Transcriptome Profiling Reveals a Novel Mechanism of Antiviral Immunity Upon Sacbrood Virus Infection in Honey Bee Larvae (Apis cerana)

Chronic bee paralysis virus exploits host antimicrobial peptides and alters gut microbiota composition to facilitate viral infection

The significance of gut microbiota in regulating animal immune response to viral infection is increasingly recognized. However, how chronic bee paralysis virus (CBPV) exploits host immune to disturb microbiota for its proliferation remains elusive. Through histopathological examination, we discovered that the hindgut harbored the highest level of CBPV, and displayed visible signs of damages. The metagenomic analysis showed that a notable reduction in the levels of Snodgrassella alvi and Lactobacillus apis, and a significant increase in the abundance of the opportunistic pathogens such as Enterobacter hormaechei and Enterobacter cloacae following CBPV infection. Subsequent co-inoculation experiments showed that these opportunistic pathogens facilitated the CBPV proliferation, leading to accelerated mortality in bees and exacerbation of bloated abdomen symptoms after CBPV infection. The expression level of antimicrobial peptide (AMP) was found to be significantly up-regulated by over 1000 times in response to CBPV infection, as demonstrated by subsequent transcriptome and quantitative real-time PCR investigations. In particular, through correlation analysis and a bacteriostatic test revealed that the AMPs did not exhibit any inhibitory effect against the two opportunistic pathogens. However, they did demonstrate inhibitory activity against S. alvi and L. apis. Our findings provide different evidence that the virus infection may stimulate and utilize the host's AMPs to eradicate probiotic species and facilitate the proliferation of opportunistic bacteria. This process weakens the intestinal barrier and ultimately resulting in the typical bloated abdomen.

Stress Response in the Honeybee (Apis mellifera L.) Gut Induced by Chlorinated Paraffins at Residue Levels Found in Bee Products

Chlorinated paraffins (CPs) have become global pollutants and are of considerable concern as a result of their persistence and long-distance transmission in the environment and toxicity to mammals. However, their risks to pollinating insects are unknown. Honeybees are classical pollinators and sensitive indicators of environmental pollution. Herein, the effects of CPs on the gut microenvironment and underlying mechanisms were evaluated and explored using Apis mellifera L. Both short- and medium-chain CPs had significant sublethal effects on honeybees at a residue dose of 10 mg/L detected in bee products but did not significantly alter the composition or diversity of the gut microbiota. However, this concentration did induce significant immune, detoxification, and antioxidation responses and metabolic imbalances in the midgut. The mechanisms of CP toxicity in bees are complicated by the complex composition of these chemicals, but this study indicated that CPs could substantially affect intestinal physiology and metabolic homeostasis. Therefore, CPs in the environment could have long-lasting impacts on bee health. Future studies are encouraged to identify novel bioindicators of CP exposure to detect early contamination and uncover the detailed mechanisms underlying the adverse effects of CPs on living organisms, especially pollinating insects.

Transcriptomic Responses Underlying the High Virulence of Black Queen Cell Virus and Sacbrood Virus following a Change in Their Mode of Transmission in Honey Bees (Apis mellifera)

BACKGROUND

Over the last two decades, honey bees (Apis mellifera) have suffered high rates of colony losses that have been attributed to a variety of factors, chief among which are viral pathogens, such as deformed wing virus (DWV), whose virulence has increased because of vector-based transmission by the invasive, ectoparasitic varroa mite (Varroa destructor). A shift in the experimental mode of transmission of the black queen cell virus (BQCV) and sacbrood virus (SBV) from fecal/food-oral (direct horizontal) to vector-mediated (indirect horizontal) transmission also results in high virulence and viral titers in pupal and adult honey bees. Agricultural pesticides represent another factor that acts independently or in interaction with pathogens, and they are also thought to cause colony loss. Understanding the molecular mechanisms underlying the higher virulence following a vector-based mode of transmission provides deeper insight into honey bee colony losses, as does determining whether or not host-pathogen interactions are modulated by exposure to pesticides.

