Circulating Hemocytes from Larvae of the Japanese Rhinoceros Beetle Allomyrina dichotoma (Linnaeus) (Coleoptera: Scarabaeidae) and the Cellular Immune Response to Microorganisms

Morphological and functional characterization of circulating hemocytes in Tribolium castaneum larvae

Hemocytes are pivotal in the immune response of insects against invasive pathogens. However, our knowledge of hemocyte types and their specific function in Tribolium castaneum, an increasingly important Coleoptera model insect in various research fields, remains limited. Presently, a combination of morphological criteria and dye-staining properties were used to characterize hemocyte types from T. castaneum larvae, and 4 distinct types were identified: granulocytes, oenocytoids, plasmatocytes and prohemocytes. Following different immune challenges, the total hemocyte counts declined rapidly in the initial phase (at 2 h), then increased over time (at 4 and 6 h) and eventually returned to the naive state by 24 h post-injection. Notably, the morphology of granulocytes underwent dramatic changes, characterized by an expansion of the surface area and an increased production of pseudopods, and with the number of granulocytes rising significantly through mitotic division. Granulocytes and plasmatocytes, the main hemocyte types in T. castaneum larvae, can phagocytose bacteria or latex beads injected into the larval hemolymph in vivo. Furthermore, these hemocytes participate in the encapsulation and melanization processes in vitro, forming capsules to encapsulate and melanize nickel-nitrilotriacetic acid (Ni-NTA) beads. This study provides the first comprehensive characterization of circulating hemocytes in T. castaneum larvae, offering valuable insights into cell-mediated immunity in response to bacterial infection and the injection of latex beads. These results deepen our understanding of the cellular response mechanisms in T. castaneum larvae and lay a solid foundation for subsequent investigations of the involvement of T. castaneum hemocytes in combating pathogens.

Granulocyte dynamics: a key player in the immune priming effects of crickets

This study investigates immune priming effects associated with granulocytes in crickets through a comprehensive analysis. Kaplan-Meier survival analysis reveals a significant contrast in survival rates, with the heat-killed Bacillus thuringiensis (Bt)-primed group exhibiting an impressive ~80% survival rate compared to the PBS buffer-primed group with only ~10% survival 60 hours post live Bt infection. Hemocyte analysis underscores elevated hemocyte counts, particularly in granulocytes of the killed Bt-primed group, suggesting a correlation between the heat-killed Bt priming and heightened immune activation. Microscopy techniques further explore granulocyte morphology, unveiling distinctive immune responses in the killed Bt-primed group characterized by prolonged immune activation, heightened granulocyte activity, phagocytosis, and extracellular trap formation, contributing to enhanced survival rates. In particular, after 24 hours of injecting live Bt, most granulocytes in the PBS buffer-primed group exhibited extracellular DNA trap cell death (ETosis), while in the killed Bt-primed group, the majority of granulocytes were observed to maintain highly activated extracellular traps, sustaining the immune response. Gene expression analysis supports these findings, revealing differential regulation of immune-related genes such as antibacterial humoral response, detection of bacterial lipopeptides, and cellular response to bacteria lipopeptides. Additionally, the heat-killed Bt-primed group, the heat-killed E. coli-primed group, and the PBS-primed group were re-injected with live Bt 2 and 9 days post priming. Two days later, only the PBS-primed group displayed low survival rates. After injecting live Bt 9 days later, the heat-killed E. coli-primed group surprisingly showed a similarly low survival rate, while the heat-killed Bt-primed group exhibited a high survival rate of ~60% after 60 hours, with actively moving and healthy crickets. In conclusion, this research provides valuable insights into both short-term and long-term immune priming effects in crickets, contributing to our understanding of invertebrate immunity with potential applications in public health.

