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■ Viral Infections ■ Bacterial Infections ■ Protozoan Diseases ■ Diseases Caused by Parasitic Worms (Helminths) ■ Emerging Infectious Diseases
Neisseria gonorrheae Attaching to Urethral Epithelial Cells
Immune Response to
Infectious Diseases
I
susceptible host, a series of coordinated events must circumvent both innate and adaptive immunity. One of the first and most important features of host innate immunity is the barrier provided by the epithelial surfaces of the skin and the lining of the gut. The difficulty of penetrat- ing these epithelial barriers ensures that most pathogens never gain productive entry into the host. In addition to pro- viding a physical barrier to infection, the epithelia also pro- duce chemicals that are useful in preventing infection. The secretion of gastric enzymes by specialized epithelial cells lowers the pH of the stomach and upper gastrointestinal tract, and other specialized cells in the gut produce antibac- terial peptides. A major feature of innate immunity is the presence of the normal gut flora, which can competitively inhibit the bind- ing of pathogens to gut epithelial cells. Innate responses can also block the establishment of infection. For example, the cell walls of some gram-positive bacteria contain a peptido- glycan that activates the alternative complement pathway, resulting in the generation of C3b, which opsonizes bacteria and enhances phagocytosis (see Chapter 13). Some bacteria produce endotoxins such as LPS, which stimulate the pro- duction of cytokines such as TNF-, IL-1, and IL-6 by macrophages or endothelial cells. These cytokines can acti- vate macrophages. Phagocytosis of bacteria by macrophages and other phagocytic cells is another highly effective line of innate defense. However, some types of bacteria that com- monly grow intracellularly have developed mechanisms that allow them to resist degradation within the phagocyte. Viruses are well known for the stimulation of innate responses. In particular, many viruses induce the production of interferons, which can inhibit viral replication by induc- ing an antiviral response. Viruses are also controlled by NK cells. As described in Chapter 14, NK cells frequently form the first line of defense against viral infections. Generally, pathogens use a variety of strategies to escape destruction by the adaptive immune system. Many patho- gens reduce their own antigenicity either by growing within host cells, where they are sequestered from immune attack, or by shedding their membrane antigens. Other pathogens camouflage themselves by mimicking the surfaces of host cells, either by expressing molecules with amino acid se- quences similar to those of host cell-membrane molecules or by acquiring a covering of host membrane molecules. Some pathogens are able to suppress the immune response selec-
tively or to regulate it so that a branch of the immune system is activated that is ineffective against the pathogen. Contin- ual variation in surface antigens is another strategy that enables a pathogen to elude the immune system. This anti- genic variation may be due to the gradual accumulation of mutations, or it may involve an abrupt change in surface antigens. Both innate and adaptive immune responses to patho- gens provide critical defense, but infectious diseases, which have plagued human populations throughout history, still cause the death of millions each year. Although widespread use of vaccines and drug therapy has drastically reduced mortality from infectious diseases in developed countries, such diseases continue to be the leading cause of death in the Third World. It is estimated that over 1 billion people are infected worldwide, resulting in more than 11 million deaths every year (Figure 17-1). Despite these alarming numbers, estimated expenditures for research on infectious diseases prevalent in the Third World are less than 5% of total health- research expenditures worldwide. Not only is this a tragedy for these countries, but some of these diseases are begin- ning to emerge or re-emerge in developed countries. For
chapter 17
example, some United States troops returned from the Per- sian Gulf with leishmaniasis; cholera cases have recently increased worldwide, with more than 100,000 cases reported in KwaZulu-Natal, South Africa, during the summer of 2001;
and a new drug-resistant strain of Mycobacterium tuberculo- sis is spreading at an alarming rate in the United States. In this chapter, the concepts described in earlier chapters, antigenicity (Chapter 3) and immune effector mechanisms (Chapters 12–16), as well as vaccine development (which will be considered in Chapter 18) are applied to selected infec- tious diseases caused by viruses, bacteria, protozoa, and helminths—the four main types of pathogens.
Viral Infections
A number of specific immune effector mechanisms, together with nonspecific defense mechanisms, are called into play to eliminate an infecting virus (Table 17-1). At the same time, the virus acts to subvert one or more of these mechanisms to prolong its own survival. The outcome of the infection de- pends on how effectively the host’s defensive mechanisms resist the offensive tactics of the virus. The innate immune response to viral infection is primar- ily through the induction of type I interferons (IFN- and IFN-) and the activation of NK cells. Double stranded RNA (dsRNA) produced during the viral life cycle can induce the expression of IFN- and IFN- by the infected cell. Macro- phages, monocytes, and fibroblasts also are capable of syn- thesizing these cytokines, but the mechanisms that induce the production of type I interferons in these cells are not completely understood. IFN- and IFN- can induce an antiviral response or resistance to viral replication by bind- ing to the IFN / receptor. Once bound, IFN- and IFN- activate the JAK-STAT pathway, which in turn induces the transcription of several genes. One of these genes encodes an
390 PART I V The Immune System in Health and Disease
Deaths in millions
0 Acute respiratory infections (including pneumonia and influenza)
AIDS Diarrheal TB Malaria Measles diseases
Over age five
Under age five
FIGURE 17-1 Leading infectious disease killers. Data collected and compiled by the World Health Organization in 2000 for deaths in
- HIV-infected individuals who died of TB are included among AIDS deaths.
TABLE 17-1 Mechanisms of humoral and cell-mediated immune responses to viruses
Response type Effector molecule or cell Activity
Humoral Antibody (especially, secretory IgA) Blocks binding of virus to host cells, thus preventing infection or reinfection IgG, IgM, and IgA antibody Blocks fusion of viral envelope with host-cells plasma membrane IgG and IgM antibody Enhances phagocytosis of viral particles (opsonization) IgM antibody Agglutinates viral particles Complement activated by IgG or Mediates opsonization by C3b and lysis IgM antibody of enveloped viral particles by membrane- attack complex
Cell-mediated IFN- secreted by TH or TC cells Has direct antiviral activity Cytotoxic T lymphocytes (CTLs) Kill virus-infected self-cells NK cells and macrophages Kill virus-infected cells by antibody- dependent cell-mediated cytotoxicity (ADCC)
adoptive transfer: adoptive transfer of a CTL clone specific for influenza virus strain X protects mice against influenza virus X but not against influenza virus strain Y.
