Drosophila immunity research on the move

Drosophila has been established as useful model for infectious diseases because it allows large numbers of whole animals to be studied and provides powerful genetic tools and conservation with signaling and pathogenesis mechanisms in vertebrates. During the past twenty years, significant progress has been made on the characterization of innate immune responses against various pathogenic organisms in flies (Fig. 1). In this year's Drosophila Research Conference, which was held in San Diego (March 30-April 3) and sponsored by the Genetics Society of America, the immunity and pathogenesis session comprised seven platform presentations and 34 posters that highlighted the latest advances in Drosophila infection and immunity field. The presented work covered a wide range of studies from immune signaling pathways and the molecular basis of humoral and cellular immune mechanisms to the role of endosymbionts in fly immune function and effects of immune priming. Here, we give an overview of the presented work and we explain how these findings will open new avenues in Drosophila immunity research.     

Analysis of signal-dependent changes in the proteome of Drosophila blood cells during an immune response

Innate immunity is based on the recognition of cell-surface molecules of infecting agents. Microbial substances, such as peptidoglycan, lipopolysaccharide, and beta-1,3-glucans, produce functional responses in Drosophila hemocytes that contribute to innate immunity. We have used two-dimensional gel electrophoresis and MS to resolve lipopolysaccharide-induced changes in the protein profile of a Drosophila hemocytic cell line. We identified 24 intracellular proteins that were up- or down-regulated, or modified, in response to immune challenge. Several proteins with predicted immune functions, including lysosomal proteases, actin-binding/remodeling proteins, as well as proteins involved in cellular responses to oxidative stress, were affected by the immune assault. Intriguingly, a number of the proteins identified in this study have recently been implicated in phagocytosis in higher vertebrates. We suggest that phagocytosis is activated in Drosophila hemocytes by the presence of microbial substances, and that this activation constitutes an evolutionarily conserved arm of innate immunity. In addition, a number of proteins involved in calcium-regulated signaling, mRNA processing, and nuclear transport were affected, consistent with a possible role in reprogramming of gene expression. In conclusion, the present proteome analysis identified many proteins previously not linked to innate immunity, demonstrating that differential protein profiling of Drosophila hemocytes is a valuable tool for identification of new players in immune-related cellular processes.     

Intracellular recognition of pathogens and autophagy as an innate immune host defence

Pathogen recognition is the first and crucial step in innate immunity. Molecular families involved in the recognition of pathogens and activation of the innate immune responses in immunoreactive cells include the Toll-like receptor family in mammals and the peptidoglycan recognition protein (PGRP) family in Drosophila, which sense microorganisms in an extracellular or luminal compartment. Other emerging families are the intracellular recognition molecules for bacteria, such as nucleotide binding and oligomerization domain-like receptors in mammals and PGRP--LE in Drosophila, several of which have been shown to detect structures of bacterial peptidoglycan in the host cell cytosol. Exciting advances in recent studies on autophagy indicate that macroautophagy (referred to here as autophagy) is selectively induced by intracellular recognition molecules and has a crucial role in the elimination of intracellular pathogens, including bacteria, viruses and parasites. This review discusses recent studies related to intracellular recognition molecules and innate immune responses to intracellular pathogens, and highlights the role of autophagy in innate immunity.     

Establishment of ex vivo systems to identify compounds acting on innate immune responses and to determine their target molecules using transgenic Drosophila

