Transcriptomic and Quantitative Proteomic Analyses Provide Insights Into the Phagocytic Killing of Hemocytes in the Oyster Crassostrea gigas

Morphologic, cytometric, quantitative transcriptomic and functional characterisation provide insights into the haemocyte immune responses of Pacific abalone (Haliotis discus hannai)

In recent years, the abalone aquaculture industry has been threatened by the bacterial pathogens. The immune responses mechanisms underlying the phagocytosis of haemocytes remain unclear in Haliotis discus hannai. It is necessary to investigate the immune mechanism in response to these bacterial pathogens challenges. In this study, the phagocytic activities of haemocytes in H. discus hannai were examined by flow cytometry combined with electron microscopy and transcriptomic analyses. The results of Vibrio parahaemolyticus, Vibrio alginolyticus and Staphylococcus aureu challenge using electron microscopy showed a process during phagosome formation in haemocytes. The phagocytic rate (PP) of S. aureus was higher than the other five foreign particles, which was about 63%. The PP of Vibrio harveyi was about 43%, the PP peak of V. alginolyticus in haemocyte was 63.7% at 1.5 h. After V. parahaemolyticus and V. alginolyticus challenge, acid phosphatase, alkaline phosphatase, total superoxide dismutase, lysozyme, total antioxidant capacity, catalase, nitric oxide synthase and glutathione peroxidase activities in haemocytes were measured at different times, differentially expressed genes (DEGs) were identified by quantitative transcriptomic analysis. The identified DEGs after V. parahaemolyticus challenge included haemagglutinin/amebocyte aggregation factor-like, supervillin-like isoform X4, calmodulin-like and kyphoscoliosis peptidase-like; the identified DEGs after V. alginolyticus challenge included interleukin-6 receptor subunit beta-like, protein turtle homolog B-like, rho GTPase-activating protein 6-like isoform X2, leukocyte surface antigen CD53-like, calponin-1-like, calmodulin-like, troponin C, troponin I-like isoform X4, troponin T-like isoform X18, tumor necrosis factor ligand superfamily member 10-like, rho-related protein racA-like and haemagglutinin/amebocyte aggregation factor-like. Some immune-related KEGG pathways were significantly up-regulated or down-regulated after challenge, including thyroid hormone synthesis, Th17 cell differentiation signalling pathway, focal adhesion, melanogenesis, leukocyte transendothelial migration, inflammatory mediator regulation of TRP channels, ras signalling pathway, rap1 signalling pathway. This study is the first step towards understanding the H. discus hannai immune system by adapting several tools to gastropods and providing a first detailed morpho-functional study of their haemocytes.

Acetylcholinesterase and dopamine inhibition suppress the filtration rate, burrowing behaviours, and immunological responses induced by bisphenol A in the hemocytes and gills of date mussels, Lithophaga lithophaga

Bisphenol A (BPA), a common industrial chemical with estrogenic activity, has recently gained attention due to its well-documented negative effects on humans and other organisms in the environment. The potential immunotoxicity and neurotoxicity of BPA remain poorly understood in marine invertebrate species. Therefore, the impacts of exposure to BPA on a series of behaviours, immune responses, oxidative stress, neural biomarkers, histology, and the ultrastructure of gills were investigated in the date mussel, Lithophaga lithophaga. After 28 days of exposure to 0.25, 1, 2, and 5 µg/L BPA, hemolymphs from controls and exposed date mussels were collected, and the effects of BPA on immunological parameters were evaluated. Moreover, oxidative stress and neurochemical levels were measured in the gills of L. lithophaga. BPA reduced filtration rates and burrowing behaviour, whereas a 2 µg/L BPA resulted in an insignificant increase after 24 h. The exposure of date mussels to BPA significantly increased total hemocyte counts, a significant reduction in the diameter and phagocytosis of hemocytes, as well as gill lysozyme level. BPA increased lipid peroxidation levels and SOD activity in gills exposed to 2 and 5 µg/L BPA, but decreased GSH levels and SOD activity in 0.25 and 1 µg/L BPA-treated date mussels. Dose-dependent dynamics were observed in the inhibition of acetylcholinesterase activity and dopamine levels. Histological and scanning electron microscope examination revealed cilia erosion, necrosis, inflammation, and hyperplasia formation in the gills. Overall, our findings suggest a relationship between BPA exposure and changes in the measured immune parameters, oxidative stress, and neurochemical disturbances, which may be factored into the mechanisms underlying BPA toxicity in marine molluscs, providing a scientific foundation for marine BPA risk assessment and indicating immunosuppression in BPA-exposed date mussels.

