Recognition of Coxiella burnetii by toll-like receptors and nucleotide-binding oligomerization domain-like receptors

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.

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.

Recent Advances on the Innate Immune Response to Coxiella burnetii

Coxiella burnetii is an obligate intracellular Gram-negative bacterium and the causative agent of a worldwide zoonosis known as Q fever. The pathogen invades monocytes and macrophages, replicating within acidic phagolysosomes and evading host defenses through different immune evasion strategies that are mainly associated with the structure of its lipopolysaccharide. The main transmission routes are aerosols and ingestion of fomites from infected animals. The innate immune system provides the first host defense against the microorganism, and it is crucial to direct the infection towards a self-limiting respiratory disease or the chronic form. This review reports the advances in understanding the mechanisms of innate immunity acting during C. burnetii infection and the strategies that pathogen put in place to infect the host cells and to modify the expression of specific host cell genes in order to subvert cellular processes. The mechanisms through which different cell types with different genetic backgrounds are differently susceptible to C. burnetii intracellular growth are discussed. The subsets of cytokines induced following C. burnetii infection as well as the pathogen influence on an inflammasome-mediated response are also described. Finally, we discuss the use of animal experimental systems for studying the innate immune response against C. burnetii and discovering novel methods for prevention and treatment of disease in humans and livestock.

Avoidance of the NLRP3 Inflammasome by the Stealth Pathogen, Coxiella burnetii

Coxiella burnetii, a highly adapted obligate intracellular bacterial pathogen and the cause of the zoonosis Q fever, is a reemerging public health threat. C. burnetii employs a Type IV secretion system (T4SS) to establish and maintain its intracellular niche and modulate host immune responses including the inhibition of apoptosis. Interactions between C. burnetii and caspase-1-mediated inflammasomes are not fully elucidated. This study confirms that C. burnetii does not activate caspase-1 during infection of mouse macrophages in vitro. C. burnetii-infected cells did not develop NLRP3 and ASC foci indicating its ability to avoid cytosolic detection. C. burnetii is unable to inhibit the pyroptosis and IL-1β secretion that is induced by potent inflammasome stimuli but rather enhances these caspase-1-mediated effects. We found that C. burnetii upregulates pro-IL-1β and robustly primes NLRP3 inflammasomes via TLR2 and MyD88 signaling. As for wildtype C. burnetii, T4SS-deficient mutants primed and potentiated NLRP3 inflammasomes. An in vivo model of pulmonary infection in C57BL/6 mice was developed. Mice deficient in NLRP3 or caspase-1 were like wildtype mice in the development and resolution of splenomegaly due to red pulp hyperplasia, and histologic lesions and macrophage kinetics, but had slightly higher pulmonary bacterial burdens at the greatest measured time point. Together these findings indicate that C. burnetii primes but avoids cytosolic detection by NLRP3 inflammasomes, which are not required for the clinical resistance of C57BL/6 mice. Determining mechanisms employed by C. burnetii to avoid cytosolic detection via NLRP3 inflammasomes will be beneficial to the development of preventative and interventional therapies for Q fever.

Pattern Recognition Receptors in Innate Immunity to Obligate Intracellular Bacteria

Host pattern recognition receptors (PRRs) are crucial for sensing pathogenic microorganisms, launching innate responses, and shaping pathogen-specific adaptive immunity during infection. Rickettsia spp., Orientia tsutsugamushi, Anaplasma spp., Ehrlichia spp., and Coxiella burnetii are obligate intracellular bacteria, which can only replicate within host cells and must evade immune detection to successfully propagate. These five bacterial species are zoonotic pathogens of clinical or agricultural importance, yet, uncovering how immune recognition occurs has remained challenging. Recent evidence from in-vitro studies and animal models has offered new insights into the types and kinetics of PRR activation during infection with Rickettsia spp., A. phagocytophilum, E. chaffeensis, and C. burnetii, respectively. However, much less is known in these regards for O. tsutsugamushi infection, until the recent discovery for the role of the C-type lectin receptor Mincle during lethal infection in mice and in primary macrophage cultures. This review gives a brief summary for clinical and epidemiologic features of these five bacterial infections, focuses on fundamental biologic facets of infection, and recent advances in host recognition. In addition, we discuss knowledge gaps for innate recognition of these bacteria in the context of disease pathogenesis.

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.

