Immunity to Parasitic Infection

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Current Approaches to the Development of a Vaccine Against

Current Approaches to the Development of a Vaccine Against

List of Contributors

London School of Hygiene and Tropical Medicine, Department of Immunology and Infection, Keppel Street, London, WC1E 7HT, United Kingdom. Department of Pediatrics, Emory University School of Medicine, Atlanta VA Medical Center, 1670 Clairmont Road, Dectaur, GA 30033, USA.

Terrazas

Introduction

Immunoparasitology: the making of a modern

Alan Sher, PhD

Origins

Acquired resistance to 'swamp fever' (malaria) was also recognized by European colonizers of tropical countries who came to realize their extreme susceptibility compared to that of indigenous populations. Although earlier than protozoa were recognized as human parasites, the existence of acquired immunity to helminths was less obvious and awaited the next phase in the development of the field.

Development of immunoparasitology as an experimental science

The period in which most of the major parasitic infections of man were identified (ie, the second half of the 19th century and the beginning of the 20th century) coincided with the era in which the role of the immune system in host defense was first elucidated. Together with his wife, Lucy, he performed experimental trypanosome and malaria infections in rodents, documenting both acquired immunity and the role of antibodies in protection against parasite variants.

Figure 1 Some pioneers and early champions of the field of immunoparasitology.

The immunological renaissance

Later, as director of the Wellcome Trust, Ogilvie played a key role in promoting the development of modern parasitological research in Britain. In the United States, Jack Remington played an analogous role in establishing the critical function of cell-mediated immunity in host resistance to Toxoplasma infection (Chapter 4).

Early breakthroughs and disappointments in parasite vaccine development

Another important British investigator was James Howard who, in addition to building a highly innovative parasitology group at the Burroughs Wellcome Research Labs in Beckenham, UK, carried out pioneering studies with FY Liew on the function of CD4+ T cells in host resistance and susceptibility. to Leishmania major in the murine model (Chapter 7). These transitional pioneers helped open the field of parasitology to modern immunological approaches and laid the groundwork for the major discoveries of the following decades.

Effector mechanisms and effector choice in parasitic infection

With the discovery that both immediate and delayed hypersensitivity reactions are mediated by the same T cell subpopulation defined by the presence of the CD4 molecule, the field was presented with an interesting paradox. Thus, work on parasitic infection models has greatly contributed to the development of the concept of immunological effector choice.

Parasites define roles for regulatory T cells

Once again, many of the seminal in vivo observations supporting this concept were made in mouse parasite infection models. As discussed below, studies of parasitic infection later led to the discovery that effector cells themselves can be induced to display regulatory activity.

Parasites help define the roles of regulatory cytokines and the plasticity of CD4+ T cell subsets

However, with the discovery that CD25+Foxp3 T-regulatory (Treg) cells play a major role in dampening immune responses, it became clear that these CD4+. Parasites help define the roles of regulatory cytokines and the plasticity of CD4+ T-cell subsets.

Parasites as triggers of the innate immune response

Lessons from helminth immunology about allergic and fibrotic disease

In this context, the study of immunopathology induced by schistosome eggs in the liver and lung (Chapter 16) has provided important basic information on the formation and fibrosis of Th2-driven granulomas. In the case of the latter topic, it has also revealed key roles for IL-13 and other cytokine mediators in the fibrotic process.

Returning from mouse to man

The realization that the host carefully modulates helminth Th2 responses and consequently triggers minimal pathology has helped to promote a version of the "hygiene hypothesis," which proposes that the elimination of helminth infection as a consequence of economic development led to a worldwide increase in allergic disease (Chapter 23). In fact, the field of helminth immunology is currently experiencing a resurgence, largely based on the insights it has provided into the role of Th2 responses at tissue sites and the mechanism of their regulation.

Andrade’s challenge and the future of immunoparasitology

1997-8 Initiating roles for dendritic cells in the host response to protozoan infection demonstrated by Reis and Sousa and Kaye. This introductory chapter has chronicled the emergence of immunoparasitology in the latter part of the 20th century, as well as its contribution to modern immunology.

