Abstract

Silencing Cactus, a suppressor of Toll signaling, in Anopheles gambiae, eliminates Plasmodium ookinetes by enhancing local release of hemocyte-derived microvesicles that promote activation of the mosquito complement-like system. We report that Cactus silencing dramatically increases the proportion of megacytes, a new effector hemocyte subpopulation of large granulocytes, from 5 to 79% of circulating granulocytes. Transcriptomic and morphological analysis, as well as cell counts, in situ hybridization and expression of cell-specific markers, indicate that Cactus silencing triggers granulocyte differentiation into megacytes, a process mediated by the Rel1 transcription factor of the Toll pathway. Megacytes are very plastic cells that can extend long filopodia, tend to form clusters in vivo, and are massively recruited to the basal midgut surface in response to bacterial feeding and Plasmodium infection. We show that hemocyte differentiation to the megacyte lineage greatly enhances mosquito immunity against Plasmodium.