Crustacean hematopoiesis

Unveiling a new hemocyte subpopulation in white shrimp (Penaeus vannamei) and the characterization of immune response in hemocyte subpopulation

Hemocytes are the primary cellular components of the shrimp immune system, playing a crucial role in host defense. However, a comprehensive understanding of their subpopulations and specific functions remains incomplete. In this study, four hemocyte subpopulations, designated as H1, H2, H3, and H4, were identified in Penaeus vannamei using transmission electron microscopy (TEM) and flow cytometry. The H1 subpopulation was the most abundant cells, the smallest in size, lacked granules, and had the highest nucleus-to-cytoplasm (N:C) ratio, identifying it as prohemocytes (immature cells). The H2 subpopulation fits the criteria of hyaline hemocytes. They are relatively small, have a large nucleus, and contain no or very few cytoplasmic granules. The H3 subpopulation was the least abundant cell. These cells are larger than HCs, have a moderate N:C ratio, and contain a few granules, identifying them as semi-granulocytes. The H4 subpopulation, representing granulocytes, had the largest cell size and the lowest N:C ratio and was characterized by the presence of large granules in the cytoplasm. Non-specific immune responses were investigated through various parameters and gene expression profiling. Each hemocyte subpopulation exhibited distinct immune functions. Prohemocytes strongly expressed notch-1, suggesting a role in hemocyte proliferation. Hyalinocytes exhibited strong phagocytic activity and produced superoxide anions. Semigranulocytes exhibited high expression of lysozyme and anti-lipopolysaccharide factor. Granulocytes showed high expression of propo-1, propo-2, and antimicrobial peptide genes. Following Vibrio parahaemolyticus injection, the H1 subpopulation significantly increased at 6 h post-infection before returning to baseline levels, whereas the H4 subpopulation followed an opposite trend. These findings suggest that both H1 and H4 hemocytes play critical roles in the immune response against V. parahaemolyticus.

Microplastics from cigarette filters: Comparative effects on selected terrestrial and aquatic invertebrates

Cigarette filters comprise plasticised cellulose acetate, a synthetic polymer categorized as bioplastic. They represent a significant source of microplastics (MPs), in particular microfibers, and associated chemicals, yet their impact on organisms, especially soil invertebrates, is not well-studied. This research examines the effects of MPs milled from smoked (SCF-MPs) and unsmoked cigarette filters (UCF-MPs) on terrestrial invertebrates (Porcellio scaber, Tenebrio molitor) and aquatic invertebrates (Daphnia magna, Brachionus calyciflorus). SCF-MPs and UCF-MPs were found to be 9.06 ± 4.1 μm and 12.71 ± 6.82 μm in size, respectively. Both samples contained triacetin and potentially toxic metals, while SCF-MPs also contained nicotine and a larger number of trace organic compounds. While exposure to SCF-MPs or UCF-MPs (up to 1.5 % MPs, w/w in soil) did not affect the survival of either terrestrial invertebrate, several physiological responses were observed. These included changes in immune parameters, energy-related biomarker levels, and altered glutathione S-transferase and acetylcholinesterase activities. Both types of MPs were acutely toxic to aquatic invertebrates, reducing the survival rates of B. calyciflorus (10 mg L-1 of either particle after 48h exposure) and D. magna (100 mg L-1 of SCF-MPs after 48h exposure). SCF-MPs generally caused more pronounced effects than UCF-MPs. This study highlights the need for effective environmental management to address both smoked and unsmoked cigarette filters.

The hematopoietic tissue of the freshwater crayfish, Pacifastacus leniusculus: organization and expression analysis

The hematopoietic tissue (HPT) and anterior proliferation center (APC) are the main hemocyte-producing organs of the freshwater crayfish, Pacifastacus leniusculus. To deepen our understanding of immune responses to various pathogens, it is essential to identify distinct hemocyte subpopulations with specific functions and to further explore how these cells are generated. Here we provide an in-depth histological study of the HPT and APC in order to localize cell types in different developmental stages, and to provide some information regarding the hemocyte differentiation in the crayfish. We localized mRNA expression of previously identified genes in the HPT/APC and hemocytes by RNA-FISH. The expression of hemolectin and transglutaminase 1 was shown to be co-localized in a high number of the HPT cells, while transglutaminase 2 was expressed in different cell types mainly associated with epithelium or endothelium. Furthermore, by double RNA-FISH for hemolectin and a previously unidentified PDGF-like factor, combined with immunostaining for prophenoloxidase, we could identify several different subtypes of hemocytes, indicating that the immune function of hemocytes in crayfish is more diversified and complex than previously appreciated.

CqProfilin enhances WSSV infection by promoting viral intracellular transport through binding to both viral nucleocapsid and actin cytoskeleton

White spot syndrome virus (WSSV) is a large nuclear-replicating DNA virus of crustaceans such as shrimp and crayfish; however, the molecular mechanisms facilitating its transport from the invasion site to the cell nucleus have not yet been well elucidated. In this study, a CqProfilin (CqPFN) with a conserved PROF domain was identified from the red claw crayfish Cherax quadricarinatus. CqPFN was ubiquitously expressed in all examined tissues and hemocyte, with the highest levels in the hemocyte, followed by hematopoietic tissue (Hpt) from which the hemocyte were derived in crayfish. The transcript of WSSV genes such as IE1 and VP28 was obviously decreased both in vivo in hemocyte and Hpt, as well as in vitro in cultured Hpt cells, after CqPFN gene silencing; in contrast, the expression of viral genes was significantly increased by the introduction of a recombinant CqPFN protein in Hpt cells in vitro. Moreover, CqPFN was clearly colocalized with the main viral nucleocapsid protein VP664 and F-actin cytoskeleton, respectively, during the early stage of WSSV infection in Hpt cells. In addition, CqPFN was confirmed to interact with a truncated VP6642,405-2,535 and another viral nucleocapsid protein VP15 of WSSV and Cqβ-Actin from Hpt by co-immunoprecipitation assays. Further studies found that VP664 also colocalized with F-actin in the Hpt cell cytoplasm after WSSV infection, suggesting that the actin cytoskeleton was involved in the intracellular transport of incoming viral nucleocapsid. Taken together, CqPFN might combine with the actin cytoskeleton to promote WSSV infection through binding with viral nucleocapsid proteins VP664 and VP15, promoting intracellular transport of viral incoming nucleocapsid for further releasing genome into the nucleus for transcription. Collectively, these results provided an understanding of the WSSV pathogenesis, which will contribute to the development of an antiviral strategy against WSSV disease.

Exosome-mediated viral nucleic acid presentation in a crustacean expounds innate immunity from a novel perspective

As an enduring hot topic in the field of innate immunity, apoptosis is widely considered an effective approach to eliminate pathogenic microbes and plays a crucial role during host-pathogen interactions. Recently, researchers have found that the virus-containing host cells could transmit apoptotic signals to the surrounding uninfected cells during infection, but the mechanism remains unclear. Here, we found that exosomes secreted by WSSV-infected mud crab hemocytes contain viral nucleic acid wsv277, which could be transported to the recipient cells and further expressed viral protein with phosphokinase activity. Besides, by using transcriptome, proteome, ChIP-seq, and coIP techniques, the results revealed that wsv277 could activate the transcription and translation of apoptotic genes via interacting with CBF and EF-1α so as to suppress the spread of virus infection by inducing apoptosis of the surrounding cells. Therefore, for the first time, our study proved that the components of DNA virus could be encapsulated into exosomes and elucidated the mechanism of apoptotic signal transduction between cells from the perspective of exosomes.

IMPORTANCE

Our study revealed that the components of DNA virus could be packaged and transmitted through the exosomes of lower invertebrates, which strongly demonstrated the diversity of exosome-mediated viral immunity and its universality in animals. Furthermore, we elucidated the mechanism of apoptotic signal transduction between cells from the perspective of exosomes and revealed a novel strategy for the host to cope with viral infection.