Acknowledgments

Sacks, D & Noben-Trauth, N (2002). The immunology of susceptibility and resistance to Leishmania major in mice. Nature Reviews. 2007). Conventional T-bet (+) Foxp3 (-) Th1 cells are the major source of host-protective regulatory IL-10 during intracellular protozoan infection. The Journal of Experimental Medicine.

Section

Notes on the Immune System

Tracey J. Lamb

The immune system

Immunologic memory response—especially the adaptive arm of the immune response—occurs in the secondary lymphoid organs that drain the site of infection. There is a high degree of 'cross-talk' between the innate and adaptive arms of the immune system.

Figure 1.1 Innate and adaptive immune cells of the human body. All cells are derived from self-renewing haematopoietic stem cells in the bone marrow, and they arise from myeloid or lymphoid progenitors

Innate immune processes

• Inflammation
• The acute phase response
• Anti-microbial peptides

The innate arm of the immune system recognizes pathogens non-specifically and generates immediate generic mechanisms for pathogen clearance. The adaptive arm of the immune system is more specific to individual pathogens and takes a number of days to develop.

Figure 1.3 Some of the main pattern recognition receptors found on antigen presenting cells.

The complement cascade

Although the lectin pathway of complement activation is little studied, it is known to play a role in defense against some protozoan parasites, such as Cryptosporidium (see Chapter 5). Unwanted complement activation can be harmful to the host tissues, so it is necessary to regulate the process of complement activation.

Innate recognition

Upon complement activation, deposition of C5b on the surface of the invading pathogen leads to lysis via MAC assembly. C1 inhibitor (C1-INH) controls complement activation through the classical and lectin-binding pathways, associating with the C1 complex and causing the dissociation of C1r and C1s from C1q (see Figure 1.4).

Pattern recognition receptors

• Signalling events activated upon ligation of PRRs

TLR2 Surface Zymosan (glucan from yeast cell wall) GPI anchors (Plasmodium, Trypanosomes) TLR3 Endosomal nucleic acids: double-stranded RNA. These then associate with TNFR-associated factor (TRAF)-6, which in turn activates TGF-␤-activated kinase 1 (TAK-1) and the transcription of pro-inflammatory genes such as TNF-␣ (via NF-␬B activation ) and IL-12 (via MAP kinase activation).

Table 1.2 Some innate receptors commonly used for recognition of pathogens.

Innate immune cells

• Macrophages
• Granulocytes
• Dendritic cells
• Natural killer (NK) cells

Thus, ligation of TLR4 leads to the transcription of proinflammatory cytokines and the initiation of the innate immune response. Langerhans cells are specialized DCs located in the skin and are determined by the expression of the C-type lectin Langerin (CD207).

Figure 1.6 Phagocytosis leads to uptake and digestion of extracellular pathogens and their products.

Communication in the immune system

Some of the inhibitory receptors expressed on NK cells (for example, some of the natural killer immunoglobulin-like receptors (KIRs)) ligate with MHC-I molecules, delivering inhibitory signals that prevent activation of NK cells. The efficiency of NK cells in the immune response can be enhanced by interactions with phagocytic cells such as DCs and macrophages.

Adaptive immunity

IL-18 Macrophages Activation of NK cells; production of IFN-␥ by NK cells; promoting Th1 responses. Maximizing the number and efficiency of these memory cells ('immunological memory') is the goal of vaccination against parasitic infection (Chapter 25).

Table 1.3 Some of the main cytokines involved in parasitic infections.

The role of the MHC in the immune response

Cross-presentation, where endocytic material 'crosses over' into the MHC-I loading pathway, occurs through mechanisms that are not fully understood. However, this scheme is simplified; the process of cross-presentation (see below), allowing CD8+ T cells to be 'primed' by DCs, demonstrates that the extracellularly derived pathogenic proteins can be endocytosed and 'crossed over' to the MHC-I loading pathway in the endoplasmic reticulum .

