Diversification of Ig superfamily genes in an invertebrate

Digenean-gastropod host associations inform on aspects of specific immunity in snails

Gastropod immunology is informed importantly by the study of the frequent encounters snails endure with digeneans (digenetic trematodes). One of the hallmarks of gastropod-digenean associations is their specificity: any particular digenean parasite species is transmitted by a limited subset of snail taxa. We discuss the nature of this specificity, including its immunological basis. We then review studies of the model gastropod Biomphalaria glabrata indicating that the baseline responses of snails to digeneans can be elevated in a specific manner. Studies incorporating molecular and functional approaches are then highlighted, and are further suggestive of the capacity for specific gastropod immune responses. These studies have led to the compatibility polymorphism hypothesis: the interactions between diversified fibrinogen-related proteins (FREPs) and diverse carbohydrate-decorated polymorphic parasite antigens determine recognition and trigger specific immunity. Complex glycan structures are also likely to play a role in the host specificity typifying snail-digenean interactions. We conclude by noting the dynamic and consequential interactions between snails and digeneans can be considered as drivers of diversification of digenean parasites and in the development and maintenance of specific immunity in gastropods.

Fibrinogen-Related Proteins (FREPs) in Mollusks

Anti-parasite responses of the snail Biomphalaria glabrata involve antigen-reactive plasma lectins termed fibrinogen-related proteins (FREPs) comprising a C-terminal fibrinogen (FBG) domain and one or two upstream immunoglobulin domains. FREPs are highly polymorphic; they derive from several gene families with multiple loci and alleles that are diversified by exon loss, alternative splicing, and random somatic mutation (gene conversion and point mutations). Individual B. glabrata snails have dynamically distinct FREP sequence repertoires. The immune relevance of B. glabrata FREPs is indicated by FREP binding to polymorphic antigens of (snail-specific) digenean parasites and altered resistance of B. glabrata to digeneans following RNAi knockdown of FREPs. The compatibility polymorphism hypothesis proposes that FREP mutation increases the range of germline-encoded immune recognition in B. glabrata to counter antigenically-varied parasites. Somatic mutation may result from sequence exchange among tandemly arranged FREP genes in the genome, and analysis of sequence variants also suggests involvement of cytidine deaminase-like activity or epigenetic regulation. Without current indications of selection or retention of effective sequence variants toward immunological memory, FREP diversification is thought to afford B. glabrata immunity that is anticipatory but not adaptive. More remains to be learned about this system; other mollusks elaborate diversified lectins consisting of single FBG domains, and bona fide FREPs were reported from additional gastropod species, but these may not be diversified. Future comparative immunological studies and gene discovery driven by next-generation sequencing will further clarify taxonomic distribution of FREP diversification and the underlying mutator mechanisms as a component of immune function in mollusks.

A family of variable immunoglobulin and lectin domain containing molecules in the snail Biomphalaria glabrata

Technical limitations have hindered comprehensive studies of highly variable immune response molecules that are thought to have evolved due to pathogen-mediated selection such as fibrinogen-related proteins (FREPs) from Biomphalaria glabrata. FREPs combine upstream immunoglobulin superfamily (IgSF) domains with a C-terminal fibrinogen-related domain (FreD) and participate in reactions against trematode parasites. From RNAseq data we assembled a de novo reference transcriptome of B. glabrata to investigate the diversity of FREP transcripts. This study increased over two fold the number of bonafide FREP subfamilies and revealed important sequence diversity within FREP12 subfamily. We also report the discovery of related molecules that feature one or two IgSF domains associated with different C-terminal lectin domains, named C-type lectin-related proteins (CREPs) and Galectin-related protein (GREP). Together, the highly similar FREPs, CREPs and GREP were designated VIgL (Variable Immunoglobulin and Lectin domain containing molecules).

Single sea urchin phagocytes express messages of a single sequence from the diverse Sp185/333 gene family in response to bacterial challenge

Immune systems in animals rely on fast and efficient responses to a wide variety of pathogens. The Sp185/333 gene family in the purple sea urchin, Strongylocentrotus purpuratus, consists of an estimated 50 (±10) members per genome that share a basic gene structure but show high sequence diversity, primarily due to the mosaic appearance of short blocks of sequence called elements. The genes show significantly elevated expression in three subpopulations of phagocytes responding to marine bacteria. The encoded Sp185/333 proteins are highly diverse and have central effector functions in the immune system. In this study we report the Sp185/333 gene expression in single sea urchin phagocytes. Sea urchins challenged with heat-killed marine bacteria resulted in a typical increase in coelomocyte concentration within 24 h, which included an increased proportion of phagocytes expressing Sp185/333 proteins. Phagocyte fractions enriched from coelomocytes were used in limiting dilutions to obtain samples of single cells that were evaluated for Sp185/333 gene expression by nested RT-PCR. Amplicon sequences showed identical or nearly identical Sp185/333 amplicon sequences in single phagocytes with matches to six known Sp185/333 element patterns, including both common and rare element patterns. This suggested that single phagocytes show restricted expression from the Sp185/333 gene family and infers a diverse, flexible, and efficient response to pathogens. This type of expression pattern from a family of immune response genes in single cells has not been identified previously in other invertebrates.

Review of Dscam-mediated immunity in shrimp and other arthropods

Although true adaptive immunity is only found in vertebrates, there is increasing evidence that shrimp and other arthropods exhibit immune specificity and immune memory. The invertebrate immune response is now called "innate immunity with specificity" or "immune priming", and its underlying mechanisms are still unclear. However, while vertebrate antibodies have no invertebrate homolog, the Down syndrome cell adhesion molecule (Dscam), which is a hypervariable protein created by alternative splicing, can function as a pathogen-specific recognizing molecule in arthropods. Here we review our current understanding of the Dscam-mediated immune responses in arthropods, especially in shrimp, and show that Dscam may be involved in both general innate immunity and the pathogen-specific immune response.