Expression of C/EBP and Kr-h1 transcription factors under immune stimulation in the noble crayfish

Transcription factors (TFs) have an important role in the regulation of the gene expression network. The role of TFs in the immune response of freshwater crayfish is poorly understood, but leveraging the regulatory mechanisms of immune response could augment the resistance against the invasive oomycete pathogen, Aphanomyces astaci. Previous studies indicated that the TFs CCAAT/enhancer-binding protein (C/EBP) and putative Krüppel homolog-1 protein (Kr-h1) might play a role in immune and stress response of the noble crayfish (Astacus astacus). Here, we aimed to further characterise these two gene products to gain a better understanding of their evolutionary origin, domain organisation and expression patterns across different crayfish tissues. Furthermore, we conducted an immune stimulation experiment to observe the potential changes in the gene expression of C/EBP and Kr-h1 under immune challenge in different crayfish tissues. Our results showed that both C/EBP and Kr-h1 are closely related to other C/EBPs and Kr-h1s in Malacostraca. Gene expression analysis revealed that both TFs are present in all analysed tissues, with higher expression of C/EBP in the gills and Kr-h1 in the abdominal muscle. Immune stimulation with laminarin (mimicking β-1-3-glucan in the oomycete cell wall) showed an activation of the crayfish immune system, with an overall increase in the total haemocyte count (THC) compared to untreated control and crayfish buffered saline (CBS) treatment. On the gene expression level, an up-regulation of the C/EBP gene was detected in the laminarin treated group in hepatopancreas and heart, while no changes were observed for the Kr-h1 gene. Our results indicate an early change in C/EBP expression in multiple tissues during immune stimulation and suggest its involvement in the immune response of the noble crayfish.

Response of terrestrial crustacean Porcellio scaber and mealworm Tenebrio molitor to non-degradable and biodegradable fossil-based mulching film microplastics

Agricultural mulching films are potential sources of microplastics (MPs) in soil. As an alternative to conventional non-degradable mulching films, a variety of different biodegradable mulching films are used. However, it is not yet known whether MPs from biodegradable mulching films pose a lower risk to terrestrial invertebrates compared to MPs from conventional mulching films. In this study, the effects of MPs produced from two conventional polyethylene (PE-1 and PE-2) and two biodegradable (starch-based poly(butylene adipate co-terephthalate); PBAT-BD-1, and PBAT-BD-2) fossil-based mulching films on terrestrial crustacean woodlice Porcellio scaber and mealworm Tenebrio molitor were compared. A key finding was that no clear differences in induced responses between biodegradable and conventional MPs were detected. No adverse effects on P. scaber after two weeks and on T. molitor after four weeks of exposure were observed up to 5 % (w/w dry soil) of either MP type. However, some sublethal physiological changes in metabolic rate and immune parameters were found in P. scaber after two weeks of exposure indicating a response of organisms to the presence of MP exposure in soil. In addition, it was demonstrated that both types of MPs might affect the soil water holding capacity and pH. In conclusion, we confirmed that biodegradable MPs can induce responses in organisms hence further studies testing the environmental hazard of biodegradable MPs are justified.

Comparison analysis of circulating hemocytes in decapod crustaceans

Hemocytes are the primary immune cells of crustaceans. Few comparison studies have been done among different crustaceans and some key parameters of circulating hemocytes have not been investigated. Here, we compared the circulating hemocytes in six decapod crustaceans, Cherax quadrinatus, Procambarus clarkii, Penaeus vannamei, Penaeus monodon, Eriocheir sinensis, and Scylla paramamosain. Although the hemocytes of different species vary in size, they share common morphological characteristics. Based on their morphological features, circulating hemocytes can be basically classified into granular cells (GCs), semi-granular cells (SGCs), and hyaline cells (HCs). In the six decapods analyzed in this study, the proportion of GCs varied from 10 % to 30 %. P. vannamei, P. monodon, and P. clarkii had fewer GCs in circulation than the other three species. Correspondingly, proliferation was detected only in a small portion of cells in P. vannamei, P. monodon, and P. clarkii under physical conditions. The hemocyte renewal rates for P. clarkii, E. sinensis, and C. quadrinatus were 6.1 %, 5.1 %, and 1.5 % per day, while no steady new hemocyte production was found in S. paramamosain within six days. These data give a general picture of the similarities and differences of circulating hemocytes in decapods and provide a base for an in-depth study of their immune system.

Myeloid differentiation protein 2 regulates the innate immunity and the disease resistant against Vibrio alginolyticus in Scylla paramamosain

Myeloid differentiation protein 2 (MD2), generally functions as a coreceptor of Toll-like receptor 4 (TLR4), facilitating the activation of TLR4 and the recognition of lipopolysaccharides (LPS) in host organisms. While the role of MD2 in immune activation is well-documented across various species, the specific role of the MD2 homolog in Scylla paramamosain (SpMD2) remains unidentified. In this study, we applied RNA interference to reduce SpMD2 expression, aiming to elucidate its role in immune system of mud crabs. Notably, SpMD2 interference leded to decrease in the hemocyte counts and phagocytic activity, along with increase in apoptosis rates and level of reactive oxygen species (ROS). Furthermore, the activities of key enzymes related to immune, such as superoxide dismutase (SOD), catalase (CAT), phenoloxidase (PO), peroxidase (POD), lysozyme (LZM), and acid phosphatase (ACP), were reduced by SpMD2 knockdown. Following infection with Vibrio alginolyticus, increase of SpMD2 expression level was observed. This was accompanied by alterations in the expression levels of genes related to immune in mud crabs. Challenge experiment with Vibrio alginolyticus showed a higher mortality rate after SpMD2 interference. Our study underscore the critical role of SpMD2 in enhancing the innate immunity and disease resistant in S. paramamosain, advancing our understanding of the innate immune regulatory mechanisms in crustaceans.

Decoding the role of HIF-1α in immunoregulation in Litopenaeus vannamei under hypoxic stress

Hypoxia poses a significant challenge to aquatic organisms, especially Litopenaeus vannamei (L. vannamei), which play a vital role in the global aquaculture industry. Hypoxia-inducible factor 1α (HIF-1α) is a pivotal regulator of the organism's adaptation to hypoxic conditions. To understand of how HIF-1α affects the immunity of L. vannamei under hypoxic conditions, we conducted a thorough study involving various approaches. These included observing tissue morphology, analyzing the expression of immune-related genes, assessing the activities of immune-related enzymes, and exploring immune-related pathways. Our study revealed that RNA interference (RNAi)-mediated knockdown of HIF-1α markedly reduced HIF-1α expression in the gill (75-95 %), whereas the reduction ranged from 2 to 43 % in the hepatopancreas. Knockdown of HIF-1α resulted in increased damage to both gill and hepatopancreatic tissues in hypoxic conditions. Additionally, immune-related genes, including Astakine (AST), Hemocyanin (HC), and Ferritin (FT), as well as immune-related enzymes such as Acid Phosphatase (ACP), Alkaline Phosphatase (AKP), and Phenoloxidase (PO), exhibited intricate regulatory patterns in response to hypoxia stress following the knockdown of HIF-1α. Transcriptome analysis revealed that HIF-1α knockdown significantly impacts multiple signaling pathways, including the JAK-STAT signaling pathway, Th17 cell differentiation pathways, PI3K-Akt signaling pathway, ErbB signaling pathway, MAPK signaling pathway, chemokine signaling pathway, ribosomal pathways, apoptosis, lysosomes and arachidonic acid metabolism. These alterations disrupt the organism's immune balance and interfere with normal metabolic processes, potentially leading to various immune-related diseases. We speculate that the weakened immune response resulting from HIF-1 inhibition is due to the reduced metabolic capacity, and the existence of a direct regulatory relationship between them requires further exploration. This study greatly advances our understanding of the vital role that HIF-1α plays in regulating immune responses in shrimp under hypoxic conditions, thereby deepening our comprehension of this critical biological mechanism.