Figure 1.9 MHC processing pathways. MHC-II is loaded in specialised vesicles (Class II vesicles) (1) that fuse with

T cell activation and cellular-mediated immunity

• Three signals are required for CD4+ T cell activation
• Cross-presentation and cross-priming of CD8+ T cells
• CD4+ T cell phenotypes
• Other T cells of the innate immune system

Co-stimulation is critical to induce the expression of IL-2 and to up-regulate expression of the ␣-chain of the IL-2 receptor. Priming of CD8+ T cells also requires 'help' from CD4+ T helper cells in addition to ligation of the TCR with pMHC-I on DCs.

Table 1.4 Cytokines required for polarisation of CD4+ T cell subsets.

B cells and the humoral response

• B cell activation against T-dependent antigens
• Antibody isotypes
• Fc receptor recognition via Fc receptors

The heavy chain constant region determines the isotype of the antibody and is the part of the antibody that binds to Fc receptors. Cross-linking of Fc receptors activates effector pathways for destruction of the opsonized pathogen.

Figure 1.13 Activation of B cells towards T-dependent antigen. Ligation of the BCR by antigen (1) leads to internalisation and digestion of the antigen

Cell trafficking around the body

Interactions between integrins and selectins initiate rolling adhesion before cell adhesion molecules support tighter binding of the immune cell to the endothelium. Leukocyte extravasation, i.e., passage between endothelial cells and into tissue in a process known as diapedesis, completes the route of activated immune cells to the site of infection.

Cellular immune effector mechanisms

• Phagocytosis and pathogen digestion
• Cellular-mediated lysis of pathogens
• Granuloma formation as a method of containment

Once immune cells are recruited to the site of infection, they must attach to the endothelium and leave the bloodstream to mediate effector functions in the infected tissue. Immune cells are attracted to chemokines produced as a result of the inflammatory immune response generated by cells already in the infected tissue.

Hypersensitivity reactions

• Type 1 hypersensitivity reactions (immediate)

In this case, the prolonged induction of Th2 inflammation, and the involvement of M2 macrophages, can lead to fibrosis in the liver.

IMMEDIATE

IMMEDIATE IgG dependent lysis of target cells via

IMMEDIATE

DELAYED

• Type 2 hypersensitivity reaction (immediate)
• Type 3 hypersensitivity reactions (immediate)
• Type 4 hypersensitivity reaction (delayed)

Rutschman, Ret al. (2001). Top: Stat6-dependent substrate depletion regulates nitric oxide production. Journal of Immunology. Wilson, MS et al. (2005). Suppression of allergic airway inflammation by helminth-induced regulatory T cells. Journal of Experimental Medicine.

Introduction to Protozoan Infections

The protozoa

Within these supergroups, the positions of the main species that infect humans are also shown with a pathogen cartoon. We have limited our discussion to the three protozoan supergroups that contain the major human pathogens: the Amoebozoa, Excavata, and Harosa.

Amoebozoa

• Entamoebidae
• Entamaoba histolytica

In normal, asymptomatic infections, trophozoites pass in stool formations to the lower parts of the colon. Chromatoid bodies containing large amounts of RNA are prominent during the development of the cyst (Figures 2.2B and 2.2C).

Table 2.1 There are 30 species of protozoa that commonly infect humans. Many of these species are commensals or opportunistic pathogens that only develop into medically important parasites in immunocompromised individuals

Excavata

• Metamonad, Diplomonadida
• Metamonad, Parabasalia
• Discoba, Euglenozoa

A TEM image of a Leishmania promastigote kinetoplast and kDNA complex at the base of the kinetosome and flagella. The kinetoplast is connected to the two kinetosomes at the base of the flagella (Figures 2.5A and 2.5B).

Figure 2.3 Giardia lamblia basic anatomy. (A) A light microscopy image of Giemsa-stained G

Harosa

• Aveolata

Invasion of hepatocytes is facilitated by secretion of proteins from the secretory organelles of the apical complex. These are the sexual stages of the parasite and the only transmissible stages of Plasmodium that are contagious to mosquitoes.