WSSV-induced crayfish Dscam shows durable immune behavior

One of the major gaps in our understanding of arthropod specific immune priming concerns the mechanism[s] by which the observed long-term (>2 weeks) protective effects might be mediated. Hypervariable Dscam (Down syndrome cell adhesion molecule) might support arthropod innate immunity with specificity for more extended periods. We show here that, in the relatively long-lived arthropod Cherax quadricarinatus, CqDscam does not behave like a typical, immediately-acting, short-lived innate immune factor: CqDscam was not induced within hours after challenge with a lethal virus, but instead was only up-regulated after 2-5 days. This initial response faded within ∼ 2 weeks, but another maximum was reached ∼ 1 month later. At around 2 months after the initial challenge, the virus-induced CqDscam bound to the virus virion and acted to neutralize the virus However, although CqDscam helped crayfish to survive during persistent infection, it nevertheless failed to provide any enhanced protection against a subsequent WSSV challenge. Thus, CqDscam is capable of supporting extended anti-virus immune memory in arthropods. Also, during a persistent virus infection, the balance of "immune firepower" in crayfish appears to be altered such that the general immune factors become depleted while CqDscam becomes relatively predominant.

The specifically enhanced cellular immune responses in Pacific oyster (Crassostrea gigas) against secondary challenge with Vibrio splendidus

The increasing experimental evidences suggest that there are some forms of specific acquired immunity in invertebrates, but the underlying mechanism is not fully understood. In the present study, Pacific oyster (Crassostrea gigas) stimulated primarily by heat-killed Vibrio splendidus displayed stronger immune responses at cellular and molecular levels when they encountered the secondary challenge of live V. splendidus. The total hemocyte counts (THC) increased significantly after the primary stimulation of heat-killed V. splendidus, and it increased even higher (p < 0.01) and reached the peak earlier (at 6 h) after the secondary challenge with live V. splendidus compared with that of the primary stimulation. The number of new generated circulating hemocytes increased dramatically (p < 0.01) at 6 h after the pre-stimulated oysters received the secondary stimulation with live V. splendidus, and the phagocytic rate was also enhanced significantly (p < 0.01) at 12 h after the secondary stimulation. Meanwhile, the enhanced phagocytosis of hemocytes was highly specific for V. splendidus and they could distinguish Vibrio anguillarum, Vibrio coralliilyticus, Yarrowia lipolytica, and Micrococcus luteus efficiently. In addition, the mRNA expression of 12 candidate genes related to phagocytosis and hematopoiesis were also monitored, and the expression levels of CgIntegrin, CgPI3K (phosphatidylinositol 3-kinase), CgRho J, CgMAPKK (mitogen-activated protein kinase kinase), CgRab32, CgNADPH (nicotinamide adenine dinucleotide phosphate) oxidase, CgRunx1 and CgBMP7 (bone morphogenetic protein 7) in the hemocytes of pre-stimulated oysters after the secondary stimulation of V. splendidus were higher (p < 0.01) than that after the primary stimulation, but there was no statistically significant changes for the genes of CgPKC (protein kinase C), CgMyosin, CgActin, and CgGATA 3. These results collectively suggested that the primary stimulation of V. splendidus led to immune priming in oyster with specifically enhanced phagocytosis and rapidly promoted regeneration of circulating hemocytes when the primed oysters encountered the secondary challenge with V. splendidus.

Cloning and characterization of two different ficolins from the giant freshwater prawn Macrobrachium rosenbergii

Ficolins, a kind of lectin containing collagen-like and fibrinogen-related domains (FReDs, also known as FBG or FREP), are involved in the first line of host defense against pathogens. In this study, two ficolins, namely, MrFico1 and MrFico2, from the giant freshwater prawn Macrobrachium rosenbergii were identified. In contrast to other ficolins, these two ficolins have no collagen-like domain, but such ficolins contain a coiled region and a FReD domain. Phylogenetic analysis showed that MrFico1 and MrFico2, together with two ficolin-like proteins from Pacifastacus leniusculus, belonged to one group. Quantitative RT-PCR (qRT-PCR) showed that both MrFico1 and MrFico2 were expressed in hepatopancreas, stomach and intestine, with the highest expression in stomach for MrFico1, compared to the highest expression in hepatopancreas for MrFico2. qRT-PCR analysis also showed that MrFico1 was obviously upregulated upon Vibrio anguillarium challenge, while MrFico2 was upregulated after challenged by V. anguillarium or white spot syndrome virus. Bacterium-binding experiment showed that MrFico1 and MrFico2 could bind to different microbes, and sugar-binding assay revealed that these two ficolins could also bind to lipopolysaccharide and peptidoglycan, the glycoconjugates of bacteria surface. Moreover, these two ficolins could agglutinate bacteria in a calcium-dependent manner, and the results of bacteria clearance experiment showed that both ficolins could facilitate the clearance of injected bacteria in the prawn. Our results suggested that MrFico1 and MrFico2 may function as pattern-recognition receptors in the immune system of M. rosenbergii.

Generation of individual diversity: a too neglected fundamental property of adaptive immune system

The fitness gains resulting from development of the adaptive immune system (AIS) during evolution are still the subject of hot debate. A large random repertoire of antigenic receptors is costly to develop and could be the source of autoimmune reactions. And yet, despite their drawbacks, AIS-like systems seem to have been independently acquired in several phyla of metazoans with very different anatomies, longevities, and lifestyles. This article is a speculative attempt to explore the selective pressures, which favored this striking convergent evolution. It is well known that the AIS enables an organism to produce a specific immune response against all natural or artificial antigenic structures. However, it is frequently neglected that this response is highly variable among individuals. In practice, each individual possesses a "private" adaptive immune repertoire. This individualization of immune defenses implies that invasion and escape immune mechanisms developed by pathogens will certainly not always be successful as the specific targets and organization of the immune response are somewhat unpredictable. In a population, where individuals display heterogeneous immune responses to infection, the probability that a pathogen is able to infect all individuals could be reduced compared to a homogeneous population. This suggests that the individual diversity of the immune repertoire is not a by-product of the AIS but of its fundamental properties and could be in part responsible for repeated selection and conservation of the AIS during metazoan evolution. The capacity of the AIS to improve the management of cooperative or parasitic symbiotic relationships at the individual level could be a secondary development due to its progressive integration into the innate immune system. This hypothesis constitutes a new scenario for AIS emergence and explains the selection of MHC restriction and MHC diversification.

