Chapter 17 Infectious Diseases and Vaccines

Parasitic Infections

Infections caused by parasites account for an enormous disease burden worldwide, especially in developing countries within tropical or subtropical regions. In these locales sanitation and living conditions are not always ideal, increasing the spread of all types of infectious disease. Thanks to climate, tropical regions are also common breeding grounds for the arthropod vectors that carry parasitic infection, such as mosquitoes, flies, and ticks. Further complicating this system, many of these parasites can infect nonhuman primates and other mammals, allowing both human-to-human and animal-to-human (zoonotic) spread.

The term parasite encompasses a vast array of infectious protozoans (unicellular) and metazoans (helminths or worms). The diversity of the parasitic universe makes it difficult to generalize about this group. For instances, most protozoan parasites, although eukaryotic, inhabit intracellular spaces in their human host for at least one of their life cycle stages. Conversely, helminths are multicellular eukaryotes that can be quite large in their adult stages; up to 1 m in length! These organisms typically live and reproduce exclusively outside host cells (region E in Overview Figure 17-3), sometimes occupying host body cavities, like the gut.

One of the biggest challenges posed to the immune response by most parasites is their complicated life cycle, leading to changes in antigenic structure and location over time. Therefore, the most effective immune response will depend on the type of organism, the location of the infection, and the life cycle stage of the parasite.

Protozoan Parasites Are a Diverse Set of Unicellular Eukaryotes

Many of the most burdensome and least treatable tropical diseases are caused by protozoan parasites, itself a broad category of all parasitic infections. The only common features of this group are that all are unicellular eukaryotes, and many are motile. Some, but not all, are pathogenic. Many can be free-living and found in contaminated water (e.g., Giardia or Toxoplasma). Other protozoan parasites move from their arthropod vector hosts, such as mosquitoes and flies (e.g., the parasites that cause malaria and African sleeping sickness, respectively), to their mammalian hosts when the infected insects draw blood during feeding. These often complicated gymnastics of movement between hosts or environmental sites, combined with multiple life stages within any one host, make immune detection and eradiation extremely challenging.

There is no one common protozoan parasitic infection cycle. However, there are some protozoan parasites with particular significance to human health and disease that have been well characterized. For instance, many protozoan parasites progress through multiple antigenic forms and/or locations during their life cycle in the human host, leaving the immune response one step behind. When parasites are in the bloodstream, gut, or interstitial fluid of their human host, humoral immunity is the most effective response. However, these stages can be very transient or include evasion strategies, presenting little opportunity for clonal selection of lymphocytes or antibody attachment. Those parasites that undergo intracellular life cycle stages require cell-mediated immune reactions as a defense. However, these can be but short stops in a series of life cycle “jumps” to another site, and each presents the host with new antigenic structures to attack and new immune pathways to initiate. This challenges not only the immune response but also our ability to design effective treatments and vaccines.

Some important immunologic lessons have been learned from the study of protozoan parasites. For example, the trypanosomes that cause African sleeping sickness use a novel evasive strategy that employs up to 1000 possible variants of protein coat to outrun the immune response (Clinical Focus Box 17-4). The individual immune response to another trypanosome, leishmania, can head in one of two polarized directions, depending on host and pathogen characteristics; a T_H1-driven response that effectively limits pathology or a T_H2-mediated pathway leading to rampant dissemination and progressive disease. Finally, our struggle against malaria, arguably the protozoan parasite that has taken the greatest toll in recent memory, is confounded by a complicated life cycle involving multiple extracellular and intracellular stages of infection, like a red blood cell stage that is particularly refractory to immune detection. This pathogen illustrates several unique challenges presented to the immune system and highlights many of the obstacles to vaccine design common to protozoan parasites.

CLINICAL FOCUS BOX 17-4

African Sleeping Sickness: Novel Immune Evasion Strategies Employed by Trypanosomes

Two species of African trypanosomes, a protozoan parasite, cause African sleeping sickness, a chronic, debilitating disease transmitted to humans and cattle by the bite of the tsetse fly. In the bloodstream, this flagellated protozoan differentiates into a long, slender form that continues to divide every 4 to 6 hours. The disease progresses from an early, systemic stage in which trypanosomes multiply in the blood to a neurologic stage in which the parasite infects cells of the central nervous system, leading to meningoencephalitis and eventual loss of consciousness—thus the name.

