Number and types of hemocytes in Sunetta scripta and Villorita cyprinoides var. cochinensis (Bivalvia), and leukocytosis subsequent to bacterial challenge

Bivalve Haemocyte Subpopulations: A Review

Bivalve molluscs stand out for their ecological success and their key role in the functioning of aquatic ecosystems, while also constituting a very valuable commercial resource. Both ecological success and production of bivalves depend on their effective immune defence function, in which haemocytes play a central role acting as both the undertaker of the cellular immunity and supplier of the humoral immunity. Bivalves have different types of haemocytes, which perform different functions. Hence, identification of cell subpopulations and their functional characterisation in immune responses is essential to fully understand the immune system in bivalves. Nowadays, there is not a unified nomenclature that applies to all bivalves. Characterisation of bivalve haemocyte subpopulations is often combined with 1) other multiple parameter assays to determine differences between cell types in immune-related physiological activities, such as phagocytosis, oxidative stress and apoptosis; and 2) immune response to different stressors such as pathogens, temperature, acidification and pollution. This review summarises the major and most recent findings in classification and functional characterisation of the main haemocyte types of bivalve molluscs.

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.

Hematological and Biochemical Assessment of Two Species of Freshwater Mussels, Quadrula quadrula and Amblema plicata, Following Translocation

Nondestructive, sublethal, and sensitive health monitoring tools are needed to assess the health of freshwater mussels (family Unionidae). Recent developments to standardize hemocyte characterization have assisted in the hematologic assessment of wild and captive freshwater mussels. In this study, preliminary baseline hematological reference ranges were established for wild mapleleaf mussels Quadrula quadrula (n = 14) and threeridge mussels Amblema plicata (n = 20) collected from the Muskingum River in Devola, Ohio. Mussels were collected from the wild, and hemolymph was sampled from each mussel in the field upon capture (baseline sample). They were then transported live to a propagation facility. Subsequent hemolymph samples were collected at 2 and 4 weeks and quarterly thereafter for 11 months following translocation. Hemocyte counts, hemocyte morphology, and hemolymph chemistry (Na⁺ , Cl^- , Mg²⁺ , P^3- , K⁺ , Ca²⁺ , glucose, alanine aminotransferase, aspartate aminotransferase, and alkaline phosphatase]) were measured from each sample on each sampling occasion. Hemocyte counts were consistently greater in Q. quadrula than in A. plicata following transfer to captivity. Baseline hemocyte morphology and hemolymph chemistry varied between species. This study provides a foundation of reference ranges for hemocyte characterization for Q. quadrula, and A. plicata and a preliminary understanding of how hemocyte character might be expected to change when wild mussels are translocated into captivity, and thus be a useful technique for monitoring the health of freshwater mussels.

Does a blue crab putative insulin-like peptide binding protein (ILPBP) play a role in a virus infection?

Insulin-like peptides (ILPs) have regulatory roles in reproduction, development and metabolism in invertebrates. The mode of ILP actions has not been well studied in invertebrates in regard to the role of binding partners, i.e., ILP binding protein (ILPBP). In this study, the full-length cDNA of Callinectes sapidus ILPBP (Cas-ILPBP, 960 bp) has been isolated using RACE cloning, having short 5' and 3' UTRs of 30 and 162 bp, respectively. The predicted precursor of Cas-ILPBP (255 aa) contains, in order a signal peptide (23 aa), an insulin-like growth factor (IGF) binding (IB) domain (79 aa), a kazal-type serine protease inhibitor (KI) domain (36 aa) and an immunoglobulin (Ig) domain (101 aa). Phylogenetic analysis shows that Cas-ILPBP is grouped with the ILPBPs of other crustacean species, and it shares the closest relationship with the ILPBP from another crab species, Scylla paramamosain. Transcripts of Cas-ILPBP are found in all examined tissues, with the highest levels in the nervous tissues (eyestalk ganglia, brain and thoracic ganglia complex) and followed by midgut, the pericardial organ, abdominal muscle and the heart. As Cas-ILPBP contains a putative Ig domain, it is hypothesized that this protein may be involved in immunity, particularly in the adult females infected with a reo-like virus (CsRV1). The expression levels of Cas-ILPBP are examined in several tissues (hemocytes, midgut, eyestalk ganglia) from the animals carrying varying levels of CsRV1 at 17 and 23 °C water temperatures. Cas-ILPBP levels in the midgut are most significantly affected by high levels of CsRV1 infection. Reduction in Cas-ILPBP levels in the midguts is noted from the animals infected with high levels of CsRV1 that show reduced or stop feeding activity, indicating that it may play an important role in midgut functions such as digestion and nutrient absorption.