A new fibrinogen-related protein from Argopecten irradians (AiFREP-2) with broad recognition spectrum and bacteria agglutination activity

Fibrinogen-related proteins (FREPs) are a kind of pattern recognition receptors (PRRs) containing fibrinogen-like (FBG) domains, and they play curial roles in the innate immune response. In the present study, a new FREP protein was identified from bay scallop Argopecten irradians (designated as AiFREP-2). The full-length cDNA of AiFREP-2 was of 1299 bp with an open reading frame of 762 bp encoding a polypeptide of 253 amino acids, including a signal sequence and an FBG domain. The FBG domain in AiFREP-2 was highly similar to those of ficolins, tenascins and other FREPs. The mRNA expression of AiFREP-2 could be detected in all the examined tissues with the highest level in gill. The mRNA expression of AiFREP-2 in hemocytes was significantly up-regulated post the stimulation of lipopolysaccharide (LPS), peptidoglycan (PGN) and β-glucan (GLU) (P < 0.01). The recombinant AiFREP-2 (rAiFREP-2) could bind not only different PAMP ligands including LPS, PGN and GLU, but also various microbes including Gram-negative bacteria (Vibrio anguillarum), Gram-positive bacteria (Staphylococcus aureus) and fungus (Pichia pastoris and Yarrowia lipolytica). Additionally, rAiFREP-2 exhibited obvious agglutination activity towards Gram-negative bacteria V. anguillarum and Gram positive bacteria S. aureus. The results indicated that AiFREP-2 was involved in the immune response against Gram-negative bacteria, Gram-positive bacteria and fungus as a PRR in bay scallop, and the information was helpful to understand the innate immune defense mechanisms of mollusks.

Immunological memory within the innate immune system

Immune memory has traditionally been the domain of the adaptive immune system, present only in antigen-specific T and B cells. The purpose of this review is to summarize the evidence for immunological memory in lower organisms (which are not thought to possess adaptive immunity) and within specific cell subsets of the innate immune system. A special focus will be given to recent findings in both mouse and humans for specificity and memory in natural killer (NK) cells, which have resided under the umbrella of innate immunity for decades. The surprising longevity and enhanced responses of previously primed NK cells will be discussed in the context of several immunization settings.

Characterization of a novel hemolytic activity of human IgG fractions arising from diversity in protein and oligosaccharide components

Human IgG is a well-established multifunctional antigen specific immunoglobulin molecule of the adaptive immune system. However, an antigen nonspecific immunological function of human IgG has never been reported. In this study, human IgG was isolated using ammonium sulfate fractional precipitation and diethylaminoethanol (DEAE) cellulose 52 ion exchange chromatography, from which h-IgG and hs-IgG fractions were purified on the basis of their differential binding to rabbit anti-shrimp hemocyanin antibody (h) and rabbit anti-shrimp hemocyanin's small subunit antibody (hs), respectively. We found that h-IgG had a higher hemolytic activity than hs-IgG against erythrocytes from humans, rabbits, mice and chickens, whereas the control IgG showed negligible activity. h-IgG could interact directly with erythrocyte membranes, and this interaction was suppressed by high molecular weight osmoprotectants, showing that it may follow a colloid-osmotic mechanism. In comparative proteomics and glycomics studies, h-IgG and hs-IgG yielded 20 and 5 significantly altered protein spots, respectively, on a 2-D gel. The mean carbohydrate content of h-IgG and hs-IgG was approximately 3.6- and 2-fold higher than that of IgG, respectively, and the α-d-mannose/α-d-glucose content was in the order of h-IgG>hs-IgG>IgG. In this study, a novel antigen nonspecific immune property of human IgG was investigated, and the diversity in the protein constituents and glycosylation levels may have functional signficance.

Variable immune molecules in invertebrates

Recently it has become evident that invertebrates may mount a highly variable immune response that is dependent on which pathogen is involved. The molecular mechanisms behind this diversity are beginning to be unravelled and in several invertebrate taxa immune proteins exhibiting a broad range of diversity have been found. In some cases, evidence has been gathered suggesting that this molecular diversity translates into the ability of an affected invertebrate to mount a defence that is specifically aimed at a particular pathogen.

The past, present, and future of immune repertoire biology - the rise of next-generation repertoire analysis

T and B cell repertoires are collections of lymphocytes, each characterized by its antigen-specific receptor. We review here classical technologies and analysis strategies developed to assess immunoglobulin (IG) and T cell receptor (TR) repertoire diversity, and describe recent advances in the field. First, we describe the broad range of available methodological tools developed in the past decades, each of which answering different questions and showing complementarity for progressive identification of the level of repertoire alterations: global overview of the diversity by flow cytometry, IG repertoire descriptions at the protein level for the identification of IG reactivities, IG/TR CDR3 spectratyping strategies, and related molecular quantification or dynamics of T/B cell differentiation. Additionally, we introduce the recent technological advances in molecular biology tools allowing deeper analysis of IG/TR diversity by next-generation sequencing (NGS), offering systematic and comprehensive sequencing of IG/TR transcripts in a short amount of time. NGS provides several angles of analysis such as clonotype frequency, CDR3 diversity, CDR3 sequence analysis, V allele identification with a quantitative dimension, therefore requiring high-throughput analysis tools development. In this line, we discuss the recent efforts made for nomenclature standardization and ontology development. We then present the variety of available statistical analysis and modeling approaches developed with regards to the various levels of diversity analysis, and reveal the increasing sophistication of those modeling approaches. To conclude, we provide some examples of recent mathematical modeling strategies and perspectives that illustrate the active rise of a "next-generation" of repertoire analysis.

