Chapter 20. Immunity to Parasitic and Fungal Infections

Topics Covered IMMUNITY TO PARASITIC AND FUNGAL INFECTIONS • • • • • • • • • Chapter 20 Overview of parasitic diseases Immune response to parasitic infections Immunity to malaria Immunity to schistosomiasis Immune effectors in parasitic infections How parasites evade the immune response General features of fungal pathogens Immunity to fungal infections 1 Immunity to Malaria 2 Immunity to Malaria • Immunity to sporozoites injected by mosquito • Overproduction of TH1 cytokines – Mediated by antibody that prevents infection of liver cells – Important for parasite control – Produces life threatening complications of cerebral malaria, anemia, and other symptoms • Immunity to parasites in liver cells – Mediated by CD4+ and CD8+ T cells producing IFN-! – Not lytic activity but IFN-! prevents replication of parasite • Sequestration of parasites in the brain leads to the production and release of proinflammatory cytokines • Immunity to parasites in red blood cells – Exponential growth stage and symptoms of disease – TH1 cells produce IL-2 and IFN-! (proinflammatory cytokines) • M" activated and destroy infected RBCs – TH2 cells drive specific Ab production – TH1 cytokines IFN-!, IL-2, TNF, IL-1 – Mediate the brain lesions and neurologic damage – NO produced by cerebral endothelium interferes with neurotransmission • blocks invasion of new RBCs • Destroys infected RBCs through complement activation • Enhances phagocytosis by M" through FcR 3 4 1 Dichotomy of protection and pathology associated with the TH1 response to Plasmodium Immunity to Schistosomiasis • Correlative evidence: – high IL-4, IL-5, and IgE individuals remain un-reinfected – High IFN-g in individuals who were reinfected • Disease is due to TH2-directed granulomatous response to worm egg antigens – B cells proliferate and produce IL-10 – IL-10 stops costimulation of APCs – Downmodulates the size of the granuloma 5 Immunity to Schistosomiasis 6 Immune Effectors in Parasitic Infections • Most important cytokine activated cell is the M" – In control and elimination of parasites • Nitric Oxide (NO) main mechanism for M" killing of parasites – IFN-! activates M" to generate NO – TNF-# also enhances NO production – TGF-$ and IL-10 inhibit NO production • Direct killing of schistosome larvae by IFN-! activated M" • ADCC killing of schistosome larvae by eosinophils and Ab IgE – Eosinophils use FcR to bind IgE coated larvae – Esosinophils degranulate and release ROI and other toxins that kill over 24 hours 7 8 2 Immune Effectors in Parasitic Infections: Immunity to liver stage of malaria Schistosome larvae in vitro experiments • Vaccination with irradiated sporozoites in the lab – Provides protection against reinfection • Ab against sporozoites • CMI to merozoites at liver stage – CTLs lyse infected liver cells • TH1 cells produce IFN-! – Inhibits growth of parasite inside the cell – Along with IL-6 (by liver cells), TNF, IL-1 • IL-6 and IFN-! – induce production of NO by infected cells and liver M" (Kupffer cells) – Induce MHC class II on liver cells -killing by CD4+ cells also • IL-6, IL-1, and TNF 9 Immunity to liver stage of malaria – induce liver cells to release C-reactive protein (CRP) – CRP binds to sporozoite and inhibits development to merozoite 10 Ab isotypes and parasitic infection • Correlation between high IgE levels and disease symptoms in lymphatic filariasis (elephantitis) • Individual hosts with circulating microfilariae have high IgG4 and low levels of IgE – No disease symptoms • Individual hosts that have cleared the microfilariae have high levels of IgE and low levels of IgG4 – but exhibit elephantitis • Suggests IgE and not the parasite causes the lymphatic immunopathology 11 12 3 Cytokine and Ab profiles during lymphatic filariasis Immune evasion by parasites • Parasites have evolved many ways to evade host immune defenses • Seclusion intracellularly • Molecules that inhibit nonspecific host effector mechanisms – Complement • Shield themselves with host derived molecules • Antigenic variation of surface molecules • Induce host cells to take on an immunosupressive role 13 Immune evasion by parasites: Seclusion intracellularly 14 Immune evasion by parasites: Molecules that inhibit nonspecific host effector mechanisms • Avoid antibody and complement by hiding inside cells • Parasites use different independent strategies to avoid destruction within the cell • Activated immune cells – Produce hydrogen peroxide, superoxide ions, hydroxyl radicals that are toxic to pathogen – Leishmania activate complement and opsonization • Are taken up by macrophage, are enclosed in the phagolysosome • But produce antioxidant enzymes and inhibitors of lysosomal enzymes – T. gondii are taken up by phagocytes • Prevents phagosome from fusing with lysosome – T. cruzi (Chagas’ disease) not normal phagocytosis • Causes lysosomes to cluster and enters directly – Trichinella spiralis transform muscle cell to a specialized nurse cell • Only intracellular human helminth 15 • Parasites produce antioxidant enzymes – Oxygen scavenging enzymes to protect themselves – Major enzymes are superoxide dismutase, catalase, and glutathione peroxidase – Superoxide dismutase is produced in extraordinary amounts by some parasites • All protozoan and helminth parasites examined so far contain at least one of these antioxidant enzymes 16 4 Immune evasion by parasites: Immune evasion by parasites: Molecules that inhibit Complement Antigenic variation of surface molecules • Leishmania avoid membrane attack complex (MAC) by having extended surface proteins • Many examples by pathogens • the most dramatic is T. brucei (african sleeping sickness) – that bind complement and induce phagocytosis of parasite – But do not allow MAC formation because they are too far away from the membrane surface • – Entire surface is covered with a dense coating of a single protein called VSG (variant surface glycoprotein) – Waves