Behavioural and immunological responses to an immune challenge in Octopus vulgaris

Behavioural and immunological changes consequent to stress and infection are largely unexplored in cephalopods, despite the wide employment of species such as Octopus vulgaris in studies that require their manipulation and prolonged maintenance in captivity. Here we explore O. vulgaris behavioural and immunological (i.e. haemocyte number and serum lysozyme activity) responses to an in vivo immune challenge with Escherichia coli lipopolysaccharides (LPS). Behavioural changes of immune-treated and sham-injected animals were observed in both sight-allowed and isolated conditions, i.e. visually interacting or not with a conspecific. Immune stimulation primarily caused a significant increase in the number of circulating haemocytes 4h after the treatment, while serum lysozyme activity showed a less clear response. However, the effect of LPS on the circulating haemocytes begins to vanish 24h after injection. Our observations indicate a significant change in behaviour consequent to LPS administration, with treated octopuses exhibiting a decrease of general activity pattern when kept in the isolated condition. A similar decrease was not observed in the sight-allowed condition, where we noticed a specific significant reduction only in the time spent to visually interact with the conspecific. Overall, significant, but lower, behavioural and immunological effects of injection were detected also in sham-injected animals, suggesting a non-trivial susceptibility to manipulation and haemolymph sampling. Our results gain importance in light of changes of the regulations for the use of cephalopods in scientific procedures that call for the prompt development of guidelines, covering many aspects of cephalopod provision, maintenance and welfare.

Phagocytosis and respiratory burst activity of haemocytes from the ivory snail, Babylonia areolata

Haemocytes from the ivory snail, Babylonia areolata phagocytized Saccharomyces cerevisiae and Vibrio parahaemolyticus after 30 min. Haemocytes phagocytized V. parahaemolyticus at a greater rate than they phagocytized S. cerevisiae. The phagocytic rate (PP) of V. parahaemolyticus by granulocytes to was a little higher than that of S. cerevisiae. The phagocytic index (PI) of V. parahaemolyticus by granulocytes was significantly higher than that of S. cerevisiae. The same was true of hyalinocytes. The PP of granulocytes was significantly higher than that of hyalinocytes for each pathogen. No difference in PI was observed in granulocytes and hyalinocytes. Two defense mechanisms of B. areolata were quantified using flow cytometry. Haemocyte phagocytosis was quantified using fluorescent microbeads and respiratory burst activity was measured using H2O2 increases detected by 2', 7'-dichlorofluorescein diacetate. Both phagocytosis and respiratory burst activity of the haemocytes increased over time. After 90 min the phagocytic rate no longer increased. In the case of respiratory burst, the greatest increase in fluorescence occurred between 30 and 120 min, no further increase was seen after 120 min. These results showed unequivocally that a native (unstimulated) haemocyte oxidative burst was active in B. areolata. The aim of this study was to further the knowledge of immunology in gastropods.

Differential localization and bacteriostasis of Vibrio campbellii among tissues of the Eastern oyster, Crassostrea virginica

In bivalve mollusks the roles of individual tissues in antimicrobial defense remain unclear. In this study, Crassostrea virginica were injected in the adductor muscle with 10(5) live Vibrio campbellii. Major tissues were dissected at 10, 30, 60 or 120 min postinjection (PI); in each tissue undegraded (intact) bacteria were quantified by real-time PCR and culturable bacteria were enumerated by selective plating. At 10 min PI, accumulation of bacteria varied among tissues from approximately 2.4 x 10(3) (labial palps, digestive gland) to 24.2 x 10(3) (gonads) intact Vibrio g(-1). Neither distribution nor accumulation of intact bacteria changed with time except in the hemolymph. In most tissues, more than 80% of intact bacteria were culturable at 10 min PI and culturability decreased with time. In contrast, only 19% of intact bacteria in gonadal tissue could be cultured at 10 min PI, pointing to a major role for the gonadal tissues in antibacterial defense of molluscs.

