Gene expression following low dose inhalational Francisella tularensis (SchuS4) exposure in Balb/c mice and the potential role of the epithelium and cell adhesion

Mechanisms of Disease: Host-Pathogen Interactions between Burkholderia Species and Lung Epithelial Cells

Members of the Burkholderia species can cause a range of severe, often fatal, respiratory diseases. A variety of in vitro models of infection have been developed in an attempt to elucidate the mechanism by which Burkholderia spp. gain entry to and interact with the body. The majority of studies have tended to focus on the interaction of bacteria with phagocytic cells with a paucity of information available with regard to the lung epithelium. However, the lung epithelium is becoming more widely recognized as an important player in innate immunity and the early response to infections. Here we review the complex relationship between Burkholderia species and epithelial cells with an emphasis on the most pathogenic species, Burkholderia pseudomallei and Burkholderia mallei. The current gaps in knowledge in our understanding are highlighted along with the epithelial host-pathogen interactions that offer potential opportunities for therapeutic intervention.

Prior infection with Type A Francisella tularensis antagonizes the pulmonary transcriptional response to an aerosolized Toll-like receptor 4 agonist

Background

Francisella infection attenuates immune cell infiltration and expression of selected pro-inflammatory cytokines in response to endogenous LPS, suggesting the bacteria is actively antagonizing at least some part of the response to Toll-like receptor 4 (TLR4) engagement. The ability of different Francisella strains to inhibit the ability of E. coli LPS to induce a pulmonary inflammatory response, as measured by gene expression profiling, was examined to define the scope of modulation and identify of inflammatory genes/pathways that are specifically antagonized by a virulent F. tularensis infection.

Results

Prior aerosol exposure to F. tularensis subsp. tularensis, but not the live attenuated strain (LVS) of F. tularensis subsp. holarctica or F. novicida, significantly antagonized the transcriptional response in the lungs of infected mice exposed to aerosolized E. coli LPS. The response to E. coli LPS was not completely inhibited, suggesting that the bacteria is targeting further downstream of the TLR4 molecule. Analysis of the promotors of LPS-responsive genes that were perturbed by Type A Francisella infection identified candidate transcription factors that were potentially modulated by the bacteria, including multiple members of the forkhead transcription factor family (FoxA1, Foxa2, FoxD1, Foxd3, Foxf2, FoxI1, Fox03, Foxq1), IRF1, CEBPA, and Mef2. The annotated functional roles of the affected genes suggested that virulent Francisella infection suppressed cellular processes including mRNA processing, antiviral responses, intracellular trafficking, and regulation of the actin cytoskeleton. Surprisingly, despite the broad overall suppression of LPS-induced genes by virulent Francisella, and contrary to what was anticipated from prior studies, Type A Francisella did not inhibit the expression of the majority of LPS-induced cytokines, nor the expression of many classic annotated inflammatory genes.

Conclusions

Collectively, this analysis demonstrates clear differences in the ability of different Francisella strains to modulate TLR4 signaling and identifies genes/pathways that are specifically targeted by virulent Type A Francisella.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2022-2) contains supplementary material, which is available to authorized users.

The impact of "omic" and imaging technologies on assessing the host immune response to biodefence agents

Understanding the interactions between host and pathogen is important for the development and assessment of medical countermeasures to infectious agents, including potential biodefence pathogens such as Bacillus anthracis, Ebola virus, and Francisella tularensis. This review focuses on technological advances which allow this interaction to be studied in much greater detail. Namely, the use of “omic” technologies (next generation sequencing, DNA, and protein microarrays) for dissecting the underlying host response to infection at the molecular level; optical imaging techniques (flow cytometry and fluorescence microscopy) for assessing cellular responses to infection; and biophotonic imaging for visualising the infectious disease process. All of these technologies hold great promise for important breakthroughs in the rational development of vaccines and therapeutics for biodefence agents.

B cell subsets are activated and produce cytokines during early phases of Francisella tularensis LVS infection

Francisella tularensis, a facultative intracellular Gram-negative bacterium, causes the illness tularemia. The infection of mice with live vaccine strain is considered to be a model of human tularemia. F. tularensis infects predominantly such phagocytic cells as macrophages or neutrophils, but it also infects non-phagocytic hepatocytes, epithelial cells, and murine and human B cell lines. Based on work with the murine tularemia model, we report here that F. tularensis LVS infects peritoneal CD19(+) cells - exclusively B-1a cells - early after intraperitoneal infection in vivo. The peritoneal and consequently spleen CD19(+) cells are activated by the F. tularensis LVS infection to express the activation markers from MHC class II, CD25, CD54, CD69, and the co-stimulatory molecules CD80 and CD86. As early as 12 h post-infection, the peritoneal CD19(+) cells produce IFN-γ, IL-1β, IL-4, IL-6, IL-12, IL-17, IL-23, and TNF-α. The spleen CD19(+) cells respond to infection with some delay. Moreover, the F. tularensis infected A20 B cell line activates CD3(+) spleen cells isolated from naïve mice. Thus, the data presented here suggest that B cells have all the attributes to actively participate in the induction and regulation of the adaptive immune response during early stages of F. tularensis infection.