Innate immunity is an evolutionarily conserved self-defense mechanism against microbial infections. In Drosophila, induction of antimicrobial peptides is a major immune response that is regulated by two distinct signaling pathways called the IMD pathway and the Toll pathway, similar to the tumor necrosis factor-alpha signaling and Toll-like receptor/interleukin-1 signaling pathways, respectively, in mammals. In mammals, innate immunity interacts with adaptive immunity and has a key role in the regulated immune response. Therefore, innate immunity is a pharmaceutical target for the development of immune regulators. Previously, based on the striking conservation between the mechanisms that regulate Drosophila immunity and human innate immunity, we established an ex vivo culture in which compounds acting on innate immunity can be evaluated using a reporter gene that reflects activation of the IMD pathway [Yajima et al. [Yajima, M., Takada, M., Takahashi, N., Kikuchi, H., Natori, S., Oshima, Y., Kurata, S., 2003. A newly established in vitro culture using transgenic Drosophila reveals functional coupling between the phospholipase A2-generated fatty acid cascade and lipopolysaccharide-dependent activation of the immune deficiency (imd) pathway in insect immunity. The Biochemical Journal 371(Pt 1), 205-210] Biochem J 371, 205-210]. Here, we combined the ex vivo culture with a reporter gene that reflects the heat shock response and demonstrated that the resulting systems are useful for screening compounds that act specifically on innate immunity, including mammalian innate immune responses. Identification of target molecules is essential for the development of more potent medicines with fewer side effects. In this study, we also established ex vivo systems capable of identifying target molecules of the identified compounds using targeted activation of the IMD pathway.     

Drosophila immunity research on the move

Drosophila has been established as useful model for infectious diseases because it allows large numbers of whole animals to be studied and provides powerful genetic tools and conservation with signaling and pathogenesis mechanisms in vertebrates.¹ During the past twenty years, significant progress has been made on the characterization of innate immune responses against various pathogenic organisms in flies (Fig. 1).^2,3,4 In this year’s Drosophila Research Conference, which was held in San Diego (March 30 – April 3) and sponsored by the Genetics Society of America, the immunity and pathogenesis session comprised seven platform presentations and 34 posters that highlighted the latest advances in Drosophila infection and immunity field. The presented work covered a wide range of studies from immune signaling pathways and the molecular basis of humoral and cellular immune mechanisms to the role of endosymbionts in fly immune function and effects of immune priming. Here, we give an overview of the presented work and we explain how these findings will open new avenues in Drosophila immunity research.     

Is innate enough? The innate immune response in Drosophila

In recent years, the innate immune system has emerged from the shadow of adaptive immune responses as a major area of research in its own right. One of the most significant model systems that has been used to investigate this phenomenon has been the fruit fly, Drosophila melanogaster. Exploration of the differential immune response presented by Drosophila led to the discovery of important signalling events and transduction pathways, which were thereafter shown to be specific for the type of infecting pathogen. These factors and pathways were subsequently found to have homologues in many other organisms, including those with adaptive immune responses. In light of the present status of studies in innate immunity, this review describes the current state of understanding of the Drosophila immune response.     

果蝇先天性免疫研究进展

果蝇是生命科学与人类疾病研究的重要模式生物,虽然不具有人类高度专一的获得性免疫,但也有对病原微生物感染作出快速有效反应的先天性免疫应答系统,主要包括体液免疫,细胞免疫和黑化反应。文章结合国外最新研究,详细介绍果蝇体液免疫中控制抗菌肽合成的Toll信号通路和Imd信号通路中涉及的蛋白及其相互作用,并对果蝇细胞免疫中的吞噬、包埋功能和黑化反应作简要阐述。研究表明,果蝇的Toll和Imd信号通路分别与人类的TLR4和TNRF-1信号通路存在着惊人的相似之处,说明果蝇与人类在免疫调控通路方面可能存在着共同的进化起源。     

