Pattern Recognition Receptors in Lepidopteran Insects and Their Biological Functions

Introduction to Pattern Recognition Receptors in Lepidoptera

Lepidoptera represent one of the largest groups of herbivorous insects and play essential ecological and economic roles in natural and agricultural ecosystems. Many species contribute to pollination and serve as food sources for predators, while others are considered major agricultural pests that significantly reduce crop productivity. Certain economically valuable species, including the silkworm Bombyx mori and the ghost moth Thitarodes xiaojinensis, are widely used in silk production and medicinal industries. Because of their biological diversity and adaptive immune systems, lepidopteran insects have become important experimental models for studying genetics, physiology, development, host–pathogen interactions, and innate immunity.

The innate immune system of insects relies heavily on Pattern Recognition Receptors (PRRs), which are germline-encoded proteins specialized in detecting invading microorganisms. PRRs recognize conserved microbial structures known as Pathogen-Associated Molecular Patterns (PAMPs) located on bacterial, fungal, viral, or parasitic surfaces. Once activated, PRRs initiate a wide range of immune defenses, including phagocytosis, melanization, encapsulation, nodulation, agglutination, and the synthesis of antimicrobial peptides (AMPs). These mechanisms collectively protect insects against microbial infection and parasitic invasion.

Several major PRR families have been identified in lepidopteran insects, including:

  • Peptidoglycan Recognition Proteins (PGRPs)
  • β-1,3-Glucan Recognition Proteins (βGRPs) or Gram-Negative Binding Proteins (GNBPs)
  • C-Type Lectins (CTLs)
  • Galectins (GALEs)
  • Fibrinogen-Related Proteins (FREPs)
  • Thioester Proteins (TEPs)
  • Scavenger Receptors (SCRs)

Genomic and immunotranscriptomic analyses have identified PRR genes in numerous lepidopteran species such as Bombyx mori, Manduca sexta, Helicoverpa armigera, Plutella xylostella, and Thitarodes xiaojinensis. These discoveries have improved understanding of insect immunity and host defense signaling pathways.

C-Type Lectins (CTLs) in Lepidopteran Immune Responses

C-Type Lectins (CTLs) are one of the most extensively studied PRR families in Lepidoptera. These carbohydrate-binding proteins contain specialized carbohydrate-recognition domains (CRDs) that interact with microbial glycans in a calcium-dependent manner. CTLs are essential regulators of insect immune responses and participate in:

  • Microbial agglutination
  • Hemocyte-mediated phagocytosis
  • Encapsulation
  • Melanization
  • Nodulation
  • Activation of antimicrobial defense pathways

Comparative genomic studies revealed significant diversity in CTL gene numbers among lepidopteran insects. For example, only seven CTL genes were identified in Plutella xylostella, whereas Manduca sexta possesses more than thirty CTL-related genes. Most lepidopteran CTLs contain either a single CRD or dual tandem CRDs. The dual-CRD CTLs, also called immulectins, are highly characteristic of Lepidoptera and are rarely found in other insect orders.

Phylogenetic analyses suggest that some CTLs evolved through strong selective pressure and horizontal gene transfer events, particularly in species of the genus Spodoptera. These evolutionary adaptations likely contributed to enhanced pathogen recognition and immune specialization.

CTLs in the Cotton Bollworm Helicoverpa armigera

The cotton bollworm Helicoverpa armigera is one of the most destructive agricultural pests worldwide. Immunotranscriptomic analyses identified approximately 26 CTL genes in this species, including both single-CRD and dual-CRD lectins.

HaCTL1 and Antimicrobial Defense

HaCTL1 was among the first CTLs characterized in H. armigera. This dual-CRD lectin is constitutively expressed in hemocytes and becomes strongly upregulated following bacterial, fungal, or viral infection. Recombinant HaCTL1 can agglutinate Gram-positive bacteria, Gram-negative bacteria, and fungi in a calcium-dependent manner.

Functional studies demonstrated that HaCTL1 enhances hemocyte phagocytosis and inhibits microbial proliferation, particularly against Bacillus thuringiensis. Different CRDs within HaCTL1 exhibit selective pathogen-binding activities, indicating that structural organization strongly influences immune specificity.

HaCTL3 and Encapsulation Responses

HaCTL3 plays a critical role in encapsulation and melanization during parasitic infection. Expression of HaCTL3 increases significantly after immune challenge with Gram-negative bacteria such as Escherichia coli. Recombinant HaCTL3 recognizes microbial molecules including:

  • Peptidoglycans
  • Lipopolysaccharides
  • Maltose
  • Trehalose

HaCTL3 also binds directly to the entomopathogenic nematode Ovomermis sinensis. This interaction stimulates encapsulation and melanization responses that reduce parasite survival. Further studies revealed that HaCTL3 interacts with β-integrin receptors on hemocytes, suggesting an important role in cellular immune signaling.

HaCTL7 and Melanization Mechanisms

HaCTL7 contains two CRDs with distinct immune activities. Its C-terminal CRD is primarily responsible for microbial agglutination, encapsulation, and melanization. Recombinant HaCTL7 enhances hemocyte aggregation and stimulates melanization in larval hemolymph.

Interestingly, the N-terminal CRD appears more involved in hemocyte recognition and phagocytic interactions, highlighting functional specialization within the same protein.

HaCTL14 and Antifungal Immunity

HaCTL14 is strongly associated with antifungal defense against Beauveria bassiana. The protein contains a mannose-recognition motif and promotes fungal agglutination in a calcium-dependent manner.

