PRIMALVAC

var2CSA as a Pregnancy Associated Malaria Vaccine Candidate

Press Release August 2012

Background

Pregnancy Associated Malaria (PAM) is caused by Plasmodium falciparum-infected erythrocytes (PE) that bind to the placental receptor Chondroitin Sulfate A (CSA) and sequester in the placenta¹, where they cause disease and death for the mother and her off-spring.  PAM is responsible for as many as 200,000 fetal and infant deaths, and at least 10,000 maternal deaths every year, representing a major mortality risk.  A PAM vaccine would thus save hundreds of thousands of lives each year^2,3.  Such a vaccine could be combined with other vaccines targeting the same population such as the human papillomavirus vaccine and/or the Rubella vaccine.

The project focuses on the distinct form of the parasite that infects the placenta causing disease and death in mothers and infants.  Evidence strongly supports var2CSA, a member of the Plasmodium falciparum Erythrocyte Membrane Protein 1 (PfEMP1) adhesins encoded by the var gene family, as the leading candidate for a pregnancy malaria vaccine^4-6.  Indeed, var2CSA is preferentially expressed by placental parasites and the protein binds to CSA⁷.  Antisera to var2CSA react to the surface of CSA-binding parasites⁸.  Women acquire antibodies against var2CSA expressed by placental parasites over successive pregnancies, as they become resistant to pregnancy malaria⁹.  These data provide a rational basis for accelerating vaccine development aimed at blocking the adhesion of CSA-binding parasites to the placenta.

Var2CSA is a 350 kDa transmembrane protein with a 300 kDa extracellular region composed of six Duffy-binding-like (DBL) domains and a cysteine-rich interdomain region module, as well as short inter-domain regions^10,11.  It has been shown that, unlike the individual DBL domains, the full-length var2CSA extracellular region binds with high affinity and specificity to CSA^12,13, and that the main binding site lies within the DBL1X-3X segment of var2CSA^8,12,14.  It has been found that DBL3X is the principal target of the inhibitory antibodies.  Immunisation of the full length var2CSA protein as well as multiple domain constructs induced antibodies that efficiently abrogate parasite adhesion to CSA^8,14.  Interestingly, naturally acquired antibodies and those induced by vaccination against the domain between the N-terminal sequence and the DBL2X segment^15,16 target overlapping strain-transcendent anti-adhesion epitopes.  Taken together, these results indicate that DBL1X-3X region is an important target for inhibitory antibodies, and that strategies aimed at blocking PE adhesion to CSA should focus on the N-terminal region of var2CSA.

PRIMALVAC Group                                                   Infected erythrocytes

Objectives

This project aims at developing a vaccine to prevent PAM and improve pregnancy outcomes using translational research.

The main objective is to obtain proof of concept that a var2CSA based vaccine inducing long lasting or rapidly boosted cross reactive and inhibitory antibodies can be designed for human use.  Recombinant forms of var2CSA will thus be generated, and their activity as immunogens that elicit functional and cross-reactive antibodies against placental parasite forms will be assessed.  The candidate antigens that best meet strict immunogenicity criteria will be transitioned to preclinical and clinical development.

Major Milestones

- Evaluation of various expression systems (Q3 2012)
- Batch of recombinant protein for immunogenicity testing (Q4 2012)
- Vaccine candidate for further preclinical and clinical development selected (Q2 2013)
- Development of Quality Control (QC) assays (Q4 2013)
- Batch release of the Master Cell Bank expressing the var2CSA clone (Q4 2013)
- Clinical phase I batch available (Q4 2014)
- Toxicity studies draft report (Q4 2014)
- Approval from Independent Ethics Committee and Regulatory Authorities (Q1 2015)
- Expected start of phase I clinical trial: (Q1 2015)
- Go/no-go decision for the next phase of clinical trial (Q4 2015)

References:

