Dr. Murai Éva szerk.: Parasitologia Hungarica 22. (Budapest, 1989)

THE ROLE OF MUCUS IN WORM REJECTION In the last decade, the anaphylactic release of mucus as a part of intestinal immunopathology has attracted much interest and research. It has been recognized that the superficial intestinal layer of mucus is an integral part of the gut associated immune system. This area has been the subject of three recent reviews by MILLER et al. (1985, 19861 and MILLER (1987). MILLER and his associates also conducted extensive studies to explore the relationship between worm Induced intestinal hypersensitivity reactions and enhanced mucus secretion. The visco-elastic mucous gel lining the gastrointestinal epithelium acts as a barrier against antigen uptake and worm establishment. Worms of a primary infection can penetrate through this barrier. Challenge worms, however, fail to cross the mucus layer, and induce anamnestic rise in mucus production as well as increased mucosal permeability. Mucus trapping of intestinal worms has been suggested to be in a cause-and-effect relationship between enhanced mucus secretion and worm expulsion (LEE and OGILVTE 1981, 1982; MILLER, HUNTLEY and DAWSON 1981a; MILLER, HUNTLEY and WALLACE 1981b; MILLER and HUNTLEY 1982a, 1982b). Immune exclusion was believed to be effected by nonspecific physical entrapment of nematodes in the barrier of the intestinal mucus (MILLER et al. 1981b; MILLER et al. 1983), or, alternatively, the in vitro studies of DOUCH et al. (1983, 1984) suggested Inflammatory mediators of the mucus, such as leukotrienes, to possess nematode paralysing potential. In vitro bioassays of the mucus and dlgesta from the small Intestine of sheep made resistant to Trichostrongylus colubriformls by continuing larval Infections have indicated the presence of a paralysing activity against the infective larvae (KIMAMBO and MacRAE 1988). These observations suggest that substances secreted into the lumen of the small intestine or abomasum could account for the inability of infective larvae to establish themselves in resistant sheep. It was also experimentally demonstrated that the expulsion of N. braslliensis from the rat coincides not only with quantitative, but also with remarkable qualitative changes in the histochemical composition (from neutral to acid) of mucins in the goblet cells (KONINKX et al. 19881. In rats infected with N. brasiliensis it was found that mucus trapping occurred in primed rats only. A treatment with corticosteroids (betamethasone) before challenge abrogates both immune exclusion and mucus trapping. On the other hand in the same study it was also found that only about 20% of the worms recovered in the intestinal perfusate of Immune recipient rats were enveloped in thick globules of mucus, and most of the 4-day-old juvenile challenge worms were expelled before significant trapping was detected (MILLER et al. 1981b; MILLER and HUNTLEY 1982a). Our own observations were in line with those of MILLER et al., showing that during expulsion from hyperimmune recipient rats some 80% of adult Nippostrongyles remained sufficiently active to be recovered by the isolation method, and only about 20% were trapped In the intestinal mucus. In naive control recipient rats mucus trapping occurred for less than 10% of the worms (KASSAI et al. 1987). BELL et al. (1984) quantitatively analyzed the rapid expulsion of T. spiralis from immune rats. Their results also showed that intestinal mucus trapping was not an essential or required component of worm rejection. Thus, it is pertinent to conclude that although intestinal mucus may play a contributory role in worm expulsion, mucus trapping does not seem to be the major effector mechanism in volved.