Genes that fight infection: what the Drosophila genome says about animal immunity

Genome-wide analysis of the Drosophila immune response by using oligonucleotide microarrays

To identify new Drosophila genes involved in the immune response, we monitored the gene expression profile of adult flies in response to microbial infection by using high-density oligonucleotide microarrays encompassing nearly the full Drosophila genome. Of 13,197 genes tested, we have characterized 230 induced and 170 repressed by microbial infection, most of which had not previously been associated with the immune response. Many of these genes can be assigned to specific aspects of the immune response, including recognition, phagocytosis, coagulation, melanization, activation of NF-kappaB transcription factors, synthesis of antimicrobial peptides, production of reactive oxygen species, and regulation of iron metabolism. Additionally, we found a large number of genes with unknown function that may be involved in control and execution of the immune response. Determining the function of these genes represents an important challenge for improving our knowledge of innate immunity. Complete results may be found at http://www.fruitfly.org/expression/immunity/.

Drosophila immunity: genes on the third chromosome required for the response to bacterial infection

We have screened the third chromosome of Drosophila melanogaster for mutations that prevent the normal immune response. We identified mutant lines on the basis of their failure to induce transcription of an antibacterial peptide gene in response to infection or their failure to form melanized clots at the site of wounding. These mutations define 14 genes [immune response deficient (ird) genes] that have distinct roles in the immune response. We have identified the molecular basis of several ird phenotypes. Two genes, scribble and kurtz/modulo, affect the cellular organization of the fat body, the tissue responsible for antimicrobial peptide production. Two ird genes encode components of the signaling pathways that mediate responses to bacterial infection, a Drosophila gene encoding a homolog of I kappa B kinase (DmIkk beta) and Relish, a Rel-family transcription factor. These genetic studies should provide a basis for a comprehensive understanding of the genetic control of immune responses in Drosophila.

Mutations in the Drosophila dTAK1 gene reveal a conserved function for MAPKKKs in the control of rel/NF-kappaB-dependent innate immune responses

In mammals, TAK1, a MAPKKK kinase, is implicated in multiple signaling processes, including the regulation of NF-kappaB activity via the IL1-R/TLR pathways. TAK1 function has largely been studied in cultured cells, and its in vivo function is not fully understood. We have isolated null mutations in the Drosophila dTAK1 gene that encodes dTAK1, a homolog of TAK1. dTAK1 mutant flies are viable and fertile, but they do not produce antibacterial peptides and are highly susceptible to Gram-negative bacterial infection. This phenotype is similar to the phenotypes generated by mutations in components of the Drosophila Imd pathway. Our genetic studies also indicate that dTAK1 functions downstream of the Imd protein and upstream of the IKK complex in the Imd pathway that controls the Rel/NF-kappaB like transactivator Relish. In addition, our epistatic analysis places the caspase, Dredd, downstream of the IKK complex, which supports the idea that Relish is processed and activated by a caspase activity. Our genetic demonstration of dTAK1's role in the regulation of Drosophila antimicrobial peptide gene expression suggests an evolutionary conserved role for TAK1 in the activation of Rel/NF-kappaB-mediated host defense reactions.

Increased sexual activity reduces male immune function in Drosophila melanogaster

Despite the benefits of resistance, susceptibility to infectious disease is commonplace. Although specific susceptibility may be considered an inevitable consequence of the co-evolutionary arms race between parasite and host, a more general constraint may arise from the cost of an immune response. This "cost" hypothesis predicts a tradeoff between immune defense and other components of fitness. In particular, a tradeoff between immunity and sexually selected male behavior has been proposed. Here we provide experimental support for the direct phenotypic tradeoff between sexual activity and immunity by studying the antibacterial immune response in Drosophila melanogaster. Males exposed to more females showed a reduced ability to clear a bacterial infection, an effect that we experimentally link to changes in sexual activity. Our results suggest immunosuppression is an important cost of reproduction and that immune function and levels of disease susceptibility will be influenced by sexual selection.

Toll receptors in innate immunity

Innate immunity is the first-line host defense of multicellular organisms that rapidly operates to limit infection upon exposure to infectious agents. In addition, the cells and molecules operating during this early stage of the immune response in vertebrates have a decisive impact on the shaping of the subsequent adaptive response. Genetic studies initially performed in the fruitfly Drosophila and later in mice have revealed the importance of proteins of the Toll family in the innate immune response. We present here our current understanding of the role of this evolutionary ancient family of proteins that are thought to function as cytokine receptors (Toll in Drosophila) or pattern-recognition receptors (TLRs in mammals) and activate similar, albeit non-identical, signal-transduction pathways in flies and mammals.

