In vitro studies of hematopoiesis in the silkworm: cell proliferation in and hemocyte discharge from the hematopoietic organ

Integrin β2 and β3: Two plasmatocyte markers deepen our understanding of the development of plasmatocytes in the silkworm Bombyx mori

Insect hemocytes play important biological roles at developmental stages, metamorphosis, and innate immunity. As one of the most abundant cell types, plasmatocytes can participate in various innate immune responses, especially in encapsulation and node formation. Here, 2 molecular markers of plasmatocytes, consisting of integrin β2 and β3, were identified and used to understand the development of plasmatocytes. Plasmatocytes are widely distributed in the hematopoietic system, including circulating hemolymph and hematopoietic organs (HPOs). HPOs constantly release plasmatocytes with high proliferative activity in vitro; removal of HPOs leads to a dramatic reduction in the circulating plasmatocytes, and the remaining plasmatocytes gradually lose their ability to proliferate in vivo. Our results demonstrated that the release of plasmatocytes from HPOs is regulated by insulin-mediated signals and their downstream pathways, including PI3K/Akt and MAPK/Erk signals. The insulin/PI3K/Akt signaling pathway can significantly irritate the hematopoiesis, and its inhibitor LY294002 could inhibit the hemocytes discharged from HPOs. While the insulin/MAPK/Erk signaling pathway plays a negative regulatory role, inhibiting its activity with U0126 can markedly promote the discharge of plasmatocytes from HPOs. Our results indicate that the circulating plasmatocytes are mainly generated and discharged by HPOs. This process is co-regulated by the PI3K/Akt and MAPK/Erk signals in an antagonistic manner to adjust the dynamic balance of the hemocytes. These findings can enhance our understanding of insect hematopoiesis.

Hemocyte Clusters Defined by scRNA-Seq in Bombyx mori: In Silico Analysis of Predicted Marker Genes and Implications for Potential Functional Roles

Within the hemolymph, insect hemocytes constitute a heterogeneous population of macrophage-like cells that play important roles in innate immunity, homeostasis and development. Classification of hemocytes in different subtypes by size, morphology and biochemical or immunological markers has been difficult and only in Drosophila extensive genetic analysis allowed the construction of a coherent picture of hemocyte differentiation from pro-hemocytes to granulocytes, crystal cells and plasmatocytes. However, the advent of high-throughput single cell technologies, such as single cell RNA sequencing (scRNA-seq), is bound to have a high impact on the study of hemocytes subtypes and their phenotypes in other insects for which a sophisticated genetic toolbox is not available. Instead of averaging gene expression across all cells as occurs in bulk-RNA-seq, scRNA-seq allows high-throughput and specific visualization of the differentiation status of individual cells. With scRNA-seq, interesting cell types can be identified in heterogeneous populations and direct analysis of rare cell types is possible. Next to its ability to profile the transcriptomes of individual cells in tissue samples, scRNA-seq can be used to propose marker genes that are characteristic of different hemocyte subtypes and predict their functions. In this perspective, the identities of the different marker genes that were identified by scRNA-seq analysis to define 13 distinct cell clusters of hemocytes in larvae of the silkworm, Bombyx mori, are discussed in detail. The analysis confirms the broad division of hemocytes in granulocytes, plasmatocytes, oenocytoids and perhaps spherulocytes but also reveals considerable complexity at the molecular level and highly specialized functions. In addition, predicted hemocyte marker genes in Bombyx generally show only limited convergence with the genes that are considered characteristic for hemocyte subtypes in Drosophila.

A review on the biological effects of nanomaterials on silkworm (Bombyx mori)

The production of high-quality silkworm silk is of importance in sericulture in addition to the production of biomass, silk proteins, and animal feed. The distinctive properties of nanomaterials have the potential to improve the development of various sectors including medicine, cosmetics, and agriculture. The application of nanotechnology in sericulture not only improves the survival rate of the silkworm, promotes the growth and development of silkworm, but also improves the quality of silk fiber. Despite the positive contributions of nanomaterials, there are a few concerns regarding the safety of their application to the environment, in humans, and in experimental models. Some studies have shown that some nanomaterials exhibit toxicity to tissues and organs of the silkworm, while other nanomaterials exhibit therapeutic properties. This review summarizes some reports on the biological effects of nanomaterials on silkworm and how the application of nanomaterials improves sericulture.