Enhancement of immunity and disease resistance in Litopenaeus vannamei through injection of tyramine formulated with polyethylene glycol

This study investigates the prolonged effect of immune disease resistance in Litopenaeus vannamei through the administration of tyramine (TA) formulated with polyethylene glycol (PEG). Facing the challenges of intensive farming, environmental stress, and global climate changes, innovative approaches to improve shrimp health are essential. The research focuses on the role of biogenic amines in stress response and immune regulation, demonstrating that TA, especially when combined with PEG, significantly prolongs immunity and resistance against Vibrio alginolyticus. The experimental design included administering TA, PEG, and TA-PEG, followed by evaluations of immunity, lactate and glucose levels, and immune-related gene expressions. Results showed notable prolonged effects in total hemocyte count, phenoloxidase activity, and phagocytic activity in the TA-PEG group, indicating enhanced immune activation period. Additionally, the expression of prophenoloxidase system-related genes was significantly upregulated in the TA-PEG group. Furthermore, the TA-PEG group exhibited a significantly higher survival rate in a susceptibility test against V. alginolyticus. The results of this study confirm that the combined use of PEG can effectively extend the immunostimulatory duration of TA.

High variation of virulence in Aphanomyces astaci strains lacks association with pathogenic traits and mtDNA haplogroups

Introduced into Europe from North America 150 years ago alongside its native crayfish hosts, the invasive pathogen Aphanomyces astaci is considered one of the main causes of European crayfish population decline. For the past two centuries, this oomycete pathogen has been extensively studied, with the more recent efforts focused on containing and monitoring its spread across the continent. However, after the recent introduction of new strains, the newly-discovered diversity of A. astaci in North America and several years of coevolution with its European host, a new assessment of the traits linked to the pathogen's virulence is much needed. To fill this gap, we investigated the presence of phenotypic patterns (i.e., in vitro growth and sporulation rates) possibly associated with the pathogen's virulence (i.e., induced mortality in crayfish) in a collection of 14 A. astaci strains isolated both in North America and in Europe. The results highlighted a high variability in virulence, growth rate and motile spore production among the different strains, while the total-sporulation rate was more similar across strains. Surprisingly, growth and sporulation rates were not significantly correlated with virulence. Furthermore, none of the analysed parameters, including virulence, was significantly different among the major A. astaci haplogroups. These results indicate that each strain is defined by a characteristic combination of pathogenic features, specifically assembled for the environment and host faced by each strain. Thus, canonical mitochondrial markers, often used to infer the pathogen's virulence, are not accurate tools to deduce the phenotype of A. astaci strains. As the diversity of A. astaci strains in Europe is bound to increase due to translocations of new carrier crayfish species from North America, there is an urgent need to deepen our understanding of A. astaci's virulence variability and its ability to adapt to new hosts and environments.

Effects of high temperature and LPS injections on the hemocytes of the crab Neohelice granulata

Temperature fluctuations, particularly elevated temperatures, can significantly affect immune responses. These fluctuations can influence the immune system and alter its response to infection signals, such as lipopolysaccharide (LPS). Therefore, this study was designed to investigate how high temperatures and LPS injections collectively influence the immune system of the crab Neohelice granulata. Two groups were exposed to 20 °C (control) or 33 °C for four days. Subsequently, half were injected with 10 μL of physiological crustacean (PS), while the rest received 10 μL of LPS [0.1 mg.kg-1]. After 30 min, the hemolymph samples were collected. Hemocytes were then isolated and assessed for various parameters using flow cytometry, including cell integrity, DNA fragmentation, total hemocyte count (THC), differential hemocyte count (DHC), reactive oxygen species (ROS) level, lipid peroxidation (LPO), and phagocytosis. Results showed lower cell viability at 20 °C, with more DNA damage in the same LPS-injected animals. There was no significant difference in THC, but DHC indicated a decrease in hyaline cells (HC) at 20 °C following LPS administration. In granular cells (GC), an increase was observed after both PS and LPS were injected at the same temperature. In semi-granular cells (SGC), there was a decrease at 20 °C with the injection of LPS, while at a temperature of 33 °C, the SGC there was a decrease only in SGC injected with LPS. Crabs injected with PS and LPS at 20 °C exhibited higher levels of ROS in GC and SGC, while at 33 °C, the increase was observed only in GC and SGC cells injected with LPS. A significant increase in LPO was observed only in SGC cells injected with PS and LPS at 20 °C and 33 °C. Phagocytosis decreased in animals at 20 °C with both injections and exposed to 33 °C only in those injected with LPS. These results suggest that elevated temperatures induce changes in immune system parameters and attenuate the immune responses triggered by LPS.

Transforming Aquaculture through Vaccination: A Review on Recent Developments and Milestones

Aquaculture has rapidly emerged as one of the fastest growing industries, expanding both on global and on national fronts. With the ever-increasing demand for proteins with a high biological value, the aquaculture industry has established itself as one of the most efficient forms of animal production, proving to be a vital component of global food production by supplying nearly half of aquatic food products intended for human consumption. As in classic animal production, the prevention of diseases constitutes an enduring challenge associated with severe economic and environmental repercussions. Nevertheless, remarkable strides in the development of aquaculture vaccines have been recently witnessed, offering sustainable solutions to persistent health-related issues challenging resilient aquaculture production. These advancements are characterized by breakthroughs in increased species-specific precision, improved vaccine-delivery systems, and innovations in vaccine development, following the recent advent of nanotechnology, biotechnology, and artificial intelligence in the -omics era. The objective of this paper was to assess recent developments and milestones revolving around aquaculture vaccinology and provide an updated overview of strengths, weaknesses, opportunities, and threats of the sector, by incorporating and comparatively discussing various diffuse advances that span across a wide range of topics, including emerging vaccine technologies, innovative delivery methods, insights on novel adjuvants, and parasite vaccine development for the aquaculture sector.

Longitudinal tracking of hemocyte populations in vivo indicates lineage relationships and supports neural progenitor identity in adult neurogenesis

Adult neurogenesis, which takes place in both vertebrate and invertebrate species, is the process by which new neurons are born and integrated into existing functional neural circuits, long after embryonic development. Most studies in mammals suggest that self-renewing stem cells are the source of the new neurons, although the extent of self-renewal is a matter of debate. In contrast, research in the crayfish Procambarus clarkii has demonstrated that the neural progenitors producing adult-born neurons are capable of both self-renewing and consuming (non-self-renewing) divisions. However, self-renewing divisions are relatively rare, and therefore the production of adult-born neurons depends heavily on progenitors that are not replenishing themselves. Because the small pool of neural progenitors in the neurogenic niche is never exhausted throughout the long lives of these animals, we hypothesized that there must also be an extrinsic source of these cells. It was subsequently demonstrated that the neural progenitors originate in hemocytes (blood cells) produced by the immune system that travel in the circulation before ultimately integrating into niches where the neural lineage begins. The current study examines the developmental lineage of the three hemocyte types - hyaline (HC), semigranular (SGC) and granular (GC) cells - with the goal of understanding the origins of the progenitor cells that produce adult-born neurons. Longstanding qualitative metrics for hemocyte classification were validated quantitatively. Then, in a longitudinal study, proliferation markers were used to label the hemocytes in vivo, followed by sampling the circulating hemocyte population over the course of two months. Hemolymph samples were taken at intervals to track the frequencies of the different hemocyte types. These data reveal sequential peaks in the relative frequencies of HCs, SGCs and GCs, which were identified using qualitative and quantitative measures. These findings suggest that the three hemocyte types comprise a single cellular lineage that occurs in the circulation, with each type as a sequential progressive stage in hemocyte maturation beginning with HCs and ending with GCs. When combined with previously published data, this timeline provides additional evidence that HCs serve as the primary neural progenitor during adult neurogenesis in P. clarkii.