METHODS

Through an experimental design with controlled laboratory, we investigated the effects of the modes of transmission of BQCV and SBV (feeding vs. vector-mediated via injection) alone or in combination with chronic exposure to sublethal and field-realistic concentrations of flupyradifurone (FPF), a novel agricultural insecticide, on honey bee survival and transcription responses by using high-throughput RNA sequencing (RNA-seq) analysis.

RESULTS

Co-exposure to viruses via feeding (VF) or injection (VI) and FPF insecticide had no statistically significant interactive effect on their survival compared to, respectively, VF or VI treatments alone. Transcriptomic analysis revealed a distinct difference in the gene expression profiles of bees inoculated with viruses via injection (VI) and exposed to FPF insecticide (VI+FPF). The number of differentially expressed genes (DEGs) at log2 (fold-change) > 2.0 in VI bees (136 genes) or/and VI+FPF insecticide (282 genes) was very high compared to that of VF bees (8 genes) or the VF+FPF insecticide treatment (15 genes). Of these DEGs, the expression in VI and VI+FPF bees of some immune-related genes, such as those for antimicrobial peptides, Ago2, and Dicer, was induced. In short, several genes encoding odorant binding proteins, chemosensory proteins, odor receptors, honey bee venom peptides, and vitellogenin were downregulated in VI and VI+FPF bees.

CONCLUSIONS

Given the importance of these suppressed genes in honey bees' innate immunity, eicosanoid biosynthesis, and olfactory associative function, their inhibition because of the change in the mode of infection with BQCV and SBV to vector-mediated transmission (injection into haemocoel) could explain the high virulence observed in these viruses when they were experimentally injected into hosts. These changes may help explain why other viruses, such as DWV, represent such a threat to colony survival when transmitted by varroa mites.

Viral Co-Infections and Antiviral Immunity in Honey Bees

Over the past few decades, honey bees have been facing an increasing number of stressors. Beyond individual stress factors, the synergies between them have been identified as a key factor in the observed increase in colony mortality. However, these interactions are numerous and complex and call for further research. Here, in line with our need for a systemic understanding of the threats that they pose to bee health, we review the interactions between honey bee viruses. As viruses are obligate parasites, the interactions between them not only depend on the viruses themselves but also on the immune responses of honey bees. Thus, we first summarise our current knowledge of the antiviral immunity of honey bees. We then review the interactions between specific pathogenic viruses and their interactions with their host. Finally, we draw hypotheses from the current literature and suggest directions for future research.

Parasite and Pesticide Impacts on the Bumblebee (Bombus terrestris) Haemolymph Proteome

Pesticides pose a potential threat to bee health, especially in combination with other stressors, such as parasites. However, pesticide risk assessment tests pesticides in isolation from other stresses, i.e., on otherwise healthy bees. Through molecular analysis, the specific impacts of a pesticide or its interaction with another stressor can be elucidated. Molecular mass profiling by MALDI BeeTyping® was used on bee haemolymph to explore the signature of pesticidal and parasitic stressor impacts. This approach was complemented by bottom-up proteomics to investigate the modulation of the haemoproteome. We tested acute oral doses of three pesticides—glyphosate, Amistar and sulfoxaflor—on the bumblebee Bombus terrestris, alongside the gut parasite Crithidia bombi. We found no impact of any pesticide on parasite intensity and no impact of sulfoxaflor or glyphosate on survival or weight change. Amistar caused weight loss and 19–41% mortality. Haemoproteome analysis showed various protein dysregulations. The major pathways dysregulated were those involved in insect defences and immune responses, with Amistar having the strongest impact on these dysregulated pathways. Our results show that even when no response can be seen at a whole organism level, MALDI BeeTyping® can detect effects. Mass spectrometry analysis of bee haemolymph provides a pertinent tool to evaluate stressor impacts on bee health, even at the level of individuals.

