Coxiella burnetii Subverts p62/Sequestosome 1 and Activates Nrf2 Signaling in Human Macrophages

Serine ubiquitination of SQSTM1 regulates NFE2L2-dependent redox homeostasis

The KEAP1-NFE2L2 axis is essential for the cellular response against metabolic and oxidative stress. KEAP1 is an adaptor protein of CUL3 (cullin 3) ubiquitin ligase that controls the cellular levels of NFE2L2, a critical transcription factor of several cytoprotective genes. Oxidative stress, defective autophagy and pathogenic infections activate NFE2L2 signaling through phosphorylation of the autophagy receptor protein SQSTM1, which competes with NFE2L2 for binding to KEAP1. Here we show that phosphoribosyl-linked serine ubiquitination of SQSTM1 catalyzed by SidE effectors of Legionella pneumophila controls NFE2L2 signaling and cell metabolism upon Legionella infection. Serine ubiquitination of SQSTM1 sterically blocks its binding to KEAP1, resulting in NFE2L2 ubiquitination and degradation. This reduces NFE2L2-dependent antioxidant synthesis in the early phase of infection. Levels of serine ubiquitinated SQSTM1 diminish in the later stage of infection allowing the expression of NFE2L2-target genes; causing a differential regulation of the host metabolome and proteome in a NFE2L2-dependent manner.Abbreviation: ARE: antioxidant response element; Dup: deubiquitinase specific for phosphoribosyl-linked serine ubiquitination; ER: endoplasmic reticulum; h.p.i: hours post infection; HIF1A/HIF-1α: hypoxia inducible factor 1 subunit alpha; KEAP1: kelch like ECH associated protein 1; KIR: KEAP1-interacting region; LIR: LC3-interacting region; NES: nuclear export signal; NFKB/NF-κB: nuclear factor kappa B; NLS: nuclear localization signal; NFE2L2/Nrf2: NFE2 like bZIP transcription factor 2; PB1 domain: Phox1 and Bem1 domain; PR-Ub: phosphoribosyl-linked serine ubiquitination; ROS: reactive oxygen species; SQSTM1/p62: sequestosome 1; tBHQ: tertiary butylhydroquinone; TUBE2: tandem ubiquitiin binding entity 2; UBA domain: ubiquitin-associated domain.

Embracing multiple infection models to tackle Q fever: A review of in vitro, in vivo, and lung ex vivo models

Multiple animal and cell culture models are employed to study pathogenesis of Coxiella burnetii, the causative agent of acute and chronic human Q fever. C. burnetii is a lung pathogen that is aerosolized in contaminated products and inhaled by humans to cause acute disease that can disseminate to other organs and establish chronic infection. Cellular models of Q fever include a variety of tissue-derived cell lines from mice and humans such as lung alveolar ex vivo cells. These models have the advantage of being cost-effective and reproducible. Similarly, animal models including mice and guinea pigs are cost-effective, although only immunocompromised SCID mice display a severe disease phenotype in response to Nine Mile I and Nine Mile II isolates of C. burnetii while immunocompetent guinea pigs display human-like symptoms and robust immune responses. Non-human primates such as macaques and marmosets are the closest model of human disease but are costly and largely used for adaptive immune response studies. All animal models are used for vaccine development but many differences exist in the pathogen's ability to establish lung infection when considering infection routes, bacterial isolates, and host genetic background. Similarly, while cellular models are useful for characterization of host-pathogen mechanisms, future developments should include use of a lung infection platform to draw appropriate conclusions. Here, we summarize the current state of the C. burnetii lung pathogenesis field by discussing the contribution of different animal and cell culture models and include suggestions for continuing to move the field forward.

TLR-mediated aggresome-like induced structures comprise antimicrobial peptides and attenuate intracellular bacterial survival

Immune cells employ diverse mechanisms for host defense. Macrophages, in response to TLR activation, assemble aggresome-like induced structures (ALIS). Our group has shown TLR4-signaling transcriptionally upregulates p62/sequestome1, which assembles ALIS. We have demonstrated that TLR4-mediated autophagy is, in fact, selective-autophagy of ALIS. We hypothesize that TLR-mediated autophagy and ALIS contribute to host-defense. Here we show that ALIS are assembled in macrophages upon exposure to different bacteria. These structures are associated with pathogen-containing phagosomes. Importantly, we present evidence of increased bacterial burden, where ALIS assembly is prevented with p62-specific siRNA. We have employed 3D-super-resolution structured illumination microscopy (3D-SR-SIM) and mass-spectrometric (MS) analyses to gain insight into the assembly of ALIS. Ultra-structural analyses of known constituents of ALIS (p62, ubiquitin, LC3) reveal that ALIS are organized structures with distinct patterns of alignment. Furthermore, MS-analyses of ALIS identified, among others, several proteins of known antimicrobial properties. We have validated MS data by testing the association of some of these molecules (Bst2, IFITM2, IFITM3) with ALIS and the phagocytosed-bacteria. We surmise that AMPs enrichment in ALIS leads to their delivery to bacteria-containing phagosomes and restricts the bacteria. Our findings in this paper support hitherto unknown functions of ALIS in host-defense.

