A large repertoire of parasite epitopes matched by a large repertoire of host immune receptors in an invertebrate host/parasite model

Combining a transcriptomic approach and a targeted metabolomics approach for deciphering the molecular bases of compatibility phenotype in the snail Biomphalaria glabrata toward Schistosoma mansoni

Biomphalaria glabrata is a freshwater snail and the obligatory intermediate host of Schistosoma mansoni parasite, the etiologic agent of intestinal Schistosomiasis, in South America and Caribbean. Interestingly in such host-parasite interactions, compatibility varies between populations, strains or individuals. This observed compatibility polymorphism is based on a complex molecular-matching-phenotype, the molecular bases of which have been investigated in numerous studies, notably by comparing between different strains or geographical isolates or clonal selected snail lines. Herein we propose to decipher the constitutive molecular support of this interaction in selected non-clonal resistant and susceptible snail strain originating from the same natural population from Brazil and thus having the same genetic background. Thanks to a global RNAseq transcriptomic approach on whole snail, we identified a total of 328 differentially expressed genes between resistant and susceptible phenotypes among which 129 were up-regulated and 199 down-regulated. Metabolomic studies were used to corroborate the RNAseq results. The activation of immune genes and specific metabolic pathways in resistant snails might provide them with the capacity to better respond to parasite infection.

Cross-talk and mutual shaping between the immune system and the microbiota during an oyster's life

The Pacific oyster Crassostrea gigas lives in microbe-rich marine coastal systems subjected to rapid environmental changes. It harbours a diversified and fluctuating microbiota that cohabits with immune cells expressing a diversified immune gene repertoire. In the early stages of oyster development, just after fertilization, the microbiota plays a key role in educating the immune system. Exposure to a rich microbial environment at the larval stage leads to an increase in immune competence throughout the life of the oyster, conferring a better protection against pathogenic infections at later juvenile/adult stages. This beneficial effect, which is intergenerational, is associated with epigenetic remodelling. At juvenile stages, the educated immune system participates in the control of the homeostasis. In particular, the microbiota is fine-tuned by oyster antimicrobial peptides acting through specific and synergistic effects. However, this balance is fragile, as illustrated by the Pacific Oyster Mortality Syndrome, a disease causing mass mortalities in oysters worldwide. In this disease, the weakening of oyster immune defences by OsHV-1 µVar virus induces a dysbiosis leading to fatal sepsis. This review illustrates the continuous interaction between the highly diversified oyster immune system and its dynamic microbiota throughout its life, and the importance of this cross-talk for oyster health. This article is part of the theme issue 'Sculpting the microbiome: how host factors determine and respond to microbial colonization'.

The genome and transcriptome of the snail Biomphalaria sudanica s.l.: immune gene diversification and highly polymorphic genomic regions in an important African vector of Schistosoma mansoni

BACKGROUND

Control and elimination of schistosomiasis is an arduous task, with current strategies proving inadequate to break transmission. Exploration of genetic approaches to interrupt Schistosoma mansoni transmission, the causative agent for human intestinal schistosomiasis in sub-Saharan Africa and South America, has led to genomic research of the snail vector hosts of the genus Biomphalaria. Few complete genomic resources exist, with African Biomphalaria species being particularly underrepresented despite this being where the majority of S. mansoni infections occur. Here we generate and annotate the first genome assembly of Biomphalaria sudanica sensu lato, a species responsible for S. mansoni transmission in lake and marsh habitats of the African Rift Valley. Supported by whole-genome diversity data among five inbred lines, we describe orthologs of immune-relevant gene regions in the South American vector B. glabrata and present a bioinformatic pipeline to identify candidate novel pathogen recognition receptors (PRRs).

RESULTS

De novo genome and transcriptome assembly of inbred B. sudanica originating from the shoreline of Lake Victoria (Kisumu, Kenya) resulted in a haploid genome size of ~ 944.2 Mb (6,728 fragments, N50 = 1.067 Mb), comprising 23,598 genes (BUSCO = 93.6% complete). The B. sudanica genome contains orthologues to all described immune genes/regions tied to protection against S. mansoni in B. glabrata, including the polymorphic transmembrane clusters (PTC1 and PTC2), RADres, and other loci. The B. sudanica PTC2 candidate immune genomic region contained many PRR-like genes across a much wider genomic region than has been shown in B. glabrata, as well as a large inversion between species. High levels of intra-species nucleotide diversity were seen in PTC2, as well as in regions linked to PTC1 and RADres orthologues. Immune related and putative PRR gene families were significantly over-represented in the sub-set of B. sudanica genes determined as hyperdiverse, including high extracellular diversity in transmembrane genes, which could be under pathogen-mediated balancing selection. However, no overall expansion in immunity related genes was seen in African compared to South American lineages.

CONCLUSIONS

The B. sudanica genome and analyses presented here will facilitate future research in vector immune defense mechanisms against pathogens. This genomic/transcriptomic resource provides necessary data for the future development of molecular snail vector control/surveillance tools, facilitating schistosome transmission interruption mechanisms in Africa.

Immuno-molecular profile for Biomphalaria glabrata/Schistosoma mansoni interaction

The complex innate immune defense of Biomphalaria glabrata, the intermediate host of Schistosoma mansoni, governs the successful development of the intramolluscan stages of the parasite. The interaction between the snail and the parasite involves a complex immune molecular crosstalk between several parasite antigens and the snail immune recognition receptors, evoking different signals and effector molecules. This work seeks to discuss the immune-related molecules that influence compatibility in Biomphalaria glabrata/Schistosoma mansoni interaction and the differential expression of these molecules between resistant and susceptible snails. It also includes the current understanding of the immune molecular determinants that govern the compatibility in sympatric and allopatric interactions, and the expression of these molecules after immune priming and the secondary immune response. Herein, the differences in the immune-related molecules in the interaction of other Biomphalaria species with Schistosoma mansoni compared to the Biomphalaria glabrata model snail are highlighted. Understanding the diverse immune molecular determinants in the snail/schistosome interaction can lead to alternative control strategies for schistosomiasis.

The genome and transcriptome of the snail Biomphalaria sudanica s.l.: Immune gene diversification and highly polymorphic genomic regions in an important African vector of Schistosoma mansoni

Background

Control and elimination of schistosomiasis is an arduous task, with current strategies proving inadequate to break transmission. Exploration of genetic approaches to interrupt Schistosoma mansoni transmission, the causative agent for human intestinal schistosomiasis in sub-Saharan Africa and South America, has led to genomic research of the snail vector hosts of the genus Biomphalaria. Few complete genomic resources exist, with African Biomphalaria species being particularly underrepresented despite this being where the majority of S. mansoni infections occur. Here we generate and annotate the first genome assembly of Biomphalaria sudanica sensu lato, a species responsible for S. mansoni transmission in lake and marsh habitats of the African Rift Valley. Supported by whole-genome diversity data among five inbred lines, we describe orthologs of immune-relevant gene regions in the South American vector B. glabrata and present a bioinformatic pipeline to identify candidate novel pathogen recognition receptors (PRRs).

Results

De novo genome and transcriptome assembly of inbred B. sudanica originating from the shoreline of Lake Victoria (Kisumu, Kenya) resulted in a haploid genome size of ~944.2 Mb (6732 fragments, N50=1.067 Mb), comprising 23,598 genes (BUSCO=93.6% complete). The B. sudanica genome contains orthologues to all described immune genes/regions tied to protection against S. mansoni in B. glabrata. The B. sudanica PTC2 candidate immune genomic region contained many PRR-like genes across a much wider genomic region than has been shown in B. glabrata, as well as a large inversion between species. High levels of intra-species nucleotide diversity were seen in PTC2, as well as in regions linked to PTC1 and RADres orthologues. Immune related and putative PRR gene families were significantly over-represented in the sub-set of B. sudanica genes determined as hyperdiverse, including high extracellular diversity in transmembrane genes, which could be under pathogen-mediated balancing selection. However, no overall expansion in immunity related genes were seen in African compared to South American lineages.

Conclusions

The B. sudanica genome and analyses presented here will facilitate future research in vector immune defense mechanisms against pathogens. This genomic/transcriptomic resource provides necessary data for the future development of molecular snail vector control/surveillance tools, facilitating schistosome transmission interruption mechanisms in Africa.

The mechanisms and factors that induce trained immunity in arthropods and mollusks

Besides dividing the organism's immune system into adaptive and innate immunity, it has long been thought that only adaptive immunity can establish immune memory. However, many studies have shown that innate immunity can also build immunological memory through epigenetic reprogramming and modifications to resist pathogens' reinfection, known as trained immunity. This paper reviews the role of mitochondrial metabolism and epigenetic modifications and describes the molecular foundation in the trained immunity of arthropods and mollusks. Mitochondrial metabolism and epigenetic modifications complement each other and play a key role in trained immunity.

Status Quo and Future Perspectives of Molecular and Genomic Studies on the Genus Biomphalaria-The Intermediate Snail Host of Schistosoma mansoni

Schistosomiasis, or also generally known as bilharzia or snail fever, is a parasitic disease that is caused by trematode flatworms of the genus Schistosoma. It is considered by the World Health Organisation as the second most prevalent parasitic disease after malaria and affects more than 230 million people in over 70 countries. People are infected via a variety of activities ranging from agricultural, domestic, occupational to recreational activities, where the freshwater snails Biomphalaria release Schistosoma cercariae larvae that penetrate the skin of humans when exposed in water. Understanding the biology of the intermediate host snail Biomphalaria is thus important to reveal the potential spread of schistosomiasis. In this article, we present an overview of the latest molecular studies focused on the snail Biomphalaria, including its ecology, evolution, and immune response; and propose using genomics as a foundation to further understand and control this disease vector and thus the transmission of schistosomiasis.

