Rapid phagocytic response and sessile phagocytic foci formation by mosquito hemocytes following infection

 

Mosquitoes counter infection with an innate immune response that relies heavily on the action of immune cells (hemocytes). In non-mosquito insects it has been shown that hemocytes exist both in circulation and adhered to tissues (sessile). Using a novel in vivo staining technique, intravital video imaging, and microdissection we conducted an extensive microscopic analysis of hemocyte behavior following immune stimulation in Anopheles gambiae. Qualitative whole-body studies showed that sessile hemocytes occur throughout the mosquito but tend to concentrate in specific foci following immune challenge. Correlative analyses revealed that the major regions of foci formation are near the cardiac ostia (periostial region), and while naïve mosquitoes maintain sessile hemocytes in these regions, the number of periostial hemocytes increase over a consistent time scale following immune activation. Various techniques confirmed that, following an infection, periostial hemocytes rapidly phagocytose and begin degrading pathogens. Periostial foci formation is induced by bacterial infection in a load dependent manner, and by Plasmodium sporozoite migration. Injections of inert microspheres, peptidoglycan or β (1, 3)-glucan also induce periostial hemocyte aggregation, demonstrating that phagocytosis is sufficient, but not necessary to trigger this process. Together, these data demonstrate a novel cellular immune response in mosquitoes.