Protective and Pathogenic Roles of CD8+ T Lymphocytes in Murine Orientia tsutsugamushi Infection

Orientia tsutsugamushi Ank5 promotes NLRC5 cytoplasmic retention and degradation to inhibit MHC class I expression

How intracellular bacteria subvert the major histocompatibility complex (MHC) class I pathway is poorly understood. Here, we show that the obligate intracellular bacterium Orientia tsutsugamushi uses its effector protein, Ank5, to inhibit nuclear translocation of the MHC class I gene transactivator, NLRC5, and orchestrate its proteasomal degradation. Ank5 uses a tyrosine in its fourth ankyrin repeat to bind the NLRC5 N-terminus while its F-box directs host SCF complex ubiquitination of NLRC5 in the leucine-rich repeat region that dictates susceptibility to Orientia- and Ank5-mediated degradation. The ability of O. tsutsugamushi strains to degrade NLRC5 correlates with ank5 genomic carriage. Ectopically expressed Ank5 that can bind but not degrade NLRC5 protects the transactivator during Orientia infection. Thus, Ank5 is an immunoevasin that uses its bipartite architecture to rid host cells of NLRC5 and reduce surface MHC class I molecules. This study offers insight into how intracellular pathogens can impair MHC class I expression.

Lack of the IFN-γ signal leads to lethal Orientia tsutsugamushi infection in mice with skin eschar lesions

Scrub typhus is an acute febrile disease due to Orientia tsutsugamushi (Ot) infection and can be life-threatening with organ failure, hemorrhage, and fatality. Yet, little is known as to how the host reacts to Ot bacteria at early stages of infection; no reports have addressed the functional roles of type I versus type II interferon (IFN) responses in scrub typhus. In this study, we used comprehensive intradermal (i.d.) inoculation models and two clinically predominant Ot strains (Karp and Gilliam) to uncover early immune events. Karp infection induced sequential expression of Ifnb and Ifng in inflamed skin and draining lymph nodes at days 1 and 3 post-infection. Using double Ifnar1-/-Ifngr1-/- and Stat1-/- mice, we found that deficiency in IFN/STAT1 signaling resulted in lethal infection with profound pathology and skin eschar lesions, which resembled to human scrub typhus. Further analyses demonstrated that deficiency in IFN-γ, but not IFN-I, resulted in impaired NK cell and macrophage activation and uncontrolled bacterial growth and dissemination, leading to metabolic dysregulation, excessive inflammatory cell infiltration, and exacerbated tissue damage. NK cells were found to be the major cellular source of innate IFN-γ, contributing to the initial Ot control in the draining lymph nodes. In vitro studies with dendritic cell cultures revealed a superior antibacterial effect offered by IFN-γ than IFN-β. Comparative in vivo studies with Karp- and Gilliam-infection revealed a crucial role of IFN-γ signaling in protection against progression of eschar lesions and Ot infection lethality. Additionally, our i.d. mouse models of lethal infection with eschar lesions are promising tools for immunological study and vaccine development for scrub typhus.

Intranasal Immunization With Nanoparticles Containing an Orientia tsutsugamushi Protein Vaccine Candidate and a Polysorbitol Transporter Adjuvant Enhances Both Humoral and Cellular Immune Responses

Scrub typhus, a mite-borne infectious disease, is caused by Orientia tsutsugamushi. Despite many attempts to develop a protective strategy, an effective preventive vaccine has not been developed. The identification of appropriate Ags that cover diverse antigenic strains and provide long-lasting immunity is a fundamental challenge in the development of a scrub typhus vaccine. We investigated whether this limitation could be overcome by harnessing the nanoparticle-forming polysorbitol transporter (PST) for an O. tsutsugamushi vaccine strategy. Two target proteins, 56-kDa type-specific Ag (TSA56) and surface cell Ag A (ScaA) were used as vaccine candidates. PST formed stable nano-size complexes with TSA56 (TSA56-PST) and ScaA (ScaA-PST); neither exhibited cytotoxicity. The formation of Ag-specific IgG2a, IgG2b, and IgA in mice was enhanced by intranasal vaccination with TSA56-PST or ScaA-PST. The vaccines containing PST induced Ag-specific proliferation of CD8+ and CD4+ T cells. Furthermore, the vaccines containing PST improved the mouse survival against O. tsutsugamushi infection. Collectively, the present study indicated that PST could enhance both Ag-specific humoral immunity and T cell response, which are essential to effectively confer protective immunity against O. tsutsugamushi infection. These findings suggest that PST has potential for use in an intranasal vaccination strategy.

Orientia tsutsugamushi: A life between escapes

The life cycle of the mite-borne, obligate intracellular pathogen Orientia tsutsugamushi (Ot), the causative agent of human scrub typhus, differs in many aspects from that of other members of the Rickettsiales order. Particularly, the nonlytic cellular exit of individual Ot bacteria at the plasma membrane closely resembles the budding of enveloped viruses but has only been rudimentarily studied at the molecular level. This brief article is focused on the current state of knowledge of escape events in the life cycle of Ot and highlights differences in strategies of other rickettsiae.

A whole blood intracellular cytokine assay optimised for field site studies demonstrates polyfunctionality of CD4+ T cells in acute scrub typhus

BACKGROUND

Assessment of cellular immune responses by combining intracellular cytokine staining and immunophenotyping using flow cytometry enables the simultaneous measurement of T cell phenotype and effector function in response to pathogens and vaccines. The use of whole blood samples rather than peripheral blood mononuclear cells avoids both the need for immediate processing and loss of functional antigen presenting cells due to processing and cryopreservation. Using whole blood provides the possibility to stimulate peripheral T cells in situ, and is more suitable for studies where sample volume is limited, such as those involving children, the elderly and critically ill patients. The aim of this study was to provide a robust tool for the assessment of antigen-specific T cell responses in a field site setting with limited resources.

