Macrophages recycle phagocytosed bacteria to fuel immunometabolic responses

Macrophages specialize in phagocytosis, a cellular process that eliminates extracellular matter, including microbes, through internalization and degradation. Despite the critical role of phagocytosis during bacterial infection, the fate of phagocytosed microbial cargo and its impact on host cell is poorly understood. Here, we reveal that ingested bacteria constitute an alternative nutrient source that skews immunometabolic host responses. Tracing stable isotope-labelled bacteria, we found that phagolysosomal degradation of bacteria provides carbon atoms and amino acids that are recycled into various metabolic pathways, including glutathione and itaconate biosynthesis, and satisfy macrophage bioenergetic needs. Metabolic recycling of microbially-derived nutrients is regulated by the nutrient sensing mTORC1 and intricately tied to microbial viability. Dead bacteria, as opposed to live ones, sustain the cellular adenosine monophosphate (AMP) pool and subsequently activate AMP protein kinase (AMPK) to inhibit mTORC1. Consequently, killed bacteria strongly fuel metabolic recycling, but elicit decreased reactive oxygen species (ROS) production and a reduced IL-1β secretion compared to viable bacteria. These results reveal a novel insight into the fate of engulfed microbes and highlights a microbial viability-associated metabolite that triggers host metabolic and immune responses. Our findings hold promise for shaping immunometabolic intervention in various immune-related pathologies.

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