Novel monoclonal antibodies targeting distinct sites on placental binding P. falciparum antigen VAR2CSA synergistically enhance parasite phagocytosis

Abstract

Placental malaria, due to the sequestration of Plasmodium falciparum-infected erythrocytes (IE), causes adverse pregnancy outcomes. The sequestration is mediated by VAR2CSA, a protein that binds to placental chondroitin sulfate A (CSA). VAR2CSA antibodies protect against adverse pregnancy outcomes; however, no licensed VAR2CSA-based vaccine or therapeutic exists to date. We identified and expressed VAR2CSA-specific IgG1 monoclonal antibodies (mAbs) using B cells of malaria-exposed Papua New Guinean women. VAR2CSA mAbs were characterised by their ability to recognise eight heterologous CSA-binding P. falciparum strains, to neutralise CSA binding and/or to induce phagocytosis of IEs by THP-1 monocytes. We identified 16 mAbs, all of which targeted just two of the six domains of VAR2CSA, DBL3X and DBL5{varepsilon}. Cross-reactivity varied between mAbs, but was highest among mAbs to DBL5{varepsilon}, with four of eight of these mAbs binding to all eight strains. Although individual mAbs did not promote phagocytosis, combinations of mAbs recognising distinct epitopes either on the same domain or over different domains did. None of the mAbs inhibited IEs from binding to CSA. Our findings suggest that a combination of mAbs recognising more than one epitope would be needed for a therapeutic aiming to promote parasite clearance by phagocytosis; that DBL5{varepsilon} could be considered for a VAR2CSA vaccine that aims to elicit cross-reactive antibodies that promote phagocytosis; and that identification of binding-inhibitory mAbs requires thoughtful B-cell bait design.