Virulent Francisella tularensis destabilize host mRNA to rapidly suppress inflammation

Macrophage fate: to kill or not to kill?

Macrophages are dynamic innate immune cells that either reside in tissue, serving as sentinels, or recruited as monocytes from bone marrow into inflamed and infected tissue. In response to cues in the tissue microenvironment (TME), macrophages polarize on a continuum toward M1 or M2 with diverse roles in progression and resolution of disease. M1-like macrophages exhibit proinflammatory functions with antimicrobial and anti-tumorigenic activities, while M2-like macrophages have anti-inflammatory functions that generally resolve inflammatory responses and orchestrate a tissue healing process. Given these opposite phenotypes, proper spatiotemporal coordination of macrophage polarization in response to cues within the TME is critical to effectively resolve infectious disease and regulate wound healing. However, if this spatiotemporal coordination becomes disrupted due to persistent infection or dysregulated coagulation, macrophages' inappropriate response to these cues will result in the development of diseases with clinically unfavorable outcomes. Since plasticity and heterogeneity are hallmarks of macrophages, they are attractive targets for therapies to reprogram toward specific phenotypes that could resolve disease and favor clinical prognosis. In this review, we discuss how basic science studies have elucidated macrophage polarization mechanisms in TMEs during infections and inflammation, particularly coagulation. Therefore, understanding the dynamics of macrophage polarization within TMEs in diseases is important in further development of targeted therapies.

Protective potential of outer membrane vesicles derived from a virulent strain of Francisella tularensis

Francisella tularensis secretes tubular outer membrane vesicles (OMVs) that contain a number of immunoreactive proteins as well as virulence factors. We have reported previously that isolated Francisella OMVs enter macrophages, cumulate inside, and induce a strong pro-inflammatory response. In the current article, we present that OMVs treatment of macrophages also enhances phagocytosis of the bacteria and suppresses their intracellular replication. On the other hand, the subsequent infection with Francisella is able to revert to some extent the strong pro-inflammatory effect induced by OMVs in macrophages. Being derived from the bacterial surface, isolated OMVs may be considered a "non-viable mixture of Francisella antigens" and as such, they present a promising protective material. Immunization of mice with OMVs isolated from a virulent F. tularensis subsp. holarctica strain FSC200 prolonged the survival time but did not fully protect against the infection with a lethal dose of the parent strain. However, the sera of the immunized animals revealed unambiguous cytokine and antibody responses and proved to recognize a set of well-known Francisella immunoreactive proteins. For these reasons, Francisella OMVs present an interesting material for future protective studies.

Francisella tularensis disrupts TLR2-MYD88-p38 signaling early during infection to delay apoptosis of macrophages and promote virulence in the host

Francisella tularensis is a zoonotic pathogen and the causative agent of tularemia. F. tularensis replicates to high levels within the cytosol of macrophages and other host cells while subverting the host response to infection. Critical to the success of F. tularensis is its ability to delay macrophage apoptosis to maintain its intracellular replicative niche. However, the host-signaling pathway(s) modulated by F. tularensis to delay apoptosis are poorly characterized. The outer membrane channel protein TolC is required for F. tularensis virulence and its ability to suppress apoptosis and cytokine expression during infection of macrophages. We took advantage of the F. tularensis ∆tolC mutant phenotype to identify host pathways that are important for activating macrophage apoptosis and that are disrupted by the bacteria. Comparison of macrophages infected with wild-type or ∆tolC F. tularensis revealed that the bacteria interfere with TLR2-MYD88-p38 signaling at early times post infection to delay apoptosis, dampen innate host responses, and preserve the intracellular replicative niche. Experiments using the mouse pneumonic tularemia model confirmed the in vivo relevance of these findings, revealing contributions of TLR2 and MYD88 signaling to the protective host response to F. tularensis, which is modulated by the bacteria to promote virulence. IMPORTANCE Francisella tularensis is a Gram-negative intracellular bacterial pathogen and the causative agent of the zoonotic disease tularemia. F. tularensis, like other intracellular pathogens, modulates host-programmed cell death pathways to ensure its replication and survival. We previously identified the outer membrane channel protein TolC as required for the ability of F. tularensis to delay host cell death. However, the mechanism by which F. tularensis delays cell death pathways during intracellular replication is unclear despite being critical to pathogenesis. In the present study, we address this gap in knowledge by taking advantage of ∆tolC mutants of F. tularensis to uncover signaling pathways governing host apoptotic responses to F. tularensis and which are modulated by the bacteria during infection to promote virulence. These findings reveal mechanisms by which intracellular pathogens subvert host responses and enhance our understanding of the pathogenesis of tularemia.