Experimental Infection Models and Their Usefulness for White Spot Syndrome Virus (WSSV) Research in Shrimp

White spot syndrome virus (WSSV) is marked as one of the most economically devastating pathogens in shrimp aquaculture worldwide. Infection of cultured shrimp can lead to mass mortality (up to 100%). Although progress has been made, our understanding of WSSV's infection process and the virus-host-environment interaction is far from complete. This in turn hinders the development of effective mitigation strategies against WSSV. Infection models occupy a crucial first step in the research flow that tries to elucidate the infectious disease process to develop new antiviral treatments. Moreover, since the establishment of continuous shrimp cell lines is a work in progress, the development and use of standardized in vivo infection models that reflect the host-pathogen interaction in shrimp is a necessity. This review critically examines key aspects of in vivo WSSV infection model development that are often overlooked, such as standardization, (post)larval quality, inoculum type and choice of inoculation procedure, housing conditions, and shrimp welfare considerations. Furthermore, the usefulness of experimental infection models for different lines of WSSV research will be discussed with the aim to aid researchers when choosing a suitable model for their research needs.

Considerations for cultivated crustacean meat: potential cell sources, potential differentiation and immortalization strategies, and lessons from crustacean and other animal models

Cultivated crustacean meat (CCM) is a means to create highly valued shrimp, lobster, and crab products directly from stem cells, thus removing the need to farm or fish live animals. Conventional crustacean enterprises face increasing pressures in managing overfishing, pollution, and the warming climate, so CCM may provide a way to ensure sufficient supply as global demand for these products grows. To support the development of CCM, this review briefly details crustacean cell culture work to date, before addressing what is presently known about crustacean muscle development, particularly the molecular mechanisms involved, and how this might relate to recent work on cultivated meat production in vertebrate species. Recognizing the current lack of cell lines available to establish CCM cultures, we also consider primary stem cell sources that can be obtained non-lethally including tissues from limbs which are readily released and regrown, and putative stem cells in circulating hemolymph. Molecular approaches to inducing myogenic differentiation and immortalization of putative stem cells are also reviewed. Finally, we assess the current status of tools available to CCM researchers, particularly antibodies, and propose avenues to address existing shortfalls in order to see the field progress.

Immunological lectins in shrimp Penaeus vannamei challenged with infectious myonecrosis virus (IMNV) under low-salinity conditions

Lectins are proteins capable of recognizing and binding to glycan in a specific way. In invertebrates, lectins are a crucial group of Pattern Recognition Proteins (PRRs), activating cellular and humoral responses in the innate immune system. The shrimp Penaeus vannamei is the main crustacean cultivated worldwide, however, the productivity of cultures is strongly affected by diseases, mainly viral ones, such as Infectious Myonecrosis (IMN). Thus, we investigated the participation of five lectins (LvAV, LvCTL4, LvCTL5, LvCTLU, and LvLdlrCTL) in IMNV-challenged shrimp. We verified upregulation gene profiles of lectins after IMNV-challenge, especially in hepatopancreas and gills, in addition to an increase in total hemocytes count (THC) after to 12 h post-infection (hpi). The bioinformatics characterization also revealed several sites of post-translational modification (PTM), such as phosphorylation and glycosylation, which possibly influence the action and stabilization of these lectins. We conclude that LvLdlrCTL and LvCTL5 are the lectins with greater participation in the activation of the immune system against IMNV, showing the greatest potential for PTM, higher upregulation levels, and overlapping with the THC and IMNV viral load.

Single-cell RNA-seq revealed heterogeneous responses and functional differentiation of hemocytes against white spot syndrome virus infection in Litopenaeus vannamei

Shrimp hemocytes are the vital immune cells participating in innate immune response to defend against viruses. However, the lack of specific molecular markers for shrimp hemocyte hindered the insightful understanding of their functional clusters and differential roles in combating microbial infections. In this study, we used single-cell RNA sequencing to map the transcriptomic landscape of hemocytes from the white spot syndrome virus (WSSV)-infected Litopenaeus vannamei and conjointly analyzed with our previous published single-cell RNA sequencing technology data from the healthy hemocytes. A total of 16 transcriptionally distinct cell clusters were identified, which occupied different proportions in healthy and WSSV-infected hemocytes and exerted differential roles in antiviral immune response. Following mapping of the sequencing data to the WSSV genome, we found that all types of hemocytes could be invaded by WSSV virions, especially the cluster 8, which showed the highest transcriptional levels of WSSV genes and exhibited a cell type-specific antiviral response to the viral infection. Further evaluation of the cell clusters revealed the delicate dynamic balance between hemocyte immune response and viral infestation. Unsupervised pseudo-time analysis of hemocytes showed that the hemocytes in immune-resting state could be significantly activated upon WSSV infection and then functionally differentiated to different hemocyte subsets. Collectively, our results revealed the differential responses of shrimp hemocytes and the process of immune-functional differentiation post-WSSV infection, providing essential resource for the systematic insight into the synergistic immune response mechanism against viral infection among hemocyte subtypes.

IMPORTANCE

Current knowledge of shrimp hemocyte classification mainly comes from morphology, which hinder in-depth characterization of cell lineage development, functional differentiation, and different immune response of hemocyte types during pathogenic infections. Here, single-cell RNA sequencing was used for mapping hemocytes during white spot syndrome virus (WSSV) infection in Litopenaeus vannamei, identifying 16 cell clusters and evaluating their potential antiviral functional characteristics. We have described the dynamic balance between viral infestation and hemocyte immunity. And the functional differentiation of hemocytes under WSSV stimulation was further characterized. Our results provided a comprehensive transcriptional landscape and revealed the heterogeneous immune response in shrimp hemocytes during WSSV infection.

Hemolymph exosomes inhibit Spiroplasma eriocheiris infection by promoting Tetraspanin-mediated hemocyte phagocytosis in crab

Exosomes released from infected cells are thought to play an important role in the dissemination of pathogens, as well as in host-derived immune molecules during infection. As an intracellular pathogen, Spiroplasma eriocheiris is harmful to multiple crustaceans. However, the immune mechanism of exosomes during Spiroplasma infection has not been investigated. Here, we found exosomes derived from S. eriocheiris-infected crabs could facilitate phagocytosis and apoptosis of hemocytes, resulting in increased crab survival and suppression of Spiroplasma intracellular replication. Proteomic analysis revealed the altered abundance of EsTetraspanin may confer resistance to S. eriocheiris, possibly by mediating hemocyte phagocytosis in Eriocheir sinensis. Specifically, knockdown of EsTetraspanin in E. sinensis increased susceptibility to S. eriocheiris infection and displayed compromised phagocytic ability, whereas overexpression of EsTetraspanin in Drosophila S2 cells inhibited S. eriocheiris infection. Further, it was confirmed that intramuscular injection of recombinant LEL domain of EsTetraspanin reduced the mortality of S. eriocheiris-infected crabs. Blockade with anti-EsTetraspanin serum could exacerbate S. eriocheiris invasion of hemocytes and impair hemocyte phagocytic activity. Taken together, our findings prove for the first time that exosomes modulate phagocytosis to resist pathogenic infection in invertebrates, which is proposed to be mediated by exosomal Tetraspanin, supporting the development of preventative strategies against Spiroplasma infection.

Effects of the tumor necrosis factor on hemocyte proliferation and bacterial infection in Chinese mitten crab (Eriocheir sinensis)

Tumor necrosis factor (TNF) is an important cytokine that can regulate a variety of cellular responses by binding tumor necrosis factor receptor (TNFR). We studied whether the TNF of Eriocheir sinensis can regulate hemocyte proliferation. The results showed that the EsTNF and EsTNFR were constitutively expressed in all tested tissues, including the heart, hepatopancreas, muscles, gills, stomachs, intestines, and hemocytes. We found that low levels of EsTNF and EsTNFR transcripts were present in hemocytes. The gene expression levels were significantly increased in the hemocytes after being stimulated by Staphylococcus aureus or Vibrio parahaemolyticus. We also found some genes related to cell proliferation were expressed at a higher level in pulsing rTNF-stimulated hemocytes compared with the control group. We also knocked down the EsTNFR gene with RNAi technology. The results showed that the expression level of these genes related to cell proliferation was significantly down-regulated compared with the control group when the TNF does not bind TNFR. We used Edu technology to repeat the above experiments and the results were similar. Compared with the control group, the hemocytes stimulated by rTNF showed more significant proliferation, and the proliferation rate was significantly down-regulated after knocking down the EsTNFR gene. Therefore, we indicate that TNF binding TNFR can affect the proliferation of E. sinensis hemocytes, which might be manifested by affecting the expression of some proliferation-related genes.

