Mask modulates Toll signaling by regulating the levels of Cactus in <i>Drosophila</i>

Innate immunity serves as the first line of defense against pathogens, requiring precise regulation to maintain immune homeostasis. The Toll signaling pathway is a central regulator of immune responses in both Drosophila and mammals, with its activation culminating in the nuclear translocation of NF-κB factors Dorsal and Dorsal-related immunity factor (Dif), thereby inducing antimicrobial peptide (AMP) expression. Here, we identify Multiple ankyrin repeats single KH domain (Mask) as a novel regulator of Toll signaling in Drosophila. Mask is known to function in the Hippo and JAK/STAT pathways, but its role in immune regulation remains largely unexplored. Our study demonstrates that Mask overexpression induces melanotic mass formation and enhances lamellocyte production and crystal cell accumulation, hallmark features of an activated immune response. Mechanistically, we show that Mask influences nuclear accumulation of Dorsal by modulating Cactus protein levels, leading to upregulated AMP expression. Additionally, we dissect the functional contributions of different Mask domains in Toll-mediated immune activation. Our findings suggest Mask as a crucial component in Toll pathway regulation, providing new insights into immune signaling mechanisms.