Move the Characteristics to Their Correct Category to Review the Properties of B Cells and T Cells

The surface immunoglobulin that serves as the B-prison cell antigen receptor (BCR) has two roles in B-prison cell activation. Start, similar the antigen receptor on T cells, it transmits signals directly to the jail cell's interior when it binds antigen (run into Section 6-1). Second, the B-cell antigen receptor delivers the antigen to intracellular sites where it is degraded and returned to the B-cell surface as peptides leap to MHC class Ii molecules (run across Chapter 5). The peptide:MHC grade Two complex can be recognized by antigen-specific armed helper T cells, stimulating them to make proteins that, in plow, cause the B cell to proliferate and its progeny to differentiate into antibody-secreting cells. Some microbial antigens can activate B cells straight in the absenteeism of T-cell help. The ability of B cells to respond directly to these antigens provides a rapid response to many important bacterial pathogens. However, somatic hypermutation and switching to sure immunoglobulin isotypes depend on the interaction of antigen-stimulated B cells with helper T cells and other cells in the peripheral lymphoid organs. Antibodies induced by microbial antigens alone are therefore less variable and less functionally versatile than those induced with T-cell help.

9-one. The humoral allowed response is initiated when B cells that bind antigen are signaled by helper T cells or by certain microbial antigens alone

Information technology is a general rule in adaptive immunity that naive antigen-specific lymphocytes are difficult to actuate past antigen lone. Naive T cells require a co-stimulatory signal from professional person antigen-presenting cells; naive B cells require accessory signals that tin come either from an armed helper T cell or, in some cases, straight from microbial constituents.

Antibody responses to protein antigens require antigen-specific T-jail cell help. B cells can receive help from armed helper T cells when antigen bound by surface immunoglobulin is internalized and returned to the cell surface every bit peptides bound to MHC class II molecules. Armed helper T cells that recognize the peptide:MHC complex then evangelize activating signals to the B jail cell. Thus, poly peptide antigens bounden to B cells both provide a specific signal to the B cell by cross-linking its antigen receptors and allow the B cell to attract antigenspecific T-cell help. These antigens are unable to induce antibiotic responses in animals or humans who lack T cells, and they are therefore known as thymus-dependent or TD antigens (Fig. 9.ii, top ii panels).

Figure ix.2

A 2d signal is required for B-jail cell activation by either thymus-dependent or thymus-independent antigens. The beginning signal required for B-cell activation is delivered through its antigen receptor (meridian panel). For thymus-dependent antigens, the second (more...)

The B-cell co-receptor circuitous of CD19:CD21:CD81 (run into Section half-dozen-8) can greatly enhance B-cell responsiveness to antigen. CD21 (too known as complement receptor 2, CR2) is a receptor for the complement fragment C3d (meet Section 2-11). When mice are immunized with hen egg lysozyme coupled to iii linked molecules of the complement fragment C3dg, the modified lysozyme induces antibody without added adjuvant at doses upward to 10,000 times smaller than unmodified hen egg lysozyme. Whether binding of CD21 enhances B-jail cell responsiveness past increasing B-cell signaling, by inducing co-stimulatory molecules on the B cell, or past increasing the receptormediated uptake of antigen, is non yet known. As we volition run across later in this affiliate, antibodies already bound to antigens can activate the complement system, thus coating the antigen with C3d and producing a more potent antigen, which in turn leads to more than efficient B-cell activation and antibody production.

Although armed peptide-specific helper T cells are required for B-cell responses to poly peptide antigens, many microbial constituents, such equally bacterial polysaccharides, tin can induce antibody product in the absence of helper T cells. These microbial antigens are known as thymus-independent or TI antigens because they induce antibody responses in individuals who have no T lymphocytes. The second signal required to activate antibody product to TI antigens is either provided directly by recognition of a mutual microbial constituent (encounter Fig. nine.ii, lesser panel) or past a nonthymus-derived accessory prison cell in conjunction with massive cross-linking of B-prison cell receptors, which would occur when a B jail cell binds repeating epitopes on the bacterial cell. Thymus-independent antibiotic responses provide some protection confronting extracellular bacteria, and we will render to them later.

9-2. Armed helper T cells activate B cells that recognize the same antigen

T-cell dependent antibody responses require the activation of B cells by helper T cells that answer to the same antigen; this is called linked recognition. This means that before B cells can exist induced to make antibody to an infecting pathogen, a CD4 T cell specific for peptides from this pathogen must get-go be activated to produce the appropriate armed helper T cells. This presumably occurs past interaction with an antigen-presenting dendritic jail cell (see Department 8-1). Although the epitope recognized by the armed helper T jail cell must therefore be linked to that recognized past the B prison cell, the 2 cells demand not recognize identical epitopes. Indeed, nosotros saw in Chapter v that T cells tin recognize internal peptides that are quite singled-out from the surface epitopes on the same protein recognized by B cells. For more complex natural antigens, such as viruses, the T cell and the B cell might not even recognize the aforementioned protein. It is, however, crucial that the peptide recognized past the T cell be a physical function of the antigen recognized past the B jail cell, which can thus produce the appropriate peptide after internalization of the antigen bound to its B-prison cell receptors.