Identification, diversity, and evolution analysis of thioester-containing protein family in Pacific oyster (Crassostrea gigas) and immune response to biotic and abiotic stresses

Thioester-containing proteins (TEPs) play a vital role in the innate immune response to biotic and abiotic stresses. In this study, the TEPs in C. gigas were identified, and their gene structure, phylogenetic relationships, collinearity relationships, expression profiles, sequence diversity, and alternative splicing were analyzed. Eight Tep genes were identified in C. gigas genome. Functional analysis and evolutionary relationships indicated a high level of homology to other mollusks TEPs. The transcriptome quantitative analysis results showed that the Tep genes in C. gigas respond to heat stress and Vibrio stress. Alternative splicing analysis revealed four Tep genes (designated A2M_1, CD109_3, CD109_5, complement C3) encode multiple alternative splice variants. Analysis of gene structure and multiple alignments revealed that seven CD109_5 variants are produced through the alternative splicing of the 19th exon, which encodes the highly variable central region. Sequence diversity analysis revealed thirteen missense variants within the 19th exon region of these seven CD109_5 alternative splice variants. Furthermore, the differential alternative splicing analysis showed significant induction of CD109_5, A2M_1 and A2M_2 variants after infection with V. parahaemolyticus. This study explores the Tep genes of C. gigas, providing insights into the molecular mechanisms underlying the involvement of C. gigas TEPs in innate immunity.

Molecular mechanisms underlying the vulnerability of Pacific abalone (Haliotis discus hannai) to Vibrio harveyi infection at higher water temperature

Climate change is one of the most important threats to farmed abalone worldwide. Although abalone is more susceptible to vibriosis at higher water temperatures, the molecular mode of action underlying this has not been fully elucidated. Therefore, this study aimed to address the high susceptibility of Halitotis discus hannai to V. harveyi infection using abalone hemocytes exposed to low and high temperatures. Abalone hemocytes were divided into four groups, 20C, 20V, 25C, and 25V, depending on co-culture with (V)/without (C) V. harveyi (MOI = 12.8) and incubation temperature (20 °C or 25 °C). After 3 h of incubation, hemocyte viability and phagocytic activity were measured, and RNA sequencing was performed using Illumina Novaseq. The expression of several virulence-related genes in V. harveyi was analyzed using real-time PCR. The viability of hemocytes was significantly decreased in the 25V group compared to cells in the other groups, whereas phagocytic activity at 25 °C was significantly higher than at 20 °C. Although a number of immune-associated genes were commonly upregulated in abalone hemocyte exposed to V. harveyi, regardless of temperature, pathways and genes regarding pro-inflammatory responses (interleukin-17 and tumor necrosis factor) and apoptosis were significantly overexpressed in the 25V group compared to the 25C group. Notably, in the apoptosis pathway, genes encoding executor caspases (casp3 and casp7) and pro-apoptotic factor, bax were significantly up-regulated only in the 25V group, while the apoptosis inhibitor, bcl2L1 was significantly up-regulated only in the 20V group compared to the control group at the respective temperatures. The co-culture of V. harveyi with abalone hemocytes at 25 °C up-regulated several virulence-related genes involved in quorum sensing (luxS), antioxidant activity (katA, katB, and sodC), motility (flgI), and adherence/invasion (ompU) compared to those at 20 °C. Therefore, our results showed that H. discus hannai hemocytes exposed to V. harveyi at 25 °C were highly stressed by vigorously activated inflammatory responses and that the bacterial pathogen overexpressed several virulence-related genes at the high temperature tested. The transcriptomic profile of both abalone hemocytes and V. harveyi in the present study provide insight into differential host-pathogen interactions depending on the temperature conditions and the molecular backgrounds related to increased abalone vulnerability upon global warming.

