Protection from within

Symbiotic bacteria assist in maintaining homeostasis of the animal immune system. However, the molecular mechanisms that underlie symbiont-mediated host immunity are largely unknown. Tsetse flies (Glossina spp.) house maternally transmitted symbionts that regulate the development and function of their host’s immune system. Herein we demonstrate that the obligate mutualist, Wigglesworthia, up-regulates expression of odorant binding protein six in the gut of intrauterine tsetse larvae. This process is necessary and sufficient to induce systemic expression of the hematopoietic RUNX transcription factor lozenge and the subsequent production of crystal cells, which actuate the melanotic immune response in adult tsetse. Larval Drosophila’s indigenous microbiota, which is acquired from the environment, regulates an orthologous hematopoietic pathway in their host. These findings provide insight into the molecular mechanisms that underlie enteric symbiont-stimulated systemic immune system development, and indicate that these processes are evolutionarily conserved despite the divergent nature of host-symbiont interactions in these model systems. DOI: http://dx.doi.org/10.7554/eLife.19535.001
eLife digest Bacteria live within all animals. While a small number of these microbes can cause disease, most promote the health and wellbeing of their host. Microbes that support their host and benefit from the close association are often referred to as symbionts. Animals can be negatively affected and even become diseased if their symbionts are disrupted. As a result, a more complete understanding of the molecular interactions between animal hosts and their beneficial microbes will lead to better treatments for a number of diseases. Tsetse flies are insects that harbor two bacterial symbionts, which are transferred from pregnant females to their larval offspring. If the offspring mature without these microbes, they fail to develop cells called hemocytes. These cells are normally found in the insect’s equivalent of blood – a fluid called hemolymph – and they comprise an important component of the insect’s immune system. Adult tsetse flies that lack hemocytes are susceptible to certain infections. These findings indicate that the bacterial symbionts induce the production of hemocytes in tsetse fly larvae via an unknown mechanism. Benoit et al. now reveal that the bacterial symbionts trigger tsetse flies to produce a small protein called “odorant binding protein 6”. This protein controls the generation of one specific type of hemocyte called crystal cells in developing larvae. Crystal cells are largely responsible for triggering the production of melanin, a protein involved in killing disease-causing microbes and inhibiting the loss of hemolymph from wound sites in the insect’s exoskeleton. Benoit et al. discovered that bacterial symbionts associated with the larvae of fruit flies also support the development of their host’s immune system. Although these symbionts are acquired from the external environment rather than from the insect’s parent, they too control the production of an odorant binding protein and crystal cells in their larval host. Collectively, these findings confirm that bacterial symbionts are critically important for the development of the immune systems of insects, and they show that this process has been conserved throughout evolution. Future studies are likely to focus on identifying which molecules from the symbionts stimulate their hosts to produce new hemolymph cells. Furthermore, identifying which tissues and cell types in the animal hosts are targets for these molecules will provide a more complete picture of the pathways that lead to the production of new hemolymph cells. DOI: http://dx.doi.org/10.7554/eLife.19535.002