394 PART I The Immune System in Health and Disease Some influenza a strains and antigenic shift is thought to occur through genetic TABLE 17-2 their hemagglutinin(H)and sortment between influenza virions from humans and from neuraminidase(N) subtype various animals, including horses, pigs, and ducks(Figure 17-6b). The fact that influenza contains eight separate Antigenic strands of ssRNA makes possible the reassortment of the Virus strain designation RNA strands of human and animal virions within a single cell infected with both viruses. Evidence for in vivo genetic Human A/Puerto Rico/8/34 HONT reassortment between influenza a viruses from humans and A/Fort Monmouth/1/47 HINT domestic pigs was obtained in 1971. After infecting a pig A/Singapore/1/57 H2N2 simultaneously with human Hong Kong influenza(H3N2) A/Hong Kong/1/68 H3N2 and with swine influenza(HIN1), investigators were able to A/USSR/80/77 recover virions expressing H3N1. In some cases, an apparent A/Brazil/11/78 HINT antigenic shift may represent the re-emergence of a previous A/Bangkok/1/79 strain that has remained hidden for several decades. In May A/Taiwan/1/86 of 1977, a strain of influenza, A/USSR/77(HIN1), appeared that proved to be identical to a strain that had caused an epi A/Shanghai/16/89 H3N2 demic 27 years earlier. The virus could have been preserved A/Johannesburg/33/95 H3N2 over the years in a frozen state or in an animal reservoir A/Wuhan/359/95 H3N2 When such a re-emergence occurs, the ha and na antigens expressed are not really new; however, they will be seen by A/Hong Kong/156/97 H5N1 the immune system of anyone not previously exposed to that strain(people under the age of twenty-seven in the 1977 Swine HINT epidemic, for example)as if they were new because no mem- A/Sw/Taiwan/70 H3N2 ory cells specific for these antigenic subtypes will exist in the susceptible population. Thus, from an immunologic Horse(equine) A/Eq/Prague/1/56 point of view, the re-emergence of an old influenza A strain A/Eq/Miami/1/63 H3N8 Birds A/Fowl/ Dutch/27 H7N7 A/Turkey/Ontario/68 A/Chicken/Hong Kong/258/97 H5N1 e25 Between pandemic-causing a enic drift, generating minor variations which account for strain differences within a sub type. The immune response contributes to the emergence 品星 of these different influenza strains as individuals infected with a given influenza strain mount an effective immune response, the strain is eliminated. However, the accumula tion of point mutations sufficiently alters the antigenicity of some variants so that they are able to escape immune elimi- nation( Figure 17-6a). These variants become a new strain of influenza, causing another local epidemic cycle. The role of PB2 PBI PA NP HAI NA MI M2 NSI NSZ antibody in such immunologic selection can be demon strated in the laboratory by mixing an influenza strain with a FIGURE 17-5 Amino acid sequence variation in 10 influenza viral monoclonal antibody specific for that strain and then cultur- proteins from two H3N2 strains and one HiN1 strain. The surface ing the virus in cells. The antibody neutralizes all unaltered glycoproteins hemagglutinin(HA1)and neuraminidase(NA)show viral particles and only those viral particles with mutations significant sequence variation; in contrast, the sequences of internal resulting in altered antigenicity escape neutralization and are viral proteins, such as matrix proteins(M1 and M2) and nucleopro- able to continue the infection. Within a short time in culture, tein(NP), are largely conserved. Adapted from G G. Brownlee, 1986, a new influenza strain can be shown to emerge in Options for the Control of Influenza, Alan R. Liss
Between pandemic-causing antigenic shifts, the influenza virus undergoes antigenic drift, generating minor antigenic variations, which account for strain differences within a subtype. The immune response contributes to the emergence of these different influenza strains. As individuals infected with a given influenza strain mount an effective immune response, the strain is eliminated. However, the accumulation of point mutations sufficiently alters the antigenicity of some variants so that they are able to escape immune elimination (Figure 17-6a). These variants become a new strain of influenza, causing another local epidemic cycle. The role of antibody in such immunologic selection can be demonstrated in the laboratory by mixing an influenza strain with a monoclonal antibody specific for that strain and then culturing the virus in cells. The antibody neutralizes all unaltered viral particles and only those viral particles with mutations resulting in altered antigenicity escape neutralization and are able to continue the infection. Within a short time in culture, a new influenza strain can be shown to emerge. Antigenic shift is thought to occur through genetic reassortment between influenza virions from humans and from various animals, including horses, pigs, and ducks (Figure 17-6b). The fact that influenza contains eight separate strands of ssRNA makes possible the reassortment of the RNA strands of human and animal virions within a single cell infected