Laboratory maintenance of Rickettsia rickettsii

Stomach as the target organ of Rickettsia heilongjiangensis infection in C57BL/6 mice identified by click chemistry

Spotted fever group rickettsiae (SFGR) are obligate intracellular bacteria that cause spotted fever. The limitations of gene manipulation pose great challenges to studying the infection mechanisms of Rickettsia. By combining bioorthogonal metabolism and click chemistry, we developed a method to label R. heilongjiangensis via azide moieties and achieved rapid pathogen localization without complex procedures. Moreover, we constructed a C57BL/6 mice infection model by simulating tick bites and discovered that the stomach is the target organ of R. heilongjiangensis infection through in vivo imaging systems, which explained the occurrence of gastrointestinal symptoms following R. heilongjiangensis infection in some cases. This study offers a unique perspective for subsequent investigations into the pathogenic mechanisms of SFGR and identifies a potential target organ for R. heilongjiangensis.

Rickettsia asembonensis Isolated from Four Human Cases with Acute Undifferentiated Febrile Illness in Peru

Rickettsioses, often underreported, pose public health challenges. Rickettsia asembonensis is a potential emerging pathogen that was previously detected in humans, animals, and a variety of arthropods. While its pathogenicity in humans remains unclear, it poses a potential public health threat. Here, we present an extended epidemiological, diagnostic, and genetic analysis of the information provided in a preliminary report on the investigation of rickettsiae in Peru. In particular, we report the detection of R. asembonensis in blood specimens collected from four human patients with an acute undifferentiated fever of a seven- to nine-day duration, all of whom tested negative for other vector-borne pathogens. Additionally, we describe the replicative capacity of the R. asembonensis isolates in cell cultures.

Rickettsia rickettsii virulence determinants RARP2 and RapL mitigate IFN- signaling in primary human dermal microvascular endothelial cells

We compared the growth characteristics of a virulent Rickettsia rickettsii strain (Sheila Smith) to an attenuated R. rickettsii stain (Iowa) and a non-pathogenic species (R. montanensis) in primary human dermal microvascular endothelial cells (HDMEC). All replicated in Vero cells, however, only the Sheila Smith strain productively replicated in HDMECs. The Iowa strain showed minimal replication over a 24-h period, while R. montanensis lost viability and induced lysis of the HDMECs via a rapid programmed cell death response. Both the virulent and attenuated R. rickettsii strains, but not R. montanensis, induced an interferon-1 response, although the response was of lesser magnitude and delayed in the Sheila Smith strain. IFN-β secretion correlated with increased host cell lysis, and treatment with anti-IFNAR2 antibody decreased lysis from Iowa-infected but not Sheila Smith-infected cells. Both Sheila Smith- and Iowa-infected cells eventually lysed, although the response from Sheila Smith was delayed and showed characteristics of apoptosis. We, therefore, examined whether reconstitution of the Iowa strain with two recently described putative virulence determinants might enhance survival of Iowa within HDMECs. Reconstitution with RARP2, which is inhibitory to anterograde trafficking through the Golgi apparatus, reduced IFN-β secretion but had no effect on cell lysis. RapL, which proteolytically processes surface exposed autotransporters and enhances replication of Iowa in Guinea pigs, suppressed both IFN-β production and host cell lysis. These findings suggest distinct mechanisms by which virulent spotted fever group rickettsiae may enhance intracellular survival and replication.IMPORTANCEWe examined a naturally occurring non-pathogenic rickettsial species, R. montanensis, a laboratory-attenuated R. rickettsii strain (Iowa), and a fully virulent R. rickettsii strain (Sheila Smith) for growth in human dermal microvascular endothelial cells. The two avirulent strains replicated poorly or not at all. Only the virulent Sheila Smith strain replicated. IFN-β production correlated with the inhibition of R. rickettsii Iowa. Reconstitution of Iowa with either of two recently described putative virulence determinants altered the IFN-β response. A rickettsial ankyrin repeat protein, RARP2, disrupts the trans-Golgi network and inhibits IFN-β secretion. An autotransporter peptidase, RapL, restores proteolytic maturation of outer membrane autotransporters and diminishes the IFN-β response to enhance cell survival and permit replication of the recombinant strain. These studies point the way toward discovery of mechanisms for innate immune response avoidance by virulent rickettsia.

Prompt diagnosis and appropriate treatment of Japanese spotted fever: A report of three cases

Background

Japanese Spotted Fever (JSF) is a Spotted Fever Group (SFG) rickettsiosis caused by Rickettsia japonica. More than 300 cases are diagnosed annually in Japan, and the number of reported cases has been increasing. Correct diagnoses depend on the triad of symptoms and signs, including fever, rash, and eschar, which can be seen at the site of vector bites. JSF is not life-threatening if treated appropriately without diagnostic delay but there are some fatal cases every year. This negligence leads to disseminated intravascular coagulation (DIC) and multiple organ failure (MOF), and poor prognoses, consequently. Prompt diagnosis of JSF is difficult when the aforementioned triad of signs and symptoms is not initially present.

Case report

This report describes three JSF cases: an 87-year-old woman with fever, shock, pancytopenia, DIC, and MOF; a 79-year-old man with fever and difficulty in movement; and a 78-year-old man with fever, general fatigue, and appetite loss. All patients had a rash and eschar, which led to prompt diagnosis and appropriate treatment immediately. All patients were treated without any complications.

Why should an emergency physician be aware of this?

As mentioned above, JFS can be fatal with delayed diagnoses and treatment initiations. The key for a prompt diagnosis is to recognize the triad of symptoms and signs, which are not often present initially, and it makes JSF diagnosis challenging. Repeated comprehensive physical examinations are essential for prompt diagnosis and improve prognosis of JSF.

Anaphylatoxin signaling activates macrophages to control intracellular Rickettsia proliferation

IMPORTANCE

Pathogenic Rickettsia species are extremely dangerous bacteria that grow within the cytoplasm of host mammalian cells. In most cases, these bacteria are able to overpower the host cell and grow within the protected environment of the cytoplasm. However, a dramatic conflict occurs when Rickettsia encounter innate immune cells; the bacteria can "win" by taking over the host, or the bacteria can "lose" if the host cell efficiently fights the infection. This manuscript examines how the immune complement system is able to detect the presence of Rickettsia and alert nearby cells. Byproducts of complement activation called anaphylatoxins are signals that "activate" innate immune cells to mount an aggressive defensive strategy. This study enhances our collective understanding of the innate immune reaction to intracellular bacteria and will contribute to future efforts at controlling these dangerous infections.

Ehrlichia chaffeensis co-opts phagocytic hemocytes for systemic dissemination in the Lone Star tick, Amblyomma americanum

Hematophagous arthropods can acquire and transmit several pathogens of medical importance. In ticks, the innate immune system is crucial in the outcome between vector-pathogen interaction and overall vector competence. However, the specific immune response(s) elicited by the immune cells known as hemocytes remains largely undefined in Ehrlichi a chaffeensis and its competent tick vector, Amblyomma americanum . Here, we show that granulocytes, professional phagocytic cells, are integral in eliciting immune responses against commensal and pathogen infection. The chemical depletion of granulocytes led to decreased phagocytic efficiency of tissues-associated hemocytes. We demonstrate E. chaffeensis can infect circulating hemocytes, and both cell-free plasma and hemocytes from E. chaffeensis- infected ticks can establish Ehrlichia infection in recipient ticks. Lastly, we provide evidence to show granulocytes play a dual role in E. chaffeensis infection. Depleting granulocytic hemocytes increased Ehrlichia load in the salivary gland and midgut tissues. In contrast, granulocyte depletion led to a reduced systemic load of Ehrlichia . This study has identified multiple roles for granulocytic hemocytes in the control and systemic dissemination of E. chaffeensis infection.

Rickettsia Deregulates Genes Coding for the Neurotoxic Cell Response Pathways in Cerebrocortical Neurons In Vitro

Rickettsial infections of the central nervous system (CNS) are manifested by severe neurological symptoms and represent a serious life-threatening condition. Despite the considerable health danger, only a few studies have been conducted focusing on the pathogenesis induced by Rickettsia sp. in CNS. To investigate the signaling pathways associated with the neurotoxic effects of rickettsiae, we employed an experimental model of cerebrocortical neurons combined with molecular profiling and comprehensive bioinformatic analysis. The cytopathic effect induced by Rickettsia akari and Rickettsia slovaca was demonstrated by decreased neuronal viability, structural changes in cell morphology, and extensive fragmentation of neurites in vitro. Targeted profiling revealed the deregulation of genes involved in the neuroinflammatory and neurotoxic cell response pathways. Although quantitative analysis showed differences in gene expression response, functional annotation revealed that the biological processes are largely shared between both Rickettsia species. The identified enriched pathways are associated with cytokine signaling, chemotaxis of immune cells, responses to infectious agents, interactions between neurons, endothelial and glial cells, and regulation of neuronal apoptotic processes. The findings of our study provide new insight into the etiopathogenesis of CNS infection and further expand the understanding of molecular signaling associated with neuroinvasive Rickettsia species.

