T cells are essential for bacterial clearance, and gamma interferon, tumor necrosis factor alpha, and B cells are crucial for disease development in Coxiella burnetii infection in mice

Molecular mechanisms of Coxiella burnetii formalin-fixed cellular vaccine reactogenicity

Local and systemic reactogenic responses to Q-VAX have prevented licensing of this vaccine outside of Australia. These reactogenic responses occur in previously sensitized individuals and have not been well defined at the cellular level, in part because many studies have been done in guinea pigs that have limited molecular tools. We previously characterized a mouse model of reactogenicity where local reaction sites showed an influx of CD8+ and IFNγ-expressing IL17a+ CD4+ T cells consistent with a Th1 delayed-type hypersensitivity. In this study, we determined, using depletion and adoptive transfer experiments, that both anti-Coxiella antibodies and CD4+ T cells were essential for localized reactions at the site of vaccination. Furthermore, IFNγ depletion showed significant histological changes at the local reaction sites demonstrating the essential nature of this cytokine to reactogenicity. In addition to the cells and cytokines required for this response, we determined that whole cell vaccine (WCV) material remained at the site of vaccination for at least 26 weeks post-injection. Transmission electron microscopy (TEM) of these sites demonstrated intact rod-shaped bacteria at 2 weeks post-injection and partially degraded bacteria within macrophages at 26 weeks post-injection. Finally, because small cell variants (SCVs) are an environmentally stable form, we determined that local reactions were more severe when the WCV material was prepared with higher levels of SCVs compared to typical WCV or with higher levels of large cell variant (LCV). These studies support the hypothesis that antigen persistence at the site of injection contributes to this reactogenicity and that anti-Coxiella antibodies, CD4+ T cells, and IFNγ each contribute to this process.

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.

Molecular Mechanisms of Coxiella burnetii Formalin Fixed Cellular Vaccine Reactogenicity

Local and systemic reactogenic responses to Q-VAX® have prevented licensing of this vaccine outside of Australia. These reactogenic responses occur in previously sensitize individuals and have not been well defined at the cellular level, in part because many studies have been done in guinea pigs that have limited molecular tools. We previously characterized a mouse model of reactogenicity where local reactions sites showed an influx of CD8+ and IFNγ-expressing IL17a+ CD4+ T cells consistent with a Th1 delayed-type hypersensitivity. In this study we determined using depletion and adoptive transfer experiments that both anti- Coxiella antibodies and CD4+ T cells were essential for localized reactions at the site of vaccination. Furthermore, IFNγ depletion showed significant histological changes at the local reaction sites demonstrating the essential nature of this cytokine to reactogenicity. In addition to the cells and cytokines required for this response, we determined WCV material remained at the site of vaccination for at least 26 weeks post-injection. Transmission electron microscopy of these sites demonstrated intact rod-shaped bacteria at 2 weeks post-injection and partially degraded bacteria within macrophages at 26 weeks post-injection. Finally, since SCVs are an environmentally stable form, we determined that local reactions were more severe when the WCV material was prepared with higher levels of SCVs compared to typical WCV or with higher levels of LCV. These studies support the hypothesis that antigen persistence at the site of injection contributes to this reactogenicity and that anti- Coxiella antibodies, CD4+ T cells, and IFNγ each contribute to this process.

Caspase-8 activity mediates TNFα production and restricts Coxiella burnetii replication during murine macrophage infection

Coxiella burnetii is an obligate intracellular bacteria that causes the global zoonotic disease Q Fever. Treatment options for chronic infection are limited, and the development of novel therapeutic strategies requires a greater understanding of how C. burnetii interacts with immune signaling. Cell death responses are known to be manipulated by C. burnetii, but the role of caspase-8, a central regulator of multiple cell death pathways, has not been investigated. In this research, we studied bacterial manipulation of caspase-8 signaling and the significance of caspase-8 to C. burnetii infection, examining bacterial replication, cell death induction, and cytokine signaling. We measured caspase, RIPK, and MLKL activation in C. burnetii-infected tumor necrosis factor alpha (TNFα)/cycloheximide-treated THP-1 macrophage-like cells and TNFα/ZVAD-treated L929 cells to assess apoptosis and necroptosis signaling. Additionally, we measured C. burnetii replication, cell death, and TNFα induction over 12 days in RIPK1-kinase-dead, RIPK3-kinase-dead, or RIPK3-kinase-dead-caspase-8-/- bone marrow-derived macrophages (BMDMs) to understand the significance of caspase-8 and RIPK1/3 during infection. We found that caspase-8 is inhibited by C. burnetii, coinciding with inhibition of apoptosis and increased susceptibility to necroptosis. Furthermore, C. burnetii replication was increased in BMDMs lacking caspase-8, but not in those lacking RIPK1/3 kinase activity, corresponding with decreased TNFα production and reduced cell death. As TNFα is associated with the control of C. burnetii, this lack of a TNFα response may allow for the unchecked bacterial growth we saw in caspase-8-/- BMDMs. This research identifies and explores caspase-8 as a key regulator of C. burnetii infection, opening novel therapeutic doors.

Tissue distribution of Coxiella burnetii and antibody responses in macropods co-grazing with livestock in Queensland, Australia

Coxiella burnetii, the causative agent of Q fever, is a zoonotic bacteria of global public health significance. The organism has a complex, diverse, and relatively poorly understood animal reservoir but there is increasing evidence that macropods play some part in the epidemiology of Q fever in Australia. The aim of this cross-sectional survey was to estimate the animal- and tissue-level prevalence of coxiellosis amongst eastern grey (Macropus giganteus) and red (Osphranter rufus) kangaroos co-grazing with domestic cattle in a Q fever endemic area in Queensland. Serum, faeces and tissue samples from a range of organs were collected from 50 kangaroos. A total of 537 tissue samples were tested by real-time PCR, of which 99 specimens from 42 kangaroos (84% of animals, 95% confidence interval [CI], 71% to 93%) were positive for the C. burnetii IS1111 gene when tested in duplicate. Twenty of these specimens from 16 kangaroos (32%, 95% CI 20% to 47%) were also positive for the com1 or htpAB genes. Serum antibodies were present in 24 (57%, 95% CI 41% to 72%) of the PCR positive animals. There was no statistically significant difference in PCR positivity between organs and no single sample type consistently identified C. burnetii positive kangaroos. The results from this study identify a high apparent prevalence of C. burnetii amongst macropods in the study area, albeit seemingly with an inconsistent distribution within tissues and in relatively small quantities, often verging on the limits of detection. We recommend Q fever surveillance in macropods should involve a combination of serosurveys and molecular testing to increase chances of detection in a population, noting that a range of tissues would likely need to be sampled to confirm the diagnosis in a suspect positive animal.

Role of Type 4B Secretion System Protein, IcmE, in the Pathogenesis of Coxiella burnetii

Coxiella burnetii is an obligate intracellular Gram-negative bacterium that causes Q fever, a life-threatening zoonotic disease. C. burnetii replicates within an acidified parasitophorous vacuole derived from the host lysosome. The ability of C. burnetii to replicate and achieve successful intracellular life in the cell cytosol is vastly dependent on the Dot/Icm type 4B secretion system (T4SSB). Although several T4SSB effector proteins have been shown to be important for C. burnetii virulence and intracellular replication, the role of the icmE protein in the host-C. burnetii interaction has not been investigated. In this study, we generated a C. burnetii Nine Mile Phase II (NMII) mutant library and identified 146 transposon mutants with a single transposon insertion. Transposon mutagenesis screening revealed that disruption of icmE gene resulted in the attenuation of C. burnetii NMII virulence in SCID mice. ELISA analysis indicated that the levels of pro-inflammatory cytokines, including interleukin-1β, IFN-γ, TNF-α, and IL-12p70, in serum from Tn::icmE mutant-infected SCID mice were significantly lower than those in serum from wild-type (WT) NMII-infected mice. Additionally, Tn::icmE mutant bacteria were unable to replicate in mouse bone marrow-derived macrophages (MBMDM) and human macrophage-like cells (THP-1). Immunoblotting results showed that the Tn::icmE mutant failed to activate inflammasome components such as IL-1β, caspase 1, and gasdermin-D in THP-1 macrophages. Collectively, these results suggest that the icmE protein may play a vital role in C. burnetii virulence, intracellular replication, and activation of inflammasome mediators during NMII infection.

