Recognition of nonself is necessary to activate Drosophila's immune response against an insect parasite

BACKGROUND

Innate immune responses can be activated by pathogen-associated molecular patterns (PAMPs), danger signals released by damaged tissues, or the absence of self-molecules that inhibit immunity. As PAMPs are typically conserved across broad groups of pathogens but absent from the host, it is unclear whether they allow hosts to recognize parasites that are phylogenetically similar to themselves, such as parasitoid wasps infecting insects.

RESULTS

Parasitoids must penetrate the cuticle of Drosophila larvae to inject their eggs. In line with previous results, we found that the danger signal of wounding triggers the differentiation of specialized immune cells called lamellocytes. However, using oil droplets to mimic infection by a parasitoid wasp egg, we found that this does not activate the melanization response. This aspect of the immune response also requires exposure to parasite molecules. The unidentified factor enhances the transcriptional response in hemocytes and induces a specific response in the fat body.

CONCLUSIONS

We conclude that a combination of danger signals and the recognition of nonself molecules is required to activate Drosophila's immune response against parasitic insects.

Single-Nucleus Sequencing of Silkworm Larval Brain Reveals the Key Role of Lysozyme in the Antiviral Immune Response in Brain Hemocytes

INTRODUCTION

The brain is considered as an immune-privileged organ, yet innate immune reactions can occur in the central nervous system of vertebrates and invertebrates. Silkworm (Bombyx mori) is an economically important insect and a lepidopteran model species. The diversity of cell types in the silkworm brain, and how these cell subsets produce an immune response to virus infection, remains largely unknown.

METHODS

Single-nucleus RNA sequencing (snRNA-seq), bioinformatics analysis, RNAi, and other methods were mainly used to analyze the cell types and gene functions of the silkworm brain.

RESULTS

We used snRNA-seq to identify 19 distinct clusters representing Kenyon cell, glial cell, olfactory projection neuron, optic lobes neuron, hemocyte-like cell, and muscle cell types in the B. mori nucleopolyhedrovirus (BmNPV)-infected and BmNPV-uninfected silkworm larvae brain at the late stage of infection. Further, we found that the cell subset that exerts an antiviral function in the silkworm larvae brain corresponds to hemocytes. Specifically, antimicrobial peptides were significantly induced by BmNPV infection in the hemocytes, especially lysozyme, exerting antiviral effects.

CONCLUSION

Our single-cell dataset reveals the diversity of silkworm larvae brain cells, and the transcriptome analysis provides insights into the immune response following virus infection at the single-cell level.

Immunostimulation of Parasteatoda tepidariorum (Araneae: Theridiidae) in juvenile and adult stages. Immunity reactions to injury with foreign body and Bacillus subtilis infection

To assess the immune potential of spiders, in the present study juvenile and adult females of Parasteatoda tepidariorum were exposed to Bacillus subtilis infection, injury by a nylon monofilament and a combination of both. The expression level of selected immune-related genes: defensin 1 (PtDEF1), lysozyme 1 (PtLYS1), lysozyme C (PtLYSC), lysozyme M1 (PtLYSM1), autophagy-related protein 101 (PtATG101), dynamin (PtDYN) and heat shock proteins (HSP70) (PtHSPB, PtHSPB2A, PtHSPB2B), production of lysozyme and HSP70 proteins, and hemocytes viability were measured. The obtained results indicated expression of the lysozyme, autophagy-related protein and HSP70 genes in both ontogenetic stages of P. tepidariorum. It has been also shown that the simultaneous action of mechanical and biological factors causes higher level of lysozyme and HSP70, cell apoptosis intensity and lower level of hemocytes viability than in the case of exposure to a single immunostimulant. Moreover, mature females showed stronger early immune responses compared to juveniles.

Host-pathogen interactions upon Candida auris infection: fungal behaviour and immune response in Galleria mellonella

Candida auris has globally emerged as a multidrug-resistant fungus linked to healthcare-associated outbreaks. There is still limited evidence on its virulence, pathogenicity determinants, and complex host-pathogen interactions. This study analyzes the in vivo fungal behaviour, immune response, and host-pathogen interactions upon C. auris infection compared to C. albicans and C. parapsilosis in G. mellonella. This was performed by immunolabelling fungal structures and larval plasmatocytes and using a quantitative approach incorporating bioinformatic morphometric techniques into the study of microbial pathogenesis. C. auris presents a remarkably higher immunogenic activity than expected at its moderate degree of tissue invasion. It induces a greater inflammatory response than C. albicans and C. parapsilosis at the expense of plasmatocyte nodule formation, especially in non-aggregative strains. It specifically invades the larval respiratory system, in a pattern not previously observed in other Candida species, and presents inter-phenotypic tissue tropism differences. C. auris filaments in vivo less frequently than C. albicans or C. parapsilosis mostly through pseudohyphal growth. Filamentation might not be a major pathogenic determinant in C. auris, as less virulent aggregative phenotypes form pseudohyphae to a greater extent. C. auris has important both interspecific and intraspecific virulence and phenotype heterogeneity, with aggregative phenotypes of C. auris sharing characteristics with low pathogenic species such as C. parapsilosis. Our work suggests that C. auris owns an important morphogenetic plasticity that distinguishes it from other yeasts of the genus. Routine phenotypic identification of aggregative or non-aggregative phenotypes should be performed in the clinical setting as it may impact patient management.

Innate immunity in the edible ascidian Halocynthia roretzi developing soft tunic syndrome: Hemolymph can eliminate the causative flagellates and discriminate allogeneic hemocytes

The infection of the kinetoplastid flagellate Azumiobodo hoyamushi causes soft tunic syndrome that often results in mass mortality in the aquaculture of the edible ascidian Halocynthia roretzi. In the diseased ascidian individuals, the flagellates are exclusively found in the tunic matrix that entirely cover the epidermis, and never invade into internal tissues, such as a mantle. The present study for the first time demonstrated that the ascidian blood plasma and hemolymph have an activity to agglutinate and disintegrate the flagellates, suggesting the innate immunity protects the internal tissue from the invasion of A. hoyamushi. This activity is indifferent between the healthy and the diseased individuals. Allo-specific recognition and cytotoxic reaction among ascidian hemocytes, so-called contact reaction, occur among the individuals of healthy-healthy, healthy-diseased, and diseased-diseased combination, and therefore, the hemocytes from diseased individuals still retain the allo-reactivity. Moreover, the allo-reactive combinations are not changed under the presence of the flagellates, indicating the flagellates neither suppress nor induce the effector system of the contact reaction. These results suggest that the infection of A. hoyamushi does not impair the innate immunity in the ascidian hemolymph.

SpBAG3 assisted WSSV infection in mud crab (Scylla paramamosain) by inhibiting apoptosis

The function of B-cell lymphoma-2 (Bcl-2) family proteins can be divided into two categories: anti-apoptotic and pro-apoptotic. As an anti-apoptotic protein, Bcl2-associated athanogene 3 (BAG3) plays a key role in regulating apoptosis, development, cell movement, and autophagy, and mediating the adaptability of cells to stimulation. However, SpBAG3 has not been reported in mud crab (Scylla paramamosain), and the regulatory effect of SpBAG3 on apoptosis in mud crab and its function in antiviral immunity is still unknown. In this study, SpBAG3 was found, and characterized, which encoded a total of 175 amino acid (molecular mass 19.3 kDa), including a specific conserved domain of the BAG family. SpBAG3 was significantly down-regulated at 0-48 h post-infection with WSSV in vivo. The antiviral effect of SpBAG3 was investigated using RNA interference. The results indicated that SpBAG3 might be involved in assisting the replication of WSSV in the host. SpBAG3 could change the mitochondrial membrane potential (△ψm), and affect cell apoptosis through mitochondrial apoptotic pathways. Therefore, the results of this study suggested that SpBAG3 could assist WSSV infection by inhibiting the apoptosis of the hemocytes in mud crab.

Mantle tissue in the pearl oyster Pinctada fucata secretes immune components via vesicle transportation

Molluscan bivalves secrete shell matrices into the extrapallial space (EPS) to guide the precipitation of rigid shells. Meanwhile, immune components are present in the EPS and shell matrices, which are pivotal in resistant to invaded pathogens, thus ensuring the shell formation process. However, the origin of these components remains unclear. In this study, we revealed numerous vesicles were secreted from the outer mantle epithelial cells by using light and electron microscopes. The secreted vesicles were isolated by gradient centrifugation and confirmed by transmission electron microscopy. Proteomics analysis showed that the secreted vesicles were composed of cytoplasmic and immune components, most of which do not have signal peptides, indicating that they were secreted by a non-classical pathway. Moreover, real-time PCR revealed that some immune components were highly expressed in the mantle tissue, compared to the hemocytes. FTIR analysis verified the presence of lipids in the shell matrices, indicating that the vesicles have integrated into the shell layers. Taken together, our results suggested that mantle epithelial cells secreted some important immune components into the EPS via secreted vesicle transportation, thus cooperating with the hemocytes to play a vital role in immunity during shell formation.

Immune responses of oyster hemocyte subpopulations to in vitro and in vivo zinc exposure

Oysters are an excellent biomonitor of coastal pollution and the hyper-accumulator of toxic metals such as copper and zinc (Zn). One unique feature of molluscs is their hemocytes which are mainly involved in immune defenses. Different subpopulations of hemocytes have been identified, but their functions in metal transport and detoxification are not clear. In this study, we examined the immune responses of different subpopulations of oyster Crassostrea hongkongensis hemocytes under different periods of Zn exposure by using flow cytometer and confocal microscopy. In vitro exposure to Zn resulted in acute immune responses by increasing the reactive oxygen species (ROS) production and phagocytosis and decreased number of granulocytes and mitochondrial membrane potential (MMP) within 3 h. Granulocyte mortality and lysosomal pH increased whereas glutathione (GSH) decreased within 1 h of in vitro exposure, indicating the immune stimulation of granulocytes. Within the first 7 days of in vivo exposure, immunocompetence of granulocytes was inhibited with increasing granulocyte mortality but decreasing ROS production and phagocytosis. However, with a further extension of Zn exposure to 14 days, both phagocytosis and lysosomal content increased with an increasing number of granulocytes, indicating the increase of hemocyte-mediated immunity. Our study demonstrated that granulocytes played important roles in oyster immune defenses while other subpopulations may also participate in immune functions. The degranulation and granulation due to transition between semigranulocytes and granulocytes after Zn exposure were important in metal detoxification. The study contributed to our understanding of the immune phenomena and the adaptive capability of oysters in metal contaminated environments.

