10. Pathogens vs Immune System

Learning Outcomes: L19

You should be able to discuss and provide specific examples of (with key molecules and outcomes): Pathogen immunosuppression Pathogen immune avoidance Pathogen immune evasion

Pathogen Survival Strategies

Parasites must combat or avoid immune systems of hosts: Immune suppression- disabling the opposition Immune avoidance- intracellular parasites Immune evasion- masters of disguise

Immune Sup: Bacteria Manipulate Host

E.coli bacteria manipulating host: extra Pathogenicity Island (PAI) DNA in pathogenic strain: Different genes encoded on PAI in bacteria: Toxins Adhesins (host specificity/tissue tropism) Siderophores (living in free iron limiting conditions) Immune modulatory factors eg IgA protease Type 3 secretion system (T3SS)- pathogen delivery system and its effector molecules to modulate host cell properties

Examples of proteins encoded by PAIs: Gram -ve bacteria T3SS Complex molecular syringe to insert proteins in the host cell  Alters host cell in diverse ways, depending on specific effector proteins  E.coli immunomodulator expression (Shiga toxin) ​​​​​​​Extracellular cytotoxin that targets host ribosome (modifies rRNA), inhibits translation  Induces apoptotic signalling pathway  Hemolytic uremic syndrome (HUS)- kidney damage 

Immune Sup: Bacteria Manipulate Host

Immune Suppression

Pathogens obstruct T cell activation of host: Bacteria: Heliobacter pylori (H.pylori) Causes gastric ulcers  Can block transcription of T cell growth factor IL-2 Viruses: Human Immunodeficiency Virus (HIV) Prevents phosphorylation of TCR cascade Prevents formation of Immunological synapse- presentation of antigens between cells

Immune Supression: HIV Replication

HIV mainly targets infection of T cells and macrophages Progressively destroys immune cells, impairs their function and reduces numbers  Weakens immune system-> increased susceptibility to other opportunistic infections Inability to battle secondary infections-> death and disease How they infect: HIV attaches to CD4 on surface of T cells Macrophages: interaction between viral gp120 with CD4 and CCR5 (coreceptor) T cells: interaction between viral gp120 with CD4 and CXCR4 (co-receptor)

Immune Sup: Helminths Manipulate Host

Helminths manipulate host immune response: Large extracellular helminths interact with host through their immunomodulators- surface molecules and secreted molecules Helminths and hygiene hypothesis: Helminth infection ubiquitous throughout human evolution- so immune system evolved in presence of worms (used to immunosuppression in their presence) Helminth infection now rare in western world- lead to increase in inflammatory diseases, autoimmunity disorders in their absence: . Allergies, autoimmunity (T1D, MS), intestinal inflammation, metabolic syndrome, obesity Hygiene hypothesis states rise in "Western inflammatory diseases" is the direct result of reduced pathogen exposure- i.e helminths essential for correct education of our immune system

Immune Sup: Helminths Manipulate Host

Large extracellular helminths modify host with their secreted molecules= Excretory/secretory (ES) molecules  Worms produce parasite immunomodulatory molecules These molecules bias host immune system/ limit inflammation (suppressing immune cells) Worm prevents/ limits immune attack i.e chronic infection  Commensal bacteria can promote survival of worms or compete against them 

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Immune Sup: Helminths Manipulate Host

Does worm infection prevent inflammatory disease? Asthma- allergic lung inflammation (using mouse model to represent) Induce inflammation by giving mouse ovalbumin protein-> generates OVA specific Th2 cells OVA delivery to lungs-> accute inflammation mimicing asthma Inject worms before hand, do they prevent?    