RNA interference experiments demonstrated that suppression of HaCTL14 significantly reduces melanization pathway activity by altering the expression of:

  • Serine proteases
  • Serpins
  • Prophenoloxidase-related genes

These findings confirm that HaCTL14 is an important regulator of age-dependent antifungal immunity in H. armigera larvae.

CTLs in Other Lepidopteran Species

CTLs in Spodoptera exigua

The beet armyworm Spodoptera exigua possesses approximately 25 CTL genes. Some of these lectins are highly similar to bracovirus lectins and are believed to have originated through horizontal gene transfer.

Several recombinant CTLs from S. exigua exhibit potent agglutination activity against both Gram-positive and Gram-negative bacteria. Certain CTLs also display antiviral properties by reducing susceptibility to nucleopolyhedroviruses and densoviruses.

These findings indicate that CTLs contribute not only to antibacterial defense but also to antiviral immunity in lepidopteran insects.

CTLs in Mythimna separata

The rice armyworm Mythimna separata produces a dual-CRD lectin known as encapsulation-promoting lectin (MseEPL). This protein regulates cellular immunity according to the size of invading particles.

Experimental studies showed that recombinant MseEPL promotes encapsulation of large particles while suppressing phagocytosis of smaller particles. This demonstrates the remarkable specificity of insect immune recognition systems.

CTLs in Pieris rapae

The cabbage white butterfly Pieris rapae expresses a lectin called PrCTL that participates in antibacterial defense and anti-parasitoid immunity.

Silencing of PrCTL leads to reduced:

  • Phagocytosis
  • Encapsulation
  • Phenoloxidase activity
  • Antimicrobial peptide expression

PrCTL also binds directly to parasitoid wasp eggs, indicating a major role in parasite recognition and immune activation.

Downstream Immune Functions of CTLs

Antimicrobial Immunity

Transcriptomic analyses consistently show that CTL expression changes significantly following bacterial, fungal, viral, and parasitic infections. In several species, CTLs are induced rapidly after pathogen exposure and contribute to antimicrobial peptide synthesis and microbial clearance.

Certain CTLs can either promote or suppress immune responses depending on pathogen type and physiological context.

Cellular Immunity

Cellular immune responses in Lepidoptera mainly involve granulocytes and plasmatocytes. CTLs regulate:

  • Hemocyte adhesion
  • Encapsulation
  • Cytoskeletal remodeling
  • Phagocytic activity

Proteins such as HaCTL3 interact with β-integrins on hemocyte membranes to activate downstream encapsulation mechanisms.

Melanization and PPO Activation

The melanization pathway is a critical insect defense mechanism involving activation of prophenoloxidase (PPO) into active phenoloxidase (PO).

Several CTLs, including HaCTL3, HaCTL7, and HaCTL14, stimulate melanization by regulating:

  • Clip-domain serine proteases
  • Serpins
  • PPO activation cascades

These pathways ultimately lead to melanin production, which helps eliminate pathogens and parasites.

Physiological Roles of CTLs Beyond Immunity

In addition to immunity, CTLs participate in several physiological and developmental processes. Hormones such as 20-hydroxyecdysone (20E) regulate CTL gene expression during:

  • Molting
  • Metamorphosis
  • Diapause
  • Reproduction

Some CTLs are developmentally regulated and show stage-specific expression patterns, suggesting important links between endocrine signaling and immune regulation.

Other Important PRRs in Lepidopteran Insects

Peptidoglycan Recognition Proteins (PGRPs)

PGRPs recognize bacterial peptidoglycans and activate immune signaling pathways such as the Toll and IMD pathways.

These proteins contribute to:

  • Antibacterial defense
  • PPO activation
  • Phagocytosis
  • Encapsulation
  • Regulation of gut microbiota
  • Antiviral immunity

Certain PGRPs also participate in developmental regulation and metabolic homeostasis.

β-1,3-Glucan Recognition Proteins (βGRPs/GNBPs)

βGRPs detect fungal β-1,3-glucans and initiate antifungal immune responses. They stimulate:

  • PPO activation
  • Antimicrobial peptide production
  • Resistance against fungal pathogens

Expression of βGRPs increases significantly following fungal infection in several lepidopteran species.

Galectins (GALEs)

Galectins are β-galactoside-binding proteins involved in immune recognition and developmental regulation. Some lepidopteran galectins can agglutinate bacteria, while others influence insect susceptibility to toxins or pathogens through interactions with midgut glycans and peritrophic membranes.

Future Perspectives in Lepidopteran PRR Research

Lepidopteran insects have evolved highly specialized PRR systems that allow adaptation to diverse microbial and environmental challenges. Understanding PRR-mediated immunity offers promising opportunities for developing sustainable biological pest control strategies.

Future research should focus on:

  • Identifying pathogen surface molecules recognized by PRRs
  • Characterizing PRR interacting proteins
  • Understanding microbiota immune interactions
  • Clarifying signaling pathways downstream of PRR activation
  • Exploring PRR involvement in insect development and physiology

Additional studies on lesser-known PRRs such as FREPs, TEPs, and scavenger receptors may reveal novel mechanisms of insect immunity and host pathogen coevolution.

Overall, PRRs represent essential components of lepidopteran innate immunity and provide valuable molecular targets for agricultural biotechnology, biological pest management, and insect immunology research.