Hemocytin, the special aggregation factor connecting insect hemolymph immunity, a potential target of insecticidal immunosuppresant

Insects possess an effective innate immunity that enables them to adapt to their intricate living environment and fend off various pathogens (or parasites). This innate immunity comprises both humoral and cellular immunity, which synergistically orchestrate immune responses. Hemocytin, a lectin with a distinctive structure, plays a crucial role in insect hemolymph immunity. Hemocytin is involved in the early immune response, facilitating processes such as coagulation, nodulation, and encapsulation in the hemolymph. It prevents hemolymph overflow and microbial pathogens invasion resulting from epidermal damage, and also aids in the recognition and elimination of invaders. However, the research on hemocytin is still limited. Our previous findings demonstrated that destruxin A effectively inhibits insect hemolymph immunity by interacting with hemocytin, suggesting that hemocytin could be a potential target for insecticides development. Therefore, it is crucial to gain a deeper understanding of hemocytin. This review integrates recent advancements in the study of the structure and function of insect hemocytin and also explores the potential of hemocytin as a target for insecticides. This review aims to enhance our comprehension of insect innate immunity and provide innovative ideas for the development of environmentally friendly pesticides.

Octanoic Acid-An Insecticidal Metabolite of Conidiobolus coronatus (Entomopthorales) That Affects Two Majors Antifungal Protection Systems in Galleria mellonella (Lepidoptera): Cuticular Lipids and Hemocytes

The food flavour additive octanoic acid (C8:0) is also a metabolite of the entomopathogenic fungus Conidiobolus coronatus, which efficiently infects and rapidly kills Galleria mellonella. GC-MS analysis confirmed the presence of C8:0 in insecticidal fraction FR3 extracted from C. coronatus filtrate. Topical administration of C8:0 had a dose-dependent effect on survival rates of larvae but not on pupation or adult eclosion times of the survivors. Topically applied C8:0 was more toxic to adults than larvae (LD100 for adults 18.33 ± 2.49 vs. 33.56 ± 2.57 µg/mg of body mass for larvae). The administration of C8:0 on the cuticle of larvae and adults, in amounts corresponding to their LD50 and LD100 doses, had a considerable impact on the two main defense systems engaged in protecting against pathogens, causing serious changes in the developmental-stage-specific profiles of free fatty acids (FFAs) covering the cuticle of larvae and adults and damaging larval hemocytes. In vitro cultures of G. mellonella hemocytes, either directly treated with C8:0 or taken from C8:0 treated larvae, revealed deformation of hemocytes, disordered networking, late apoptosis, and necrosis, as well as caspase 1-9 activation and elevation of 8-OHdG level. C8:0 was also confirmed to have a cytotoxic effect on the SF-9 insect cell line, as determined by WST-1 and LDH tests.

Galleria mellonella as a Good Model to Study Acinetobacter baumannii Pathogenesis

The invertebrate model, Galleria mellonella, has been widely used to study host-pathogen interactions due to its cheapness, ease of handling, and similar mammalian innate immune system. G. mellonella larvae have been proven to be useful and a reliable model for analyzing pathogenesis mechanisms of multidrug resistant Acinetobacter baumannii, an opportunistic pathogen difficult to kill. This review describes the detailed experimental design of G. mellonella/A. baumannii models, and provides a comprehensive comparison of various virulence factors and therapy strategies using the G. mellonella host. These investigations highlight the importance of this host-pathogen model for in vivo pathogen virulence studies. On the long term, further development of the G. mellonella/A. baumannii model will offer promising insights for clinical treatments of A. baumannii infection.

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.

Determination of the acute toxic effects of zinc oxide nanoparticles (ZnO NPs) in total hemocytes counts of Galleria mellonella (Lepidoptera: Pyralidae) with two different methods

Zinc oxide nanoparticles (ZnO NPs) are now commonly used in many consumer products (detergents, antibacterial products, protective creams). The aim of the study is to determine the ecotoxicological effects of ZnO NPs on the survival and in the total hemocyte counts of Galleria mellonella L. 1758 (Lepidoptera: Pyralidae) with two different methods (automated cell counter and hemocytometer). A toxicity test was performed to determine the lethal concentrations of ZnO NPs on larvae by force feeding method. After 24 h the treatment, LC₅₀ was 6.03 μg/10 µl and LC₉₉ was 12.86 μg/10 µl for force fed larvae according to probit assay. The NP that induced changes in the total hemocytes counts were counted by optical microscopy (larvae exposed to four different doses of ZnO NPs) and by automated cell counter. Analyses of total hemocyte counts of the insect were performed with four doses (0.5, 1, 2.5, 5 μg/10 µl) <LC₅₀ at 24 h upon feeding larvae revealed that a number of hemocytes did not show significant changes in all treatments compared with control in the optical microscopy counting. The similar statistically insignificant counting results were also seen in the automated cell counting results. The percentage of the dead cells (10.01%) in the 5 μg/10 µl group was significantly higher than the control group (3.03%) and showed a statistically significant difference at 24 h in the optical microscopy count with trypan blue viability test.

