The GATA factor Serpent is required for the onset of the humoral immune response in Drosophila embryos

Innate immunity in Drosophila is characterized by the inducible expression of antimicrobial peptides, We have investigated the development and regulat ion of immune responsiveness in Drosophila embryos after infection. Immune competence, as monitored by the induction of Cecropin A1-lacZ constructs, w as observed first in the embryonic yolk. This observation suggests that the yolk plays an important role in the humoral immune response of the develop ing embryo by synthesizing antimicrobial peptides, Around midembryogenesis, the response in the yolk was diminished. Simultaneously, Cecropin expressi on became inducible in a large number of cells in the epidermis, demonstrat ing that late-stage embryos can synthesize their own antibiotics in the epi dermis. This production likely serves to provide the hatching larva with an active antimicrobial barrier and protection against systemic infections. C ecropin expression in the yolk required the presence of a GATA site in the promoter as well as the involvement of the GATA-binding transcription facto r Serpent (dGATAb). In contrast neither the GATA site nor Serpent were nece ssary for Cecropin expression in the epidermis. Thus, the inducible immune responses in the yolk and in the epidermis can be uncoupled and call for di stinct sets of transcription factors, Our data suggest that Serpent is invo lved in the distinction between a systemic response in the yolk/fat body an d a local immune response in epithelial cells. In addition, the present stu dy shows that signal transduction pathways controlling innate and epithelia l defense reactions can be dissected genetically in Drosophila embryos.