Single nucleotide polymorphism (SNP) rs3751143 in P2RX7 is associated with therapy failure in chronic Q fever while rs7125062 in MMP1 is associated with fewer complications

OBJECTIVES

Chronic Q fever is a persistent infection with the intracellular bacterium Coxiella burnetii. Development of chronic Q fever is associated with single nucleotide polymorphisms (SNPs) in genes encoding for pattern recognition receptors, for phagolysosomal pathway components and for matrix metalloproteinases (MMPs). We evaluated the association of SNPs in these innate-immunity and MMP genes with clinical outcomes.

METHODS

SNPs were selected from previous association studies and analysed in a cohort of patients with chronic Q fever. The primary outcome was all-cause mortality; secondary outcomes were therapy failure and chronic Q fever-related complications. Subdistribution hazard ratios (SHR) were calculated.

RESULTS

Nineteen SNPs were analysed in 134 patients with proven and 29 with probable chronic Q fever. In multivariable analysis, none of the selected SNPs was associated with all-cause mortality. However, SNP rs3751143 located in P2RX7 appeared to be associated with therapy failure (SHR 2.42; 95% confidence interval, 1.16-5.05; p 0.02), which is in line with other reports, showing that a loss of function of the P2X7 receptor leads to inefficient killing of intracellular organisms. In addition, SNP rs7125062 located in MMP1, involved in the cleavage of extracellular matrix, was associated with fewer chronic Q fever-related complications such as acute aneurysms (SHR 0.49; 95% confidence interval, 0.29-0.83; p 0.008).

CONCLUSIONS

A polymorphism in P2RX7, known to lead to loss of function of the receptor and inefficient killing of intracellular organisms, and a polymorphism in MMP1 were respectively associated with more therapy failures and fewer complications such as acute aneurysms in patients with chronic Q fever.

Genetic epidemiology of the Alpine ibex reservoir of persistent and virulent brucellosis outbreak

While it is now broadly accepted that inter-individual variation in the outcomes of host-pathogen interactions is at least partially genetically controlled, host immunogenetic characteristics are rarely investigated in wildlife epidemiological studies. Furthermore, most immunogenetic studies in the wild focused solely on the major histocompatibility complex (MHC) diversity despite it accounts for only a fraction of the genetic variation in pathogen resistance. Here, we investigated immunogenetic diversity of the Alpine ibex (Capra ibex) population of the Bargy massif, reservoir of a virulent outbreak of brucellosis. We analysed the polymorphism and associations with disease resistance of the MHC Class II Drb gene and several non-MHC genes (Toll-like receptor genes, Slc11A1) involved in the innate immune response to Brucella in domestic ungulates. We found a very low neutral genetic diversity and a unique MHC Drb haplotype in this population founded few decades ago from a small number of individuals. By contrast, other immunity-related genes have maintained polymorphism and some showed significant associations with the brucellosis infection status hence suggesting a predominant role of pathogen-mediated selection in their recent evolutionary trajectory. Our results highlight the need to monitor immunogenetic variation in wildlife epidemiological studies and to look beyond the MHC.

From cell culture to cynomolgus macaque: infection models show lineage-specific virulence potential of Coxiella burnetii

Coxiella burnetii is an obligate intracellular pathogen that causes the zoonotic disease Q fever in humans, which can occur in either an acute or a chronic form with serious complications. The bacterium has a wide host range, including unicellular organisms, invertebrates, birds and mammals, with livestock representing the most significant reservoir for human infections. Cell culture models have been used to decipher the intracellular lifestyle of C. burnetii, and several infection models, including invertebrates, rodents and non-human primates, are being used to investigate host-pathogen interactions and to identify bacterial virulence factors and vaccine candidates. However, none of the models replicate all aspects of human disease. Furthermore, it is becoming evident that C. burnetii isolates belonging to different lineages exhibit differences in their virulence in these models. Here, we compare the advantages and disadvantages of commonly used infection models and summarize currently available data for lineage-specific virulence.

