Hemocytes and hemocytopoiesis in Silkworms

In-depth transcriptomic analysis of Anopheles gambiae hemocytes uncovers novel genes and the oenocytoid developmental lineage

BACKGROUND

Hemocytes are immune cells that patrol the mosquito hemocoel and mediate critical cellular defense responses against pathogens. However, despite their importance, a comprehensive transcriptome of these cells was lacking because they constitute a very small fraction of the total cells in the insect, limiting the study of hemocyte differentiation and immune function.

RESULTS

In this study, an in-depth hemocyte transcriptome was built by extensive bulk RNA sequencing and assembly of hemocyte RNAs from adult A. gambiae female mosquitoes, based on approximately 2.4 billion short Illumina and about 9.4 million long PacBio high-quality reads that mapped to the A. gambiae PEST genome (P4.14 version). A total of 34,939 transcripts were annotated including 4,020 transcripts from novel genes and 20,008 novel isoforms that result from extensive differential splicing of transcripts from previously annotated genes. Most hemocyte transcripts identified (89.8%) are protein-coding while 10.2% are non-coding RNAs. The number of transcripts identified in the novel hemocyte transcriptome is twice the number in the current annotation of the A. gambiae genome (P4.14 version). Furthermore, we were able to refine the analysis of a previously published single-cell transcriptome (scRNAseq) data set by using the novel hemocyte transcriptome as a reference to re-define the hemocyte clusters and determine the path of hemocyte differentiation. Unsupervised pseudo-temporal ordering using the Tools for Single Cell Analysis software uncovered a novel putative prohemocyte precursor cell type that gives rise to prohemocytes. Pseudo-temporal ordering with the Monocle 3 software, which analyses changes in gene expression during dynamic biological processes, determined that oenocytoids derive from prohemocytes, a cell population that also gives rise to the granulocyte lineage.

CONCLUSION

A high number of mRNA splice variants are expressed in hemocytes, and they may account for the plasticity required to mount efficient responses to many different pathogens. This study highlights the importance of a comprehensive set of reference transcripts to perform robust single-cell transcriptomic data analysis of cells present in low abundance. The detailed annotation of the hemocyte transcriptome will uncover new facets of hemocyte development and function in adult dipterans and is a valuable community resource for future studies on mosquito cellular immunity.

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.

Effects of the copper oxide nanoparticles (CuO NPs) on Galleria mellonella hemocytes

In this study, 38 nm-sized and flake-like-shaped CuO NPs (10, 50, 100, 150 μg/10 µl/larva) were force-fed to fourth instar (100 ± 20 mg) Galleria mellonella (Lepidoptera: Pyralidae) larvae under the laboratory conditions. The effects of CuO NPs on total hemocyte counts (THCs) and the frequency of viable, mitotic, apoptotic, necrotic, and micronucleated hemocyte indices were detected with the double-staining protocol by hematoxylin and eosin (H&E) stains. The total hemocyte counts (THCs) did not change significantly in G. mellonella larvae at all concentrations for 24 h and 72 h post-force-feeding treatment. The ratio of viable hemocytes decreased at 50, 100, 150 μg/10 µl concentrations in 24 h and 72 h when compared with untreated larvae. The increases in the percentage of mitotic and micronucleated hemocytes were statistically significant at 150 μg/10 µl in 24 h. The results showed that high concentrations (>10 μg/10 µl) of CuO NPs increased the percentage of apoptotic hemocytes in 24 h. 100 and 150 μg/10 µl of CuO NPs caused a significant increase in the percentage of necrotic hemocytes in 24 h. The decrease in the percentage of mitotic hemocytes at 10, 100 and 150 μg/10 µl in 72 h was statistically significant. Apoptotic hemocytes increased and were found to be higher at 100 and 150 μg/10 µl of CuO NPs in 72 h in comparison with the untreated larvae. Finally, we observed an increase in the percentage of necrotic hemocytes at 150 μg/10 µl in 72 h.

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.

