantitatively and qualitatively from responses to subsequent exposures, called secondary responses ( Fig. 7.3 ). The amounts of antibody produced in the primary immune response are smaller than the amounts produced in secondary responses. In secondary responses to protein antigens, there is increased heavy-chain isotype switching and affinity maturation, because repeated stimulation by a protein antigen leads to an increase in the number and activity of helper T lymphocytes.
With this introduction, we now discuss B cell activation and antibody production, beginning with the responses of B cells to the initial encounter with antigen.
Stimulation of B Lymphocytes by Antigen
Humoral immune responses are initiated when antigen-specific B lymphocytes in the spleen, lymph nodes, and mucosal lymphoid tissues recognize antigens . Some of the antigens in tissues or in the blood are transported to and concentrated in the B cell–rich follicles and marginal zones of these peripheral lymphoid organs. In lymph nodes, macrophages lining the subcapsular sinus may capture antigens and take them to the adjacent follicles, where the bound antigens are displayed to B cells. B lymphocytes specific for an antigen use their membrane-bound Ig as receptors that recognize the antigen directly, without any need for processing of the antigen. B cells are capable of recognizing the native antigen, so the antibodies that are subsequently secreted (which have the same specificity as the B cell antigen receptors) are able to bind to the native microbe or microbial product.
The recognition of antigen triggers signaling pathways that initiate B cell activation. As with T lymphocytes, B cell activation also requires signals in addition to antigen recognition, and many of these second signals are produced during innate immune reactions to microbes. In the following sections, we describe the mechanisms of B cell activation by antigen and other stimuli, followed by a discussion of the functional consequences of antigen recognition.
Antigen-Induced Signaling in B Cells
Antigen-induced clustering of membrane Ig receptors triggers biochemical signals that activate B cells ( Fig. 7.4 ). The process of B lymphocyte activation is, in principle, similar to the activation of T cells (see Chapter 5 , Fig. 5.9 ). In B cells, antigen receptor–mediated signal transduction requires the bringing together (cross-linking) of two or more membrane Ig molecules. Antigen receptor cross-linking occurs when two or more antigen molecules in an aggregate, or repeating epitopes of one antigen molecule, bind to adjacent membrane Ig molecules of a B cell. Polysaccharides, lipids, and other nonprotein antigens often contain multiple identical epitopes in each molecule and are therefore able to bind to numerous Ig receptors on a B cell at the same time. Even protein antigens may be expressed in an array on the surface of microbes and are thus able to cross-link antigen receptors of a B cell.
Signals initiated by antigen receptor cross-linking are transduced by receptor-associated proteins . Membrane IgM and IgD, the antigen receptors of naive B lymphocytes, have highly variable extracellular antigen-binding regions (see Chapter 4 ). However, these membrane receptors have short cytoplasmic tails, so although they recognize antigens, they do not themselves transduce signals. The receptors are noncovalently associated with two proteins, called Igα and Igβ, to form the B cell receptor (BCR) complex , analogous to the T cell receptor (TCR) complex of T lymphocytes. The cytoplasmic domains of Igα and Igβ each contain a conserved immunoreceptor tyrosine-based activation motif (ITAM), similar to those found in signaling subunits of many other activating receptors in the immune system (e.g., CD3 and ζ proteins of the TCR complex; see Chapter 5 ). When two or more antigen receptors of a B cell are brought together by antigen-induced cross-linking, the tyrosines in the ITAMs of Igα and Igβ are phosphorylated by tyrosine kinases associated with the BCR complex. These phosphotyrosines recruit the Syk tyrosine kinase (equivalent to ZAP-70 in T cells), which is activated and in turn phosphorylates tyrosine residues on adaptor proteins. These phosphorylated proteins then recruit and activate a number of downstream molecules, mainly enzymes that initiate signaling cascades that activate transcription factors.
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. Some of the important proteins are described below.
