A variety of parasite substances may come in contact with host tissues during abomasal parasite infection. These molecules, which are excreted after digestion or secreted from secretory glands, mainly those located in the parasite’s oesophageal region, belong to the E/S products and can be obtained in vitro by incubation of larvae or adult parasites. The E/S products released by abomasal
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nematodes into their environment contain chemotaxins, metabolic end-products, enzymes, immunomodulators and growth factors.
Among the excreted metabolites are free fatty acids (BRYANT 1993), propan-1-ol, acetate, propionate, small amounts of ethanol, lactate and succinate (WARD 1974;
WARD and HUSKISSON 1978; WARD et al. 1981), polar and non-polar lipids (KAPUR and SOOD 1991) and ammonia (BARRETT 1981). Ammonia as the principal nitrogenous excretory product is potentially involved in the pathophysiology of the parietal cell because of its known cytotoxicity and ability to reduce 14C-aminopyrine accumulation (HAGEN et al. 1997).
Enzymes reported in the E/S products of a range of gastrointestinal helminths include acetylcholinesterase (LEE and HODSDEN 1963; OGILVIE et al. 1973;
KNOX and JONES 1990; GRIFFITHS and PRITCHARD 1994; HUBY et al. 1999a), serine, cysteine, aspartic and metalloproteases (KARANU et al. 1993; TODOROVA 2000), elastase (KNOX and JONES 1990), N-acetyl-β-D-glucosaminidase and an acid phosphohydrolase (GAMBLE and MANSFIELD 1996). Acetylcholinesterase is higher in the E/S products of adult than larval stages (KNOX and JONES 1990) and has been suggested to prevent peristalsis and parasite expulsion (OPPERMAN and CHANG 1992), inhibit mucus secretion (PHILIPP 1984) or alter the immune response to the parasite (RHOADS 1984). Proteolytic enzymes present in the E/S products may aid penetration of host tissues (MATTHEWS 1977), act as an anticoagulant (HOTEZ and CERAMI 1983; KNOX and JONES 1990), degrade protein for ingestion (VON BRAND 1973) or inactivate complement and cytokines released by leukocytes (LEID et al. 1987). Different classes of proteases were shown to be secreted in a stage-specific manner in L3, L4 and adult stages in a number of gastrointestinal parasites in cattle and sheep (KNOX and JONES 1990; DE COCK et al. 1993). A number of proteases have been identified in adult H. contortus E/S products, specifically associated with the onset of blood feeding, including several cysteine proteases capable of degrading blood (KNOX et al. 1992; RHOADS and FETTERER 1995).
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Many of the antigens in E/S products are expressed only in certain parasitic stages (BALIC et al. 2000a). Other antigenic parasite components are its surface molecules which are also specific for the different parasitic stages (FETTERER and RHOADS 1993), and can be released during moulting or during surface shedding which has been described for some parasites (BEHNKE et al. 1992).
These surface molecules can also be present in varying amounts in the E/S preparations. Some internal parasite molecules make contact with host cells only when parasite death and degradation occur within tissues. These ‘somatic antigens’, together with the other parasite products mentioned above, are contained in the parasite extracts obtained by mechanical disruption of whole worms.
SCHALLIG et al. (1994) identified in the E/S products of adult H. contortus at least 15 polypeptides, with molecular weights (mol.wt.) ranging from 10,000 to >100,000, which induced an immune response in infected sheep, as demonstrated by specific IgG levels and lymphocyte proliferation. Two of these polypeptides, of 15,000 and 24,000 mol.wt., were cloned (SCHALLIG et al. 1997b) and vaccination of sheep with these antigens caused a substantial degree of protection against subsequent H. contortus infections (SCHALLIG et al. 1997a). TAKATS et al. (1995) demonstrated that the 24,000 antigen is produced in the oesophagus of adult H.
contortus and is probably secreted during feeding. A stage-specific 31,000 glycoprotein localised in the secretory organelles in the oesophageal glands of O.
circumcincta L3 was recognised by sera of resistant sheep and was a major component in E/S products obtained from incubation of O. circumcincta L3 in vitro (MCGILLIVERY et al. 1989).