Vaccine development: obligate intracellular bacteria new tools, old pathogens: the current state of vaccines against obligate intracellular bacteria

Obligate intracellular bacteria have remained those for which effective vaccines are unavailable, mostly because protection does not solely rely on an antibody response. Effective antibody-based vaccines, however, have been developed against extracellular bacteria pathogens or toxins. Additionally, obligate intracellular bacteria have evolved many mechanisms to subvert the immune response, making vaccine development complex. Much of what we know about protective immunity for these pathogens has been determined using infection-resolved cases and animal models that mimic disease. These studies have laid the groundwork for antigen discovery, which, combined with recent advances in vaccinology, should allow for the development of safe and efficacious vaccines. Successful vaccines against obligate intracellular bacteria should elicit potent T cell memory responses, in addition to humoral responses. Furthermore, they ought to be designed to specifically induce strong cytotoxic CD8+ T cell responses for protective immunity. This review will describe what we know about the potentially protective immune responses to this group of bacteria. Additionally, we will argue that the novel delivery platforms used during the Sars-CoV-2 pandemic should be excellent candidates to produce protective immunity once antigens are discovered. We will then look more specifically into the vaccine development for Rickettsiaceae, Coxiella burnetti, and Anaplasmataceae from infancy until today. We have not included Chlamydia trachomatis in this review because of the many vaccine related reviews that have been written in recent years.

Synthetic Particulate Subunit Vaccines for the Prevention of Q Fever

Coxiella burnetti is an intracellular bacterium that causes Q fever, a disease of worldwide importance. Q-VAX® , the approved human Q fever vaccine, is a whole cell vaccine associated with safety concerns. Here a safe particulate subunit vaccine candidate is developed that is ambient-temperature stable and can be cost-effectively manufactured. Endotoxin-free Escherichia coli is bioengineered to efficiently self-assemble biopolymer particles (BPs) that are densely coated with either strings of 18 T-cell epitopes (COX-BP) or two full-length immunodominant antigens (YbgF-BP-Com1) all derived from C. burnetii. BP vaccine candidates are ambient-temperature stable. Safety and immunogenicity are confirmed in mice and guinea pig (GP) models. YbgF-BP-Com1 elicits specific and strong humoral immune responses in GPs with IgG titers that are at least 1 000 times higher than those induced by Q-VAX® . BP vaccine candidates are not reactogenic. After challenge with C. burnetii, YbgF-BP-Com1 vaccine leads to reduced fever responses and pathogen burden in the liver and the induction of proinflammatory cytokines IL-12 and IFN-γ inducible protein (IP-10) when compared to negative control groups. These data suggest that YbgF-BP-Com1 induces functional immune responses reducing infection by C. burnetii. Collectively, these findings illustrate the potential of BPs as effective antigen carrier for Q fever vaccine development.

Caspase-8 activity mediates TNFα production and restricts Coxiella burnetii replication during murine macrophage infection

Coxiella burnetii is an obligate intracellular bacteria which causes the global zoonotic disease Q Fever. Treatment options for infection are limited, and development of novel therapeutic strategies requires a greater understanding of how C. burnetii interacts with immune signaling. Cell death responses are known to be manipulated by C. burnetii, but the role of caspase-8, a central regulator of multiple cell death pathways, has not been investigated. In this research, we studied bacterial manipulation of caspase-8 signaling and the significance of caspase-8 to C. burnetii infection, examining bacterial replication, cell death induction, and cytokine signaling. We measured caspase, RIPK, and MLKL activation in C. burnetii-infected TNFα/CHX-treated THP-1 macrophage-like cells and TNFα/ZVAD-treated L929 cells to assess apoptosis and necroptosis signaling. Additionally, we measured C. burnetii replication, cell death, and TNFα induction over 12 days in RIPK1-kinase-dead, RIPK3-kinase-dead, or RIPK3-kinase-dead-caspase-8-/- BMDMs to understand the significance of caspase-8 and RIPK1/3 during infection. We found that caspase-8 is inhibited by C. burnetii, coinciding with inhibition of apoptosis and increased susceptibility to necroptosis. Furthermore, C. burnetii replication was increased in BMDMs lacking caspase-8, but not in those lacking RIPK1/3 kinase activity, corresponding with decreased TNFα production and reduced cell death. As TNFα is associated with the control of C. burnetii, this lack of a TNFα response may allow for the unchecked bacterial growth we saw in caspase-8-/- BMDMs. This research identifies and explores caspase-8 as a key regulator of C. burnetii infection, opening novel therapeutic doors.

Interdisciplinary studies on Coxiella burnetii: From molecular to cellular, to host, to one health research

The Q-GAPS (Q fever GermAn interdisciplinary Program for reSearch) consortium was launched in 2017 as a German consortium of more than 20 scientists with exceptional expertise, competence, and substantial knowledge in the field of the Q fever pathogen Coxiella (C.) burnetii. C. burnetii exemplifies as a zoonotic pathogen the challenges of zoonotic disease control and prophylaxis in human, animal, and environmental settings in a One Health approach. An interdisciplinary approach to studying the pathogen is essential to address unresolved questions about the epidemiology, immunology, pathogenesis, surveillance, and control of C. burnetii. In more than five years, Q-GAPS has provided new insights into pathogenicity and interaction with host defense mechanisms. The consortium has also investigated vaccine efficacy and application in animal reservoirs and identified expanded phenotypic and genotypic characteristics of C. burnetii and their epidemiological significance. In addition, conceptual principles for controlling, surveilling, and preventing zoonotic Q fever infections were developed and prepared for specific target groups. All findings have been continuously integrated into a Web-based, interactive, freely accessible knowledge and information platform (www.q-gaps.de), which also contains Q fever guidelines to support public health institutions in controlling and preventing Q fever. In this review, we will summarize our results and show an example of how an interdisciplinary consortium provides knowledge and better tools to control a zoonotic pathogen at the national level.

Cytometry profiling of ex vivo recall responses to Coxiella burnetii in previously naturally exposed individuals reveals long-term changes in both adaptive and innate immune cellular compartments

Introduction

Q fever, caused by the intracellular bacterium Coxiella burnetii, is considered an occupational and biodefense hazard and can result in debilitating long-term complications. While natural infection and vaccination induce humoral and cellular immune responses, the exact nature of cellular immune responses to C. burnetii is incompletely understood. The current study seeks to investigate more deeply the nature of long-term cellular recall responses in naturally exposed individuals by both cytokine release assessment and cytometry profiling.

Methods

Individuals exposed during the 2007-2010 Dutch Q fever outbreak were grouped in 2015, based on a C. burnetii-specific IFNγ release assay (IGRA), serological status, and self-reported clinical symptoms during initial infection, into asymptomatic IGRA-negative/seronegative controls, and three IGRA-positive groups (seronegative/asymptomatic; seropositive/asymptomatic and seropositive/symptomatic). Recall responses following in vitro re-stimulation with heat-inactivated C. burnetii in whole blood, were assessed in 2016/2017 by cytokine release assays (n=55) and flow cytometry (n=36), and in blood mononuclear cells by mass cytometry (n=36).

Results

Cytokine release analysis showed significantly elevated IL-2 responses in all seropositive individuals and elevated IL-1β responses in those recovered from symptomatic infection. Comparative flow cytometry analysis revealed significantly increased IFNγ, TNFα and IL-2 recall responses by CD4 T cells and higher IL-6 production by monocytes from symptomatic, IGRA-positive/seropositive individuals compared to controls. Mass cytometry profiling and unsupervised clustering analysis confirmed recall responses in seropositive individuals by two activated CD4 T cell subsets, one characterized by a strong Th1 cytokine profile (IFNγ+IL-2+TNFα+), and identified C. burnetii-specific activation of CD8 T cells in all IGRA-positive groups. Remarkably, increased C. burnetii-specific responses in IGRA-positive individuals were also observed in three innate cell subpopulations: one characterized by an IFNγ+IL-2+TNFα+ Th1 cytokine profile and lack of canonical marker expression, and two IL-1β-, IL-6- and IL-8-producing CD14+ monocyte subsets that could be the drivers of elevated secretion of innate cytokines in pre-exposed individuals.

Discussion

These data highlight that there are long-term increased responses to C. burnetii in both adaptive and innate cellular compartments, the latter being indicative of trained immunity. These findings warrant future studies into the protective role of these innate responses and may inform future Q fever vaccine design.

Q fever immunology: the quest for a safe and effective vaccine

Q fever is an infectious zoonotic disease, caused by the Gram-negative bacterium Coxiella burnetii. Transmission occurs from livestock to humans through inhalation of a survival form of the bacterium, the Small Cell Variant, often via handling of animal parturition products. Q fever manifests as an acute self-limiting febrile illness or as a chronic disease with complications such as vasculitis and endocarditis. The current preventative human Q fever vaccine Q-VAX poses limitations on its worldwide implementation due to reactogenic responses in pre-sensitized individuals. Many strategies have been undertaken to develop a universal Q fever vaccine but with little success to date. The mechanisms of the underlying reactogenic responses remain only partially understood and are important factors in the development of a safe Q fever vaccine. This review provides an overview of previous and current experimental vaccines developed for use against Q fever and proposes approaches to develop a vaccine that establishes immunological memory while eliminating harmful reactogenic responses.