Macrophages Demonstrate Guanylate-Binding Protein-Dependent and Bacterial Strain-Dependent Responses to Francisella tularensis

Francisella tularensis is a facultative intracellular bacterium and the etiological agent of tularemia, a zoonotic disease. Infection of monocytic cells by F. tularensis can be controlled after activation with IFN-γ; however, the molecular mechanisms whereby the control is executed are incompletely understood. Recently, a key role has been attributed to the Guanylate-binding proteins (GBPs), interferon-inducible proteins involved in the cell-specific immunity against various intracellular pathogens. Here, we assessed the responses of bone marrow-derived murine macrophages (BMDM) and GBP-deficient BMDM to F. tularensis strains of variable virulence; the highly virulent SCHU S4 strain, the human live vaccine strain (LVS), or the widely used surrogate for F. tularensis, the low virulent F. novicida. Each of the strains multiplied rapidly in BMDM, but after addition of IFN-γ, significant GBP-dependent control of infection was observed for the LVS and F. novicida strains, whereas there was no control of the SCHU S4 infection. However, no differences in GBP transcription or translation were observed in the infected cell cultures. During co-infection with F. novicida and SCHU S4, significant control of both strains was observed. Patterns of 18 cytokines were very distinct between infected cell cultures and high levels were observed for almost all cytokines in F. novicida-infected cultures and very low levels in SCHU S4-infected cultures, whereas levels in co-infected cultures for a majority of cytokines showed intermediate levels, or levels similar to those of F. novicida-infected cultures. We conclude that the control of BMDM infection with F. tularensis LVS or F. novicida is GBP-dependent, whereas SCHU S4 was only controlled during co-infection. Since expression of GBP was similar regardless of infecting agent, the findings imply that SCHU S4 has an inherent ability to evade the GBP-dependent anti-bacterial mechanisms.

Francisella tularensis Outer Membrane Vesicles Participate in the Early Phase of Interaction With Macrophages

Francisella tularensis is known to release unusually shaped tubular outer membrane vesicles (OMV) containing a number of previously identified virulence factors and immunomodulatory proteins. In this study, we present that OMV isolated from the F. tularensis subsp. holarctica strain FSC200 enter readily into primary bone marrow-derived macrophages (BMDM) and seem to reside in structures resembling late endosomes in the later intervals. The isolated OMV enter BMDM generally via macropinocytosis and clathrin-dependent endocytosis, with a minor role played by lipid raft-dependent endocytosis. OMVs proved to be non-toxic and had no negative impact on the viability of BMDM. Unlike the parent bacterium itself, isolated OMV induced massive and dose-dependent proinflammatory responses in BMDM. Using transmission electron microscopy, we also evaluated OMV release from the bacterial surface during several stages of the interaction of Francisella with BMDM. During adherence and the early phase of the uptake of bacteria, we observed numerous tubular OMV-like protrusions bulging from the bacteria in close proximity to the macrophage plasma membrane. This suggests a possible role of OMV in the entry of bacteria into host cells. On the contrary, the OMV release from the bacterial surface during its cytosolic phase was negligible. We propose that OMV play some role in the extracellular phase of the interaction of Francisella with the host and that they are involved in the entry mechanism of the bacteria into macrophages.