Innate immune responses of Drosophila melanogaster are altered by spaceflight

Alterations and impairment of immune responses in humans present a health risk for space exploration missions. The molecular mechanisms underpinning innate immune defense can be confounded by the complexity of the acquired immune system of humans. Drosophila (fruit fly) innate immunity is simpler, and shares many similarities with human innate immunity at the level of molecular and genetic pathways. The goals of this study were to elucidate fundamental immune processes in Drosophila affected by spaceflight and to measure host-pathogen responses post-flight. Five containers, each containing ten female and five male fruit flies, were housed and bred on the space shuttle (average orbit altitude of 330.35 km) for 12 days and 18.5 hours. A new generation of flies was reared in microgravity. In larvae, the immune system was examined by analyzing plasmatocyte number and activity in culture. In adults, the induced immune responses were analyzed by bacterial clearance and quantitative real-time polymerase chain reaction (qPCR) of selected genes following infection with E. coli. The RNA levels of relevant immune pathway genes were determined in both larvae and adults by microarray analysis. The ability of larval plasmatocytes to phagocytose E. coli in culture was attenuated following spaceflight, and in parallel, the expression of genes involved in cell maturation was downregulated. In addition, the level of constitutive expression of pattern recognition receptors and opsonins that specifically recognize bacteria, and of lysozymes, antimicrobial peptide (AMP) pathway and immune stress genes, hallmarks of humoral immunity, were also reduced in larvae. In adults, the efficiency of bacterial clearance measured in vivo following a systemic infection with E. coli post-flight, remained robust. We show that spaceflight altered both cellular and humoral immune responses in Drosophila and that the disruption occurs at multiple interacting pathways.     

Inducible antibacterial defense system in C. elegans

The term innate immunity refers to a number of evolutionary ancient mechanisms that serve to defend animals and plants against infection. Genetically tractable model organisms, especially Drosophila, have contributed greatly to advances in our understanding of mammalian innate immunity. Essentially, nothing is known about immune responses in the nematode Caenorhabditis elegans. Using high-density cDNA microarrays, we show here that infection of C. elegans by the Gram-negative bacterium Serratia marcescens provokes a marked upregulation of the expression of many genes. Among the most robustly induced are genes encoding lectins and lysozymes, known to be involved in immune responses in other organisms. Certain infection-inducible genes are under the control of the DBL-1/TGFbeta pathway. We found that dbl-1 mutants exhibit increased susceptibility to infection. Conversely, overexpression of the lysozyme gene lys-1 augments the resistance of C. elegans to S. marcescens. These results constitute the first demonstration of inducible antibacterial defenses in C. elegans and open new avenues for the investigation of evolutionary conserved mechanisms of innate immunity.     

进化基因组学在昆虫天然免疫研究中的应用前景

整合基因组学和进化论而发展起来的进化基因组学正在逐渐改变传统昆虫学的研究模式。对昆虫天然免疫的研究已不再仅仅依靠实验学方法。3种全基因组序列被破译的模式昆虫(黑腹果蝇、冈比亚按蚊和意大利蜜蜂)将为这些研究引入新的方向。该文将以模式昆虫为代表,简要介绍如何利用进化上的趋同和趋异概念建立一特定昆虫物种的抗微生物肽基因蓝图;以及如何利用基因组数据和进化分析方法鉴定控制昆虫Toll信号通路关键组分Spatzle配体的进化优势位点。     

A specific primed immune response in Drosophila is dependent on phagocytes

Drosophila melanogaster, like other invertebrates, relies solely on its innate immune response to fight invading microbes; by definition, innate immunity lacks adaptive characteristics. However, we show here that priming Drosophila with a sublethal dose of Streptococcus pneumoniae protects against an otherwise-lethal second challenge of S. pneumoniae. This protective effect exhibits coarse specificity for S. pneumoniae and persists for the life of the fly. Although not all microbial challenges induced this specific primed response, we find that a similar specific protection can be elicited by Beauveria bassiana, a natural fly pathogen. To characterize this primed response, we focused on S. pneumoniae-induced protection. The mechanism underlying this protective effect requires phagocytes and the Toll pathway. However, activation of the Toll pathway is not sufficient for priming-induced protection. This work contradicts the paradigm that insect immune responses cannot adapt and will promote the search for similar responses overlooked in organisms with an adaptive immune response.     