Role of Innate Immune Signals in B Cell Activation
B lymphocytes express a receptor for a complement system protein that provides second signals for the activation of these cells ( Fig. 7.5A ). The complement system, introduced in Chapter 2 , is a collection of plasma proteins that are activated by microbes and by antibodies attached to microbes and function as effector mechanisms of host defense (see Chapter 8 ). When the complement system is activated by a microbe as part of the innate immune response, the microbe becomes coated with proteolytic fragments of the most abundant complement protein, C3. One of these fragments is called C3d. B lymphocytes express a receptor for C3d called complement receptor type 2 (CR2, or CD21). B cells that are specific for a microbe’s antigens recognize the antigens by their BCRs and simultaneously recognize the bound C3d via the CR2 receptor. Engagement of CR2 greatly enhances antigen-dependent activation responses of B cells by enhancing tyrosine phosphorylation of ITAMs. This role of complement in humoral immune responses illustrates the fundamental tenet of the two-signal hypothesis that was introduced in Chapter 2 , that microbes or innate immune responses to microbes provide signals in addition to antigen that are necessary for lymphocyte activation. In humoral immunity, complement activation represents one way in which innate immunity facilitates B lymphocyte activation.
Microbial products also directly activate B cells by engaging innate pattern recognition receptors (see Fig. 7.5B ). B lymphocytes, similar to dendritic cells and other leukocytes, express numerous Toll-like receptors (TLRs; see Chapter 2 ). Pathogen-associated molecular patterns bind to TLRs on the B cells, which triggers activating signals that work in concert with signals from the antigen receptor. This combination of signals stimulates B cell proliferation, differentiation, and Ig secretion, thus promoting antibody responses against microbes.
Functional Consequences of B Cell Activation by Antigen
B cell activation by multivalent antigen (and other signals) may initiate the proliferation and differentiation of the cells and prepares them to interact with helper T lymphocytes if the antigen is a protein ( Fig. 7.6 ). The activated B lymphocytes may begin to synthesize more IgM and to produce some of this IgM in a secreted form. Thus, antigen stimulation induces the early phase of the humoral immune response. 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, as discussed later, but not most soluble proteins. Therefore, by themselves, protein antigens typically do not stimulate high levels of B cell proliferation and differentiation. However, protein antigens induce changes in B cells that enhance their ability to interact with helper T lymphocytes.
Activated B cells endocytose protein antigen that binds specifically to the BCR, resulting in degradation of the antigen and display of peptides bound to class II MHC molecules, which can be recognized by helper T cells. Activated B cells migrate out of the follicles and toward the anatomic compartment where helper T cells are concentrated. Thus, the B cells are poised to interact with and respond to helper T cells, which were derived from naive T cells previously activated by the same antigen presented by dendritic cells.
The next section describes the interactions of helper T cells with B lymphocytes in antibody responses to T-dependent protein antigens. Responses to T-independent antigens are discussed at the end of the chapter.
Functions of Helper T Lymphocytes in Humoral Immune Responses
For a protein antigen to stimulate an antibody response, B lymphocytes and helper T lymphocytes specific for that antigen must come together in lymphoid organs and interact in a way that stimulates B cell proliferation and differentiation. We know this process works efficiently because protein antigens elicit antibody responses within 3 to 7 days after antigen exposure. The efficiency of antigen-induced T-B cell interaction raises many questions. How do B cells and T cells specific for epitopes of the same antigen find one another, considering that naive B and T 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? As discussed next, the answers to these questions are now well understood.
The process of T-B cell interaction and T cell–dependent antibody responses is initiated by recognition of different epitopes of the same protein antigen by the two cell types and occurs in a series of sequential steps ( Fig. 7.7 ):
Naive CD4 + T cells are activated in the T cell zone of a secondary lymphoid organ by antigen (in the form of processed peptides bound to class II MHC molecules) presented by dendritic cells, and differentiate into functional (cytokine-producing) helper T cells.
Naive B cells are activated in the follicles of the same lymphoid organ by an exposed epitope on the same protein (in its native conformation) that is transported to the follicle.
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.
Some of the cells migrate back into follicles to form germinal centers, where the more specialized antibody responses are induced.