METHODOLOGY/PRINCIPLE FINDINGS

We optimised a flow cytometry-based whole blood intracellular cytokine assay (WBA) with respect to duration of antigen stimulation and intracellular protein retention time. We demonstrate the ability of the WBA to capture polyfunctional T cell responses in the context of acute scrub typhus infection, by measuring IFN-γ, TNF and IL-2 in CD4+ and CD8+ T cells in response to the causative agent O. tsutsugamushi (OT). Using an optimised OT antigen preparation, we demonstrate the presence of polyfunctional antigen-specific memory CD4+ T cells in the blood of scrub typhus patients.

CONCLUSIONS/SIGNIFICANCE

In conclusion, this flow cytometry-based WBA is well-suited for use at field study sites, and enables the assessment of polyfunctional T cell responses to infectious agents and vaccines through delineation of antigen-specific cytokine secretion at the single cell level.

A scrub typhus vaccine presents a challenging unmet need

Scrub typhus caused by the obligately intracellular bacterium, Orientia tsutsugamushi, is a major cause of life-threatening acute undifferentiated febrile illness in eastern Asia and the islands of the Western Pacific and Indian oceans. Since the estimation of an incidence of 1 million cases annually two decades ago, the number of cases has increased substantially in endemic regions, reappeared where the disease was forgotten, and spread northward. Trombiculid mites are both reservoir and vector. Despite 80 years of efforts to develop a vaccine, there is none. Protective immunity is mediated by antibodies and CD8 and CD4 T cells. Previous efforts have failed because of gaps in understanding immunity to O. tsutsugamushi, particularly the requirements for vaccine-induced immunity, lack of knowledge regarding immune memory in scrub typhus, and lack of attention to addressing the issue of cross-protection between strains. There are numerous strains of O. tsutsugamushi, and modestly durable immunity is strain-specific. Antibodies to the strain that caused infection are protective against challenges with the homologous strain but, despite reactivity with other immunodominant antigens, the immune serum does not protect against heterologous strains. Among the antigens detected by western immunoblot in immune sera (22-, 47-, 56-, 58-, and 110 kDa proteins), only the 56 kDa protein stimulates strong protection. This protein contains four hypervariable regions which are likely, on the basis of limited data, to be the targets of neutralizing antibodies. However, a method that definitively detects neutralizing antibody has yet to be developed. Only one study has used genomic data to pursue the discovery of protective antigens. Three conserved autotransporters were identified, and only immunization with ScaA provided protection against the homologous strain, but only 40% of animals were protected against challenge with a heterologous strain. A multiplex vaccine containing conformational antigens of the hypervariable regions of the 56 kDa protein of the strains of the greatest clinical and epidemiological importance, as well as conserved regions of the 56 kDa protein, ScaA, and other protective antigens identified by future genomic and bioinformatics methods should be developed and tested.

Host transcriptomic profiling of CD-1 outbred mice with severe clinical outcomes following infection with Orientia tsutsugamushi

Orientia tsutsugamushi is an obligately intracellular bacterium with endothelial tropism and can cause mild to lethal scrub typhus in humans. No vaccine is available for this reemerging and severely neglected infection. Previous scrub typhus studies have utilized inbred mice, yet such models have intrinsic limitations. Thus, the development of suitable mouse models that better mimic human diseases is in great need for immunologic investigation and future vaccine studies. This study is aimed at establishing scrub typhus in outbred CD-1 mice and defining immune biomarkers related to disease severity. CD-1 mice received O. tsutsugamushi Karp strain via the i.v. route; major organs were harvested at 2-12 days post-infection for kinetic analyses. We found that for our given infection doses, CD-1 mice were significantly more susceptible (90-100% lethal) than were inbred C57BL/6 mice (0-10% lethal). Gross pathology of infected CD-1 mouse organs revealed features that mimicked human scrub typhus, including pulmonary edema, interstitial pneumonia, perivascular lymphocytic infiltrates, and vasculitis. Alteration in angiopoietin/receptor expression in inflamed lungs implied endothelial dysfunction. Lung immune gene profiling using NanoString analysis displayed a Th1/CD8-skewed, but Th2 repressed profile, including novel biomarkers not previously investigated in other scrub typhus models. Bio-plex analysis revealed a robust inflammatory response in CD-1 mice as evidenced by increased serum cytokine and chemokine levels, correlating with immune cell recruitment during the severe stages of the disease. This study provides an important framework indicating a value of CD-1 mice for delineating host susceptibility to O. tsutsugamushi, immune dysregulation, and disease pathogenesis. This preclinical model is particularly useful for future translational and vaccine studies for severe scrub typhus.

Distinct Role of TNFR1 and TNFR2 in Protective Immunity Against Orientia tsutsugamushi Infection in Mice

Infection with Orientia tsutsugamushi, an obligate intracellular bacterium, can cause mild or severe scrub typhus. Some patients develop acute lung injury, multi-organ failure, and fatal infection; however, little is known regarding key immune mediators that mediate infection control or disease pathogenesis. Using murine models of scrub typhus, we demonstrated in this study the requirement of TNF-TNFR signaling in protective immunity against this infection. Mice lacking both TNF receptors (TNFR1 and TNFR2) were highly susceptible to O. tsutsugamushi infection, displaying significantly increased tissue bacterial burdens and succumbing to infection by day 9, while most wild-type mice survived through day 20. This increased susceptibility correlated with poor activation of cellular immunity in inflamed tissues. Flow cytometry of lung- and spleen-derived cells revealed profound deficiencies in total numbers and activation status of NK cells, neutrophils, and macrophages, as well as CD4 and CD8 T cells. To define the role of individual receptors in O. tsutsugamushi infection, we used mice lacking either TNFR1 or TNFR2. While deficiency in either receptor alone was sufficient to increase host susceptibility to the infection, TNFR1 and TNFR2 played a distinct role in cellular responses. TNF signaling through TNFR1 promoted inflammatory responses and effector T cell expansion, while TNFR2 signaling was associated with anti-inflammatory action and tissue homeostasis. Moreover, TNFRs played an intrinsic role in CD8+ T cell activation, revealing an indispensable role of TNF in protective immunity against O. tsutsugamushi infection.