Immunoglobulin superfamily protein Dscam exhibited molecular diversity by alternative splicing in hemocytes of crustacean, Eriocheir sinensis

Be absent of adaptive immunity which have both specificity and memory, invertebrates seem to have evolved alternative adaptive immune strategies to resist various intruding pathogens. Whereas vertebrates could generate a wide range of immunological receptors with somatic rearrangement, invertebrates possibly depend on alternative splicing of pattern-recognition receptors (PRRs). Recently, it has been suggested that a member of the immunoglobulin superfamily (IgSF), Down syndrome cell adhesion molecule (Dscam), plays a crucial role in the alternative adaptive immune system of invertebrates. At present, we successfully isolated and characterized the first crab Dscam from Eriocheir sinensis. EsDscam has typical domain architecture compared with other Dscam orthologs, including one signal-peptide, 10 immunoglobulin (Ig) domains, 6 fibronectin type III domains (FNIII), one transmembrane domain (TM) and one cytoplasmic tail. We had detected four hypervariable regions of EsDscam in the N-terminal halves of Ig2 (25) and Ig3 domain (30), the complete Ig7 (18) and also the transmembrane domain (2), potentially generate 27,000 unique isoforms at least. Transcription of EsDscam were both a) detected in all tissues, especially in immune system, digestive system and nerve system; b) significantly induced in hemocytes post lipopolysaccharides (LPS), peptidoglycans (PG) and β-1, 3-glucans (Glu) injection. Importantly, we had detected membrane-bound and secreted Dscam isoforms in E. sinensis, and showed that secreted isoforms were extensively transcribed post different PAMPs challenge respectively. Results from immuno-localization assay revealed that EsDscam evenly distributed in the cell surface of hemocytes. These findings indicated that EsDscam is a hypervariable PRR in the innate immune system of the E. sinensis.

Immune response of the Caribbean sea fan, Gorgonia ventalina, exposed to an Aplanochytrium parasite as revealed by transcriptome sequencing

Coral reef communities are undergoing marked declines due to a variety of stressors including disease. The sea fan coral, Gorgonia ventalina, is a tractable study system to investigate mechanisms of immunity to a naturally occurring pathogen. Functional studies in Gorgonia ventalina immunity indicate that several key pathways and cellular components are involved in response to natural microbial invaders, although to date the functional and regulatory pathways remain largely un-described. This study used short-read sequencing (Illumina GAIIx) to identify genes involved in the response of G. ventalina to a naturally occurring Aplanochytrium spp. parasite. De novo assembly of the G. ventalina transcriptome yielded 90,230 contigs of which 40,142 were annotated. RNA-Seq analysis revealed 210 differentially expressed genes in sea fans exposed to the Aplanochytrium parasite. Differentially expressed genes involved in immunity include pattern recognition molecules, anti-microbial peptides, and genes involved in wound repair and reactive oxygen species formation. Gene enrichment analysis indicated eight biological processes were enriched representing 36 genes, largely involved with protein translation and energy production. This is the first report using high-throughput sequencing to characterize the host response of a coral to a natural pathogen. Furthermore, we have generated the first transcriptome for a soft (octocoral or non-scleractinian) coral species. Expression analysis revealed genes important in invertebrate innate immune pathways, as well as those whose role is previously un-described in cnidarians. This resource will be valuable in characterizing G. ventalina immune response to infection and co-infection of pathogens in the context of environmental change.

Molluscan cells in culture: primary cell cultures and cell lines

In vitro cell culture systems from molluscs have significantly contributed to our basic understanding of complex physiological processes occurring within or between tissue-specific cells, yielding information unattainable using intact animal models. In vitro cultures of neuronal cells from gastropods show how simplified cell models can inform our understanding of complex networks in intact organisms. Primary cell cultures from marine and freshwater bivalve and gastropod species are used as biomonitors for environmental contaminants, as models for gene transfer technologies, and for studies of innate immunity and neoplastic disease. Despite efforts to isolate proliferative cell lines from molluscs, the snail Biomphalaria glabrata Say, 1818 embryonic (Bge) cell line is the only existing cell line originating from any molluscan species. Taking an organ systems approach, this review summarizes efforts to establish molluscan cell cultures and describes the varied applications of primary cell cultures in research. Because of the unique status of the Bge cell line, an account is presented of the establishment of this cell line, and of how these cells have contributed to our understanding of snail host-parasite interactions. Finally, we detail the difficulties commonly encountered in efforts to establish cell lines from molluscs and discuss how these difficulties might be overcome.

Where does innate immunity stop and adaptive immunity begin?

The regulation of alternative splicing in the immune effector Dscam reported by Dong et al. (2012) in this issue of Cell Host & Microbe raises important questions about the nature of immune responses. Can we clearly define "adaptive" as being different from "innate" immunity, or is it time for a more flexible description?

Drosophila immune priming against Pseudomonas aeruginosa is short-lasting and depends on cellular and humoral immunity

Immune responses are traditionally divided into the innate and the adaptive arm, both of which are present in vertebrates, while only the innate arm is found in invertebrates. Immune priming experiments in Drosophila melanogaster and other invertebrates during the last decade have challenged this dogma, questioning the boundaries between innate and adaptive immunity. Studies on repeated inoculation of Drosophila with microbes reveal a long-lasting cellular immunity adaptation against particular microorganisms. Here we study the lasting effect of immune priming against infection with Pseudomonas aeruginosa, an opportunistic human pathogen that is lethal to the common fruit fly. Drosophila priming with heat-killed or low in virulence P. aeruginosa extends fly survival during a secondary lethal infection with a virulent strain of the same species. The protective immune response can last for more than 10 days after exposure to a persistent low-in-virulence live infection, but it is eliminated 7 days after the host is primed with heat-killed bacteria. Moreover, not only the cellular, but also the systemic NF-κB-mediated immune responses contribute to immune priming. Thus each microbe might elicit different mechanisms of immune priming that may or may not last for long.

