Intestinal Serum amyloid A suppresses systemic neutrophil activation and bactericidal activity in response to microbiota colonization.

Caitlin C Murdoch,Olaf Mueller,David M Tobin,John F Rawls,Molly A Matty,Scott T Espenschied

doi:10.1371/journal.ppat.1007381

Abstract

The intestinal microbiota influences the development and function of myeloid lineages such as neutrophils, but the underlying molecular mechanisms are unresolved. Using gnotobiotic zebrafish, we identified the immune effector Serum amyloid A (Saa) as one of the most highly induced transcripts in digestive tissues following microbiota colonization. Saa is a conserved secreted protein produced in the intestine and liver with described effects on neutrophils in vitro, however its in vivo functions remain poorly defined. We engineered saa mutant zebrafish to test requirements for Saa on innate immunity in vivo. Zebrafish mutant for saa displayed impaired neutrophil responses to wounding but augmented clearance of pathogenic bacteria. At baseline, saa mutants exhibited moderate neutrophilia and altered neutrophil tissue distribution. Molecular and functional analyses of isolated neutrophils revealed that Saa suppresses expression of pro-inflammatory markers and bactericidal activity. Saa’s effects on neutrophils depended on microbiota colonization, suggesting this protein mediates the microbiota’s effects on host innate immunity. To test tissue-specific roles of Saa on neutrophil function, we over-expressed saa in the intestine or liver and found that sufficient to partially complement neutrophil phenotypes observed in saa mutants. These results indicate Saa produced by the intestine in response to microbiota serves as a systemic signal to neutrophils to restrict aberrant activation, decreasing inflammatory tone and bacterial killing potential while simultaneously enhancing their ability to migrate to wounds.

Highlights

The vertebrate intestine is densely colonized with complex communities of micro-organisms, collectively referred to as the intestinal microbiota
To test tissue-specific roles of Serum amyloid A (Saa) on neutrophil function, we over-expressed saa in the intestine or liver and found that sufficient to partially complement neutrophil phenotypes observed in saa mutants. These results indicate Saa produced by the intestine in response to microbiota serves as a systemic signal to neutrophils to restrict aberrant activation, decreasing inflammatory tone and bacterial killing potential while simultaneously enhancing their ability to migrate to wounds
Saa mRNA was not upregulated at 6 hours post amputation in WT larvae (S1M Fig), demonstrating acute saa induction did not affect neutrophil activity

Summary

Introduction

The vertebrate intestine is densely colonized with complex communities of micro-organisms, collectively referred to as the intestinal microbiota. Studies using gnotobiotic animals have demonstrated that microbiota colonization is required for the normal development of an innate immune system capable of mounting appropriate responses to diverse challenges such as infection and injury [1]. Despite being spatially confined to the intestinal lumen by physical and chemical barriers such as the intestinal epithelium and mucus, the microbiota influences both local and systemic host immune development and function [2, 3]. Intestinal epithelial cells (IECs) serve as the primary host interface with the intestinal microbiota and secrete a myriad of factors following microbial colonization [4]. Previous studies have identified a secreted host factor, Serum amyloid A (Saa), that is potently up-regulated in the intestine following microbial colonization in zebrafish and mice [7,8,9,10,11]. SAA’s high degree of evolutionary conservation coupled with its strong induction following inflammatory stimuli suggests important roles for SAA in health and disease

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Results

Conclusion