Induction of neutralizing antibody in mice by immunization with recombinant 56 kDa protein of Orientia tsutsugamushi

Vaccination against Bacterial Infections: Challenges, Progress, and New Approaches with a Focus on Intracellular Bacteria

Many bacterial infections are major health problems worldwide, and treatment of many of these infectious diseases is becoming increasingly difficult due to the development of antibiotic resistance, which is a major threat. Prophylactic vaccines against these bacterial pathogens are urgently needed. This is also true for bacterial infections that are still neglected, even though they affect a large part of the world’s population, especially under poor hygienic conditions. One example is typhus, a life-threatening disease also known as “war plague” caused by Rickettsia prowazekii, which could potentially come back in a war situation such as the one in Ukraine. However, vaccination against bacterial infections is a challenge. In general, bacteria are much more complex organisms than viruses and as such are more difficult targets. Unlike comparatively simple viruses, bacteria possess a variety of antigens whose immunogenic potential is often unknown, and it is unclear which antigen can elicit a protective and long-lasting immune response. Several vaccines against extracellular bacteria have been developed in the past and are still used successfully today, e.g., vaccines against tetanus, pertussis, and diphtheria. However, while induction of antibody production is usually sufficient for protection against extracellular bacteria, vaccination against intracellular bacteria is much more difficult because effective defense against these pathogens requires T cell-mediated responses, particularly the activation of cytotoxic CD8+ T cells. These responses are usually not efficiently elicited by immunization with non-living whole cell antigens or subunit vaccines, so that other antigen delivery strategies are required. This review provides an overview of existing antibacterial vaccines and novel approaches to vaccination with a focus on immunization against intracellular bacteria.

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.

An Inducible Expression System for Recombinant Sca Proteins with an Autotransporter Domain from Orientia Tsutsugamushi in Escherichia coli

BACKGROUND

Orientia tsutsugamushi (Ot) is an obligate, intracellular, gram-negative bacterium causing scrub typhus. Some of its encoded proteins play key roles in the adhesion and internalization of the Ot strain into host cells and are suitable resources for vaccine development and tools for scrub typhus diagnosis. Surface cell antigen (Sca) proteins, classified as autotransporter (AT) proteins, are one of the largest protein families involved in bacterial pathogenesis and can be promising candidates for vaccine development. These proteins are typically large and contain inhibitory domains; therefore, recombinant proteins without such domains have been evaluated for this purpose. However, the expression for recombinant Sca proteins containing the AT domain, which might largely affect their protective role against scrub typhus, has not been analyzed and optimized.

OBJECTIVE

In this study, we optimized expression and purification conditions for individual Ot Sca protein fragments [ScaA (27-1461), ScaC (257-526), ScaD (26-998), and ScaE (35-760)] harboring the AT domain.

METHODS

To this end, we subcloned sequences of codon-optimized DNA encoding Sca protein fragments into the Escherichia coli expression vector. In addition, the expression condition for individual Sca fragments was optimized, and the proteins were purified using one-step Ni-NTA column method. The purified fractions were re-folded by serial dilution method, followed by BCA quantification and densitometric analysis to estimate the yield and purity of proteins.

RESULTS

We prepared platforms for expression of recombinant Sca protein fragments [ScaA (27-1461), ScaC (257-526), ScaD (26-998), and ScaE (35-760)] containing an AT domain without the signal peptide and transmembrane (TM) domain. The protein yield per liter of culture with >70% of purity was ScaC (257-576), ScaE (35-760), ScaD (26-998), and ScaA (27-1461) in order.

CONCLUSION

Our results could be used to develop Sca AT-domain based vaccines and tools for scrub typhus diagnosis with rapid and cost-effective ways.

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.

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.