Viruses Can Evade Host Defense
Mechanisms
Despite their restricted genome size, a number of viruses have been found to encode proteins that interfere at various levels with specific or nonspecific host defenses. Presumably, the advantage of such proteins is that they enable viruses to replicate more effectively amidst host antiviral defenses. As described above, the induction of IFN- and IFN- is a major innate defense against viral infection, but some viruses have developed strategies to evade the action of IFN-/. These include hepatitis C virus, which has been shown to overcome the antiviral effect of the interferons by blocking or inhibiting the action of PKR (see Figure 17-2). Another mechanism for evading host responses, utilized in particular by herpes simplex viruses (HSV) is inhibition of antigen presentation by infected host cells. HSV-1 and HSV-2 both express an immediate-early protein (a protein synthesized shortly after viral replication) called ICP47, which very effectively inhibits the human transporter mole- cule needed for antigen processing (TAP; see Figure 8-8). Inhibition of TAP blocks antigen delivery to class I MHC re- ceptors on HSV-infected cells, thus preventing presentation of viral antigen to CD8+^ T cells. This results in the trapping of empty class I MHC molecules in the endoplasmic reticu- lum and effectively shuts down a CD8+^ T-cell response to HSV-infected cells. The targeting of MHC molecules is not unique to HSV. Other viruses have been shown to down-regulate class I MHC expression shortly after infection. Two of the best- characterized examples, the adenoviruses and cytomegalo- virus (CMV), use distinct molecular mechanisms to reduce the surface expression of class I MHC molecules, again in- hibiting antigen presentation to CD8+^ T cells. Some viruses— CMV, measles virus, and HIV—have been shown to reduce levels of class II MHC molecules on the cell surface, thus blocking the function of antigen-specific antiviral helper T cells. Antibody-mediated destruction of viruses requires com- plement activation, resulting either in direct lysis of the viral particle or opsonization and elimination of the virus by phagocytic cells. A number of viruses have strategies for evad- ing complement-mediated destruction. Vaccinia virus, for example, secretes a protein that binds to the C4b complement component, inhibiting the classical complement pathway; and herpes simplex viruses have a glycoprotein component that binds to the C3b complement component, inhibiting both the classical and alternative pathways. A number of viruses escape immune attack by constantly changing their antigens. In the influenza virus, continual antigenic variation results in the frequent emergence of new infectious strains. The absence of protective immunity to
these newly emerging strains leads to repeated epidemics of influenza. Antigenic variation among rhinoviruses, the causa- tive agent of the common cold, is responsible for our inabil- ity to produce an effective vaccine for colds. Nowhere is anti- genic variation greater than in the human immunodeficiency virus (HIV), the causative agent of AIDS. Estimates suggest that HIV accumulates mutations at a rate 65 times faster than does influenza virus. Because of the importance of AIDS, a section of Chapter 19 addresses this disease. A large number of viruses evade the immune response by causing generalized immunosuppression. Among these are the paramyxoviruses that cause mumps, the measles virus, Epstein-Barr virus (EBV), cytomegalovirus, and HIV. In some cases, immunosuppression is caused by direct viral in- fection of lymphocytes or macrophages. The virus can then either directly destroy the immune cells by cytolytic mecha- nisms or alter their function. In other cases, immunosup- pression is the result of a cytokine imbalance. For example, EBV produces a protein, called BCRF1, that is homologous to IL-10; like IL-10, BCRF1 suppresses cytokine production by the TH1 subset, resulting in decreased levels of IL-2, TNF, and IFN-.
Influenza Has Been Responsible for Some of the Worst Pandemics in History
The influenza virus infects the upper respiratory tract and major central airways in humans, horses, birds, pigs, and even seals. In 1918–19, an influenza pandemic (worldwide epidemic) killed more than 20 million people, a toll surpass- ing the number of casualties in World War I. Some areas, such as Alaska and the Pacific Islands, lost more than half of their population during that pandemic.
PROPERTIES OF THE INFLUENZA VIRUS Influenza viral particles, or virions, are roughly spherical or ovoid in shape, with an average diameter of 90–100 nm. The virions are surrounded by an outer envelope—a lipid bilayer acquired from the plasma membrane of the infected host cell during the process of budding. Inserted into the envelope are two glycoproteins, hemagglutinin (HA) and neuraminidase (NA), which form radiating projections that are visible in electron micrographs (Figure 17-3). The hemagglutinin pro- jections, in the form of trimers, are responsible for the attachment of the virus to host cells. There are approximately 1000 hemagglutinin projections per influenza virion. The hemagglutinin trimer binds to sialic acid groups on host-cell glycoproteins and glycolipids by way of a conserved amino acid sequence that forms a small groove in the hemagglu- tinin molecule. Neuraminidase, as its name indicates, cleaves N -acetylneuraminic (sialic) acid from nascent viral glyco- proteins and host-cell membrane glycoproteins, an activity that presumably facilitates viral budding from the infected host cell. Within the envelope, an inner layer of matrix pro- tein surrounds the nucleocapsid, which consists of eight dif-
392 PART I V The Immune System in Health and Disease
ferent strands of single-stranded RNA (ssRNA) associated with protein and RNA polymerase (Figure 17-4). Each RNA strand encodes one or more different influenza proteins. Three basic types of influenza (A, B, and C), can be distin- guished by differences in their nucleoprotein and matrix pro- teins. Type A, which is the most common, is responsible for the major human pandemics. Antigenic variation in hemagglu- tinin and neuraminidase distinguishes subtypes of type A in- fluenza virus. According to the nomenclature of the World Health Organization, each virus strain is defined by its animal host of origin (specified, if other than human), geographical origin, strain number, year of isolation, and antigenic descrip- tion of HA and NA (Table 17-2). For example, A/Sw/Iowa/ 15/30 (H1N1) designates strain-A isolate 15 that arose in swine in Iowa in 1930 and has antigenic subtypes 1 of HA and NA. Notice that the H and N spikes are antigenically distinct in these two strains. There are 13 different hemagglutinins and 9 neu- raminidases among the type A influenza viruses. The distinguishing feature of influenza virus is its vari- ability. The virus can change its surface antigens so com- pletely that the immune response to infection with the virus that caused a previous epidemic gives little or no protection against the virus causing a subsequent epidemic. The anti- genic variation results primarily from changes in the hemag- glutinin and neuraminidase spikes protruding from the viral envelope (Figure 17-5). Two different mechanisms generate antigenic variation in HA and NA: antigenic drift and anti- genic shift. Antigenic drift involves a series of spontaneous point mutations that occur gradually, resulting in minor changes in HA and NA. Antigenic shift results in the sudden
emergence of a new subtype of influenza whose HA and pos- sibly also NA are considerably different from that of the virus present in a preceding epidemic. The first time a human influenza virus was isolated was in 1934; this virus was given the subtype designation H0N (where H is hemagglutinin and N is neuraminidase). The H0N1 subtype persisted until 1947, when a major antigenic shift generated a new subtype, H1N1, which supplanted the previous subtype and became prevalent worldwide until 1957, when H2N2 emerged. The H2N2 subtype prevailed for the next decade and was replaced in 1968 by H3N2. Antigenic shift in 1977 saw the re-emergence of H1N1. The most recent antigenic shift, in 1989, brought the re-emergence of H3N2, which remained dominant throughout the next several years. However, an H1N1 strain re-emerged in Texas in 1995, and current influenza vaccines contain both H3N2 and H1N strains. With each antigenic shift, hemagglutinin and neu- raminidase undergo major sequence changes, resulting in major antigenic variations for which the immune system lacks memory. Thus, each antigenic shift finds the population immunologically unprepared, resulting in major outbreaks of influenza, which sometimes reach pandemic proportions.