For example, past recognizing an epitope on a viral poly peptide glaze, a B cell can internalize a complete virus particle. Subsequently internalization, the virus particle is degraded and peptides from internal viral proteins besides as coat proteins can exist displayed past MHC class II molecules on the B-cell surface. Helper T cells that accept been primed earlier in an infection past macrophages or dendritic cells presenting these internal peptides tin can so activate the B cell to make antibodies that recognize the glaze protein (Fig. 9.three).

Effigy 9.3

B cells and helper T cells must recognize epitopes of the same molecular complex in order to interact. An epitope on a viral coat protein is recognized by the surface immunoglobulin on a B prison cell and the virus is internalized and degraded. Peptides derived (more...)

The specific activation of the B jail cell past a T cell sensitized to the same antigen or pathogen depends on the power of the antigen-specific B cell to concentrate the appropriate peptide on its surface MHC class Two molecules. B cells that bind a particular antigen are up to 10,000 times more efficient at displaying peptide fragments of that antigen on their MHC form II molecules than are B cells that do not demark the antigen. Armed helper T cells volition thus help only those B cells whose receptors bind an antigen containing the peptide they recognize.

The requirement for linked recognition has important consequences for the regulation and manipulation of the humoral immune response. I is that linked recognition helps ensure self tolerance, equally volition be described in Affiliate xiii. An important awarding of linked recognition is in the design of vaccines, such every bit that used to immunize infants against Haemophilus influenzae type B. This bacterial pathogen can infect the lining of the brain, called the meninges, causing meningitis and, in severe cases, neurological damage or death. Protective amnesty to this pathogen is mediated past antibodies against its capsular polysaccharide. Although adults make very effective thymus-contained responses to these polysaccharide antigens, such responses are weak in the immature immune system of the infant. To make an effective vaccine for employ in infants, therefore, the polysaccharide is linked chemically to tetanus toxoid, a foreign poly peptide against which infants are routinely and successfully vaccinated (see Affiliate 14). B cells that demark the polysaccharide component of the vaccine tin be activated by helper T cells specific for peptides of the linked toxoid (Fig. 9.iv).

Figure nine.four

Protein antigens attached to polysaccharide antigens allow T cells to help polysaccharide-specific B cells. Haemophilus influenzae type B vaccine is a conjugate of bacterial polysaccharide and the tetanus toxoid protein. The B cell recognizes and binds (more than...)

Linked recognition was originally discovered through studies of the production of antibodies to haptens (see Appendix I, Section A-1). Haptens are small chemical groups that cannot elicit antibiotic responses on their own because they cannot cantankerous-link B-cell receptors and they cannot recruit T-prison cell help. When coupled at high density to a carrier protein, however, they become immunogenic, because the protein will carry multiple hapten groups that tin now cross-link B-prison cell receptors. In improver, T-prison cell dependent responses are possible because T cells can be primed to peptides derived from the protein. Coupling of a hapten to a protein is responsible for the allergic responses shown past many people to the antibiotic penicillin, which reacts with host proteins to course a coupled hapten that tin stimulate an antibody response, equally we will learn in Affiliate 12.

9-3. Antigenic peptides bound to cocky MHC class Ii molecules trigger armed helper T cells to brand membrane-bound and secreted molecules that can activate a B cell

Armed helper T cells activate B cells when they recognize the appropriate peptide:MHC class 2 complex on the B-cell surface (Fig. 9.5). As with armed T_H1 cells interim on macrophages, recognition of peptide:MHC class 2 complexes on B cells triggers armed helper T cells to synthesize both cellbound and secreted effector molecules that synergize in activating the B cell. Ane specially important T-cell effector molecule is a membrane-bound molecule of the tumor necrosis factor (TNF) family unit known every bit CD40 ligand (CD40L, also known as CD154) because it binds to the B-prison cell surface molecule CD40. CD40 is a member of the TNF-receptor family of cytokine receptors (see Section 8-xx) however, it does non contain a 'decease domain.' Information technology is involved in directing all phases of the B-cell response. Binding of CD40 by CD40L helps to drive the resting B cell into the prison cell cycle and is essential for B-cell responses to thymus-dependent antigens.

Figure ix.v

Armed helper T cells stimulate the proliferation so the differentiation of antigen-binding B cells. The specific interaction of an antigen-bounden B cell with an armed helper T cell leads to the expression of the B-cell stimulatory molecule CD40 (more...)

B cells are stimulated to proliferate in vitro when they are exposed to a mixture of artificially synthesized CD40L and the cytokine interleukin-iv (IL-iv). IL-iv is likewise made by armed T_Htwo cells when they recognize their specific ligand on the B-cell surface, and IL-four and CD40L are idea to synergize in driving the clonal expansion that precedes antibiotic production in vivo. IL-4 is secreted in a polar manner by the T_H2 cell and is directed at the site of contact with the B jail cell (Fig. 9.6) and so that it acts selectively on the antigen-specific target B prison cell.The combination of B-jail cell receptor and CD40 ligation, forth with IL-iv and other signals derived from direct T-cell contact, leads to B-cell proliferation. Some of these contact signals have recently been elucidated. They involve other TNF/TNF-receptor family unit members, including CD30 and CD30 ligand and BLyS (B lymphocyte stimulator) and its receptor on B cells, TACI. Later on several rounds of proliferation, B cells tin can further differentiate into antibiotic-secreting plasma cells. Ii boosted cytokines, IL-five and IL-6, both secreted by helper T cells, contribute to these later on stages of B-cell activation.