A Recombinant 47-kDa Outer Membrane Protein Induces an Immune Response against Orientia tsutsugamushi Strain Boryong

We investigated the 47-kDa outer membrane protein (OMP), which is a periplasmic serine protease and an antigenic major surface protein of Orientia tsutsugamushi, as a vaccine candidate. We developed a conventional subunit vaccine expressing recombinant 47-kDa OMP (rec47) and a DNA vaccine (p47). In mouse immunization experiments, intranasal immunization with rec47 alone or with rec47 plus heat-labile enterotoxin B subunit from Escherichia coli or plus cholera toxin (CT) as adjuvants induced a higher amount of rec47-specific antibodies than intramuscular immunization with p47 alone or with p47 plus pBOOST2-samIRF7/3 (pB) as adjuvant. Moreover, the combination of rec47 and CT induced a strong cellular immune response to 47-kDa OMP, as demonstrated by a spleen cell proliferation assay, and also induced Th1- and Th2-type cytokine production, as demonstrated by a cytokine enzyme-linked immunosorbent assay. Intranasal immunization with rec47 plus CT was the most effective method for the induction of humoral and cell-mediated immune responses. Furthermore, relatively strong protection against homologous O. tsutsugamushi strain Boryong challenge was observed in mice immunized with rec47 plus CT. Therefore, 47-kDa OMP is an attractive candidate for developing a prophylactic vaccine against scrub typhus by O. tsutsugamushi infection.

Diversification of Orientia tsutsugamushi genotypes by intragenic recombination and their potential expansion in endemic areas

Background

Scrub typhus is a mite-borne febrile disease caused by O. tsutsugamushi infection. Recently, emergence of scrub typhus has attracted considerable attention in several endemic countries in Asia and the western Pacific. In addition, the antigenic diversity of the intracellular pathogen has been a serious obstacle for developing effective diagnostics and vaccine.

Methodology/Principal findings

To understand the evolutionary pathway of genotypic diversification of O. tsutsugamushi and the environmental factors associated with the epidemiological features of scrub typhus, we analyzed sequence data, including spatiotemporal information, of the tsa56 gene encoding a major outer membrane protein responsible for antigenic variation. A total of 324 tsa56 sequences covering more than 85% of its open reading frame were analyzed and classified into 17 genotypes based on phylogenetic relationship. Extensive sequence analysis of tsa56 genes using diverse informatics tools revealed multiple intragenic recombination events, as well as a substantially higher mutation rate than other house-keeping genes. This suggests that genetic diversification occurred via frequent point mutations and subsequent genetic recombination. Interestingly, more diverse bacterial genotypes and dominant vector species prevail in Taiwan compared to other endemic regions. Furthermore, the co-presence of identical and sub-identical clones of tsa56 gene in geographically distant areas implies potential spread of O. tsutsugamushi genotypes.

Conclusions/Significance

Fluctuation and diversification of vector species harboring O. tsutsugamushi in local endemic areas may facilitate genetic recombination among diverse genotypes. Therefore, careful monitoring of dominant vector species, as well as the prevalence of O. tsutsugamushi genotypes may be advisable to enable proper anticipation of epidemiological changes of scrub typhus.

Scrub typhus, caused by Orientia tsutsugamushi infection, is a mite-borne febrile illness endemic in the Asia-Pacific region. Recent emergence and continuous local outbreaks in many of the endemic countries make it a serious public health issue. In addition, the antigenic diversity of the tsa56 gene, encoding a major outer membrane protein, hampers the development of effective diagnostics and vaccine. Here, we extensively analyzed tsa56 sequences and their spatiotemporal information to elucidate the evolutionary pathway of genotypic diversification, as well as the environmental basis associated with the epidemiological changes of scrub typhus. Based on various informatics analyses, we found that genetic diversification of tsa56 might have been attained via frequent point mutations and subsequent genetic recombination among diverse genotypes. Prevalence of numerous bacterial genotypes and dominant vector species in Taiwan also suggest that the subtropical area located at the center of endemicity, may serve as a local mixing ground for genotype diversification. In addition, detection of identical and sub-identical clones of tsa56 genes in geographically distant countries indicates a potential spreading of bacterial genotypes. Continuous monitoring of dominant vector species and the associated O. tsutsugamushi genotypes might be required for developing better diagnostics and an effective vaccine for scrub typhus.

Differences in Clinical and Laboratory Features for Different Genotypes of Orientia tsutsugamushi in Guangzhou, Southern China

PURPOSE

To describe the genotypes distribution of Orientia tsutsugamushi, in Guangzhou, and explore differences in clinical and laboratory features among genotypes.

METHOD

We collected blood or eschar samples of all scrub typhus cases reported in 2013 from 10 district hospitals positive for PCR. We further tested the PCR-positive samples for DNA sequencing and phylogenetic analysis. We collected the demographic, clinical, and laboratory data by questionnaire and medical records.

