Current Status of Immune Deficiency Pathway in Tenebrio molitor Innate Immunity

Involvement of bacteria in the development of fungal infections in the Colorado potato beetle

Entomopathogenic fungi may interact with insects' symbiotic bacteria during infection. We hypothesized that topical infection with Beauveria bassiana may alter the microbiota of the Colorado potato beetle (CPB) and that these modifications may alter the course of mycoses. We used a model with two concentrations of conidia: (1) high concentration that causes rapid (acute) pathogenesis with fast mortality followed by bacterial decomposition of insects; (2) lower concentration that leads to prolonged pathogenesis ending in conidiation on cadavers. The fungal infections increased loads of enterobacteria and bacilli on the cuticle surface and in hemolymph and midgut, and the greatest increase was detected during the acute mycosis. By contrast, stronger activation of IMD and JAK-STAT signaling pathways in integuments and fat body was observed during the prolonged mycosis. Relatively stable (nonpathogenic) conditions remained in the midgut during both scenarios of mycosis with slight changes in bacterial communities, the absence of mesh and stat expression, a decrease in reactive oxygen species production, and slight induction of Toll and IMD pathways. Oral administration of antibiotic and predominant CPB bacteria (Enterobacteriaceae, Lactococcus, Pseudomonas) led to minor and mainly antagonistic effects in survival of larvae infected with B. bassiana. We believe that prolonged mycosis is necessary for successful development of the fungus because such pathogenesis allows the host to activate antibacterial reactions. Conversely, after infection with high concentrations of the fungus, the host's resources are insufficient to fully activate antibacterial defenses, and this situation makes successful development of the fungus impossible.

Tenebrio molitor (Coleoptera: Tenebrionidae) as an alternative host for the study of pathogenicity in Candida auris

BACKGROUND

Candida auris, a multidrug-resistant fungal pathogen, has emerged as a significant global health threat due to its high transmission and mortality rates, especially in healthcare settings.

OBJECTIVE

This study aimed to establish the larvae of the coleopteran Tenebrio molitor (mealworm) as an in vivo model to evaluate the virulence of different C. auris strains.

METHODS

T. molitor larvae were inoculated with varying doses and strains of C. auris. Mortality rates were monitored, melanization responses, and phenoloxidase activity were assessed. Histopathological analyses were conducted to observe tissue invasion by the yeast cells. Additionally, a biofilm formation test was included as a complementary analysis to determine if biofilm production would influence the virulence of the C. auris strains.

RESULTS

A dose-dependent increase in mortality was observed, with the highest fungal load leading to the highest mortality rates. The study also revealed significant differences in virulence among the strains, with those from Kuwait and the reference strain CBS 10913 showing the highest pathogenicity. Melanization rates were significantly higher in infected larvae, indicating an active immune response. The histopathological analysis revealed the presence of C. auris cells within the tissue of T. molitor larvae. However, the biofilm formation complementary test did not show a significant difference in virulence among the different clades of C. auris.

CONCLUSION

The T. molitor model effectively demonstrated the pathogenic potential and virulence differences of C. auris strains. Strains from Kuwait and the reference strain CBS 10913 exhibited the highest virulence, causing 100 % mortality within 24 h. The model also highlighted significant biofilm formation and melanization responses, correlating with fungal burden. This insect model provides a valuable and cost-effective tool for preliminary virulence screening of clinical yeast strains, offering insights into host-pathogen interactions and the potential for evaluating antifungal treatments in vivo.

