intracellular parasite immune evasion - leishmania

Question 1

threats avoided by intracellular parasites

(compared to extracellular parasites)

Answer 1

• complement system and MAC-mediated lysis
• phagocytosis
• host antibodies

Question 2

additional threats faced by intracellular parasites

Answer 2

• lysosomal enzymes
• toxic metabolites
• immune responses targeting the infeted cell

Question 3

challenges faced by intracellular parasites

Answer 3

• how to invade host cell
• how to evade host clearance mechanisms
• how to obtain nutrients
  
  • parasitophorous vacuole doesn’t provide full acces to the cytoplasm

Question 4

host cell clearance mechanisms

Answer 4

• lysosomal degradation
• apoptosis
• autophagy
• ROI
• innate immune system (TLRs)
• adaptive immunity

Question 5

leishmania invasion strategy

Answer 5

• use of host molecules to facilitate entry into specialised cell (macrophage)
  
  • antibodies or complement
• encourages its own phagocytosis
• easy entry but harsh internal environment

Question 6

leishmania’s intracellular niche

Answer 6

• modified phagolysosome
• abundant nutrients
• highly acidic

Question 7

apicomplexan host cell invasion

(not leishamania)

Answer 7

• adhesins stored in micronemes translocated to apical surface
• bind host receptors and actin-myosin cytoskeleton of parasite
• create junction that moves towards posterior end of parasite
• parasite pushed into invaginating vacuole
• adhesin cleavage and junction dissolution
• non-fusigenic vacuole formed

Question 8

fusigenic vs non-fusigenic vacuole

Answer 8

• fusigenic merges with internal compartment of the host
• non-fusigenic doesn’t merge with host
  
  • simply sits within the cell

Question 9

leishmania

Answer 9

• kinetoplastid (not apicomplexan)
• transmitted by phlebotamine sandfly
  
  • bites reservoir host (usually dogs) and transmits to humans
• leishmaniasis
  
  • visceral (severe and systemic) or cutaneous
• sores that can remain dormant and reappear

Question 10

infective form of leishmania

Answer 10

• metacyclic promastigotes
• injected into bloodstream during bites

Question 11

phagocytosis of leishmania

Answer 11

• complement deposition on parasite in bloodstream
• phagocyte attracted by chemoattractants (C5a)
• C3b and iC3b binds CR1 and CR3 receptors on phagocytes → phagocytosis
• resides in phagolysosome
  
  • loss of flagellum → amastigote
• proliferation → host cell bursts
  
  • reinvasion or picked up by sandfly

Question 12

differentiation of promastigote

Answer 12

• triggered by pH drop and elevated temperature
• amastigote much more resistant to low pH

Question 13

promastigote coat

Answer 13

• lipophosphoglycan (LPG)
• specific to promastigote
• no antigenic variation
• confers resistance to lytic effect of complement
• connected via GPI anchor
• LPG-deficient leishmania:
  
  • can’t colonise sandfly
  • poorly virulent in macrophages

Question 14

leishmania host cell remodelling

Answer 14

• uses LPG
• promastigote remodels surrounding vacuole to prevent lysosomal fusion with the phagosome
• not completely blocked but slowed down
  
  • enough to allow differentiation into amastigote

Question 15

promastigote entry

Answer 15

• different receptor-mediated pathways but phagocytosis is the major one
• C3b binds CR1
• GP63 on parasite converts C3b into iC3b to block MAC formation (can’t recruit B factor)
• LPG may bind mannose receptor
• GP63 binds fibronectin
  
  • binds fibronectin receptors

Question 16

amastigote phagocytosis

Answer 16

• later in infection after proliferation within macrophage
• host beginning to create antibodies
  
  • IgG receptor (FcγRs) recognition exploited by amastigote
• in addition to C3b and iC3b

Question 17

Trojan horse mechanism

Answer 17

• leishmania invasion of neutrophils
• after bite, neturophils mgirate to site
• CR3 and c-type lectins facilitate phagocytosis
• parasite damages the neutrophil instead of colonising
  
  • no proliferation
• unhealthy neutrophil phagocytosed by macrophages

Question 18

provision of nutrients to leishmania

Answer 18

• ER vesicles, lysosomes and endocytic vesicles fuse with parasitophorous vacuole
• creates membrane to engulf parasite
  
  • phagolysosome formation
  • also provides nutrients

Question 19

LAMP-1

Answer 19

• lysosomal marker used to identify the compartment containg the parasite as a phagolysosome

Question 20

LPG and the phagosome

Answer 20

• LPG inserted into lipid rafts of the phagosomal membrane
• remodels the compartment to prevent lysosome fusion
  
  • inhibits recognition by the lysosome
• LPG deficient L. donovani promastigotes mature but don’t remodel
  
  • lysosomal fusion and parasite death
• promastigotes only