Molecular characterization of thioester-containing proteins in Biomphalaria glabrata and their differential gene expression upon Schistosoma mansoni exposure

Schistosomiasis is a disease caused by trematode parasites of the genus Schistosoma that affects approximately 200 million people worldwide. Schistosomiasis has been a persistent problem in endemic areas as there is no vaccine available, currently used anti-helmintic medications do not prevent reinfection, and most concerning, drug resistance has been documented in laboratory and field isolates. Thus, alternative approaches to curtail this human disease are warranted. Understanding the immunobiology of the obligate intermediate host of these parasites, which include the freshwater snail Biomphalaria glabrata, may facilitate the development of novel methods to stop or reduce transmission to humans. Molecules from the thioester-containing protein (TEP) superfamily have been shown to be involved in immunological functions in many animals including corals and humans. In this study we identified, characterized, and compared TEP transcripts and their expression upon S. mansoni exposure in resistant and susceptible strains of B. glabrata snails. Results showed the expression of 11 unique TEPs in B. glabrata snails. These transcripts present high sequence identity at the nucleotide and putative amino acid levels between susceptible and resistant strains. Further analysis revealed differences in several TEPs’ constitutive expression levels between resistant and susceptible snail strains, with C3-1, C3-3, and CD109 having higher constitutive expression levels in the resistant (BS90) strain, whereas C3-2 and TEP-1 showed higher constitutive expression levels in the susceptible (NMRI) strain. Furthermore, TEP-specific response to S. mansoni miracidia exposure reiterated their differential expression, with resistant snails upregulating the expression of both TEP-4 and TEP-3 at 2 h and 48 h post-exposure, respectively. Further understanding the diverse TEP genes and their functions in invertebrate animal vectors will not only expand our knowledge in regard to this ancient family of immune proteins, but also offer the opportunity to identify novel molecular targets that could aid in the efforts to develop control methods to reduce schistosomiasis transmission.

Why and how do protective symbionts impact immune priming with pathogens in invertebrates?

Growing evidence demonstrates that invertebrates display adaptive-like immune abilities, commonly known as "immune priming". Immune priming is a process by which a host improves its immune defences following an initial pathogenic exposure, leading to better protection after a subsequent infection with the same - or different - pathogens. Nevertheless, beneficial symbionts can enhance similar immune priming processes in hosts, such as when they face repeated infections with pathogens. This "symbiotic immune priming" protects the host against pathogenic viruses, bacteria, fungi, or eukaryotic parasites. In this review, we explore the extent to which protective symbionts interfere and impact immune priming against pathogens from both a mechanical (proximal) and an evolutionary (ultimate) point of view. We highlight that the immune priming of invertebrates is the cornerstone of the tripartite interaction of hosts/symbionts/pathogens. The main shared mechanism of immune priming (induced by symbionts or pathogens) is the sustained immune response at the beginning of host-microbial interactions. However, the evolutionary outcome of immune priming leads to a specific discrimination, which provides enhanced tolerance or resistance depending on the type of microbe. Based on several studies testing immune priming against pathogens in the presence or absence of protective symbionts, we observed that both types of immune priming could overlap and affect each other inside the same hosts. As protective symbionts could be an evolutionary force that influences immune priming, they may help us to better understand the heterogeneity of pathogenic immune priming across invertebrate populations and species.

Experimental Infection of the Biomphalaria glabrata Vector Snail by Schistosoma mansoni Parasites Drives Snail Microbiota Dysbiosis

Host-parasite interaction can result in a strong alteration of the host-associated microbiota. This dysbiosis can affect the fitness of the host; can modify pathogen interaction and the outcome of diseases. Biomphalaria glabrata is the snail intermediate host of the trematode Schistosoma mansoni, the agent of human schistosomiasis, causing hundreds of thousands of deaths every year. Here, we present the first study of the snail bacterial microbiota in response to Schistosoma infection. We examined the interplay between B. glabrata, S. mansoni and host microbiota. Snails were infected and the microbiota composition was analysed by 16S rDNA amplicon sequencing approach. We demonstrated that the microbial composition of water did not affect the microbiota composition. Then, we characterised the Biomphalaria bacterial microbiota at the individual scale in both naive and infected snails. Sympatric and allopatric strains of parasites were used for infections and re-infections to analyse the modification or dysbiosis of snail microbiota in different host-parasite co-evolutionary contexts. Concomitantly, using RNAseq, we investigated the link between bacterial microbiota dysbiosis and snail anti-microbial peptide immune response. This work paves the way for a better understanding of snail/schistosome interaction and should have critical consequences in terms of snail control strategies for fighting schistosomiasis disease in the field.

New Insights Into Biomphalysin Gene Family Diversification in the Vector Snail Biomphalaria glabrata

Aerolysins initially characterized as virulence factors in bacteria are increasingly found in massive genome and transcriptome sequencing data from metazoans. Horizontal gene transfer has been demonstrated as the main way of aerolysin-related toxins acquisition in metazoans. However, only few studies have focused on their potential biological functions in such organisms. Herein, we present an extensive characterization of a multigene family encoding aerolysins - named biomphalysin - in Biomphalaria glabrata snail, the intermediate host of the trematode Schistosoma mansoni. Our results highlight that duplication and domestication of an acquired bacterial toxin gene in the snail genome result in the acquisition of a novel and diversified toxin family. Twenty-three biomphalysin genes were identified. All are expressed and exhibited a tissue-specific expression pattern. An in silico structural analysis was performed to highlight the central role played by two distinct domains i) a large lobe involved in the lytic function of these snail toxins which constrained their evolution and ii) a small lobe which is structurally variable between biomphalysin toxins and that matched to various functional domains involved in moiety recognition of targets cells. A functional approach suggests that the repertoire of biomphalysins that bind to pathogens, depends on the type of pathogen encountered. These results underline a neo-and sub-functionalization of the biomphalysin toxins, which have the potential to increase the range of effectors in the snail’s immune arsenal.

Hemocyte siRNA uptake is increased by 5' cholesterol-TEG addition in Biomphalaria glabrata, snail vector of schistosome

Biomphalaria glabrata is one of the snail intermediate hosts of Schistosoma mansoni, the causative agent of intestinal schistosomiasis disease. Numerous molecular studies using comparative approaches between susceptible and resistant snails to S. mansoni infection have helped identify numerous snail key candidates supporting such susceptible/resistant status. The functional approach using RNA interference (RNAi) remains crucial to validate the function of such candidates. CRISPR-Cas systems are still under development in many laboratories, and RNA interference remains the best tool to study B. glabrata snail genetics. Herein, we describe the use of modified small interfering RNA (siRNA) molecules to enhance cell delivery, especially into hemocytes, the snail immune cells. Modification of siRNA with 5′ Cholesteryl TriEthylene Glycol (Chol-TEG) promotes cellular uptake by hemocytes, nearly eightfold over that of unmodified siRNA. FACS analysis reveals that more than 50% of hemocytes have internalized Chol-TEG siRNA conjugated to Cy3 fluorophores, 2 hours only after in vivo injection into snails. Chol-TEG siRNA targeting BgTEP1 (ThioEster-containing Protein), a parasite binding protein, reduced BgTEP1 transcript expression by 70–80% compared to control. The level of BgTEP1 protein secreted in the hemolymph was also decreased. However, despite the BgTEP1 knock-down at both RNA and protein levels, snail compatibility with its sympatric parasite is not affected suggesting functional redundancy among the BgTEP genes family in snail-schistosoma interaction.

Immune Evasion Strategies of Schistosomes

Human schistosomes combat the unique immune systems of two vastly different hosts during their indirect life cycles. In gastropod molluscs, they face a potent innate immune response composed of variable immune recognition molecules and highly phagocytic hemocytes. In humans, a wide variety of innate and adaptive immune processes exist in proximity to these parasites throughout their lifespan. To survive and thrive as the second most common parasitic disease in humans, schistosomes have evolved many techniques to avoid and combat these targeted host responses. Among these techniques are molecular mimicry of host antigens, the utilization of an immune resistant outer tegument, the secretion of several potent proteases, and targeted release of specific immunomodulatory factors affecting immune cell functions. This review seeks to describe these key immune evasion mechanisms, among others, which schistosomes use to survive in both of their hosts. After diving into foundational observational studies of the processes mediating the establishment of schistosome infections, more recent transcriptomic and proteomic studies revealing crucial components of the host/parasite molecular interface are discussed. In order to combat this debilitating and lethal disease, a comprehensive understanding of schistosome immune evasion strategies is necessary for the development of novel therapeutics and treatment plans, necessitating the discussion of the numerous ways in which these parasitic flatworms overcome the immune responses of both hosts.

RNA-seq: The early response of the snail Physella acuta to the digenetic trematode Echinostoma paraensei

To analyze the response of the snail Physella acuta to Echinostoma paraensei, a compatible digenetic trematode, Illumina RNA-seq data were collected from snails with early infection (5 snails at 2 days post-exposure [DPE]) and established infection (4 snails, 8 DPE), and 7 control (unexposed) snails. A reference transcriptome (325,563 transcripts, including 98% of eukaryotic universal single-copy orthologs; BUSCO) and a draft P. acuta genome (employing available genomic Illumina reads; 799,945 scaffolds, includes 88% BUSCO genes) were assembled to guide RNA-seq analyses. Parasite exposure of P. acuta led to 10,195 differentially expressed (DE) genes at 2 DPE and 8,876 DE genes at 8 DPE with only 18% of up-regulated and 22% of down-regulated sequences shared between these time points. Gene ontology (GO) analysis yielded functional annotation of only 1.2% of DE genes but did not indicate major changes in biological activities of P. acuta between 2 and 8 DPE. Increased insights were achieved by analysis of expression profiles of 460 immune-relevant DE transcripts, identified by BLAST and InterProScan. Physella acuta has expanded gene families that encode immune-relevant domains, including CD109/TEP, GTPase IMAP, Limulus agglutination factor (dermatopontin), FReD (≥82 sequences with fibrinogen-related domains), and transcripts that combine C-type lectin (C-LECT) and C1q domains, novel among metazoa. Notably, P. acuta expressed sequences from these immune gene families at all time points, but the assemblages of unique transcripts from particular immune gene families differed between 2 and 8 DPE. The shift in profiles of DE immune genes, from early exposure to parasite establishment, suggests that compatible P. acuta initially respond to infection but switch to express immune genes that likely are less effective against E. paraensei but counter other types of (opportunistic) pathogens and parasites. We propose that the latter expression profile is part of an extended phenotype of E. paraensei, imposed upon P. acuta through parasite manipulation of the host, following successful parasite establishment in the snail after 2 DPE.