A study of innate immune kinetics reveals a role for a chloride transporter in a virulent Francisella tularensis type B strain

Tularemia is a zoonotic disease of global proportions. Francisella tularensis subspecies tularensis (type A) and holarctica (type B) cause disease in healthy humans, with type A infections resulting in higher mortality. Repeated passage of a type B strain in the mid-20th century generated the Live Vaccine Strain (LVS). LVS remains unlicensed, does not protect against high inhalational doses of type A, and its exact mechanisms of attenuation are poorly understood. Recent data suggest that live attenuated vaccines derived from type B may cross-protect against type A. However, there is a dearth of knowledge regarding virulent type B pathogenesis and its capacity to stimulate the host's innate immune response. We therefore sought to increase our understanding of virulent type B in vitro characteristics using strain OR96-0246 as a model. Adding to our knowledge of innate immune kinetics in macrophages following infection with virulent type B, we observed robust replication of strain OR96-0246 in murine and human macrophages, reduced expression of pro-inflammatory cytokine genes from "wild type" type B-infected macrophages compared to LVS, and delayed macrophage cell death suggesting that virulent type B may suppress macrophage activation. One disruption in LVS is in the gene encoding the chloride transporter ClcA. We investigated the role of ClcA in macrophage infection and observed a replication delay in a clcA mutant. Here, we propose its role in acid tolerance. A greater understanding of LVS attenuation may reveal new mechanisms of pathogenesis and inform strategies toward the development of an improved vaccine against tularemia.

A predicted Francisella tularensis DXD-motif glycosyltransferase blocks immune activation

Pathogens enhance their survival during infections by manipulating host defenses. Francisella tularensis evades innate immune responses, which we have found to be dependent on an understudied gene ybeX (FTL_0883/FTT_0615c). To understand the function of YbeX, we sought protein interactors in F. tularensis subsp. holarctica live vaccine strain (LVS). An unstudied Francisella protein co-immunoprecipitated with recombinant YbeX, which is a predicted glycosyltransferase with a DXD-motif. There are up to four genomic copies of this gene with identical sequence in strains of F. tularensis pathogenic to humans, despite ongoing genome decay. Disruption mutations were generated by intron insertion into all three copies of this glycosyltransferase domain containing gene in LVS, gdcA1-3. The resulting strains stimulated more cytokines from macrophages in vitro than wild-type LVS and were attenuated in two in vivo infection models. GdcA was released from LVS during culture and was sufficient to block NF-κB activation when expressed in eukaryotic cells. When co-expressed in zebrafish, GdcA and YbeX were synergistically lethal to embryo development. Glycosyltransferases with DXD-motifs are found in a variety of pathogens including NleB, an Escherichia coli type-III secretion system effector that inhibits NF-κB by antagonizing death receptor signaling. To our knowledge, GdcA is the first DXD-motif glycosyltransferase that inhibits NF-κB in immune cells. Together, these findings suggest DXD-motif glycosyltransferases may be a conserved virulence mechanism used by pathogenic bacteria to remodel host defenses.

Unique Francisella Phosphatidylethanolamine Acts as a Potent Anti-Inflammatory Lipid

Virulent Francisella tularensis subsp. tularensis (Ftt) is a dynamic, intracellular, bacterial pathogen. Its ability to evade and rapidly suppress host inflammatory responses is considered a key element for its profound virulence. We previously established that Ftt lipids play a role in inhibiting inflammation, but we did not determine the lipid species mediating this process. Here, we show that a unique, abundant, phosphatidylethanolamine (PE), present in Francisella, contributes to driving the suppression of inflammatory responses in human and mouse cells. Acyl chain lengths of this PE, C24: 0 and C10: 0, were key to the suppressive capabilities of Francisella PE. Addition of synthetic PE 24: 0-10: 0 resulted in the accumulation of PE in host cells for up to 24 h of incubation, and recapitulated the inhibition of inflammatory responses observed with native Ftt PE. Importantly, this novel PE significantly inhibited inflammatory responses driven by a medically and globally important flavivirus, dengue fever virus. Thus, targeting these lipids and/or the pathways that they manipulate represents a new strategy to combat immunosuppression engendered by Ftt, but they also show promise as a novel therapeutic intervention for significant viral infections.

