Serine Protease Lectin-2 (SPL2) is a negative modulator of hemolymph melanization and involved in modulation of melanization response to malaria infection in Aedes aegypti.

Mosquitoes have an efficient immune system protecting themselves from pathogens. Despite the presence of powerful immune mechanisms able of controlling proliferation of the malaria parasite, mosquitoes are susceptible to malaria infection in variable degrees depending on mosquito/Plasmodium species combination and genetic variability. One of immune mechanisms involved in anti-Plasmodium resistance is melanization. The key enzymes of melanization, hemolymph prophenoloxidases (PPOs), are proteolytically activated at 24 hours after a blood-meal in the hemolymph of the mosquito Aedes aegypti. This PPO activation is diminished if mosquito takes an infectious blood-meal containing Plasmodium gallinaceum, suggesting the presence of a modulation mechanism of the immune melanization in the presence of the parasite. We have characterized a compound gene of serine protease and lectin, Serine Protease Lectin-2 (SPL2), as an acute immune-responsible gene regulated by IMD pathway. SPL2 gene expression and hemolymph PPO activation were inversely correlated after a Gram (-) bacterial infection. Furthermore, RNAi gene targeting of SPL2 resulted in the constitutive PPO activation, suggesting that SPL2 is an immune modulator, which restricts non-specific, off-site activation of melanization during immune response, thereby ensuring its activation in a precise temporal and spatial manner. The SPL2 gene was specifically up-regulated by malaria parasite infection. In SPL2 RNAi-depleted mosquitoes, the parasite-mediated suppression of PPO activation was prevented, and these mosquitoes became refractory to the parasite. These results suggest the role of SPL2 in the parasite-mediated modulation of immune melanization.