Anopheles gambiae, the major vector for the protozoan malaria parasite Plasmodium falciparum, mounts powerful antiparasitic responses that cause marked parasite loss during midgut invasion. Here, we showed that these antiparasitic defenses were composed of pre- and postinvasion phases and that the preinvasion phase was predominantly regulated by Rel1 and Rel2 members of the NF-kappaB transcription factors. Concurrent silencing of Rel1 and Rel2 decreased the basal expression of the major antiparasitic genes TEP1 and LRIM1 and abolished resistance of Anopheles to the rodent malaria parasite P. berghei. Conversely, depletion of a negative regulator of Rel1, Cactus, prior to infection, enhanced the basal expression of TEP1 and of other immune factors and completely prevented parasite development. Our findings uncover the crucial role of the preinvasion defense in the elimination of parasites, which is at least in part based on circulating blood molecules.

Plasmodium development within Anopheles mosquitoes is a vulnerable step in the parasite transmission cycle, and targeting this step represents a promising strategy for malaria control. The thioester-containing complement-like protein TEP1 and two leucine-rich repeat (LRR) proteins, LRIM1 and APL1, have been identified as major mosquito factors that regulate parasite loads. Here, we show that LRIM1 and APL1 are required for binding of TEP1 to parasites. RNAi silencing of the LRR-encoding genes results in deposition of TEP1 on Anopheles tissues, thereby depleting TEP1 from circulation in the hemolymph and impeding its binding to Plasmodium. LRIM1 and APL1 not only stabilize circulating TEP1, they also stabilize each other prior to their interaction with TEP1. Our results indicate that three major antiparasitic factors in mosquitoes jointly function as a complement-like system in parasite killing, and they reveal a role for LRR proteins as complement control factors.