Trained immunity of alveolar macrophages depends on metabolic rewiring and type 1 interferon signaling (RNA-Seq)

Environmental microbial triggers shape the development and functionality of the immune system. Alveolar macrophages (AMs), tissue-resident macrophages of the lungs, are in constant and direct contact with inhaled particles and microbes. Such exposures likely impact AM reactivity to subsequent challenges by immunological imprinting mechanisms referred to as trained immunity. Here, we investigated whether a ubiquitous microbial compound has the potential to induce AM training in vivo. We showed that intranasal exposure to ambient amounts of lipopolysaccharide (LPS) protected mice from bacterial pneumonia six days later and discovered a pronounced AM memory response, characterized by enhanced reactivity upon pneumococcal challenge. Exploring the mechanistic basis of AM training, we identified a critical role of type 1 interferon signaling and found that trained AMs displayed a substantially modulated metabolite and lipid composition. Inhibition of fatty acid oxidation and glutaminolysis significantly attenuated the training effect, suggesting a key function of metabolic rewiring in LPS-induced AM memory. Collectively, our findings demonstrate the profound impact of ambient microbial exposure on pulmonary immune memory and highlight tissue-specific features of trained immunity. RNA-seq analysis of trained and control AMs following secondary (ex vivo) challenge: Wild type mice received LPS (Sigma; E.coli O55:B5) or endotoxin-free saline intranasally (i.n.). Six days later, LPS-exposed ("trained") and saline-exposed ("control") AMs were isolated by broncho-alveolar lavage from biological replicates (n=5 per group). Cells were counted, seeded (2 wells for each biological replicate) and left to adhere. After 2 h, non-adherent cells were removed and each biological replicate was incubated with heat-inactivated Streptococcus pneumoniae (HISP) in medium or with medium only for 3 h, generating a total of 20 samples.