The Toll/NF-κB signaling pathway is required for epidermal wound repair in Drosophila.

The Toll/NF-κB pathway, first identified in studies of dorsal-ventral polarity in the early Drosophila embryo, is well known for its role in the innate immune response. Here, we reveal that the Toll/NF-κB pathway is essential for wound closure in late Drosophila embryos. Toll mutants and Dif dorsal (NF-κB) double mutants are unable to repair epidermal gaps. Dorsal is activated on wounding, and Dif and Dorsal are required for the sustained down-regulation of E-cadherin, an obligatory component of the adherens junctions (AJs), at the wound edge. This remodeling of the AJs promotes the assembly of an actin-myosin cable at the wound margin; contraction of the actin cable, in turn, closes the wound. In the absence of Toll or Dif and dorsal (dl), both E-cadherin down-regulation and actin-cable formation fail, thus resulting in open epidermal gaps. Given the conservation of the Toll/NF-κB pathway in mammals and the epithelial expression of many components of the pathway, this function in wound healing is likely to be conserved in vertebrates.

Drosophila Rel proteins are central regulators of a robust, multi-organ immune network.

Survival of all animals depends on effective protection against infection. In Drosophila, opportunistic infection kills larvae if they lack the Rel/NF-kappaB proteins Dorsal and Dif. We have used tissue-specific expression of Dif and Dorsal to reveal that these Rel proteins act in three different tissues to defend larvae from infection. Dif and Dorsal act in circulating blood cells, where they are required autonomously to promote blood-cell survival and phagocytosis of microorganisms. We show that a major transcriptional target of Dorsal and Dif in blood cells is Drosophila IAP1, a gene protecting these cells from death. We find that in addition to their autonomous role in blood-cell survival, Dif and Dorsal also act in the fat body to produce factors that promote blood-cell viability. These Rel proteins act in the epidermis to prevent infection by maintaining a barrier to microbial entry. Dorsal or Dif in any one of the three tissues is sufficient to defend the animal from opportunistic infection. Thus Drosophila has a multi-pronged system of defense and each branch of this network requires Rel proteins. Based on similarities between Drosophila and mammals, we propose that a Rel-dependent network is an ancient and robust framework of animal immune systems.

Rel/NF-kappaB double mutants reveal that cellular immunity is central to Drosophila host defense.

Studies on Drosophila immunity have focused on the humoral response, whereas less is known about the Drosophila cellular immunity. Here we show that mutants that lack the Drosophila Rel/NF-kappaB proteins Dorsal and Dif have very few blood cells, are constitutively infected by opportunistic microbes, and die from infection as larvae. When the double mutants are grown in microbe-free conditions, the animals are rescued from chronic infection and many survive to adult stages. Thus, Dif and Dorsal are required for survival because they protect the animal from infection by microbes from the environment. Specific expression of Dif or dorsal in the blood cell lineage is sufficient to restore blood cell number, clear microbes, and allow survival to the adult stage. These findings demonstrate that the cellular immune response is essential for the ability of Drosophila to survive in their standard laboratory environment, and that Dif and Dorsal control crucial aspects of the cellular immune response, including blood cell survival and the ability to fight off microbial infection.