The larval haematopoietic organs of Manduca sexta (Insecta, Lepidoptera): An insight into plasmatocyte development and larval haematopoiesis

Haematopoietic organs (HOs) in Lepidoptera are widely recognised as the source for at least two haemocyte types. With new specific markers for oenocytoids and spherule cells and a method to identify prohaemocytes, the haemocytes formed in and released by the HOs of Manduca sexta are characterised. Differentiation of HO-cells to haemocytes other than plasmatocytes and prohaemocytes neither occurs in the organ itself nor in cells released in vitro by the HOs. Differential labelling patterns evidence the existence of plasmatocyte subpopulations and prohaemocytes, which might represent a gradual differentiation of haemocytes within the organs. Prohaemocytes can be identified by PNA-labelling of the cell membrane. These prohaemocytes are found in circulation and in the HOs and are released by the organs. Circulating prohaemocytes possess characteristics for granular cells, plasmatocytes or oenocytoids while HO derived prohaemocytes share characteristics only with plasmatocytes. Ablation of the HOs diminishes the plasmatocyte and prohaemocyte number, indicating a true larval haematopoietic function.

Hemocyte Changes During Immune Melanization in Bombyx Mori Infected with Escherichia coli

Hemolymph melanization is a conserved immune response in insects and other arthropods. However, the physiological process of the hemolymph system in the melanization response is hardly studied. Here, alterations of hemocytes in immune melanization were observed by Escherichia coli infection in Bombyx mori. Results first showed that there were cells aggregating into clusters. However, it vanished, and only part of clustered hemocytes were melanized during the period of intense immunity. The hemocyte numbers immediately decreased following an immune challenge, slowly increased to a peak, then reduced and finally returned to normalization. Granulocytes participated in cells aggregation at the early and later immune stage, while plasmatocytes were responsible for hemocytes agglomerate and melanization for the longest time, and more oenocytoids appeared at the peak stage of melanization. Moreover, hemocytes played a crucial role in resisting invasion of pathogens by agglomerate and melanization, and the circulatory system maintained higher hemocyte numbers and stronger antibacterial activity in fifth than fourth instar larvae after infection. In vitro immune melanization was most likely preferentially implemented in an independent process. These were the main characteristics reflecting the physiological process of hemolymph immune melanization, which provided an important foundation for further study of the complete mechanisms in the immunity of silkworm.

Boric acid-induced immunotoxicity and genotoxicity in model insect Galleria mellonella L. (Lepidoptera: Pyralidae)

Boric acid (BA) is widely used in various industrial process and can be accessed to nontarget organisms. This study aimed to investigate the insecticidal effects of BA and its toxic activities with respect to immunologic and genotoxic effects using Galleria mellonella larvae as a model. BA concentrations (78.125-10,000 ppm) were administrated to the larvae using the feeding method. Concentration-dependent mortality was observed in all larval groups. Probit analysis revealed LC₃₀ , LC₅₀ , and LC₇₀ values to be 112.4, 320.1, and 911.4 ppm, respectively. These concentrations were used in all bioassays. Drastic reductions in total hemocyte counts along with changes in differential hemocyte counts were observed following BA treatment. Cell viability assays showed dose-dependent reductions in viable cells and an increase in the necrotic and apoptotic ratios after BA treatment. However, mitotic indices of larval hemocytes did not change at all BA concentrations. The cytotoxic effect of BA led to a significant reduction in cellular immune responses such as encapsulation, melanization, and nodulation activities of treated larvae. While BA increased micronucleus ratios at the highest concentration, comet parameters indicating DNA damage increased in G. mellonella larval hemocytes at all concentrations. These report that BA suppresses the immune system of G. mellonella and also poses risks of genotoxicity at high concentrations.

Different toxicity of cadmium telluride, silicon, and carbon nanomaterials against hemocytes in silkworm, Bombyx mori

Exposure to CdTe QDs, SiNPs, or C–NCDs exerted different toxic effects on silkworm hemocytes via the induction of different PCD processes.

