Antigen recognition in the gastrointestinal tract: death to the dogma

Immune parameters in the intestine of wild and reared unvaccinated and vaccinated Atlantic salmon (Salmo salar L.)

Forming a barrier to the outside world, the gut mucosa faces the challenge of absorbing nutrients and fluids while initiating immune reactions towards potential pathogens. As a continuation to our previous publication focusing on the regional intestinal morphology in wild caught post smolt and spawning Atlantic salmon, we here investigate selected immune parameters and compare wild, reared unvaccinated and vaccinated post smolts. We observed highest transcript levels for most immune-related genes in vaccinated post smolts followed by reared unvaccinated and finally wild post smolts, indicating that farming conditions like commercial feed and vaccination might contribute to a more alerted immune system in the gut. In all groups, higher levels of immune transcripts were observed in the second segment of mid-intestine and in the posterior segment. In the life stages and conditions investigated here, we found no indication of a previously suggested population of intestinal T cells expressing MHC class II nor RAG1 expression.

Effect of oestradiol and pathogen-associated molecular patterns on class II-mediated antigen presentation and immunomodulatory molecule expression in the mouse female reproductive tract

Cells of the female reproductive tract (FRT) can present antigen to naive and memory T cells. However, the effects of oestrogen, known to modulate immune responses, on antigen presentation in the FRT remain undefined. In the present study, DO11.10 T-cell antigen receptor transgenic mice specific for the class II MHC-restricted ovalbumin (OVA) 323-339 peptide were used to study the effects of oestradiol and pathogen-associated molecular patterns on antigen presentation in the FRT. We report here that oestradiol inhibited antigen presentation of OVA by uterine epithelial cells, uterine stromal cells and vaginal cells to OVA-specific memory T cells. When ovariectomized animals were treated with oestradiol for 1 or 3 days, antigen presentation was decreased by 20-80%. In contrast, incubation with PAMP increased antigen presentation by epithelial cells (Pam(3)Cys), stromal cells (peptidoglycan, Pam(3)Cys) and vaginal cells (Pam(3)Cys). In contrast, CpG inhibited both stromal and vaginal cell antigen presentation. Analysis of mRNA expression by reverse transcription PCR indicated that oestradiol inhibited CD40, CD80 and class II in the uterus and CD40, CD86 and class II in the vagina. Expression in isolated uterine and vaginal cells paralleled that seen in whole tissues. In contrast, oestradiol increased polymeric immunoglobulin receptor mRNA expression in the uterus and decreased it in the vagina. These results indicate that antigen-presenting cells in the uterus and vagina are responsive to oestradiol, which inhibits antigen presentation and co-stimulatory molecule expression. Further, these findings suggest that antigen-presenting cells in the uterus and vagina respond to selected Toll-like receptor agonists with altered antigen presentation.

Class II MHC expression on rat intestinal intraepithelial lymphocytes

The expression of class II major histocompatibility complex (MHC) on rat intestinal intraepithelial lymphocytes (IELs) and mesenteric lymph node cells (MLNCs) were examined by immunofluorescence and flow cytometry. As expected, MLNCs contain eight small populations of CD4+ class II MHC+ T-cells in addition to classical antigen presenting cells. In contrast, rat IELs include a significant population of class II MHC+ T-cells, predominantly in the CD8+ CD4-alpha beta TCR+ subset. IEL samples with a relatively high percentage of class II MHC+ cells also include some CD4+ class II MHC+ cells; IEL samples with a low percentage of class II MHC+ cells also include some CD4- CD8- class II MHC+ cells. The role of lymphocyte subpopulations in the intestinal epithelium may need to be revisited in consideration of these findings.

