11-1. Antigenic variation allows pathogens to escape from immunity

One way in which an infectious agent can evade immune surveillance is by altering its antigens; this is particularly important for extracellular pathogens, against which the principal defense is the production of antibody against their surface structures. There are three ways in which antigenic variation can occur. First, many infectious agents exist in a wide variety of antigenic types. There are, for example, 84 known types of Streptococcus pneumoniae, an important cause of bacterial pneumonia. Each type differs from the others in the structure of its polysaccharide capsule. The different types are distinguished by serological tests and so are often known as serotypes. Infection with one serotype of such an organism can lead to type-specific immunity, which protects against reinfection with that type but not with a different serotype. Thus, from the point of view of the adaptive immune system, each serotype of S. pneumoniae represents a distinct organism. The result is that essentially the same pathogen can cause disease many times in the same individual (Fig. 11.1).

Figure 11.1

Host defense against Streptococcus pneumoniae is type specific. The different strains of S. pneumoniae have antigenically distinct capsular polysaccharides. The capsule prevents effective phagocytosis until the bacterium is opsonized by specific antibody (more...)

A second, more dynamic mechanism of antigenic variation is seen in the influenza virus. At any one time, a single virus type is responsible for most infections throughout the world. The human population gradually develops protective immunity to this virus type, chiefly by directing neutralizing antibody against the major surface protein of the influenza virus, its hemagglutinin. Because the virus is rapidly cleared from individual hosts, its survival depends on having a large pool of unprotected individuals among whom it spreads very readily. The virus might therefore be in danger of running out of potential hosts if it had not evolved two distinct ways of changing its antigenic type (Fig. 11.2).

Figure 11.2

Two types of variation allow repeated infection with type A influenza virus. Neutralizing antibody that mediates protective immunity is directed at the viral surface protein hemagglutinin (H), which is responsible for viral binding to and entry into cells. (more...)

The first of these, antigenic drift, is caused by point mutations in the genes encoding hemagglutinin and a second surface protein, neuraminidase. Every 2–3 years, a variant arises with mutations that allow the virus to evade neutralization by antibodies in the population; other mutations affect epitopes that are recognized by T cells and, in particular, CD8 T cells, so that cells infected with the mutant virus also escape destruction. Individuals who were previously infected with, and hence are immune to, the old variant are thus susceptible to the new variant. This causes an epidemic that is relatively mild because there is still some cross-reaction with antibodies and T cells produced against the previous variant of the virus, and therefore most of the population have some level of immunity (see Section 10-25).

Major influenza pandemics resulting in widespread and often fatal disease occur as the result of the second process, which is termed antigenic shift. This happens when there is reassortment of the segmented RNA genome of the influenza virus and related animal influenza viruses in an animal host, leading to major changes in the hemagglutinin protein on the viral surface. The resulting virus is recognized poorly, if at all, by antibodies and by T cells directed against the previous variant, so that most people are highly susceptible to the new virus, and severe infection results.

The third mechanism of antigenic variation involves programmed rearrangements in the DNA of the pathogen. The most striking example occurs in African trypanosomes, where changes in the major surface antigen occur repeatedly within a single infected host. Trypanosomes are insect-borne protozoa that replicate in the extracellular tissue spaces of the body and cause sleeping sickness in humans. The trypanosome is coated with a single type of glycoprotein, the variant-specific glycoprotein (VSG), which elicits a potent protective antibody response that rapidly clears most of the parasites. The trypanosome genome, however, contains about 1000 VSG genes, each encoding a protein with distinct antigenic properties. Only one of these is expressed at any one time by being placed into an active ‘expression site’ in the genome. The VSG gene expressed can be changed by gene rearrangement that places a new VSG gene into the expression site (Fig. 11.3). So, by having their own system of gene rearrangement that can change the VSG protein produced, trypanosomes keep one step ahead of an immune system capable of generating many distinct antibodies by gene rearrangement. A few trypanosomes with such changed surface glycoproteins thus evade the antibodies made by the host, and these soon grow and cause a recurrence of disease (see Fig. 11.3, bottom panel). Antibodies are then made against the new VSG, and the whole cycle repeats. This chronic cycle of antigen clearance leads to immune-complex damage and inflammation, and eventually to neurological damage, finally resulting in coma. This gives African trypanosomiasis its common name of sleeping sickness. These cycles of evasive action make trypanosome infections very difficult for the immune system to defeat, and they are a major health problem in Africa. Malaria is another major disease caused by a protozoan parasite that varies its antigens to evade elimination by the immune system.

