Infection of human monocyte-derived macrophages with Coxiella burnetii

Expression of human TLR4/myeloid differentiation factor 2 directs an early innate immune response associated with modest increases in bacterial burden during Coxiella burnetii infection

Human TLR4 (hTLR4) and mouse TLR4 molecules respond differently to hypo-acylated LPS. The LPS of Coxiella burnetii is hypo-acylated and heavily glycosylated and causes a minimal response by human cells. Thus, we hypothesized that mice expressing hTLR4 molecules would be more susceptible to C. burnetii infection. Our results show that transgenic mice expressing hTLR4 and the human myeloid differentiation factor 2 (MD-2) adaptor protein (hTLR4/MD-2) respond similarly to wild type mice with respect to overall disease course. However, differences in bacterial burdens in tissues were noted, and lung pathology was increased in hTLR4/MD2 compared to wild type mice. Surprisingly, bone marrow chimera experiments indicated that hTLR4/MD-2 expression on non-hematopoietic cells, rather than the target cells for C. burnetii infection, accounted for increased bacterial burden. Early during infection, cytokines involved in myeloid cell recruitment were detected in the plasma of hTLR4/MD2 mice but not wild type mice. This restricted cytokine response was accompanied by neutrophil recruitment to the lung in hTLR4/MD2 mice. These data suggest that hTLR4/MD-2 alters early responses during C. burnetii infection. These early responses are precursors to later increased bacterial burdens and exacerbated pathology in the lung. Our data suggest an unexpected role for hTLR4/MD-2 in non-hematopoietic cells during C. burnetii infection.

High-Content Imaging Reveals Expansion of the Endosomal Compartment during Coxiella burnetii Parasitophorous Vacuole Maturation

Coxiella burnetii is an obligate intracellular pathogen and the causative agent of human Q fever. Replication of the bacterium within a large parasitophorous vacuole (PV) resembling a host phagolysosome is required for pathogenesis. PV biogenesis is a pathogen driven process that requires engagement of several host cell vesicular trafficking pathways to acquire vacuole components. The goal of this study was to determine if infection by C. burnetii modulates endolysosomal flux to potentially benefit PV formation. HeLa cells, infected with C. burnetii or left uninfected, were incubated with fluorescent transferrin (Tf) for 0–30 min, and the amount of Tf internalized by cells quantitated by high-content imaging. At 3 and 5 days, but not 1 day post-infection, the maximal amounts of fluorescent Tf internalized by infected cells were significantly greater than uninfected cells. The rates of Tf uptake and recycling were the same for infected and uninfected cells; however, residual Tf persisted in EEA.1 positive compartments adjacent to large PV after 30 min of recycling in the absence of labeled Tf. On average, C. burnetii-infected cells contained significantly more CD63-positive endosomes than uninfected cells. In contrast, cells containing large vacuoles generated by Chlamydia trachomatis exhibited increased rates of Tf internalization without increased CD63 expression. Our results suggest that C. burnetii infection expands the endosomal system to increase capacity for endocytic material. Furthermore, this study demonstrates the power of high-content imaging for measurement of cellular responses to infection by intracellular pathogens.

Murine Alveolar Macrophages Are Highly Susceptible to Replication of Coxiella burnetii Phase II In Vitro

Coxiella burnetii is a Gram-negative bacterium that causes Q fever in humans. Q fever is an atypical pneumonia transmitted through inhalation of contaminated aerosols. In mammalian lungs, C. burnetii infects and replicates in several cell types, including alveolar macrophages (AMs). The innate immunity and signaling pathways operating during infection are still poorly understood, in part because of the lack of relevant host cell models for infection in vitro In the study described here, we investigated and characterized the infection of primary murine AMs by C. burnetii phase II in vitro Our data reveal that AMs show a pronounced M2 polarization and are highly permissive to C. burnetii multiplication in vitro Murine AMs present an increased susceptibility to infection in comparison to primary bone marrow-derived macrophages. AMs support more than 2 logs of bacterial replication during 12 days of infection in culture, similar to highly susceptible host cells, such as Vero and THP-1 cells. As a proof of principle that AMs are useful for investigation of C. burnetii replication, we performed experiments with AMs from Nos2(-/-) or Ifng(-/-) mice. In the absence of gamma interferon and nitric oxide synthase 2 (NOS2), AMs were significantly more permissive than wild-type cells. In contrast, AMs from Il4(-/-) mice were more restrictive to C. burnetii replication, supporting the importance of M2 polarization for the permissiveness of AMs to C. burnetii replication. Collectively, our data account for understanding the high susceptibility of alveolar macrophages to bacterial replication and support the use of AMs as a relevant model of C. burnetii growth in primary macrophages.

