Exposure of Intestinal Epithelial Cells to UV-killed Lactobacillus GG but Not Bifidobacterium breve Enhances the Effector Immune Response in vitro

Background: Intestinal bacteria and intestinal epithelial cells (IEC) may modulate the mucosal immune response. In this study, immune modulation by Lactobacillus GG (LGG) and Bifidobacterium breve (Bb1, Bb2) in the presence or absence of IEC was addressed in an in vitro transwell co-culture model. Methods: UV-killed LGG,Bb1, Bb2 or Toll-like receptor (TLR) 2 or nucleotide oligomerization domain (NOD) 2 ligands were added directly to unstimulated or anti-CD3/CD28-stimulated PBMC, or applied apically to human IEC (HT-29) co-cultured with PBMC. A mixture of live bacteria was used as reference. The effect on T helper 1 (IFN-γ, IL-12), T helper 2 (IL-13), inflammatory (TNF-α) and regulatory (IL-10) cytokine secretion was determined. Results: Both UV-killed LGG and Bb enhanced IL-12, IFN-γ, TNF-α and IL-10, and reduced IL-13 secretion when added directly to stimulated PBMC, similar to live bacteria. IEC reduced IL-13, IFN-γ and IL-10 secretion by stimulated PBMC. Apically added LGG, TLR2 and NOD2 ligands,but not Bb, enhanced IFN-γ, IL-12 and/or TNF-α secretion. Bacteria did not induce cytokine secretion when added to HT-29/unstimulated PBMC co-cultures, whereas direct incubations with PBMC did. Conclusion: UV-killed LGG as well as Bb supported a T helper 1 and/or regulatory phenotype when added directly to activated PBMC, similar to live bacteria. In contrast, LGG, TLR2 or NOD2 ligands – but not Bb – enhanced T helper 1 type cytokine secretion when added to IEC, while IL-10 secretion remained suppressed. Co-cultures combining IEC and PBMC may reveal differences between bacterial strains relevant for the in vivo situation.