The IMD and JNK immune signaling pathways regulate the aggregation of hemocytes on the heart of Anopheles gambiae mosquitoes

The immune and circulatory systems are functionally integrated in insects. In the African malaria mosquito, Anopheles gambiae, hemocytes (immune cells) reside at the locations of the body that experience the highest hemolymph (blood) flow, especially near the heart’s ostia (valves). These hemocytes – called periostial hemocytes – actively phagocytose and destroy pathogens that are about to enter the heart. Infection further induces hemocytes to aggregate on the heart, enhancing this heart-associated immunity. The mechanics of this process are well understood, but the genetic factors that drive periostial hemocyte aggregation remain mostly unknown. Here, we utilized RNA sequencing and found that multiple components of the IMD and JNK pathways are highly expressed in periostial hemocytes during infection. Silencing the transcription factor of the IMD pathway, Rel2, reduces infection-induced periostial hemocyte aggregation, whereas knocking down its negative regulator, Caspar, increases this response. Similarly, overexpressing the JNK pathway – via knockdown of its negative regulator Puckered – increases the number of periostial hemocytes and its associated immune responses on the heart. Together, this shows that both the IMD and JNK pathways positively regulate periostial hemocyte aggregation in A. gambiae, highlighting their involvement in the interaction between the immune and circulatory systems in mosquitoes.