Evolution of vertebrate immunity

All multicellular organisms protect themselves against pathogens using sophisticated immune defenses. Functionally interconnected humoral and cellular facilities maintain immune homeostasis in the absence of overt infection and regulate the initiation and termination of immune responses directed against pathogens. Immune responses of invertebrates, such as flies, are innate and usually stereotyped; those of vertebrates, encompassing species as diverse as jawless fish and humans, are additionally adaptive, enabling more rapid and efficient immune reactivity upon repeated encounters with a pathogen. Many of the attributes historically defining innate and adaptive immunity are in fact common to both, blurring their functional distinction and emphasizing shared ancestry and co-evolution. These findings provide indications of the evolutionary forces underlying the origin of somatic diversification of antigen receptors and contribute to our understanding of the complex phenotypes of human immune disorders. Moreover, informed by phylogenetic considerations and inspired by improved knowledge of functional networks, new avenues emerge for innovative therapeutic strategies.

Evidence for specific genotype-dependent immune priming in the lophotrochozoan Biomphalaria glabrata snail

Historically, the prevailing view in the field of invertebrate immunity was that invertebrates that do not possess acquired adaptive immunity rely on innate mechanisms with low specificity and no memory. Several recent studies have shaken this paradigm and suggested that the immune defenses of invertebrates are more complex and specific than previously thought. Mounting evidence has shown that at least some invertebrates (mainly Ecdysozoa) show high levels of specificity in their immune responses to different pathogens, and that subsequent reexposure may result in enhanced protection (recently called 'immune priming'). Here, we investigated immune priming in the Lophotrochozoan snail species Biomphalaria glabrata, following infection by the trematode pathogen Schistosoma mansoni. We confirmed that snails were protected against a secondary homologous infection whatever the host strain. We then investigated how immune priming occurs and the level of specificity of B. glabrata immune priming. In this report we confirmed that immune priming exists and we identified a genotype-dependent immune priming in the fresh-water snail B. glabrata.

Early differential gene expression in haemocytes from resistant and susceptible Biomphalaria glabrata strains in response to Schistosoma mansoni

The outcome of infection in the host snail Biomphalaria glabrata with the digenean parasite Schistosoma mansoni is determined by the initial molecular interplay occurring between them. The mechanisms by which schistosomes evade snail immune recognition to ensure survival are not fully understood, but one possibility is that the snail internal defence system is manipulated by the schistosome enabling the parasite to establish infection. This study provides novel insights into the nature of schistosome resistance and susceptibility in B. glabrata at the transcriptomic level by simultaneously comparing gene expression in haemocytes from parasite-exposed and control groups of both schistosome-resistant and schistosome-susceptible strains, 2 h post exposure to S. mansoni miracidia, using an novel 5K cDNA microarray. Differences in gene expression, including those for immune/stress response, signal transduction and matrix/adhesion genes were identified between the two snail strains and tests for asymmetric distributions of gene function also identified immune-related gene expression in resistant snails, but not in susceptible. Gene set enrichment analysis revealed that genes involved in mitochondrial electron transport, ubiquinone biosynthesis and electron carrier activity were consistently up-regulated in resistant snails but down-regulated in susceptible. This supports the hypothesis that schistosome-resistant snails recognize schistosomes and mount an appropriate defence response, while in schistosome-susceptible snails the parasite suppresses this defence response, early in infection.

Correlating structure and function during the evolution of fibrinogen-related domains

Fibrinogen-related domains (FReDs) are found in a variety of animal proteins with widely different functions, ranging from non-self recognition to clot formation. All appear to have a common surface where binding of one sort or other occurs. An examination of 19 completed animal genomes--including a sponge and sea anemone, six protostomes, and 11 deuterostomes--has allowed phylogenies to be constructed that show where various types of FReP (proteins containing FReDs) first made their appearance. Comparisons of sequences and structures also reveal particular features that correlate with function, including the influence of neighbor-domains. A particular set of insertions in the carboxyl-terminal subdomain was involved in the transition from structures known to bind sugars to those known to bind amino-terminal peptides. Perhaps not unexpectedly, FReDs with different functions have changed at different rates, with ficolins by far the fastest changing group. Significantly, the greatest amount of change in ficolin FReDs occurs in the third subdomain ("P domain"), the very opposite of the situation in most other vertebrate FReDs. The unbalanced style of change was also observed in FReDs from non-chordates, many of which have been implicated in innate immunity.

HMGB in mollusk Crassostrea ariakensis Gould: structure, pro-inflammatory cytokine function characterization and anti-infection role of its antibody

BACKGROUND

Crassostrea ariakensis Gould is a representative bivalve species and an economically important oyster in China, but suffers severe mortalities in recent years that are caused by rickettsia-like organism (RLO). Prevention and control of this disease is a priority for the development of oyster aquaculture. It has been proven that mammalian HMGB (high mobility group box) can be released extracellularly and acts as an important pro-inflammatory cytokine and late mediator of inflammatory reactions. In vertebrates, HMGB's antibody (anti-HMGB) has been shown to confer significant protection against certain local and systemic inflammatory diseases. Therefore, we investigated the functions of Ca-HMGB (oyster HMGB) and anti-CaHMGB (Ca-HMGB's antibody) in oyster RLO/LPS (RLO or LPS)-induced disease or inflammation.

