2018 Volume 21 Pages 18-22
Bacteroides fragilis is a member of normal intestinal flora and is known as an opportunistic infectious bacterium causing intraperitoneal abscess or septicemia. It has been shown that the bacterium activates the intestinal innate immune system via Toll-like receptor 2 (TLR2), and the relation with intestinal disease has been drawing attention.
The cell surface of this bacterium is composed of capsular polysaccharide (CPS), lipopolysaccharide (LPS) and lipoprotein (LP). Capsular polysaccharide A (PSA), a type of CPS, is a zwitterionic polysaccharide and is known to induce regulatory T cells in the intestinal tract and inhibit enteritis. In recent years, it has been reported that PSA activates TLR2 which is an innate immune receptor. However, the structure of PSA is largely different from the general recognition structure of TLR2, and the possibility of contamination could be considered. In this study, we separated the PSA fraction from B. fragilis and examined the substances involved in TLR2 activation.
B. fragilis cells were subjected to hot water-phenol extraction and followed by hydrophobic chromatography separation to obtain a PSA fraction having the ability to activate TLR2. The fraction was further subjected to SDS-PAGE separation based on their molecular weight. TLR2-stimulating activity was scarcely observed in the high molecular mass area where PSA was present, and high activities were observed in the low molecular mass area. The active substances were found to be proteins because they were digested with proteolytic enzymes. These results indicate that contaminating proteins are substances responsible for TLR2 activation in the PSA fraction derived from B. fragilis.