Antigenic analysis of Chinese strains of spotted fever group rickettsiae by protein immunoblotting

Multispacer Typing (MST) of Spotted Fever Group Rickettsiae Isolated from Humans and Rats in Chengmai County, Hainan Province, China

Spotted fever caused by spotted fever group rickettsiae (SFGR) is found throughout China. During 2007–2008, 28 human SFGR isolates and 34 rat SFGR isolates including 15 isolates from Rattus fulvescens, 5 isolates from R. edwardsi, 7 isolates from Callosciurus erythraeus roberti and 7 isolates from Dremomys rufigenis) were obtained from L929 cell culture. Previous research indicated that the 62 strains of SFGR mentioned above shared not only the same serophenotype but also 100% of identity sequences of 16S rRNA, gltA, ompA, groEL and 17KD, which enabled us to apply multispacer typing (MST) to the 62 SFGR isolates in the study. Six primer pairs, which were used for typing of Rickettsia rickettsii and Rickettsia conorii, were chosen, and the results exhibited greater nucleotide polymorphisms among the 62 isolates tested. A total of 48 distinct genotypes were identified. The dominant genotype, represented by h3 isolates, accounted for 21.7% (13/60) of the isolates tested, and the remaining 47 genotypes were all unique. Phylogenetic analysis showed that all the 48 genotypes could be classified in the same clade, while the genetically related strain, R. heilongjiangensis, was close but not the same as the cluster. We concluded that the genetically diverse of spotted fever group rickettsiae strains are endemic in Chengmai County, Hainan Province, China.

Detection of A. phagocytophilum and E. chaffeensis in patient and mouse blood and ticks by a duplex real-time PCR assay

Human granulocytic anaplasmosis (HGA) and human monocytic ehrlichiosis (HME) are emerging, tick-borne, zoonotic infectious diseases caused by Anaplasma phagocytophilum and Ehrlichia chaffeensis, respectively. Early diagnosis is essential for rapid clinical treatment to avoid misdiagnosis and severe patient outcomes. Simple, sensitive and reliable diagnostic methods are urgently needed. In this study, we developed a duplex real-time PCR assay targeting the A. phagocytophilum ankA gene and the E. chaffeensis TRP120 gene, respectively. The lowest limit of detection of the duplex real-time PCR assay was 100 copies of the targeted A. phagocytophilum ankA gene and the E. chaffeensis TRP120 gene per reaction, and the specificity was 100%. Detection in blood DNA samples from the acute stage of illness for 22 HGA cases and 8 HME cases indicated that the duplex real-time PCR assay was more sensitive than the nested PCR assay. The infection of Citellusundulatus Pallas with A. phagocytophilum and E. chaffeensis was first confirmed in Xinjiang Province and the positive rate was 3.1% for A. phagocytophilum, 6.3% for E. chaffeensis and 3.1% for co-infection with both pathogens. The rates of A. phagocytophilum and E. chaffeensis infection of D. silvarum ticks collected from Shanxi Province were 8.2% and 14.8%, respectively, and the co-infection rate was 3.3%. The rates of A. phagocytophilum and E. chaffeensis infection in H. longicornis ticks collected from Shandong Province were 1.6% and 6.3%, respectively, and the co-infection rate was 1.6%.

Rapid, simple, and sensitive detection of the ompB gene of spotted fever group rickettsiae by loop-mediated isothermal amplification

Background

Spotted fever caused spotted fever group rickettsiae (SFGR) is prevalent throughout China. In this study, we describe a rapid, simple, and sensitive loop-mediated isothermal amplification (LAMP) assay targeting the ompB gene of spotted fever group rickettsiae ideal for application in China. The LAMP assay has the potential to detect spotted fever group rickettsiae early in infection and could therefore serve as an alternative to existing methods.