Pathogenesis of Important Virulence Factors of Porphyromonas gingivalis via Toll-Like Receptors

Periodontitis is a common intraoral infection and is inextricably linked to systemic diseases. Recently, the regulation between host immunologic response and periodontal pathogens has become a hotspot to explain the mechanism of periodontitis and related systemic diseases. Since Porphyromonas gingivalis (P. gingivalis) was proved as critical periodontal pathogen above all, researches focusing on the mechanism of its virulence factors have received extensive attention. Studies have shown that in the development of periodontitis, in addition to the direct release of virulent factors by periodontal pathogens to destroy periodontal tissues, over-low or over-high intrinsic immune and inflammatory response mediated by Toll-like receptors (TLRs) can lead to more lasting destruction of periodontal tissues. It is very necessary to sort out how various cytopathic factors of P. gingivalis mediate inflammation and immune responses between the host through TLRs so as to help precisely prevent, diagnose, and treat periodontitis in clinic. This review summarizes the role of three most widely studied pathogenic factors produced by P. gingivalis (lipopolysaccharide, gingipains, pili) and their interactions with TLRs at the cellular and molecular level in the progress of periodontitis.

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.

Viable Coxiella burnetii Induces Differential Cytokine Responses in Chronic Q Fever Patients Compared to Heat-Killed Coxiella burnetii

Cytokine responses of chronic Q fever patients to the intracellular bacterium Coxiella burnetii have mostly been studied using ex vivo stimulation of immune cells with heat-killed C. burnetii due to the extensive measures needed to work with viable biosafety level 3 agents. Whether research with heat-killed C. burnetii can be translated to immune responses to viable C. burnetii is imperative for the interpretation of previous and future studies with heat-killed C. burnetii Peripheral blood mononuclear cells (PBMCs) of chronic Q fever patients (n = 10) and healthy controls (n = 10) were stimulated with heat-killed or viable C. burnetii of two strains, Nine Mile and the Dutch outbreak strain 3262, for 24 h, 48 h, and 7 days in the absence or presence of serum containing anti-C. burnetii antibodies. When stimulated with viable C. burnetii, PBMCs of chronic Q fever patients and controls produced fewer proinflammatory cytokines (interleukin-6 [IL-6], tumor necrosis factor alpha, and IL-1β) after 24 h than after stimulation with heat-killed C. burnetii In the presence of Q fever seronegative serum, IL-10 production was higher after stimulation with viable rather than heat-killed C. burnetii; however, when incubating with anti-C. burnetii antibody serum, the effect on IL-10 production was reduced. Levels of adaptive, merely T-cell-derived cytokine (gamma interferon, IL-17, and IL-22) and CXCL9 production were not different between heat-killed and viable C. burnetii stimulatory conditions. Results from previous and future research with heat-killed C. burnetii should be interpreted with caution for innate cytokines, but heat-killed C. burnetii-induced adaptive cytokine production is representative of stimulation with viable bacteria.

Genetic deficiency of NOD2 confers resistance to invasive aspergillosis

Invasive aspergillosis (IA) is a severe infection that can occur in severely immunocompromised patients. Efficient immune recognition of Aspergillus is crucial to protect against infection, and previous studies suggested a role for NOD2 in this process. However, thorough investigation of the impact of NOD2 on susceptibility to aspergillosis is lacking. Common genetic variations in NOD2 has been associated with Crohn's disease and here we investigated the influence of these  genetic variations on the anti-Aspergillus host response. A NOD2 polymorphism reduced the risk of IA after hematopoietic stem-cell transplantation. Mechanistically, absence of NOD2 in monocytes and macrophages increases phagocytosis leading to enhanced fungal killing, conversely, NOD2 activation reduces the antifungal potential of these cells. Crucially, Nod2 deficiency results in resistance to Aspergillus infection in an in vivo model of pulmonary aspergillosis. Collectively, our data demonstrate that genetic deficiency of NOD2 plays a protective role during Aspergillus infection.

Disentangling Immediate Adaptive Introgression from Selection on Standing Introgressed Variation in Humans

Recent studies have reported evidence suggesting that portions of contemporary human genomes introgressed from archaic hominin populations went to high frequencies due to positive selection. However, no study to date has specifically addressed the postintrogression population dynamics of these putative cases of adaptive introgression. Here, for the first time, we specifically define cases of immediate adaptive introgression (iAI) in which archaic haplotypes rose to high frequencies in humans as a result of a selective sweep that occurred shortly after the introgression event. We define these cases as distinct from instances of selection on standing introgressed variation (SI), in which an introgressed haplotype initially segregated neutrally and subsequently underwent positive selection. Using a geographically diverse data set, we report novel cases of selection on introgressed variation in living humans and shortlist among these cases those whose selective sweeps are more consistent with having been the product of iAI rather than SI. Many of these novel inferred iAI haplotypes have potential biological relevance, including three that contain immune-related genes in West Siberians, South Asians, and West Eurasians. Overall, our results suggest that iAI may not represent the full picture of positive selection on archaically introgressed haplotypes in humans and that more work needs to be done to analyze the role of SI in the archaic introgression landscape of living humans.