Impact of fluorescent silicon nanoparticles on circulating hemolymph and hematopoiesis in an invertebrate model organism

Silicon nanoparticles (SiNPs) have attractive potential applications in biological and medical fields, and yet their impact on animals is still controversial, and there have been no reports of their effects on hematopoiesis. In this study, the effects of SiNPs on hemocytes and hematopoiesis were investigated by administering SiNPs via a vascular injection into an invertebrate model, the silkworm. Our results show that the ability of SiNPs to enter different types of circulating hemocytes and their impact on those hemocytes differed significantly. Rapid accumulation of SiNPs was observed in granulocytes, oenocytoids, and spherulocytes, which have immune functions in the circulating hemolymph, whereas SiNPs did not easily enter prohemocytes, which can differentiate into granulocytes, oenocytoids, and spherulocytes and replenish them. The SiNPs that entered the hemocytes initiated autophagy and apoptosis via the lysosomal/mitochondrial pathway. High-dose SiNPs weakly stimulated lysosomal activity in hematopoietic organs, but did not lead to a significant increase in reactive oxygen species or severe autophagy or apoptosis in the organ tissues. We suggest that the damage caused by high-dose SiNPs to hematopoiesis is self-healing, because few SiNPs entered the hematopoietic stem cells in the circulating hemolymph, so the damage to the hematopoietic tissues was limited.

The relative abundance of hemocyte types in a polyphagous moth larva depends on diet

Hemocytes are crucial cells of the insect immune system because of their involvement in multiple immune responses including coagulation, phagocytosis and encapsulation. There are various types of hemocytes, each having a particular role in immunity, such that variation in their relative abundance affects the outcome of the immune response. This study aims to characterize these various types of hemocytes in larvae of the grapevine pest insect Eupoecilia ambiguella, and to assess variation in their concentration as a function of larval diet and immune challenge. Four types of hemocytes were found in the hemolymph of 5th instar larvae: granulocytes, oenocytoids, plasmatocytes and spherulocytes. We found that the total concentration of hemocytes and the concentration of each hemocyte type varied among diets and in response to the immune challenge. Irrespective of the diet, the concentration of granulocytes increased following a bacterial immune challenge, while the concentration of plasmatocytes and spherulocytes differentially varied between larval diets. The concentration of oenocytoids did not vary among diets before the immune challenge but varied between larval diets in response to the challenge. These results suggest that the resistance of insect larvae to different natural enemies critically depends on the effect of larval diet on the larvae's investment into the different types of hemocytes.

Insect immunology and hematopoiesis

Insects combat infection by mounting powerful immune responses that are mediated by hemocytes, the fat body, the midgut, the salivary glands and other tissues. Foreign organisms that have entered the body of an insect are recognized by the immune system when pathogen-associated molecular patterns bind host-derived pattern recognition receptors. This, in turn, activates immune signaling pathways that amplify the immune response, induce the production of factors with antimicrobial activity, and activate effector pathways. Among the immune signaling pathways are the Toll, Imd, Jak/Stat, JNK, and insulin pathways. Activation of these and other pathways leads to pathogen killing via phagocytosis, melanization, cellular encapsulation, nodulation, lysis, RNAi-mediated virus destruction, autophagy and apoptosis. This review details these and other aspects of immunity in insects, and discusses how the immune and circulatory systems have co-adapted to combat infection, how hemocyte replication and differentiation takes place (hematopoiesis), how an infection prepares an insect for a subsequent infection (immune priming), how environmental factors such as temperature and the age of the insect impact the immune response, and how social immunity protects entire groups. Finally, this review highlights some underexplored areas in the field of insect immunobiology.

A continuous cell line, SYSU-OfHe-C, from hemocytes of Ostrinia furnacalis possesses immune ability depending on the presence of larval plasma

A continuous cell line, SYSU-OfHe-C, from larval hemocytes of corn borer, Ostrinia furnacalis was established. With increasing passages, the cells grew increasingly faster, and approximately 45% of the cells were in division at passage 55. The culture was mainly composed of two types of cells, granulocytes and plasmatocytes, which showed different division and proliferation behaviors, but possessed similar phagocytic ability. Its spreading ability was significantly weaker than that of hemocytes from naïve larva; however, it could be promoted by larval plasma. Furthermore, its encapsulation ability was also promoted by larval plasma to form multilayer capsules on Sephadex A-25 beads. Finally, the expression of several immune-related genes was verified after provocation by microbes or Sephadex beads. These results indicated that the cell line possessed immune ability depending on the presence of plasma of naïve larvae and are beneficial to studies of insect cellular systems.