Roles of hemocyte subpopulations in silver nanoparticle transformation and toxicity in the oysters Crassostrea hongkongensis

Hemocytes are the main immune cells in bivalve mollusks and one of the sensitive targets for nanoparticle toxicity. Bivalve hemocytes consist of multiple functional heterogeneous cell types, but their different roles in immune system against foreign particles remain largely unknown. In order to clarify the different immune responses of hemocyte subpopulations to silver nanoparticles (AgNPs) and Ag ions, in this study, the Hong Kong oyster (Crassostrea hongkongensis) hemocytes were employed and separated into three subpopulations based on their cell size and granularity, including agranulocytes (R1), semigranulocytes (R2), and granulocytes (R3). We first demonstrated that AgNPs could rapidly enter into the oyster hemocytes within 3 h by phagocytosis process and resulted in different immune responses in hemocyte subpopulations. The most affected cell subtype by AgNPs was the granulocytes, followed by semigranulocytes, whereas agranulocytes were not affected following exposure to AgNPs. Interestingly, AgNPs induced the granule formation in semigranulocytes and further increased the proportion of granulocytes, whereas their ionic counterparts had no such effects on hemocyte composition, indicating the different detoxification mechanisms for nanoparticulate and ionic form. Following AgNP exposure, the dissolved Ag ions were accumulated in lysosomes and caused lysosomal dysfunction, indicating that lysosomes were the main targets for AgNP toxicity and the dissolved Ag ions were the main contributor of AgNP toxicity. Furthermore, AgNP exposure induced reactive oxygen production and impeded the lysosome function and phagocytosis in granulocytes, with impaired immunity system in oysters. Our study identified the different immune responses of oyster hemocyte subpopulations to AgNPs based on the in vitro short-term exposure assays, which may be applied to rapidly evaluate the ecotoxicological risks of different nanoparticles in aquatic systems.

RGD-Labeled Hemocytes With High Migration Activity Display a Potential Immunomodulatory Role in the Pacific Oyster Crassostrea gigas

Immunocyte migration to infection sites is important for host cellular defense, but the main types of migrating hemocytes and their mechanisms against pathogen invasions are unclear in invertebrates. In the present study, a population of hemocytes in the Pacific oyster Crassostrea gigas labeled with a fluorescein isothiocyanate (FITC)-conjugated Arg-Gly-Asp (RGD)-containing peptide was sorted. RGD⁺ hemocytes were characterized by a smaller cell size and cytoplasmic-nucleo ratio, fewer cytoplasmic granules, and higher levels of myeloperoxidase, reactive oxygen species, and intracellular free calcium concentration. RGD⁺ hemocytes exhibited a high level of migration activity, which was further induced after V. splendidus infection. Transcriptome analysis revealed that RGD⁺ hemocytes highly expressed a series of migration-related genes, which together with migration-promoting genes were significantly upregulated after V. splendidus infection. The neuroendocrine system was also proven to regulate the migration activity of RGD⁺ hemocytes, especially with the excitatory neuroendocrine factor dopamine, which promoted migration activity as confirmed by receptor blocking assays. Meanwhile, RGD⁺ hemocytes could highly express immunomodulatory factor interleukin (IL)-17s and their receptor genes, which was positively related to the production of antimicrobial peptides in whole hemocytes after V. splendidus infection. Collectively, this study identified a specific hemocyte population, i.e., RGD⁺ hemocytes, that shows high migration activity in response to pathogen infection and exerts a potential immunomodulatory role by highly expressing IL-17s that might enhance the hemocytes’ antimicrobial peptide production in oysters.

Proteomic changes associated with predator-induced morphological defenses in oysters

Inducible prey defenses occur when organisms undergo plastic changes in phenotype to reduce predation risk. When predation pressure varies persistently over space or time, such as when predator and prey co-occur over only part of their biogeographic ranges, prey populations can become locally adapted in their inducible defenses. In California estuaries, native Olympia oyster (Ostrea lurida) populations have evolved disparate phenotypic responses to an invasive predator, the Atlantic oyster drill (Urosalpinx cinerea). In this study, oysters from an estuary with drills, and oysters from an estuary without drills, were reared for two generations in a laboratory common garden, and subsequently exposed to cues from Atlantic drills. Comparative proteomics was then used to investigate molecular mechanisms underlying conserved and divergent aspects of their inducible defenses. Both populations developed smaller, thicker, and harder shells after drill exposure, and these changes in shell phenotype were associated with up-regulation of calcium transport proteins that could influence biomineralization. Inducible defenses evolve in part because defended phenotypes incur fitness costs when predation risk is low. Immune proteins were down-regulated by both oyster populations after exposure to drills, implying a trade-off between biomineralization and immune function. Following drill exposure, oysters from the population that co-occurs with drills grew smaller shells than oysters inhabiting the estuary not yet invaded by the predator. Variation in the response to drills between populations was associated with isoform-specific protein expression. This trend suggests that a stronger inducible defense response evolved in oysters that co-occur with drills through modification of an existing mechanism.