Multivalent vaccines demonstrate immunogenicity and protect against Coxiella burnetii aerosol challenge

Vaccines are among the most cost-effective public health measures for controlling infectious diseases. Coxiella burnetii is the etiological agent of Q fever, a disease with a wide clinical spectrum that ranges from mild symptoms, such as fever and fatigue, to more severe disease, such as pneumonia and endocarditis. The formalin-inactivated whole-cell vaccine Q-VAX® contains hundreds of antigens and confers lifelong protection in humans, but prior sensitization from infection or vaccination can result in deleterious reactogenic responses to vaccination. Consequently, there is great interest in developing non-reactogenic alternatives based on adjuvanted recombinant proteins. In this study, we aimed to develop a multivalent vaccine that conferred protection with reduced reactogenicity. We hypothesized that a multivalent vaccine consisting of multiple antigens would be more immunogenic and protective than a monovalent vaccine owing to the large number of potential protective antigens in the C. burnetii proteome. To address this, we identified immunogenic T and B cell antigens, and selected proteins were purified to evaluate with a combination adjuvant (IVAX-1), with or without C. burnetii lipopolysaccharide (LPS) in immunogenicity studies in vivo in mice and in a Hartley guinea pig intratracheal aerosol challenge model using C. burnetii strain NMI RSA 493. The data showed that multivalent vaccines are more immunogenic than monovalent vaccines and more closely emulate the protection achieved by Q-VAX. Although six antigens were the most immunogenic, we also discovered that multiplexing beyond four antigens introduces detectable reactogenicity, indicating that there is an upper limit to the number of antigens that can be safely included in a multivalent Q-fever vaccine. C. burnetii LPS also demonstrates efficacy as a vaccine antigen in conferring protection in an otherwise monovalent vaccine formulation, suggesting that its addition in multivalent vaccines, as demonstrated by a quadrivalent formulation, would improve protective responses.

Humoral and cellular immune responses in sheep following administration of different doses of an inactivated phase I vaccine against Coxiella burnetii

An inactivated Coxiella burnetii Phase I (PhI) vaccine (Coxevac®) is licensed in several European countries for goats and cattle to prevent coxiellosis. The vaccine is also applied to sheep, although detailed information about the ovine immune response and vaccine dose is missing. Eighteen gimmers from a C. burnetii unsuspected flock were randomly divided into three groups of six. Group 1 (Cox1) and 2 (Cox2) were vaccinated twice with 1 ml and 2 ml Coxevac®, respectively, three weeks apart (primary vaccination). The same procedure was applied with Cox3 (2 ml sodium chloride, control group). A third injection (booster) was performed after nine months. Potential side effects were determined by measuring the rectal body temperature and skin thickness at the injection site. Blood samples were collected to detect phase-specific IgM and IgG antibodies and interferon-ɣ (IFN-ɣ) release by immunofluorescence assay and ELISAs, respectively. Moreover, a cell infection neutralization assay determined the appearance of neutralizing sera. Body temperatures increased for one day post vaccination, and the skin swelled only slightly. Regardless of the vaccine volume, immunized sheep reacted first with an IgM and IgG PhII response. Ten weeks after the primary vaccination, IgG PhI antibodies predominated. Boosting eight months after primary vaccination resulted in a robust IgG PhI increase and strong IFN-ɣ response. In the vaccinated animals, the neutralizing effect is more widespread after the administration of 1 ml than after the treatment with 2 ml. In summary, differences between 1 and 2 ml Coxevac® are minor, and a vaccine volume of 1 ml seems to be sufficient. A booster after the primary vaccination is apparently necessary to stimulate the cell-mediated immune response in naïve sheep.

Evaluation of Alternative Doxycycline Antibiotic Regimes in an Inhalational Murine Model of Q Fever

The timing of the initiation of antibiotic treatment has been shown to impact the clinical outcome of many bacterial infections, including Q fever. Delayed, suboptimal or incorrect antibiotic treatment has been shown to result in poor prognosis, resulting in the progression of acute disease to long-term chronic sequalae. Therefore, there is a requirement to identify an optimal, effective therapeutic regimen to treat acute Q fever. In the study, the efficacies of different doxycycline monohydrate regimens (pre-exposure prophylaxis, post-exposure prophylaxis or treatment at symptom onset or resolution) were evaluated in an inhalational murine model of Q fever. Different treatment lengths (7 or 14 days) were also evaluated. Clinical signs and weight loss were monitored during infection and mice were euthanized at different time points to characterize bacterial colonization in the lungs and the dissemination of bacteria to other tissues including the spleen, brain, testes, bone marrow and adipose. Post-exposure prophylaxis or doxycycline treatment starting at symptoms onset reduced clinical signs, and also delayed the systemic clearance of viable bacteria from key tissues. Effective clearance was dependent on the development of an adaptive immune response, but also driven by sufficient bacterial activity to maintain an active immune response. Pre-exposure prophylaxis or post-exposure treatment at the resolution of clinical signs did not improve outcomes. These are the first studies to experimentally evaluate different doxycycline treatment regimens for Q fever and illustrate the need to explore the efficacy of other novel antibiotics.

Syntaxin 11 Contributes to the Interferon-Inducible Restriction of Coxiella burnetii Intracellular Infection

There is a limited understanding of host defense mechanisms targeting intracellular pathogens that proliferate in a lysosome. Coxiella burnetii is a model bacterial pathogen capable of replicating in the hydrolytic and acidic environment of the lysosome. It has been shown that gamma interferon (IFNγ)-stimulated host cells restrict C. burnetii replication by a mechanism that involves host IDO1 depletion of tryptophan. Host cells deficient in IDO1 activity, however, retain the ability to restrict C. burnetii replication when stimulated with IFNγ, which suggests additional mechanisms of host defense. This study identified syntaxin 11 (STX11) as a host protein that contributes to IFNγ-mediated suppression of C. burnetii replication. STX11 is a SNARE protein; SNARE proteins are proteins that mediate fusion of host vesicles with specific subcellular organelles. Depletion of STX11 using either small interfering RNA (siRNA)- or CRISPR-based approaches enhanced C. burnetii replication intracellularly. Stable expression of STX11 reduced C. burnetii replication in epithelial cells and macrophages, which indicates that this STX11-dependent cell-autonomous response is operational in multiple cell types and can function independently of other IFNγ-induced factors. Fluorescently tagged STX11 localized to the Coxiella-containing vacuole (CCV), and STX11 restriction was found to involve an interaction with STX8. Thus, STX11 regulates a vesicle fusion pathway that limits replication of this intracellular pathogen in a lysosome-derived organelle. IMPORTANCE Cell intrinsic defense mechanisms are used by eukaryotic cells to restrict the replication and dissemination of pathogens. This study identified a human protein called syntaxin 11 (STX11) as a host restriction factor that inhibits the intracellular replication of Coxiella burnetii. Syntaxins regulate the delivery of cargo inside vesicles by promoting specific membrane fusion events between donor and acceptor vesicles. Data presented here demonstrate that STX11 regulates an immunological defense pathway that controls replication of pathogens in lysosome-derived organelles, which provides new insight into the function of this SNARE protein.

TGF-β/IFN-γ Antagonism in Subversion and Self-Defense of Phase II Coxiella burnetii-Infected Dendritic Cells

Dendritic cells (DCs) belong to the first line of innate defense and come into early contact with invading pathogens, including the zoonotic bacterium Coxiella burnetii, the causative agent of Q fever. However, the pathogen-host cell interactions in C. burnetii-infected DCs, particularly the role of mechanisms of immune subversion beyond virulent phase I lipopolysaccharide (LPS), as well as the contribution of cellular self-defense strategies, are not understood. Using phase II Coxiella-infected DCs, we show that impairment of DC maturation and MHC I downregulation is caused by autocrine release and action of immunosuppressive transforming growth factor-β (TGF-β). Our study demonstrates that IFN-γ reverses TGF-β impairment of maturation/MHC I presentation in infected DCs and activates bacterial elimination, predominantly by inducing iNOS/NO. Induced NO synthesis strongly affects bacterial growth and infectivity. Moreover, our studies hint that Coxiella-infected DCs might be able to protect themselves from mitotoxic NO by switching from oxidative phosphorylation to glycolysis, thus ensuring survival in self-defense against C. burnetii. Our results provide new insights into DC subversion by Coxiella and the IFN-γ-mediated targeting of C. burnetii during early steps in the innate immune response.