Hemocyte morphology of worker subcastes of the leaf-cutting ant Atta sexdens rubropilosa (Hymenoptera: Formicidae)

Hemocytes are cells present in the hemolymph of insects that play a role in combating invasive pathogens, ensuring defense by the immune system in these organisms. While the types of hemocytes are well known in some insect representatives, data on these cells in Hymenoptera are limited to certain bees and wasps, with little information available for ants. Among ants, the genus Atta has environmental and economic importance, forming highly organized colonies consisting of the queen and workers, with the latter subdivided into subcastes: gardeners, waste removers, foragers, and soldiers, which are exposed to different pathogens. This study describes the morphology of hemocytes in the worker subcastes of Atta sexdens rubropilosa. Hemolymph samples from the ant were submitted to light, confocal, and scanning electron microscopy analyses. Five types of hemocytes were identified in the hemolymph of all ant subcastes, including prohemocytes, oenocytoids, spherulocytes, plasmatocytes, and granulocytes. They exhibited nuclei with a predominance of decondensed chromatin. The granulocytes were the most abundant cell type in the subcastes, followed by prohemocytes, plasmatocytes, oenocytoids, and spherulocytes. Phagocytosis assays reveal that plasmatocytes and granulocytes are the main phagocytic cells in all castes evaluated. This study fills an important gap in understanding the immune response in this ant species.

Morphofunctional characterization of hemocytes in black soldier fly larvae

In insects, the cell-mediated immune response involves an active role of hemocytes in phagocytosis, nodulation, and encapsulation. Although these processes have been well documented in multiple species belonging to different insect orders, information concerning the immune response, particularly the hemocyte types and their specific function in the black soldier fly Hermetia illucens, is still limited. This is a serious gap in knowledge given the high economic relevance of H. illucens larvae in waste management strategies and considering that the saprophagous feeding habits of this dipteran species have likely shaped its immune system to efficiently respond to infections. The present study represents the first detailed characterization of black soldier fly hemocytes and provides new insights into the cell-mediated immune response of this insect. In particular, in addition to prohemocytes, we identified five hemocyte types that mount the immune response in the larva, and analyzed their behavior, role, and morphofunctional changes in response to bacterial infection and injection of chromatographic beads. Our results demonstrate that the circulating phagocytes in black soldier fly larvae are plasmatocytes. These cells also take part in nodulation and encapsulation with granulocytes and lamellocyte-like cells, developing a starting core for nodule/capsule formation to remove/encapsulate large bacterial aggregates/pathogens from the hemolymph, respectively. These processes are supported by the release of melanin precursors from crystal cells and likely by mobilizing nutrient reserves in newly circulating adipohemocytes, which could thus trophically support other hemocytes during the immune response. Finally, the regulation of the cell-mediated immune response by eicosanoids was investigated.

Citrobacter freundii, a natural associate of the Colorado potato beetle, increases larval susceptibility to Bacillus thuringiensis

BACKGROUND

We assume that certain representatives of gut microflora mediate immune changes during dysbiosis, accelerating septicemia caused by Bacillus thuringiensis.

RESULTS

Co-introduction of Citrobacter freundii with Bacillus thuringiensis var. tenebrionis (morrisoni) (Bt) led to an increase in Colorado potato beetle (CPB) larval mortality to 69.0% (1.3-5×) and a synergistic effect was observed from day 1 to day 6. Ultrathin sections of the CPB midgut showed autophagosome formation and partial destruction of gut microvilli under the influence of Bt, which was accompanied by pronounced hypersecretion of the endoplasmic reticulum with apocrine vesicle formation and oncotic changes in cells under the action of C. freundii. The destruction of gut tissues was accompanied by suppression of detoxification processes under the action of the bacteria and a decrease (2.8-3.5×) in the concentration of lipid oxidation products during Bt infection. In the first hours post combined treatment, we registered a slight increase in the total hemocyte count (THC) especially a predomination (1.4×) of immune-competent plasmatocytes. Oral administration of symbiotic and entomopathogenic bacteria to the CPB larvae significantly decreased the THC (1.4×) after 24 h and increased (1.1-1.5×) the detoxifying enzymes level in the lymph. These changes are likely to be associated with the destruction of hemocytes and the need to remove the toxic products of reactive oxygen species.

CONCLUSION

The obtained results indicate that feeding of C. freundii and B. thuringiensis to the CPB larvae is accompanied by tissue changes that significantly affect the cellular and humoral immunity of the insect, increasing its susceptibility to Bt. © 2022 Society of Chemical Industry.

