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Although Toll-like receptors have been widely identified and functionally characterized in mammalian models and Drosophila, the immunological function of these receptors in other insects remains unclear. Here, we explored the relevant innate immune response of Tenebrio molitor (T. molitor) Toll-3 against Gram-negative bacteria, Gram-positive bacteria, and fungal infections. Our findings indicated that TmToll-3 expression was mainly induced by Candida albicans infections in the fat bodies, gut, Malpighian tubules, and hemolymph of young T. molitor larvae. Surprisingly, Escherichia coli systemic infection caused mortality after TmToll-3 knockdown via RNA interference (RNAi) injection, which was not observed in the control group. Further analyses indicated that in the absence of TmToll-3, the final effector of the Toll signaling pathway, antimicrobial peptide (AMP) genes and relevant transcription factors were significantly downregulated after E. coli challenge. Our results indicated that the expression of almost all AMP genes was suppressed in silenced individuals, whereas the expression of relevant genes was positively regulated after fungal injection. Therefore, this study revealed the immunological involvement of TmToll-3 in T. molitor in response to systematic infections.

Antimicrobial peptides (AMPs) are considered to be candidate effectors for eliciting humoral immune responses against infectious pathogens in the host. Cecropins are α‐helical peptides of 30–40 amino acids, which are known to permeabilize bacterial membranes, and play authoritative roles in the innate immune system of insects. In the present study, we identified the full‐length open reading frame (ORF) encoding the Tenebrio molitor cecropin‐2 (TmCec2) gene using the Tribolium castaneum cecropin‐2 (TcCec2) gene to query a T. molitor Rnaseq database. Phylogenetic analysis revealed close identity of TmCec2 with TcCec2. TmCec2 was ubiquitously expressed in the insect during all developmental stages, with the highest expression observed in the adult. Tissue‐specific TmCec2 expression was highest in larval hemocytes and in the adult integument and hemocytes. Microbial challenge experiments revealed that TmCec2 was highly induced in response to gram‐positive and gram‐negative bacteria, and fungi. These data provide credible evidence for a putative role of TmCec2 in insect innate immunity against a plethora of pathogens.

Antimicrobial peptides (AMPs) are effector candidates that elicit humoral immunity in invertebrates. AMPs facilitate bacterial clearance by either physically disrupting the microbial membranes or the intracellular targets. In the Coleopteran pest, Tenebrio molitor, transcriptional regulation of AMPs has been studied in the context of innate immune signaling cascades and antimicrobial immunity. Here, we report the transcriptional response of three AMP genes, Coleoptericin A, B, and C (TmCole A, B and C) in T. molitor in response to bacterial (Escherichia coli, Staphylococcus aureus), and fungal (Candida albicans) challenges. These genes were expressed essentially in the gut and hemocytes followed by the integument tissue of the T. molitor larva. Further, these genes were highly expressed in the late‐larval, pupal, and early adult stages. Furthermore, while all of these transcripts were highly upregulated in the fat body and Malpighian tubules after bacterial challenge, TmCole B and TmCole C were induced in the gut after E. coli challenge. Fungal stimulation was not required for the upregulation of the transcription of Coleoptericin genes in T. molitor.

Defensins are a major family of antimicrobial peptides that serve as the innate immune defense of both vertebrates and invertebrates. Due to their antimicrobial, chemotactic, and regulatory activities, Defensins have been exploited for their therapeutic potential. Insect Defensins are cysteine‐rich and contain an N‐terminal loop, α‐helix, and antiparallel β‐sheet, forming a “cysteine‐stabilized alpha beta (CSαβ)” or “loop–helix‐sheet” structure. In this study, we identified the full‐length open reading frame (ORF) sequences of Defensin (TmDef) and Defensin‐like (TmDef‐like) genes from the mealworm beetle Tenebrio molitor using in silico methods. TmDef and TmDef‐like genes encode the peptides of 72 and 71 amino acid residues, respectively. TmDefensin is comprised of a Defensin domain and the TmDefensin‐like is comprised of a signal peptide of 21 amino acid residues. Phylogenetic analysis revealed close similarities of TmDefensin with the Defensin of Acalolepta luxuriosa of the longhorn beetle family. The expression of TmDef mRNA was found to be greater than that of TmDef‐like mRNA and was mostly expressed in the pupal and adult stages. Tissue distribution showed high expression of TmDef‐like mRNA in larval hemocytes, gut, integument, and fat body, while in adults, the expression was high in gut and hemocytes. Following bacterial and fungal stimulation in vivo, TmDef was upregulated at 24 h post‐infection in whole body, fat body, and hemocytes of the larvae. Even TmDef‐like mRNA was upregulated in the gut and hemocytes at 12 and 9 h post‐infection respectively. These results suggest that TmDef and TmDef‐like genes play important roles in protecting T. molitor from microbial contact.

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.

Vertebrates rely on the most sophisticated adaptive immunity to defend themselves against various pathogens. This includes immunologic memory cells, which mount a stronger and more effective immune response against an antigen after its first encounter. Unlike vertebrates, invertebrates' defense completely depends on the innate immunity mechanisms including humoral and cell-mediated immunity. Furthermore, the invertebrate equivalent of the memory cells was discovered only recently. Since the discovery of transgenerational immune priming (TGIP) in crustaceans, numerous findings have proven the IP in invertebrate classes such as insects. TGIP can be induced through maternal priming pathways such as transcriptional regulation of antimicrobial peptides, and also paternal IP including the induction of proPO system activity. We appraise the diversity and specificity of IP agents to provide sustained immunologic memory in insects, particularly T. molitor in the review. An understanding of IP (more so TGIP) response in T. molitor will deepen our knowledge of invertebrate immunity, and boost the mass-rearing industry by reducing pathogen infection rates.