Autophagy as an innate immunity response against pathogens: a Tango dance

Intracellular infections as well as changes in the cell nutritional environment are main events that trigger cellular stress responses. One crucial cell response to stress conditions is autophagy. During the last 30 years, several scenarios involving autophagy induction or inhibition over the course of an intracellular invasion by pathogens have been uncovered. In this review, we will present how this knowledge was gained by studying different microorganisms. We intend to discuss how the cell, via autophagy, tries to repel these attacks with the objective of destroying the intruder, but also how some pathogens have developed strategies to subvert this. These two fates can be compared with a Tango, a dance originated in Buenos Aires, Argentina, in which the partner dancers are in close connection. One of them is the leader, embracing and involving the partner, but the follower may respond escaping from the leader. This joint dance is indeed highly synchronized and controlled, perfectly reflecting the interaction between autophagy and microorganism.

The intricate dance: host autophagy and Coxiella burnetii infection

Q fever is a zoonotic disease caused by Coxiella burnetii, an obligatory intracellular bacterial pathogen. Like other intracellular pathogens, C. burnetii is able to survive and reproduce within host cells by manipulating host cellular processes. In particular, the relationship between C. burnetii infection and host autophagy, a cellular process involved in degradation and recycling, is of great interest due to its intricate nature. Studies have shown that autophagy can recognize and target intracellular pathogens such as Legionella and Salmonella for degradation, limiting their replication and promoting bacterial clearance. However, C. burnetii can actively manipulate the autophagic pathway to create an intracellular niche, known as the Coxiella-containing vacuole (CCV), where it can multiply and evade host immune responses. C. burnetii promotes the fusion of CCVs with lysosomes through mechanisms involving virulence factors such as Cig57 and CvpF. This review summarizes the latest findings on the dynamic interaction between host autophagy and C. burnetii infection, highlighting the complex strategies employed by both the bacterium and the host. A better understanding of these mechanisms could provide important insights into the development of novel therapeutic interventions and vaccine strategies against C. burnetii infections.

Coxiella co-opts the Glutathione Peroxidase 4 to protect the host cell from oxidative stress-induced cell death

The causative agent of human Q fever, Coxiella burnetii, is highly adapted to infect alveolar macrophages by inhibiting a range of host responses to infection. Despite the clinical and biological importance of this pathogen, the challenges related to genetic manipulation of both C. burnetii and macrophages have limited our knowledge of the mechanisms by which C. burnetii subverts macrophages functions. Here, we used the related bacterium Legionella pneumophila to perform a comprehensive screen of C. burnetii effectors that interfere with innate immune responses and host death using the greater wax moth Galleria mellonella and mouse bone marrow-derived macrophages. We identified MceF (Mitochondrial Coxiella effector protein F), a C. burnetii effector protein that localizes to mitochondria and contributes to host cell survival. MceF was shown to enhance mitochondrial function, delay membrane damage, and decrease mitochondrial ROS production induced by rotenone. Mechanistically, MceF recruits the host antioxidant protein Glutathione Peroxidase 4 (GPX4) to the mitochondria. The protective functions of MceF were absent in primary macrophages lacking GPX4, while overexpression of MceF in human cells protected against oxidative stress-induced cell death. C. burnetii lacking MceF was replication competent in mammalian cells but induced higher mortality in G. mellonella, indicating that MceF modulates the host response to infection. This study reveals an important C. burnetii strategy to subvert macrophage cell death and host immunity and demonstrates that modulation of the host antioxidant system is a viable strategy to promote the success of intracellular bacteria.

Establishing the intracellular niche of obligate intracellular vacuolar pathogens

Obligate intracellular pathogens occupy one of two niches - free in the host cell cytoplasm or confined in a membrane-bound vacuole. Pathogens occupying membrane-bound vacuoles are sequestered from the innate immune system and have an extra layer of protection from antimicrobial drugs. However, this lifestyle presents several challenges. First, the bacteria must obtain membrane or membrane components to support vacuole expansion and provide space for the increasing bacteria numbers during the log phase of replication. Second, the vacuole microenvironment must be suitable for the unique metabolic needs of the pathogen. Third, as most obligate intracellular bacterial pathogens have undergone genomic reduction and are not capable of full metabolic independence, the bacteria must have mechanisms to obtain essential nutrients and resources from the host cell. Finally, because they are separated from the host cell by the vacuole membrane, the bacteria must possess mechanisms to manipulate the host cell, typically through a specialized secretion system which crosses the vacuole membrane. While there are common themes, each bacterial pathogen utilizes unique approach to establishing and maintaining their intracellular niches. In this review, we focus on the vacuole-bound intracellular niches of Anaplasma phagocytophilum, Ehrlichia chaffeensis, Chlamydia trachomatis, and Coxiella burnetii.