Figure 9.6

When an armed helper T cell encounters an antigen-binding B cell, it becomes polarized and secretes IL-4 and other cytokines at the point of cell-cell contact. On binding antigen on the B cell through its T-jail cell receptor, the helper T cell is induced (more...)

nine-iv. Isotype switching requires expression of CD40L past the helper T prison cell and is directed past cytokines

Antibodies are remarkable not just for the diversity of their antigen-binding sites simply also for their versatility equally effector molecules. The specificity of an antibody response is determined by the antigen-binding site, which consists of the two variable 5 domains, 5_H and 5_L; however, the effector action of the antibody is determined by the isotype of its heavy-concatenation C region (meet Department 4-15). A given heavy-concatenation V domain can become associated with the C region of whatsoever isotype through the process of isotype switching (run across Section four-16). We volition see later on in this chapter how antibodies of each isotype contribute to the elimination of pathogens. The Deoxyribonucleic acid rearrangements that underlie isotype switching and confer this functional diversity on the humoral allowed response are directed by cytokines, especially those released by armed effector CD4 T cells.

All naive B cells express prison cell-surface IgM and IgD, nevertheless IgM makes upward less than ten% of the immunoglobulin found in plasma, where the most arable isotype is IgG. Much of the antibody in plasma has therefore been produced by B cells that have undergone isotype switching. Piddling IgD antibiotic is produced at whatsoever time, so the early stages of the antibody response are dominated by IgM antibodies. Afterwards, IgG and IgA are the predominant isotypes, with IgE contributing a small but biologically important part of the response. The overall predominance of IgG results, in office, from its longer lifetime in the plasma (encounter Fig. four.16).

Isotype switching does not occur in individuals who lack functional CD40L, which is necessary for productive interactions betwixt B cells and helper T cells; such individuals make only small amounts of IgM antibodies in response to thymus-dependent antigens and have abnormally loftier levels of IgM ( Hyper IgM Immunodeficiency, in Case Studies in Immunology, see Preface for details) in their plasma. These IgM antibodies may be induced by thymus-independent antigens expressed by the pathogens that chronically infect these patients, who endure from severe humoral immunodeficiency, equally we will run across in Affiliate 11.

Almost of what is known about the regulation of isotype switching by helper T cells has come from experiments in which mouse B cells are stimulated with bacterial lipopolysaccharide (LPS) and purified cytokines in vitro. These experiments bear witness that dissimilar cytokines preferentially induce switching to different isotypes. Some of these cytokines are the same as those that bulldoze B-cell proliferation in the initiation of a B-cell response. In the mouse, IL-iv preferentially induces switching to IgG1 and IgE, whereas transforming growth factor (TGF)-β induces switching to IgG2b and IgA. T_H2 cells brand both of these cytokines as well as IL-five, which induces IgA secretion by cells that take already undergone switching. Although T_Hone cells are relatively poor initiators of antibiotic responses, they participate in isotype switching by releasing interferon (IFN)-γ, which preferentially induces switching to IgG2a and IgG3. The part of cytokines in directing B cells to make the dissimilar antibody isotypes is summarized in Fig. 9.7.

Figure 9.seven

Unlike cytokines induce switching to dissimilar isotypes. The individual cytokines induce (violet) or inhibit (cherry-red) product of sure isotypes. Much of the inhibitory effect is probably the result of directed switching to a different isotype. These (more...)

Cytokines induce isotype switching by stimulating the formation and splicing of mRNA transcribed from the switch recombination sites that lie 5′ to each heavy-chain C gene (run into Fig. 4.20). When activated B cells are exposed to IL-iv, for case, transcription from a site upstream of the switch regions of C_γ1 and C_ε can be detected a 24-hour interval or two before switching occurs (Fig. nine.8). Contempo information suggest that the product of a spliced switch transcript has a role in directing switching, but the mechanism is not yet clear. Each of the cytokines that induces switching seems to induce transcription from the switch regions of ii different heavy-chain C genes, promoting specific recombination to one or other of these genes only. Such a directed mechanism is supported by the observation that private B cells often undergo switching to the same C cistron on both chromosomes, even though the antibody heavy chain is only being expressed from i of the chromosomes. Thus, helper T cells regulate both the product of antibody past B cells and the isotype that determines the effector role of the antibody.

Figure 9.8

Isotype switching is preceded by transcriptional activation of heavy-concatenation C-region genes. Resting naive B cells transcribe the μ and δ genes at a low rate, giving rise to surface IgM and IgD. Bacterial lipopolysaccharide (LPS), which (more...)

nine-5. Antigen-binding B cells are trapped in the T-cell zone of secondary lymphoid tissues and are activated by see with armed helper T cells

One of the most puzzling features of the antibiotic response is how an antigenspecific B cell manages to run across a helper T cell with an appropriate antigen specificity. This question arises because the frequency of naive lymphocytes specific for any given antigen is estimated to exist betwixt 1 in 10,000 and one in 1,000,000. Thus, the chance of an encounter betwixt a T lymphocyte and a B lymphocyte that recognize the same antigen should be betwixt 1 in 10⁸ and 1 in 10¹². Achieving such an run across is a far more difficult challenge than getting effector T cells activated, because, in the latter case, only one of the 2 cells involved has specific receptors. Moreover, T cells and B cells generally occupy quite singled-out zones in peripheral lymphoid tissue (run across Fig. ane.viii). Every bit in naive T-cell activation (meet Chapter 8), the answer seems to lie in the antigen-specific trapping of migrating lymphocytes.

