The intestinal mucus barrier to parasites and bacteria

Some non-mucin components of mucus and their possible biological roles

Non-mucin components have essential roles in the protective functions of mucous secretions. Secretory IgA (SIgA) antibodies probably act by blocking the attachment of pathogenic microorganisms to mucosal cells. In addition SIgA1 may render bacteria more 'mucophilic', possibly by virtue of the 'mucus-like' stretch that the immunoglobulin molecule possesses. Lysozyme will attack cell walls of susceptible bacteria. As the enzyme associates strongly with mucus glycoproteins the mucus layer is provided with powerful bactericidal properties. Lactoferrin, normally unsaturated, sequesters any free iron in secretions, so exerting a bacteriostatic action on iron-requiring microorganisms. In addition it may protect mucus glycoproteins from iron-catalysed active oxygen species. This mucoprotective action would be overcome during infections. Attention is also directed towards a possible copper-mediated limited degradation by hydrogen peroxide. Surfactants and free lipid have long been recognized as components of normal bronchial mucus. For example, some lipid is tightly but non-covalently bound to a hydrophobic region of bronchial mucin. More intriguing is the presence of small amounts of covalently bound lipid in normal human gastric mucin. In addition, normal human gastric mucus contains significant amounts of a galactose-rich polysaccharide. The function of this is not known but it may act as a cross-linking strand in the mucus gel structure or as a renewable cell membrane component, perhaps interacting between glycocalyx and the mucus layer.

The beta-N-acetylhexosaminidase of Entamoeba histolytica is composed of two homologous chains and has been localized to cytoplasmic granules

We have purified a beta-N-acetylhexosaminidase from trophozoites of Entamoeba histolytica to homogeneity. In SDS-PAGE, the enzyme yielded a single protein band at an apparent M(r) of 64,000. The elution behaviour of the native enzyme upon molecular sieve chromatography corresponded to a molecular mass of approximately 132,000 suggesting that the enzyme is a dimer. Upon sedimentation velocity centrifugation, hexosaminidase activity sedimented at 12S, implying aggregation to a higher molecular mass complex with an apparent M(r) of approximately 400,000. Based on the N-terminal sequence of the purified enzyme and on data extracted from the E. histolytica genomic data base, we amplified and cloned two genes (EhHEXA and EhHEXB) coding for two presumptive, highly similar hexosaminidase chains which we designated as Ehhexalpha and Ehhexbeta. Northern blot analysis indicated that the two genes were expressed to a similar level, and Western blotting with chain-specific antisera showed that the trophozoites synthesize both proteins. By cell fractionation, the hexosaminidase was found to be a major component of cytoplasmic granules; these contain tissue-destructive factors and are released after collagen-induced exocytosis to the cell surface. In agreement with this observation, immunocytochemistry with an antiserum cross-reacting with both hexosaminidase chains revealed strong fluorescence in surface patches, which we interpret as released granules, and in vesicles throughout the cell. Its localization in cytoplasmic granules strengthens the notion that the hexosaminidase complex may contribute to amoebic pathogenicity.

Histochemical characteristics of the goblet cell mucins and their role in defence mechanisms against Nippostrongylus brasiliensis infection in the small intestine of mice

The histochemical characteristics of the small intestinal goblet cell mucins of mice and their protective role during the expulsion of Nippostrongylus brasiliensis adult worms were examined. Lectin histochemistry revealed that, before infection, strongly positive stainability of Helix pomatia agglutinin (HPA) and Limax flavus agglutinin (LFA) was observed in all of the small intestinal goblet cell mucins of mice. When 'normal' or 'damaged' N. brasiliensis adult worms were implanted separately into the small intestine of euthymic mice, 50% of them were established in the intestine on day one after implantation, but were subsequently expelled within one week in association with HPA and LFA positive goblet cell hyperplasia. On the other hand, in dexamethasone treated euthymic mice, 70% of implanted 'normal' worms were expelled by day 7 and 80% of implanted 'damaged' worms were expelled by day 4 in association with HPA and LFA positive goblet cell mucins but no goblet cell hyperplasia. This is consistent with the fact that, in spite of no increase in goblet cell number in hypothymic nu/nu mice, all of the implanted 'damaged' worms were expelled within day 5 and 80% of implanted 'normal' worms were expelled by day 14 post implantation, possibly associated with strongly expressed terminal GalNAc and sialic acid residues of the small intestinal goblet cell mucins prior to infection.