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Result: Intestinal helminth infection prevents lung allergic reaction Worm induces Tregs that limit inflammation Naive cells- no reaction  OVA injected- big inflammatory response (high number of eosinophils)  Worms before- supressed inflammation  Worm infections also prevent inflammation in models of: Diabetes, intestinal inflammation, obesity, MS

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Immune Sup: Helminths Manipulate Host

Immune Sup: Helminths Manipulate Host

Summary of helmith-induced immune modulation  Studies to isolate ​​​​​immunomodulators to treat allergic reactions would (treat patient without infecting them with helmith) Suppresion of immune cells promote worm survival but also prevent western autoimmune diseases

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Immune Sup: Helminths Manipulate Host

Hygiene hypothesis- evidence from human studies: Observational study- Cross-sectional comparison of human population with various levels of worm infection and/ or inflammatory diseases- Does worm infection correlate with reduced inflammatory disease? Yes Interventionist study- Compare incidence of inflammatory disease before and after deworming program- Does deworming lead to increased inflammatory disease? Deliberate human infection study- Helminth therapy- Does experimental infection with a worm reduce inflammatory disease in a patient cohort Helminth immunomodulatory molecules (vs live infection)- All benefits of worm-induced immunosuppression without the pathogen

Immune Avoid: Parasites manipulate Host

Viruses manipulate host with immunoevasins to block their proteins presented to CD8+ T cells TAP= transporter, delivers peptide from cytosol to ER lumen TAP binds tapasin and peptide (binds tapasin) is pulled through by TAP-I and TAP-II proteins and loaded on MHC-I  Molecules/viruses block uptake of peptides, can't be presented: ICP47 blocks peptide binding TAP US6 inhibits TAP ATPase activity UL49.5 inhibits TAP peptide transport  Adenovirus E19- competitive inhibitor of tapasin  HCMV US2- degradation of MHC-1 by removing newly synthesis MHC-1 from ER to cytosol

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Viruses Block Peptide Presentation

Immune Avoid: Parasites manipulate Host

Leishmania control innate immune response: 2 forms: promastigote (human infective, dont divide), amastigote (human infection, divide and cause disease) Many receptors implicated in leismania uptake- DC and macrophages are main targets for infection

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Leishmania control innate immune response: Innate phagocytes can internalise Leishmania and are destroyed by it Neutrophils can be initially infected and present themselves to macrophages (indirect mac infection) Differentiates into disease causing form  

Immune Avoid: Parasites manipulate Host

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Immune Avoid: Parasites manipulate Host

Leishmania control innate immune response: Sandfly commensal bacteria promote Leishmania human infection  Sandfly gut microbes prime host cell inflammasome Sand flies have commensal bacterial in their saliva/gut, transmit bacteria with saliva + promastigote secretory gel, activate inflammasome-> draws out immune cells (neutrophil infiltration)    Inflammasome derived IL-1β keeps neutrophils at bite sites Antibiotics: Treating sandfly with antibiotics before transmission blocks neutrophil infiltration- Antibiotic clears the commensal bacteria Impairs Leishmania dissemination (parasite transmission without inflammation- lower level of transmission) 

Commensals Promote Leishmania Infection

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Leishmania control innate immune response: Inhibition of phagosome maturation GP63= protease on surface of transmissable form of parasite GP63 cleaves VAMP8 which is necessary for fusion of phaogosome and gp91phox vesicle that matures phagosome (increases oxidation->alkaline pH, decreased proteolytic activity allows processing) so it can present parasite on MHC No cross presentation, no stimulation of T cells Surface of parasite impacts on phagosome and whether parasitic vesicle can fuse with phaogosome

Immune Avoid: Parasites manipulate Host

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Inhibition of Phagosome Maturation

Immune Avoid: Parasites manipulate Host

Leishmania control innate immune response: Models for delivery of Leishmania virulence factors When in parasitic vcauole, release virulence factors into host cell cytoplasm via: Exosomes- eg contain immunomodulatory factors that fuse out into host cytoplasm and remodel actin and block lysosome (keep pH at good level for survival)  Lipid rafts  Translocases flipases 

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Immune Avoid: Parasites manipulate Host

