Antimicrobial Peptide Production

A vital feature of the insect immune response is the induction of AMP synthesis by the fat body and circulating haemocytes and secretion of these AMPs into the haemo-lymph to destroy invading microbes (Ratcliff, 1985). This aspect of the innate immune response is also important in vertebrates; however, its presence in insects is crucial as insects lack any adaptive response. The activation of the Toll and IMD pathways initiates the transcription of many AMPs, which can act by directly damaging microbial cell surfaces. The insect immune response can discriminate between pathogens and is able to mount the appropriate responses and produce the most suitable AMPs when it encounters different micro-organisms. This is partly performed by the initiation of specific pathways by different microbes; Gram-positive bacteria and fungi activate the Toll pathway and the appropriate AMPs, and Gram-negative bacteria activate the IMD pathway and also the AMPs associated with activation of this pathway, which are appropriate for destroying these specific micro-organisms. Lemaitre et al. (1996) and Rutschmann et al. (2002) found that Drosophila deficient in the Toll pathway are susceptible to fungal and Gram-positive infections, while Lemaitre et al. (1995) found that Drosophila deficient in the IMD pathway were susceptible to Gram-negative infections.

There have been many classes of AMPs identified in insects with similarities to vertebrate AMPs (Vilmos et al., 1998; Salzet, 2001). In Drosophila there are seven classes of AMPs, which are divided into three groups depending on their microbial targets. Drosomycins and metchnikowins are active against fungi, defensins against Gram-positive bacteria, and attacins, cecropins, diptericins, and drosocins against Gram-negative bacteria. All of the AMPs produced by insects are ampiphatic molecules that act in a detergent-like manner, forming pores in micro-bial membranes. Attacins have a very small spectrum of antibacterial and antifun-gal activity, and it is believed that the primary function of attacins is to facilitate the action of lysozyme and cecropins (Engstrom et al., 1984). Proline-rich pep-tides, glycin-rich peptides, and diptericin are small AMPs (Hoffmann, 1995), which peptides appear to function by increasing membrane permeability and cause lysis of Gram-negative bacteria. Lysozyme is a cationic protein with a broad spectrum of activity against Gram-positive bacteria. Lysozyme acts by hydrolysing cell wall components and it found in the granules of neutrophils, monocytes, macrophages, blood plasma, tears, saliva, and airway secretions. Lysozyme effectively hydrolyses P-(1,4) glycosidic bonds in peptidoglucan of bacterial cell walls (Suzuki and Rode, 1969; Thorne et al., 1976). LPS-binding proteins facilitate bacterial clearance in insects by promoting nodule formation. LPS-binding proteins are known to activate the vertebrate TLR pathway and may act on similar pathways in insects (see Kavanagh & Reeves, 2004). Transferrin has an iron-binding domain and may function by sequestering iron from pathogens thus inhibiting their growth (Lowenberger, 2001). Human lactoferrin is a member of the transferring family and displays antibacterial activities by limiting the availability of environmental iron (Bullen, 1981). Defensins are cysteine-rich cationic peptides with antimicrobial activities in insects and are active against a variety of bacteria, fungi and some viruses (Lehrer et al., 1989, 1993; Ganz et al., 1985). Defensins represent an early defence mechanism, with production occurring within 3 h of infection. These AMPs lyse bacterial cells by forming voltage-dependent ion channels in the cytoplasmic membrane of the bacteria, leading to leakage of potassium and other ions (Hoffmann, 1995). Cociancich et al. (1993) reported that an insect defensin disrupted the permeability barrier of the cytoplasmic membrane of Micrococcus luteus, resulting in a loss of cytoplamsic potassium, a partial depolarisation of the inner membrane, a decrease in cytoplasmic ATP and an inhibition of respiration. Additionally, Johns et al. (2001) isolated a small inducible cationic peptide with 83% similarity to scorpion defensin, from the American dog tick Dermacentor variabilis. Interestingly, the active peptide constituted 0.1% of the total protein in the haemolymph plasma and was present as early as 1 h post-Bacillus subtilis and B. burgdorferi infection. This gives an idea of the promptness of the appearance of AMPs in an infection and suggests their importance in initial defence. The fact that defensins are produced by a wide variety of organisms (Broekaert et al., 1995; Hoffmann & Reichhart, 1997), suggests their early evolutionary origin (Ganz & Lehrer, 1995). In vertebrates, AMPs are stored in neutrophil granules where they are released upon cell activation into the extracellular matrix. There is evidence of a similar phenomenon occurring in insects, where lysozyme is stored within intracellular vesicles and released in a degranulation process upon haemocyte activation (Munoz et al., 2002).

In addition to AMP, there have been cytokine-like molecules identified in various invertebrate haemocytes such as the echinoderm Asterias forbesi (Beck et al., 1986), tunicates (Beck et al., 1989), molluscan haemocytes (Ottaviani et al., 1993), and haemocytes of Calliphora vomitoria (Franchini et al., 1996). The only cytokines identified in invertebrates so far are IL-1, IL-2, IL-6, and TNF. These are all pro-inflammatory cytokines and have remained highly conserved throughout evolution. Vertebrate IL-1 is produced by circulating monocytes and macrophages and has many functions. IL-1 is involved in cell growth regulation and differentiation in bone marrow and the thymus. IL-1 is also involved in the stimulation of cytokine secretion, secretion of components of the complement system, acute phase proteins and prostaglandin 2 (PGE2). TNF is also mainly produced by circulating monocytes and macrophages, and is also a multifunctional cytokine. TNF, besides its tumour-suppressing capabilities, regulates inflammatory reactions by interacting with IL-1 and IL-6. Other activities of TNF are its ability to increase the phagocytosis and cytotoxicity activities of monocytes, macrophages, and granulocytes. TNF also causes the expression of adhesion molecules on endothelial cells and granulocytes, the growth and differentiation of B cells, the production and secretion of cytokines and the expression of MHC-molecules in fibroblasts and endothelial cells. Both of these molecules, IL-1, and TNF, have been identified in G. mellon-ella plasmatocytes and granulocytes and were observed to be released upon bacterial stimulation (Wittwer et al., 1999). As IL-1 is produced by the TLR/IL-1R pathway in vertebrate lymphocytes and TNF is produced by activation of the TNF activation pathway, and considering the fact that homologues of both these pathways exist in invertebrates, namely the Toll pathway and IMD pathway (Lamaitre et al., 1995; De Gregario et al., 2002; Medzhitov & Janeway, 1998), it is tempting to suggest that the IL-1-like molecule and TNF-like molecule are produced through activation of the invertebrate Toll pathway and the IMD pathway, respectively. The invertebrate cytokines have functional similarity to vertebrate cytokines, in that they are able to modulate natural-killer cell activity of molluscan haemocytes (Franceschi et al., 1991) and to induce the release of biogenic amines and to affect cell migration, phagocytosis, and induction of nitric oxide synthase (Ottaviani et al., 1997).

Cure Your Yeast Infection For Good

Cure Your Yeast Infection For Good

The term vaginitis is one that is applied to any inflammation or infection of the vagina, and there are many different conditions that are categorized together under this ‘broad’ heading, including bacterial vaginosis, trichomoniasis and non-infectious vaginitis.

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