Diptericin-like protein: an immune response gene regulated by the anti-bacterial gene induction pathway in Drosophila

Insects produce various anti-microbial peptides in response to injury and infection. In Drosophila, diptericin has previously been studied as an anti-bacterial immune response gene. Here, we report the cloning of the diptericin-like protein (dptlp) gene as a paralog of Drosophila diptericin. By comparison of their sequences, we found that the dptlp gene has all of the functional domains conserved in the diptericin gene and other anti-bacterial proteins. The dptlp gene was rapidly induced by bacterial infections and showed different time-dependent gene expression patterns from those of diptericin. Like diptericin, dptlp was specifically produced from the fat body, and its expression was strictly dependent on bacterial infections. In addition, the dptlp gene expression was almost completely abolished in the imd mutant, which implicates that its expression is regulated by the anti-bacterial arm of the Drosophila innate immune regulatory pathways. In support of this, we found GATA, interferon consensus responding element, and kappa B binding sites, which is known to be important for the proper expression of anti-bacterial genes, in the proximal promoter region of the dptlp gene. Taken together, our findings support that dptlp is a novel anti-bacterial peptide whose expression is regulated by the anti-bacterial immune response mechanism.

Drosophila immunity: two paths to NF-kappaB

Recent studies of Drosophila immune responses have defined the immune deficiency (IMD) signaling pathway that mediates defense against Gram-negative bacterial infection. Like the Toll pathway, the IMD pathway regulates antimicrobial peptide gene expression via a Rel/nuclear factor (NF)-kappaB-like transcription factor. However, the two pathways do not appear to share any intermediate components. Maintaining distinct immune response pathways might be one mechanism by which flies mount adapted immune responses.

The evolution and genetics of innate immunity

The immune system provides protection from a wide range of pathogens. One component of immunity, the phylogenetically ancient innate immune response, fights infections from the moment of first contact and is the fundamental defensive weapon of multicellular organisms. The Toll family of receptors has a crucial role in immune defence. Studies in fruitflies and in mammals reveal that the defensive strategies of invertebrates and vertebrates are highly conserved at the molecular level, which raises the exciting prospects of an increased understanding of innate immunity.

Common mechanisms for pathogens of plants and animals

The vast evolutionary gulf between plants and animals--in terms of structure, composition, and many environmental factors--would seem to preclude the possibility that these organisms could act as receptive hosts to the same microorganism. However, some pathogens are capable of establishing themselves and thriving in members of both the plant and animal kingdoms. The identification of functionally conserved virulence mechanisms required to infect hosts of divergent evolutionary origins demonstrates the remarkable conservation in some of the underlying virulence mechanisms of pathogenesis and is changing researchers' thinking about the evolution of microbial pathogenesis.

Expression and evolution of the Drosophila attacin/diptericin gene family

We describe the genes for three new glycine-rich antimicrobial peptides in Drosophila, two attacins (AttC and AttD) and one diptericin (DptB). Their structures support the proposal that these glycine-rich antimicrobial peptides evolved from a common ancestor and are probably also related to proline-rich peptides such as drosocin. AttC is similar to the nearby AttA and AttB genes. AttD is more divergent and located on a different chromosome. Intriguingly, AttD may encode an intracellular attacin. DptB is linked in tandem to the closely related Diptericin. However, the DptB gene product contains a furin-like cleavage site and may be processed in an attacin-like fashion. All attacin and diptericin genes are induced after bacterial challenge. This induction is reduced in imd mutants, and unexpectedly also in Tl(-) mutants. The 18w mutation particularly affects the induction of AttC, which may be a useful marker for 18w signaling.

Tissue-specific inducible expression of antimicrobial peptide genes in Drosophila surface epithelia

The production of antimicrobial peptides is an important aspect of host defense in multicellular organisms. In Drosophila, seven antimicrobial peptides with different spectra of activities are synthesized by the fat body during the immune response and secreted into the hemolymph. Using GFP reporter transgenes, we show here that all seven Drosophila antimicrobial peptides can be induced in surface epithelia in a tissue-specific manner. The imd gene plays a critical role in the activation of this local response to infection. In particular, drosomycin expression, which is regulated by the Toll pathway during the systemic response, is regulated by imd in the respiratory tract, thus demonstrating the existence of distinct regulatory mechanisms for local and systemic induction of antimicrobial peptide genes in Drosophila.

The Drosophila caspase Dredd is required to resist gram-negative bacterial infection

The Drosophila innate immune system discriminates between pathogens and responds by inducing the expression of specific antimicrobial peptide-encoding genes through distinct signaling cascades. Fungal infection activates NF-kappaB-like transcription factors via the Toll pathway, which also regulates innate immune responses in mammals. The pathways that mediate antibacterial defenses, however, are less defined. We have isolated loss-of-function mutations in the caspase encoding gene dredd, which block the expression of all genes that code for peptides with antibacterial activity. These mutations also render flies highly susceptible to infection by gram-negative bacteria. Our results demonstrate that Dredd regulates antibacterial peptide gene expression, and we propose that Dredd, Immune Deficiency and the P105-like rel protein Relish define a pathway that is required to resist gram-negative bacterial infections.