Rickettsia parkeri hijacks tick hemocytes to manipulate cellular and humoral transcriptional responses

Introduction

Blood-feeding arthropods rely on robust cellular and humoral immunity to control pathogen invasion and replication. Tick hemocytes produce factors that can facilitate or suppress microbial infection and pathogenesis. Despite the importance of hemocytes in regulating microbial infection, understanding of their basic biology and molecular mechanisms remains limited.

Methods

Here we combined histomorphology and functional analysis to identify five distinct phagocytic and non-phagocytic hemocyte populations circulating within the Gulf Coast tick Amblyomma maculatum.

Results and discussion

Depletion of phagocytic hemocytes using clodronate liposomes revealed their function in eliminating bacterial infection. We provide the first direct evidence that an intracellular tick-borne pathogen, Rickettsia parkeri, infects phagocytic hemocytes in Am. maculatum to modify tick cellular immune responses. A hemocyte-specific RNA-seq dataset generated from hemocytes isolated from uninfected and R. parkeri-infected partially blood-fed ticks generated ~40,000 differentially regulated transcripts, >11,000 of which were immune genes. Silencing two differentially regulated phagocytic immune marker genes (nimrod B2 and eater-two Drosophila homologs), significantly reduced hemocyte phagocytosis.

Conclusion

Together, these findings represent a significant step forward in understanding how hemocytes regulate microbial homeostasis and vector competence.

Real-time PCR validation to estimate the number of rickettsias in the biological material under study

Using the example of the clinical strain of R. sibirica «Bayevo 105/87», the possibility of quantitative determination of rickettsias in clinical samples from patients with Siberian tick-borne typhus by real-time polymerase chain reaction (PCR-RT) was evaluated. Cultivation was carried out in the yolk sacs of developing chicken embryos, from which a piece of the yolk sac or chorion was taken. A total of 125 samples were examined. A set of reagents "RealBest DNA Rickettsia species (kit1)" was used for PCR-RT. The obtained values of the threshold amplification cycle (Ct) were compared with the results of microscopy of smear preparations stained by the Zdrodovsky method, the values of which were divided into ranks: the I rank - single rickettsias in individual fields of vision, the II rank - single rickettsias in each field of vision, the III rank - from 10 to 25 rickettsias in each field of vision, the IV rank - from 25 to 50 rickettsias in each field of view. The median Ct value for rank I was 17.6 (16.37; 18.58), for the II - 16.0 (15.0; 16.41), for the III - 15.0 (14.0; 15.1) and for the IV - 15.0 (13.7; 14.64). A significant average correlation was established between the number of rickettsias in the preparation under microscopy and the value of the threshold cycle in PCR RT (r=-0, 4849542; p=9.968e-09). When determining the correlation between the pathomorphological characteristic and the value of the threshold cycle, its absence was established. The detection of rickettsias in the blood vessels of the chorion of developing chicken embryos was of interest. In 10 samples, the yolk sac and chorion were taken for the study, and in parallel they were examined by PCR-RT. The use of modern, more sensitive molecular biological methods allows for quantitative analysis of DNA in the chorion, while preserving the volumes of the most valuable material - the yolk sac.

Evaluating the Clinical and Immune Responses to Spotted Fever Rickettsioses in the Guinea Pig-Tick-Rickettsia System

The guinea pig was the original animal model developed for investigating spotted fever rickettsiosis (SFR). This model system has persisted on account of the guinea pig's conduciveness to tick transmission of SFR agents and ability to recapitulate SFR in humans through clinical signs that include fever, unthriftiness, and in some cases the development of an eschar. The guinea pig is the smallest animal model for SFR that allows the collection of multiple blood and skin samples antemortem for longitudinal studies. This unit provides the basic protocols necessary to establish, maintain, and utilize a guinea pig-tick-Rickettsia model for monitoring the course of infection and immune response to an infection by spotted fever group Rickettsia (SFGR) that can be studied at biosafety level 2 (BSL-2) and arthropod containment level 2 (ACL-2); adaptations must be made for BSL-3 agents. The protocols cover methods for tick feeding and colony development, laboratory infection of ticks, tick transmission of Rickettsia to guinea pigs, and monitoring of the course of infection through clinical signs, rickettsial burden, and immune response. It should be feasible to adapt these methods to study other tick-borne pathogens. © 2022 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Tick transmission of SFGR to guinea pigs Support Protocol 1: Laboratory infection of ticks by injection Alternate Protocol 1: Needle inoculation of SFGR to guinea pigs Basic Protocol 2: Monitoring the course of guinea pig rickettsial infection: clinical signs Basic Protocol 3: Monitoring the course of guinea pig rickettsial infection: collection of biological specimens Support Protocol 2: Guinea pig anesthesia Basic Protocol 4: Monitoring rickettsial burden in guinea pigs by multiplex qPCR Basic Protocol 5: Monitoring guinea pig immune response to infection: blood leukocytes by flow cytometry Basic Protocol 6: Monitoring immune response to guinea pig rickettsial infection: leukocyte infiltration of skin at the tick bite site by flow cytometry Basic Protocol 7: Monitoring the immune response to guinea pig rickettsial infection: antibody titer by ELISA Support Protocol 4: Coating ELISA Plates Alternate Protocol 2: Monitoring immune response to guinea pig rickettsial infection: antibody titer by immunofluorescence assay.

Procedure for spotted fever group Rickettsia isolation from limited clinical blood specimens

BACKGROUND

Current isolation techniques for spotted fever group Rickettsia from clinical samples are laborious and are limited to tissue, blood and blood derivatives with volumes ideally greater than 1 mL. We validated the use of simplified methodologies for spotted fever group Rickettsia culture isolation that overcome sample volume limitations and provide utility in clinical diagnostics and research studies.

METHODOLOGY/PRINCIPAL FINDINGS

A modified cell culture method is evaluated for the isolation of Rickettsia ssp. from human diagnostic samples. Culture sampling method, culture platform, and growth phase analysis were evaluated to determine best practices for optimal culture isolation conditions. Rickettsial isolates (R. conorii, R. rickettsii, and R. parkeri) were grown in Vero E6 cells over a course of 5 to 7 days at low inoculum treatments (~40 bacterial copies) to standardize the sampling strategy at a copy number reflective of the bacteremia in acute diagnostic samples. This methodology was verified using small volumes (50 μL) of 25 unprocessed clinical whole blood, plasma, and serum samples from acute samples of patients suspected of having Rocky Mountain Spotted Fever, of which 10 were previously confirmed positive via the PanR8 qPCR assay, 13 had no detectable Rickettsia DNA by the PanR8 qPCR assay, and 2 were not previously tested; these samples resulted in the cultivation of 7 new R. rickettsii isolates.

CONCLUSIONS/SIGNIFICANCE

We observed that rickettsial isolate growth in culture is reproducibly identified by real-time PCR testing of culture media within 72 hours after inoculation. Additionally, specimen sedimentation prior to isolation to remove red blood cells was found to decrease the amount of total organism available in the inoculum. A small volume culture method was established focusing on comparative qPCR detection rather than bacterial visualization, taking significantly shorter time to detect, and requiring less manipulation compared to traditional clinical isolate culture methods.

Rickettsia Aglow: A Fluorescence Assay and Machine Learning Model to Identify Inhibitors of Intracellular Infection

Rickettsia is a genus of Gram-negative bacteria that has for centuries caused large-scale morbidity and mortality. In recent years, the resurgence of rickettsial diseases as a major cause of pyrexias of unknown origin, bioterrorism concerns, vector movement, and concerns over drug resistance is driving a need to identify novel treatments for these obligate intracellular bacteria. Utilizing an uvGFP plasmid reporter, we developed a screen for identifying anti-rickettsial small molecule inhibitors using Rickettsia canadensis as a model organism. The screening data were utilized to train a Bayesian model to predict growth inhibition in this assay. This two-pronged methodology identified anti-rickettsial compounds, including duartin and JSF-3204 as highly specific, efficacious, and noncytotoxic compounds. Both molecules exhibited in vitro growth inhibition of R. prowazekii, the causative agent of epidemic typhus. These small molecules and the workflow, featuring a high-throughput phenotypic screen for growth inhibitors of intracellular Rickettsia spp. and machine learning models for the prediction of growth inhibition of an obligate intracellular Gram-negative bacterium, should prove useful in the search for new therapeutic strategies to treat infections from Rickettsia spp. and other obligate intracellular bacteria.

