Active Suppression of the Pulmonary Immune Response by Francisella tularensis Schu4

Abstract

Francisella tularensis is an obligate, intracellular bacterium that causes acute, lethal disease following inhalation. As an intracellular pathogen F. tularensis must invade cells, replicate, and disseminate while evading host immune responses. The mechanisms by which virulent type A strains of Francisella tularensis accomplish this evasion are not understood. Francisella tularensis has been shown to target multiple cell types in the lung following aerosol infection, including dendritic cells (DC) and macrophages. We demonstrate here that one mechanism used by a virulent type A strain of F. tularensis (Schu4) to evade early detection is by the induction of overwhelming immunosuppression at the site of infection, the lung. Following infection and replication in multiple pulmonary cell types, Schu4 failed to induce the production of proinflammatory cytokines or increase the expression of MHCII or CD86 on the surface of resident DC within the first few days of disease. However, Schu4 did induce early and transient production of TGF-β, a potent immunosuppressive cytokine. The absence of DC activation following infection could not be attributed to the apoptosis of pulmonary cells, because there were minimal differences in either annexin or cleaved caspase-3 staining in infected mice compared with that in uninfected controls. Rather, we demonstrate that Schu4 actively suppressed in vivo responses to secondary stimuli (LPS), e.g., failure to recruit granulocytes/monocytes and stimulate resident DC. Thus, unlike attenuated strains of F. tularensis, Schu4 induced broad immunosuppression within the first few days after aerosol infection. This difference may explain the increased virulence of type A strains compared with their more attenuated counterparts.

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

“…3 and data not shown). We along with others have observed induction of antiinflammatory cytokines and type I IFN by F. tularensis following infection of antigen-presenting cells (7,8,26,51). As expected, DCs responded to E. coli LPS by secreting IL-12, TNF-␣, IL-1␤, and TGF-␤ into culture supernatant (Fig.…”

“…Thus, the profound suppression mediated by F. tularensis in DCs was not obtained at the cost of an initial burst of cytokines that might alert the immune system. Importantly, the broad suppression of DC responsiveness (in terms of cytokine secretion) is similar to that observed in the lungs of mice following aerosol infection with Schu S4 (7). Together, these data suggested that Schu S4 suppresses the host immune response in a global sense; that is, interference with host responsiveness is not restricted to infected cells but extends to uninfected cells that may aid in the resolution of infection.…”

“…Green fluorescent protein (GFP)-expressing Schu S4 was generated as previously described (12). Schu S4 was cultured in modified Mueller-Hinton broth ( [MMH] Mueller-Hinton broth supplemented with CaCl 2 , MgCl, 0.1% glucose, 0.025% ferric pyrophosphate and 2% Isovitalex) at 37°C with constant shaking overnight, aliquoted into 1-ml samples, frozen at Ϫ80°C, and thawed just prior to use, as previously described (7). Titers were determined in frozen stocks by enumerating viable bacteria from serial dilutions plated on MMH agar as previously described (9,19).…”

“…F. tularensis is no exception in its ability to modify DC responsiveness to infection. For example, in the mouse model, both attenuated and virulent strains of F. tularensis fail to induce the production of cytokines required for mobilization of key effector cells (7,8,14,50). However, only virulent F. tularensis also prevents increased expression of key surface receptors required for antigen presentation in the lung in vivo (7).…”

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.…”