Roles of Toll-Like Receptor 2 (TLR2), TLR4, and MyD88 during Pulmonary Coxiella burnetii Infection

Coxiella burnetii, the causative agent of Q fever, is an obligate intracellular, primarily pulmonary, bacterial pathogen. Although much is known about adaptive immune responses against this bacterium, our understanding of innate immune responses against C. burnetii is not well defined, particularly within the target tissue for infection, the lung. Previous studies examined the roles of the innate immune system receptors Toll-like receptor 2 (TLR2) and TLR4 in peripheral infection models and described minimal phenotypes in specific gene deletion animals compared to those of their wild-type controls (S. Meghari et al., Ann N Y Acad Sci 1063:161-166, 2005,http://dx.doi.org/10.1196/annals.1355.025; A. Honstettre et al., J Immunol 172:3695-3703, 2004,http://dx.doi.org/10.4049/jimmunol.172.6.3695) . Here, we assessed the roles for TLR2, TLR4, and MyD88 in pulmonary C. burnetii infection and compared responses to those that occurred in TLR2- and TLR4-deficient animals following peripheral infection. As observed previously, neither TLR2 nor TLR4 was needed for limiting bacterial growth after peripheral infection. In contrast, TLR2 and, to a lesser extent, TLR4 limited growth (or dissemination) of the bacterium in the lung and spleen after pulmonary infection. TLR2, TLR4, and MyD88 were not required for the general inflammatory response in the lungs after pulmonary infection. However, MyD88 signaling was important for infection-induced morbidity. Finally, TLR2 expression on hematopoietic cells was most important for limiting bacterial growth in the lung. These results expand on our knowledge of the roles for TLR2 and TLR4 in C. burnetii infection and suggest various roles for these receptors that are dictated by the site of infection.

Transcriptional Profiling of Coxiella burnetii Reveals Extensive Cell Wall Remodeling in the Small Cell Variant Developmental Form

A hallmark of Coxiella burnetii, the bacterial cause of human Q fever, is a biphasic developmental cycle that generates biologically, ultrastructurally, and compositionally distinct large cell variant (LCV) and small cell variant (SCV) forms. LCVs are replicating, exponential phase forms while SCVs are non-replicating, stationary phase forms. The SCV has several properties, such as a condensed nucleoid and an unusual cell envelope, suspected of conferring enhanced environmental stability. To identify genetic determinants of the LCV to SCV transition, we profiled the C. burnetii transcriptome at 3 (early LCV), 5 (late LCV), 7 (intermediate forms), 14 (early SCV), and 21 days (late SCV) post-infection of Vero epithelial cells. Relative to early LCV, genes downregulated in the SCV were primarily involved in intermediary metabolism. Upregulated SCV genes included those involved in oxidative stress responses, arginine acquisition, and cell wall remodeling. A striking transcriptional signature of the SCV was induction (>7-fold) of five genes encoding predicted L,D transpeptidases that catalyze nonclassical 3-3 peptide cross-links in peptidoglycan (PG), a modification that can influence several biological traits in bacteria. Accordingly, of cross-links identified, muropeptide analysis showed PG of SCV with 46% 3-3 cross-links as opposed to 16% 3-3 cross-links for LCV. Moreover, electron microscopy revealed SCV with an unusually dense cell wall/outer membrane complex as compared to LCV with its clearly distinguishable periplasm and inner and outer membranes. Collectively, these results indicate the SCV produces a unique transcriptome with a major component directed towards remodeling a PG layer that likely contributes to Coxiella's environmental resistance.