Culture Isolate of Rickettsia felis from a Tick

Although the cat flea, Ctenocephalides felis, has been identified as the primary vector of Rickettsia felis, additional flea, tick, mite, and louse species have also been associated with this bacterium by molecular means; however, the role of these arthropods in the transmission of R. felis has not been clarified. Here, we succeeded in culture isolation of R. felis from a host-seeking castor bean tick, Ixodes ricinus, the most common tick in Slovakia. The bacterial isolation was performed on XTC-2 cells at 28 °C using the shell-vial technique. An evaluation of the growth properties was performed for both the XTC-2 and Vero cell lines. We observed R. felis in the infected host cells microscopically by Gimenez staining and immunofluorescence assay. The R. felis isolate was purified by gradient ultracentrifugation and visualized by electron microscopy. Fragments of the genes gltA, ompA, ompB, htrA, rpoB, sca4, rffE, and rrs were amplified and compared with the corresponding sequences of the type strain URRWXCal2 and other R. felis culture -isolated strains. We did not detect any nucleotide polymorphisms; however, plasmid pRFδ, characteristic of the standard strain, was absent in our isolate. Herein, we describe the first successful isolation and characterization of a tick-derived R. felis strain “Danube”, obtained from an I. ricinus nymph.

Spotted Fever Group Rickettsia Trigger Species-Specific Alterations in Macrophage Proteome Signatures with Different Impacts in Host Innate Inflammatory Responses

The molecular details underlying differences in pathogenicity between Rickettsia species remain to be fully understood. Evidence points to macrophage permissiveness as a key mechanism in rickettsial virulence. Different studies have shown that several rickettsial species responsible for mild forms of rickettsioses can also escape macrophage-mediated killing mechanisms and establish a replicative niche within these cells. However, their manipulative capacity with respect to host cellular processes is far from being understood. A deeper understanding of the interplay between mildly pathogenic rickettsiae and macrophages and the commonalities and specificities of host responses to infection would illuminate differences in immune evasion mechanisms and pathogenicity. We used quantitative proteomics by sequential windowed data independent acquisition of the total high-resolution mass spectra with tandem mass spectrometry (SWATH-MS/MS) to profile alterations resulting from infection of THP-1 macrophages with three mildly pathogenic rickettsiae: Rickettsia parkeri, Rickettsia africae, and Rickettsia massiliae, all successfully proliferating in these cells. We show that all three species trigger different proteome signatures. Our results reveal a significant impact of infection on proteins categorized as type I interferon responses, which here included several components of the retinoic acid-inducible gene I (RIG-1)-like signaling pathway, mRNA splicing, and protein translation. Moreover, significant differences in protein content between infection conditions provide evidence for species-specific induced alterations. Indeed, we confirm distinct impacts on host inflammatory responses between species during infection, demonstrating that these species trigger different levels of beta interferon (IFN-β), differences in the bioavailability of the proinflammatory cytokine interleukin 1β (IL-1β), and differences in triggering of pyroptotic events. This work reveals novel aspects and exciting nuances of macrophage-Rickettsia interactions, adding additional layers of complexity between Rickettsia and host cells' constant arms race for survival. IMPORTANCE The incidence of diseases caused by Rickettsia has been increasing over the years. It has long been known that rickettsioses comprise diseases with a continuous spectrum of severity. There are highly pathogenic species causing diseases that are life threatening if untreated, others causing mild forms of the disease, and a third group for which no pathogenicity to humans has been described. These marked differences likely reflect distinct capacities for manipulation of host cell processes, with macrophage permissiveness emerging as a key virulence trait. However, what defines pathogenicity attributes among rickettsial species is far from being resolved. We demonstrate that the mildly pathogenic Rickettsia parkeri, Rickettsia africae, and Rickettsia massiliae, all successfully proliferating in macrophages, trigger different proteome signatures in these cells and differentially impact critical components of innate immune responses by inducing different levels of beta interferon (IFN-β) and interleukin 1β (IL-1β) and different timing of pyroptotic events during infection. Our work reveals novel nuances in rickettsia-macrophage interactions, offering new clues to understand Rickettsia pathogenicity.

The Retropepsin-Type Protease APRc as a Novel Ig-Binding Protein and Moonlighting Immune Evasion Factor of Rickettsia

Rickettsiae are obligate intracellular Gram-negative bacteria transmitted by arthropod vectors. Despite their reduced genomes, the function(s) of the majority of rickettsial proteins remains to be uncovered. APRc is a highly conserved retropepsin-type protease, suggested to act as a modulator of other rickettsial surface proteins with a role in adhesion/invasion. However, APRc’s function(s) in bacterial pathogenesis and virulence remains unknown. This study demonstrates that APRc targets host serum components, combining nonimmune immunoglobulin (Ig)-binding activity with resistance to complement-mediated killing. We confirmed nonimmune human IgG binding in extracts of different rickettsial species and intact bacteria. Our results revealed that the soluble domain of APRc is capable of binding to human (h), mouse, and rabbit IgG and different classes of human Ig (IgG, IgM, and IgA) in a concentration-dependent manner. APRc-hIgG interaction was confirmed with total hIgG and normal human serum. APRc-hIgG displayed a binding affinity in the micromolar range. We provided evidence of interaction preferentially through the Fab region and confirmed that binding is independent of catalytic activity. Mapping the APRc region responsible for binding revealed the segment between amino acids 157 and 166 as one of the interacting regions. Furthermore, we demonstrated that expression of the full-length protease in Escherichia coli is sufficient to promote resistance to complement-mediated killing and that interaction with IgG contributes to serum resistance. Our findings position APRc as a novel Ig-binding protein and a novel moonlighting immune evasion factor of Rickettsia, contributing to the arsenal of virulence factors utilized by these intracellular pathogens to aid in host colonization.

Microbiome of Haemaphysalis longicornis Tick in Korea

Ticks can transmit pathogenic bacteria, protozoa, and viruses to humans and animals. In this study, we investigated the microbiomes of Haemaphysalis longicornis according to sex and life stages. The Shannon index was significantly higher for nymphs than adult ticks. Principal coordinates analysis showed that the microbiome composition of female adult and male adult ticks were different. Notably, Coxiella-like bacterium (AB001519), known as a tick symbiont, was found in all nymphs and female adult ticks, but only one out of 4 male adult ticks had Coxiella-like bacterium (AB001519). In addition, Rickettsia rickettsii, Coxiella burnetii, and Anaplasma bovis were detected in this study.

Contribution of classical complement activation and IgM to the control of Rickettsia infection

Pathogenic Rickettsia are obligate intracellular bacteria and the etiologic agents of many life‐threatening infectious diseases. Due to the serious nature of these infections, it is imperative to both identify the responsive immune sensory pathways and understand the associated immune mechanisms that restrict Rickettsia proliferation. Previous studies have demonstrated that the mammalian complement system is both activated during Rickettsia infection and contributes to the immune response to infection. To further define this component of the mammalian anti‐Rickettsia immune response, we sought to identify the mechanism(s) of complement activation during Rickettsia infection. We have employed a series of in vitro and in vivo models of infection to investigate the role of the classical complement activation pathway during Rickettsia infection. Depletion or elimination of complement activity demonstrates that both C1q and pre‐existing IgM contribute to complement activation; thus implicating the classical complement system in Rickettsia‐mediated complement activation. Elimination of the classical complement pathway from mice increases susceptibility to R. australis infection with both increased bacterial loads in multiple tissues and decreased immune activation markers. This study highlights the role of the classical complement pathway in immunity against Rickettsia and implicates resident Rickettsia‐responsive IgM in the response to infection.

An in vitro antiviral activity of iodine complexes against SARS-CoV-2

Since the emergence of COVID-19 pandemic in China in late 2019, scientists are striving hard to explore non-toxic, viable anti-SARS-CoV-2 compounds or medicines. We determined In vitro anti-SARS-CoV-2 activity of oral formulations (syrup and capsule)of an Iodine-complex (Renessans). First, cell cytotoxicity of Renessans on the Vero cells was determined using MTT assay. Afterwards, the antiviral activity of Renessans was determined using viral inhibition assays and TCID50. For this, nontoxic concentrations of the Renessans were used. The results showed that Renessans is nontoxic to the cells up to 50 µg/mL. At 1.5 µg/mL concentration, SARS-CoV-2 production was significantly reduced to 101.43 TCID50 and 101.58 TCID50 for the syrup and capsule, respectively, as compare to virus infected control cells 106.08 TCID50 and we found the dose dependent inhibition of virus replication in the presence of Renessans. Renessans inhibited SARS-CoV-2 with an EC50 value of 0.425 µg/mL and 0.505 µg/mL for syrup and capsule, respectively. Furthermore, there was no virus detected at concentration of 50 µg/mL of Renessans. This study indicates that Renessans, containing iodine, have potential activity against SARS-CoV-2 which needs to be further investigated in human clinical trials.