QuilA® adjuvanted Coxevac® sustains Th1-CD8+-type immunity and increases protection in Coxiella burnetii-challenged goats

Coxevac® is the EMA-approved veterinary vaccine for the protection of cattle and goats against Q fever, a zoonotic bacterial disease due to Coxiella burnetii. Since Coxevac® reduces bacterial shedding and clinical symptoms but does not prevent infection, novel, ready-to-use vaccine formulations are needed to increase its immunogenicity. Here, a goat vaccination-challenge model was used to evaluate the impact of the commercially available saponin-based QuilA® adjuvant on Coxevac® immunity. Upon challenge, the QuilA®-Coxevac® group showed a stronger immune response reflected in a higher magnitude of total IgG and an increase in circulating and splenic CD8⁺ T-cells compared to the Coxevac® and challenged-control groups. The QuilA®-Coxevac® group was characterized by a targeted Th1-type response (IFNγ, IP10) associated with increased transcripts of CD8⁺ and NK cells in spleens and γδ T cells in bronchial lymph nodes. Coxevac® vaccinated animals presented an intermediate expression of Th1-related genes, while the challenged-control group showed an immune response characterized by pro-inflammatory (IL1β, TNFα, IL12), Th2 (IL4 and IL13), Th17 (IL17A) and other immunoregulatory cytokines (IL6, IL10). An intriguing role was observed for γδ T cells, which were of TBX21- and SOX4-types in the QuilA®-Coxevac® and challenged control group, respectively. Overall, the addition of QuilA® resulted in a sustained Th1-type activation associated with an increased vaccine-induced bacterial clearance of 33.3% as compared to Coxevac® only. QuilA® could be proposed as a readily-applied veterinary solution to improve Coxevac® efficacy against C. burnetii infection in field settings.

Impact of Coxiella burnetii vaccination on humoral immune response, vaginal shedding, and lamb mortality in naturally pre-infected sheep

Introduction

Sheep are considered to be one of the main reservoirs for Coxiella burnetii, a gram-negative bacterium with high zoonotic potential. Infected sheep shed tremendous amounts of the pathogen through birth products which caused human Q fever epidemics in several countries. Information about the impact of an inactivated C. burnetii Phase I vaccine on humoral immune response, vaginal shedding, and lamb mortality in naturally pre-infected sheep is scarce.

Methods

Two identically managed and naturally C. burnetii-infected sheep flocks were examined for two lambing seasons (2019 and 2020). One flock (VAC) received a primary vaccination against Q fever before mating and the second flock served as control (CTR). In each flock, one cohort of 100 ewes was included in follow-up investigations. Serum samples at eight different sampling dates were analyzed by C. burnetii phase-specific ELISAs to differentiate between the IgG Phase I and II responses. Vaginal swabs were collected within three days after parturition and examined by a C. burnetii real-time PCR (IS1111). Lamb losses were recorded to calculate lamb mortality parameters.

Results

After primary vaccination, almost all animals from cohort VAC showed a high IgG Phase I response up until the end of the study period. In cohort CTR, the seropositivity rate varied from 35.1% to 66.3%, and the Phase I and Phase II pattern showed an undulating trend with higher IgG Phase II activity during both lambing seasons. The number of vaginal shedders was significantly reduced in cohort VAC compared to cohort CTR during the lambing season in 2019 (p < 0.0167). There was no significant difference of vaginal shedders in 2020. The total lamb losses were low in both cohorts during the two investigated lambing seasons (VAC 2019: 6.8%, 2020: 3.2%; CTR 2019: 1.4%, 2020: 2.7%).

Discussion

Neither the C. burnetii vaccine nor the C. burnetii infection seem to have an impact on lamb mortality. Taken together, the inactivated C. burnetii Phase I vaccine induced a strong IgG Phase I antibody response in naturally pre-infected sheep. It might also reduce vaginal shedding in the short term but seems to have little beneficial impact on lamb mortality.

Macrophages inhibit Coxiella burnetii by the ACOD1-itaconate pathway for containment of Q fever

Infection with the intracellular bacterium Coxiella (C.) burnetii can cause chronic Q fever with severe complications and limited treatment options. Here, we identify the enzyme cis-aconitate decarboxylase 1 (ACOD1 or IRG1) and its product itaconate as protective host immune pathway in Q fever. Infection of mice with C. burnetii induced expression of several anti-microbial candidate genes, including Acod1. In macrophages, Acod1 was essential for restricting C. burnetii replication, while other antimicrobial pathways were dispensable. Intratracheal or intraperitoneal infection of Acod1^-/- mice caused increased C. burnetii burden, weight loss and stronger inflammatory gene expression. Exogenously added itaconate restored pathogen control in Acod1^-/- mouse macrophages and blocked replication in human macrophages. In axenic cultures, itaconate directly inhibited growth of C. burnetii. Finally, treatment of infected Acod1^-/- mice with itaconate efficiently reduced the tissue pathogen load. Thus, ACOD1-derived itaconate is a key factor in the macrophage-mediated defense against C. burnetii and may be exploited for novel therapeutic approaches in chronic Q fever.

Chronic Q Fever as Recurrent Osteoarticular Infection in Children: Case Report and Literature Review

Q fever osteomyelitis has been rarely reported in children. This infection has an unclear pathophysiology and the optimal therapy is unknown. We report a 2-year-old girl with Coxiella burnetti recurrent multifocal osteomyelitis: femur, metatarsal, cuneiform, and calcaneus. We highlight the complicated diagnosis and management of this case and the importance of considering Q fever in children with chronic-recurrent multifocal osteomyelitis.

BTN3A Targeting Vγ9Vδ2 T Cells Antimicrobial Activity Against Coxiella burnetii-Infected Cells

Vγ9Vδ2 T cells have been reported to participate to the immune response against infectious diseases such as the Q fever caused by Coxiella burnetii infection. Indeed, the number and proportion of Vγ9Vδ2 T cells are increased during the acute phase of Q fever. Human Vγ9Vδ2 T cell responses are triggered by phosphoantigens (pAgs) produced by pathogens and malignant cells, that are sensed via the membrane receptors butyrophilin-3A1 (BTN3A1) and -2A1 (BTN2A1). Here, by using CRISPR-Cas9 inactivation in THP-1 cells, we show that BTN3A and BTN2A are required to Vγ9Vδ2 T cell response to C. burnetii infection, though not directly involved in the infection process. Furthermore, C. burnetii-infected monocytes display increased BTN3A and BTN2A expression and induce Vγ9Vδ2 T cell activation that can be inhibited by specific antagonist mAb. More importantly, we show that the antimicrobial functions of Vγ9Vδ2 T cells towards C. burnetii are enhanced in the presence of an BTN3A activating antibody. This supports the role of Vγ9Vδ2 T cells in the control of C. burnetii infection and argues in favor of targeting these cells as an alternative treatment strategy for infectious diseases caused by intracellular bacteria.

Natural Exposure- and Vaccination-Induced Profiles of Ex Vivo Whole Blood Cytokine Responses to Coxiella burnetii

Q fever is a zoonotic disease caused by the highly infectious Gram-negative coccobacillus, Coxiella burnetii (C. burnetii). The Q fever vaccine Q-VAX® is characterised by high reactogenicity, requiring individuals to be pre-screened for prior exposure before vaccination. To date it remains unclear whether vaccine side effects in pre-exposed individuals are associated with pre-existing adaptive immune responses to C. burnetii or are also a function of innate responses to Q-VAX®. In the current study, we measured innate and adaptive cytokine responses to C. burnetii and compared these among individuals with different pre-exposure status. Three groups were included: n=98 Dutch blood bank donors with unknown exposure status, n=95 Dutch village inhabitants with known natural exposure status to C. burnetii during the Dutch Q fever outbreak of 2007-2010, and n=96 Australian students receiving Q-VAX® vaccination in 2021. Whole blood cytokine responses following ex vivo stimulation with heat-killed C. burnetii were assessed for IFNγ, IL-2, IL-6, IL-10, TNFα, IL-1β, IP-10, MIP-1α and IL-8. Serological data were collected for all three cohorts, as well as data on skin test and self-reported vaccine side effects and clinical symptoms during past infection. IFNγ, IP-10 and IL-2 responses were strongly elevated in individuals with prior C. burnetii antigen exposure, whether through infection or vaccination, while IL-1β, IL-6 and TNFα responses were slightly increased in naturally exposed individuals only. High dimensional analysis of the cytokine data identified four clusters of individuals with distinct cytokine response signatures. The cluster with the highest levels of adaptive cytokines and antibodies comprised solely individuals with prior exposure to C. burnetii, while another cluster was characterized by high innate cytokine production and an absence of C. burnetii-induced IP-10 production paired with high baseline IP-10 levels. Prior exposure status was partially associated with these signatures, but could not be clearly assigned to a single cytokine response signature. Overall, Q-VAX® vaccination and natural C. burnetii infection were associated with comparable cytokine response signatures, largely driven by adaptive cytokine responses. Neither individual innate and adaptive cytokine responses nor response signatures were associated retrospectively with clinical symptoms during infection or prospectively with side effects post-vaccination.