Figure 11.3

Antigenic variation in trypanosomes allows them to escape immune surveillance. The surface of a trypanosome is covered with a variant-specific glycoprotein (VSG). Each trypanosome has about 1000 genes encoding different VSGs, but only the gene in a specific (more...)

Antigenic variation also occurs in bacteria: DNA rearrangements help to account for the success of two important bacterial pathogens—Salmonella typhimurium, a common cause of salmonella food poisoning, and Neisseria gonorrhoeae, which causes gonorrhea, a major sexually transmitted disease and an increasing public health problem in the United States. S. typhimurium regularly alternates its surface flagellin protein by inverting a segment of its DNA containing the promoter for one flagellin gene. This turns off expression of the gene and allows the expression of a second flagellin gene, which encodes an antigenically distinct protein. N. gonorrhoeae has several variable antigens, the most striking of which is the pilin protein, which, like the variable surface glycoproteins of the African trypanosome, is encoded by several variant genes, only one of which is active at any given time. Silent versions of the gene from time to time replace the active version downstream of the pilin promoter. All of these mechanisms help the pathogen to evade an otherwise specific and effective immune response.

11-2. Some viruses persist in vivo by ceasing to replicate until immunity wanes

Viruses usually betray their presence to the immune system once they have entered cells by directing the synthesis of viral proteins, fragments of which are displayed on the surface MHC molecules of the infected cell, where they are detected by T lymphocytes. To replicate, a virus must make viral proteins, and rapidly replicating viruses that produce acute viral illnesses are therefore readily detected by T cells, which normally control them. Some viruses, however, can enter a state known as latency in which the virus is not being replicated. In the latent state, the virus does not cause disease but, because there are no viral peptides to flag its presence, the virus cannot be eliminated. Such latent infections can be reactivated and this results in recurrent illness.

Herpes viruses often enter latency. Herpes simplex virus, the cause of cold sores, infects epithelia and spreads to sensory neurons serving the area of infection. After an effective immune response controls the epithelial infection, the virus persists in a latent state in the sensory neurons. Factors such as sunlight, bacterial infection, or hormonal changes reactivate the virus, which then travels down the axons of the sensory neuron and reinfects the epithelial tissues (Fig. 11.4). At this point, the immune response again becomes active and controls the local infection by killing the epithelial cells, producing a new sore. This cycle can be repeated many times. There are two reasons why the sensory neuron remains infected: first, the virus is quiescent in the nerve and therefore few viral proteins are produced, generating few virus-derived peptides to present on MHC class I; second, neurons carry very low levels of MHC class I molecules, which makes it harder for CD8 T cells to recognize infected neurons and attack them. This low level of MHC class I expression might be beneficial, as it reduces the risk that neurons, which regenerate very slowly if at all, will be attacked in appropriately by CD8 T cells. It also makes neurons unusually vulnerable to persistent infections. Another example of this is provided by herpes zoster (or varicella zoster), the virus that causes chickenpox. This virus remains latent in one or a few dorsal root ganglia after the acute illness is over and can be reactivated by stress or immunosuppression to spread down the nerve and reinfect the skin. The reinfection causes the reappearance of the classic rash of varicella in the area of skin served by the infected dorsal root, a disease commonly called shingles. Herpes simplex reactivation is frequent, but herpes zoster usually reactivates only once in a lifetime in an immunocompetent host.

Figure 11.4

Persistence and reactivation of herpes simplex virus infection. The initial infection in the skin is cleared by an effective immune response, but residual infection persists in sensory neurons such as those of the trigeminal ganglion, whose axons innervate (more...)

