We investigated whether certain strains of lactic acid bacteria (LAB) could antagonize specific T-helper functions in vitro and thus have the potential to prevent inflammatory intestinal immunopathologies. All strains tested induced various levels of both interleukin-12 (IL-12) and IL-10 in murine splenocytes. In particular, Lactobacillus paracasei (strain NCC2461) induced the highest levels of these cytokines. Since IL-12 and IL-10 have the potential to induce and suppress Th1 functions, respectively, we addressed the impact of this bacterium on the outcome of CD4 ؉ T-cell differentiation. For this purpose, bacteria were added to mixed lymphocyte cultures where CD4؉ T-cells from naive BALB/c mice were stimulated weekly in the presence of irradiated allogeneic splenocytes. In these cultures, L. paracasei NCC2461 strongly inhibited the proliferative activity of CD4 ؉ T cells in a dose-dependent fashion. This was accompanied by a marked decrease of both Th1 and Th2 effector cytokines, including gamma interferon, IL-4, and IL-5. In contrast, IL-10 was maintained and transforming growth factor ␤ (TGF-␤) was markedly induced in a dose-dependent manner. The bacteria were not cytotoxic, because cell viability was not affected after two rounds of stimulation. Thus, unidentified bacterial components from L. paracasei NCC2461 induced the development of a population of CD4 ؉ T cells with low proliferative capacity that produced TGF-␤ and IL-10, reminiscent of previously described subsets of regulatory cells implicated in oral tolerance and gut homeostasis.

Background: Mechanisms of systemic IgE suppression by oral tolerance have been extensively studied, but less is known about oral tolerance induction in mice challenged at mucosal sites. We have previously shown in systemically challenged mice that high-dose tolerance suppressed specific but not bystander IgE. In an attempt to mimic oral tolerance in food-allergic patients, we have investigated how IgE suppression could be induced in mice sensitized orally against β-lactoglobulin (BLG). Methods: Mice were immunized orally against BLG using cholera toxin as adjuvant. Before oral sensitization, mice were administered milk whey proteins, either in the form of a single high-dose gavage, or by prolonged ad libitum administration of various doses. Results: Orally sensitized mice mounted a BLG-specific IgE response. In contrast to systemically challenged mice, a single high-dose gavage of whey protein given prior to the onset of oral sensitization resulted in the suppression of both specific and bystander IgE. When mice were fed moderate to low doses of milk whey proteins daily ad libitum in the drinking water during 3 weeks prior to oral sensitization, all doses effectively suppressed antigen-specific IgE. However, bystander IgE suppression was observed only at the lowest doses. When mice were tolerized during 4 days instead of 3 weeks, IgE titers remained unchanged. Conclusions: In orally sensitized mice, bystander IgE suppression depended on the dose of tolerogen, but also on its mode of administration. Mucosally induced IgE responses were suppressed by a mechanism that was distinct from that operating in the periphery.

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