The versatile defender: exploring the multifaceted role of p62 in intracellular bacterial infection

As a highly conserved, multifunctional protein with multiple domains, p62/SQSTM1 plays a crucial role in several essential cellular activities, particularly selective autophagy. Recent research has shown that p62 is crucial in eradicating intracellular bacteria by xenophagy, a selective autophagic process that identifies and eliminates such microorganisms. This review highlights the various roles of p62 in intracellular bacterial infections, including both direct and indirect, antibacterial and infection-promoting aspects, and xenophagy-dependent and independent functions, as documented in published literature. Additionally, the potential applications of synthetic drugs targeting the p62-mediated xenophagy mechanism and unresolved questions about p62's roles in bacterial infections are also discussed.

ACKR4a induces autophagy to block NF-κB signaling and apoptosis to facilitate Vibrio harveyi infection

Autophagy and apoptosis are two recognized mechanisms of resistance to bacterial invasion. However, bacteria have likewise evolved the ability to evade immunity. In this study, we identify ACKR4a, a member of an atypical chemokine receptor family, as a suppressor of the NF-κB pathway, which cooperates with Beclin-1 to induce autophagy to inhibit NF-κB signaling and block apoptosis, facilitating Vibrio harveyi infection. Mechanistically, V. harveyi-induced Ap-1 activates ACKR4a transcription and expression. ACKR4a forms a complex with Beclin-1 and MyD88, respectively, inducing autophagy and transporting MyD88 into the lysosome for degradation to suppress inflammatory cytokine production. Meanwhile, ACKR4a-induced autophagy blocks apoptosis by inhibiting caspase8. This study proves for the first time that V. harveyi uses both autophagy and apoptosis to evade innate immunity, suggesting that V. harveyi has evolved the ability to against fish immunity.

Breathe In, Breathe Out: Metabolic Regulation of Lung Macrophages in Host Defense Against Bacterial Infection

Lung macrophages are substantially distinct from other tissue-resident macrophages. They act as frontier sentinels of the alveolar-blood interface and are constantly exposed to various pathogens. Additionally, they precisely regulate immune responses under homeostatic and pathological conditions to curtail tissue damage while containing respiratory infections. As a highly heterogeneous population, the phenotypes and functions of lung macrophages with differing developmental ontogenies are linked to both intrinsic and extrinsic metabolic processes. Importantly, targeting these metabolic pathways greatly impacts macrophage functions, which in turn leads to different disease outcomes in the lung. In this review, we will discuss underlying metabolic regulation of lung macrophage subsets and how metabolic circuits, together with epigenetic modifications, dictate lung macrophage function during bacterial infection.

Coxiella burnetii Plasmid Effector B Promotes LC3-II Accumulation and Contributes To Bacterial Virulence in a SCID Mouse Model

Coxiella burnetii, the causative agent of zoonotic Q fever, is characterized by replicating inside the lysosome-derived Coxiella-containing vacuole (CCV) in host cells. Some effector proteins secreted by C. burnetii have been reported to be involved in the manipulation of autophagy to facilitate the development of CCVs and bacterial replication. Here, we found that the Coxiella plasmid effector B (CpeB) localizes on vacuole membrane targeted by LC3 and LAMP1 and promotes LC3-II accumulation. Meanwhile, the C. burnetii strain lacking the QpH1 plasmid induced less LC3-II accumulation, which was accompanied by smaller CCVs and lower bacterial loads in THP-1 cells. Expression of CpeB in the strain lacking QpH1 led to restoration in LC3-II accumulation but had no effect on the smaller CCV phenotype. In the severe combined immune deficiency (SCID) mouse model, infections with the strain expressing CpeB led to significantly higher bacterial burdens in the spleen and liver than its parent strain devoid of QpH1. We also found that CpeB targets Rab11a to promote LC3-II accumulation. Intratracheally inoculated C. burnetii resulted in lower bacterial burdens and milder lung lesions in Rab11a conditional knockout (Rab11a^-/- CKO) mice. Collectively, these results suggest that CpeB promotes C. burnetii virulence by inducing LC3-II accumulation via a pathway involving Rab11a.

