D-alanine Inhibits Murine Intestinal Inflammation by Suppressing IL-12 and IL-23 Production in Macrophages

Abstract

Background and Aims: Free D-amino acids, which have different functions from L-amino acids, have recently been discovered in various tissues. However, studies on the potential interactions between intestinal inflammation and D-amino acids are limited. We examined the inhibitory effects of D-alanine on the pathogenesis of intestinal inflammation. Methods: We investigated serum D-amino acid levels in 40 patients with ulcerative colitis and 34 healthy volunteers. For 7 days [d], acute colitis was induced using dextran sulphate sodium in C57BL/6J mice. Plasma D-amino acid levels were quantified in mice with dextran sulphate sodium-induced colitis, and these animals were administered D-alanine via intraperitoneal injection. IFN-γ, IL-12p35, IL-17A, and IL-23p19 mRNA expression in the colonic mucosa was measured using real-time polymerase chain reaction [PCR]. In vitro proliferation assays were performed to assess naïve CD4+ T cell activation under Th-skewing conditions. Bone marrow cells were stimulated with mouse macrophage-colony stimulating factor to generate mouse bone marrow-derived macrophages. Results: Serum D-alanine levels were significantly lower in patients with ulcerative colitis than in healthy volunteers. Dextran sulphate sodium-treated mice had significantly lower plasma D-alanine levels than control mice. D-alanine-treated mice had significantly lower disease activity index than control mice. IFN-γ, IL-12p35, IL-17A, and IL-23p19 mRNA expression levels were significantly lower in D-alanine-administered mice than in control mice. D-alanine suppressed naïve T cell differentiation into Th1 cells in vitro, and inhibited the production of IL-12p35 and IL-23p19 in bone marrow-derived macrophages. Conclusions: Our results suggest that D-alanine prevents dextran sulphate sodium-induced colitis in mice and suppresses IL-12p35 and IL-23p19 production in macrophages.