Evaluation of a Human T Cell-Targeted Multi-Epitope Vaccine for Q Fever in Animal Models of Coxiella burnetii Immunity

T cell-mediated immunity plays a central role in the control and clearance of intracellular Coxiella burnetii infection, which can cause Q fever. Therefore, we aimed to develop a novel T cell-targeted vaccine that induces pathogen-specific cell-mediated immunity to protect against Q fever in humans while avoiding the reactogenicity of the current inactivated whole cell vaccine. Human HLA class II T cell epitopes from C. burnetii were previously identified and selected by immunoinformatic predictions of HLA binding, conservation in multiple C. burnetii isolates, and low potential for cross-reactivity with the human proteome or microbiome. Epitopes were selected for vaccine inclusion based on long-lived human T cell recall responses to corresponding peptides in individuals that had been naturally exposed to the bacterium during a 2007-2010 Q fever outbreak in the Netherlands. Multiple viral vector-based candidate vaccines were generated that express concatemers of selected epitope sequences arranged to minimize potential junctional neo-epitopes. The vaccine candidates caused no antigen-specific reactogenicity in a sensitized guinea pig model. A subset of the vaccine epitope peptides elicited antigenic recall responses in splenocytes from C57BL/6 mice previously infected with C. burnetii. However, immunogenicity of the vaccine candidates in C57BL/6 mice was dominated by a single epitope and this was insufficient to confer protection against an infection challenge, highlighting the limitations of assessing human-targeted vaccine candidates in murine models. The viral vector-based vaccine candidates induced antigen-specific T cell responses to a broader array of epitopes in cynomolgus macaques, establishing a foundation for future vaccine efficacy studies in this large animal model of C. burnetii infection.

Immunogenicity and Reactogenicity in Q Fever Vaccine Development

Coxiella burnetii is an obligate intracellular bacterium which, in humans, causes the disease Q fever. Although Q fever is most often a mild, self-limiting respiratory disease, it can cause a range of severe syndromes including hepatitis, myocarditis, spontaneous abortion, chronic valvular endocarditis, and Q fever fatigue syndrome. This agent is endemic worldwide, except for New Zealand and Antarctica, transmitted via aerosols, persists in the environment for long periods, and is maintained through persistent infections in domestic livestock. Because of this, elimination of this bacterium is extremely challenging and vaccination is considered the best strategy for prevention of infection in humans. Many vaccines against C. burnetii have been developed, however, only a formalin-inactivated, whole cell vaccine derived from virulent C. burnetii is currently licensed for use in humans. Unfortunately, widespread use of this whole cell vaccine is impaired due to the severity of reactogenic responses associated with it. This reactogenicity continues to be a major barrier to access to preventative vaccines against C. burnetii and the pathogenesis of this remains only partially understood. This review provides an overview of past and current research on C. burnetii vaccines, our knowledge of immunogenicity and reactogenicity in C. burnetii vaccines, and future strategies to improve the safety of vaccines against C. burnetii.

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.

Coxiella burnetii inhibits host immunity by a protein phosphatase adapted from glycolysis

Coxiella burnetii is a bacterial pathogen that replicates within host cells by establishing a membrane-bound niche called the Coxiella-containing vacuole. Biogenesis of this compartment requires effectors of its Dot/Icm type IV secretion system. A large cohort of such effectors has been identified, but the function of most of them remain elusive. Here, by a cell-based functional screening, we identified the effector Cbu0513 (designated as CinF) as an inhibitor of NF-κB signaling. CinF is highly similar to a fructose-1,6-bisphosphate (FBP) aldolase/phosphatase present in diverse bacteria. Further study reveals that unlike its ortholog from Sulfolobus tokodaii, CinF does not exhibit FBP phosphatase activity. Instead, it functions as a protein phosphatase that specifically dephosphorylates and stabilizes IκBα. The IκBα phosphatase activity is essential for the role of CinF in C. burnetii virulence. Our results establish that C. burnetii utilizes a protein adapted from sugar metabolism to subvert host immunity.

Host Gasdermin D restrains systemic endotoxemia by capturing Proteobacteria in the colon of high-fat diet-feeding mice

Gasdermin D (GSDMD) functions as a key pyroptotic executor through its secreted N-terminal domain (GSDMD-N). However, the functional relevance and mechanistic basis of the precise roles of host colonic GSDMD in high-fat diet (HFD)-induced gut dysbiosis and systemic endotoxemia remain elusive. In this study, we demonstrate that HFD feeding triggers GSDMD-N secretion of both T-lymphocytes and enterocytes in mouse colons. GSDMD deficiency aggravates HFD-induced systemic endotoxemia, gut barrier impairment, and colonic inflammation. More importantly, active GSDMD-N kills the Proteobacteria phylum via directly interacting with Cardiolipin. Mechanistically, we identify that the Glu236 (a known residue for GSDMD protein cleavage) is a bona fide important site for the bacterial recognition of GSDMD. Collectively, our findings explain the mechanism by which colonic GSDMD-N maintains low levels of HFD-induced metabolic endotoxemia. A GSDMD-N mimetic containing an exposed Glu236 site could be an attractive strategy for the treatment of HFD-induced metabolic endotoxemia.

Soluble antigens derived from Coxiella burnetii elicit protective immunity in three animal models without inducing hypersensitivity

Q fever is caused by the intracellular bacterium Coxiella burnetii, for which there is no approved vaccine in the United States. A formalin-inactivated whole-cell vaccine (WCV) from virulent C. burnetii NMI provides single-dose long-lived protection, but concerns remain over vaccine reactogenicity. We therefore sought an alternate approach by purifying native C. burnetii antigens from the clonally derived avirulent NMII strain. A soluble bacterial extract, termed Sol II, elicits high-titer, high-avidity antibodies and induces a CD4 T cell response that confers protection in naive mice. In addition, Sol II protects against pulmonary C. burnetii challenge in three animal models without inducing hypersensitivity. An NMI-derived extract, Sol I, enhances protection further and outperforms the WCV gold standard. Collectively, these data represent a promising approach to design highly effective, non-reactogenic Q fever vaccines.

Coxiella burnetti prosthetic joint infection in an immunocompromised woman: iterative surgeries, prolonged ofloxacin-rifampin treatment and complex reconstruction were needed for the cure

Background

Q fever is a zoonotic disease caused by the bacterium Coxiella burnetii, a strictly intracellular pathogen that can cause acute and chronic infection. Chronic Q fever can occur in immunocompetent as well as in immuno-compromised hosts, as a persistent localized infection. The main localizations are endocardial, vascular and, less frequently, osteoarticular. The most frequent osteoarticular form is spondyliscitis. Recommended treatment is combined doxycycline and hydroxychloroquine for 18 months, with cotrimoxazole as another option. Coxiella burnetti infection has been implicated in rare cases of prosthetic joint infection (PJI), and the medical and surgical management and outcome in such cases have been little reported.

Case presentation

We report an unusual case of chronic Q fever involving a hip arthroplasty in an immunocompromised woman treated with tumor necrosis factor (TNF)-α blockers for rheumatoid arthritis. Numerous surgical procedures (explantation, “second look”, femoral resection and revision by megaprosthesis), modification of the immunosuppressant therapy and switch from doxycycline-hydroxychloroquine to prolonged ofloxacin-rifampin combination therapy were needed to achieve reconstruction and treat the PJI, with a follow-up of 7 years.

Conclusions

Coxiella burnetti PJI is a complex infection that requires dedicated management in an experienced reference center. Combined use of ofloxacin-rifampin can be effective.

The Feasibility of Using Coxiella burnetii Avirulent Nine Mile Phase II Viable Bacteria as a Live Attenuated Vaccine Against Q fever

This study aimed to explore if viable C. burnetii avirulent Nine Mile phase II (NMII) can elicit protective immunity against virulent NM phase I (NMI) infection. Interestingly, mice immunized with viable NMII elicited significant protection against NMI infection at different time points post-immunization. Viable NMII induced a dose-dependent NMI-specific IgG response in mice, but all doses of NMII-immunized mice conferred a similar level of protection. Comparing different routes of immunization indicated that intranasally immunized mice showed significantly higher levels of protection than other immunization routes. The observation that viable NMII induced a similar level of long-term protection against NMI challenge as the formalin-inactivated NMI vaccine (PIV) suggests that viable NMII bacteria can induce a similar level of long-term protection against virulent NMI challenge as the PIV. Viable NMII also induced significant protection against challenge with virulent Priscilla and Scurry strains, suggesting that viable NMII can elicit broad protection. Immune sera and splenocytes from viable NMII-immunized mice are protective against NMI infection, but immune serum-receiving mice did not control NMI replication. Additionally, viable NMII conferred a comparable level of protection in wild-type, CD4⁺ T cell-deficient, and CD8⁺ T cell-deficient mice, and partial protection in B cell-deficient mice. However, NMII-immunized T cell-deficient mice were unable to prevent C. burnetii replication. Thus, both B cells and T cells are required for viable NMII-induced protective immunity but T cells may play a critical role. Collectively, this study demonstrates the feasibility of using avirulent NMII as a live attenuated vaccine against human Q fever.