JH modulates a cellular immunity of Tribolium castaneum in a Met-independent manner

Juvenile hormone (JH) regulates diverse physiological processes in insects during entire developmental stages. Especially, the identification of Methoprene-tolerant (Met), a JH nuclear receptor, allows us to better understand molecular actions of JH to control gene expressions related with metamorphosis. However, several physiological processes including cellular immune response and some molecular actions of JH have been suspected to be mediated via its non-genomic actions. To prove its non-genomic action, JH nuclear signals were suppressed by RNA interference (RNAi) of Met or its downstream gene, Krüppel homolog 1 (Kr-h1), in the red flour beetle, Tribolium castaneum. These RNAi-treated larvae failed to undergo a normal development and suffered precocious metamorphosis. Hemocytes of T. castaneum exhibited their spreading behavior on extracellular matrix and nodule formation in response to bacterial challenge. When the larvae were treated with either RNAi of Met or Kr-h1, the hemocytes of the treated larvae were responsive to JH without any significant difference with those of control larvae. These results suggest that the response of hemocytes to JH is not mediated by its nuclear signal. On the other hand, the JH modulation of hemocyte behaviors of T. castaneum was significantly influenced by membrane and cytosolic protein activities, in which ethoxyzolamide (a specific inhibitor of carbonic anhydrase), calphostin C (a specific inhibitor of protein kinase C) or ouabain (a specific inhibitor of Na(+)-K(+) ATPase) significantly suppressed the responsiveness of hemocytes to JH.

Establishment of transgenic silkworms expressing GAL4 specifically in the haemocyte oenocytoid cells

Insect haemocytes play significant roles in innate immunity. The silkworm, a lepidopteran species, is often selected as the model for studies into the functions of haemocytes in immunity; however, our understanding of the role of haemocytes remains limited because the lack of haemocyte promoters for transgene expression makes genetic manipulations difficult. In the present study, we aimed to establish transgenic silkworm strains expressing GAL4 in their haemocytes. First, we identified three genes with strong expression in haemocytes, namely, lp44, Haemocyte Protease 1 (HP1) and hemocytin. Transgenic silkworms expressing GAL4 under the control of the putative promoters of these genes were then established and expression was examined. Although GAL4 expression was not detected in haemocytes of HP1-GAL4 or hemocytin-GAL4 strains, lp44-GAL4 exhibited a high level of GAL4 expression, particularly in oenocytoids. GAL4 expression was also detected in the midgut but in no other tissues, indicating that GAL4 expression in this strain is mostly oenocytoid-specific. Thus, we have identified a promoter that enables oenocytoid expression of genes of interest. Additionally, the lp44-GAL4 strain could also be used for other types of research, such as the functional analysis of genes in oenocytoids, which would facilitate advances in our understanding of insect immunity.

Developmental regulation and antifungal activity of a growth-blocking peptide from the beet armyworm Spodoptera exigua

Insect cytokine growth-blocking peptides (GBPs) are involved in growth regulation and the innate immune response. However, the microbial binding and antimicrobial activities of GBPs remain unclear. Here, we investigate the developmental role and antifungal activity of a GBP from the beet armyworm Spodoptera exigua (SeGBP). Sequence analysis predicted that mature SeGBP consists of 24 amino acid residues, including 2 cysteine residues. During S. exigua development, SeGBP is constitutively expressed in the fat body during the larval and adult stages but not in pupae. SeGBP expression is up-regulated by 20-hydroxyecdysone and down-regulated by juvenile hormone analog. Recombinant SeGBP purified from baculovirus-infected insect cells retards the growth of S. exigua larvae. Additionally, SeGBP expression is acutely induced in the fat body after injection with Escherichia coli, Bacillus thuringiensis, or Beauveria bassiana. Recombinant SeGBP can bind to B. bassiana but not to E. coli or B. thuringiensis. Consistent with these findings, SeGBP shows antifungal activity against B. bassiana. Therefore, these results provide insight into the role of SeGBP during the innate immune response following microbial infection, and furthermore, they suggest a novel function for SeGBP as a direct antifungal agent against entomopathogenic fungi, such as B. bassiana.

Insulin-like and IGF-like peptides in the silkmoth Bombyx mori: discovery, structure, secretion, and function

A quarter of a century has passed since bombyxin, the first insulin-like peptide identified in insects, was discovered in the silkmoth Bombyx mori. During these years, bombyxin has been studied for its structure, genes, distribution, hemolymph titers, secretion control, as well as physiological functions, thereby stimulating a wide range of studies on insulin-like peptides in other insects. Moreover, recent studies have identified a new class of insulin family peptides, IGF-like peptides, in B. mori and Drosophila melanogaster, broadening the base of the research area of the insulin-related peptides in insects. In this review, we describe the achievements of the studies on insulin-like and IGF-like peptides mainly in B. mori with short histories of their discovery. Our emphasis is that bombyxins, secreted by the brain neurosecretory cells, regulate nutrient-dependent growth and metabolism, whereas the IGF-like peptides, secreted by the fat body and other peripheral tissues, regulate stage-dependent growth of tissues.

