2. Higher Parasitemia in Falciparum Malaria
all erythrocytes invaded
Pv/Po = reticulocytes
Pm = senescent RBC
up to 36 merozoites
sequestration of infected erythrocytes
trophozoite and schizont stages
primarily in brain, heart, lungs, and gut
complications
immune evasion (spleen avoidance)

3. avoidance of spleen
low oxygen tensions
better invasion

4. Severe Falciparum Malaria
Complications
Features Indicating Poor Prognosis
cerebral malaria
anemia
hyperpyrexia
hypoglycemia
acidosis
GI and liver syndromes
pulmonary edema
blackwater fever
algid malaria (shock)
impaired consciousness
repeated convulsions
respiratory distress
shock
acidosis/hyperlactemia
jaundice or other liver malfunctions
renal impairment
high parasitemia (>500,000/mm3)

5. Cerebral Malaria severe complication of falciparum malaria
mortality of 30-50%
a diffuse encephalopathy with loss of consciousness
severe headache followed by drowsiness, confusion and coma
consciousness ranges from stupor to coma
unresponsive to pain, visual, and verbal stimuli
convulsions frequently observed
onset can be gradual or sudden
associated with sequestration in micro-vasculature of brain

6. P. falciparum expresses ‘knobs’ on the surface of infected erythrocytes. Knobs mediate cytoadherence to endothelial cells.
the knobs are electron dense protuberances found on the erythrocyte membrane of infected cells
these knobs are believed to mediate cytoadherence--or binding to endothelial cells of the capillaries
this results in sequestration of the mature parasites in the deep tissues
This sequestration has two major advantages: 1) metabolic low O2 tensions, and 2) spleen advoidance
this sequestration is also responsible for the increased pathology associated with Pf.
for the remaining lecture I will discuss the molecular and cellular biology of knobs and this process of cytoadherence
lets first look at knobs ….

7. Several Parasite Proteins Are Associated with Knobs
KAHRP and PfEMP2 are believed to interact with the submembrane cytoskeleton of the host erythrocyte
reorganization of the membrane skeleton may result in knob formation
PfEMP1 crosses the erythrocyte membrane and is exposed on the surface
proteins synthesized by the parasite are transported to the host erythrocyte membrene
some of these proteins have been localized to the knobs
in particular KAHRP and PfEMP2 are believed to interact with the submembrane cytoskeleton
as discussed earlier the cytoskeleton is responsible for cell shape
a reorganization of this cytoskeleton could result in knob formation
neither of these proteins are exposed on the surface
PfEMP1 is exposed on the erythrocyte surface
interestingly it has an acidic domain at its C-terminus, or the portion on the cytoplasmic side. the KAHRP is very basic. Binding has been demonstrated., suggesting that PfEMP1 is anchored into the knob through interactions with KAHRP
the exposure of PfEMP1 suggests that it could mediate binding to endothelial cells
the gene for PfEMP-1 has been cloned and sequenced ...

8. PfEMP-1 Structure family of 40-50 var genes
conserved intracellular C-terminus
acidic terminal segment (ATS)
binds cytoskeleton + KAHRP
transmembrane domain
variable extracellular domain composed of modules
2-7 copies of Duffy-binding like domains
5 sequence types (a, b, g, d, e)
1-2 cys-rich interdomain regions
all have DBL1a + CIDR
participates in cytoadherence
there is not just one PfEMP-1 gene, but it is a multigene family, called var, with an estimated members
as shown on the previous slide, one end of the protein (the C-terminus) probably interacts with the erythrocyte submembrane skeleton
this intracellular domain is conserved
there is a domain with transmembrane characteristics
the extracellular domain is variable, but contains recognizable motifs
1-5 copies of Duffy-binding like domains.
exhibit similar cys and hydrophobic residues even though not highly conserved
there is also cys-rich interdomain region
because of its location speculated to be a ligand for cytoadherence
a ligand suggests the presence of a receptor--many have been identified
most widely studied is CD kDa glycoprotein found on monocytes, platelets and endothelial cells
many var gene products bind to CD36, widely used for in vitro studies
ICAM most likely to be involved in cerebral pathology
some binding sites mapped to domains on PfEMP-1
receptor/ligand interactions usually specific????

9. Possible Host Receptors
CD36
Ig super-family
ICAM-1
VCAM-1
PECAM-1
E-selectin
thrombospondin
chondroitin sulfate A
hyaluronic acid
Rosetting Receptors
CR-1
glycosaminoglycan
blood group A

11. hypoxia, metabolic effects
Sequestration
Hypothesis
cytoadherence 
cerebral ischemia
hypoxia, metabolic effects
coma
death

13. Problems with Sequestration Hypothesis
rapid reversibility
lack of ischemic damage
low levels of permanent neurological damage
sequestration occurs in non-cerebral malaria cases

14. Cytokine Theory

15. Cytokine Theory Problem
minimal lymphocyte infil-tration or inflammation

16. Severe falciparum malaria
potentially high parasitemias
sequestration
complex (and not fully understood) host-parasite interactions