Enhanced Immune Protection of Mud Crab Scylla paramamosain in Response to the Secondary Challenge by Vibrio parahaemolyticus

“Immune priming” plays a vital part in the immune system of invertebrates, protecting against recurrent infections by pathogens, and can provide some ideas for the prevention and treatment of invertebrate diseases. Many invertebrates have been demonstrated recently to have immune priming, but the relevant mechanisms are not known. Expression of immune system–related genes in the hemocytes and hepatopancreas of the mud crab (Scylla paramamosain) before and after repeated stimulation with Vibrio parahaemolyticus were analyzed by real-time fluorescence quantitative polymerase chain reaction. Some molecules that may participate in the immune priming of S. paramamosain were screened out, and their possible roles in immune priming were interpreted. Crabs injected first with heat-killed V. parahaemolyticus (HkVp group) or physiologic (0.9%) saline (PS group) were rechallenged at 168 h with live V. parahaemolyticus (HkVp+Vp group and PS+Vp group, respectively). The log-rank test shows a significant difference in survival rate between the HkVp+Vp group and the other groups after the ICH (p < 0.05). Expression of genes involved in the toll-like receptor (TLR) signaling pathway and some antimicrobial peptide genes were detected. By, respectively, comparing gene quantification at different time points in hemocytes and the hepatopancreas, the molecules that may play a part in the early stage of the immune priming of S. paramamosain in the hemocytes are found to be down syndrome cell adhesion molecule (Dscam), Hyastatin, Cactus, Arasin, antilipopolysaccharide factor 3 (ALF3), ALF4, ALF5, and ALF6 as well as later acting molecules, such as Crustin, Dorsal, Pelle, and myeloid differentiation factor 88 (MyD88). The molecules that functioned throughout the entire period are TLR and Spaetzle. In the hepatopancreas, the molecules that may play a part in the early stages of immune priming are Dscam, Hyastatin, Arasin, ALF6, Pelle, Spaetzle, Dorsal and, in the later stage, ALF4. The molecules that functioned throughout the entire period are TLR, Crustin, Cactus, MyD88, ALF3, and ALF5. In summary, the immune function of S. paramamosain is enhanced after it receives the same repetitive stimulation by V. parahaemolyticus, indicating immune priming in S. paramamosain. Our study enriches research on immune priming in invertebrates and lays the foundation for further studies revealing the molecular mechanism of immune priming in crabs.

Genomic and transcriptional analysis of genes containing fibrinogen and IgSF domains in the schistosome vector Biomphalaria glabrata, with emphasis on the differential responses of snails susceptible or resistant to Schistosoma mansoni

Achieving a deeper understanding of the factors controlling the defense responses of invertebrate vectors to the human-infecting pathogens they transmit will provide needed new leads to pursue for control. Consequently, we provide new genomic and transcriptomic insights regarding FReDs (containing a fibrinogen domain) and FREPs (fibrinogen domain and one or two IgSF domains) from the planorbid snail Biomphalaria glabrata, a Neotropical vector of Schistosoma mansoni, causative agent of human intestinal schistosomiasis. Using new bioinformatics approaches to improve annotation applied to both genome and RNA-Seq data, we identify 73 FReD genes, 39 of which are FREPs. We provide details of domain structure and consider relationships and homologies of B. glabrata FBG and IgSF domains. We note that schistosome-resistant (BS-90) snails mount complex FREP responses following exposure to S. mansoni infection whereas schistosome-susceptible (M line) snails do not. We also identify several coding differences between BS-90 and M line snails in three FREPs (2, 3.1 and 3.2) repeatedly implicated in other studies of anti-schistosome responses. In combination with other results, our study provides a strong impetus to pursue particular FREPs (2, 3.1, 3.2 and 4) as candidate resistance factors to be considered more broadly with respect to schistosome control efforts, including involving other Biomphalaria species vectoring S. mansoni in endemic areas in Africa.

A Sustained Immune Response Supports Long-Term Antiviral Immune Priming in the Pacific Oyster, Crassostrea gigas

Over the last decade, innate immune priming has been evidenced in many invertebrate phyla. If mechanistic models have been proposed, molecular studies aiming to substantiate these models have remained scarce. We reveal here the transcriptional signature associated with immune priming in the oyster Crassostrea gigas Oysters were fully protected against Ostreid herpesvirus 1 (OsHV-1), a major oyster pathogen, after priming with poly(I·C), which mimics viral double-stranded RNA. Global analysis through RNA sequencing of oyster and viral genes after immune priming and viral infection revealed that poly(I·C) induces a strong antiviral response that impairs OsHV-1 replication. Protection is based on a sustained upregulation of immune genes, notably genes involved in the interferon pathway and apoptosis, which control subsequent viral infection. This persistent antiviral alert state remains active over 4 months and supports antiviral protection in the long term. This acquired resistance mechanism reinforces the molecular foundations of the sustained response model of immune priming. It further opens the way to applications (pseudovaccination) to cope with a recurrent disease that causes dramatic economic losses in the shellfish farming industry worldwide.IMPORTANCE In the last decade, important discoveries have shown that resistance to reinfection can be achieved without a functional adaptive immune system, introducing the concept of innate immune memory in invertebrates. However, this field has been constrained by the limited number of molecular mechanisms evidenced to support these phenomena. Taking advantage of an invertebrate species, the Pacific oyster (Crassostrea gigas), in which we evidenced one of the longest and most effective periods of protection against viral infection observed in an invertebrate, we provide the first comprehensive transcriptomic analysis of antiviral innate immune priming. We show that priming with poly(I·C) induced a massive upregulation of immune-related genes, which control subsequent viral infection, and it was maintained for over 4 months after priming. This acquired resistant mechanism reinforces the molecular foundations of the sustained response model of immune priming. It opens the way to pseudovaccination to prevent the recurrent diseases that currently afflict economically or ecologically important invertebrates.

Biomphalaria glabrata immunity: Post-genome advances

The freshwater snail, Biomphalaria glabrata, is an important intermediate host in the life cycle for the human parasite Schistosoma mansoni, the causative agent of schistosomiasis. Current treatment and prevention strategies have not led to a significant decrease in disease transmission. However, the genome of B. glabrata was recently sequenced to provide additional resources to further our understanding of snail biology. This review presents an overview of recently published, post-genome studies related to the topic of snail immunity. Many of these reports expand on findings originated from the genome characterization. These novel studies include a complementary gene linkage map, analysis of the genome of the B. glabrata embryonic (Bge) cell line, as well as transcriptomic and proteomic studies looking at snail-parasite interactions and innate immune memory responses towards schistosomes. Also included are biochemical investigations on snail pheromones, neuropeptides, and attractants, as well as studies investigating the frontiers of molluscan epigenetics and cell signaling were also included. Findings support the current hypotheses on snail-parasite strain compatibility, and that snail host resistance to schistosome infection is dependent not only on genetics and expression, but on the ability to form multimeric molecular complexes in a timely and tissue-specific manner. The relevance of cell immunity is reinforced, while the importance of humoral factors, especially for secondary infections, is supported. Overall, these studies reflect an improved understanding on the diversity, specificity, and complexity of molluscan immune systems.

Coordination of humoral immune factors dictates compatibility between Schistosoma mansoni and Biomphalaria glabrata

Immune factors in snails of the genus Biomphalaria are critical for combating Schistosoma mansoni, the predominant cause of human intestinal schistosomiasis. Independently, many of these factors play an important role in, but do not fully define, the compatibility between the model snail B. glabrata, and S. mansoni. Here, we demonstrate association between four previously characterized humoral immune molecules; BgFREP3, BgTEP1, BgFREP2 and Biomphalysin. We also identify unique immune determinants in the plasma of S. mansoni-resistant B. glabrata that associate with the incompatible phenotype. These factors coordinate to initiate haemocyte-mediated destruction of S. mansoni sporocysts via production of reactive oxygen species. The inclusion of BgFREP2 in a BgFREP3-initiated complex that also includes BgTEP1 almost completely explains resistance to S. mansoni in this model. Our study unifies many independent lines of investigation to provide a more comprehensive understanding of the snail immune system in the context of infection by this important human parasite.

Glabralysins, Potential New β-Pore-Forming Toxin Family Members from the Schistosomiasis Vector Snail Biomphalaria glabrata

Biomphalaria glabrata is a freshwater Planorbidae snail. In its environment, this mollusk faces numerous microorganisms or pathogens, and has developed sophisticated innate immune mechanisms to survive. The mechanisms of recognition are quite well understood in Biomphalaria glabrata, but immune effectors have been seldom described. In this study, we analyzed a new family of potential immune effectors and characterized five new genes that were named Glabralysins. The five Glabralysin genes showed different genomic structures and the high degree of amino acid identity between the Glabralysins, and the presence of the conserved ETX/MTX2 domain, support the hypothesis that they are pore-forming toxins. In addition, tertiary structure prediction confirms that they are structurally related to a subset of Cry toxins from Bacillus thuringiensis, including Cry23, Cry45, and Cry51. Finally, we investigated their gene expression profiles in snail tissues and demonstrated a mosaic transcription. We highlight the specificity in Glabralysin expression following immune stimulation with bacteria, yeast or trematode parasites. Interestingly, one Glabralysin was found to be expressed in immune-specialized hemocytes, and two others were induced following parasite exposure.