Harman and norharman, metabolites of the entomopathogenic fungus Conidiobolus coronatus (Entomophthorales), affect the serotonin levels and phagocytic activity of hemocytes, insect immunocompetent cells, in Galleria mellonella (Lepidoptera)

Background

Although the β-carboline alkaloids harman and norharman are considered as plant metabolites, they can also be secreted by fungi such as the entomopathogen Conidiobolus coronatus. Norharman and harman are also known to be reversible competitive monamine oxidase inhibitors, which increase serotonin concentrations in tissues. In addition, these alkaloids are able to bind to serotonin receptors, an important immune regulatory molecule in both vertebrates and invertebrates. In insects, serotonin modulates hemocyte phagocytosis, nodule formation and the populations of hemocyte classes. The present study examines whether harman and norharman may influence the phagocytic activity of insect hemocytes by regulating serotonin levels.

Results

Significantly greater serotonin levels and hemocyte phagocytic activity were observed after 24 h of exposure to food contaminated with harman and norharman. Similar responses were noticed 1 h after topical application or addition to in vitro hemocyte cultures. Observations and measurements performed 24 h later revealed decreased responses, suggesting decomposition and/or exertion of alkaloids and/or serotonin. Harman and norharman influenced the activity of Galleria mellonella plasmatocytes and the granulocyte cytoskeleton. Disturbances in hemocyte network formation, abnormal cell shape, naked nuclei, cell aggregates, fragments of disintegrated cells, interrupted cell membrane continuity and actin condensation in cells were observed.

Conclusion

Our findings may have a considerable impact on research concerning insect physiology, parasitology, immunology and biocontrol of pests. They confirm for the first time that harman and norharman (metabolites of the entomopathogenic fungus C. coronatus) elevate serotonin levels in G. mellonella hemocytes, thus potentially stimulating their phagocytic activity. Our studies shed light on the mechanisms underlying the interaction between innate insect immune responses and entomopathogen metabolites.

Electronic supplementary material

The online version of this article (10.1186/s13578-019-0291-1) contains supplementary material, which is available to authorized users.

Transgenic characterization of two silkworm tissue-specific promoters in the haemocyte plasmatocyte cells

Haemocytes play crucial roles in insect metabolism, metamorphosis, and innate immunity. As a model of lepidopteran insects, the silkworm is a useful model to study the functions of both haematopoiesis and haemocytes. Tissue-specific promoters are excellent tools for genetic manipulation and are widely used in fundamental biological research. Herein, two haemocyte-specific genes, Integrin β2 and Integrin β3, were confirmed. Promoter activities of Integrin β2 and Integrin β3 were evaluated by genetic manipulation. Quantitative real-time PCR and western blotting suggested that both promoters can drive enhanced green fluorescent protein (EGFP) specifically expressed in haemocytes. Further evidence clearly demonstrated that the transgenic silkworm exhibited a high level of EGFP signal in plasmatocytes, but not in other detected haemocyte types. Moreover, EGFP fluorescence signals were observed in the haematopoietic organ of both transgenic strains. Thus, two promoters that enable plasmatocytes to express genes of interest were confirmed in our study. It is expected that the results of this study will facilitate advances in our understanding of insect haematopoiesis and immunity in the silkworm, Bombyx mori.

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.

Metabolomics reveals insect metabolic responses associated with fungal infection

The interactions between insects and pathogenic fungi are complex. We employed metabolomic techniques to profile insect metabolic dynamics upon infection by the pathogenic fungus Beauveria bassiana. Silkworm larvae were infected with fungal spores and microscopic observations demonstrated that the exhaustion of insect hemocytes was coupled with fungal propagation in the insect body cavity. Metabolomic analyses revealed that fungal infection could significantly alter insect energy and nutrient metabolisms as well as the immune defense responses, including the upregulation of carbohydrates, amino acids, fatty acids, and lipids, but the downregulation of eicosanoids and amines. The insect antifeedant effect of the fungal infection was evident with the reduced level of maclurin (a component of mulberry leaves) in infected insects but elevated accumulations in control insects. Insecticidal and cytotoxic mycotoxins like oosporein and beauveriolides were also detected in insects at the later stages of infection. Taken together, the metabolomics data suggest that insect immune responses are energy-cost reactions and the strategies of nutrient deprivation, inhibition of host immune responses, and toxin production would be jointly employed by the fungus to kill insects. The data obtained in this study will facilitate future functional studies of genes and pathways associated with insect-fungus interactions.

