The role of the gut in oral tolerance induction

The gut handles large amounts of potentially immunogenic material before it reaches the peripheral immune system. The degradative and transport pathways of the gastrointestinal tract have been implicated in the induction of oral tolerance through utilisation of these processes by the local immune system.

Two mechanisms by which the gut could influence tolerance induction are:

1. Digestion of antigen in the lumen of the gut thereby permitting tolerogenic epitopes to become exposed and subsequently presented to the local immune system (Hanson and Morimoto 1980, Parks and Weigle 1980, Strobel et al 1983, Bruce and Ferguson 1986a and b).

2. Active antigen uptake, processing and presentation to the local immune system by columnar epithelial cells (Bland and Warren 1986a and b, Mayer and Shlien

1987).

Evidence fo r hypothesis L

It has been demonstrated that transfer of tolerance can occur if serum from OVA fed mice is transferred into naive recipients one hour after a feed (Strobel et al 1983). This tolerance appears to correspond with the presence of OVA as demonstrated by Bruce and Ferguson (1986b) who showed, that when OVA was removed from "tolerogenic" serum by affinity chromatography the ability of the OVA depleted serum to induce oral tolerance was abrogated. Further support for this hypothesis is provided by the observation of Hanson et al (1979b) who injected mice with mouse anti-OVA antibodies during feeding with OVA. This resulted in an intermediate tolerant state suggesting some loss of tolerance induction following removal of OVA from the circulation of the immunised mice by immune complexing of antigen by injected antibody.

The suppression described above is not observed if serum is transferred 5 minutes after a feed even though measurable levels of OVA are present (Peng et al 1990). Furtherm ore if serum obtained 5 minutes after feeding is concentrated so that it contains an equivalent dose of antigen as serum obtained 60 minutes after feeding (i.e. tolerogenic serum) tolerance is still not transferred (Peng et al 1990). If the same dose of antigen is administeredparenterallyand allowed to circulate for one hour in the donor before transfer of serum to the recipients, there is no subsequent tolerance induction (Bruce and Ferguson 1986a). Therefore there appears to be a qualitative difference in the nature of the antigen present which enables it to act as a tolerogen rather than an immunogen, this qualitative difference in circulating antigen appears to be unique to antigen which has entered the circulation via the gut. The serum transfer model induces tolerance of DTH but not of antibody suggesting some divergence in the mechanisms and thresholds of tolerance induction for different limbs of the immune response (Strobel et al 1983 and Strobel and Ferguson 1986). Further evidence of gut involvement is provided by an unconfirmed report that prior

treatment of mice with trasylol, a trypsin and chymotrypsin inhibitor, abolished the induction of oral tolerance (Hanson and Morimoto 1980). It was suggested that this may have been due to decreased production of an altered form of antigen or, that a swamping effect occurred in the local immune system due to increased uptake of intact protein from the gut (Hanson and Morimoto 1980). It has also been observed that direct injection of antigen into the lumen of the lower ileum was two fold less efficient at inducing tolerance than gastric or duodenal injection suggesting that enzymes present in the small intestine were essential for the production of an OVA tolerogen (Hanson and Morimoto 1980). However this finding is contested by the reports of Strobel (1984) and Nagatomi et al (1980) who both reported that antigen administered rectally could also induce a tolerant state indistinguishable to oral tolerance.

Another commonly used antigen in the induction of oral tolerance is BSA. It has been shown that proteolytic digestion of BSA leads to production of tolerogenic fragments (Muckerheide et al 1977, Dosa et al 1979, Ferguson et al 1983c) and that these fragments bind to specific T suppressor cells from orally tolerised mice (Zhang

et al 1987). These results appear to indicate that digestion of protein by the gut may play a significant contributory role in the suppression of immune responses following oral administration of an antigen. However care should be taken not to extrapolate from the results obtained with one particular protein antigen to provide a general hypothesis since the effect of gastrointestinal modification of antigens may vary dramatically depending on which antigen is used.

Evidence fo r hypothesis 2.

One function of intestinal epithelial cells is to transport food antigens, after digestion in the lumen of the gut, from their luminal surfaces to the basal membrane and consequently into the circulation for distribution around the body. They could, therefore, be involved in selective antigen handling and presentation to the immune system. The observation that intestinal epithelial cells constitutively express class II MHC on their surface in mammals (Parr and MacKenzie 1979, Wiman et al 1979,

Scott et al 1980, Mason et al 1981) suggested a role in antigen presentation (Geppert and Lipsky 1985) to the underlying lymphocytes of the lamina propria and lEL perhaps to specifically induce oral tolerance. There is some evidence of intracellular accumulation within the vesicular endosome compartment suggesting uptake of antigen (Mayrhofer et al 1983). Recycling of the receptor has not been observed but the restricted surface distribution would only allow release of the ligand in an endosome basolateral membrane direction (Bland 1988). All these observations would suggest that intestinal epithelial cells could be efficient antigen presenters and have been demonstrated to present soluble protein antigens in vitro to T cells and may selectively induce suppressor T cells (Bland and Warren 1986b, Bland and Whiting 1989).

The level of expression is increased in association with inflammatory responses in the mucosae (Mason et al 1981) probably as a result of rIFN production by lEL (Kiyono et al 1991, Taguchi et al 1991) or lamina propria lymphocytes (Barclay and Mason 1982) . Expression of Class II antigen is absent throughout fetal development in the rodent and becomes detectable at various times after birth depending on the species studied. The rat does not express class II on its enterocytes until weaning (Bland and Warren 1986a) whereas mice express such antigens after the first week of life (Natalli et al 1981).

There are many reasons why enterocytes could be involved in immune regulation in the mucosal immune system, e.g.;

(1) Their position at the interface between the outside environment and cells of mucosal immune system.

(2) Close spatial relationship with lEL and lamina propria lymphoid cells (Brandt- zaeg et al 1988).

(3) They harbour terminal peptidases in the microvilli of their luminal surface. (4) Constitutive expression of class II MHC (reviewed by Bland 1988).

It is unclear whether antigen undergoes processing while being transported across the enterocyte (Bland 1988). However such cells are able in vitro to present antigen to specific T cells (Bland and Warren 1986a) and this presentation can be blocked by anti I-A and I-E antibodies (Bland and Warren 1986a). Furthermore it has been reported that enterocytes selectively stimulate functional T suppressor cells (Bland and Warren 1986b). However for presentation to T cells most protein antigens require processing to small oligopeptides so that they can be accommodated in the peptide groove of the MHC molecule (Allen 1987). Therefore the antigens which enterocytes are transporting and presenting via class II must be small fragments, perhaps digested by the term inal peptidases present on their lum inal surface. However, the serum transfer model of oral tolerance requires relatively large frag­ ments of OVA, which still retain conformational epitopes recognisable by polyclonal anti-OVA antibodies (Strobel et al 1983). This would not correspond with the conventional I oligopeptides: required for antigen presentation by Class II (Allen 1987).