Immune Response to Infectious Diseases CH APTER 17 393
Matrix protein
Lipid bilayer
Hemagglutinin
Neuraminidase
Nucleocapsid
NS1, NS
M1, M
PB
PB
PA
HA
NP
NA
0 10 20 30 40 50 Nanometers
FIGURE 17-3 Electron micrograph of influenza virus reveals roughly spherical viral particles enclosed in a lipid bilayer with protruding hemagglutinin and neuraminidase glycoprotein spikes. [Courtesy of G. Murti, Department of Virology, St. Jude Children’s Research Hospital, Memphis, Tenn.]
FIGURE 17-4 Schematic representation of influenza structure. The envelope is covered with neuraminidase and hemagglutinin spikes. In- side is an inner layer of matrix protein surrounding the nucleocapsid, which consists of eight ssRNA molecules associated with nucleopro- tein. The eight RNA strands encode ten proteins: PB1, PB2, PA, HA (hemagglutinin), NP (nucleoprotein), NA (neuraminidase), M1, M2, NS1, and NS2.
O.1 m
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 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 the disease. Instead, the serum antibody appears to play a sig- nificant 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 experimen- tally 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 re- sponses 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 gas- 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
Immune Response to Infectious Diseases CH APTER 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 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 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 re- gions—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 re- sponse. 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.
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 in- oculums 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 res- piratory 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 organ- isms—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 anaphy- latoxins, inducing local mast-cell degranulation and thus vasodilation and the extravasation of lymphocytes and neu- trophils from the blood into tissue space (see Figure 17-8). Other complement split products serve as chemotactic fac- tors 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 neu- tralize it; the antibody-toxin complexes are then cleared by phagocytic cells in the same manner as any other antigen- antibody complex. While innate immunity is not very effective against intra- cellular bacterial pathogens, intracellular bacteria can acti- vate NK cells, which, in turn, provide an early defense against these bacteria. Intracellular bacterial infections tend to in- duce a cell-mediated immune response, specifically, delayed- type 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 mole- cules that attach to both the bacterium and the ciliated epithelial cells of the upper respiratory tract. Secretory IgA antibodies specific for such bacterial struc- tures can block bacterial attachment to mucosal epithelial cells and are the main host defense against bacterial attach- ment. 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 ex- ample, 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 pneu- moniae, 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 I V The Immune System in Health and Disease
Go to www.whfreeman.com/ immunology Animation Vaccine Strategies See Pathenogenesis
During infection, the host produces antibody against the infecting serotype. This antibody protects against reinfection with the same serotype but will not protect against infection by a different serotype. In this way, S. pneumoniae can cause disease many times in the same individual. On other bacteria, such as Streptococcus pyogenes, a surface protein projection called the M protein inhibits phagocytosis. Some pathogenic staphylococci are able to assemble a protective coat from host proteins. These bacteria secrete a coagulase enzyme that pre- cipitates a fibrin coat around them, shielding them from phagocytic cells.
Mechanisms for interfering with the complement system help other bacteria survive. In some gram-negative bacteria, for example, long side chains on the lipid A moiety of the cell-wall core polysaccharide help to resist complement- mediated lysis. Pseudomonas secretes an enzyme, elastase, that inactivates both the C3a and C5a anaphylatoxins, there- by diminishing the localized inflammatory reaction. A number of bacteria escape host defense mechanisms by their ability to survive within phagocytic cells. Some, such as Listeria monocytogenes, do this by escaping from the phago- lysosome to the cytoplasm, which is a more favorable environ- ment for their growth. Other bacteria, such as Mycobacterium avium, block lysosomal fusion with the phagolysosome; and some mycobacteria are resistant to the oxidative attack that takes place within the phagolysosome.
Immune Responses Can Contribute to Bacterial Pathogenesis In some cases, disease is caused not by the bacterial pathogen itself but by the immune response to the pathogen. As described in Chapter 12, pathogen-stimulated overproduc- tion of cytokines leads to the symptoms of bacterial septic shock, food poisoning, and toxic-shock syndrome. For in- stance, cell-wall endotoxins of some gram-negative bacteria activate macrophages, resulting in release of high levels of IL-1 and TNF-, which can cause septic shock. In staphylo- coccal food poisoning and toxic-shock syndrome, exotoxins produced by the pathogens function as superantigens, which can activate all T cells that express T-cell receptors with a par- ticular V domain (see Table 10-4). The resulting overpro- duction of cytokines by activated TH cells causes many of the symptoms of these diseases.
398 PART I V The Immune System in Health and Disease
TABLE 17-3 Host immune responses to bacterial infection and bacterial evasion mechanisms
Infection process Host defense Bacterial evasion mechanisms
Attachment to host Blockage of attachment by Secretion of proteases that cleave secretory IgA dimers cells secretory IgA antibodies ( Neisseria meningitidis, N. gonorrhoeae, Haemophilus influenzae ) Antigenic variation in attachment structures (pili of N. gonorrhoeae )
Proliferation Phagocytosis (Ab- and Production of surface structures (polysaccharide capsule, M protein, C3b-mediated opsonization) fibrin coat) that inhibit phagocytic cells Mechanisms for surviving within phagocytic cells Induction of apoptosis in macrophages ( Shigella flexneri ) Complement-mediated lysis and Generalized resistance of gram-positive bacteria to complement- localized inflammatory response mediated lysis Insertion of membrane-attack complex prevented by long side chain in cell-wall LPS (some gram-negative bacteria) Invasion of host tissues Ab-mediated agglutination Secretion of elastase that inactivates C3a and C5a ( Pseudomonas )
Toxin-induced damage Neturalization of toxin by antibody Secretion of hyaluronidase, which enhances bacterial invasiveness to host cells
FIGURE 17-9 Electron micrograph of Neisseria gonorrhoeae at- taching to urethral epithelial cells. Pili (P) extend from the gonococ- cal surface and mediate the attachment. [From M. E. Ward and P. J. Watt, 1972, J. Inf. Dis. 126: 601.]