RESULT

A total of 589 samples were tested and 40 were PCR positive. Four genotypes were identified: Karp (52.2%), Kato (22.5%), Gilliam (12.5%), and TA763 (7.5%). Two new strains (ZCX90 and CHX5 strain) were previously unidentified in Guangzhou. Gilliam type cases were more likely to have continuous fever (60.0%). Only Kato genotype cases presented with splenomegaly. For laboratory features, Kato group had higher level of white blood cell count (16.44 × 10¹²/L) than normal range and other groups. Total bilirubin and direct bilirubin in Gilliam group (10.10 μM and 3.05 μM, respectively) were both lower than other groups and normal value.

CONCLUSION

Continuous fever and splenomegaly may be the distinctive symptom of Gilliam and Kato genotype cases, respectively. Karp genotype remained dominant in Guangzhou, but attention should be paid to the emergence of new types or variation of existing genotypes.

Antigenic Relationships among Human Pathogenic Orientia tsutsugamushi Isolates from Thailand

Background

Scrub typhus is a common cause of undiagnosed febrile illness in certain tropical regions, but can be easily treated with antibiotics. The causative agent, Orientia tsutsugamushi, is antigenically variable which complicates diagnosis and efforts towards vaccine development.

Methodology/Principal Findings

This study aimed to dissect the antigenic and genetic relatedness of O. tsutsugamushi strains and investigate sero-diagnostic reactivities by titrating individual patient sera against their O. tsutsugamushi isolates (whole-cell antigen preparation), in homologous and heterologous serum-isolate pairs from the same endemic region in NE Thailand. The indirect immunofluorescence assay was used to titrate Orientia tsutsugamushi isolates and human sera, and a mathematical technique, antigenic cartography, was applied to these data to visualise the antigenic differences and cross-reactivity between strains and sera. No functional or antigen-specific analyses were performed. The antigenic variation found in clinical isolates was much less pronounced than the genetic differences found in the 56kDa type-specific antigen genes. The Karp-like sera were more broadly reactive than the Gilliam-like sera.

Conclusions/Significance

Antigenic cartography worked well with scrub typhus indirect immunofluorescence titres. The data from humoral responses suggest that a Karp-like strain would provide broader antibody cross-reactivity than a Gilliam-like strain. Although previous exposure to O. tsutsugamushi could not be ruled out, scrub typhus patient serum antibody responses were characterised by strong homologous, but weak heterologous antibody titres, with little evidence for cross-reactivity by Gilliam-like sera, but a broader response from some Karp-like sera. This work highlights the importance of antigenic variation in O. tsutsugamushi diagnosis and determination of new serotypes.

Scrub Typhus is a common and potentially severe febrile illness in certain tropical regions in Asia. This infection is treatable with specific antibiotics if diagnosed correctly, but there is no effective vaccine available at present. The bacterium causing this disease is called Orientia tsutsugamushi; it is transmitted by small mites and it has variable surface proteins, which make diagnosis and vaccine development difficult. In this study, we tested how well the antibodies in the blood of patients with scrub typhus recognise different strains of bacteria, and used these results to create a map of the relationships between the bacteria and sera. From examining this map we can see that some sera have activity against a wider range of bacteria than others. These methods and findings will help with selecting bacteria strains and evaluating immune responses, which will potentially help us to improve diagnosis and vaccine development.

Prevalence and Phylogenetic Analysis of Orientia tsutsugamushi in Small Mammals in Hanoi, Vietnam

Rodents are important reservoirs of many human pathogens transmitted via arthropod vectors. Arthropod-borne bacteria belonging to the family Rickettsiaceae cause acute febrile diseases in humans worldwide, but the real burdens of rickettsial diseases appear to be underestimated in Hanoi, Vietnam, because differential diagnosis on the basis of clinical signs and symptoms is confounded by the presence of other tropical infectious diseases with similar signs and symptoms. To know the prevalence of bacteria of the family Rickettsiaceae among small mammals in Hanoi, 519 animals thriving in the public places were captured and examined for the presence of bacterial sequences using duplex PCR. Nucleotide sequences specific for Orientia tsutsugamushi were detected in seven samples (1.3%). Out of seven animals, two were captured in a market, whereas five were in hospitals. None of the captured small mammals tested positive for the genus Rickettsia. The nucleotide sequence analysis of the genes encoding the 47-kDa high-temperature requirement A (47-kDa HtrA) and 56-kDa type-specific antigen (TSA) showed that these seven isolates were indistinguishable from each other. O. tsutsugamushi isolated in this study was closely related phylogenetically to the Gilliam strain, which was originally isolated at the border of Assam and Burma, rather than to those isolated in the central to southern part of Vietnam. It should be emphasized that Vietnamese hospitals were heavily infested by small rodents and some of them harbored O. tsutsugamushi. Strict hygienic control should be implemented to mitigate the potential risk posed by O. tsutsugamushi in hospital settings.

