Lecture 16 + 17 - Malaria Flashcards
Age group most represented in malaria deaths
Children under 5
Phylum of malaria
Apicomplexia
Genus of malaria
Plasmodium
Plasmodium species that infect humans 1) 2) 3) 4)
1) Falciparum
2) Vivax
3) Ovalae
4) Malariae
Primate - human plasmodium pathogen
Plasmodium knowlesi
How does plasmodium gain access to hepatocytes?
Kupfer cell
3 places where plasmodium live
Mosquito, human hepatocytes, human erythrocytes
Example of the strength of anti-malarial selective pressure
Sickle-cell anaemia alleles are maintained in sub-Saharan Africa, as being heterozygous for this reduces susceptibility to malaria
Difference in P. vivax and P. falciparum infection
Vivax causes more severe morbidity, but is less likely to kill
Number of malaria deaths per year
~650,000
What are the gold crystals observed in malaria-infected erythrocytes?
Haemozin
Parasite eats haemoglobin, but haem is toxic.
Crystallises haem to haemozoin
Malaria definitive host
Mosquito (where sexual reproduction of gametocytes occurs)
Why are malarial infections comparable between Latin America and sub-Saharan Africa, but sub-Saharan Africa has much higher mortality rates?
P. vivax more common in Latin America.
P. falciparum more common in sub-Saharan Africa
Estimated number of malaria cases per year
300 - 500 million
Amount of household spending on malaria in Africa
Estimated 10% of household spending (mosquito nets, medication ,etc)
P. falciparum life cycle 1) a, b, c 2) a, b 3) a, b 4)
1) Mosquito stage (in mosquito gut)
a) Gamete
b) Zygote
c) Ookinete
MOSQUITO BITES HUMAN
2) Hepatocyte stage
a) Sporozoites infect hepatocytes
b) Merozoites form, exit hepatocytes
3) Blood stage
a) Merozoites infect erythrocytes, divide rapidly
b) Gametocytes form
4) Mosquito ingests gametocytes from human host, sexual reproduction in mosquito
When does malaria cause illness?
During merozoite stage
Hepatocyte stage is asymptomatic
Number of infected erythrocytes directly after hepatocyte stage Vs number of infected erythrocytes after merozoite expansion
10^3 infected erythrocytes Vs 10^11 infected erythrocytes
What does merozoite release coincide with?
Periodic fever, cytokine storm
Basic overview of three stages of malarial infection
Mosquito stage - Sexual reproduction
Hepatocyte stage - Asexual reproduction
Erythrocyte stage - Asexual reproduction, major amplification stage
Where does the sexual stage occur, and for how long?
1 - 2 weeks in female Anopheles mosquito
How long does it take for injected sporozoites to enter hepatocytes from skin?
~30 minutes
How long does the asexual liver stage last for?
~1-2 weeks
How long after infection does disease occur?
~1 month
How long does the asexual erythrocyte stage last for?
~2-3 days, is quite synchronous
Symptoms of malaria
Fever, chills, anaemia, coma
Proportion of malaria deaths caused by P. falciparum
95%
How does P. vivax cause relapsing malaria?
Hypnozoites in the liver
What role does cytoadherence play?
Malaria doesn’t want infected blood cells to enter the spleen, as they would be destroyed there.
Instead, cause infected erythrocytes to adhere to vascular wall.
What coincides most with malarial death?
Infected erythrocytes adhering to walls of blood vessels in brain.
Exact mechanism of death not known. Not straight ischaemia
What adheres infected erythrocytes to the vascular wall?
PfEMP1
PfEMP1 role
Binds to molecules on microvascular endothelium
Surface protein in the brain microvasculature that PfEMP1 binds to
ICAM1
Number of different genomic copies of PfEMP1 in a malarial genome
Over 60
Name of genes encoding PfEMP1
var genes
How are different var genes expressed?
Epigenetic silencing of all var genes except for 1.
A parasite can change var gene expression
What correlates with different waves of parasitaemia?
Expression of different PfEMP1 genes
Express different ones when an effective antibody response is launched against one
Where are most var genes located?
