Download Helminths: Pathogenesis and Defenses

Helminths: Pathogenesis
and Defenses
Derek Wakelin
GENERAL CONCEPTS
Classification
Helminth is a general term for a parasitic worm. The helminths include the Platyhelminthes
or flatworms (flukes and tapeworms) and the Nematoda or roundworms.
Characteristics
All helminths are relatively large (> 1 mm long); some are very large (> 1 m long). All
have well-developed organ systems and most are active feeders. The body is either
flattened and covered with plasma membrane (flatworms) or cylindrical and covered with
cuticle (roundworms). Some helminths are hermaphrodites; others have separate sexes.
Epidemiology
Helminths are worldwide in distribution; infection is most common and most serious in
poor countries. The distribution of these diseases is determined by climate, hygiene, diet,
and exposure to vectors.
Infection
The mode of transmission varies with the type of worm; it may involve ingestion of eggs or
larvae, penetration by larvae, bite of vectors, or ingestion of stages in the meat of
intermediate hosts. Worms are often long-lived.
Pathogenesis
Many infections are asymptomatic; pathologic manifestations depend on the size, activity,
and metabolism of the worms. Immune and inflammatory responses also cause pathology.
Host Defenses
Nonspecific defense mechanisms limit susceptibility. Antibody- and cell-mediated
responses are important, as is inflammation. Parasites survive defenses through many
evasion strategies.
INTRODUCTION
Helminths - worms - are some of the world's commonest parasites (see Ch. 86). They
belong to two major groups of animals, the flatworms or Platyhelminthes (flukes and
tapeworms) and the roundworms or Nematoda. All are relatively large and some are very
large, exceeding one meter in length.
Their bodies have well-developed organ systems, especially reproductive organs, and most
helminths are active feeders. The bodies of flatworms are flattened and covered by a
plasma membrane, whereas roundworms are cylindrical and covered by a tough cuticle.
Flatworms are usually hermaphroditic whereas roundworms have separate sexes; both have
an immense reproductive capacity.
The most serious helminth infections are acquired in poor tropical and subtropical areas,
but some also occur in the developed world; other, less serious, infections are worldwide in
distribution. Exposure to infection is influenced by climate, hygiene, food preferences, and
contact with vectors. Many potential infections are eliminated by host defenses; others
become established and may persist for prolonged periods, even years. Although infections
are often asymptomatic, severe pathology can occur. Because worms are large and often
migrate through the body, they can damage the host's tissues directly by their activity or
metabolism. Damage also occurs indirectly as a result of host defense mechanisms. Almost
all organ systems can be affected.
Host defense can act through nonspecific mechanisms of resistance and through specific
immune responses. Antibody-mediated, cellular, and inflammatory mechanisms all
contribute to resistance. However, many worms successfully avoid host defenses in a
variety of ways, and can survive in the face of otherwise effective host responses.
Infection
Transmission of Infection
Helminths are transmitted to humans in many different ways (Fig. 87-1). The simplest is by
accidental ingestion of infective eggs (Ascaris, Echinococcus, Enterobius, Trichuris) or
larvae (some hookworms). Other worms have larvae that actively penetrate the skin
(hookworms, schistosomes, Strongyloides). In several cases, infection requires an
intermediate host vector. In some cases the intermediate vector transmits infective stages
when it bites the host to take a blood meal (the arthropod vectors of filarial worms); in
other cases, the larvae are contained in the tissues of the intermediate host and are taken in
when a human eats that host (Clonorchis in fish, tapeworms in meat and fish, Trichinella in
meat). The levels of infection in humans therefore depend on standards of hygiene (as eggs
and larvae are often passed in urine or feces), on the climate (which may favor survival of
infective stages), on the ways in which food is prepared, and on the degree of exposure to
insect vectors.
FIGURE 87-1 Entry and localization of pathogenic helminths.
Host Factors Influencing Susceptibility
Human behavior is a major factor influencing susceptibility to infection. If the infective
stages of helminths are present in the environment, then certain ways of behaving,
particularly with regard to hygiene and food, will result in greater exposure. Because
helminths, with few exceptions (Strongyloides, Trichinella, some tapeworm larvae), do not
increase their numbers by replication within the same host, the level of infection is directly
related to the number of infective stages encountered. Obviously, not every exposure results
in the development of a mature infection. Many infective organisms are killed by the host's
nonspecific defense mechanisms. Of those that do become established, many are destroyed
or eliminated by specific defenses. The number of worms present at any one time therefore
represents a dynamic balance between the rate of infection and the efficiency of defense.
This balance (which reflects the host's overall susceptibility) is altered by changes in the
host's behavior and ability to express forms of defense. Children are more susceptible to
many helminths than are adults, and frequently are the most heavily infected members of a
community. The waning of immune competence with age may also result in increased
levels of infection. Individuals differ genetically in their ability to resist infection, and it is
well known that in infected populations, some individuals are predisposed to heavier
infections than others. Changes in diet may affect susceptibility, as do the hormonalimmune changes accompanying pregnancy and lactation. An important cause of increased
susceptibility is the immune suppression that accompanies concurrent infections with some
other pathogens and the development of certain tumors. Similarly, immunosuppressive
therapies (irradiation, immunosuppressant drugs) may enhance susceptibility to helminth
infection. A particular hazard in immunocompromised patients is the development of
disseminated strongyloidiasis, in which large numbers of larvae develop in the body by
autoinfection from relatively small numbers of adult Strongyloides stercoralis. It is
interesting that the human immunodeficiency virus does not result in an overall increase in
susceptibility to helminth infection.
Parasite Factors Influencing Susceptibility
The ability of hosts to control infection is offset by the ability of parasites to avoid the
host's defenses and increase their survival. In addition to their ability to evade specific
immune defenses (see below), many worms are unaffected by the host's attempts to limit
their activities or to destroy them simply because they are large and mobile. Many
important species measure several centimeters in length or diameter (Ascaris, hookworms,
hydatid cysts, Trichuris) and others may exceed one meter in length (tapeworms). Size
alone renders many defense mechanisms inoperative, as does the tough cuticle of adult
roundworms. The ability of worms to move actively through tissues enables them to escape
inflammatory foci.
Many of the pathogenic consequences of worm infections are related to the size, movement
and longevity of the parasites, as the host is exposed to long-term damage and immune
stimulation, as well as to the sheer physical consequences of being inhabited by large
foreign bodies.
Pathogenesis
Direct Damage from Worm Activity
The most obvious forms of direct damage are those resulting from the blockage of internal
organs or from the effects of pressure exerted by growing parasites (Fig. 87-2). Large
Ascaris or tapeworms can physically block the intestine, and this may occur after some
forms of chemotherapy; migrating Ascaris may also block the bile duct. Granulomas that
form around schistosome eggs may block the flow of blood through the liver, and this may
lead to pathological changes in that organ and elsewhere. Blockage of lymph flow, leading
to elephantiasis, is associated with the presence of adult Wuchereria in lymphatics. Pressure
atrophy is characteristic of larval tapeworm infections (hydatid cyst, the larva of
Echinococcus granulosus) where the parasite grows as a large fluid-filled cyst in the liver,
brain, lungs, or body cavity. The multilocular hydatid cysts caused by Echinococcus
multilocularis have a different growth form, metastasizing within organs and causing
necrosis. The larvae of Taenia solium, the pork tapeworm, frequently develop in the central
nervous system (CNS) and eyes. Some of the neurological symptoms of the resulting
condition, called cysticercosis, are caused by the pressure exerted by the cysts.
FIGURE 87-2 Pathogenesis: direct damage caused by large helminths.
Intestinal worms cause a variety of pathologic changes in the mucosa, some reflecting
physical and chemical damage to the tissues, others resulting from immunopathologic
responses. Hookworms (Ancylostoma and Necator) actively suck blood from mucosal
capillaries. The anticoagulants secreted by the worms cause the wounds to bleed for
prolonged periods, resulting in considerable blood loss. Heavy infections in malnourished
hosts are associated with anemia and protein loss. Protein-losing enteropathies may also
result from the inflammatory changes induced by other intestinal worms. Diversion of host
nutrients by competition from worms is probably unimportant, but interference with normal
digestion and absorption may well aggravate undernutrition. The tapeworm
Diphyllobothrium latum can cause vitamin B12 deficiency through direct absorption of this
factor.