Morphometry of cellular behavior of coelomocytes from starfish Asterias amurensis

A comprehensive statistical analysis using a wide range of linear and non-linear morphological parameters enabled identification of the main stages in the in vitro dynamics of cell behavior of immune cells of the marine invertebrate Asterias amurensis (Echinodermata, Asteroidea). Three stages may be distinguished in the cell behavior, which are characterized by the differences in complexity of the cell boundary microsculpture as well as by the size and asymmetry of the cell and convex hull of the cell. The first stage (5 min after placing cells onto a substrate) is characterized by more complex cell morphology and an increase in the process number and spreading area. The second stage (15 min) is characterized by simplification of cell morphology, retraction of some processes, and rounding of cells upon continued cell spreading. At the third stage (60 min), new large processes with rounded contours emerge due to partial retraction of the flattened cell surface. Each stage is characterized by statistically significant differences in several linear and nonlinear parameters of the external morphology for all cell types.

Insights Into the Immune Response of the Black Soldier Fly Larvae to Bacteria

In insects, a complex and effective immune system that can be rapidly activated by a plethora of stimuli has evolved. Although the main cellular and humoral mechanisms and their activation pathways are highly conserved across insects, the timing and the efficacy of triggered immune responses can differ among different species. In this scenario, an insect deserving particular attention is the black soldier fly (BSF), Hermetia illucens (Diptera: Stratiomyidae). Indeed, BSF larvae can be reared on a wide range of decaying organic substrates and, thanks to their high protein and lipid content, they represent a valuable source of macromolecules useful for different applications (e.g., production of feedstuff, bioplastics, and biodiesel), thus contributing to the development of circular economy supply chains for waste valorization. However, decaying substrates bring the larvae into contact with different potential pathogens that can challenge their health status and growth. Although these life strategies have presumably contributed to shape the evolution of a sophisticated and efficient immune system in this dipteran, knowledge about its functional features is still fragmentary. In the present study, we investigated the processes underpinning the immune response to bacteria in H. illucens larvae and characterized their reaction times. Our data demonstrate that the cellular and humoral responses in this insect show different kinetics: phagocytosis and encapsulation are rapidly triggered after the immune challenge, while the humoral components intervene later. Moreover, although both Gram-positive and Gram-negative bacteria are completely removed from the insect body within a few hours after injection, Gram-positive bacteria persist in the hemolymph longer than do Gram-negative bacteria. Finally, the activity of two key actors of the humoral response, i.e., lysozyme and phenoloxidase, show unusual dynamics as compared to other insects. This study represents the first detailed characterization of the immune response to bacteria of H. illucens larvae, expanding knowledge on the defense mechanisms of this insect among Diptera. This information is a prerequisite to manipulating the larval immune response by nutritional and environmental factors to increase resistance to pathogens and optimize health status during mass rearing.

Haemocyte-mediated immunity in insects: Cells, processes and associated components in the fight against pathogens and parasites

The host defence of insects includes a combination of cellular and humoral responses. The cellular arm of the insect innate immune system includes mechanisms that are directly mediated by haemocytes (e.g., phagocytosis, nodulation and encapsulation). In addition, melanization accompanying coagulation, clot formation and wound healing, nodulation and encapsulation processes leads to the formation of cytotoxic redox-cycling melanin precursors and reactive oxygen and nitrogen species. However, demarcation between cellular and humoral immune reactions as two distinct categories is not straightforward. This is because many humoral factors affect haemocyte functions and haemocytes themselves are an important source of many humoral molecules. There is also a considerable overlap between cellular and humoral immune functions that span from recognition of foreign intruders to clot formation. Here, we review these immune reactions starting with the cellular mechanisms that limit haemolymph loss and participate in wound healing and clot formation and advancing to cellular functions that are critical in restricting pathogen movement and replication. This information is important because it highlights that insect cellular immunity is controlled by a multilayered system, different components of which are activated by different pathogens or during the different stages of the infection.

Cell Line Platforms Support Research into Arthropod Immunity

Simple Summary

Many insect and tick species are serious pests, because insects damage crop plants and, along with ticks, transmit a wide range of human and animal diseases. One way of controlling these pests is by impairing their immune system, which protects them from bacterial, fungal, and viral infections. An important tool for studying immunity is using long-lasting cell cultures, known as cell lines. These lines can be frozen and thawed at will to be used in automated tests, and they provide consistent results over years. Questions that can be asked using cell lines include: How do insects or ticks recognize when they have been infected and by what organism? What kinds of defensive strategies do they use to contain or kill infectious agents? This article reviews research with insect or tick cell lines to answer these questions, as well as other questions relating to immunity. This review also discusses future research strategies for working with cell lines.