Immune responses of mussel hemocyte subpopulations are differentially regulated by enzymes of the PI 3-K, PKC, and ERK kinase families

Various hemocyte cell types have been described in invertebrates, but for most species a functional characterization of different hemocyte cell types is still lacking. In order to characterize some immunological properties of mussel (Mytilus galloprovincialis) hemocytes, cells were separated by flow cytometry and their capacity for phagocytosis, production of reactive oxygen species (ROS), and production of nitric oxide (NO), was examined. Phosphatidylinositol 3-kinase (PI 3-K), protein kinase C (PKC), and extracellular signal-regulated kinase (ERK) inhibitors were also used to biochemically characterize these cell responses. Four morphologically distinct subpopulations, designated R1-R4, were detected. R1, R2, and R3 cells presented different levels of phagocytosis towards zymosan, latex beads, and two bacteria species. Similarly, R1 to R3, but not R4, cells produced ROS, while all subpopulations produced NO, in response to zymosan. Internalization of all phagocytic targets was blocked by PI 3-K inhibition. In addition, internalization of latex particles, but not of bacteria, was partially blocked by PKC or ERK inhibition. Interestingly, phagocytosis of zymosan was impaired by PKC, or ERK inhibitors, only in R2 cells. Zymosan-induced ROS production was blocked by PI 3-K inhibition, but not by PKC, or ERK inhibition. In addition, zymosan-stimulated NO production was affected by PI 3-K inhibition in R1 and R2, but not in R3 or R4 cells. NO production in all cell types was unaffected by PKC inhibition, but ERK inhibition blocked it in R2 cells. These data reveal the existence of profound functional and biochemical differences in mussel hemocytes and indicate that M. galloprovincialis hemocytes are specialized cells fulfilling specific tasks in the context of host defense.

Sydney rock oyster (Saccostrea glomerata) hemocytes: Morphology and function

In this study, three major hemocyte types were identified in the Sydney rock oyster. They were characterized primarily by light and electron microscopy based on the presence or absence of granules and nucleus to cytoplasm ratios. Hemoblast-like cells were the smallest cell type 4.0+/-0.4microm and comprised 15+/-3% of the hemocyte population. They had large nuclei and scanty basic cytoplasm. This cell type also had some endoplasmic reticuli and mitochondria. The second major type were hyalinocytes. Hyalinocytes represented 46+/-6% of all hemocytes. They were large cells (7.1+/-1.0microm) that had low nucleus:cytoplasm ratios and agranular basic or acidic cytoplasm. Hyalinocytes had the ability to phagocytose yeast cells and formed the core of hemocyte aggregates associated with agglutination. Four discrete sub-populations of hyalinocytes were identified. The third major cell type were the granulocytes, comprising 38+/-1% of the hemocyte population. These cells were large (9.3+/-0.3microm) and were characterized by cytoplasm containing many acidic or basic granules. Granulocytes were more phagocytic than hyalinocytes and they formed the inner layer of hemocytes during the encapsulation of fungal hyphae. Five discrete sub-populations of granulocytes were identified based on the types of granules in their cytoplasm. Flow cytometry showed that the hemocytes of rock oysters could be divided into between two and four major cell types based on their light scattering properties. The most common of the cell types identified by flow cytometry corresponded to hyalinocytes and granulocytes. Cytochemical assays showed that most enzymes associated with immunological activity were localized in granulocytes. Their granules contained acid phosphatase, peroxidase, phenoloxidase, superoxide and melanin. Hyalinocytes were positive only for acid phosphatase. All of these observations suggest that Sydney rock oysters have a broad variety of functionally specialized hemocytes, many of which are involved in host defense.