Draft Genome Sequences of Two Novel Amoeba-Resistant Intranuclear Bacteria, "Candidatus Berkiella cookevillensis" and "Candidatus Berkiella aquae"

“Candidatus Berkiella cookevillensis” and “Candidatus Berkiella aquae” are obligate intranuclear endosymbionts of freshwater amoebae. Here, we present the draft genome sequences of these two bacteria, with total sizes of 2,990,361 bp and 3,626,027 bp, respectively.

Characterization of a lipopolysaccharide-targeted monoclonal antibody and its variable fragments as candidates for prophylaxis against the obligate intracellular bacterial pathogen Coxiella burnetii

Our previous study demonstrated that treatment of Coxiella burnetii with the phase I lipopolysaccharide (PI-LPS)-targeted monoclonal antibody (MAb) 1E4 significantly inhibited C. burnetii infection in mice, suggesting that 1E4 is a protective MAb. To determine whether passive transfer of antibodies (Abs) can provide protection against C. burnetii natural infection, we examined if passive transfer of 1E4 would protect SCID mice against C. burnetii aerosol infection. The results indicated that 1E4 conferred significant protection against aerosolized C. burnetii, suggesting that 1E4 may be useful for preventing C. burnetii natural infection. To further understand the mechanisms of 1E4-mediated protection and to test the possibility of using humanized 1E4 to prevent C. burnetii infection, we examined whether the Fab fragment of 1E4 (Fab1E4), a recombinant murine single-chain variable fragment (muscFv1E4), and a humanized single-chain variable fragment (huscFv1E4) retained the ability of 1E4 to inhibit C. burnetii infection. The results indicated that Fab1E4, muscFv1E4, and huscFv1E4 were able to inhibit C. burnetii infection in mice but that their ability to inhibit C. burnetii infection was lower than that of 1E4. In addition, treatment of C. burnetii with Fab1E4, muscFv1E4, or huscFv1E4 can block C. burnetii infection of macrophages. Interestingly, treatment of C. burnetii with huscFv1E4 can significantly reduce C. burnetii infectivity in human macrophages. This report provides the first evidence to demonstrate that the humanized variable fragments of an LPS-specific MAb can neutralize C. burnetii infection and appears to be a promising step toward the potential use of a humanized MAb as emergency prophylaxis against C. burnetii exposure.

Yersinia pestis survival and replication within human neutrophil phagosomes and uptake of infected neutrophils by macrophages

Yersinia pestis, the bacterial agent of plague, is transmitted by fleas. The bite of an infected flea deposits Y. pestis into the dermis and triggers recruitment of innate immune cells, including phagocytic PMNs. Y. pestis can subvert this PMN response and survive at the flea-bite site, disseminate, and persist in the host. Although its genome encodes a number of antiphagocytic virulence factors, phagocytosis of Y. pestis by PMNs has been observed. This study tests the hypotheses that Y. pestis, grown at the ambient temperature of the flea vector (21°C), where the major antiphagocytic virulence factors are not produced, can survive and replicate within human PMNs and can use PMNs as a route to infect macrophages subsequently. We show that Y. pestis is localized within PMN phagosomes, predominately as individual bacteria, and that intracellular bacteria can survive and replicate. Within 12 h of infection, ~70% of infected PMNs had PS on their surface and were plausibly competent for efferocytosis. With the use of live cell confocal imaging, we show that autologous HMDMs recognize and internalize infected PMNs and that Y. pestis survives and replicates within these HMDMs following efferocytosis. Addition of HMDMs to infected PMNs resulted in decreased secretion of inflammatory cytokines (compared with HMDMs incubated directly with pCD1(-) Y. pestis) and increased secretion of the anti-inflammatory cytokine IL-1ra. Thus, Y. pestis can survive and replicate within PMNs, and infected PMNs may be a route for noninflammatory infection of macrophages.