A trade-off switch of two immunological memories in Caenorhabditis elegans reinfected by bacterial pathogens

Recent studies have suggested that innate immune responses exhibit characteristics associated with memory linked to modulations in both vertebrates and invertebrates. However, the diverse evolutionary paths taken, particularly within the invertebrate taxa, should lead to similarly diverse innate immunity memory processes. Our understanding of innate immune memory in invertebrates primarily comes from studies of the fruit fly Drosophila melanogaster, the generality of which is unclear. Caenorhabditis elegans typically inhabits soil harboring a variety of fatal microbial pathogens; for this invertebrate, the innate immune system and aversive behavior are the major defensive strategies against microbial infection. However, their characteristics of immunological memory remains infantile. Here we discovered an immunological memory that promoted avoidance and suppressed innate immunity during reinfection with bacteria, which we revealed to be specific to the previously exposed pathogens. During this trade-off switch of avoidance and innate immunity, the chemosensory neurons AWB and ADF modulated production of serotonin and dopamine, which in turn decreased expression of the innate immunity-associated genes and led to enhanced avoidance via the downstream insulin-like pathway. Therefore, our current study profiles the immune memories during C. elegans reinfected by pathogenic bacteria and further reveals that the chemosensory neurons, the neurotransmitter(s), and their associated molecular signaling pathways are responsible for a trade-off switch between the two immunological memories.     

CD8+ T-cell priming regulated by cytokines of the innate immune system

Host protection against a variety of pathogens and tumours requires the efficient induction of CD8(+) T-cell responses. Yet, it has proven difficult to develop vaccines that effectively stimulate this type of cellular immunity. One well-defined obstacle is antigen accessibility to the MHC class I processing pathway. However, cytokines that are produced by cells of the innate immune system also have a key role in CD8(+) T-cell responses, by enhancing 'cross-presentation' and/or inducing CD8(+) T-cell priming and differentiation. Here, we discuss how innate cytokine responses regulate CD8(+) T-cell immunity, and argue that a greater understanding of these processes will be essential for effective tailoring of vaccine-induced cellular immune responses.     

Pattern recognition molecules and innate immunity to parasites

Recent pioneering advances in understanding how plants, insects and worms eliminate pathogens has led to the realization that innate immunity plays a vital role in protecting humans from infection. This comprehensive review examines the molecules involved in innate immune responses, how they act to control parasites and if their engagement can explain many immune features characteristic of parasitic infections.     

Intracellular innate resistance to bacterial pathogens

Mammalian innate immunity stimulates antigen-specific immune responses and acts to control infection prior to the onset of adaptive immunity. Some bacterial pathogens replicate within the host cell and are therefore sheltered from some protective aspects of innate immunity such as complement. Here we focus on mechanisms of innate intracellular resistance encountered by bacterial pathogens and how some bacteria can evade destruction by the innate immune system. Major strategies of intracellular antibacterial defence include pathogen compartmentalization and iron limitation. Compartmentalization of pathogens within the host endocytic pathway is critical for generating high local concentrations of antimicrobial molecules, such as reactive oxygen species, and regulating concentrations of divalent cations that are essential for microbial growth. Cytosolic sensing, autophagy, sequestration of essential nutrients and membrane attack by antimicrobial peptides are also discussed.     

DNA识别受体PYHIN家族及相关病毒免疫逃逸机制的研究

宿主细胞内的DNA识别受体可识别病毒核酸分子并激活细胞天然免疫反应,从而产生抗病毒效应;同时,病毒也进化出相应机制来逃避或抑制这种免疫反应。本文总结了宿主细胞内DNA识别受体PYHIN家族识别病毒核酸并激活细胞天然免疫反应的特点和分子机制,并讨论了病毒逃避宿主天然免疫应答的方式。     

Evolutionary perspective on innate immune recognition

Analysis of human and Drosophila genomes demonstrates an ancient origin of innate immunity and the diversity of the mechanisms of innate immune recognition.