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 enzyme-linked 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. Whole-blood 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.

Fig. 1

MHCII molecules are upregulated in IAP-KO mice (a) com-pared with WT mice (b)

Fig. 2

WT and IAP-KO mice that did not receive S. typhimurium gavage (−) did not experience weight loss. However, with S. typhimurium gavage (?), WT mice had an associated greater percentage weight loss compared with IAP-KO mice (a), and also exhibited [2–fold increased bacterial translocation to mesenteric nodes and remote organs (b). (**P \.005, *P \.05)

Fig. 3

There was no significant difference in white blood cell count after intraperitoneal LPS injection (a) but a 50% decrease in platelet count in WT mice (b). Serum IL-6 was significantly increased (1.8-fold) in WT compared with IAP-KO mice (c). (***P \ .0005, *P \ .05)

Fig. 4

Whole-blood stimulation with varying amounts of LPS (lL) and Pam-3-Cys (lL) resulted in significantly decreased IL-6 (a) and a trend towards decreased TNF-alpha secretion in WT mice compared with IAP-KO mice (b). (**P \ .005, *P \ .05)