A novel granulocyte-specific α integrin is essential for cellular immunity in the silkworm Bombyx mori

Haemocytes play crucial roles in immune responses and survival in insects. Specific cell markers have proven effective in clarifying the function and haematopoiesis of haemocytes. The silkworm Bombyx mori is a good model for studying insect haemocytes; however, little is known about haemocyte-specific markers or their functions in silkworm. In this study, we identified the α subunit of integrin, BmintegrinαPS3, as being specifically and highly expressed in silkworm haemocytes. Immunofluorescence analysis validated the specificity of BmintegrinαPS3 in larval granulocytes. Further analyses indicated that haemocytes dispersed from haematopoietic organs (HPOs) into the circulating haemolymph could differentiate into granulocytes. In addition, the processes of encapsulation and phagocytosis were controlled by larval granulocytes. Our work demonstrated that BmintegrinαPS3 could be used as a specific marker for granulocytes and could be applied to future molecular cell biology studies.

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.

Participation of haemocytes in fat body degradation via cathepsin L expression

Insect haemocytes are known to participate in innate immunity via the phagocytosis of pathogens. However, the function of haemocytes in tissue remodelling is less understood. We report here that haemocytes play roles in fat body degradation by expressing a cysteine proteinase cathepsin L in the lepidopteran Helicoverpa armigera. During metamorphosis, haemocytes undergo morphological changes by increasing their cell size and transforming their granulocytes into macrogranulocytes. The population of haemocytes also changes with increased number of granulocytes and decreased plasmatocytes. The expression level of cathepsin L in haemocytes, mainly in granulocytes and plasmatocytes, increases. The steroid hormone 20-hydroxyecdysone is able to promote the transformation of granulocytes into macrogranulocytes, and up-regulate the expression level of cathepsin L. The knock-down of the cathepsin L gene by RNA interference in haemocytes in vitro results in deficient granulocytes transforming into macrogranulocytes. Haemocytes are able to enter the decomposed fat body during metamorphosis. The over-expression of the proteinase domain C1A of cathepsin L results in cell apoptosis. Haemocytes, especially macrogranulocytes, undergo apoptosis and cathepsin L is released into haemolymph and the fat body during metamorphosis for fat body decomposition and degradation. These results suggest that cathepsin L is related to the transformation of granulocytes to macrogranulocytes to enter the fat body, and induce haemocyte apoptosis for further tissue degradation.

Hemocytes and humoral factors in silkworm blood are cooperatively involved in sheep erythrocyte aggregation

Sheep red blood cells (SRBCs) rapidly aggregated when injected into the blood (hemolymph) of living silkworms. SRBCs also rapidly aggregated when incubated with hemolymph in vitro. SRBCs did not aggregate when incubated with single hemolymph components, hemocytes and cell-free plasma separated by centrifugation, whereas incubation with the mixture of components induced SRBC aggregation, suggesting that both hemocytes and plasma are required for the reaction. Treatment of hemocytes with sodium azide inhibited SRBC aggregation. On the other hand, SRBCs pre-incubated with hemocytes aggregated in the plasma, even in the presence of sodium azide. SRBC aggregation was not observed when the SRBCs were physically separated from the hemocytes by a polycarbonate filter. These findings suggest that SRBCs are directly attacked by hemocytes and become sensitive to humoral factors that cause SRBC aggregation.