Diagnostic Value of CD4/CD8 in Scrub Typhus

Scrub typhus is often misdiagnosed in febrile patients, leading to antibiotic abuse and multiple complications. We conducted a retrospective record review at the Fourth Affiliated Hospital of Guangxi Medical University in China. Data were collected on 52 patients with a confirmed diagnosis of scrub typhus and complete clinical data. In addition, data were collected on 52 patients with bloodstream infection, 25 patients with HIV infection, 112 patients with common community-acquired pneumonia (CCAP), and 36 patients with severe community-acquired pneumonia (SCAP) to serve as control groups. The peripheral blood CD4 and CD8 counts, CD4/CD8 ratio, C-reactive protein, procalcitonin, alanine aminotransferase, aspartate aminotransferase, creatinine, and β2 microglobulin levels; and the white blood cell count and neutrophil percentage were compared between the scrub typhus and the control groups. The value of these biomarkers in the diagnosis of scrub typhus was assessed using receiver-operating characteristic curve analysis. The scrub typhus group had a significantly lower CD4 count and CD4/CD8 ratio than the bloodstream infection, CCAP, and SCAP groups, and a significantly greater CD4 count and CD4/CD8 ratio than the HIV infection group. In contrast, the scrub typhus group had a significantly greater CD8 count than the bloodstream infection and CCAP and SCAP groups, and it had a lower level of CD8 than the HIV infection group. The areas under the curve of CD4/CD8 were more than 0.93 in the receiver-operating characteristic curve analysis. These findings suggest that the CD4/CD8 ratio is a useful ancillary test for diagnosing scrub typhus.

Buerger's Disease May be a Chronic Rickettsial Infection with Superimposed Thrombosis: Literature Review and Efficacy of Doxycycline in Three Patients

Buerger's disease (BD) is a chronic inflammatory vasculitis of unknown etiology. The infectious etiology of BD was proposed by Buerger in 1914. Furthermore, there are scattered reports insisting that BD may be related to rickettsial infection, first asserted by Goodman since 1916, followed by Giroud and other French investigators from the 1940s through the 1960s, Nicolau in the 1960s, Bartolo (1980s), and Fazeli (2010s). However, their causal relationship has hardly been accepted because rickettsial infections are known to be acute febrile, vector-borne illnesses, whereas BD is a chronic afebrile illness. In this article we review the relevant literature on the chronic nature of Rickettsia and Orientia infections and on the rickettsial etiology of BD. Excellent initial responses to doxycycline in three patients with BD are briefly described. Based on these findings, we hypothesize that BD patients acquired a rickettsial infection far before the onset of BD. Over years, the infected area expands to become a segment of the infected vessel. Subsequently, thrombus develops on the luminal surface of the infected endothelial cells, which produces the vascular obstructive manifestations of BD. Collectively, it is postulated that BD is a chronic infection with a member of the family Rickettsiaceae with superimposed thrombosis.

Vaccine Design and Vaccination Strategies against Rickettsiae

Rickettsioses are febrile, potentially lethal infectious diseases that are a serious health threat, especially in poor income countries. The causative agents are small obligate intracellular bacteria, rickettsiae. Rickettsial infections are emerging worldwide with increasing incidence and geographic distribution. Nonetheless, these infections are clearly underdiagnosed because methods of diagnosis are still limited and often not available. Another problem is that the bacteria respond to only a few antibiotics, so delayed or wrong antibiotic treatment often leads to a more severe outcome of the disease. In addition to that, the development of antibiotic resistance is a serious threat because alternative antibiotics are missing. For these reasons, prophylactic vaccines against rickettsiae are urgently needed. In the past years, knowledge about protective immunity against rickettsiae and immunogenic determinants has been increasing and provides a basis for vaccine development against these bacterial pathogens. This review provides an overview of experimental vaccination approaches against rickettsial infections and perspectives on vaccination strategies.

CCR2 Deficiency Impairs Ly6Clo and Ly6Chi Monocyte Responses in Orientia tsutsugamushi Infection

Orientia (O.) tsutsugamushi, the causative agent of scrub typhus, is a neglected, obligate intracellular bacterium that has a prominent tropism for monocytes and macrophages. Complications often involve the lung, where interstitial pneumonia is a typical finding. The severity of scrub typhus in humans has been linked to altered plasma concentrations of chemokines which are known to act as chemoattractants for myeloid cells. The trafficking and function of monocyte responses is critically regulated by interaction of the CC chemokine ligand 2 (CCL2) and its CC chemokine receptor CCR2. In a self-healing mouse model of intradermal infection with the human-pathogenic Karp strain of O. tsutsugamushi, we investigated the role of CCR2 on bacterial dissemination, development of symptoms, lung histology and monocyte subsets in blood and lungs. CCR2-deficient mice showed a delayed onset of disease and resolution of symptoms, higher concentrations and impaired clearance of bacteria in the lung and the liver, accompanied by a slow infiltration of interstitial macrophages into the lungs. In the blood, we found an induction of circulating monocytes that depended on CCR2, while only a small increase in Ly6Chi monocytes was observed in CCR2-/- mice. In the lung, significantly higher numbers of Ly6Chi and Ly6Clo monocytes were found in the C57BL/6 mice compared to CCR2-/- mice. Both wildtype and CCR2-deficient mice developed an inflammatory milieu as shown by cytokine and inos/arg1 mRNA induction in the lung, but with delayed kinetics in CCR2-deficient mice. Histopathology revealed that infiltration of macrophages to the parenchyma, but not into the peribronchial tissue, depended on CCR2. In sum, our data suggest that in Orientia infection, CCR2 drives blood monocytosis and the influx and activation of Ly6Chi and Ly6Clo monocytes into the lung, thereby accelerating bacterial replication and development of interstitial pulmonary inflammation.