Functional heterogeneity of immune defenses in molluscan oysters Crassostrea hongkongensis revealed by high-throughput single-cell transcriptome

Oyster is the worldwide aquaculture molluscan and evolves a complex immune defense system, with hemocytes as the major immune system for its host defense. However, the functional heterogeneity of hemocyte has not been characterized, which markedly hinders our understanding of its defense role. Here, we used the single-cell transcriptome profiling (scRNA-seq), which provides a high-resolution visual insight into its dynamics, to map the hemocyte and assess its heterogeneity in a molluscan oyster Crassostrea hongkongensis. By combining with the cell type specific RNA-seq, thirteen subpopulations belonging to granulocyte, semi-granulocyte, and hyalinocyte were revealed. The granulocytes mainly participated in immune response and autophagy process. Pseudo-temporal ordering of granulocytes identified two different cell-lineages. The hematopoietic transcription factors regulated networks controlling their differentiations were also identified. We further identified one subpopulation of granulocytes in immune activate states with the cell cycle and immune responsive genes expressions, which illustrated the functional heterogeneity of the same cell type. Collectively, our scRNA-seq analysis demonstrated the hemocytes diversity of molluscans. The results are important in our understanding of the immune defense evolution and functional differentiation of hemocytes in Phylum Mollusca.

Pan-neuronal expression of human mutant huntingtin protein in Drosophila impairs immune response of hemocytes

Huntington's disease (HD) is a late-onset; progressive, dominantly inherited neurological disorder marked by an abnormal expansion of polyglutamine (poly Q) repeats in Huntingtin (HTT) protein. The pathological effects of mutant Huntingtin (mHTT) are not restricted to the nervous system but systemic abnormalities including immune dysregulation have been evidenced in clinical and experimental settings of HD. Indeed, mHTT is ubiquitously expressed and could induce cellular toxicity by directly acting on immune cells. However, it is still unclear if selective expression of mHTT exon1 in neurons could induce immune responses and hemocytes' function. In the present study, we intended to monitor perturbations in the hemocytes' population and their physiological functions in Drosophila, caused by pan-neuronal expression of mHTT protein. A measure of hemocyte count and their physiological activities caused by pan-neuronal expression of mHTT protein highlighted the extent of immune dysregulation occurring with disease progression. We found that pan-neuronal expression of mHTT significantly alters crystal cells and plasmatocyte count in larvae and adults with disease progression. Interestingly, plasmatocytes isolated from diseased conditions exhibit a gradual decline in phagocytic activity ex vivo at progressive stages of the disease as compared to age-matched control groups. In addition, diseased flies displayed elevated reactive oxygen species (ROS) in circulating plasmatocytes at the larval stage and in sessile plasmatocytes of hematopoietic pockets at terminal stages of disease. These findings strongly implicate that neuronal expression of mHTT alone is sufficient to induce non-cell-autonomous immune dysregulation in vivo.

miR-2 contributes to WSSV infection by targeting Caspase 2 in mud crab (Scylla paramamosain)

Caspase 2 is widely studied for its function in the regulation of apoptosis in mammals. Despite the fundamental role of apoptosis during the anti-viral immune response, the relationship between Caspase 2 and virus infection has not been extensively explored in invertebrates. Also, whether or not miRNAs involve this process remains unclear. To address this issue, the miRNA-mediated regulation of Caspase 2 in mud crab (Scylla paramamosain) (Sp-Caspase 2) was characterized in this study. Sp-Caspase 2 contains an open reading frame (ORF) of 969 bp encoding 322 deduced amino acids and possesses a conserved CASc domain. The results suggested that Sp-Caspase 2 could suppress white spot syndrome virus infection via apoptosis induction. The further data showed that Sp-Caspase 2 was directly targeted by miR-2 in mud crab. Silencing or overexpression of miR-2 could affect apoptosis and WSSV replication through the regulation of Sp-Caspase 2 expression. Taken together, these results demonstrated the crucial role of the miR-2-Caspase 2 pathway in the innate immunity of mud crabs and revealed a novel mechanism in the anti-viral immune response in marine invertebrates.

A calmodulin targeted by miRNA scaffold659_26519 regulates IL-17 expression in the early immune response of oyster Crassostrea gigas

Calmodulin (CaM) is a highly conserved second messenger protein transducing calcium signals by binding and modulating intracellular calcium ions (Ca²⁺), and involves in the Ca²⁺-dependent physical processes including host defense in vertebrates. In the present study, a CaM homologue (designated as CgCaM) was identified from Pacific oyster Crassostrea gigas. The open reading frame of CgCaM cDNA was of 471 bp encoding a polypeptide of 156 amino acid residues. There were four EFh domains predicted in CgCaM, which shared high homologies with those in CaMs from oyster C. virginica and other invertebrates. The mRNA transcripts of CgCaM were constitutively expressed in all the tested tissues including labellum, mantle, gonad, gills, adductor muscle, haemocytes and hepatopancreas, with the highest expression level in haemocytes. The mRNA expression level of CgCaM in haemocytes decreased significantly (0.31-fold of that in blank, p < 0.05) at 3 h after LPS stimulation, while the intracellular Ca²⁺ (1.57-fold of that in blank, p < 0.05) and the mRNA expression of cytokine CgIL17-1 (4.87-fold of that in blank, p < 0.05) both increased in haemocytes. Meanwhile, an oyster miRNA scaffold659_26519 was identified, and it was proved to target the 3'-untranslated regions (3'-UTR) of CgCaM mRNA by luciferase reporter assay. The expression of scaffold659_26519 increased significantly at 3 h (43.523-fold of that of blank, p < 0.05) and 6 h (55.91-fold of that of blank, p < 0.05) after LPS stimulation. When the expression of scaffold659_26519 was inhibited by transfection with its inhibitor in vitro, the expression of CgIL17-1 declined significantly to 0.58-fold of that in LPS stimulation group. These findings indicated that the miRNA scaffold659_26519 targeted CaM was involved in the early inflammatory response of oyster immunity, and provided a new evidence for CaM-mediated immune mechanism in molluscs.

Galleria mellonella (Lepidoptera: Pyralidae) Hemocytes Release Extracellular Traps That Confer Protection Against Bacterial Infection in the Hemocoel

Extracellular traps (ETs) released from vertebrate and invertebrate immune cells consist of chromatin and toxic granule contents that are capable of immobilizing and killing microbes. This recently described innate immune response is not well documented in insects. The present study found that ETs were released by hemocytes of Galleria mellonella (Linnaeus) (Lepidoptera: Pyralidae) in vivo and ex vivo after bacterial stimulation. ET release (ETosis), hemolymph coagulation, and melanization likely contributed to the immobilization and killing of the bacteria. The injection of G. mellonella hemocyte deoxyribonucleic acid (DNA) in the presence of bacteria increased bacterial clearance rate and prolonged insect survival. Taken together, these results indicate the presence of insect hemocyte extracellular traps (IHETs) that protect the insect against microbial infection in the hemocoel and represent the first documentation of ETs in insects in vivo.

14-3-3ζ and 14-3-3ε are involved in innate immune responses in Pacific abalone (Haliotis discus hannai)

The 14-3-3 proteins play important roles in various cellular processes by binding to different ligands, but little is known about these proteins in mollusks. In this study, two 14-3-3 cDNAs were identified from the Pacific abalone Haliotis discus hannai (designated 14-3-3ζ and 14-3-3ε), possessing 59.40% identity with each other. Both genes were predominantly expressed in the gills of unchallenged abalones, and their mRNA signals could also be detected in several other tissues, including the mantle, hepatopancreas and ovary. However, after Vibrio harveyi challenge, hemocytes were induced significantly (p < 0.01). Meanwhile, phagocytosis was inhibited, but apoptosis, reactive oxygen species formation, and caspase 3 expression were significantly induced (p < 0.01), and they were all suppressed with 14-3-3ζ knockdown (p < 0.01). The differences were that silencing 14-3-3ε reverted the decline in the phagocytic rate derived from bacterial infection, while ROS formation was not influenced significantly. In addition, the expression levels of several antimicrobial peptide and proinflammatory cytokine genes were also decreased with the silencing of 14-3-3 genes. However, with the knockdown of 14-3-3ζ, the expression of 14-3-3ε was further significantly increased (p < 0.01), and vice versa. Overall, our results suggested that 14-3-3ζ and 14-3-3ε should play important roles in innate immunity against V. harveyi infection.

