A Role for Intestinal Alkaline Phosphatase in the Maintenance of Local Gut Immunity

Abstract

Background and Aims-Intestinal alkaline phosphatase (IAP) is a gut mucosal defense factor known to dephosphorylate lipopolysaccharide (LPS); however, the role of IAP in the gut response to luminal bacteria remains poorly defined. We investigated immune responses of wild-type (WT) and IAP-knockout (IAP-KO) mice to LPS and Salmonella typhimurium challenges. Methods-Cryostat sectioning and standard indirect immunohistochemical staining for major histocompatibility complex (MHC) class II molecules were performed on liver tissue from WT and IAP-KO mice. WT and IAP-KO mice were orally gavaged with S. typhimurium; bacterial translocation to mesenteric nodes, liver, and spleen was determined by tissue homogenization and plating. In other experiments, WT and IAP-KO mice received intraperitoneal injections of LPS, with subsequent quantification of complete blood counts and serum interleukin (IL)-6 by enzymelinked immunosorbent assay (ELISA). WT and IAP-KO whole blood were plated and stimulated with LPS and Pam-3-Cys, followed by cytokine assays. Results-Immunohistologic liver examinations showed increased expression of MHC class II molecules in IAP-KO mice. Following S. typhimurium challenge, WT mice appeared moribund compared with IAP-KO mice, with increased bacterial translocation. WT mice had [50% decrease (P \ .005) in platelets and 1.8-fold (P \ .05) increased serum IL-6 compared with IAP-KO mice in response to LPS injections. IAP-KO whole-blood stimulation with LPS and Pam-3-Cys resulted in increased IL-6 and tumor necrosis factor (TNF)-alpha secretion compared with WT. Conclusions-IAP-KO mice exhibit characteristics consistent with local LPS tolerance. Wholeblood response of IAP-KO mice did not reflect systemic tolerance. These data suggest that IAP is a local immunomodulating factor, perhaps regulating LPS-toll-like receptor 4 (TLR4) interaction between commensal microflora and intestinal epithelium.

Intestinal alkaline phosphatase promotes gut bacterial growth by reducing the concentration of luminal nucleotide triphosphates

“…In the present study we provide evidence that the brush-border enzyme IAP promotes the growth of commensal bacteria. IAP exerts a number of beneficial effects on the gut, including the maintenance of local gut immunity (8), enhancing barrier function, reducing excessive inflammation, and inhibiting metabolic endotoxemia (4,12,14,21,26). The present study delineates another key beneficial impact of the IAP enzyme, promoting the growth of the commensal flora.…”

“…Much recent progress has been in made in delineating the physiological and pharmacological properties of the IAP enzyme. IAP has the ability to dephosphorylate a variety of bacterial and hostderived ligands (LPS, CpG DNA, flagellin, UDP), each of which works through a specific receptor to exert its inflammatory impact on target cells (8,39). Here we show that IAP reverses the growth-inhibitory effects of various nucleotide triphosphates, indicating that these nucleotides are also IAP targets (see Figs.…”

Intestinal alkaline phosphatase and sodium butyrate may be beneficial in attenuating LPS-induced intestinal inflammation

“…Recently, the role of IAP in detoxifying LPS and reducing the inflammatory response has been investigated (Koyama et al, 2002;Bates et al, 2007;Bol-Schoenmakers et al, 2010;Chen et al, 2011). It has been demonstrated that early weaning of piglets reduces the expression of IAP mRNA, and may predispose animals to enteric infections and diarrhea events, resulting in poor health and impaired growth (Lackeyram et al, 2010).…”

Cellular function and molecular structure of ecto-nucleotidases

“…Recent evidence suggests that IAP expressed on apical surfaces of microvillus membranes of enterocytes detoxifies LPS in the intestine, thereby preventing inflammation, sustaining barrier function, and maintaining homeostasis within intestinal flora [501][502][503][504]. This hypothesis is supported by the analysis of IAP knockout mice [505,506]. In addition, LPS-dephosphorylating activity of gut-derived IAP has been suggested to be part of the endogenous hepatocyte defense system against LPS [507].…”