A New Assessment of Thioester-Containing Proteins Diversity of the Freshwater Snail Biomphalaria glabrata

Thioester-containing proteins (TEPs) superfamily is known to play important innate immune functions in a wide range of animal phyla. TEPs are involved in recognition, and in the direct or mediated killing of several invading organisms or pathogens. While several TEPs have been identified in many invertebrates, only one TEP (named BgTEP) has been previously characterized in the freshwater snail, Biomphalaria glabrata. As the presence of a single member of that family is particularly intriguing, transcriptomic data and the recently published genome were used to explore the presence of other BgTEP related genes in B. glabrata. Ten other TEP members have been reported and classified into different subfamilies: Three complement-like factors (BgC3-1 to BgC3-3), one α-2-macroblobulin (BgA2M), two macroglobulin complement-related proteins (BgMCR1, BgMCR2), one CD109 (BgCD109), and three insect TEP (BgTEP2 to BgTEP4) in addition to the previously characterized BgTEP that we renamed BgTEP1. This is the first report on such a level of TEP diversity and of the presence of macroglobulin complement-related proteins (MCR) in mollusks. Gene structure analysis revealed alternative splicing in the highly variable region of three members (BgA2M, BgCD109, and BgTEP2) with a particularly unexpected diversity for BgTEP2. Finally, different gene expression profiles tend to indicate specific functions for such novel family members.

Gene drives for schistosomiasis transmission control

Schistosomiasis is one of the most important and widespread neglected tropical diseases (NTD), with over 200 million people infected in more than 70 countries; the disease has nearly 800 million people at risk in endemic areas. Although mass drug administration is a cost-effective approach to reduce occurrence, extent, and severity of the disease, it does not provide protection to subsequent reinfection. Interventions that target the parasites’ intermediate snail hosts are a crucial part of the integrated strategy required to move toward disease elimination. The recent revolution in gene drive technology naturally leads to questions about whether gene drives could be used to efficiently spread schistosome resistance traits in a population of snails and whether gene drives have the potential to contribute to reduced disease transmission in the long run. Responsible implementation of gene drives will require solutions to complex challenges spanning multiple disciplines, from biology to policy. This Review Article presents collected perspectives from practitioners of global health, genome engineering, epidemiology, and snail/schistosome biology and outlines strategies for responsible gene drive technology development, impact measurements of gene drives for schistosomiasis control, and gene drive governance. Success in this arena is a function of many factors, including gene-editing specificity and efficiency, the level of resistance conferred by the gene drive, how fast gene drives may spread in a metapopulation over a complex landscape, ecological sustainability, social equity, and, ultimately, the reduction of infection prevalence in humans. With combined efforts from across the broad global health community, gene drives for schistosomiasis control could fortify our defenses against this devastating disease in the future.

Allelic variation in a single genomic region alters the hemolymph proteome in the snail Biomphalaria glabrata

Freshwater snails are obligate intermediate hosts for numerous parasitic trematodes, most notably schistosomes. Schistosomiasis is a devastating human and veterinary illness, which is primarily controlled by limiting the transmission of these parasites from their intermediate snail hosts. Understanding how this transmission occurs, as well as the basic immunobiology of these snails may be important for controlling this disease in the future. Allelic variation in the Guadeloupe resistance complex (GRC) of Biomphalaria glabrata partially determines their susceptibility to parasitic infection, and can influence the microbiome diversity and microbial defenses in the hemolymph of these snails. In the present study, we examine the most abundant proteins present in the hemolymph of snails that are resistant or susceptible to schistosomes, as determined by their GRC genotype. Using proteomic analysis, we found that snails with different GRC genotypes have differentially abundant hemolymph proteins that are not explained by differences in transcription. There are 13 revealed hemolymph proteins that differ significantly between resistant and susceptible genotypes, nearly 40% of which are involved in immune responses. These findings build on the mounting evidence that genes in the GRC region have multiple physiological roles, and likely contribute more extensively to the general immune response than previously believed. These data also raise the intriguing possibility that the GRC region controls resistance to schistosomes, not directly, but indirectly via its effects on the snail's proteome and potentially its microbiome.

Derivatives of the lectin complement pathway in Lophotrochozoa

A plethora of non-overlapping immune molecular mechanisms in metazoans is the most puzzling issue in comparative immunobiology. No valid evolutionary retrospective on these mechanisms has been developed. In this study, we aimed to reveal the origin and evolution of the immune complement-like system in Lophotrochozoa. For this, we analyzed publicly available transcriptomes of prebilaterian and lophotrochozoan species, mapping lineage-specific molecular events on the phylogenetic tree. We found that there were no orthologs of mannose-binding lectin (MBL) and ficolins (FCN) in Lophotrochozoa but C1q-like proteins (C1qL), bearing both a collagen domain and a globular C1q domain, were omnipresent in them. This suggests that among all complement-like activators the C1qL-specific domain architecture was an evolutionarily first. Two novel protostomian MASP-Related Molecules, MReM1 and MReM2, might hypothetically compensate for the loss of a prebilaterian MASP-orthologous gene and act in complex with C1qL and C1qDC as a "proto-activator" of an ancient "proto-complement". We proposed a new model of the complement evolution predicting that numerous lineage-specific complement-like systems should have evolved from a stem "antique" molecular complex. First evolved in the common ancestor of coelomic animals, the "antique" humoral complex consisted of a TEP molecule, the common ancestor of TEP-associated proteases (C2/Bf/Сf/Lf), the common ancestor of MASP-like proteases (MASP/C1r/C1s, MReM1/MReM2) and multimeric recognition proteins (C1q-, MBL- and FCN-homologs). Further evolutionary specialization and expansion of the complex was independent and lineage-specific, examples being the mammalian complement system and the Apogastropoda complement-like complex. The latter includes an impressive array of multimeric recognition proteins, the variable immunoglobulin and lectin domain containing molecules (VIgL), homologous to C1q, MBL, FCN and other lectins. Four novel polymorphic subfamilies of VIgLs were found to be expressed in Apogastropoda: C1q-related proteins (QREP), zona pellucida-related proteins (ZREP), Scavenger Receptor Cys-Rich-related proteins (SREP) and HPA-lectin related proteins (HREP). The transcriptional response of fibrinogen-related proteins of VIgL family (LlFREP), LlQREP and LlSREP to infestation of common periwinkle, Littorina littorea, with digenean parasite Himasthla elongata correlates with that of LlMReM1, supporting the model suggested in this study.

Sympatric versus allopatric evolutionary contexts shape differential immune response in Biomphalaria / Schistosoma interaction

Selective pressures between hosts and their parasites can result in reciprocal evolution or adaptation of specific life history traits. Local adaptation of resident hosts and parasites should lead to increase parasite infectivity/virulence (higher compatibility) when infecting hosts from the same location (in sympatry) than from a foreign location (in allopatry). Analysis of geographic variations in compatibility phenotypes is the most common proxy used to infer local adaptation. However, in some cases, allopatric host-parasite systems demonstrate similar or greater compatibility than in sympatry. In such cases, the potential for local adaptation remains unclear. Here, we study the interaction between Schistosoma and its vector snail Biomphalaria in which such discrepancy in local versus foreign compatibility phenotype has been reported. Herein, we aim at bridging this gap of knowledge by comparing life history traits (immune cellular response, host mortality, and parasite growth) and molecular responses in highly compatible sympatric and allopatric Schistosoma/Biomphalaria interactions originating from different geographic localities (Brazil, Venezuela and Burundi). We found that despite displaying similar prevalence phenotypes, sympatric schistosomes triggered a rapid immune suppression (dual-RNAseq analyses) in the snails within 24h post infection, whereas infection by allopatric schistosomes (regardless of the species) was associated with immune cell proliferation and triggered a non-specific generalized immune response after 96h. We observed that, sympatric schistosomes grow more rapidly. Finally, we identify miRNAs differentially expressed by Schistosoma mansoni that target host immune genes and could be responsible for hijacking the host immune response during the sympatric interaction. We show that despite having similar prevalence phenotypes, sympatric and allopatric snail-Schistosoma interactions displayed strong differences in their immunobiological molecular dialogue. Understanding the mechanisms allowing parasites to adapt rapidly and efficiently to new hosts is critical to control disease emergence and risks of Schistosomiasis outbreaks.

Schistosomiasis, the second most widespread human parasitic disease after malaria, is caused by helminth parasites of the genus Schistosoma. More than 200 million people in 74 countries suffer from the pathological, and societal consequences of this disease. To complete its life cycle, the parasite requires an intermediate host, a freshwater snail of the genus Biomphalaria for its transmission. Given the limited options for treating Schistosoma mansoni infections in humans, much research has focused on developing methods to control transmission by its intermediate snail host. Biomphalaria glabrata. Comparative studies have shown that infection of the snail triggers complex cellular and humoral immune responses resulting in significant variations in parasite infectivity and snail susceptibility, known as the so-called polymorphism of compatibility. However, studies have mostly focused on characterizing the immunobiological mechanisms in sympatric interactions. Herein we used a combination of molecular and phenotypic approaches to compare the effect of infection in various sympatric and allopatric evolutionary contexts, allowing us to better understand the mechanisms of host-parasite local adaptation. Learning more about the immunobiological interactions between B. glabrata and S. mansoni could have important socioeconomic and public health impacts by changing the way we attempt to eradicate parasitic diseases and prevent or control schistosomiasis in the field.

