Chapter 10:  Adaptive Immunity to Infection

10-1. The course of an infection can be divided into several distinct phases

The process of infection can be broken down into stages, each of which can be blocked by different defense mechanisms. In the first stage, a new host is exposed to infectious particles shed by an infected individual. The number, route, mode of transmission, and stability of an infectious agent outside the host determines its infectivity. Some pathogens, such as anthrax, are spread by spores that are highly resistant to heat and drying, while others, such as the human immunodeficiency virus (HIV), are spread only by the exchange of bodily fluids or tissues because they are unable to survive as infectious agents outside the body.

Figure 10.2

Infections and the responses to them can be divided (more...)

Figure 10.2

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   Infections and the responses to them can be divided into a series of stages

These are illustrated here for an infectious microorganism entering across an epithelium, the commonest route of entry. The infectious organism must first adhere to epithelial cells and then cross the epithelium. A local nonadaptive response helps contain the infection and delivers antigen to local lymph nodes, leading to adaptive immunity and clearance of the infection. The role of γ:δ T cells is uncertain, as indicated by the question mark.

Fig. 10.2

Only when a microorganism has successfully established a site of infection in the host does disease occur, and little damage will be caused unless the agent is able to spread from the original site of infection or can secrete toxins that can spread to other parts of the body. Extracellular pathogens spread by direct extension of the focus of infection through the lymphatics or the bloodstream. Usually, spread by the bloodstream occurs only after the lymphatic system has been overwhelmed by the burden of infectious agent. Obligate intracellular pathogens must spread from cell to cell; they do so either by direct transmission from one cell to the next or by release into the extracellular fluid and reinfection of both adjacent and distant cells. Many common food poisoning organisms cause pathology without spreading into the tissues. They establish a site of infection on the epithelial surface in the lumen of the gut and cause no direct pathology themselves, but they secrete toxins that cause damage either in situ or after crossing the epithelial barrier and entering the circulation.

Most infectious agents show a significant degree of host specificity, causing disease only in one or a few related species. What determines host specificity for every agent is not known, but the requirement for attachment to a particular cell-surface molecule is one critical factor. As other interactions with host cells are also commonly needed to support replication, most pathogens have a limited host range. The molecular mechanisms of host specificity comprise an area of research known as molecular pathogenesis, which falls outside the scope of this book.

Fig. 10.2

Fig. 10.2

After many types of infection there is little or no residual pathology following an effective primary response. In some cases, however, the infection or the response to it causes significant tissue damage. In other cases, such as infection with cytomegalovirus or Mycobacterium tuberculosis, the infection is contained but not eliminated and can persist in a latent form. If the adaptive immune response is later weakened, as it is in acquired immune deficiency syndrome (AIDS), these diseases reappear as virulent systemic infections. We will focus on the strategies used by certain pathogens to evade or subvert adaptive immunity and thereby establish a persistent infection in the first part of Chapter 11.

In addition to clearing the infectious agent, an effective adaptive immune response prevents reinfection. For some infectious agents, this protection is essentially absolute, while for others infection is reduced or attenuated upon reexposure.

10-2. Infectious diseases are caused by diverse living agents that replicate in their hosts

Figure 10.3

A variety of microorganisms can cause disease

Figure 10.3

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   A variety of microorganisms can cause disease

Pathogenic organisms are of five main types: viruses, bacteria, fungi, protozoa, and worms. Some common pathogens in each group are listed in the column on the right.

Fig. 10.3

Figure 10.4

Pathogens can be found in various compartments of the (more...)

Figure 10.4

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   Pathogens can be found in various compartments of the body, where they must be combated by different host defense mechanisms

Virtually all pathogens have an extracellular phase where they are vulnerable to antibody-mediated effector mechanisms. However, intracellular phases are not accessible to antibody, and these are attacked by T cells.

Fig. 10.4

Many microorganisms replicate in extracellular spaces, either within the body or on the surface of epithelia. Extracellular bacteria are usually susceptible to killing by phagocytes and thus pathogenic species have developed means of resisting engulfment. The encapsulated gram-positive cocci, for instance, grow in extracellular spaces and resist phagocytosis by means of their polysaccharide capsule. This means they are not immediately eliminated by tissue phagocytes on infecting a previously unexposed host. However, if this mechanism of resistance is overcome by opsonization by complement and specific antibody, they are readily killed after ingestion by phagocytes. Thus, these extracellular bacteria are cleared by means of the humoral immune response (see Chapter 9).

Figure 10.5

Pathogens can damage tissues in a variety of different (more...)

Figure 10.5

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   Pathogens can damage tissues in a variety of different ways

The mechanisms of damage, representative infectious agents, and the common names of the diseases associated with each are shown. Exotoxins are released by microorganisms and act at the surface of host cells, for example, by binding to receptors. Endotoxins, which are intrinsic components of microbial structure, trigger phagocytes to release cytokines that produce local or systemic symptoms. Many pathogens are cytopathic, directly damaging the cells they infect. Finally, adaptive immune response to the pathogen can generate antigen:antibody complexes that can activate neutrophils and macrophages, antibodies that can cross-react with host tissues, or T cells that kill infected cells. All of these have some potential to damage the host's tissues. In addition, neutrophils, the most abundant cells early in infection, release many proteins and small-molecule inflammatory mediators that both control infection and cause tissue damage (not shown).

Fig. 10.5

Fig. 10.5

As we learned in Chapter 2, for a pathogen to invade the body, it must first bind to or cross the surface of an epithelium. When the infection is due to intestinal pathogens such as Salmonella typhi, the causal agent of typhoid fever, or Vibrio cholerae, which causes cholera, the adaptive immune response occurs in the specialized mucosal immune system associated with the gastrointestinal tract, as described later in this chapter. Some intestinal pathogens even target the M cells of the gut mucosal immune system, which are specialized to transport antigens across the epithelium, as a means of entry.

Many pathogens cannot be entirely eliminated by the immune response. But neither are most pathogens universally lethal. Those pathogens that have persisted for many thousands of years in the human population are highly evolved to exploit their human hosts, and cannot alter their pathogenicity without upsetting the compromise they have achieved with the human immune system. Rapidly killing every host it infects is no better for the long-term survival of a pathogen than being wiped out by the immune response before it has had time to infect another individual. In short, we have learned to live with our enemies, and they with us. However, we must be on the alert at all times for new pathogens and new threats to health. The human immunodeficiency virus that causes AIDS serves as a warning to mankind that we remain constantly vulnerable to the emergence of new infectious agents.

Summary

The mammalian body is susceptible to infection by many pathogens, which must first make contact with the host and then establish a focus of infection in order to cause infectious disease. To establish an infection, the pathogen must first colonize the skin or the internal mucosal surfaces of the respiratory, gastrointestinal, or urogenital tracts and then overcome or bypass the innate immune defenses associated with the epithelia and underlying tissues. If it succeeds in doing this, it will provoke an adaptive immune response that will take effect after several days and will usually clear the infection. Pathogens differ greatly in their lifestyles and means of pathogenesis, requiring an equally diverse set of defensive responses from the host immune system.

10-3. The nonspecific responses of innate immunity are necessary for an adaptive immune response to be initiated

The establishment of a focus of infection in tissues and the response of the innate immune system to it produce changes in the immediate environment of the infection. In a bacterial infection, the first thing that usually happens is that the infected tissue becomes inflamed (see Fig. 1.12). As we learnt in Chapter 2, this is initially the result of the activation of the resident macrophages by bacterial components such as lipopolysaccharide (LPS) acting through Toll-like receptors on the macrophage. The cytokines and chemokines secreted by the activated macrophages, especially the cytokine tumor necrosis factor-α (TNF-α), induce numerous changes in the endothelial cells of nearby blood capillaries, a process known as endothelial cell activation. The cytokines cause the release of Weibel-Palade bodies from within the endothelial cells, which deliver P-selectin to the endothelial cell surface. Cytokines and chemokines also induce the synthesis and translation of RNA encoding E-selectin, which thus also appears on the endothelial cell surface.

These two selectins cause leukocytes to adhere to and roll on the endothelial surface in large numbers. Among these will be polymorphonuclear leukocytes, mainly neutrophils, and monocytes. The cytokines also induce the production of the adhesion molecule VCAM-1 on the endothelial cells, which binds to adhesion molecules on the leukocytes. This strengthens the interaction between leukocytes and endothelial cells, and aids the neutrophils and monocytes to enter the infected tissue in large numbers to form an inflammatory focus (see Fig. 2.36). As monocytes mature into tissue macrophages and become activated in their turn, more and more inflammatory cells are attracted into the infected tissue and the inflammatory response is maintained and reinforced. The inflammatory response can be thought of as putting up a flag on the endothelial cells to signal the presence of infection, but as yet, the response is entirely nonspecific for the pathogen antigens.

