The Influence of Sub-Unit Composition and Expression System on the Functional Antibody Response in the Development of a VAR2CSA Based Plasmodium falciparum Placental Malaria Vaccine

Abstract

The disease caused by Plasmodium falciparum (Pf) involves different clinical manifestations that, cumulatively, kill hundreds of thousands every year. Placental malaria (PM) is one such manifestation in which Pf infected erythrocytes (IE) bind to chondroitin sulphate A (CSA) through expression of VAR2CSA, a parasite-derived antigen. Protection against PM is mediated by antibodies that inhibit binding of IE in the placental intervillous space. VAR2CSA is a large antigen incompatible with large scale recombinant protein expression. Vaccines based on sub-units encompassing the functionally constrained receptor-binding domains may, theoretically, circumvent polymorphisms, reduce the risk of escape-mutants and induce cross-reactive antibodies. However, the sub-unit composition and small differences in the borders, may lead to exposure of novel immuno-dominant antibody epitopes that lead to non-functional antibodies, and furthermore influence the folding, stability and yield of expression. Candidate antigens from the pre-clinical development expressed in High-Five insect cells using the baculovirus expression vector system were transitioned into the Drosophila Schneider-2 cell (S2) expression-system compliant with clinical development. The functional capacity of antibodies against antigens expressed in High-Five cells or in S2 cells was equivalent. This enabled an extensive down-selection of S2 insect cell-expressed antigens primarily encompassing the minimal CSA-binding region of VAR2CSA. In general, we found differential potency of inhibitory antibodies against antigens with the same borders but of different var2csa sequences. Likewise, we found that subtle size differences in antigens of the same sequence gave varying levels of inhibitory antibodies. The study shows that induction of a functional response against recombinant subunits of the VAR2CSA antigen is unpredictable, demonstrating the need for large-scale screening in order to identify antigens that induce a broadly strain-transcending antibody response.

Non-conventional expression systems for the production of vaccine proteins and immunotherapeutic molecules

“…Additionally, 2 malaria vaccines, from the Jenner Institute and Copenhagen University, respectively, are soon expected to enter phase I. 88 Drosophila cells, even if still marginal can be considered as a viable or at least complementary option to traditional insect cell lines.…”

Antibody responses to the full-length VAR2CSA and its DBL domains in Cameroonian children and teenagers

“…In malaria endemic areas, pregnant women produce antibodies (Ab) to VAR2CSA over successive pregnancies [25] that inhibit the binding of IE to CSA in vitro [26, 27], reduce maternal anaemia [28], and improve pregnancy outcome [25, 29, 30]. VAR2CSA-based recombinant subunit vaccine candidates are currently under clinical evaluation [31, 32]. …”

Comparison of the specificity of antibodies to VAR2CSA in Cameroonian multigravidae with and without placental malaria: a retrospective case–control study

“…Over successive pregnancies, women produce Ab to VAR2CSA that inhibit the binding of IE to CSA in vitro [ 3 , 4 ], reduce maternal anaemia [ 5 ], lower placental parasitaemia at delivery [ 6 , 7 ], increase the length of gestation [ 8 ], and improve infant birth weight [ 8 ]. VAR2CSA is a primary vaccine candidate for PM and is currently under clinical investigation [ 9 ]. Since full-length VAR2CSA is 350 kDa, researchers are exploring various sequences of VAR2CSA Duffy-binding-like (DBL) domains to develop a vaccine, including the N-terminal ID1-ID2a sequence that contains the CSA-minimal binding site [ 10 ], DBL1-2 and DBL3x [ 11 ] and DBL1x-DBL3x [ 12 ] and C-terminal domains 4 (DBL4ε) and 5 (DBL5ε) [ 13 ].…”