Antioxidant Defenses of Francisella tularensis Modulate Macrophage Function and Production of Proinflammatory Cytokines

Abstract

Francisella tularensis, the causative agent of a fatal human disease known as tularemia, has been used in the bioweapon programs of several countries in the past, and now it is considered a potential bioterror agent. Extreme infectivity and virulence of F. tularensis is due to its ability to evade immune detection and to suppress the host's innate immune responses. However, Francisella-encoded factors and mechanisms responsible for causing immune suppression are not completely understood. Macrophages and neutrophils generate reactive oxygen species (ROS)/reactive nitrogen species as a defense mechanism for the clearance of phagocytosed microorganisms. ROS serve a dual role; at high concentrations they act as microbicidal effector molecules that destroy intracellular pathogens, and at low concentrations they serve as secondary signaling messengers that regulate the expression of various inflammatory mediators. We hypothesized that the antioxidant defenses of F. tularensis maintain redox homeostasis in infected macrophages to prevent activation of redox-sensitive signaling components that ultimately result in suppression of pro-inflammatory cytokine production and macrophage microbicidal activity. We demonstrate that antioxidant enzymes of F. tularensis prevent the activation of redox-sensitive MAPK signaling components, NF-B signaling, and the production of pro-inflammatory cytokines by inhibiting the accumulation of ROS in infected macrophages. We also report that F. tularensis inhibits ROS-dependent autophagy to promote its intramacrophage survival. Collectively, this study reveals novel pathogenic mechanisms adopted by F. tularensis to modulate macrophage innate immune functions to create an environment permissive for its intracellular survival and growth.Francisella tularensis is a Gram-negative intracellular pathogen and the causative agent of a fatal human disease known as tularemia. F. tularensis is classified into four subspecies as follows: F. tularensis subspecies tularensis; F. tularensis subspecies holarctica; F. tularensis subspecies mediasiatica, and F. tularensis subspecies novicida. All classifications are based on virulence, genetics, and metabolic characteristics. F. tularensis subspecies tularensis (type A) is the most virulent of all four Francisella subspecies. About 70% of tularemia cases in North America are a result of type A Francisella with an infectivity dose of less than 10 colony-forming units (cfu) in humans (1). The live vaccine strain (LVS) 3 is a derivative of Russian S15 strain of F. tularensis subspecies holarctica (type B). F. tularensis LVS is not approved for mass vaccinations in the United States due to adverse reactions in vaccinated individuals (2). F. tularensis LVS is relatively avirulent in humans and therefore commonly used as a surrogate for the more virulent SchuS4 strain to study tularemia pathogenesis. F. tularensis subspecies mediasiatica and novicida are rarely associated with human tularemia and have been isolated in Asia and in North America and Australia, re...

Effects of engineered nanomaterial exposure on macrophage innate immune function

“…While oxidative stress (ROS production) is well known to be a precursor to cytotoxicity and genotoxicity generally, which could in turn affect innate immune functions, ROS production also plays an important role in killing of intracellular bacteria, including FT [72–75]. Indeed, FT has been shown to downregulate host cellular ROS responses and to produce superoxide disumtases in order to provide protection against ROS within macrophages [73–75].…”

“…Indeed, FT has been shown to downregulate host cellular ROS responses and to produce superoxide disumtases in order to provide protection against ROS within macrophages [73–75]. It is thus possible that changes in ROS production caused by ENM exposure could alter FT killing by macrophages.…”

Bacterial Pathogens versus Autophagy: Implications for Therapeutic Interventions

“…First, F. tularensis surface polysaccharidic O-antigen acts as a shield against ubiquitinylation and subsequent xenophagic recognition during infection of cultured murine macrophages[24]. Second, antioxidant superoxide dismutase enzymes of F. tularensis inhibit ROS-mediated induction of xenophagy and LC3-associated phagocytosis (LAP) during infection of cultured murine macrophages[25]. In addition, F. tularensis exploits an ATG5-independent autophagy pathway in fibroblasts and macrophages to obtain nutrients released from degraded cellular constituents to promote its replication[26].…”

Mitogen-activated protein kinases (MAPKs) are modulated during Francisella tularensis infection, but inhibition of extracellular-signal-regulated kinases (ERKs) is of limited therapeutic benefit

“…Previously, it has been demonstrated that the up-regulation of p38, IkB and c-Jun occurs following infection of murine macrophages with F. tularensis strain LVS [15]. In addition, it has been demonstrated that the uptake of Francisella novicida by macrophages is dependent upon ERK activation, and modulation of the MAPK pathways is important for intracellular survival of F. tularensis LVS [23, 24]. Furthermore, within an in-vivo setting it was found that the inhibition of GSK3B (a serine/threonine protein kinase) with lithium chloride significantly reduces the production of pro-inflammatory cytokines such as IFN-γ, IL-12p40, and TNF-α in F. tularensis LVS-infected mice, and increases survival from 60 % in untreated mice to 90 % following treatment [25].…”