The Epstein-Barr virus (EBV), yet another herpes virus, enters latency in B cells after a primary infection that often passes without being diagnosed. In a minority of infected individuals, the initial acute infection of B cells is more severe, causing a disease known as infectious mononucleosis or glandular fever (Acute Infectious Mononucleosis, in Case Studies in Immunology, see Preface for details). EBV infects B cells by binding to CR2 (CD21), a component of the B-cell co-receptor complex. The infection causes most of the infected cells to proliferate and produce virus, leading in turn to the proliferation of antigen-specific T cells and the excess of mononuclear white cells in the blood that gives the disease its name. The infection is controlled eventually by specific CD8 T cells, which kill the infected proliferating B cells. A fraction of B lymphocytes become latently infected, however, and EBV remains quiescent in these cells. Latently infected cells express a viral protein, EBNA-1, which is needed to maintain the viral genome, but EBNA-1 interacts with the proteasome (see Section 5-3) to prevent its own degradation into peptides that would elicit a T-cell response.

Latently infected B cells can be isolated by taking B cells from individuals who have apparently cleared their EBV infection and placing them in tissue culture: in the absence of T cells, the latently infected cells that have retained the EBV genome transform infected B cells sometimes undergo malignant transformation, giving rise to a B-cell lymphoma called Burkitt's lymphoma (see Section 7-33). This is a rare event, and it seems likely that a crucial part of this process is a failure of T-cell surveillance. Further support for this hypothesis comes from the increased risk of EBV-associated B-cell lymphomas developing in patients with acquired and inherited immuno-deficiencies of T-cell function (see Sections 11-15 and 11-26).

11-3. Some pathogens resist destruction by host defense mechanisms or exploit them for their own purposes

Some pathogens induce a normal immune response but have evolved specialized mechanisms for resisting its effects. For instance, some bacteria that are engulfed in the normal way by macrophages have evolved means of avoiding destruction by these phagocytes; indeed, they use macrophages as their primary host. Mycobacterium tuberculosis, for example, is taken up by macrophages but prevents the fusion of the phagosome with the lysosome, protecting itself from the bactericidal actions of the lysosomal contents.

Other microorganisms, such as Listeria monocytogenes, escape from the phagosome into the cytoplasm of the macrophage, where they can multiply readily. They then spread to adjacent cells in tissues without emerging from the cell into the extracellular environment. They do this by hijacking the host cytoskeletal protein actin, which assembles into filaments at the rear of the bacterium. The actin filaments drive the bacteria forward into vacuolar projections to adjacent cells; these vacuoles are then lysed by the Listeria, releasing the bacteria directly into the cytoplasm of the adjacent cell. In this way they avoid attack by antibodies. Cells infected with L. monocytogenes are, however, susceptible to killing by cytotoxic T cells. The protozoan parasite Toxoplasma gondii can apparently generate its own vesicle, which isolates it from the rest of the cell because it does not fuse with any cellular vesicle. This might actually enable T. gondii to avoid making peptides derived from its proteins accessible for loading onto MHC molecules, and thus remain invisible to the immune system.

Two prominent spirochetal infections, Lyme disease and syphilis, avoid elimination by antibodies through less well understood mechanisms and establish a persistent and extremely damaging infection in tissues. Lyme disease is caused by the spirochete bacterium Borrelia burgdorferi, whereas syphilis, the more widespread and much the better understood of the two diseases, is caused by Treponema pallidum. T. pallidum is believed to avoid recognition by antibodies by coating its surface with host molecules until it has invaded tissues such as the central nervous system, where it is less easily reached by antibodies.

Finally, many viruses have evolved mechanisms to subvert various arms of the immune system. These range from capturing cellular genes for cytokines or cytokine receptors, to synthesizing complement-regulatory molecules, inhibiting MHC class I synthesis or assembly, or producing decoy proteins that mimic so-called TIR domains that we learned about in Section 6-15. This area is one of the most rapidly expanding areas in the field of host-pathogen relationships. Examples of how members of the herpes and poxvirus families subvert host responses are shown in Fig. 11.5.

Figure 11.5

Mechanisms of subversion of the host immune system by viruses of the herpes and pox families.