A second crucial effect of infection is the activation of potential professional antigen-presenting cells—the dendritic cells—that reside in most tissues. These take up antigen in the infected tissues and, as for macrophages, they are activated through innate immune receptors that respond to common pathogen constituents. For example, the combination of LPS and lipopolysaccharide-binding protein (LBP) binding to the cell-surface receptors CD14 and the Toll-like receptor TLR-4 induces the dendritic cells to mature into potent antigen-presenting cells. Activated dendritic cells increase their synthesis of MHC class II molecules and, most importantly, begin to express the co-stimulatory molecules CD80 and CD86 on their surface. As described in Chapter 8, these antigen-presenting cells are carried away from the infected tissue in lymph, along with their antigen cargo, to enter secondary lymphoid tissues, in which they can initiate the adaptive immune response. They arrive in large numbers at the draining lymph nodes, or other nearby lymphoid tissue, attracted by the chemokines ELC, MIP-3β, and SLC that are produced by lymph node stromal and high vascular endothelial cells.

Once dendritic cells arrive in the lymphoid tissues, they appear to have reached their final destination. They eventually die in these tissues, but before this their role is to activate antigen-specific naive T lymphocytes. Naive lymphocytes are continually passing through the lymph nodes, which they enter from the blood across the walls of high endothelial venules, as we will describe below. Those naive T cells that are able to recognize antigen on the surface of dendritic cells are activated and both divide and mature into effector cells that reenter the circulation. When there is a local infection, the changes induced by inflammation in the walls of nearby venules, as we will see later, induce these effector T cells to leave the blood vessel and migrate to the site of infection.

Thus the local release of cytokines and chemokines at the site of infection has far-reaching consequences. As well as recruiting neutrophils and macrophages, which are not specific for antigen, the changes induced in the blood vessel walls also enable newly activated effector T lymphocytes to enter infected tissue.

10-4. An adaptive immune response is initiated when circulating T cells encounter their corresponding antigen in draining lymphoid tissues and become activated

The importance of the peripheral lymphoid organs in the initiation of adaptive immune responses was first shown by ingenious experiments in which a skin flap was isolated from the body wall so that it had a blood circulation but no lymphatic drainage. Antigen placed in the flap of skin did not elicit a T-cell response, showing that T cells do not become sensitized in the infected tissue itself. We now know that naive T lymphocytes are activated in the peripheral lymphoid organs by antigens brought there by dendritic cells. The immune response to pathogens that enter through the skin rather than across mucosal surfaces is generally believed to occur in the lymph nodes, which are sites of intersection of two pathways of circulation, those of the lymph and the blood (see Fig. 1.8).

As described in Chapter 8, immature dendritic cells in tissues take up antigens and are stimulated by infection to migrate to draining lymph nodes. Antigens introduced directly into the bloodstream are picked up by antigen-presenting cells in the spleen, and lymphocytes are activated in the splenic white pulp (see Fig. 1.9). The trapping of antigen by antigen-presenting cells that migrate to these lymphoid tissues, and the continuous recirculation of naive T cells through these tissues, ensure that rare antigen-specific T cells will encounter their specific antigen on an antigen-presenting cell surface. The unique architecture of the peripheral lymphoid organs virtually guarantees contact of foreign antigen with specific T-cell receptors in the lymph nodes and spleen or in mucosa-associated lymphoid tissues (MALT) (see Fig. 1.10).

Figure 10.7

Lymphocytes in the blood enter lymphoid tissue by crossing (more...)

Figure 10.7

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   Lymphocytes in the blood enter lymphoid tissue by crossing the walls of high endothelial venules

The first step is the binding of L-selectin on the lymphocyte to sulfated carbohydrates of GlyCAM-1 and CD34 on the high endothelial cells. Local chemokines activate LFA-1 on the lymphocyte and cause it to bind tightly to ICAM-1 on the endothelial cell, allowing migration across the endothelium. For the lymphocyte to cross the high endothelial barrier successfully, migration has to lead to activation of matrix metalloproteinases, as with the migration of neutrophils out of the blood (see Fig. 2.36).

Fig. 10.7

Fig. 10.7

Figure 10.8

Trapping and activation of antigen-specific naive T (more...)

Figure 10.8

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   Trapping and activation of antigen-specific naive T cells in lymphoid tissue

Naive T cells entering the lymph node from the blood encounter many antigen-presenting dendritic cells in the lymph node cortex. T cells that do not recognize their specific antigen in the cortex leave via the efferent lymphatics and reenter the blood. T cells that do recognize their specific antigen bind stably to the dendritic cell and are activated through their T-cell receptors, resulting in the production of armed effector T cells. Lymphocyte recirculation and recognition is so effective that all of the specific naive T cells in circulation can be trapped by antigen in one node within 2 days. By 5 days after the arrival of antigen, activated effector T cells are leaving the lymph node in large numbers via the efferent lymphatics.

Fig. 10.8

10-5. Cytokines made in the early phases of an infection influence the functional differentiation of CD4 T cells

The differentiation of naive CD4 T cells into the two major classes of CD4 effector T cell occurs during the initial response of these cells to antigen in the peripheral lymphoid tissues. This step, at which a naive CD4 T cell becomes either an armed T_H1 cell or an armed T_H2 cell, has a critical impact on the outcome of an adaptive immune response, determining whether it will be dominated by macrophage activation or by antibody production.

Figure 10.9

The differentiation of naive CD4 T cells into different (more...)

Figure 10.9

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   The differentiation of naive CD4 T cells into different subclasses of armed effector T cells is influenced by cytokines elicited by the pathogen

Many pathogens, especially intracellular bacteria and viruses, activate dendritic cells and NK cells to produce IL-12 and IFN-γ, which cause proliferating CD4 T cells to differentiate into T_H1 cells. IL-4 can inhibit these responses. IL-4, produced by an NK 1.1⁺ T cell in response to parasitic worms or other pathogens, acts on proliferating CD4 T cells to cause them to become T_H2 cells. The mechanisms by which these cytokines induce the selective differentiation of CD4 T cells is now the subject of intensive study. They may act either when the CD4 T cell is first activated by an antigen-presenting cell or during the subsequent proliferative phase.

Fig. 10.9

One possible source of the IL-4 needed to generate T_H2 cells is a specialized subset of CD4 T cells that express the NK1.1 marker normally associated with NK cells; these cells are called NK 1.1⁺ T cells. They have a nearly invariant α:β T-cell receptor; in fact, essentially the same receptor seems to be used in the NK 1.1⁺ T cells of mice and their counterparts in humans. Unlike that of other CD4 T cells, the development of the NK 1.1⁺ T cells does not depend on the expression of MHC class II molecules. Instead, they recognize an MHC class IB molecule, CD1, which is not encoded within the MHC (see Section 5-18). In mice there are two CD1 genes (CD1.1 and CD1.2), whereas in humans there are five (CD1a-e), of which only CD1d is homologous to the murine CD1.1 and CD1.2. CD1 molecules are expressed by thymocytes, professional antigen-presenting cells, and intestinal epithelium.

Fig. 10.9

The differential capacity of pathogens to interact with dendritic cells, macrophages, NK cells, and NK 1.1⁺ T cells can therefore influence the overall balance of cytokines present early in the immune response, and thus determine whether T_H1 or T_H2 cells develop preferentially to bias the adaptive immune response toward a cellular or a humoral response. This can, in turn, determine whether the pathogen is eliminated or survives within the host; some pathogens may even have evolved to interact with the innate immune system so as to generate responses that are beneficial to them rather than to the host.

10-6. Distinct subsets of T cells can regulate the growth and effector functions of other T-cell subsets

Figure 10.10

The two subsets of CD4 T cells each produce cytokines (more...)

Figure 10.10

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   The two subsets of CD4 T cells each produce cytokines that can negatively regulate the other subset

T_H2 cells make IL-10, which acts on macrophages to inhibit T_H1 activation, perhaps by blocking macrophage IL-12 synthesis, and TGF-β, which acts directly on the T_H1 cells to inhibit their growth (left panels). T_H1 cells make IFN-γ, which blocks the growth of T_H2 cells (right panels). These effects allow either subset to dominate a response by suppressing outgrowth of cells of the other subset. A similar dichotomy in cytokine profile is seen in CD8 T cells (not shown), leading to the nomenclature T_C1 and T_C2 cells.