Immunological assays of hemocytes in the Northern Quahog Mercenaria mercenaria

The northern quahog Mercenaria mercenaria (commonly named hard clam) is an important aquaculture and fishery species along the Atlantic west coast. Environmental stresses, such as heat shock, fluctuating salinity, and harmful algal blooms are major challenges for clam aquaculture. In response to environmental stresses, hemocytes would change dynamically for defense and immunity. The goal of this study was to characterize basic immunological assays of hemocytes in the northern quahog by use of flow cytometry. The objectives were to: 1) develop a non-lethal method for hemolymph collection and dilution; 2) verify the capability of flow cytometry for hemocyte count and type identification through comparison with microscopic observation; 3) validate hemocyte viability assay based on plasma membrane integrity, and 4) develop hemocyte phagocytosis assay by use of fluorescein labeled microbeads. A non-lethal hemocyte collection method was developed using needle insertion through the ligament. Osmolality measurement of serum was the same as that of culture seawater. The pH measurement of serum (7.2) was significantly different from that of culture seawater (8.4). By microscopic observation, three types of hemocytes were identified with granulocytes, the dominant cell type (70 ± 16%), agranulocyte (14 ± 4%), and blast-like cell (16 ± 4%), and no differences were found from the measurements by flow cytometer on FSC/SSC plot (cell size/granularity). The viability of hemocytes based on plasma membrane integrity was 88 ± 6% ranging from 70 to 97% (n = 60, three populations), and viability protocol was further validated with the pre-set expected viability (p ≥ 0.424). Phagocytosis assay of hemocytes with fluorescence beads showed a mean capacity of 10 ± 5% (n = 60, three populations). Incubation time (up to 6 h) or bead concentrations (2:1 or 5:1 to hemocytes) did not affect the phagocytosis measurement. Overall, this study reported the basic characteristics of hemolymph (serum and hemocytes) of northern quahogs. It is expected that the assay methodologies will be applied to evaluation of hemocyte responses to environmental stresses for clam aquaculture.

Haemocyte-mediated immunity in insects: Cells, processes and associated components in the fight against pathogens and parasites

The host defence of insects includes a combination of cellular and humoral responses. The cellular arm of the insect innate immune system includes mechanisms that are directly mediated by haemocytes (e.g., phagocytosis, nodulation and encapsulation). In addition, melanization accompanying coagulation, clot formation and wound healing, nodulation and encapsulation processes leads to the formation of cytotoxic redox-cycling melanin precursors and reactive oxygen and nitrogen species. However, demarcation between cellular and humoral immune reactions as two distinct categories is not straightforward. This is because many humoral factors affect haemocyte functions and haemocytes themselves are an important source of many humoral molecules. There is also a considerable overlap between cellular and humoral immune functions that span from recognition of foreign intruders to clot formation. Here, we review these immune reactions starting with the cellular mechanisms that limit haemolymph loss and participate in wound healing and clot formation and advancing to cellular functions that are critical in restricting pathogen movement and replication. This information is important because it highlights that insect cellular immunity is controlled by a multilayered system, different components of which are activated by different pathogens or during the different stages of the infection.

Hemocyte migration and expression of four Sox genes during wound healing in Pacific abalone, Haliotis discus hannai

In molluscs, migration of hemocytes and epithelial cells is believed to play central roles in wound healing. Here, we assessed cellular and molecular mechanisms of wound healing in Pacific abalone, a marine gastropod. Light and electron microscopy in the wounds showed early accumulation of putative hemocytes, collagen deposition by fibroblasts, and further coverage of this tissue by migration of adjacent epithelial cells. Cell labelling technique allowed us to track hemocytes, which migrated to wound surface within 24 h. The migrated cells first expressed PCNA and SoxF weakly, and then the epithelial cells expressed abundant PCNA and SoxB1, SoxB2, and SoxC. These findings imply that abalone SoxF is involved in hemocyte migration or their differentiation into fibroblasts, and suggest that the migrated epithelia acquire stem cell-like property and undergo active proliferation. This study is the first to show direct evidence of hemocyte migration to wounds and expression of Sox genes in molluscan wound healing.

Probiotic, Lactobacillus pentosus BD6 boost the growth and health status of white shrimp, Litopenaeus vannamei via oral administration

This study aims to assess and determine the oral-administration of probiotic, Lactobacillus pentosus BD6 on growth performance, immunity and disease resistance of white shrimp, Litopenaeus vannamei. Lac. pentosus BD6 effectively inhibited the growth of aquatic pathogens, which was used in the test. Shrimp were fed with the control diet (without probiotic supplement) for 60 days and the probiotic-containing diets at 10⁷, 10⁸, 10⁹, and 10¹⁰ cfu kg^-1, respectively. Shrimp fed with the diet containing probiotic at the doses of 10^9-10 cfu kg^-1 showed significant increase in growth performance as well as feed efficiency than that of the control. After a challenge test with Vibrio alginolyticus, shrimp fed with a probiotic diet at a dose of 10¹⁰ cfu kg^-1 showed a significantly lower mortality as compared to the control and that of shrimp fed the diet containing probiotic at the levels up to 10^7-8 cfu kg^-1. In addition, a therapeutic potential of Lac. pentosus BD6 was discovered because the cumulative mortalities of shrimp fed with probiotic and pathogen V. parahaemolyticus simultaneously were significantly lower when compared to control shrimp. Probiotic in diet at a dose of 10^9-10 cfu kg^-1 significantly increased PO activity of shrimp, while shrimp receiving probiotic at the doses of 10^8-10 cfu kg^-1 showed significant increase in lysozyme activity and phagocytic activity. Shrimp fed with the diet containing probiotic at the level of 10¹⁰ cfu kg^-1 also indicated higher gene expression of prophenoloxidase (proPO) I, but not proPO II, lipopolysaccharide and β-1,3-glucan-binding protein and penaeidin 4. Analysis of the bacterial microbiota of the shrimp intestine revealed that oral administration of probiotic increased the relative abundance of beneficial bacteria and reduced the abundance of harmful pathogenic bacteria in the gut flora of shrimp. Despite no statistically significant difference, an analysis of microbial diversity recorded higher species richness, Shannon-Weaver diversity index and evenness in the probiotic group, compared to the control group. It was concluded that Lac. pentosus BD6 has great antibacterial ability against a wide range of pathogens and has therapeutic potential to reduce the mortality of shrimp infected with V. parahaemolyticus. Additionally, dietary Lac. pentosus BD6 at the level of 10¹⁰ cfu kg^-1 was recommended to improve growth performance, immunity and disease resistance of shrimp against V. alginolyticus.

Differential Expression of Long Non-Coding RNA (lncRNA) in Mediterranean Mussel (Mytilus galloprovincialis) Hemocytes under Immune Stimuli

The Mediterranean mussel is one of the most economically relevant bivalve mollusk species in Europe and China. The absence of massive mortalities and their resistance to pathogens affecting other cultured bivalves has been under study in recent years. The transcriptome response of this species to different immune stimuli has been extensively studied, and even the complexity of its genome, which has recently been sequenced, has been suggested as one of the factors contributing to this resistance. However, studies concerning the non-coding RNA profiles remain practically unexplored-especially those corresponding to the lncRNAs. To the best of our knowledge, this is the second characterization and study of lncRNAs in this bivalve species. In this work, we identified the potential repertoire of lncRNAs expressed in mussel hemocytes, and using RNA-Seq we analyzed the lncRNA profile of mussel hemocytes stimulated in vitro with three different immune stimuli: LPS, poly I:C, and β-glucans. Compared to unstimulated hemocytes, LPS induced the highest modulation of lncRNAs, whereas poly I:C and β-glucans induced a similar discrete response. Based on the potential cis-regulatory activity of the lncRNAs, we identified the neighboring protein-coding genes of the regulated lncRNAs to estimate-at least partially-the processes in which they are implicated. After applying correlation analyses, it seems that-especially for LPS-the lncRNAs could participate in the regulation of gene expression, and substantially contribute to the immune response.

Exosomal miR-224 contributes to hemolymph microbiota homeostasis during bacterial infection in crustacean

It is well known that exosomes could serve as anti-microbial immune factors in animals. However, despite growing evidences have shown that the homeostasis of the hemolymph microbiota was vital for immune regulation in crustaceans, the relationship between exosomes and hemolymph microbiota homeostasis during pathogenic bacteria infection has not been addressed. Here, we reported that exosomes released from Vibrio parahaemolyticus-infected mud crabs (Scylla paramamosain) could help to maintain the homeostasis of hemolymph microbiota and have a protective effect on the mortality of the host during the infection process. We further confirmed that miR-224 was densely packaged in these exosomes, resulting in the suppression of HSP70 and disruption of the HSP70-TRAF6 complex, then the released TRAF6 further interacted with Ecsit to regulate the production of mitochondrial ROS (mROS) and the expression of Anti-lipopolysaccharide factors (ALFs) in recipient hemocytes, which eventually affected hemolymph microbiota homeostasis in response to the pathogenic bacteria infection in mud crab. To the best of our knowledge, this is the first document that reports the role of exosome in the hemolymph microbiota homeostasis modulation during pathogen infection, which reveals the crosstalk between exosomal miRNAs and innate immune response in crustaceans.

Exosomes are small membrane vesicles of endocytic origin which are widely involved in the regulation of a variety of pathological processes in mammals. Yet, although the antibacterial function of exosomes has been discovered for many years, the relationship between exosomes and hemolymph microbiota homeostasis remains unknown. In the present study, we identified the miRNAs packaged by exosomes that were possibly involved in Vibrio parahaemolyticus infection by modulating hemolymph microbiota homeostasis in crustacean mud crab Scylla paramamosain. Moreover, it was found that miR-224 was densely packaged in exosomes after Vibrio parahaemolyticus challenge, resulting in the suppression of HSP70 and disruption of the HSP70-TRAF6 complex in recipient hemocytes, then the released TRAF6 was further interacted with Ecsit to regulate ROS and ALFs levels, which eventually affected hemolymph microbiota homeostasis to cope with pathogenic bacteria infection. Our finding is the first to reveal the relationship between exosomes and hemolymph microbiota homeostasis in animals, which shows a novel molecular mechanism of invertebrate resistance to pathogenic microbial infection.

