Blood-borne Parasites

• type of parasite of blood borne trypanosomes - protozoa causes sleeping sickness
• type of parasite of blood borne plasmodium - protozoa causes malaria
• T. brucei also causes disease in cattle (nagana) and horses and some wild animals 
• trypanosomes The spread by the tsetse flies.

  →  this parasite lives in the blood and the extracellular fluid

  → doesn’t invade any cells 

  → size of a red blood cell 
• trypanosomes lifecycle

  1. tsetse flies takes blood meals
  2. injects metacyclic trypomastigotes and converts it into trypomastigotes
  3. the binary fission
  4. actue phase undetectable in blood
  5. another fly takes it in
• trypanosomes :: Acute stage : it causes cytokine storm, predominantly fever
• Trypanosomes : Chronic infection : can give rise to this process called sleeping sickness - due to invasion of the brain but always extracellular fluid 
• Trypanosomes Immunity: So if we have parasites in the blood like this, then the immunity isn't going to be cell based immunity.-  mostly antibodies
• →   it's essential that, you know, that different isotopes have different properties.

  → igM star structure 

  → y shaped molecule is IgE

  →  y shaped molecule is  IgG  

  → IgA is a dimer - secreted into the mucosa
• → igM is tend to produced in a t independent manner

  → not dependant on t- cell help 

  → igM is in red and produced the same response every time 

  → levels of IgM are not boosted by vaccination or repeated infection

  → BUT Antigen specific. IgG and IgE are t - cell dependent and they get these increases in concentration after vaccination or repeated infections because of the specific memory cells.

  
• How do antibodies kill parasites? 

  Activate complement (Membrane Attack Complex) – lyse targets 

  Act as opsonin's (facilitate phagocytosis) 

  Neutralise essential antigens
• → IgM : actually effective at killing trypanosomes and that's because

  → it can coat the parasite and activate complement and the MAC complement

  → punches holes into the parasite and kills it

  → igM does not act as an opsonin and so it's not going to be as effective as IgG
•  Trypanosomes : antibodies involved its IgG and the cells involved are phagocytes.

  → Because the parasite is significantly smaller than the phagocytes, the site can engulf it and kill it intracellularly 

  → igG is better than igM  because simply it has higher concentrations of the blood at its core,

  → also acts as an opsonin 

  → also activates complement 
• IGE is mostly for helminths not for trypanosomes. noramall what happens  In this case, the cells involved are the mast cells and eosinophils they express a different FC receptor FC epsilon.

  → They granulation releases factors onto the surface of the worm, which partially kills it.

  → chemokinesis of other immune cells 
• So IgE doesn't work in Trypanosomaif you have lots of mass cell granulating, an immense allergic reaction and would cause anaphylaxis.

  →
• how does Trypanosomes replicate - binary fission
• trypanosomes  The surface of the parasite is mostly covered by just 1 protein 

  → protein called 

  → Variable surface glycoprotein VSG
• how does  Variable surface glycoprotein VSG

  help Trypanosomes survive - the VSG coat protects more important proteins from the host’s antibodies

  → less efficient at producing IgG to glycoproteins

  ;Glycan instead of peptide therfore no t- cell help
•  antibodies do destroy T.Brucei 

  → Part of the problem is that large numbers of parasites have been produced

  → parasite expansionary increases 

  →  simply because the replication rate exceeds the ability of the immune response to destroy parasites

  → However, soon the host starts producing antibodies in sufficient numbers to kill the parasites.

  → Ultimately, the antibodies are produced in excess and the killing of parasites is greater than the production of new parasites
• BUT 

  → parasites are very good at escaping antibody mediated killing : antigenic variation
• • Trypanosoma brucei exhibits coat variation through multiple copies of the same gene producing a variable surface glycoprotein (VSG).
  • Only one VSG is expressed at a time, allowing the parasite to switch expression to a different VSG gene via epigenetics.
  • Antibodies to a previous VSG do not recognize the "new VSG," allowing the parasite to grow unchecked until new antibodies are produced.
• • Female mosquito takes a blood meal, injecting saliva to prevent coagulation.
  • Introduces sporozoites, a form of the parasite, into the bloodstream.
  • Sporozoites quickly find a liver cell to invade.- replicates in liver cells
  • After a few days, they break out and invade red blood cells to replicate.
  • Parasitaemia (percentage of infected erythrocytes) reaches about 2%.
  • A small percentage develop into sexual forms, but require ingestion by a mosquito to progress.
  • Mature sexual forms, or gametes, are produced in the mosquito.
• what role does IgA play in Trypanosomes - IgA secreted anti body and this parasite lives in the blood . IgA low level in blood
• what role do cytotoxic t cells CD8 play in trypanosomes -  They recognise cells as infected by an intracelluar parasite or a virus- This is a parasite that's not intracellular. They're not going to do anything.
• what role do t helper 2 cells CD4 play in trypanosomes - the two cells produce cytokines that help antibody productiona ndSo yes, in this case you're going to get more antibodies or you produce antibodies quicker and that will help get
• What are Plasmodia parasites?