To die or not to die: Programmed cell death responses and their interactions with Coxiella burnetii infection

Coxiella burnetii is a Gram-negative, obligate intracellular, macrophage-tropic bacterium and the causative agent of the zoonotic disease Q fever. The epidemiology of Q fever is associated with the presence of infected animals; sheep, goats, cattle, and humans primarily become infected by inhalation of contaminated aerosols. In humans, the acute phase of the disease is characterized primarily by influenza-like symptoms, and approximately 3-5% of the infected individuals develop chronic infection. C. burnetii infection activates many types of immune responses, and the bacteria's genome encodes for numerous effector proteins that interact with host immune signaling mechanisms. Here, we will discuss two forms of programmed cell death, apoptosis and pyroptosis. Apoptosis is a form of non-inflammatory cell death that leads to phagocytosis of small membrane-bound bodies. Conversely, pyroptosis results in lytic cell death accompanied by the release of proinflammatory cytokines. Both apoptosis and pyroptosis have been implicated in the clearance of intracellular bacterial pathogens, including C. burnetii. Finally, we will discuss the role of autophagy, the degradation of unwanted cellular components, during C. burnetii infection. Together, the review of these forms of programmed cell death will open new research questions aimed at combating this highly infectious pathogen for which treatment options are limited.

Keeping the host alive - lessons from obligate intracellular bacterial pathogens

Mammals have evolved sophisticated host cell death signaling pathways as an important immune mechanism to recognize and eliminate cell intruders before they establish their replicative niche. However, intracellular bacterial pathogens that have co-evolved with their host have developed a multitude of tactics to counteract this defense strategy to facilitate their survival and replication. This requires manipulation of pro-death and pro-survival host signaling pathways during infection. Obligate intracellular bacterial pathogens are organisms that absolutely require an eukaryotic host to survive and replicate, and therefore they have developed virulence factors to prevent diverse forms of host cell death and conserve their replicative niche. This review encapsulates our current understanding of these host-pathogen interactions by exploring the most relevant findings of Anaplasma spp., Chlamydia spp., Rickettsia spp. and Coxiella burnetii modulating host cell death pathways. A detailed comprehension of the molecular mechanisms through which these obligate intracellular pathogens manipulate regulated host cell death will not only increase the current understanding of these difficult-to-study pathogens but also provide insights into new tools to study regulated cell death and the development of new therapeutic approaches to control infection.

Neurotransmitter System-Targeting Drugs Antagonize Growth of the Q Fever Agent, Coxiella burnetii, in Human Cells

Coxiella burnetii is a highly infectious, intracellular, Gram-negative bacterial pathogen that causes human Q fever, an acute flu-like illness that can progress to chronic endocarditis. C. burnetii is transmitted to humans via aerosols and has long been considered a potential biological warfare agent. Although antibiotics, such as doxycycline, effectively treat acute Q fever, a recently identified antibiotic-resistant strain demonstrates the ability of C. burnetii to resist traditional antimicrobials, and chronic disease is extremely difficult to treat with current options. These findings highlight the need for new Q fever therapeutics, and repurposed drugs that target eukaryotic functions to prevent bacterial replication are of increasing interest in infectious disease. To identify this class of anti-C. burnetii therapeutics, we screened a library of 727 FDA-approved or late-stage clinical trial compounds using a human macrophage-like cell model of infection. Eighty-eight compounds inhibited bacterial replication, including known antibiotics, antipsychotic or antidepressant treatments, antihistamines, and several additional compounds used to treat a variety of conditions. The majority of identified anti-C. burnetii compounds target host neurotransmitter system components. Serotoninergic, dopaminergic, and adrenergic components are among the most highly represented targets and potentially regulate macrophage activation, cytokine production, and autophagy. Overall, our screen identified multiple host-directed compounds that can be pursued for potential use as anti-C. burnetii drugs.

IMPORTANCECoxiella burnetii causes the debilitating disease Q fever in humans. This infection is difficult to treat with current antibiotics and can progress to long-term, potentially fatal infection in immunocompromised individuals or when treatment is delayed. Here, we identified many new potential treatment options in the form of drugs that are either FDA approved or have been used in late-stage clinical trials and target human neurotransmitter systems. These compounds are poised for future characterization as nontraditional anti-C. burnetii therapies.