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1. Fried M, Duffy PE. Adherence of Plasmodium falciparum to chondroitin sulfate A in the human placenta. Science. 1996 Jun 7;272(5267):1502-4.
2. Brabin BJ, Romagosa C, Abdelgalil S, Menendez C, Verhoeff FH, McGready R, et al. The sick placenta-the role of malaria. Placenta. 2004 May;25(5):359-78.
3. Umbers AJ, Aitken EH, Rogerson SJ. Malaria in pregnancy: small babies, big problem. Trends Parasitol. 2011 Apr;27(4):168-75.
4. Baruch DI, Pasloske BL, Singh HB, Bi X, Ma XC, Feldman M, et al. Cloning the P. falciparum gene encoding PfEMP1, a malarial variant antigen and adherence receptor on the surface of parasitized human erythrocytes. Cell. 1995 Jul 14;82(1):77-87
5. Su XZ, Heatwole VM, Wertheimer SP, Guinet F, Herrfeldt JA, Peterson DS, et al. The large diverse gene family var encodes proteins involved in cytoadherence and antigenic variation of Plasmodium falciparum-infected erythrocytes. Cell. 1995 Jul 14;82(1):89-100
6. Smith JD, Chitnis CE, Craig AG, Roberts DJ, Hudson-Taylor DE, Peterson DS, et al. Switches in expression of Plasmodium falciparum var genes correlate with changes in antigenic and cytoadherent phenotypes of infected erythrocytes. Cell. 1995 Jul 14;82(1):101-10.
7. Salanti A, Staalsoe T, Lavstsen T, Jensen AT, Sowa MP, Arnot DE, et al. Selective upregulation of a single distinctly structured var gene in chondroitin sulphate A-adhering Plasmodium falciparum involved in pregnancy-associated malaria. Mol Microbiol. 2003 Jul;49(1):179-91.
8. Avril M, Hathaway MJ, Srivastava A, Dechavanne S, Hommel M, Beeson JG, et al. Antibodies to a full-length VAR2CSA immunogen are broadly strain-transcendent but do not cross-inhibit different placental-type parasite isolates. PLoS ONE. 2011;6(2):e16622.
9. Fried M, Nosten F, Brockman A, Brabin BJ, Duffy PE. Maternal antibodies block malaria. Nature. 1998 Oct 29;395(6705):851-2
10. Bockhorst J, Lu F, Janes JH, Keebler J, Gamain B, Awadalla P, et al. Structural polymorphism and diversifying selection on the pregnancy malaria vaccine candidate VAR2CSA. Mol Biochem Parasitol. 2007 Oct;155(2):103-12
11. Rask TS, Hansen DA, Theander TG, Gorm Pedersen A, Lavstsen T. Plasmodium falciparum erythrocyte membrane protein 1 diversity in seven genomes--divide and conquer. PLoS Comput Biol. 2010;6(9).
12. Srivastava A, Gangnard S, Dechavanne S, Amirat F, Lewit Bentley A, Bentley GA, et al. var2CSA minimal CSA binding region is located within the N-terminal region. PLoS One. 2011;6(5):e20270.
13. Srivastava A, Gangnard S, Round A, Dechavanne S, Juillerat A, Raynal B, et al. Full-length extracellular region of the var2CSA variant of PfEMP1 is required for specific, high-affinity binding to CSA. Proc Natl Acad Sci U S A. 2010 Mar 16;107(11):4884-9.
14. Dahlback M, Jorgensen LM, Nielsen MA, Clausen TM, Ditlev SB, Resende M, et al. The chondroitin sulfate A-binding site of the VAR2CSA protein involves multiple N-terminal domains. J Biol Chem. 2011 May 6;286(18):15908-17.
15. Bordbar B, Gnidehou S, Ndam NT, Doritchamou J, Moussiliou A, Quiviger M, et al. Electroporation-mediated genetic vaccination for antigen mapping: Application to Plasmodium falciparum VAR2CSA protein. Bioelectrochemistry. 2011 Dec 30
16. Bigey P, Gnidehou S, Doritchamou J, Quiviger M, Viwami F, Couturier A, et al. The NTS-DBL2X region of VAR2CSA induces cross-reactive antibodies that inhibit adhesion of several Plasmodium falciparum isolates to chondroitin sulfate A. J Infect Dis. 2011 Oct 1;204(7):1125-33.