Many helminths undertake extensive migrations through body tissues, which both damage
tissues directly and initiate hypersensitivity reactions. The skin, lungs, liver, and intestines
are the organs most affected. Petechial hemorrhages, pneumonitis, eosinophilia, urticaria
and pruritus, organomegaly, and granulomatous lesions are among the signs and symptoms
produced during these migratory phases.
Feeding by worms upon host tissues is an important cause of pathology, particularly when
it induces hyperplastic and metaplastic changes in epithelia. For example, liver fluke
infections lead to hyperplasia of the bile duct epithelium. Chronic inflammatory changes
around parasites (for example, the granulomas around schistosome eggs in the bladder
wall) have been linked with neoplasia, but the nature of the link is not known. The
continuous release by living worms of excretory-secretory materials, many of which are
known to have direct effects upon host cells and tissues, may also contribute to pathology.
Indirect Damage from Host Response
As with all infectious organisms, it is impossible to separate the pathogenic effects caused
strictly by mechanical or chemical tissue damage from those caused by the immune
response to the parasite. All helminths are "foreign bodies" not only in the sense of being
large and invasive but also in the immunologic sense: they are antigenic and therefore
stimulate immunity. An excellent illustration of this interrelation between direct and
indirect damage is seen in the pathology associated with schistosome infections, especially
with Schistosoma mansoni (Fig. 87-3). Hypersensitivity-based, granulomatous responses to
eggs trapped in the liver cause a physical obstruction to blood flow, which leads to liver
pathology. Hypersensitivity-based inflammatory changes probably also contribute to the
lymphatic blockage associated with filarial infections (Brugia, Wuchereria).
FIGURE 87-3 Pathogenesis: indirect damage caused by immunopathologic responses
(for example, in Schistosomiasis).
Immune-mediated inflammatory changes occur in the skin, lungs, liver, intestine, CNS, and
eyes as worms migrate through these structures. Systemic changes such as eosinophilia,
edema, and joint pain reflect local allergic responses to parasites. The pathologic
consequences of immune-mediated inflammation are seen clearly in intestinal infections
(especially Strongyloides and Trichinella infections). Structural changes, such as villous
atrophy, develop. The permeability of the mucosa changes, fluid accumulates in the gut
lumen, and intestinal transit time is reduced. Prolonged changes of this type may lead to a
protein-losing enteropathy. The inflammatory changes that accompany the passage of
schistosome eggs through the intestinal wall also cause severe intestinal pathology. Heavy
infections with the whipworm Trichuris in the large bowel can lead to inflammatory
changes, resulting in blood loss and rectal prolapse.
The severity of these indirect changes is a result of the chronic nature of the infection. The
fact that many worms are extremely long-lived means that many inflammatory changes
become irreversible, producing functional changes in tissues. Three examples are the
hyperplasia of bile ducts in long-term liver fluke infections, the extensive fibrosis
associated with chronic schistosomiasis, and the skin atrophy associated with
onchocerciasis. Severe pathology may also result when worms stray into abnormal body
sites.
Defenses Against Infection
Nonspecific Resistance
Infective stages attempting to enter via the mouth or through the skin are opposed by the
same non-specific defenses that protect humans from invasion of other pathogens.
Following oral ingestion, parasites must survive passage through the acid stomach to reach
the small bowel. The natural parasites of humans are adapted to do this, but opportunistic
parasites may be killed. Similarly, natural parasites are adapted to the environmental
conditions of the bowel (and in many cases require them as cues for development), but
accidental parasites may find them inappropriate. Penetration into the intestinal wall may
trigger inflammatory responses that immobilize and kill the worm. This may itself lead to
serious pathology (as in Anisakis infection). Worms entering through the skin must survive
the skin secretions, penetrate the epidermal layers, and avoid inflammatory trapping in the
dermis. Invasion of humans by the larvae of dog and cat hookworms (Ancylostoma spp.)
results in dermatitis and "creeping eruption" as the worms become the focus of
inflammatory reactions that form trails in the skin.
Once in the tissues, worms need the correct sequence of environmental signals to mature.
Absent or incomplete signals constitute a form of nonspecific resistance that may partially
or completely prevent further development. The parasite may not die, however; indeed,
prolonged survival at a larval stage may result in pathology from the continuing
inflammatory response (e.g. Toxocara infection).
Specific Acquired Immunity
There is no doubt that specific immunity is responsible for the most effective forms of host
defense, although the dividing line between nonspecific and specific mechanisms is
difficult to draw with precision (Fig. 87-4). All helminths stimulate strong immune
responses, which can easily be detected by measuring specific antibody or cellular
immunity. Although these responses are useful for diagnosing infection, they frequently
appear not to be protective. The high prevalence of helminth infection in endemic areas
(sometimes approaching 100 per cent), and the fact that individuals may remain infected for
many years and can easily be reinfected after they are cured by chemotherapy, suggest that
protective immunity against helminths is weak or absent in humans. However, some degree
of immunity does appear to operate, because the intensity of infection often declines with
age, and many individuals in endemic areas remain parasitologically negative and/or
clinically normal. Evidence from laboratory studies provides some clues as to the
mechanisms involved. Antibodies that bind to surface antigens may focus complement- or
cell-mediated effectors that can damage the worm. Macrophages and eosinophils are the
prime cytotoxic effector cells, and IgM, IgG and IgE are the important immunoglobulins.
Antibodies may also block enzymes released by the worm, thus interfering with its ability
to penetrate tissues or to feed. Inflammatory changes may concentrate effector cells around
worms, and the release of cellular mediators may then disable and kill the worm.
Encapsulation of trapped worms by inflammatory cells may also result in the death of the
worm, although this is not always the case. Intestinal worms can be dislodged by the
structural and physiologic changes that occur in the intestine during acute inflammation. It
has long been suspected that IgE-mediated hypersensitivity reactions, involving mast cells
and basophils, contribute to this process, but the evidence is still circumstantial. Despite the
abundance of IgA in the intestinal lumen, there is no conclusive evidence that it is involved
in protective immunity in humans, although some field and laboratory data suggest it is.
FIGURE 87-4 Host defense and parasite escape. A schematic diagram of the
development and expression of acquired immunity to helminths and of the ways in which
parasites escape the immune response.
Avoidance of Host Defenses
Despite their immunogenicity, many helminths survive for extended periods in the bodies
of their hosts. Some of the reasons have already been mentioned (size, motility), but we
now know that worms employ many sophisticated devices to render host defenses
ineffective (Fig. 87-4). Some worms (schistosomes) disguise their outer surface by
acquiring host molecules which reduce their antigenicity; intrinsic membrane changes also
make these worms resistant to immune attack. Filarial nematodes acquire serum albumin on
their cuticle, which may act as a disguise. Many worms release substances that depress
lymphocyte function, inactivate macrophages, or digest antibodies. Larval cestodes appear
to prolong their survival by producing anticomplement factors which protect their outer
layers from lytic attack. Antigenic variation in the strict sense is not known to occur, but
many species show a stage-specific change of antigens as they develop, and this
phenomenon may delay the development of effective immune mechanisms. All helminths
release relatively large amounts of antigenic materials, and this voluminous production may
divert immune responses or even locally exhaust immune potential. Irrelevant antibodies
produced by the host may block the activity of potentially protective antibodies, as has been
shown to be the case in schistosome infections.
It is striking that many helminth infections are associated with a degree of immune
suppression, which may affect specific or general responsiveness. Many explanations have
been proposed for this immune suppression, including antigen overload, antigenic
competition, induction of suppressor cells, and production of lymphocyte-specific
suppressor factors. Reduced immune responsiveness may not only prolong the survival of
the original infecting worm species but increase the host's susceptibility to other pathogens.
Epidemiologic evidence also raises the possibility that infections acquired early in life before or shortly after birth - may induce a form of immune tolerance, allowing heavy
worm burdens to accumulate in the body.
The subtlety with which parasitic worms manipulate the host's immune system not only
increases their importance as pathogens but also creates formidable problems for their
control and eradication.
Protozoa: Pathogenesis and
Defenses
John Richard Seed
General Concepts
Resistance
Resistance is the ability of a host to defend itself against a pathogen. Resistance to
protozoan parasites involves three interrelated mechanisms: nonspecific factors, cellular
immunity, and humoral immunity.
Pathology
Protozoal infection results in tissue damage leading to disease. In chronic infections the
tissue damage is often due to an immune response to the parasite and/or to host antigens as
well as to changes in cytokine profiles. Alternatively, it may be due to toxic protozoal
products and/or to mechanical damage.