Methods

A set of universal primers which are specific 7 common species of spotted fever group rickettsiae in China were designed using PrimerExplorer V4 software based on conserved sequences of ompB gene. The sensitivity, specificity and reproducibility of the LAMP were evaluated. The LAMP assay for detecting SFGR was compared with conventional PCR assays for sensitivity and specificity in early phase blood samples obtained from 11 infected human subjects.

Results

The sensitivity of the LAMP assay was five copies per reaction (25 μL total volume), and the assay did not detect false-positive amplification across 42 strains of 27 members of the order Rickettsiales and 17 common clinical pathogens. The LAMP assay was negative to typhus group rickettsiae including R. prowazekii and R. typhi for no available conserved sequences of ompB was obtained for designing primers.

To evaluate the clinical applicability of the LAMP assay, a total of 11 clinical samples, 10 samples confirmed serologically (3 cases), ecologically (1 case), by real-time polymerase chain reaction (PCR; 2 cases), ecologically and by real-time PCR (1 case), and serologically and by real-time PCR (3 cases) were analyzed by the ompB LAMP assay. Data were validated using a previously established nested PCR protocol and real-time PCR. A positive LAMP result was obtained for 8 of the 10 confirmed cases (sensitivity, 73%; specificity, 100%), while none of these samples were positive by nested PCR (sensitivity, 0%; specificity, 100%).

Conclusions

The LAMP assay described here is the most reliable among the three methods tested and would be an ideal choice for development as a rapid and cost-effective means of detecting SFGR in China.

Production of monoclonal antibodies against Rickettsia massiliae and their use in antigenic and epidemiological studies

Rickettsiae are gram-negative, obligate intracellular bacteria which have historically been divided into three groups: the typhus group, the scrub typhus group, and the spotted fever group (SFG). Recently, several new SFG rickettsiae have been characterized, and most of these species are associated with ticks and have, as yet, no known pathogenicity toward humans. Rickettsia massiliae, which is widely distributed in Europe and Africa, is one such rickettsia. In order to investigate the antigenic relationships between R. massiliae and other rickettsial species and to develop a more convenient methodology for identifying R. massiliae, we produced monoclonal antibodies against the type strain (Mtu1T) of R. massiliae by fusing immunized splenocytes with SP2/0-Ag14 myeloma cells. A panel of 16 representatives were selected from the 163 positive hybridomas identified on initial screening, and their secreted monoclonal antibodies were further characterized. The reactivities of these 16 monoclonal antibodies with a large panel of rickettsial species were assessed by the microimmunofluorescence assay. All species of the SFG rickettsiae reacted with the monoclonal antibodies directed against epitopes on lipopolysaccharide, which is the common antigen among the SFG rickettsiae. Some closely related species of the SFG, such as Bar29, "R. aeschlimanni," and R. rhipicephali, showed strong cross-reactivities with the monoclonal antibodies directed against epitopes on the two major high-molecular-mass heat-labile proteins (106 and 120 kDa). In addition, species-specific monoclonal antibodies demonstrated that R. massiliae is antigenically different from other rickettsial species. Moreover, these species-specific monoclonal antibodies were successfully used for identifying R. massiliae in the ticks collected from southern France, and are therefore potentially useful tools in the identification and investigation of R. massiliae in ticks in large-scale field work.

Characterization of and application of monoclonal antibodies against Rickettsia africae, a newly recognized species of spotted fever group rickettsia

