Phagocytic hemocytes, independent of pericardial cells, modulate cellular immune responses on the dorsal vessel of mosquitoes, including the infection-induced reduction of the heart rate

Infection induces the aggregation of hemocytes on the dorsal vessel of mosquitoes. These hemocytes, called periostial hemocytes, phagocytose pathogens and produce immune factors on the abdominal portion of the dorsal vessel, called the heart. One of these immune factors, nitric oxide, is a pleiotropic free radical that is an antimicrobial and a heartbeat reducer. But nitric oxide is not just produced by hemocytes. It is also synthesized by pericardial cells that flank the heart, and other tissues. To determine whether it is the periostial hemocytes or the pericardial cells that modulate the heart following infection, we chemically ablated the hemocytes using clodronate liposomes and measured immune responses and heart physiology. We demonstrate that clodronate liposomes ablate the sessile hemocytes, including the periostial hemocytes, while leaving the pericardial cells and heart integrity unaffected. Moreover, ablating hemocytes abolishes the phagocytosis of bacteria, alters the deposition of melanized bacteria, and decreases nitric oxide synthase activity on the heart. Importantly, hemocyte ablation eliminates the infection induced reduction of the heart rate, mainly by modifying the anterograde heart rate. Therefore, periostial hemocytes drive immune responses on the heart and infection-induced changes to circulatory physiology.