Francisella tularensis Confronts the Complement System

Francisella tularensis has developed a number of effective evasion strategies to counteract host immune defenses, not the least of which is its ability to interact with the complement system to its own advantage. Following exposure of the bacterium to fresh human serum, complement is activated and C3b and iC3b can be found covalently attached to the bacterial surface. However, the lipopolysaccharide and capsule of the F. tularensis cell wall prevent complement-mediated lysis and endow the bacterium with serum resistance. Opsonization of F. tularensis with C3 greatly increases its uptake by human neutrophils, dendritic cells and macrophages. Uptake occurs by an unusual looping morphology in human macrophages. Complement receptor 3 is thought to play an important role in opsonophagocytosis by human macrophages, and signaling through this receptor can antagonize Toll-like receptor 2-initiated macrophage activation. Complement C3 also determines the survival of infected human macrophages and perhaps other cell types. C3-opsonization of F. tularensis subsp. tularensis strain SCHU S4 results in greatly increased death of infected human macrophages, which requires more than complement receptor engagement and is independent of the intracellular replication by the pathogen. Given its entry into the cytosol of host cells, F. tularensis has the potential for a number of other complement-mediated interactions. Studies on the uptake C3-opsonized adenovirus have suggested the existence of a C3 sensing system that initiates cellular responses to cytosolic C3b present on invading microbes. Here we propose that C3 peptides enter the cytosol of human macrophages following phagosome escape of F. tularensis and are recognized as intruding molecular patterns that signal host cell death. With the discovery of new roles for intracellular C3, a better understanding of tularemia pathogenesis is likely to emerge.

Subversion of innate immune responses by Francisella involves the disruption of TRAF3 and TRAF6 signalling complexes

The success of pathogens depends on their ability to circumvent immune defences. Francisella tularensis is one of the most infectious bacteria known. The remarkable virulence of Francisella is believed to be due to its capacity to evade or subvert the immune system, but how remains obscure. Here, we show that Francisella triggers but concomitantly inhibits the Toll-like receptor, RIG-I-like receptor, and cytoplasmic DNA pathways. Francisella subverts these pathways at least in part by inhibiting K63-linked polyubiquitination and assembly of TRAF6 and TRAF3 complexes that control the transcriptional responses of pattern recognition receptors. We show that this mode of inhibition requires a functional type VI secretion system and/or the presence of live bacteria in the cytoplasm. The ability of Francisella to enter the cytosol while simultaneously inhibiting multiple pattern recognition receptor pathways may account for the notable capacity of this bacterium to invade and proliferate in the host without evoking a self-limiting innate immune response.

Primary macrophages and J774 cells respond differently to infection with Mycobacterium tuberculosis

Macrophages play an essential role in the early immune response to Mycobacterium tuberculosis and are the cell type preferentially infected in vivo. Primary macrophages and macrophage-like cell lines are commonly used as infection models, although the physiological relevance of cell lines, particularly for host-pathogen interaction studies, is debatable. Here we use high-throughput RNA-sequencing to analyse transcriptome dynamics of two macrophage models in response to M. tuberculosis infection. Specifically, we study the early response of bone marrow-derived mouse macrophages and cell line J774 to infection with live and γ-irradiated (killed) M. tuberculosis. We show that infection with live bacilli specifically alters the expression of host genes such as Rsad2, Ifit1/2/3 and Rig-I, whose potential roles in resistance to M. tuberculosis infection have not yet been investigated. In addition, the response of primary macrophages is faster and more intense than that of J774 cells in terms of number of differentially expressed genes and magnitude of induction/repression. Our results point to potentially novel processes leading to immune containment early during M. tuberculosis infection, and support the idea that important differences exist between primary macrophages and cell lines, which should be taken into account when choosing a macrophage model to study host-pathogen interactions.