with both viruses. Evidence for in vivo genetic reassortment between influenza A viruses from humans and domestic pigs was obtained in 1971. After infecting a pig simultaneously with human Hong Kong influenza (H3N2) and with swine influenza (H1N1), investigators were able to recover virions expressing H3N1. In some cases, an apparent antigenic shift may represent the re-emergence of a previous strain that has remained hidden for several decades. In May of 1977, a strain of influenza, A/USSR/77 (H1N1), appeared that proved to be identical to a strain that had caused an epidemic 27 years earlier. The virus could have been preserved over the years in a frozen state or in an animal reservoir. When such a re-emergence occurs, the HA and NA antigens expressed are not really new; however, they will be seen by the immune system of anyone not previously exposed to that strain (people under the age of twenty-seven in the 1977 epidemic, for example) as if they were new because no memory cells specific for these antigenic subtypes will exist in the susceptible population. Thus, from an immunologic point of view, the re-emergence of an old influenza A strain 394 PART IV The Immune System in Health and Disease Amino acid change, % NS2 5 Viral proteins PB2 PB1 PA NP HA1 NA M1 M2 NS1 61 69 10 15 20 25 30 FIGURE 17-5 Amino acid sequence variation in 10 influenza viral proteins from two H3N2 strains and one H1N1 strain. The surface glycoproteins hemagglutinin (HA1) and neuraminidase (NA) show significant sequence variation; in contrast, the sequences of internal viral proteins, such as matrix proteins (M1 and M2) and nucleoprotein (NP), are largely conserved. [Adapted from G. G. Brownlee, 1986, in Options for the Control of Influenza, Alan R. Liss.] TABLE 17-2 Some influenza A strains and their hemagglutinin (H) and neuraminidase (N) subtype Antigenic Species Virus strain designation subtype Human A/Puerto Rico/8/34 H0N1 A/Fort Monmouth/1/47 H1N1 A/Singapore/1/57 H2N2 A/Hong Kong/1/68 H3N2 A/USSR/80/77 H1N1 A/Brazil/11/78 H1N1 A/Bangkok/1/79 H3N2 A/Taiwan/1/86 H1N1 A/Shanghai/16/89 H3N2 A/Johannesburg/33/95 H3N2 A/Wuhan/359/95 H3N2 A/Texas/36/95 H1N1 A/Hong Kong/156/97 H5N1 Swine A/Sw/Iowa/15/30 H1N1 A/Sw/Taiwan/70 H3N2 Horse (equine) A/Eq/Prague/1/56 H7N7 A/Eq/Miami/1/63 H3N8 Birds A/Fowl/Dutch/27 H7N7 A/Tern/South America/61 H5N3 A/Turkey/Ontario/68 H8N4 A/Chicken/Hong Kong/258/97 H5N1
Immune Response to Infectious Diseases CHAPTER 17 395 humoral responses, CTLs can play a role in immune re- Virus r Bacterial Infections Immunity to bacterial infections is achieved by means of ntibody unless the bacterium is capable of intracellular growth, in which case delayed-type hypersensitivity has an important role. Bacteria enter the body either through a Human wine number of natural routes( e. g, the respiratory tract, the gas influenza trointestinal tract, and the genitourinary tract) or through normally inaccessible routes opened up by breaks in mucous membranes or skin. Depending on the number of organisms Tip/interface FIGURE 17-6 Two mechanisms generate variations in influenza surface antigens (a) In antigenic drift, the accumulation of point mu- tations eventually yields a variant protein that is no longer recognized by antibody to the original antigen. (b) Antigenic shift may occur by re- assortment of an entire ssrna between human and animal virions in- fecting the same cell. Only four of the eight RNA strands are depict have the same effect as an antigenic shift that generates a Hinge HOST RESPONSE TO INFLUENZA INFECTION a helix Humoral antibody specific for the HA molecule is produced during an influenza infection. This antibody confers protec- tion against influenza, but its specificity is strain-specific and is readily bypassed by antigenic drift. antigenic drift in the HA molecule results in amino acid substitutions in several antigenic domains at the molecule's distal end( Figure 17-7) Two of these domains are on either side of the conserved sialic-acid-binding cleft, which is necessary for binding of virions to target cells. Serum antibodies specific for these two regions are important in blocking initial viral infectivity These antibody titers peak within a few days of infection and then decrease over the next 6 months; the titers then plateau and remain fairly stable for the next several years. This anti body does not appear to be required for recovery from in- fluenza, as patients with agammaglobulinemia recover from FIGURE 17-7 Structure of hemagglutinin molecule Sialic acid on the disease. Instead, the serum antibody appears to play a sig- host cells interacts with the binding cleft, which is bounded by re- nificant role in resistance to reinfection by the same strain. gions-designated the loop and tip/interface--where antigenic drift When serum-antibody levels are high for a particular Ha is prevalent(blue areas). Antibodies to these regions are important molecule, both mice and humans are resistant to infection by in blocking viral infections. Continual changes in amino acid residues virions expressing that HA molecule. If mice are infected in these regions allow the influenza virus to evade the antibody re- with influenza virus and antibody production is experimen- sponse. Small red dots represent residues that exhibit a high degree tally suppressed, the mice recover from the infection but of variation among virus strains. /Adapted from D. C. Wiley et al can be reinfected with the same viral strain In addition to 1987, Nature 289 373. towww.whfreeman.com/immunology95molecularVis Viral Antigens See Introduction and Flu virus Hemagglutinin