Fig. 10.10

This interplay of cytokines is important in human disease, but it has been explored at present mainly in certain mouse models, where such polarized responses are easier to study. For example, when BALB/c mice are experimentally infected with the protozoan parasite Leishmania, their CD4 T cells fail to differentiate into T_H1 effector cells; instead, the mice preferentially make T_H2 cells in response to this pathogen. These T_H2 cells are unable to activate macrophages to inhibit leishmanial growth, resulting in susceptibility to disease. By contrast, C57BL/6 mice respond by producing T_H1 cells that protect the host by activating infected macrophages to kill the Leishmania. The preferential activation of T_H2 rather than T_H1 cells in BALB/c mice can be reversed if IL-4 is blocked in the first days of infection by injecting anti-IL-4 antibody, but this treatment is ineffective after a week or so of infection.

Because cytokines seem to regulate the balance between T_H1 and T_H2 cells, one might expect that it would be possible to shift this balance by administering appropriate cytokines. IL-2 and IFN-γ have been used to stimulate cell-mediated immunity in diseases such as lepromatous leprosy, and can cause both a local resolution of the lesion and a systemic change in T-cell responses. IL-12, which is a potent inducer of T_H1 cells, might be an even more attractive potential therapy.

CD8 T cells are also able to regulate the immune response by producing cytokines. It has become clear recently that effector CD8 T cells can, in addition to their familiar cytolytic function, also respond to antigen by secreting cytokines typical of either T_H1 or T_H2 cells. Such CD8 T cells, called T_C1 or T_C2 by analogy to the T_H subsets, seem to be responsible for the development of leprosy in its lepromatous rather than its tuberculoid form, which we discuss in detail in Chapter 11. Patients with the less destructive tuberculoid leprosy make T_C1 cells, whose cytokines induce T_H1 cells, which can activate macrophages to rid the body of its burden of leprosy bacilli. Patients with lepromatous leprosy have CD8 T cells that suppress the T_H1 response by making IL-10 and TGF-β. Thus, the suppression of CD4 T cells by CD8 T cells that has been observed in various situations can be explained by their expression of different sets of cytokines.

10-7. The nature and amount of antigenic peptide can also affect the differentiation of CD4 T cells

Figure 10.11

The nature and amount of ligand presented to a CD4 (more...)

Figure 10.11

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   The nature and amount of ligand presented to a CD4 T cell during primary stimulation can determine its functional phenotype

CD4 T cells presented with low levels of a ligand that binds the T-cell receptor poorly differentiate preferentially into T_H2 cells making IL-4 and IL-5. Such T cells are most active in stimulating naive B cells to differentiate into plasma cells and make antibody. T cells presented with a high density of a ligand that binds the T-cell receptor strongly differentiate into T_H1 cells that secrete IL-2, TNF-β, and IFN-γ, and are most effective in activating macrophages.

Fig. 10.11

This difference could be very important in several circumstances. For instance, allergy is caused by the production of IgE antibody, which, as we learned in Chapter 9, requires high levels of IL-4 but does not occur in the presence of IFN-γ, a potent inhibitor of IL-4-driven class switching to IgE. Antigens that elicit IgE-mediated allergy are generally delivered in minute doses, and they elicit T_H2 cells that make IL-4 and no IFN-γ. It is also relevant that allergens do not elicit any of the known innate immune responses, which produce cytokines that tend to bias CD4 T-cell differentiation toward T_H1 cells. Finally, allergens are delivered to humans in minute doses across a thin mucosa, such as that of the lung. Something about this route of sensitization allows even potent generators of T_H1 responses like Leishmania major to induce T_H2 responses.

Most protein antigens that elicit CD4 T-cell responses stimulate the production of both T_H1 and T_H2 cells. This reflects the presence in most proteins of several different peptide sequences that can bind to MHC class II molecules and be presented to CD4 T cells. Some of these peptides are likely to bind to MHC class II molecules with high affinity, and consequently will be present at high density on the antigen-presenting cell, whereas others are likely to bind MHC class II molecules with low affinity and be present only at low density. Naive T cells specific for peptide antigens that have high affinity for MHC molecules are therefore likely to encounter a high density of their ligand, whereas others might only encounter a low density, and these differences could affect the subsequent response of the T cell. Indeed, it can be shown experimentally that some peptides in a protein tend to elicit the production of T_H2 cells, whereas other peptides tend to elicit T_H1 cells.

10-8. Armed effector T cells are guided to sites of infection by chemokines and newly expressed adhesion molecules

Figure 10.12

Armed effector T cells change their surface molecules, (more...)

Figure 10.12

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   Armed effector T cells change their surface molecules, allowing them to home to sites of infection

Naive T cells home to lymph nodes through the binding of L-selectin to sulfated carbohydrates displayed by various proteins, such as CD34 and GlyCAM-1 on the high endothelial venule (HEV, top panel). If they encounter antigen and differentiate into armed effector T cells, many lose expression of L-selectin, leave the lymph node about 4–5 days later, and now express VLA-4 and increased levels of LFA-1. These bind to VCAM-1 and ICAM-1, respectively, on peripheral vascular endothelium at sites of inflammation (bottom panel). On differentiating into effector cells, T cells also alter their splicing of the mRNA encoding the cell-surface molecule CD45. The CD45RO isoform expressed by effector T cells lacks one or more exons that encode extracellular domains present in the CD45RA isoform expressed by naive T cells, and makes effector T cells more sensitive to stimulation by specific antigen.

Fig. 10.12

Differential expression of adhesion molecules can direct different subsets of armed effector T cells to specific sites. Some, for example, migrate to the lamina propria of the gut, which involves the binding of both L-selectin and the α₄:β₇ integrin expressed on the T cell to separate sites on MAdCAM-1. T cells that home to the epithelium of the gut express a novel integrin called α_e:β₇ and bind to the E-cadherin expressed on epithelial cells. Cells that home to the skin, in contrast, express the cutaneous lymphocyte antigen (CLA), a glycosylated isoform of P-selectin glycolipid-1, and bind to E-selectin. As we will discuss later in this chapter, the peripheral immune system is compartmentalized such that different populations of lymphocytes migrate through different lymphoid compartments and—after activation—through the different tissues they serve. The selective expression of different homing receptors that bind to tissue-specific ‘addressins’ is the mechanism by which this is achieved.

Not all infections trigger innate immune responses that activate local endothelial cells, and it is not so clear how armed effector T cells are guided to the sites of infection in these cases. However, activated T cells seem to enter all tissues in very small numbers, perhaps via adhesive interactions such as the binding of P-selectin to P-selectin glycolipid-1, and could thus encounter their antigens even in the absence of a previous inflammatory response.

Effector T cells that recognize pathogen antigens in the tissues produce cytokines such as TNF-α, which activates endothelial cells to express E-selectin, VCAM-1, and ICAM-1, and chemokines such as RANTES (see Fig. 2.33), which can then act on effector T cells to activate their adhesion molecules. The increased levels of VCAM-1 and ICAM-1 on endothelial cells bind VLA-4 and LFA-1, respectively, on armed effector T cells, recruiting more of these cells into tissues that contain the antigen. At the same time, monocytes and polymorphonuclear leukocytes are recruited to these sites by adhesion to E-selectin. TNF-α and IFN-γ released by the activated T cells also act synergistically to change the shape of endothelial cells, allowing increased blood flow, increased vascular permeability, and increased emigration of leukocytes, fluid, and protein into a site of infection.

Thus one or a few specific effector T cells encountering antigen in a tissue can initiate a potent local inflammatory response that recruits both a greater number of specific armed effector cells and many more nonspecific inflammatory cells to that site. By contrast, effector T cells that enter the tissues but do not recognize their antigen are rapidly lost. They either enter afferent lymph in the tissues and return to the bloodstream, or undergo apoptosis. Most of the T cells in the afferent lymph that drains tissues are memory or effector T cells, which characteristically express the CD45RO isoform of the cell-surface molecule CD45 and lack L-selectin. Effector T cells and memory T cells have a similar phenotype, as we will discuss later, and both seem to be committed to migration through potential sites of infection. As well as allowing effector T cells to clear all sites of infection, this pattern of migration allows them to contribute, along with memory cells, to protecting the host against reinfection with the same pathogen (see Sections 10-11 and 10-12).

10-9. Antibody responses develop in lymphoid tissues under the direction of armed helper T cells

Migration out of lymphoid tissues is clearly important for the effector actions of armed CD8 cytotoxic T cells and armed T_H1 cells. However, the most important functions of helper CD4 T cells, predominantly T_H2 cells, depend on their interactions with B cells, and these interactions occur in the lymphoid tissues themselves. B cells specific for a protein antigen cannot be activated to proliferate, form germinal centers, or differentiate into plasma cells until they encounter a helper T cell that is specific for one of the peptides derived from that antigen. Humoral immune responses to protein antigens thus cannot occur until after antigen-specific helper T cells have been generated.