Escape Mechanisms
Escape mechanisms are strategies by which parasites avoid the killing effect of the immune
system in an immunocompetent host. Escape mechanisms used by protozoal parasites
include the following.
Antigenic Masking: Antigenic masking is the ability of a parasite to escape immune
detection by covering itself with host antigens.
Blocking of Serum Factors: Some parasites acquire a coating of antigen-antibody
complexes or noncytotoxic antibodies that sterically blocks the binding of specific antibody
or lymphocytes to the parasite surface antigens.
Intracellular Location: The intracellular habitat of some protozoan parasites protects them
from the direct effects of the host's immune response. By concealing the parasite antigens,
this strategy also delays detection by the immune system.
Antigenic Variation: Some protozoan parasites change their surface antigens during the
course of an infection. Parasites carrying the new antigens escape the immune response to
the original antigens.
Immunosuppression: Parasitic protozoan infections generally produce some degree of
host immunosuppression. This reduced immune response may delay detection of antigenic
variants. It may also reduce the ability of the immune system to inhibit the growth of and/or
to kill the parasites.
INTRODUCTION
Resistance to parasitic protozoa appears to be similar to resistance against other infectious
agents, although the mechanisms of resistance in protozoan infections are not yet as well
understood. Resistance can be divided into two main groups of mechanisms: (1)
nonspecific mechanism(s) or factor(s) such as the presence of a nonspecific serum
component that is lethal to the parasite; and (2) specific mechanism(s) involving the
immune system (Fig. 78-1). Probably the best studied nonspecific mechanisms involved in
parasite resistance are the ones that control the susceptibility of red blood cells to invasion
or growth of plasmodia, the agents of malaria. Individuals who are heterozygous or
homozygous for the sickle cell hemoglobin trait are considerably more resistant to
Plasmodium falciparum than are individuals with normal hemoglobin. Similarly,
individuals who lack the Duffy factor on their red blood cells are not susceptible to P vivax.
Possibly both the sickle cell trait and absence of the Duffy factor have become established
in malaria-endemic populations as a result of selective pressure exerted by malaria.
Epidemiologic evidence suggests that other inherited red blood cell abnormalities, such as
thalassanemia and glucose-6-phosphate dehydrogenase deficiency, may contribute to
survival of individuals in various malaria-endemic geographical regions. A second welldocumented example of a nonspecific factor involved in resistance is the presence in the
serum of humans of a trypanolytic factor that confers resistance against Trypanosoma
brucei brucei, an agent of trypanosomiasis (sleeping sickness) in animals. There is evidence
that other nonspecific factors, such as fever and the sex of the host, may also contribute to
the host's resistance to various protozoan parasites. Although nonspecific factors can play a
key role in resistance, usually they work in conjunction with the host's immune system
(Fig. 78-1).
FIGURE 78-1 Some interrelationships between host factors involved in resistance to
protozoan infections.
Different parasites elicit different humoral and/or cellular immune responses. In malaria
and trypanosome infections, antibody appears to play a major role in immunity. In both T
cruzi and T brucei gambiense infections, antibody-dependent cytotoxic reactions against
the parasite have been reported. Although antibody has been shown to be responsible for
clearing the African trypanosomes from the blood of infected animals, recent evidence
suggests that the survival time of infected mice does not necessarily correlate with the
ability of the animal to produce trypanosome-specific antibody. In other words, resistance
as measured by survival time may not solely involve the specific humoral immune system.
Recent data suggest that cellular immunity is required for resistance to malaria. for
example, vaccine trials with a sporozoite antigen indicated that both an active cellular
response and sporozoite-specific antibody may be needed for successful immunization.
Cellular immunity is believed to be the single most important defense mechanism in
leishmaniasis and toxoplasmosis. In animals infected with Toxoplasma, the activated
macrophage has been shown to play an important role in resistance. Accordingly, resistance
to the protozoan parasites most likely involves nonspecific factors as well as specific
humoral and/or cellular mechanisms. Cytokines are involved in the control of both the
immune response and pathology. It has become apparent that there are subsets of both
helper (h) and cytotoxic (c) T-cells that produce different profiles of cytokines. For
example, the Th-1 subset produces gamma interferon (IFN-), and interleukin-2 (IL-2) and
is involved in cell-mediated immunity. In contrast the Th-2 subset produces IL-4 and IL-6,
and is responsible for antibody-mediated immunity. The induction of a particular T-cell
subset is key to recovery and resistance. The Th-1 subset and increased IFN- are important
in resistance to Leishmania, T cruzi and Toxoplasma infections, whereas the Th-2 response
is more important in parasitic infections in which antibody is a key factor. It is important to
recognize that the cytokines produced by one T-cell subset can up or downregulate the
response of other T-cell subsets. IL-4 will downregulate Th-1 cells and exacerbate infection
and/or susceptibility of mice to Leishmania. The cytokines produced by T and other cell
types do not act directly on the parasites but influence other host cell types. The response of
cells to cytokines includes a variety of physiological changes, such as changes in glucose,
fatty acid and protein metabolism. For example, IL-1 and tumor necrosis factor will
increase gluconeogenesis, and glucose oxidation. It should be noted that cytokines
influence the metabolism not only of T-cells, but also a variety of other cell types and organ
systems. Cytokines can also stimulate cell division and, therefore, clonal expansion of T
and B-cell subsets. This can lead to increased antibody production and/or cytotoxic T-cell
numbers. The list of cytokines and their functions is growing rapidly, and it would appear
that these chemical messages influence all phases of the immune response. they are also
clearly involved in the multitude of physiological responses (fever, decreased food intake,
etc.) observed in an animal's response to a pathogen, and in the pathology that results.
Unlike most viral and bacterial infections, protozoan diseases are often chronic, lasting
months or years. When associated with a strong host immune response, this type of chronic
infection is apt to result in a high incidence of immunopathology. The question also arises
of how these parasites survive in an immunocompetent animal. The remainder of this
chapter treats the mechanisms responsible for pathology, particularly immunopathology, in
protozoan disease, and the mechanisms by which parasites evade the immune responses of
the host. Finally, because of the very rapid advances in our knowledge of the host-parasite
relationship (due primarily to the development of techniques in molecular biology), it is
necessary to briefly mention the potential for developing vaccines to the pathogenic
protozoa.
PATHOLOGY
The protozoa can elicit humoral responses in which antigen-antibody complexes in the
region of antibody excess activate Hageman blood coagulation factor (Factor XII), which in
turn activates the coagulation, fibrinolytic, kinin and complement systems. It has been
suggested that this type of immediate hypersensitivity is responsible for various clinical
syndromes in African trypanosomiasis, including blood hyperviscosity, edema, and
hypotension. Similar disease mechanisms would be expected in other infections by
protozoa involving a strong humoral immune response (Table 78-1).
Immune complexes have been found circulating in serum and deposited in the kidneys and
other tissues of humans and animals infected with protozoans. These parasite antigenantibody complexes, plus complement, have been eluted from kidney tissue in cases of
malaria and African trypanosomiasis. Antigen and antibody have been directly visualized in
the glomeruli of infected animals by light and electron microscopy. Inflammatory cell
infiltrates accompany these deposits, and signs of glomerulonephritis are usually seen.
African trypanosomes and presumably their antigens are also found in a variety of
extravascular locations. Immune complexes, cellular infiltrates, and tissue damage have
been detected in these tissues.
Another important form of antibody-mediated pathology is autoimmunity. Autoantibodies
to a number of different host antigens (for example, red blood cells, laminin, collagen, and
DNA) have been demonstrated. These autoantibodies may play a role in the pathology of
parasitic diseases in two ways. First the antibodies may exert a direct cytotoxic effect on the
host cells; for example, autoantibodies that coat red blood cells produce hemolytic anemia.
Alternatively, autoantibodies may be pathogenic through a buildup of antigen-antibody
complexes in the kidneys or other tissues, leading to glomerulonephritis or other forms of
immediate hypersensitivity. A particularly good example of a protozoan infection in which
autoimmunity appears to be an important contributor to pathogenesis is T cruzi infection. In
this case, there is substantial evidence that host and parasite share cross-reacting antigens.
Antibodies and cytotoxic lymphocytes to these antigens appear to be harmful to host tissue.
