Bifidobacteria BbC50 Fermentation Products Induce Human Cd4+ Regulatory T Cells with Antigen-Specific Activation and Bystander Suppression

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-10 secretion by human dendritic cells. As IL-10 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 co-cultured 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-γ 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-10 and TGF-β, 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.

Induction of porcine regulatory cells by mycophenolic Acid-treated dendritic cells

Tolerance induction in murine allogeneic transplantation is relatively easy, often by induction of regulatory T cells (Treg). Unfortunately, the implementation of these models in clinical situations has not yielded reliable protocols of tolerance induction in humans. Our project sought to create a preclinical model of tolerance induction in large animals. Our current efforts seek to induce and characterize porcine Treg, obtaining dendritic cells (DC) able to preferentially stimulate them. DCs were differentiated from blood monocytes with porcine recombinant interleukin-4 (IL-4) and granulocyte-macrophage colony-stimulating factor (GM-CSF) for 6 days. These DCs were then stimulated by human CD40 ligand-transfected L cells with or without mycophenolic acid (MPA) for 48 hours. We analyzed surface marker expression, cytokine synthesis, and ability to stimulate allogeneic peripheral blood mononuclear cells (PBMC). The porcine lymphocytes underwent 4 rounds of 1-week stimulation with allogeneic DC treated or not with MPA. At the end of this coculture we analyzed their capacity to suppress allogeneic PBMC proliferation induced by mature DC. Our results showed that porcine DCs pretreated with MPA display a low expression of B7 costimulatory molecules, produce low levels of IL-12, and induce weak proliferation of allogeneic lymphocytes. Moreover, after 4 rounds of stimulation with MPA-treated DCs, PBMCs were able to inhibit an alloreactive response. These preliminary results suggested induction of a regulatory T-cell population that we are currently seeking to characterize.

Mycophenolic Acid Inhibits p38 Mitogen-Activated Protein Kinase in Human Monocyte-Derived Dendritic Cells Stimulated by Lipopolysaccharide

Dendritic cell (DC) maturation, a crucial stage in the immune response, can be induced by various stimuli, such as lipopolysaccharide (LPS). Maturation signals trigger up-regulation of costimulatory molecule expression, increasing the ability of DCs to prime T helper cells. We and others have previously reported that mycophenolic acid (MPA) inhibits DC maturation and activation. However, the mechanisms remain unknown. The primary effect of MPA is inhibition of inosine monophosphate dehydrogenase (IMPDH), an enzyme involved in the de novo synthesis of guanosine nucleotide. The process of DC maturation is highly dependent on mitogen-activated protein kinase (MAPK) phosphorylation, especially p38MAPK. We therefore decided to study whether MPA affects these processes. Human monocyte-derived DCs were activated by LPS in the presence or absence of MPA. To assess whether the depletion of guanine affected p38MAPK phosphorylation, increasing doses of exogenous guanosine were added before stimulation. The results by flow cytometry showed that MPA inhibited p38MAPK phosphorylation by 25%. Interestingly, exogenous guanosine did not reverse the MPA inhibition. Our results suggested that MPA inhibits p38MAPK activity independent of IMPDH in human DCs. This effect of MPA may explain its capacity to inhibit maturation marker expression on DCs.