Rickettsia africae is a newly described species which causes African tick bite fever. Mediterranean spotted fever caused by R. conorii is endemic in the same regions of Africa as tick bite fever, and differentiation of the two syndromes by characterization of their etiological agents is important for epidemiological studies. R. africae and R. conorii are, however, difficult to distinguish, and therefore, our aim was to produce monoclonal antibodies to address this problem. Monoclonal antibodies were produced against R. africae by fusing splenocytes from BALB/C mice immunized with purified rickettsial organisms and SP2/0-Ag14 myeloma cells. A total of 355 hybridomas producing monoclonal antibodies to R. africae were identified by initial screening with six different antigens by microimmunofluorescence assay. A panel of 23 representative monoclonal antibodies were selected and subcloned. This panel was screened with a further 17 different spotted fever group (SFG) rickettsial reference antigens. Of these 23 monoclonal antibodies, 1 cross-reacted with only R. parkeri, whereas the others cross-reacted with more than two different antigens. Immunoblotting indicated that all the monoclonal antibodies were directed against the epitopes on two major high-molecular-mass heat-labile proteins, of which the molecular masses were 128 and 135 kDa, respectively. This monoclonal antibody panel was used successfully to identify R. africae in the blood culture of an infected patient, in infected cells within shell vials, and in infected ticks collected from Africa. Furthermore, the cross-reactivity of each SFG rickettsia with each of these 23 monoclonal antibodies was scored and was used to build a dendrogram of taxonomic relatedness between R. africae and the other SFG rickettsiae on the basis of Jaccard coefficients and unweighted pair group method with arithmetic mean analysis. The relatedness was generally consistent with that obtained by other methods of comparison.

Evidence for a high prevalence of spotted fever group rickettsial infections in diverse ecologic zones of Inner Mongolia

A 3-year study of spotted fever group rickettsial ecology in Inner Mongolia revealed that nearly half of the human population tested had antibodies to Rickettsia sibirica detected by complement fixation test. Infected persons, ticks and a high proportion of seropositive livestock and wild rodents were found in all five vegetation zones (desert, steppe, forest, forest-grassland and grassland).

Species-specific BALB/c mouse antibodies to rickettsiae studied by western blotting

BALB/c mice were inoculated intraperitoneally either once only, or up to four times at weekly intervals, with viable Rickettsia rickettsii, Rickettsia conorii or the Israeli spotted fever group rickettsia. Sera collected one week after the last inoculation were tested for the presence of antibodies reactive with the above organisms by indirect fluorescent antibody testing and Western blot. With repeated inoculations there was a general progressive rise in homologous and heterologous immunofluorescence titers although the increase after the first inoculation was always the greatest. For each rickettsia, the homologous titers were higher than the heterologous titers. Western blots showed that the reactive antibodies were against rickettsial high molecular mass species specific protein antigens and homologous species-specific antibody reactions were detectable earlier than heterologous cross-reacting antibody reactions. Antibodies in mice sera did not react with the group specific lipopolysaccharide-like antigens of the rickettsiae although such reactivity was strong in Western blots with sera from patients suffering from acute Rickettsia conorii infections. Our findings suggest that the intraperitoneal route of inoculation of BALB/c mice can be used for the differentiation of spotted fever group rickettsiae.

Differentiation among spotted fever group rickettsiae species by analysis of restriction fragment length polymorphism of PCR-amplified DNA

Restriction fragment length polymorphism (RFLP) analysis of PCR-amplified genes was used to study spotted fever group (SFG) rickettsiae, extending the previous work of Regnery et al. (R.L. Regnery, C.L. Spruill, and B.D. Plikaytis, J. Bacteriol. 173:1576-1589, 1991). Twenty-six strains of SFG rickettsia were studied, including several recognized species which have never been studied (R. parkeri, R. helvetica, and R. japonica) as well as strains which are not currently classified. Two previously used primer pairs derived from the R. prowazekii citrate syntase gene and the R. rickettsii 190-kDa protein antigen gene were studied, as were primer pairs obtained from the R. rickettsii 120-kDa protein antigen gene. By using three amplifications and three enzyme digestions, it was possible to differentiate between almost all of the known SFG rickettsia species and to differentiate between several strains of the R. conorii complex. Two human pathogens, "R. africae" and the Israeli tick typhus rickettsia, were first separated by using BG-12 pair primer amplification and then RsaI restriction endonuclease digestion. The proposed simplified model of identification may be useful in studying the geographical distributions of SFG rickettsiae.

