We investigated whether commensal bacteria modulate activation and maturation of bone marrow-derived DC and their ability to prime CD4 + T cells. We used Escherichia coli mpk, which induces colitis in gnotobiotic IL-2-deficient (IL-2 -/-) mice, and Bacteroides vulgatus mpk, which prevents E. coli-induced colitis. Stimulation of DC with E. coli induced TNF-a, IL-12 and IL-6 secretion and expression of activation markers. Moreover, stimulation of DC with E. coli increased T cell activation and led to Th1 polarization. Stimulation with B. vulgatus led only to secretion of IL-6, and DC were driven into a semi-mature state with low expression of activation markers and did not promote Th1 responses. B. vulgatus-induced semi-mature DC were non-responsive to stimulation by E. coli in terms of maturation, T cell priming and TNF-a but not IL-6 production. The non-responsiveness of B. vulgatus-stimulated DC was abrogated by addition of anti-IL-6 mAb or mimicked with rIL-6. These data suggest that B. vulgatusinduced IL-6 drives DC into a semi-mature state in which they are non-responsive to proinflammatory activation by E. coli. This in vitro mechanism might contribute to the prevention of E. coli-triggered colitis development by B. vulgatus in vivo; high IL-6 mRNA expression was consistently found in B. vulgatus-colonized or B. vulgatus/E. coli co-colonized IL-2 -/-mice and was associated with absence of colitis.

Stimulation of naive mouse dendritic cells (DC) with LPS or Pam3CSK4 (P3C) induces production of TNF-α via TLR4- or TLR2-signaling. Although tolerance in macrophages has been studied in detail, we investigated the role of TLR agonist concentration and IL-6 for tolerance in DC. P3C- or LPS-primed DC were nonresponsive to P3C or LPS restimulation in terms of TNF-α but not IL-6 production. The mechanisms involved in tolerance were dependent on the concentration of the TLR ligand used for DC priming. DC primed with LPS or P3C at high concentrations developed a maturation dependent, IL-6 independent tolerance associated with inhibition of TLR signaling upstream of IκB as indicated by decreased IκB degradation. In contrast, priming of DC with LPS or P3C at low concentrations resulted in IL-6-dependent tolerance, which was abolished in IL-6 deficient DC, and was not accompanied by maturation of DC or by down-regulation of TLR2 or TLR4. In homotolerogenic DC primed with LPS or P3C at high concentrations, degradation of IκB upon restimulation with LPS or P3C was inhibited suggesting tolerance mechanism(s) upstream of IκB; in contrast, cross-tolerance in DC primed with LPS or P3C at low concentrations was not associated with reduced IκB degradation suggesting tolerance mechanisms downstream of IκB. Our data indicate that in naive DC TLR4- and TLR2-stimulation results in homo- and cross-tolerance; the mechanisms involved in tolerance depend on the concentration of the TLR agonist used for DC priming and are governed by IL-6 and maturation.

Our results lead to the conclusion that L. fermentum inhibits the Y. enterocolitica-induced IL-8 production by a possibly secreted phospholipid of <10 kDa molecular weight. These data suggest that L. fermentum may have probiotic properties modulating intestinal inflammatory responses and might offer new therapeutic strategies in the treatment of intestinal inflammatory diseases.