The humoral immune response of the lepidopteran model insect, silkworm Bombyx mori L., to microbial pathogens

Abstract

Insects are valuable models for studying innate immunity and its role in combating infections. The silkworm Bombyx mori L., a well-studied insect model, is susceptible to a range of pathogens, including bacteria, fungi, viruses, and microsporidia. Their susceptibility makes it a suitable model for investigating host-pathogen interactions and immune responses against infections and diseases. This review focuses on the humoral immune response and the production of antimicrobial peptides (AMPs), the phenoloxidase (PO) system, and other soluble factors that constitute the primary defense of silkworms against microbial pathogens. The innate immune system of silkworms relies on pattern recognition receptors (PRRs) to recognize pathogen-associated molecular patterns (PAMPs), which then activate various immune pathways including Imd, Toll, JAK/STAT, and RNA interference (RNAi). Their activation triggers the secretion of AMPs, enzymatic defenses (lysozyme and PO), and the generation of reactive oxygen species (ROS). Collectively, these pathways work together to neutralize and eliminate pathogens, thereby contributing to the defense mechanism of silkworms. Understanding the innate immunity of silkworms can uncover conserved molecular pathways and key immune components shared between insects and vertebrates. Additionally, it can provide valuable insights for improving sericulture practices, developing strategies to control diseases affecting silk production, and providing a theoretical foundation for developing pest control measures.

Keywords: Antimicrobial peptides; Humoral immunity; Microbial pathogens; Phenoloxidase system; Signaling pathways; Silkworm.

Fig. 1

Schematic representation of the modulation of the silkworm immune system upon pathogen challenge. The immune response of silkworms to pathogen challenge involves the recognition of pathogen-associated molecular patterns (PAMPs) by pattern recognition receptors (PRRs), leading to the activation of innate immunity (both cellular and humoral) in silkworms. The humoral innate immune system in silkworms consists of the Imd, Toll, and JAK/STAT pathways, which are responsible for various defense mechanisms against pathogens. These mechanisms include the secretion of antimicrobial peptides (AMPs), production of reactive oxygen species (ROS), activation of phenoloxidase (PO), melanization, and activation of RNA interference (RNAi) pathways targeting different pathogens. The pathway components involved in these processes include serine proteases (SPs), ROS, peptidoglycan (PGN), lipopolysaccharide (LPS), phenoloxidase-activating enzyme (PPAE), and prophenoloxidase (PPO).

Fig. 2

Schematic overview of the (A) Imd pathway and (B) Toll pathway of silkworms in response to microbial pathogens including bacteria, fungi, microsporidia, and viruses. These pathways are initiated upon recognition of microbial components by pathogen recognition receptors (PRRs). This recognition activates the downstream components and the NF-κB transcription factor that eventually trigger the expression of antimicrobial peptides (AMPs) and other effector molecules. See text for further details.

Fig. 3

Schematic overview of the (A) JAK/STAT pathway and (B) Phenoloxidase (PO) pathway of silkworms in response to microbial pathogens. Recognition of pathogen-associated molecular pattern (PAMPs) by pathogen recognition receptors (PRRs) triggers the signaling cascades that activate these pathways. JAK/STAT pathway regulates the transcription of antimicrobial peptides (AMPs), while thew PO pathway triggers melanization, and together they play a crucial role in silkworms’ defense against invading microorganisms.