De novo spermidine synthesis by hypervirulent Francisella tularensis alters polyamine metabolism in infected human macrophages and suppresses proinflammatory response

Background Francisella tularensis , a zoonotic and facultative intracellular pathogen, causes tularemia. This Tier 1 select agent replicates in infected macrophages. Only subtype A.I strains, which are some of the most pathogenic bacteria known, can synthesize spermidine de novo . The contribution of this trait to macrophage immunomodulation was investigated in this study. Methods Primary human macrophages were infected with F. tularensis . Polyamine precursor and polyamine profiles were obtained by using ultra-performance liquid chromatography coupled with tandem mass spectrometry. Relative levels of macrophage transcripts and proteins involved in arginine and polyamine metabolism were assessed using reverse transcription-quantitative PCR and immunoblot analysis, respectively. M1 proinflammatory and M2 anti-inflammatory surface markers were evaluated using fluorescence-activated cell sorting and secreted cytokines were quantified utilizing fluorescence multiplex assays. Results Hypervirulent F. tularensis A.I strains SCHU S4 and MA00–2987 replicated considerably faster in infected macrophages relative to their isogenic Δ speE mutant and type B LVS. SCHU S4 infected macrophages had reduced spermidine and elevated spermine levels, whereas the converse was the case for SCHU S4 Δ speE infected macrophages. SCHU S4 infected and uninfected macrophages shared a higher abundance of M2 anti-inflammatory CD206 to M1 proinflammatory CD86 surface markers and secreted substantially lower amounts of proinflammatory cytokines TNF-α, IL-1β, and IL-6 relative to SCHU S4 Δ speE infected macrophages. Conclusion Together these results indicate that de novo spermidine synthesis by hypervirulent F. tularensis A.I strains in infected human macrophages promotes intracellular persistence, modulates polyamine metabolism, and suppresses M1 proinflammatory activation, underscoring the importance of polyamines in macrophage function.