MDP-NOD2 stimulation induces HNP-1 secretion, which contributes to NOD2 antibacterial function

Background: Human neutrophil peptide 1 (HNP-1) is a defensin with antibacterial activity secreted by various cells as a component of the innate immune host defense. NOD2 is a cytoplasmic protein that recognizes bacterial derived muramyl dipeptide, and is involved in bacterial clearance. The aim of the present study was to investigate the relationship between antibacterial activity of NOD2 and HNP-1 expression in epithelial cell lines. Methods: Gentamicin protection assay using Salmonella typhimurium was performed in Caco-2 cells. The mRNA level was determined by quantitative reverse-transcription polymerase chain reaction (RT-PCR) and defensin expression was assessed by Western blot and enzyme-linked immunosorbent assay (ELISA). Nuclear factor-kB activation was assessed, using pIV luciferase and Renilla Plasmids. A NOD2 mutant was generated by site-directed mutagenesis. Results: Among the defensins tested, only HNP-1 expression, is induced in colonic epithelial model HCT 116 cells after MDP-LD stimulation. HNP-1 secretion is significantly increased after MDPLD stimulation in the cell supernatant of intestinal epithelial cells expressing endogenous NOD2, but not in cells that lack endogenous NOD2 expression. HNP-1 is required for NOD2-dependent NF-kB activation after MDP-LD stimulation since hnp-1 siRNA transfection abrogated the response to MDP-LD stimulation. The antibacterial function of NOD2 against S. typhimurium was impaired when expression of HNP-1 was blocked by siRNA. Conclusions: HNP-1 secretion depends on NOD2 stimulation by MDP-LD and contributes to antibacterial activity in intestinal epithelial cells expressing endogenous NOD2, but not NOD2 3020insC mutant associated with increased susceptibility to Crohn's disease.