Utilization of Galleria mellonella larvae to characterize the development of Staphylococcus aureus infection

Staphylococcus aureus is a human opportunistic pathogen that causes a wide range of superficial and systemic infections in susceptible patients. Here we describe how an inoculum of S. aureus activates the cellular and humoral response of Galleria mellonella larvae while growing and disseminating throughout the host, forming nodules and ultimately killing the host. An inoculum of S. aureus (2×10⁶ larva^{- 1} ) decreased larval viability at 24 (80±5.77 %), 48 (55.93±5.55 %) and 72 h (10.23±2.97 %) and was accompanied by significant proliferation and dissemination of S. aureus between 6 and 48 h and the formation of nodules in the host. The hemocyte (immune cell) densities increased between 4 and 24 h and hemocytes isolated from larvae after 24 h exposure to heat-killed S. aureus (2×10⁶ larva^{- 1} ) showed altered killing kinetics as compared to those from control larvae. Alterations in the humoral immune response of larvae 6 and 24 h post-infection were also determined by quantitative shotgun proteomics. The proteome of 6 h-infected larvae was enriched for antimicrobial proteins, proteins of the prophenoloxidase cascade and a range of peptidoglycan recognition proteins. By 24 h there was a significant increase in the abundance of a range of antimicrobial peptides with anti-staphylococcal activity and proteins associated with nodule formation. The results presented here indicate how S. aureus interacts with the larval immune response, induces the expression of a variety of immune-related peptides and also forms nodules which are a hallmark of soft tissue infections during human infection.

The antioxidants dimethylsulfoxide and dimethylthiourea affect the immediate adhesion responses of larval haemocytes from 3 lepidopteran insect species

Antioxidants, dimethylsulfoxide (DMSO) and dimethylthiourea (DMTU), at concentrations not affecting the viability of blood cells (haemocytes) from the larval stage of 3 lepidopteran insects - Galleria mellonella, Lymantria dispar, and Malacosoma disstria - differed in their influence on the innate binding of haemocytes to glass, bacteria to haemocytes, and on humoral responses to alien materials. In vitro DMSO had little effect, whereas DMTU substantially impaired the adhesion of the haemocyte types, the plasmatocytes and granular cells, to slides as well as the attachment of Bacillus subtilis to these haemocytes. Although both antioxidants increased lysozyme and phenoloxidase activities, there was no correlation of enzyme activity and haemocyte adhesion responses, possibly reflecting sequestered radicals. Nitric oxide and hydroxyl radicals offset the DMTU effect. In the absence of antioxidants, inactivate protein kinases A (PKA) and C (PKC) enhanced haemocyte aggregation. In general, DMSO, as opposed to DMTU, did not alter the effects of PKA and PKC activators and inhibitors on haemocyte aggregation or of PKC and PKA activities. High concentrations of DMSO and all levels of DMTU, although inhibiting PKA and PKC, inhibited haemocyte adhesion to slides. Comparable results occurred for DMTU-treated haemocytes incubated with B. subtilis. In vivo DMSO, unlike DMTU, did not impair plasmatocyte or granular cell responses to foreign materials, including bacterial removal from the haemolymph and nodulation.

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.

Hemocytes of the centipede Scutigera coleoptrata (Chilopoda, Notostigmophora) with notes on their interactions with the tracheae

The hemocytes of Scutigera coleoptrata were investigated by light and electron microscopy. Four types of hemocytes were identified: prohemocytes, plasmatocytes, granulocytes, and spherulocytes. Only granulocytes could be distinguished from the three other types by May-Grünwald staining, as this is the only hemocyte type demonstrating an eosinophilic reaction. Shape and size give further indications for distinguishing the cell types. In addition, differentiation is possible on the basis of their ultrastructure. However, only a combination of all three methods (staining and light and electron microscopy) allows clear separation of the cell types. As transitional stages between the cell types occur in S. coleoptrata, it is likely that prohemocytes, plasmatocytes, and granulocytes are ontogenetic stages of a single cell lineage. Special cell components and their possible functions are described. Plasmatocytes exocytose tubular structures that probably play a role in coagulation processes. These tubular structures develop in the grana of plasmatocytes. Also, a special arrangement of microtubules and microfilaments was demonstrated. For the first time interactions between hemocytes and tracheae are documented within the Chilopoda. It is assumed that the hemocytes meet their oxygen requirements directly from the tracheae. Phylogenetic implications of the results are discussed.