Figure 10.13

The specialized regions of lymphoid tissue provide (more...)

Figure 10.13

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   The specialized regions of lymphoid tissue provide an environment where antigen-specific naive B cells can interact with armed helper T cells specific for the same antigen

The initial encounter of antigen-specific naive B cells with the appropriate helper T cells occurs in the T-cell areas in lymphoid tissue and stimulates the proliferation of B cells in contact with the helper T cells to form a primary focus, as shown in the first three panels. This also results in some isotype switching in the B cells. Some of the activated B-cell blasts then migrate to medullary cords, where they divide, differentiate into plasma cells, and secrete antibody for a few days (third panel). Other B-cell blasts migrate into primary lymphoid follicles, where they proliferate rapidly to form a germinal center under the influence of antigen and of helper T cells (fourth panel). The germinal center is the site of somatic hypermutation and selection of high-affinity B cells that are able to bind antigen better than lower-affinity cells and thus to survive either because they are protected from apoptotic signals delivered by T cells or/and because they are more capable of presenting antigen to T cells and thereby receiving positive signals such as IL-4 and CD40L (fifth panel). FDC, follicular dendritic cells.

Fig. 10.13

Fig. 10.13

Fig. 10.13

Fig. 10.13

The proliferation, somatic hypermutation, and selection that occur in the germinal centers during a primary antibody response have been described in Chapter 9. The adhesion and chemokine molecules that govern the migratory behavior of B cells are likely to be very important to this process but, as yet, little is known of their nature or of the ligands to which they bind. The chemokine/receptor pair BLC/CXCR5, which controls B-cell migration into the follicle, may be important, particularly for B cells homing to the germinal center. Another chemokine receptor, CCR7, which is strongly expressed on T cells and weakly expressed on B cells, may play a role in temporarily directing B cells to the interface with the T-cell zone. The ligands for CCR7—MIP-3β and SLC—are highly expressed in the T zone (see Section 7-30) and could attract B cells that have regulated their CCR7 receptor upward.

10-10. Antibody responses are sustained in medullary cords and bone marrow

Figure 10.14

Plasma cells are dispersed in medullary cords and bone (more...)

Figure 10.14

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   Plasma cells are dispersed in medullary cords and bone marrow

In these sites they secrete antibody at high rates directly into the blood for distribution to the rest of the body. In the upper micrograph, plasma cells in lymph node medullary cords are stained green (with fluorescein anti-IgA) if they are secreting IgA, and red (with rhodamine anti-IgG) if they are secreting IgG. The plasma cells in these local extrafollicular sites are short lived (2–4 days). The lymphatic sinuses are outlined by green granular staining selective for IgA. In the lower micrograph, longer-lived plasma cells (3 weeks to 3 months or more) in the bone marrow are revealed with antibodies specific for light chains (fluorescein anti-λ and rhodamine anti-κ stain). Plasma cells secreting immunoglobulins containing λ light chains shown, on this micrograph, as yellow. Those secreting immunoglobulins containing κ light chains stain red. Photographs courtesy of P. Brandtzaeg.

Fig. 10.14

Fig. 10.14

10-11. The effector mechanisms used to clear an infection depend on the infectious agent

Figure 10.15

Different effector mechanisms are used to clear primary (more...)

Figure 10.15

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   Different effector mechanisms are used to clear primary infections with different pathogens and to protect against subsequent reinfection

The pathogens are listed in order of increasing complexity, and the defense mechanisms used to clear a primary infection are identified by the red shading of the boxes where these are known. Yellow shading indicates a role in protective immunity. Paler shades indicate less well established mechanisms. Much has to be learned about such host-pathogen interactions. It is clear that classes of pathogens elicit similar protective immune responses, reflecting similarities in their lifestyles.

Figure 10.16

Protective immunity consists of preformed immune reactants (more...)

Figure 10.16

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   Protective immunity consists of preformed immune reactants and immunological memory

Antibody levels and effector T-cell activity gradually decline after an infection is cleared. An early reinfection is rapidly cleared by these immune reactants. There are few symptoms but levels of immune reactants increase. Reinfection at later times leads to rapid increases in antibody and effector T cells owing to immunological memory, and infection can be mild or even inapparent.

Figure 10.15

Fig. 10.16

Fig. 10.15

Preformed reactants can also allow the immune system to respond more rapidly and efficiently to a second exposure to a pathogen. Thus, when opsonizing antibodies such as IgG1 are present (see Section 9-12), opsonization and phagocytosis of pathogens will be more efficient. If specific IgE is present, then pathogens will also be able to activate mast cells, rapidly initiating an inflammatory response through the release of histamine and leukotrienes.

10-12. Resolution of an infection is accompanied by the death of most of the effector cells and the generation of memory cells

When an infection is effectively repelled by the adaptive immune system, two things occur. The first is the removal of most of the effector cells, as part of the restoration of tissue integrity. The immune system has well-developed mechanisms for getting rid of cells that have outlasted their usefulness. Most unwanted effector cells die by apoptosis, a process used by all multicellular eukaryotic organisms to remove unwanted cells.

The actions of effector cells remove the specific stimulus that originally recruited them. In the absence of this stimulus, they then undergo ‘death by neglect,’ removing themselves by apoptosis. The dying cells are rapidly cleared by macrophages, which recognize the membrane lipid phosphatidylserine. This lipid is normally found only on the inner surface of the plasma membrane, but in apoptotic cells it rapidly redistributes to the outer surface, where it can be recognized by specific receptors on phagocytes. Thus, not only does the ending of infection lead to the removal of the pathogen, it also leads to the loss of most of the pathogen-specific effector cells.

However, some of the effector cells are retained, and these provide the raw material for memory T-cell and B-cell responses. These are crucially important to the operation of the adaptive immune system, as we will argue in Chapter 15. The memory T cells, which we will consider at the end of this chapter, are retained virtually forever. However, the mechanisms underlying the decision to induce apoptosis in the majority of effector cells and retain only a few are not known. It seems likely that the answer will lie in the cytokines produced by the environment or by the T cells themselves.

Summary

The adaptive immune response is required for effective protection of the host against pathogenic microorganisms. The response of the innate immune system to pathogens helps initiate the adaptive immune response, as interactions with these pathogens lead to the production of cytokines and the activation of dendritic cells to activated antigen-presenting cell status. The antigens of the pathogen are transported to local lymphoid organs by these migrating antigen-presenting cells and presented to antigen-specific naive T cells that continuously recirculate through the lymphoid organs. T-cell priming and the differentiation of armed effector T cells occur here on the surface of antigen-loaded dendritic cells, and the armed effector T cells either leave the lymphoid organ to effect cell-mediated immunity in sites of infection in the tissues, or remain in the lymphoid organ to participate in humoral immunity by activating antigen-binding B cells. Which response occurs is determined by the differentiation of CD4 T cells into T_H1 or T_H2 cells, which is in turn determined by the cytokines produced in the early nonadaptive phase. CD4 T-cell differentiation is also affected by ill-defined characteristics of the activating antigen and by its overall abundance. CD8 T cells play an important role in protective immunity, especially in protecting the host against infection by viruses and intracellular infections by Listeria and other microbial pathogens that have special means for entering the host cell's cytoplasm. Ideally, the adaptive immune response eliminates the infectious agent and provides the host with a state of protective immunity against reinfection with the same pathogen.

10-13. Mucosa-associated lymphoid tissue is located in anatomically defined microcompartments throughout the gut

The mucosa-associated lymphoid tissues lining the gut are known as gut-associated lymphoid tissue or GALT. The tonsils and adenoids form a ring, known as Waldeyer's ring, at the back of the mouth at the entrance of the gut and airways. They represent large aggregates of mucosal lymphoid tissue, which often become extremely enlarged in childhood because of recurrent infections, and which in the past were victims of a vogue for surgical removal. A reduced IgA response to oral polio vaccination has been seen in individuals who have had their tonsils and adenoids removed, which illustrates the importance of this subcompartment of the mucosal immune system.

The other principal sites within the gut mucosal immune system for the induction of immune responses are the Peyer's patches of the small intestine, the appendix (which is another frequent victim of the surgeon's knife), and solitary lymphoid follicles of the large intestine and rectum. Peyer's patches are an extremely important site for the induction of immune responses in the small intestine and have a distinctive structure, forming domelike structures extending into the lumen of the intestine (see Fig. 1.10). The overlying layer of follicle-associated epithelium of the Peyer's patches contains specialized epithelial cells. These have microfolds on their luminal surface, instead of the microvilli present on the absorptive epithelial cells of the intestine, and are known as microfold cells or M cells. They are much less prominent than the absorptive gut epithelial cells, known as enterocytes, and form a membrane overlying the lymphoid tissue within the Peyer's patch. M cells lack a thick surface glycocalyx and do not secrete mucus. Hence they are adapted to interact directly with molecules and particles within the lumen of the gut.