Regulatory mechanism of silkworm hemocyte adhesion to organs

Circulating hemocytes in the body fluid of the silkworm are increased during the larval-larval molting period. We investigated hemocyte adhesion to organs mediating the selectin-selectin ligands during the feeding period and the larval-larval molting period using the lectin staining method, sugar chain digestion test with glycoside hydrolases, and the hemocyte adhesion inhibition test using monosaccharides. The results of these tests suggested that the selectin ligand involved in hemocyte adhesion was the Sialyl Lewis x-type, and the structure was changed from the feeding period to the larval-larval molting period. Beta-galactosidase appears to be an enzyme that eliminates N-acetylgalactosamine and sialylated N-acetylgalactosamine from the terminal of Sialyl Lewis x. Beta-galactosidase activation in skin basement membranes, muscle, fat bodies, midguts, and hemocytes increased markedly during the larval-larval molting period, and at that time, hemocytes were detached from organs. Adding 20-hydroxyecdysone or its analog, tebufenozide to cultured fat bodies increased β-galactosidase activity in these tissues. Therefore, 20-hydroxyecdysone may induce a structural change in Sialyl Lewis x type sugar chains on the cell surface of silkworm's organs by increasing the β-galactosidase activity to detach hemocytes from organs and increase the number of circulating hemocytes during the larval-larval molting period.

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.

Appearance of apoptotic cells and granular cells in the silkworm midgut lumen during larval-pupal ecdysis

To study midgut degradation and programmed cell death, we performed methyl green-pyronin staining and Giemsa staining of the midgut of silkworms during metamorphosis. Midgut epithelial cells underwent pyknosis and cytoplasmic shrinkage on the second day of spinning. In the prepupal stage, all midgut epithelial cells desquamated into the midgut lumen, rapidly forming apoptotic bodies. The number of apoptotic bodies in the midgut decreased rapidly from the prepupal stage to the third day of the pupal stage. DNA fragmentation at the time of apoptotic body formation was confirmed by the comet assay. In the midgut lumen from the prepupal stage to the first through third days of the pupal stage in which apoptotic bodies were observed, granular cells were present. Their morphology was similar to that in the body fluid and, during the pupal stage, intracellular granules increased in size and number with time, giving the appearance of a foamy cell. In this stage, numerous granular cells were observed under the basement membrane of the midgut, and phagocytosed apoptotic bodies were seen within granular cells in the midgut lumen. Granular cells may be actively involved in the clearance of apoptotic bodies from the midgut during larval-pupal ecdysis.

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.

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.

A review of insect stem cell types

Insect stem cells have been described from both embryonic and adult tissues from a diversity of insect species, although much of the focus in insect stem cell research has been on Drosophila. Insects are a vast and diverse group and it is surprising that a critical aspect of their development like stem cells has not received more attention. In this review we discuss the current state of knowledge of insect stem cell types. We examine what stem cell types have been identified from insects, and briefly discuss what is known about their regulation.

Insect haemocytes: what type of cell is that?

Classification of insect larvae circulating haemocytes is the subject of controversy, and the terminology used to designate each cellular type is often different from one species to another. However, a survey of the literature on insect haemocytes suggests that there are resemblances for most of the cell types and functions, in different insect species. In this review paper, we compare the structure and functions of circulating haemocytes in those insect species that are, by far, the most often used species for insect physiology studies, i.e. lepidopteran species and Drosophila. We show that there is high degree of homology of haemocyte types and suggest possible synonymies in terminology among species from these taxa.

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.

Hemocyte differentiation in the hematopoietic organs of the silkworm, Bombyx mori: prohemocytes have the function of phagocytosis

Hemocytes isolated from the larval hematopoietic organs of the silkworm were classified following staining with acridine orange and propidium iodide. Among the hemocytes isolated from the hematopoietic organs of whole fifth larval and wandering stages, most were prohemocytes (60%-70%) and oenocytoids (30%-40%). Granulocytes comprised only about 0.5%-1% at the wandering stage and were even rarer at other stages; no spherulocytes or plasmatocytes were found. Therefore, hemocyte differentiation inside larval hematopoietic organs is not as extensive as previously thought. Following 10-30 min in vitro culture of hemocytes isolated from larval hematopoietic organs, many young granulocytes and plasmatocytes appeared. Furthermore, during phagocytosis assays, prohemocytes were seen to adopt the morphology of plasmatocytes, containing fragments of phagocytosed cells. Our results underline the similarities between Drosophila and Bombyx hematopoiesis.