Honeybee (Apis mellifera) resistance to deltamethrin exposure by Modulating the gut microbiota and improving immunity

Honeybees (Apis mellifera) are important economic insects and play important roles in pollination and maintenance of ecological balance. However, the use of pesticides has posed a substantial threat to bees in recent years, with the more widely used deltamethrin being the most harmful. In this study, we found that deltamethrin exposure significantly reduced bee survival in a dose-dependent manner (p = 0.025). In addition, metagenomic sequencing further revealed that DM exposure significantly reduced the diversity of the bee gut microbiota (Chao1, p < 0.0001; Shannon, p < 0.0001; Simpson, p < 0.0001) and decreased the relative abundance of core species of the gut microbiota. Importantly, in studies of GF-bees, we found that the colonization of important gut bacteria such as Gilliamella apicola and Lactobacillus kunkeei significantly increased bee resistance to DM (survival rate increased from 16.7 to 66.7%). Interestingly, we found that the immunity-genes Defensin-2 and Toll were significantly upregulated in bees after the colonization of gut bacteria. These results suggest that gut bacteria may protect against DM stress by improving host immunity. Our findings provide an important rationale for protecting honeybees from pollutants from the perspective of gut microbes.

Stably transmitted defined microbial community in honeybees preserves Hafnia alvei inhibition by regulating the immune system

The gut microbiota of honeybees is highly diverse at the strain level and essential to the proper function and development of the host. Interactions between the host and its gut microbiota, such as specific microbes regulating the innate immune system, protect the host against pathogen infections. However, little is known about the capacity of these strains deposited in one colony to inhibit pathogens. In this study, we assembled a defined microbial community based on phylogeny analysis, the 'Core-20' community, consisting of 20 strains isolated from the honeybee intestine. The Core-20 community could trigger the upregulation of immune gene expressions and reduce Hafnia alvei prevalence, indicating immune priming underlies the microbial protective effect. Functions related to carbohydrate utilization and the phosphoenolpyruvate-dependent sugar phosphotransferase system (PTS systems) are represented in genomic analysis of the defined community, which might be involved in manipulating immune responses. Additionally, we found that the defined Core-20 community is able to colonize the honeybee gut stably through passages. In conclusion, our findings highlight that the synthetic gut microbiota could offer protection by regulating the host immune system, suggesting that the strain collection can yield insights into host-microbiota interactions and provide solutions to protect honeybees from pathogen infections.

Current Knowledge on Bee Innate Immunity Based on Genomics and Transcriptomics

As important pollinators, bees play a critical role in maintaining the balance of the ecosystem and improving the yield and quality of crops. However, in recent years, the bee population has significantly declined due to various pathogens and environmental stressors including viruses, bacteria, parasites, and increased pesticide application. The above threats trigger or suppress the innate immunity of bees, their only immune defense system, which is essential to maintaining individual health and that of the colony. In addition, bees can be divided into solitary and eusocial bees based on their life traits, and eusocial bees possess special social immunities, such as grooming behavior, which cooperate with innate immunity to maintain the health of the colony. The omics approach gives us an opportunity to recognize the distinctive innate immunity of bees. In this regard, we summarize innate bee immunity from a genomic and transcriptomic perspective. The genetic characteristics of innate immunity were revealed by the multiple genomes of bees with different kinds of sociality, including honeybees, bumblebees, wasps, leaf-cutter bees, and so on. Further substantial transcriptomic data of different tissues from diverse bees directly present the activation or suppression of immune genes under the infestation of pathogens or toxicity of pesticides.