Rickettsia conorii survival in THP-1 macrophages involves host lipid droplet alterations and active rickettsial protein production

Rickettsia conorii is a Gram-negative, cytosolic intracellular bacterium that has classically been investigated in terms of endothelial cell infection. However, R. conorii and other human pathogenic Rickettsia species have evolved mechanisms to grow in various cell types, including macrophages, during mammalian infection. During infection of these phagocytes, R. conorii shifts the host cell's overall metabolism towards an anti-inflammatory M2 response, metabolically defined by an increase in host lipid metabolism and oxidative phosphorylation. Lipid metabolism has more recently been identified as a key regulator of host homeostasis through modulation of immune signalling and metabolism. Intracellular pathogens have adapted mechanisms of hijacking host metabolic pathways including host lipid catabolic pathways for various functions required for growth and survival. In the present study, we hypothesised that alterations of host lipid droplets initiated by lipid catabolic pathways during R. conorii infection is important for bacterial survival in macrophages. Herein, we determined that host lipid droplet modulation is initiated early during R. conorii infection, and these alterations rely on active bacteria and lipid catabolic pathways. We also find that these lipid catabolic pathways are essential for efficient bacterial survival. Unlike the mechanisms used by other intracellular pathogens, the catabolism of lipid droplets induced by R. conorii infection is independent of upstream host peroxisome proliferator-activated receptor-alpha (PPARα) signalling. Inhibition of PPARɣ signalling and lipid droplet accumulation in host cells cause a significant decrease in R. conorii survival suggesting a negative correlation with lipid droplet production and R. conorii survival. Together, these results strongly suggest that the modulation of lipid droplets in macrophage cells infected by R. conorii is an important and underappreciated aspect of the infection process. TAKE AWAYS: Host lipid droplets are differentially altered in early and replicative stages of THP-1 macrophage infection with R. conorii. Lipid droplet alterations are initiated in a bacterial-dependent manner and do not require host peroxisome proliferator-activated receptors α or ɣ activation. Pharmacological inhibition of host lipid catabolic processes during R. conorii infection indicates a requirement of lipid catabolism for bacterial survival and initiation of lipid droplet modulation. A significant increase in host lipid droplets during infection has a negative impact on R. conorii survival in THP-1 macrophages.

Isolation of Rickettsia rickettsii in Rocky Mountain Spotted Fever Outbreak, Panama

We report new cases of Rocky Mountain spotted fever in patients from Kinkantu, Ngäbe-Bugle indigenous comarca, Panama. We isolated Rickettsia rickettsii in cell culture after intraperitoneal inoculation of guinea pigs with tissues from a deceased patient. Our results indicate that Rocky Mountain spotted fever is emerging in this region.

Subversion of Host Innate Immunity by Rickettsia australis via a Modified Autophagic Response in Macrophages

We recently reported that the in vitro and in vivo survivals of Rickettsia australis are Atg5-dependent, in association with an inhibited level of anti-rickettsial cytokine, IL-1β. In the present study, we sought to investigate how R. australis interacts with host innate immunity via an Atg5-dependent autophagic response. We found that the serum levels of IFN-γ and G-CSF in R. australis-infected Atg5^flox/floxLyz-Cre mice were significantly less compared to Atg5^flox/flox mice, accompanied by significantly lower rickettsial loads in tissues with inflammatory cellular infiltrations including neutrophils. R. australis infection differentially regulated a significant number of genes in bone marrow-derived macrophages (BMMs) in an Atg5-depdent fashion as determined by RNA sequencing and Ingenuity Pathway Analysis, including genes in the molecular networks of IL-1 family cytokines and PI3K-Akt-mTOR. The secretion levels of inflammatory cytokines, such as IL-1α, IL-18, TNF-α, and IL-6, by R. australis-infected Atg5^flox/floxLyz-Cre BMMs were significantly greater compared to infected Atg5^flox/flox BMMs. Interestingly, R. australis significantly increased the levels of phosphorylated mTOR and P70S6K at a time when the autophagic response is induced. Rapamycin treatment nearly abolished the phosphorylated mTOR and P70S6K but did not promote significant autophagic flux during R. australis infection. These results highlight that R. australis modulates an Atg5-dependent autophagic response, which is not sensitive to regulation by mTORC1 signaling in macrophages. Overall, we demonstrate that R. australis counteracts host innate immunity including IL-1β-dependent inflammatory response to support the bacterial survival via an mTORC1-resistant autophagic response in macrophages.

Pathogenic, but Not Nonpathogenic, Rickettsia spp. Evade Inflammasome-Dependent IL-1 Responses To Establish an Intracytosolic Replication Niche

Rickettsia species (spp.) are strict obligate intracellular bacteria, some of which are pathogenic in their mammalian host, including humans. One critical feature of these stealthy group of pathogens is their ability to manipulate hostile cytosolic environments to their benefits. Although our understanding of Rickettsia cell biology and pathogenesis is evolving, the mechanisms by which pathogenic Rickettsia spp. evade host innate immune detection remain elusive. Here, we show that disease severity in wild-type (WT) C57BL/6J mice infected with Rickettsia typhi (the etiologic agent of murine typhus) and Rickettsia rickettsii (the etiologic agent of Rocky Mountain spotted fever), but not with the nonpathogenic species Rickettsia montanensis, correlated with levels of bacterial burden as detected in the spleens of mice, as well as the serum concentrations of proinflammatory cytokine interleukin-1α (IL-1α) and, to a lesser extent, IL-1β. Antibody-mediated neutralization of IL-1α confirmed a key role in controlling mortality rates and bacterial burdens of rickettsia-infected WT mice. As macrophages are a primary source of both IL-1α and IL-1β cytokines, we determined the mechanism of the antirickettsial activities using bone marrow-derived macrophages. We found that pathogenic R. typhi and R. rickettsii, but not nonpathogenic R. montanensis, eluded pro-IL-1α induction and benefited predominantly from the reduced IL-1α secretion, via a caspase-11-gasdermin D (Gsdmd)-dependent pathway, to facilitate intracytosolic replication. Adoptive transfer experiments identified that IL-1α secretion by macrophages was critical for controlling rickettsiosis in WT mice. In sum, we identified a previously unappreciated pathway by which pathogenic, unlike nonpathogenic, rickettsiae preferentially target the caspase-11-Gsdmd-IL-1α signaling axis in macrophages, thus supporting their replication within the host. IMPORTANCE Currently, no vaccines are available to prevent rickettsioses, while vector-borne rickettsial infections in humans are on the rise globally. In fact, the insufficient understanding of how pathogenic Rickettsia species circumvent host immune defense mechanisms has significantly hindered the development of more effective therapeutics. Here, we identified a previously unappreciated role for the caspase-11-Gsdmd-IL-1α signaling axis in limiting the replication of pathogenic R. rickettsia and R. typhi species in murine macrophages and wild-type (WT) C57BL/6J mice. Adoptive transfer studies further identified IL-1α-secreting macrophages as critical mediators in controlling rickettsial infection in WT mice. Collectively, these findings provide insight into the potential mechanism of how pathogenic, but not nonpathogenic, Rickettsia spp. benefit from a reduction in the caspase-11-Gsdmd-mediated release of IL-1α to support host colonization.

Significant Growth by Rickettsia Species within Human Macrophage-Like Cells Is a Phenotype Correlated with the Ability to Cause Disease in Mammals

Rickettsia are significant sources of tick-borne diseases in humans worldwide. In North America, two species in the spotted fever group of Rickettsia have been conclusively associated with disease of humans: Rickettsia rickettsii, the causative agent of Rocky Mountain spotted fever, and Rickettsia parkeri, the cause of R. parkeri rickettsiosis. Previous work in our lab demonstrated non-endothelial parasitism by another pathogenic SFG Rickettsia species, Rickettsia conorii, within THP-1-derived macrophages, and we have hypothesized that this growth characteristic may be an underappreciated aspect of rickettsial pathogenesis in mammalian hosts. In this work, we demonstrated that multiple other recognized human pathogenic species of Rickettsia, including R. rickettsii, R. parkeri, Rickettsia africae, and Rickettsiaakari can grow within target endothelial cells as well as within PMA-differentiated THP-1 cells. In contrast, Rickettsia bellii, a Rickettsia species not associated with disease of humans, and R. rickettsii strain Iowa, an avirulent derivative of pathogenic R. rickettsii, could invade both cell types but proliferate only within endothelial cells. Further analysis revealed that similar to previous studies on R. conorii, other recognized pathogenic Rickettsia species could grow within the cytosol of THP-1-derived macrophages and avoided localization with two different markers of lysosomal compartments; LAMP-2 and cathepsin D. R. bellii, on the other hand, demonstrated significant co-localization with lysosomal compartments. Collectively, these findings suggest that the ability of pathogenic rickettsial species to establish a niche within macrophage-like cells could be an important factor in their ability to cause disease in mammals. These findings also suggest that analysis of growth within mammalian phagocytic cells may be useful to predict the pathogenic potential of newly isolated and identified Rickettsia species.