The ability of some bacteria to survive intracellularly with- in infected cells can result in chronic antigenic activation of CD4+^ T cells, leading to tissue destruction by a cell-mediated response with the characteristics of a delayed-type hypersen- sitivity reaction (see Chapter 14). Cytokines secreted by these activated CD4+^ T cells can lead to extensive accumulation and activation of macrophages, resulting in formation of a granuloma. The localized concentrations of lysosomal en- zymes in these granulomas can cause extensive tissue necro- sis. Much of the tissue damage seen with M. tuberculosis is due to a cell-mediated immune response.
Diphtheria ( Corynebacterium diphtheriae )
May Be Controlled by Immunization with
Inactivated Toxoid
Diphtheria is the classic example of a bacterial disease caused by a secreted exotoxin to which immunity can be induced by immunization with an inactivated toxoid. The causative agent, a gram-positive, rodlike organism called Corynebac- terium diphtheriae, was first described by Klebs in 1883 and was shown a year later by Loeffler to cause diphtheria in guinea pigs and rabbits. Autopsies on the infected animals revealed that, while bacterial growth was limited to the site of inoculation, there was widespread damage to a variety of organs, including the heart, liver, and kidneys. This finding led Loeffler to speculate that the neurologic and cardiologic manifestations of the disease were caused by a toxic sub- stance elaborated by the organism. Loeffler’s hypothesis was validated in 1888, when Roux and Yersin produced the disease in animals by injecting them with a sterile filtrate from a culture of C. diphtheriae. Two years later, von Behring showed that an antiserum to the toxin was able to prevent death in infected animals. He pre- pared a toxoid by treating the toxin with iodine trichloride and demonstrated that it could induce protective antibodies in animals. However, the toxoid was still quite toxic and therefore unsuitable for use in humans. In 1923, Ramon found that exposing the toxin to heat and formalin rendered it nontoxic but did not destroy its antigenicity. Clinical trials showed that formaldehyde-treated toxoid conferred a high level of protection against diphtheria. As immunizations with the toxoid increased, the number of cases of diphtheria decreased rapidly. In the 1920s, there were approximately 200 cases of diphtheria per 100,000 pop- ulation in the United States. In 1989, the Centers for Disease Control reported only three cases of diphtheria in the entire United States. Recently in the former Soviet Union, there has been an alarming epidemic of diphtheria due to a reduction in vaccinations. Natural infection with C. diphtheriae occurs only in hu- mans. The disease is spread from one individual to another by airborne respiratory droplets. The organism colonizes the nasopharyngeal tract, remaining in the superficial layers of the respiratory mucosa. Growth of the organism itself causes
little tissue damage, and only a mild inflammatory reaction develops. The virulence of the organism is due completely to its potent exotoxin. The toxin causes destruction of the underlying tissue, resulting in the formation of a tough fibri- nous membrane (“pseudomembrane”) composed of fibrin, white blood cells, and dead respiratory epithelial cells. The membrane itself can cause suffocation. The exotoxin also is responsible for widespread systemic manifestations. Pro- nounced myocardial damage (often leading to congestive heart failure) and neurologic damage (ranging from mild weakness to complete paralysis) are common. The exotoxin that causes diphtheria symptoms is encoded by the tox gene carried by phage . Within some strains of C. diphtheriae, phage can exist in a state of lysogeny, in which the -prophage DNA persists within the bacterial cell. Only strains that carry lysogenic phage are able to produce the exotoxin. The diphtheria exotoxin contains two disulfide- linked chains, a binding chain and toxin chain. The binding chain interacts with ganglioside receptors on susceptible cells, facilitating internalization of the exotoxin. Toxicity re- sults from the inhibitory effect of the toxin chain on protein synthesis. The diphtheria exotoxin is extremely potent; a sin- gle molecule has been shown to kill a cell. Removal of the binding chain prevents the exotoxin from entering the cell, thus rendering the exotoxin nontoxic. As described in Chap- ter 4, an immunotoxin can be prepared by replacing the binding chain with a monoclonal antibody specific for a tumor-cell surface antigen; in this way the toxin chain can be targeted to tumor cells (see Figure 4-23). Today, diphtheria toxoid is prepared by treating diphthe- ria toxin with formaldehyde. The reaction with formalde- hyde cross-links the toxin, resulting in an irreversible loss in its toxicity while enhancing its antigenicity. The toxoid is administered together with tetanus toxoid and inactivated Bordetella pertussis in a combined vaccine that is given to children beginning at 6–8 weeks of age. Immunization with the toxoid induces the production of antibodies (antitoxin), which can bind to the toxin and neutralize its activity. Be- cause antitoxin levels decline slowly over time, booster doses are recommended at 10-year intervals to maintain antitoxin levels within the protective range. Interestingly, antibodies specific for epitopes on the binding chain of the diphtheria exotoxin are critical for toxin neutralization because these antibodies block internalization of the active toxin chain.
Tuberculosis ( Mycobacterium tuberculosis ) Is Primarily Controlled by CD
T Cells Tuberculosis is the leading cause of death in the world from a single infectious agent, killing about 3 million individuals every year and accounting for 18.5% of all deaths in adults between the ages of 15 and 59. About 1.79 billion people, roughly one-third of the world’s population, are infected with the causative agent M. tuberculosis and are at risk of develop- ing the disease. Long thought to have been eliminated as a
Immune Response to Infectious Diseases CH APTER 17 399
after treatment begins. To avoid the side effects associated with the usual antibiotic therapy, many patients, once they feel better, stop taking the medications long before the recom- mended treatment period is completed. Because briefer treat- ment may not eradicate organisms that are somewhat resistant to the antibiotics, a multidrug-resistant strain can emerge. Noncompliance with required treatment regimes, one of the most troubling aspects of the large number of current tuber- culosis cases, clearly compromises efforts to contain the spread of the disease. Presently, the only vaccine for M. tuberculosis is an attenu- ated strain of M. bovis called BCG (Bacillus Calmette-Guerin). The vaccine appears to provide fairly effective protection against extrapulmonary tuberculosis but has been inconsis- tent against pulmonary tuberculosis. In different studies, BCG has provided protection in anywhere from 0% to 80% of vac- cinated individuals; in some cases, BCG vaccination has even increased the risk of infection. Moreover, after BCG vaccina- tion, the tuberculin skin test cannot be used as an effective monitor of exposure to M. tuberculosis. Because of the variable effectiveness of the BCG vaccine and the inability to monitor for exposure with the skin test after vaccination, this vaccine is not used in the United States. However, the alarming increase in multidrug-resistant strains has stimulated renewed efforts to develop a more effective tuberculosis vaccine.