Figure 10.17

M cells take up antigens from the lumen of the gut (more...)

Figure 10.17

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   M cells take up antigens from the lumen of the gut by endocytosis

The cell membrane at the base of these cells is folded around lymphocytes and dendritic cells within the Peyer's patches. Antigens are transported through M cells by the process of transcytosis and delivered directly to antigen-presenting cells and lymphocytes of the mucosal immune system.

Fig. 10.17

Because M cells are much more accessible than enterocytes to particles within the gut, a number of pathogens target M cells to gain access to the subepithelial space, even though such pathogens then find themselves in the heart of the adaptive immune system of the intestine, the Peyer's patches. We will consider one of these pathogens in Section 10-19.

10-14. The mucosal immune system contains a distinctive repertoire of lymphocytes

Figure 10.18

Anatomy of mucosal immune responses

Figure 10.18

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   Anatomy of mucosal immune responses

The left panel shows the afferent immune response. Antigen from pathogenic micro-organisms is presented beneath mucosal surfaces to naive lymphocytes within organized mucosal lymphoid tissue, for example Peyer's patches. Activated lymphocytes leave this tissue via draining lymph nodes and reenter the circulation through the thoracic duct. The right panel shows the efferent immune response in which primed lymphocytes reenter mucosal tissues throughout the body from the circulation, thereby disseminating a mucosal immune response.

Fig. 10.18

The distinctiveness of the mucosal immune system from the rest of the peripheral lymphoid system is further underlined by the different lymphocyte repertoires in the different compartments. The T cells of the gut can be divided into two types. One type bears the conventional α:β T-cell receptors in conjunction with either CD4 or CD8, and participates in conventional T-cell responses to foreign antigens as discussed in earlier chapters. The second class is made up of T cells with unusual surface phenotypes such as TCRγ:δ and CD8α:α TCRα:β. The receptors of these T cells do not bind to the normal MHC:peptide ligands but instead bind to a number of different ligands, including MHC class IB molecules. These highly specialized T cells are abundant in the epithelium of the gut and have a restricted repertoire of T-cell receptor specificities. Unlike conventional T cells, many of these cells do not undergo positive and negative selection in the thymus (see Chapter 7), and express receptors with sequences that have undergone no or minimal divergence from their germline-encoded sequences. These cells may be classified in phylogenetic terms as being at the interface between innate and adaptive immunity.

Figure 10.19

T cells of the mucosal immune system bearing γ:δ (more...)

Figure 10.19

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   T cells of the mucosal immune system bearing γ:δ T-cell receptors and an activating NK receptor recognize and kill injured enterocytes

Infection or other injury to enterocytes, the epithelial cells lining the lumen of the gut, stimulates a stress response, which causes expression on the cell surface of two atypical MHC class IB molecules, known as MIC-A and MIC-B. Intraepithelial T cells carrying the NK receptor NKG2D bind MIC-A and MIC-B and induce apoptosis in the injured enterocytes. The dying enterocyte is removed from the epithelium and the local tissue injury is repaired.

Fig. 10.19

The V_δ1-containing receptor on these T cells may also play a part in allowing them to survey tissues for injured cells. Some human T cells expressing this receptor bind to CD1c, one of the isotypes of the CD1 family of MHC class I-like molecules that we encountered in Section 10-5. This protein, which shows increased expression on activated monocytes and dendritic cells, presents endogenous lipid and glycolipid antigens to some types of T cell. In response to antigen presentation by CD1c, these T cells secrete IFN-γ, which may have an important role in polarizing the response of conventional T cells bearing α:β receptors toward a T_H1 response. This is closely analogous, although opposite in effect, to the polarization toward T_H2 cells induced by secretion of IL-4 by NK 1.1⁺ T cells responding to CD1d discussed in Section 10-5.

A second group of specialized mucosal T cells, so far only characterized in mice, express α:β T-cell receptors together with a CD8 α:α homodimer, instead of the normal CD8 α:β heterodimer that characterizes MHC class I-restricted cytotoxic T cells. These cells can be found in the gut of mice lacking conventional MHC class I molecules, which shows that their development is not dependent on positive selection in the thymus by peptides bound to classical MHC class I molecules. They are, however, absent in mice lacking expression of β₂-microglobulin, which is necessary for the expression of MHC class IB molecules. The ligand recognized by these T cells in mice has been identified as the nonpolymorphic MHC class IB molecule known as Qa-2. These cells are likely to represent a further class of T cells that have a major role in maintaining the integrity of the gut mucosa by recognizing and destroying injured mucosal cells.

10-15. Secretory IgA is the antibody isotype associated with the mucosal immune system

The dominant antibody isotype of the mucosal immune system is IgA. This class of antibody is found in humans in two isotypic forms, IgA1 and IgA2. The expression of IgA differs between the two main compartments in which it is found—blood and mucosal secretions. In the blood, IgA is mainly found as a monomer and the ratio of IgA1 to IgA2 is approximately 4:1. In mucosal secretions, IgA is almost exclusively produced as a dimer and the ratio of IgA1 to IgA2 is approximately 3:2. A number of common intestinal pathogens possess proteolytic enzymes that can digest IgA1, whereas IgA2 is much more resistant to digestion. The higher proportion of plasma cells secreting IgA2 in the gut lamina propria may therefore be the consequence of selective pressure by pathogens against individuals with low IgA2 levels in the gut. The mechanism of isotype switching to IgA is discussed in Section 9-14.

Figure 10.20

The major antibody isotype present in the lumen of (more...)

Figure 10.20

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   The major antibody isotype present in the lumen of the gut is secretory polymeric IgA

This is synthesized by plasma cells in the lamina propria and transported into the lumen of the gut through epithelial cells at the base of the crypts. Polymeric IgA binds to the mucus layer overlying the gut epithelium and acts as an antigen-specific barrier to pathogens and toxins in the gut lumen.

Fig. 10.20

10-16. Most antigens presented to the mucosal immune system induce tolerance

We are continuously exposed to a huge array of foreign antigens in the form of foods, but these do not normally induce an adaptive immune response. For example, IgA antibodies with high affinity to food antigens do not normally develop. This lack of response occurs despite the fact that the repertoire of lymphocyte antigen receptors has not been negatively selected to remove those specific for food antigens. This is because, like any other foreign antigen, food antigens do not play a part in the central mechanisms of lymphocyte tolerance to self, which are established in the thymus and bone marrow (see Chapter 7).

The feeding of foreign antigens leads typically to a state of specific and active unresponsiveness, a phenomenon known as oral tolerance. Thus, no antibody response follows the feeding of a foreign protein such as ovalbumin, although a strong antibody response to this protein can be induced by injecting it subcutaneously, especially if an adjuvant is given as well. However, the feeding of ovalbumin is followed by a prolonged period during which the administration of ovalbumin by injection, even in the presence of adjuvant, elicits no antibody response. This suppression is antigen-specific, because antibody responses to other injected antigens are not affected. These experiments show that there are antigen-specific mechanisms for suppressing peripheral immune responses to antigens delivered by mouth. The mechanisms of oral tolerance are partly understood but, before considering them, we will first discuss the contrasting immune responses that are seen in response to bacterial infections of the gut.

10-17. The mucosal immune system can mount an immune response to the normal bacterial flora of the gut

We each harbor more than 400 species of commensal bacteria, which are present in the largest numbers in the colon and ileum. Despite the fact that these bacteria collectively weigh approximately 1 kg and outnumber us by approximately 10¹⁴ to 1, for most of the time we cohabit with our intestinal bacterial flora in a happy symbiotic relationship.

Figure 10.21

Treatment with antibiotics causes massive death of (more...)

Figure 10.21

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   Treatment with antibiotics causes massive death of the commensal bacteria that normally colonize the colon

This allows pathogenic bacteria to proliferate and occupy an ecological niche that is normally occupied by harmless commensal bacteria. Clostridium difficile is an example of such a pathogen that produces toxins that may cause severe bloody diarrhea in patients treated with antibiotics.

