Immunology Select

This issue's Immunology Select highlights recent studies that provide insight into the timing and regulation of both innate and adaptive immune responses. Recent work in the fruit fly shows that enhanced stem cell proliferation in the intestine maintains tissue homeostasis in the face of bacterial infection. Other recent findings include a new factor involved in regulatory T cell function, a role for RNA interference in systemic antiviral immunity, an examination of the course of infection of highly pathogenic influenza viruses, and a clever means by which a bacterial pathogen masquerades as host tissue. Although the intestine is a frequent route for the entry of pathogenic microorganisms, the changes in the gut after infection and how gut homeostasis is maintained are incompletely understood. Buchon et al. (2009) recently reported a comprehensive analysis of the changes in gene expression that occur in the Drosophila gut in response to infection by the bacterium Erwinia carotovora. They demonstrate that bacterial infections trigger expression of a combination of immune, stress, and developmental signaling pathways. In the immune system, they show that these responses are not monolithic: signaling through the Imd and Jak/Stat pathways is increased, whereas signaling through the Toll pathway is not. Subsequent analysis showed that both the Imd and Jak/Stat signaling pathways appear to have a particularly important role in the production of antimicrobial peptides. Infection by E. carotovora also leads to an increase in stem cell proliferation and production of epithelial cells. These changes are likely to be mediated by altered expression of components of growth factor and morphogen signaling pathways, including elements of the Hedgehog, Notch, and epidermal growth factor pathways. The authors suggest that these pathways are upregulated in order to maintain intestinal homeostasis, countering the cell death that occurs as a secondary consequence of the reactive oxygen species released during the immune response against the bacterial invaders. These findings provide the basis for numerous future studies that will further explore the timing and connectivity of these interwoven pathways. Regulatory T (Treg) cells control the duration and intensity of immune responses by constraining the activity of effector T cells. Zheng et al. (2009) now identify a transcription factor that confers upon Treg cells the ability to control the responses of a specific type of effector T cell, known as a TH2 cell. They build on their prior discovery that Foxp3, a pivotal regulator of the Treg cell lineage, controls expression …