Molecular characterisation of immunological memory following homologous or heterologous challenges in the schistosomiasis vector snail, Biomphalaria glabrata

Invertebrate immune response may be primed by a current infection in a sustained manner, leading to the failure of a secondary infection with the same pathogen. The present study focuses on the Schistosomiasis vector snail Biomphalaria glabrata, in which a specific genotype-dependent immunological memory was demonstrated as a shift from a cellular to a humoral immune response. Herein, we investigate the complex molecular bases associated with this genotype-dependant immunological memory response. We demonstrate that Biomphalaria regulates a polymorphic set of immune recognition molecules and immune effector repertoires to respond to different strains of Schistosoma parasites. These results suggest a combinatorial usage of pathogen recognition receptors (PRRs) that distinguish different strains of parasites during the acquisition of immunological memory. Immunizations also show that snails become resistant after exposure to parasite extracts. Hemolymph transfer and a label-free proteomic analysis proved that circulating hemolymph compounds can be produced and released to more efficiently kill the newly encountered parasite of the same genetic lineage.

Proteomic Analysis of Biomphalaria glabrata Hemocytes During in vitro Encapsulation of Schistosoma mansoni Sporocysts

Circulating hemocytes of the snail Biomphalaria glabrata, a major intermediate host for the blood fluke Schistosoma mansoni, represent the primary immune effector cells comprising the host's internal defense system. Within hours of miracidial entry into resistant B. glabrata strains, hemocytes infiltrate around developing sporocysts forming multi-layered cellular capsules that results in larval death, typically within 24–48 h post-infection. Using an in vitro model of hemocyte-sporocyst encapsulation that recapitulates in vivo events, we conducted a comparative proteomic analysis on the responses of hemocytes from inbred B. glabrata strains during the encapsulation of S. mansoni primary sporocysts. This was accomplished by a combination of Laser-capture microdissection (LCM) to isolate sections of hemocyte capsules both in the presence and absence of sporocysts, in conjunction with mass spectrometric analyses to establish protein expression profiles. Comparison of susceptible NMRI snail hemocytes in the presence and absence of sporocysts revealed a dramatic downregulation of proteins in during larval encapsulation, especially those involved in protein/CHO metabolism, immune-related, redox and signaling pathways. One of 4 upregulated proteins was arginase, competitor of nitric oxide synthetase and inhibitor of larval-killing NO production. By contrast, when compared to control capsules, sporocyst-encapsulating hemocytes of resistant BS-90 B. glabrata exhibited a more balanced profile with enhanced expression of shared proteins involved in protein synthesis/processing, immunity, and redox, and unique expression of anti-microbial/anti-parasite proteins. A final comparison of NMRI and BS-90 host hemocyte responses to co-cultured sporocysts demonstrated a decrease or downregulation of 77% of shared proteins by NMRI cells during encapsulation compared to those of the BS-90 strain, including lipopolysaccharide-binding protein, thioredoxin reductase 1 and hemoglobins 1 and 2. Overall, using this in vitro model, results of our proteomic analyses demonstrate striking differences in proteins expressed by susceptible NMRI and resistant BS-90 snail hemocytes to S. mansoni sporocysts during active encapsulation, with NMRI hemocytes exhibiting extensive downregulation of protein expression and a lower level of constitutively expressed immune-relevant proteins (e.g., FREP2) compared to BS-90. Our data suggest that snail strain differences in hemocyte protein expression during the encapsulation process account for observed differences in their cytotoxic capacity to interact with and kill sporocysts.

Innate Immune Memory in Invertebrate Metazoans: A Critical Appraisal

The ability of developing immunological memory, a characteristic feature of adaptive immunity, is clearly present also in innate immune responses. In fact, it is well known that plants and invertebrate metazoans, which only have an innate immune system, can mount a faster and more effective response upon re-exposure to a stimulus. Evidence of immune memory in invertebrates comes from studies in infection immunity, natural transplantation immunity, individual, and transgenerational immune priming. These studies strongly suggest that environment and lifestyle take part in the development of immunological memory. However, in several instances the formal correlation between the phenomenon of immune memory and molecular and functional immune parameters is still missing. In this review, we have critically examined the cellular and humoral aspects of the invertebrate immune memory responses. In particular, we have focused our analysis on studies that have addressed immune memory in the most restrictive meaning of the term, i.e., the response to a challenge of a quiescent immune system that has been primed in the past. These studies highlight the central role of an increase in the number of immune cells and of their epigenetic re-programming in the establishment of sensu stricto immune memory in invertebrates.

Immune repertoire in the transcriptome of Littorina littorea reveals new trends in lophotrochozoan proto-complement evolution

The evolution of complement system in invertebrates is poorly investigated. While the repertoire of complement genes in several Ecdysozoa lineages is found substantially different from that of Deuterostomia, the composition and function of the complement in the second protostome lineage, Lophotrochozoa, remains unclear. Here we report the general description of new transcriptomic data on the common periwinkle, Littorina littorea, and trace the evolutionary trajectories of the ancestral proto-complement repertoire. The repertoire is defined as immune cascade providing the minimum set of C3-associated molecules required for C3b amplification, opsonization of the targets and their phagocytosis: thioester protein (TEP) C3, serine protease C2/factor B (Bf) and complement receptors (CR). The reference transcriptome of L. littorea was built from the dual-species RNA-seq experiment with the periwinkle and its tissue digenean parasite Himasthla elongata. Five TEPs, including the ortholog of the C3, are found expressed in the in the mollusk's inflamed tissues. The homolog of the complement receptors CR1/CR2 is also expressed, however the ortholog of Bf is not. The extensive phylogenetic analysis showed that the C3 ortholog and the complement receptors are retained in all key lophotrochozoan taxa: Mollusca, Annelida and Brachiopoda. However, the Bf ortholog was lost at least three times independently in different lineages: i) Cephalopoda, ii) a common ancestor of all Gastropoda and iii) one of the Annelida lineage, Clitellata. Both C3 and Bf molecules were retained in bivalve species, brachiopods and annelid worms from the Polychaeta lineage. Hypothetically, the function of the lost Bf in these animals can be compensated by Factor L (Lf) - the serine protease first found in L. littorea and homologous to both, the Bf and the arthropod factor C (Cf). The contrast differences in proto-complement repertoire between the sister mollusk' taxa, Bivalvia and Gastropoda (the conserved and modified sets, respectively), can underlie differences in their susceptibility to digenean infection.

Comparative immunological study of the snail Physella acuta (Hygrophila, Pulmonata) reveals shared and unique aspects of gastropod immunobiology

The freshwater snail Physella acuta was selected to expand the perspective of comparative snail immunology. Analysis of Physella acuta, belonging to the Physidae, taxonomic sister family to Planorbidae, affords family-level comparison of immune features characterized from Biomphalaria glabrata, the model snail often used to interpret general gastropod immunity. To capture constitutive and induced immune sequences, transcriptomes of an individual Physella acuta snail, 12 h post injection with bacteria (Gram -/+) and one sham-exposed snail were recorded with 454 pyrosequencing. Assembly yielded a combined reference transcriptome containing 24,288 transcripts. Additionally, genomic Illumina reads were obtained (∼15-fold coverage). Recovery of transcripts for two macin-like antimicrobial peptides (AMPs), 12 aplysianins, four LBP/BPIs and three physalysins indicated that Physella acuta shares a similar organization of antimicrobial defenses with Biomphalaria glabrata, contrasting a modest AMP arsenal with a diverse set of antimicrobial proteins. The lack of predicted transmembrane domains in all seven Physella acuta PGRP transcripts supports the notion that gastropods do not employ cell-bound PGRP receptors, different from ecdysozoan invertebrates yet similar to mammals (vertebrate deuterostomes). The well-documented sequence diversification by Biomphalaria glabrata FREPs (immune lectins comprising immunoglobulin superfamily domains and fibrinogen domains), resulting from somatic mutations of a large FREP gene family is hypothesized to be unique to Planorbidae; Physella acuta revealed just two bonafide FREP genes and these were not diversified. Furthermore, the flatworm parasite Echinostoma paraensei, confirmed here to infect both snail species, did not evoke from Physella acuta the abundant expression of FREP proteins at 2, 4 and 8 days post exposure that was previously observed from Biomphalaria glabrata. The Physella acuta reference transcriptome also revealed 24 unique transcripts encoding proteins consisting of a single fibrinogen-related domain (FReDs), with a short N-terminal sequence encoding either a signal peptide, transmembrane domain or no predicted features. The Physella acuta FReDs are candidate immune genes based on implication of similar sequences in immunity of bivalve molluscs. Overall, comparative analysis of snails of sister families elucidated the potential for taxon-specific immune features and investigation of strategically selected species will provide a more comprehensive view of gastropod immunity.

BgTEP: An Antiprotease Involved in Innate Immune Sensing in Biomphalaria glabrata

Insect thioester-containing protein (iTEP) is the most recently defined group among the thioester-containing protein (TEP) superfamily. TEPs are key components of the immune system, and iTEPs from flies and mosquitoes were shown to be major immune weapons. Initially characterized from insects, TEP genes homologous to iTEP were further described from several other invertebrates including arthropods, cniderians, and mollusks albeit with few functional characterizations. In the freshwater snail Biomphalaria glabrata, a vector of the schistosomiasis disease, the presence of a TEP protein (BgTEP) was previously described in a well-defined immune complex involving snail lectins (fibrinogen-related proteins) and schistosome parasite mucins (SmPoMuc). To investigate the potential role of BgTEP in the immune response of the snail, we first characterized its genomic organization and its predicted protein structure. A phylogenetic analysis clustered BgTEP in a well-conserved subgroup of mollusk TEP. We then investigated the BgTEP expression profile in different snail tissues and followed immune challenges using different kinds of intruders during infection kinetics. Results revealed that BgTEP is particularly expressed in hemocytes, the immune-specialized cells in invertebrates, and is secreted into the hemolymph. Transcriptomic results further evidenced an intruder-dependent differential expression pattern of BgTEP, while interactome experiments showed that BgTEP is capable of binding to the surface of different microbes and parasite either in its full length form or in processed forms. An immunolocalization approach during snail infection by the Schistosoma mansoni parasite revealed that BgTEP is solely expressed by a subtype of hemocytes, the blast-like cells. This hemocyte subtype is present in the hemocytic capsule surrounding the parasite, suggesting a potential role in the parasite clearance by encapsulation. Through this work, we report the first characterization of a snail TEP. Our study also reveals that BgTEP may display an unexpected functional dual role. In addition to its previously characterized anti-protease activity, we demonstrate that BgTEP can bind to the intruder surface membrane, which supports a likely opsonin role.

