Lactate metabolism is exploited by <i>Francisella tularensis</i> via its O-antigen capsule to limit macrophage-mediated activation and cell death

Intracellular pathogens manipulate host metabolic systems to establish a replicative niche and evade immune responses. However, mechanisms by which metabolism can be altered for successful infection remain poorly understood. Here, we identify a metabolism-based strategy used by the intracellular pathogen <i>Francisella tularensis</i> subsp. <i>tularensis</i> (FTT) to condition its host cell for optimal replication. FTT, through its associated O-antigen capsule, augmented lactate oxidation in macrophages, thereby improving mitochondrial bioenergetics, reducing redox imbalances and production of pro-inflammatory cytokines, and maintaining cellular viability. Heavy isotope tracing and extracellular flux analysis confirmed lactate incorporation into the TCA cycle with increased mitochondrial ATP production, and these outcomes were associated with upregulation of lactate import and conversion systems, specifically monocarboxylate transporter 4 (MCT4) and LDHB, respectively. Targeting MCT4 using the small molecule inhibitor MSC-4381, controlled FTT infection-mediated manipulation of mitochondrial function and increased cell death in infected macrophages, thereby controlling infection. MSC-4381 also directly controlled FTT infection in defined medium implicating a critical role of short-chain carbon intermediates among the bacteria. Ultimately, these data demonstrate that the lactate oxidation metabolic node, including MCT4 activity, is integral for successful FTT infection and contributes to the pleiotropic effects of the FTT capsule on immune evasion.IMPORTANCE<i>Francisella tularensis</i> subsp. <i>tularensis</i> (FTT) is an extremely virulent pathogen for which antibiotic intervention must be delivered early, often before proper diagnosis and effective, durable vaccines do not exist. Part of what makes FTT so problematic is that it is proficient at manipulating host metabolic pathways that govern immune activation. However, specific mechanisms by which this manipulation occurs have remained elusive. Discovering new ways in which FTT modulates host cell metabolism will reveal new therapeutic targets to help treat this difficult-to-control infection. We show here that FTT, through its O-antigen polysaccharide capsule, increased lactate oxidation, which in turn reduced inflammatory cytokine production, redox imbalance, and cell death. Targeting proteins responsible for lactate transport controlled FTT infection and blocked the capsule's ability to manipulate the host cell. These data are the first to show lactate oxidation as a potential target for pathogen success and may be a therapeutic target for FTT infection.