When an antigen is introduced into an animate being, it is captured and candy by professional antigen-presenting cells, peculiarly the dendritic cells that migrate from the tissues into the T-cell zones of local lymph nodes. Recirculating naive T cells laissez passer by such cells continuously and those rare T cells whose receptors demark peptides derived from the antigen are trapped very efficiently. This trapping clearly involves the specific antigen receptor on the T cell, although it is stabilized by the activation of adhesion molecules and chemokines as we learned in Sections viii-iii and eight-4. Ingenious experiments using mice transgenic for rearranged immunoglobulin genes show that, in the presence of the appropriate antigen, B cells with antigen-specific receptors are as well trapped in the T-cell zones of lymphoid tissue past a like mechanism. On encountering antigen, migrating antigen-bounden B cells are arrested by the activation of adhesion molecules and the date of chemokine receptors such as CCR7, a receptor for MIP-3β and SLC.

Trapping of B cells in the T-prison cell zones provides an elegant solution to the problem posed at the commencement of this section. T cells are themselves trapped and activated to helper status in the T-cell zones, and when B cells migrate into lymphoid tissue through high endothelial venules they first enter these same T-prison cell zones. Most of the B cells move quickly through the T-cell zone into the B-cell zone (the main follicle), merely those B cells that accept bound antigen are trapped. Thus, antigen-binding B cells are selectively trapped in precisely the correct location to maximize the chance of encountering a helper T prison cell that can actuate them. Interaction with armed helper T cells activates the B cell to establish a primary focus of clonal expansion (Fig. 9.9). Hither, at the border between T-cell and B-prison cell zones, both types of lymphocyte volition proliferate for several days to constitute the first phase of the primary humoral immune response.

Figure 9.nine

Antigen-binding cells are trapped in the T-cell zone. Upon entry into lymphoid tissues through a loftier endothelial venule (HEV), T cells and B cells domicile to different regions, as described in Affiliate vii. Antigen-specific T cells remain in the T-prison cell zone (more...)

Subsequently several days, the primary focus of proliferation begins to involute. Many of the lymphocytes comprising the focus undergo apoptosis. All the same, some of the proliferating B cells differentiate into antibodysynthesizing plasma cells and migrate to the crimson pulp of the spleen or the medullary cords of the lymph node. The differentiation of a B cell into a plasma cell is accompanied by many morphological changes that reflect its commitment to the product of large amounts of secreted antibody. The backdrop of resting B cells and plasma cells are compared in Fig. 9.10. Plasma cells have arable cytoplasm dominated by multiple layers of rough endoplasmic reticulum (run into Fig. ane.19). The nucleus shows a feature design of peripheral chromatin condensation, a prominent perinuclear Golgi apparatus is visible, and the cisternae of the endoplasmic reticulum are rich in immunoglobulin, which makes upwardly 10–twenty% of all the protein synthesized. MHC class Ii molecules are not expressed, and so plasma cells can no longer present antigen to helper T cells, although these T cells may all the same provide important signals for plasma jail cell differentiation and survival, like IL-6 and CD40L. Surface immunoglobulin is even so expressed on plasma cells at low levels, and recent evidence suggests that the survival of plasma cells may exist determined in part by their ability to continue to demark antigen. Plasma cells accept a range of life-spans. Some survive for only days to a few weeks subsequently their last differentiation, whereas others are very long-lived and account for the persistence of antibiotic responses.

Figure 9.10

Plasma cells secrete antibody at a loftier charge per unit but tin no longer respond to antigen or helper T cells. Resting naive B cells carry surface immunoglobulin (usually IgM and IgD) and MHC form II molecules on their surface. Their Five genes do not carry somatic (more than...)

9-6. The second stage of the primary B-jail cell immune response occurs when activated B cells migrate to follicles and proliferate to form germinal centers

There is another fate for some of the B cells and T cells that proliferate in the primary focus. Some of these cells migrate into a primary lymphoid follicle (Fig. 9.11) where they continue to proliferate and ultimately form a germinal center (Fig. nine.12). Germinal centers are composed mainly of proliferating B cells, merely antigen-specific T cells brand upwardly about ten% of germinal center lymphocytes and provide indispensable assistance to the B cells. The germinal centre is substantially an isle of prison cell division that sets upwardly amidst a bounding main of resting B cells in the principal follicles; germinal eye B cells readapt the resting B cells toward the periphery of the follicle, forming a drapery zone of resting cells around the center. Principal follicles comprise resting B cells amassed around a dumbo network of processes extending from a specialized prison cell type, the follicular dendritic cell (FDC). Follicular dendritic cells attract both naive and activated B cells into the follicles by secreting the chemokine BLC (see Section 7-xxx).

Figure ix.eleven

Activated B cells course germinal centers in lymphoid follicles. Some B cells activated in the primary focus drift to grade a germinal heart inside a principal follicle. Germinal centers are sites of rapid B-cell proliferation and differentiation. Follicles in (more than...)

Figure 9.12

Germinal centers are formed when activated B cells enter lymphoid follicles. The germinal heart is a specialized microenvironment in which B-jail cell proliferation, somatic hypermutation, and selection for antigen bounden all occur. Chop-chop proliferating (more...)