Influence of the agrochemical benomyl on Cryptococcus gattii-plant interaction in vitro and in vivo

Cryptococcus gattii, an environmental fungus, is one of the agents of cryptococcosis. The influence of agrochemicals on fungal resistance to antifungals is widely discussed. However, the effects of benomyl (BEN) on fungal interaction with different hosts is still to be understood. Here we studied the influence of adaptation to BEN in the interaction with a plant model, phagocytes and with Tenebrio molitor. First, the strain C. gattii L24/01 non-adapted (NA), adapted (A) to BEN, and adapted with further culture on drug-free media (10p) interact with Nicotiana benthamiana, with a peak in the yeast burden on the 7th day post-inoculation. C. gattii L24/01 A and 10p provided lower fungal burden, but these strains increased cell diameter and capsular thickness after the interaction, together with decreased fungal growth. The strains NA and A showed reduced ergosterol levels, while 10p exhibited increased activity of laccase and urease. L24/01 A recovered from N. benthamiana was less engulfed by murine macrophages, with lower production of reactive oxygen species. This phenotype was accompanied by increased ability of this strain to grow inside macrophages. Otherwise, L24/01 A showed reduced virulence in the T. molitor larvae model. Here, we demonstrate that the exposure to BEN, and interaction with plants interfere in the morphophysiology and virulence of the C. gattii.

Invertebrate Immunity, Natural Transplantation Immunity, Somatic and Germ Cell Parasitism, and Transposon Defense

While the vertebrate immune system consists of innate and adaptive branches, invertebrates only have innate immunity. This feature makes them an ideal model system for studying the cellular and molecular mechanisms of innate immunity sensu stricto without reciprocal interferences from adaptive immunity. Although invertebrate immunity is evolutionarily older and a precursor of vertebrate immunity, it is far from simple. Despite lacking lymphocytes and functional immunoglobulin, the invertebrate immune system has many sophisticated mechanisms and features, such as long-term immune memory, which, for decades, have been exclusively attributed to adaptive immunity. In this review, we describe the cellular and molecular aspects of invertebrate immunity, including the epigenetic foundation of innate memory, the transgenerational inheritance of immunity, genetic immunity against invading transposons, the mechanisms of self-recognition, natural transplantation, and germ/somatic cell parasitism.

Recent trends in insect gut immunity

The gut is a crucial organ in insect defense against various pathogens and harmful substances in their environment and diet. Distinct insect gut compartments possess unique functionalities contributing to their physiological processes, including immunity. The insect gut's cellular composition is vital for cellular and humoral immunity. The peritrophic membrane, mucus layer, lumen, microvilli, and various gut cells provide essential support for activating and regulating immune defense mechanisms. These components also secrete molecules and enzymes that are imperative in physiological activities. Additionally, the gut microbiota initiates various signaling pathways and produces vitamins and minerals that help maintain gut homeostasis. Distinct immune signaling pathways are activated within the gut when insects ingest pathogens or hazardous materials. The pathway induced depends on the infection or pathogen type; include immune deficiency (imd), Toll, JAK/STAT, Duox-ROS, and JNK/FOXO regulatory pathways. These pathways produce different antimicrobial peptides (AMPs) and maintain gut homeostasis. Furthermore, various signaling mechanisms within gut cells regulate insect gut recovery following infection. Although some questions regarding insect gut immunity in different species require additional study, this review provides insights into the insect gut's structure and composition, commensal microorganism roles in Drosophila melanogaster and Tenebrio molitor life cycles, different signaling pathways involved in gut immune systems, and the insect gut post-infection recovery through various signaling mechanisms.

Effects of TmTak1 silencing on AMP production as an Imd pathway component in Tenebrio molitor

Mealworms beetles, Tenebrio molitor, are the limelight next-generation food for humans due to their high nutrient contents. Since Tenebrio molitor is used as feed for pets and livestock in addition to their ability to decompose polystyrene and plastic waste, it is recognized as an insect with an industrial core value. Therefore, it is important to study the immune mechanism related to the development and infection of mealworms for mass breeding purposes. The immune deficiency (Imd) signaling is one of the main pathways with pivotal roles in the production of antimicrobial peptides (AMPs). Transforming growth factor-β activated kinase (TAK1) is one of the Imd pathway components, forms a complex with TAK1 binding protein 2 (TAB2) to ultimately help activate the transcription factor Relish and eventually induce host to produce AMPs. Relatively, little has been revealed about TAK1 in insect models, especially in the T. molitor. Therefore, this study was conducted to elucidate the function of TmTak1 in T. molitor. Our results showed that the highest and lowest mRNA expression of TmTak1 were found in egg and young larvae respectively. The tissue-specific expression patterns were reported in the gut of T. molitor larvae and the fat bodies of adults. Systemic microbial challenge illustrated TmTak1 high expression following the fungal infection in all dissected tissues except for the whole body. However, silencing TmTak1 experiments showed that the survivability of T. molitor larvae affected significantly following Escherichia coli infection. Accordingly, AMP induction after TmTak1 knock down was mainly reported in the integument and the fat bodies.