Reconstitution by bone marrow grafting of the defective protective capacity at the migratory phase but not at the intestinal phase of Nippostrongylus brasiliensis infection in W/Wv mice

After a primary infection by subcutaneous inoculation with the infective larvae (L3) of Nippostrongylus brasiliensis, the intestinal worm burden was higher and expulsion was slower in W/Wv mice than in +/+ mice. When the course of infection was segregated into the migratory and intestinal phases, protection during the migratory phase examined by the larval recovery from the lungs and that during the intestinal phase measured by worm burden after intraduodenal implantation with adult worms were both defective in W/Wv mice. The higher susceptibility of W/Wv mice during the migratory phase was normalized by bone marrow reconstitution. On the other hand, higher susceptibility of W/Wv mice during the intestinal phase, which was measured by worm burden 24 h after intraduodenal implantation of the larvae recovered from the lungs of rats, was not normalized by bone marrow grafting. Furthermore, slower expulsion seen in W/Wv mice after intraduodenal implantation with adult worms was not hastened by bone marrow reconstitution. These results indicate that the protective mechanisms against N. brasiliensis operating during the migratory phase and those during the intestinal phase were different in terms of bone marrow dependency and that non-myeloid cells utilizing c-kit ligand/receptor system to express their functions are involved in the mucosal defence against N. brasiliensis.

T lymphocyte dependent enteropathy in murine Trichinella spiralis infection

Mice infected with Trichinella spiralis developed significant enteropathy, comprising villus atrophy, crypt hyperplasia, goblet cell hyperplasia and a decrease in intra-epithelial lymphocyte numbers by 10 days post-infection, when most of the parasites had been expelled from the gut. However, worm expulsion was prevented by treatment with cyclosporin A and, despite a continued parasite burden, cyclosporin A treated animals had no villus atrophy or changes in inflammatory cell numbers. These results confirm that the expulsion of T. spiralis from the mouse gut is accompanied by a significant intestinal lesion and that both of these phenomena are T-cell mediated.

Pre-epithelial mucus layer in the colon of conventional and germ-free rats

The pre-epithelial mucus layer (PML) and epithelial mucins were studied by mucin histochemistry in 10 microns-thick celloidinstabilized cryostat sections in the proximal and distal colon of conventional and germ-free rats aged 120 and 350 days. No continuous PML was found in the proximal colon. A continuous mucus blanket, of fairly homogenous thickness, was observed in the distal colon, where the PML-thickness was 40 +/- 24 microns at 120 days of age and 44 +/- 22 microns at 350 days of age in conventional rats, and 25 +/- 17 microns (120 days) and 22 +/- 10 microns (350 days) in germ-free rats. The stainability of the PML by periodic acid-Schiff and Alcian Blue at pH 2.5 and 1.0 was stronger in conventional rats than in germ-free rats, indicating higher concentrations of mucosubstances and of acid and sulphated mucins, respectively. The PML of the conventional rat distal colon showed a stratified structure of up to eight sublayers. In the distal colon of germ-free rats, the whole gut wall thickness was reduced 47% compared to the conventional rat (germ-free; 185 +/- 73 microns, conventional: 350 +/- 115 microns). No stratification of the PML was observed. The presence of intestinal microflora obviously had a strong influence on the thickness, compactness, mucin content, mucin composition and structure of the pre-epithelial mucus layer.

Gastrointestinal mucus, a medium for survival and for elimination of parasitic nematodes and protozoa

Mucus is a sticky visco-elastic material which coats all mucosal surfaces. Florey, in 1955, noted the following three functions for gastrointestinal mucus: protection of the underlying mucosa from chemical and physical injury, lubrication of the mucosal surface to facilitate passage of luminal contents, and removal of parasites by binding and entrapment. In the 31 years since Florey's review, detailed analyses of the composition of mucus and of the biochemistry of mucin glycoproteins, as well as measurements of the physical properties of mucus from different organs and sites have yielded information at the molecular level which provide additional support for his views on its function (Allen, 1981; Forstner, Wesley & Forstner, 1982).