The early events in the primary focus pb to the prompt secretion of specific antibiotic that serves as firsthand protection to the infected individual. The germinal center reaction, on the other manus, provides for a more constructive afterward response, should the pathogen plant a chronic infection or the host become reinfected. To this cease, B cells undergo a number of important modifications in the germinal middle These include somatic hypermutation (see Chapter iv), which alters the V regions of B cells, analogousness maturation, which selects for survival of B cells with loftier affinity for the antigen, and isotype switching (run across Sections 9-4 and 4-xvi), which allows these selected B cells to express a variety of effector functions in the form of antibodies of different isotypes. The selected B cells will either differentiate into memory B cells, the function of which will exist described in Chapter 10, or into plasma cells, which volition brainstorm to secrete higher-analogousness and isotype-switched antibody during the latter role of the primary immune response.

The germinal center is a site of intense jail cell proliferation, with B cells dividing every 6 to 8 hours. Initially, these rapidly proliferating B cells dramatically reduce their expression of surface immunoglobulin, especially of IgD. These B cells are termed centroblasts. As fourth dimension goes on, some B cells reduce their charge per unit of division and begin to express higher levels of surface immunoglobulin. These are termed centrocytes. The centroblasts at commencement proliferate in the night zone of the germinal center (run across Fig. 9.12), so chosen because the proliferating cells are densely packed. With further development, B cells begin to fill the light zone of the germinal center, an area of the follicle that is more than richly supplied with follicular dendritic cells and less densely packed with cells. It was idea originally that only the centroblasts in the dark zone proliferated, whereas centrocytes in the calorie-free zone did not divide. Indeed, this may be the case in chronic germinal centers found in inflamed tonsils that accept been surgically removed. However, in newly forming germinal centers in mice, it is now credible that proliferation can occur in both calorie-free and dark zones, and that proliferative cells in the dark zone can express moderate amounts of immunoglobulin on their surface. Then the distinction betwixt dark and calorie-free zones as areas of B-cell proliferation or quiescence does not strictly employ to chief germinal centers, at to the lowest degree in mice. Follicular dendritic cells, which originally were well-nigh prominent in the lite zone, appear to react to germinal center germination and begin to extend more prominently throughout the germinal heart equally it develops. The event is that a mature germinal center at twenty-four hours 15 after immunization more resembles a light zone, with few of the classic dark zone characteristics. This view of germinal centre evolution may help to explain how B cells with high affinity for immunizing antigen are selected, as we now hash out.

9-vii. Germinal center B cells undergo V-region somatic hypermutation and cells with mutations that improve analogousness for antigen are selected

The procedure of somatic hypermutation, as one of the four mechanisms that create immunoglobulin diverseness, was described in Chapter 4. Hither we draw the signals that initiate hypermutation and the biological consequences of mutation for those cells. Somatic hypermutation is usually restricted to B cells that are proliferating in germinal centers. This was beginning shown by FACS sorting of germinal heart B cells (run into Appendix I, Section A-22) and sequencing of the V genes of cell lines derived from them; after, it was shown more directly by sequencing the V genes that were amplified by PCR of DNA isolated from germinal center B cells that had been micro-dissected from histologic sections. However, in vitro studies accept shown that B cells can be induced to undergo hypermutation outside of germinal centers when their B-jail cell receptors are cantankerous-linked and they receive assistance, including cytokines and CD40L stimulation, from activated T cells. In fact, mice that lack germinal centers attributable to a mutation in the lymphotoxin-α gene (see Department seven-xxx) still back up B-jail cell hypermutation, although where this takes place is unknown.

Unlike the other mechanisms of immunoglobulin diversification (see Department 4-6), which generate B cells with radically differing B-jail cell receptors, somatic hypermutation has the potential to create a series of related B cells that differ subtly in their specificity and analogousness for antigen. This is considering somatic hypermutation generally involves private point mutations that change merely a single amino acid. Immunoglobulin Five-region genes accumulate mutations at a rate of about one base pair change per x³ base of operations pairs per cell division. The mutation rates of all other somatic cell DNA are much lower: around one base pair modify per 10¹⁰ base pairs per cell division. As each of the expressed heavy- and calorie-free-chain Five-region genes is encoded past about 360 base pairs, and about three out of every four base changes results in an contradistinct amino acid, every second B cell volition acquire a mutation in its receptor at each partition. These mutations also impact some DNA flanking the rearranged V gene only they generally do not extend into the C-region exons. Thus, random point mutations are somehow targeted to the rearranged V genes in a B cell.

The point mutations accumulate in a stepwise way as B-cell clones expand in the germinal center. Generally, a B cell volition non larn more than one or two new mutations in each generation. Mutations can touch the ability of a B cell to bind antigen and thus will affect the fate of the B cell in the germinal centre, equally diagrammed in Fig. 9.13. Near mutations have a negative impact on the power of the B-cell receptor to bind the original antigen. For instance, some mutations will abolish receptor office altogether by introducing a stop codon that prevents proper translation; other deleterious mutations alter framework region amino acids that are essential for correct immunoglobulin folding; and notwithstanding others alter amino acids in the complementarity-determining regions that are responsible for contacting antigen. These deleterious mutations are disastrous for the cells that harbor them; these cells are eliminated by apoptosis either because they can no longer make a B-cell receptor or because they cannot compete with sibling cells that bind antigen more strongly. Deleterious mutation is evidently a frequent event, every bit germinal centers are filled with apoptotic B cells that are quickly engulfed past macrophages, resulting in tingible body macrophages, which contain dark-staining nuclear debris in their cytoplasm and are a longrecognized histologic feature of germinal centers.

