The IMD pathway regulates lysozyme-like proteins (LLPs) in the silkmoth Antheraea mylitta

Infection of the entomopathogenic fungus Metarhizium rileyi suppresses cellular immunity and activates humoral antibacterial immunity of the host Spodoptera frugiperda

BACKGROUND

Metarhizium rileyi is an entomopathogenic fungus with promising potential for controlling agricultural pests, including Spodoptera frugiperda. Following penetration of the host through the cuticle, M. rileyi cells transform into in vivo blastospores or hyphal bodies, propagating within the hemocoel. However, the strategies and molecular mechanisms by which M. rileyi survives upon exposure to the powerful insect immune system remain unclear.

RESULTS

We determined the pathogenicity of M. rileyi and found that either conidial immersion or blastospore injection significantly decreased S. frugiperda survival in a dose-dependent manner. Injection of M. rileyi blastospores decreased the number of S. frugiperda hemocytes and impaired host cellular reactions such as nodulation, encapsulation and phagocytosis. Blastospore injection led to increased antibacterial activity in plasma at 48 h post-injection (hpi). RNA-sequencing analyses identified a large number of antimicrobial peptide genes upregulated in the fat body of M. rileyi-infected larvae at 48 hpi, which may be attributable to the activation of Toll and IMD signaling pathway.

CONCLUSION

This study demonstrates that the compromised cellular immunity of the insect host is due to the marked decrease in hemocytes and impaired cellular cytoskeletons, which may facilitate early infection by M. rileyi. Late in the course of infection, the enhanced antibacterial activity of plasma, which may be in response to intestinal evading bacteria, cannot inhibit hyphal growth in hemolymph. Our data provide a comprehensive resource for exploring the molecular mechanism employed by M. rileyi to overcome S. frugiperda immunity. © 2022 Society of Chemical Industry.

Identification of four Spätzle genes (MnSpz1, MnSpz2, MnSpz2-isoform, and MnSpz3) and their roles in the innate immunity of Macrobrachium nipponense

Spätzle, an extracellular ligand of the Toll receptor, is involved in the innate immunity of crustaceans. In this study, four Spätzle genes were cloned from Macrobrachium nipponense and designed as MnSpz1, MnSpz2, MnSpz2-isoform, and MnSpz3. The coding region of the four Spätzle genes all contained one intron and two exons, and they were predicted to be produced by gene duplication based on sequence similarities and phylogenetic tree. The predicted MnSpz1, MnSpz2, and MnSpz3 proteins all contained a signal peptide and a Spätzle domain. No signal peptide but a Spätzle domain existed in MnSpz2-isoform because of frameshift mutation caused by 50 bp nucleotide deletion compared with MnSpz2. Quantitative real-time polymerase chain reaction (RT-qPCR) analysis showed that MnSpz1, MnSpz2, and MnSpz3 were expressed in all the detected tissues of M. nipponense, and MnSpz2 was found to be the major isoform in the heart, gills, stomach, and intestine. After stimulation by Vibrio parahaemolyticus, Staphylococcus aureus, or White spot syndrome virus (WSSV), the expression levels of MnSpz1, MnSpz2, and MnSpz3 changed. Given the high similarities among MnSpz1-3, RNA interference (RNAi) using dsRNA of MnSpz1 inhibited the expression of the three Spätzle genes (MnSpz1, MnSpz2 and MnSpz3). Silencing of MnSpz1-3 down-regulated the expression levels of nine antimicrobial peptide (AMP) genes in M. nipponense. After Knockdown of MnSpzs, the number of V. parahaemolyticus, S. aureus and WSSV copies in M. nipponense increased significantly in vivo. Our results suggest that Spätzles are involved in the innate immunity of M. nipponense. The expansion of MnSpz genes through gene duplication is beneficial to enhance the innate immune defense ability of M. nipponense.