METHODOLOGY/PRINCIPAL FINDINGS

Sequencing analysis revealed Ca-HMGB shares conserved structures with mammalians. Tissue-specific expression indicates that Ca-HMGB has higher relative expression in hemocytes. Significant continuous up-regulation of Ca-HMGB was detected when the hemocytes were stimulated with RLO/LPS. Recombinant Ca-HMGB protein significantly up-regulated the expression levels of some cytokines. Indirect immunofluorescence study revealed that Ca-HMGB localized both in the hemocyte nucleus and cytoplasm before RLO challenge, but mainly in the cytoplasm 12 h after challenge. Western blot analysis demonstrated Ca-HMGB was released extracellularly 4-12 h after RLO challenge. Anti-CaHMGB was added to the RLO/LPS-challenged hemocyte monolayer and real-time RT-PCR showed that administration of anti-CaHMGB dramatically reduced the rate of RLO/LPS-induced up-regulation of LITAF at 4-12 h after treatment. Flow cytometry analysis indicated that administration of anti-CaHMGB reduced RLO/LPS-induced hemocyte apoptosis and necrosis rates.

CONCLUSIONS/SIGNIFICANCE

Ca-HMGB can be released extracellularly and its subcellular localization varies when stimulated with RLO. Ca-HMGB is involved in oyster immune reactions and functions as a pro-inflammatory cytokine. Anti-CaHMGB can significantly suppress RLO/LPS-induced inflammatory responses and hemocyte necrosis and apoptosis, suggesting that Ca-HMGB is a potential target to prevent and control RLO/LPS-induced disease or inflammation.

Diversity in recognition of glycans by F-type lectins and galectins: molecular, structural, and biophysical aspects

Although lectins are "hard-wired" in the germline, the presence of tandemly arrayed carbohydrate recognition domains (CRDs), of chimeric structures displaying distinct CRDs, of polymorphic genes resulting in multiple isoforms, and in some cases, of a considerable recognition plasticity of their carbohydrate binding sites, significantly expand the lectin ligand-recognition spectrum and lectin functional diversification. Analysis of structural/functional aspects of galectins and F-lectins-the most recently identified lectin family characterized by a unique CRD sequence motif (a distinctive structural fold) and nominal specificity for l-Fuc-has led to a greater understanding of self/nonself recognition by proteins with tandemly arrayed CRDs. For lectins with a single CRD, however, recognition of self and nonself glycans can only be rationalized in terms of protein oligomerization and ligand clustering and presentation. Spatial and temporal changes in lectin expression, secretion, and local concentrations in extracellular microenvironments, as well as structural diversity and spatial display of their carbohydrate ligands on the host or microbial cell surface, are suggestive of a dynamic interplay of their recognition and effector functions in development and immunity.

Knowing your friends: invertebrate innate immunity fosters beneficial bacterial symbioses

The innate immune system is present in all animals and is a crucial first line of defence against pathogens. However, animals also harbour large numbers of beneficial microorganisms that can be housed in the digestive tract, in specialized organs or on tissue surfaces. Although invertebrates lack conventional antibody-based immunity, they are capable of eliminating pathogens and, perhaps more importantly, discriminating them from other microorganisms. This Review examines the interactions between the innate immune systems of several model invertebrates and the symbionts of these organisms, and addresses the central question of how these long-lived and specific associations are established and maintained.

Creative deaminases, self-inflicted damage, and genome evolution

Organisms minimize genetic damage through complex pathways of DNA repair. Yet a gene family--the AID/APOBECs--has evolved in vertebrates with the sole purpose of producing targeted damage in DNA/RNA molecules through cytosine deamination. They likely originated from deaminases involved in A>I editing in tRNAs. AID, the archetypal AID/APOBEC, is the trigger of the somatic diversification processes of the antibody genes. Its homologs may have been associated with the immune system even before the evolution of the antibody genes. The APOBEC3s, arising from duplication of AID, are involved in the restriction of exogenous/endogenous threats such as retroviruses and mobile elements. Another family member, APOBEC1, has (re)acquired the ability to target RNA while maintaining its ability to act on DNA. The AID/APOBECs have shaped the evolution of vertebrate genomes, but their ability to mutate nucleic acids is a double-edged sword: AID is a key player in lymphoproliferative diseases by triggering mutations and chromosomal translocations in B cells, and there is increasing evidence suggesting that other AID/APOBECs could be involved in cancer development as well.

Molecular characterization of a novel tetraspanin from the oyster, Crassostrea ariakensis: variation, localization and relationship to oyster host defense

We identified a tetraspanin family member gene, named Ca-TSP, in the oyster Crassostrea ariakensis and found that the transcription profiles of Ca-TSP were variable in the oyster hemocytes. Three distinct patterns of variation of Ca-TSP were observed. Using immunofluorescence and immunoelectron microscopy, we show that Ca-TSP was present in granules and in vesicular structures of the oyster hemocyte. Sequence analysis, structural features and immunogold electron microscopy showed that Ca-TSP is an integral membrane glycoprotein of granules of hemocyte and may be a novel CD63-like gene of the tetraspanin family of molluscs. The gene expression analysis of Ca-TSP using isolated oyster hemocytes, was done following challenge of the oysters with LPS and Poly I:C. The Ca-TSP mRNA levels increased in hemocytes in the first 12 h after LPS and Poly I:C stimulation, and decreased after the addition of H(2)O(2). Western blot analysis using anti-Ca-TSP antibody indicated that gene expression and protein levels were similar. The recombinant Ca-TSP was found to significantly inhibit hemocytes aggregation. Our results suggested that Ca-TSP participates in the innate immunity of the oyster.

Compatibility polymorphism in snail/schistosome interactions: From field to theory to molecular mechanisms

Coevolutionary dynamics in host-parasite interactions potentially lead to an arms race that results in compatibility polymorphism. The mechanisms underlying compatibility have remained largely unknown in the interactions between the snail Biomphalaria glabrata and Schistosoma mansoni, one of the agents of human schistosomiasis. This review presents a combination of data obtained from field and laboratory studies arguing in favor of a matching phenotype model to explain compatibility polymorphism. Investigations focused on the molecular determinants of compatibility have revealed two repertoires of polymorphic and/or diversified molecules that have been shown to interact: the parasite antigens S. mansoni polymorphic mucins and the B. glabrata fibrinogen-related proteins immune receptors. We hypothesize their interactions define the compatible/incompatible status of a specific snail/schistosome combination. This line of thought suggests concrete approaches amenable to testing in field-oriented studies attempting to control schistosomiasis by disrupting schistosome-snail compatibility.

