Changes in hemolymph characteristics of ark shell Scapharaca broughtonii dealt with Vibrio anguillarum challenge in vivo and various of anticoagulants in vitro

Transcriptome analysis provides new insights into the immune response of Ruditapes philippinarum infected with Vibrio alginolyticus

Manila clam (Ruditapes philippinarum) is a bivalve species with commercial value, but it is easily infected by pathogenic microorganisms in aquaculture, which restricts the shellfish industry. Notably, the impact of Vibrio alginolyticus on clam culture is obvious. In this study, RNA-seq was performed to analyze clam hepatopancreas tissue in 48 h (challenge group, G48h) and 96 h (challenge group, G96h) after infection with V. alginolyticus and 0 h after injection of PBS (control group, C). The results showed that a total of 1670 differentially expressed genes were detected in the G48h vs C group, and 1427 differentially expressed genes were detected in the G96h vs C group. In addition, KEGG analysis showed that DEGs were significantly enriched in pathways such as Lysosome and Mitophagy. Moreover, 15 immune related DEGs were selected for qRT-PCR analysis to verify the accuracy of RNA-seq, and the results showed that the expression level of DEGs was consistent with that of RNA-seq. Therefore, the results obtained in this study provides a preliminary understanding of the immune defense of R. philippinarum and molecular insights for genetic breeding of V. alginolyticus resistance in Manila clam.

Flow cytometric characterization of the hemocytes of blood cockles Anadara broughtonii (Schrenck, 1867), Anadara kagoshimensis (Lischke, 1869), and Tegillarca granosa (Linnaeus, 1758) as a biomarker for coastal environmental monitoring

Marine bivalves are often used as a sentinel species in coastal environmental monitoring since changes in the environmental quality are often well preserved in their cells and tissues. Anadara and Tegillarca species of Arcidae, the blood cockles, are considered to be good sentinel species in monitoring coastal pollution and ecosystem health because they are distributed widely in the subsurface of intertidal mudflats. Internal cellular defense of the blood cockles to physical and biological stresses is mediated by the circulating hemocytes, while their hemocyte types and functions are poorly studied. In this study, we first characterized morphology and immune-related activities of hemocytes of three common blood cockles Anadara broughtonii, A. kagoshimensis, and Tegillarca granosa using flow cytometry. Based on cell morphology and immunological functions, we described five types of hemocytes identically in the three blood cockles: erythrocytes type-I (erythrocytes-I), erythrocytes type-II (erythrocytes-II), granulocytes, hyalinocytes, and blast-like cells. Erythrocytes were round cells containing hemoglobin with numerous granules in the cytoplasm and these cells consist of two central populations. Erythrocytes-I were the most abundant cells accounting for 80-89% of the total circulating hemocytes and exhibited a certain level of lysosome and oxidative capacity. Erythrocytes-II were the largest cells and displayed high lysosome content and the most active oxidative capacity. Both erythrocytes-I and erythrocytes-II did not show phagocytosis capacity. Granulocytes were intermediated-sized hemocytes characterized by granules in the cytoplasm and long pseudopodia on the cell surface, and these cells were mainly engaged in the cellular defense exhibiting the largest lysosome content, the most active phagocytosis, and high oxidative capacity. Contrary to granulocytes, hyalinocytes were comparatively small and round cells and exhibited no granules in the cytoplasm. Hyalinocytes displayed a certain level of lysosome and phagocytosis and oxidative capacities. Blast-like cells characterized by the smallest size and small quantity of cytoplasm and exhibited an absence of phagocytosis and extremely low oxidative capacity, suggesting that this population is not directly involved in the cell-mediated immune activities. In conclusion, flow cytometry indicated that three blood cockles had five types of hemocytes, and the erythrocytes and granulocytes were mainly involved in the immunological activities.

Applications of flow cytometry in molluscan immunology: Current status and trends

Flow cytometry (FCM) is routinely used in fundamental and applied research, clinical practice, and clinical trials. In the last three decades, this technique has also become a routine tool used in immunological studies of molluscs to analyse physical and chemical characteristics of haemocytes. Here, we briefly review the current implementation of FCM in the field of molluscan immunology. These applications cover a diverse range of practices from straightforward total cell counts and cell viability to characterize cell subpopulations, and further extend to analyses of DNA content, phagocytosis, oxidative stress and apoptosis. The challenges and prospects of FCM applications in immunological studies of molluscs are also discussed.

Ark shell Scapharca broughtonii hemocyte response against Vibrio anguillarum challenge