Abstract

Innate immune responses are essential to maintaining insect and tick health and are the primary defense against pathogenic viruses, bacteria, and fungi. Cell line research is a powerful method for understanding how invertebrates mount defenses against pathogenic organisms and testing hypotheses on how these responses occur. In particular, immortal arthropod cell lines are valuable tools, providing a tractable, high-throughput, cost-effective, and consistent platform to investigate the mechanisms underpinning insect and tick immune responses. The research results inform the controls of medically and agriculturally important insects and ticks. This review presents several examples of how cell lines have facilitated research into multiple aspects of the invertebrate immune response to pathogens and other foreign agents, as well as comments on possible future research directions in these robust systems.

Investigating immune responses of the house cricket, Acheta domesticus to pathogenic Eschericia coli K1

BACKGROUND

Insects models are excellent models of the innate immune system, as they are free from the influences of the vertebrate adaptive immunity. Crickets are hemimetabolous insects belonging to the order Orthopteran order that have not been as extensively characterized as other holometabolous insects, and may provide new insights to the insect immune responses. In this study, we aim to characterize the innate immune responses of the common house cricket, Acheta domesticus in response to a human pathogenic bacterium E. coli K1.

METHODS

Crickets were injected with sterile buffer, live E. coli K1 or heat-killed E. coli K1. Physiological effects such as mortality and weight change of the crickets were determined 24-, 48 and 72-hours post injection while immunological effects such as hemocyte counts, bacteremia, phenoloxidase and lysozyme activity of the crickets were measured at 2- and 24-hours post-injection.

RESULTS

The injection of E. coli K1 in crickets resulted in >85% mortality 3-days post injection, accompanied by significant weight loss. E. coli K1 injection caused a significant increase in both phenoloxidase and lysozyme activities in cricket hemolymphs 24-hours post injection. Live E. coli K1 injected crickets resulted in a significant reduction in circulating hemocytes 24-hours post injection which was not observed in other treatment groups. This was consistent with the resolution of bacteremia observed 24-hours post infection in live E. coli K1 injected crickets.

CONCLUSION

Our study provides new insights on the innate immune response to pathogenic E. coli K1 in a cricket model.

Morphological Characterisation of Haemocytes in the Mealworm Beetle Tenebrio molitor (Coleoptera, Tenebrionidae)

The immunocompetence of the mealworm beetle Tenebrio molitor has been well investigated at molecular and physiological levels, but information on morphological and functional characteristics of its immune cells (haemocytes) is still scarce and fragmentary. This study provides an updated overview of the morphology of circulating immune cells from mealworm beetle adults, using light and transmission electron microscopy. Based on their affinities for May-Grünwald Giemsa stain, haemocytes were defined as either eosinophilic, basophilic or neutral. Ultrastructural descriptions allowed to detect four main cell types in the haemolymph: prohaemocytes, plasmatocytes, granular cells and oenocytoids. The morphological plasticity of haemocytes and the evidence of mitotic circulating cells, intermediate cell stages, as well as autophagic activities suggest haemocyte proliferation, turnover and transdifferentiation as constantly active processes in the haemolymph. Cytochemical tests revealed differences in the distribution of carbohydrates among cell types underling the great plasticity of the immune response and the direct involvement of circulating immune cells in the resource allocation. In addition, our results provide a detailed morphological description of vesicle trafficking, macro- and microautophagy, apoptotic and necrotic processes, confirming the suitability of T. molitor haemocytes as a model for studying evolutionarily conserved cellular mechanisms.