Fig. 10.21

There are some circumstances in which the normal bacterial inhabitants of the gut cause disease, for example, following breakdown of the integrity of the mucosa lining the gut. This may occur following poor blood flow in the gut, or following endotoxemia (see Chapter 2). In these circumstances, normally innocuous gut bacteria, such as nonpathogenic Escherichia coli, can cross the mucosa, invade the bloodstream, and cause fatal systemic infection. This illustrates the vital importance of the barrier to infection provided by the mucosal surfaces of the body. The normal gut flora also becomes an important cause of systemic infection in patients with immunodeficiency. This illustrates the role of the adaptive immune system in host defense against the flora of the gut, but also shows that this response does not result in the elimination of bacteria from the lumen of the gut, but rather a state resembling symbiosis.

The scale of the normal immune response to gut bacteria is illustrated by the study of animals delivered by Caesarian section into a sterile environment in which there is no colonization of the gut by microorganisms. These are known as germ-free or gnotobiotic animals. These animals have marked reductions in the size of all secondary lymphoid organs and reduced levels of antibodies of all isotypes.

10-18. Enteric pathogens cause a local inflammatory response and the development of protective immunity

In spite of the array of innate immune mechanisms in the gut and stiff competition from the indigenous flora, the gut is a frequent site of infection by pathogenic microorganisms. These include many species of viruses, enteric pathogenic bacteria including Salmonella, Yersinia, Shigella, and Listeria, and protozoa such as Entamoeba histolytica and Cryptosporidium. These organisms cause disease in many different ways, but there are certain common features of infection that are crucial to understanding how these pathogens stimulate an immune response by the host, in contrast to the immunological tolerance shown to the foreign antigens ingested in food.

The most important consequence of infection in the gut, as elsewhere in the body, is the development of an inflammatory response. The release of cytokines and chemokines in this response is key to the induction of an adaptive immune response. The inflammatory mediators stimulate the maturation of dendritic cells and other antigen-presenting cells, so that they express the co-stimulatory molecules that provide the additional signals for activation and expansion of naive lymphocytes.

Some intestinal pathogens infect enterocytes, the absorptive cells that line much of the intestine. Enterocytes do not act as passive victims of infection but signal infection by releasing cytokines and chemokines. These include the chemokine IL-8, which is a potent neutrophil chemoattractant, and CC chemokines such as MCP-1, MIP-1α and β, and RANTES, which are chemoattractants for monocytes, eosinophils, and T lymphocytes (see Fig. 2.33). In this way, the onset of infection triggers an influx of inflammatory cells and lymphocytes, leading to the induction of an immune response to the antigens of the infectious agent. Injury and stress to the enterocytes lining the gut may also stimulate the expression of nonclassical MHC molecules, such as MIC-A and MIC-B (see Section 10-14). These act as ligands to the receptors on γ:δ T cells at the base of the crypts, which kill the infected mucosal cell, thereby promoting repair and recovery of the injured mucosa.

A number of pathogens directly exploit the M cell as a means of invasion. Some viruses are transported through the M cell by transcytosis and from the subepithelial space are able to establish systemic infection. For example, from this site polio and retroviruses enter intestinal neuronal cells and spread to the central nervous system. HIV, which we discuss in detail in Chapter 11, may use a similar route into the lymphoid tissue of the rectal mucosa, where it first encounters and infects macrophages.

Many of the most important enteric bacteria that cause infections in humans gain entry to the body through M cells. Invasion by this route delivers bacteria straight to the lymphoid system of the host. Depending on the pathogenicity of the organism and the strength of the host adaptive immune response, infections that breach the gut mucosa may be cleared with little tissue injury, cause a local inflammatory response, or invade the bloodstream or lymphatics and result in a systemic infection. Bacteria that specifically target M cells include Salmonella typhi, the causative agent of typhoid, and S. typhimurium, a major cause of bacterial food-poisoning. These bacteria cause a brisk local and systemic inflammatory response associated with the induction of T_H1-type T-cell responses and antibody responses of the IgG and IgA classes.

10-19. Infection by Helicobacter pylori causes a chronic inflammatory response, which may cause peptic ulcers, carcinoma of the stomach, and unusual lymphoid tumors

Figure 10.22

Helicobacter pylori infects the stomachs of many (more...)

Figure 10.22

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   Helicobacter pylori infects the stomachs of many millions of people

In some, it stimulates the G cells of the stomach to secrete gastrin, which stimulates excess acid production by the stomach, causing peptic ulceration. In others, it causes chronic inflammation of the mucosa, leading to atrophy and loss of acid production. The chronic inflammation of the stomach wall in these individuals may lead to the development of gastric carcinoma.

Fig. 10.22

10-20. In the absence of inflammatory stimuli, the normal response of the mucosal immune system to foreign antigens is tolerance

We have seen that there are two possible and opposite outcomes of exposure to foreign antigens entering through the mucosa of the gut. These are tolerance in the case of food antigens, which is contrasted with a vigorous antibody and T-cell response after exposure to pathogens. The essential difference between antigenic challenge by food compared with that by pathogens is that the pathogens cause inflammation, whereas food does not. Both the antigens within food and the antigens within pathogens are presented by antigenpresenting cells to T lymphocytes, but the contexts in which these two sources of antigen are presented are quite different.

Three different responses of T cells to the presentation of peptides derived from foods and other antigens delivered via the mucosa may account for the phenomenon of oral tolerance. The first is the deletion of antigen-specific T cells by the induction of apoptosis, which has been found to occur in experimental animals in response to oral intake of very large, and probably nonphysiological, doses of antigen. This is probably not the most important mechanism of oral tolerance, although it may contribute.

Figure 10.23

T-cell anergy following feeding of antigen

Figure 10.23

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   T-cell anergy following feeding of antigen

Mice were injected with CD4 T cells bearing a transgenic receptor specific for an ovalbumin peptide. Two days later they were fed ovalbumin or a control protein. Four days later mice were injected with the relevant ovalbumin peptide with adjuvant. Eight days later the draining lymph node was harvested and the number of ovalbumin-specific transgenic T cells was measured and their proliferative response was assessed to stimulation by ovalbumin peptide in vitro. Mice fed with ovalbumin demonstrated a small reduction, compared with control-fed mice, in the number of transgenic T cells recovered, showing some deletion of T cells by orally fed antigen. However, many transgenic T cells remained in the ovalbumin-fed mice and these were refractory to stimulation by antigen in vitro, compared with the control-fed mice, which showed vigorous proliferative responses. These transgenic T cells had become anergic following oral intake of antigen.

Fig. 10.23

The third response involves the development of regulatory T cells, which can actively suppress antigen-specific responses following rechallenge with antigen. One subset of T cells has been described that produces IL-4, IL-10, and TGF-β on stimulation with antigen. These cells have been called T_H3 cells. A similar population secretes TGF-β in an IL-10-dependent manner and has been named T_R1 (T regulatory 1 cells). This pattern of cytokine secretion in response to antigen-specific stimulation inhibits the development of T_H1 responses and is associated with low levels of antibody and virtually absent inflammatory T-cell responses. The γ:δ T cells that are abundant throughout the mucosal immune system may also have a role in oral tolerance, because tolerance appears to be reduced in mice lacking this subset of T lymphocytes.

Antigen-specific suppression is a form of oral tolerance that can be transferred experimentally to recipient animals by lymphocytes derived from animals that have been fed antigen. When an animal that has been injected with such lymphocytes is exposed to the same antigen for the first time, these regulatory cells respond to the antigen and inhibit the responses of naive T cells in the recipient animal. This contrasts with anergic T cells, which cannot transfer oral tolerance following transfer to a naive animal.

In order to understand the phenomenon of oral tolerance, it is essential to understand how orally delivered antigens are presented to T cells. Two routes of antigen presentation of soluble food antigens have been characterized that may induce T-cell responses favoring tolerance rather than immune activation. The first is presentation of soluble food antigens by the antigen-presenting cells of the gut and other peripheral lymphoid organs. In the absence of inflammatory stimuli, antigen presentation by dendritic cells favors the induction of tolerance rather than T-cell activation. Dendritic cells in Peyer's patches have been shown to express IL-10 and IL-4, in contrast to similar cells in peripheral lymph nodes which express IFN-γ and IL-12. However, this heterogeneity of cytokine responses does not fully explain tolerance to food antigens. These may be detected in the bloodstream after feeding and there is evidence that the induction of tolerance to food antigens takes place in lymph nodes and spleen as well as in the mucosal lymphoid system. The second possible route of presentation of food antigens is by the enterocytes of the gut, which express MHC class I and MHC class II molecules in the absence of co-stimulatory molecules and thus may induce anergy on presenting antigens to intraepithelial lymphocytes.