Scapharca broughtonii is one of the most important Arcidae aquaculture species in the Asia-Pacific region. We aimed to investigate the immune responses of hemocytes from ark shell S. broughtonii hemolymph against pathogens. Hemocyte ultrastructure and immunological activity in response to Vibrio anguillarum challenge were observed by scanning and transmission electron microscopy. Before ultrastructure observation, we used the API ZYM semi-quantitative kit to evaluate the levels of hydrolytic enzymes in the plasma and hemocytes following V. anguillarum infection. An enzyme-linked immunosorbent assay kit was used to investigate the variation in the lysozyme activity and hemocytes following bacterial infection. The results showed that hemocytes were the main defense cells against bacterial infection, whereas plasma played a role in the transport and support of hemocytes. It was presumed that an important function of lysozymes and hydrolytic enzymes in lysosomes was for bacterial digestion. Three major types of hemocytes were observed, namely, red blood cells (RBCs), white blood cells (WBCs), and thrombocytes (TCs). Scanning electron microscopy showed that the normal RBCs appeared pie-shaped with 10 μm diameter and 4 μm central thickness, whereas WBCs were spherical in shape with varying sizes, 4-8 μm diameter, and included small lymphocytes. TCs were long, spindle-shaped, and 12-20 μm in length. The cell membrane surface was smooth and even for all cells before pathogen challenge. Under transmission electron microscopy, RBCs displayed a limited ability to devour and digest bacteria adherent to the cell surface following infection. Many hemoglobin particles were observed in the RBC cytoplasm. WBCs were very active against bacterial invasion and showed a strong ability to digest and decompose infected and wrapped V. anguillarum through phagocytosis and lysosome fusion. Digestive vacuoles rapidly became transparent and were thought to contain increasing quantities of pathogen-induced lysozymes. WBCs that devoured pathogenic bacteria were prone to deformation as well as adhesion to each other. TCs were rich in endoplasmic reticulum (ER) content in their cytoplasm and were widely connected in a net-shaped structure. Mitochondria in TCs formed clusters upon invasion of V. anguillarum in the hemolymph. TCs disintegrated to release the ER into the plasma to form a mesh that facilitated clotting. The ability of circulating hemocytes to quickly modify their morphologies and stainability suggests that S. broughtonii is endowed with highly dynamic hemocyte populations capable of coping with environmental changes and rapidly growing pathogens.

Ostreid Herpesvirus-1 Infects Specific Hemocytes in Ark Clam, Scapharca broughtonii

High levels of ostreid herpesvirus 1 (OsHV-1) were detected in hemocytes of OsHV-1 infected mollusks. Mollusk hemocytes are comprised of different cell types with morphological and functional heterogeneity. Granular cells are considered the main immunocompetent hemocytes. This study aimed to ascertain if OsHV-1 infects specific types of hemocytes in ark clams. Types of hemocytes were first characterized through microexamination and flow cytometry. In addition to a large group of red cells, there were three types of recognizable granular cells in ark clams. Type II granular cells were mostly found with OsHV-1 infection by transmission electron microscope (TEM) examination, and represented the hemocyte type that was susceptible to OsHV-1 infection. The subcellular location of OsHV-1 particles in apoptotic type II granular cells was further analyzed. Some OsHV-1 particles were free inside the apoptotic cells, which may contribute to OsHV-1 transmission among cells in the host, some particles were also found enclosed inside apoptotic bodies. Apoptosis is an important part of the host defense system, but might also be hijacked by OsHV-1 as a strategy to escape host immune attack. Following this investigation, a primary culture of type II granular cells with OsHV-1 infection would facilitate the research on the interaction between OsHV-1 and mollusk hosts.

Molecular cloning, expression and biochemical characterization of hemoglobin gene from ark shell Scapharca broughtonii

Hemoglobin, the main component of haemolymph, is widely distributed in animals. Although its important oxygen transport functions has been extensively reported, studies on the immune function of hemoglobin in mollusc are few. Research on immune of hemoglobin of ark shell Scapharca broughtonii attracted more and more attention due to its ownership of erythrocyte comparing with many other shellfish. In this study, the hemoglobin cDNA of S. broughtonii was cloned by EST and RACE methods (named as SbHb). Sequence analysis revealed that the cDNA was 946 bp in length, including an open reading frame (ORF) of 459 bp which encoded a polypeptide of 152 amino acid residues, and a 5'-untranslated region (UTR) of 313 bp, a 3'-UTR of 174 bp. Sequence and homology analysis showed that the SbHb shared similarity with that of other related species. The mRNA expression profiles of SbHb in tested tissues analyzed by quantitative real-time PCR (qRT-PCR) revealed that the mRNA of SbHb could be all detected in foot, gill, mantle, adductor muscle, haemocytes and hepatopancreas, and the highest level was found in the haemocytes, which is 163.2 times higher than that in adductor muscle. Vibrio anguillarum stimulation and hypoxia treatment both had significant impact on the expression of SbHb, which showed the same trends as increasing first to the highest at 16 h after treatment and then followed by declining. Recombinant protein of SbHb (rSbHb) was successfully obtained by prokaryotic expression, and further function analysis indicated obviously that the rSbHb had very strong phenoloxidase-like activity (PO-like activity) and it could remarkably inhibit growth of gram-negative bacteria V. anguillarum. All the data suggested that the SbHb plays a significant role in the process of antibacterial and anoxia tolerance reaction in S. broughtonii, providing the evidence that SbHb is a key immune factor.

Impact of zinc oxide nanoparticles and ocean acidification on antioxidant responses of Mytilus coruscus

Increased production of engineered nanoparticles (NPs) has raised extensive concerns about the potential toxic effects on marine organisms. Extensive evidences documented the impact of ocean acidification (OA) on the physiology and fitness of bivalves. In the present study, we investigated the biochemical responses of the mussel Mytilus coruscus exposed to both nano-ZnO and low pH relevant for ocean acidification conditions for 14 d followed by a 7-d recovery period. Most biochemical indexes (superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), acid phosphatase (ACP) and alkaline phosphatase (ALP)) measured in gills and hemocytes were increased when the mussels were subject to low pH or high concentration of nano-ZnO, suggesting oxidative stress responses. No significant interactions between the two stressors were observed for most measured parameters. After a 1 week recovery period, low pH and nano-ZnO had less marked impact for SOD, GPx, ACP and ALP in hemocytes as compared to the end of the 14 d exposure. However, no recovery was observed in gills. Overall, our results suggest that both low pH and nano-ZnO induce an anti-oxidative response in Mytilus coruscus with gills being more sensitive than hemocytes.