SpTransformer proteins from the purple sea urchin opsonize bacteria, augment phagocytosis, and retard bacterial growth

The purple sea urchin, Strongylocentrotus purpuratus, has a complex and robust immune system that is mediated by a number of multi-gene families including the SpTransformer (SpTrf) gene family (formerly Sp185/333). In response to immune challenge from bacteria and various pathogen-associated molecular patterns, the SpTrf genes are up-regulated in sea urchin phagocytes and express a diverse array of SpTrf proteins. We show here that SpTrf proteins from coelomocytes and isolated by nickel affinity (cNi-SpTrf) bind to Gram-positive and Gram-negative bacteria and to Baker's yeast, Saccharomyces cerevisiae, with saturable kinetics and specificity. cNi-SpTrf opsonization of the marine bacteria, Vibrio diazotrophicus, augments phagocytosis, however, opsonization by the recombinant protein, rSpTrf-E1, does not. Binding by cNi-SpTrf proteins retards growth rates significantly for several species of bacteria. SpTrf proteins, previously thought to be strictly membrane-associated, are secreted from phagocytes in short term cultures and bind V. diazotrophicus that are located both outside of and within phagocytes. Our results demonstrate anti-microbial activities of native SpTrf proteins and suggest variable functions among different SpTrf isoforms. Multiple isoforms may act synergistically to detect a wide array of pathogens and provide flexible and efficient host immunity.

Viral exposure effects on life-history, flight-related traits, and wing melanisation in the Glanville fritillary butterfly

Infections represent a constant threat for organisms and can lead to substantial fitness losses. Understanding how individuals, especially from natural populations, respond towards infections is thus of great importance. Little is known about immunity in the Glanville fritillary butterfly (Melitaea cinxia). As the larvae live gregariously in family groups, vertical and horizontal transmission of infections could have tremendous effects on individuals and consequently impact population dynamics in nature. We used the Alphabaculovirus type strain Autographa californica multiple nucleopolyhedrovirus (AcMNPV) and demonstrated that positive concentration-dependent baculovirus exposure leads to prolonged developmental time and decreased survival during larval and pupal development, with no sex specific differences. Viral exposure did not influence relative thorax mass or wing morphometric traits often related to flight ability, yet melanisation of the wings increased with viral exposure, potentially influencing disease resistance or flight capacity via thermal regulation. Further research is needed to explore effects under sub-optimal conditions, determine effects on fitness-related traits, and investigate a potential adaptive response of increased melanisation in the wings due to baculovirus exposure.

Comparative Functional Study of Thioester-containing Related Proteins in the Recently Sequenced Genome of Biomphalaria glabrata

BACKGROUND

There is paucity of information on functional relationship and characterization of Biomphalaria glabrata thioester-containing proteins (BgTEP) to other well-annotated homologues. We performed functional characterization studies of BgTEP to homologues in Anopheles gambiae and in disparate invertebrates.

METHODS

Genomic sequences of TEPs were retrieved after annotation of the B. glabrata genome. In addition, TEP sequences deposited in NCBI protein database were also retrieved and utilized for sequence analysis. BgTEP relatedness to its other homologues as well as functional domain and protein-protein interaction analysis was performed.

RESULTS

Our analysis resulted in the identification of TEPs in a number of organisms including, B. glabrata, A. gambiae, and Chlamys farreri. In addition, we identified 19 TEP sequences spread across 10 animal species. The B. glabrata genome contains 14190 unannotated proteins after filtration with about 85% of its proteome annotated. The phylogenetics, functional domain and protein-protein interaction analyses suggest an immunological role for BgTEP in B. glabrata.

CONCLUSION

The predicted role of thioester-containing proteins to be involved in immunological role in B. glabrata may have a strong effect on resistance to infection.

Specific Pathogen Recognition by Multiple Innate Immune Sensors in an Invertebrate

Detection of pathogens by all living organisms is the primary step needed to implement a coherent and efficient immune response. This implies a mediation by different soluble and/or membrane-anchored proteins related to innate immune receptors called PRRs (pattern-recognition receptors) to trigger immune signaling pathways. In most invertebrates, their roles have been inferred by analogy to those already characterized in vertebrate homologs. Despite the induction of their gene expression upon challenge and the presence of structural domains associated with the detection of pathogen-associated molecular patterns in their sequence, their exact role in the induction of immune response and their binding capacity still remain to be demonstrated. To this purpose, we developed a fast interactome approach, usable on any host–pathogen couple, to identify soluble proteins capable of directly or indirectly detecting the presence of pathogens. To investigate the molecular basis of immune recognition specificity, different pathogens (Gram-positive bacterium, Micrococcus luteus; Gram-negative, Escherichia coli; yeast, Saccharomyces cerevisiae; and metazoan parasites, Echinostoma caproni or Schistosoma mansoni) were exposed to hemocyte-free hemolymph from the gastropod Biomphalaria glabrata. Twenty-three different proteins bound to pathogens were identified and grouped into three different categories based on their primary function. Each pathogen was recognized by a specific but overlapping set of circulating proteins in mollusk’s hemolymph. While known PRRs such as C-type lectins were identified, other proteins not known to be primarily involved in pathogen recognition were found, including actin, tubulin, collagen, and hemoglobin. Confocal microscopy and specific fluorescent labeling revealed that extracellular actin present in snail hemolymph was able to bind to yeasts and induce their clotting, a preliminary step for their elimination by the snail immune system. Aerolysin-like proteins (named biomphalysins) were the only ones involved in the recognition of all the five pathogens tested, suggesting a sentinel role of these horizontally acquired toxins. These findings highlight the diversity and complexity of a highly specific innate immune sensing system. It paves the way for the use of such approach on a wide range of host–pathogen systems to provide new insights into the specificity and diversity of immune recognition by innate immune systems.

Integrated multi-omic analyses in Biomphalaria-Schistosoma dialogue reveal the immunobiological significance of FREP-SmPoMuc interaction

The fresh water snail Biomphalaria glabrata is one of the vectors of the trematode pathogen Schistosoma mansoni, which is one of the agents responsible of human schistosomiasis. In this host-parasite interaction, co-evolutionary dynamic results into an infectivity mosaic known as compatibility polymorphism. Integrative approaches including large scale molecular approaches have been conducted in recent years to improve our understanding of the mechanisms underlying compatibility. This review presents the combination of integrated Multi-Omic approaches leading to the discovery of two repertoires of polymorphic and/or diversified interacting molecules: the parasite antigens S. mansoni polymorphic mucins (SmPoMucs) and the B. glabrata immune receptors fibrinogen-related proteins (FREPs). We argue that their interactions may be major components for defining the compatible/incompatible status of a specific snail/schistosome combination.

Schistosomiasis from a Snail's Perspective: Advances in Snail Immunity

The snail's immune response is an important determinant of schistosome infection success, acting in concert with host, parasite, and environmental factors. Coordinated by haemocytes and humoral factors, it possesses immunological hallmarks such as pattern recognition receptors, and predicted gastropod-unique factors like the immunoglobulin superfamily domain-containing fibrinogen-related proteins. Investigations into mechanisms that underpin snail-schistosome compatibility have advanced quickly, contributing functional insight to many observational studies. While the snail's immune response is important to continue studying from the perspective of evolutionary immunology, as the foundational determinants of snail-schistosome compatibility continue to be discovered, the possibility of exploiting the snail for schistosomiasis control moves closer into reach. Here, we review the current understanding of immune mechanisms that influence compatibility between Schistosoma mansoni and Biomphalaria glabrata.

An evolutionary perspective on the systems of adaptive immunity

We propose an evolutionary perspective to classify and characterize the diverse systems of adaptive immunity that have been discovered across all major domains of life. We put forward a new function-based classification according to the way information is acquired by the immune systems: Darwinian immunity (currently known from, but not necessarily limited to, vertebrates) relies on the Darwinian process of clonal selection to 'learn' by cumulative trial-and-error feedback; Lamarckian immunity uses templated targeting (guided adaptation) to internalize heritable information on potential threats; finally, shotgun immunity operates through somatic mechanisms of variable targeting without feedback. We argue that the origin of Darwinian (but not Lamarckian or shotgun) immunity represents a radical innovation in the evolution of individuality and complexity, and propose to add it to the list of major evolutionary transitions. While transitions to higher-level units entail the suppression of selection at lower levels, Darwinian immunity re-opens cell-level selection within the multicellular organism, under the control of mechanisms that direct, rather than suppress, cell-level evolution for the benefit of the individual. From a conceptual point of view, the origin of Darwinian immunity can be regarded as the most radical transition in the history of life, in which evolution by natural selection has literally re-invented itself. Furthermore, the combination of clonal selection and somatic receptor diversity enabled a transition from limited to practically unlimited capacity to store information about the antigenic environment. The origin of Darwinian immunity therefore comprises both a transition in individuality and the emergence of a new information system - the two hallmarks of major evolutionary transitions. Finally, we present an evolutionary scenario for the origin of Darwinian immunity in vertebrates. We propose a revival of the concept of the 'Big Bang' of vertebrate immunity, arguing that its origin involved a 'difficult' (i.e. low-probability) evolutionary transition that might have occurred only once, in a common ancestor of all vertebrates. In contrast to the original concept, we argue that the limiting innovation was not the generation of somatic diversity, but the regulatory circuitry needed for the safe operation of amplifiable immune responses with somatically acquired targeting. Regulatory complexity increased abruptly by genomic duplications at the root of the vertebrate lineage, creating a rare opportunity to establish such circuitry. We discuss the selection forces that might have acted at the origin of the transition, and in the subsequent stepwise evolution leading to the modern immune systems of extant vertebrates.