This type of experimental data, combined with the fact that the parasite itself seems not to
cause the tissue pathology, lead one to conclude that autoimmunity may play a key role in
pathogenesis.
Cellular hypersensitivity is also observed in protozoan diseases (Table 78-1). For example,
in leishmaniasis (caused by Leishmania tropica), the lesions appear to be caused by a cellmediated immune response and have many, if not all, of the characteristics of granulomas
observed in tuberculosis or schistosomiasis. In these lesions, a continuing immune response
to pathogens that are able to escape the host's defense mechanisms causes further influx of
inflammatory cells, which leads to sustained reactions and continued pathology at the sites
of antigen deposition. During a parasitic infection, various host cell products (cytokines,
lymphokines, etc.) are released from activated cells of the immune system. These mediators
influence the action of other cells and may be directly involved in pathogenesis. An
example is tumor necrosis factor (TNF), which is released by lymphocytes. TNF may be
involved in the muscle wasting observed in the chronic stages of African trypanosomiasis.
TNF has also been implicated in the cachexia and wasting in Leishmania donovani
infection, cerebral malaria in P falciparum in children and decreased survival in T cruziinfected mice. It is apparent that mediators involved in resistance to protozoan parasites
may also lead to pathology during a chronic infection (Fig. 78-1). There appears to be a
delicate balance between the factors involved in resistance to infectious agents and those
which ultimately produce pathology and clinical disease.
Numerous authors have suggested that toxic products produced by parasitic protozoa are
responsible for at least some aspects of pathology (Table 78-1). For example, the
glycoproteins on the surface of trypanosomes have been found to fix complement. This
activation of complement presumably results in the production of biologically active and
toxic complement fragments. In addition, trypanosomes are known to release proteases and
phospholipases when they lyse. These enzymes can produce host cell destruction,
inflammatory responses, and gross tissue pathology. Furthermore, it has been hypothesized
that the trypanosomes contain a B-cell mitogen that may alter the immune response of the
host by eliciting a polyclonal B-cell response that leads to immunosuppression. Finally it
has recently been shown that the African trypanosomes also contain an endotoxin which is
presumably released during antibody- mediated lysis. Parasitic protozoa have also been
reported to synthesize (or contain) low-molecular-weight toxins. For example, the
trypanosomes produce several indole catabolites; at pharmacologic doses, some of these
catabolites can produce pathologic effects, such as fever, lethargy, and even
immunosuppression. Similarly, enzymes, B-cell mitogen, etc., are presumably released by
many if not all of the other parasitic protozoa. There has been limited work on the role of
these protozoal products in pathogenesis. However, parasitic protozoa are generally not
known to produce toxins with potencies comparable to those of the classic bacterial toxins
(such as the toxins responsible for anthrax and botulism). One possible exception is the
African trypanosomes which are suggested to contain an endotoxin.
IMMUNE ESCAPE
Parasite escape mechanisms may include a number of different phenomena (Table 78-2). In
antigenic masking, the parasite becomes coated with host components and so fails to be
recognized as foreign. In blocking, noncytotoxic antibody combines with parasite antigens
and inhibits the binding of cytotoxic antibodies or cells. The parasite may pass part of its
life cycle in an intracellular location, for example, in erythrocytes or macrophages, in which
it is sheltered from intracellular digestion and from the cytotoxic action of antibody and/ or
lymphocytes. Some parasites practice antigenic variation, altering their surface antigens
during the course of an infection and thus evading the host's immune responses. Finally, the
parasite may cause immunosuppression, reducing the host's immune response either to the
parasite specifically or to foreign antigens in general. These strategies are discussed in more
detail below.
Masking and Mimicry
Various species of trypanosomes have host immunoglobulins associated with their cell
surfaces. There are several reports that these antibodies are not bound to the trypanosomes
through their variable regions, but presumably through the Fc portion of their molecule.
These antibodies may mask the parasite-that is, prevent immune recognition by the host.
However, no evidence other than the presence of immunoglobulins on the surface of the
trypanosomes supports this hypothesis. Mimicry, in which the parasite has the genetic
information to synthesize antigens identical to those of its host, has not been demonstrated
in parasitic protozoa.
Blocking
It has been hypothesized that in some cases antigen-antibody complexes in serum of
infected animals bind to the parasite's surface, mechanically blocking the actions of
cytotoxic antibodies or lymphocytes and directly inhibiting the actions of lymphocytes.
This type of immune escape mechanism has been proposed for tumor cells and for the
parasitic helminths. Because the trypanosomes carry immunoglobulins on their cell
surfaces, they may use a similar mechanism; however, no direct evidence has yet been
reported.
Intracellular location
Many protozoan parasites grow and divide within host cells. For example, Plasmodium
parasites grow first in hepatocytes and then in red blood cells. Leishmania and Toxoplasma
organisms are capable of growing in macrophages; one genus of parasitic protozoa,
Theilera, not only multiplies in lymphocytes but appears even to stimulate the
multiplication of the infected lymphocytes. Although some parasites, such as Plasmodium,
are restricted to a limited number of host cell types, others, such as T cruzi and
Toxoplasma, appear to be able to grow and divide in a variety of different host cells.
An intracellular refuge may protect a parasite from the harmful or lethal effects of antibody
or cellular defense mechanisms. For example, Plasmodium may be susceptible to the
actions of antibody only during the brief extracellular phases of its life cycle (the sporozoite
and merozoite stages). It should be remembered that Plasmodium actually resides in a
membrane-bound vacuole in the host cell. Thus, plasmodia are shielded from the external
environment by at least two host membranes (the outer cell membrane and an inner vacuole
membrane). Although intracellular plasmodia are very well protected from the host's
immune response early in their growth, this strategy does create physiologic problems for
the parasite. For example, the parasite must obtain its nutrients for growth through three
membranes (two host and one parasite), and must eliminate its waste products through the
same three membranes. Plasmodia solve this problem by appropriately modifying the host
cell membranes. Parasitic proteins are incorporated into the red blood cell outer membrane.
The host eventually responds to these antigens, and this response ultimately leads to the
increased removal of infected host cells.
The existence of extracellular phases in the malaria life cycle is important, since
immunization against these stages is the rationale for the development of our current
vaccine candidates. The protective antigens on these extracellular stages have been purified
as potential antigens for a vaccine. However, this approach has problems. For example, the
sporozoite stage is exposed to protective antibody for only a brief period, and even a single
sporozoite that escapes immune elimination will lead to an infection. Second, the antigenic
variability of different isolates and the ability of different strains to undergo antigenic
variation are not fully known. Therefore, the effectiveness of the vaccine candidates must
still be demonstrated. However a large synthetic peptide containing antigenic sequences
from 3 different proteins of P falciparum has been shown to reduce the clinical incidence of
malaria by 31% in field trials. There is therefore optimism that a vaccine against P
falciparum may be available in the near future.
A number of parasitic protozoa reside in macrophages. Although these organisms are
protected from external immune threats, they must still evade digestion by the macrophage.
Three strategies have been suggested. First, the parasite may prevent the fusion of
lysosomes with the phagocytic vacuole. The actual mechanism responsible for this
inhibition is not yet understood, but it has been shown to occur in cells infected with
Toxoplasma. A second mechanism is represented by the ability of T cruzi to escape from
the phagocytic vacuole into the cytoplasm of the macrophage. Finally, it is possible that
some parasites can survive in the presence of lysosomal enzymes, as can the leprosy
bacillus. One of the best-studied examples of a protozoan parasite able to survive in the
phagolysosome is Leishmania. It has been suggested that the resistance of this parasite to
the host's hydrolytic enzymes is due to surface components that inhibit the host's enzymes
and/or to the presence of parasitic enzymes that hydrolyze the host's enzymes. As
previously noted, at least one protozoan parasite, Theilera, is capable of growing directly in
lymphocytes. Therefore, this parasite may escape the host's immune response by growing
inside the very cells required for the response.
Antigenic Variation
Three major groups of parasitic protozoa are known to be able to change the antigenic
properties of their surface coat. The African trypanosomes can completely replace the
antigens in their glycocalyx each time the host exhibits a new humoral response. These
alterations in serotype are one important way in which the African trypanosomes escape
their host's defense mechanism. Although less well-characterized, similar changes are
reported to occur in Plasmodium, Babesia, and Giardia.