From neglected to dissected: How technological advances are leading the way to the study of Coxiella burnetii pathogenesis

Coxiella burnetii is an obligate intracellular bacterial pathogen responsible for severe worldwide outbreaks of the zoonosis Q fever. The remarkable resistance to environmental stress, extremely low infectious dose and ease of dissemination, contributed to the classification of C. burnetii as a class B biothreat. Unique among intracellular pathogens, C. burnetii escapes immune surveillance and replicates within large autophagolysosome-like compartments called Coxiella-containing vacuoles (CCVs). The biogenesis of these compartments depends on the subversion of several host signalling pathways. For years, the obligate intracellular nature of C. burnetii imposed significant experimental obstacles to the study of its pathogenic traits. With the development of an axenic culture medium in 2009, C. burnetii became genetically tractable, thus allowing the implementation of mutagenesis tools and screening approaches to identify its virulence determinants and investigate its complex interaction with host cells. Here, we review the key advances that have contributed to our knowledge of C. burnetii pathogenesis, leading to the rise of this once-neglected pathogen to an exceptional organism to study the intravacuolar lifestyle.

Lycopene alleviates hepatic ischemia reperfusion injury via the Nrf2/HO-1 pathway mediated NLRP3 inflammasome inhibition in Kupffer cells

Background

Lycopene is a naturally occurring carotenoid found in many fruits and vegetables, which has antioxidant effects. Although lycopene’s protective effect has been observed on ischemia reperfusion (IR) injury in different organs, the effect of lycopene on Kupffer cells (KCs) has not been clearly elucidated in IR-induced acute hepatic inflammatory injury.

Methods

Mice were administered with either olive oil (10 mL/kg body weight) as the control or lycopene (20 mg/kg body weight) by gavage for 2 weeks before undergoing hepatic IR injury.

Results

In this study, we observed that the levels of aspartate aminotransferases (AST), alanine aminotransferase (ALT), and the percentages of hepatocellular apoptosis in mice pretreated with lycopene were significantly lower than control mice. Lycopene inhibited F4/80+ macrophage and Ly6G+ neutrophil accumulation, which further decreased the levels of tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), and interleukin 6 (IL-6). Interestingly, lycopene induced increased autophagy in KCs, which was evidenced by elevated autophagosomes and the increased protein level of LC3B. In these KCs, lycopene-induced upregulation of autophagy inhibited NOD-like receptor family pyrin domain-containing 3 protein (NLRP3) inflammasome activation, which was demonstrated by the reduced mRNA and protein levels of NLRP3, cleaved caspase-1, an apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC), and IL-1β. Furthermore, 3-methyladenine, an autophagy inhibitor, abolished lycopene’s inhibitory effect on the NLRP3 inflammasome in KCs, which led to increased hepatic IR injury. Intriguingly, we identified that the protein levels of nuclear factor erythroid 2-related factor 2 (Nrf2) and heme oxygenase 1 (HO-1) were elevated in KCs isolated from IR-stressed mice pretreated with lycopene. Nrf2-siRNA or HO-1-siRNA could block the autophagy activation enhanced by lycopene in KCs, resulting in the activation of the NLRP3 inflammasome and aggravated hepatic IR injury.

Conclusions

Our findings demonstrated that lycopene promoted Nrf2/HO-1 pathway activation and further suppressed the NLRP3 inflammasome via enhancing KC autophagy, which alleviated hepatic IR injury.

Take my breath away: studying pathogen invasion of the human lung using primary tissue models

The human pulmonary environment is complex, containing a matrix of cells, including fibroblasts, epithelial cells, interstitial macrophages, alveolar macrophages and neutrophils. When confronted with foreign material or invading pathogens, these cells mount a robust response. Nevertheless, many bacterial pathogens with an intracellular lifecycle stage exploit this environment for replication and survival. These include, but are not limited to, Coxiella burnetii, Legionella pneumophila, Yersinia pestis, Mycobacterium tuberculosis and Staphylococcus aureus. Currently, few human disease-relevant model systems exist for studying host-pathogen interactions during these bacterial infections in the lung. Here, we present two novel infection platforms, human alveolar macrophages (hAMs) and human precision-cut lung slices (hPCLS), along with an up-to-date synopsis of research using said models. Additionally, alternative uses for these systems in the absence of pathogen involvement are presented, such as tissue banking and further characterization of the human lung environment. Overall, hAMs and hPCLS allow novel human disease-relevant investigations that other models, such as cell lines and animal models, cannot completely provide.

Coxiella burnetii: international pathogen of mystery

Coxiella burnetii is an intracellular bacterium that causes acute and chronic Q fever. This unique pathogen has been historically challenging to study due to obstacles in genetically manipulating the organism and the inability of small animal models to fully mimic human Q fever. Here, we review the current state of C. burnetii research, highlighting new approaches that allow the mechanistic study of infection in disease relevant settings.