Sub-telomeric regions of chromosomes
Why do different P. falciparum parasites have a greater variety of var genes than expected?
1)
2)
3)
1) Chromosomes cluster at at nuclear periphery of parasite
2) Recombination between heterologous chromosomes that are clumped together, aligned
3) Results in great variation of var genes and expressed PfEMP1s
Number of proteins that P. falciparum exports into erythrocyte cytoplasm
Hundreds
KAHRP function
Knob formation.
Knob allows parasite to hang onto microvascular endothelium under flow pressure
Evidence for KAHRP function
With KAHRP, infected RBC surface is rough, cell sticks to microvascular endothelium
Without KAHRP, infected RBC surface is smooth, doesn’t stay attached to vascular wall
Motif allowing P. falciparum to export proteins into RBC cytoplasm
PEXEL motif
Amount of P. falciparum proteome estimated to be exported out of parasite
~5%
PEXEL motif role
Allow proteins to cross parasitophorous vacuole membrane
Two important sites in P. falciparum protein export
1) Endoplasmic reticulum
2) Parasitophorous vacuole membrane
What is PEXEL?
A protease cleavage site
Cleaved by Plasmepsin IV
What does plasmepsin IV cleave?
PEXEL motif, in the endoplasmic reticulum
Where is PEXEL cleaved by plasmepsin IV?
RxLxE/Q/D –> RxL | xE/Q/D
In the ER
Where is plasmepsin IV present?
In the ER of P. falciparum
PTEX
A putative translocon of PEXEL proteins
Criteria for PTEX 1) 2) 3) 4)
1) Plasmodium-specific, in the correct location
2) Essential to blood-stages
3) Energy source, a protein-unfolding mechanism
4) Binds transiting cargo PEXEL proteins
PTEX structure
1)
2)
3)
1) 5 proteins
2) HSP101 is an energy source, unfolds proteins so that they can fit through pore
3) EXP2 is a pore protein
PTEX location
Parasitiphorous vacuole membrane in RBC
How long does it take for a merozoite to enter an erythrocyte?
~30 seconds
Possible anti-malarial drug targets
1)
2)
1) Plasmepsin IV
2) PTEX
Types of anti-malarial vaccine
1)
2)
3)
1) Pre-erythrocytic
2) Transmission-blocking
3) Blood-stage (anti-merozoite)
Example of possible pre-erythrocytic vaccine
RTS, S
In stage III clinical trials
Difficulty for anti-merozoite antibodies
Only have about 1.5 minutes when merozoites are susceptible to action.
~1.5 minutes between when merozoites exit former host cell, enter erythrocyte
Primary interaction target of possible vaccine
Binding to cell
Secondary interaction target of possible vaccines 1) 2) 3) 4) 5)
1) Actin/myosin motor
2) Surface protein shedding
3) Secondary ligand shedding
4) Resealing
5) Recovery from echinocytosis
Problems with making an anti-malaria vaccine 1) 2) 3) 4)
1) Empirical vaccine techniques aren’t working
2) Very little functional knowledge of individual antigens
3) Many antigens are identified, but which to target?
4) Vaccine must cover three major invasion pathways
Target of P. vivax on erythrocytes
Duffy protein
Adaptation of West Africans against P. vivax
Don’t express Duffy protein on erythrocyte membrane
As a result, no P. vivax in West Africa
Families of surface proteins that bind erythrocytes in P. falciparum
1) EBA (erythrocyte-binding antigen) (5 members)
2) PfRh (5 members)
What does the W2mef strain of P. falciparum use to bind erythrocytes?
Can use EBA175 to bind sialic acid on GlyA
What does W2mef strain do when RBCs have been depleted of sialic acid by neuraminidase?
Expresses a silaic acid-independent invasion phenotype using a different surface ligand (Rh4)
Important targets of inhibitory antibodies
EBA, Rh proteins
Why does P. falciparum use several invasion pathways?
1)
2)
1) Accommodate RBC polymorphism
2) Immune avoidance
Major invasion pathways now identified
Three major invasion pathways