Characterization of hemocytes proliferation in larval silkworm, Bombyx mori

Hemocytes play multiple important roles during insect growth and development. Five types of hemocytes have been identified in the silkworm, Bombyx mori: prohemocyte, plasmatocyte, granulocyte, spherulocyte, and oenocytoid. We used the S-phase marker bromodeoxyuridine (BrdU) antibody along with the mitosis marker phosphohistone H3 (PHH3) antibody to monitor proliferation of hemocytes in vivo. The results indicate that silkworm hematopoiesis not only occurs in the circulatory system but also in hematopoietic organs (HPOs). During the 5th instar, the hemocyte proliferation in the circulatory system reaches a peak at the pre-wandering stage. Following infection by Escherichia coli, circulating hemocytes increase their cell divisions as demanded by the cellular immune response. All hemocytes, except spherulocytes, have the capacity to multiply in vivo. The BrdU label-retaining assay shows that a small portion of cells from the circulatory system and the HPOs are continuously labelled up to 9days and 4days respectively. A small number of long-term label retaining cells (LRCs) quiescently locate in circulatory system. All results indicate that there are a few quiescent stem cells or some progenitors in the larval circulatory system and HPO that produce new hemocytes and continuously release them into the circulating system.

Hematopoiesis at the onset of metamorphosis: terminal differentiation and dissociation of the Drosophila lymph gland

The Drosophila melanogaster hematopoietic organ, called lymph gland, proliferates and differentiates throughout the larval period. The lymph gland of the late larva is comprised of a large primary lobe and several smaller secondary lobes. Differentiation into two types of hemocytes, plasmatocytes and crystal cells, is confined to the outer layer (cortical zone) of the primary lobe; the center of the primary lobe (medullary zone), as well as the secondary lobes, contain only proliferating prohemocytes. A small cluster of cells located at the posterior tip of the primary lobe serves as a signaling center (PSC) that inhibits precocious differentiation of the medullary zone. The larval lymph gland is stabilized by layers of extracellular matrix (basement membranes) that surround individual hemocytes, groups of hemocytes, as well as the lymph gland as a whole. In this paper, we investigated the events shaping the lymph gland in the early pupa. The lymph gland dissociates and hemocytes disperse during the first 12 h after puparium formation (APF), leaving behind empty husks of basement membrane. Prior to lymph gland dissociation, cells of the medullary zone differentiate, expressing the early differentiation marker Peroxidasin (Pxn), as well as, in part, the late differentiation marker P1. Cells of the PSC spread throughout the pupal lymph gland prior to their dispersal. Cells of the secondary lobes undergo a rapid phase of proliferation that lasts until 8 h APF, followed by expression of Pxn and dispersal. These hemocytes do not express P1, indicating that they disperse prior to full maturation.

Phenol oxidases from Rhodnius prolixus: temporal and tissue expression pattern and regulation by ecdysone

Rhodnius prolixus 5th instar nymphs have significant PO enzymatic activity in the anterior midgut, fat body and hemolymph. The tissue with the major amount of PO activity is the anterior midgut while those with higher specific activities are the fat body and hemolymph. In this work the temporal pattern of PO enzymatic activity in different tissues was investigated. In fat body, PO peaks occur at 7, 12 and 16 days after a blood meal. In hemolymph, PO diminishes until day 7, and then recovers by day 14. In the anterior midgut tissue, PO peaks on day 9 and just before ecdysis; a similar pattern was observed in the anterior midgut contents. Some of these activities are dependent on the release of ecdysone, as feeding blood meal containing azadirachtin suppresses them and ecdysone treatment counteracts this effect. These results suggest that during the development of the 5th instar, the insect has natural regulating cycles of basal PO expression and activation, which could be related to the occurrence of natural infections. The differences in temporal patterns of activity and the effects of azadirachtin and ecdysone in each organ suggest that, at least in R. prolixus, different tissues are expressing different PO genes.

Two hemocyte lineages exist in silkworm larval hematopoietic organ

Background

Insects have multiple hemocyte morphotypes with different functions as do vertebrates, however, their hematopoietic lineages are largely unexplored with the exception of Drosophila melanogaster.