Q Fever Vaccine Development: Current Strategies and Future Considerations

Q fever is a zoonotic disease caused by the intracellular pathogen Coxiella burnetii. This disease typically manifests as a self-limiting, febrile illness known as acute Q fever. Due to the aerosol transmissibility, environmental persistence, and infectivity of C. burnetii, this pathogen is a notable bioterrorism threat. Despite extensive efforts to develop next-generation human Q fever vaccines, only one vaccine, Q-Vax®, is commercially available. Q-Vax® is a phase I whole-cell vaccine, and its licensed use is limited to Australia, presumably due to the potential for a post-vaccination hypersensitivity response. Pre-clinical Q fever vaccine development is a major area of interest, and diverse approaches have been undertaken to develop an improved Q fever vaccine. Following a brief history of Q fever vaccine development, current approaches will be discussed along with future considerations for an improved Q fever vaccine.

Acute Q Fever in an Ankylosing Spondyloarthritis Patient Treated with Etanercept

Q fever is a rare zoonotic infection caused by Coxiella burnetii. Tumor necrosis factor-alpha (TNF-α) has an important role in the early control of this infection. However, TNF-α blockers increase the risk of infectious diseases. We present herein a patient who developed acute Q fever under anti-TNF-α who had a good evolution after anti-TNF stoppage and treatment with doxycycline.

Proteome-wide analysis of Coxiella burnetii for conserved T-cell epitopes with presentation across multiple host species

Background

Coxiella burnetii is the Gram-negative bacterium responsible for Q fever in humans and coxiellosis in domesticated agricultural animals. Previous vaccination efforts with whole cell inactivated bacteria or surface isolated proteins confer protection but can produce a reactogenic immune responses. Thereby a protective vaccine that does not cause aberrant immune reactions is required. The critical role of T-cell immunity in control of C. burnetii has been made clear, since either CD8⁺ or CD4⁺ T cells can empower clearance. The purpose of this study was to identify C. burnetii proteins bearing epitopes that interact with major histocompatibility complexes (MHC) from multiple host species (human, mouse, and cattle).

Results

Of the annotated 1815 proteins from the Nine Mile Phase I (RSA 493) assembly, 402 proteins were removed from analysis due to a lack of inter-isolate conservation. An additional 391 proteins were eliminated from assessment to avoid potential autoimmune responses due to the presence of host homology. We analyzed the remaining 1022 proteins for their ability to produce peptides that bind MHCI or MHCII. MHCI and MHCII predicted epitopes were filtered and compared between species yielding 777 MHCI epitopes and 453 MHCII epitopes. These epitopes were further examined for presentation by both MHCI and MHCII, and for proteins that contained multiple epitopes. There were 31 epitopes that overlapped positionally between MHCI and MHCII across host species. Of these, there were 9 epitopes represented within proteins containing ≥ 5 total epitopes, where an additional 24 proteins were also epitope dense. In all, 55 proteins were found to contain high scoring T-cell epitopes. Besides the well-studied protein Com1, most identified proteins were novel when compared to previously studied vaccine candidates.

Conclusion

These data represent the first proteome-wide evaluation of C. burnetii peptide epitopes. Furthermore, the inclusion of human, mouse, and bovine data capture a range of hosts for this zoonotic pathogen plus an important model organism. This work provides new vaccine targets for future vaccination efforts and enhances opportunities for selecting multiple T-cell epitope types to include within a vaccine.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12859-021-04181-w.

The prognostic value of serological titres for clinical outcomes during treatment and follow-up of patients with chronic Q fever

OBJECTIVES

We assessed the prognostic value of phase I IgG titres during treatment and follow-up of chronic Q fever.

METHODS

We performed a retrospective cohort study to analyse the course of phase I IgG titres in chronic Q fever. We used a multivariable time-varying Cox regression to assess our primary (first disease-related event) and secondary (therapy failure) outcomes. In a second analysis, we evaluated serological characteristics after 1 year of therapy (fourfold decrease in phase I IgG titre, absence of phase II IgM and reaching phase I IgG titre of ≤1:1024) with multivariable Cox regression.

RESULTS

In total, 337 patients that were treated for proven (n = 284, 84.3%) or probable (n = 53, 15.7%) chronic Q fever were included. Complications occurred in 190 (56.4%), disease-related mortality in 71 (21.1%) and therapy failure in 142 (42.1%) patients. The course of phase I IgG titres was not associated with first disease-related event (HR 1.00, 95% CI 0.86-1.15) or therapy failure (HR 1.02, 95% CI 0.91-1.15). Similar results were found for the serological characteristics for the primary (HR 0.97, 95% CI 0.62-1.51; HR 1.12, 95% CI 0.66-1.90; HR 0.99, 95% CI 0.57-1.69, respectively) and secondary outcomes (HR 0.86, 95% CI 0.57-1.29; HR 1.37, 95% CI 0.86-2.18; HR 0.80, 95% CI 0.48-1.34, respectively).

DISCUSSION

Coxiella burnetii serology does not reliably predict disease-related events or therapy failure during treatment and follow-up of chronic Q fever. Alternative markers for disease management are needed, but, for now, management should be based on clinical factors, PCR results, and imaging results.

Subunit Vaccines Using TLR Triagonist Combination Adjuvants Provide Protection Against Coxiella burnetii While Minimizing Reactogenic Responses

Q fever is caused by the obligate intracellular bacterium, Coxiella burnetii, a designated potential agent of bioterrorism because of its route of transmission, resistance to disinfectants, and low infectious dose. The only vaccine licensed for human use is Q-VAX® (Seqirus, licensed in Australia), a formalin-inactivated whole-cell vaccine, which produces severe local and systemic reactogenic responses in previously sensitized individuals. Accordingly, the U.S. Food and Drug Administration and other regulatory bodies around the world, have been reluctant to approve Q-VAX for widespread use. To obviate these adverse reactions, we prepared recombinant protein subunit vaccine candidates containing purified CBU1910, CBU0307, CBU0545, CBU0612, CBU0891, and CBU1398 proteins and TLR triagonist adjuvants. TLR triagonist adjuvants combine different TLR agonists to enhance immune responses to vaccine antigens. We tested both the protective efficacy and reactogenicity of our vaccine candidates in Hartley guinea pigs using intratracheal infection with live C. burnetii. While all of our candidates showed varying degrees of protection during challenge, local reactogenic responses were significantly reduced for one of our vaccine candidates when compared with a formalin-inactivated whole-cell vaccine. Our findings show that subunit vaccines combined with novel TLR triagonist adjuvants can generate protective immunity to C. burnetii infection while reducing reactogenic responses.

Humoral immune response to Q fever vaccination of three sheep flocks naturally pre-infected with Coxiella burnetii

Qfever is a zoonotic disease caused by the bacterium Coxiella burnetii; Coxiella-infected ruminants are the main reservoir shedding the pathogen during abortion or parturition through birth products. Germany has a long history of small-scale Q fever epidemics in the human population mostly associated with lambing sheep. Therefore, fast and efficient control measures are essentially required to prevent transmission from infected sheep flocks to humans. In our present study, three sheep flocks were vaccinated with an inactivated C.burnetii phase I vaccine after a field infection with C.burnetii was diagnosed. Serum samples and vaginal swabs were collected at different time points to evaluate the extent of the outbreak and the consequences of the vaccination. The serum samples were examined by phase-specific IgG phase I and phase II ELISAs and a commercial ELISA, simultaneously detecting both phase variations. Moreover, vaginal swabs were analysed by qPCR. The fourth flock with no Q fever history and non-vaccinated animals were used as a control group to evaluate the phase-specific ELISAs. The inactivated C.burnetii phase I vaccine induced an IgG phase II response and boosted the humoral immune reaction against natural pre-infections. Furthermore, the longevity of vaccine-induced antibodies seems to depend on previous infections. Around 16 months after primary vaccination, mainly IgG phase I antibodies were detectable. Vaccination did not prevent shedding at the next lambing season. Most interestingly, the phase-specific ELISAs revealed more C.burnetii positive animals than the blended ELISA-Assay. Taken together, phase-specific ELISAs are suitable tools to provide insights into natural- or vaccine-induced humoral immune responses to C.burnetii in sheep.

A Novel Marmoset (Callithrix jacchus) Model of Human Inhalational Q Fever

Common marmosets (Callithrix jacchus) were shown to be susceptible to inhalational infection with Coxiella burnetii, in a dose-dependent manner, producing a disease similar to human Q fever, characterized by a resolving febrile response. Illness was also associated with weight loss, liver enzyme dysfunction, characteristic cellular activation, circulating INF-γ and bacteraemia. Viable C. burnetii was recovered from various tissues during disease and from 75% of the animal's lungs on 28 days post challenge, when there were no overt clinical features of disease but there was histological evidence of macrophage and lymphocyte infiltration into the lung resulting in granulomatous alveolitis. Taken together, these features of disease progression, physiology and bacterial spread appear to be consistent with human disease and therefore the common marmoset can be considered as a suitable model for studies on the pathogenesis or the development of medical counter measures of inhalational Q fever.