Coxiella effector protein CvpF subverts RAB26-dependent autophagy to promote vacuole biogenesis and virulence

Coxiella burnetii, the etiological agent of the zoonosis Q fever, replicates inside host cells within a large vacuole displaying autolysosomal characteristics. The development of this compartment is mediated by bacterial effectors, which interfere with a number of host membrane trafficking pathways. By screening a Coxiella transposon mutant library, we observed that transposon insertions in cbu0626 led to intracellular replication and vacuole biogenesis defects. Here, we demonstrate that CBU0626 is a novel member of the Coxiella vacuolar protein (Cvp) family of effector proteins, which is translocated by the Dot/Icm secretion system and localizes to vesicles with autolysosomal features as well as Coxiella-containing vacuoles (CCVs). We thus renamed this effector CvpF for Coxiella vacuolar protein F. CvpF specifically interacts with the host small GTPase RAB26, leading to the recruitment of the autophagosomal marker MAP1LC3B/LC3B (microtubule associated protein 1 light chain 3 beta) to CCVs. Importantly, cvpF::Tn mutants were highly attenuated compared to wild-type bacteria in the SCID mouse model of infection, highlighting the importance of CvpF for Coxiella virulence. These results suggest that CvpF manipulates endosomal trafficking and macroautophagy/autophagy induction for optimal C. burnetii vacuole biogenesis.

Abbreviations: ACCM: acidified citrate cystein medium; AP: adaptor related protein complex; CCV: Coxiella-containing vacuole; Cvp: Coxiella vacuolar protein; GDI: guanosine nucleotide dissociation inhibitor; GDF: GDI dissociation factor; GEF: guanine exchange factor; LAMP1: lysosomal associated membrane protein 1; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; MTORC1: mechanistic target of rapamycin kinase MTOR complex 1; PBS: phosphate-buffered saline; PMA: phorbol myristate acetate; SQSTM1/p62: sequestosome 1; WT: wild-type.

Biogenesis of the Spacious Coxiella-Containing Vacuole Depends on Host Transcription Factors TFEB and TFE3

Coxiella burnetii is an obligate intracellular bacterial pathogen that replicates inside the lysosome-derived Coxiella-containing vacuole (CCV). To establish this unique niche, C. burnetii requires the Dot/Icm type IV secretion system (T4SS) to translocate a cohort of effector proteins into the host cell, which modulate multiple cellular processes. To characterize the host-pathogen interactions that occur during C. burnetii infection, stable-isotope labeling by amino acids in cell culture (SILAC)-based proteomics was used to identify changes in the host proteome during infection of a human-derived macrophage cell line. These data revealed that the abundances of many proteins involved in host cell autophagy and lysosome biogenesis were increased in infected cells. Thus, the role of the host transcription factors TFEB and TFE3, which regulate the expression of a network of genes involved in autophagy and lysosomal biogenesis, were examined in the context of C. burnetii infection. During infection with C. burnetii, both TFEB and TFE3 were activated, as demonstrated by the transport of these proteins from the cytoplasm into the nucleus. The nuclear translocation of these transcription factors was shown to be dependent on the T4SS, as a Dot/Icm mutant showed reduced nuclear translocation of TFEB and TFE3. This was supported by the observation that blocking bacterial translation with chloramphenicol resulted in the movement of TFEB and TFE3 back into the cytoplasm. Silencing of the TFEB and TFE3 genes, alone or in combination, significantly reduced the size of the CCV, which indicates that these host transcription factors facilitate the expansion and maintenance of the organelle that supports C. burnetii intracellular replication.

Coxiella burnetii Type 4B Secretion System-dependent manipulation of endolysosomal maturation is required for bacterial growth

Upon host cell infection, the obligate intracellular bacterium Coxiella burnetii resides and multiplies within the Coxiella–Containing Vacuole (CCV). The nascent CCV progresses through the endosomal maturation pathway into a phagolysosome, acquiring endosomal and lysosomal markers, as well as acidic pH and active proteases and hydrolases. Approximately 24–48 hours post infection, heterotypic fusion between the CCV and host endosomes/lysosomes leads to CCV expansion and bacterial replication in the mature CCV. Initial CCV acidification is required to activate C. burnetii metabolism and the Type 4B Secretion System (T4BSS), which secretes effector proteins required for CCV maturation. However, we found that the mature CCV is less acidic (pH~5.2) than lysosomes (pH~4.8). Further, inducing CCV acidification to pH~4.8 causes C. burnetii lysis, suggesting C. burnetii actively regulates pH of the mature CCV. Because heterotypic fusion with host endosomes/lysosomes may influence CCV pH, we investigated endosomal maturation in cells infected with wildtype (WT) or T4BSS mutant (ΔdotA) C. burnetii. In WT-infected cells, we observed a significant decrease in proteolytically active, LAMP1-positive endolysosomal vesicles, compared to mock or ΔdotA-infected cells. Using a ratiometric assay to measure endosomal pH, we determined that the average pH of terminal endosomes in WT-infected cells was pH~5.8, compared to pH~4.75 in mock and ΔdotA-infected cells. While endosomes progressively acidified from the periphery (pH~5.5) to the perinuclear area (pH~4.7) in both mock and ΔdotA-infected cells, endosomes did not acidify beyond pH~5.2 in WT-infected cells. Finally, increasing lysosomal biogenesis by overexpressing the transcription factor EB resulted in smaller, more proteolytically active CCVs and a significant decrease in C. burnetii growth, indicating host lysosomes are detrimental to C. burnetii. Overall, our data suggest that C. burnetii inhibits endosomal maturation to reduce the number of proteolytically active lysosomes available for heterotypic fusion with the CCV, possibly as a mechanism to regulate CCV pH.

