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)

avoidance of spleen low oxygen tensions better invasion

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)

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

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 ….

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 ...

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 40-50 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 CD36--88 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????

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

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

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

Cytokine Theory

Cytokine Theory Problem minimal lymphocyte infil-tration or inflammation

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