Comparison of Lethal and Nonlethal Mouse Models of Orientia tsutsugamushi Infection Reveals T-Cell Population-Associated Cytokine Signatures Correlated with Lethality and Protection

The antigenic diversity of Orientia tsutsugamushi as well as the interstrain difference(s) associated with virulence in mice impose the necessity to dissect the host immune response. In this study we compared the host response in lethal and non-lethal murine models of O. tsutsugamushi infection using the two strains, Karp (New Guinea) and Woods (Australia). The models included the lethal model: Karp intraperitoneal (IP) challenge; and the nonlethal models: Karp intradermal (ID), Woods IP, and Woods ID challenges. We monitored bacterial trafficking to the liver, lung, spleen, kidney, heart, and blood, and seroconversion during the 21-day challenge. Bacterial trafficking to all organs was observed in both the lethal and nonlethal models of infection, with significant increases in average bacterial loads observed in the livers and hearts of the lethal model. Multicolor flow cytometry was utilized to analyze the CD4+ and CD8+ T cell populations and their intracellular production of the cytokines IFNγ, TNF, and IL2 (single, double, and triple combinations) associated with both the lethal and nonlethal murine models of infection. The lethal model was defined by a cytokine signature of double- (IFNγ-IL2) and triple-producing (IL2-TNF-IFNγ) CD4+ T-cell populations; no multifunctional signature was identified in the CD8+ T-cell populations associated with the lethal model. In the nonlethal model, the cytokine signature was predominated by CD4+ and CD8+ T-cell populations associated with single (IL2) and/or double (IL2-TNF) populations of producers. The cytokine signatures associated with our lethal model will become depletion targets in future experiments; those signatures associated with our nonlethal model are hypothesized to be related to the protective nature of the nonlethal challenges.

Regulation of Serum Exosomal MicroRNAs in Mice Infected with Orientia tsutsugamushi

Exosomes are small extracellular vesicles that carry proteins, lipids, and nucleic acids. They are circulated in many body fluids and play an important role in intercellular communications. MicroRNAs (miRNAs), as major components of exosomes, are often regulated in many diseases including bacterial and viral infections. Functionally, exosome-carried miRNAs interact with various immune cells and affect their behavior. Little is known whether exosomal miRNAs are regulated during scrub typhus, a potentially lethal infection caused by intracellular bacteria, Orientiatsutsugamushi. In the present study, we utilized a scrub typhus mouse model and collected serum at various time points post infection. A custom quantitative PCR array covering 92 murine miRNAs was used to profile serum exosomal miRNAs. A total of 12 miRNAs were found to be significantly up- or down-regulated at least at one time point post infection when compared to uninfected animals. Further analysis identified multiple miRNAs in the let-7 family that were consistently down-regulated at early and late phase of infection. Functionally, serum exosomes isolated from infected mice displayed strong proinflammatory effect when incubated with bone marrow-derived macrophages. Our data revealed dynamic regulations of serum exosomal miRNA during scrub typhus infection, which could significantly influence host immune responses and disease outcome.

Orientia tsutsugamushi: The dangerous yet neglected foe from the East

Orientia tsutsugamushi (OT), the causative agent of the vector-borne Scrub typhus zoonotic disease in humans, is a unique microorganism that exists in the Asia-Pacific region since a long time. In spite of its occurrence, the organism had been neglected until recent years. Humans are the accidental dead-end hosts of O. tsutsugamushi and display manifestations which are both severe and misleading. The vast antigenic diversity of OT and non-pathognomic symptoms of Scrub typhus, create hurdles in the clinical management of the disease and impede the OT-research. Many countries in the Asia-Pacific region have reported the resurgence of OT- infections and have raised concerns for its expanding distribution. This has triggered the development of advanced techniques for diagnosis and research on exploring a successful vaccine candidate to reduce the burden of the disease. Thus, the aim of this systematic review is to provide an update on the recent advances in the OT-research and highlight the key areas that have remained obscure and demand attention.

Intranasal Vaccination with Outer-Membrane Protein of Orientia tsutsugamushi induces Protective Immunity Against Scrub Typhus

Scrub typhus develops after the individual is bitten by a trombiculid mite infected with Orientia tsutsugamushi. Since it has been reported that pneumonia is frequently observed in patients with scrub typhus, we investigated whether intranasal (i.n.) vaccination with the outer membrane protein of O. tsutsugamushi (OMPOT) would induce a protective immunity against O. tsutsugamushi infection. It was particular interest that when mice were infected with O. tsutsugamushi, the bacteria disseminated into the lungs, causing pneumonia. The i.n. vaccination with OMPOT induced IgG responses in serum and bronchoalveolar lavage (BAL) fluid. The anti-O. tsutsugamushi IgA Abs in BAL fluid after the vaccination showed a high correlation of the protection against O. tsutsugamushi. The vaccination induced strong Ag-specific Th1 and Th17 responses in the both spleen and lungs. In conclusion, the current study demonstrated that i.n. vaccination with OMPOT elicited protective immunity against scrub typhus in mouse with O. tsutsugamushi infection causing subsequent pneumonia.

The neglected challenge: Vaccination against rickettsiae

Over the last decades, rickettsioses are emerging worldwide. These diseases are caused by intracellular bacteria. Although rickettsioses can be treated with antibiotics, a vaccine against rickettsiae is highly desired for several reasons. Rickettsioses are highly prevalent, especially in poor countries, and there are indications of the development of antibiotic resistance. In addition, some rickettsiae can persist and cause recurrent disease. The development of a vaccine requires the understanding of the immune mechanisms that are involved in protection as well as in immunopathology. Knowledge about these immune responses is accumulating, and efforts have been undertaken to identify antigenic components of rickettsiae that may be useful as a vaccine. This review provides an overview on current knowledge of adaptive immunity against rickettsiae, which is essential for defense, rickettsial antigens that have been identified so far, and on vaccination strategies that have been used in animal models of rickettsial infections.