Coxiella burnetii-Infected NK Cells Release Infectious Bacteria by Degranulation

Natural killer (NK) cells are critically involved in the early immune response against various intracellular pathogens, including Coxiella burnetii and Chlamydia psittaci Chlamydia-infected NK cells functionally mature, induce cellular immunity, and protect themselves by killing the bacteria in secreted granules. Here, we report that infected NK cells do not allow intracellular multiday growth of Coxiella, as is usually observed in other host cell types. C. burnetii-infected NK cells display maturation and gamma interferon (IFN-γ) secretion, as well as the release of Coxiella-containing lytic granules. Thus, NK cells possess a potent program to restrain and expel different types of invading bacteria via degranulation. Strikingly, though, in contrast to Chlamydia, expulsed Coxiella organisms largely retain their infectivity and, hence, escape the cell-autonomous self-defense mechanism in NK cells.

T-Bet Controls Susceptibility of Mice to Coxiella burnetii Infection

T-bet is a transcription factor known to initiate and coordinate the gene expression program during Th1 differentiation, which is crucial for clearance of intracellular pathogens. Q fever is a worldwide zoonosis caused by Coxiella burnetii. This bacterium is transmitted to humans by aerosol. Indeed, the inhibition of the Coxiella-specific adaptive Th1 immune response leads to persistent infection and organ injury. How deficiency of T-bet affects host infection by C. burnetii has not been investigated. Here, using mice with a deletion of the T-bet gene and an airborne mode of infection to reproduce the natural conditions of C. burnetii infection, we show that infected T-bet^–/– mice were more affected than wild-type mice. The lack of T-bet leads to defective bacterial control, intense replication, persistent infection, and organ injury manifesting as an increased number of granulomas. The absence of T-bet was also associated with an impaired immune response. Indeed, the production of the immunomodulatory cytokines interleukin (IL)-6 and IL-10 was increased, whereas the expression of microbicidal genes by splenocytes was impaired. Moreover, the absence of T-bet exhibited impaired production of interferon-γ, the principal cytokine released by Th1 effector cells. Thus, our study highlights the key role of T-bet in the control of C. burnetii infection in mice and leads to a reappraisal of granulomas in the pathogenesis of Q fever disease.

Novel multiparameter correlates of Coxiella burnetii infection and vaccination identified by longitudinal deep immune profiling

Q-fever is a flu-like illness caused by Coxiella burnetii (Cb), a highly infectious intracellular bacterium. There is an unmet need for a safe and effective vaccine for Q-fever. Correlates of immune protection to Cb infection are limited. We proposed that analysis by longitudinal high dimensional immune (HDI) profiling using mass cytometry combined with other measures of vaccination and protection could be used to identify novel correlates of effective vaccination and control of Cb infection. Using a vaccine-challenge model in HLA-DR transgenic mice, we demonstrated significant alterations in circulating T-cell and innate immune populations that distinguished vaccinated from naïve mice within 10 days, and persisted until at least 35 days post-vaccination. Following challenge, vaccinated mice exhibited reduced bacterial burden and splenomegaly, along with distinct effector T-cell and monocyte profiles. Correlation of HDI data to serological and pathological measurements was performed. Our data indicate a Th1-biased response to Cb, consistent with previous reports, and identify Ly6C, CD73, and T-bet expression in T-cell, NK-cell, and monocytic populations as distinguishing features between vaccinated and naïve mice. This study refines the understanding of the integrated immune response to Cb vaccine and challenge, which can inform the assessment of candidate vaccines for Cb.

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.

Major Histocompatibility Complex Class II-Restricted, CD4+ T Cell-Dependent and -Independent Mechanisms Are Required for Vaccine-Induced Protective Immunity against Coxiella burnetii

To understand the role of major histocompatibility complex class I (MHC-I) and MHC-II in vaccine-mediated protection against Coxiella burnetii, we evaluated the protective efficacy of a formalin-inactivated C. burnetii Nine Mile phase I vaccine (PIV) in β2-microglobulin-deficient (B2m KO) and MHC-II-deficient (MHC-II KO) mice. Vaccination reduced disease severity in wild-type (WT) and B2m KO mice but failed to reduce bacterial burden in MHC-II KO mice. This suggests that the MHC-II antigen presentation pathway is required for PIV-mediated protection against C. burnetii infection. MHC-I and MHC-II affect antibody isotype switching, since both PIV-vaccinated B2m KO and MHC-II KO mice produced less Coxiella-specific IgG than PIV-vaccinated WT mice. Interestingly, MHC-II and CD4 deficiencies were not equivalent in terms of splenomegaly and bacterial clearance. This demonstrates a partial role for CD4+ T cells while revealing MHC-II-restricted, CD4-independent mechanisms. Adoptive transfer of CD4+ T cells from PIV-vaccinated WT mice to naive CD4-deficient (CD4 KO) mice demonstrated that antigen-experienced CD4+ T cells are sufficient to generate protection. Conversely, transfer of naive CD4+ T cells to PIV-vaccinated CD4 KO mice exacerbates disease. Using Tbet-deficient (Tbet KO) mice, we showed a partial role for Th1 subset CD4+ T cells in vaccine protection. Furthermore, Th1-independent roles for Tbet were suggested by significant differences in disease between PIV-vaccinated Tbet KO and CD4 KO mice. Interferon gamma was shown to contribute to the host inflammatory response but not bacterial clearance. Collectively, these findings suggest that vaccine-induced protective immunity against a murine model of experimental Q fever requires MHC-II-restricted, CD4+ T cell-dependent and -independent mechanisms that can be exploited for a new-generation human Q fever vaccine.

Eosinophils Affect Antibody Isotype Switching and May Partially Contribute to Early Vaccine-Induced Immunity against Coxiella burnetii

Coxiella burnetii is an obligate intracellular Gram-negative bacterium which causes human Q fever. An acidified citrate cysteine medium (ACCM-2) has been developed which mimics the intracellular replicative niche of C. burnetii and allows axenic growth of the bacteria. To determine if C. burnetii cultured in ACCM-2 retains immunogenicity, we compared the protective efficacies of formalin-inactivated C. burnetii Nine Mile phase I (PIV) and phase II (PIIV) vaccines derived from axenic culture 7, 14, and 28 days postvaccination. PIV conferred significant protection against virulent C. burnetii as early as 7 days postvaccination, which suggests that ACCM-2-derived PIV retains immunogenicity and protectivity. We analyzed the cellular immune response in spleens from PIV- and PIIV-vaccinated mice by flow cytometry at 7 and 14 days postvaccination and found significantly more granulocytes in PIV-vaccinated mice than in PIIV-vaccinated mice. Interestingly, we found these infiltrating granulocytes to be SSChigh CD11b+ CD125+ Siglec-F+ (where SSChigh indicates a high side scatter phenotype) eosinophils. There was no change in the number of eosinophils in PIV-vaccinated CD4-deficient mice compared to the level in controls, which suggests that eosinophil accumulation is CD4+ T cell dependent. To evaluate the importance of eosinophils in PIV-mediated protection, we vaccinated and challenged eosinophil-deficient ΔdblGATA mice. ΔdblGATA mice had significantly worse disease than their wild-type counterparts when challenged 7 days postvaccination, while no significant difference was seen at 28 days postvaccination. Nevertheless, ΔdblGATA mice had elevated serum IgM with decreased IgG1 and IgG2a whether mice were challenged at 7 or 28 days postvaccination. These results suggest that eosinophils may play a role in early vaccine protection against C. burnetii and contribute to antibody isotype switching.

Full-Term Human Placental Macrophages Eliminate Coxiella burnetii Through an IFN-γ Autocrine Loop

The intracellular bacterium Coxiella burnetii is responsible for Q fever, an infectious disease that increases the risk of abortion, preterm labor, and stillbirth in pregnant women. It has been shown that C. burnetii replicates in BeWo trophoblast cell line and inhibits the activation and maturation of decidual dendritic cells. Although tissue macrophages are known to be targeted by C. burnetii, no studies have investigated the interplay between placental macrophages and C. burnetii. Here, CD14⁺ macrophages from 46 full-term placentas were isolated by positive selection. They consisted of a mixed population of maternal and fetal origin as shown by genotype analysis. We showed that C. burnetii organisms infected placental macrophages after 4 h. When these infected macrophages were incubated for an additional 9-day culture, they completely eliminated organisms as shown by quantitative PCR. The ability of placental macrophages to form multinucleated giant cells was not affected by C. burnetii infection. The transcriptional immune response of placental macrophages to C. burnetii was investigated using quantitative real-time RT-PCR on 8 inflammatory and 10 immunoregulatory genes. C. burnetii clearly induced an inflammatory profile. Interestingly, the production by placental macrophages of interferon-γ, a cytokine known to be involved in efficient immune responses, was dramatically increased in response to C. burnetii. In addition, a clear correlation between interferon-γ production and C. burnetii elimination was found, suggesting that macrophages from full-term placentas eliminate C. burnetii under the control of an autocrine production of interferon-γ.

