No abstract

Bacterial access to the gut immune system is a crucial process to promote host immune responses. The probiotic L-92 strain of Lactobacillus acidophilus exerts anti-allergic immunomodulatory effects upon oral administration in mice. Here, we show that microfold cells (M cells) are responsible for L-92 internalization for evoking L-92-mediated immune responses. L-92 specifically bound to uromodulin, a glycosylphosphatidylinositol-anchored protein expressed exclusively on M cells among intestinal epithelial cells. Internalization of L-92 into M cells was significantly reduced in uromodulin-deficient (Umod-/-) mice compared to Umod+/+ mice. Furthermore, the binding of L-92 to uromodulin was significantly decreased after removal of surface layer protein A (SlpA) from the bacteria. Our study thus revealed a crucial role of uromodulin on the M-cell surface for the uptake of SlpA-positive lactic acid bacteria into M cells, possibly leading to subsequent delivery of the bacteria to dendritic cells closely associated with M cells for immunomodulation. Our study also shed light on the possibility that SlpA and uromodulin could be used as vehicle and target, respectively, for efficient mucosal vaccine delivery.

Lantibiotics are peptide-derived antibacterial substances produced by some Gram-positive bacteria and characterized by the presence of unusual amino acids, like lanthionines and dehydrated amino acids. Because lantibiotic producers may be attacked by self-produced lantibiotics, they express immunity proteins on the cytoplasmic membrane. An ATP-binding cassette (ABC) transport system mediated by the LanFEG protein complex is a major system in lantibiotic immunity. Multiple-sequence alignment analysis revealed that LanF proteins contain the E loop, a variant of the Q loop, which is a well-conserved motif in the nucleotide-binding domains (NBDs) of general ABC transporters. To elucidate E loop function, we introduced a mutation in the NukF protein, which is involved in the nukacin-ISK-1 immunity system. Amino acid replacement of glutamic acid in the E loop with glutamine (E85Q) resulted in slight decreases in the immunity level and transport activity. Additionally, the E85A mutation severely impaired the immunity level and transport activity. On the other hand, ATPase activities of purified E85Q and E85A mutants were almost similar to that of the wild type. These results suggested that the E loop found in ABC transporters involved in lantibiotic immunity plays a significant role in the function of these transporters, especially in the structural change of transmembrane domains.