The intracellular bacterium Rickettsia rickettsii exerts an inhibitory effect on the apoptosis of tick cells

BACKGROUND

Rickettsia rickettsii is a tick-borne obligate intracellular bacterium that causes Rocky Mountain spotted fever, a life-threatening illness. To obtain an insight into the vector-pathogen interactions, we assessed the effects of infection with R. rickettsii on the proteome cells of the tick embryonic cell line BME26.

METHODS

The proteome of BME26 cells was determined by label-free high-performance liquid chromatography coupled with tandem mass spectrometry analysis. Also evaluated were the effects of infection on the activity of caspase-3, assessed by the hydrolysis of a synthetic fluorogenic substrate in enzymatic assays, and on the exposition of phosphatidyserine, evaluated by live-cell fluorescence microscopy after labeling with annexin-V. Finally, the effects of activation or inhibition of caspase-3 activity on the growth of R. rickettsii in BME26 cells was determined.

RESULTS

Tick proteins of different functional classes were modulated in a time-dependent manner by R. rickettsii infection. Regarding proteins involved in apoptosis, certain negative regulators were downregulated at the initial phase of the infection (6 h) but upregulated in the middle of the exponential phase of the bacterial growth (48 h). Microorganisms are known to be able to inhibit apoptosis of the host cell to ensure their survival and proliferation. We therefore evaluated the effects of infection on classic features of apoptotic cells and observed DNA fragmentation exclusively in noninfected cells. Moreover, both caspase-3 activity and phosphatidylserine exposition were lower in infected than in noninfected cells. Importantly, while the activation of caspase-3 exerted a detrimental effect on rickettsial proliferation, its inhibition increased bacterial growth.

CONCLUSIONS

Taken together, these results show that R. rickettsii modulates the proteome and exerts an inhibitory effect on apoptosis in tick cellsthat seems to be important to ensure cell colonization.

Comparative transcriptomic analysis of Rickettsia conorii during in vitro infection of human and tick host cells

BACKGROUND

Pathogenic Rickettsia species belonging to the spotted fever group are arthropod-borne, obligate intracellular bacteria which exhibit preferential tropism for host microvascular endothelium in the mammalian hosts, resulting in disease manifestations attributed primarily to endothelial damage or dysfunction. Although rickettsiae are known to undergo evolution through genomic reduction, the mechanisms by which these pathogens regulate their transcriptome to ensure survival in tick vectors and maintenance by transovarial/transstadial transmission, in contrast to their ability to cause debilitating infections in human hosts remain unknown. In this study, we compare the expression profiles of rickettsial sRNAome/transcriptome and determine the transcriptional start sites (TSSs) of R. conorii transcripts during in vitro infection of human and tick host cells.

RESULTS

We performed deep sequencing on total RNA from Amblyomma americanum AAE2 cells and human microvascular endothelial cells (HMECs) infected with R. conorii. Strand-specific RNA sequencing of R. conorii transcripts revealed the expression 32 small RNAs (Rc_sR's), which were preferentially expressed above the limit of detection during tick cell infection, and confirmed the expression of Rc_sR61, sR71, and sR74 by quantitative RT-PCR. Intriguingly, a total of 305 and 132 R. conorii coding genes were differentially upregulated (> 2-fold) in AAE2 cells and HMECs, respectively. Further, enrichment for primary transcripts by treatment with Terminator 5'-Phosphate-dependent Exonuclease resulted in the identification of 3903 and 2555 transcription start sites (TSSs), including 214 and 181 primary TSSs in R. conorii during the infection to tick and human host cells, respectively. Seventy-five coding genes exhibited different TSSs depending on the host environment. Finally, we also observed differential expression of 6S RNA during host-pathogen and vector-pathogen interactions in vitro, implicating an important role for this noncoding RNA in the regulation of rickettsial transcriptome depending on the supportive host niche.

CONCLUSIONS

In sum, the findings of this study authenticate the presence of novel Rc_sR's in R. conorii, reveal the first evidence for differential expression of coding transcripts and utilization of alternate transcriptional start sites depending on the host niche, and implicate a role for 6S RNA in the regulation of coding transcriptome during tripartite host-pathogen-vector interactions.

Optimized fixation of actin filaments for improved indirect immunofluorescence staining of rickettsiae

Objective

The objective was to investigate fixative solutions: 3.7% formaldehyde, 4% paraformaldehyde, 4% paraformaldehyde in the cytoskeletal buffer and 4% paraformaldehyde in PHEM buffer (containing PIPES, HEPES, EGTA and MgCl₂), applicable for immunofluorescence assay.

Results

Herein we optimized this serological technique, testing four fixative solutions, for the sensitive detection of rickettsial antigens, and preservation of intracellular structures of the host cells, particularly filamentous actin. Rickettsial antigens were presented equally well both with formaldehyde and all paraformaldehyde-based fixations, but only protocol with 4% paraformaldehyde in PHEM buffer allowed accurate imaging of actin filaments, and simultaneously allows monitoring of rickettsiae using actin-based motility during infection inside the host cells.

Diagnosis of spotted fever group Rickettsia infections: the Asian perspective

Spotted fever group rickettsiae (SFG) are a neglected group of bacteria, belonging to the genus Rickettsia, that represent a large number of new and emerging infectious diseases with a worldwide distribution. The diseases are zoonotic and are transmitted by arthropod vectors, mainly ticks, fleas and mites, to hosts such as wild animals. Domesticated animals and humans are accidental hosts. In Asia, local people in endemic areas as well as travellers to these regions are at high risk of infection. In this review we compare SFG molecular and serological diagnostic methods and discuss their limitations. While there is a large range of molecular diagnostics and serological assays, both approaches have limitations and a positive result is dependent on the timing of sample collection. There is an increasing need for less expensive and easy-to-use diagnostic tests. However, despite many tests being available, their lack of suitability for use in resource-limited regions is of concern, as many require technical expertise, expensive equipment and reagents. In addition, many existing diagnostic tests still require rigorous validation in the regions and populations where these tests may be used, in particular to establish coherent and worthwhile cut-offs. It is likely that the best strategy is to use a real-time quantitative polymerase chain reaction (qPCR) and immunofluorescence assay in tandem. If the specimen is collected early enough in the infection there will be no antibodies but there will be a greater chance of a PCR positive result. Conversely, when there are detectable antibodies it is less likely that there will be a positive PCR result. It is therefore extremely important that a complete medical history is provided especially the number of days of fever prior to sample collection. More effort is required to develop and validate SFG diagnostics and those of other rickettsial infections.

Whole-Genome Sequence of Rickettsia parkeri Strain Atlantic Rainforest, Isolated from a Colombian Tick

Rickettsia parkeri is classified as a member of the alphaproteobacterial microorganisms, genus Rickettsia Here, we report the complete genome sequence of Rickettsia parkeri strain Atlantic Rainforest, which was isolated from an Amblyomma ovale tick collected in the municipality of Necoclí, Colombia.

Approaches for the successful isolation and cell culture of American Rickettsia species

Rickettsia are intracellular vector-borne bacteria, which are the etiologic agent of severe infections that could inflict death to their host. The intracellular behaviour of Rickettsia makes the study of its genetics, proteomics and cellular processes very difficult. Hence, isolation remains an important experimental technique that permits the obtention of important yields of bacteria, useful for a broad range of experiments. Isolation of Rickettsia using passages in animals or embryonated eggs has been described for long time; however, it was until the 1990s that faster and more feasible approaches for cell culture were developed. Current isolation approaches are mainly based on shell vial culture, that varies according to the media, atmosphere or temperature conditions. These variations have allowed the establishment of isolates from different pathogenic and non-pathogenic Rickettsia species, using arthropod, animal or human samples. Purification method of bacteria has also witnessed changes alongside the quantification of its load in the resulting isolates, from the laborious and time consuming plaque assays, to the routinary use of real-time polymerase chain reaction (qPCR), which is faster and more accurate. This review discusses various approaches that have been used for the isolation and purification of different Rickettsia species along with the mention of some successful examples. It indicated that a successful strategy for the isolation of Rickettsia requires a careful selection of media, cell lines and culture conditions which now are not as time consuming as used to be.