It has been estimated that African trypanosomes have approximately 1,000 different genes
coding for surface antigens. These genes are located on various chromosomes; however, to
be expressed, the gene must be located at the end of a chromosome (telomeric site). The
rate at which variation occurs in a tsetse-fly-transmitted population appears quite high. It
has been shown that 1 in 10 cells appears to be capable of switching its surface antigen. The
order in which the surface coat genes are expressed is not predictable. Much information is
available on the nucleotide sequence of the genes coding the coat proteins; however, neither
the factor(s) that induces a cell to switch its surface antigens nor the specific genetic
mechanisms) involved in the switch are fully understood. The antibody response does not
induce the genetic switch, but merely selects variants with new surface antigens out of the
original population. Considerably less information is available on the phenomenon of
antigenic variation in malaria or babesiosis. However, antigen variation could be a major
problem in reference to the development of a blood stage (merozoite) vaccine for malaria.
Finally, antigenic variation has been observed in Giardia lamblia. A number of different
gene families coding for surface proteins in Giardia have been identified. Antigenic
variation has been suggested to assist Giardia in escaping the host's immune response.
Immunosuppression
Immunosuppression of the host has been observed with almost every parasitic organism
carefully examined to date. In some cases the suppression is specific, involving only the
host's response to the parasite. In other cases the suppression is much more general,
involving the response to various heterologous and nonparasite antigens. It has not yet been
proven that this immunosuppression allows the parasites to survive in a normally
immunocompetent host. However, one can postulate that immunosuppression could permit
a small number of parasites to escape immune surveillance, thus favoring establishment of
a chronic infection. This mechanism might be particularly effective in parasites thai
undergo antigenic variation, since it could allow the small number of parasites with new
surface antigens to go undetected initially. Immunosuppression experimentally induced by
various extraneous agents has certainly been shown to produce higher parasitemias, higher
infection rates, or both. Therefore, the hypothesis that parasite-induced immmosuppression
increases the chance for a parasite to complete its life cycle makes sense.
It should be noted that immunosuppression can be pathogenic itself. A reduced response to
heterologous antigens could favor secondary infections. Humans suffering from malaria or
trypanosomiasis have been shown to be immunosuppressed to a variety of heterologous
antigens. Secondary infections may often be involved in death from African
trypanosomiasis.
A variety of mechanisms have been suggested to explain the immunosuppression observed
in protozoan infections. The most common mechanisms proposed are (1) the presence in
the infected host of parasite or host substances that nonspecifically stimulate the growth of
antibody-producing B cells, rather than stimulating the proliferation of specific antiparasite
B-cells; (2) proliferation of suppressor T-celis and/or macrophages that inhibit the immune
system by excretion of regulatory cytokines; and (3) production by the parasite of specific
immune suppressor substances.
Non-specific Host Defenses:
Non-specific host defenses are those that do not rely on antigen-antibody reactions and
which operate (more or less) without regard to the pathogen involved. Antibody mediated
defenses on the other hand are quite specific to certain pathogens, and even to certain
specific molecules associated with specific pathogens. We will consider the immune
systen (antibody defenses) in the next unit.
Non-specific defense mechanisms include a variety of physical, chemical, and biological
barriers to infection.
Physical defenses:
epithelial surfaces - intact (undamaged) skin and mucous membranes present a formidible
barrier that few organisms can penetrate. The larvae of hookworms and the cercaria of the
blood flukes represent a few of the rare exceptions.
Cross-sections of human skin (Credit: National Institutes of Health)
mucus - creates a sticky layer on tissue surfaces that entraps potential invading organisms.
the "ciliary escalator" - this refers to the system of ciliated epithelial surfaces of the upper
respiratory tract. The beating of the cilia moves the thin sheet of mucus associated with it
upward (or downward from the nose and sinuses) towards an accumulation point in the
throat where it triggers a cough reflex that causes the accumulation of mucus and the dust
and bacteria trapped in it to be expectorated or swallowed.
Respiratory epithelium: 1-cilia; 2-mucus
cell
Scanning EM of respiratory cilia
Image: Linkpublishing
flushing action - the flow of tears, sweat, and urine all serve to flush bacteria away.
coughing and sneezing - serve to flush (with air rather than water) invading organisms
away.
Chemical defenses:
Acidic pH - of skin (due to fatty acids in sebum); of the stomach (due to gastric
hydrochloric acid); of the vagina (due to the normal flora of lactobacilli that produce lactic
acid); and the slightly acidic pH of urine all serve to suppress the growth of most bacteria
or even kill them outright in the case of the stomach where the pH is extremly low (acidic).
Osmotic pressure - The salt left behind on skin as sweat evaporates creates high osmotic
pressure which suppresses the growth of most bacteria.
Lysozymes are smallish enzymes that are widely distributed throughout the biological
world. They attack the polysacharide polymers of the bacterial cell wall and render the cell
subject to osmotic lysis. Lysozyme is found in egg white, and in human blood, mucus,
tears, sweat, and saliva. It is also produced by phagocytic white blood cells (neutrophils and
monocytes) and by some bacteriophages. Like penicillin, it was discovered by Alexander
Fleming. Egg white lysozyme was the first enzyme to have its structure fully determined.
Interferons are a family of low molecular weight proteins that protect cells from viral
attack (among other functions). There are 3 groups of human interferons (alpha, beta, and
gamma interferon). Interferons are produced by virus-infected cells and also by
macrophages and some lymphocytes, which are stimulated to produce interferon via
activation by antigens. The effect of interferon on healthy cells is to stimulate them to
produce antiviral proteins (AVP) which protects them from viral infection.
Bile salts in bile suppress the growth of gram positive bacteria. Bile salts are steroids with
detergent properties. Bile salts conjugated to certain amino acids have recently been shown
to have antiviral activity, including anti-HIV activity.
Digestive enzymes in the small intestine create a very harsh chemical environment that is
hostile to most bacterial cells.
Complement - a complex series of proteins in body fluids that cause damage to the
bacterial plasma membrane. In humans, complement is produced by the liver. Complement
may also serve as an opsonin- a substance that coats invading cells and prepares them for
destruction by phagocytes (see below). Many antibody-antigen reactions require the
participation of complement as well.
The complement system consists of about thirty plasma proteins (designated C1 through C9
with a number of sub-types) that function either as enzymes or as binding proteins. It plays
an important role in host defence against pathogens and also participates in the process of
inflammation. In addition to the protein components, the system also includes specific
receptors on the surfaces of immune system cells to which the complement proteins (or
their fragments) bind. There are also several regulatory proteins that protect the host's
cells from accidental attack by its own complement.
The complement system can be activated by either of two different pathways: the classical
pathway and the alternative pathway. The classical pathway is activated by the binding
of certain antibody molecules to an antigen and is therefore antibody-dependent. The
alternative pathway is activated by the invading pathogens themselves and does not require
antibody.
Complement activation by either of the two pathways initiates a cascade reaction in which
each protein initiates the production of the next protein in the cascade sequence, and
ultimately produces a complex of proteins called the membrane attack complex (MAC).
The MAC produces a large hole in the membrane of the invading microorganism. With its
membrane integrity destroyed the invader loses the ability to regulate its intracellular
chemical environment and is killed by osmotic lysis.
Complement mediated lysis occurs in bacteria, enveloped (animal) viruses, and other cells
such as tumor cells, lymphocytes, platelets, and erythrocytes (red blood cells). The
destruction of red blood cells in transfusion reactions is an example of complement
mediated lysis.
During the course of the reaction cascade, other molecular products are produced that have
other effects including the release of histamine and other inducers of the inflammatory
response. These chemicals are released by basophils and mast cells (tissue basophils). Other
effects inclkude the stimulation and attraction of neutrophils, and the coating of invading
cells to mark them for destruction by phagocyles (opsonization).
Some complement components also appear to play a role in the body's elimination of large
and insoluble immune complexes (proteinaceous aggregates of antigen, antibody, and
complement). Receptors for C3B(a complement protein) on red blood cells allow these
cells to transport immune complexes to the liver where they are destroyed.
Complement cascade resulting in the formation of
membrane hole in invading cell (credit: Nat. Cancer Inst. )
Biological (cellular) defenses:
Phagocytosis: certain white blood cells (polymorphonuclear neutrophils and
monocytes) and other cells called macrophages (tissue cells derived from monocytes) are
able to engulf and destroy bacteria and some other invading organisms. Phagocytosis is
actually a rather complex process involving at least 5 steps:
1. locating the cell to be destroyed. This is usually accomplished by some form of
chemotaxis , which involves attraction of the phagocyte by a chemical substance produced
either by the microorganism itself or by other cells of the immune system..