can have the same effect as an antigenic shift that generates a new subtype. HOST RESPONSE TO INFLUENZA INFECTION Humoral antibody specific for the HA molecule is produced during an influenza infection. This antibody confers protection against influenza, but its specificity is strain-specific and is readily bypassed by antigenic drift. Antigenic drift in the HA molecule results in amino acid substitutions in several antigenic domains at the molecule’s distal end (Figure 17-7). Two of these domains are on either side of the conserved sialic-acid–binding cleft, which is necessary for binding of virions to target cells. Serum antibodies specific for these two regions are important in blocking initial viral infectivity. These antibody titers peak within a few days of infection and then decrease over the next 6 months; the titers then plateau and remain fairly stable for the next several years. This antibody does not appear to be required for recovery from influenza, as patients with agammaglobulinemia recover from the disease. Instead, the serum antibody appears to play a significant role in resistance to reinfection by the same strain. When serum-antibody levels are high for a particular HA molecule, both mice and humans are resistant to infection by virions expressing that HA molecule. If mice are infected with influenza virus and antibody production is experimentally suppressed, the mice recover from the infection but can be reinfected with the same viral strain. In addition to humoral responses, CTLs can play a role in immune responses to influenza. Bacterial Infections Immunity to bacterial infections is achieved by means of antibody unless the bacterium is capable of intracellular growth, in which case delayed-type hypersensitivity has an important role. Bacteria enter the body either through a number of natural routes (e.g., the respiratory tract, the gastrointestinal tract, and the genitourinary tract) or through normally inaccessible routes opened up by breaks in mucous membranes or skin. Depending on the number of organisms Immune Response to Infectious Diseases CHAPTER 17 395 Antigenic Virus drift Host cell Antigenic shift Human influenza Swine influenza (a) (b) FIGURE 17-6 Two mechanisms generate variations in influenza surface antigens. (a) In antigenic drift, the accumulation of point mutations eventually yields a variant protein that is no longer recognized by antibody to the original antigen. (b) Antigenic shift may occur by reassortment of an entire ssRNA between human and animal virions infecting the same cell. Only four of the eight RNA strands are depicted. Tip/interface Binding cleft Loop Hinge α helix β pleated sheet FIGURE 17-7 Structure of hemagglutinin molecule. Sialic acid on host cells interacts with the binding cleft, which is bounded by regions—designated the loop and tip/interface—where antigenic drift is prevalent (blue areas). Antibodies to these regions are important in blocking viral infections. Continual changes in amino acid residues in these regions allow the influenza virus to evade the antibody response. Small red dots represent residues that exhibit a high degree of variation among virus strains. [Adapted from D. C. Wiley et al., 1981, Nature 289:373.] Go to www.whfreeman.com/immunology Molecular Visualization Viral Antigens See Introduction and Flu Virus Hemagglutinin
396 PART I The Immune System in Health and Disease entering and their virulence, different levels of host defense macrophages to kill ingested pathogens more effectively(see are enlisted. If the inoculum size and the virulence are both Figure 14-15) low, then localized tissue phagocytes may be able to eliminate the bacteria with an innate, nonspecific defense. Larger in- Bacteria Can Effectively Evade Host oculus or organisms with greater virulence tend to induce Defense Mechanisms an adaptive, specific immune response. There are four primary steps in bacterial infection Immune Responses to Extracellular ttachment to host cells and Intracellular Bacteria Can Differ Proliferation Infection by extracellular bacteria induces production of humoral antibodies, which are ordinarily secreted by plasma Invasion of host tissue cells in regional lymph nodes and the submucosa of the res Toxin-induced damage to host cells piratory and gastrointestinal tracts. The humoral immune response is the main protective response against extracellular Host-defense mechanisms act at each of these steps, and bacteria. The antibodies act in several ways to protect the many bacteria have evolved ways to circumvent some of these host from the invading