The identification of nuclear factor Akirin with immune defense role in silkworm, Bombyx mori

Akirins, highly conserved nuclear factors, regulate diverse physiological processes such as innate immunity. The biological functions of Akirins have extensively been studied in vertebrates and many invertebrates; however, there is no report so far on lepidopteran insects. In the present study, we identified and characterized a novel Akirin from the silkworm, Bombyx mori (designated as BmAkirin), and explored its potential roles in innate immunity. The expression analysis revealed the unequal mRNA levels of BmAkirin in all the tested tissues; however, the gene's transcription level was highest in testis, followed by ovaries and hemocytes. It also had significant expression levels at the early stages of embryonic development. Expression of BmAkirin in fat bodies and hemocytes exhibited an increase in various degrees when challenged with virus, fungus, Gram-negative bacteria, and Gram-positive bacteria. Immunofluorescence analysis showed BmAkirin protein was prominently localized in the nucleus. Knockdown of BmAkirin strongly reduced the expression of AMPs and decreased the survival ability of larva upon immune stimulation. Moreover, the bacterial clearance ability of larvae was also decreased following the depletion of BmAkirin. Collectively, our results demonstrate that BmAkirin plays an indispensable role in the innate immunity of Bombyx mori (B. mori) by positively modulating AMPs expression in vivo.

Mitochondria are essential for antibacterial extracellular trap formation mediated by zymosan in hemocytes of Ruditapes philippinarum

The formation of extracellular traps (ETs) is an important innate immune mechanism that serves to combat different invading pathogens. In this study, zymosan significantly induced the formation of ETs in the hemocytes of Ruditapes philippinarum, and this effect was accompanied by translocation of the mitochondria to the cell surface. Zymosan stimulation clearly induced an increase in intracellular ROS and MPO production and an overexpression of ROS-related genes (PI3K, AKT and HIF). In response to the ROS burst, the mitochondrial membrane potential decreased, and the mitochondrial permeability transition pore opened. Conversely, mitochondrial superoxide inhibitor (Mito-TEMPO) significantly inhibited the formation of ETs, suggesting that mitochondrial ROS were necessary for the formation of ETs. In addition, we found that zymosan-induced ETs showed antibacterial activities against gram-negative and gram-positive bacteria, such as Vibrio anguillarum, Vibrio harveyi, Escherichia coli and Micrococcus luteus. Taken together, these findings elucidated a new antibacterial approach for R. philippinarum and highlighted the role of mitochondria in the formation of zymosan-induced ETs.

Proteomics of purified lamellocytes from Drosophila melanogaster HopTum-l identifies new membrane proteins and networks involved in their functions

In healthy Drosophila melanogaster larvae, plasmatocytes and crystal cells account for 95% and 5% of the hemocytes, respectively. A third type of hemocytes, lamellocytes, are rare, but their number increases after oviposition by parasitoid wasps. The lamellocytes form successive layers around the parasitoid egg, leading to its encapsulation and melanization, and finally the death of this intruder. However, the total number of lamellocytes per larva remains quite low even after parasitoid infestation, making direct biochemical studies difficult. Here, we used the Hop^Tum-l mutant strain that constitutively produces large numbers of lamellocytes to set up a purification method and analyzed their major proteins by 2D gel electrophoresis and their plasma membrane surface proteins by 1D SDS-PAGE after affinity purification. Mass spectrometry identified 430 proteins from 2D spots and 344 affinity-purified proteins from 1D bands, for a total of 639 unique proteins. Known lamellocyte markers such as PPO3 and the myospheroid integrin were among the components identified with specific chaperone proteins. Affinity purification detected other integrins, as well as a wide range of integrin-associated proteins involved in the formation and function of cell-cell junctions. Overall, the newly identified proteins indicate that these cells are highly adapted to the encapsulation process (recognition, motility, adhesion, signaling), but may also have several other physiological functions (such as secretion and internalization of vesicles) under different signaling pathways. These results provide the basis for further in vivo and in vitro studies of lamellocytes, including the development of new markers to identify coexisting populations and their respective origins and functions in Drosophila immunity.

A possible role of tachykinin-related peptide on an immune system activity of mealworm beetle, Tenebrio molitor L

Tachykinin-related peptides (TRPs) are important neuropeptides. Here we show that they affect the insect immune system, especially the cellular response. We also identify and predict the sequence and structure of the tachykinin-related peptide receptor (TRPR) and confirm the presence of expression of gene encoding TRPR on Tenebrio molitor haemocytes. After application of the Tenmo-TRP-7 in T. molitor the number of circulating haemocytes increased and the number of haemocytes participating in phagocytosis of latex beads decreased in a dose- and time-dependent fashion. Also, Tenmo-TRP-7 affects the adhesion ability of haemocytes. Six hours after injection of Tenmo-TRP-7, a decrease of haemocyte surface area was observed under both tested Tenmo-TRP-7 concentrations (10^-7 and 10^-5 M). The opposite effect was reported 24 h after injection, which indicates that the influence of Tenmo-TRP-7 on modulation of haemocyte behaviour differs at different stages of stress response. Tenmo-TRP-7 application also resulted in increased phenoloxidase activity 6 and 24 h after injection. The assessment of DNA integrity of haemocytes showed that the injection of Tenmo-TRP-7 at 10^-7 M led to a decrease in DNA damage compared to control individuals. This effect was only visible 6 h after Tenmo-TRP-7 application. After 24 h, Tenmo-TRP-7 injection increased DNA damage. We also confirmed the expression of immune-related genes in nervous tissue of T. molitor. Transcripts for genes encoding receptors participating in pathogen recognition processes and antimicrobial peptides were detected in T. molitor brain, retrocerebral complex and ventral nerve cord. These results may indicate a role of the insect nervous system in pathogen recognition and modulation of immune response similar to vertebrates. Taken together, our results support the notion that tachykinin-related peptides probably play an important role in the regulation of the insect immune system. Moreover, some resemblances with action of tachykinin-related peptides and substance P showed that insects can be potential model organisms for analysis of hormonal regulation of conserved innate immune mechanisms.

Effects of the toxic dinoflagellates Prorocentrum lima and Ostreopsis cf. ovata on immune responses of cultured oysters Crassostrea gasar

Oyster production in Brazil has been highlighted as an important economic activity and is directly impacted by the quality of the environment, which is largely the result of human interference and climate change. Harmful algal blooms occur in aquatic ecosystems worldwide, including coastal marine environments which have been increasing over the last decades as a result of global change and anthropogenic activities. In this study, the native oysters Crassostrea gasar from Northeast of Brazil were exposed to two toxic benthic dinoflagellate species, Prorocentrum lima and Ostreopsis cf. ovata. Their respective effects on C. gasar physiology and defense mechanisms were investigated. Oyster hemocytes were first exposed in vitro to different concentrations of both dinoflagellate species to assess their effects on hemocyte functions, such as phagocytosis, production of reactive oxygen species, as well as mortality. Results highlighted an alteration of hemocyte phagocytosis and viability in presence of O. cf. ovata, whereas P. lima did not affect the measured hemocyte functions. In a second experiment, oysters were exposed for 4 days in vivo to toxic culture of O. cf. ovata to assess its effects on hemocyte parameters, tissues damages and pathogenic Perkinsus spp. infection. An increase in hemocyte mortality was also observed in vivo, associated with a decrease of ROS production. Histopathological analyses demonstrated a thinning of the epithelium of the digestive tubules of the digestive gland, inflammatory reaction and a significant increase in the level of infection by Perkinsus spp. in oysters exposed to O. cf. ovata. These results indicate that oysters C. gasar seem to be pretty resilient to an exposure to P. lima and may be more susceptible to O. cf. ovata. Furthermore, the latter clearly impaired oyster physiology and defense mechanisms, thus highlighting that harmful algal blooms of O. cf. ovata could potentially lead to increased susceptibility of C. gasar oysters to parasite infections.

The prostanoids, thromboxanes, mediate hemocytic immunity to bacterial infection in the lepidopteran Spodoptera exigua

We report on a new insect prostanoid in a lepidopteran insect, Spodoptera exigua. Thromboxane B₂ (TXB₂) was detected by LC-MS/MS in extracts of larval epidermis, midgut, fat body and hemocytes, with highest amounts in hemocytes (about 300 ng/g tissue with substantial variation). Thromboxane A₂ (TXA₂) is an unstable intermediate that is non-enzymatically hydrolyzed into the stable TXB_2. In S. exigua, both thromboxanes mediate at least two cellular immune responses to bacterial infection, hemocyte-spreading behavior and nodule formation. At the molecular level, a TXA₂ synthase (SeTXAS) was identified from a group of 139 S. exigua cytochrome P450 monooxygenases. SeTXAS was highly similar to mammalian TXAS genes and is expressed in all developmental stages and four tested larval tissues. Immune challenge significantly enhanced SeTXAS expression, especially in hemocytes. RNA interference (RNAi) injections using gene-specific double stranded RNA led to reduced SeTXAS expression and suppressed the cellular immune responses, which were rescued following TXA₂ or TXB₂ injections. Unlike other PGs, TXA₂ or TXB₂ did not influence oocyte development in adult females. We infer that thromboxanes are present in insect tissues, where they mediate innate immune responses.

B-type allatostatin regulates immune response of hemocytes in mud crab Scylla paramamosain

B-type allatostatins (AST-B) are neuropeptides that have important physiological roles in arthropods, they have also been identified in a number of crustacean species. Recent research on neuroendocrine-immune (NEI) regulatory system in invertebrates has exploded, it reveals that the NEI network plays an indispensable role in optimizing the immune response and maintaining homeostasis. Herein, mud crab Scylla paramamosain provides a primitive and ancient model to study crosstalk between the neuroendocrine and immune systems. In this study, qRT-PCR analysis showed that the nervous system was the main production site for Sp-AST-B mRNA in S. paramamosain, while its receptor gene (Sp-AST-BR) mRNA could be detected in all the analyzed tissues including hemocytes. This reveals that AST-B might act as a pleiotropic neuropeptide. In situ hybridization further confirmed that granular cells of hemocyte subpopulations express Sp-AST-BR. Time-course analysis revealed that bacteria-analog LPS or virus-analog Poly (I:C) challenge significantly induced Sp-AST-B expression in the thoracic ganglion, and the expression of Sp-AST-BR in hemocytes were also positively changed. Furthermore, mud crabs treated with a synthetic AST-B peptide significantly increased the mRNA levels of AST-BR, nuclear factor-κB (NF-κB) transcription factor (Dorsal and Relish), pro-inflammatory cytokine (IL-16) and immune-effector molecules, and also dramatically enhanced the nitric oxide (NO) production and phagocytic activity in hemocytes. Meanwhile dsRNA-mediated knockdown of Sp-AST-B remarkably suppressed the NO concentrations, phagocytic activity and the expression of immune related genes, resulting in markedly impaired ability of crabs to inhibit bacterial proliferation in vivo. Combined, these data demonstrate that AST-B induced innate immune in the mud crab.