Effigy 9.13

Subsequently T-jail cell-dependent activation, B cells undergo rounds of mutation and option for higher-affinity mutants in the germinal heart, ultimately resulting in loftier-affinity retentiveness B cells and antibody secreted from plasma cells. B cells are first activated (more than...)

More rarely, mutations will improve the affinity of a B-cell receptor for antigen. Cells that harbor these mutations are efficiently selected and expanded. Whether this is due to prevention of cell death and/or enhancement of cell division is still unclear. In either instance, it is clear that pick is incremental. Afterward each circular of mutation, B cells begin to express the new receptor, and it determines the jail cell's fate, whether favorable or unfavorable. If favorable, the prison cell undergoes another round of division and mutation and the expression and selection process is repeated. In this style, the affinity and specificity of positively selected B cells is continually refined during the germinal center response. The fact that both centroblasts and centrocytes proliferate and tin can limited immunoglobulin explains how mutation and positive selection tin can take identify simultaneously throughout the germinal center without the need for migration dorsum and along between the nighttime and light zones. Evidence of positive and negative selection is seen in the pattern of somatic hyper-mutations in 5 regions of B cells that have survived passage through the germinal middle (see Section 4-9). The existence of negative pick is shown by the relative scarcity of amino acid replacements in the framework regions, reflecting the loss of cells that had mutated any one of the many residues that are disquisitional for immunoglobulin V-region folding. Negative selection is an important force in the germinal center, near likely eliminating about 1 in every two cells. Were information technology not for substantial negative selection, B cells dividing three to four times per day in a single germinal center would quickly create enough progeny to overwhelm the entire organism; more than a billion cells could be created in 10 days in a unmarried germinal heart. Instead, a germinal center actually contains a few thousand B cells at its tiptop.

The marking of positive option, on the other hand, is an aggregating of numerous amino acid replacements in the complementarity-determining regions (encounter Fig. 4.nine). The consequence of these cycles of proliferation, mutation, and option, which all happen within the germinal center, is that the average analogousness of the population of responding B cells for its antigen increases over fourth dimension, largely explaining the observed miracle of analogousness maturation of the antibody response. The selection process can be quite stringent: although fifty to 100 B cells may seed the germinal center, almost of these leave no progeny, and by the time the germinal center reaches maximum size, it is typically composed of the descendants of merely 1 or a few B cells.

9-8. Ligation of the B-cell receptor and CD40, together with direct contact with T cells, are all required to sustain germinal heart B cells

Germinal center B cells are inherently prone to dice and, in gild to survive, they must receive specific signals. It was originally discovered in vitro that germinal center B cells could be kept alive by simultaneously cross-linking their B-cell receptors and ligating their prison cell-surface CD40. In vivo, these signals are delivered by antigen and T cells, respectively. Additional signals are also required for survival, which are delivered by direct contact with T cells. The nature of these signals is still obscure, but one signaling system involving the TNF-family member BLyS (the T-prison cell signal) and TACI (its receptor on B cells) has recently been found to be essential for the maintenance of germinal centers.

The source of antigen in the germinal middle has been the matter of some controversy. Antigen tin be trapped and stored for long periods of fourth dimension in the form of immune complexes on follicular dendritic cells (Figs 9.fourteen and ix.15) and it was therefore assumed that this was the antigen that sustained germinal eye B-prison cell proliferation. While this may be truthful under certain circumstances, in that location is now evidence that antigen on follicular dendritic cells is not required to sustain a normal germinal heart response. Indeed, the function of the antigen depot on these cells is unknown, although it could be to maintain long-lived plasma cells. Where does the antigen that sustains the germinal center come from? Under normal circumstances, it is most likely that live pathogens carried to the lymphoid tissues and multiplying there will keep to provide antigens until they are eliminated past the immune response, afterwards which the germinal center decays. Immunizations with protein antigens are usually given in a form that slowly releases the antigen over fourth dimension, which mimics the situation with live pathogens. Indeed, information technology is difficult to stimulate germinal center formation by immunization without either a alive replicating pathogen or a sustained release of antigen in adjuvant (come across Appendix I, Section A-four).

Figure 9.14

Immune complexes demark to the surface of follicular dendritic cells. Radiolabeled antigen localizes to, and persists in, lymphoid follicles of draining lymph nodes (see light micrograph and the schematic representation below, showing a germinal center (more than...)

Figure 9.fifteen

Immune complexes spring to follicular dendritic cells form iccosomes, which are released and can be taken up by B cells in the germinal center. Follicular dendritic cells have a prominent prison cell body and many dendritic processes. Immune complexes, spring (more...)

How the various signals that maintain the germinal middle exert their effects on B cells is not completely understood. The combined signals from the B-cell receptor and CD40 seem to upregulate a protein called Bcl-X_L, a relative of Bcl-two, which promotes B-cell survival (meet Affiliate 6). In that location are doubtless many other signals all the same to exist discovered that promote B-cell differentiation.

nine-9. Surviving germinal center B cells differentiate into either plasma cells or memory cells

The purpose of the germinal centre reaction is to enhance the afterwards part of the primary allowed response. Some germinal eye cells differentiate first into plasmablasts and then into plasma cells. Plasmablasts proceed to carve up rapidly merely have begun to specialize to secrete antibody at a loftier rate; they are destined to become nondividing, terminally differentiated plasma cells and thus stand for an intermediate phase of differentiation. These plasma cells will drift to the os marrow, where a subset of them will live for a long period of time. Plasma cells obtain signals from bone marrow stromal cells that are essential for their survival. These plasma cells provide a source of long-lasting high-affinity antibody.