Farming, Q fever and public health: agricultural practices and beyond

Since the Neolithic period, humans have domesticated herbivores to have food readily at hand. The cohabitation with animals brought various advantages that drastically changed the human lifestyle but simultaneously led to the emergence of new epidemics. The majority of human pathogens known so far are zoonotic diseases and the development of both agricultural practices and human activities have provided new dynamics for transmission. This article provides a general overview of some factors that influence the epidemic potential of a zoonotic disease, Q fever. As an example of a disease where the interaction between the environment, animal (domestic or wildlife) and human populations determines the likelihood of the epidemic potential, the management of infection due to the Q fever agent, Coxiella burnetii, provides an interesting model for the application of the holistic One Health approach.

From Q Fever to Coxiella burnetii Infection: a Paradigm Change

Coxiella burnetii is the agent of Q fever, or "query fever," a zoonosis first described in Australia in 1937. Since this first description, knowledge about this pathogen and its associated infections has increased dramatically. We review here all the progress made over the last 20 years on this topic. C. burnetii is classically a strict intracellular, Gram-negative bacterium. However, a major step in the characterization of this pathogen was achieved by the establishment of its axenic culture. C. burnetii infects a wide range of animals, from arthropods to humans. The genetic determinants of virulence are now better known, thanks to the achievement of determining the genome sequences of several strains of this species and comparative genomic analyses. Q fever can be found worldwide, but the epidemiological features of this disease vary according to the geographic area considered, including situations where it is endemic or hyperendemic, and the occurrence of large epidemic outbreaks. In recent years, a major breakthrough in the understanding of the natural history of human infection with C. burnetii was the breaking of the old dichotomy between "acute" and "chronic" Q fever. The clinical presentation of C. burnetii infection depends on both the virulence of the infecting C. burnetii strain and specific risks factors in the infected patient. Moreover, no persistent infection can exist without a focus of infection. This paradigm change should allow better diagnosis and management of primary infection and long-term complications in patients with C. burnetii infection.

Analysis of the Caenorhabditis elegans innate immune response to Coxiella burnetii

The nematode Caenorhabditis elegans is well established as a system for characterization and discovery of molecular mechanisms mediating microbe-specific inducible innate immune responses to human pathogens. Coxiella burnetii is an obligate intracellular bacterium that causes a flu-like syndrome in humans (Q fever), as well as abortions in domesticated livestock, worldwide. Initially, when wild type C. elegans (N2 strain) was exposed to mCherry-expressing C. burnetii (CCB) a number of overt pathological manifestations resulted, including intestinal distension, deformed anal region and a decreased lifespan. However, nematodes fed autoclave-killed CCB did not exhibit these symptoms. Although vertebrates detect C. burnetii via TLRs, pathologies in tol-1(-) mutant nematodes were indistinguishable from N2, and indicate nematodes do not employ this orthologue for detection of C. burnetii. sek-1(-) MAP kinase mutant nematodes succumbed to infection faster, suggesting that this signaling pathway plays a role in immune activation, as previously shown for orthologues in vertebrates during a C. burnetii infection. C. elegans daf-2(-) mutants are hyper-immune and exhibited significantly reduced pathological consequences during challenge. Collectively, these results demonstrate the utility of C. elegans for studying the innate immune response against C. burnetii and could lead to discovery of novel methods for prevention and treatment of disease in humans and livestock.

Development of an Ex Vivo Tissue Platform To Study the Human Lung Response to Coxiella burnetii