The T cell factor, pangolin, from Litopenaeus vannamei play a positive role in the immune responses against white spot syndrome virus infection

As a downstream interactor of β-catenin, Pangolin which is the homologous protein of the T cell factor/lymphoid enhancer factor (TCF/LEF) in vertebrates is less understood in the research field of immunity. In this study, two isoforms of Litopenaeus vannamei Pangolin (LvPangolin1 and LvPangolin2) were identified. Phylogenetic tree analysis revealed that all of the Pangolin proteins from invertebrates were represented the same lineage. The mRNA expression profiles of the LvPangolin1 and LvPangolin2 genes differed across different tissues. The expression of LvPangolin1 and the amount of LvPangolin1and LvPangolin2 combined (LvPangolinComb) were significantly increased in the haemocyte, intestine and gill but reduced in the hepatopancreas after white spot syndrome virus (WSSV) challenge. The inhibition of LvPangolin1 but not LvPangolinComb significantly reduced the survival rates of L. vannamei after WSSV infection, while significantly higher WSSV viral loads in both LvPangolin1-inhibited and LvPangolinComb-inhibited L. vannamei were observed. Knockdown of LvPangolin by RNAi could distinctly decrease the expression of antimicrobial peptide (AMP) genes and their related transcription factors. All of these results indicate that LvPangolin plays a positive role in the response to WSSV infection and that this may be mediated through regulating the immune signalling pathways which control the expression of AMPs with antiviral abilities.

Cky811 protein expressed by polydnavirus and venom gland of Cotesia kariyai regulates the host Mythimna separata larvae immune response function of C-type lectin responsible for foreign substance recognition which suppresses its melanization and encapsulation

Cotesia kariyai (Ck) larvae implanted into the body cavity of the Mythimna separata (armyworm) larvae get melanized and encapsulated after adhesion by hemocytes called hyperspread cells (HSCs). The present study showed that HSCs could not adhere to the implanted Ck larvae in armyworm larvae after injection of Ck polydnavirus (CkPDV) + venom (V), thus melanization and encapsulation could not occur. A C-type lectin called Mys-IML of the host armyworm larvae was considered to be involved in the recognition of foreign substances which always expressed in hemocytes. The CkPDV DNA encodes a C-type lectin called Cky811 that has high amino acid homology to Mys-IML. HSCs did not adhere when CkPDV + V was mixed with the hemolymph of armyworm larvae on glass slides and incubated in vitro, but the addition of anti-Cky811 antibody enabled HSCs to adhere. The messenger RNA (mRNA) expression of Mys-IML in armyworm larvae injected with CkPDV + V became undetectable by 6 h. On the contrary, Cky811 mRNA was well expressed in the hemocytes of armyworm larvae injected with CkPDV + V from 0.5 to 6 h. Cky811 protein was also detected in the crude extracts from Ck venom gland + Ck venom reservoir, suggesting that these proteins regulate foreign substance recognition by the armyworm within 0.5 h. These results suggest that CkPDV + V suppresses mRNA expression of Mys-IML, and that Cky811 protein expressed in hemocytes regulates foreign substance recognition of Mys-IML, resulting in inhibition of the downstream reaction steps: HSCs adhesion, melanization, and encapsulation.

A Comprehensive Review on Crustaceans' Immune System With a Focus on Freshwater Crayfish in Relation to Crayfish Plague Disease

Freshwater crayfish immunity has received great attention due to the need for urgent conservation. This concern has increased the understanding of the cellular and humoral defense systems, although the regulatory mechanisms involved in these processes need updating. There are, however, aspects of the immune response that require clarification and integration. The particular issues addressed in this review include an overall description of the oomycete Aphanomyces astaci, the causative agent of the pandemic plague disease, which affects freshwater crayfish, and an overview of crustaceans' immunity with a focus on freshwater crayfish. It includes a classification system of hemocyte sub-types, the molecular factors involved in hematopoiesis and the differential role of the hemocyte subpopulations in cell-mediated responses, including hemocyte infiltration, inflammation, encapsulation and the link with the extracellular trap cell death pathway (ETosis). In addition, other topics discussed include the identity and functions of hyaline cells, the generation of neoplasia, and the emerging topic of the role of sessile hemocytes in peripheral immunity. Finally, attention is paid to the molecular execution of the immune response, from recognition by the pattern recognition receptors (PRRs), the role of the signaling network in propagating and maintaining the immune signals, to the effector elements such as the putative function of the Down syndrome adhesion molecules (Dscam) in innate immune memory.

The primitive interferon-like system and its antiviral function in molluscs

The phylum mollusca is a very important group in the animal kingdom for the large number and diversified species. Recently, interest in molluscan immunity has increased due to their phylogenetic position and importance in worldwide aquaculture and aquatic environment. As the main aquaculture animal, most molluscs live in the water environment and they have to cope with many pathogen challenges, in which virus is one of the primary causes for the mass mortality. In vertebrates, interferon (IFN) system is generally recognized as the first line of defence against viral infection, while the antiviral mechanisms in molluscs remain to be clearly illuminated. Recently, some IFN-like proteins and IFN-related components have been characterized from molluscs, such as pattern recognition receptors (PRRs), interferon regulatory factors (IRFs), IFN-like receptors, JAK/STAT and IFN-stimulated genes (ISGs), which reinforce the existence of IFN-like system in molluscs. This system can be activated by virus or poly (I:C) challenges and further regulate the antiviral response of haemocytes in molluscs. This review summarizes the research progresses of IFN-like system in molluscs with the emphases on the uniformity and heterogeneity of IFN-like system of molluscs compared to that of other animals, which will be helpful for elucidating the antiviral modulation in molluscs and understanding the origin and evolution of IFN system.

Chitin binding protein from the kuruma shrimp Marsupenaeus japonicus facilitates the clearance of Vibrio anguillarum

Peritrophic membrane (PM) refers to a vital physical barrier enabling shrimp to resist pathogen invasion. It primarily consists of chitin and proteins, mostly chitin-binding protein (CBP). CBPs have been identified from microorganisms to higher organisms. In the present study, a CBP, designated MjCBP, was reported from Marsupenaeus japonicus. The open reading frame of MjCBP was 1854 bp, encoding a protein with 618 amino acids (MH544098). To be specific, the theoretical pI and molecular mass of mature MjCBP reached 5.43 and 66064.00 Da, respectively. MjCBP consisted of seven type Ⅱ chitin-binding domains (ChtB D2), which was up-regulated after being challenged with Vibrio anguillarum and then agglutinating several bacteria. In addition, MjCBP and the first chitin-binding domain (CBD1) could bind to several Gram-positive and Gram-negative bacteria via the binding process to lipopolysaccharides and peptidoglycans, whereas CBD1 was not capable of agglutinating bacteria. Moreover, the anterior and posterior segments of CBD1 were synthesized in vitro, and the posterior segment could bind to lipopolysaccharides. However, both segments fail to agglutinate bacteria. Furthermore, MjCBP and CBD1 facilitated the clearance of V. anguillarum in vivo, and the silencing of MjCBP via RNA interference reduced the ability of bacterial clearance. As revealed from the mentioned results, MjCBP acts as an opsonin or pattern recognition receptor to achieve antibacterial immune response in shrimp.

FOXO regulates the expression of antimicrobial peptides and promotes phagocytosis of hemocytes in shrimp antibacterial immunity

Invertebrates rely on innate immunity, including humoral and cellular immunity, to resist pathogenic infection. Previous studies showed that forkhead box transcription factor O (FOXO) participates in mucosal immune responses of mammals and the gut humoral immune regulation of invertebrates. However, whether FOXO is involved in systemic and cellular immunity regulation in invertebrates remains unknown. In the present study, we identified a FOXO from shrimp (Marsupenaeus japonicus) and found that it was expressed at relatively basal levels in normal shrimp, but was upregulated significantly in shrimp challenged by Vibrio anguillarum. FOXO played a critical role in maintaining hemolymph and intestinal microbiota homeostasis by promoting the expression of Relish, the transcription factor of the immune deficiency (IMD) pathway for expression of antimicrobial peptides (AMPs) in shrimp. We also found that pathogen infection activated FOXO and induced its nuclear translocation by reducing serine/threonine kinase AKT activity. In the nucleus, activated FOXO directly regulated the expression of its target Amp and Relish genes against bacterial infection. Furthermore, FOXO was identified as being involved in cellular immunity by promoting the phagocytosis of hemocytes through upregulating the expression of the phagocytotic receptor scavenger receptor C (Src), and two small GTPases, Rab5 and Rab7, which are related to phagosome trafficking to the lysosome in the cytoplasm. Taken together, our results indicated that FOXO exerts its effects on homeostasis of hemolymph and the enteric microbiota by activating the IMD pathway in normal shrimp, and directly or indirectly promoting AMP expression and enhancing phagocytosis of hemocytes against pathogens in bacteria-infected shrimp. This study revealed the different functions of FOXO in the mucosal (local) and systemic antibacterial immunity of invertebrates.

