Developing Vaccines against ParasitesFailures and Future strategies

1
Developing Vaccines against Parasites-Failures
and Future strategies

• Why do parasites survive in the host?
• Animal defense system
• Innate Immunity
  Acquired Immunity
• Secretory molecules
  
• Cells
  Recognition of parasite
  molecules
• 
  (Ags)
• 
  Activation of Adoptive Immune system
• 
  
• 
  Responses
• 
  
• 
  Primary Secondary
• Parasites survive by overcoming these defenses

2
Current control Measures

• Anthelmintics Treatment
  Pasture Management
• Drawbacks
• Drug Resistance
• Environmental Issues
• Chemicals in Food Chain
• Cost Involvement Control cost ?
• Drug Sales of GI parasites of
  animals 1000 Million pounds/y
• Way Out ?
• Vaccine Development Parasite Vaccines are
  difficult to develop than Bacterial and Viral
  vaccines

3
Available Live Veterinary Protozoal Vaccines

• Organisms Host
  Features
• Eimeria spp Poultry sporulated
  oocysts (low doses, orally feed, water)
• E. Tenella Poultry
  Precocious and egg-passaged lines
• Theileria parva cattle Low
  doses of infection followed by drug therapy
• T. Annulata cattle
  Culture derived schizonts
• Toxoplasma gondii Sheep S48 strain
  (passage in mice oocyst formation
• 
  cryptic)
• Babesia bovis cattle Infected
  blood from splenectomized calves
• B. bigemina

4
Available killed/subunit veterinary protozoal
vaccines

• Organisms Host
  Features
• Neospora canninum Cattle
  Killed tachzoites
• Giardia duodenalis Dogs
  cultured trophozoites
• Sarcocystis neurona Horses In vitro
  cultured merozoites, chemically
  inactivated
• Babesia canis Dogs In
  vitro cultured supernatant antigens
• Leishmania donovani Dogs Native
  fucose-mannose ligand antigen complex
• Eimeria maxima Poultry
  Gametocyte antigens

5
Available Veterinary Metazoan Vaccines

• Very few metazoan vaccines. Why ?
• Huge physical
  size
• Internalization by Phagocyte difficult
  Killing by T cells ineffective
• Organisms Host
  Features
• Dictyocaulus viviparus cattle irradiated
  infective L3 larvae (truncated
  development)
• Echinococcus granulosus EG 95
  protein
• Taenia ovis dog
  Ag(s) of activated oncospheres

6
Drawbacks of present parasite vaccines and ways
to improve them

• Maintenance of infection in animals
• Animal ethic issues
• Time consuming
• Cost involvement
• Future strategies
• Identification of parasite molecules that elicit
  strong immune response and protection very
  crucial
• Hurdles
• Antigenic variation (Trypanosomes)
  Intracellular protozoan (neo- proteins)

7

• Multicellular
  parasites
• Hurdles
• Complex life cycle
• Lack of in vitro culture system
• GI parasites (Minimal interaction with host
  immune system)
• Tissue dwelling (evoke immune response)
• Antigen
  Classification
• Natural
  Hidden
• (immunity gets boosted during natural infection)

8
Identification and isolation of Antigen (s)

• Natural antigens
• Conventional techniques
• Screening cDNA libraries
• Hidden antigens
• Gut associated (GI parasites) functional
  analysis. Target the one which is critical/
  essential for the parasite.
• Excretory/ secretory products natural antigens
  as well as functional targets
• Source of antigen (s) parasites, these are best
  in terms of protection but recovery is limited
  and infections are to be maintained in the
  animals
• Alternatives recombinant protein antigens,
  generation of antigen producing cell lines from
  the parasites, anti-idiotype antibodies, DNA
  vaccines

9
Recombinant antigens advantages and drawbacks

• Advantages Easy to perform, bulk production of
  protein antigen in prokaryotic system
• Disadvantages Batch to batch variations due to
  protein aggregation and defective protein
  folding. Also eukaryotic system is not suitable
  for glycoprotein antigens (insect cells derived
  carbohydrate rich extract as substitute for
  sugars)
• Expression of glycoprotein antigens in eukaryotic
  or insect cells suitable but protection less than
  the natural antigens derived from the parasites.
  Reasons unique sugar moieties in parasite
  antigens (fucose linked glycans)
• Recent advancements expression of GI nematode
  GP antigens in C. elegans

10
Emerging Technologies

• Generation of parasite specific cell lines
  expressing antigens very important for the
  metazoan parasites because of lack of cell
  culture system. Cells lines derived from the
  infective larvae of H. contortus were maintained
  for almost 4 years.
• Knowledge gap - can the larvae cell lines
  be transformed into adult parasitic cell lines ?
• 2. Anti-idiotype antibodies vaccine these
  antibodies should be structurally similar to the
  antigen. Applications where carbohydrate moiety
  is involved in immunogenicity.
• DNA vaccines work by administration of DNA
  fragment in an expression vector coding for a
  specific antigen.
• Advantages plasmid stable minimal cold
  conditions required, safe, devoid of pathogens or
  other components
• Major concern integration of plasmid
  DNA into genomic DNA, anti-DNA antibodies
  formation

11
Understanding synergistic factors in vaccine
development

• Success of a vaccine mechanism of host
  immune mechanism
• Cytokines, cell derived molecules play
  critical part in immunity.
• Role of cytokines is poorly studied in large
  animals understanding the role of various
  cytokines in parasitic infections is warranted.
• Adjuvant choice varies with each antigen,
  recent adjuvants include liposome, DDA (dimethyl
  dioctadecyl ammonium bromide) etc
• Conclusion It is hoped that with recent
  technological advances, parasites vaccines could
  be a reality in near future. These vaccines may
  represent a cocktail of molecules one or more
  antigens along with the cytokines.