11-4. Immunosuppression or inappropriate immune responses can contribute to persistent disease

Many pathogens suppress immune responses in general. For example, staphylococci produce toxins, such as the staphylococcal enterotoxins and toxic shock syndrome toxin-1 (Toxic Shock Syndrome, in Case Studies in Immunology, see Preface for details), that act as superantigens. Superantigens are proteins that bind the antigen receptors of very large numbers of T cells (see Section 7-26), stimulating them to produce cytokines that cause significant suppression of all immune responses. The details of this suppression are not understood. The stimulated T cells proliferate and then rapidly undergo apoptosis, leaving a generalized immunosuppression together with the deletion of many peripheral T cells.

Many other pathogens cause mild or transient immunosuppression during acute infection. These forms of suppressed immunity are poorly understood but important, as they often make the host susceptible to secondary infections by common environmental microorganisms. A crucially important example of immune suppression follows trauma, burns, or even major surgery. The burned patient has a clearly diminished capability to respond to infection, and generalized infection is a common cause of death in these patients. The reasons for this are not fully understood.

Measles virus infection, in spite of the widespread availability of an effective vaccine, still accounts for 10% of the global mortality of children under 5 years old and is the eighth leading cause of death worldwide. Malnourished children are the main victims and the cause of death is usually secondary bacterial infection, particularly pneumonia caused by measles-induced immunosuppression. The immunosuppression that follows measles infection can last for several months and is associated with reduced T- and B-cell function. There is reduced or absent delayed-type hypersensitivity and, during this period of acquired immunodeficiency, children have markedly increased susceptibility to mycobacterial infection, reflecting the important role of macrophage activation by T_H1 cells in host defense against mycobacteria. An important mechanism for measles-induced immunosuppression is the infection of dendritic cells by measles virus. Infected dendritic cells cause unresponsiveness of T lymphocytes by mechanisms that are not yet understood, and it seems likely that this is the proximate cause of the immunosuppression induced by measles virus.

The most extreme case of immune suppression caused by a pathogen is the acquired immune deficiency syndrome caused by infection with HIV. The ultimate cause of death in AIDS is usually infection with an opportunistic pathogen, a term used to describe a microorganism that is present in the environment but does not usually cause disease because it is well controlled by normal host defenses. HIV infection leads to a gradual loss of immune competence, allowing infection with organisms that are not normally pathogenic.

Leprosy, which we discussed in Section 8-13, is a more complex case, in which the causal bacterium, Mycobacterium leprae, is associated either with the suppression of cell-mediated immunity or with a strong cell-mediated antibacterial response. This leads to two major forms of the disease—lepromatous and tuberculoid leprosy. In lepromatous leprosy, cell-mediated immunity is profoundly depressed, M. leprae are present in great profusion, and cellular immune responses to many antigens are suppressed. This leads to a phenotypic state in such patients called anergy, here meaning the absence of delayed-type hypersensitivity to a wide range of antigens unrelated to M. leprae. In tuberculoid leprosy, by contrast, there is potent cell-mediated immunity with macrophage activation, which controls but does not eradicate infection. Few viable microorganisms are found in tissues, the patients usually survive, and most of the symptoms and pathology are caused by the inflammatory response to these persistent microorganisms (Fig. 11.6). The difference between the two forms of disease might lie in a difference in the ratio of T_H1 to T_H2 cells, and this is thought to be caused by cytokines produced by CD8 T cells, as we learned in Section 10-6.

Figure 11.6

T-cell and macrophage responses to Mycobacterium leprae are sharply different in the two polar forms of leprosy. Infection with M. leprae, which stain as small dark red dots in the photographs, can lead to two very different forms of disease. In tuberculoid (more...)

11-5. Immune responses can contribute directly to pathogenesis

Tuberculoid leprosy is just one example of an infection in which the pathology is caused largely by the immune response. This is true to some degree in most infections; for example, the fever that accompanies a bacterial infection is caused by the release of cytokines by macrophages. One medically important example of immunopathology is the wheezy broncheolitis caused by respiratory syncytial virus (RSV). Broncheolitis caused by RSV is the major cause of admission of young children to hospital in the Western world, with as many as 90,000 admissions and 4500 deaths each year in the United States alone. The first indication that the immune response to the virus might have a role in the pathogenesis of this disease came from the observation that young infants vaccinated with an alum-precipitated killed virus preparation suffered a worse disease than unvaccinated children. This occurred because the vaccine failed to induce neutralizing antibodies but succeeded in producing T_H2 cells. On infection, the T_H2 cells released interleukin (IL)-3, IL-4, and IL-5, which induced bronchospasm, increased mucus secretion, and tissue eosinophilia. Mice can be infected with RSV and develop a disease similar to that seen in humans.

