Effect of infection by Plasmodium falciparum on the melanization immune response of Anopheles gambiae.

Melanization is an immune response of mosquitoes that could potentially limit Plasmodium development. That mosquitoes rarely melanize Plasmodium falciparum in natural populations might result from immuno-suppression by the parasite, as has been observed in Aedes aegypti mosquitoes infected by Plasmodium gallinaceum. We tested this possibility in Anopheles gambiae mosquitoes infected by P. falciparum by comparing the ability to melanize a Sephadex bead of infected mosquitoes, of mosquitoes that had fed on infectious blood without becoming infected, and of control mosquitoes fed on uninfected blood. Rather than being immuno-suppressed, infected mosquitoes tended to have a stronger melanization response than mosquitoes in which the infection failed and than control mosquitoes, possibly because of immune activation after previous exposure to invading parasites. This finding suggests that P. falciparum relies on immune evasion rather than immuno-suppression to avoid being melanized and confirms that natural malaria transmission systems differ from laboratory models of mosquito-Plasmodium interactions.