Malaria pathogenesis

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Malaria pathogenesis
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Apicomplexan invasion

• Active, parasite driven process
• Depends on parasite actin/myosin motility
  (conveyor belt model)
• Involves secretion of micronemes (attachment,
  motility), rhoptries (PV MJ formation) and
  dense granules (makes PV into a suitable home)
• Sets up a parasitophorous vacuole which initially
  is derived from the host cell cell-membrane
• A moving junction is formed which screens out
  host membrane proteins from the PV, the PV is
  fusion incompetent and the parasite protected

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Malaria

• 40 of worlds population is at risk
• 300 - 500 million cases per year
• 1.5 to 2.0 million deaths per year (most of these
  are children under 6 years old in Africa)

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Malaria life cycle

• Sex and sporogony in the mosquito
• Extraerythrocytic schizogony in the liver
• Merogony in the blood phase
• Gamogony in the blood and mosquito gut

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Sporozoites

• Only Anopheles mosquitoes transmit human malaria
• 15 sporozoites are injected per average blood
  meal (lt 10 will initiate infection 100)

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Sporozoites

• Once the sporozoites have entered the skin they
  penetrate a blood vessel and enter the
  bloodstream
• Sporozoites rapidly invade hepatocytes (lt 1 h,
  probably only minutes?)

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The erythrocytic cycle
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Malaria the disease
WHO-TDR
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Malaria the disease

• 9-14 day incubation period
• Fever, chills, headache, back and joint pain
• Gastrointestinal symptoms (nausea, vomiting, etc.)

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Malaria the disease
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Malaria the disease

• Malaria tertiana 48h between fevers (P. vivax
  and ovale)
• Malaria quartana 72h between fevers (P.
  malariae)
• Malaria tropica irregular high fever (P.
  falciparum)

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Malaria the disease

• Symptoms intensify
• Irregular high fever
• Anxiety, delirium and other mental problems
• Sweating, increased pulse rate, severe exhaustion
• Worsening GI symptoms
• Enlarged spleen and liver

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Malaria the disease
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Pathogenesis of malaria

• In highly endemic areas high mortality among
  children due to severe anemia, children who
  survive beyond the first years show decreasing
  parasitemia and disease (this immunity is not
  sterile and depends on constant exposure)
• In areas with less infection pressure malaria is
  an epidemic disease with varying intensity.
  Adults and children are equally susceptible and
  death in adults is mostly due to cerebral malaria

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Pathogenesis of malaria
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Pathogenesis of malaria

• Histologically cerebral malaria is characterized
  by multiple hemorrhages
• Several (not mutually exclusive) hypotheses have
  been put forward to explain the observed
  pathology
• We will consider two immuno-toxin and
  sequestration

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Cytokines toxins

• Malaria produces a strong Th-1 type response (Dr.
  Moores lecture will provide further detail)
• Elevated serum levels of IFNg and TNFa
• Cytokines can induce (mimic) many of the symptoms
  and signs of malaria (shivering, headache,
  chills, spiking fever, sweating, vasodilation,
  hypoglycemia)

• Hatchedchill
• Blackrigor
• Clearsweating

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Cytokines toxins

• In bacteria complex glycolipids like LPS can act
  as immuno-toxin inducing a strong TH1
  over-reaction
• Free and membrane bound Plasmodium GPI anchors
  have been shown to induce TNF in vitro
• GPI anchors might be a candidate malaria toxin

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Cytokines toxins

• Mice have been immunized with synthetic GPI
  glycan coupled to carrier protein prior to
  challenge with P. berghii
• Immunized mice show some (initial) protection (a)
  independent of parasitemia control (b)
• Immunized mice show less hemorrhage (c),
  pulmonary oedema (d) and acidosis (e)

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Pathogenesis of cerebral malaria

• High cytokine levels could be toxic on their own
• High levels of cytokine also enhance the second
  process thought to be responsible for cerebral
  malaria sequestration of infected RBCs

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Sequestration cytoadherence

• The second model suggests sequestration is the
  main culprit
• In falciparum infections only early stages
  (rings) are found in the peripheral blood
• Trophozoites and schizonts are sequestered to the
  post-capilary venules by attachment to the
  endothelium

Ring stages
Trophopzoites schizonts
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Sequestration cytoadherence

• Cytoadherence correlates with pathogenesis (but
  high cytokine levels induce expression of
  endothelial adhesins enhancing attachment)
• Infected RBCs will adhere to the endothelium as
  well as to each other

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Sequestration cytoadherence

• Rosetting (adhesion of infected RBCs to other
  RBCs) and clumping (adhesion between infected
  cells) was first observed in in vitro culture
• Rosetting was also found in 50 of field isolates
  and correlated strongly with the severity of the
  observed disease

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Pathogenesis of falciparum malaria

• Parasite infected RBC become sticky and adhere
  to endothelial cells
• This phenomenon takes about 10-12 hours to
  develop after parasite invasion
• Under high flow (here modeled using a
  microfluidic device) this first results in
  rolling and then in attachment

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Sequestration cytoadherence

• Cytoadherence and rosetting correlates with the
  presence of knobs (left column) on the surface
  of the infected RBS
• Knobs are made up of parasite derived proteins
• There are also knob less strains displaying
  adherence

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Sequestration cytoadherence

• PfEMP1 (P. falciparum erythrocyte membrane
  protein) is found in knobs and is responsible for
  cytoadherence and rosetting
• PfEMP1 is a large membrane protein anchored in
  the RBC membrane with the bulk extending into the
  blood stream
• Two conserved binding domains and a series of
  highly variable domains

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Sequestration cytoadherence

• How do parasite proteins travel to the surface of
  the RBC?
• This is a considerable challenge as RBC lack
  functional secretory apparatus
• Why do patients fail to mount an effective immune
  response against antigens that are presented this
  prominently?

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Knobs and cytoadherence
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Knobs and cytoadherence

• The parasite exports PfEMP1 and other proteins
  (this picture is showing Knob associated protein)
  into the RBC and its surface to form knobs
• F in early rings protein is in the parasites,
  G,H in trophs it is found first within the RBC
  cytoplasm and then at the RBC membrane (I).

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Export is sensitive to BFA
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Export into the host cell