In this work we evaluated the specific immune activation properties of different Bifidobacterium strains, some of the most relevant intestinal microorganisms. To this end, we examined the in vitro effect of 12 Bifidobacterium strains belonging to 4 different species, Bifidobacterium longum, Bifidobacterium breve, Bifidobacterium bifidum and Bifidobacterium animalis subsp. lactis, on the maturation pattern of human monocyte-derived dendritic cells (DCs), as well as in their ability to induce cytokine secretion. In addition, we determined peripheral blood mononuclear cell (PBMC) proliferation and cytokine expression after exposure to bacterial strains. All bifidobacteria tested were able to induce full DC maturation but showed differences in the levels of cytokine production, especially IL-12, IL-10, TNFalpha and IL-1beta, suggesting that specific cytokine ratios could be used to predict the type of Th response that they may promote. In fact, analysis of cytokine production by PBMC showed that most of the tested B. animalis and B. longum strains induced the secretion of large amounts of IFNgamma and TNFalpha, in agreement with the Th1 profile suggested by DC cytokine production. Remarkably, three of four B. bifidum strains induced poor secretion of these cytokines and significant amounts of IL-17, the main product of Th17 cells, in accordance with the high IL-1beta/IL-12 ratio observed after DC stimulation. In conclusion, this work shows species and strain-specific immune effects of bifidobacteria and describes a valuable method for screening possible probiotic strains with different immunomodulatory properties. Notably, some B. bifidum strains seem to promote Th17 polarization, which could be useful for future probiotic applications.
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