Identification of Outer Membrane Vesicles Derived from Orientia tsutsugamushi

Orientia tsutsugamushi, a causative pathogen of Scrub typhus, is a gram-negative intracellular bacterium. Outer membrane vesicles (OMVs) are produced from the membrane of bacteria and play many roles related to the survival of the pathogen. However, there have been no reports confirming whether O. tsutsugamushi indeed produce OMVs. O. tsutsugamushi boryong was cultured in ECV-304 cells for the purification of OMVs. Western blot analysis and immunoenrichment using anti-O. tsutsugamushi monoclonal antibody and electron microscopy were employed for identification and characterization of OMVs. We confirm the presence of OMVs derived from O. tsutsugamushi, and also found that those OMVs contain a major surface antigen of 56-kDa protein and variant immunogenic antigens.

Immunization with an autotransporter protein of Orientia tsutsugamushi provides protective immunity against scrub typhus

BACKGROUND

Scrub typhus is an acute febrile disease caused by Orientia tsutsugamushi infection. Recently, the rapid increase of scrub typhus incidence in several countries within the endemic region has become a serious public health issue. Despite the wide range of preventative approaches that have been attempted in the past 70 years, all have failed to develop an effective prophylactic vaccine. Currently, the selection of the proper antigens is one of the critical barriers to generating cross-protective immunity against antigenically-variable strains of O. tsutsugamushi.

METHODOLOGY/PRINCIPAL FINDINGS

We examined the potential role of ScaA protein, an autotransporter protein of O. tsutsugamushi, in bacterial pathogenesis and evaluated the protective attributes of ScaA immunization in lethal O. tsutsugamushi infection in mice. Our findings demonstrate that ScaA functions as a bacterial adhesion factor, and anti-ScaA antibody significantly neutralizes bacterial infection of host cells. In addition, immunization with ScaA not only provides protective immunity against lethal challenges with the homologous strain, but also confers significant protection against heterologous strains when combined with TSA56, a major outer membrane protein of O. tsutsugamushi.

CONCLUSIONS/SIGNIFICANCE

Immunization of ScaA proteins provides protective immunity in mice when challenged with the homologous strain and significantly enhanced protective immunity against infection with heterologous strains. To our knowledge, this is the most promising result of scrub typhus vaccination trials against infection of heterologous strains in mouse models thus far.

Approaches to vaccines against Orientia tsutsugamushi

Scrub typhus is a severe mite-borne infection caused by Orientia tsutsugamushi, an obligately intracellular bacterium closely related to Rickettsia. The disease explains a substantial proportion of acute undifferentiated febrile cases that require hospitalization in rural areas of Asia, the North of Australia, and many islands of the Pacific Ocean. Delayed antibiotic treatment is common due to the lack of effective commercially available diagnostic tests and the lack of specificity of the early clinical presentation. The systemic infection of endothelial cells that line the vasculature with Orientia can lead to many complications and fatalities. In survivors, immunity does not last long, and is poorly cross-reactive among numerous strains. In addition, chronic infections are established in an unknown number of patients. All those characteristics justify the pursuit of a prophylactic vaccine against O. tsutsugamushi; however, despite continuous efforts to develop such a vaccine since World War II, the objective has not been attained. In this review, we discuss the history of vaccine development against Orientia to provide a clear picture of the challenges that we continue to face from the perspective of animal models and the immunological challenges posed by an intracellular bacterium that normally triggers a short-lived immune response. We finish with a proposal for development of an effective and safe vaccine for scrub typhus through a new approach with a strong focus on T cell-mediated immunity, empirical testing of the immunogenicity of proteins encoded by conserved genes, and assessment of protection in relevant animal models that truly mimic human scrub typhus.