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.

CgRab1 regulates Cgcathepsin L1 expression and participates in the phagocytosis of haemocytes in oyster Crassostrea gigas

Rab protein plays an important role in a variety of cellular activities, especially the fusion process of the inner membrane during endocytosis. In the present study, a Rab1 protein (CgRab1) was identified from the Pacific oyster Crassostrea gigas. The full-length cDNA sequence of CgRab1 was of 2248 bp with an open reading frame of 618 bp, encoding a polypeptide of 205 amino acids containing a Rab domain. The deduced amino acid sequence of CgRab1 shared 94.2% and 89.3% identity with Rab1 from Pomacea canaliculata and Homo sapiens respectively. In the phylogenetic tree, CgRab1 was firstly clustered with the Rab1s from Aplysia californica and Pomacea canaliculata to form a sister group with Rab1 from invertebrates. The recombinant CgRab1 protein (rCgRab1) was able to bind GTP. The mRNA transcripts of CgRab1 were highly expressed in oyster haemocytes, and its expression level in oyster haemocytes was significantly up-regulated at 24 h after the stimulations with Vibro splendidus, which was 2.43-fold (p < 0.01) of that in the control group. After the expression of CgRab1 was knocked down (0.38-fold of that in EGFP-RNAi experimental group) by RNAi，the protein expression of Cgcathepsin L1 were reduced (0.63-fold, p < 0.01) compared with that in EGFP-RNAi experimental group. The phagocytic rate and phagocytic index of haemocytes in CgRab1-RNAi oysters decreased after V. splendidus stimulation, which was 0.50-fold (p < 0.01) and 0.58-fold (p < 0.01) of that in EGFP-RNAi experimental group. These results indicated that CgRab1 was involved in the process of haemocytes phagocytosis by regulating the expression of Cgcathepsin L1 in oyster C. gigas.

Single-cell analysis of mosquito hemocytes identifies signatures of immune cell subtypes and cell differentiation

Mosquito immune cells, known as hemocytes, are integral to cellular and humoral responses that limit pathogen survival and mediate immune priming. However, without reliable cell markers and genetic tools, studies of mosquito immune cells have been limited to morphological observations, leaving several aspects of their biology uncharacterized. Here, we use single-cell RNA sequencing (scRNA-seq) to characterize mosquito immune cells, demonstrating an increased complexity to previously defined prohemocyte, oenocytoid, and granulocyte subtypes. Through functional assays relying on phagocytosis, phagocyte depletion, and RNA-FISH experiments, we define markers to accurately distinguish immune cell subtypes and provide evidence for immune cell maturation and differentiation. In addition, gene-silencing experiments demonstrate the importance of lozenge in defining the mosquito oenocytoid cell fate. Together, our scRNA-seq analysis provides an important foundation for future studies of mosquito immune cell biology and a valuable resource for comparative invertebrate immunology.

Development of a High-Efficiency Immunomagnetic Enrichment Method for Detection of Human Norovirus via PAMAM Dendrimer/SA-Biotin Mediated Cascade-Amplification

Human norovirus is a common cause of acute gastroenteritis worldwide, and oysters have been found to be the main carriers for its spread. The lack of efficient pre-treatment methods has been a major bottleneck limiting the detection of viruses in oysters. In this study, we established a novel immunomagnetic enrichment method using polyamidoamine (PAMAM) dendrimer/SA-biotin-mediated cascade amplification for reverse transcriptase quantitative real-time polymerase chain reaction (RT-qPCR) detection. We compared the capture efficiency of traditional immunomagnetic enrichment, biotin-amplified immunomagnetic enrichment, and PAMAM dendrimer/SA-biotin-mediated cascade-amplification immunomagnetic enrichment. The optimal capture efficiency of the novel method was 44.26 ± 1.45%, which increased by 183.17% (P < 0.01) and 18.09% (P < 0.05) compared with the first two methods, respectively. Three methods were all applied in detecting norovirus in 44 retail oysters, the detection rate of the PAMAM dendrimer/SA-biotin-mediated method was 25.0%, which was higher than those of traditional IME (15.90%) and SA-biotin-amplified IME (18.80%) by 9.1 and 6.2%, respectively. In conclusion, the novel method can be applied for the rapid detection of norovirus in oysters, which can help reduce the cost and time of detection and improve detection rates.

