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

Abstract

The host defence of insects includes a combination of cellular and humoral responses. The cellular arm of the insect innate immune system includes mechanisms that are directly mediated by haemocytes (e.g., phagocytosis, nodulation and encapsulation). In addition, melanization accompanying coagulation, clot formation and wound healing, nodulation and encapsulation processes leads to the formation of cytotoxic redox‐cycling melanin precursors and reactive oxygen and nitrogen species. However, demarcation between cellular and humoral immune reactions as two distinct categories is not straightforward. This is because many humoral factors affect haemocyte functions and haemocytes themselves are an important source of many humoral molecules. There is also a considerable overlap between cellular and humoral immune functions that span from recognition of foreign intruders to clot formation. Here, we review these immune reactions starting with the cellular mechanisms that limit haemolymph loss and participate in wound healing and clot formation and advancing to cellular functions that are critical in restricting pathogen movement and replication. This information is important because it highlights that insect cellular immunity is controlled by a multilayered system, different components of which are activated by different pathogens or during the different stages of the infection.

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

“…The data reported herein highlight that the two branches of the immune system have different activation dynamics in this insect as phagocytosis and encapsulation are rapidly triggered once the larva is exposed to the foreign antigen, while the humoral components intervene later. This evidence - although important because it fills a gap in our knowledge of BSF larvae - is common among insects, that are able to activate different defense mechanisms against infections via different speeds, specificities, and routes ( 17 , 97 ). The possible variation in numbers and populations of circulating hemocytes, and the rate of their recruitment to overcome the intruders likely affect the extent and time course of phagocytosis and encapsulation among different insects and within the same species depending on the foreign invader ( 17 ).…”

“…This evidence - although important because it fills a gap in our knowledge of BSF larvae - is common among insects, that are able to activate different defense mechanisms against infections via different speeds, specificities, and routes ( 17 , 97 ). The possible variation in numbers and populations of circulating hemocytes, and the rate of their recruitment to overcome the intruders likely affect the extent and time course of phagocytosis and encapsulation among different insects and within the same species depending on the foreign invader ( 17 ). Similarly, the variation in the evoking potency of the bacterial cell wall PAMPs, e.g., peptidoglycan or lipopolysaccharides of Gram-positive and Gram-negative bacteria, respectively, and the efficiency of the subsequent response mechanisms may contribute, in part, to the speed and specificity of immune phenomena in different challenged insects, e.g., Orthoptera ( 98 ), Lepidoptera ( 99 ), Hymenoptera ( 100 ), Coleoptera ( 101 ) and other Diptera ( 102 ).…”

“…Although hemocytes are responsible for producing humoral molecules ( 67 , 68 ), they are mainly involved in phagocytosis, encapsulation, and nodulation ( 17 ). In Diptera, phagocytosis is triggered very rapidly after an immune challenge [from 5 to 15 minutes from the infection depending on the type of bacteria ( 69 – 72 )] and can last up to 24 hours ( 70 , 71 ).…”

“…Insect immunity with its cellular and humoral arms has evolved under substantial selective pressure to protect the host against foreign invaders ( 17 , 97 ). The biological significance of the present study lies in its more detailed appreciation of how the speed and kinetics of immune activation influence the ability of the host (i.e., BSF larvae, whose immune system is poorly studied despite the great interest in this insect for its bioconversion ability) to counteract a specific category of invaders (i.e., Gram-positive and Gram-negative bacteria).…”

“…The immune system of insects possesses a sophisticated set of cellular and humoral innate mechanisms that can be rapidly activated in the presence of infections. 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 ).…”