Synergy between Probiotic Lactobacillus casei and Milk to Maintain Barrier Integrity of Intestinal Epithelial Cells

We hypothesized that Lactobacillus casei BL23 and milk work synergistically to prevent damage to epithelial barrier integrity induced by pro-inflammatory cytokines. To test this, barrier disruption was induced in polarized Caco-2 monolayers by sequential, basolateral treatment with IFN-γ and TNF-α. Apical application of either 25% v/v reconstituted skim milk (RSM) or ultra high temperature (UHT) milk (2% fat) prior to cytokine exposure reduced losses to transepithelial electrical resistance (TER). Permeability to fluorescein isothiocyanate-dextran (FD-4; 4 kDa) was also significantly reduced in the presence of 25% v/v UHT milk ( P < 0.05) but not RSM. Protection against increases in paracellular permeability was even greater when cell-free preparations of L. casei BL23 fermented UHT milk or fermented RSM were applied. The permeability coefficients of cells incubated with BL23 fermented UHT milk were equivalent to the untreated controls ( P = 0.12) and those cells also produced 247.6 ± 35.5 pg/mL IL-8, quantities significantly lower than found for cytokine-treated controls (353.9 ± 40.0 pg/mL). The benefits of the fermented milk were also confirmed by the reduced expression of TNF receptor 2 (TNFR2), myosin light-chain kinase (MLCK), and claudin-encoding genes relative to the controls. By comparison, apical application of viable L. casei onto the Caco-2 cells did not result in protection from the barrier-disruptive actions of IFN-γ and TNF-α. These results indicate that milk can maintain intestinal barrier integrity during pro-inflammatory cytokine exposure and that this is enhanced by modifications to milk matrix caused by prior incubation with L. casei BL23.