of parasitemia correspond to a clonal population of parasites expressing a single VSG type – Antibodies are generated against this VSG and destroy the parasites – New clonal population arises with different VSG type and persist until a new antibody response is generated – Almost 1,000 genes encoding different variants of VSG T. cruzi express a molecule that prevents assembly of the complement cascade – No opsonization by complement and No MAC formation • Shistosome larvae insert the host factor that normally blocks assembly of the complement cascade – No opsonization by complement and No MAC formation • Taenia solium (pig tapeworm) produces paramyosin that blocks assembly of the complement cascade – No opsonization by complement and No MAC formation 17 • These genes are activated one at a time to effect antigenic variation • Gene switching occurs at a low spontaneous rate • At any given time, at least one parasite is likely expressing a different VSG than is targeted by antibody – can escape and cause another round of parasitemia 18 Immune evasion by parasites: Immune evasion by parasites: Antigenic variation of surface molecules Antigenic variation of surface molecules • Plasmodium falciparum (Malaria) • Antigenic variation of PfEMP1 expressed on surface of infected RBC • Lots of genes encode these proteins – 2-6% of the parasite genome • Variant switching occurs at a high rate • Emergence of new variants allows chronic infection • Explains why previously infected individuals can be reinfected 19 20 5 Immune evasion by parasites: Immune evasion by parasites: host immunosuppression Antigenic variation of surface molecules • T. cruzi, T gondii, and Leishmania induce expression of TGF-$ – Downregulates both TH1 and TH2 responses • T gondii, and Leishmania enter M" without inducing IL-12 – No TH1 response • Shistosome eggs induce IL-10 – Diminishes IFN-! mediated M" activation 21 22 Immunology of Fungal Infections Fungal Pathogens • Eukaryotic organisms that live on dead organic material • Like bacteria, most are harmless • Small number cause disease in humans – Called mycoces • • • • Different from “classical” parasites Do not depend on interaction with host for survival Only cause disease by accidentally infecting humans Fungi occur in 2 forms – Unicellular yeasts – Molds that grow in branching chains • Common superficial infections to life threatening systemic diseases • Hyphae – Immunocompromised hosts • The most pathogenic fungal genera are • Very difficult to treat • Lack of effective safe anti-fungal drugs • Amphoterin B is highly effective for systemic mycoses – Has serious side effects 23 – Aspergillis • Grow as molds – Cryptococcus, and Histoplasma • Grow as molds in nature and in vitro tissue culture plates • Grow as yeast-like budding cells in infected human tissue 24 6 Invasive pulmonary Aspergillis in a patient with Leukemia Aspergillis on an agar culture plate 25 26 Cryptococcus neoformans yeast cells in spinal fluid of AIDS patient with cryptococcus meningitis Aspergillis on an agar culture plate 27 28 7 Cryptococcus neoformans yeast cells in the liver of a patient with disseminated cryptococcus Fungal Diseases • Classified into 3 clinical groups 1. Superficial mycoses – Most common infections • • Skin, hair, and nails – Athlete’s foot and ringworm Mucosal surfaces – Candida albicans normally present in mouth, vagina and intestinal tract 2. Subcutaneous mycoses • • Caused by puncture wounds Localized abscesses 3. Systemic mycoses • Histoplasmosis, cyrptococcosis, and coccidioidomycosis – – – Begin as lung infections acquired by inhaling spores Mild influenza-like symptoms Fatal without treatment • 29 Innate immune responses to Fungi At risk are immunosuppressed individuals – Chemotherapy, steroid therapy, AIDS patients 30 Effector cells in fungal infections • Physical barriers: skin and mucosa • Phagocytic cells • PMNs are most effective killers • Drawn to site of fungal infection by chemotactic factors produced by fungus • Or by fungal membrane activation of complement (not ab mediated) • Fungi can stimulate IL-1 and TNF-# • Chemical factors in serum and skin secretions • Phagocytic and nonphagocytic cells – Neutrophils (PMNs) are most important phagocyte – Enhances infiltration of PMNs • If these are insufficient • PMNS kill by – T cell mediated responses are required for effective control – Oxygen dependent and independent mechanisms 31 32 8 ACQUIRED IMMUNITY TO FUNGAL INFECTIONS Effector cells in fungal infections • PMNS kill by Oxygen dependent – Generation of toxic chem via oxidative burst – Or release granules with enzymes that generate hypochlorous acid • TH cells and macrophages are important • PMNS kill by Oxygen-independent mechanisms – Proteases – Defensins are antimicrobial peptides • Probably little role for Ab • PMNs produce IL-12 and activate TH1 response • TH1 response leads to disease resolution • NK cells directly by cytolysin granules and indirectly by activating Macrophages • Alveolar Macrophages • TH2 response leads to exacerbation of fungal infection – Kill inhaled spores – Aspergillus are readily killed – Coccidioides and Histoplasma are resistant to M" killing • No phagosome/lysosome joining in Coccidioides • Histoplasma Grows within M" if they are not activated by specific immune response Copyright 2004 ASM Press – IFN-! and IL-12 – IL-4 • Show same responses in mice as Leishmania model 33 TH1 vs. TH2 responses to fungal infections Coccidiodes immitis 34 Dermatophyte immunity Athlete’s foot, Ringworm • Infections that result in high inflammatory responses – are more likely to be cleared (TH1 response) – those that do not are more likely to be chronic (TH2 response) Coccidiodes immitis • Dermatophyte antigens are important allergens? – If they elicit a TH2 response – Immediate type hypersensitivity Coccidiodes immitis • Allergic bronchopulmonary aspergillosis Coccidiodes immitis – Have activated TH2 cells and asthma-like symptoms 35 36 9