Phylogenetic analysis of the 56-kDa type-specific protein genes of Orientia tsutsugamushi in Central Korea

There are several antigenic variants of Orientia tsutsugamushi. The 56-kDa type-specific antigen (TSA) is responsible for the antigenic variation. Nucleotide sequences of the 56-kDa TSA obtained from 44 eschar samples of Korean scrub typhus patients and from 40 representative strains retrieved from the GenBank database were analyzed phylogenetically. Clinical patient data were assessed based on the genotyping results. Of the 44 nucleotide sequences, 32 (72.7%) clustered with the Boryong genotype, which is the major genotype in Korea. Eleven nucleotide sequences (25%) clustered with the Kawasaki genotype, not identified in Korea until 2010. One nucleotide sequence was consistent with the Karp genotype. The clinical course of the patients infected with each genotype showed no differences. Diagnostic performance of the immunofluorescence assay (IFA) using the 56-kDa TSA from Gilliam, Karp and Boryong as test antigens were not different for the Boryong and Kawasaki genotypes. Although Boryong is still the predominant genotype, the results suggest that Kawasaki genotype is quite prevalent in Chungbuk province of Korea.

Application of ImageJ program to the enumeration of Orientia tsutsugamushi organisms cultured in vitro

The ImageJ program was applied to the enumeration of Orientia tsutsugamushi organisms in cell culture using indirect immunofluorescence assay (IFA). The highest correlation (r=0.984) was observed between manual counting methods and the ImageJ program (MaxEntropy threshold algorithm). This software-based methodology is cheaper, more standardised and better reproducible than a manual-based approach.

Analysis of the cross-reactivity of various 56 kDa recombinant protein antigens with serum samples collected after Orientia tsutsugamushi infection by ELISA

Orientia tsutsugamushi, the etiologic agent of scrub typhus, has a highly expressed and immunodominant 56-kD outer membrane protein. This protein is one of the leading candidates for diagnosis and vaccine development for scrub typhus. Previous studies using recombinant 56-kD protein (r56s) derived from Karp strain (Kpr56) in a mouse model have shown good homologous protection but only moderate to poor heterologous protection. We evaluated the cross-reactivity of recombinant 56-kD proteins from Karp, Kato, Gilliam, TA763, and three chimeric 56-kD proteins. Not all r56s are equally reactive with strain-specific serum samples. These data provide a first glance of how reactive these r56s are toward the antiserum of different strains and which r56 exhibits the broadest reactivity. A formulation of this combination has the potential to provide broad protection against the heterologous challenge and to be used in a highly sensitive diagnostic assay.

Broad-coverage molecular epidemiology of Orientia tsutsugamushi in Thailand

Orientia tsutsugamushi, an obligate intracellular bacterium closely related to the genus Rickettsia, is the causative agent of scrub typhus, a major cause of febrile illness in rural areas of Asia-Pacific region. Scrub typhus is transmitted by the bite of infected mites of the genus Leptotrombidium. The region of the 56-kDa TSA gene spanning from variable domain I (VDI) to variable domain IV (VDIV) was sequenced and used for genotyping 77 O. tsutsugamushi samples from human patients confirmed with scrub typhus from 2001 to 2003 and 2009 to 2010 in different regions of Thailand. These sequences were also compared to previously published 56-kDa TSA sequences. Only 4 genotypes out of 8 previously reported in Thailand were identified, i.e. Karp, JG-v, TA763 and Kato, respectively. Two strains were not associated with known genotypes but were closely related to Taiwanese strains. The Karp genotype was confirmed as the predominant clade. The JG-v and TA763 genotypes, in contrast to other studies, also were found. The genotype TA716 was not found, except for one strain previously described.