Reexamination of phenoloxidase in larval circulating hemocytes of the silkworm, Bombyx mori

We have developed a modified method to detect phenoloxidase activity on hemocytes by using freshly prepared l-DOPA (1 mg/ml in 35% ethanol) to fix and incubate larval hemocytes. This method is more sensitive than the common method, in which hemocytes were fixed in 4% formaldehyde and then incubated with 2 mg/ml l-DOPA in water separately. Phenoloxidase assayed using this modified method can be inhibited by phenyltiourea (phenoloxidase inhibitor). After incubation with l-DOPA solution in ethanol, most prohemocytes, all plasmatocytes and young granulocytes are stained brown due to oxidation of l-DOPA into pigments, indicating that they have phenoloxidase. Oenocytoids are dimly stained because many of their cell inclusions have been released during the treatment. Large propidium-iodide-negative prohemocytes have strong phenoloxidase activity and are easily misunderstood as propidium-iodide-positive oenocytoids if the fluorescent method is not used for identification. Thus, in addition to oenocytoids and plasmatocytes, some prohemocytes and granulocytes in the silkworm also have phenoloxidase.

Mechanism of reduction in the number of the circulating hemocytes in the Pseudaletia separata host parasitized by Cotesia kariyai

Larval endoparasitoids can avoid the immune response of the host by the function of polydnavirus (PDV) and venom. PDV infects hemocytes and affects the hemocyte function of the host. In this paper, we investigated how PDV and venom affect the hemocyte population of the host. Cotesia kariyai, the larval endoparasitoid, lowers the hemocyte population of the noctuid host larvae soon after parasitization. The reduction in the number of circulating hemocytes is caused by the breakdown of the circulating hemocytes and of the hematopoietic organ which generates the circulating hemocytes. The decrease in the number of hemocytes shortly after parasitization is a response to the venom. However, the decrease in hemocyte population on and after 6 h post-parasitization appears to be caused by the PDV. Apoptosis in circulating hemocytes was observed on and after 6 h post-injection of PDV plus venom. It was revealed through cytometry that mitosis of circulating hemocytes was halted within 24 h after the injection of PDV plus venom. Apoptosis in the hematopoietic organ was induced 12 h after the injection of PDV plus venom. Furthermore, the plasma from the hosts injected with PDV plus venom depressed the number of hemocytes released from the hemotopoiteic organs.

Classification of larval circulating hemocytes of the silkworm, Bombyx mori, by acridine orange and propidium iodide staining

Circulating hemocytes of the silkworm can be classified by fluorescence microscopy following staining with acridine orange and propidium iodide. Based on their fluorescence characteristics, three groups of circulating hemocytes can be distinguished. The first group, granulocytes and spherulocytes, is positive for acridine orange and contain bright green fluorescent granules when observed by fluorescence microscopy. In granulocytes, these green granules are heterogeneous and relatively small. In contrast, in spherulocytes, the green granules appear more homogenous and larger. The second group of hemocytes consists of prohemocytes and plasmatocytes. These cells appear faint green following staining with acridine orange and do not contain any green fluorescent granules in the cytoplasm. Prohemocytes are round, and their nuclei are dark and clear within a background of faint green fluorescence. Inside the nucleus there are one or two small bright green fluorescent bodies. Plasmatocytes are irregularly shaped and their nuclei are invisible. Oenocytoids belong to the third group, and their nuclei are positive for propidium iodide. Therefore, all five types of circulating hemocytes of the silkworm, including many peculiar ones that are difficult to identify by light microscopy, can now be easily classified by fluorescence microscopy following staining with acridine orange and propidium iodide. In addition, we show that hemocytes positive for acridine orange and propidium iodide are in fact living cells based on assays for hemocyte composition, phagocytosis, and mitochondrial enzyme activity.

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

The lepidopteran hematopoietic process is poorly understood. We therefore examined the fundamental properties of hematopoiesis in the silkworm Bombyx mori using hematopoietic organ culture. In a medium containing larval plasma taken from the fourth day of the final larval stadium, over 50,000 hemocytes per hematopoietic organ were discharged within 48 h, with the number of cells comprising the hematopoietic organ simultaneously increasing from approximately 20,000 to 40,000. However, in the absence of plasma, cell numbers comprising the hematopoietic organ were unchanged and the number of discharged cells was much less. Hematopoietic organs cultured with plasma showed strong mitotic indices in a BrdU incorporation assay, but did not when cultured without plasma, indicating that plasma contains hematopoietic factor(s). The hematopoietic stimulation ability of larval plasma was observed from the last day of the penultimate larval stadium to the prepupal stage. The response of the hematopoietic organs to larval plasma was highest at the beginning of the final larval stadium and decreased with aging. Most cells discharged from the hematopoietic organ were plasmatocytes and prohemocytes, irrespective of location and developmental stage. Using this in vitro culture method, we tested the effects of 20-hydroxyecdysone (20E) and juvenile hormone-I (JH-I) on B. mori hematopoiesis. 20E showed a weak, but significant, hematopoietic activity, whereas JH-I did not, suggesting that a part of larval hematopoiesis is endocrinally regulated.

