developing parasite vaccines

Question 1

anti-parasitic vaccines

Answer 1

• none commercially available for human disease
• some for animals
  
  • giardia in dogs
  • tapeworm in pigs

Question 2

malaria vaccine

Answer 2

• huge efforts
  
  • massive socio-economic burden ($12 billion yearly in africa alone)

Question 3

features of plasmodium

Answer 3

• unicellular eukaryote
• complex multi-host lifecycle
• sporozoite form is transmitted from mosquito to human
• merozoite form infects human RBCs
• gametocytes medaite transmission to mosquitoes
  
  • all potential vaccine targets

Question 4

potential mechanisms of malaria vaccines

Answer 4

• prevent infection - liver stage
• prevent disease - blood stage
• prevent transmission - mosquito stage

Question 5

features of an effective vaccine

Answer 5

• induction of strong and appropriate immune responses
  
  • durable and broad enough
• safe, well tolerated
• easy to administer and transport
• cost effective

Question 6

potential for plasmodium life cycle intervention

Answer 6

• antibody response to sporozoites before liver cell invasion
• CD8+ T cell repsonse against parasite infected liver cells
• antibody/CD4+ mediated destruction of free floating merozoites in blood
• antibody mediated response to sexual stage forms in mosquito

Question 7

vaccine strategies

Answer 7

• whole organism approach
• subunit vaccine approach

Question 8

vaccine classes

Answer 8

• pre-erythrocytic malaria vaccines (PEV)
• blood-stage vaccines
• transmission blocking vaccines (TBVs)

Question 9

whole organism vaccine approach

Answer 9

• make preparations of parasites from infected people/mosquitoes
• attenuate with radiation, chemicals, crispr/cas9 editing
• increased susceptibility to immune response

Question 10

subunit vaccine approach

Answer 10

• create recombinant expression of single, relevant antigens
• deliver with adjuvant
• usually stage specific
• difficult to design, make and are expensive
• issues with injecting peopel with recombinant parasite

Question 11

PEVs

Answer 11

• aim to stimulate strong T cell responses against liver stage
• and/or antibodies against sporozoites
• prevent blood stage infection and transmission
• some evidence of reduced liver burden
  
  • can ameliorate blood stage clinical disease

Question 12

challenge models

Answer 12

• good ones available for PEVs in small animals and humans
• mosquito bite challenge
  
  • 5 infected mosquitoes in cup exposed to person
  • after all have bitten, see how person is affected
  • expesnive, need volunteers, is 5 enough (different parasite numbers)
• i.v. challenge
  
  • purify sporozoites from salivary glands and inject
  • but 95% of parasites dead

Question 13

RTS,S

Answer 13

• vaccine nearest clinical release
• PEV
• subunit vaccine
• targets circumsporozoite protein - 60% of surface, liver cell binding

Question 14

structure of RTS,S

Answer 14

• recombinant expression of B cell and CD4+ epitopes
• fused to hep B surface antigen
  
  • binds non-conjugated antigens
• RTS = fusion protein
  
  • part of CSP with T/B cell epitopes
• S = HBsAg
• RTS and S spontaneously combine to form virus like particles

Question 15

RTS,S adjuvant

Answer 15

• AS01
• liposome-based with MPL
• safe and mdoerately effective

Question 16

RTS,S efficacy

Answer 16

• higher in children compared to infants after 12 months
  
  • 56% - 31%
  • unknwon why
• efficacy wanes quickly
  
  • almost completely gone after 3 years

Question 17

drawbacks of RTS,S

Answer 17

• waning efficacy
• high initial outlay
• could shift disease burden to older children in areas of high transmission

Question 18

irradiated sporozoites

Answer 18

• whole organism vaccine, 1960s
• attenuation by irradiation
• mobile sporozoites invade liver cells but don’t develop further
• effective but discontinued in 1970s
  
  • difficult to get enough sporozoites to clinical quality
  • advances in subunit techniques
• Sanaria

Question 19

Sanaria vaccine

Answer 19

• PfSPZ, whole parasite, 2013
• dissected sporozoites out of mosquitoes for vaccine use
• doesn’t always work but highly effective when it does
• more sporozoites → more effective
• is it feasible?
• iv delivery most effective - difficult
• also liquid N2 storage

Question 20

blood stage vaccines

Answer 20

• stimulate T cell and/or antibody responses to blood stage
  
  • antigens on iRBC or parasite surface
• focus on metabolism, cytoadherence and mainly invasion
• reduces pathology - fewer parasites in bloodstream
• high parasite numbers in circulation make it difficult
  
  • also genetic variation

Question 21

blood stage vaccine targets

Answer 21

• MSP1 - merozoite surface protein 1
• AMA1 - microneme secreted merozoite surface protein

Question 22

MSP1

Answer 22

• secreted to coat surface from micronemes
• GPI anchored
• undergoes proteolysis to form fragments
• essential for attachment to RBC
• successful in mice and primates but not humans

Question 23

AMA1

Answer 23

• transported to surface when schizonts rupture
• conserved, interats with RON in RBC
• associated with antibodies in natural immunity
• highly polymorphic
  
  • difficult vaccine design
• no success in humans so far

Question 24

TBVs

Answer 24

• prevent mosquitoes getting malaria in the first place
• only 1-5 parasites in msoquito - much easier target
• larger window of opportunity
  
  • extracellular for 24h in mosquito
• highly invariant targets
  
  • no pressure from host immune system

Question 25

successful TBV features

Answer 25

• inhibit parasite development by inducing a circulating antibody
  
  • antibody ingestion via bloodmeal
  • 24h IgG and complement window in midgut before IgG degradation
• target antigens on sexual stage surface
• induce high and long enough titres for at least one transmission season (6 months)

Question 26

SMFA

Answer 26

• standard membrane feeding assay for TBVs
• infected human blood with gametocytes
• mix with test antibody and keep warm
• introduce membrane and let females feed
• oocysts in midgut counted
• assess intensity and prevalence

Question 27

TBV antigens

Answer 27

• 5 identified and proven to work
• ookinete surface proteins
  
  • P25 and P28 -reduce transmission to 0.01% in knockouts
• male gamete surface proteins
  
  • P48/45 - fertilisation
  • P230 - may hold P48/45 on surface
  • HAP2 - fertilisation
• need more for success
  
  • screening proteomes

Question 28

current TBV status

Answer 28

• immunisation of humans and animals has induced antibodies with high efficacy
• can result in eradication in animal lab populations
• coverage? field trials? more antigens? better delivery?
• combination of multiple vaccine classes?
• need to immunise whole population

Question 29

non-falciparum malaria vaccines

Answer 29

• P. vivax another major cause of malaria
• forms dormant heptic hypnozoites
  
  • can lead to relapse years later
• greater global distribution
  
  • lower temperatrues
• restricted by Duffy antigen emergence
  
  • removed receptor of RBC invasion
• neglected but may become problem