Involvement of both granular cells and plasmatocytes in phagocytic reactions in the greater wax moth, Galleria mellonella

Although it has been previously found by most authors that only plasmatocytes are involved in phagocytosis of non-self in the greater wax moth, Galleria mellonella, in the present study we demonstrate that in vitro, both granular cells and plasmatocytes are involved in this reaction, using monolayers of these haemocytes prepared from larval haemolymph by a differential cell fractionation method. The adhesion of granular cells to glassware and phagocytosis by granular cells of FITC-labelled silica beads were both greatly reduced by the presence of p-NPGB, a serine proteinase inhibitor, which is known to inhibit the activation of the prophenoloxidase cascade, but the reactions were only partly influenced by PTU, an inhibitor of phenoloxidase. These results suggest that an enhancing factor for both reactions is phenoloxidase itself or a component induced during the course of activation of the prophenoloxidase cascade, but not the melanised substance produced by the action of this reaction. For plasmatocytes, attachment to non-self was totally blocked by the absence of CaCl(2) or by the presence of EDTA at concentrations greater than 20 mM, and phagocytosis was greatly enhanced by CaCl(2), but suppressed by EDTA. These results suggest that calcium is a factor required for adhesion of plasmatocytes, and that it also functions to enhance their phagocytic action.

The ultrastructure of hemocytopoietic organs in the desert scorpion, Paruroctonus

The light and electron microscopes were used to examine possible hemocytopoietic tissue in the desert scorpion, Paruroctonus mesaensis. Results agree with earlier light microscopic studies that cells are released into the blood from the two lateral lymphoid organs and the supraneural gland. The former are sacciform structures attached by their anterior ends to the diaphragm. The supraneural gland forms the thickened wall of the supraneural artery in the mesosoma from the first to the third abdominal ganglia. The lateral lymphoid glands have an acellular stroma in which are embedded granular and agranular cells. The stroma is apparently formed by specialized cells which release membranous cell fragments that become the matrix of the gland. Cells are released into the body cavity from the periphery of the two organs. The supraneural gland has a fibrous stroma in which are embedded a variety of cell types. The cells appear to be released in greatest abundance into the blood in the lumen of the gland. The gland has cells with opaque granules (0.9-1.4 micron diameter) and agranular cells of variable shape. The most abundant cell, possibly the stem-cell for the others, is about 10 micron diameter and often has processes of variable length. In addition, muscle cells at various stages of differentiation are found at the inner margin of the gland. These cells have thick and thin myofilaments (24-32 and 5-8 nm diameter) and dense bodies which sometimes become organized into sarcomeres with Z-bands before the cells are released into the gland lumen. The function of these muscle cells is unknown, but possibly they contribute to the maintenance of blood pressure and the release of cells into the blood from the inner margin of the gland.

Hemocytes and hemocytopoiesis in Silkworms

A brief review is presented of the current state of ultrastructure, cytochemistry, and physiology of the hemocytes and meso- and metathoracic peri-imaginal-wing organs in silkworms. According to the accepted morphological classification, five circulating types of hemocytes are recognized in Bombyx mori as well as in Antheraea pernyi. They are prophemocytes or stem cells, plasmatocytes or pre-differentiated cells and three specialized cells, granulocytes, spherule cells and oenocytoids. During post-embryonic development the last four types are the most common in the circulating hemolymph. Plasmatocytes are considered to be pluripotent cells from which granulocytes, spherule cells and oenocytoids are derived. Contrary to the situation in most insects the plasmatocytes are not phagocytic in Antheraea. The granulocytes are efficient phagocytes. Both plasmatocytes and granulocytes are involved in pinocytosis. Another possible function of the granulocytes is hemolymph coagulation. The function of the spherule cells which contain a paracrystalline material (muco- or glycoproteins) is by no means clear. The phenoloxidase activity found within the cytosol of oenocytoids appears effective against the natural monophenol and diphenol substrates. The involvement of oenocytoids in the complex metabolism of phenols and particularly in the production of plasma phenolases has been reported. The mitotic division of five circulating hemocyte types is well known and was long regarded as the only mechanism of postembryonic hemocyte production. We present for silkworms, experimental evidence of the hemocytopoietic function of the meso- and metathoracic organs surrounding the imaginal wing discs. Ablation experiments demonstrate that the mitotic activity of free hemocytes is unable to maintain the normal hemocytogram in the absence of the two paris of organs. These organs are typically divided into cell islets ensheathed by a connective tissue membrane. Two types of islets may be classified by the disposition of the cells : the compact islets or aggregations of stem cells and the reticulate islets which are mainly composed of hemocytes at different steps of differentiation. The relative number of prohemocytes in the total hemocyte population ranges from 84 to 97 p. cent in organs of Antheraea pernyi. This well-defined cell type appears to be the major hemocyte type in hemocytopoietic organs. In Antheraea, the mitotic index (the relative number of mitotic hemocytes in the total cell population) varies from 0.5 to about 3 p. cent. Finally, our data direct attention to cyclic functional changes such as mitotic divisions and hemocyte differentiation which run parallel to the molting cycle.

