1.3.3.7.1 Effects on Secretion
ES products of abomasal nematodes have been shown to stimulate pepsinogen secretion, inhibit the ECL cell and acid secretion, but not secretion of gastrin. Merkelbach et al. (2002) found that adult H. contortus ES products were able to inhibit acid secretion by rabbit gastric glands in vitro. The active ES component(s) for this effect was found to be smaller than 5kDa. Ammonia was suggested as a possible active component as parallel testing with ammonium chloride in equivalent concentrations to those in the ES preparations showed a similar response. Hertzberg et al. (1999a) demonstrated that adult H. contortus ES products were able to inhibit the secretory activity of ECL cells in vitro. They suggested that parietal cell stimulation, and therefore acid secretion, was suppressed by the inhibition of the ECL cells by parasite ES products. In vitro, pepsinogen release from abomasal tissue was demonstrated after exposure to ES products from T. circumcincta, but only in tissues from animals that had been previously exposed to parasites (Scott and McKellar, 1998). It was suggested that hypersensitivity reactions to antigens present in ES products were responsible. Neither Lawton et al. (2002) nor Haag et al. (2005) were able to find evidence for a gastrin stimulant in ES products of several life cycle stages of T. circumcincta or
H. contortus, respectively (see also 1.3.1).
1.3.3.7.2 Immunomodulatory Effects
A variety of molecules with immunomodulatory effects have been identified in extracts and ES products of numerous helminths (Johnston et al., 2009). These immunomodulatory effects include induction of eosinophil and neutrophil chemotaxis, inhibition of eosinophils or monocytes, reduction or increase in cytokine levels and induction of growth factor release.
An inhibitor of monocytes has been found in adult H. contortus ES products (Rathore et al., 2006). The 66kDa ES antigen was able to reduce the production of hydrogen peroxide and nitric oxide from monocytes in vitro by as yet undefined mechanisms. This may be another
Chapter 1: Literature Review 38 possible antioxidative strategy in addition to the antioxidant enzymes described above (1.3.3.3).
An eosinophil chemoattractant is produced by both H. contortus and T. circumcincta
(Reinhardt, 2004; Wildblood et al., 2005). Extracts from L3, L4 and adult parasites had a
dose-dependent stimulatory effect on the migration of eosinophils. Similar effects were obtained with ES products from L3 as well as live L3 (Wildblood et al., 2005). A lectin in ES
products from O. ostertagi was responsible for eosinophil chemotaxis (Klesius, 1993) whereas in H. contortus it was likely due to a galectin or galectin mixture (Turner et al., 2008). Galectins were also found in ES products of T. circumcincta L4 and different stages of
B. malayi (Craig et al., 2006; Hewitson et al., 2008; Moreno and Geary, 2008). In addition, using cDNA cloning, Greenhalgh et al. (2000) detected three novel galectins, which were stage specifically expressed at different concentrations in H. contortus. Turner et al. (2005) suggested that it is possible that parasite galectins mimic the action of host galectins, which have been shown to have diverse functions, including eosinophil chemoattractant activity (galectin-9 (Hirashima, 2000)), modulation of cell adhesion and apoptosis (galectin-8 (Hadari et al., 2000; Zick et al., 2004)). Neutrophil chemotaxins have been detected in a range of parasites including H. contortus (Reinhardt, 2004), T. circumcincta (Reinhardt, 2004),
Ascaris (Tanaka et al., 1979) and O. volvulus, where the activity has been shown to be dependent on Wolbachia endobacteria (Brattig et al., 2001). It was suggested that the recruitment of inflammatory cells might be beneficial for the parasites by damaging parietal cells thus reducing acid secretion and pepsinogen activation (Simpson, 2000) and/or tissue damage being beneficial for parasitic invasion (Wildblood et al., 2005). Alternatively, they could just evoke an inflammatory response.
1.3.3.7.3 Effects on Cell Proliferation
A few studies examined the effects of ES products on cell proliferation (Hoste et al., 1995; Huby et al., 1995; 1999). T. colubriformis ES products increased cell proliferation in a dose- dependent manner in most cell lines tested, including epithelial cells of digestive (RIC, IEC-6, IRD-98 and HT29-D4) and non-digestive origin (MDCK) and inhibition only in epithelial cells of non-digestive origin (CHO and 3T3) (Hoste et al., 1995;Huby et al., 1999).
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T. circumcinta ES products also had a stimulatory effect on HT29-D4 cells, but no significant effect was observed for H. contortus (Huby et al., 1995). In contrast, Abner et al.
(2002) found a dose-dependent cytotoxic response of T. suis ES products on intestinal epithelial cells, particularly IPEC-1 and INT407. Furthermore, Przemeck et al. (2005) and Huber et al. (2005) noted an increased rate of cell detachment after exposure of ES products from L3 and adult H. contortus and T. circumcincta to HeLa cells (epithelial,
human cervix adenocarcinoma cells), but this was not quantitated.
1.3.3.7.4 Cell Vacuolation
ES products from L3 and adult H. contortus and T. circumcincta have been shown to cause
vacuolation in HeLa cells in a stage- and parasite-density dependent manner (Huber et al., 2005; Przemeck et al., 2005). The effect of vacuolation was enhanced by adding 8mM ammonium chloride to the medium (Przemeck et al., 2005). The ammonia in ES products was therefore considered as a possible contributor of the vacuolation activity in the ES products of abomasal nematodes, similar to H. pylori-induced vacuoles. VacA, the vacuolating cytotoxin from H. pylori, forms anion-selective channels, which are internalised and insert into late endosomes. This changes the permeability of late endosomes and enhances the vacuolar ATPase proton pumping activity to compensate for the increased anion concentration. Ammonia generated by H. pylori urease enters the acidic organelles and is protonated to ammonium and trapped. Osmotic influx of water leads to vesicle swelling, forming a vacuole (Montecucco and de Bernard, 2003; Cover and Blanke, 2005). However, observations by Huber et al. (2005) indicated that ES component(s) other than ammonia might be involved. Larval and adult ES preparations contained the same concentrations of ammonia, but larval ES preparations induced less severe vacuolation. Additionally, incorporated Neutral red (NR), which is a lipophilic free base and commonly used for quantitative analysis of VacA induced vacuolation by H. pylori (Cover et al., 1991; Garner and Cover, 1996), was observed to be localised in the cytosol and peri-nuclear regions and not inside the numerous small vacuoles. In contrast, in vacuoles induced by H. pylori VacA, NR accumulates almost exclusively within the vacuoles, which were fewer in number and significantly larger in size than those induced by ES products.
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