Pendimethalin-based herbicide impairs cellular immune response and haemocyte morphology in a beneficial ground beetle

Herbicides have become the most commonly applied agrochemicals in agroecosystems. Thus, basic knowledge of their physiological effects on insects is needed, especially for understanding their impact on beneficial insect species. In this study, we evaluated the effect of a pendimethalin-based herbicide (PND) on the cellular immune response of the carabid beetle Harpalus (Pseudoophonus) rufipes (De Geer 1774) (Coleoptera, Carabidae), acting as biocontrol agent in agroecosystems. Total and differential haemocyte counts and phagocytosis assay, performed by injecting in vivo carboxylate-modified polystyrene latex beads, were measured in beetles exposed to a recommended field dose (4L per ha) of PND to evaluate the exposure effects over the time. The pattern of haemocyte subpopulations and the decrease of the phagocytic index after the exposure to PND suggested a lowering of P. rufipes ability to face an infection performing a cell-mediated response. PND was also found to cause cytotoxic effects on the haemocyte ultrastructure. Ultrastructural alterations such as irregular shape, large vacuolization of the cytoplasm, and condensation of marginated chromatin were recorded from 2d of exposure. The loss of RER, Golgi apparatus, mitochondria integrity and the swelling of the outer nuclear membrane found in some haemocytes suggested an interference of PND with the membrane permeability. Results indicated that the exposure to PND impairs the distribution, morphology and physiological functions of haemocytes causing a decrease of P. rufipes immunocompetence. Moreover, the sensitivity to herbicide exposure makes this species a suitable model and a useful bioindicator for monitoring exposure effects on non-target species. This study provides useful information to protect and preserve biodiversity of insects in agroecosystems.

Ex vivo characterization of the circulating hemocytes of bed bugs and their responses to bacterial exposure

Bed bugs (Cimex spp.) are urban pests of global importance. Knowledge of the immune system of bed bugs has implications for understanding their susceptibility to biological control agents, their potential to transmit human pathogens, and the basic comparative immunology of insects. Nonetheless, the immunological repertoire of the family Cimicidae remains poorly characterized. Here, we use microscopy, flow cytometry, and RNA sequencing to provide a basal characterization of the circulating hemocytes of the common bed bug, Cimex lectularius. We also examine the responses of these specialized cells to E. coli exposure using the same techniques. Our results show that circulating hemocytes are comprised of at least four morphologically distinct cell types that are capable of phagocytosis, undergo degranulation, and exhibit additional markers of activation following stimulation, including size shift and DNA replication. Furthermore, transcriptomic profiling reveals expression of predicted Toll/IMD signaling pathway components, antimicrobial effectors and other potentially immunoresponsive genes in these cells. Together, our data demonstrate the conservation of several canonical cellular immune responses in the common bed bug and provide a foundation for additional mechanistic immunological studies with specific pathogens of interest.

Hemocyte-hemocyte adhesion by granulocytes is associated with cellular immunity in the cricket, Gryllus bimaculatus

In this study, more than 1,000 cricket (Gryllus bimaculatus) hemocytes were classified based on their size and morphology. These hemocytes were classified into six types: granulocytes, plasmatocytes, prohemocytes, spherulocytes, coagulocytes, and oenocytoids. Hemocyte cultures was observed in real time to determine which hemocytes were associated with cellular immune responses against potential pathogens. Granulocytes were identified as the professional immune cell that mediates nodulation, encapsulation, and phagocytosis of pathogens. Granulocytes have been shown to actively produce various sticky nets (amoeba-like hairs and extracellular traps) from their plasma membranes that they use to gather other hemocytes and to implement cellular immune responses. The activation of lysosomes in granulocytes started at 4 h, peaked at 12 h, and returned to baseline by 24 h post-infection. At 48 h post-infection, cells could be found within the cytoplasm of granulocytes and reactivated lysosomes surrounding these cells were visible. This result seems to reflect a phenomenon in which necrotic granulocytes are removed by other healthy granulocytes. This unique mechanism of cellular immunity is therefore a way to efficiently and effectively remove pathogens and simultaneously maintain healthy hemocytes.

TcaB, an insecticidal protein from Photorhabdus akhurstii causes cytotoxicity in the greater wax moth, Galleria mellonella