A histological study of wound healing and hemocyte function in the wax-moth Galleria mellonella

The stages in wound healing of the wax-moth, Galleria mellonella, were examined using routine histological techniques. Final instar larvae were wounded and maintained for up to 72 hours before fixation, dehydration and embedding in Paraplast wax or Araldite. Immediately after wounding both fat-body and hemolymph were forced through the wound to form a plug. Concomitantly, the hemolymph coagulated forming vast networks of strand-like material attached to the hemocytes underlying the wound. After one to two hours this plug melanized and the cells became highly necrotic. Six hours after wounding, there was a massive influx of hemocytes which eventually attached to the melanized layer over the wound to form a multicellular sheath. Twelve to twenty-four hours later, the epidermal cells underlying the broken cuticle detached and migrated across the wound to form a new intact layer. This layer secreted an amorphous material which is probably the precursor of the new cuticle. Hemocyte functions during wound healing and other cellular defenses are discussed.

Studies on the in vivo cellular reactions of insects: an ultrastructural analysis of nodule formation in Galleria mellonella

Cellular aggregates or nodules which formed in Galleria mellonella larvae in response to injections of killed Bacillus cereus were examined during their first 24 hr of development. The initial aggregation stage was very rapid, occurring within 1 min, and was brought about by the release of an adhesive flocculent material by granular haemocytes in contact with the bacteria. This material, which subsequently became increasingly electron dense owing to melanin deposition, consolidated and by 5 min ramified all the intercellular spaces. The granular cells degenerated, releasing portions of cytoplasm form their peripheries, and eventurally appeared in the matrix as voids containing loosely flocculent material. No bacterial breakdown was seen, however. From 2-4 hr, plasmatocytes began to encapsulate the necrotic mass, and by 24 hr a complete sheath, composed of three discrete regions of cells, was formed. These findings are discussed in relation to the various proposed mechanisms of nodule and capsule formation.

The granular cells of Galleria mellonella during clotting and phagocytic reactions in vitro

Light and electron-microscopic observations of the blood-cells (hemocytes) of the wax-moth Galleria mellonella showed that hemolymph coagulation was initiated by the rapid release of material from the granular cells. During incubation for short terms in vitro these cells showed progressive degranulation as material derived from the granules was discharged into the hemolymph. Attempts to determine the nature of this material by staining with ruthenium red proved mainly unsuccessful. When challenged with bacteria in vitro the granular cells failed to phagocytose these particles and instead the bacteria became embedded in the granular material surrounding these cells. The mode of coagulation reported here is compared with previous reports of the role of invertebrate hemocytes in hemolymph clotting.

[Cytochemical observations of hemocytes in decapod brachyuran crustacea (author's transl)]

Hemocytes from two species of crabs, Eriocheir sinensis and Carcinus maenas, have been submitted to different cytochemical reactions in order to bring out the nature of granules' content, either by specific coloration or be enzymatic digestion. Light as well as electron microscopy has been used. The granules appeared to be made essentially of basic proteins, rich in arginin and disulfide bridges but poor in tyrosin. These proteins are linked with nonglycogenic carbohydrates. Glycogen deposits of varied size are restricted to the cytoplasm in all the different cell-types but have never been detected inside the granules themselves. Lipids are poorly represented. The bulk of the granule material is made of neutral mucopolysaccharides although minute amounts of acid mucosubstances could be found in some instances.