Polarized lung inflammation and Tie2/angiopoietin-mediated endothelial dysfunction during severe Orientia tsutsugamushi infection

Orientia tsutsugamushi infection can cause acute lung injury and high mortality in humans; however, the underlying mechanisms are unclear. Here, we tested a hypothesis that dysregulated pulmonary inflammation and Tie2-mediated endothelial malfunction contribute to lung damage. Using a murine model of lethal O. tsutsugamushi infection, we demonstrated pathological characteristics of vascular activation and tissue damage: 1) a significant increase of ICAM-1 and angiopoietin-2 (Ang2) proteins in inflamed tissues and lung-derived endothelial cells (EC), 2) a progressive loss of endothelial quiescent and junction proteins (Ang1, VE-cadherin/CD144, occuludin), and 3) a profound impairment of Tie2 receptor at the transcriptional and functional levels. In vitro infection of primary human EC cultures and serum Ang2 proteins in scrub typhus patients support our animal studies, implying endothelial dysfunction in severe scrub typhus. Flow cytometric analyses of lung-recovered cells further revealed that pulmonary macrophages (MΦ) were polarized toward an M1-like phenotype (CD80+CD64+CD11b+Ly6G-) during the onset of disease and prior to host death, which correlated with the significant loss of CD31+CD45- ECs and M2-like (CD206+CD64+CD11b+Ly6G-) cells. In vitro studies indicated extensive bacterial replication in M2-type, but not M1-type, MΦs, implying the protective and pathogenic roles of M1-skewed responses. This is the first detailed investigation of lung cellular immune responses during acute O. tsutsugamushi infection. It uncovers specific biomarkers for vascular dysfunction and M1-skewed inflammatory responses, highlighting future therapeutic research for the control of this neglected tropical disease.

Immunization with a recombinant antigen composed of conserved blocks from TSA56 provides broad genotype protection against scrub typhus

Scrub typhus is an acute febrile disease caused by Orientia tsutsugamushi infection. Despite the wide range of approaches explored during the last seventy years, an effective prophylactic vaccine is not yet available. Here, we developed a novel recombinant antigen derived from conserved regions of 56 kDa type-specific antigen (TSA56), a major outer membrane protein responsible for genetic heterogeneity and antigenicity, and evaluated it as a protective vaccine antigen. Our findings demonstrate that immunization with conserved blocks of TSA56 (cTSA56) not only provides protective immunity against lethal challenges with the homologous genotype, but also confers significantly better protection against heterologous genotypes than TSA56. Adoptive transfer of CD4⁺ or CD8⁺ T cells from immunized mice provided significantly enhanced protection against lethal challenge, whereas immune B cells failed to do so, indicating that cellular immunity against the conserved epitopes plays a protective role. Moreover, immunization with a 10-mer peptide mixture, screened from CD8⁺ T cell epitopes within the conserved region of TSA56, provided enhanced protection against lethal challenge with O. tsutsugamushi. Therefore, this novel recombinant antigen is a promising candidate for scrub typhus vaccine against a wide range of O. tsutsugamushi genotypes.

Dysregulated Th1 Immune and Vascular Responses in Scrub Typhus Pathogenesis

Scrub typhus is an emerging, insect-transmitted disease caused by Orientia tsutsugamushi, a Gram- and LPS-negative bacterium that replicates freely within professional phagocytes and endothelial cells. Scrub typhus is prevalent with high mortality rates, but information regarding its molecular pathogenesis, microbial virulence determinants, and key immune responses is limited. Improved animal models have recently been developed that respectively resemble the pathological features of self-limiting or severe scrub typhus in humans. Strong activation of Th1 and CD8, but not Th2 and regulatory T, immune responses, accompanied by altered angiopoietin/Tie2-related regulation, are hallmarks of lethal infection in murine models. This review, based primarily on recent advances from clinical and experimental studies, highlights tissue- and endothelial cell-specific biomarkers that are indicative of immune dysregulation. The potential roles of neutrophils and damage-associated molecular pattern molecules at late stages of disease are discussed in the context of vascular leakage, pulmonary and renal injury, and scrub typhus pathogenesis.

The Obligate Intracellular Bacterium Orientia tsutsugamushi Targets NLRC5 To Modulate the Major Histocompatibility Complex Class I Pathway

Orientia tsutsugamushi is an obligate intracellular bacterium that infects mononuclear and endothelial cells to cause the emerging global health threat scrub typhus. The ability of O. tsutsugamushi to survive in monocytes facilitates bacterial dissemination to endothelial cells, which can subsequently lead to several potentially fatal sequelae. As a strict intracellular pathogen that lives in the cytoplasm of host cells, O. tsutsugamushi has evolved to counter adaptive immunity. How the pathogen does so and the outcome of this strategy in monocytes versus endothelial cells are poorly understood. This report demonstrates that O. tsutsugamushi reduces cellular levels of NOD-, LRR-, and CARD-containing 5 (NLRC5), a recently identified specific transactivator of major histocompatibility complex class I (MHC-I) component gene expression, to inhibit MHC-I biosynthesis. Importantly, the efficacy of this approach varies with the host cell type infected. In nonprofessional antigen-presenting HeLa and primary human aortic endothelial cells, the O. tsutsugamushi-mediated reduction of NLRC5 results in lowered MHC-I component transcription and, consequently, lower total and/or surface MHC-I levels throughout 72 h of infection. However, in infected THP-1 monocytes, which are professional antigen-presenting cells, the reductions in NLRC5 and MHC-I observed during the first 24 h reverse thereafter. O. tsutsugamushi is the first example of a microbe that targets NLRC5 to modulate the MHC-I pathway. The differential ability of O. tsutsugamushi to modulate this pathway in nonprofessional versus professional antigen-presenting cells could influence morbidity and mortality from scrub typhus.

Batf2 differentially regulates tissue immunopathology in Type 1 and Type 2 diseases

Basic leucine zipper transcription factor 2 (Batf2) activation is detrimental in Type 1-controlled infectious diseases, demonstrated during infection with Mycobacterium tuberculosis (Mtb) and Listeria monocytogenes Lm. In Batf2-deficient mice (Batf2^-/-), infected with Mtb or Lm, mice survived and displayed reduced tissue pathology compared to infected control mice. Indeed, pulmonary inflammatory macrophage recruitment, pro-inflammatory cytokines and immune effectors were also decreased during tuberculosis. This explains that batf2 mRNA predictive early biomarker found in active TB patients is increased in peripheral blood. Similarly, Lm infection in human macrophages and mouse spleen and liver also increased Batf2 expression. In striking contrast, Type 2-controlled schistosomiasis exacerbates during infected Batf2^-/- mice with increased intestinal fibro-granulomatous inflammation, pro-fibrotic immune cells, and elevated cytokine production leading to wasting disease and early death. Together, these data strongly indicate that Batf2 differentially regulates Type 1 and Type 2 immunity in infectious diseases.