The surface of the Trypanosoma parasite is covered with a variable surface glycoprotein (VSG). Several unusual genetic processes generate extensive variation in these surface structures, enabling the organism to escape immunologic clearance. An individual trypanosome carries a large repertoire of VSG genes, each encoding a different VSG primary sequence, but the trypanosome expresses only a single VSG gene at one time. Trypanosoma brucei, for example, carries more than 1000 VSG genes in its genome. Activation of a VSG gene results in duplication of the gene and its transposition to a transcriptionally active expression site (ES) at the telomeric end of a specific chromosome (Figure 1a). Activation of a new VSG gene displaces the previous gene from the telomeric ES. Trypanosomes have multiple transcriptionally active ES sites, so that a number of VSG genes can potentially be expressed; unknown control mechanisms limit expression to a single VSG expression site at any time.

FIGURE 1 Successive waves of parasitemia after infection with Trypanosoma result from antigenic shifts in the parasite’s variant surface glycoprotein (VSG). (a) Antigenic shifts in trypanosomes occur by the duplication of gene segments encoding variant VSG molecules and their translocation to an expression site located close to the telomere. (b) Antibodies develop against each variant as the numbers of these parasites rise, but each new variant that arises is unaffected by the humoral antibodies induced by the previous variant.

The gene segments of the actual parasite are arranged in the following order, from the 5-dash end to the 3-dash end: V S G 1, V S G 2, V S G 3, V S G 4, V S G n and V S G 1 at the expression site. The first antigenic shift involves the duplication and translocation of V S G 2 segment to the expression site. The gene segments in the first mutated parasite are arranged in the following order, from the 5-dash end to the 3-dash end: V S G 1, V S G 2, V S G 3, V S G 4, V S G n and V S G 2 at the expression site. The second antigenic shift involves the duplication and translocation of V S G 3 segment to the expression site. The gene segments in the second mutated parasite are arranged in the following order, starting from the left: V S G 1, V S G 2, V S G 3, V S G 4, V S G n and V S G 3 at the expression site. The graph has a curve with three peaks. The first peak corresponds to 1.2 million of trypanosomes per milliliter of blood and 26 weeks after tsetse fly bite. This is when variant 1 antibodies develop. The second peak corresponds to 1 million of trypanosomes per milliliter of blood and 27 weeks after tsetse fly bite. This is when variant 2 antibodies develop. The third peak corresponds to 1.3 million of trypanosomes per milliliter of blood and 28 weeks after tsetse fly bite. This is when variant 3 antibodies develop.

As parasite numbers increase after infection, an effective humoral response develops to the VSG covering the surface of the parasites. These antibodies eliminate most of the parasites from the bloodstream, both by complement-mediated lysis and by opsonization and subsequent phagocytosis. However, about 1% of the organisms bear an antigenically different VSG because of transposition of a different VSG gene into the ES. These parasites escape the initial antibody response, begin to proliferate in the bloodstream, and go on to populate the next wave of parasitemia in the host. The successive waves of parasitemia reflect a unique mechanism of antigenic shift by which the trypanosomes sequentially evade the immune response to their surface antigens. Each new variant that arises in the course of a single infection escapes the humoral antibodies generated in response to the preceding variant, and so waves of parasitemia recur (Figure 1b). The new variants arise not by clonal outgrowth from a single escape variant cell, but from the expansion of multiple cells that have activated the same VSG gene in the current wave of parasitic growth. It is not known how this process is coordinated. This continual shifting of surface epitopes has made vaccine development extremely difficult.

Parasitic Worms (Helminths) Typically Generate Weak Immune Responses

Metazoan parasites, or helminths (worms), are responsible for a range of diseases in humans and animals. Adult forms of helminths are large, multicellular organisms that can often be seen with the naked eye. The three main types of parasitic worms are nematodes (roundworms), cestodes (tapeworms), and trematodes (flukes). Most enter their animal hosts through the intestinal tract; helminth eggs can contaminate food, water, feces, and soil. Some, like schistosomes, are transmitted directly through the skin (Clinical Focus Box 17-5).

CLINICAL FOCUS BOX 17-5

Schistosomiasis: Low Antigenicity and Large Size Pose Unique Challenges to Immune Detection and Elimination of Helminths

More than 300 million people are infected with the helminthic parasite Schistosoma species, which causes the chronic, debilitating, and sometimes fatal disease schistosomiasis. Infection occurs through contact with free-swimming infectious larvae that are released from an infected snail and bore into the skin, frequently while individuals wade through contaminated water. As they mature, these parasites migrate in the body, with the final site of infection varying by species. The females produce eggs, some of which are excreted and infect more snails. Most symptoms of schistosomiasis are initiated by the eggs, which invade tissues and cause hemorrhage. A chronic state can develop in which the unexcreted eggs induce cell-mediated DTH reactions, resulting in large granulomas that can obstruct the venous blood flow to the liver or bladder.