2. attachment to the invading cell is facilitated by antibodies, complement, and specific
binding sites on the surface of the phagocyte.
3. ingestion of the invading cell by internalizing it within the phagocyte by a process of
endocytosis. A membranous phagosome (food vacuole) is thus formed around the
invading cell.
4. killing the invading cell with chemical substances which may include various enzymes,
hydrogen peroxide, superoxide anion, and hypochlorite ion. Lysosomes (containing up to
60 different digestive enzymes) fuse with and discharge their contents into the phagosome
to form a phagolysosome.
5. digestion of invading cell by hydrolytic (digestive) lysosomal enzymes.
For an animation of the process of phagocytosis, press HERE. (Graphic: Community College of
Baltimore Co.)
Human white blood cells (wbc's or leucocytes): There are typically 5,000 to 10,000 of
these per cubic millimeter of human blood ( 5-10 billion per liter ). The white cell count
typically increases in response to infection. Leucocytes play a role in both specific
(antibody-based) and non-specific host defense mechanisms.
Stained Human Blood Cells (RBC= red blood Scanning EM of Human Blood Cells (Image:
cells)
NIH)
There are two broad types of white cells: granulocytes (neutrophils, eosinophils, and
basophils) and agranulocytes (lymphocytes and monocytes). The 5 basic types of
leukocytes are:
Polymorphonuclear neutrophils (PMNs) - about 55-70% of circulating wbc's are PMNs.
These are phagocytic cells that play an important role in the defense against bacterial
pathogens. (Image)
Eosinophils represent about 5 % of circulating wbc's. These are capable of phagocytosis
under some conditions, but are more directly concerned with the extracellular destruction of
invading organisms, particularly parasites such as helminths. High eosinophil numbers are
typically associated with helminth infections. (Image)
Basophils - constitute 1% or less of circulating wbc's. Basophils produce a variety of
biologically active chemicals including histamine (involved in allergic response) and
heparin (an anticoagulant). (Image)
Lymphocytes - about 20% of circulating wbc's are lymphocytes. These cells are largely
responsible for both humoral and cellular immunity. There are two general types of
lymphocytes, B cells (plasma cells) that produce soluble (humoral) antibodies, and T cells
which are responsible for cellular immunity. There are 5 different types of T-cells, each
with its own set of functions (discussed later): (Image)
Helper cells (Th)
Suppressor cells (Ts)
Delayed hypersensitivity cells (Td)
Cytoxic [killer] cells (Tc)
Natural killer cells (Nk) - destroy tumor cells, transplanted tissue, and cells infected with
intracellular bacteria such as the Rickettsia and Chlamydias. Both Tc and Nk cells produce
a protein called perforin which bores a hole in the membrane of the attacked cell, much
like the membrane attack complex( MAC) of complement. While lymphocytes are
generally considered to be part of the "specific" immunity system (involving antigens and
antibodies), the Nk cells do not require an antigen to activate them, and so are perhaps best
considered to be part of the body's non-specific defense system.
Monocytes constitute about 5-8 % of circulating wbc's. Like, PMNs, these cells are capable
of phagocytosis. In the tissues, these cells can be transformed into macrophages which
phagocytize invading cells and process and "present" antigens to B-lymphocytes,
stimulating them to produce antibodies. (Image)
inflamation - a complex interwoven series of processes that increase the blood supply to an
infected site. The symptoms of inflamation: swelling, warming, reddening, and pain are all
the result of the increased blood supply brought about by capillary dilation. A series of
chemical mediators including histamine and the prostaglandins are also involved in
inflamatory processes.
fever - elevated body temperature protects the body from infection in several ways:


creates an unfavorable thermal environment for bacteria whose thermal optimum is
37o C.
speeds-up chemical reactions that are part of the chemical defense mechanism(s)
Nonspecific Defenses
Ferdinando Dianzani
Samuel Baron
General Concepts
Most viral infections are limited by defenses that are antigen nonspecific and/or specific.
Nonspecific defenses act sooner than specific defenses. Some are always in place (anatomic
barriers, nonspecific inhibitors, and phagocytic cells); others are evoked by the infection
(fever, inflammation, and interferon).
Anatomic Barriers
Anatomic barriers are located at body surfaces (skin and mucosa) or within the body
(endothelial cells and basement membranes). They are partly effective in preventing virus
spread but may be breached by large numbers of virus, by trauma, by increased
permeability, by replication of virus in endothelial cells, or by transportation of virus in
leukocytes.
Nonspecific Inhibitors
Body fluids and tissues normally contain soluble viral inhibitors. Most prevent viral
attachment, some directly inactivate viruses, and others act intracellularly. These inhibitors
may be overwhelmed by sufficient virus.
Phagocytosis
Viruses may be phagocytosed to different degrees by polymorphonuclear leukocytes and
macrophages. The effect of phagocytosis may be virus inactivation, persistence, or
multiplication; consequently, the result may be clearance of virus, transportation to distant
sites, or enhanced infection.
Fever
Replication of most viruses is reduced by even a modest rise in temperature. During viral
infection, fever can be initiated by several endogenous pyrogens, such as interleukins-1 and
-6, interferon, prostaglandin E2, and tumor necrosis factor.
Inflammation
Inflammation inhibits viral replication through (1) elevated local temperature, (2) reduced
oxygen tension, (3) metabolic alterations, and (4) acid production. The effects of these
mechanisms are often additive.
Viral Interference and Interferon
Viral interference occurs when infection by one virus renders cells resistant to the same or
other superinfecting viruses. Interference is usually mediated by newly induced host cell
proteins designated as the interferon systems. Secreted interferon binds to cells and induces
them to block various stages of viral replication. Interferon also (1) inhibits growth of some
normal and tumor cells and of many intracellular parasites, such as rickettsiae and protozoa;
(2) modulates the immune response; and (3) affects cell differentiation. There are three
main types of interferon, alpha, beta, and gamma interferons. Alpha interferon is produced
mainly by certain leukocytes (dendritic cells, macrophages and B cells), beta interferon by
epithelial cells and fibroblasts, and gamma interferon by T and natural killer cells. Two
other interferon types are related to alpha interferon. Omega interferons share about seventy
percent identity with alpha interferons. Tau interferons also are related structurally to alpha
interferons but are unusual by (a) being produced for only a few days by normal placental
trophoblasts and (b) not being inducible by viruses.
INTRODUCTION
Most viral infections are limited by nonspecific defenses, which (1) restrict initial virus
multiplication to manageable levels, (2) initiate recovery from established infections that is
then completed by a combination of these early nonspecific and subsequent antigenspecific immune defenses, and (3) enable the host to cope with the peak numbers of virus
that, if presented as the infecting dose, could be lethal. Although immune and nonimmune
(nonspecific) defenses operate together to control viral infections, this chapter considers
only nonspecific defenses. Some nonspecific defenses exist independently of infection
(e.g., genetic factors, anatomic barriers, nonspecific inhibitors in body fluids, and
phagocytosis). Others (e.g., fever, inflammation, and interferon) are produced by the host in
response to infection. All nonspecific defenses begin to act before the specific defense
responses develop and can potentiate some of the established immune effector mechanisms.
The fact that viruses replicate intracellularly and the ability of some viruses to spread by
inducing cell fusion partly protect viruses against such extracellular defenses as
neutralizing antibody, phagocytosis, and nonspecific inhibitors. However, because they
replicate within the cell, viruses are vulnerable to intracellular alterations caused by host
responses to infection. Nonspecific responses that alter the intracellular environment
include fever, inflammation, and interferon.
These multiple defenses function with great complexity because of their interactions with
one another. This complexity is compounded by the varying effectiveness of the defenses
that results from the diversity of viruses, hosts, and sites and stages of infection.
Defense Mechanisms that Precede Infection
Anatomic Barriers
Anatomic barriers to viruses exist at the body surfaces and within the body. At the body
surfaces, the dead cells of the epidermis and any live cells that may lack viral receptors
resist virus penetration and do not permit virus replication. However, this barrier is easily
breached, for example, by animal bites (rabies virus), insect bites (togaviruses), and minor
traumas (wart virus). At mucosal surfaces, only the mucus layer stands between invading
virus and live cells. The mucus layer forms a physical barrier that entraps foreign particles
and carries them out of the body; it also contains nonspecific inhibitors (see following
section). The mucus barrier is not absolute, however, since sufficient quantities of many
viruses can overwhelm it and infect by this route. In fact, most viruses use mucous surfaces
as the portal of entry and initial replication site.