Effects of silkworm paralytic peptide on in vitro hematopoiesis and plasmatocyte spreading

Bombyx mori paralytic peptide (BmPP), a multifunctional cytokine-like molecule, is expressed in the hematopoietic organ-wing imaginal disc complex, suggesting that BmPP is involved in both immune response and the hematopoietic process. We studied the effects of BmPP on plasmatocytes and hematopoietic organs of the silkworm. BmPP (1 microM) stimulated spreading of circulating plasmatocytes, but the percentage of spread plasmatocytes was only 20%. Over 10 nM of BmPP, however, elicited prominent spreading in 70% of young plasmatocytes discharged from cultured hematopoietic organs. Cells in hematopoietic organs that were enzymatically dispersed did not spread even after adding 100 nM of BmPP, indicating that plasmatocytes acquired BmPP-sensitivity immediately after discharge. When cultured in a medium containing larval plasma, hematopoietic organs grew markedly and discharged a large number of hemocytes, over 95% of which were morphologically plasmatocytes. The hemocyte discharge was blocked in the medium containing BmPP dose-dependently, although hematopoietic organ growth was not suppressed. These results suggest that BmPP plays important roles both in hematopoietic regulation and in the hemocyte immune reaction of the silkworm.

Hemocyte types and total and differential counts in unparasitized and parasitized Anastrepha obliqua (Diptera, Tephritidae) larvae

The hemocyte types, in addition to total and differential hemocyte counts were studied in parasitized and unparasitized Anastrepha obliqua larvae at the beginning and at the end of the third instar. In both developmental phases, in parasitized and unparasitized larvae, prohemocytes, plasmatocytes, granulocytes, adipohemocytes, spherulocytes and oenocytoids cells were observed. Mitotic figures indicate prohemocytes as stem cells. Prohemocytes, plasmatocytes and granulocytes are the most numerous cells in the hemolymph of A. obliqua. Difference in the total number of hemocytes was observed between unparasitized and parasitized larvae at the end of the third instar, but not at the beginning.

Insect hemocytes and their role in immunity

The innate immune system of insects is divided into humoral and cellular defense responses. Humoral defenses include antimicrobial peptides, the cascades that regulate coagulation and melanization of hemolymph, and the production of reactive intermediates of oxygen and nitrogen. Cellular defenses refer to hemocyte-mediated responses like phagocytosis and encapsulation. In this review, we discuss the cellular immune responses of insects with emphasis on studies in Lepidoptera and Diptera. Insect hemocytes originate from mesodermally derived stem cells that differentiate into specific lineages identified by morphology, function, and molecular markers. In Lepidoptera, most cellular defense responses involve granular cells and plasmatocytes, whereas in Drosophila they involve primarily plasmatocytes and lamellocytes. Insect hemocytes recognize a variety of foreign targets as well as alterations to self. Both humoral and cell surface receptors are involved in these recognition events. Once a target is recognized as foreign, hemocyte-mediated defense responses are regulated by signaling factors and effector molecules that control cell adhesion and cytotoxicity. Several lines of evidence indicate that humoral and cellular defense responses are well-coordinated with one another. Cross-talk between the immune and nervous system may also play a role in regulating inflammation-like responses in insects during infection.