In vitro Evaluation of Programmed Cell Death in the Immune System of Pacific Oyster Crassostrea gigas by the Effect of Marine Toxins

Programmed cell death (PCD) is an essential process for the immune system's development and homeostasis, enabling the remotion of infected or unnecessary cells. There are several PCD's types, depending on the molecular mechanisms, such as non-inflammatory or pro-inflammatory. Hemocytes are the main component of cellular immunity in bivalve mollusks. Numerous infectious microorganisms produce toxins that impair hemocytes functions, but there is little knowledge on the role of PCD in these cells. This study aims to evaluate in vitro whether marine toxins induce a particular type of PCD in hemocytes of the bivalve mollusk Crassostrea gigas during 4 h at 25°C. Hemocytes were incubated with two types of marine toxins: non-proteinaceous toxins from microalgae (saxitoxin, STX; gonyautoxins 2 and 3, GTX2/3; okadaic acid/dynophysistoxin-1, OA/DTX-1; brevetoxins 2 and 3, PbTx-2,-3; brevetoxin 2, PbTx-2), and proteinaceous extracts from bacteria (Vibrio parahaemolyticus, Vp; V. campbellii, Vc). Also, we used the apoptosis inducers, staurosporine (STP), and camptothecin (CPT). STP, CPT, STX, and GTX 2/3, provoked high hemocyte mortality characterized by apoptosis hallmarks such as phosphatidylserine translocation into the outer leaflet of the cell membrane, exacerbated chromatin condensation, DNA oligonucleosomal fragments, and variation in gene expression levels of apoptotic caspases 2, 3, 7, and 8. The mixture of PbTx-2,-3 also showed many apoptosis features; however, they did not show apoptotic DNA oligonucleosomal fragments. Likewise, PbTx-2, OA/DTX-1, and proteinaceous extracts from bacteria Vp, and Vc, induced a minor degree of cell death with high gene expression of the pro-inflammatory initiator caspase-1, which could indicate a process of pyroptosis-like PCD. Hemocytes could carry out both PCD types simultaneously. Therefore, marine toxins trigger PCD's signaling pathways in C. gigas hemocytes, depending on the toxin's nature, which appears to be highly conserved both structurally and functionally.

A Tale of Two Transcriptomic Responses in Agricultural Pests via Host Defenses and Viral Replication

Autographa californica Multiple Nucleopolyhedrovirus (AcMNPV) is a baculovirus that causes systemic infections in many arthropod pests. The specific molecular processes underlying the biocidal activity of AcMNPV on its insect hosts are largely unknown. We describe the transcriptional responses in two major pests, Spodoptera frugiperda (fall armyworm) and Trichoplusia ni (cabbage looper), to determine the host-pathogen responses during systemic infection, concurrently with the viral response to the host. We assembled species-specific transcriptomes of the hemolymph to identify host transcriptional responses during systemic infection and assessed the viral transcript abundance in infected hemolymph from both species. We found transcriptional suppression of chitin metabolism and tracheal development in infected hosts. Synergistic transcriptional support was observed to suggest suppression of immune responses and induction of oxidative stress indicating disease progression in the host. The entire AcMNPV core genome was expressed in the infected host hemolymph with a proportional high abundance detected for viral transcripts associated with replication, structure, and movement. Interestingly, several of the host genes that were targeted by AcMNPV as revealed by our study are also targets of chemical insecticides currently used commercially to control arthropod pests. Our results reveal an extensive overlap between biological processes represented by transcriptional responses in both hosts, as well as convergence on highly abundant viral genes expressed in the two hosts, providing an overview of the host-pathogen transcriptomic landscape during systemic infection.

A truncated intracellular Dicer-like molecule involves in antiviral immune recognition of oyster Crassostrea gigas

The enzyme Dicer is best known for its role as an endoribonuclease in the small RNA pathway, playing a crucial role in recognizing viral double-stranded RNA (dsRNA) and inducing down-stream cascades to mediate anti-virus immunity. In the present study, a truncated Dicer-like gene was identified from oyster Crassostrea gigas, and its open reading frame (ORF) encoded a polypeptide (designed as CgDCL) of 530 amino acids. The CgDCL contained one N-terminal DEAD domain and a C-terminal helicase domain, but lack the conserved PAZ domain, ribonuclease domain (RIBOc) and dsRNA binding domain. The mRNA transcripts of CgDCL were detected in all the examined tissues with high expression levels in lip, gills and haemocytes, which were 62.06-fold, 48.91-fold and 47.13-fold (p < 0.05) of that in mantle, respectively. In the primarily cultured oyster haemocytes, the mRNA transcripts of CgDCL were significantly induced at 12 h after poly(I:C) stimulation, which were 4.04-fold (p < 0.05) of that in control group. The expression level of CgDCL mRNA in haemocytes was up-regulated significantly after dsRNA and recombinant interferon-like protein (rCgIFNLP) injection, which was 12.87-fold (p < 0.01) and 3.22-fold (p < 0.05) of that in control group, respectively. CgDCL proteins were mainly distributed in the cytoplasm of haemocytes. The recombinant CgDCL protein displayed binding activity to dsRNA and poly(I:C), but no obvious dsRNA cleavage activity. These results collectively suggest that truncated CgDCL from C. gigas was able to be activated by poly(I:C), dsRNA and CgIFNLP, and functioned as an intracellular recognition molecule to bind nucleic acid of virus, indicating a potential mutual cooperation between RNAi and IFN-like system in anti-virus immunity of oysters.

A DM9-containing protein from oyster Crassostrea gigas (CgDM9CP-3) mediating immune recognition and encapsulation

DM9 domain containing protein (DM9CP) is a recently identified pattern recognition molecules exiting in most organisms except plants. In the present study, a novel DM9-containing protein (CgDM9CP-3) was identified from Pacific oyster Crassostrea gigas with an open reading frame of 438 bp, encoding a polypeptide of 145 amino acids containing two tandem DM9 repeats. The deduced amino acid sequence of CgDM9CP-3 shared 52.4% and 58.6% identity with CgDM9CP-1 and CgDM9CP-2, respectively. The mRNA transcripts of CgDM9CP-3 were highest expressed in oyster gills and its protein was mainly distributed in cytomembrane of haemocytes. After the stimulations with Vibrio splendidus and mannose, the mRNA expression of CgDM9CP-3 in oyster gills was significantly up-regulated and reached the peak level at 12 h and 24 h (p < 0.05), which was 7.80-fold (p < 0.05) and 42.82-fold (p < 0.05) of that in the control group, respectively. The recombinant CgDM9CP-3 protein (rCgDM9CP-3) was able to bind LPS, PGN and d-Mannose, fungi Pichia pastoris and Yarrowia lipolytica, as well as gram-negative bacteria Escherichia coli, Vibrio anguillarum and V. splendidus in a Ca²⁺-dependent manner. Moreover, it could enhance the encapsulation of haemocytes and exhibited agglutination activity towards fungi P. pastoris and Y. lipolytica in vitro with Ca²⁺. These results suggested that CgDM9CP-3 not only acted as a PRR involved in the pathogen recognition, but also enhanced cellular encapsulation in oyster C. gigas.

Immunopathogenesis of hematopoietic tissues in response to Vibrio parahaemolyticus (VPAHPND) infection in Macrobrachium rosenbergii

In crustacean, hemocytes are known as crucial components of crustaceans' innate immunity against pathogens. Drastic hemocytes reduction during infectious disease is apparently related to disease severity and calls for a health status evaluation and aquaculture management. The molecular pathogenesis of hemocytes loss during bacterial infection was elucidated with VP_AHPND challenged in M. rosenbergii. We report herein a correlation between hemocyte loss and the pathogenicity and aggressive immune response in hematopoietic tissues of moribund M. rosenbergii. In this study, adult freshwater prawn was administered an LC₅₀ dose of VP_AHPND; bacterial clearance ensued, and success was reached within 24 h. Hemocytes increased in survival, yet drastically decreased in moribund prawn. Pathological analysis of hematopoietic tissue of moribund prawn showed apparent abnormal signs, including the presence of bacteria, a small number of mitotic cells, cellular swelling, loosening of connective tissue, and karyorrhectic nuclei cells. A significant upregulation of a core apoptotic machinery gene, caspase-3, was detected in hematopoietic tissue of moribund shrimp, but not in those of Escherichia coli DH5α (non-pathogenic bacteria) and VP_AHPND survival prawn. The highest level was found in the moribund group, which confirms the occurrence of apoptosis in this hematopoietic tissue. Further, our results suggest that hematopoietic tissue damage may arise from inflammation triggered by an aggressive immune response. Immune activation was indicated by the comparison of immune-related gene expression between controls, E. coli (DH5α)-infected (non-pathogenic), and VP_AHPND-infected survival groups with moribund prawn. RT-PCR revealed a significant upregulation of all genes in hematopoietic tissues and hemocytes within 6-12 h and declined by 24 h. This evident related to the almost VP_AHPND are clearance in survival and E. coli (DH5α) challenged group in contrast with drastic high expression was determined in moribund group. We conclude that a reduction of renewing circulating hemocytes in fatally VP_AHPND-infected prawn was caused by an acute self-destructive immune response by hematopoietic cells.