Kinetics and magnitude of antibody responses against the conserved 47-kilodalton antigen and the variable 56-kilodalton antigen in scrub typhus patients

Western blot analysis of Orientia tsutsugamushi whole-cell lysates with scrub typhus patient sera has identified at least five protein antigens of O. tsutsugamushi with molecular sizes of 22 kDa, 47 kDa, 56 kDa, 58 kDa, and 110 kDa. In this study, sera from serial bleedings of 108 patients were used to study the kinetics and the magnitude of specific antibody responses against the 47-kDa and 56-kDa antigens. Recombinant protein of the conserved 47-kDa antigen (r47b) or a mixture of truncated 56-kDa antigen (r56s) from three prototype strains was used as the antigen in an enzyme-linked immunosorbent assay (ELISA). Our results showed that 76% and 93% of these patients had elevated IgM and IgG against r47b, respectively, and 98% and 100% had elevated IgM and IgG against r56s, respectively. The kinetics of antibody responses against r47b and r56s can be grouped into three patterns. In the first type of response, IgM and IgG against r47b and r56s appeared about the same time. The IgM and IgG titers against r56s were much higher than those against r47b. In the second type of response, induction of IgM appeared to be similar to that in the first type. The major difference to the first type is that the IgG titers against r47b were induced at least 1 week later than those against the r56s. The third type showed strong IgG responses against both r47b and r56s, and low or no IgM responses indicated a secondary infection. This is the first systematic investigation of antibody response kinetics against the conserved 47-kDa antigen versus the variable 56-kDa antigen in scrub typhus patients.

Subversion of host cell signaling by Orientia tsutsugamushi

Progress has been made in deciphering the mechanisms on Orientia tsutsugamushi-host interaction. The genome sequencing, microarray and proteomic analyses of this ancient bacterium have provided a wealth of new information. This paper reviews the general characteristics of O. tsutsugamushi and recent developments especially in signaling events involved in the bacteria--host interaction.

Molecular epidemiology of Orientia tsutsugamushi in Cambodia and Central Vietnam reveals a broad region-wide genetic diversity

Scrub typhus is an acute infectious disease caused by an obligate intracellular bacterium Orientia tsutsugamushi following the bite of infected trombiculid mites of the genus Leptotrombidium. This zoonotic disease is a major cause of febrile illness in the Asia-Pacific region, with a large spectrum of clinical manifestations from unapparent or mild disease to fatal disease. O. tsutsugamushi is characterized by a very high genomic plasticity and a large number of antigenic variants amongst strains. The 56-kDa type specific antigen (TSA) gene, encoding the major antigenic protein, was used as reference to investigate the genetic relationships between the strains and to genotype O. tsutsugamushi isolates. The open reading frame of the 56-kDa TSA gene of 41 sequences (28 Cambodian and 13 Vietnamese strains) from patient samples were sequenced and used for genotyping. The 28 Cambodian isolates clustered into 5 major groups, including Karp (43.5%), JG-v (25%), Kato/TA716 (21.5%), TA763 (3.5%) and Gilliam (3.5%). Karp (77%), TA763 (15.5%) and JG-v (7.5%) strains were identified amongst the 13 Vietnamese isolates. This is the first countrywide genotyping description in Cambodia and in Central Vietnam. These results demonstrate the considerable diversity of genotypes in co-circulation in both countries. The genotyping result might raise awareness amongst Cambodian and Vietnamese clinicians of the high genetic diversity of circulating O. tsutsugamushi strains and provides unique and beneficial data for serological and molecular diagnosis of scrub typhus infections as well as raw materials for future studies and vaccine development.

Fibronectin facilitates the invasion of Orientia tsutsugamushi into host cells through interaction with a 56-kDa type-specific antigen

BACKGROUND

Orientia tsutsugamushi, the causative agent of scrub typhus, is an obligate intracellular bacterium. The pathogen's mechanism of cellular invasion is poorly characterized.

METHODS

Through ligand immunoblots, glutathione S-transferase (GST) pull-down assays, and in vitro inhibition assays of intracellular invasion, a bacterial ligand was identified and was shown to interact with fibronectin (Fn) to enhance the intracellular invasion of O. tsutsugamushi.

RESULTS

O. tsutsugamushi can bind to immobilized Fn in vitro, and exogenous Fn stimulates bacterial invasion of mammalian host cells. Bacterial invasion in the presence of Fn was abrogated by the addition of Arg-Gly-Asp peptides or by an anti-alpha5beta1 integrin antibody. Through a ligand immunoblot and GST pull-down assay, a 56-kDa type-specific antigen (TSA56) was identified as the bacterial ligand responsible for the interaction with Fn. Antigenic domain III and the adjacent C-terminal region (aa 243-349) of TSA56 interacted with Fn. Furthermore, we found that the enhanced invasion of the pathogen was abrogated by the addition of purified recombinant peptides derived from TSA56.