Figure 2.7 Apicomplexa basic anatomy. (A) A transmission electron microscopy (TEM) image of a Toxoplasma gondii tachyzoite

Protozoa that are now fungi

Taxonomy and the evolution of the parasitic protozoa

These rapidly evolving sequences would always cluster together or with the bacterial sequences used as comparators to the eukaryotic sequences to anchor the phylogenetic tree. The tree presented in Figure 2.1 shows a snapshot of the current global view of protozoan taxonomy and evolution.

Genomic and post genomic exploration of protozoan biology

After the demise of the Archaea, single-gene phylogenies were abandoned in favor of those that combined multiple features, such as two or more genes, and ultra-structural data to form a consensus view. More importantly, methods for genetic manipulation of many of the major protozoan parasites are now routinely used.

Table 2.2 A selection of protozoa genome projects that have been completed (C) or are nearing completion (P)

Summary

For example, transcriptomic and proteomic analyzes have become important tools, allowing parasite gene expression and/or protein content to be assessed at any stage of the life cycle or from limited clinical samples. Subcellular proteomics now allows the biology of important organelles such as the apical complex, hydrogenosomes or apicoplasts to be explored at a level of detail not previously possible.

General information on protozoa

Cavalier-Smith, T (2009). Kingdoms Protozoa and Chromista and the Eozoic Root of the Eukaryotic Tree. Biology Letters Malaria parasites require TLR9 signaling for immune evasion by activating regulatory T cells. One 6(2), e Genetic diversity of Toxoplasma gondii in animals and humans. Philosophical Transactions of The Royal Society Of London.

Apicomplexa: Malaria

• Malaria
• The life cycle of malaria
• Mouse models of malaria
• Recognition of malaria parasites
• Innate effector mechanisms
• Pre-erythrocytic stages
• Asexual erythrocytic cycle
• Adaptive immunity
• Immunity to pre-erythrocytic stages
• Immunity to the asexual erythrocytic cycle
• Memory responses
• Immune evasion
• Subversion of T cell responses by liver-stage parasites
• Immune evasion in the erythrocytic cycle
• Immunopathology
• Thermoregulation
• Severe malarial anaemia
• Metabolic acidosis and respiratory distress
• Cerebral malaria

Activation of ␥␦ T cells in malaria infection requires exogenous cytokine stimulation of other cells of the immune system. One of the primary splenic DC subsets in the mouse to prime CD4+ T cells to a Th1 phenotype in P .

Figure 3.1 The life cycle of malaria. Malaria sporozoites are deposited in the vascular beds of the skin by a mosquito bite;

Apicomplexa

Toxoplasma gondii

Emma Wilson

• Introduction
• Life cycle and pathogenesis
• Clinical manifestations
• Innate immune responses
• Innate recognition
• Neutrophils
• Monocytes and macrophages
• NK cells
• Dendritic cells
• Evasion strategies
• Adaptive immune responses
• T. gondii -specific T cells
• Costimulation
• T cell polarisation
• T cell memory
• CNS infection
• Conclusions
• gondii-specific T cells

One of the questions when studying Toxoplasma is how the parasite spreads rapidly within the host. 1999). The role of CD28 in generating effector and memory responses necessary for resistance to Toxoplasma gondii. Journal of Immunology.

Figure 4.1 Toxoplasma gondii life cycle. Toxoplasma exists in one of three states: 1) sexually as sporozoites within a dormant highly resistant oocyst; or asexually as 2) bradyzoites within a latent tissue cyst; or 3) as fast replicating tachyzoites

Cryptosporidium

• Life cycle
• Clinical presentation
• General immune responses in cryptosporidiosis
• Innate effector mechanisms
• Toll-like receptors
• Chemokines and chemokine receptors
• Mannose-binding lectin
• Type 1 interferons
• Anti-microbial peptides
• NK cells
• Macrophages and dendritic cells
• Adaptive immunity
• T cells: CD4+ cells
• T cells: CD8+ cells
• Cytokines
• Antibody response
• Memory responses
• Antigens eliciting the immune response
• Immune evasion
• Immunopathology in the gut and intestinal tract