organisms, including removal of the host defenses ( Table 17-3) bacteria and inactivation of bacterial toxins(Figure 17-8) Some bacteria have surface structures or molecules that Extracellular bacteria can be pathogenic because they induce enhance their ability to attach to host cells. a number of a localized inflammatory response or because they produce gram-negative bacteria, for instance, have pili (long hairlike toxins. The toxins, endotoxin or exotoxin, can be cytotoxic projections), which enable them to attach to the membrane but also may cause pathogenesis in other ways. An excellent of the intestinal or genitourinary tract(Figure 17-9). Other example of this is the toxin produced by diphtheria, which bacteria, such as Bordetella pertussis, secrete adhesion mole exerts a toxic effect on the cell by blocking protein synthesis. cules that attach to both the bacterium and the ciliated Endotoxins, such as lipopolysaccharides(LPS), are generally epithelial cells of the upper respiratory tract components of bacterial cell walls, while exotoxins, such Secretory IgA antibodies specific for such bacterial struc diphtheria toxin, are secreted by the bacteria. ures can block bacterial attachment to mucosal epithelial Antibody that binds to accessible antigens on the surface cells and are the main host defense against bacterial attach of a bacterium can, together with the C3b component of ment. However, some bacteria(e. g, Neisseria gonorrhoeae, complement, act as an opsonin that increases ph agocytoSIS emo US uenzae, and Neisseria meningiti and thus clearance of the bacterium(see Figure 17-8). In the the igA response by secreting proteases that cleave secretory case of some bacteria-notably, the gram-negative organ IgA at the hinge region; the resulting Fab and Fc fragments isms-complement activation can lead directly to lysis of the have a shortened half-life in mucous secretions and are not organism. Antibody-mediated activation of the complement able to agglutinate microorganisms system can also induce localized production of immune Some bacteria evade the igA response of the host by ffector molecules that help to develop an amplified and changing these surface antigens. In N. gonorrhoeae, for ex more effective inflammatory response. For example, the ample, pilin, the protein component of the pili, has a highly complement split products C3a, C4a, and C5a act as anaphy- variable structure Variation in the pilin amino acid sequence latoxins, inducing local mast-cell degranulation and thus is generated by gene rearrangements of its coding sequence vasodilation and the extravasation of lymphocytes and neu- The pilin locus consists of one or two expressed genes and trophils from the blood into tissue space(see Figure 17-8). 10-20 silent genes. Each gene is arranged into six regions Other complement split products serve as chemotactic fac- called minicassettes. Pilin variation is generated by a process tors for neutrophils and macrophages, thereby contributing of gene conversion, in which one or more minicassettes from to the buildup of phagocytic cells at the site of infection. the silent genes replace a minicassette of the expression gene. Antibody to a bacteria toxin may bind to the toxin and n This process generates enormous antigenic variation, which tralize it; the antibody-toxin complexes are then cleared by may contribute to the pathogenicity of N. gonorrhoeae by phagocytic cells in the same manner as any other antigen- increasing the likelihood that expressed pili will bind firmly antibody complex to epithelial cells. In addition, the continual changes in the While innate immunity is not very effective against intra- pilin sequence allow the organism to evade neutralization cellular bacterial pathogens, intracellular bacteria can acti- by IgA ate NK cells, which, in turn, provide an early defense against Some bacteria possess surface structures that serve to these bacteria. Intracellular bacterial infections tend to in- inhibit phagocytosis. a classic example is Streptococcus pneu duce a cell-mediated immune response, specifically, delayed- moniae, whose polysaccharide capsule prevents phagocytosis type hypersensitivity. In this response, cytokines secreted by very effectively. There are 84 serotypes of S pneumoniae that CD4* T cells are important-notably IFN-Y, which activates differ from one another by distinct capsular polysaccharides Gotowww.whfreeman.com/immunology mation Vaccine Strategies See Pathenogenesis