Intestinal immunization of mice with antigen conjugated to anti-MHC class II antibodies

We have explored a new technique for immunization of the intestinal tract of mice, using protein antigens bound to antibodies with specificity for murine MHC class II molecules (MHC-II). Either of two protein antigens, hen avidin (AV) or hen egg lysozyme (HEL) were covalently conjugated to anti-MHC-II antibodies and the purified conjugates were given orally (p.o.) or by direct intraduodenal (i.d.) injection into the intestinal lumen of mice. A secondary immunization p.o. with the same conjugate or with the non-conjugated antigen in the presence of cholera toxin (CTX) resulted in production of both intestinal secretory IgA and serum IgA antibody by those mice. In addition, serum IgG antibodies were produced. Conjugates with appropriate MHC-II specificity targeted the antigen because they induced more IgA and IgG antibody than conjugates with irrelevant antibody specificity or antigen alone, and because they induced antibody in mice that were genetic low responders to antigen. The results indicate the feasibility of oral subunit type vaccines with antibody targeting technology.

The role of the local immune response in the pathogenesis of peptic ulcer formation

Mucosal immune responses are designed to provide local protection against infection, without inducing excessive amounts of inflammation that would alter epithelial integrity or function. It has become clear that the epithelium not only serves as a barrier to exclude pathogens, but also initiates host responses to infection. Gastric epithelial cells infected with Helicobacter pylori can respond within hours to produce inflammatory mediators that recruit and activate neutrophils. The gastric epithelium can also be recognized by local T-cells, resulting in their activation and ability to induce epithelial damage. During infection with H. pylori, there is a remarkable increase in the level of local IgG antibodies, which may also recognize and damage the epithelium. Thus, activated neutrophils, T-cells and auto-antibodies may contribute to a weakened epithelial barrier that allows luminal acid and other factors to contribute to peptic ulceration. The epithelium appears to play a key role in the initiation of the local inflammatory and immune responses that may contribute to the more serious sequelae associated with H. pylori infection.

Cholera toxin and cholera B subunit as oral-mucosal adjuvant and antigen vector systems

Cholera toxin (CT) and the analogous heat-labile enterotoxin (LT) from Escherichia coli have several immunomodulating effects which alone or in combination might explain their strong adjuvant action in stimulating mucosal IgA and other immune responses to admixed unrelated antigens after oral immunization. These effects include, depending on animal species and experimental systems, enhanced antigen presentation by a variety of cell types; promotion of isotype differentiation in B cells leading to increased IgA formation; and complex stimulatory as well as inhibitory effects on T-cell proliferation and lymphokine production. This adjuvant activity appears to be closely linked to the ADP-ribosylating action of CT and LT associated with enhanced cyclic AMP formation in the affected cells, and thus it may prove difficult to eliminate the enterotoxic activity without loss of adjuvanticity. However, through a separate mechanism, as an antigen-carrier system providing specific binding to epithelium including the M cells of intestinal Peyer's patches, both CT and its non-toxic binding subunit moiety (CTB) have been shown to markedly enhance the mucosal immune response to various foreign antigens or epitopes covalently linked to these molecules. This gives promise for the future use of CTB or related non-toxic binding derivatives as vehicles to facilitate induction of mucosal immune responses to a broad range of antigens for human vaccination purposes.

CD1d is involved in T cell-intestinal epithelial cell interactions

We assessed the role of the nonclassical class I molecule, CD1d, in the interaction between intestinal epithelial cells and T cells. In a mixed lymphocyte reaction (MLR) system where the stimulator cells were irradiated normal intestinal cells, the anti-CD1d monoclonal antibody (mAb) 3C11 inhibited T cell proliferation. In contrast, no inhibition was seen when mAb 3C11 was added to conventional MLR cultures (non T cell stimulators). Furthermore, no inhibition was seen when either airway epithelial cells were used as stimulator cells or lamina propria lymphocytes were used as responder cells. These latter two conditions along with a conventional MLR favor CD4+ T cell proliferation. However, we have previously shown that normal intestinal epithelial cells stimulate CD8+ T cells under similar culture conditions. Thus, CD1d expressed on intestinal epithelial cells may be an important ligand in CD8+ T cell-epithelial cell interactions.