IKKβ regulates antimicrobial innate immune responses in the yellow mealworm, Tenebrio molitor

Toll and IMD pathways regulate antimicrobial innate immune responses in insect model systems. The transcriptional activation of antimicrobial peptides (AMPs) confers humoral immunity in the host against invaded pathogens. The IKK kinase complex (IKKα, IKKβ, and the regulatory subunit IKKγ/NEMO) centrally regulates the NF-κB response to various stimuli. It triggers an appropriate antimicrobial immune response in the host. In this study, a TmIKKβ (or TmIrd5) homolog was screened from the RNA-seq database of the coleopteran beetle, Tenebrio molitor. A single exon characterizes the TmIKKβ gene, and the open reading frame (ORF) comprises of 2112 bp that putatively encodes a polypeptide of 703 amino acid residues. TmIKKβ contains a serine/threonine kinase domain and is phylogenetically close to Tribolium castaneum IKKβ homolog (TcIKKβ). TmIKKβ transcripts were highly expressed in the early pupal (P1) and adult (A5) stages. Among the tissues, TmIKKβ showed higher expression in the integument of the last instar larvae and the fat body and hemocytes of 5-day-old adults. TmIKKβ mRNA was upregulated post-E. coli challenge to the host. Moreover, RNAi-based TmIKKβ mRNA silencing increased host larvae' susceptibility against E. coli, S. aureus and C. albicans. TmIKKβ RNAi in the fat body led to a downregulation in mRNA expression of ten out of fourteen AMP genes, including TmTenecin1, -2, and -4; TmDefensin, and -like; TmColeoptericinA, and -B; and TmAttacin1a, -1b, and -2, suggesting the requirement of the gene in antimicrobial innate immune responses. Further, a decrease in the mRNA expression of NF-κB factors such as TmRelish, TmDorsal1, and TmDorsal2 in the fat body of T. molitor larvae was observed post-microorganisms challenge. Thus, TmIKKβ regulates antimicrobial innate immune responses in T. molitor.

Immunological Roles of TmToll-2 in Response to Escherichia coli Systemic Infection in Tenebrio molitor

The antimicrobial roles of Toll-like receptors have been mainly identified in mammalian models and Drosophila. However, its immunological function in other insects has yet to be fully clarified. Here, we determined the innate immune response involvement of TmToll-2 encountering Gram-negative, Gram-positive, and fungal infection. Our data revealed that TmToll-2 expression could be induced by Escherichia coli, Staphylococcus aureus, and Candida albicans infections in the fat bodies, gut, Malpighian tubules, and hemolymph of Tenebrio molitor young larvae. However, TmToll-2 silencing via RNAi technology revealed that sole E. coli systemic infection caused mortality in the double-strand RNA TmToll-2-injected group compared with that in the control group. Further investigation indicated that in the absence of TmToll-2, the final effector of Toll signaling pathway, antimicrobial peptide (AMP) genes and relevant transcription factors were significantly downregulated, mainly E. coli post-insult. We showed that the expression of all AMP genes was suppressed in the main immune organ of insects, namely, fat bodies, in silenced individuals, while the relevant expressions were not affected after fungal infection. Thus, our research revealed the immunological roles of TmToll-2 in different organs of T. molitor in response to pathogenic insults.