The Short-term Effect of Peptidoglycan Injected into the Intestinal Lumen against Host Defense Responses

Numerous indigenous bacteria in the animal intestine are regulated by host defense responses.  In our previous studies, some host defense responses, such as the acceleration of epithelial cell migration and the transient secretion of bactericidal peptides from Paneth cells, were induced in the ileal intestinal villi with hyper-proliferation of indigenous bacteria.  In the present study, 1 mg/ml peptidoglycan (PGN), the dominant constituent of the bacterial cell wall, especially in Gram-positive bacteria, was administered into the lumen of intestinal loop made in a fasted rat jejunum and reacted for 30 min.  Then, the effect of PGN against host defense responses was investigated.  The results showed that Toll-like receptor (TLR)-2, a receptor for PGN, was detected in the striated borders of the villous columnar epithelial cells in the rat jejunum.  The number of proliferating epithelial cells in the intestinal crypt was greater in the PGN-injected group than the negative control group.  On the other hand, the disappearance of lysozyme-immunopositivity and the formation of vacuoles in Paneth cells were not increased by PGN injection.  From these findings, PGN might induce acceleration of the epithelial cell cycle-mediated host defense response in a short period of time, probably via TLR-2 expressed in the intestinal villi.