In vitro and in vivo infectious potential of coxiella burnetii: a study on Belgian livestock isolates

Q-fever is a zoonosis caused by the gram-negative obligate intracellular pathogen Coxiella burnetii. Since its discovery, and particularly following the recent outbreaks in the Netherlands, C. burnetii appeared as a clear public health concern. In the present study, the infectious potential displayed by goat and cattle isolates of C. burnetii was compared to a reference strain (Nine Mile) using both in vitro (human HeLa and bovine macrophage cells) and in vivo (BALB/c mice) models. The isolates had distant genomic profiles with one - the goat isolate - being identical to the predominant strain circulating in the Netherlands during the 2007–2010 outbreaks. Infective doses were established with ethidium monoazide-PCR for the first time here applied to C. burnetii. This method allowed for the preparation of reproducible and characterized inocula thanks to its capacity to discriminate between live and dead cells. Globally, the proliferative capacity of the Nine Mile strain in cell lines and mice was higher compared to the newly isolated field strains. In vitro, the bovine C. burnetii isolate multiplied faster in a bovine macrophage cell line, an observation tentatively explained by the preferential specificity of this strain for allogeneic host cells. In the BALB/c mouse model, however, the goat and bovine isolates multiplied at about the same rate indicating no peculiar hypervirulent behavior in this animal model.

Formalin-inactivated Coxiella burnetii phase I vaccine-induced protection depends on B cells to produce protective IgM and IgG

To further understand the mechanisms of formalin-inactivated Coxiella burnetii phase I (PI) vaccine (PIV)-induced protection, we examined if B cell, T cell, CD4(+) T cell, or CD8(+) T cell deficiency in mice significantly affects the ability of PIV to confer protection against a C. burnetii infection. Interestingly, compared to wild-type (WT) mice, PIV conferred comparable levels of protection in CD4(+) T cell- or CD8(+) T cell-deficient mice and partial protection in T cell-deficient mice but did not provide measurable protection in B cell-deficient mice. These results suggest that PIV-induced protection depends on B cells. In addition, anti-PI-specific IgM was the major detectable antibody (Ab) in immune sera from PIV-vaccinated CD4(+) T cell-deficient mice, and passive transfer of immune sera from PIV-vaccinated CD4(+) T cell-deficient mice conferred significant protection. These results suggest that T cell-independent anti-PI-specific IgM may contribute to PIV-induced protection. Our results also suggested that PIV-induced protection may not depend on complement activation and Fc receptor-mediated effector functions. Furthermore, our results demonstrated that both IgM and IgG from PIV-vaccinated WT mouse sera were able to inhibit C. burnetii infection in vivo, but only IgM from PIV-vaccinated CD4(+) T cell-deficient mouse sera inhibited C. burnetii infection. Collectively, these findings suggest that PIV-induced protection depends on B cells to produce protective IgM and IgG and that T cell-independent anti-PI-specific IgM may play a critical role in PIV-induced protection against C. burnetii infection.

Advances in genetic manipulation of obligate intracellular bacterial pathogens

Infections by obligate intracellular bacterial pathogens result in significant morbidity and mortality worldwide. These bacteria include Chlamydia spp., which causes millions of cases of sexually transmitted disease and blinding trachoma annually, and members of the α-proteobacterial genera Anaplasma, Ehrlichia, Orientia, and Rickettsia, agents of serious human illnesses including epidemic typhus. Coxiella burnetii, the agent of human Q fever, has also been considered a prototypical obligate intracellular bacterium, but recent host cell-free (axenic) growth has rescued it from obligatism. The historic genetic intractability of obligate intracellular bacteria has severely limited molecular dissection of their unique lifestyles and virulence factors involved in pathogenesis. Host cell restricted growth is a significant barrier to genetic transformation that can make simple procedures for free-living bacteria, such as cloning, exceedingly difficult. Low transformation efficiency requiring long-term culture in host cells to expand small transformant populations is another obstacle. Despite numerous technical limitations, the last decade has witnessed significant gains in genetic manipulation of obligate intracellular bacteria including allelic exchange. Continued development of genetic tools should soon enable routine mutation and complementation strategies for virulence factor discovery and stimulate renewed interest in these refractory pathogens. In this review, we discuss the technical challenges associated with genetic transformation of obligate intracellular bacteria and highlight advances made with individual genera.