A Type I Interferon and IL-10 Induced by Orientia tsutsugamushi Infection Suppresses Antigen-Specific T Cells and Their Memory Responses

Despite the various roles of type I interferon (type I IFN) responses during bacterial infection, its specific effects in vivo have been poorly characterized in scrub typhus caused by Orientia tsutsugamushi infection. Here, we show that type I IFNs are primarily induced via intracellular nucleic acids sensors, including RIG-I/MAVS and cGAS/STING pathways, during O. tsutsugamushi invasion. However, type I IFN signaling did not significantly affect pathogenesis, mortality, or bacterial burden during primary infection in vivo, when assessed in a mice model lacking a receptor for type I IFNs (IFNAR KO). Rather, it significantly impaired the induction of antigen-specific T cells and reduced memory T cell responses. IFNAR KO mice that recovered from primary infection showed stronger antigen-specific T cell responses, especially Th1, and more efficiently controlled bacteremia during secondary infection than wild type mice. Enhanced IL-10 expression by macrophages in the presence of type I IFN signaling might play a significant role in the suppression of antigen-specific T cell responses as neutralization or knock-out (KO) of IL-10 increased T cell responses in vitro. Therefore, induction of the type I IFN/IL-10 axis by O. tsutsugamushi infection might play a significant role in the suppression of T cell responses and contribute to the short longevity of cell-mediated immunity, often observed in scrub typhus patients.

Dissemination of Orientia tsutsugamushi, a Causative Agent of Scrub Typhus, and Immunological Responses in the Humanized DRAGA Mouse

Scrub typhus is caused by Orientia tsutsugamushi, an obligated intracellular bacterium that affects over one million people per year. Several mouse models have been used to study its pathogenesis, disease immunology, and for testing vaccine candidates. However, due to the intrinsic differences between the immune systems in mouse and human, these mouse models could not faithfully mimic the pathology and immunological responses developed by human patients, limiting their value in both basic and translational studies. In this study, we have tested for the first time, a new humanized mouse model through footpad inoculation of O. tsutsugamushi in DRAGA (HLA-A2.HLA-DR4.Rag1KO.IL2RγcKO.NOD) mice with their human immune system reconstituted by infusion of HLA-matched human hematopoietic stem cells from umbilical cord blood. Upon infection, Orientia disseminated into various organs of DRAGA mice resulted in lethality in a dose-dependent manner, while all C3H/HeJ mice infected by the same route survived. Tissue-specific lesions associated with inflammation and/or necroses were observed in multiple organs of infected DRAGA mice. Consistent with the intracellular nature of Orientia, strong Th1, but subdued Th2 responses were elicited as reflected by the human cytokine profiles in sera from infected mice. Interestingly, the percentage of both activated and regulatory (CD4⁺FOXP3⁺) human T cells were elevated in spleen tissues of infected mice. After immunization with irradiated whole cell Orientia, humanized DRAGA mice showed a significant activation of human T cells as evidenced by increased number of human CD4⁺ and CD8⁺ T cells. Specific human IgM and IgG antibodies were developed after repetitive immunization. The humanized DRAGA mouse model represents a new pre-clinical model for studying Orientia-human interactions and also for testing vaccines and novel therapeutics for scrub typhus.

An Update on Host-Pathogen Interplay and Modulation of Immune Responses during Orientia tsutsugamushi Infection

The obligate intracellular bacterium Orientia tsutsugamushi is the causative agent of scrub typhus in humans, a serious mite-borne disease present in a widespread area of endemicity, which affects an estimated 1 million people every year. This disease may exhibit a broad range of presentations, ranging from asymptomatic to fatal conditions, with the latter being due to disseminated endothelial infection and organ injury. Unique characteristics of the biology and host-pathogen interactions of O. tsutsugamushi, including the high antigenic diversity among strains and the highly variable, short-lived memory responses developed by the host, underlie difficulties faced in the pursuit of an effective vaccine, which is an imperative need. Other factors that have hindered scientific progress relative to the infectious mechanisms of and the immune response triggered by this bacterium in vertebrate hosts include the limited number of mechanistic studies performed on animal models and the lack of genetic tools currently available for this pathogen. However, recent advances in animal model development are promising to improve our understanding of host-pathogen interactions. Here, we comprehensively discuss the recent advances in and future perspectives on host-pathogen interactions and the modulation of immune responses related to this reemerging disease, highlighting the role of animal models.

Orientia tsutsugamushi Is Highly Susceptible to the RNA Polymerase Switch Region Inhibitor Corallopyronin A In Vitro and In Vivo

Scrub typhus is a potentially lethal infection caused by the obligate intracellular bacterium Orientia tsutsugamushi Reports on the emergence of doxycycline-resistant strains highlight the urgent need to develop novel antiinfectives against scrub typhus. Corallopyronin A (CorA) is a novel α-pyrone compound synthesized by the myxobacterium Corallococcus coralloides that was characterized as a noncompetitive inhibitor of the switch region of the bacterial RNA polymerase (RNAP). We investigated the antimicrobial action of CorA against the human-pathogenic Karp strain of O. tsutsugamushiin vitro and in vivo The MIC of CorA against O. tsutsugamushi was remarkably low (0.0078 μg/ml), 16-fold lower than that against Rickettsia typhi In the lethal intraperitoneal O. tsutsugamushi mouse infection model, a minimum daily dose of 100 μg CorA protected 100% of infected mice. Two days of treatment were sufficient to confer protection. In contrast to BALB/c mice, SCID mice succumbed to the infection despite treatment with CorA or tetracycline, suggesting that antimicrobial treatment required synergistic action of the adaptive immune response. Similar to tetracycline, CorA did not prevent latent infection of O. tsutsugamushiin vivo However, latency was not caused by acquisition of antimicrobial resistance, since O. tsutsugamushi reisolated from latently infected BALB/c mice remained fully susceptible to CorA. No mutations were found in the CorA-binding regions of the β and β' RNAP subunit genes rpoB and rpoC Inhibition of the RNAP switch region of O. tsutsugamushi by CorA is therefore a novel and highly potent target for antimicrobial therapy for scrub typhus.

