Cytoadherence of knobless Plasmodium falciparum-infected erythrocytes and its inhibition by a human monoclonal antibody

Red blood cells infected with mature stages of the malaria parasite Plasmodium falciparum bind to the endothelial lining of capillaries and venules. This sequestration is important for the survival of the parasite but may have severe consequences for the host. For example, it is involved in the causation of cerebral malaria which carries 25% mortality. Knob-like protrusions present on the surface of infected erythrocytes have been considered necessary but not sufficient for this cytoadherence. Here we describe the adhesion to endothelial cells of infected erythrocytes which do not have knobs. A human monoclonal antibody (33G2) which was specific for an epitope containing regularly spaced dimers of glutamic acid present in the repeated amino-acid sequences of some defined P. falciparum antigens was found to inhibit cyto-adherence and may therefore be an important reagent for elucidating the molecular basis of parasite sequestration.     

A recombinant dromedary antibody fragment (VHH or nanobody) directed against human Duffy antigen receptor for chemokines

Fy blood group antigens are carried by the Duffy antigen receptor for chemokines (DARC), a red cells receptor for Plasmodium vivax broadly implicated in human health and diseases. Recombinant VHHs, or nanobodies, the smallest intact antigen binding fragment derivative from the heavy chain-only antibodies present in camelids, were prepared from a dromedary immunized against DARC N-terminal extracellular domain and selected for DARC binding. A described VHH, CA52, does recognize native DARC on cells. It inhibits P. vivax invasion of erythrocytes and displaces interleukin-8 bound to DARC. The targeted epitope overlaps the well-defined DARC Fy6 epitope. K _D of CA52?CDARC equilibrium is sub-nanomolar, hence ideal to develop diagnostic or therapeutic compounds. Immunocapture by immobilized CA52 yielded highly purified DARC from engineered K562 cells. This first report on a VHH with specificity for a red blood cell protein exemplifies VHHs?? potentialities to target, to purify, and to modulate the function of cellular markers.