Protozoan Diseases
Protozoans are unicellular eukaryotic organisms. They are responsible for several serious diseases in humans, includ- ing amoebiasis, Chagas’ disease, African sleeping sickness, malaria, leishmaniasis, and toxoplasmosis. The type of im- mune response that develops to protozoan infection and the effectiveness of the response depend in part on the location of the parasite within the host. Many protozoans have life-cycle stages in which they are free within the bloodstream, and it is during these stages that humoral antibody is most effective. Many of these same pathogens are also capable of intracellular growth; during these stages, cell-mediated immune reactions are effective in host defense. In the development of vaccines for protozoan diseases, the branch of the immune system that is most likely to confer protection must be carefully considered.
Malaria ( Plasmodium Species) Infects
600 Million People Worldwide
Malaria is one of the most devastating diseases in the world today, infecting nearly 10% of the world population and causing 1–2 million deaths every year. Malaria is caused by various species of the genus Plasmodium, of which P. falci- parum is the most virulent and prevalent. The alarming development of multiple-drug resistance in Plasmodium and the increased resistance of its vector, the Anopheles mosquito, to DDT underscore the importance of developing new stra- tegies to hinder the spread of malaria.
PLASMODIUM LIFE CYCLE AND PATHOGENESIS OF MALARIA Plasmodium progresses through a remarkable series of devel- opmental and maturational stages in its extremely complex life cycle. Female Anopheles mosquitoes, which feed on blood meals, serve as the vector for Plasmodium, and part of the parasite’s life cycle takes place within the mosquito. (Because male Anopheles mosquitoes feed on plant juices, they do not transmit Plasmodium. ) Human infection begins when sporozoites, one of the Plasmodium stages, are introduced into an individual’s blood- stream as an infected mosquito takes a blood meal (Figure 17-11). Within 30 min, the sporozoites disappear from the
Immune Response to Infectious Diseases CH APTER 17 401
Sporozoites
Liver
Merozoites
Gametocytes
In mosquito gut
RBC
FIGURE 17-11 The life cycle of Plasmodium. Sporozoites enter the bloodstream when an infected mosquito takes a blood meal. The sporozoites migrate to the liver, where they multiply, transforming liver hepatocytes into giant multinucleate schizonts, which release thousands of merozoites into the bloodstream. The merozoites in- fect red blood cells, which eventually rupture, releasing more mero- zoites. Eventually some of the merozoites differentiate into male and female gametocytes, which are ingested by a mosquito and differen- tiate into gametes. The gametes fuse to form a zygote that differenti- ates to the sporozoite stage within the salivary gland of the mosquito.
blood as they migrate to the liver, where they infect hepato- cytes. Sporozoites are long, slender cells that are covered by a 45-kDa protein called circumsporozoite (CS) antigen, which appears to mediate their adhesion to hepatocytes. The binding site on the CS antigen is a conserved region in the carboxyl-terminal end (called region II) that has a high degree of sequence homology with known cell-adhesion molecules. Within the liver, the sporozoites multiply extensively and undergo a complex series of transformations that culminate in the formation and release of merozoites in about a week. It has been estimated that a liver hepatocyte infected with a sin- gle sporozoite can release 5,000–10,000 merozoites. The re- leased merozoites infect red blood cells, initiating the symp- toms and pathology of malaria. Within a red blood cell, merozoites replicate and undergo successive differentiations; eventually the cell ruptures and releases new merozoites, which go on to infect more red blood cells. Eventually some of the merozoites differentiate into male and female gameto- cytes, which may be ingested by a female Anopheles mosquito during a blood meal. Within the mosquito’s gut, the male and female gametocytes differentiate into gametes that fuse to form a zygote, which multiplies and differentiates into sporo- zoites within the salivary gland. The infected mosquito is now set to initiate the cycle once again. The symptoms of malaria are recurrent chills, fever, and sweating. The symptoms peak roughly every 48 h, when suc- cessive generations of merozoites are released from infected red blood cells. An infected individual eventually becomes weak and anemic and shows splenomegaly. The large num- bers of merozoites formed can block capillaries, causing intense headaches, renal failure, heart failure, or cerebral damage—often with fatal consequences. There is speculation that some of the symptoms of malaria may be caused not by Plasmodium itself but instead by excessive production of cytokines. This hypothesis stemmed from the observation that cancer patients treated in clinical trials with recombi- nant tumor necrosis factor (TNF) developed symptoms that mimicked malaria. The relation between TNF and malaria symptoms was studied by infecting mice with a mouse- specific strain of Plasmodium, which causes rapid death by cerebral malaria. Injection of these mice with antibodies to TNF was shown to prevent the rapid death.
HOST RESPONSE TO PLASMODIUM INFECTION
In regions where malaria is endemic, the immune response to Plasmodium infection is poor. Children less than 14 years old mount the lowest immune response and consequently are most likely to develop malaria. In some regions, the child- hood mortality rate for malaria reaches 50%, and worldwide the disease kills about a million children a year. The low im- mune response to Plasmodium among children can be demonstrated by measuring serum antibody levels to the sporozoite stage. Only 22% of the children living in endemic areas have detectable antibodies to the sporozoite stage,
whereas 84% of the adults have such antibodies. Even in adults, the degree of immunity is far from complete, how- ever, and most people living in endemic regions have lifelong low-level Plasmodium infections. A number of factors may contribute to the low levels of immune responsiveness to Plasmodium. The maturational changes from sporozoite to merozoite to gametocyte allow the organism to keep changing its surface molecules, result- ing in continual changes in the antigens seen by the immune system. The intracellular phases of the life cycle in liver cells and erythrocytes also reduce the degree of immune activa- tion generated by the pathogen and allow the organism to multiply while it is shielded from attack. Furthermore, the most accessible stage, the sporozoite, circulates in the blood for only about 30 min before it infects liver hepatocytes; it is unlikely that much immune activation can occur in such a short period of time. And even when an antibody response does develop to sporozoites, Plasmodium has evolved a way of overcoming that response by sloughing off the surface CS- antigen coat, thus rendering the antibodies ineffective.