We will discuss each of these mechanisms of tolerance further in Chapter 13, where we consider how the loss of tolerance to self tissues may contribute to the development of autoimmune disease. As we will see in Chapter 14, one of the strategies for treating allergy and autoimmune disease is to attempt to manipulate the nature of the antigen-specific response to stimulate T cells with the properties of such regulatory T cells.

Summary

The immune system can be divided into a series of functional anatomical compartments, of which the two most important are the peripheral lymphoid system made up of the conventionally studied spleen and lymph nodes, and the mucosal lymphoid system. Specific homing mechanisms for lymphocytes to each of these compartments serve to maintain a separate population of lymphocytes in each. The mucosal surfaces of the body are highly vulnerable to infection and possess a complex array of innate and adaptive mechanisms of immunity. The adaptive immune system of the mucosa-associated lymphoid tissues differs from that of the rest of the peripheral lymphoid system in several respects. The types and distribution of T cells differ, with significantly greater numbers of γ:δ T cells in the gut mucosa compared with peripheral lymph nodes and blood. The major antibody type secreted across the epithelial cells lining mucosal surfaces is secretory polymeric IgA. The mucosal lymphoid system is exposed to a vast array of foreign antigens from foods, from the commensal bacteria of the gut, and from pathogenic microorganisms and parasites. No immune response can normally be detected to food antigens. Indeed, soluble antigens taken by mouth may induce antigen-specific tolerance or antigen-specific suppression. In contrast, pathogenic microorganisms induce strong protective T_H1 responses. It is an important challenge to understand these contrasting specific immune responses. The key distinction between tolerance and the development of powerful protective adaptive immune responses is the context in which peptide antigen is presented to T lymphocytes in the mucosal immune system. In the absence of inflammation, presentation of peptide to T cells by MHC molecules on antigen-presenting cells occurs in the absence of co-stimulation. By contrast, pathogenic microorganisms induce inflammatory responses in the tissues, which stimulate the maturation and expression of co-stimulatory molecules on antigen- presenting cells. This form of antigen presentation to T cells favors development of a protective T_H1 response.

10-21. Immunological memory is long-lived after infection or vaccination

Most children in the United States are now vaccinated against measles virus; before vaccination was widespread, most were naturally exposed to this virus and suffered from an acute, unpleasant, and potentially dangerous viral illness. Whether through vaccination or infection, children exposed to the virus acquire long-term protection from measles. The same is true of many other acute infectious diseases: this state of protection is a consequence of immunological memory.

The basis of immunological memory has been hard to explore experimentally. Although the phenomenon was first recorded by the ancient Greeks and has been exploited routinely in vaccination programs for over 200 years, it is just now becoming clear that memory reflects a persistent population of specialized memory cells that is independent of the continued persistence of the original antigen that induced them. This mechanism of maintaining memory is consistent with the finding that only individuals who were themselves previously exposed to a given infectious agent are immune, and that memory is not dependent on repeated exposure to infection as a result of contacts with other infected individuals. This was established by observations made on remote island populations, where a virus such as measles can cause an epidemic, infecting all people living on the island at that time, after which the virus disappears for many years. On reintroduction from outside the island, the virus does not affect the original population but causes disease in those people born since the first epidemic. This means that immunological memory need not be maintained by repeated exposure to infectious virus.

Instead, it is most likely that memory is sustained by long-lived antigen-specific lymphocytes that were induced by the original exposure and that persist until a second encounter with the pathogen. It was thought that retained antigen, bound in immune complexes on follicular dendritic cells, might be crucial in maintaining these cells, but recent experiments suggest otherwise. While most of the memory cells are in a resting state, careful studies have shown that a small percentage are dividing at any one time. What stimulates this infrequent cell division is unclear. However, cytokines such as those produced either constitutively or during the course of antigen-specific immune responses directed at noncross-reactive antigens could be responsible. One such cytokine, IL-15, has been implicated in maintaining CD8 memory T cells. Regardless of cell division, the number of memory cells for a given antigen is highly regulated, remaining practically constant during the memory phase.

Immunological memory can be measured experimentally in various ways. Adoptive transfer assays (seeAppendix I, Section A-42) of lymphocytes from animals immunized with simple, nonliving antigens have been favored for such studies, as the antigen cannot proliferate. When an animal is first immunized with a protein antigen, helper T-cell memory against that antigen appears abruptly and is at its maximal level after 5 days or so. Antigen-specific memory B cells appear some days later, because B-cell activation cannot begin until armed helper T cells are available, and B cells must then enter a phase of proliferation and selection in lymphoid tissue. By one month after immunization, memory B cells are present at their maximal levels. These levels are then maintained with little alteration for the lifetime of the animal. In these experiments, the existence of memory cells is measured purely in terms of the transfer of specific responsiveness from an immunized, or ‘primed,’ animal to an irradiated, immunoincompetent and nonimmunized recipient. In the following sections, we will look in more detail at the changes that occur in lymphocytes after antigen priming, and discuss the mechanisms that might account for these changes.

10-22. Both clonal expansion and clonal differentiation contribute to immunological memory in B cells

Figure 10.24

The generation of secondary antibody responses from (more...)

Figure 10.24

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   The generation of secondary antibody responses from memory B cells is distinct from the generation of the primary antibody response

These responses can be studied and compared by isolating B cells from immunized and unimmunized donor mice, and stimulating them in culture in the presence of armed antigen-specific effector T cells. The primary response usually consists of antibody molecules made by plasma cells derived from a relatively large number of different precursor B cells. The antibodies are of relatively low affinity, with few somatic mutations. The secondary response derives from far fewer high-affinity precursor B cells, which have undergone significant clonal expansion. Their receptors and antibodies are of high affinity for the antigen and show extensive somatic mutation. Thus, there is usually only a 10- to 100-fold increase in the frequency of activatable B cells after priming; however, the quality of the antibody response is altered radically, such that these precursors induce a far more intense and effective response.

Figure 10.25

The affinity as well as the amount of antibody increases (more...)

Figure 10.25

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   The affinity as well as the amount of antibody increases with repeated immunization

The upper panel shows the increase in the level of antibody with time after primary, followed by secondary and tertiary, immunization; the lower panel shows the increase in the affinity of the antibodies. The increase in affinity (affinity maturation) is seen largely in IgG antibody (as well as in IgA and IgE, which are not shown) coming from mature B cells that have undergone isotype switching and somatic hypermutation to yield higher-affinity antibodies. Although some affinity maturation occurs in the primary antibody response, most arises in later responses to repeated antigen injections. Note that these graphs are on a logarithmic scale.

Fig. 10.24

Fig. 10.24

Fig. 10.25

The distinction between primary and secondary antibody responses is most clearly seen in those cases where the primary response is dominated by antibodies that are closely related and show few if any somatic hypermutations. This occurs in inbred mouse strains in response to certain haptens that are recognized by a limited set of naive B cells. The antibodies produced are encoded by the same V_H and V_L genes in all animals of the strain, suggesting that these variable regions have been selected during evolution for recognition of determinants on pathogens that happen to cross-react with some haptens. As a result of the uniformity of these primary responses, changes in the antibody molecules produced in secondary responses to the same antigens are easy to observe. These differences include not only numerous somatic hypermutations in antibodies containing the dominant variable regions but also the addition of antibodies containing V_H and V_L gene segments not detected in the primary response. These are thought to derive from B cells that were activated at low frequency during the primary response, and thus not detected, but that differentiated into memory B cells.

10-23. Repeated immunizations lead to increasing affinity of antibody owing to somatic hypermutation and selection by antigen in germinal centers

Upon reexposure to the same antigen, a secondary immune response will ensue. In some ways, this resembles the primary immune response, with the initial proliferation of B cells and T cells at the interface between the T- and B-cell zones. The secondary response is characterized by early and vigorous generation of plasma cells, thus accounting for early profuse IgG production. Some B cells that have not yet undergone terminal differentiation can migrate into the follicle and become germinal center B cells. There, these B cells enter a second proliferative phase, during which the DNA encoding their immunoglobulin V domains again undergoes somatic hypermutation before the B cells differentiate into antibody-secreting plasma cells (see Section 9-7).

Figure 10.26

The mechanism of affinity maturation in an antibody (more...)

Figure 10.26

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   The mechanism of affinity maturation in an antibody response

At the beginning of a primary response, B cells with receptors of a wide variety of affinities (K_A), most of which will bind antigen with low affinity, take up antigen, present it to helper T cells, and become activated to produce antibody of varying and relatively low affinity (top panel). These antibodies then bind and clear antigen, so that only those B cells with receptors of the highest affinity can continue to capture antigen and interact effectively with helper T cells. Such B cells will therefore be selected to undergo further expansion and clonal differentiation and the antibodies they produce will dominate a secondary response, (middle panel). These higher affinity antibodies will in turn compete for antigen and select for the activation of B cells bearing receptors of still higher affinity in the tertiary response (bottom panel).