An immune response does develop to the schistosomes, but it is usually not sufficient to eliminate the adult worms. Instead, the worms survive for up to 20 years, causing prolonged morbidity. Adult schistosome worms have several unique mechanisms that protect them from immune defenses. These include decreasing the expression of antigens on their outer membrane and enclosing themselves in a glycolipid-and-glycoprotein coat derived from the host, masking the presence of their own antigens. Among the antigens observed on the adult worm are the host’s own ABO blood-group and histocompatibility antigens. The immune response is, of course, diminished by this covering made of the host’s self antigens, which contributes to the lifelong persistence of these organisms.

The major contributors to protective immunity against schistosomiasis are controversial. The immune response to infection with S. mansoni, the most common cause of the disease, is dominated by T_H2-like mediators, with high titers of anti-schistosome IgE antibodies, localized increases in degranulating mast cells, and an influx of eosinophils (Figure 1, top). These cells can then bind the antibody-coated parasite, using their Fc receptors for IgE or IgG, inducing degranulation and death of the parasite via antibody-dependent cell-mediated cytotoxicity (ADCC; see Chapter 12). One eosinophil mediator, called basic protein, has been found to be particularly toxic to helminths. However, immunization studies in mice suggest that a T_H1 response, characterized by IFN-γ and macrophage accumulation, may actually be more effective for inducing protective immunity (Figure 1, bottom). In fact, inbred strains of mice with deficiencies in mast cells or IgE can still develop protective immunity to S. mansoni following vaccination. Based on these observations, it has been suggested that the ability to induce an ineffective T_H2-like response may have evolved in schistosomes as a clever defense mechanism to ensure that IL-10 and other T_H1 inhibitors are produced in response to infection, therefore blocking initiation of a more effective T_H1-dominated pathway.

FIGURE 1 The immune response generated against Schistosoma mansoni. The response includes an IgE humoral component (top) and a cell-mediated component involving CD4⁺ T cells (bottom). C = complement; ECF = eosinophil chemotactic factor; NCF = neutrophil chemotactic factor; PAF = platelet-activating factor.

An adult worm is shown at the center of the illustration. I g E antibodies released by a plasma cell binds to a mast cell. The complementary antigens of the adult worm interact with C 3 a and C 5 a and migrate toward the mast cell leading to the release of eosinophil chemotactic factor and neutrophil chemotactic factor. The basic proteins of the activated eosinophil interact with the antigens on the outer membrane of the adult worm. The C 3 b binds neutrophil with the outer membrane of the adult worm. The platelet-activating factor released by the neutrophils activates the platelets act as mediators to inhibit inflammation. The complementary antigens of the adult worm interact with C3a and C5a that migrate toward a mast cell causing degranulation. These granules act as mediators that help macrophages migrate through the vasodilated epithelial membrane. The macrophage undergoes Chemotaxis releasing C 3 b and binding the outer membrane of the adult worm. The I F N-gamma cytokines released by the T helper cells activate the macrophage.

Although helminths are exclusively extracellular and therefore more accessible to the immune system than protozoans (see Overview Figure 17-3, region E versus V), most infected individuals carry relatively few individual parasites at any one time. Also, unlike protozoan parasites, helminths do not multiply within their human hosts. This results in fewer foreign epitopes that can be recognized by the immune system and weak engagement to each, generating relatively poor immune reactivity. Adult helminths are also too big for phagocytic cells to engulf. This means that the best approach may be expulsion rather than the typical humoral opsonization and digestion response. In that case, IgE-mediated responses that result in mast cell degranulation can help, ejecting the worm from the body via the release of histamines and leukotrienes that induce muscle contractions and mucus production (e.g., coughing, vomiting, or explosive diarrhea). One common feature of effective immune responses against metazoan parasites is a reliance on T_H2-type responses, including ILC2s, IL-4 production, T_H2-cell activation, and the production of IgE over IgG. Interestingly, a paucity of early life exposures to helminthic parasites, as occurs in highly developed and urban settings, is credited with a lower threshold for T_H2-type response induction, resulting in overproduction of IgE: heightened type I, IgE-mediated, allergic responses to random, benign environmental antigens (see Chapter 15).