DESIGN OF MALARIA VACCINES An effective vaccine for malaria should maximize the most effective immune defense mechanisms. Unfortunately, little is known of the roles that humoral and cell-mediated responses play in the development of protective immunity to this disease. Current approaches to design of malaria vac- cines focus largely on the sporozoite stage. One experimental vaccine, for example, consists of Plasmodium sporozoites at- tenuated by x-irradiation. In one study, nine volunteers were repeatedly immunized by the bite of P. falciparum –infected, irradiated mosquitoes. Later challenge by the bites of mos- quitoes infected with virulent P. falciparum revealed that six of the nine recipients were completely protected. These re- sults are encouraging, but translating these findings into mass immunization remains problematic. Sporozoites do not grow well in cultured cells, so an enormous insectory would be required to breed mosquitoes in which to prepare enough irradiated sporozoites to vaccinate just one small village. Current vaccine strategies are aimed at producing syn- thetic subunit vaccines consisting of epitopes that can be rec- ognized by T cells and B cells. While no effective vaccine has been developed, this is an active area of investigation.
African Sleeping Sickness ( Trypanosoma Species) Two species of African trypanosomes, which are flagellated protozoans, can cause sleeping sickness, a chronic, debilitat- ing disease transmitted to humans and cattle by the bite of the tsetse fly. In the bloodstream, a trypanosome differentiates into a long, slender form that continues to divide every 4–6 h. The disease progresses through several stages, beginning with an early (systemic) stage in which trypanosomes multiply in the blood and progressing to a neurologic stage in which the
402 PART I V The Immune System in Health and Disease
subsequent phagocytosis. However, about 1% of the organ- isms, which bear an antigenically different VSG, escape the initial antibody response. These surviving organisms now begin to proliferate in the bloodstream, and a new wave of parasitemia is observed. The successive waves of parasitemia reflect a unique mechanism of antigenic shift by which the trypanosomes can evade the immune response to their gly- coprotein antigens. This process is so effective that each new variant that arises in the course of a single infection is able to escape the humoral antibodies generated in response to the preceding variant, so that waves of parasitemia recur (Figure 17-12a). Several unusual genetic processes generate the extensive variation in trypanosomal VSG that enables the organism to escape immunologic clearance. An individual trypanosome carries a large repertoire of VSG genes, each encoding a dif- ferent VSG primary sequence. Trypanosoma brucei, for ex- ample, contains more than 1000 VSG genes in its genome, clustered at multiple chromosomal sites. A trypanosome expresses only a single VSG gene at a time. Activation of a VSG gene results in duplication of the gene and its transposi- tion to a transcriptionally active expression site (ES) at the telomeric end of specific chromosomes (Figure 17-12b). Activation of a new VSG gene displaces the previous gene from the telomeric expression site. A number of chromo- somes in the trypanosome have transcriptionally active expression sites at the telomeric ends, so that a number of VSG genes can potentially be expressed, but unknown con- trol mechanisms limit expression to a single VSG expression site at a time. There appears to be some order in the VSG variation dur- ing infection. Each new variant arises not by clonal out- growth from a single variant cell but instead from the growth of multiple cells that have activated the same VSG gene in the current wave of parasite growth. It is not known how this process is regulated among individual trypanosomes. The continual shifts in epitopes displayed by the VSG make the development of a vaccine for African sleeping sickness ex- tremely difficult.
Leishmaniasis Is a Useful Model for
Demonstrating Differences in Host
Responses
The protozoan parasite Leishmania major provides a power- ful and illustrative example of how host responses can differ between individuals. These differences can lead to either clearance of the parasite or fatality from the infection. Leish- mania is a protozoan that lives in the phagosomes of macro- phages. Resistance to the infection correlates well with the production of IFN- and the development of a TH1 re- sponse. Elegant studies in mice have demonstrated that strains that are resistant to Leishmania develop a TH1 re- sponse and produce IFN- upon infection. Such strains of mice become highly susceptible to Leishmania -induced fatal-
ity if they lose either IFN- or the IFN- receptor, further underscoring the importance of IFN- in containing the infection. A few strains of mice, such as BALB/c, are highly susceptible to Leishmania, and these animals frequently suc- cumb to infection. These mice mount a TH2-type response to Leishmania infection; they produce high levels of IL-4 and essentially no IFN-. Thus, one difference between an effec- tive and an ineffective defense against the parasite is the development of a TH1 response or a TH2 response. Recent studies demonstrate that one difference between the resistant strains of mice and BALB/c is that a small restricted subset of BALB/c CD4+^ T cells are capable of recognizing a particular epitope on L. major, and this subset produces high levels of IL-4 early in the response to the parasite. This skews the response toward a predominantly TH2 type. Understanding how these different T-helper responses affect the outcome of infection could provide a more rational approach to the de- sign of effective treatments and successful vaccines for other pathogens.
Diseases Caused by Parasitic
Worms (Helminths)
Unlike protozoans, which are unicellular and often grow within human cells, helminths are large, multicellular organ- isms that reside in humans but do not ordinarily multiply there and are not intracellular pathogens. Although hel- minths are more accessible to the immune system than pro- tozoans, most infected individuals carry few of these para- sites; for this reason, the immune system is not strongly engaged and the level of immunity generated to helminths is often very poor. Parasitic worms are responsible for a wide variety of dis- eases in both humans and animals. More than a billion peo- ple are infected with Ascaris, a parasitic roundworm that infects the small intestine, and more than 300 million people are infected with Schistosoma, a trematode worm that causes a chronic debilitating infection. Several helminths are impor- tant pathogens of domestic animals and invade humans who ingest contaminated food. These helminths include Taenia, a tapeworm of cattle and pigs, and Trichinella, the roundworm of pigs that causes trichinosis. Several Schistosoma species are responsible for the chronic, debilitating, and sometimes fatal disease schistoso- miasis (formerly known as bilharzia ). Three species, S. man- soni, S. japonicum, and S. haematobium, are the major pathogens in humans, infecting individuals in Africa, the Middle East, South America, the Caribbean, China, South- east Asia, and the Philippines. A rise in the incidence of schis- tosomiasis in recent years has paralleled the increasing worldwide use of irrigation, which has expanded the habitat of the freshwater snail that serves as the intermediate host for schistosomes.