The Coxiella burnetii cryptic plasmid is enriched in genes encoding type IV secretion system substrates

The intracellular bacterial pathogen Coxiella burnetii directs biogenesis of a phagolysosome-like parasitophorous vacuole (PV), in which it replicates. The organism encodes a Dot/Icm type IV secretion system (T4SS) predicted to deliver to the host cytosol effector proteins that mediate PV formation and other cellular events. All C. burnetii isolates carry a large, autonomously replicating plasmid or have chromosomally integrated plasmid-like sequences (IPS), suggesting that plasmid and IPS genes are critical for infection. Bioinformatic analyses revealed two candidate Dot/Icm substrates with eukaryotic-like motifs uniquely encoded by the QpH1 plasmid from the Nine Mile reference isolate. CpeC, containing an F-box domain, and CpeD, possessing kinesin-related and coiled-coil regions, were secreted by the closely related Legionella pneumophila Dot/Icm T4SS. An additional QpH1-specific gene, cpeE, situated in a predicted operon with cpeD, also encoded a secreted effector. Further screening revealed that three hypothetical proteins (CpeA, CpeB, and CpeF) encoded by all C. burnetii plasmids and IPS are Dot/Icm substrates. By use of new genetic tools, secretion of plasmid effectors by C. burnetii during host cell infection was confirmed using β-lactamase and adenylate cyclase translocation assays, and a C-terminal secretion signal was identified. When ectopically expressed in HeLa cells, plasmid effectors trafficked to different subcellular sites, including autophagosomes (CpeB), ubiquitin-rich compartments (CpeC), and the endoplasmic reticulum (CpeD). Collectively, these results suggest that C. burnetii plasmid-encoded T4SS substrates play important roles in subversion of host cell functions, providing a plausible explanation for the absolute maintenance of plasmid genes by this pathogen.

Coxiella burnetii Nine Mile II proteins modulate gene expression of monocytic host cells during infection

BACKGROUND

Coxiella burnetii is an intracellular bacterial pathogen that causes acute and chronic disease in humans. Bacterial replication occurs within enlarged parasitophorous vacuoles (PV) of eukaryotic cells, the biogenesis and maintenance of which is dependent on C. burnetii protein synthesis. These observations suggest that C. burnetii actively subverts host cell processes, however little is known about the cellular biology mechanisms manipulated by the pathogen during infection. Here, we examined host cell gene expression changes specifically induced by C. burnetii proteins during infection.

RESULTS

We have identified 36 host cell genes that are specifically regulated when de novo C. burnetii protein synthesis occurs during infection using comparative microarray analysis. Two parallel sets of infected and uninfected THP-1 cells were grown for 48 h followed by the addition of chloramphenicol (CAM) to 10 μg/ml in one set. Total RNA was harvested at 72 hpi from all conditions, and microarrays performed using Phalanx Human OneArray slides. A total of 784 (mock treated) and 901 (CAM treated) THP-1 genes were up or down regulated ≥2 fold in the C. burnetii infected vs. uninfected cell sets, respectively. Comparisons between the complementary data sets (using >0 fold), eliminated the common gene expression changes. A stringent comparison (≥2 fold) between the separate microarrays revealed 36 host cell genes modulated by C. burnetii protein synthesis. Ontological analysis of these genes identified the innate immune response, cell death and proliferation, vesicle trafficking and development, lipid homeostasis, and cytoskeletal organization as predominant cellular functions modulated by C. burnetii protein synthesis.