Sacbrood Virus: A Growing Threat to Honeybees and Wild Pollinators

Sacbrood virus (SBV) is one of the many viruses that infect both the Western honeybee (Apis mellifera) and the Eastern honeybee (Apis cerana). Recently, the interspecies transmission of SBV has been discovered, especially among wild pollinators. This newly discovered evolutionary occurrence regarding SBV indicates a much wider host range than previously believed, causing further concern about the future sustainability of agriculture and the resilience of ecosystems. Over the past few decades, vast numbers of studies have been undertaken concerning SBV infection in honeybees, and remarkable progress has been made in our understanding of the epidemiology, pathogenesis, transmission, and manifestations of SBV infection in honeybees and other pollinators. Meanwhile, some methods, including Chinese medicine, have been established to control and prevent sacbrood disease in A. cerana in Asian countries. In this review, we summarize the existing knowledge of SBV and address the gaps in the knowledge within the existing literature in the hope of providing future directions for the research and development of management strategies for controlling the spread of this deadly disease.

Adenosine and L-proline can possibly hinder Chinese Sacbrood virus infection in honey bees via immune modulation

Sacbrood virus (SBV) infects larvae of honey bees, resulting in infected larvae becoming fluid-filled sacs. Our previous studies showed that the extract of herbal medicine, Radix Isatidis, could inhibit Chinese SBV (CSBV) infection in Asian honey bees (Apis cerana). Here, two compounds, adenosine and L-proline, which were previously reported to be associated with immune modulation, were identified in R. Isatidis extract and then selected for an evaluation of their antiviral effect on CSBV infection in A. cerana. Our results revealed that both adenosine and L-proline could significantly mitigate the impact of CSBV on the growth and development of infected larvae and modulate hosts' immune responses by downregulating the expression of immune genes in infected larvae. The results gained from this study suggest that adenosine and L-proline could possibly interfere CSBV infection via immune modulation to avoid exacerbations and nonspecific damage to infected larvae's own tissues.

Specific Strains of Honeybee Gut Lactobacillus Stimulate Host Immune System to Protect against Pathogenic Hafnia alvei

Honeybee gut microbiota plays an important role in host physiology and metabolism. Recent studies have shown that the influence of the resident microorganisms in the regulation of honeybee immune system is profound, which protects against the pathogen Serratia marcescens. However, only few of the core gut members in the regulation of immune functions have been studied. Here, we explored how different bee gut bacterial species aided in the clearance of the pathogenic Hafnia alvei, which causes bee septicemia with a high mortality rate. We found that both Gilliamella apicola W8136 and Lactobacillus apis W8172 protect honeybees from the opportunistic pathogen, while two other strains from Gilliamella and Lactobacillus did not affect the invasion of H. alvei. Transcriptomic analysis revealed that gut species induced different expression profiles in the gut. Specifically, two regulator genes from the Toll pathway, PGRP-S3 recognizing Gram-positive and Spätzle that bind to the Toll protein for the downstream signal transduction, were elevated by L. apis. Correspondingly, multiple genes encoding antibacterial proteins were also stimulated by L. apis. Interestingly, we found an increased expression of apidaecin, which also exhibited a high in vitro inhibitory effect on H. alvei. To elucidate the difference of strains in the host’s immune regulation, comparative genomic analyses indicate that the S-layer proteins unique to L. apis are potentially involved in honeybee Toll signaling and the activation of antibacterial protein production.

IMPORTANCE Honeybees are essential pollinators supporting global agricultural economies and food supplies. Recent honeybee decline has been linked to several factors, while pathogen infection is considered one of the most significant contributing factors. Although a limited number of bacterial pathogens have been identified, Hafnia alvei is one of the pathogens causing septicemia in adult bees. In this study, we showed that two bee gut members, Gilliamella and Lactobacillus, can clear H. alvei from invasion. Mono-colonization of specific strains can stimulate the host Toll signaling pathway and the downstream expression of AMPs. Specifically, apidaecin upregulated by the gut symbionts is more effective against the pathogen. Moreover, our genomic analysis suggests that the surface-layer proteins specific to Lactobacillus strains are an important driver of Toll signaling, highlighting the variation of bee gut strains in regulating the host immune system.