Coxiella burnetii is an intracellular bacterial pathogen that causes human Q fever, an acute debilitating flu-like illness that can also present as chronic endocarditis. Disease typically occurs following inhalation of contaminated aerosols, resulting in an initial pulmonary infection. In human cells, C. burnetii generates a replication niche termed the parasitophorous vacuole (PV) by directing fusion with autophagosomes and lysosomes. C. burnetii requires this lysosomal environment for replication and uses a Dot/Icm type IV secretion system to generate the large PV. However, we do not understand how C. burnetii evades the intracellular immune surveillance that triggers an inflammatory response. We recently characterized human alveolar macrophage (hAM) infection in vitro and found that avirulent C. burnetii triggers sustained interleukin-1β (IL-1β) production. Here, we evaluated infection of ex vivo human lung tissue, defining a valuable approach for characterizing C. burnetii interactions with a human host. Within whole lung tissue, C. burnetii preferentially replicated in hAMs. Additionally, IL-1β production correlated with formation of an apoptosis-associated speck-like protein containing a caspase activation and recruitment domain (ASC)-dependent inflammasome in response to infection. We also assessed potential activation of a human-specific noncanonical inflammasome and found that caspase-4 and caspase-5 are processed during infection. Interestingly, although inflammasome activation is closely linked to pyroptosis, lytic cell death did not occur following C. burnetii-triggered inflammasome activation, indicating an atypical response after intracellular detection. Together, these studies provide a novel platform for studying the human innate immune response to C. burnetii.

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.

Primary Role for Toll-Like Receptor-Driven Tumor Necrosis Factor Rather than Cytosolic Immune Detection in Restricting Coxiella burnetii Phase II Replication within Mouse Macrophages

Coxiella burnetii replicates within permissive host cells by employing a Dot/Icm type IV secretion system (T4SS) to translocate effector proteins that direct the formation of a parasitophorous vacuole. C57BL/6 mouse macrophages restrict the intracellular replication of the C. burnetii. Nine Mile phase II (NMII) strain. However, eliminating Toll-like receptor 2 (TLR2) permits bacterial replication, indicating that the restriction of bacterial replication is immune mediated. Here, we examined whether additional innate immune pathways are employed by C57BL/6 macrophages to sense and restrict NMII replication. In addition to the known role of TLR2 in detecting and restricting NMII infection, we found that TLR4 also contributes to cytokine responses but is not required to restrict bacterial replication. Furthermore, the TLR signaling adaptors MyD88 and Trif are required for cytokine responses and restricting bacterial replication. The C. burnetii NMII T4SS translocates bacterial products into C57BL/6 macrophages. However, there was little evidence of cytosolic immune sensing of NMII, as there was a lack of inflammasome activation, T4SS-dependent cytokine responses, and robust type I interferon (IFN) production, and these pathways were not required to restrict bacterial replication. Instead, endogenous tumor necrosis factor (TNF) produced upon TLR sensing of C. burnetii NMII was required to control bacterial replication. Therefore, our findings indicate a primary role for TNF produced upon immune detection of C. burnetii NMII by TLRs, rather than cytosolic PRRs, in enabling C57BL/6 macrophages to restrict bacterial replication.

Genetic Variation in Pattern Recognition Receptors and Adaptor Proteins Associated With Development of Chronic Q Fever

BACKGROUND

Q fever is an infection caused by Coxiella burnetii. Persistent infection (chronic Q fever) develops in 1%-5% of patients. We hypothesize that inefficient recognition of C. burnetii and/or activation of host-defense in individuals carrying genetic variants in pattern recognition receptors or adaptors would result in an increased likelihood to develop chronic Q fever.

METHODS

Twenty-four single-nucleotide polymorphisms in genes encoding Toll-like receptors, nucleotide-binding oligomerization domain-like receptor-2, αvβ3 integrin, CR3, and adaptors myeloid differentiation primary response protein 88 (MyD88), and Toll interleukin 1 receptor domain-containing adaptor protein (TIRAP) were genotyped in 139 patients with chronic Q fever and in 220 controls with cardiovascular risk-factors and previous exposure to C. burnetii. Associations between these single-nucleotide polymorphisms and chronic Q fever were assessed by means of univariate logistic regression models. Cytokine production in whole-blood stimulation assays was correlated with relevant genotypes.

RESULTS

Polymorphisms in TLR1 (R80T), NOD2 (1007fsX1), and MYD88 (-938C>A) were associated with chronic Q fever. No association was observed for polymorphisms in TLR2, TLR4, TLR6, TLR8, ITGAV, ITGB3, ITGAM, and TIRAP. No correction for multiple testing was performed because only genes with a known role in initial recognition of C. burnetii were included. In the whole-blood assays, individuals carrying the TLR1 80R-allele showed increased interleukin 10 production with C. burnetii exposure.

CONCLUSIONS

Polymorphisms in TLR1 (R80T), NOD2 (L1007fsX1), and MYD88 (-938C>A) are associated with predisposition to development of chronic Q fever. For TLR1, increased interleukin 10 responses to C. burnetii in individuals carrying the risk allele may contribute to the increased risk of chronic Q fever.