Fig. 10.26

10-24. Memory T cells are increased in frequency and have distinct activation requirements and cell-surface proteins that distinguish them from armed effector T cells

Figure 10.27

Encounter with antigen generates effector T cells and (more...)

Figure 10.27

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   Encounter with antigen generates effector T cells and long-lived memory T cells

On priming with antigen, a naive T cell divides and differentiates. Most of the progeny are relatively short-lived effector cells. However, some become long-lived memory T cells, which may be sustained by cytokines.

Fig. 10.27

A major problem in experiments aimed at establishing the existence of memory T cells is that most assays for T-cell effector function take several days, during which the putative memory T cells are reinduced to armed effector cell status. Thus, these assays do not distinguish preexisting effector cells from memory T cells. This problem does not apply to cytotoxic T cells, however, as cytotoxic effector T cells can program a target cell for lysis in 5 minutes. Memory CD8 T cells need to be reactivated to become cytotoxic, but they can do so without undergoing DNA synthesis, as shown by studies carried out in the presence of mitotic inhibitors. Recently, it has become possible to track particular clones of antigen-specific CD8 T cells by staining them with tetrameric MHC:peptide complexes (seeAppendix I, Section A-28). It has been found that the number of antigen-specific CD8 T cells increases dramatically during an infection, and then drops by up to 100-fold; nevertheless, this final level is distinctly higher than before priming. These cells continue to express some markers characteristic of activated cells, like CD44, but stop expressing other activation markers, like CD69. In addition, they express more Bcl-2, a protein that promotes cell survival and may be responsible for the long half-life of memory CD8 cells. These cells are more sensitive to restimulation by antigen than are naive cells, and more quickly and more vigorously produce cytokines such as IFN-γ in response to such stimulation.

Figure 10.28

Many cell-surface molecules have altered expression (more...)

Figure 10.28

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   Many cell-surface molecules have altered expression on memory T cells

This is seen most clearly in the case of CD45, where there is a change in the isoforms expressed (see Fig. 10.29). Many of these changes are also seen on cells that have been activated to become armed effector T cells. The changes increase the adhesion of the T cell to antigen-presenting cells and to endothelial cells, particularly at sites of inflammation. They also increase the sensitivity of the memory T cell to antigen stimulation. The loss of CD62L on most or all memory T cells means that they are no longer able to enter lymph nodes across the high endothelial venules.Instead they migrate though the tissues and enter lymphoid compartments via the afferent lymph vessels.

Figure 10.29

Memory CD4 T cells express altered CD45 isoforms that (more...)

Figure 10.29

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   Memory CD4 T cells express altered CD45 isoforms that regulate the interaction of the T-cell receptor with its co-receptors

CD45 is a transmembrane tyrosine phosphatase with three variable exons (A, B, and C) that encode part of its external domain. In naive T cells, high molecular weight isoforms (CD45RA) are found that do not associate with either the T-cell receptor (TCR/CD3) or co-receptors (CD4). In memory T cells, the variable exons are removed by alternative splicing of CD45 RNA, and this isoform, known as CD45RO, associates with both the T-cell receptor and the co-receptor. This receptor complex seems to transduce signals more effectively than the receptor on naive T cells.

Fig. 10.28

Fig. 10.29

It thus seems reasonable to designate these cells as memory CD4 T cells, and to surmise that naive CD4 T cells can differentiate into armed effector T cells or into memory T cells that can later be activated to effector status. Recent experiments show that CD4 cells can differentiate into two types of memory cell, with distinct activation characteristics. One type are called effector memory cells because they can rapidly mature into effector CD4 T cells and secrete large amounts of IFN-γ, IL-4, and IL-5 early after restimulation. These cells lack the chemokine receptor CCR7 but express high levels of β-1 and β-2 integrins, as well as receptors for inflammatory chemokines. This profile suggests these effector memory cells are specialized for quickly entering inflamed tissues. The other type are called central memory cells. They express CCR7 and thus would be expected to recirculate more easily to T zones of secondary lymphoid tissues, as do naive T cells. These central memory cells are very sensitive to T-cell receptor cross-linking and quickly upregulate CD40L in response to it; however, they take longer to differentiate into effector cells and thus do not secrete as much cytokine as do effector memory cells early after restimulation. Interestingly, CD8 T cells can also be divided into analogous central and effector memory subsets.

As with memory CD8 T cells, the field will soon be revolutionized by direct staining of CD4 T cells with peptide:MHC class II oligomers (seeAppendix I, Section A-28). This technique allows one not only to identify antigen-specific CD4 T cells but also, using intracellular cytokine staining (seeAppendix I, Section A-27), to determine whether they are T_H1 or T_H2 cells. These improvements in the identification and phenotyping of CD4 T cells will rapidly increase our knowledge of these hitherto mysterious cells, and could contribute valuable information on naive, memory, and effector CD4 T cells.

10-25. In immune individuals, secondary and subsequent responses are mediated solely by memory lymphocytes and not by naive lymphocytes

In the normal course of an infection, a pathogen first proliferates to a level sufficient to elicit an adaptive immune response and then stimulates the production of antibodies and effector T cells that eliminate the pathogen from the body. Most of the armed effector T cells then die, and antibody levels gradually decline after the pathogen is eliminated, because the antigens that elicited the response are no longer present at the level needed to sustain it. We can think of this as feedback inhibition of the response. Memory T and B cells remain, however, and maintain a heightened ability to mount a response to a recurrence of infection with the same pathogen.

The antibody and memory T cells remaining in an immunized individual also prevent the activation of naive B and T cells by the same antigen. Such a response would be wasteful, given the presence of memory cells that can respond much more quickly. The suppression of naive lymphocyte activation can be shown by passively transferring antibody or memory T cells to naive recipients; when the recipient is then immunized, naive lymphocytes do not respond to the original antigen, but responses to other antigens are unaffected. This has been put to practical use to prevent the response of Rh^- mothers to their Rh⁺ children; if anti-Rh antibody is given to the mother before she reacts to her child's red blood cells, her response will be inhibited. The mechanism of this suppression is likely to involve the antibody-mediated clearance and destruction of the child's red blood cells, thus preventing naive B cells and T cells from mounting an immune response. Memory B-cell responses are not inhibited by antibody against the antigen, so the Rh^- mothers at risk must be identified and treated before a response has occurred. This is because memory B cells are much more sensitive, because of their high affinity and alterations in their B-cell receptor signaling requirements, to smaller amounts of antigen that cannot be efficiently cleared by the passive anti-Rh antibody. The ability of memory B cells to be activated to produce antibody even when exposed to preexisting antibody also allows secondary antibody responses to occur in individuals who are already immune.

Adoptive transfer of immune T cells (seeAppendix I, Section A-42) to naive syngeneic mice also prevents the activation of naive T cells by antigen. This has been shown most clearly for cytotoxic T cells. It is possible that, once reactivated, the memory CD8 T cells regain cytotoxic activity sufficiently rapidly to kill the antigen-presenting cells that are required to activate naive CD8 T cells, thereby inhibiting the latter's activation.

Figure 10.30

When individuals who have already been infected with (more...)

Figure 10.30

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   When individuals who have already been infected with one variant of influenza virus are infected with a second variant they make antibodies only to epitopes that were present on the initial virus

A child infected for the first time with an influenza virus at 2 years of age makes a response to all epitopes (left panel). At age 5 years, the same child exposed to a variant influenza virus responds preferentially to those epitopes shared with the original virus, and makes a smaller than normal response to new epitopes on the virus (middle panel). Even at age 20 years, this commitment to respond to epitopes shared with the original virus, and the subnormal response to new epitopes, is retained (right panel). This phenomenon is called ‘original antigenic sin.’

Fig. 10.30

Summary

Protective immunity against reinfection is one of the most important consequences of adaptive immunity operating through the clonal selection of lymphocytes. Protective immunity depends not only on preformed antibody and armed effector T cells, but most importantly on the establishment of a population of lymphocytes that mediate long-lived immunological memory. The capacity of these cells to respond rapidly to restimulation with the same antigen can be transferred to naive recipients by primed B and T cells. The precise changes that distinguish naive, effector, and memory lymphocytes are now being characterized and, with the advent of receptor-specific reagents, the relative contributions of clonal expansion and differentiation to the memory phenotype are rapidly being clarified. Memory B cells can also be distinguished by changes in their immunoglobulin genes because of isotype switching and somatic hypermutation, and secondary and subsequent immune responses are characterized by antibodies with increasing affinity for the antigen.