Categorization of hemocytes of three gastropod species Trachea vittata (Muller), Pila globosa (Swainson) and Indoplanorbis exustus (Dehays)

Light microscopic observations were made on the hemocytes of three gastropod species namely Trachea vittata, Indoplanorbis exustus and Pila globosa. It revealed two basic types of hemocytes. They are agranulocytes and granulocytes. Agranulocytes are hyalinocytes which are round, unspread hemocytes and have a large nucleo-cytoplasmic ratio. Granulocytes are spreading hemocytes, forming numerous pseudopodia. For the purpose of differential counting, we present a categorization of the granulocytes into three sub-categories based on cell dimensions, nucleo-cytoplasmic ratio, distribution of granules in the cytoplasm and position of the nucleus. The smaller granulocytes are younger cells, and are termed Granulocytes I (Progranulocytes). The larger ones are fully developed cells that have been differentiated into Granulocyte II (basophilic) and Granulocyte III (eosinophilic).

Effects of the pathogenic Vibrio tapetis on defence factors of susceptible and non-susceptible bivalve species: II. Cellular and biochemical changes following in vivo challenge

This work compared the effect of challenge with Vibrio tapetis, the etiologic agent of brown ring disease (BRD) in clams, and other bacterial strains on defence-related factors in four bivalve species: Ruditapes philippinarum (highly susceptible to BRD), R. decussatus (slightly susceptible to BRD), Mercenaria mercenaria and Crassostrea virginica (both non-susceptible to BRD). Results show that bacterial challenge modulated defence-related factors, namely total and differential haemocyte counts, percentage of viable haemocytes, and lysozyme activity, both in haemolymph and extrapallial fluid. Injection with bacteria induced a response that was dependent upon the bacterial and bivalve species investigated, and upon the site of inoculation: external (pallial cavity), pseudo-internal (extrapallial space), or internal compartment (adductor muscle). The most conspicuous changes were systematically measured in R. philippinarum injected with V. tapetis, indicating a bacterial pathogenicity particular to the host in which it causes a specific disease syndrome. Alterations of defence-related factors were maximal in haemolymph of clams injected with V. tapetis in the muscle, and in the extrapallial fluid when the bacteria were injected into the pallial or the extrapallial cavity. Resistance to the development of the BRD symptom was not related to the extent of the haemocyte reaction measured following in vivo challenge.

Incidence of hemocytes and parasites in coastal populations of blue mussels (Mytilus edulis)--testing correlations with area, season, and distance to industrial plants

Blue mussel hemocytes (cells with immunoresponse activities) are suggested as indicators of anthropogenic contamination. We compared hemocyte numbers, granulocytoma (aggregated hemocytes), and parasites among populations of mussels from different areas of Skagerrak (a north and a south), seasons (summer and autumn), and impact levels (close or far from industrial activities). Seasonal hemocyte numbers were larger in the north compared to the south. Northern unimpacted populations had higher hemocyte numbers than populations close to industries, while no differences were found in the south. More uneven tissue distributions were found in populations far from industries in the north area and in populations close to industries in the south area. Parasites were more common in northern mussels than in southern, but no relationship to impact level was found. Mussels with granulocytoma, however, were found in all populations from the impacted sites while in none of the other populations suggesting granulocytoma as a possible indicator of industrial impact.

Haemocytes of the clam Tapes philippinarum (Adams & Reeve, 1850): morphofunctional characterisation

Tapes philippinarum is a bivalve mollusc of the Pacific Ocean, successfully imported for human consumption into the northern Adriatic Sea (Europe). For better knowledge of its considerable adaptive ability in comparison with similar autochthonous species, a morpho-functional characterisation of its haemocytes was carried out with the establishment of short-term cell cultures (60 min at 25 degrees C). Various methods of cytochemical staining identified four cell types in the haemolymph: granulocytes (48.05% +/- 1.43), hyalinocytes (32.18% +/- 0.99), haemoblasts (18.97% +/- 0.63) and serous cells (0.8% +/- 0.19). The granulocytes, possessing cytoplasmic granules with differing dye affinity, included basophils, neutrophils and acidophils. Such granules stained vitally with Neutral Red, and correspond to lysosomes. Hydrolytic and oxidative enzymes were mainly detectable after stimulation in the presence of yeast cells. Both granulocytes and hyalinocytes were positive for alkaline phosphatase, non-specific esterase, peroxidase, and cytochrome C oxidase, whereas only granulocytes were positive for beta-glucuronidase, acid esterase, and arylsulphatase. Both cell types were competent phagocytes towards yeast and plasma had an opsonising effect. Moreover, the respiratory burst accompanied phagocytosis with superoxide anion production, recognisable through cytoplasmic deposits of formazan after treatment with nitro blue tetrazolium. Haemoblasts were small undifferentiated cells which, due to their morphology and positivity to the anti-CD34 antibody, show the typical features of stem cells. Serous cells, probably arising from Keber's gland and belonging to another differentiation pathway, contained non-sulphate acid mucopolysaccharides and play an important role in early defence mechanisms, taking part in the formation of clots.

