# Gut microbiome-derived propionate reprograms alveolar macrophages metabolically and regulates lung injury responses in mice

## TL;DR

A fiber-rich diet changes lung immune cells and affects lung injury responses in mice through gut bacteria-produced propionate.

## Contribution

The study shows that gut microbiome-derived propionate can reprogram lung macrophages and influence lung injury outcomes.

## Key findings

- A fiber-rich diet reprogrammed alveolar macrophages and reduced inflammation after sterile lung injury.
- The effect of the diet could be transferred to germ-free mice via fecal microbiome transplantation.
- Propionate, a short-chain fatty acid, altered lung macrophage metabolism without relying on FFAR or chromatin remodeling.

## Abstract

Responses to lung injury can vary between individuals with the diet and gut microbiome representing two underappreciated sources for this variability. The gut microbiome can influence lung injury outcomes through the gut‒lung axis, but exactly how diet and its effects on the microbiota are involved remains unclear. We hypothesized that dietary fiber interventions would favor the presence of short-chain fatty acid (SCFA)-producing fermentative bacteria presence in the gut microbiome, thereby influencing the resting lung immunometabolic tone as well as influencing downstream responses to lung injury and infection. To test this hypothesis, we fed mice fiber-rich (FR) and fiber-free (FF) diets, and observed changes in the steady-state transcriptional programming of alveolar macrophages (AM). Next, we examined the effects of the FR and FF diets on murine responses to sterile and infectious lung injury in vivo while simultaneously profiling the gut microbiota and SCFA levels transmitted along the gut‒lung axis. Finally, we validated our in vivo observations with mechanistic studies of the metabolic, signaling, and chromatin-modifying effects of specific SCFAs on lung AM ex vivo and in vitro. Overall, our fiber-rich diet reprogrammed AMs and attenuated lung inflammation after sterile injury while exacerbating lung infection. This effect of FR diets could be transferred to germ-free (GF) mice by fecal microbiome transplantation (FMT) and depended on the ability of the microbiota to produce propionate. Mechanistically, SCFAs altered the metabolic programming of AMs and lung tissue ex vivo without a clear role for free fatty acid receptors (FFAR) or chromatin remodeling. These findings demonstrate that the gut‒lung axis can regulate resting lung metabolic tone through dietary fiber intake and the enrichment of SCFA-producing gut bacteria, as well as influence sterile and non-sterile lung injury responses. These results provide evidence to support the development of therapeutic dietary interventions to preserve or enhance specific aspects of host pulmonary immunity.