404 PART I V The Immune System in Health and Disease
Infection occurs through contact with free-swimming infectious larvae, called cercariae, which are released from an infected snail at the rate of 300–3000 per day. When cercariae contact human skin, they secrete digestive enzymes that help them to bore into the skin, where they shed their tails and are transformed into schistosomules. The schistosomules enter the capillaries and migrate to the lungs, then to the liver, and finally to the primary site of infection, which varies with the species. S. mansoni and S. japonicum infect the intestinal mesenteric veins; S. haematobium infects the veins of the uri- nary bladder. Once established in their final tissue site, schis- tosomules mature into male and female adult worms. The worms mate and the females produce at least 300 spiny eggs a day. Unlike protozoan parasites, schistosomes and other helminths do not multiply within their hosts. The eggs pro- duced by the female worm do not mature into adult worms in humans; instead, some of them pass into the feces or urine and are excreted to infect more snails. The number of worms in an infected individual increases only through repeated ex- posure to the free-swimming cercariae, and so most infected individuals carry rather low numbers of worms. Most of the symptoms of schistosomiasis are initiated by the eggs. As many as half of the eggs produced remain in the host, where they invade the intestinal wall, liver, or bladder and cause hemorrhage. A chronic state can then develop in which the adult worms persist and the unexcreted eggs in- duce cell-mediated delayed-type hypersensitive reactions, resulting in large granulomas that are gradually walled off by fibrous tissue. Although the eggs are contained by the forma- tion of the granuloma, often the granuloma itself obstructs the venous blood flow to the liver or bladder. Although an immune response does develop to the schis- tosomes, in most individuals it is not sufficient to eliminate the adult worms, even though the intravascular sites of schis- tosome infestation should make the worm an easy target for immune attack. Instead, the worms survive for up to 20 years. The schistosomules would appear to be the forms most sus- ceptible to attack, but because they are motile, they can evade the localized cellular buildup of immune and inflammatory cells. Adult schistosome worms also have several unique mechanisms that protect them from immune defenses. The adult worm has been shown to decrease the expression of antigens on its outer membrane and also to enclose itself in a glycolipid and glycoprotein coat derived from the host, masking the presence of its own antigens. Among the anti- gens observed on the adult worm are the host’s own ABO blood-group antigens and histocompatibility antigens! The immune response is, of course, diminished by this covering made of the host’s self-antigens, which must contribute to the lifelong persistence of these organisms. The relative importance of the humoral and cell- mediated responses in protective immunity to schistosomia- sis is controversial. The humoral response to infection with S. mansoni is characterized by high titers of antischistosome IgE antibodies, localized increases in mast cells and their sub-
sequent degranulation, and increased numbers of eosino- phils (Figure 17-13, top ). These manifestations suggest that cytokines produced by a TH2-like subset are important for the immune response: IL-4, which induces B cells to class- switch to IgE production; IL-5, which induces bone-marrow precursors to differentiate into eosinophils; and IL-3, which (along with IL-4) stimulates growth of mast cells. Degranula- tion of mast cells releases mediators that increase the infiltra- tion of such inflammatory cells as macrophages and eosino- phils. The eosinophils express Fc receptors for IgE and IgG and bind to the antibody-coated parasite. Once bound to the parasite, an eosinophil can participate in antibody- dependent cell-mediated cytotoxicity (ADCC), releasing me- diators from its granules that damage the parasite (see Figure 14-12). One eosinophil mediator, called basic protein, is par- ticularly toxic to helminths. Immunization studies with mice, however, suggest that this humoral IgE response may not provide protective im- munity. When mice are immunized with S. mansoni vaccine, the protective immune response that develops is not an IgE response, but rather a TH1 response characterized by IFN- production and macrophage accumulation (Figure 17-13, bottom ). Furthermore, inbred strains of mice with deficien- cies in mast cells or IgE develop protective immunity from vaccination, whereas inbred strains with deficiencies in cell- mediated CD4+^ T-cell responses fail to develop protective immunity in response to the vaccine. These studies suggest that the CD4+^ T-cell response may be the most important in immunity to schistosomiasis. It has been suggested that the ability to induce an ineffective TH2-like response may have evolved in schistosomes as a clever defense mechanism to ensure that TH2 cells produced sufficient levels of IL-10 to inhibit protective immunity mediated by the TH1-like subset in the CD4+^ T response. Antigens present on the membrane of cercariae and young schistosomules are promising vaccine components because these stages appear to be most susceptible to im- mune attack. Injecting mice and rats with monoclonal anti- bodies to cercariae and young schistosomules passively transferred resistance to infection with live cercariae. When these protective antibodies were used in affinity columns to purify schistosome membrane antigens from crude mem- brane extracts, it was found that mice immunized and boosted with these purified antigens exhibited increased resistance to a later challenge with live cercariae. Schisto- some cDNA libraries were then established and screened with the protective monoclonal antibodies to identify those cDNAs encoding surface antigens. Experiments using cloned cercariae or schistosomule antigens are presently under way to assess their ability to induce protective immu- nity in animal models. However, in developing an effective vaccine for schistosomiasis, a fine line separates a beneficial immune response, which at best limits the parasite load, from a detrimental response, which in itself may become pathologic.