Host cell depletion of tryptophan by IFNγ-induced Indoleamine 2,3-dioxygenase 1 (IDO1) inhibits lysosomal replication of Coxiella burnetii

Most intracellular pathogens that reside in a vacuole prevent transit of their compartment to lysosomal organelles. Effector mechanisms induced by the pro-inflammatory cytokine Interferon-gamma (IFNγ) can promote the delivery of pathogen-occupied vacuoles to lysosomes for proteolytic degradation and are therefore important for host defense against intracellular pathogens. The bacterial pathogen Coxiella burnetii is unique in that, transport to the lysosome is essential for replication. The bacterium modulates membrane traffic to create a specialized autophagolysosomal compartment called the Coxiella-containing vacuole (CCV). Importantly, IFNγ signaling inhibits intracellular replication of C. burnetii, raising the question of which IFNγ-activated mechanisms restrict replication of a lysosome-adapted pathogen. To address this question, siRNA was used to silence a panel of IFNγ-induced genes in HeLa cells to identify genes required for restriction of C. burnetii intracellular replication. This screen demonstrated that Indoleamine 2,3-dioxygenase 1 (IDO1) contributes to IFNγ-mediated restriction of C. burnetii. IDO1 is an enzyme that catabolizes cellular tryptophan to kynurenine metabolites thereby reducing tryptophan availability in cells. Cells deficient in IDO1 function were more permissive for C. burnetii replication when treated with IFNγ, and supplementing IFNγ-treated cells with tryptophan enhanced intracellular replication. Additionally, ectopic expression of IDO1 in host cells was sufficient to restrict replication of C. burnetii in the absence of IFNγ signaling. Using differentiated THP1 macrophage-like cells it was determined that IFNγ-activation resulted in IDO1 production, and that supplementation of IFNγ-activated THP1 cells with tryptophan enhanced C. burnetii replication. Thus, this study identifies IDO1 production as a key cell-autonomous defense mechanism that limits infection by C. burnetii, which suggests that peptides derived from hydrolysis of proteins in the CCV do not provide an adequate supply of tryptophan for bacterial replication.

Chemokine Receptor 7 Is Essential for Coxiella burnetii Whole-Cell Vaccine-Induced Cellular Immunity but Dispensable for Vaccine-Mediated Protective Immunity

BACKGROUND

Protective immunity against Coxiella burnetii infection is conferred by vaccination with virulent (PI-WCV), but not avirulent (PII-WCV) whole-cell inactivated bacterium. The only well-characterized antigenic difference between virulent and avirulent C. burnetii is they have smooth and rough lipopolysaccharide (LPS), respectively.

METHODS

Mice were vaccinated with PI-WCV and PII-WCV. Humoral and cellular responses were evaluated using protein chip microarrays and ELISpots, respectively. Dendritic cell (DC) maturation after stimulation with PI-WVC and PII-WVC was evaluated using flow cytometry. Vaccine-challenge studies were performed to validate the importance of the receptor CCR7.

RESULTS

Other than specific antibody response to PI-LPS, similar antibody profiles were observed but IgG titers were significantly higher after vaccination with PI-WCV. Furthermore, higher frequency of antigen-specific CD4+ T cells was detected in mice immunized with PI-WCV. PI-WCV-stimulated DCs displayed significantly higher levels of CCR7 and migratory ability to secondary lymphoid organs. Challenge-protection studies in wild-type and CCR7-deficient mice confirmed that CCR7 is critical for PI-WCV-induced cellular immunity.

CONCLUSIONS

PI-WVC stimulates protective immunity to C. burnetii in mice through stimulation of migratory behavior in DCs for protective cellular immunity. Additionally, the humoral immune response to LPS is an important component of protective immunity.

Promiscuous Coxiella burnetii CD4 Epitope Clusters Associated With Human Recall Responses Are Candidates for a Novel T-Cell Targeted Multi-Epitope Q Fever Vaccine

Coxiella burnetii, the causative agent of Q fever, is a Gram-negative intracellular bacterium transmitted via aerosol. Regulatory approval of the Australian whole-cell vaccine Q-VAX® in the US and Europe is hindered by reactogenicity in previously exposed individuals. The aim of this study was to identify and rationally select C. burnetii epitopes for design of a safe, effective, and less reactogenic T-cell targeted human Q fever vaccine. Immunoinformatic methods were used to predict 65 HLA class I epitopes and 50 promiscuous HLA class II C. burnetii epitope clusters, which are conserved across strains of C. burnetii. HLA binding assays confirmed 89% of class I and 75% of class II predictions, and 11 HLA class II epitopes elicited IFNγ responses following heterologous DNA/DNA/peptide/peptide prime-boost immunizations of HLA-DR3 transgenic mice. Human immune responses to the predicted epitopes were characterized in individuals naturally exposed to C. burnetii during the 2007–2010 Dutch Q fever outbreak. Subjects were divided into three groups: controls with no immunological evidence of previous infection and individuals with responses to heat-killed C. burnetii in a whole blood IFNγ release assay (IGRA) who remained asymptomatic or who experienced clinical Q fever during the outbreak. Recall responses to C. burnetii epitopes were assessed by cultured IFNγ ELISpot. While HLA class I epitope responses were sparse in this cohort, we identified 21 HLA class II epitopes that recalled T-cell IFNγ responses in 10–28% of IGRA+ subjects. IGRA+ individuals with past asymptomatic and symptomatic C. burnetii infection showed a comparable response pattern and cumulative peptide response which correlated with IGRA responses. None of the peptides elicited reactogenicity in a C. burnetii exposure-primed guinea pig model. These data demonstrate that a substantial proportion of immunoinformatically identified HLA class II epitopes show long-lived immunoreactivity in naturally infected individuals, making them desirable candidates for a novel human multi-epitope Q fever vaccine.

MyD88 Is Required for Efficient Control of Coxiella burnetii Infection and Dissemination

The intracellular pathogen Coxiella (C.) burnetii causes Q fever, a usually self-limiting respiratory infection that becomes chronic and severe in some patients. Innate immune recognition of C. burnetii and its role in the decision between resolution and chronicity is not understood well. However, TLR2 is important for the response to C. burnetii in mice, and genetic polymorphisms in Myd88 have been associated with chronic Q fever in humans. Here, we have employed MyD88-deficient mice in infection models with the attenuated C. burnetii Nine Mile phase II strain (NMII). Myd88^−/− macrophages failed to restrict the growth of NMII in vitro, and to upregulate production of the cytokines TNF, IL-6, and IL-10. Following intraperitoneal infection, NMII bacterial burden was significantly higher on day 5 and 20 in organs of Myd88^−/− mice. After infection via the natural route by intratracheal injection, a higher bacterial load in the lung and increased dissemination of NMII to other organs was observed in MyD88-deficient mice. While wild-type mice essentially cleared NMII on day 27 after intratracheal infection, it was still readily detectable on day 42 in multiple organs in the absence of MyD88. Despite the elevated bacterial load, Myd88^−/− mice had less granulomatous inflammation and expressed significantly lower levels of chemoattractants, inflammatory cytokines, and of several IFNγ-induced genes relevant for control of intracellular pathogens. Together, our results show that MyD88-dependent signaling is essential for early control of C. burnetii replication and to prevent systemic spreading. The continued presence of NMII in the organs of Myd88^−/− mice constitutes a new mouse model to study determinants of chronicity and resolution in Q fever.

Frequency of Adverse Events Following Q Fever Immunisation in Young Adults

Q fever is a zoonosis of concern in many countries. Vaccination is the most effective means of prevention, and since 1989, Australia has had a licensed Q fever vaccine, Q-VAX^®. This vaccine was also used in the Netherlands in 2011 following the largest recorded Q fever outbreak globally. There is a paucity of available data regarding adverse events following immunisation (AEFI) for young adult females. Such data are important for informing future vaccination recommendations both within Australia and internationally. This study collected Q fever vaccine (Q-VAX^®) AEFI data in veterinary and animal science students at Australian universities. Students were enrolled at the time of vaccination and were emailed a link to an online AEFI survey one week later. Of the 60% (499/827) that responded, 85% were female and the median age was 18 years. Local injection site reactions (ISRs) occurred in 98% (95%; CI 96–99%) of respondents, of which 30% (95% CI 24–32%) were severe. Systemic AEFI occurred in 60% (95%; CI 55–64%) of respondents within the seven days following immunisation. Medical attention was sought by 19/499 (3.8%) respondents, of whom one sought treatment at a hospital emergency department. Females were more likely than males to experience any local ISR (odds ratio [OR] 9.3; 95% CI 2.5–33.8; p < 0.001), ISRs of greater severity (OR 2.5; 95% CI 1.5–4.2; p < 0.001), and any systemic AEFI (OR 1.9; 95% CI 1.1–3.1; p = 0.016). These safety data suggest that a high frequency of adverse events following immunisation should be expected in young adults, particularly females. However, the consequences of Q fever disease are potentially far more debilitating.