Evasion of Immunity 2

1. Evasion of Immunity 2 Immunity to specific parasites & parasite immune evasion strategies. Dr. Jo Hamilton Parasitology BS

2. Introduction. In the last session we discussed vertebrate and invertebrate immunity. In this session we will: • Examine vertebrate and invertebrate immune responses to different groups of parasites. • Explore the strategies that have evolved in parasites to overcome their hosts’ defences.

3. Objectives and learning outcomes. • By the end of this session students should be: • Familiar with vertebrate and invertebrate immune responses to different groups of parasites. • Familiar with a range of strategies used by parasites to evade their hosts’ immune mechanisms. • Able to give examples of parasites and link them to their immune evasion strategies.

4. Introduction. • Successful parasites have evolved strategies for survival & development in both invertebrate and vertebrate hosts.

5. Immunoparasitology (Parasite immunology). • Host - susceptible if parasite survives. • Host - insusceptibleif parasite killed by innate immunity. • E.g. Humans are insusceptible to the larval stages of bird schistosomes (e.g. Trichobilharzia). These parasites are quickly killed off, the associated inflammation & itching is called cercarial dermatitis (or ‘swimmers itch’). In the natural duck host, larval stages develop into established infection with adult worms.

6. Immunoparasitology (Parasite immunology). • Spontaneous-cureoccurs when parasite establishes itself but is eventually expelled, e.g., Nippostrongylus brasiliensis (the rat hookworm). • The adult Nippostrongylus, releases protective antigens that are not stage specific to the adult. That is, the resulting antibodies recognise targets both on the adult worm and on the migrating infective larvae. • Under conditions of trickle infection, possible to get persistent population of parasites in gut which are able to survive the adverse immunological conditions. Morphologically though these worms are stunted and appear to be less immunogenic than normal worms.

7. Immunoparasitology (Parasite immunology). • Parasites of major medical & veterinary importance successfully adapted to innate & acquired immune responses of host. E.g. malaria (Plasmodium spp.) & Fasciola hepatica in sheep. • Susceptibility of a host to a given parasite can depend genetic background, age, nutritional & hormonal status etc. of an individual.

8. Immunoparasitology (Parasite immunology). • In nearly every case, immune response mounted to both protozoal and helminth infections. • Evidence- • (1) the prevalence of infection declines with age. • (2) immunodepressed individuals quickly succumb to infection. • (3) acquired immunity has been demonstrated in lab models.

9. Immunopathology. Parasites can cause direct damage to host by: • Competing for nutrients (e.g. tapeworms). • Disrupting tissues (e.g. Hydatid disease) or destroying cells (e.g. malaria, hookworm, schistosomiasis; feeding on or causing destruction of cells = anaemia). • Mechanical blockage (e.g. Ascaris in intestine). • However, severe disease often has a specific immune or inflammatory component.

10. Immunopathology. Some examples include: • Cerebral malaria - TNF, IFN & other proinflammatory cytokines in brain. • Hepatosplenic schistosomiasis - anti-egg immune responses initiate hepatic fibrosis. • Onchocerciasis - anti-microfilarial responses in eye = blindness, perhaps inducing autoimmune response via cross-reactive antigens in eye & microfilariae, this immune response is not protective as is against stage specific surface antigens, no cross-reaction with infective L3 larvae.

11. Immunopathology cont’d. • Anaphylactic shock - rupture of hydatid cyst. Immediate hypersensitivity initiated by systemic release of parasite antigens reacting with IgE & mast cells = degranulation & release of mediators, e.g. histamine. • Nephropathy - immune complexes (parasite antigens, antibody + complement) in kidney (e.g. malaria, schistosomiasis).

12. Vertebrate Immune responses to Protozoan parasites. Innate immune responses. • In vertebrates, extracellular protozoa are eliminated by phagocytosis and complement activation. • T cell responses. • Extracellular protozoa - Th2 cytokines released for antibody production. • Intracellular protozoa - Cytotoxic lymphocytes (CTL’s) kill infected cells. Th1 cytokines produced to activate macrophages, CTL’s & DTH response also involved.

13. Vertebrate Immune responses to Protozoan parasites. Combination of innate and acquired immune responses. • Antibody + Complement, e.g. lysis of blood dwelling trypanosomes. Antibody / complement plus neutrophils or macrophages against malaria merozoites. Activated macrophages can be effective against many intracellular protozoa, e.g. Leishmania, Toxoplasma, Trypanosoma cruzi. CD8+ cytotoxic T cells respond parasite infected host cells, e.g. Plasmodium infected liver cell.

14. Vertebrate Immune responses to Protozoan parasites. Acquired immune responses. • Antibody responses. - Extracellular protozoa are eliminated by opsonization, complement activation and ADCC. - Intracellular protozoa are prevented from entering the host cells by a process of neutralisation e.g. neutralising antibody against malaria sporozoites, blocks cell receptor for entry into liver cells.

15. Invertebrate Immune responses to Protozoan parasites. • Melanotic encapsulation. Malarial mosquito vector, Anopheles gambiae, melanotic encapsulation of young Plasmodium oocysts takes place. In general, the reactions set in motion by phenoloxidase activity result in chemical as well as physical protection, because oxidations leading to melanin formation also generate free radicals & toxic quinone intermediate radicals.

