A hemocyte-like cell line established from the malaria vector Anopheles gambiae expresses six prophenoloxidase genes

Cell lines from the malaria vector Anopheles gambiae have been established as a tool for the study of the mosquito innate immune system in vitro. Here, we describe the first continuous insect cell line that produces prophenoloxidase (PPO). This cell line (4a-3B) expresses constitutively six PPO genes, three of which are novel (PPO4, PPO5, and PPO6). The PPO genes show distinct temporal expression profiles in the intact mosquito, spanning stages from the embryo to the adult in an overlapping manner. Transient induction of larva-specific PPO genes in blood-fed adult females suggests that the developmental hormone 20-hydroxyecdysone may be involved in PPO gene regulation. Indeed, exposure of 4a-3B cells to 20-hydroxyecdysone in culture results in induction of those PPO genes that are mainly expressed in early developmental stages, and repression of PPO5, which is preferentially expressed at the adult stage. The cell line shows bacteria-induced immune transcripts that encode defensin and Gram-negative bacteria-binding protein, but no induction of PPO transcripts. This cell line most likely derives from a hemocyte lineage, and represents an appropriate in vitro model for the study of the humoral and cellular immune defenses of A gambiae.