Photorhabdus akhurstii can produce a variety of proteins that aid this bacterium and its mutualistic nematode vector, Heterorhabditis indica to kill the insect host. Herein, we characterized (by heterologously expressing in E. coli) an open reading frame (1713 bp) of the toxin complex protein, TcaB from P. akhurstii strains IARI-SGHR2 and IARI-SGMS1 and assessed its toxic effect on G. mellonella larvae. The intra-hemocoel injection of purified TcaB (molecular weight-63 kDa) caused fourth instar larval bodies to blacken and die with LD₅₀ values of 67.25 (IARI-SGHR2) and 52.08 (IARI-SGMS1) ng per larva at 12 h. Additionally, oral administration of the toxin caused larval mortality with LD₅₀ values of 709.55 (IARI-SGHR2) and 598.44 (IARI-SGMS1) ng per g diet per larva at 7 days post feeding. Injection of purified TcaB caused loss of viability of fourth instar G. mellonella hemocytes at 6 h post incubation; cells displayed morphological changes typical of apoptosis, including cell shrinkage, membrane blebbing, nuclear condensation and disintegration. Injection of TcaB also elevated the phenoloxidase activity in insect hemolymph which triggers an extensive immune response that potentially leads to larval death. Similar to other bacterial toxins TcaB possesses potent biological activity which may enable it to be used as an efficient agent for pest management.

Acute and oral exposure to imidacloprid does not affect the number of circulating hemocytes in the stingless bee Melipona quadrifasciata post immune challenge

In the present work, the effects of the neonicotinoid imidacloprid formulation on the total hemocyte counts (THC) and differential hemocyte counts (DHC) were investigated in foraging workers of the stingless bee Melipona quadrifasciata under (or not) the challenge with the bacteria Escherichia coli. The THC was not altered with the insecticide exposure and/or bacterial infection. However, the DHC of the bees changed with the imidacloprid exposure and/or bacterial infection. The number of prohemocytes (stem cells) increased in bees exposed to imidacloprid, but it did not change after the bacterial infection. The number of plasmatocytes (phagocytic cells) increased in imidacloprid-exposed and uninfected bees and decreased in infected bees regardless of the exposure to imidacloprid. On the other hand, there was a reduction of granulocytes, the most active immune cells, after imidacloprid exposure and an increase of granulocytes after the infection. Previous studies have shown that the neonicotinoids exposure can impair the immune system of bees. Our findings showed that the relative number of granulocytes declined with imidacloprid exposure, but the overall capacity of hemocyte responses in terms of total numbers after bacterial infection persisted even after the insecticide exposure.

Hemocyte Morphology and Cellular Immune Response in Termite (Reticulitermes speratus)

Because termites (Reticulitermes speratus) are very small, it is difficult to conduct experiments involving pathogen injection and hemocyte collection. Therefore, to observe hemocyte-mediated immune responses against foreign substances, in vitro hemocyte culture is essential. After collecting about 3 μl of hemolymph, hemocytes were cultured for 7 d, during which the cells maintained full function. Four types of hemocyte were identified, namely, granulocytes, plasmatocytes, oenocytoids, and prohemocytes, among which granulocytes are the main immune hemocytes that fight invasion by foreign substances. Most hemocytes were alive and/or functioning after 7 d of culture, but then either died or lost function.

Immune tolerance to an intestine-adapted bacteria, Chryseobacterium sp., injected into the hemocoel of Protaetia brevitarsis seulensis

To explore the interaction of gut microbes and the host immune system, bacteria were isolated from the gut of Protaetia brevitarsis seulensis larvae. Chryseobacterium sp., Bacillus subtilis, Arthrobacter arilaitensis, Bacillus amyloliquefaciens, Bacillus megaterium, and Lysinibacillus xylanilyticus were cultured in vitro, identified, and injected in the hemocoel of P. brevitarsis seulensis larvae, respectively. There were no significant changes in phagocytosis-associated lysosomal formation or pathogen-related autophagosome in immune cells (granulocytes) from Chryseobacterium sp.-challenged larvae. Next, we examined changes in the transcription of innate immune genes such as peptidoglycan recognition proteins and antimicrobial peptides following infection with Chryseobacterium sp. PGRP-1 and -2 transcripts, which may be associated with melanization generated by prophenoloxidase (PPO), were either highly or moderately expressed at 24 h post-infection with Chryseobacterium sp. However, PGRP-SC2 transcripts, which code for bactericidal amidases, were expressed at low levels. With respect to antimicrobial peptides, only coleoptericin was moderately expressed in Chryseobacterium sp.-challenged larvae, suggesting maintenance of an optimum number of Chryseobacterium sp. All examined genes were expressed at significantly higher levels in larvae challenged with a pathogenic bacterium. Our data demonstrated that gut-inhabiting bacteria, the Chryseobacterium sp., induced a weaker immune response than other pathogenic bacteria, E. coli K12.