2Higher Parasitemia in Falciparum Malaria all erythrocytes invadedPv/Po = reticulocytesPm = senescent RBCup to 36 merozoitessequestration of infected erythrocytestrophozoite and schizont stagesprimarily in brain, heart, lungs, and gutcomplicationsimmune evasion (spleen avoidance)
3avoidance of spleenlow oxygen tensionsbetter invasion
4Severe Falciparum Malaria ComplicationsFeatures Indicating Poor Prognosiscerebral malariaanemiahyperpyrexiahypoglycemiaacidosisGI and liver syndromespulmonary edemablackwater feveralgid malaria (shock)impaired consciousnessrepeated convulsionsrespiratory distressshockacidosis/hyperlactemiajaundice or other liver malfunctionsrenal impairmenthigh parasitemia (>500,000/mm3)
5Cerebral Malaria severe complication of falciparum malaria mortality of 30-50%a diffuse encephalopathy with loss of consciousnesssevere headache followed by drowsiness, confusion and comaconsciousness ranges from stupor to comaunresponsive to pain, visual, and verbal stimuliconvulsions frequently observedonset can be gradual or suddenassociated with sequestration in micro-vasculature of brain
6P. 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 cellsthese knobs are believed to mediate cytoadherence--or binding to endothelial cells of the capillariesthis results in sequestration of the mature parasites in the deep tissuesThis sequestration has two major advantages: 1) metabolic low O2 tensions, and 2) spleen advoidancethis 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 cytoadherencelets first look at knobs ….
7Several Parasite Proteins Are Associated with Knobs KAHRP and PfEMP2 are believed to interact with the submembrane cytoskeleton of the host erythrocytereorganization of the membrane skeleton may result in knob formationPfEMP1 crosses the erythrocyte membrane and is exposed on the surfaceproteins synthesized by the parasite are transported to the host erythrocyte membrenesome of these proteins have been localized to the knobsin particular KAHRP and PfEMP2 are believed to interact with the submembrane cytoskeletonas discussed earlier the cytoskeleton is responsible for cell shapea reorganization of this cytoskeleton could result in knob formationneither of these proteins are exposed on the surfacePfEMP1 is exposed on the erythrocyte surfaceinterestingly 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 KAHRPthe exposure of PfEMP1 suggests that it could mediate binding to endothelial cellsthe gene for PfEMP-1 has been cloned and sequenced ...
8PfEMP-1 Structure family of 40-50 var genes conserved intracellular C-terminusacidic terminal segment (ATS)binds cytoskeleton + KAHRPtransmembrane domainvariable extracellular domain composed of modules2-7 copies of Duffy-binding like domains5 sequence types (a, b, g, d, e)1-2 cys-rich interdomain regionsall have DBL1a + CIDRparticipates in cytoadherencethere is not just one PfEMP-1 gene, but it is a multigene family, called var, with an estimated membersas shown on the previous slide, one end of the protein (the C-terminus) probably interacts with the erythrocyte submembrane skeletonthis intracellular domain is conservedthere is a domain with transmembrane characteristicsthe extracellular domain is variable, but contains recognizable motifs1-5 copies of Duffy-binding like domains.exhibit similar cys and hydrophobic residues even though not highly conservedthere is also cys-rich interdomain regionbecause of its location speculated to be a ligand for cytoadherencea ligand suggests the presence of a receptor--many have been identifiedmost widely studied is CD kDa glycoprotein found on monocytes, platelets and endothelial cellsmany var gene products bind to CD36, widely used for in vitro studiesICAM most likely to be involved in cerebral pathologysome binding sites mapped to domains on PfEMP-1receptor/ligand interactions usually specific????
9Possible Host Receptors CD36Ig super-familyICAM-1VCAM-1PECAM-1E-selectinthrombospondinchondroitin sulfate Ahyaluronic acidRosetting ReceptorsCR-1glycosaminoglycanblood group A