16. Vertebrate Immune responses to helminth infections. • Most helminths extracellular & too large for phagocytosis. • For the larger worms, e.g. some gastrointestinal nematodes host develops inflammation and hypersensitivity. Eosinophils & IgE activated to initiate inflammatory response in the intestine or lungs to expel the worms. These histamine elicited reactions are similar to allergic reactions.

17. Vertebrate Immune responses to helminth infections. • The acute response after previous exposure can involve an IgE and eosinophil mediated systemic inflammation which results in expulsion of the worms.

18. Vertebrate Immune responses to helminth infections. • Chronic exposure to worm antigens can cause chronic inflammation: • Delayed type hypersensitivity (DTH), Th1 / activated macrophages which can result in granulomas. • Th2 / B cell responses increase IgE, mast cells & eosinophils activate inflammation.

19. Vertebrate Immune responses to helminth infections. • Helminths commonly induce Th2 responses characterised by cytokine pattern with IL-4, IL-5, IL-6, IL-9, IL-13 & eosinophils & antibody responses including in particular, IgE. • Characteristic ADCC (Antibody-dependent cell-mediated cytotoxicity) reactions i.e. killer cells (e.g. macrophages, neutrophils, eosinophils) directed against target parasite by specific antibody. E.g. Eosinophil killing of parasite larvae by IgE (or some IgG subclasses).

20. Invertebrate immune responses to helminth infections. • Melanotic encapsulation.This mechanism is used tocontain filarial larvae (nematodes) in mosquitoes.

21. Parasite Immune Evasion –Evasion strategies. • Parasites need time in host to complete complex development, to sexually reproduce & to ensure vector transmission. • Chronic infections (from a few months to many years) are normal, therefore parasite needs to avoid immune elimination. • Parasites have evolved immune evasion strategies.

22. Protozoan immune evasion strategies. 1. Anatomical seclusion in the vertebrate host. • Parasites may live intracellularly. By replicating inside host cell parasites avoid immune response. • Plasmodium lives inside Red Blood Cells (RBC’S) which have no nucleus, when infected not recognised by CTL’s & NK cells. Other stages of Plasmodium live inside liver cells. • Leishmania parasites and Trypanosoma cruzi live inside macrophages.

23. Protozoan immune evasion strategies. 2. Anatomical seclusion in the invertebrate host. • Plasmodium ookinetes develop in serosal membrane & are beyond reach of phagocytic cells (haemocytes).

24. Protozoan immune evasion strategies. 3. Antigenic variation. • In Plasmodium, different stages of the life cycle express different antigens. We will describe evasion strategies of Plasmodium in more detail in the next lecture. • Antigenic variation also occurs in the extracellular protozoan, Giardia lamblia.

25. Protozoan immune evasion strategies. 3. Antigenic variation cont’d. • African Trypanosomes have one surface glycoprotein that covers the parasite. • This protein is immunodominant for antibody responses. • Trypanosomes have “gene cassettes” of variant surface glycoproteins (VSG’s) which allow them to switch to different VSG. • VSG is switched regularly. The effect of this is that host mounts immune response to current VSG but parasite is already switching VSG to another type which is not recognised by the host.

26. Protozoan immune evasion strategies. 3. Antigenic variation cont’d. • A parasite expressing the new VSG will escape antibody detection and replicate to continue the infection. • This allows the parasite to survive for months or years. • Up to 2000 genes involved in this process.

27. After Ross, P. (1910), Proc. Royal Soc. London, B82, 411 Protozoan immune evasion strategies. 3. Antigenic variation cont’d. • The fluctuations in parasitaemia in a patient with trypanosomiasis. Characteristic of both animal & human trypanosomiasis. • After each peak, the trypanosome population is antigenically different from that of earlier or later peaks. • We will cover antigenic variation in the African trypanosomes in more detail in the next lecture.

28. Protozoan immune evasion strategies. • 4. Shedding or replacement of surface e.g. Entamoeba histolytica. • 5. Immunosupression – manipulation of the immune response e.g. Plasmodium. • 6. Anti-immune mechanisms - Leishmania produce anti-oxidases to counter products of macrophage oxidative burst.

29. Helminth immune evasion strategies. Helminth immune evasion mechanisms in the vertebrate host. • 1. Large size.Difficult for immune system to eliminate large parasites. Primary response is inflammation to initiate expulsion, often worms are not eliminated.

30. Helminth immune evasion strategies. Helminth immune evasion mechanisms in the vertebrate host. • 2. Coating with host proteins. Tegument of cestode & trematode worms, is able to adsorb host components, e.g. RBC Ags, thus giving the worm the immunological appearance of host tissue. Schistosomes take up host blood proteins, e.g. blood group antigens & MHC class I & II molecules, therefore, the worms are seen as “self”. We will describe schistosome evasion strategies in more detail in the next lecture.