Immunological Responses of Marine Bivalves to Contaminant Exposure: Contribution of the -Omics Approach

The increasing number of data studies on the biological impact of anthropogenic chemicals in the marine environment, together with the great development of invertebrate immunology, has identified marine bivalves as a key invertebrate group for studies on immunological responses to pollutant exposure. Available data on the effects of contaminants on bivalve immunity, evaluated with different functional and molecular endpoints, underline that individual functional parameters (cellular or humoral) and the expression of selected immune-related genes can distinctly react to different chemicals depending on the conditions of exposure. Therefore, the measurement of a suite of immune biomarkers in hemocytes and hemolymph is needed for the correct evaluation of the overall impact of contaminant exposure on the organism's immunocompetence. Recent advances in -omics technologies are revealing the complexity of the molecular players in the immune response of different bivalve species. Although different -omics represent extremely powerful tools in understanding the impact of pollutants on a key physiological function such as immune defense, the -omics approach has only been utilized in this area of investigation in the last few years. In this work, available information obtained from the application of -omics to evaluate the effects of pollutants on bivalve immunity is summarized. The data shows that the overall knowledge on this subject is still quite limited and that to understand the environmental relevance of any change in immune homeostasis induced by exposure to contaminants, a combination of both functional assays and cutting-edge technology (transcriptomics, proteomics, and metabolomics) is required. In addition, the utilization of metagenomics may explain how the complex interplay between the immune system of bivalves and its associated bacterial communities can be modulated by pollutants, and how this may in turn affect homeostatic processes of the host, host–pathogen interactions, and the increased susceptibility to disease. Integrating different approaches will contribute to knowledge on the mechanism responsible for immune dysfunction induced by pollutants in ecologically and economically relevant bivalve species and further explain their sensitivity to multiple stressors, thus resulting in health or disease.

Identification of a Novel Pattern Recognition Receptor DM9 Domain Containing Protein 4 as a Marker for Pro-Hemocyte of Pacific Oyster Crassostrea gigas

DM9 refers to an uncharacterized protein domain that is originally discovered in Drosophila melanogaster. Two proteins with DM9 repeats have been recently identified from Pacific oyster Crassostrea gigas as mannose-specific binding pattern-recognition receptors (PRRs). In the present study, a novel member of DM9 domain containing protein (designated as CgDM9CP-4) was identified from C. gigas. CgDM9CP-4, about 16 kDa with only two tandem DM9 domains, was highly enriched in hemocytes and gill. The transcripts level of CgDM9CP-4 in circulating hemocytes were decreased after LPS, PGN and Vibrio splendidus stimulations. The recombinant protein of CgDM9CP-4 (rCgDM9CP-4) displayed a broad binding spectrum towards various pathogen-associated molecular patterns (PAMPs) (LPS, PGN, β-glucan and Mannose) and microorganisms (Staphylococcus aureus, Micrococcus luteus, V. splendidus, V. anguillarum, Escherichia coli, Pichia pastoris and Yarrowia lipolytica). CgDM9CP-4 was mostly expressed in gill and some of the hemocytes. Flow cytometry analysis demonstrated that the CgDM9CP-4-positive hemocytes accounted for 7.3% of the total hemocytes, and they were small in size and less in granularity. CgDM9CP-4 was highly expressed in non-phagocytes (~82% of total hemocytes). The reactive oxygen species (ROS) and the expression levels of cytokines in CgDM9CP-4-positive hemocytes were much lower than that in CgDM9CP-4-negative hemocytes. The mRNA expression level of CgDM9CP-4 in hemocytes was decreased after RNAi of hematopoietic-related factors (CgGATA, CgRunt, CgSCL, and CgNotch). In addition, CgDM9CP-4-positive cells were found to be much more abundant in hemocytes from gill than that from hemolymph, with most of them located in the gill filament. All these results suggested that CgDM9CP-4 was a novel member of PRR that expressed in undifferentiated pro-hemocytes to mediate immune recognition of pathogens.

