Rapid Phagocytosis and Melanization of Bacteria and Plasmodium Sporozoites by Hemocytes of the Mosquito Aedes Aegypti

Abstract

Mosquitoes are vectors of many deadly and debilitating pathogens. In the current study, we used light and electron microscopies to study the immune response of Aedes aegypti hemocytes to bacterial inoculations, Plasmodium gallinaceum natural infections, and latex bead injections. After challenge, mosquitoes mounted strong phagocytic and melanization responses. Granulocytes phagocytosed bacteria singly or pooled them inside large membrane-delimited vesicles. Phagocytosis of bacteria, Plasmodium sporozoites, and latex beads was extensive; we estimated that individual granulocytes have the capacity to phagocytose hundreds of bacteria and thousands of latex particles. Oenocytoids were also seen to internalize bacteria and latex particles, although infrequently and with low capacity. Besides phagocytosis, mosquitoes cleared bacteria and sporozoites by melanization. Interestingly, the immune response toward 2 species of bacteria was different; most Escherichia coli were phagocytosed, but most Micrococcus luteus were melanized. Similar to E. coli, most Plasmodium sporozoites were phagocytosed. The immune response was rapid; phagocytosis and melanization of bacteria began as early as 5 min after inoculation. The magnitude and speed of the cellular response suggest that hemocytes, acting in concert with the humoral immune response, are the main force driving the battle against foreign invaders.

Synergy and specificity of two Na+–aromatic amino acid symporters in the model alimentary canal of mosquito larvae

“…In addition to their role in housekeeping protein synthesis, insects use large amounts of aromatic substrates during ecdysis (Sugumaran, 2000), cuticle hardening and tanning (Sugumaran and Nelson, 1998;Vincent, 2002), egg chorion formation (Li, 1994); wound healing (Galko and Krasnow, 2004;Shi et al, 2006), immunity (Siva-Jothy andThompson, 2002), melanotic encapsulation of pathogens (Hillyer et al, 2003;Koella and Sorensen, 2002), neurotransmission (Caveney and Donly, 2002;Osborne, 1996) and hormonal signaling (Gade and Goldsworthy, 2003;Kelly et al, 1994). Hence the demand for aromatic substrates is expected to be accompanied by an elevated expression of aromatic NATs.…”

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).…”

Pathogen-insect interaction candidate molecules for transmission-blocking control strategies of vector borne diseases

“…30 Attempts to increase mosquito resistance to Plasmodium by inducing the overexpression of immune molecules have shown variable success. The induction of NF-kB Rel2 transcription factor (IMD pathway) in midgut and fat body of An.…”