A single-chain antibody fragment specific for the Plasmodium berghei ookinete protein Pbs21 confers transmission blockade in the mosquito midgut

Mouse monoclonal antibody 13.1 (mAb 13.1) directed against Pbs21, a 21-kDa sexual-stage surface protein of Plasmodium berghei, is known to inhibit ooc yst development from gametocytes and ookinetes in the mosquito midgut. To e xamine the properties and potential uses of a single-chain antibody fragmen t (scFv) for blocking transmission of malaria parasites to mosquitoes, we h ave cloned and sequenced the genes encoding variable regions of the immunog lobulin heavy and light chains (V-H and V-L) of mAb 13.1. The V-H and V-L g enes were assembled as an scFv gene, and expressed in a baculovirus express ion system. Following purification of 13.1 scFv, Western blotting and inhib ition ELISA assays confirmed that 13.1 scFv retained the binding specificit y of the parent mAb 13.1 for Pbs21. Furthermore, 13.1 scFv bound to the sur face of P. berghei ookinetes, and blocked oocyst development in the mosquit o midgut by at least 93%, as assessed by oocyst counts in mosquitoes. We su ggest that the 13.1 scFv gene could be useful not only in studying the mech anism of transmission blockade, but also in generating, by mosquito germlin e transformation, a model system to evaluate the production of mosquitoes r efractory to malaria. (C) 1999 Elsevier Science B.V. All rights reserved.