CONCLUSIONS

Collectively, these data indicate that C. burnetii proteins actively regulate the expression of specific host cell genes and pathways. This is in addition to host cell genes that respond to the presence of the pathogen whether or not it is actively synthesizing proteins. These findings indicate that C. burnetii modulates the host cell gene expression to avoid the immune response, preserve the host cell from death, and direct the development and maintenance of a replicative PV by controlling vesicle formation and trafficking within the host cell during infection.

Coxiella burnetii phase I and II variants replicate with similar kinetics in degradative phagolysosome-like compartments of human macrophages

Coxiella burnetii infects mononuclear phagocytes, where it directs biogenesis of a vacuolar niche termed the parasitophorous vacuole (PV). Owing to its lumenal pH (approximately 5) and fusion with endolysosomal vesicles, the PV is considered phagolysosome-like. However, the degradative properties of the mature PV are unknown, and there are conflicting reports on the maturation state and growth permissiveness of PV harboring virulent phase I or avirulent phase II C. burnetii variants in human mononuclear phagocytes. Here, we employed infection of primary human monocyte-derived macrophages (HMDMs) and THP-1 cells as host cells to directly compare the PV maturation kinetics and pathogen growth in cells infected with the Nine Mile phase I variant (NMI) or phase II variant (NMII) of C. burnetii. In both cell types, phase variants replicated with similar kinetics, achieving roughly 2 to 3 log units of growth before they reached stationary phase. HMDMs infected by either phase variant secreted similar amounts of the proinflammatory cytokines interleukin-6 and tumor necrosis factor alpha. In infected THP-1 cells, equal percentages of NMI and NMII PVs decorate with the early endosomal marker Rab5, the late endosomal/lysosomal markers Rab7 and CD63, and the lysosomal marker cathepsin D at early (8 h) and late (72 h) time points postinfection (p.i.). Mature PVs (2 to 4 days p.i.) harboring NMI or NMII contained proteolytically active cathepsins and quickly degraded Escherichia coli. These data suggest that C. burnetii does not actively inhibit phagolysosome function as a survival mechanism. Instead, NMI and NMII resist degradation to replicate in indistinguishable digestive PVs that fully mature through the endolysosomal pathway.

Antibody-mediated immunity to the obligate intracellular bacterial pathogen Coxiella burnetii is Fc receptor- and complement-independent

Background

The obligate intracellular bacterial pathogen Coxiella burnetii causes the zoonosis Q fever. The intracellular niche of C. burnetii has led to the assumption that cell-mediated immunity is the most important immune component for protection against this pathogen. However, passive immunization with immune serum can protect naïve animals from challenge with virulent C. burnetii, indicating a role for antibody (Ab) in protection. The mechanism of this Ab-mediated protection is unknown. Therefore, we conducted a study to determine whether Fc receptors (FcR) or complement contribute to Ab-mediated immunity (AMI) to C. burnetii.

Results

Virulent C. burnetii infects and replicates within human dendritic cells (DC) without inducing their maturation or activation. We investigated the effects of Ab opsonized C. burnetii on human monocyte-derived and murine bone marrow-derived DC. Infection of DC with Ab-opsonized C. burnetii resulted in increased expression of maturation markers and inflammatory cytokine production. Bacteria that had been incubated with naïve serum had minimal effect on DC, similar to virulent C. burnetii alone. The effect of Ab opsonized C. burnetii on DC was FcR dependent as evidenced by a reduced response of DC from FcR knockout (FcR k/o) compared to C57Bl/6 (B6) mice. To address the potential role of FcR in Ab-mediated protection in vivo, we compared the response of passively immunized FcR k/o mice to the B6 controls. Interestingly, we found that FcR are not essential for AMI to C. burnetii in vivo. We subsequently examined the role of complement in AMI by passively immunizing and challenging several different strains of complement-deficient mice and found that AMI to C. burnetii is also complement-independent.