CONCLUSION

Fn facilitates the invasion of O. tsutsugamushi through its interaction with TSA56.

Survival of two Orientia tsutsugamushi bacterial strains that infect mouse macrophages with varying degrees of virulence

Orientia tsutsugamushi, an intracellular parasitic bacterium, comprises numerous strains of differing virulence. When BALB/c mice were infected intraperitoneally with this pathogen, a virulent strain known as Karp was found to multiply in the intraperitoneal macrophages and kill the mouse. In contrast, an avirulent strain, Kuroki, was shown to invade macrophages but be eliminated from the cells, allowing mouse survival. O. tsutsugamushi invades its host cell cytoplasm through phagocytosis and disruption of phagosomal membranes but some bacteria are then killed by phago-lysosomes within 1h of infection. Microscopic observations could not differentiate the Karp and Kuroki strains during entry and subsequent cell killing by phago-lysosomes. However, the Kuroki cells failed to divide and were markedly deformed following cytoplasmic invasion at several days post-infection. These findings suggest that macrophages have a mechanism to eliminate O. tsutsugamushi in the cytoplasm, if the invading bacteria escape phagosomal clearance, and that it is this mechanism that Kuroki does not survive. Additionally, significant levels of nitric oxide (NO) are produced in macrophages by Kuroki, but not by Karp. An NO synthase inhibitor, however, does not increase the growth of Kuroki, suggesting that NO is induced in a strain-dependent manner but does not effect proliferation.

Role of Syndecan-4 in the cellular invasion of Orientia tsutsugamushi

Cell surface heparan sulfate proteoglycans (HSPGs) play a critical role in the cellular invasion of intracellular bacteria and are presumed to have a role in the infection of host cells by Orientia tsutsugamushi. Previously, we showed that O. tsutsugamushi infection decreased markedly after treating host cells with heparinase, which suggests that HSPGs play an important role in oriential infection. We tested oriential infection in REF-Syn4 cells over-expressing syndecan-4, and in REF-Syn4AS cells in which the expression of syndecan-4 was down regulated by transfecting with anti-sense syndecan-4 cDNA. Oriential infection was found to be dependent on the expression level of syndecan-4 on the cell surface. Furthermore, the infectivity of O. tsutsugamushi was specifically reduced by treating O. tsutsugamushi with the purified recombinant core protein of syndecan-4 (Syn4E). These results suggest that the core protein of syndecan-4 and the heparin/heparan sulfate chain of syndecan play an important role in oriential infection by O. tsutsugamushi.

Recombinant 56-kilodalton major outer membrane protein antigen of Orientia tsutsugamushi Shanxi and its antigenicity

The gene encoding the 56-kDa protein of Orientia tsutsugamushi Shanxi was amplified by a nested PCR and cloned into the expression vector pQE30. The 56-kDa protein of O. tsutsugamushi Shanxi (Sxh56) was expressed as a fusion protein with the His(6)-binding protein of Escherichia coli by deleting the signal peptide-encoding sequence from the 5' end of the open reading frame. The recombinant protein formed inclusion bodies when expressed in E. coli M15. The recombinant protein was examined for reactivity with mouse sera against three antigenic prototypes of O. tsutsugamushi by an immunoblot assay. The recombinant Sxh56 reacted only to polyclonal antiserum to O. tsutsugamushi Gilliam in an enzyme-linked immunosorbent assay (ELISA) and in an immunoblot assay. Recombinant Sxh56 was purified by Ni-nitrilotriacetic acid affinity chromatography and injected into mice to evaluate its ability to stimulate immune responses. High levels of immunoglobulin G and T-cell proliferation appeared in mice immunized with the recombinant protein. The recombinant Sxh56 was used in an ELISA to evaluate the ability of the method to detect antibodies to O. tsutsugamushi in human and animal sera. Thirty sera from mice infected with O. tsutsugamushi Gilliam or Shanxi and 55 sera from normal mice were detected in the ELISA with recombinant Sxh56, and the sensitivity and specificity were 96.67 and 100%, respectively. One hundred fifty-one positive sera and 412 negative sera to O. tsutsugamushi Gilliam were detected in an indirect immunofluorescence assay with the recombinant protein, and the sensitivity and specificity were 96.36 and 88.08%, respectively. These results strongly suggest that the recombinant Sxh56 is a suitable type-specific immunodiagnostic antigen and vaccine candidate.