Another example of a pathogenic immune response is the response to the eggs of the schistosome. Schistosomes are parasitic worms that lay eggs in the hepatic portal vein. Some of the eggs reach the intestine and are shed in the feces, spreading the infection; others lodge in the portal circulation of the liver, where they elicit a potent immune response leading to chronic inflammation, hepatic fibrosis, and eventually liver failure. This process reflects the excessive activation of T_H1 cells, and can be modulated by T_H2 cells, IL-4, or CD8 T cells, which can also act by producing IL-4.

In the case of the mouse mammary tumor virus (MMTV), a retrovirus that causes mammary tumors in mice, the immune response is required for the infective cycle of the pathogen (Fig. 11.7). MMTV is transferred from the mother's mammary gland to her pups in milk. The virus then enters the B lymphocytes of the new host, where it must replicate to be transported to the mammary epithelium to continue its life cycle. As it is a retrovirus, however, MMTV can replicate only in dividing cells. The virus ensures that infected B cells will proliferate by causing them to express on their surface a superantigen encoded within the MMTV genome. This superantigen enables the B cells to bypass the requirement for specific antigen and stimulate large numbers of CD4 T cells with the appropriate T-cell receptor V_β domain (see Section 5-15), causing them to produce cytokines and express CD40 ligand, which in turn stimulates the B cells to divide. The virus can then replicate in the B cells and infect the host's mammary epithelial cells.

Figure 11.7

Activation of T cells by the MMTV superantigen in mice is crucial for the virus life cycle. MMTV is transferred from mother to pup in milk, and crosses the gut epithelium to reach the lymphoid tissue of its new host and thus infect B lymphocytes. The (more...)

One way to block this cycle of transmission is by deleting the particular subset of T cells carrying the V_β domain recognized by the viral superantigen. This has been done experimentally by taking mice that are normally susceptible to a particular MMTV virus, and using the superantigen gene from this virus to construct transgenic mice. As we learned in Section 7-26, superantigens that are expressed in the thymus induce the clonal deletion of developing T cells. Thus the expressed transgene induced the loss of T cells bearing the appropriate V_β domains. The B cells in these transgenic mice could be infected by the MMTV virus but could not activate any of the remaining T cells. Thus the infected B cells were not stimulated to divide, and could not support MMTV replication. Consequently, the transgenic mice, unlike their nontransgenic littermates, were unable to transmit the relevant strain of MMTV.

This mode of protection against MMTV might explain the finding that most mouse strains have MMTV genomes stably integrated into their DNA. These defective endogenous retroviruses have lost certain essential genes and are unable to produce virions, but they have retained the genes encoding their superantigens, which are expressed on the cells of the host. Although a section of the T-cell repertoire is lost as a result of carrying these endogenous retroviruses, the mice are protected against infection with nondefective MMTV encoding the same superantigen. There are several different strains of MMTV whose superantigens bind to different V_β domains, and these are matched by different endogenous MMTV strains. Mice containing different endogenous MMTV genomes delete different parts of their T-cell receptor repertoire, reducing the risk that whole mouse populations will be susceptible to a given MMTV strain. No human diseases dependent on such mechanisms have yet been described.

Summary

Infectious agents can cause recurrent or persistent disease by avoiding normal host defense mechanisms or by subverting them to promote their own replication. There are many different ways of evading or subverting the immune response. Antigenic variation, latency, resistance to immune effector mechanisms, and suppression of the immune response all contribute to persistent and medically important infections. In some cases, the immune response is part of the problem; some pathogens use immune activation to spread infection, others would not cause disease if it were not for the immune response. Each of these mechanisms teaches us something about the nature of the immune response and its weaknesses, and each requires a different medical approach to prevent or to treat infection.