CqPP2A inhibits white spot syndrome virus infection by up-regulating antimicrobial substances expression in red claw crayfish Cherax quadricarinatus

Protein phosphatase 2A (PP2A) is an important serine/threonine phosphatase, a highly conserved enzyme widely expressed in eukaryotic cells, which accounts for a majority of the serine/threonine phosphatase activity in cells implicated in regulation of immune signaling pathways and antiviral response. However, most of studies about PP2A have been conducted in mammals but few in crustaceans. In this study, two subunits of PP2A (named as CqPP2Ab and CqPP2Ac) were characterized to be involved in white spot syndrome virus (WSSV) infection in the haematopoietic tissue (Hpt) cells from red claw crayfish Cherax quadricarinatus. The open reading frame (ORF) of CqPP2Ab was 1341 bp encoding 446 amino acids with seven WD40 domains, and the ORF of CqPP2Ac was 930 bp encoding 309 amino acids with a PP2Ac domain. Tissue distribution analysis showed that the mRNA transcript of CqPP2Ab and CqPP2Ac were both widely expressed in all the tested tissues with the highest expression in hemocyte, followed by high expression in Hpt. The gene expressions of CqPP2Ab and CqPP2Ac were both significantly down-regulated at 6 h post WSSV infection (6 hpi) in Hpt cells. Importantly, the expression of viral immediate early gene IE1 and late viral gene envelope protein VP28 were both significantly increased post WSSV infection after gene silencing of CqPP2Ab or CqPP2Ac in Hpt cells, suggesting that CqPP2Ab and CqPP2Ac could inhibit WSSV infection in Hpt cells, probably by increasing the antimicrobial substances expression in consideration to the significantly reduced expression of anti-lipopolysaccharide factor, crustin, and lysozyme after gene silencing of CqPP2Ab or CqPP2Ac, respectively. These findings provide a new light on the mechanism of WSSV infection and the antiviral response in crustaceans.

Hemocyte extracellular traps of Manila clam Ruditapes philippinarum: Production characteristics and antibacterial effects

Extracellular traps (ETs) have been found to be an important strategy of mammals to immobilize and kill invading microorganisms. In the present study, we observed the formation of ETs in the hemocytes of marine mollusks Ruditapes philippinarum in response to challenge from bacteria Vibrio anguillarum, and examined the potential factors and signaling pathways underling this process. We detected an increase of reactive oxygen species (ROS) and myeloperoxidase (MPO) production during ETosis, accompanied by significantly up-regulated expression of ROS-related and MPO genes. The suppression of ETs structures by nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitor (diphenyleneiodonium chloride, DPI) and MPO inhibitor (aminobenzoic acid hydrazide, ABAH) further confirmed the essential roles ROS and MPO played in ETosis. Furthermore, ET production could be inhibited by phosphotidylinsitol-3-kinase (PI3K) inhibitor (LY294002) and extracellular regulated protein kinase (ERK) inhibitor (U0126), suggesting the idea that both the PI3K and ERK pathways were suggested to function during ETosis. In addition, the ETosis process was accompanied by enhancement of glycolysis-related enzymatic activities, e.g., pyruvate kinase (PK) and hexokinase (HK), and over-expression of the glycolysis-related genes, e.g., PK, HK and glucose transport protein (GLUT), indicating high involvement of glycolysis in the ETosis process. Furthermore, our scanning electron microscopy (SEM) observation and antibacterial activities test successfully showed the patterns how clam ETs entrapped and killed the invading V. anguillarum. Taken together, our results revealed that ETosis with bactericidal effect increased ROS, MPO and glycolysis level and carried out in a ROS-, MPO-, PI3K-ERK-dependent manner.

Identification and analysis of two lebocins in the oriental armyworm Mythimna separata

The insect immune system can produce defensive molecules, such as antimicrobial peptides (AMPs), to eliminate invading pathogens. Here, we report the identification of two cDNAs (MseLeb1, MseLeb2) that encode lepidopteral lebocin preproproteins in the oriental armyworm, Mythimna separata. Their open reading frames are 483/492 bp that encode 161/164 aa peptides. MseLeb1 is mainly expressed in the fat body and epidermis, while MseLeb2 is mainly expressed in the fat body, Malpighian tube, and epidermis. They were significantly induced by Escherichia coli, Staphylococcus aureus, and Beauveria bassiana in hemocytes. The preproproteins can be processed after RXXR motifs into mature peptides. Multiple sequence alignment indicates that MseLeb1 (18-42, 121-161) are potentially active peptides. Five peptides were synthesized for analyses: 18-42, 121-161, 121-154, 121-151, 121-146. Synthetic peptides showed agglutinating activity, but no hemolytic activity. Bacterial growth assay, colony formation assay, and electron microscopy revealed that synthetic peptides can inhibit bacterial growth and disrupt bacterial cell wall. B. bassiana conidia and blastospores were lysed by synthetic peptides. These results indicate that MseLeb1 and MseLeb2 are immune responsive lebocins, and the mature peptides have antibacterial and antifungal activities.

Current progress on identification of virus pathogens and the antiviral effectors in echinoderms

Echinoderms are important marine organisms that live in a wide range from the intertidal zone to the abyssal zone. Members of this phylum are prone to dramatic population fluctuations that may trigger dramatic shifts in ecosystem structure. Despite the extremely complex nature of the marine environment, the immune systems of echinoderms induce a complex innate immune response to prokaryotic and eukaryotic pathogens. Previous studies showed that many echinoderm disease outbreaks were associated with specific bacteria, whereas recent scientific investigations using newly developed technologies revealed the amazing diversity of viruses in seawater. Viruses are potential pathogens of several infectious diseases of marine echinoderms. We reviewed the discovery of viruses in echinoderms and discussed the relationship between viruses and diseases for the first time. We further summarized the research progress of the potential immune-related genes and signal pathways induced by viruses and poly (I:C). Additionally, numbers of studies showed that active substances extracted from echinoderms, or the compounds synthesized from these substances, have significant antihuman virus ability. This result suggests that the active substances derived from echinoderms provide potential antiviral protection for the organism, which may provide future research directions for the antiviral immunity of echinoderms. Thus, this review also collected information on the antiviral activities of biologically active substances from echinoderms, which may pave the way for new trends in antiviral immunity for echinoderms and antiviral drugs in humans.

Novel globular C1q domain-containing protein (PmC1qDC-1) participates in shell formation and responses to pathogen-associated molecular patterns stimulation in Pinctada fucata martensii

The C1q protein, which contains the globular C1q (gC1q) domain, is involved in the innate immune response, and is found abundantly in the shell, and it participates in the shell formation. In this study, a novel gC1q domain-containing gene was identified from Pinctada fucata martensii (P. f. martensii) and designated as PmC1qDC-1. The full-length sequence of PmC1qDC-1 was 902 bp with a 534 bp open reading frame (ORF), encoding a polypeptide of 177 amino acids. Quantitative real-time PCR (qRT-PCR) result showed that PmC1qDC-1 was widely expressed in all tested tissues, including shell formation-associated tissue and immune-related tissue. PmC1qDC-1 expression was significantly high in the blastula and gastrula and especially among the juvenile stage, which is the most important stage of dissoconch shell formation. PmC1qDC-1 expression was located in the outer epithelial cells of mantle pallial and mantle edge and irregular crystal tablets were observed in the nacre upon knockdown of PmC1qDC-1 expression at mantle pallial. Moreover, the recombined protein PmC1qDC-1 increased the rate of calcium carbonate precipitation. Besides, PmC1qDC-1 expression was significantly up-regulated in the mantle pallial at 6 h and was significantly up-regulated in the mantle edge at 12 h and 24 h after shell notching. The expression level of PmC1qDC-1 in mantle edge was significantly up-regulated at 48 h after LPS stimulation and was significantly up-regulated at 12 h, 24 h and 48 h after poly I:C stimulation. Moreover, PmC1qDC-1 expression was significantly up-regulated in hemocytes at 6 h after lipopolysaccharide (LPS) and poly I:C challenge. These findings suggest that PmC1qDC-1 plays a crucial role both in the shell formation and the innate immune response in pearl oysters, providing new clues for understanding the shell formation and defense mechanism in mollusk.

A Pattern Recognition Receptor C-type Lectin-S6 (CTL-S6) is Involved in the Immune Response in the Silkworm (Lepidoptera: Bombycidae)

Insect innate immunity is initiated by the special recognition and binding of the foreign pathogens, which is accomplished by the pattern recognition receptors (PRRs). As an important type of PRRs, C-type lectins (CTLs) play various roles in insect innate immunity, including pathogen recognition, stimulation of prophenoloxidase, regulation of cellular immunity and so on. In this study, we have cloned the full-length cDNA of a CTL gene named CTL-S6 from the silkworm, Bombyx mori. The open reading frame (ORF) of B. mori CTL-S6 encodes 378 amino acids, which contain a secretion signal peptide. The mRNA of CTL-S6 exhibited the highest transcriptional level in the midgut. Its transcriptional level increased dramatically in fat body and hemocytes upon Escherichia coli or Micrococcus luteus challenge. Purified recombinant CTL-S6 could bind to bacterial cell wall components, including peptidoglycan (PGN, from Bacillus subtilis) and lipopolysaccharide (LPS, from E. coli 0111:B4), and recombinant CTL-S6 was involved in the encapsulation and melanization of hemocytes. Furthermore, the addition of recombinant CTL-S6 to the hemolymph of silkworm resulted in a significant increase in phenoloxidase activity. Overall, our results indicated that B. mori CTL-S6 may serve as a PRR for the recognition of foreign pathogens, prophenoloxidase pathway stimulation and involvement in the innate immunity.

Distinct vitellogenin domains differentially regulate immunological outcomes in invertebrates

The classical role of Vitellogenin (Vg) is providing energy reserves for developing embryos, but its roles appear to extend beyond this nutritional function, and its importance in host immune defense is garnering increasing research attention. However, Vg-regulated immunological functions are dependent on three different domains within different species and remain poorly understood. In the present study, we confirmed three conserved VG domains-LPD_N, DUF1943, and VWD-in the Chinese mitten crab (Eriocheir sinensis), highlighting functional similarities of Vg in vertebrates and invertebrates. Of these three domains, DUF1943 and VWD showed definitive bacterial binding activity via interaction with the signature components on microbial surfaces, but this activity was not exhibited by the LPD_N domain. Antibacterial assays indicated that only the VWD domain inhibits bacterial proliferation, and this function may be conserved between different species due to the conserved amino acid residues. To further explore the relationship between Vg and polymeric immunoglobulin receptor (pIgR), we expressed EspIgR and the three E. sinensis Vg (EsVg) domains in HEK293T cells, and coimmunoprecipitation assay demonstrated that only the DUF1943 domain interacts with EspIgR. Subsequent experiments demonstrated that EsVg regulates hemocyte phagocytosis by binding with EspIgR through the DUF1943 domain, thus promoting bacterial clearance and protecting the host from bacterial infection. To the best of our knowledge, our work is the first to report distinct domains in Vg inducing different immunological outcomes in invertebrates, providing new evidence that pIgR acts as a phagocytic receptor for Vg.