Conclusion

Despite our data showing FcR-dependent stimulation of DC in vitro, Ab-mediated immunity to C. burnetii in vivo is FcR-independent. We also found that passive immunity to this pathogen is independent of complement.

The Coxiella burnetii ankyrin repeat domain-containing protein family is heterogeneous, with C-terminal truncations that influence Dot/Icm-mediated secretion

Coxiella burnetii is an obligate intracellular bacterium that directs biogenesis of a parasitophorous vacuole (PV) for replication. Effectors of PV maturation are likely translocated into the host cytosol by a type IV secretion system (T4SS) with homology to the Dot/Icm apparatus of Legionella pneumophila. Since secreted bacterial virulence factors often functionally mimic the activities of host proteins, prokaryotic proteins with eukaryotic features are considered candidate T4SS substrates. Genes encoding proteins with eukaryotic-type ankyrin repeat domains (Anks) were identified upon genome sequencing of the C. burnetii Nine Mile reference isolate, which is associated with a case of human acute Q fever. Interestingly, recent genome sequencing of the G and K isolates, derived from human chronic endocarditis patients, and of the Dugway rodent isolate revealed remarkable heterogeneity in the Ank gene family, with the Dugway isolate harboring the largest number of full-length Ank genes. Using L. pneumophila as a surrogate host, we identified 10 Dugway Anks and 1 Ank specific to the G and K endocarditis isolates translocated into the host cytosol in a Dot/Icm-dependent fashion. A 10-amino-acid C-terminal region appeared to be necessary for translocation, with some Anks also requiring the chaperone IcmS for secretion. Ectopically expressed Anks localized to a variety of subcellular regions in mammalian cells, including microtubules, mitochondria, and the PV membrane. Collectively, these data suggest that C. burnetii isolates translocate distinct subsets of the Ank protein family into the host cytosol, where they modulate diverse functions, some of which may be unique to C. burnetii pathotypes.

Host cell-free growth of the Q fever bacterium Coxiella burnetii

The inability to propagate obligate intracellular pathogens under axenic (host cell-free) culture conditions imposes severe experimental constraints that have negatively impacted progress in understanding pathogen virulence and disease mechanisms. Coxiella burnetii, the causative agent of human Q (Query) fever, is an obligate intracellular bacterial pathogen that replicates exclusively in an acidified, lysosome-like vacuole. To define conditions that support C. burnetii growth, we systematically evaluated the organism's metabolic requirements using expression microarrays, genomic reconstruction, and metabolite typing. This led to development of a complex nutrient medium that supported substantial growth (approximately 3 log(10)) of C. burnetii in a 2.5% oxygen environment. Importantly, axenically grown C. burnetii were highly infectious for Vero cells and exhibited developmental forms characteristic of in vivo grown organisms. Axenic cultivation of C. burnetii will facilitate studies of the organism's pathogenesis and genetics and aid development of Q fever preventatives such as an effective subunit vaccine. Furthermore, the systematic approach used here may be broadly applicable to development of axenic media that support growth of other medically important obligate intracellular pathogens.

Sustained activation of Akt and Erk1/2 is required for Coxiella burnetii antiapoptotic activity

Coxiella burnetii is an obligate intracellular bacterial pathogen that directs biogenesis of a lysosome-like, parasitophorous vacuole in mammalian cells. We recently reported that C. burnetii inhibits apoptotic cell death in macrophages, presumably as a mechanism to sustain the host for completion of its lengthy infectious cycle. In the current study, we further investigated C. burnetii manipulation of host cell signaling and apoptosis by examining the effect of C. burnetii infection on activation of 15 host proteins involved in stress responses, cytokine production, and apoptosis. C. burnetii infection of THP-1 human macrophage-like cells caused increased levels of phosphorylated c-Jun, Hsp27, Jun N-terminal protein kinase, and p38 at 2 h postinfection (hpi), and this activation rapidly decreased to near basal levels by 24 hpi. The prosurvival kinases Akt and Erk1/2 (extracellular signal-regulated kinases 1 and 2) were also activated at 2 to 6 hpi; however, the phosphorylation of these proteins increased coincident with C. burnetii replication through at least 72 hpi. Sustained phosphorylation of Akt and Erk1/2 was abolished by treatment of infected cells with rifampin, indicating their activation is a C. burnetii-directed event requiring pathogen RNA synthesis. Moreover, pharmacological inhibition of Akt or Erk1/2 significantly decreased C. burnetii antiapoptotic activity. Collectively, these results indicate the importance of C. burnetii modulation of host signaling and demonstrate a critical role for Akt and Erk1/2 in successful intracellular parasitism and maintenance of host cell viability.