Immune Response to Infectious Diseases CH APTER 17 405
infectious diseases. The re-emergence of these diseases should not be surprising if we consider that bacteria can adapt to living in almost any environment. If they can adapt to living at the high temperatures of the thermal vents deep within the oceans, it is not difficult to accept that they can evolve to evade antimicrobial drugs. (An additional risk from intentionally disseminated diseases is discussed in the Clini- cal Focus.) Tuberculosis is a well-known re-emerging disease. Fifteen years ago, public health officials were convinced that tuber- culosis would soon disappear as a major health consideration in the United States. Then, because of a number of events, including the AIDS epidemic, thousands of infected individ- uals developed TB strains resistant to the conventional bat- tery of antibiotics. These individuals then passed on the newly emerged, antibiotic-resistant strains of M. tuberculosis to others. While the rate of infection with M. tuberculosis in the United States increased sharply during the early part of the 1990s, by 1995 the incidence had begun to decline again. However, the worldwide incidence of the disease is still in-
creasing, and the World Health Organization predicts that, between 1998 and 2020, one billion more people will become infected and over 70 million will die from this disease if pre- ventive measures are not adopted. Another re-emerging disease is diphtheria. This disease was almost non-existent throughout Europe in recent years because of vaccination; in 1994, however, scattered cases were reported in some of the republics of the former Soviet Union. By 1995, there were over 50,000 cases reported in the same region, and thousands died from diphtheria infection. The social upheaval and instability that came with the breakup of the Soviet Union was almost certainly a major factor in the re-emergence of this disease, because of the resultant lapses in public health measures—perhaps most important was the loss of immunization programs. Since 1995, immunization programs have been re-established and the trend has re- versed, with only 13,687 cases of diphtheria reported in Rus- sian republics in 1996, 6932 in 1998, and 1573 in 2000. Other diseases have appeared seemingly from nowhere and, as far as we know, are new pathogens. These include
Immune Response to Infectious Diseases CH APTER 17 407
TABLE 17-4 Emerging pathogens recognized since 1973
Year Pathogen Disease
1973 Rotavirus Major cause of infantile diarrhea globally 1974 Hepatitis C Non-A, non-B hepatitis commonly transmitted via transfusions 1976 Cryptosporidium parvum Acute chronic diarrhea 1977 Ebola virus Ebola haemorrhagic fever Legionella pneumophilia Legionnaires’ disease Hantavirus Haemorrhagic fever with renal syndrome Campylobacter jejuni Enteric diseases distributed globally 1980 Human T-lymphotrophic virus I (HTLV-1) T-cell lymphoma 1981 Toxin-producing strains of Toxic shock syndrome Staphylococcus aureus 1982 Escherichia coli 0157:H7 Haemorrhagic colitis HTLV-II Hairy cell leukemia Borrelia burgdorferi Lyme disease 1983 HIV AIDS Helicobacter pylori Peptic ulcers 1988 Hepatitis E Enteric non-A, non-B hepatitis 1990 Guanarito virus Venezuelan haemorrhagic fever 1991 Encephalitozzon hellem Conjunctivitis, disseminated disease 1992 Vibrio cholerae 0139 New strain of epidemic cholera Bartonella henselae Cat scratch disease 1994 Sabia virus Brazilian haemorrhagic fever 1995 Human herpes virus-8 Associated with Kaposi sarcoma in AIDS patients 1996 TSE causing agent New variant of Creutzfeldt-Jakob disease (mad cow disease) 1997 Influenza A subtype H5N1 Avian influenza 1999 Influenza A subtype H9N2 New strain of human influenza Nipah virus Encephalitis West Nile virus Encephalitis SOURCE: Adapted from M. F. Good et. al., 1988, Annual Review of Immunology, Vol. 6.
such pathogens as the widely publicized Ebola virus and Legionella pneumophilia, the bacterial causative agent for Legionnaires’disease. Ebola was first recognized after an out- break in Africa, in 1976. By 1977, the virus that causes this
disease had been isolated and classified as a filovirus, a type of RNA virus that includes Marburg virus, a close relative of Ebola. Ebola causes a particularly severe haemorrhagic fever that kills more than 50% of those infected. Because of the
408 PART I V The Immune System in Health and Disease
spread of the virus requires direct contact with infected fluids. More worrisome are pathogens that can be spread by aerosol contact, such as anthrax, and toxins that can be added to food or water supplies, such as botulinum toxin. It is ironic that one of the most feared bioterrorism agents is smallpox, the tar- get of the first vaccine. Smallpox is caused by the virus Variola major; 30% or more of those infected with this virus die within a month of exposure. Survivors may be hor- ribly scarred. Smallpox can spread rapidly, even before symptoms are visible. As de- scribed in Chapter 1, the vaccine for small- pox is a virus ( Vaccinia ) related to variola, which in most cases causes a localized pustule that resolves within 3 weeks. Smallpox disappeared as a consequence of widespread vaccination—the last re- ported case of natural infection was in
- As the disease was eradicated, vac- cination was discontinued. In the United States, vaccination ceased in 1972. Pro- duction of the vaccine ceased and the re- maining doses were put into storage. Reasons for discontinuing smallpox vaccination include side effects that affect approximately 40 individuals per million vaccinees. These can be life threatening and take the form of encephalitis or dis- seminated skin infection. In addition, re- cently vaccinated individuals can spread the virus to others, especially those with compromised immunity. The occasional negative reactions to vaccinia can be treated by the administration of immu- noglobulin isolated from sera of persons previously vaccinated, but this so-called
Vaccina IG, or VIG, is no longer produced and little remains available. Facing the threat of smallpox as a bioterrorism agent means that vaccination must be reconsid- ered. It is unlikely that the vaccine pro- duced today will be the same one used earlier. Vaccine was produced by infection of the scarified skin of calves and virus was collected by scraping the infected area. Most likely a new vaccine candidate will be produced under controlled condi- tions in a tissue-cultured cell line that is certified free of any contaminating viruses. Furthermore, the actual virus used may be a more highly attenuated form of vaccinia. Stocks of VIG must be replenished before a mass vaccination effort is begun. Most of the viruses on the select agent list are not easy to disseminate. Agents of bioterrorism prepared in a form that allows easy dispersal are re- ferred to as weaponized. While nightmare scenarios include customized viral agents engineered in the laboratory, the more likely weaponized pathogens are bacteria. An accidental release of anthrax ( Bacillus anthracis ) in Sverdlovsk in the former Soviet Union infected 79 per- sons, of whom 68 died, pointing to the deadly potential of this organism. In late 2001, mail containing anthrax (see the accompanying figure) infected a number of persons in multiple postal centers as the letters progressed to their destina- tions, giving a glimpse of how widely and rapidly a bioweapon might be spread through modern infrastructure. Bacillus anthracis is a common veteri- nary pathogen, and like smallpox was the subject of early vaccine efforts, in this case by Louis Pasteur. Human infection was found mainly in those working with hair or hides from animals, especially goats. In- fection occurs via three different routes:
■ (^) Inhalation causes severe flu-like illness with high mortality unless diagnosed and treated immediately
The use of human patho- gens as weapons has a long history. Lord Jeffery Amherst used smallpox against native American populations before the Revolutionary War, and there are reports of attempts to spread plague and anthrax in both the distant and recent past. A few years ago, members of a dis- sident cult in Oregon introduced salmo- nella into the salad bars of several restaurants in an attempt cause sickness and death. The more recent discovery of anthrax spores mailed to congressmen and news offices accelerates our interest in possible agents of bioterrorism. Pathogens and toxins with potential for use as weapons are called “select agents” and include bacteria, bacterial toxins, and certain viruses (see table). The threat from such agents depends on both the severity of the disease it causes and the ease with which it can be dissemi- nated. For example, Ebola virus causes a fulminating hemorraghic disease, but
C L I N I C A L F O C U S
The Threat of Infection
from Potential Agents
of Bioterrorism
Category A agents of bioterrorism Anthrax ( Bacillus anthracis ) Botulism ( Clostridium botulinum toxin) Plague ( Yersinia pestis ) Smallpox ( Variola major ) Tularemia ( Francisella tularensis ) Viral hemorrhagic fevers (filoviruses [e.g., Ebola, Marburg] and arenaviruses [e.g., Lassa, Machupo])
