In Drosophila, the Toll pathway plays an important role in the immune defense against Gram-positive bacteria and fungi. Molecular determinants coming from those pathogens are directly detected by pattern recognition receptors (PPRs) circulating in the hemolymph. It has been proposed that several serine protease cascades are activated by PRRs, leading to the activation of a cleaved form of the cytokine-like molecule Spätzle, the ligand of the Toll receptor. The results obtained during my master project demonstrate an essential role for ModSP, a modular serine protease acting in the activation of the Toll pathway upon Gram-positive and fungal infections. We demonstrate that ModSP integrates signals coming from GNBP3 and PGRP-SA recognition molecules and that ModSP sends this signal to Grass, a serine protease already known to activate SPE and thereby Spätzle. Further biochemical experiments show the interaction between ModSP and GNBP1 demonstrating an apical role of this serine protease in the proteolytic cascades leading to Toll pathway activation. We also find that ModSP is expressed in specific vesicles released from the fat bodies into the hemolymph. Also, preliminary studies suggest that ModSP does not participate in the melanization reaction, a secondary but important insect immune mechanism. Biochemical analysis done by some collaborators indicates that ModSP does not cleave Grass and that ModSP exhibits a high level of auto-proteolysis when this molecule is expressed. Our data reveal a conserved role of modular serine protease in the regulation of immune proteolytic cascade in insects