Candidate antigens for Q fever serodiagnosis revealed by immunoscreening of a Coxiella burnetii protein microarray

Q fever is a widespread zoonosis caused by Coxiella burnetii. Diagnosis of Q fever is usually based on serological testing of patient serum. The diagnostic antigen of test kits is formalin-fixed phase I and phase II organisms of the Nine Mile reference strain. Deficiencies of this antigen include (i) potential for cross-reactivity with other pathogens; (ii) an inability to distinguish between C. burnetii strains; and (iii) a need to propagate and purify C. burnetii, a difficult and potentially hazardous process. Consequently, there is a need for sensitive and specific serodiagnostic tests utilizing defined antigens, such as recombinant C. burnetii protein(s). Here we describe the use of a C. burnetii protein microarray to comprehensively identify immunodominant antigens recognized by antibody in the context of human C. burnetii infection or vaccination. Transcriptionally active PCR products corresponding to 1,988 C. burnetii open reading frames (ORFs) were generated. Full-length proteins were successfully synthesized from 75% of the ORFs by using an Escherichia coli-based in vitro transcription and translation system (IVTT). Nitrocellulose microarrays were spotted with crude IVTT lysates and probed with sera from acute Q fever patients and individuals vaccinated with Q-Vax. Immune sera strongly reacted with approximately 50 C. burnetii proteins, including previously identified immunogens, an ankyrin repeat-domain containing protein, and multiple hypothetical proteins. Recombinant protein corresponding to selected array-reactive antigens was generated, and the immunoreactivity was confirmed by enzyme-linked immunosorbent assay. This sensitive and high-throughput method for identifying immunoreactive C. burnetii proteins will aid in the development of Q fever serodiagnostic tests based on recombinant antigen.

Sustained axenic metabolic activity by the obligate intracellular bacterium Coxiella burnetii

Growth of Coxiella burnetii, the agent of Q fever, is strictly limited to colonization of a viable eukaryotic host cell. Following infection, the pathogen replicates exclusively in an acidified (pH 4.5 to 5) phagolysosome-like parasitophorous vacuole. Axenic (host cell free) buffers have been described that activate C. burnetii metabolism in vitro, but metabolism is short-lived, with bacterial protein synthesis halting after a few hours. Here, we describe a complex axenic medium that supports sustained (>24 h) C. burnetii metabolic activity. As an initial step in medium development, several biological buffers (pH 4.5) were screened for C. burnetii metabolic permissiveness. Based on [(35)S]Cys-Met incorporation, C. burnetii displayed optimal metabolic activity in citrate buffer. To compensate for C. burnetii auxotrophies and other potential metabolic deficiencies, we developed a citrate buffer-based medium termed complex Coxiella medium (CCM) that contains a mixture of three complex nutrient sources (neopeptone, fetal bovine serum, and RPMI cell culture medium). Optimal C. burnetii metabolism occurred in CCM with a high chloride concentration (140 mM) while the concentrations of sodium and potassium had little effect on metabolism. CCM supported prolonged de novo protein and ATP synthesis by C. burnetii (>24 h). Moreover, C. burnetii morphological differentiation was induced in CCM as determined by the transition from small-cell variant to large-cell variant. The sustained in vitro metabolic activity of C. burnetii in CCM provides an important tool to investigate the physiology of this organism including developmental transitions and responses to antimicrobial factors associated with the host cell.