31. Helminth immune evasion strategies. Helminth immune evasion mechanisms in the vertebrate host. • 3. Molecular mimicry. The parasite is able to mimic a host structure or function. E.g. schistosomes have E-selectin that may help in adhesion or invasion. • 4. Anatomical seclusion - Uniquely, even one nematode worm larva does this; Trichinella spiralis can live inside mammalian muscle cells for many years. • 5. Shedding or replacement of surface e.g. trematodes, hookworms.

32. Helminth immune evasion strategies. Helminth immune evasion mechanisms in the vertebrate host. • 6. Immunosupression – manipulation of the immune response. High burdens of nematode infection often carried with no outward sign of infection. • Growing evidence that parasite secreted products include anti-inflammatory agents which act to suppress the recruitment and activation of effector leukocytes. E.g. a hookworm protein which binds the ß integrin CR3 & inhibits neutrophil extravasation.

33. Helminth immune evasion strategies. Helminth immune evasion mechanisms in the vertebrate host. • 6. Immunosupression cont’d. There is other evidence of secreted products which block chemokine-receptor interactions, & an acetylhydrolase from N. brasiliensis has been discovered which inactivates the pro-inflammatory molecule Platelet-activating Factor (PAF).

34. Helminth immune evasion strategies. Helminth immune evasion mechanisms in the vertebrate host. • 7. Anti-immune mechanisms e.g. liver fluke larvae secretes enzyme that cleaves Ab. • 8. Migration e.g. Hookworms, move about gut avoiding local inflammatory reactions. 

35. Helminth immune evasion strategies. Helminth immune evasion mechanisms in the vertebrate host. • 9. Production of parasite enzymes - Filarial parasites secrete a number of anti-oxidant enzymes such as glutathione peroxidase & superoxide dismutase which most likely contribute to their observed resistance to antibody-dependent cellular cytotoxicity and oxidative stress. • Genes for these enzymes cloned & expressed with aim of producing effective vaccines.

36. Helminth immune evasion strategies. Helminth immune evasion mechanisms in the vertebrate host. • 9. Production of parasite enzymes cont’d -Many nematodes which colonise alimentary tract of host secrete acetylcholinesterases (AChEs), enzymes generally associated with termination of neuronal impulses via hydrolysis of acetylcholine at synapses and neuromuscular junctions. This unusual phenomenon has been known for some time, yet the physiological function of the enzymes remains undetermined.

37. Helminth immune evasion strategies. Helminth immune evasion mechanisms in the invertebrate host. • Anatomical seclusion – Acanthocephalaacanthors maintain host tissue layer around them. Acanthor only becomes melanised if developing larva dies. • Molecular mimicry – sporocysts of Schistosoma in the intermediate moluscan host produce surface molecules that are similar to molecules present in the haemolymph of the snail host. The parasite is thus seen as “self”.

38. Helminth immune evasion strategies. Helminth immune evasion mechanisms in the invertebrate host. • Immunosupression – developing microfilariae of Brugia pahangi & Dirofilaria immitis suppress the immune response of the mosquito.

39. Hymentopteran immune evasion strategies. Hymentopteran immune evasion mechanisms in the invertebrate host. • Anatomical seclusion.Many parasitic wasps lay eggs in ventral ganglion of insect or spider hosts thus avoiding action of phagocytic cells. • Immunosupression. Some parasitic ichneumonid wasps lay eggs in larvae of lepdopterans. Eggs are not attacked by the immune system as long as they stay alive E.g. Nemeritis wasp lays eggs in the almond moth Ephesita.

40. Evasion strategies of parasites of invertebrates. • 1. Immature hosts. Many parasites take advantage of immature hosts in which there are less circulating haemocytes. • 2. Incorporation of host antigen. This evasion strategy is used to make the parasite appear as “self” to the hosts’ immune system. • E.g. The pedicellaria, tiny claw-like structures on surface of echinoderms. Used to prevent ectoparasites from settling. Mucous on the surface of these ‘claws’ inhibits the biting response. Many ectoparasites coat themselves in mucous to prevent being ‘bitten’

41. Evasion strategies of parasites of invertebrates. • 2. Incorporation of host antigen cont’d. E.g. Clown fish produce mucous that does not contain sialic acid, this prevents them being stung by tentacles of sea anemone with which it lives. However, lack of sialic acid makes the fish more susceptible to bacterial infections.

42. Evasion strategies of ectoparasites of vertebrates. Ectoparasites also employ strategies to evade host defences & whilst they are not immune evasion strategies they are worth briefly mentioning. • Rapid feedingof blood-sucking insects to avoid host defensive movements. • Use of ‘hooks/claws’ e.g. claws on tarsi of head lice etc. used to hold on to hair – allows parasite to survive grooming activities of host.

43. Summary. By the end of this session you should be: • Familiar with vertebrate and invertebrate immune responses to different groups of parasites. • Familiar with a range of strategies used by parasites to evade their hosts’ immune mechanisms. • Able to give examples of these parasites and link them to their immune evasion strategies.

44. Next session. We will: • Explore selected parasite immune evasion mechanisms in more detail. • We will examine the immune evasion strategies of the schistosomes in both their intermediate and definitive hosts, Plasmodium, Trypanosoma cruzi & the African trypanosomes.