Probiotic bacteria have been shown to have health benefits in various situations (inflammation, allergy, infection). We previously showed that a bacteria-free fermentation product of Bifidobacterium breve C50 (BbC50sn) induced high IL-IO secretion by human dendritic cells. As IL-IO is a regulatory cytokine, the aim of the present study was to examine whether DCs cultured in the presence of BbC50sn could induce regulatory T cells in an allogeneic context. Purified CD4+CD25-human T cells were cocultured with allogeneic BbC50sn-treated dendritic cells for 4 weeks. The T cell population (BbC50sn-T) was analysed both at phenotypical and functional [ability to inhibit a mixed lymphocyte reaction (MLR)] levels. We showed that T lymphocytes acquired phenotype characteristics of regulatory T cells after 4 weeks of co-culture with BbC50sn-DCs, and inhibited in vitro T lymphocyte proliferation and IFN-y production in an MLR. Transwell experiments demonstrated that this suppressive activity was not T cell contact-dependent but probably mediated by a soluble factor. Although BbC50sn-T cells secreted significant amounts of IL-IO and TGF-p, their suppressive effect is most likely not mediated through these cytokines. This is, to our knowledge, the first demonstration of in vitro regulatory T cell induction by a bacteria-free fermentation product in an allogeneic context. Dendritic cells (DCs) are professional antigenpresenting cells that have a key role in the immune response, These cells are present throughout the body, including the mucosa where they orchestrate innate and adaptive immune responses by means of their pattern recognition receptors such as Toll Like Receptors (TLR). DCs can either prime naive T cells to elicit an effector immune response against infectious agents or promote immune tolerance to innocuous antigens such as food and commensal bacteria (l). A breakdown in commensal bacteria tolerance resulting from DC dysfunction in the
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