Within the body, anatomic barriers to virus spread are formed by the layer of endothelial
cells that separates blood from tissues (e.g., the bloodbrain barrier). Under normal
conditions, these barriers have a low permeability for viruses unless the virus can penetrate
them by replicating in the capillary endothelial cells or in circulating leukocytes. These
internal barriers may explain, in part, the high level of viremia required to infect organs
such as the brain, placenta, and lungs.
Nonspecific Inhibitors
A number of viral inhibitors occur naturally in most body fluids and tissues. They vary
chemically (lipids, polysaccharides, proteins, lipoproteins, and glycoproteins) and in the
degree of viral inhibition and types of viruses affected. Some inhibitors are related to the
viral receptors of the cell surface, but most are of unknown origin. Many inhibitors act by
preventing virus from attaching to cells, others by directly inactivating virus, and a few by
inhibiting virus replication. In the gastrointestinal tract, some susceptible viruses are
inactivated by acid, bile salts, and enzymes. Whereas most inhibitors block only one or a
few viruses, some have a broad antiviral spectrum. Although the effectiveness of the
inhibitors has not been fully established in vivo, their importance as host defenses is
suggested by their antiviral activity in tissue culture and in vivo and by the direct
correlation between the degree of virulence of some viruses and their degree of resistance
to certain inhibitors. Examples are the serum and mucus inhibitors of influenza viruses
during experimental infections. However, even sensitive viruses may overwhelm these
inhibitors when the infecting dose of virus is sufficiently high. Therefore, the presence of
these inhibitors may explain the relatively high dose of virus required to initiate infection in
vivo, compared with the dose needed in cell cultures.
Phagocytosis
The limited information available suggests that phagocytosis is less effective against viral
infections than against bacterial infections. However, few of the factors that control uptake
of virions or infected cells by phagocytes and their digestion by lysosomal enzymes have
been studied systemically. Different viruses are affected differently by the various
phagocytic cells. Some viruses are not engulfed, whereas others are engulfed but may not
be inactivated. In fact, some viruses, such as human immunodeficiency virus (HIV), may
even multiply in the phagocytes (e.g., macrophages), which may serve as a persistent
reservoir of virus (Fig. 49-1). The virulence of several strains of HIV and herpesviruses
correlates with their ability to multiply in macrophages. Infected macrophages may carry
virus across the blood-brain barrier. Interestingly, cytomegalovirus has been reported to
replicate in granulocytes. Macrophages seem to be more effective against viruses than are
granulocytes, and some viruses seem to be more susceptible to phagocytosis than others.
Macrophages and polymorphonuclear leukocytes can afford important protection by
markedly reducing the viremia caused by virus strains susceptible to phagocytosis.
FIGURE 49-1 Possible outcomes of phagocytosis of a virus.
Viruses may stimulate macrophages to produce monokines, which can reduce viral
multiplication. For example, macrophage-produced alpha interferon (IFN-a) inhibits viral
multiplication both directly and also indirectly by activating natural killer cells. Interleukin
1 (IL-1), produced by macrophages, can interfere with viral multiplication in a number of
ways: (1) by inducing T lymphocytes to produce interleukin-2, which in turn induces
gamma interferon (IFN-g), which can induce alpha and beta interferons; (2) by inducing the
production of beta interferon (IFN-b) by fibroblasts and epithelial cells; (3) by inducing
fever, which inhibits viral replication; (4) by enhancing macrophage-mediated cytolysis of
infected cells; and (5) by inducing production of tumor necrosis factor (TNF), which
inhibits virus multiplication both directly and indirectly by inducing interferon and other
cytokines and augmenting inflammation, phagocytosis and cytotoxic activity.
Therefore, depending on the situation, macrophages acting as phagocytes may reduce the
number of viruses, help spread the infection, augment or depress immune defenses, or have
little effect.
Defense Mechanisms Evoked by Infection
Fever
Viral replication is influenced strongly by temperature. Fever can be induced during viral
infection by at least three independent endogenous pyrogens: interleukins-1 and 6,
interferon, prostaglandin E2, and tumor necrosis factor. Even a modest increase can cause
strong inhibition: a temperature rise from 37°C to 38°C drastically decreases the yield of
many viruses. This phenomenon has been observed in tissue culture as well as in many
experimental (including primate) and natural infections. Artificial induction of fever
reduces mortality in mice infected with viruses (Fig. 49-2). Artificial lowering of the
temperature during infection may increase mortality, as in suckling mice infected with
coxsackieviruses and taken away from the warmth of their mother's nest. Fever also
augments the generation of cytotoxic T lymphocytes.
FIGURE 49-2 Protection of mice by elevated temperature or antibody administered
before or after intracerebral infection with the picornavirus EMC type.
Several observations suggest strongly that fever reduces virus multiplication during human
viral infections. Retrospective studies have shown that the incidence and severity of
paralysis among children infected with polioviruses were significantly greater in patients
treated with antipyretic drugs (e.g., aspirin) than in untreated children. Also consistent with
these findings is the observation that virus strains that replicate best at fever temperature are
usually virulent, whereas virus strains that replicate poorly at fever temperature are usually
low in virulence and therefore often are used as live virus vaccines.
Temperatures as low as 33°C are normal at body surfaces exposed to air; viruses that infect
these sites and replicate optimally at these temperatures establish only local infections that
do not spread to deeper tissues, where the body temperature is higher. For example,
rhinoviruses that cause common colds replicate optimally at 33°C to 34°C (found in
normally ventilated nasal passages); however, they are inhibited at 37°C (found when
swelling of the edematous mucosa and secretions interrupt air flow). An interesting
question is whether this temperature increase is important for recovery from coryza. The
same general considerations of temperature probably apply to other human viral infections
such as measles, rubella, and mumps, although, unfortunately, suitable and controlled
studies have not been conducted. Nevertheless, available information suggests that
antipyretic drugs be used conservatively.
Inflammation
Several antiviral mechanisms are generated by the local inflammatory response to virusinduced cell damage or to virus-stimulated mediators such as activated complement. The
major components of the inflammatory process are circulatory alterations, edema,
leukocyte accumulation and perhaps prostaglandins A and J. The resulting phenomena are
elevated local temperature, reduced oxygen tension in the involved tissues, altered cell
metabolism, and increased levels of CO2 and organic acids. All of these alterations, which
occur in a cascading and interrelated fashion, drastically reduce the replication of many
viruses. For instance, the altered energy metabolism of the infected and surrounding cells,
as well as the accumulating lymphocytes, can generate local hyperthermia. At superficial
sites where the temperature is normally lower, hyperthermia can also be generated by
hyperemia during the early stages of inflammation. As inflammation progresses, hyperemia
becomes passive, thereby greatly reducing blood flow and decreasing oxygen tension. Two
factors account for this decrease in oxygen tension: limited influx of erythrocytes, and
lower diffusion of oxygen through edema fluid. In turn, the decreased oxygen tension
causes less ATP production, thus reducing the energy available for viral synthesis and
increasing anaerobic glycolysis, which increases the accumulation of CO2 and organic acids
in the tissues. These acid catabolites may decrease the local pH to levels that inhibit the
replication of many viruses. Local acidity also may increase by accumulation and
subsequent degradation of the leukocytes in the affected area. It is possible that other less
well-defined factors are also significant .
Therefore, the local inflammation resulting from viral infection clearly activates several
metabolic, physicochemical, and physiologic changes; acting individually or together, these
changes interfere with virus multiplication. Although further animal and human studies are
required, this interpretation is supported by the finding that anti-inflammatory drugs
(corticosteroids) often increase the severity of infection in animals. Therefore, these drugs
should be used with caution in treating viral diseases.
Viral Interference and Interferon
Viral Interference
Generally, infection by one virus renders host cells resistant to other, superinfecting
viruses. This phenomenon, called viral interference, occurs frequently in cell cultures and
in animals (including humans). Although interference occurs between most viruses, it may
be limited to homologous viruses under certain conditions. Some types of interference are
caused by competition among different viruses for critical replicative pathways
(extracellular competition for cell surface receptors, intracellular competition for
biosynthetic machinery and genetic control). Similar interference may result from
competition between defective (nonmultiplying) and infective viruses that may be produced
concurrently. Another type of interferencethe most important type in natural infectionsis
directed by the host cells themselves. These infected cells may respond to viral infection by
producing interferon proteins, which can react with uninfected cells to render them resistant
to infection by a wide variety of viruses.