Virulence of Vibrio alginolyticus Accentuates Apoptosis and Immune Rigor in the Oyster Crassostrea hongkongensis

Vibrio species are ubiquitously distributed in marine environments, with important implications for emerging infectious diseases. However, relatively little is known about defensive strategies deployed by hosts against Vibrio pathogens of distinct virulence traits. Being an ecologically relevant host, the oyster Crassostrea hongkongensis can serve as an excellent model for elucidating mechanisms underlying host-Vibrio interactions. We generated a Vibrio alginolyticus mutant strain (V. alginolyticus^△vscC ) with attenuated virulence by knocking out the vscC encoding gene, a core component of type III secretion system (T3SS), which led to starkly reduced apoptotic rates in hemocyte hosts compared to the V. alginolyticus^WT control. In comparative proteomics, it was revealed that distinct immune responses arose upon encounter with V. alginolyticus strains of different virulence. Quite strikingly, the peroxisomal and apoptotic pathways are activated by V. alginolyticus^WT infection, whereas phagocytosis and cell adhesion were enhanced in V. alginolyticus^△vscC infection. Results for functional studies further show that V. alginolyticus^WT strain stimulated respiratory bursts to produce excess superoxide (O2^•-) and hydrogen peroxide (H₂O₂) in oysters, which induced apoptosis regulated by p53 target protein (p53tp). Simultaneously, a drop in sGC content balanced off cGMP accumulation in hemocytes and repressed the occurrence of apoptosis to a certain extent during V. alginolyticus^△vscC infection. We have thus provided the first direct evidence for a mechanistic link between virulence of Vibrio spp. and its immunomodulation effects on apoptosis in the oyster. Collectively, we conclude that adaptive responses in host defenses are partially determined by pathogen virulence, in order to safeguard efficiency and timeliness in bacterial clearance.

Effects of Phyllanthus amarus extract on nonspecific immune responses, growth, and resistance to Vibrio alginolyticus in white shrimp Litopenaeus vannamei

This study investigates the effects of Phyllanthus amarus extract (PAE) on immune responses, growth, and resistance to Vibrio alginolyticus in white shrimp (Litopenaeus vannamei). In vitro PAE treatment did not alter the cell viability of haemocytes and significantly enhanced immune parameters such as phenoloxidase (PO) activity, phagocytic activity, and superoxide anion (O₂^-) production. We conducted two feeding trials to examine the effects of PAE on the growth, disease resistance, and innate immune parameters of white shrimp. In the first in vivo trial, shrimps (4.01 ± 0.03 g) were fed a diet containing 0 g (control), 10 g (PAE10), 20 g (PAE20), or 40 g (PAE40) of PAE per kilogram of feed for 56 days. After the feeding period, the PAE20 group showed a significantly higher weight gain and specific growth rate than shrimp fed the control diet. Furthermore, after challenge with V. alginolyticus, shrimp fed a diet containing PAE showed significantly higher survival than those fed the control diet. The second in vivo trial (28 days) was performed to identify the mechanisms of enhanced immunity in PAE-fed shrimp. Shrimp fed the PAE20 diet generally had the highest total haemocyte count, PO activity, phagocytic activity, and O₂^- production, followed by the PAE40 and PAE10 groups. Thus, our results suggest that administration of 20 g of PAE per kilogram of feed can enhance immunity, growth, and resistance to V. alginolyticus in white shrimp.

A Litopenaeus vannamei TRIM32 gene is involved in oxidative stress response and innate immunity

Tripartite motif (TRIM) family proteins are named by the presence of tripartite motifs in their amino terminal domains. Apart from the amino terminal, their carboxyl terminal contain variable domains which mediate diverse functions of the TRIM proteins. It had been found that TRIM proteins played important roles in distinct biological processes, such as innate immunity, anti-tumor immunity, cell cycle regulation and so on. In the present study, we cloned a TRIM32 (LvTRIM32) gene from Litopenaeus vannamei. LvTRIM32 was highly expressed in hemocytes, gills and epidermis, and subcellular localization analysis indicated that it was widely distributed in S2 cells. In vitro ubiquitination assays indicated that LvTRIM32 had E3 ubiquitin ligase activity. Results of real-time RT-PCR assay showed that LvTRIM32 was induced in shrimp hemocytes upon oxidative stress. It was also proved that the promoter activity of LvTRIM32 was enhanced by NF-E2-related factor, and knocked-down expression of LvTRIM32 depressed the expression of malic enzyme and epoxide hydrolase. Downregulated LvTRIM32 suppressed the cumulative mortality of shrimp under oxidative stress. Moreover, it was found that LvTRIM32 could be induced in shrimp hemocytes upon immunostimulation, and downregulated LvTRIM32 increased the cumulative mortality of shrimp infected with white spot syndrome virus (WSSV) or Vibrio alginolyticus. Collecting results suggested that LvTRIM32 was a member of shrimp antioxidant stress system, and it was also involved in WSSV- or V. alginolyticus-infection resistance.

Both quiescent and proliferating cells circulate in the blood of the invasive apple snail Pomacea canaliculata

Gastropod hematopoiesis occurs at specialized tissues in some species, but the evidence also suggests that hemocyte generation is maybe widespread in the connective tissues or the blood system in others. In Ampullariidae (Caenogastropoda), both the kidney and the lung contain putative hematopoietic cells, which react to immune challenges. In the current study, we wanted to explore if hematopoiesis occurs in the blood of Pomacea canaliculata. Thus, we obtained circulating hemocytes from donor animals and tested their ability to proliferate in the blood of conspecific recipients. We tracked cell proliferation by labeling the donors' hemocytes with the fluorescent cell proliferation marker carboxyfluorescein diacetate succinimidyl ester (CFSE). Transferred CFSE-labeled hemocytes survived and proliferated into the recipients' circulation for at least 17 days. We also determined the cell cycle status of circulating hemocytes by using the propidium iodide (PI) and acridine orange (AO) staining methods. Flow cytometry analyses showed that most PI-stained hemocytes were in the G1 phase (~96%), while a lower proportion of cells were through the G2/S-M transition (~4%). When we instead used AO-staining, we further distinguished a subpopulation of cells (~5%) of low size, complexity-granularity, and RNA content. We regarded this subpopulation as quiescent cells. In separate experimental sets, we complemented these findings by assessing in circulating hemocytes two evolutionary conserved features of quiescent, undifferentiated cells. First, we used JC-1 staining to determine the mitochondrial membrane potential (Ψ_m) of circulating hemocytes, which is expected to be low in quiescent cells. Most hemocytes (~87%) showed high aggregation of JC-1, which indicates a high Ψ_m. Besides that, a small hemocyte subpopulation (~11%) showed low aggregation of the dye, thus indicating a low Ψ_m. It is known that the transition from a quiescent to a proliferating state associates with an increase of the Ψ_m. The specificity of these changes was here controlled by membrane depolarization with the Ψ_m disruptor CCCP. Second, we stained hemocytes with Hoechst33342 dye to determine the efflux activity of ABC transporters, which participate in the multixenobiotic resistance system characteristic of undifferentiated cells. Most hemocytes (>99%) showed a low dye-efflux activity, but a small proportion of cells (0.06-0.12%) showed a high dye-efflux activity, which was significantly inhibited by 100 and 500 μM verapamil, and thus is indicative of an undifferentiated subpopulation of circulating hemocytes. Taken together, our results suggest that, among circulating hemocytes, there are cells with the ability to proliferate or to stay in a quiescent state and behave as progenitor cells later, either in the circulation or the hematopoietic tissues/organs.

Immune priming: the secret weapon of the insect world

Insects are a highly successful group of animals that inhabit almost every habitat and environment on Earth. Part of their success is due to a rapid and highly effective immune response that identifies, inactivates, and eliminates pathogens. Insects possess an immune system that shows many similarities to the innate immune system of vertebrates, but they do not possess an equivalent system to the antibody-mediated adaptive immune response of vertebrates. However, some insect do display a process known as immune priming in which prior exposure to a sublethal dose of a pathogen, or pathogen-derived material, leads to an elevation in the immune response rendering the insect resistant to a subsequent lethal infection a short time later. This process is mediated by an increase in the density of circulating hemocytes and increased production of antimicrobial peptides. Immune priming is an important survival strategy for certain insects while other insects that do not show this response may have colony-level behaviors that may serve to limit the success of pathogens. Insects are now widely used as in vivo models for studying microbial pathogens of humans and for assessing the in vivo efficacy of antimicrobial agents. Knowledge of the process of immune priming in insects is essential in these applications as it may operate and augment the perceived in vivo antimicrobial activity of novel compounds.Abbreviations: 1,3-dibenzyl-4,5-diphenyl-imidazol-2-ylidene silver(I) acetate; SBC3: antimicrobial peptides; AMPs: dorsal-related immunity factor; DIF: Down syndrome cell adhesion molecule; Dscam: Lipopolysaccharide; LPS: Pathogen-associated molecular patterns; PAMPS: Patterns recognition receptors; PRR: Prophenoloxidase; PO: Toll-like receptors; TLRs: Toll/IL-1R; TIR, Transgenerational Immune Priming; TgIP: Tumor necrosis factor-α; TNF-α.