Direct and Indirect Impairment of Human Dendritic Cell Function by VirulentFrancisella tularensisSchu S4

Abstract

The gram-negative, facultative intracellular bacterium Francisella tularensis causes acute, lethal pneumonic disease following infection with only 10 CFU. The mechanisms used by the bacterium to accomplish this in humans are unknown. Here, we demonstrate that virulent, type A F. tularensis strain Schu S4 efficiently infects and replicates in human myeloid dendritic cells (DCs). Despite exponential replication over time, Schu S4 failed to stimulate transforming growth factor ␤, interleukin-10 (IL-10), IL-6, IL-1␤, IL-12, tumor necrosis factor alpha, alpha interferon (IFN-␣), and IFN-␤ throughout the course of infection. Schu S4 also suppressed the ability of directly infected DCs to respond to different Toll-like receptor agonists. Furthermore, we also observed functional inhibition of uninfected bystander cells. This inhibition was mediated, in part, by a heat-stable bacterial component. Lipopolysaccharide (LPS) from Schu S4 was present in Schu S4-conditioned medium. However, Schu S4 LPS was weakly inflammatory and failed to induce suppression of DCs at concentrations below 10 g/ml, and depletion of Schu S4 LPS did not significantly alleviate the inhibitory effect of Schu S4-conditioned medium in uninfected human DCs. Together, these data show that type A F. tularensis interferes with the ability of a central cell type of the immune system, DCs, to alert the host of infection both intra-and extracellularly. This suggests that immune dysregulation by F. tularensis operates on a broader and more comprehensive scale than previously appreciated.Francisella tularensis is a gram-negative, facultative intracellular bacterium and the causative agent of tularemia. Although the bacteria was identified nearly 100 years ago, the nature of its interaction with host cells and tissues has remained largely undefined until recent times. The unfortunate realization of the potential of F. tularensis as a biological weapon has resulted in a resurgence of research on tularemia. Much of our recent understanding of tularemia pathogenesis has come from manipulation of the mouse model of Francisella infections. While these murine studies have yielded many important advances in our understanding of tularemia pathogenesis, very little is understood about how Francisella, especially the most virulent type A strains such as Schu S4, interacts with human cells.Dendritic cells (DCs) serve as a central cell type in the immune system, bridging the innate and adaptive immune response to effectively eradicate invading pathogens.

Identification of a Novel Francisella tularensis Factor Required for Intramacrophage Survival and Subversion of Innate Immune Response

“…To date, a limited number of F. tularensis factors including intracellular growth locus C, RipA, and antioxidant enzyme catalase (KatG) of F. tularensis LVS have been shown to cause innate immune subversion through inhibition of MAPK and NF-B signaling (16 -19). In contrast, F. tularensis SchuS4-mediated cytokine suppression is independent of intracellular growth locus C (20), and the roles of RipA or KatG in the immune subversion have not been fully established.…”

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

“…Recent studies have shown that F. tularensis infection leads to rapid bacterial replication in the host. Despite an exponential increase in bacterial numbers, activation of macrophages, dendritic cells and neutrophils is markedly suppressed (37)(38)(39). In addition, F. tularensis exerts profound suppressive effects on the production of pro-inflammatory cytokines by restricting signal transduction pathways (40,41).…”

Repression of Inflammasome by Francisella tularensis during Early Stages of Infection

“…Being an intracellular pathogen, Francisella can survive and replicate in a variety of cell types including macrophages. F. tularensis has been shown to modulate macrophage functions and suppress proinflammatory cytokines (5)(6)(7)(8). The virulence properties and mechanisms underlying innate immune subversion of F. tularensis are unique and remain largely undefined.…”