Bombyx mori prohemocyte division and differentiation in individual microcultures

We followed the fate of microcultured Bombyx mori prohemocytes in vitro. Prohemocytes isolated from larval hemolymph (day 1 of 4th instar) were maintained for 4-11 days in serum-free MGM-450 medium and some of them underwent mitotic division. Over 60% of the non-dividing prohemocytes differentiated to plasmatocytes or granulocytes. Some of the granulocytes subsequently transformed to spherulocytes. Of the dividing prohemocytes, 59.2% of the daughter cells differentiated into other types of hemocytes such as plasmatocytes, granulocytes and spherulocytes, and the remainder divided into new prohemocytes. Four of these renewed prohemocytes generated daughter cells composed of plasmatocytes and granulocytes. These results suggest that prohemocytes possess the properties of stem cells, and that plasmatocytes and spherulocytes may be terminally differentiated cells, whereas granulocytes, at least in part, may be a transient form of spherulocyte. Oenocytoids were not produced, suggesting that the lineage of oenocytoids differs from that of other types of hemocytes and that it is determined before release from hemopoietic organs.

Hematopoiesis in larval Pseudoplusia includens and Spodoptera frugiperda

Maintenance of circulating hemocytes in larval Lepidoptera has been attributed to both mitosis of hemocytes already in circulation and the release of hemocytes from hematopoietic organs. In this study, we compared hematopoiesis in the noctuids Pseudoplusia includens and Spodoptera frugiperda. For both species, hemocyte densities per microl of blood increased with instar. Differential hemocyte counts indicated that plasmatocytes were the most abundant hemocyte type during early instars but granular cells were the most abundant hemocyte type in the last instar. Hematopoietic organs were located in the meso- and metathorax of S. Frugiperda and P. Includens. These organs contained large numbers of hemocytes in S. Frugiperda, but contained few hemocytes in P. Includens. The majority of the hemocytes recovered from hematopoietic organs were identified as plasmatocytes. Using hemocyte type-specific markers and bromodeoxyuridine (BrdU) incorporation experiments, we determined that all hemocyte types with the exception of oenocytoids synthesize DNA. BrdU labeling indices for both species also fluctuated with the molting cycle. Ligation experiments suggested that hematopoietic organs are an important source of circulating plasmatocytes in S. Frugiperda but not in P. Includens. Injection of heat killed bacteria into larvae induced higher levels of BrdU labeling than injection of sterile saline, suggesting that infection and wounding induce different levels of hemocyte proliferation. Arch.

Monoclonal antibodies bind distinct classes of hemocytes in the moth Pseudoplusia includens

Insect hemocytes have historically been identified on the basis of morphology, ultrastructure and hypothesized function. Among insects in the order Lepidoptera, five hemocyte classes are usually recognized: granular cells, plasmatocytes, spherule cells, oenocytoids and prohemocytes. We have generated a panel of monoclonal antibodies (mAbs) against hemocytes of the moth Pseudoplusia includens. In this study, hemocyte identification using 16 different mAbs was compared to identification methods using morphological characters. Three main categories of mAb binding activity were identified: (1) mAbs that specifically labeled only one morphological class of hemocytes, (2) mAbs that labeled granular cells and spherule cells, and (3) mAbs that labeled plasmatocytes and oenocytoids. With one exception, none of the antibodies bound to other tissues in P. includens. However, certain mAbs that specifically labeled granular cells and/or spherule cells in separated hemocyte populations also labeled plasmatocytes co-cultured with granular cells or cultured in granular cell conditioned medium. Overall, our results suggest that granular cells are antigenically related to spherule cells, and that plasmatocytes are antigenically related to oenocytoids. The use of mAbs as hemocyte markers are discussed.

Bombyx mori L. ribosomal proteins: resolution, nomenclature, molecular weights and in vivo phosphorylation

Bombyx mori L. ribosomal proteins have been analyzed by four related two-dimensional polyacrylamide gel electrophoretic systems (Madjar et al. 1977a). In the small and large subunits are present 32 and 45 proteins, respectively, whose numbering is proposed. No significant differences in composition or migration could be detected between proteins in membrane-bound ribosomes and free ribosomes. The molecular weights of the proteins vary from 60,000 to less than 10,000. In vivo phosphorylation was investigated by labeling with 32P-orthophosphate. Autoradiograms of four two dimensional gels unambiguously show five labeled ribosomal proteins: S1, S7, L6, L29, and L40.