A CD63 Homolog Specially Recruited to the Fungi-Contained Phagosomes Is Involved in the Cellular Immune Response of Oyster Crassostrea gigas

Cluster of differentiation 63 (CD63), a four-transmembrane glycoprotein in the subfamily of tetraspanin, has been widely recognized as a gateway from the infection of foreign invaders to the immune defense of hosts. Its role in Pacific oyster Crassostrea gigas is, however, yet to be discovered. This work makes contributions by identifying CgCD63H, a CD63 homolog with four transmembrane domains and one conservative CCG motif, and establishing its role as a receptor that participates in immune recognition and hemocyte phagocytosis. The presence of CgCD63H messenger RNA (mRNA) in hepatopancreas, labial palps, gill, and hemocytes is confirmed. The expression level of mRNA in hemocytes is found significantly (p < 0.01) upregulated after the injection of Vibrio splendidus. CgCD63H protein, typically distributed over the plasma membrane of oyster hemocytes, is recruited to the Yarrowia lipolytica-containing phagosomes after the stimulation of Y. lipolytica. The recombinant CgCD63H protein expresses binding capacity to glucan (GLU), peptidoglycan (PGN), and lipopolysaccharide (LPS) in the presence of lyophilized hemolymph. The phagocytic rate of hemocytes toward V. splendidus and Y. lipolytica is significantly inhibited (p < 0.01) after incubation with anti-CgCD63H antibody. Our work further suggests that CgCD63H functions as a receptor involved in the immune recognition and hemocyte phagocytosis against invading pathogen, which can be a marker candidate for the hemocyte typing in C. gigas.

Vibrio-bivalve interactions in health and disease

In the marine environment, bivalve mollusks constitute habitats for bacteria of the Vibrionaceae family. Vibrios belong to the microbiota of healthy oysters and mussels, which have the ability to concentrate bacteria in their tissues and body fluids, including the hemolymph. Remarkably, these important aquaculture species respond differently to infectious diseases. While oysters are the subject of recurrent mass mortalities at different life stages, mussels appear rather resistant to infections. Thus, Vibrio species are associated with the main diseases affecting the worldwide oyster production. Here, we review the current knowledge on Vibrio-bivalve interaction in oysters (Crassostrea sp.) and mussels (Mytilus sp.). We discuss the transient versus stable associations of vibrios with their bivalve hosts as well as technical issues limiting the monitoring of these bacteria in bivalve health and disease. Based on the current knowledge of oyster/mussel immunity and their interactions with Vibrio species pathogenic for oyster, we discuss how differences in immune effectors could contribute to the higher resistance of mussels to infections. Finally, we review the multiple strategies evolved by pathogenic vibrios to circumvent the potent immune defences of bivalves and how key virulence mechanisms could have been positively or negatively selected in the marine environment through interactions with predators.

Transcriptomic Evidence Reveals the Molecular Basis for Functional Differentiation of Hemocytes in a Marine Invertebrate, Crassostrea gigas

Hemocytes play unequivocally central roles in host immune defense of bivalve mollusks, though the exact mechanisms underlying their functional differentiation are only partially understood. To this end, granulocytes and hyalinocytes were sorted via flow cytometry from hemocytes of the Pacific oyster Crassostrea gigas, and consequently quantitative transcriptomic analysis revealed a striking array of differentially expressed genes (DEGs), which were globally upregulated in granulocytes, dedicating to functional differentiation among oyster hemocytes. Our network of DEGs illustrated actively engaged signaling pathways, with Cdc42/Cdc42l being a core regulator of pathway network, which was validated by a dramatically reduced capacity for hemocyte phagocytosis in the presence of Cdc42 inhibitors. Additionally, a number of transcription factors were identified among DEGs, including ELK, HELT, and Fos, which were predominantly expressed in granulocytes. The AP-1 transcription factor Fos was confirmed to facilitate functional differentiation of hemocytes in an assay on binding to target genes by the AP-1 binding site, consistent with downstream phagocytosis and ROS production. Importantly, Cdc42/Cdc42l were also regulated by the expression of Fos, providing a possible regulatory mechanism-guided hemocyte functional differentiation. Findings in this study have bridged a knowledge gap on the mechanistic underpinnings of functional differentiation of hemocytes in a marine invertebrate C. gigas, which promise to facilitate research on the evolution of immune defense and functional differentiation of phagocyte in higher-order and more recent phyla.