The obligate intracellular bacterium Coxiella burnetii causes human Q fever, which manifests as a flu-like illness but can develop into a life-threatening and difficult to treat endocarditis. C. burnetii, in contrast to many other intracellular bacteria, thrives within a lysosome-like vacuole in host cells. However, we previously found that the C. burnetii vacuole is not as acidic as lysosomes and increased acidification kills the bacteria, suggesting that C. burnetii regulates the pH of its vacuole. Here, we discovered that C. burnetii blocks endolysosomal maturation and acidification during host cell infection, resulting in fewer lysosomes in the host cell. Moreover, increasing lysosomes in the host cells inhibited C. burnetii growth. Together, our study suggests that C. burnetii regulates vacuole acidity and blocks endosomal maturation in order to produce a permissive intracellular niche.

Defying Death - How Coxiella burnetii Copes with Intentional Host Cell Suicide

The obligate intracellular pathogen Coxiella burnetii is the causative agent of the worldwide zoonotic disease Q fever. This Gram-negative bacterium infects macrophages where it establishes a replicative niche in an acidic and phagolysosome-like vacuole. Establishing and maintaining the niche requires a functional type IV secretion system (T4SS) which translocates multiple effector proteins into the host cell. These effector proteins act by manipulating diverse cellular processes allowing the bacterium to establish an infection and complete its complex biphasic developmental cycle. The lengthy nature of this life cycle suggests that C. burnetii has to successfully deal with cellular defense processes. Cell death is one mechanism infected cells frequently utilize to control or to at least minimize the impact of an infection. To date, four effector proteins have been identified in C. burnetii, which interfere with the induction of cell death. Three, AnkG, CaeA, and CaeB, affect intrinsic apoptosis, CaeA additionally extrinsic apoptosis. The proteins target different steps of the apoptotic pathway and are not conserved among isolates suggesting redundancy as an important feature of cell death inhibition. The fourth effector protein, IcaA, interferes with the non-canonical pathway of pyroptosis, an important inflammatory cell death pathway for controlling infectious disease. Autophagy is relevant for the C. burnetii life-cycle, but to which extent autophagic cell death is a factor in bacterial survival and proliferation is still not clear. To convincingly understand how bacterial manipulation of autophagy affects cell death either directly or indirectly will require further experiments. Collectively, C. burnetii modulates the extrinsic and intrinsic apoptotic pathways and non-canonical pyroptosis to inhibit host cell death, thereby providing a stable, intracellular niche for the course of the pathogen's infectious cycle.

Characterization of Early Stages of Human Alveolar Infection by the Q Fever Agent Coxiella burnetii

Human Q fever is caused by the intracellular bacterial pathogen Coxiella burnetii Q fever presents with acute flu-like and pulmonary symptoms or can progress to chronic, severe endocarditis. After human inhalation, C. burnetii is engulfed by alveolar macrophages and transits through the phagolysosomal maturation pathway, resisting the acidic pH of lysosomes to form a parasitophorous vacuole (PV) in which to replicate. Previous studies showed that C. burnetii replicates efficiently in primary human alveolar macrophages (hAMs) in ex vivo human lung tissue. Although C. burnetii replicates in most cell types in vitro, the pathogen does not grow in non-hAM cells of human lung tissue. In this study, we investigated the interaction between C. burnetii and other pulmonary cell types apart from the lung environment. C. burnetii formed a prototypical PV and replicated efficiently in human pulmonary fibroblasts and in airway, but not alveolar, epithelial cells. Atypical PV expansion in alveolar epithelial cells was attributed in part to defective recruitment of autophagy-related proteins. Further assessment of the C. burnetii growth niche showed that macrophages mounted a robust interleukin 8 (IL-8), neutrophil-attracting response to C. burnetii and ultimately shifted to an M2-polarized phenotype characteristic of anti-inflammatory macrophages. Considering our findings together, this study provides further clarity on the unique C. burnetii-lung dynamic during early stages of human acute Q fever.

