The cytokine-induced activation cascade of NF-kappaB in mammals and the activation of the morphogen dorsal in Drosophila embryos show striking structural and functional similarities (Toll/IL-1, Cactus/I-kappaB, and dorsal/NF-kappaB). Here we demonstrate that these parallels extend to the immune response of Drosophila. In particular, the intracellular components of the dorsoventral signaling pathway (except for dorsal) and the extracellular Toll ligand, spätzle, control expression of the antifungal peptide gene drosomycin in adults. We also show that mutations in the Toll signaling pathway dramatically reduce survival after fungal infection. Antibacterial genes are induced either by a distinct pathway involving the immune deficiency gene (imd) or by combined activation of both imd and dorsoventral pathways.

These studies show that miR-122, a 22-nucleotide microRNA, is derived from a liver-specific noncoding polyadenylated RNA transcribed from the gene hcr. The exact sequence of miR-122 as well as the adjacent secondary structure within the hcr mRNA are conserved from mammalian species back to fish. Levels of miR-122 in the mouse liver increase to half maximal values around day 17 of embryogenesis, and reach near maximal levels of 50,000 copies per average cell before birth. Lewis et al. (2003) predicted the cationic amino acid transporter (CAT-1 or SLC7A1) as a miR-122 target. CAT-1 protein and its mRNA are expressed in all mammalian tissues but with lower levels in adult liver. Furthermore, during mouse liver development CAT-1 mRNA decreases in an almost inverse correlation with miR-122. Eight potential miR-122 target sites were predicted within the human CAT-1 mRNA, with six in the 3'-untranslated region. Using a reporter construct it was found that just three of the predicted sites, linked in a 400-nucleotide sequence from human CAT-1, acted with synergy and were sufficient to strongly inhibit protein synthesis and reduce mRNA levels. In summary, these studies followed the accumulation during development of miR-122 from its mRNA precursor, hcr, through to identification of what may be a specific mRNA target, CAT-1.

Development of immature T-cell precursors (thymocytes) to either the CD4 helper or CD8 killer T-cell lineages correlates precisely with their T-cell receptor specificity for major histocompatibility complex class II or class I molecules, respectively, indicating that the process is carefully regulated. Although intensively studied owing to its importance in determining the composition of the mature T-cell compartment and as a general model of binary lineage decisions, the underlying molecular pathways remain obscure. We have previously reported a spontaneous mouse mutant (HD (helper deficient) mice) in which lineage commitment is specifically perturbed without affecting positive selection. Here we show that a point mutation in the zinc finger transcription factor Th-POK (T-helper-inducing POZ/Krü ppel-like factor) is responsible for redirection of class-II-restricted thymocytes to the CD8 lineage in HD mice. Furthermore, we demonstrate that constitutive expression of this factor during thymic development leads to redirection of class-I-restricted thymocytes to the CD4 lineage, indicating that Th-POK is a master regulator of lineage commitment.Developing ab T cells progress through three major stages in the thymus, defined by differential expression of the CD4 and CD8 coreceptor molecules; that is, CD4 2 CD8 2 (double negative), CD4 þ CD8 þ (double positive) and CD4 þ CD8 2 or CD4 2 CD8 þ (single positive). The double-positive to single-positive transition depends on productive rearrangement of both a-and b-subunits of the T-cell receptor (TCR) and engagement of the complete ab TCR by intrathymic ligands (positive selection). Simultaneously, thymocytes diverge into the functionally distinct T-helper and T-killer lineages, defined by expression of CD4 and CD8, respectively. Mature T cells show an almost perfect correlation between CD4 or CD8 expression and their TCR specificity towards class II or class I major histocompatibility complex (MHC) molecules, respectively. Alternative instructive and stochastic/selective models have been proposed to explain this marked correlation (for recent reviews see refs 1, 2). Current thinking favours a quantitative version of the instructive model, whereby lineage choice is determined by the relative strength or duration of TCR engagement 3-9 ; however, the intracellular pathways that are involved remain unknown.Progress in the field has been hindered because lineage commitment is so intimately tied to the process of positive selection that it is difficult to study in isolation. Hence, no specific pathways have been identified that are required for lineage commitment but not positive selection. Recently, we identified a spontaneous recessive mutation in mice, the HD mutation, which appeared to identify a genetic locus specifically required for lineage commitment 10 . Notably, this mutation caused redirection of all class-II-restricted thymocytes to the CD8 lineage 11 . The existence of such a mutation demonstrated a mechanistic distinction between the pathways governing lineage ...

In this paper we report a recessive mutation, immune deficiency (imd), that impairs the inducibility of all genes encoding antibacterial peptides during the immune response of Drosophila. When challenged with bacteria, flies carrying this mutation show a lower survival rate than wild-type flies. We also report that, in contrast to the antibacterial peptides, the antifungal peptide drosomycin remains inducible in a homozygous imd mutant background. These results point to the existence of two different pathways leading to the expression of two types of target genes, encoding either the antibacterial peptides or the antifungal peptide drosomycin.The powerful innate defense of higher insects involves proteolytic cascades (coagulation and phenoloxidase cascades), phagocytosis and encapsulation of invading microorganisms, and the synthesis by the fat body of a battery of potent antimicrobial peptides (reviewed in refs. 1 and 2). In Drosophila, several genes encoding inducible antibacterial peptides [cecropins (3, 4); diptericin (5); defensin (6); drosocin (7); M. Charlet, personal communication] and one inducible antifungal peptide [drosomycin (8); L.M. and J.-M.R., unpublished results] have been cloned. Understanding the mechanism of the coordinate control of their expression after immune challenge (e.g., septic injury) is a major goal in the field. Significant similarities exist between the control of antimicrobial peptide gene expression in insects and that of acute phase response genes in mammals (reviewed in refs. 1 and 2). This is illustrated by the involvement of common cis-regulatory motifs in the promoters of most of the insect and mammalian immune genes [e.g., NF-KB and NF-IL6 response elements, interferon consensus regulatory sequences (9-11)]. Furthermore, members of the Rel/NF-KB family play a crucial role in the transactivation of mammalian acute phase response genes; similarly, Rel proteins have been recently implicated in the control of the immune genes in Drosophila (12, 13) as the genes encoding the two Rel proteins dorsal (dl) and Dif (dorsal-related immune factor) are up-regulated in the fat body upon immune challenge and both proteins are translocated in the nucleus (refs. 12 and 13; B.L. and E.N., unpublished results). The precise roles of these proteins in the immune response of Drosophila are not yet established (discussed in refs. 14 and 15).While analyzing the expression of antibacterial genes in a mutant of the phenoloxidase cascade, we have found, by serendipity, a recessive mutation, immune deficiency (imd), that impairs the inducibility of the antibacterial peptides described so far in Drosophila. When challenged with bacteria, flies carrying this mutation show a lower survival rate than wild-type flies. We also report that, in contrast to the antibacterial peptides, the antifungal peptide drosomycin remains inducible in a homozygous imd mutant background. These results point to the existence of two different pathways leading either to the expression of the genes encoding antibact...