Antibodies against thrombospondin-related anonymous protein do not inhibitPlasmodium sporozoite infectivity in vivo

Thrombospondin-related anonymous protein (TRAP), a candidate malaria vaccin e antigen, is required for Plasmodium sporozoite gliding motility and cell invasion. For the first time, the ability of antibodies against TRAP to inh ibit sporozoite infectivity in vivo is evaluated in detail. TRAP contains a n A-domain, a well-characterized adhesive motif found in integrins, We mode led here a three-dimensional structure of the TRAP A-domain of Plasmodium y oelii and located regions surrounding the MIDAS (metal ion-dependent adhesi on site), the presumed business end of the domain. Mice were immunized,vith constructs containing these A-domain regions but were not protected from s porozoite challenge. Furthermore, monoclonal and rabbit polyclonal antibodi es against the A-domain, the conserved N terminus, and the repeat region of TRAP had no effect on the gliding motility or sporozoite infectivity to mi ce. TRAP is located in micronemes, secretory organelles of apicomplexan par asites. Accordingly, the antibodies tested here stained cytoplasmic TRAP br ightly by immunofluorescence. However, very little TRAP could be detected o n the surface of sporozoites, In contrast, a dramatic relocalization of TR IP onto the parasite surface occurred when sporozoites were treated with ca lcium ionophore, This likely mimics the release of TRAP from micronemes whe n a sporozoite contacts its target cell in vivo. Contact with hepatoma cell s in culture also appeared to induce the release of TRAP onto the surface o f sporozoites, If large amounts of TRAP are released in close proximity to its cellular receptor(s), effective competitive inhibition by antibodies ma y be difficult to achieve.