Proteomic analysis of Biomphalaria glabrata plasma proteins with binding affinity to those expressed by early developing larval Schistosoma mansoni

Interactions between early developing Schistosoma mansoni larval stages and the hemolymph of its snail intermediate host represent the first molecular encounter with the snail’s immune system. To gain a more comprehensive understanding of this early parasite-host interaction, biotinylated sporocyst tegumental membrane (Mem) proteins and larval transformation proteins (LTP) were affixed to streptavidin-agarose beads and used as affinity matrices to enrich for larval-reactive plasma proteins from susceptible (NMRI) and resistant (BS-90) strains of the snail Biomphalaria glabrata. Nano-LC/MS-MS proteomic analyses of isolated plasma proteins revealed a diverse array of 94 immune-and nonimmune-related plasma proteins. Included among the immune-related subset were pattern recognition receptors (lectins, LPS-binding protein, thioester-containing proteins-TEPs), stress proteins (HSP60 and 70), adhesion proteins (dermatopontins), metalloproteases (A Disintegrin And Metalloproteinase (ADAM), ADAM-related Zn proteinases), cytotoxins (biomphalysin) and a Ca²⁺-binding protein (neo-calmodulin). Variable immunoglobulin and lectin domain (VIgL) gene family members, including fibrinogen-related proteins (FREPs), galectin-related proteins (GREPs) and C-type lectin-related proteins (CREPs), were the most prevalent of larval-reactive immune lectins present in plasma. FREPs were highly represented, although only a subset of FREP subfamilies (FREP 2, 3 and 12) were identified, suggesting potential selectivity in the repertoire of plasma lectins recognizing larval glycoconjugates. Other larval-binding FREP-like and CREP-like proteins possessing a C-terminal fibrinogen-related domain (FReD) or C-type lectin binding domain, respectively, and an Ig-fold domain also were identified as predicted proteins from the B. glabrata genome, although incomplete sequence data precluded their placement into specific FREP/CREP subfamilies. Similarly, a group of FReD-containing proteins (angiopoeitin-4, ficolin-2) that lacked N-terminal Ig-fold(s) were identified as a distinct group of FREP-like proteins, separate from the VIgL lectin family. Finally, differential appearance of GREPs in BS-90 plasma eluates, and others proteins exclusively found in eluates of the NMRI strain, suggested snail strain differences in the expression of select larval-reactive immune proteins. This hypothesis was supported by the finding that differential gene expression of the GREP in BS-90 and ADAM in NMRI snail strains generally correlated with their patterns of protein expression. In summary, this study is the first to provide a global comparative proteomic analysis of constitutively expressed plasma proteins from susceptible and resistant B. glabrata strains capable of binding early-expressed larval S. mansoni proteins. Identified proteins, especially those exhibiting differential expression, may play a role in determining immune compatibility in this snail host-parasite system. A complete listing of raw peptide data are available via ProteomeXchange using identifier PXD004942.

Transmission of the human blood fluke Schistosoma mansoni critically depends on the successful establishment of infections within species of its snail intermediate host, Biomphalaria. One of the most important barriers to infection is the host’s innate immune system, comprised of plasma proteins and immunocytes (hemocytes) circulating in the hemolymph. Although expression of plasma lectin genes appears to be associated with larval resistance in B. glabrata, few studies have attempted an in depth analysis of gene-encoded lectins, and other immune proteins, that are capable of directly binding schistosome larvae. Using affinity matrices linked to schistosome proteins expressed during early larval development, we identified and compared the parasite-reactive plasma proteins from the susceptible NMRI and resistant BS-90 strains of B. glabrata. Proteomic analyses of isolated plasma proteins revealed a diversity immune-related proteins including lectins, pathogen recognition receptors, cytotoxins, adhesion proteins, metalloproteinases, and Ca²⁺-binding proteins. Of the lectins, the variable immunoglobulin and lectin domain (VIgL) gene family of proteins comprised of fibrinogen-related proteins (FREPs), galectin-related proteins (GREPs) and C-type lectin-related proteins (CREPs), were highly represented, and consistent with their role in host immunity. Two proteins (GREP and a Zn-metalloproteinase) exhibited snail strain-associated protein and gene expression patterns suggesting their involvement in innate immune responses to larval infection. This comparative proteomic analysis of larval S. mansoni-reactive plasma proteins from susceptible and resistant B. glabrata strains represents the first of its kind and provides valuable insights into possible pathogen recognition receptors and other immune factors regulating parasite-host compatibility in this model system.

A multistrain approach to studying the mechanisms underlying compatibility in the interaction between Biomphalaria glabrata and Schistosoma mansoni

In recent decades, numerous studies have sought to better understand the mechanisms underlying the compatibility between Biomphalaria glabrata and Schistosoma mansoni. The developments of comparative transcriptomics, comparative genomics, interactomics and more targeted approaches have enabled researchers to identify a series of candidate genes. However, no molecular comparative work has yet been performed on multiple populations displaying different levels of compatibility. Here, we seek to fill this gap in the literature. We focused on B. glabrata FREPs and S. mansoni SmPoMucs, which were previously demonstrated to be involved in snail/schistosome compatibility. We studied the expression and polymorphisms of these factors in combinations of snail and schistosome isolates that display different levels of compatibility. We found that the polymorphism and expression levels of FREPs and SmPoMucs could be linked to the compatibility level of S. mansoni. These data and our complementary results obtained by RNA-seq of samples from various snail strains indicate that the mechanism of compatibility is much more complex than previously thought, and that it is likely to be highly variable within and between populations. This complexity must be taken into account if we hope to identify the molecular pathways that are most likely to be good targets for strategies aimed at blocking transmission of the parasite through the snail intermediate host.

Schistosomiasis is the second most widespread human tropical parasitic disease after malaria. It is caused by flatworms of the genus Schistosoma, and poses a considerable threat for human health in numerous Asian, African and South American countries. The World Health Organization has set the goal of eradicating schistosomiasis by 2025. However, no vaccine is available, and we currently have only one drug (praziquantel) that can effectively and efficiently treat the disease. As treatment by mass drug administration would enhance the risk of drug resistance in schistosome parasites, complementary strategies to fight this parasitic disease are urgently needed. Freshwater snails of the Biomphalaria genus act as intermediate hosts in the transmission of the schistosome species. Thus, learning more about the mechanisms of the interaction between these snails and the schistosomes could critically facilitate the identification of potential new candidate molecules that may be targeted to prevent schistosome transmission in the field.

H+ channels in embryonic Biomphalaria glabrata cell membranes: Putative roles in snail host-schistosome interactions

The human blood fluke Schistosoma mansoni causes intestinal schistosomiasis, a widespread neglected tropical disease. Infection of freshwater snails Biomphalaria spp. is an essential step in the transmission of S. mansoni to humans, although the physiological interactions between the parasite and its obligate snail host that determine success or failure are still poorly understood. In the present study, the B. glabrata embryonic (Bge) cell line, a widely used in vitro model for hemocyte-like activity, was used to investigate membrane properties, and assess the impact of larval transformation proteins (LTP) on identified ion channels. Whole-cell patch clamp recordings from Bge cells demonstrated that a Zn2+-sensitive H+ channel serves as the dominant plasma membrane conductance. Moreover, treatment of Bge cells with Zn2+ significantly inhibited an otherwise robust production of reactive oxygen species (ROS), thus implicating H+ channels in the regulation of this immune function. A heat-sensitive component of LTP appears to target H+ channels, enhancing Bge cell H+ current over 2-fold. Both Bge cells and B. glabrata hemocytes express mRNA encoding a hydrogen voltage-gated channel 1 (HVCN1)-like protein, although its function in hemocytes remains to be determined. This study is the first to identify and characterize an H+ channel in non-neuronal cells of freshwater molluscs. Importantly, the involvement of these channels in ROS production and their modulation by LTP suggest that these channels may function in immune defense responses against larval S. mansoni.

The Compatibility Between Biomphalaria glabrata Snails and Schistosoma mansoni: An Increasingly Complex Puzzle

This review reexamines the results obtained in recent decades regarding the compatibility polymorphism between the snail, Biomphalaria glabrata, and the pathogen, Schistosoma mansoni, which is one of the agents responsible for human schistosomiasis. Some results point to the snail's resistance as explaining the incompatibility, while others support a "matching hypothesis" between the snail's immune receptors and the schistosome's antigens. We propose here that the two hypotheses are not exclusive, and that the compatible/incompatible status of a particular host/parasite couple probably reflects the balance of multiple molecular determinants that support one hypothesis or the other. Because these genes are involved in a coevolutionary arms race, we also propose that the underlying mechanisms can vary. Finally, some recent results show that environmental factors could influence compatibility. Together, these results make the compatibility between B. glabrata and S. mansoni an increasingly complex puzzle. We need to develop more integrative approaches in order to find targets that could potentially be manipulated to control the transmission of schistosomiasis.