Other germinal middle cells differentiate into retentivity B cells. Memory B cells are long-lived descendents of cells that were once stimulated by antigen and had proliferated in the germinal center. These cells divide very slowly if at all; they express surface immunoglobulin, but do non secrete antibody at a loftier rate. Since the precursors of retentivity B cells once participated in a germinal center reaction, memory B cells inherit the genetic changes that occurred in germinal center cells, including somatic mutations and the cistron rearrangements that result in isotype switch (see Sections iv-nine and 4-sixteen). The signals that command which differentiation path a B cell takes, and even whether at any given point the B cell continues to divide instead of differentiating, are unclear.

Information technology has been proposed that signals from follicular dendritic cells (FDCs) are important in stimulating a B cell to become a memory cell. Nonetheless, retentiveness cells can develop in mutant mice lacking FDCs, admitting with reduced efficiency, so there may exist other sources of signals. Another possibility is that affinity for antigen controls B-prison cell differentiation, with loftier-affinity cells perhaps being preferentially stimulated to go retention cells while the lower-affinity cells are allowed to undergo further cycles of proliferation, mutation, and selection. This is just ane of the mysteries of the germinal center that immunologists have yet to solve. Immunological memory is discussed in item in Chapter 10.

ix-10. B-prison cell responses to bacterial antigens with intrinsic ability to activate B cells do non require T-jail cell help

Although antibody responses to most protein antigens are dependent on helper T cells, humans and mice with T-cell deficiencies notwithstanding brand antibodies to many bacterial antigens. This is considering the special properties of some bacterial polysaccharides, polymeric proteins, and lipopolysaccharides enable them to stimulate naive B cells in the absence of peptide-specific T-cell help. These antigens are known as thymus-contained antigens (TI antigens) because they stimulate strong antibiotic responses in athymic individuals. These nonprotein bacterial products cannot arm-twist classical T-cell responses, yet they induce antibody responses in normal individuals. However, B-cell responses to these TI antigens are influenced past the presence of T cells, peradventure indirectly through cytokines such as IL-five since they are greatly macerated in animals that have no T cells at all.

Thymus-independent antigens fall into two classes that activate B cells past 2 unlike mechanisms. TI-i antigens possess an intrinsic activity that can directly induce B-cell segmentation. At high concentration, these molecules cause the proliferation and differentiation of most B cells regardless of their antigen specificity; this is known equally polyclonal activation (Fig. ix.xvi, top two panels). TI-one antigens are thus oftentimes called B-jail cell mitogens, a mitogen being a substance that induces cells to undergo mitosis. An example of a B-jail cell mitogen and TI-one antigen is LPS, which binds to LPS-binding protein and CD14 (see Affiliate 2), which and then associate with the receptor TLR-4 on B cells. LPS activates B cells only at doses at least 100 times greater than those needed to activate dendritic cells. Thus, when B cells are exposed to concentrations of TI-i antigens that are 10³-x⁵ times lower than those used for polyclonal activation, only those B cells whose B-prison cell receptors as well specifically bind the TI-one molecules become activated. At these depression antigen concentrations, sufficient amounts of TI-one for B-cell activation can only be concentrated on the B-cell surface with the help of this specific binding (Fig. 9.16, bottom two panels). In the presence of large amounts of the TI-1 antigen, this concentrating consequence is not required, and all B cells can be stimulated.

Effigy 9.sixteen

Thymus-contained blazon 1 antigens (TI-one antigens) are polyclonal B-prison cell activators at high concentrations, whereas at low concentrations they induce an antigen-specific antibody response. At loftier concentrations, the signal delivered by the B-jail cell-activating moiety (more...)

It is likely that, every bit with whatsoever pathogen antigen, concentrations of TI-1 antigens are low during the early stages of infections in vivo; thus, only antigen-specific B cells are probable to be activated and these volition produce antibodies specific for the TI-1 antigen. Such responses have an important role in defence force against several extracellular pathogens, as they ascend earlier than thymus-dependent responses since they do non require prior priming and clonal expansion of helper T cells. Withal, TI-one antigens are inefficient inducers of isotype switching, affinity maturation, or retentiveness B cells, all of which require specific T-cell help.

9-11. B-cell responses to bacterial polysaccharides do not require peptide-specific T-cell help

The second grade of thymus-contained antigens consist of molecules such as bacterial capsular polysaccharides that accept highly repetitive structures. These thymus-independent antigens, called TI-2 antigens, comprise no intrinsic B-cell-stimulating activity. Whereas TI-ane antigens can activate both immature and mature B cells, TI-two antigens can activate only mature B cells; immature B cells, equally nosotros saw in Chapter 7, are inactivated past repetitive epitopes. This might be why infants practise not brand antibodies to polysaccharide antigens efficiently; nearly of their B cells are young. Responses to several TI-2 antigens are prominent amidst B-1 cells (too known as CD5 B cells), which comprise an autonomously replicating subpopulation of B cells, and amidst marginal zone B cells, some other unique subset of nonrecirculating B cells that line the border of the splenic white pulp (meet Affiliate seven). Although B-1 cells arise early in development, young children do not make a fully effective response to carbohydrate antigens until about 5 years of historic period. On the other mitt, marginal zone B cells are rare at birth and accrue with historic period; they may thus be responsible for well-nigh physiological TI-2 responses, which also increase with age.