Methodology/Principal Findings

To study the hematopoietic lineage of the silkworm, Bombyx mori, we investigated in vivo and in vitro differentiation of hemocyte precursors in the hematopoietic organ (HPO) into the four mature hemocyte subsets, namely, plasmatocytes, granulocytes, oenocytoids, and spherulocytes. Five days after implantation of enzymatically-dispersed HPO cells from a GFP-expressing transgenic line into the hemocoel of normal larvae, differentiation into plasmatocytes, granulocytes and oenocytoids, but not spherulocytes, was observed. When the HPO cells were cultured in vitro, plasmatocytes appeared rapidly, and oenocytoids possessing prophenol oxidase activity appeared several days later. HPO cells were also able to differentiate into a small number of granulocytes, but not into spherulocytes. When functionally mature plasmatocytes were cultured in vitro, oenocytoids were observed 10 days later. These results suggest that the hemocyte precursors in HPO first differentiate into plasmatocytes, which further change into oenocytoids.

Conclusions/Significance

From these results, we propose that B. mori hemocytes can be divided into two major lineages, a granulocyte lineage and a plasmatocyte-oenocytoid lineage. The origins of the spherulocytes could not be determined in this study. We construct a model for the hematopoietic lineages at the larval stage of B. mori.

Characterization of cell clusters in larval hemolymph of the cabbage armyworm Mamestra brassicae and their role in maintenance of hemocyte populations

Maintenance of hemocyte populations is critical for both development and immune responses. In insects, the maintenance of hemocyte populations is regulated by mitotic division of circulating hemocytes and by discharge from hematopoietic organs. We found cell clusters in the hemolymph of Mamestra brassicae larvae that are composed of small, spherical cells. Microscopic observations revealed that the cells in these clusters are similar to immature or precursor cells present in hematopoietic organs. The results of bromodeoxyuridine (BrdU) incorporation experiments demonstrate that these cells are mitotically active. Furthermore, these cells maintain their immature state and proliferate until late in the last larval instar. The results of in vitro experiments showed that most of the cells changed their morphology to one consistent with plasmatocytes or granulocytes, and that the change was promoted by addition of larval hemolymph to the culture medium, in particular when hemolymph was collected at a prepupal stage. Taken together, our results suggested that cells in clusters may be an additional source of hemocytes during larval development.

Purification and characterization of silkworm hemocytes by flow cytometry

Hemocyte functions are well-investigated in the silkworm, Bombyx mori, however, detailed analysis of each hemocyte subset has been hampered by the lack of appropriate separation method. Here we use an array of flow cytometric analyses to characterize silkworm hemocytes with various molecular probes, such as propidium iodide, green fluorescence protein, monoclonal antibodies, and fluorescent lectins. Of these, separation using propidium iodide was the simplest and provided most reliable results for the isolation of the hemocyte subsets. cDNAs were then synthesized from these sorted populations and subset-specific gene expression was examined by RT-PCR. Granulocytes, plasmatocytes, and oenocytoids expressed different classes of immune genes, suggesting that they have multiple roles in silkworm immunity. In contrast, a contribution of spherulocytes to immunity was not documented in that they failed to express most of the genes. The functions of spherulocytes are thus likely to be distinct from those of the other three hemocyte subsets.

Antagonistic effect of juvenile hormone on hemocyte-spreading behavior of Spodoptera exigua in response to an insect cytokine and its putative membrane action

Juvenile hormone (JH) acts on membrane of follicle cells to induce ovarian patency for vitellogenesis, though it regulates various other physiological processes via putative intracellular receptors. This study suggests another JH membrane action by analyzing in vitro hemocyte behavior. In response to nonself, both granular cells and plasmatocytes of Spodoptera exigua can exhibit cell shape changes through spreading behaviors. Plasmatocytes were separated from total S. exigua hemocytes by Percoll gradient and exposed in vitro to an insect cytokine, plasmatocyte-spreading peptide (PSP), identified from Pseudoplusia includens. In response, the purified plasmatocytes spread in a dose-dependent manner from picomolar to micromolar concentrations. Interestingly, the PSP responses of plasmatocytes in S. exigua varied among different larval ages during fifth instar ( approximately 5 days at 25 degrees C) in a sensitivity order of late (5 days old)<early (1 day old)<mid (3 days old). Considering the overall endocrine changes that occur during the final instar of holometabolous insects, we suspected that JH and ecdysteroid hormones were responsive for this developmental modulation of plasmatocyte sensitivity to PSP. We tested this hypothesis by exposing plasmatocytes to hormone agonists in vitro. Pyriproxyfen, a JH agonist, significantly inhibited plasmatocyte sensitivity to PSP. JH I and II had significant effects on antagonizing plasmatocyte sensitivity to PSP, but either JH III or farnesoic acid did not. In contrast, 20-hydroxyecdysone (20E) enhanced the plasmatocyte sensitivity to PSP. Ethoxyzolamide, a putative JH competitor to membrane receptor, inhibited JH action on the plasmatocyte sensitivity to PSP. Though staurosporine (a protein kinase inhibitor) alone did not influence plasmatocyte sensitivity to PSP, it antagonized the JH inhibitory effect on the plasmatocytes. Ouabain, a specific Na+ -K+ ATPase inhibitor, also masked the JH action on the plasmatocytes. These results suggest that the JH acts on the membrane of the plasmatocytes and prevents plasmatocyte spreading by reducing cell volume through activating Na+ -K+ pump via protein kinase C signal pathway.

