antigenic variation in malaria parasites

Question 1

infected RBCs

Answer 1

• sequester and accumulate in capillaries
• altered adherence
• RBC sticks to capillary walls
• rosetting and clumping
• detection of rigidity in ageing RBCs by spleen for removal avoided by sequestration

Question 2

microvascular obstruction

Answer 2

• caused by infected RBC sequestration
• hypoxia, anaerboic glycolysis, lactic acid build up
• metabolic acidosis
  
  • impaired consciousness, respiratory distress
• general decrease in oxygen carrying

Question 3

iRBC knobs

Answer 3

• stick out on iRBC surface
• associated parasite proteins
  
  • targeted through parasitophorous vacuole to host cell surface
• KAHRP
• PfEMP1
• PfEMP2

Question 4

KAHRP and PfEMP2

Answer 4

• thought to interact with submembrane cytoskeleton of host RBC
• reorganisation of membrane skeleton may result in knob formation
• KAHRP layer above PfEMP2 layer
  
  • lie on top of submembrane cytoskeleton

Question 5

PfEMP1

Answer 5

• crosses membrane to be exposed on RBC surface
• acidic C-term domain interacts with KAHRP and cytoskeletal proteins
• encoded by variable VAR gene

Question 6

VAR gene

Answer 6

• expression in merozoites
  
  • high expression in ring stage of trophozoites before schizont rupture to release merozoites
• mediates iRBC attachment to capillary endothelial cells via surface ligands
• variable N-termini
• each variant recognises different endothelial ligands
• difficult to antigenically vary something with binding role
  
  • VSG/VSP - no binding

Question 7

endothelial cell receptors

Answer 7

• VAR family allows wide recognition range of receptors
  
  • CD36
  • ICAM-1
  • VCAM-1
  • P-selectin
  • Integrins

Question 8

strategy of immune evasion

Answer 8

• host cell remodelling from within the RBC
• PfEMP1 expression → sequestration
• potential role of rosetting
• antigenic variation of PfEMP1

Question 9

rosetting

Answer 9

• PfEMP1 can bind ligands on normal RBCs
• covers the iRBC surface
• shields from recognition

Question 10

VAR genes

Answer 10

• ~50 VAR proteins, highly variable 9especially between strains)
  
  • all encode PfEMP1 variants
• rapid generation of diversity in VAR allows survival even in previously immunised host

Question 11

haploid VAR gene repertoire

Answer 11

• merozoites are haploid
• 50-60 VAR variants in each haploid parasite
• monoallelic exclusion
• chromatin mediated repression of all other VAR genes
• tight regulation to prevent immunisation to multiple variants

Question 12

cyclic infection

Answer 12

• similar to T. brucei
• host immune response by IgG provides naturally acquired immunity
• parasite switches VAR variant to bring new wave of fever

Question 13

antigenic variation of PfEMP1

Answer 13

• once on host surface it can be recognised as non-self
• variation needed for immune evasion
• fewer variants that VSG

Question 14

PfEMP1 as a receptor

Answer 14

• variants affect cytoadherence specificity
• can change location fo sequestration
  
  • e.g. brain or placenta
• different variants can occupy different niches
  
  • immune evasion and disease progression

Question 15

VAR gene location

Answer 15

• usually telomeric but can be central/internal
• probably not a key factor
• telomeric location for generation of diversity
  
  • recombination more frequent

Question 16

telomere clustering

Answer 16

• FISH identified falciparum telomeres as clusters
• at nuclear periphery
• maybe for ectopic recombination between VAR genes
• telomeres bind nuclear periphery
• SIR2 localises to periphery to maintain heterochromatin suppression
  
  • deacetylase

Question 17

heterochromatin localisation

Answer 17

• localised to periphery but moves inside nucleus upon activation (eukaryotes)
• upon VAR gene activation, movement to another peripheral region instead (falciparum)

Question 18

model for heterochromatin localisation in falciparum

Answer 18

• nucleus has ‘neighbourhoods’
• some are silencing
• VAR gene escapes silencing neighbourhood and is activated
• maybe by binding nuclear envelope proteins
• regulation unkown - ensure only 1 escapes for monoallelic expression?

Question 19

active - silent VAR gene transition

Answer 19

• dynamic
• active gene moves back to a poised state in the silencing region
• from here it can return to being active

Question 20

chromatin

Answer 20

• DNA complexed with proteins to from compressed DNA
• allows control of transcription by unpacking
• heterochromatin and euchromatin
• DNA wrapped around core histones forming nucleosomes joined by linker histones

Question 21

histone modification

Answer 21

• histone tails (N-termini) read and modified by cell
  
  • e.g. methyl transferase
• methylation marks are conserved throughout eukaryotes (falciparum)
• mark chromatin domains in plasmodium

Question 22

plasmodium heterochromatin characteristics

Answer 22

• high levels:
  
  • H3K9Me3
• depleted:
  
  • H3K9Ac

Question 23

histone modification of VAR genes

Answer 23

• silent - coated in H3K9Me3
  
  • shifted to the back of gene upon activation
  • replaced by H3K4Me3 at start
    
    • euchromatin marker

Question 24

VAR gene promoters

Answer 24

• H3K36me3 on histones
• knockout of enzyme that catalyses this modification
  
  • promoter acetlyation and derepression of the silenced gene
  • increased number of VAR transcripts

Question 25

long non coding RNAs

Answer 25

• aid monoallelic expression
• bind regions of plasmodium genome leaving a tail sticking out
• tags the area for recruitment of silencing machinery

Question 26

RNAse II

Answer 26

• co-localises with silent VAR loci in plasmodium
• may regulate expression of some genes
  
  • not all
• knockdown → derepression of VAR genes
• combination of mechanisms involved in expression control