Alterations in hemolymph and extrapallial fluid parameters in the Manila clam, Ruditapes philippinarum, challenged with the pathogen Vibrio tapetis

In a recent study, we demonstrated the presence of defense factors, competent hemocytes and high enzymatic activities (peptidases, hydrolases, lytic, etc.), in the extrapallial fluid, located between the mantle and the shell, of the Manila clam, Ruditapes philippinarum. In Europe, this species is affected by brown ring disease, an epizootic disease caused by the bacterium Vibrio tapetis. The present work focused on the effect of the development of the disease on cellular and humoral defense parameters in the hemolymph and the extrapallial fluid of experimentally infected clams. Results indicate significant changes in total and dead hemocyte counts, as well as modifications in lysozyme activity and protein content, in the hemolymph and extrapallial fluid of challenged animals. Hemocyte counts and lysozyme activity increased significantly in the hemolymph, but particularly in the extrapallial fluid, where the highest values were observed. A healing (recalcification) process was observed 7 weeks following challenge, suggesting defense system efficiency at neutralizing the pathogen. These results are discussed with emphasis on the role of extrapallial fluids in the defense process against invading microorganisms.

Seasonal variation and effects of age, food availability, size, gonadal development, and parasitism on the hemogram of mytilus galloprovincialis

The study of hemogram characteristics in a population of cultured mussels through a period of 18 months revealed that the concentration of circulating Mytilus galloprovincialis hemocytes varied, following a seasonal pattern, being positively correlated with water temperature. Most mussels had between 1 and 5 x 10(6) hemocytes/ml of hemolymph. Differential counts showed that M. galloprovincialis hemolymph contained more granulocytes than hyalinocytes. The hemolymph cell concentration was not associated with size or weight of mussels. Circulating hemocyte concentration did not decrease in mussels with reabsorbing gonad, despite the hemocytic accumulation inside and around gonad follicles. No significant difference in hemolymph cell concentration was found between two groups of mussels of different ages. Nevertheless, older mussels had higher mean values of circulating granulocytes although these differences were only significant in the winter sample. Infestation by the protistan Marteilia refringens caused a significant increase of hemocytes in the hemolymph. In addition, the occurrence of some parasites induced significant changes in the proportion of circulating hemocyte types. Results suggested that a hemocytic response was detected in hemolymph when the foreign organisms are pathogenic and evoke a heavy inflammatory reaction. Mussels with large or numerous granulocytomas had a higher number of circulating granulocytes and hyalinocytes than those without granulocytomas. Copyright 1998 Academic Press.

Lysozyme and antiprotease activity in the lesser octopus Eledone cirrhosa (Lam.) (Cephalopoda)

Antiprotease and lysozyme activities were detected in various tissue samples including the haemocytes and haemolymph of Eledone cirrhosa. Injection of live Vibrio anguillarum caused an increase in lysozyme activity in the branchial heart over 48 hours and a decrease in the lysozyme activity of haemocytes over 24 hours. Haemocytes from control PBS injected animals demonstrated increased lysozyme levels 4 hours after injection whereas it decreased after the injection of live bacteria in PBS. The lysozyme activity of the haemolymph was not affected by these procedures. Bacteria injections had no effect on the antiprotease activity of the organ samples but increased the antiprotease activity of the haemocytes compared to controls in the 4 h samples. Haemolymph antiprotease activity decreased at a greater rate following bacteria injection than in control PBS injected animals. Haemocyte numbers/ml increased for both the control and bacteria injected animals with a greater increase demonstrated for the bacteria injected animals in the 4 h sample. Concomittant with the increase in the numbers of circulating haemocytes live V. anguillarum were cleared from the circulation of E. cirrhosa in less than 4 hours.