Diversity of insect antimicrobial peptides and proteins - A functional perspective: A review

The innate immune response of insects provides a robust line of defense against pathogenic microbes and eukaryotic parasites. It consists of two types of overlapping immune responses, named humoral and cellular, which share protective molecules and regulatory mechanisms that closely coordinate to prevent the spread and replication of pathogens within the compromised insect hemocoel. The major feature of the humoral part of the insect immune system involves the production and secretion of antimicrobial peptides from the fat body, which is considered analogous to adipose tissue and liver in vertebrates. Previous research has identified and characterized the nature of antimicrobial peptides that are directed against various targets during the different stages of infection. Here we review this information focusing mostly on the diversity and mode of action of these host defense components, and their critical contribution to maintaining host homeostasis. Extending this knowledge is paramount for understanding the evolution of innate immune function and the physiological balance required to provide sufficient protection to the host against external enemies while avoiding overactivation signaling events that would severely undermine physiological stability.

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.

Regulators and signalling in insect antimicrobial innate immunity: Functional molecules and cellular pathways

Insects possess an immune system that protects them from attacks by various pathogenic microorganisms that would otherwise threaten their survival. Immune mechanisms may deal directly with the pathogens by eliminating them from the host organism or disarm them by suppressing the synthesis of toxins and virulence factors that promote the invasion and destructive action of the intruder within the host. Insects have been established as outstanding models for studying immune system regulation because innate immunity can be explored as an integrated system at the level of the whole organism. Innate immunity in insects consists of basal immunity that controls the constitutive synthesis of effector molecules such as antimicrobial peptides, and inducible immunity that is activated after detection of a microbe or its product(s). Activation and coordination of innate immune defenses in insects involve evolutionary conserved immune factors. Previous research in insects has led to the identification and characterization of distinct immune signalling pathways that modulate the response to microbial infections. This work has not only advanced the field of insect immunology, but it has also rekindled interest in the innate immune system of mammals. Here we review the current knowledge on key molecular components of insect immunity and discuss the opportunities they present for confronting infectious diseases in humans.

The Persistence of Escherichia coli Infection in German Cockroaches (Blattodea: Blattellidae) Varies Between Host Developmental Stages and is Influenced by the Gut Microbiota

The German cockroach, Blatella germanica (L.), is a suspected vector of several enteric bacterial pathogens, including Escherichia coli, among livestock and humans. However, little is known about the factors that influence bacterial transmission by cockroaches. Here, we orally infected B. germanica with various laboratory and field strains of E. coli and examined bacterial titers over time to shed new light on the factors that influence the dynamics of infection. Our results reveal that a laboratory strain of E. coli is largely cleared within 48 h while one field isolate can persist in a majority of cockroaches (80-100%) for longer than 3 d with minimal impact on cockroach longevity. We also find that the ability to clear some strains of E. coli is greater in cockroach nymphs than adults. Notably, no differential expression of the antimicrobial gene lysozyme was observed between nymphs and adults or in infected groups. However, clearance of E. coli was significantly reduced in gnotobiotic cockroaches that were reared in the absence of environmental bacteria, suggesting a protective role for the microbiota against exogenous bacterial pathogens. Together, these results demonstrate that the interactions between cockroaches and enteric bacterial pathogens are highly dynamic and influenced by a combination of microbial, host, and environmental parameters. Such factors may affect the disease transmission capacity of cockroaches in nature and should be further considered in both lab and field studies.

Partner-specific induction of Spodoptera frugiperda immune genes in response to the entomopathogenic nematobacterial complex Steinernema carpocapsae-Xenorhabdus nematophila

The Steinernema carpocapsae-Xenorhabdus nematophila association is a nematobacterial complex used in biological control of insect crop pests. The infection success of this dual pathogen strongly depends on its interactions with the host's immune system. Here, we used the lepidopteran pest Spodoptera frugiperda to analyze the respective impact of each partner in the induction of its immune responses. First, we used previously obtained RNAseq data to construct the immunome of S. frugiperda and analyze its induction. We then selected representative genes to study by RT-qPCR their induction kinetics and specificity after independent injections of each partner. We showed that both X. nematophila and S. carpocapsae participate in the induction of stable immune responses to the complex. While X. nematophila mainly induces genes classically involved in antibacterial responses, S. carpocapsae induces lectins and genes involved in melanization and encapsulation. We discuss putative relationships between these differential inductions and the pathogen immunosuppressive strategies.