Intestinal epithelial lymphocytes (IELs) are non-conventional lymphocytes located between the epithelial cells in the lumen. 2002). Susceptibility to cryptosporidium parvumin infections in cytokine and chemokine receptor knockout mice. Journal of Parasitology The role of ␥-interferon in chemokine expression in mouse ileum and in a murine intestinal epithelial cell line after cryptosporidium parvumin infection. Infection and immunity.

Figure 5.1 Diagrammatic life cycle of Cryptosporidium parvum . Trophozoites (growing stage) differentiate following sporozoite invasion and develop into Type I meronts

Diplomonadida: Giardia

Steven Singer

• The life cycle and pathogenesis of Giardia infection
• Life cycle
• Epidemiology and treatment
• Pathogenesis
• Recognition of Giardia by the immune system
• Mannose binding lectin
• Epithelial cell recognition
• Dendritic cells
• Innate effector mechanisms against Giardia
• Defensins
• The contribution of the microbial flora
• Nitric oxide
• Adaptive immunity against Giardia
• B cells and antibodies
• T cells
• Activation of mast cells by the adaptive immune response
• Memory responses
• Evidence from outbreaks of Giardiasis
• Giardiavax TM
• Anti-cyst vaccines
• Antigens eliciting the immune response
• Immune evasion
• Immunopathology
• Summary

Other roles include the production of cytokines to help recruit other effector cells of the immune response. Additional studies of memory responses in mice focused on the parasite cyst stage.

Figure 6.1 Life cycle of Giardia .

Kinetoplastids

Leishmania

Ingrid M ¨uller 1 and Pascale Kropf 2

• The pathogenesis of Leishmania infection
• Life cycle
• Parasite transmission and avoidance of immune responses
• Innate effector mechanisms: the role of neutrophils in Leishmania infection
• Adaptive immunity: lessons from L. major infections of mice
• CD4+ T cells
• Regulatory T cells
• Th17 cells
• CD8+ T cells
• Arginase promotes Leishmania parasite growth
• Memory responses

Thus, Leishmania evolved to exploit the wound-healing response to the sandfly bite for initial survival in the vertebrate host. Natural Tregs play an important role in the regulation of the immune response against Leishmania parasites.

Table 7.1 Leishmania species causing disease in humans.

Trypanosomes

Jeremy Sternberg

• The African trypanosomes ( Trypanosoma brucei ssp.)
• Life cycle of Trypanosoma brucei
• Pathogenesis of sleeping sickness
• Variant surface glycoprotein – the key to trypanosome-host interactions
• VSG and antigenic variation
• The humoral response to African trypanosomes
• T cell responses in African trypanosome infections
• Innate defence mechanisms: trypanosome lytic factor
• Immunopathology and VSG
• Summary

Turner, CM (1997). Rate of antigenic variation in fly-borne and Trypanosoma brucei infections. FEMS Microbiology Letters. Kennedy, PG (2004). Human African trypanosomiasis of the central nervous system: current issues and challenges. Journal of Clinical Investigation.

Table 8.1 African trypanosomes causing disease in humans and livestock.

Trypanosoma cruzi (Chagas disease)

Rick Tarleton

• Life cycle and transmission
• Immune control and disease
• Innate recognition of T. cruzi
• Adaptive immunity
• T helper cell responses
• CD8+ T cell responses
• Humoral immune responses
• Regulation of immune responses and parasite persistence
• Conclusions

Benchimol Barbosa, PR (2006). Oral transmission of Chagas disease: an acute form of infection responsible for regional outbreaks. International Journal of Cardiology. Schoeld, CJ et al. (2006). The future of Chagas disease control. 2007).Chagas disease in the Amazon region.

Figure 9.1 The life cycle of Trypanosoma cruzi .