Protocols for studying the crayfish plague pathogen, Aphanomyces astaci, and its host-pathogen interactions

The crayfish plague caused by the pathogen Aphanomyces astaci has decimated the European and Asian populations of freshwater crayfish and represents an important threat to the other highly susceptible crayfish species in the world, such as the Australian, Madagascar, and South American species. The development and application of molecular methods addressed to the identification of A. astaci has increased exponentially during the last decades in contrast to a slow trend of the pathogen biology and host interaction. There is still a need for a better comprehension of the A. astaci-crayfish interactions, specifically the resistance and tolerance immune mechanism. These types of studies required a robust basic knowledge on the developmental biology of the pathogen in order to reproduce life stages and to perform infection experiments. A great piece of work in this area was carried out during the 1960 s to 80 s in University of Uppsala. Thus, the purpose of this work was to update previous protocols as well as to generate new guidelines to reproduce key developmental biology stages of A. astaci, to eventually identify crayfish populations with higher resistance and tolerance to this pathogen. This work also refers to other methodologies and guidelines for the diagnosis of crayfish plague, the pathogen isolation, and the in vitro production of zoospores.

The immunomodulatory and antioxidant effects of β-glucans in invertebrates

β-glucans (βGs) are carbohydrate polymers linked by β-1,3, 1,4 or 1,6 bonds, they have been used to protect against potential pathogens and prevent lethal diseases. The immune system possesses several receptors that identify a wide range of structures and trigger cellular and humoral mechanisms. However, the mechanisms by which βGs activate the immune system of invertebrate organisms have not been fully clarified. This review is focused on evaluating the effect of βGs on innate immune system in invertebrates. βGs stimulate different cellular and humoral mechanisms, such as phagocytosis, oxygen species production, extracellular trap formation, proPO system, and antimicrobial peptide synthesis, moreover, βGs increase survival rate and decrease pathogen load in several species.

Temporal dynamics of the immune response in Astacus astacus (Linnaeus, 1758) challenged with Aphanomyces astaci Schikora, 1906

One of the main drivers of biodiversity loss in freshwater ecosystems are alien invasive species. In Europe, pathogen Aphanomyces astaci Schikora, 1906, is considered as one of the most problematic invasive species, as its introduction caused the severe decimation of the European freshwater crayfish stocks. The most affected are the populations of noble crayfish, keystone species native to European freshwaters. Unfortunately, even after decades of research, we do not understand the temporal dynamics of the noble crayfish immune response during Ap. astaci infection. Here, we studied the changes in the immune response of the noble crayfish during a time course challenge with a highly virulent strain of Ap. astaci. We recorded gross symptoms of the disease, changes in the total haemocyte count (THC), gene expression profiles of putative immune response regulators and pathogen load. Additionally, we conducted a preliminary histological analysis of the pathogen dissemination in host tissues. Based on the occurrence of symptoms we propose three stages in the crayfish plague disease progression: asymptomatic stage, symptomatic stage, and finally death of infected individual. Furthermore, based on the qPCR analysis we could differentiate three Ap. astaci growth phases: initial lag phase, followed by exponential growth phase and finally sporulation phase. We observed that all measured immune response parameters were significantly correlated to the observed increase in the pathogen load (qPCR). Altogether, our results point to the absence of a successful immune response in the noble crayfish to a challenge with a highly virulent strain of Ap. astaci. The noble crayfish immune system was not able to suppress the growth of the intruding pathogen. In general, our observations have to be considered in the context of the specific combination of crayfish plague pathogen virulence and disease resistance of the challenged crayfish population, which defines the temporal dynamics of their interaction.

Identification and characterization of mitogen-activated protein kinase kinase 4 (MKK4) from the mud crab Scylla paramamosain in response to Vibrio alginolyticus and White Spot Syndrome Virus (WSSV)

Mitogen-activated protein kinase kinase 4 (MKK4), serves as a critical component of the mitogen-activated protein kinase signaling pathway, facilitating the direct phosphorylation and activation of the c-Jun N-terminal kinase (JNK) and p38 families of MAP kinases in response to environmental stresses. In the current research, we identified two MKK4 subtypes, namely SpMKK4-1 and SpMKK4-2, from Scylla paramamosain, followed by the analysis of their molecular characteristics and tissue distributions. The expression of SpMKK4s was induced upon WSSV and Vibrio alginolyticus challenges, and the bacteria clearance capacity and antimicrobial peptide (AMP) genes' expression upon bacterial infection were significantly decreased after knocking down SpMKK4s. Additionally, the overexpression of both SpMKK4s remarkably activated NF-κB reporter plasmid in HEK293T cells, suggesting the activation of the NF-κB signaling pathway. These results indicated the participation of SpMKK4s in the innate immunity of crabs, which shed light on a better understanding of the mechanisms through which MKK4s regulate innate immunity.

Proteomics of the haemolymph of the terrestrial crustacean Porcellio scaber reveals components of its innate immunity under baseline conditions

The terrestrial crustacean Porcellio scaber is an established test organism in environmental research. We analysed the haemolymph proteome of P. scaber using a classical proteomic approach based on one-dimensional gel electrophoresis and tandem mass spectrometry. Using a publicly available protein database and our P. scaber transcriptome data, we have identified 76 proteins involved in cytoskeleton formation, protein degradation, vesicular transport, genetic information processing, detoxification, carbohydrate and lipid metabolism reflecting haemocyte metabolic activity, active intracellular transport, and intercellular communication. Compared with the data reported for other crustaceans, 28 of these P. scaber proteins have been linked to its immunity, among them hemocyanin, α-2-macroglobulin, phenoloxidase 3, superoxide dismutase, glutathione S-transferase, haemolymph clottable protein, and histones H4 and H2B. Our results thus provide a firm base for studying the innate immune response of P. scaber at the level of the haemolymph proteome. This knowledge is of particular importance in ecotoxicity studies with various environmental stressors where understanding physiological changes is important to reveal possible modes of action.

Transcriptional and metabolomic investigation of the stress response in snow crab during simulated transport condition (Chionoecetes opilio)

The molecular mechanisms underlying the stress response are poorly described in crustaceans. This includes the snow crab (Chionoecetes opilio), a commercially important stenotherm species distributed throughout the northern hemisphere. A better understanding of the stress response in C. opilio is desperately needed for commercial and conservation purposes. The purpose of this study was to investigate the transcriptional and metabolomic response of C. opilio exposed to stressors. Crabs were randomly assigned to 24 or 72 h treatment groups where they were exposed to conditions simulating live transport (handling and air exposure). A control group was kept in cold (2 °C) and well‑oxygenated saltwater. The hepatopancreas of the crabs was sampled to perform RNA-sequencing and high-performance chemical isotope labeling metabolomics. Differential gene expression analyses showed that classic crustaceans' stress markers, such as crustacean hyperglycemic hormones and heat shock proteins, were overexpressed in response to stressors. Tyrosine decarboxylase was also up-regulated in stressed crabs, suggesting an implication of the catecholamines tyramine and octopamine in the stress response. Deregulated metabolites revealed that low oxygen was an important trigger in the stress response as intermediate metabolites of the tricarboxylic acid cycle (TCA) accumulated. Lactate, which accumulated unevenly between crabs could potentially be used to predict mortality. This study provides new information on how stressors affect crustaceans and provides a basis for the development of stress markers in C. opilio.

