S-layer protein modulates the stimulatory effects of Lactobacillus acidophilus CICC 6074 by triggering PKC signaling cascade in RAW 264.7 cells

Lactic acid bacteria (LAB) are demonstrably beneficial on the digestive and immune systems. S-layer protein (SlpA), as the outermost component of the cell wall, exerts an anti-inflammatory activity in lactobacilli, while the pro-inflammatory properties of SlpA in cross-talk with immune cells have rarely been investigated. Here, we showed that SlpA isolated from Lactobacillus acidophilus CICC 6074 exhibits more pronounced pro-inflammatory behavior through activating MAPK and NF-κB signaling, which was further corroborated by an increase in the NF-κB translocation from cytoplasm to the nucleus and DNA binding activity of NF-κB, triggered by SlpA treatment. Notably, a specific PKC inhibitor, chelerythrine, significantly blocked SlpA-elicited pro-inflammatory response, indicating that SlpA could act as stimulators of the innate immune system by triggering PKC to MAPK and NF-κB signaling pathway. This study provides important insights on the immunomodulatory properties of cell surface components in lactobacilli.