Emerging and re-emerging rickettsial infections

Rickettsial organisms are a diverse group of obligate intracellular bacteria; all species known to cause human disease are dependent on an arthropod vector and many are considered zoonotic diseases. Typical vectors of rickettsia are fleas, ticks, mites or lice. Humans become infected either when bitten or upon contact of broken skin or mucous membranes by infected secretions from an arthropod vector. The emergence and re-emergence of rickettsial diseases is a serious public health concern in the United States and abroad. Herein, the clinical and pathologic features of rickettsial diseases are described in tandem with the current scientific underpinnings. The histopathology of emerging and re-emerging rickettsiosis with species-specific discussion relating to vector issues and control are explored. Concepts of endemicity are addressed in the context of climate change and its impact on vector and sylvatic reservoirs, underscoring the need for clinical vigilance and broad consideration for encounters with these potentially life threating human pathogens.

Genome Sequence of "Candidatus Rickettsia colombianensi," a Novel Tick-Associated Bacterium Distributed in Colombia

This is the first report of the genome sequence of “Candidatus Rickettsia colombianensi” strain Adcor 2, deposited in DDBJ/ENA/GenBank under the accession number RAQN00000000. The draft genome showed 36.01% similarity with that of Rickettsia monacensis strain IrR/Munich (NCBI accession number LN794217), 37.81% similarity with that of Rickettsia heilongjiangensis 054 (NCBI accession number CP002912), and 43.88% similarity with that of Rickettsia tamurae AT-1 (NCBI accession number CCMG01000001).

This is the first report of the genome sequence of “Candidatus Rickettsia colombianensi” strain Adcor 2, deposited in DDBJ/ENA/GenBank under the accession number RAQN00000000. The draft genome showed 36.01% similarity with that of Rickettsia monacensis strain IrR/Munich (NCBI accession number LN794217), 37.81% similarity with that of Rickettsia heilongjiangensis 054 (NCBI accession number CP002912), and 43.88% similarity with that of Rickettsia tamurae AT-1 (NCBI accession number CCMG01000001).

A Pathogen and a Non-pathogen Spotted Fever Group Rickettsia Trigger Differential Proteome Signatures in Macrophages

We have previously reported that Rickettsia conorii and Rickettsia montanensis have distinct intracellular fates within THP-1 macrophages, suggesting that the ability to proliferate within macrophages may be a distinguishable factor between pathogenic and non-pathogenic Spotted fever group (SFG) members. To start unraveling the molecular mechanisms underlying the capacity (or not) of SFG Rickettsia to establish their replicative niche in macrophages, we have herein used quantitative proteomics by SWATH-MS to profile the alterations resulted by the challenge of THP-1 macrophages with R. conorii and R. montanensis. We show that the pathogenic, R. conorii, and the non-pathogenic, R. montanensis, member of SFG Rickettsia trigger differential proteomic signatures in macrophage-like cells upon infection. R. conorii specifically induced the accumulation of several enzymes of the tricarboxylic acid cycle, oxidative phosphorylation, fatty acid β-oxidation, and glutaminolysis, as well as of several inner and outer membrane mitochondrial transporters. These results suggest a profound metabolic rewriting of macrophages by R. conorii toward a metabolic signature of an M2-like, anti-inflammatory activation program. Moreover, several subunits forming the proteasome and immunoproteasome are found in lower abundance upon infection with both rickettsial species, which may help bacteria to escape immune surveillance. R. conorii-infection specifically induced the accumulation of several host proteins implicated in protein processing and quality control in ER, suggesting that this pathogenic Rickettsia may be able to increase the ER protein folding capacity. This work reveals novel aspects of macrophage-Rickettsia interactions, expanding our knowledge of how pathogenic rickettsiae explore host cells to their advantage.

Amblyomma sculptum Salivary PGE2 Modulates the Dendritic Cell-Rickettsia rickettsii Interactions in vitro and in vivo

Amblyomma sculptum is an important vector of Rickettsia rickettsii, causative agent of Rocky Mountain spotted fever and the most lethal tick-borne pathogen affecting humans. To feed on the vertebrate host's blood, A. sculptum secretes a salivary mixture, which may interact with skin resident dendritic cells (DCs) and modulate their function. The present work was aimed at depicting the A. sculptum saliva-host DC network and the biochemical nature of the immunomodulatory component(s) involved in this interface. A. sculptum saliva inhibits the production of inflammatory cytokines by murine DCs stimulated with LPS. The fractionation of the low molecular weight salivary content by reversed-phase chromatography revealed active fractions eluting from 49 to 55% of the acetonitrile gradient. Previous studies suggested that this pattern of elution matches with that observed for prostaglandin E₂ (PGE₂) and the molecular identity of this lipid mediator was unambiguously confirmed by a new high-resolution mass spectrometry methodology. A productive infection of murine DCs by R. rickettsii was demonstrated for the first time leading to proinflammatory cytokine production that was inhibited by both A. sculptum saliva and PGE₂, a result also achieved with human DCs. The adoptive transfer of murine DCs incubated with R. rickettsii followed by treatment with A. sculptum saliva or PGE₂ did not change the cytokine profile associated to cellular recall responses while IgG2a-specific antibodies were decreased in the serum of these mice. Together, these findings emphasize the role of PGE₂ as a universal immunomodulator of tick saliva. In addition, it contributes to new approaches to explore R. rickettsii-DC interactions both in vitro and in vivo.

Rickettsia rickettsii Whole-Cell Antigens Offer Protection against Rocky Mountain Spotted Fever in the Canine Host

Rocky Mountain spotted fever (RMSF) is a potentially fatal tick-borne disease in people and dogs. RMSF is reported in the United States and several countries in North, Central, and South America.

Rocky Mountain spotted fever (RMSF) is a potentially fatal tick-borne disease in people and dogs. RMSF is reported in the United States and several countries in North, Central, and South America. The causative agent of this disease, Rickettsia rickettsii, is transmitted by several species of ticks, including Dermacentor andersoni, Rhipicephalus sanguineus, and Amblyomma americanum. RMSF clinical signs generally include fever, headache, nausea, vomiting, muscle pain, lack of appetite, and rash. If untreated, it can quickly progress into a life-threatening illness in people and dogs, with high fatality rates ranging from 30 to 80%. While RMSF has been known for over a century, recent epidemiological data suggest that the numbers of documented cases and the fatality rates remain high in people, particularly during the last two decades in parts of North America. Currently, there are no vaccines available to prevent RMSF in either dogs or people. In this study, we investigated the efficacies of two experimental vaccines, a subunit vaccine containing two recombinant outer membrane proteins as recombinant antigens (RCA) and a whole-cell inactivated antigen vaccine (WCA), in conferring protection against virulent R. rickettsii infection challenge in a newly established canine model for RMSF. Dogs vaccinated with WCA were protected from RMSF, whereas those receiving RCA developed disease similar to that of nonvaccinated R. rickettsii-infected dogs. WCA also reduced the pathogen loads to nearly undetected levels in the blood, lungs, liver, spleen, and brain and induced bacterial antigen-specific immune responses. This study provides the first evidence of the protective ability of WCA against RMSF in dogs.

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

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

Vector Tick Transmission Model of Spotted Fever Rickettsiosis

Many aspects of rickettsial infections have been characterized, including pathogenic and immune pathways and mechanisms of rickettsial survival within the vertebrate host and tick vector. However, very few studies are focused on the complex pathogen-vector-host interactions during tick feeding. Therefore, our objective was to develop a tick transmission model of the spotted fever group of rickettsial infections to study the initial events in disease development. The most appropriate strain of mouse was identified for evaluation as a transmission model, and the course of infection, bacterial levels, histopathologic changes, and antibody response during tick transmission in mice infested with Amblyomma maculatum ticks carrying the emerging pathogen, Rickettia parkeri, were studied. Results showed distinct clinical signs in C3H/HeN mice infected intravenously, leading to selection of this mouse strain for tick transmission studies. Active infection of animals was observed after tick vector transmission. The bacteria disseminated systemically and spread to several organs at 24 hours after tick attachment, with peak bacterial load at day 6 after tick attachment. Skin, lung, and liver showed the greatest pathologic changes, with inflammatory cellular infiltration and necrosis. These findings indicate the feasibility of using murine infection with R. parkeri by A. maculatum tick transmission as a model to study different aspects of the spotted fever group of rickettsial disease establishment.

Th1 epitope peptides induce protective immunity against Rickettsia rickettsii infection in C3H/HeN mice

Rickettsia rickettsii is the causative pathogen of Rocky Mountain spotted fever (RMSF). Adr2, YbgF and OmpB are protective antigens of R. rickettsii. In this study, 90 candidate peptides were selected from these antigens based on their high-affinity binding capacity for the MHC class II molecule H2 I-A or H2 I-E using bioinformatic methods. Six peptides were determined using ELISPOT assay to be immunodominant based on the IFN-γ recall responses of CD4⁺ T cells from mice immunized with R. rickettsii. Six nucleotide sequences encoding the immunodominant peptides were linked in series and inserted into a plasmid for expression in Escherichia coli cells, resulting in a new, recombinant polypeptide termed GWP. After immunization and challenge, the rickettsial load or histopathological lesions in the organs of mice immunized with GWP or pooled peptides was significantly lower than that in organs of mice immunized with PBS or the individual peptide OmpB₃₉₉. An in vitro neutralization test revealed that sera from mice immunized with GWP, OmpB₃₉₉, or pooled peptides reduced R. rickettsii adherence to, and invasion of, vascular endothelial cells. Furthermore, significantly higher levels of IgG, IgG1, or IgG2a were detected in sera from mice immunized with GWP or pooled peptides, and significantly higher levels of IFN-γ or TNF-α secreted by CD4⁺ T cells from R. rickettsii-infected mice were detected after immunization with GWP. Altogether, our results indicated that polypeptides, especially GWP, could induce a Th1-type immune response against R. rickettsii infection, which might contribute to the rational design of peptide-based vaccines for RMSF.