Hemocyte coagulation and phagocytic behavior in early stages of injury in crayfish (Arthropoda: Decapoda) affect their morphology

Crustacean hemocytes are important mediators of immune functions such as coagulation and phagocytosis. We employed an in situ approach to investigate the ultrastructural behavior of hemocytes during coagulation and phagocytosis in the early stages after injury caused by leg amputation, using transmission electron microscopy technique in marbled crayfish Procambarus virginalis. Hemocytes underwent drastic morphological changes during coagulation. The morphology of the cytoplasmic granules changed from electron-dense to electron-lucent forms in an expanding manner. The transformed granules containing amorphous electron-lucent material were observed to merge and discharge their contents into extracellular space for coagulation. We also observed that the contents of the nucleus participate in the process of coagulation. In addition, leg amputation induced extensive muscle degeneration and necrotic tissues were avidly taken up by the phagocytic hemocytes containing distinct phagosomes. Interestingly, we observed for the first time how the digested contents of phagocytized necrotic tissues are incorporated into granules and other cellular components that change the cell morphology by increasing the granularity of the hemocytes. Nevertheless, the degranulation of hemocytes during coagulation can also reduce their granularity. Given that morphological traits are important criteria for hemocyte classification, these morphological changes that occur during coagulation and phagocytosis must be taken into account.

The involvement of hypoxia inducible factor-1α on the proportion of three types of haemocytes in Chinese mitten crab under hypoxia stress

Hypoxia triggers diverse cell physiological processes, and the hypoxia inducible factors (HIFs) are a family of heterodimeric transcription factors that function as master regulators to respond to hypoxia in different cells. However, the knowledge about the hypoxic responses especially cell alteration mediated by HIFs under hypoxia stress is still limited in crustaceans. In the present study, a hypoxia-inducible factor-1α (HIF-1α) gene was identified (designed as EsHIF-1α). The relative mRNA expression level of EsHIF-1α was highest in hyalinocytes and lowest in granulocytes among three types of haemocytes in crabs. Hypoxia could significantly increase the EsHIF-1α protein expression level in haemocytes. Meanwhile, the proportion of hyalinocytes began to increase from 3 h post hypoxia treatment, and reached the highest level at 24 h. However, the opposite variation in proportion of granulocytes was observed under hypoxia stress. Further investigation showed that the inhibition of EsHIF-1α induced by KC7F2 (HIF-1α inhibitor) could lead to the significant decrease in the proportion of hyalinocytes under hypoxia stress, and also resulted in an increase of granulocytes proportion. While, after EsHIF-1α was activated by IOX4 (HIF-1α activator), the proportion of hyalinocytes was significantly up-regulated and the proportion of granulocytes was significantly down-regulated under post hypoxia treatment. These results collectively suggested that EsHIF-1α was involved in the regulation of proportion of three types of haemocytes induced by hypoxia stress, which provided vital insight into the understanding of the crosstalk between hypoxia and cell development in invertebrates.

scRNA-seq analysis reveals toxicity mechanisms in shrimp hemocytes subjected to nitrite stress

In shrimp, hemocytes play an important role in detoxification and immune defense, and are where nitrite accumulates during exposure to this toxic environmental pollutant. However, the heterogeneity mechanisms of toxicity have not been reported under nitrite expose in shrimp. Here, we used single-cell RNA-seq to resolve 24,000 cells, which the responses of different cell populations of hemocytes under nitrite exposure in Penaeus vannamei. We identified 394 specific nitrite-responsive genes in 9 clusters of hemocytes, and found heterogeneity in the nitrite response of the three subpopulations of hemocytes (hyaline, semi-granular and granular cells). In hyaline, the response appeared modest, whereas nitrite-related dysregulation of metabolic processes in granular and semi-granular was pronounced. Ammonia nitrogen will rapidly accumulate in hemocytes of shrimp under nitrite stress. In semi-granular, excessive ammonia will interfere with oxidative phosphorylation and antioxidant system, thus inducing the production of reactive oxygen species. In granular, the abnormality of urea cycle caused by ammonia accumulation is the main toxic factor, which by inhibits arginase and arginine kinase. Collectively, our data provide a single-cell atlas for the dissection of shrimp hemocyte complexity, and reveal the toxicity mechanisms associated with nitrite exposure.

Effects of environmental factors on the cellular and molecular parameters of the immune system in decapods

Crustaceans and in particular decapods (i.e. shrimp, crabs and lobsters) are a diverse, commercially and ecologically important group of organisms. They are exposed to a range of environmental factors whose abiotic and biotic components are prone to fluctuate beyond their optimum ranges and, in doing so, affect crustaceans' immune system and health. Changes in key environmental factors such as temperature, pH, salinity, dissolved oxygen, ammonia concentrations and pathogens can provoke stress and immune responses due to alterations in immune parameters. The mechanisms through which stressors mediate effects on immune parameters are not fully understood in decapods. Improved knowledge of the environmental factors - above all, their abiotic components - that influence the immune parameters of decapods could help mitigate or constrain their harmful effects that adversely affect the production of decapod crustaceans. The first part of this overview examines current knowledge and information gaps regarding the basic components and functions of the innate immune system of decapods. In the second part, we discuss various mechanisms provoked by environmental factors and categorize cellular and molecular immune responses to each environmental factor with special reference to decapods.

Hallmarks of crustacean immune hemocytes at single-cell resolution

In invertebrates, hemocytes are the key factors in innate immunity. However, the types of invertebrate immune hemocytes are unclassified due to the limitation of morphological classification. To determine the immune hemocytes of crustaceans, the heterogeneity of hemocytes of shrimp Marsupenaeus japonicus and crayfish Procambarus clarkii, two representative crustacean species, were characterized in this study. The results of single-cell RNA sequencing indicated that shrimp and crayfish contained 11 and 12 types of hemocytes, respectively. Each of different types of hemocytes specifically expressed the potential marker genes. Based on the responses of shrimp and crayfish to the infection of white spot syndrome virus (WSSV) and the challenge of lipopolysaccharide (LPS), four types of immune hemocytes of crustaceans were classified, including semi-granular hemocytes involved in antimicrobial peptide production, granular hemocytes responsible for the production of antimicrobial peptides, hemocytes related to cell proliferation and hemocytes in immunity-activated state. Therefore, our study provided the first classification of crustacean hemocytes as well as of immune hemocytes of crustaceans at the single-cell resolution, which would be helpful to understand the innate immunity of invertebrates.

The immunostimulatory effects of Theobroma cacao L. pod husk extract via injection and dietary administrations on Macrobrachium rosenbergii and its resistance against Lactococcus garvieae

The extract from Theobroma cacao L, pod husk served as the immunostimulant to enhance the immunity and resistance against Lactococcus garvieae of Macrobrachium rosenbergii. In this study, we employed the injection method and dietary administration method to determine the effect of cocoa pod husk (CPH) on M. rosenbergii. The non-specific immune parameters and disease resistance were evaluated after the prawn injected with 1 μg prawn^-1 CPH extract (C1), 3 μg prawn^-1 CPH extract (C3), and 5 μg prawn^-1 CPH extract (C5) for 1, 3, and 7 days. The results showed a significant increase of total haemocyte count (THC), differential haemocyte count (DHC), phenoloxidase (PO) activity, respiratory bursts (RBs), and phagocytic activity and clearance efficiency to L. garvieae. The non-specific immune parameters, physiological parameters, and disease resistance and growth performance were evaluated after the prawn fed with 1 g kg^-1 CPH extract diet (CD1), 3 g kg^-1 CPH extract diet (CD3) and 5 g kg^-1 CPH extract diet (CD5). The results showed a significant increase in all immune parameters and showed a significant decrease in physiological parameters. No significant difference was observed in growth performance of prawn fed with the CPH containing diet. Both injection and dietary method showed a significant increase in disease resistance against to L. garvieae. We therefore recommend that CPH extract can be used as a immunostimulant for prawn by dietary administration to regulate immune responses, and carbohydrate metabolism lead to enhance resistance against pathogen.