Control of intracellular Francisella tularensis by different cell types and the role of nitric oxide

Reactive nitrogen is critical for the clearance of Francisella tularensis infections. Here we assess the role of nitric oxide in control of intracellular infections in two murine macrophage cell lines of different provenance: the alveolar macrophage cell line, MH-S, and the widely used peritoneal macrophage cell line, J774A.1. Cells were infected with the highly virulent Schu S4 strain or with the avirulent live vaccine strain (LVS) with and without stimuli. Compared to MH-S cells, J774A.1 cells were unresponsive to stimulation and were able to control the intracellular replication of LVS bacteria, but not of Schu S4. In MH-S cells, Schu S4 demonstrated control over cellular NO production. Despite this, MH-S cells stimulated with LPS or LPS and IFN-γ were able to control intracellular Schu S4 numbers. However, only stimulation with LPS induced significant cellular NO production. Combined stimulation with LPS and IFN-γ produced a significant reduction in intracellular bacteria that occurred whether high levels of NO were produced or not, indicating that NO secretion is not the only defensive cellular mechanism operating in virulent Francisella infections. Understanding how F. tularensis interacts with host macrophages will help in the rational design of new and effective therapies.

The use of a three-dimensional cell culture model to investigate host-pathogen interactions of Francisella tularensis in human lung epithelial cells

Francisella tularensis inhalation results in bacterial interaction with numerous lung cell types, including those of the epithelium. This work investigates a three-dimensional cell-culture system to characterise the epithelial response to F. tularensis. Immortalised human pneumocytes (A549s) grown in rotating-wall vessel (RWV) bioreactors display an in vivo-like phenotype, which has been confirmed to be driven by specific transcriptional events (8454 genes, p ≤ 0.05). These data support the RWV model as a more in vivo-like culture system to investigate the lung epithelium, compared to monolayer counterparts. RWV-cultured A549s were infected with F. tularensis SchuS4 and LVS and intracellular replication mapped over 22 h compared to monolayer cells. The RWV-cultured A549s were more resistant to SchuS4 and LVS infection (p ≤ 0.05). Transcriptomics identified 2086 genes (p ≤ 0.05) as candidates for host-pathogen interactions which result in the observed increase in resistance of the RWV-cultured A549 cells. Gene and pathway analysis identified processes involved in MMP modulation, endocytosis, mucin production and the complement pathway amongst others. The role of these pathways during infection was further characterised using chemical inhibitors. This work has revealed several new hypotheses worthy of further testing in order to understand the epithelial host response to F. tularensis infection.

Francisella tularensis subsp. tularensis induces a unique pulmonary inflammatory response: role of bacterial gene expression in temporal regulation of host defense responses

Pulmonary exposure to Francisella tularensis is associated with severe lung pathology and a high mortality rate. The lack of induction of classical inflammatory mediators, including IL1-β and TNF-α, during early infection has led to the suggestion that F. tularensis evades detection by host innate immune surveillance and/or actively suppresses inflammation. To gain more insight into the host response to Francisella infection during the acute stage, transcriptomic analysis was performed on lung tissue from mice exposed to virulent (Francisella tularensis ssp tularensis SchuS4). Despite an extensive transcriptional response in the lungs of animals as early as 4 hrs post-exposure, Francisella tularensis was associated with an almost complete lack of induction of immune-related genes during the initial 24 hrs post-exposure. This broad subversion of innate immune responses was particularly evident when compared to the pulmonary inflammatory response induced by other lethal (Yersinia pestis) and non-lethal (Legionella pneumophila, Pseudomonas aeruginosa) pulmonary infections. However, the unique induction of a subset of inflammation-related genes suggests a role for dysregulation of lymphocyte function and anti-inflammatory pathways in the extreme virulence of Francisella. Subsequent activation of a classical inflammatory response 48 hrs post-exposure was associated with altered abundance of Francisella-specific transcripts, including those associated with bacterial surface components. In summary, virulent Francisella induces a unique pulmonary inflammatory response characterized by temporal regulation of innate immune pathways correlating with altered bacterial gene expression patterns. This study represents the first simultaneous measurement of both host and Francisella transcriptome changes that occur during in vivo infection and identifies potential bacterial virulence factors responsible for regulation of host inflammatory pathways.