Evaluation of changes to the Rickettsia rickettsii transcriptome during mammalian infection

The lifecycle of Rickettsia rickettsii includes infection of both mammalian and arthropod hosts, with each environment presenting distinct challenges to survival. As such, these pathogens likely have distinctive transcriptional strategies for infection of each host. Herein, we report the utilization of next generation sequencing (RNAseq) and bioinformatic analysis techniques to examine the global transcriptional profile of R. rickettsii within an infected animal, and to compare that data to transcription in tissue culture. The results demonstrate substantial R. rickettsii transcriptional alteration in vivo, such that the bacteria are considerably altered from cell culture. Identification of significant transcriptional changes and validation of RNAseq by quantitative PCR are described with particular emphasis on known antigens and suspected virulence factors. Together, these results suggest that transcriptional regulation of a distinct cohort of genes may contribute to successful mammalian infection.

Comparative vertical transmission of Rickettsia by Dermacentor variabilis and Amblyomma maculatum

The geographical overlap of multiple Rickettsia and tick species coincides with the molecular detection of a variety of rickettsial agents in what may be novel tick hosts. However, little is known concerning transmissibility of rickettsial species by various tick hosts. To examine the vertical transmission potential between select tick and rickettsial species, two sympatric species of ticks, Dermacentor variabilis and Amblyomma maculatum, were exposed to five different rickettsial species, including Rickettsia rickettsii, Rickettsia parkeri, Rickettsia montanensis, Rickettsia amblyommatis, or flea-borne Rickettsia felis. Fitness-related metrics including engorgement weight, egg production index, nutrient index, and egg hatch percentage were then assessed. Subsamples of egg clutches and unfed larvae, nymphs, and adults for each cohort were assessed for transovarial and transstadial transmission of rickettsiae by qPCR. Rickettsial exposure had a minimal fitness effect in D. variabilis and transovarial transmission was observed for all groups except R. rickettsii. In contrast, rickettsial exposure negatively influenced A. maculatum fitness and transovarial transmission of rickettsiae was demonstrated only for R. amblyommatis- and R. parkeri-exposed ticks. Sustained maintenance of rickettsiae via transstadial transmission was diminished from F1 larvae to F1 adults in both tick species. The findings of this study suggest transovarial transmission specificity may not be tick species dependent, and sustained vertical transmission is not common.

GFPuv-Expressing Recombinant Rickettsia typhi: a Useful Tool for the Study of Pathogenesis and CD8+ T Cell Immunology in R. typhi Infection

Rickettsia typhi is the causative agent of endemic typhus, a disease with increasing incidence worldwide that can be fatal. Because of its obligate intracellular life style, genetic manipulation of the pathogen is difficult. Nonetheless, in recent years, genetic manipulation tools have been successfully applied to rickettsiae. We describe here for the first time the transformation of R. typhi with the pRAM18dRGA plasmid that originally derives from Rickettsia amblyommatis and encodes the expression of GFPuv (green fluorescent protein with maximal fluorescence when excited by UV light). Transformed R. typhi (R. typhi^GFPuv) bacteria are viable, replicate with kinetics similar to those of wild-type R. typhi in cell culture, and stably maintain the plasmid and GFPuv expression under antibiotic treatment in vitro and in vivo during infection of mice. CB17 SCID mice infected with R. typhi^GFPuv succumb to the infection with kinetics similar to those for animals infected with wild-type R. typhi and develop comparable pathology and bacterial loads in the organs, demonstrating that the plasmid does not influence pathogenicity. In the spleen and liver of infected CB17 SCID mice, the bacteria are detectable by immunofluorescence microscopy in neutrophils and macrophages by histological staining. Finally, we show for the first time that transformed rickettsiae can be used for the detection of CD8⁺ T cell responses. GFP-specific restimulation of spleen cells from R. typhi^GFPuv-infected BALB/c mice elicits gamma interferon (IFN-γ), tumor necrosis factor alpha (TNF-α), and interleukin 2 (IL-2) secretion by CD8⁺ T cells. Thus, R. typhi^GFPuv bacteria are a novel, potent tool to study infection with the pathogen in vitro and in vivo and the immune response to these bacteria.

Novel Rickettsia raoultii strain isolated and propagated from Austrian Dermacentor reticulatus ticks

Background

Continuous culture of tick cell lines has proven a valuable asset in isolating and propagating several different vector-borne pathogens, making it possible to study these microorganisms under laboratory conditions and develop serological tests to benefit public health. We describe a method for effective, cost- and labor-efficient isolation and propagation of Rickettsia raoultii using generally available laboratory equipment and Rhipicephalus microplus cells, further demonstrating the usefulness of continuous tick cell lines. R. raoultii is one of the causative agents of tick-borne lymphadenopathy (TIBOLA) and is, together with its vector Dermacentor reticulatus, emerging in novel regions of Europe, giving rise to an increased threat to general public health.

Methods

Dermacentor reticulatus ticks were collected in the Donau-Auen (Lobau) national park in Vienna, Austria. The hemolymph of ten collected ticks was screened by PCR-reverse line blot for the presence of rickettsial DNA. A single tick tested positive for R. raoultii DNA and was used to infect Rhipicephalus microplus BME/CTVM2 cells.

Results

Sixty-five days after infection of the tick-cell line with an extract from a R. raoultii-infected tick, we observed intracellular bacteria in the cultured cells. On the basis of microscopy we suspected that the intracellular bacteria were a species of Rickettsia; this was confirmed by several PCRs targeting different genes. Subsequent sequencing showed 99–100 % identity with R. raoultii. Cryopreservation and resuscitation of R. raoultii was successful. After 28 days identical intracellular bacteria were microscopically observed.

Conclusions

R. raoultii was successfully isolated and propagated from D. reticulatus ticks using R. microplus BME/CTVM2 cells. The isolated strain shows significant molecular variation compared to currently known sequences. Furthermore we show for the first time the successful cryopreservation and resuscitation of R. raoultii.

Comparative growth of spotted fever group Rickettsia spp. strains in Vero cells

In Brazil, the spotted fever group (SFG) Rickettsia rickettsii and Rickettsia parkeri related species are the etiological agents of spotted fever rickettsiosis. However, the SFG, Rickettsia rhipicephali, that infects humans, has never been reported. The study of growth dynamics can be useful for understanding the infective and invasive capacity of these pathogens. Here, the growth rates of the Brazilian isolates R. rickettsii str. Taiaçu, R. parkeri str. At#24, and R. rhipicephali HJ#5, were evaluated in Vero cells by quantitative polymerase chain reaction. R. rhipicephali showed different kinetic growth compared to R. rickettsii and R. parkeri.

Differences in Intracellular Fate of Two Spotted Fever Group Rickettsia in Macrophage-Like Cells

Spotted fever group (SFG) rickettsiae are recognized as important agents of human tick-borne diseases worldwide, such as Mediterranean spotted fever (Rickettsia conorii) and Rocky Mountain spotted fever (Rickettsia rickettsii). Recent studies in several animal models have provided evidence of non-endothelial parasitism by pathogenic SFG Rickettsia species, suggesting that the interaction of rickettsiae with cells other than the endothelium may play an important role in pathogenesis of rickettsial diseases. These studies raise the hypothesis that the role of macrophages in rickettsial pathogenesis may have been underappreciated. Herein, we evaluated the ability of two SFG rickettsial species, R. conorii (a recognized human pathogen) and Rickettsia montanensis (a non-virulent member of SFG) to proliferate in THP-1 macrophage-like cells, or within non-phagocytic cell lines. Our results demonstrate that R. conorii was able to survive and proliferate in both phagocytic and epithelial cells in vitro. In contrast, R. montanensis was able to grow in non-phagocytic cells, but was drastically compromised in the ability to proliferate within both undifferentiated and PMA-differentiated THP-1 cells. Interestingly, association assays revealed that R. montanensis was defective in binding to THP-1-derived macrophages; however, the invasion of the bacteria that are able to adhere did not appear to be affected. We have also demonstrated that R. montanensis which entered into THP-1-derived macrophages were rapidly destroyed and partially co-localized with LAMP-2 and cathepsin D, two markers of lysosomal compartments. In contrast, R. conorii was present as intact bacteria and free in the cytoplasm in both cell types. These findings suggest that a phenotypic difference between a non-pathogenic and a pathogenic SFG member lies in their respective ability to proliferate in macrophage-like cells, and may provide an explanation as to why certain SFG rickettsial species are not associated with disease in mammals.

