Phagocytosis

In vertebrates phagocytosis occurs upon recognition and subsequent internalisation of invading microbes, by migrating phagocytes derived from the myeloid cell line which are represented by the neutrophils and the macrophages. Before phagocytosis can occur, phagocytic cells are recruited to the site of infection by sequential signal transduction events, where phagocytes must recognise the microbe as foreign (Baggiolini & Wymann, 1990). Microbes are then ingested into a phagosome where they encounter many toxic substances and degrading enzymes.

The process of phagocytosis in insects and mammals is very similar. In both cases phagocytosis is preceded by binding of opsonic ligands to the surface of the microbe, followed by recognition by specific receptors. Recognition results in engulfment of the foreign body. Activation of the ProPO cascade is required for granulocytes to bind to non-self matter and internalise it, while calcium is required for adherence of plasmatocytes (Kavanagh & Reeves, 2004). Recognition of non-self and phagocytosis is lectin-mediated in vertebrate phagocytes and in insect phagocytes (Wilson et al., 1999; Chen et al., 1999). Insects produce different lectins for recognising a range of different bacterial cell surfaces (Wilson & Ratcliff, 2000), giving the insect immune response the diversity required to recognise various microbes.

In vertebrate phagocytes, reactive oxygen species (ROS) are produced during phagocytosis and these ROSs and proteolytic enzymes are released into the phago-some from cytoplasmic granules to destroy engulfed microbes. This process has not yet been fully investigated in invertebrate systems; however, there is a lot of evidence to support the existence of similar processes in insect haemocytes. The production of ROS by insect haemocytes has been studied by nitroblue tetrazolium reduction (Glupov et al., 2001) and was observed in haemocytes of G. mellonella, Aporia crataegi, Dendrolimus sibericus, and Gryllus bimaculatus. In addition, Nappi et al. (1998) and Slepneva et al. (1999) reported evidence of both superoxide and its dismutase product hydrogen peroxide in D. melanogaster and G. mellonella haemocytes, respectively. Moreover, all the essential components of the nicotina-mide adenine dinucleotide phosphate (NADPH) oxidase responsible for producing superoxide have been identified in G. mellonella haemocyte (Bergin et al., 2005). The presence of these NADPH oxidase components and ROSs in insect haemocytes implies that a parallel process occurs following phagocytosis in haemocytes as in vertebrate phagocytes.

Vertebrate neutrophils also undergo a degranulation process when challenged with microbes too large to ingest, such as parasites and hyphae. Granules contain proteolytic enzymes, such as elastin, cathepsin G, lysozyme, defensin, transfer-rin, and myeloperoxidase, which are released into either the phagosome where they actively destroy the ingested microbe or are exocytosed into the extracellular matrix, in a process referred to as degranulation. Once these enzymes are in close contact with the microbe they begin to effectively digest the invading microbes. A similar degranulation process occurs in insect haemocytes. Smith and Soderhall (1983) reported that the freshwater crayfish, Astacus astacus haemocytes undergo a similar process of degranulation in the presence of various bacteria. They also suggested that proteins of the phenoloxidase cascade are strong non-self signals for the haemocytes and may initiate this degranulation process. Many of these proteolytic enzymes have been identified in insect haemocytes. Lysozyme has been found in many insects, including Hylphora cecropia (Hultmark et al., 1980), Manduca sexta (Spies et al., 1986), D. melanogaster (Regel et al., 1998), Musca domestica (Ito et al., 1995), H. virescens (Lockey et al., 1996), and Aedes aegypti (Rossignol et al., 1986), G. mellonella, Bombyx mori, and Agrius convolvuli (Yu et al., 2002). Lysozyme has been found within insect haemocytes and the intra-haemolymph lysozyme levels have been shown to increase upon infection indicating the possible exocytosis of intracellular lysozyme upon contact of the haemocyte with a microbe (Wilson & Ratcliff, 2000). Other enzymes associated with neutrophils granules have been identified in insects, such as defensin from

G. mellonella (Lee et al., 2004) and Dermacentor variabilis (Johns et al., 2001), transferrin from the termite Mastotermes darwiniensis (Thompson et al., 2003) and from D. melanogaster (Yoshida et al., 2000). The myeloperoxi-dase homologue peroxynectin has also been isolated from many insects (Lin et al., 2006). The presence of a homologue of MPO in insect haemocytes is of great significance as MPO released from the granules into the phagosome converts hydrogen peroxide to one of the most toxic products found within the phagolyso-some, hypochlorous acid.

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