TI-2 antigens about probably deed by extensively cantankerous-linking the B-cell receptors of mature B cells specific for the antigen (Fig. 9.17, left panels). Excessive receptor cross-linking, notwithstanding, renders mature B cells unresponsive or anergic, just as it does young B cells. Thus, epitope density seems to exist critical in the activation of B cells past TI-2 antigens: at also low a density, receptor cross-linking is insufficient to activate the prison cell; at also high a density, the B cell becomes anergic.

Figure 9.17

B-jail cell activation by thymus-independent type 2 antigens (TI-2 antigens) requires, or is greatly enhanced by, cytokines. Multiple cross-linking of the B-cell receptor by TI-2 antigens can lead to IgM antibody production (left panels), but there is evidence that (more...)

Although responses to TI-two antigens can occur in nude mice (which lack a thymus), depletion of all T cells by knocking out the TCRβ and TCRδ loci eliminates responses to TI-ii antigens. Moreover, responses to TI-2 antigens can exist augmented in vivo by transferring small numbers of T cells to these T-cell scarce mice. How T cells contribute to TI-two responses is non clear. One possibility is that T cells can recognize and become activated by TI-2 antigens through jail cell-surface molecules shared by all T cells (Fig. 9.17, right panels). Alternatively, the help might come up from γ:δ T cells or from CD4 CD8 double-negative α:β T cells. The T-cell receptors on these cells recognize certain polysaccharides leap to unconventional MHC class I or course I-like molecules such equally CD1. Such T cells can develop outside the thymus, principally in the gut.

B-cell responses to TI-2 antigens provide a prompt and specific response to an important class of pathogen. Many common extracellular bacterial pathogens are surrounded past a polysaccharide capsule that enables them to resist ingestion by phagocytes. The bacteria not only escape direct devastation by phagocytes but too avert stimulating T-prison cell responses through the presentation of bacterial peptides by macrophages. Antibody that is produced rapidly in response to this polysaccharide capsule without the assist of peptide-specific T cells tin can coat these bacteria, promoting their ingestion and destruction by phagocytes by mechanisms nosotros will depict later in this chapter. The mutual encapsulated extracellular bacteria are oft known as pyogenic leaner, as they typically crusade the germination of abundant pus, which consists importantly of dead and dying neutrophils that take been recruited to the site of infection. Both IgM and IgG antibodies are induced by TI-ii antigens and are likely to exist an important part of the humoral immune response in many bacterial infections. We mentioned earlier the importance of antibodies to the capsular polysaccharide of Haemophilus influenzae type B, a TI-2 antigen, in protective immunity to this bacterium. A farther instance of the importance of TI-2 responses can be seen in patients with an immunodeficiency affliction known as the Wiskott-Aldrich syndrome ( Wiskott-Aldrich Syndrome, in Case Studies in Immunology, see Preface for details). These patients can respond, although poorly, to poly peptide antigens but neglect to make antibody against polysaccharide antigens and are highly susceptible to infection with encapsulated bacteria. Thus, the TI responses are important components of the humoral immune response to nonprotein antigens that do not engage peptide-specific T-jail cell help; the distinguishing features of thymus-dependent, TI-1, and TI-2 antibody responses are summarized in Fig. ix.xviii.

Figure 9.xviii

Properties of different classes of antigen that elicit antibody responses.

Summary

B-cell activation by many antigens, especially monomeric proteins, requires both binding of the antigen past the B-prison cell surface immunoglobulin—the B-cell receptor—and interaction of the B cell with antigen-specific helper T cells. Helper T cells recognize peptide fragments derived from the antigen internalized by the B cell and displayed by the B cells every bit peptide:MHC class Ii complexes. Helper T cells stimulate the B cell through the bounden of CD40L on the T cell to CD40 on the B jail cell, through interaction of other TNF-TNF-receptor family ligand pairs, and by the directed release of cytokines. The initial interaction occurs in the T-cell area of secondary lymphoid tissue, where both antigen-specific and helper T cells and antigen-specific B cells are trapped as a consequence of binding antigen; further interactions between T cells and B cells occur after migration into the B-prison cell zone or follicle, and germination of a germinal middle. Helper T cells induce a phase of vigorous B-prison cell proliferation, and direct the differentiation of the clonally expanded progeny of the naive B cells into either antibody-secreting plasma cells or memory B cells. During the differentiation of activated B cells, the antibody isotype can change in response to cytokines released past helper T cells, and the antigen-binding properties of the antibiotic tin can modify by somatic hypermutation of V-region genes. Somatic hypermutation and option for loftier-analogousness binding occur in the germinal centers. Helper T cells control these processes by selectively activating cells that have retained their specificity for the antigen and past inducing proliferation and differentiation into plasma cells and retentivity B cells. Some nonprotein antigens stimulate B cells in the absence of linked recognition by peptide-specific helper T cells. These thymus-contained antigens induce merely limited isotype switching and do not induce memory B cells. However, responses to these antigens have a critical role in host defense force against pathogens whose surface antigens cannot elicit peptide-specific T-cell responses.

warwickhavesiont.blogspot.com

Source: https://www.ncbi.nlm.nih.gov/books/NBK27142/

Share this post