Effect of rotenone-induced stress on physiologically active substances in adult Aphis glycines

The aim of this study was to determine the effect of rotenone stress on Aphis glycines Matsumura (Hemiptera: Aphididae) populations in different habitats of Northeast China. The changes in kinase expression activity of endogenous substances (proteins, total sugars, trehalose, cholesterol, and free amino acids), detoxifying enzymes (cytochrome P450 and glutathione S-transferase), and metabolic enzymes (proteases and phosphofructokinases) in specimens from three populations were compared before and after stress with rotenone at median lethal concentration (LC₅₀) and their response mechanisms were analyzed. Following a 24 h treatment with rotenone, the average LC₅₀ rotenone values in A. glycines specimens from field populations A and B, and a laboratory population were 4.39, 4.61, and 4.03 mg/L, respectively. The degree of changes in the kinase expression activity of endogenous substances also differed, which indicated a difference in the response of A. glycines specimens from varying habitats to LC₅₀ rotenone stress. The content of endogenous substances, detoxifying enzymes, and metabolic enzymes, except for that of free amino acids, changed significantly in all populations treated with rotenone at LC₅₀ compared with that in the control (P < 0.05). The decrease in protein and trehalose content, and the obstruction of cholesterol transportation owing to decreased feeding in stressed individuals were the causes of A. glycines death after rotenone treatment. Aphis glycines resistance to rotenone may be related to cytochrome P450 expression.

Insect Defense Proteins and Peptides

The composition of insect hemolymph can change depending on many factors, e.g. access to nutrients, stress conditions, and current needs of the insect. In this chapter, insect immune-related polypeptides, which can be permanently or occasionally present in the hemolymph, are described. Their division into peptides or low-molecular weight proteins is not always determined by the length or secondary structure of a given molecule but also depends on the mode of action in insect immunity and, therefore, it is rather arbitrary. Antimicrobial peptides (AMPs) with their role in immunity, modes of action, and classification are presented in the chapter, followed by a short description of some examples: cecropins, moricins, defensins, proline- and glycine-rich peptides. Further, we will describe selected immune-related proteins that may participate in immune recognition, may possess direct antimicrobial properties, or can be involved in the modulation of insect immunity by both abiotic and biotic factors. We briefly cover Fibrinogen-Related Proteins (FREPs), Down Syndrome Cell Adhesion Molecules (Dscam), Hemolin, Lipophorins, Lysozyme, Insect Metalloproteinase Inhibitor (IMPI), and Heat Shock Proteins. The reader will obtain a partial picture presenting molecules participating in one of the most efficient immune strategies found in the animal world, which allow insects to inhabit all ecological land niches in the world.

Immunosuppressive effects of the limonoid azadirachtin, insights on a nongenotoxic stress botanical, in flesh flies

The tetranortriterpenoid azadirachtin (Aza) is a well-known insect growth disruptor of plant origin. Although its actions on insects have been extensively studied; fragmentary reports are available from the immunological point of view. Therefore, in the present study, total (THC) and differential hemocyte counts (DHC), nodulation, phenoloxidase (PO) activity, immune-reactive lysozymes and inducible nitric oxide (NO) were assessed, as measures of immune responses, in Sarcophaga argyrostoma 3^rd instars challenged individually with M. luteus or Aza, or in combination with both compared to the control larvae. THC was significantly declined after 12 h and 24 h of treatment with Aza. DHC varied considerably; in particular, plasmatocytes were significantly decreased after 36 h and 48 h of treatment with Aza; whereas granulocytes were significantly increased. Nodulation was significantly increased with the increase of time after all treatments. Challenging with M. luteus significantly increased the activity of PO in hemocytes and plasma; whereas such activity was significantly decreased after treatment with Aza or combined Aza and M. luteus. Treatment with Aza or M. luteus alone or in couple significantly increased lysozyme activity of fat body, hemocytes and plasma. However, challenging with M. luteus significantly increased NO concentration in the same tissues. A hypothetical model of Aza as a potential mutagen is presented. However, no genotoxic effect was observed through tracking apoptosis-associated changes in Aza-treated hemocytes via flow cytometry-based apoptosis detection. Our study suggests that the integration of Aza, as an eco-friendly pesticide, with bacterial biopesticides may be a successful approach for controlling insect pests.