entering and their virulence, different levels of host defense are enlisted. If the inoculum size and the virulence are both low, then localized tissue phagocytes may be able to eliminate the bacteria with an innate, nonspecific defense. Larger inoculums or organisms with greater virulence tend to induce an adaptive, specific immune response. Immune Responses to Extracellular and Intracellular Bacteria Can Differ Infection by extracellular bacteria induces production of humoral antibodies, which are ordinarily secreted by plasma cells in regional lymph nodes and the submucosa of the respiratory and gastrointestinal tracts. The humoral immune response is the main protective response against extracellular bacteria. The antibodies act in several ways to protect the host from the invading organisms, including removal of the bacteria and inactivation of bacterial toxins (Figure 17-8). Extracellular bacteria can be pathogenic because they induce a localized inflammatory response or because they produce toxins. The toxins, endotoxin or exotoxin, can be cytotoxic but also may cause pathogenesis in other ways. An excellent example of this is the toxin produced by diphtheria, which exerts a toxic effect on the cell by blocking protein synthesis. Endotoxins, such as lipopolysaccharides (LPS), are generally components of bacterial cell walls, while exotoxins, such as diphtheria toxin, are secreted by the bacteria. Antibody that binds to accessible antigens on the surface of a bacterium can, together with the C3b component of complement, act as an opsonin that increases phagocytosis and thus clearance of the bacterium (see Figure 17-8). In the case of some bacteria—notably, the gram-negative organisms—complement activation can lead directly to lysis of the organism. Antibody-mediated activation of the complement system can also induce localized production of immune effector molecules that help to develop an amplified and more effective inflammatory response. For example, the complement split products C3a, C4a, and C5a act as anaphylatoxins, inducing local mast-cell degranulation and thus vasodilation and the extravasation of lymphocytes and neutrophils from the blood into tissue space (see Figure 17-8). Other complement split products serve as chemotactic factors for neutrophils and macrophages, thereby contributing to the buildup of phagocytic cells at the site of infection. Antibody to a bacteria toxin may bind to the toxin and neutralize it; the antibody-toxin complexes are then cleared by phagocytic cells in the same manner as any other antigenantibody complex. While innate immunity is not very effective against intracellular bacterial pathogens, intracellular bacteria can activate NK cells, which, in turn, provide an early defense against these bacteria. Intracellular bacterial infections tend to induce a cell-mediated immune response, specifically, delayedtype hypersensitivity. In this response, cytokines secreted by CD4+ T cells are important—notably IFN-, which activates macrophages to kill ingested pathogens more effectively (see Figure 14-15). Bacteria Can Effectively Evade Host Defense Mechanisms There are four primary steps in bacterial infection: ■ Attachment to host cells ■ Proliferation ■ Invasion of host tissue ■ Toxin-induced damage to host cells Host-defense mechanisms act at each of these steps, and many bacteria have evolved ways to circumvent some of these host defenses (Table 17-3). Some bacteria have surface structures or molecules that enhance their ability to attach to host cells. A number of gram-negative bacteria, for instance, have pili (long hairlike projections), which enable them to attach to the membrane of the intestinal or genitourinary tract (Figure 17-9). Other bacteria, such as Bordetella pertussis, secrete adhesion molecules that attach to both the bacterium and the ciliated epithelial cells of the upper respiratory tract. Secretory IgA antibodies specific for such bacterial structures can block bacterial attachment to mucosal epithelial cells and are the main host defense against bacterial attachment. However, some bacteria (e.g., Neisseria gonorrhoeae, Haemophilus influenzae, and Neisseria meningitidis) evade the IgA response by secreting proteases that cleave secretory IgA at the hinge region; the resulting Fab and Fc fragments have a shortened half-life in mucous secretions and are not able to agglutinate microorganisms. Some bacteria evade the IgA response of the host by changing these surface antigens. In N. gonorrhoeae, for example, pilin, the protein component of the pili, has a highly variable structure. Variation in the pilin amino acid sequence is generated by gene rearrangements of its coding sequence. The pilin locus consists of one or two expressed genes and 10–20 silent genes. Each gene is arranged into six regions called minicassettes. Pilin variation is generated by a process of gene conversion, in which one or more minicassettes from the silent genes replace a minicassette of the expression gene. This process generates enormous antigenic variation, which may contribute to the pathogenicity of N. gonorrhoeae by increasing the likelihood that expressed pili will bind firmly to epithelial cells. In addition, the continual changes in the pilin sequence allow the organism to evade neutralization by IgA. Some bacteria possess surface structures that serve to inhibit phagocytosis. A classic example is Streptococcus pneumoniae, whose polysaccharide capsule prevents phagocytosis very effectively. There are 84 serotypes of S. pneumoniae that differ from one another by distinct capsular polysaccharides. 396 PART IV The Immune System in Health and Disease Go to www.whfreeman.com/immunology Animation Vaccine Strategies See Pathenogenesis