The innate immune system is the first line of defense against infectious agents. It is essential for protection against pathogens and stimulation of long-term adaptive immune responses. Therefore, deciphering the mechanisms of the innate immune system is crucial for understanding the integrated systems of host defense against microbial infections, which is conserved from insects to humans.

Despite lacking a conventional adaptive immune system, insects can mount a robust immune response against a wide array of microbial pathogens. These innate immune mechanisms have been widely studied in Drosophila melanogaster, because of the model system’s powerful genetic, genomic and molecular tools. The Drosophila immunity relies on cellular and humoral innate immune responses to fight pathogens. The hallmark of the Drosophilahumoral immune response is the rapid induction of antimicrobial peptide genes in the fat body, the homolog of the mammalian liver. Expression of these antimicrobial peptide genes is controlled by two distinct immune signaling pathways, the Toll pathway and the IMD (immune deficiency) pathway.

The Toll pathway is activated by fungal and Gram-positive bacterial infections, whereas the IMD pathway responds to Gram-negative bacteria. Both pathways culminate in activation of the Rel/NF-кB transcription factors DIF (Dorsal-related immunity factor), Dorsal and Relish, which in turn translocate to the nucleus to induce the antimicrobial peptide genes. DIF and Dorsal are activated by the Toll pathway and control induction of antimicrobial peptide genes such as Drosomycin. The NF-кB precursor Relish, which is composed of an N-terminal Rel homology domain and a C-terminal IкB-like domain, is activated by the IMD pathway and initiates transcription of antimicrobial peptide genes such as Diptericin. Although many components of the Drosophila immune signaling pathways have been identified, the detailed mechanisms of signal trans