Tire microplastics exposure in soil induces changes in expression profile of immune-related genes in terrestrial crustacean Porcellio scaber

Tire particles pose a potential threat to terrestrial organisms because they are deposited in large quantities in the soil by tire wear abrasion, and moreover their chemical complexity poses an additional risk. Microplastics can affect several physiological processes in organisms, including those related to immunity. Therefore, we investigated the expression profile of selected immune-related genes (MnSod, Manganese Superoxide dismutase; Cat, Catalase; CypG, Cyclophilin G; Nos, Nitric oxide synthase; Ppae2a, Prophenoloxidase-activating enzyme 2a; Dscam, Down syndrome cell adhesion molecule; Myd88, Myeloid-differentiation factor 88; Toll4, Toll-like receptor 4; Mas-like, Masquerade-like protein) in haemocytes and the digestive gland hepatopancreas of terrestrial crustacean Porcellio scaber after two different time exposures (4 and 14 days) to tire particles in soil. Our results reveal for the first time the response of P. scaber after microplastic exposure at the transcriptome level. We observed time- and tissue-dependent changes in the expression of the analysed genes, with more pronounced alterations in haemocytes after 14 days of exposure. Some minor changes were also observed in hepatopancreas after 4 days. Changes in the expression profile of the analysed genes are a direct indication of a modulated immune status of the test organism, which, however, does not represent an adverse effect on the test organism under the given conditions. Nevertheless, the question remains whether the observed change in immune status affects the immunocompetence of the test organism.

Blood cell formation in crustaceans

In crustacean animals the hemocytes are key players in immunity and of crucial importance for the health of the animals. Hemocytes are mainly produced in the hematopoietic tissue and from there released into the circulation where they finally mature. In this review we summarize the latest findings about crustacean hemocyte formation. The role of the extracellular matrix and crosslinking enzyme transglutaminase is discussed. Moreover, important growth factors, transcriptional regulation and recent findings about inducers of hematopoiesis are covered. Finally, we discuss the use of different markers for classification of crustacean hemocytes.

Structural and functional diversity of lectins associated with immunity in the marine shrimp Litopenaeusvannamei

Lectins are important pattern recognition receptors (PRRs) and their immunological action is related to the recognition of glycans present in the pathogen cells surface. The lectins described for Litopenaeus vannamei are divided into C-type, L-type and galectin, which are mainly expressed in hepatopancreas and hemocytes. They are involved in several immune response pathways, such as phagocytosis, hemocytes recruitment, prophenoloxidase activation, and gene regulation. Although lectins have multiple immune functions, most experimental challenges focus only on WSSV and Vibrio sp. This article is a detailed review on the role of lectins in L. vannamei immune system, bringing together information on molecular structure, temporal and special expression and immune function, highlighting the wide participation of these molecules in shrimp innate immune system.

Single-cell RNA-seq uncovered hemocyte functional subtypes and their differentiational characteristics and connectivity with morphological subpopulations in Litopenaeus vannamei

Hemocytes play central roles in shrimp immune system, whereas whose subclasses have not yet been completely defined. At present, the morphological classification of hemocytes is inadequate to classify the complete hemocyte repertoire and elucidate the functions and differentiation and maturation processes. Based on single-cell RNA sequencing (scRNA-seq) of hemocytes in healthy Litopenaeus vannamei, combined with RNA-FISH and flow cytometric sorting, we identified three hemocyte clusters including TGase+ cells, CTL+ cells and Crustin+ cells, and further determined their functional properties, potential differentiation trajectory and correspondence with morphological subpopulations. The TGase+ cells were mainly responsible for the coagulation, exhibiting distinguishable characteristics of hyalinocyte, and appeared to be developmentally arrested at an early stage of hemocyte differentiation. The CTL+ cells and Crustin+ cells arrested at terminal stages of differentiation mainly participated in recognizing foreign pathogens and initiating immune defense responses, owning distinctive features of granule-containing hemocytes. Furthermore, we have revealed the functional sub-clusters of three hemocyte clusters and their potential differentiation pathways according to the expression of genes involved in cell cycle, cell differentiation and immune response, and the successive differentiation and maturation of hyalinocytes to granule-containing hemocytes have also mapped. The results revealed the diversity of shrimp hemocytes and provide new theoretical rationale for hemocyte classification, which also facilitate systematic research on crustacean immunity.

Marine Arthropods as a Source of Antimicrobial Peptides

Peptide therapeutics play a key role in the development of new medical treatments. The traditional focus on endogenous peptides has shifted from first discovering other natural sources of these molecules, to later synthesizing those with unique bioactivities. This review provides concise information concerning antimicrobial peptides derived from marine crustaceans for the development of new therapeutics. Marine arthropods do not have an adaptive immune system, and therefore, they depend on the innate immune system to eliminate pathogens. In this context, antimicrobial peptides (AMPs) with unique characteristics are a pivotal part of the defense systems of these organisms. This review covers topics such as the diversity and distribution of peptides in marine arthropods (crustacea and chelicerata), with a focus on penaeid shrimps. The following aspects are covered: the defense system; classes of AMPs; molecular characteristics of AMPs; AMP synthesis; the role of penaeidins, anti-lipopolysaccharide factors, crustins, and stylicins against microorganisms; and the use of AMPs as therapeutic drugs. This review seeks to provide a useful compilation of the most recent information regarding AMPs from marine crustaceans, and describes the future potential applications of these molecules.

Characterization of hemocytes and hematopoietic cells of a freshwater crayfish based on single-cell transcriptome analysis

Crustaceans constitute a species-rich and ecologically important animal group, and their circulating blood cells (hemocytes) are of critical importance in immunity as key players in pathogen recognition, phagocytosis, melanization, and antimicrobial defense. To gain a better understanding of the immune responses to different pathogens, it is crucial that we identify different hemocyte subpopulations with different functions and gain a better understanding of how these cells are formed. Here, we performed single-cell RNA sequencing of isolated hematopoietic tissue (HPT) cells and hemocytes from the crayfish Pacifastacus leniusculus to identify hitherto undescribed hemocyte types in the circulation and show that the circulating cells are more diversified than previously recognized. In addition, we discovered cell populations in the HPT with clear precursor characteristics as well as cells involved in iron homeostasis, representing a previously undiscovered cell type. These findings may improve our understanding of hematopoietic stem cell regulation in crustaceans and other animals.

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Single-cell RNA sequencing of hematopoietic cell types reveals new cell types

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One cell type contains iron homeostasis-associated transcripts

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Hemocytes and hematopoietic cells differ in their transcript profiles

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Prophenoloxidase is only expressed in hemocytes

The juvenile hormone epoxide hydrolase homolog in Penaeus vannamei plays immune-related functions

Juvenile hormone epoxide hydrolase (JHEH) participates in the degradation of juvenile hormone and also involved in the development and molting process in insects. Here, the JHEH homolog in Pennaus vannamei was cloned and found to consist of a full-length cDNA of 2543 bp and an open reading frame (ORF) of 1386 bp. Transcripts of PvJHEH1 were expressed in most tissues of healthy shrimp with the highest found in the hepatopancreas and lowest in hemocytes. Both Gram-negative (Vibrio parahaemolyticus) and Gram-positive (Streptococcus iniae) bacteria induced PvJHEH1 expression in shrimp hemocytes and hepatopancreas, suggesting the involvement of PvJHEH1 in P. vannamei immune responses. Moreover, the mRNA levels of ecdysone inducible nuclear transcription factor PvE75 and crustacean hyperglycemic hormone (PvCHH), two endocrine-related genes with roles in shrimp innate immune response, decreased significantly in shrimp hemocytes after PvJHEH1 knockdown. Shrimp survival was also affected after PvJHEH1 knockdown followed by V. parahaemolyticus challenge, indicating that JHEH1 plays an essential role in shrimp survival during bacterial infection.