Phagocyte Transcriptomic Analysis Reveals Focal Adhesion Kinase (FAK) and Heparan Sulfate Proteoglycans (HSPGs) as Major Regulators in Anti-bacterial Defense of Crassostrea hongkongensis

Invertebrates generally lack adaptive immunity and compensate for this with highly efficient innate immune machineries such as phagocytosis by hemocytes to eradicate invading pathogens. However, how extrinsically cued hemocytes marshal internal signals to accomplish phagocytosis is not yet fully understood. To this end, we established a facile magnetic cell sorting method to enrich professional phagocytes from hemocytes of the Hong Kong oyster (Crassostrea hongkongensis), an ecologically and commercially valuable marine invertebrate. Transcriptomic analysis on presorted cells shows that phagocytes maintain a remarkable array of differentially expressed genes that distinguish them from non-phagocytes, including 352 significantly upregulated genes and 479 downregulated genes. Pathway annotations reveal that focal adhesion and extracellular matrix–receptor interactions were the most conspicuously enriched pathways in phagocytes. Phagocytosis rate dramatically declined in the presence of an FAK inhibitor, confirming importance of the focal adhesion pathway in regulating phagocytosis. In addition, we also found that heparan sulfate proteoglycan (HSPG) families were lineage-specifically expanded in C. hongkongensis and abundantly expressed in phagocytes. Efficiency of phagocytosis and hemocytes aggregation was markedly reduced upon blockage of endogenous synthesis of HSPGs, thus implicating these proteins as key surface receptors in pathogen recognition and initiation of phagocytosis.

Teleost Gasdermin E Is Cleaved by Caspase 1, 3, and 7 and Induces Pyroptosis

Pyroptosis is a newly defined gasdermin (GSDM)-dependent inflammatory type of programmed cell death. Different from mammals, which have a panel of pyroptotic GSDM members (e.g., GSDMA-E), teleosts possess only GSDME. The pyroptotic activity and regulation mechanism of teleost GSDME remain to be elucidated. In this work, we investigated the activity of the teleost Cynoglossus semilaevis (tongue sole) GSDME (CsGSDME) in association with different caspases (CASPs). We found that CsGSDME exerted pyroptotic and bactericidal activities through its N-terminal domain. Unlike human GSDME, which is exclusively cleaved by CASP3, CsGSDME was cleaved by C. semilaevis CASP (CsCASP) 1 with high efficiency and by CsCASP3 and 7 with comparatively low efficiencies, and all cleavages occurred at the ₂₄₃FEVD₂₄₆ site in the interdomain linker region of CsGSDME. Mutation of Phe²⁴³ to Asp/Ala and Asp²⁴⁶ to Ala in ₂₄₃FEVD₂₄₆ altered the cleavage preference of CsCASP1, 3, and 7. Treatment with loss-of-function CsCASP mutants or inhibition of CsCASP activity resulted in failure of CsGSDME cleavage. CsCASP1-cleaved CsGSDME induced pyroptosis, whereas CsCASP3/7-cleaved CsGSDME and F243 mutants induced switching of cell death from apoptosis to pyroptosis. Analysis of 54 teleost GSDME sequences revealed a conserved tetrapeptide motif that fits well to the inherent cleavage site of CASP1. Taken together, the results of our study demonstrate a hitherto, to our knowledge, unrecognized GSDME cleavage mode in teleosts that is clearly different from that in mammals, thus providing an important insight into the activation mechanism of CASP-mediated, GSDM-executed pyroptosis in teleosts.

Dual Analysis of Virus-Host Interactions: The Case of Ostreid herpesvirus 1 and the Cupped Oyster Crassostrea gigas

Dual analyses of the interactions between Ostreid herpesvirus 1 (OsHV-1) and the bivalve Crassostrea gigas during infection can unveil events critical to the onset and progression of this viral disease and can provide novel strategies for mitigating and preventing oyster mortality. Among the currently used “omics” technologies, dual transcriptomics (dual RNA-seq) coupled with the analysis of viral DNA in the host tissues has greatly advanced the knowledge of genes and pathways mostly contributing to host defense responses, expression profiles of annotated and unknown OsHV-1 open reading frames (ORFs), and viral genome variability. In addition to dual RNA-seq, proteomics and metabolomics analyses have the potential to add complementary information, needed to understand how a malacoherpesvirus can redirect and exploit the vital processes of its host. This review explores our current knowledge of “omics” technologies in the study of host-pathogen interactions and highlights relevant applications of these fields of expertise to the complex case of C gigas infections by OsHV-1, which currently threaten the mollusk production sector worldwide.