Butyrate regulates neutrophil homeostasis and impairs early antimicrobial activity in the lung

Short-chain fatty acids (SCFAs) are metabolites that are produced following microbial fermentation of dietary fibre and impact cell metabolism and anti-inflammatory pathways both locally in the gut and systemically. In preclinical models, administration of SCFAs, such as butyrate, ameliorates a range of inflammatory disease models including allergic airway inflammation, atopic dermatitis and influenza infection. Here we report the effect of butyrate on a bacteria-induced acute neutrophil-driven immune response in the airways. Butyrate impacted discrete aspects of haematopoiesis in the bone marrow resulting in the accumulation of immature neutrophils. During Pseudomonas aeruginosa infection, butyrate treatment led to enhanced mobilization of neutrophils to the lungs as a result of increased CXCR2 expression by lung macrophages. Despite this increase in granulocyte numbers and their enhanced phagocytic capacity, neutrophils failed to control early bacterial growth. Butyrate reduced expression of nicotinamide adenine dinucleotide phosphate (NADPH), oxidase complex components required for reactive oxygen species (ROS) production, and reduced secondary granule enzymes, culminating in impaired bactericidal activity. These data reveal that SCFAs tune neutrophil maturation and effector function in the bone marrow under homeostatic conditions, potentially to mitigate against excessive granulocyte-driven immunopathology, but their consequently restricted bactericidal capacity impairs early control of Pseudomonas infection.