Human Infections by Multiple Spotted Fever Group Rickettsiae in Tennessee

Rocky Mountain spotted fever is the most common tick-borne disease in Tennessee. However, Rickettsia rickettsii has rarely been isolated from endemic ticks, suggesting rickettsioses may be caused by other species. A total of 56 human serum samples that were serologically positive for exposure to Rickettsia were obtained from commercial laboratories in 2010 and 2011. In addition, 20 paired sera from patients with encephalitis and positive Rickettsia serology were obtained from the Tennessee Unexplained Encephalitis Surveillance (TUES) study. Using an immunofluorescence assay, reactivity of the sera to R. rickettsii, Rickettsia montanensis, Rickettsia parkeri, and Rickettsia amblyommii was tested, and a comparison of endpoint titers was used to determine the probable antigen that stimulated the antibody response. Cross-absorption was conducted for 94.8% (N = 91) of the samples due to serologic cross-reactivity. Of the commercial laboratory samples, 55.4% (N = 31) had specific reactivity to R. amblyommii and 44.6% (N = 25) were indeterminate. Of the paired TUES samples, 20% (N = 4) had specific reactivity to R. amblyommii, 5% (N = 1) to R. montanensis, and 5% (N = 1) to R. parkeri Patients with specific reactivity to R. amblyommii experienced fever (75%), headache (68%) and myalgia (58%). Rash (36%) and thrombocytopenia (40%) were less common. To our knowledge, this is the first time R. amblyommii has been reported as a possible causative agent of rickettsioses in Tennessee.

Nonselective Persistence of a Rickettsia conorii Extrachromosomal Plasmid during Mammalian Infection

Scientific analysis of the genus Rickettsia is undergoing a rapid period of change with the emergence of viable genetic tools. The development of these tools for the mutagenesis of pathogenic bacteria will permit forward genetic analysis of Rickettsia pathogenesis. Despite these advances, uncertainty still remains regarding the use of plasmids to study these bacteria in in vivo mammalian models of infection, namely, the potential for virulence changes associated with the presence of extrachromosomal DNA and nonselective persistence of plasmids in mammalian models of infection. Here, we describe the transformation of Rickettsia conorii Malish 7 with the plasmid pRam18dRGA[AmTrCh]. Transformed R. conorii stably maintains this plasmid in infected cell cultures, expresses the encoded fluorescent proteins, and exhibits growth kinetics in cell culture similar to those of nontransformed R. conorii. Using a well-established murine model of fatal Mediterranean spotted fever, we demonstrate that R. conorii(pRam18dRGA[AmTrCh]) elicits the same fatal outcomes in animals as its untransformed counterpart and, importantly, maintains the plasmid throughout infection in the absence of selective antibiotic pressure. Interestingly, plasmid-transformed R. conorii was readily observed both in endothelial cells and within circulating leukocytes. Together, our data demonstrate that the presence of an extrachromosomal DNA element in a pathogenic rickettsial species does not affect either in vitro proliferation or in vivo infectivity in models of disease and that plasmids such as pRam18dRGA[AmTrCh] are valuable tools for the further genetic manipulation of pathogenic rickettsiae.

Rickettsia rickettsii outer membrane protein YbgF induces protective immunity in C3H/HeN mice

Rickettsia rickettsii is the etiological agent of Rocky Mountain spotted fever (RMSF). YbgF and TolC are outer membrane-associated proteins of R. rickettsii that play important roles in its interaction with host cells. We investigated the immunogenicity of YbgF and TolC for protection against RMSF. We immunized C3H/HeN mice with recombinant R. rickettsii YbgF (rYbgF) or TolC (rTolC). Rickettsial burden and impairment in the lungs, spleens, and livers of rYbgF-immunized mice were significantly lower than in rTolC-immunized mice. The ratio of IgG2a to IgG1 in rYbgF-immunized mice continued to increase over the course of our experiments, while that in rTolC-immunized mice was reduced. The proliferation and cytokine secretion of CD4(+) and CD8(+) T cells isolated from R. rickettsii-infected mice were analyzed following antigen stimulation. The results indicated that proliferation and interferon (IFN)-γ secretion of CD4(+) or CD8(+) T cells in R. rickettsii-infected mice were significantly greater than in uninfected mice after stimulation with rYbgF. YbgF is a novel protective antigen of R. rickettsii. Protection conferred by YbgF is dependent upon IFN-γ-producing CD4(+) and CD8(+) T cells and IgG2a, which act in synergy to control R. rickettsii infection.

Electrotransformation and Clonal Isolation of Rickettsia Species

Genetic manipulation of obligate intracellular bacteria of the genus Rickettsia is currently undergoing a rapid period of change. The development of viable genetic tools, including replicative plasmids, transposons, homologous recombination, fluorescent protein-encoding genes, and antibiotic selectable markers has provided the impetus for future research development. This unit is designed to coalesce the basic methods pertaining to creation of genetically modified Rickettsia. The unit describes a series of methods, from inserting exogenous DNA into Rickettsia to the final isolation of genetically modified bacterial clones. Researchers working towards genetic manipulation of Rickettsia or similar obligate intracellular bacteria will find these protocols to be a valuable reference.

Survival of rat cerebrocortical neurons after rickettsial infection

Neuroinvasive microorganisms are suspected to play an important role in the etiopathogenesis of neurological diseases. However, direct evidence for the pathogenic function is still missing. The main aim of this study was to investigate biochemical and morphological changes that may occur as a result of an in vitro infection of rat cerebrocortical neurons by selected members of the genus Rickettsia. Our results showed that survival of the neurons is significantly reduced after the infection. Intracellular level of ATP is gradually decreased and inversely correlates with the load of rickettsiae. Immunofluorescence revealed that rickettsiae can enter the neurons and are localized in perinuclear space and also in neuronal processes. Data obtained in this study correspond to the idea of possible involvement of rickettsiae in the etiopathogenesis of various neuropathies.

Improved Quantification, Propagation, Purification and Storage of the Obligate Intracellular Human Pathogen Orientia tsutsugamushi

Background

Scrub typhus is a leading cause of serious febrile illness in rural Southeast Asia. The causative agent, Orientia tsutsugamushi, is an obligate intracellular bacterium that is transmitted to humans by the bite of a Leptotrombidium mite. Research into the basic mechanisms of cell biology and pathogenicity of O. tsutsugamushi has lagged behind that of other important human pathogens. One reason for this is that O. tsutsugamushi is an obligate intracellular bacterium that can only be cultured in mammalian cells and that requires specific methodologies for propagation and analysis. Here, we have performed a body of work designed to improve methods for quantification, propagation, purification and long-term storage of this important but neglected human pathogen. These results will be useful to other researchers working on O. tsutsugamushi and also other obligate intracellular pathogens such as those in the Rickettsiales and Chlamydiales families.

Methodology

A clinical isolate of O. tsutsugamushi was grown in cultured mouse embryonic fibroblast (L929) cells. Bacterial growth was measured using an O. tsutsugamushi-specific qPCR assay. Conditions leading to improvements in viability and growth were monitored in terms of the effect on bacterial cell number after growth in cultured mammalian cells.

Key results

Conclusions

Here we present a standardised method for comparing the viability of O. tsutsugamushi after purification, treatment and propagation under various conditions. Taken together, we present a body of data to support improved techniques for propagation, purification and storage of this organism. This data will be useful both for improving clinical isolation rates as well as performing in vitro cell biology experiments.

Scrub typhus is a serious, neglected tropical disease that is endemic in large parts of Asia and northern Australia. It is caused by the bacterium O. tsutsugamushi, which is maintained in Leptotrombiculid mites, small arthropods that occasionally bite humans and transmit the disease. O. tsutsugamushi is an obligate intracellular bacterium, which means that it can only survive and grow when it is physically enclosed within a cell, both when it is living in its vector mite, and when it is living in the human or other mammalian host. This makes it difficult to work with in the laboratory, as it needs to be cultured together with host cells. This technical difficulty is one reason why our understanding of this human pathogen is less well-developed than for many other pathogens of equivalent incidence and severity. Here, we have performed a body of work that was designed to measure and improve methods for growing these bacteria in the laboratory, purifying the bacteria from their host cells without damaging them, and preserving bacteria for long periods of time by cryopreservation. This work will support future efforts to understand the basic science behind this and similar intracellular human pathogens.