Characterization of the rhesus macaque (Macaca mulatta) scrub typhus model: Susceptibility to intradermal challenge with the human pathogen Orientia tsutsugamushi Karp

BACKGROUND

Scrub typhus is an important endemic disease in tropical Asia caused by Orientia tsutsugamushi for which no effective broadly protective vaccine is available. The successful evaluation of vaccine candidates requires well-characterized animal models and a better understanding of the immune response against O. tsutsugamushi. While many animal species have been used to study host immunity and vaccine responses in scrub typhus, only limited data exists in non-human primate (NHP) models.

METHODOLOGY/PRINCIPLE FINDINGS

In this study we evaluated a NHP scrub typhus disease model based on intradermal inoculation of O. tsutsugamushi Karp strain in rhesus macaques (n = 7). After an intradermal inoculation with 106 murine LD50 of O. tsutsugamushi at the anterior thigh (n = 4) or mock inoculum (n = 3), a series of time course investigations involving hematological, biochemical, molecular and immunological assays were performed, until day 28, when tissues were collected for pathology and immunohistochemistry. In all NHPs with O. tsutsugamushi inoculation, but not with mock inoculation, the development of a classic eschar with central necrosis, regional lymphadenopathy, and elevation of body temperature was observed on days 7-21 post inoculation (pi); bacteremia was detected by qPCR on days 6-18 pi; and alteration of liver enzyme function and increase of white blood cells on day 14 pi. Immune assays demonstrated raised serum levels of soluble cell adhesion molecules, anti-O. tsutsugamushi-specific antibody responses (IgM and IgG) and pathogen-specific cell-mediated immune responses in inoculated macaques. The qPCR assays detected O. tsutsugamushi in eschar, spleen, draining and non-draining lymph nodes, and immuno-double staining demonstrated intracellular O. tsutsugamushi in antigen presenting cells of eschars and lymph nodes.

CONCLUSIONS/SIGNIFICANCE

These data show the potential of using rhesus macaques as a scrub typhus model, for evaluation of correlates of protection in both natural and vaccine induced immunity, and support the evaluation of future vaccine candidates against scrub typhus.

Longevity of antibody and T-cell responses against outer membrane antigens of Orientia tsutsugamushi in scrub typhus patients

Scrub typhus, caused by Orientia tsutsugamushi infection, has been a serious public health issue in the Asia-Pacific region, with rising incidence and sporadic outbreaks. However, human protective immunity against specific antigens has been poorly characterized for this bacterium. In addition, immunity produced in early vaccine trials or even after natural infections, did not last long and had poor cross-reactivity among various genotypes. Here, we systematically investigated the kinetics and magnitude of specific adaptive immunity against two membrane antigens, 56 kDa type-specific antigen (TSA56) and surface cell antigen A (ScaA), that are involved in bacterial adhesion and invasion of the host in 64 recovered scrub typhus patients. Antibody responses to the bacterial antigens in patients were generally short-lived and waned to baseline levels 2 years after recovery. The anti-TSA56 IgG responses were predominantly composed of the IgG1 and IgG3 subclasses and persisted for up to 1 year after recovery, whereas IgG specific to ScaA primarily consisted of more transient IgG1, with limited responses by other subclasses. Cellular immunity, including CD4 and CD8 T-cells specific to membrane antigens, also rapidly declined from 1 year after infection, as measured by enzyme-linked immunospot (ELISPOT) assays and flow cytometry. The short longevity of antigen-specific adaptive immunity might be attributable to limited memory responses, as observed in earlier vaccine studies using whole bacterial antigens. Finally, we identified HLA-A*0201-restricted and highly conserved CD8 T-cell epitopes in the TSA56 antigen, which may be valuable tools for assessing cellular immunity against O. tsutsugamushi and developing an effective scrub typhus vaccine.

Strong interferon-gamma mediated cellular immunity to scrub typhus demonstrated using a novel whole cell antigen ELISpot assay in rhesus macaques and humans

Scrub typhus is a febrile infection caused by the obligate intracellular bacterium Orientia tsutsugamushi, which causes significant morbidity and mortality across the Asia-Pacific region. The control of this vector-borne disease is challenging due to humans being dead-end hosts, vertical maintenance of the pathogen in the vector itself, and a potentially large rodent reservoir of unclear significance, coupled with a lack of accurate diagnostic tests. Development of an effective vaccine is highly desirable. This however requires better characterization of the natural immune response of this neglected but important disease. Here we implement a novel IFN-γ ELISpot assay as a tool for studying O. tsutsugamushi induced cellular immune responses in an experimental scrub typhus rhesus macaque model and human populations. Whole cell antigen for O. tsutsugamushi (OT-WCA) was prepared by heat inactivation of Karp-strain bacteria. Rhesus macaques were infected intradermally with O. tsutsugamushi. Freshly isolated peripheral blood mononuclear cells (PBMC) from infected (n = 10) and uninfected animals (n = 5) were stimulated with OT-WCA, and IFN-γ secreting cells quantitated by ELISpot assay at five time points over 28 days. PBMC were then assayed from people in a scrub typhus-endemic region of Thailand (n = 105) and responses compared to those from a partially exposed population in a non-endemic region (n = 14), and to a naïve population in UK (n = 12). Mean results at Day 0 prior to O. tsutsugamushi infection were 12 (95% CI 0-25) and 15 (2-27) spot-forming cells (SFC)/106 PBMC for infected and control macaques respectively. Strong O. tsutsugamushi-specific IFN-γ responses were seen post infection, with ELISpot responses 20-fold higher than baseline at Day 7 (mean 235, 95% CI 200-270 SFC/106 PBMC), 105-fold higher at Day 14 (mean 1261, 95% CI 1,097-1,425 SFC/106 PBMC), 125-fold higher at Day 21 (mean 1,498, 95% CI 1,496-1,500 SFC/106 PBMC) and 118-fold higher at Day 28 (mean 1,416, 95% CI 1,306-1,527 SFC/106 PBMC). No significant change was found in the control group at any time point compared to baseline. Humans from a scrub typhus endemic region of Thailand had mean responses of 189 (95% CI 88-290) SFC/106 PBMC compared to mean responses of 40 (95% CI 9-71) SFC/106 PBMC in people from a non-endemic region and 3 (95% CI 0-7) SFC/106 PBMC in naïve controls. In summary, this highly sensitive assay will enable field immunogenicity studies and further characterization of the host response to O. tsutsugamushi, and provides a link between human and animal models to accelerate vaccine development.