Transient expression of an EP1-like gene encoded in Cotesia plutellae bracovirus suppresses the hemocyte population in the diamondback moth, Plutella xylostella

A genome project has been launched and aims to sequence total genome of Cotesia plutellae bracovirus (CpBV). This on-going research has identified seven EP1-like (ELP) genes in the CpBV genome. A group of ELP genes has been speculated as an immunosuppressant encoded in Cotesia-associated bracoviruses. This study analyzed gene expression of these seven CpBV-ELPs in the parasitized diamondback moth, Plutella xylostella. Of these, six CpBV-ELPs were expressed in P. xylostella parasitized by C. plutellae. However, their expression levels varied in different tissues and parasitization stages. Especially, CpBV-ELP1 showed a persistent and ubiquitous expression pattern in both reverse transcriptase-polymerase chain reaction (RT-PCR) and immunofluorescence assays. When nonparasitized P. xylostella was transfected with a recombinant CpBV-ELP1 in a eukaryotic expression vector, CpBV-ELP1 was expressed for at least 3 days and the proteins were detectable in the cytoplasm of hemocytes. The transfected larvae showed significant reduction in total hemocyte numbers, compared with larvae injected with the cloning vector alone. Co-transfection with double-stranded RNA could knock down the expression of CpBV-ELP1 and prevented the reduction of the hemocyte population. This study demonstrates that CpBV-ELP1 plays a physiological role in suppressing host immune response presumably by its hemolytic activity during C. plutellae parasitization.

Nuclear translocation of immulectin-3 stimulates hemocyte proliferation

Immulectin-3 (IML-3) is a C-type lectin from the tobacco hornworm Manduca sexta that contains a motif (NWGV) similar to the BH1 motif (NWGR) of the mammalian galectin-3. IML-3 is synthesized in fat body and secreted into hemolymph, but can be translocated into hemocytes. In this study, we showed that IML-3 was predominantly localized to the nucleus of hemocytes and some metaphase, anaphase and telophase hemocytes from M. sexta larvae injected with bacterial lipopolysaccharide (LPS). IML-3 was detected in the membrane and soluble extracts of hemocytes, suggesting that it may be translocated into hemocytes via receptor-mediated endocytosis. To investigate the role of IML-3 translocation to the nucleus, we expressed recombinant wild-type IML-3 and a deletion mutant DeltaIML-3 that has the NWGV motif deleted in Drosophila S2 cells. We found that recombinant wild-type IML-3, but not DeltaIML-3, was localized to the nucleus of some S2 cells and also detected in the nuclear extract. Expression of recombinant wild-type IML-3, but not DeltaIML-3 or GFP, increased the number of proliferating S2 cells. Our results suggest that nuclear translocation of IML-3 may stimulate hemocyte proliferation.

Effects of high temperature on the hemocyte cell cycle in silkworm larvae

To understand the inhibitory effects of high temperature on insect growth at the cellular level, we investigated the influence of high temperature on the proliferation and division of larval hemocytes in the silkworm, Bombyx mori. Although the total number of hemocytes in the larval body increased enormously over time at 26 degrees C, no increase was observed at 38 degrees C. The number of mitotic hemocytes in circulation increased between days 1 and 2 of the fourth larval stage at 26 degrees C, whereas fewer hemocytes were observed at 38 degrees C. Laser scanning cytometry revealed that the DNA content of hemocytes collected from the fourth-stadium larvae was predominantly 2C, 4C, and 8C, and the proportion of each type of hemocyte changed dynamically with development during the fourth instar. Specifically, the proportion of hemocytes with a higher DNA content increased gradually during the feeding phase then decreased during the molting phase at 26 degrees C; in contrast, no decrease was observed at 38 degrees C. The heat-induced accumulation of 8C hemocytes was mainly detected in granulocytes and plasmatocytes. These data suggest that high temperatures induce a G(2) arrest in larval hemocytes.