Leishmania control innate immune response: Pre-emptive disabling of host cell signalling pathways by Leishmania exosome cargo Promastigotes manipulate host cell macrophages via exosomes and blebs from PM  Results in: Blocking microbial functions; NO TNF-α, ROS Promote anti-inflammatory response- IL-10, leading to Leishmania persistence

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Immune Evasion: Antigenic Variation

Influenza Surface proteins Haemagglutinin (HA)- attaches to host receptors Neuraminidase (NA)- breaks down sialic acid to allow budding Major serovars: 15 different HA, 9 different NA Antigenic drift/ minor mutations- epidemics, seasonal flu Antigenic shift/ major reassortment- host infected with 2 different flus simultaneously, pandemic

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Immune Evasion: Antigenic Variation

Trypanosomes Changing surface coat: switch VSG (Variant Surface Glycoprotein) expression VSG switches to avoid Ab landing, recycles coat every 12 min  If Ab lands on VSG, VSG moves Ab to flagella pocket, ingest via endocytosis, digests Ab, VSG shift back to surface How does it switch- T, brucei genome sequencing info  >1000 VSG genes in genome  Almost all are pseudogenes/ gene fragments Trypanosomes can create new genes by recombining fragments  VSG repertoire for antigenic variation larger than originally thought Parasite kills you before runs out of different VSGs

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VSG Surface Coat Switch

Immune Evasion: Antigenic Variation

Plasmodium Infect RBCs, RBC develop surface knobs that contain parasite proteins Surface knobs cause RBCs to stick to endothelial lining and block capillaries- Prevents parasite clearance by spleen  Stick to eachother and vessels- cause blood clots-> liver and brain inflammation and damage (cerebral malaria- why P.falciparum is so pathogenic)

Immune Evasion: Antigenic Variation

Plasmodium: PfEMP proteins Plasmodium spp. PfEMP1 protein is massive complex, 2 domains (CIDR and DBL) Functions of domains: Cytoadherence of RBC to microvascular endothelial cells in all organs to avoid spleen Evade specific immune responses through antigenic variation  CIDR and DBL domains are variable and can recombine with eachother-multiple copies of these genes throughout genome

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Immune Evasion: T.cruzi

Trypanosoma cruzi avoid innate and acquired immune response: Receptor-ligand interactions Receptor-ligand interactions enable T.cruzei adherence and invasion of mammalia cells and interactions drive: Phagocytosis, membrane raft-dependent endocytosis, macropinocytosis, autophagy, Ca2+ dependent lysosome exocytosis Every life cycle phase is infective- infect any nucleated cell by triggering their own endocytosis via calcium signalling, inside cell need to escape parasitic vacuole into cytoplasm to differentiate into amastigotes Have multiple receptors-> multiple receptor-ligand pairs  Complex downstream signalling

Immune Evasion: T.cruzi

Trypanosoma cruzi avoid innate and acquired immune response: Surface trans-sialidases obstruct CD8+ T cell immunity Immune cell function dependent on sialic acid- T cell activation associated with decreased sialyation of surface proteins T.cruzi TS re-sialytes molecules reducing T cell-target interactions

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Immune Evasion: T.cruzi

Trypanosoma cruzi avoid innate and acquired immune response: Lysosomal escape by T.cruzi trypomastigotes Inside vesicle potent oxidants, ROS are formed (H2O2 and peroxynitrite, ONOO-) T.cruzi peroxidases and superoxide dismutases (SOD) neutralises lysosomal reactive oxygen species (ROS) Escape requires acid-activated protein TC-TOX which forms pores and rips vacuole apart In cytosol, ROS can promote T.cruzi growth via amastigote access to iron

Trypanosoma cruzi avoid innate and acquired immune response: Congenital transmission by T.cruzi trypaomastigotes Problem for pregnant women, foetus at risk of infection  Complement acts as molecular bridge- C1q bidns to trypomastigote calreticulin and placental calreticulin, forming bridge  Ab's promote assist T.cruzi uptake- were tested to block interaction but promoted it by assisting recognition of the pathogen

Immune Evasion: T.cruzi