Immunity in society: diverse solutions to common problems

Understanding how organisms fight infection has been a central focus of scientific research and medicine for the past couple of centuries, and a perennial object of trial and error by humans trying to mitigate the burden of disease. Vaccination success relies upon the exposure of susceptible individuals to pathogen constituents that do not cause (excessive) pathology and that elicit specific immune memory. Mass vaccination allows us to study how immunity operates at the group level; denser populations are more prone to transmitting disease between individuals, but once a critical proportion of the population becomes immune, "herd immunity" emerges. In social species, the combination of behavioural control of infection--e.g., segregation of sick individuals, disposal of the dead, quality assessment of food and water--and aggregation of immune individuals can protect non-immune members from disease. While immune specificity and memory are well understood to underpin immunisation in vertebrates, it has been somewhat surprising to find similar phenomena in invertebrates, which lack the vertebrate molecular mechanisms deemed necessary for immunisation. Indeed, reports showing alternative forms of immune memory are accumulating in invertebrates. In this issue of PLoS Biology, Konrad et al. present an example of fungus-specific immune responses in social ants that lead to the active immunisation of nestmates by infected individuals. These findings join others in showing how organisms evolved diverse mechanisms that fulfil common functions, namely the discrimination between pathogens, the transfer of immunity between related individuals, and the group-level benefits of immunisation.

A somatically diversified defense factor, FREP3, is a determinant of snail resistance to schistosome infection

Schistosomiasis, a neglected tropical disease, owes its continued success to freshwater snails that support production of prolific numbers of human-infective cercariae. Encounters between schistosomes and snails do not always result in the snail becoming infected, in part because snails can mount immune responses that prevent schistosome development. Fibrinogen-related protein 3 (FREP3) has been previously associated with snail defense against digenetic trematode infection. It is a member of a large family of immune molecules with a unique structure consisting of one or two immunoglobulin superfamily domains connected to a fibrinogen domain; to date fibrinogen containing proteins with this arrangement are found only in gastropod molluscs. Furthermore, specific gastropod FREPs have been shown to undergo somatic diversification. Here we demonstrate that siRNA mediated knockdown of FREP3 results in a phenotypic loss of resistance to Schistosoma mansoni infection in 15 of 70 (21.4%) snails of the resistant BS-90 strain of Biomphalaria glabrata. In contrast, none of the 64 control BS-90 snails receiving a GFP siRNA construct and then exposed to S. mansoni became infected. Furthermore, resistance to S. mansoni was overcome in 22 of 48 snails (46%) by pre-exposure to another digenetic trematode, Echinostoma paraensei. Loss of resistance in this case was shown by microarray analysis to be associated with strong down-regulation of FREP3, and other candidate immune molecules. Although many factors are certainly involved in snail defense from trematode infection, this study identifies for the first time the involvement of a specific snail gene, FREP3, in the phenotype of resistance to the medically important parasite, S. mansoni. The results have implications for revealing the underlying mechanisms involved in dictating the range of snail strains used by S. mansoni, and, more generally, for better understanding the phenomena of host specificity and host switching. It also highlights the role of a diversified invertebrate immune molecule in defense against a human pathogen. It suggests new lines of investigation for understanding how susceptibility of snails in areas endemic for S. mansoni could be manipulated and diminished.

Schistosomiasis, a neglected tropical disease, owes its continued success to freshwater snails that support production of prolific numbers of human-infective cercariae. Encounters between schistosomes and snails do not always result in the snail becoming infected, in part because snails can mount immune responses that prevent schistosome development. Understanding the factors important for snail resistance to schistosome infection will facilitate new lines of investigation to 1) understand the underlying basis of compatibility between schistosomes and snails in endemic areas and how this affects transmission dynamics and control efforts; and 2) to reveal ways to manipulate natural snail populations to enhance their resistance to schistosome infections. Here, we present the first evidence that a snail immune molecule, fibrinogen related protein 3 (FREP3), is important for successful defense against schistosome infections in Biomphalaria snails. In addition, we demonstrate that FREP3 is a target suppressed by trematode parasites to facilitate their establishment within the snail.

Macroevolutionary Immunology: A Role for Immunity in the Diversification of Animal life

An emerging picture of the nature of immune systems across animal phyla reveals both conservatism of some features and the appearance among and within phyla of novel, lineage-specific defense solutions. The latter collectively represent a major and underappreciated form of animal diversity. Factors influencing this macroevolutionary (above the species level) pattern of novelty are considered and include adoption of different life styles, life histories, and body plans; a general advantage of being distinctive with respect to immune defenses; and the responses required to cope with parasites, many of which afflict hosts in a lineage-specific manner. This large-scale pattern of novelty implies that immunological phenomena can affect microevolutionary processes (at the population level within species) that can eventually lead to macroevolutionary events such as speciation, radiations, or extinctions. Immunologically based phenomena play a role in favoring intraspecific diversification, specialization and host specificity of parasites, and mechanisms are discussed whereby this could lead to parasite speciation. Host switching - the acquisition of new host species by parasites - is a major mechanism that drives parasite diversity and is frequently involved in disease emergence. It is also one that can be favored by reductions in immune competence of new hosts. Mechanisms involving immune phenomena favoring intraspecific diversification and speciation of host species are also discussed. A macroevolutionary perspective on immunology is invaluable in today's world, including the need to study a broader range of species with distinctive immune systems. Many of these species are faced with extinction, another macroevolutionary process influenced by immune phenomena.