Parasitism by Cotesia plutellae alters the hemocyte population and immunological function of the diamondback moth, Plutella xylostella

Cotesia plutellae, a solitary endoparasitoid wasp, parasitizes the diamondback moth, Plutella xylostella, and induces host immunosuppression and lethality in the late larval stage. This study focused on changes of cellular immunity in the parasitized P. xylostella in terms of hemocyte composition and cellular functions. In third and fourth instar larvae of nonparasitized P. xylostella, granular cells represented the main hemocyte type (60-70%) and plasmatocytes were also present at around 15% among the total hemocytes. Following parasitization by C. plutellae, the relative proportions of these two major hemocytes changed very little, but the total hemocyte counts exhibited a significant reduction. Functionally, the granular cells played a significant role in phagocytosis based on a fluorescence assay using fluorecein isothiocyanate-labeled bacteria. The phagocytic activity of the granular cells occurred as early as 5 min after incubation with the bacteria, and increased during the first 40 min of incubation. The parasitism by C. plutellae significantly inhibited phagocytosis of the granular cells. Plasmatocytes also exhibited minor phagocytic activity. Moreover, plasmatocyte phagocytosis was not inhibited by parasitism. On the other hand, hemocyte-spreading behavior in response to pathogen infection was significant only for plasmatocytes, which exhibited a characteristic spindle shape upon infection. A significant spreading of the plasmatocytes was found as early as 5 min after pathogen incubation and their ratio increased during the first 40 min. An insect cytokine, plasmatocyte-spreading peptide 1 (PSP1) from Pseudoplusia includens, was highly active in inducing plasmatocyte-spreading behavior of P. xylostella in a dose-dependent manner. P. xylostella parasitized by C. plutella was significantly inhibited in plasmatocyte-spreading in response to an active dose of PSP1. An in vivo encapsulation assay showed that the parasitized P. xylostella could not effectively form the hemocyte capsules around injected agarose beads. This research demonstrates that the parasitism of C. plutellae adversely affects the total hemocyte populations in number and function, which would contribute to host immunosuppression.

Insulin signaling is involved in hematopoietic regulation in an insect hematopoietic organ

Only a few extracellular hematopoietic factors have been identified in insects. We previously developed an in vitro culture system for the larval hematopoietic organ (HPO) of the silkworm Bombyx mori, and found that cell proliferation is linked to hemocyte discharge from the HPO. In this study, we tested hematopoietic activity of bombyxin, a peptide in the insulin family. When silkworm HPO was cultured with synthetic bombyxin-II, the number of discharged hemocytes increased in a dose-dependent manner, indicating that bombyxin promoted cell proliferation in the HPO. However, a neutralization experiment using anti-bombyxin-II antibody revealed that bombyxin is not the primary effector in larval plasma. Similarly, bovine insulin showed hematopoietic activity. Addition of molting hormone, 20-hydroxyecdysone, circumstantially enhanced the hematopoietic activity of bombyxin and insulin. Bombyxin and insulin induced phosphorylation of different sets of proteins in the HPO, suggesting that their signaling pathways are different.

Plasmatocyte sensitivity to plasmatocyte spreading peptide (PSP) fluctuates with the larval molting cycle

Plasmatocyte spreading peptide (PSP) is a cytokine from the moth Pseudoplusia includens that activates a class of hemocytes called plasmatocytes to bind and spread on foreign surfaces. Previous structure-function studies on PSP used plasmatocytes collected from P. includens larvae that were in the late stages of the last (fifth) instar. Here, we report that plasmatocyte sensitivity to PSP varied significantly during the fourth and fifth instar. PSP weakly activated plasmatocytes early in the instar when hemolymph juvenile hormone (JH) titers were relatively high and ecdysteroid titers were low, but strongly activated plasmatocytes late in the instar after JH titers declined and ecdysteroid titers rose. In contrast, plasmatocytes did not vary in their response to plasma, which contains other factors besides PSP that affect plasmatocyte function. In vitro assays indicated that 20-hydroxyecdysone (20E) dose-dependently synergized PSP activity, whereas the JH analog methoprene antagonized PSP activity. Methoprene had no effect on adhesion and spreading of granular cells, but plasmatocytes from larvae topically treated with methoprene exhibited a reduction in sensitivity to PSP. Collectively, these results indicate that plasmatocyte sensitivity to PSP fluctuates in relation to the molting cycle, and that PSP activity is affected by juvenoids and ecdysone.