Orientia tsutsugamushi infection: overview and immune responses

Orientia tsutsugamushi, an obligate intracellular bacterium, was isolated for the first time in 1930. Infections by virulent strains are characterized by fever, rash, eschar, pneumonia, myocarditis, and disseminated intravascular coagulation. Here we review the general aspects of O. tsutsugamushi and immune responses in terms of inflammation, protective immune mechanisms, and immunogenic antigens.

Neutralization epitopes on the antigenic domain II of the Orientia tsutsugamushi 56-kDa protein revealed by monoclonal antibodies

Monoclonal antibodies (MoAbs) reactive with the authentic Orientia tsutsugamushi 56-kDa protein were generated. MoAb FS10 and FS15 showed in vitro, as well as, in vivo neutralizing activity upon O. tsutsugamushi infection. Deletion mutants of the gene for 56-kDa protein of O. tsutsugamushi Boryong were expressed to map the binding region. FS10 and FS15 are bound to amino acids (aa) located in an antigenic domain II, at residues 140-160 and 187-214, respectively. Computer modeling indicated that aa 146-153 were important for antigenicity against FS10. A sequence for aa 142-150 was highly homologous between oriential strains. These results suggest that the antigenic determinant for neutralizing MoAbs is an epitope within aa 140-160. Furthermore, this region may be important for the adhesion/invasion or intracellular survival of O. tsutsugamushi within host cells.

Homotypic and heterotypic antibody responses to a 56-kilodalton protein of Orientia tsutsugamushi

We analyzed homotypic and heterotypic antibody responses to a type-specific antigen (Tsa), a 56-kDa protein of Orientia tsutsugamushi, by using sera from mice immunized with strains Gilliam, Karp, Kato, and Boryong. We generated a series of deletion constructs of the tsa gene and expressed them as MalE fusion proteins. Variable domain I (VD I) showed strong responses to homotypic antibodies. Antigenic domain II (AD II) from Boryong and Karp showed cross-reactivities to each other. VD III showed no responses to any of the antibodies. Sera from Kato-immunized mice showed only homotypic responses to AD III. On the other hand, sera of the mice immunized with Gilliam, Karp, or Boryong showed homotypic as well as heterotypic responses to this region. VD IV showed the strongest heterotypic antibody responses among the fragments tested. These data suggest that VD I is important in homotypic antibody responses and that AD II, AD III, and VD IV are important in heterotypic antibody responses of the mice to Tsa.

Disassembly of focal adhesions during apoptosis of endothelial cell line ECV304 infected with Orientia tsutsugamushi

Many bacterial pathogens induce apoptosis in their host cells. We observed the cellular effect of ECV304 cells infected with Orientia tsutsugamushi. The infected cells became rounded and floated in culture supernatant. These floating cells as well as adherent cells exhibited typical features of apoptosis, such as DNA fragmentation and TUNEL staining. As many cells detached from growth substrate, we examined the focal adhesion using the immunofluorescence assay method and observed decreased focal adhesions in heavily infected cells. As endothelial cells could undergo apoptosis by the loss of focal adhesions, this change of focal adhesions may account for the Orientia-induced apoptosis.

Apoptosis of endothelial cell line ECV304 persistently infected with Orientia tsutsugamushi

Endothelial cells are major targets of Orientia tsutsugamushi. To examine the consequences of the infection of endothelial cells with O. tsutsugamushi, we used human endothelial cell line ECV304. Persistent infection was established and infected cultures could be maintained for over seven months without the addition of normal cells. The heavily infected cells became round and floated in the culture medium, harboring large numbers of organisms inside them. Some of the infected ECV304 cells showed features of apoptotic cells, as determined by the terminal deoxytransferase-mediated dUTP nick end-labeling reaction and DNA fragmentation. We also found that O. tsutsugamushi increased transcription of the mRNAs of proinflammatory cytokines such as IL-6 and IL-8. These results show the first evidence of in vitro-persistent infection by O. tsutsugamushi, which may be related to in vivo persistence reported previously.