Specific versus non-specific immune responses in an invertebrate species evidenced by a comparative de novo sequencing study

Our present understanding of the functioning and evolutionary history of invertebrate innate immunity derives mostly from studies on a few model species belonging to ecdysozoa. In particular, the characterization of signaling pathways dedicated to specific responses towards fungi and Gram-positive or Gram-negative bacteria in Drosophila melanogaster challenged our original view of a non-specific immunity in invertebrates. However, much remains to be elucidated from lophotrochozoan species. To investigate the global specificity of the immune response in the fresh-water snail Biomphalaria glabrata, we used massive Illumina sequencing of 5′-end cDNAs to compare expression profiles after challenge by Gram-positive or Gram-negative bacteria or after a yeast challenge. 5′-end cDNA sequencing of the libraries yielded over 12 millions high quality reads. To link these short reads to expressed genes, we prepared a reference transcriptomic database through automatic assembly and annotation of the 758,510 redundant sequences (ESTs, mRNAs) of B. glabrata available in public databases. Computational analysis of Illumina reads followed by multivariate analyses allowed identification of 1685 candidate transcripts differentially expressed after an immune challenge, with a two fold ratio between transcripts showing a challenge-specific expression versus a lower or non-specific differential expression. Differential expression has been validated using quantitative PCR for a subset of randomly selected candidates. Predicted functions of annotated candidates (approx. 700 unisequences) belonged to a large extend to similar functional categories or protein types. This work significantly expands upon previous gene discovery and expression studies on B. glabrata and suggests that responses to various pathogens may involve similar immune processes or signaling pathways but different genes belonging to multigenic families. These results raise the question of the importance of gene duplication and acquisition of paralog functional diversity in the evolution of specific invertebrate immune responses.

SNPs of hemocyanin C-terminal fragment in shrimp Litopenaeus vannamei

In this study, we identified a variable region in the C-terminus of hemocyanin from the shrimp Litopenaeus vannamei (2288-2503bp, HcSC) by sequence alignments. A total of 13 SNPs were identified by PCR-SSCP and HcSC clone sequencing. The SSCP patterns of HcSC could be modulated in Vibro parahaemolyticus-treated shrimps. A novel SSCP band with four SNP sites was identified in V. parahaemolyticus-resistant shrimps. More importantly, three of these four SNPs introduced variations in amino acid sequence and possibly secondary structure of the HcSC polypeptide and resulted in a higher agglutinative activity against seven pathogenic bacteria. These results suggest that the C-terminus of shrimp L. vannamei hemocyanin possesses SNPs, which may be related to shrimp resistance to different pathogens.

Contribution of the caspase gene sequence diversification to the specifically antiviral defense in invertebrate

Vertebrates achieve adaptive immunity of all sorts against pathogens through the diversification of antibodies. However the mechanism of invertebrates' innate immune defense against various pathogens remains largely unknown. Our study used shrimp and white spot syndrome virus (WSSV) to show that PjCaspase, a caspase gene of shrimp that is crucial in apoptosis, possessed gene sequence diversity. At present, the role of gene sequence diversity in immunity has not been characterized. To address this issue, we compared the PjCaspase gene sequence diversities from WSSV-free and WSSV-resistant shrimp. The sequence analysis indicated that the PjCaspase gene from the WSSV-resistant shrimp contained a special fragment, designated as fragment 3 (221-229 aa). Down-regulation or overexpression of the PjCaspase gene containing fragment 3 led to significant inhibition or enhancement of virus-induced apoptosis, but had no effect on bacterium challenge. We found evidence that the silencing or overexpression of this gene led to a 7-fold increase or 11-fold decrease of WSSV copies, respectively. Our results suggested that the PjCaspase gene containing fragment 3 provided the molecular basis for the antiviral defense of shrimp. This study represented the first report of the role of gene sequence diversity in the immunity of an invertebrate against virus infection. Invertebrates may employ this gene sequence diversity as a system to avoid pathogen interference with their immune response.

Activation-induced cytidine deaminase structure and functions: a species comparative view

In the ten years since the discovery of activation-induced cytidine deaminase (AID) there has been considerable effort to understand the mechanisms behind this enzyme's ability to target and modify immunoglobulin genes leading to somatic hypermutation and class switch recombination. While the majority of research has focused on mouse and human models of AID function, work on other species, from lamprey to rabbit and sheep, has taught us much about the scope of functions of the AID mutator. This review takes a species-comparative approach to what has been learned about the AID mutator enzyme and its role in humoral immunity.

Invertebrate immune diversity

The arms race between hosts and pathogens (and other non-self) drives the molecular diversification of immune response genes in the host. Over long periods of evolutionary time, many different defense strategies have been employed by a wide variety of invertebrates. We review here penaeidins and crustins in crustaceans, the allorecognition system encoded by fuhc, fester and Uncle fester in a colonial tunicate, Dscam and PGRPs in arthropods, FREPs in snails, VCBPs in protochordates, and the Sp185/333 system in the purple sea urchin. Comparisons among immune systems, including those reviewed here have not identified an immune specific regulatory "genetic toolkit", however, repeatedly identified sequences (or "building materials" on which the tools act) are present in a broad range of immune systems. These include a Toll/TLR system, a primitive complement system, an LPS binding protein, and a RAG core/Transib element. Repeatedly identified domains and motifs that function in immune proteins include NACHT, LRR, Ig, death, TIR, lectin domains, and a thioester motif. In addition, there are repeatedly identified mechanisms (or "construction methods") that generate sequence diversity in genes with immune function. These include genomic instability, duplications and/or deletions of sequences and the generation of clusters of similar genes or exons that appear as families, gene recombination, gene conversion, retrotransposition, alternative splicing, multiple alleles for single copy genes, and RNA editing. These commonly employed "materials and methods" for building and maintaining an effective immune system that might have been part of that ancestral system appear now as a fragmented and likely incomplete set, likely due to the rapid evolutionary change (or loss) of host genes that are under pressure to keep pace with pathogen diversity.