Anti-sporozoite vaccines

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Several experimental malaria vaccines have been developed over the last 10 years. They were all so-called "subunit vaccines" based on a single or several Plasmodium surface antigens present on either the sporozoite or merozoite stage of the parasite.

The first vaccination trials were carried out with the so-called CSP- or circumsporozoite protein-based vaccine.

The rationale for the choice of the CSP protein as a candidate vaccine was the following:

• Sporozoites are the first stage that come into contact with the human being upon infection. Inactivation of this stage circulating in the blood before it is able to hide away inside the liver hepatocytes wowuld prevent infection.
• The circumsporozoite protein CSP is highly antigenic. Antisera against sporozoites that recognise mainly CSP have been shown to provide protection to infection by mosquito bites.

The following steps have led to to production of a CSP vaccine:

• Production of CSP antibodies. Antibodies were prepared from the serum of animals immunised against P. falciparum sporozoites
• Total anopheline mRNA was prepared from infected mosquitoes. This mRNA preparation should contain small amounts of sporozoite mRNA as well.
• Selection of mRNA. Total mRNA was used for the in vitro translation into protein. Polysomes that were syntesising sporozoite CSP then were precipitated bij the anti-CSP antobodies.
• Transcription into cDNA. The enriched sporozoite CSP mRNA was then transcribed into cDNA and inserted into a plasmid and cloned in E. coli.
• Sequencing of the cDNA. The cloned cDNA was then produced in large quantities and sequenced.
• Production of CSP peptide(s). CSP peptides were produced by two different techniques:
  • The cDNA was used for overexpression of the corresponding protein in E. coli and the protein then purified from the bacterium.
  • The sequence information was used for the chemical synthesis of peptides containing repetitive sequences.
• Vaccination. Both the recombinant and the synthetic-peptide preparation were then used in parallel vaccination trials in humans.
• The result was disappointing. Only one out of five volunteers turned out to be protected against a challenge with 5000 P. falciparum sporozoites. This was the case for both the synthetic as well as the recombinant vaccine.

CSP protein of P. falciparum has the following structure:

The sequence of the CSP protein predicts a signal sequence for targeting the peptide to the outside of the cell, as well as a hydrophobic anchor sequence for attachment to a glycolipid anchor in the plasma membrane of the parasite. The protein is highly charged and contains hydrophilic repeats that render it soluble and highly antigenic. Moreover, the sequence repeat is not well conserved from one isolate to another, suggesting that it does not have an essential function to the parasite other than shielding other surface epitopes and so serving as a smoke screen to fool the host immune system. Other Plasmodium surface proteins (RESA, FIRA, KAHRP) show the same characteristics. Thus this seems to be a general strategy that has been adopted not only by Plasmodium but also by other parasites. This may explain why antibodies against this highly antigenic protein are not very active in protecting against the parasite.

The candidate malaria vaccine SPf-66 is the only malaria vaccine that has been taken to phase II/III clinical trials (click here to learn more about the clinical phases of vaccine trials). It consists of a synthetic peptide polymer containing four different epitopes. One dderived from the repeat domain of the CSP protein, and three asexual stage epitopes, one from the merozoite stage and two from the P. falciparum blood stage. This candidate vaccine that was developed by the Columbian biochemist Manuel Patarroyo has undergone several field tests with varying degrees of success. It has been tested in Colombia, in Tanzania, in the Gambia in Africa and in Thailand. The last strictly controlled trial did not show any significant protection in children aged 2-15 years with respect to the control group.

Three main types of vaccine are currently under development:

• "Anti-sporozoite" vaccines, designed to prevent infection. Since infection with a single sporozoite can lead to the development of a severe blood stage infection an anti sporozoite vaccine must be 100% effective
• "Transmission-blocking" vaccines, designed to arrest the development of the parasite in the mosquito, thereby reducing or eliminating transmission of the disease.
• "Anti-asexual blood stage" vaccines, designed to reduce severe and complicated manifestations of the disease.

Such vaccines could lower morbidity and mortality among children under 5 years of age in Africa, the main risk group, and their development is given priority by WHO. Several such vaccine candidates are currently undergoing clinical and field testing.

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created by :Fred Opperdoes