Regulators and signalling in insect antimicrobial innate immunity: Functional molecules and cellular pathways

Abstract

Insects possess an immune system that protects them from attacks by various pathogenic microorganisms that would otherwise threaten their survival. Immune mechanisms may deal directly with the pathogens by eliminating them from the host organism or disarm them by suppressing the synthesis of toxins and virulence factors that promote the invasion and destructive action of the intruder within the host. Insects have been established as outstanding models for studying immune system regulation because innate immunity can be explored as an integrated system at the level of the whole organism. Innate immunity in insects consists of basal immunity that controls the constitutive synthesis of effector molecules such as antimicrobial peptides, and inducible immunity that is activated after detection of a microbe or its product(s). Activation and coordination of innate immune defenses in insects involve evolutionary conserved immune factors. Previous research in insects has led to the identification and characterization of distinct immune signalling pathways that modulate the response to microbial infections. This work has not only advanced the field of insect immunology, but it has also rekindled interest in the innate immune system of mammals. Here we review the current knowledge on key molecular components of insect immunity and discuss the opportunities they present for confronting infectious diseases in humans.

Haemocyte‐mediated immunity in insects: Cells, processes and associated components in the fight against pathogens and parasites

“…Haemocytes express the major classes of AMPs, subgrouped on the basis of their sequence composition and the secondary structures, including the linear amphipathic alpha‐helical AMPs like cecropins [27,28], the defensins [29], and the proline‐ and glycine‐rich AMPs [30,31]. Different antimicrobial proteins are also expressed by haemocytes in response to microbial stimulation, including lysozymes [32], transferrins [33], an array of soluble forms of microbial pattern recognition receptors like C‐type lectins, peptidoglycan recognition proteins (PGRPs), β‐1,3‐glucan recognition proteins (βGRPs) and galectins (GALEs), among others [34,35]. Therefore, aspects of haemocyte biology will firstly be emphasized below, then followed by cell‐mediated immune responses.…”

“…They act synergistically or antagonistically and are often pleiotropic; that is, one single cytokine can act on several different cell types or have multiple effects on growth and differentiation. The cellular and humoral arms of an insect immune system are not separate as often depicted but interconnected [35]. Insect immune responses are regulated by a complex network of signalling pathways mediating haemocyte‐to‐fat body, fat body‐to‐haemocyte, muscle‐to‐haemocyte and gut‐to‐fat body immune interactions that activate local and systemic immune responses at the onset of infection [110,121–123].…”

Insights Into the Immune Response of the Black Soldier Fly Larvae to Bacteria

“…Although the activation pathways are highly conserved in insects, the triggered cellular and humoral responses can differ between orders and species (18). In some Coleoptera, for example, the maximum number of hemocytes involved in the response to bacteria can be observed just a few hours after infection (22,23), while in Anopheles gambiae this requires 24 hours (24).…”

“…The main actors of the cellular response are hemocytes, circulating cells involved in phagocytosis, encapsulation, and nodulation of non-self-antigens ( 17 ). In parallel, the humoral response is triggered by the interaction of Pattern Recognition Receptors (PRRs), located on the surface of insect cells, and Pathogen-associated Molecular Patterns (PAMPs) such as peptidoglycans, lipopolysaccharides, and β-1,3 glucans that are present on the pathogen surface ( 18 ). Humoral responses include the phenoloxidase (PO) system, an enzymatic cascade whose activation culminates with hemolymph clotting and melanin production, antimicrobial peptides (AMPs), lysozyme, and reactive oxygen species ( 19 – 21 ).…”

DataSheet_1.docx

“…The main actors of the cellular response are hemocytes, circulating cells involved in phagocytosis, encapsulation, and nodulation of non-selfantigens (17). In parallel, the humoral response is triggered by the interaction of Pattern Recognition Receptors (PRRs), located on the surface of insect cells, and Pathogen-associated Molecular Patterns (PAMPs) such as peptidoglycans, lipopolysaccharides, and b-1,3 glucans that are present on the pathogen surface (18). Humoral responses include the phenoloxidase (PO) system, an enzymatic cascade whose activation culminates with hemolymph clotting and melanin production, antimicrobial peptides (AMPs), lysozyme, and reactive oxygen species (19)(20)(21).…”