Taxonomic position of the rickettsiae: current knowledge

The term rickettsiae initially encompassed all intracellular bacteria. Early rickettsial taxonomy was based on a comparison of a few phenotypic characteristics and recently, molecular studies brought new bases for rickettsial taxonomy. All rickettsial species studied so far belong to the alpha and gamma groups of the Proteobacteria. Ehrlichiae complex groups Cowdria ruminantium, Anaplasma marginale and Wolbachia pipientis and the related parthenogenesis and cytoplasmic incompatibility bacteria, whereas Rochalimaea species group with Bartonella bacilliformis. Rickettsia tsutsugamushi may form an independent lineage, whereas molecular data allow to regroup serologically defined typhus and spotted fever group rickettsiae. The true scale of Rickettsia and Coxiella genera remain to be determined.

Proteinic and genomic identification of spotted fever group rickettsiae isolated in the former USSR

Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), restriction fragment length polymorphism of polymerase chain reaction-amplified genes (RFLP-PCR), and pulsed-field gel electrophoresis (PFGE) were used to identify 25 isolates of spotted fever group rickettsia collected in the former USSR. Six Rickettsia akari isolates which were identical to the MK reference strain from the American Type Culture Collection were found. Also, 14 isolates were found to be Rickettsia sibirica and identical to reference strain 246. Two of three isolates previously considered as atypical, low-pathogenic strains of R. sibirica, were found to be strains of Rickettsia slovaca. The third, strain S, was similar in its RFLP-PCR profile to "R. africae" sp. nov. (proposed name for a rickettsia pathogenic for human beings in southern Africa) but in its SDS-PAGE and PFGE profiles was unique among spotted fever group rickettsiae. Strain M-1 was confirmed as a genetic variant of Rickettsia conorii. The Astrachan isolate, the causative agent of a tick-bite rickettsiosis at the North of the Caspian Sea, showed a previously described RFLP-PCR profile identical to that of the Israeli tick typhus rickettsia, but its SDS-PAGE and PFGE profiles different from those of the other strains tested.

Genotypic and antigenic identification of two new strains of spotted fever group rickettsiae isolated from China

Four isolates of spotted fever group rickettsiae isolated from ticks in China were compared with all known species and strains of spotted fever group rickettsiae by immunofluorescence assay, DNA polymerase chain reaction followed by restriction endonuclease fragment length polymorphism analysis, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and Western immunoblot. The Chinese isolates belonged to three types, including a novel serotype which has not been described before. One isolate obtained from tick ova of Dermacentor nuttallii in Inner Mongolia was antigenically and genotypically identical to Rickettsia sibirica. Two isolates obtained from Dermacentor sinicus collected from Beijing were identical, different from other members of spotted fever group rickettsiae but apparently closely related to R. sibirica. HA-91, a strain isolated from Hyalomma asiaticum bv. kozlovi olenew, was antigenically and genotypically unique among spotted fever group rickettsiae, and we feel that data presented here should prompt consideration of it as a new species on the basis of current rickettsial taxonomy.

Comparison of serologic typing, sodium dodecyl sulfate-polyacrylamide gel electrophoresis protein analysis, and genetic restriction fragment length polymorphism analysis for identification of rickettsiae: characterization of two new rickettsial strains

In 1990, 17 adult Rhipicephalus turanicus ticks were collected in the south of France. Two spotted fever group rickettsiae, Mtu1 and Mtu5, were isolated from the hemolymphs of two of these ticks by the centrifugation shell-vial technique by using HEL cells. These isolates were compared with reference spotted fever group rickettsial serotypes by using three identification methods: microimmunofluorescence serologic typing, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and polymerase chain reaction followed by restriction endonuclease fragment length polymorphism analysis. The results obtained by all these techniques showed that Mtu1 and Mtu5 are each previously undescribed rickettsial serotypes. A comparison of the three methods used to identify the isolates led us to the conclusion that, in large-scale epidemiological studies, the simplest way to identify isolates in ticks is to first use the polymerase chain reaction-restriction fragment length polymorphism analysis directly on triturated ticks as a screening method to detect interesting rickettsiae, and then attempt to isolate rickettsiae from ticks for identification by microimmunofluorescence and SDS-PAGE, both of which are time-consuming and expensive to carry out.

