Hemocyte-mediated phagocytosis and melanization in the mosquito Armigeres subalbatus following immune challenge by bacteria

Abstract

Mosquitoes are important vectors of disease. These insects respond to invading organisms with strong cellular and humoral immune responses that share many similarities with vertebrate immune systems. The strength and specificity of these responses are directly correlated to a mosquito's ability to transmit disease. In the current study, we characterized the hemocytes (blood cells) of Armigeres subalbatus by morphology (ultrastructure), lectin binding, enzyme activity, immunocytochemistry, and function. We found four hemocyte types: granulocytes, oenocytoids, adipohemocytes, and thrombocytoids. Granulocytes contained acid phosphatase activity and bound the exogenous lectins Helix pomatia agglutinin, Galanthus nivalis lectin, and wheat germ agglutinin. Following bacteria inoculation, granulocytes mounted a strong phagocytic response as early as 5 min postexposure. Bacteria also elicited a hemocyte-mediated melanization response. Phenoloxidase, the rate-limiting enzyme in the melanization pathway, was present exclusively in oenocytoids and in many of the melanotic capsules enveloping bacteria. The immune responses mounted against different bacteria were not identical; gram(-) Escherichia coli were predominantly phagocytosed and gram(+) Micrococcus luteus were melanized. These studies implicate hemocytes as the primary line of defense against bacteria.

Phenoloxidase Is an Important Component of the Defense against Aeromonas hydrophila Infection in a Crustacean, Pacifastacus leniusculus

“…In mosquitoes, several transcripts encoding enzymes involved in melanization are induced by a Plasmodium parasite in the midgut of Anopheles gambiae (11), and the resulting induced melanization could kill the ookinetes directly in susceptible mosquitoes and dispose of dead parasites in refractory mosquitoes (12). Phenoloxidase has also been demonstrated to interfere with microbial infection in several insect species (13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23). In some cases melanization, or rather the activity of the enzyme PO, was found to have no or little impact on the killing/ clearance of parasites (24,25) and the melanization reaction was found not to be required for survival of mosquitoes/flies after certain microbial infections (26,27).…”

Description of the Transcriptomes of Immune Response-Activated Hemocytes from the Mosquito Vectors Aedes aegypti and Armigeres subalbatus

“…But because mosquito hemocytes are difficult to collect from mosquitoes and cannot be cultured or even maintained for significant amounts of time in vitro, they have received little research attention. Hemocytes phagocytose and melanize large numbers of bacteria, fungi, and malaria parasites (15,24,25,28,29). Several subpopulations of mosquito hemocytes have been characterized, and immunocytochemical and cytochemical assays have demonstrated that these cells are differentially responsive to invading pathogens (24,27).…”

“…Several subpopulations of mosquito hemocytes have been characterized, and immunocytochemical and cytochemical assays have demonstrated that these cells are differentially responsive to invading pathogens (24,27). For example, cell types are distinguished by ultrastructural characteristics, phagocytic capacity, or the presence and activity of enzymes involved in the melanin biosynthetic pathway (24,28,29,36). It is hypothesized that hemocytes also participate in pattern recognition and mediate the production of immune peptides from the fat body (6).…”

“…These species of bacteria have been used extensively to study insect innate immunity, particularly immune peptide production (35,41). It has been shown that each bacterium elicits a distinct effector arm of the mosquito innate immune response; following inoculation of either Aedes aegypti or Armigeres subalbatus, the primary response to E. coli is phagocytosis, and that to M. luteus is melanization (28,29). Most recently, it was shown that melanization and/or phagocytosis is elicited by a variety of bacterial species, independent of the Gram type (30).…”

Innate immune response in insects: recognition of bacterial peptidoglycan and amplification of its recognition signal

“…Innate immunity in invertebrates consists of humoral and cellular components (3,4). The humoral component includes antimicrobial peptides (5), lectins (6), and the prophenoloxidase (proPO) activation cascade (7); the cellular component includes phagocytosis by circulating hemocytes (8). In Drosophila, the Toll signalling pathway is responsible for defense against Gram-positive bacteria and fungi, while the immune deficiency (Imd) pathway is activated primarily to defend against Gram-negative bacteria.…”