Fight or flight? - Flight increases immune gene expression but does not help to fight an infection

Flight represents a key trait in most insects, being energetically extremely demanding, yet often necessary for foraging and reproduction. Additionally, dispersal via flight is especially important for species living in fragmented landscapes. Even though, based on life‐history theory, a negative relationship may be expected between flight and immunity, a number of previous studies have indicated flight to induce an increased immune response. In this study, we assessed whether induced immunity (i.e. immune gene expression) in response to 15‐min forced flight treatment impacts individual survival of bacterial infection in the Glanville fritillary butterfly (Melitaea cinxia). We were able to confirm previous findings of flight‐induced immune gene expression, but still observed substantially stronger effects on both gene expression levels and life span due to bacterial infection compared to flight treatment. Even though gene expression levels of some immunity‐related genes were elevated due to flight, these individuals did not show increased survival of bacterial infection, indicating that flight‐induced immune activation does not completely protect them from the negative effects of bacterial infection. Finally, an interaction between flight and immune treatment indicated a potential trade‐off: flight treatment increased immune gene expression in naïve individuals only, whereas in infected individuals no increase in immune gene expression was induced by flight. Our results suggest that the up‐regulation of immune genes upon flight is based on a general stress response rather than reflecting an adaptive response to cope with potential infections during flight or in new habitats.

Deep, multi-stage transcriptome of the schistosomiasis vector Biomphalaria glabrata provides platform for understanding molluscan disease-related pathways

BACKGROUND

The gastropod mollusc Biomphalaria glabrata is well known as a vector for the tropical disease schistosomiasis, which affects nearly 200 million people worldwide. Despite intensive study, our understanding of the genetic basis of B. glabrata development, growth and disease resistance is constrained by limited genetic resources, constraints for which next-generation sequencing methods provide a ready solution.

METHODS

Illumina sequencing and de novo assembly using the Trinity program was used to generate a high-quality transcriptomic dataset spanning the entirety of in ovo development in schistosomiasis-free B. glabrata. This was subjected to automated (KEGG, BLAST2GO) and manual annotation efforts, allowing insight into the gene complements of this species in a number of contexts.

RESULTS

Excellent dataset recovery was observed, with 133,084 contigs produced of mean size 2219.48 bp. 80,952 (60.8 %) returned a BLASTx hit with an E value of less than 10^-3, and 74,492 (55.97 %) were either mapped or assigned a GO identity using the BLAST2GO program. The CEGMA set of core eukaryotic genes was found to be 99.6 % present, indicating exceptional transcriptome completeness. We were able to identify a wealth of disease-pathway related genes within our dataset, including the Wnt, apoptosis and Notch pathways. This provides an invaluable reference point for further work into molluscan development and evolution, for studying the impact of schistosomiasis in this species, and perhaps providing targets for the treatment of this widespread disease.

CONCLUSIONS

Here we present a deep transcriptome of an embryonic sample of schistosomiasis-free B. glabrata, presenting a comprehensive dataset for comparison to disease-affected specimens and from which conclusions can be drawn about the genetics of this widespread medical model. Furthermore, the dataset provided by this sequencing provides a useful reference point for comparison to other mollusc species, which can be used to better understand the evolution of this commercially, ecologically and medically important phylum.

Immune memory in invertebrates

Evidence for innate immune memory (or 'priming') in invertebrates has been accumulating over the last years. We here provide an in-depth review of the current state of evidence for immune memory in invertebrates, and in particular take a phylogenetic viewpoint. Invertebrates are a very heterogeneous group of animals and accordingly, evidence for the phenomenon of immune memory as well as the hypothesized molecular underpinnings differ largely for the diverse invertebrate taxa. The majority of research currently focuses on Arthropods, while evidence from many other groups of invertebrates is fragmentary or even lacking. We here concentrate on immune memory that is induced by pathogenic challenges, but also extent our view to a non-pathogenic context, i.e. allograft rejection, which can also show forms of memory and can inform us about general principles of specific self-nonself recognition. We discuss definitions of immune memory and a number of relevant aspects such as the type of antigens used, the route of exposure, and the kinetics of reactions following priming.

Epigenetic origin of adaptive phenotypic variants in the human blood fluke Schistosoma mansoni

BACKGROUND

Adaptive evolution is not possible without the generation of phenotypic variants. The origin of these variations has been a central topic in evolutionary biology. Up to now, it was commonly accepted that standing genetic variation is the only cause of phenotypic variants. However, epigenetic information is emerging as a complementary source of heritable phenotypic variation that contributes to evolution. The relative importance of genetics and epigenetics in generating heritable phenotypic variation is nevertheless a matter of debate.

RESULTS

We used a host-parasite system to address this question. The human blood fluke Schistosoma mansoni can adapt rapidly to new intermediate snail hosts. The interaction between parasite and mollusk is characterized by a compatibility polymorphism illustrating the evolutionary dynamics in this system. The principal molecular marker for compatibility (infection success) is the expression pattern of a group of polymorphic mucins (SmPoMuc) in the parasite. We show here that chromatin structure changes as the SmPoMuc promoters are the cause for SmPoMuc transcription polymorphism leading to phenotypic novelty and increase in infection success, i.e., fitness.

CONCLUSION

We establish that epigenetic changes can be the major if not only cause of adaptive phenotypic variants in Schistosoma mansoni, suggesting that epimutations can provide material for adaptive evolution in the absence of genetic variation in other systems. In addition, our results indicate that epidrugs can be used to control parasite development but also parasite evolution.

Endogenous growth factor stimulation of hemocyte proliferation induces resistance to Schistosoma mansoni challenge in the snail host

Digenean trematodes are a large, complex group of parasitic flatworms that infect an incredible diversity of organisms, including humans. Larval development of most digeneans takes place within a snail (Gastropoda). Compatibility between snails and digeneans is often very specific, such that suitable snail hosts define the geographical ranges of diseases caused by these worms. The immune cells (hemocytes) of a snail are sentinels that act as a crucial barrier to infection by larval digeneans. Hemocytes coordinate a robust and specific immunological response, participating directly in parasite killing by encapsulating and clearing the infection. Hemocyte proliferation and differentiation are influenced by unknown digenean-specific exogenous factors. However, we know nothing about the endogenous control of hemocyte development in any gastropod model. Here, we identify and functionally characterize a progranulin [Biomphalaria glabrata granulin (BgGRN)] from the snail B. glabrata, a natural host for the human blood fluke Schistosoma mansoni Granulins are growth factors that drive proliferation of immune cells in organisms, spanning the animal kingdom. We demonstrate that BgGRN induces proliferation of B. glabrata hemocytes, and specifically drives the production of an adherent hemocyte subset that participates centrally in the anti-digenean defense response. Additionally, we demonstrate that susceptible B. glabrata snails can be made resistant to infection with S. mansoni by first inducing hemocyte proliferation with BgGRN. This marks the functional characterization of an endogenous growth factor of a gastropod mollusc, and provides direct evidence of gain of resistance in a snail-digenean infection model using a defined factor to induce snail resistance to infection.

Polymorphic Mucin-Like Proteins in Schistosoma mansoni, a Variable Antigen and a Key Component of the Compatibility Between the Schistosome and Its Snail Host

The arms race between vertebrate hosts and parasites has led to diversification systems able to generate huge repertoires of immune recognition receptors and antigenic variants. Until recently, the invertebrate immunity was considered to be poorly specific, and consequently, antigenic variability was not expected to be high for their respective parasites. In the present chapter, we show how the study of the interaction between the snail Biomphalaria glabrata and its parasite Schistosome mansoni has shaken this paradigm. We show that the fate of the interaction between the snail and its parasite is at least partly the result of the concordance of highly variable repertoires of immune recognition receptors in the snail and corresponding antigenic variants in the parasite. We call these antigenic variants of the schistosome Schistosoma mansoni polymorphic mucins (SmPoMucs). We show that their high level of diversification is the result of a complex cascade of mechanisms, thus presenting evidence for antigenic variation in a parasite infecting an invertebrate species.

A Novel Toll-Like Receptor (TLR) Influences Compatibility between the Gastropod Biomphalaria glabrata, and the Digenean Trematode Schistosoma mansoni

Schistosomiasis, a devastating disease caused by parasitic flatworms of the genus Schistosoma, affects over 260 million people worldwide especially in tropical and sub-tropical regions. Schistosomes must undergo their larval development within specific species of snail intermediate hosts, a trait that is shared among almost all digenean trematodes. This unique and long-standing host-parasite relationship presents an opportunity to study both the importance of conserved immunological features in novel immunological roles, as well as new immunological adaptations that have arisen to combat a very specific type of immunological challenge. While it is well supported that the snail immune response is important for protecting against schistosome infection, very few specific snail immune factors have been identified and even fewer have been functionally characterized. Here, we provide the first functional report of a snail Toll-like receptor, which we demonstrate as playing an important role in the cellular immune response of the snail Biomphalaria glabrata following challenge with Schistosoma mansoni. This TLR (BgTLR) was identified as part of a peptide screen of snail immune cell surface proteins that differed in abundance between B. glabrata snails that differ in their compatibility phenotype to challenge by S. mansoni. The S. mansoni-resistant strain of B. glabrata (BS-90) displayed higher levels of BgTLR compared to the susceptible (M-line) strain. Transcript expression of BgTLR was found to be very responsive in BS-90 snails when challenged with S. mansoni, increasing 27 fold relative to β-actin (non-immune control gene); whereas expression in susceptible M-line snails was not significantly increased. Knockdown of BgTLR in BS-90 snails via targeted siRNA oligonucleotides was confirmed using a specific anti-BgTLR antibody and resulted in a significant alteration of the resistant phenotype, yielding patent infections in 43% of the normally resistant snails, which shed S. mansoni cercariae 1-week before the susceptible controls. Our results represent the first functional characterization of a gastropod TLR, and demonstrate that BgTLR is an important snail immune receptor that is capable of influencing infection outcome following S. mansoni challenge.