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

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. Author summary The intestine is colonized by complex microbial communities called the microbiota, which impacts diverse aspects of host physiology including development and function of the innate immune system. Neutrophils are phagocytic innate immune cells essential for host defense against infection. Neutrophil activity is strongly impacted by the microbiota but underlying mechanisms remain poorly defined. Here we show the evolutionarily-conserved secreted host protein Serum amyloid A (Saa) mediates microbiota-dependent effects on systemic neutrophil function. Saa is produced by the intestine and liver in response to the microbiota, but its in vivo functions have remained elusive. Using zebrafish, we demonstrate that Saa promotes neutrophil recruitment to peripheral injury yet restricts clearance of pathogenic bacterial infection. Analysis of isolated neutrophils revealed Saa reduces bactericidal activity and expression of pro-inflammatory genes in a microbiota-dependent manner. Transgenic expression of saa in the intestine and liver of saa mutants was sufficient to rescue mutant phenotypes, and intestinally-derived saa also alleviated defects in neutrophil recruitment to peripheral injury in germ-free zebrafish. Collectively, these data establish that Saa induced by the microbiota in the intestine signals systemically to neutrophils, tuning the extent to which they may be activated by other microbes or respond to injury.