Noncanonical Inhibition of mTORC1 by Coxiella burnetii Promotes Replication within a Phagolysosome-Like Vacuole

Coxiella burnetii is an intracellular pathogenic bacterium that replicates within a lysosomal vacuole. Biogenesis of the Coxiella-containing vacuole (CCV) requires effector proteins delivered into the host cell cytosol by the type 4B secretion system (T4BSS). Modifications to lysosomal physiology required for pathogen replication within the CCV are poorly understood. Mammalian (or mechanistic) target of rapamycin complex 1 (mTORC1) is a master kinase that regulates lysosome structure and function. Nutrient deprivation inhibits mTORC1, which promotes cell catabolism in the form of accelerated autophagy and increased lysosome biosynthesis. Here, we report that C. burnetii growth is enhanced by T4BSS-dependent inhibition of mTORC1 that does not activate autophagy. Canonical inhibition of mTORC1 by starvation or inhibitor treatment that induces autophagic flux does not benefit C. burnetii growth. Furthermore, hyperactivation of mTORC1 impairs bacterial replication. These findings indicate that C. burnetii inhibition of mTORC1 without accelerated autophagy promotes bacterial growth.

The Q fever agent Coxiella burnetii is a Gram-negative bacterium that invades macrophages and replicates inside a specialized lysosomal vacuole. The pathogen employs a type 4B secretion system (T4BSS) to deliver effector proteins into the host cell that modify the Coxiella-containing vacuole (CCV) into a replication-permissive niche. Mature CCVs are massive degradative organelles that acquire lysosomal proteins. Inhibition of mammalian (or mechanistic) target of rapamycin complex 1 (mTORC1) kinase by nutrient deprivation promotes autophagy and lysosome fusion, as well as activation of the transcription factors TFE3 and TFEB (TFE3/B), which upregulates expression of lysosomal genes. Here, we report that C. burnetii inhibits mTORC1 as evidenced by impaired localization of mTORC1 to endolysosomal membranes and decreased phosphorylation of elF4E-binding protein 1 (4E-BP1) and S6 kinase 1 in infected cells. Infected cells exhibit increased amounts of autophagy-related proteins protein 1A/1B-light chain 3 (LC3) and p62 as well as of activated TFE3. However, C. burnetii did not accelerate autophagy or block autophagic flux triggered by cell starvation. Activation of autophagy or transcription by TFE3/B increased CCV expansion without enhancing bacterial replication. By contrast, knockdown of tuberous sclerosis complex 1 (TSC1) or TSC2, which hyperactivates mTORC1, impaired CCV expansion and bacterial replication. Together, these data demonstrate that specific inhibition of mTORC1 by C. burnetii, but not amplified cell catabolism via autophagy, is required for optimal pathogen replication. These data reveal a complex interplay between lysosomal function and host cell metabolism that regulates C. burnetii intracellular growth.

Atg5 Supports Rickettsia australis Infection in Macrophages In Vitro and In Vivo

Rickettsiae can cause life-threatening infections in humans. Macrophages are one of the initial targets for rickettsiae after inoculation by ticks. However, it remains poorly understood how rickettsiae remain free in macrophages prior to establishing their infection in microvascular endothelial cells. Here, we demonstrated that the concentration of Rickettsia australis was significantly greater in infected tissues of Atg5^flox/flox mice than in the counterparts of Atg5^flox/flox Lyz-Cre mice, in association with a reduced level of interleukin-1β (IL-1β) in serum. The greater concentration of R. australis in Atg5^flox/flox bone marrow-derived macrophages (BMMs) than in Atg5^flox/flox Lyz-Cre BMMs in vitro was abolished by exogenous treatment with recombinant IL-1β. Rickettsia australis induced significantly increased levels of light chain 3 (LC3) form II (LC3-II) and LC3 puncta in Atg5-competent BMMs but not in Atg5-deficient BMMs, while no p62 turnover was observed. Further analysis found the colocalization of LC3 with a small portion of R. australis and Rickettsia-containing double-membrane-bound vacuoles in the BMMs of B6 mice. Moreover, treatment with rapamycin significantly increased the concentrations of R. australis in B6 BMMs compared to those in the untreated B6 BMM controls. Taken together, our results demonstrate that Atg5 favors R. australis infection in mouse macrophages in association with a suppressed level of IL-1β production but not active autophagy flux. These data highlight the contribution of Atg5 in macrophages to the pathogenesis of rickettsial diseases.