Molecular markers for hemocyte subpopulations in crayfish Cherax quadricarinatus

Semigranular cells (SGCs) and granular cells (GCs) are two dominant groups of circulating hemocytes in crayfish Cherax quadricarinatus. Molecular markers are required for the clear classification of the hemocytes and the research of their function and differentiation. In this study, we compared the protein content of GCs and SGCs by using two workflows: one-dimensional gel electrophoresis followed by LC-MS/MS and in-solution digestion of cell lysate followed by LC-MS/MS. Cell type-specific proteins were identified, and their expression in SGCs and GCs was further investigated by RT-PCR, Western blotting, and immunofluorescence analysis. Three molecular markers for GCs (peroxinectin, a mannose-binding protein, and prophenoloxidase-activating enzyme 2a) and three molecular markers for SGCs (a vitelline membrane outer layer protein I-like protein, a C-type lectin, and a peptidase) were identified. The application of some of the markers in Eriocheir sinensis was also analyzed. These molecular markers are useful tools for the research of crustaceans hemocytes.

A Dissenters' View on AppleSnail Immunobiology

We stand as dissenters against the acceptance of scientific knowledge that has not been built on empirical data. With this in mind, this review synthesizes selected aspects of the immunobiology of gastropods and of apple snails (Ampullariidae) in particular, from morphological to molecular and "omics" studies. Our trip went through more than two centuries of history and was guided by an evo-devo hypothesis: that the gastropod immune system originally developed in the mesenchymal connective tissue of the reno-pericardial complex, and that in that tissue some cells differentiated into hematopoietically committed progenitor cells that integrate constitutive hemocyte aggregations in the reno-pericardial territory, whether concentrated in the pericardium or the kidney in a species-specific manner. However, some of them may be freed from those aggregations, circulate in the blood, and form distant contingent aggregations anywhere in the body, but always in response to intruders (i.e., pathogens or any other immune challenge). After that, we reviewed the incipient immunology of the Ampullariidae by critically revising the findings in Pomacea canaliculata and Marisa cornuarietis, the only ampullariid species that have been studied in this respect, and we attempted to identify the effectors and the processes in which they are involved. Particularly for P. canaliculata, which is by far the most studied species, we ask which hemocytes are involved, in which tissues or organs are integrated, and what cellular reactions to intruders this species has in common with other animals. Furthermore, we wondered what humoral factors could also integrate its internal defense system. Among the cellular defenses, we give an outstanding position to the generation of hemocyte nodules, which seems to be an important process for these snails, serving the isolation and elimination of intruders. Finally, we discuss hematopoiesis in apple snails. There have been contrasting views about some of these aspects, but we envision a hematopoietic system centered in the constitutive hemocyte islets in the ampullariid kidney.

Early responses in Penaeus vannamei during experimental infection with Enterocytozoon hepatopenaei (EHP) spores by injection and oral routes

Hepatopancreatic microsporidiosis caused by Enterocytozoon hepatopenaei (EHP) is associated with severe production losses in Penaeus vannamei farming. Early responses in P. vannamei experimentally infected with EHP was assessed in this study by feeding infected hepatopancreatic tissue and by injecting purified EHP spores (∼1 × 10⁵ Spores/shrimp). Immune responses to EHP infection were assessed in the haemolymph by analysing the total haemocyte count (THC), superoxide dismutase (SOD) activity, prophenoloxidase activity (proPO), respiratory burst activity (RBA), catalase activity (CAT), lysozyme activity (LYS) and Toll gene expression in hepatopancreas at 0, 6, 12, 24, 36, 48, 60 and 72 h post-infection (hpi). Experimental infection with EHP resulted in a significant (p < 0.05) reduction in the immune parameters such THC, CAT and LYS at 6, 24 and 24 hpi respectively while there was a significant increase (p < 0.05) in the levels of SOD, proPO and RBA at 6 hpi. The expression of the Toll gene was significantly upregulated (p < 0.05) after experimental infection with EHP from 6 hpi. These findings on immune responses in P. vannamei during EHP infection will assist in the development of suitable management measures to reduce the negative impacts of EHP in P. vannamei farming. This is the first report on early responses in P. vannamei during EHP infection.

Changes in the Immunity, Histopathology, and Metabolism of Crayfish (Procambarus clarkii) in Response to Drought

Freshwater ecosystems are among the most threatened ecosystems on Earth. The freshwater biodiversity crisis has caused widespread global concern. Drought as one of the factors causing freshwater biodiversity is still poorly understood. Crayfish is often used in academic research as a biological indicator. In this study, flow cytometry, hematoxylin-eosin staining, and untargeted metabolomics were used to analyze the immune function, histopathology, and metabolism of crayfish under drought conditions. After drought exposure, the total hemocytes count (THC) was significantly decreased (from 8.9 × 105 mL−1 in the control group to 2.2 × 105 mL−1 at day 5). Phagocytosis decreased by 66% after 5 days of drought. The level of reactive oxygen species (ROS) in the hepatopancreas was upregulated. Moreover, histological disorder and metabolism changes in the hepatopancreas were obvious. These results indicate that drought suppresses immune function, disrupts the balance of oxidative and antioxidative systems, and induces tissue damage and metabolic changes in crayfish.

The Role of microRNA-133 in Hemocyte Proliferation and Innate Immunity of Scylla paramamosain

MicroRNAs (miRNAs) are important signaling regulators that are involved in regulating the innate immunity of crustacean. However, few studies focus on the role of crustacean miRNAs in the cellular immunity have been reported. In this study, we showed that the expression of miR-133 was significantly up-regulated in the mud crab Scylla paramamosain after infection by white spot syndrome virus (WSSV) or Vibrio parahaemolyticus. The anti-miRNA oligonucleotide AMO-miR-133 was used to knock down miR-133 expression in S. paramamosain. The number of WSSV copies increased significantly in WSSV-infected crabs after miR-133 knockdown. Knockdown of miR-133 also enhanced the mortality rates of WSSV-infected and V. parahaemolyticus-infected mud crabs, and it significantly enhanced the expression of the astakine, which was confirmed by real-time quantitative PCR and western blot analysis. The data also indicate that miR-133 may affect hemocyte proliferation in S. paramamosain by regulating astakine expression. miR-133 Knockdown enhanced the apoptosis or phagocytosis of crab hemocytes, and increased the mortality of mud crabs after WSSV or V. parahaemolyticus infection. These results indicate that miR-133 is involved in the host immune response to WSSV and V. parahaemolyticus infection in mud crabs. Taken together, our research provides new insights for the control of viral or vibrio diseases in S. paramamosain.

Transcriptome profiles of red swamp crayfish Procambarus clarkii hematopoietic tissue in response to WSSV challenge

The crayfish Procambarus clarkii could achieve a high cumulative mortality after WSSV infections. To better understand the immune response to WSSV in hematopoietic tissue, the present study investigated the immunological response of P. clarkii and analyzed the expression of some hematopoietic cytokines. After assembly, there was an average of 47,712,411 clean reads were obtained in control and treatment groups. A total of 35,945 unigenes were discovered with N50 length of 1554 bp. Under functional classification, enrichment, and pathway analysis using different database, there were about 257 differentially expressed genes (DEGs) identified, of which 139 were up-regulated and 118 were down-regulated. The GO function analysis of these DEGs were mostly participated in activation of immune response, complement activation, complement binding, negative regulation of humoral immune response and secretory granule membrane. Under KEGG analysis, these DEGs were involved in ECM-receptor interaction, HIF-1 signaling pathway, Glycolysis/Gluconeogenesis, Thyroid hormone signaling pathway and Glucagon signaling pathway. The real-time quantitative PCR (RT-qPCR) analysis of 9 selected genes confirmed the reliability of RNA-Seq results. The present research provide for the first time the transcriptomic profile of P. clarkii hematopoietic tissue in response to WSSV infection and reveals the astakines may play important roles in antiviral immune response. The results of the present study will further enrich the theoretical basis of the crayfish immune system and provide new ideas for disease prevention and control.