Other E/S product components may be involved in immunomodulation (LIGHTOWLERS and RICKARD 1988; KLESIUS 1993) and stimulation of cell proliferation (HUBY et al. 1995, 1999b). Immunomodulation can reduce the effectiveness of host immunity (BEHNKE et al. 1992) and specific immunodepression by parasite products was demonstrated to occur in chronic infections with several filarial nematodes (BEHNKE 1987). Non-specific
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immunodepression through impairment of antibody and cellular immune responses (e.g. suppression of lymphocyte reactivity) during Ostertagia infection in cattle has been demonstrated by KLESIUS (1988, 1993) and KLESIUS et al.
(1984). MONROY et al. (1989) found a low mol.wt. immunosuppressive factor in the E/S products of adult Nematospiroides dubius which inhibited the proliferation of mitogen-stimulated mouse spleen lymphocytes. The ovine intestinal parasite T. colubriformis was shown to secrete a molecule, which functionally resembled a human IFN-γ-induced protein (DOPHEIDE et al. 1991).
IFN-γ is a T helper 1 cell cytokine that suppresses the protective T helper 2 cell response (see Section 2.3). Through secretion of this molecule, T. colubriformis may be committing its host to a non-protective T helper 1 response, thereby avoiding immunity. By shedding surface-bound antibodies and by changing their surface glycoproteins with the different developmental stages (MEEUSEN and BRANDON 1994), parasites may also try to evade attack of the host immune system. The ability of H. contortus to shed surface glycoproteins bound by antibodies as a continuous surface layer similar to the sheath of moulting larvae has been demonstrated by ASHMANN et al. (1995).
Parasites may actively recruit granulocytes by secreting substances with direct chemotactic properties, which have been demonstrated in vitro in E/S products and/or extracts from a range of parasites. Eosinophil chemotactic factors have been found in O. ostertagi larvae (KLESIUS et al. 1985, 1986) and Anisakis sp.
larvae (TANAKA and TORISU 1978), Ascaris suum adults (TANAKA et al. 1979), Schistosoma japonicum eggs (OWHASHI and ISHII 1982) and adults (HORII et al. 1984a), plerocercoids of Spirometra erinacei (HORII et al. 1984b), Fasciola sp. (HORII et al. 1986a), Taenia taeniaeformis metacestodes (POTTER and LEID 1986), Dirofilaria immitis adults (OWHASHI et al. 1996), adult Angiostrongylus cantonensis (ISHIDA and YOSHIMURA 1992), different stages of Metastrongylus apri (SASAKI and KATSUNO 1983) and Hymenolepis nana cysticercoids (NIWA et al. 1998). Neutrophil chemotaxins have been reported from Ascaris suum adults (TANAKA et al. 1979), from adult D. immitis (HORII et al. 1986b), from plerocercoids of S. erinacei (HORII et al. 1984b), from F.
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hepatica adults (JEFFERIES et al. 1996), Oesophagostomum aculeatum larvae (HORII et al. 1985), Trichinella spp. larvae and adults (SHUPE and STEWARD 1991), S. japonicum adults (HORII et al. 1984a) and O. volvulus adult parasites (RUBIO DE KRÖMER et al. 1998).
In several arthropod species (WERREN et al. 1995) and in a range of filarial nematodes, including several Onchocerca spp. (BANDI et al. 2001), the presence of intracellular bacteria has been described (MCLAREN et al. 1975;
VINCENT et al. 1975; KOZEK 1977). Those bacteria have later been identified as members of the Wolbachia spp. (TAYLOR and HOERAUF 1999) and the neutrophil chemotactic activity demonstrated in Onchocerca sp. extracts (RUBIO DE KRÖMER et al. 1998; BRATTIG et al. 2001) is thought to be caused by endobacterial products, which may also be present in the E/S products of those parasites, causing the neutrophil accumulation around Onchocerca worms in vivo (BRATTIG et al. 2001).
Some authors suggested that a benefit for abomasal parasites in the deliberate or inadvertent recruitment of inflammatory cells may lie in the damage of parietal cells leading to reduced acid secretion and thus preventing pepsinogen being activated, making the parasite’s environment less hostile (SIMPSON 2000).