CD8+ T cells provide immune protection against murine disseminated endotheliotropic Orientia tsutsugamushi infection

Scrub typhus, caused by a Gram-negative obligately intracellular coccobacillus, Orientia tsutsugamushi, is a long neglected but important tropical disease. Orientia tsutsugamushi causes illness in one million people each year, and 1 billion people are at risk. Without appropriate diagnosis and treatment, the disease can cause severe multiorgan failure with a case fatality rate of 7–15%. The current gaps in knowledge of immunity include the unknown mechanisms of host immunity to O. tsutsugamushi. Using an intravenous (i.v.) disseminated infection mouse model, we observed that more CD8⁺ T cells than CD4⁺ T cells were present in the spleen of infected mice at 12 dpi. We also determined that T_reg cells and the proportion of T cells producing IL-10 were significantly increased from 6 dpi, which correlated with the onset of illness, body weight loss, and increased bacterial loads. We further studied CD8^-/-, MHC I^-/- and wild type control (WT) C57BL/6J mice to determine the importance of CD8⁺ T cells and MHC I molecules. After infection with an ordinarily sub-lethal dose of O. tsutsugamushi, all CD8^-/- and MHC I^-/- mice were moribund between 12 and 15 dpi, whereas all WT mice survived. Bacterial loads in the lung, kidney, liver and spleen of CD8^-/- and MHC I^-/- mice were significantly greater than those in WT mice. Interferon-γ (IFN-γ) and granzyme B mRNA levels in the liver of CD8^-/- and MHC I^-/- mice were significantly greater than in WT mice. In addition, more severe histopathologic lesions were observed in CD8^-/- mice. Finally, adoptive transfer confirmed a major role of immune CD8⁺ T cells as well as a less effective contribution by immune CD8 T cell-depleted splenocytes in protection against O. tsutsugamushi infection. These studies demonstrated the critical importance of CD8⁺ T cells in the host immune response during O. tsutsugamushi infection.

Scrub typhus is a febrile illness that can be life-threatening without appropriate treatment. It is caused by a gram-negative, obligately intracellular bacterium, Orientia tsutsugamushi. Approximately one million people annually are infected globally, mainly in the Asia-Pacific area. Unfortunately, no vaccine has been developed for the prevention of the disease. There is very little known about the pathogenesis of or host immunity to scrub typhus. The limited number of studies on the mechanisms of immunity has failed to provide a fundamental explanation of the protective host immune response during the disease due in part to the outdated and inaccurate animal models employed. Here, we examined and confirmed that CD8⁺ T cells provide critical protection during the infection. Our data also demonstrated the potential contribution of non-CD8⁺ immune cells in protection against the infection. This study of host immune responses during scrub typhus with a more accurate mouse model improves the understanding of the interaction between the host and pathogen, and provides a foundation for future investigations of pathogenesis, immune control and prevention including vaccine development.

Murine models of scrub typhus associated with host control of Orientia tsutsugamushi infection

Background

Scrub typhus, a febrile illness of substantial incidence and mortality, is caused by infection with the obligately intracellular bacterium Orientia tsutsugamushi. It is estimated that there are more than one million cases annually transmitted by the parasitic larval stage of trombiculid mites in the Asia-Pacific region. The antigenic and genetic diversity of the multiple strains of O. tsutsugamushi hinders the advancement of laboratory diagnosis, development of long-lasting vaccine-induced protection, and interpretation of clinical infection. Despite the life-threatening severity of the illness in hundreds of thousands of cases annually, 85–93% of patients survive, often without anti-rickettsial treatment. To more completely understand the disease caused by Orientia infection, animal models which closely correlate with the clinical manifestations, target cells, organ involvement, and histopathologic lesions of human cases of scrub typhus should be employed. Previously, our laboratory has extensively characterized two relevant C57BL/6 mouse models using O. tsutsugamushi Karp strain: a route-specific intradermal model of infection and persistence and a hematogenously disseminated dose-dependent lethal model.

Principal findings

To complement the lethal model, here we illustrate a sublethal model in the same mouse strain using the O. tsutsugamushi Gilliam strain, which resulted in dose-dependent severity of illness, weight loss, and systemic dissemination to endothelial cells of the microcirculation and mononuclear phagocytic cells. Histopathologic lesions included expansion of the pulmonary interstitium by inflammatory cell infiltrates and multifocal hepatic lesions with mononuclear cellular infiltrates, renal interstitial lymphohistiocytic inflammation, mild meningoencephalitis, and characteristic typhus nodules.

Significance

These models parallel characteristics of human cases of scrub typhus, and will be used in concert to understand differences in severity which lead to lethality or host control of the infection and to address the explanation for short duration of heterologous immunity in Orientia infection.

Scrub typhus is an acute febrile illness with considerable mortality, and no available vaccine, caused by the obligately intracellular bacterium, Orientia tsutsugamushi. Despite the life-threatening severity of the illness in approximately one million cases annually, 85–93% of patients survive. The lack of appropriate animal models of scrub typhus has left a void in the fundamental knowledge necessary to develop a vaccine, such as mechanisms which contribute to disease severity and immunity. Here, we report a sublethal inbred murine model for scrub typhus using the intradermal and intravenous routes of inoculation, which are comparable to the natural route of chigger-bite transmission and subsequent hematogenous spread. This model, infection of mice with O. tsutsugamushi Gilliam strain, can be employed in conjunction with the lethal model of O. tsutsugamushi Karp strain to perform in-depth mechanistic studies related to strain cross-protection, lethality, pathogenesis and specific immunological investigations of the host immune response.