  Single cell apicomplexan (eukaryotic)

  Human – P. falciparum and P. vivax 

  Animals – apes, reptiles, rodents and birds 

  Mostly host-specific (not zoonotic
• Disease is associated only with the asexual blood stage

  for plasmodia
• plasmodia  

  1. mild cases of fever, which is related to showing schizont rupture

  →  mature schizont releases factors They seem to stimulate the immune response and we get a cytokine storm and fever every 48 hours  

  2. severe forms, a life threatening cerebral malaria= coma & anemia 
•  But the first phase is the Sporozoite which is injected by a feeding mosquito,

  → Do not live for time in the blood - Short period to invade hepatocytes (Liver) (minutes)

  → This form of the parasite is covered by one major protein called

  → Circumsporozoite protein (CSP) main surface antigen

  → Anti-CSP antibodies block invasion

  → So stop it invading the liver cells
• plasmodium → For the liver stage. Once inside the liver cell, they can multiply rapidly and then develop into a form called merozoites

  → intracellular environment protects them against antibodies, so antibodies can't get across the cell membrane
• plasmodium → Vaccine R21/matrix-M Target is circumsporozoite protein (CSP) which is expressed on sporozoites and liver stages

  Vaccine works by  One is producing antibodies which stop the merozoites invading the liver cell.but it also induces cytotoxic T cells because the CSP is also expressed on the liver stage.  so cytotoxic T cells can recognise infected liver cells and kill them.
• Asexual stage : 

  Invasion :  merozoites released from liver infect red blood cells / invasion 

  → development: inside the red blood cell: can give rise to 24 daughter merozoites in 48h

  →  schizont rupture :  schizont  releases  merozoites

  → Gametogenesis: asexual forms develop into male and female  merozoites
• → plasmodium sporozoites into Merozoites

  Merozoites have a Apical pole ( apicomplexa) - involved in the invasion of parasite into host cell

  → Merozoites do not survive in the blood for a long time (die within minutes)- ie very different from T. brucei
• • BINDING many proteins in both parasite and host
  • Plasmodium Parasite binds to and invades red blood cells.
  • Binding involves specific proteins on both parasite and erythrocyte.
  • P. vivax uses Duffy antigen on erythrocytes, rare in West Africa
• • plasmodium Parasite may require multiple attempts to bind to erythrocytes.
  • Once tightly bound, the merozoite re-orientates with its apical end prominent to begin invasion.
  • Invasion involves secretory organelles like rhoptry and micronemes, which release phospholipids and proteins necessary for invasion.
  • It's an active process requiring calcium fluxes and active actin-myosin "motors."
• • Parasites modify the erythrocyte environment.
  • Undergo differentiation and asexual division.
  • Ring stage begins with an immature trophozoite.
  • Progresses into a trophozoite stage with a distinct nucleus.
  • Specific membranes form inside infected red blood cells.
  • Parasite produces a protective parasitophorous vacuole membrane, shielding against host cell defense mechanisms.
  • Tubulin network resembling an alimentary canal is formed for nutrient processing.
  • Parasite produces its own enzymes for replication.
  • In the first half of its life cycle, the parasite is essentially invisible to the immune system.
• • Schizont rupture releases major surface protein (PfEMP-1) aggregates that form knobs binding to endothelial cells.
  • This process, known as sequestration, helps the parasite to bind to and feed on host cells.
  • Sequestration theory suggests it prevents the parasite from passing through the spleen and areas of high immune surveillance.
  • However, sequestration in the brain can lead to cerebral malaria.
  • Schizonts, full of merozoites, rupture and infect new erythrocytes.
  • Factors released during schizont rupture may trigger a cytokine storm, leading to pathology
• • Main immune response controlling parasite numbers is through antibodies (IgG).
  • Antibodies aid in clearing infected erythrocytes via complement and phagocytosis.
  • They can neutralize by blocking invasion of new erythrocytes by merozoites.
  • Plasmodia employ antigenic variation to evade destruction, as most antigens are highly polymorphic.
  • Natural immunity is achieved by producing antibodies against all local strains of the parasite.
• Which antibodies plasmodium ?

  • igM  → It will activate, complement and complement will kill the parasites or red blood cell 
  • igA→ Very little idea in the bloodstream
  • igE → a danger that if you get lots of muscles too granular eating, you'll get anaphylactic shock
  • igG is key 
• cytotoxic t- cells for plasmodium - red blood cell dotn express MHC class 1