Ecological questions concerning rickettsiae

The past ten years were characterized by the appearance of several "new" transmissible spotted fever group (SFG) rickettsioses, e.g. Israeli, Japanese and Astrakhan fevers. The factors responsible for their establishment probably include the introduction of chemicals from industry, agriculture and the timber industry into natural habitats. Such factors may influence the pathogenicity of these rickettsiae. In this case, in addition to the human influence, the mechanism of the circulation of the agents under natural conditions of both abiotic (climate, etc.) and biotic (flora and fauna) components may play a decisive role. The modern management of breeding domestic animals, indoor and outdoor maintenance, seasonal migrations, new animal foods, stress, etc., can be important factors affecting the biological properties of the Q fever agent. Nonpathogenic rickettsiae, rickettsia-like symbionts and other microorganisms circulating in nature may also influence the pathogenic rickettsiae. Studies on their interrelationships in hosts and vectors may markedly contribute to the understanding of the circulation of pathogenic rickettsiae in nature. Recognition of factors causing the appearance of new rickettsial agents or differences in pathogenicity of rickettsial strains is important not only for the prognosis of rickettsial diseases but also for the prognosis of other infectious diseases.

Epidemiology of rickettsial diseases

Rickettsial diseases have a diversity of epidemiologic characteristics reflective of the variety of ecologic situations in which the obligate intracellular bacteria are transmitted to humans. For the spotted fever group (SFG) rickettsiae, Rickettsia typhi, R. tsutsugamushi, Coxiella burnetii, and the human ehrlichial agent, humans are a dead-end host who plays no role in the maintenance of the organism in nature. All rickettsioses exist as zoonoses. Moreover, all rickettsiae are found in infected arthopods, which generally serve as the natural hosts and can transmit the infection to the next generation of ticks, mites, chiggers, or fleas. From our anthropocentric viewpoint, Q fever aerosol infection from parturient animals and Brill-Zinsser disease ignited epidemics of louse-borne epidemic typhus are exceptions. However, silent cycles of C. burnetii in ticks and R. prowazekii in the flying squirrel flea may have maintained these agents in transovarial or enzootic cycles for eons before humans and their domestic animals arrived on the scene. Thus, the epidemiology of rickettsial diseases must be recognized as an unfortunate aberration of the rickettsial economy. Several excellent reviews of rickettsial ecology contain a wealth of useful information.

Causative agent of spotted fever group rickettsiosis in Japan

Since 1984, it has been known that spotted fever group rickettsiosis exists in Japan. We isolated three strains of the causative rickettsiae, designated Katayama, Misaka, and Abe, from patients with the disease and studied the characteristics of the isolates. Nude mice and cyclophosphamide-treated mice died after infection with the isolates. However, infected normal mice recovered and acquired immunity. Infected adult male guinea pigs had fever, a scrotal reaction, and seroconversion. The isolates propagated well in tissue-cultured Vero cells. Analysis by the cross-immunofluorescence antibody method showed that these isolates were closely related serologically. To reveal their immunological properties in detail, we produced 21 anti-Katayama monoclonal antibodies. Seven of these antibodies reacted with all representative strains of spotted fever group rickettsiae used in this study, and five others reacted only with the homologous strain, revealing that the Katayama strain has a strain-specific antigen(s) different from those of other spotted fever group rickettsiae. Moreover, these strain-specific antibodies also reacted with the Misaka and Abe strains. These results demonstrate that the causative agent of spotted fever group rickettsiosis in Japan is a new serotype of spotted fever group rickettsiae.