Interferon
The important role played by interferon as a defense mechanism is clearly documented by
three types of experimental and clinical observations: (1) for many viral infections, a strong
correlation has been established between interferon production and natural recovery; (2)
inhibition of interferon production or action enhances the severity of infection; and (3)
treatment with interferon protects against infection. In addition, the interferon system is one
of the earliest appearing of known host defenses, becoming operative within hours of
infection. Figure 49-3 compares the early production of interferon with the level of
antibody during experimental infection of humans with influenza virus. Clinical studies of
interferon and its inducers have shown protection against certain viruses, including hepatitis
B and C viruses, papovaviruses, rhinoviruses, and herpes simplex virus.
FIGURE 49-3 Production of virus, interferon, and antibody during experimental
infection of humans with influenza wild-type virus. Nonspecific defenses include
anatomic barriers, inhibitors, phagocytosis, fever, inflammation, and IFN. Specific defenses
include antibody and cell-mediated immunity. Data from a study by B. Murphy et al,
National Institutes of Health (personal communication).
Although interferon was first recognized as an extraordinarily potent antiviral agent, it was
found subsequently to affect other vital cell and body functions. For example, it may
enhance killing by granulocytes, macrophages, natural killer (NK) cells, and cytotoxic
lymphocytes and affect the humoral immune response and the expression of cell membrane
antigens and receptors. It may also lyse or inhibit the division of certain cells, influence cell
differentiation, and cross-activate hormone functions such as those of epinephrine and
adrenocorticotropin (ACTH). The effect of these modulations may influence many viral
infections.
Interferon Production and Types
Interferon is produced de novo by cellular protein synthesis. The three types (alpha, beta,
and gamma) differ both structurally and antigenically and have molecular weights ranging
from 16,000 to 45,000. Interferons are secreted by the cell into the extracellular fluids (Fig.
49-4). Usually, virus-induced interferon is produced at about the same time as the viral
progeny are released by the infected cell, thus protecting neighboring cells from the
spreading virus.
FIGURE 49-4 Induction of beta interferon, alpha interferon, and gamma interferon,
respectively, by foreign nucleic acids, foreign cells, and foreign antigens.
The three known types of interferon are induced by different stimuli. Beta interferon is
induced by viral and other foreign nucleic acids in most body cells (fibroblasts, epithelial
cells, and macrophages). This induction mechanism is illustrated in Figure 49-4 and the top
portion of Figure 49-5.
FIGURE 49-5 Cellular events of the induction, production, and action of interferon.
Inducers of interferon react with cells to depress the interferon gene(s) (A). This leads to
the production of mRNA for interferon (B). The mRNA is translated into the interferon
protein (C), which is secreted into the extracellular fluid(D), where it reacts with the
membrane receptors of cells (E). The interferon-stimulated cells derepress genes (F) for
effector proteins (AVP) that establish antiviral resistance and other cell changes. The
activated cells also stimulate contacted cells (G) to produce AVP by a still unknown
mechanism.
Alpha interferon can be induced by foreign cells, virus-infected cells, tumor cells, bacterial
cells, and viral envelopes that stimulate mostly circulating dendritic cells and to a lesser
degree monocytes and B lymphocytes to produce it (Fig. 49-4, middle).
Gamma interferon is produced (along with other lymphokines) by T lymphocytes induced
by foreign antigens to which the T lymphocytes have been presensitized (Fig. 49-4).
Mitogens for T cells may mimic this induction. Gamma interferon has several unusual
properties: (1 ) it exerts greater immunomodulatory activity, including activation of
macrophages, than the other interferons; (2) it exerts greater lytic effects than the other
interferons; (3) it potentiates the actions of other interferons; (4) it activates cells by a
mechanism significantly different from that of the other interferons; and (5) it inhibits
intracellular microorganisms other than viruses (e.g., rickettsia).
The 24 genes that code for interferons alpha and the single gene for beta in humans, are
located in adjacent positions on chromosome 9. The only gene for interferon gamma is
found on chromosome 12. The genes for interferons alpha and beta exhibit significant
homology but not with interferon gamma.
Genes for interferon alpha may be differentiated into two distinct clusters on the basis of
the degree of homology. As a consequence, interferon alpha comprises two families of
proteins, at least 14 of which belong to the alpha-1 type and two to the alpha-2 type (omega
and tau).
Also, interferon occurs without apparent stimulation in the plasma of patients with
autoimmune diseases (such as rheumatoid arthritis, disseminated lupus erythematosus and
pemphigus) and in patients with advanced HIV infection. In these cases, an interferon
antigenically identical to interferon alpha is present but which, unlike the latter, is partially
inactivated at pH 2 (acid-labile interferon alpha). This interferon is a synergistic
combination of interferons alpha (acid stable) and gamma (acid labile). Consequently, acid
treatment reduces the interferon activity by inactivating the synergistic interferon gamma.
Mechanism of Action
Interferon does not inactivate viruses directly. Instead, it prevents viral replication in
surrounding cells by reacting with specific receptors on the cell membranes to derepress
cellular genes that encode intracellular effector antiviral proteins, which must be
synthesized before virus replication can be inhibited (Figs. 49-5 and 49-6). Alpha and beta
interferons both bind to the same type of membrane receptor; gamma interferon binds to a
different receptor. The antiviral proteins probably inhibit viral multiplication by inhibiting
the synthesis of essential viral proteins, but alternative or additional inhibitory mechanisms
(e.g, inhibition of transcription and viral release) also occur. Viral protein synthesis may be
inhibited by several biochemical alterations of cells, which may, in theory, inhibit viral
replication at the different steps shown in Figure 49-6.
FIGURE 49-6 Molecular mechanisms of interferon antiviral actions.
It has been shown that the antiviral state may be transferred from interferon-treated cells to
adjacent untreated cells without the continued presence of interferon (Fig. 49-4); this
transfer mechanism may further amplify and spread the activity of the interferon system.
The interferon system is nonspecific in two ways: (1) various viral stimuli induce the same
type of interferon, and (2) the same type of interferon inhibits various viruses. On the other
hand, the interferon molecule is mostly specific in its action for the animal species in which
it was induced: interferon produced by animals or humans generally stimulates antiviral
activity only in cells of the same or closely related families (e.g., human interferon protects
human and monkey cells, but not chicken cells).
Interferon During Natural Infection
The importance of interferon in the response to certain natural virus infections varies. Much
depends on the effectiveness of the virus in stimulating interferon production and on its
susceptibility to the antiviral action of interferon. Interferon protects solid tissues during
virus infection; it is also disseminated through the bloodstream during viremia, thereby
protecting distant organs against the spreading infection. Cells protected against viral
replication may eliminate virus by degrading the virus genome (Fig. 49-7).
FIGURE 49-7 Nonspecific elimination of viruses by cells.
Medical Applications
Interferons have been approved in several nations for treatment of viral infections
(papillomas and condylomata, herpes simplex, and hepatitis B and C) and cancers (hairy
cell leukemia, chronic myelogenous leukemia, non-Hodgkin's lymphomas, and Kaposi's
sarcoma in AIDS patients). Clinical trials also have shown effectiveness against
cryoglobulinemia and thrombocytosis and maintenance of remission in multiple myeloma.
Interferon beta has received governmental approval for treatment of relapsing multiple
sclerosis and interferon gamma for chronic granulomatous disease. Studies of effectiveness
in other viral infections and cancers are continuing, as are studies with substances capable
of inducing endogenous interferon.
Conclusion
In conclusion, individual defense mechanisms assume roles of varying importance during
different viral infections; in most cases, the recovery process is probably carried out by the
simultaneous or sequential action of several mechanisms. The presence of multiple
defenses helps explain why suppression of one or several mechanisms does not entirely
abrogate host resistance to viral infections; however, impairment of host defenses by
medications used for symptomatic relief of viral infections may lead to more severe illness.
For example, aspirin and corticosteroids reduce the nonspecific defenses. Therefore, the
well-established principle of the ancient physician"primum non nocere" (primarily do not
harm)is still valid.