Distribution of immunogenic epitopes on the two major immunodominant proteins (rOmpA and rOmpB) of Rickettsia conorii among the other rickettsiae of the spotted fever group

Targeted knockout of the Rickettsia rickettsii OmpA surface antigen does not diminish virulence in a mammalian model system

Strains of Rickettsia rickettsii, the causative agent of Rocky Mountain spotted fever (RMSF), differ dramatically in virulence despite >99% genetic homology. Spotted fever group (SFG) rickettsiae produce two immunodominant outer membrane proteins, rickettsial OmpA (rOmpA) and rOmpB, which are conserved throughout the SFG and thought to be fundamental to pathogenesis. rOmpA is present in all virulent strains of R. rickettsii but is not produced in the only documented avirulent strain, Iowa, due to a premature stop codon. Here we report the creation of an isogenic ompA mutant in the highly virulent strain Sheila Smith by insertion of intronic RNA to create a premature stop codon 312 bp downstream of the 6,747-bp open reading frame initiation site (int312). Targeted insertion was accomplished using an LtrA group II intron retrohoming system. Growth and entry rates of Sheila Smith ompA::int312 in Vero cells remained comparable to those of the wild type. Virulence was assessed in a guinea pig model by challenge with 100 PFU of either ompA::int312 Sheila Smith or the wild type, but no significant difference in either fever peak (40.5°C) or duration (8 days) were shown between the wild type and the knockout. The ability to disrupt genes in a site-specific manner using an LtrA group II intron system provides an important new tool for evaluation of potential virulence determinants in rickettsial disease research.

R. rickettsii rOmpA is an immunodominant outer membrane autotransporter conserved in the spotted fever group. Previous studies and genomic comparisons suggest that rOmpA is involved in adhesion and may be critical for virulence. Little information is available for rickettsial virulence factors in an isogenic background, as limited systems for targeted gene disruption are currently available. Here we describe the creation of an rOmpA knockout by insertion of a premature stop codon into the 5′ end of the open reading frame using a group II intron system. An isogenic rOmpA knockout mutation in the highly virulent Sheila Smith strain did not cause attenuation in a guinea pig model of infection, and no altered phenotype was observed in cell culture. We conclude that rOmpA is not critical for virulence in a guinea pig model but may play a role in survival or transmission from the tick vector.

Protective immunity against Rickettsia heilongjiangensis in a C3H/HeN mouse model mediated by outer membrane protein B-pulsed dendritic cells

Rickettsia heilongjiangensis is an obligate intracellular bacterium that causes Far-Eastern tick-borne spotted fever. Outer membrane protein B (OmpB) is an important surface protein antigen of rickettsiae. In the present study, the ompB gene of R. heilongjiangensis was divided into four fragments, resulting in four recombinant proteins (OmpB-p1, OmpB-p2, OmpB-p3, and OmpB-p4). Each OmpB was used in vitro to stimulate murine bone marrow-derived dendritic cells (BMDCs) of C3H/HeN mice, and the OmpB-pulsed BMDCs were transferred to naïve C3H/HeN mice. On day 14 post-transfer of BMDCs, the mice were challenged with R. heilongjiangensis and the rickettsial loads in the mice were quantitatively determined on day 7 post-challenge. Mice receiving BMDCs pulsed with OmpB-p2, OmpB-p3, or OmpB-p4 exhibited significantly lower bacterial load compared with mice receiving OmpB-p1-pulsed BMDCs. CD4(+) and CD8(+) T cells isolated from the spleen of C3H/HeN mice receiving BMDCs pulsed with each OmpB were co-cultured with BMDCs pulsed with the respective cognate protein. In flow cytometric analysis, the expression level of CD69 on CD4(+) or CD8(+) T cells from mice receiving BMDCs pulsed with OmpB-p2, OmpB-p3, or OmpB-p4 was higher than that on cells from mice receiving OmpB-p1-pulsed BMDCs, while the expression level of tumor necrosis factor (TNF)-α on CD8(+) T cells and interferon (IFN)-γ on the CD4(+) and CD8(+) T cells from mice receiving OmpB-p2, -p3, or -p4 was significantly higher than on cells from mice receiving OmpB-p1-pulsed BMDCs. Our results suggest that the protective OmpBs could activate CD4(+) and CD8(+) T cells and drive their differentiation toward CD4(+) Th1 and CD8(+) Tcl cells, respectively, which produce greater amounts of TNF-α and, in particular, IFN-γ, to enhance rickettsicidal activity of host cells.

Spotted fever group--Rickettsiae in the Tyrols: evidence by seroepidemiology and PCR

The aim of our study was to assess the occurrence of Rickettsia in the inner-alpine valleys of the Eastern Alps and to determine the amount of seroreaction among the local human population. Ticks were investigated by PCR and the percentage of seropositives was determined among local blood donors by an in-house immunofluorescence assay. The local cut-off titre for screening of IgG was set at 1 : 128 with a well-characterised low-risk collective according to WHO-guidelines. Positive sera were confirmed by independent re-testing. Rickettsia is present in ticks north and south of the continental divide. Of 259 ticks investigated, 12.4% are positive for Rickettsia. Of over 1200 blood donors tested so far, 7.7% bear IgG at a titre of 1 : 128 or higher against R. helvetica. R. helvetica is present in the study area, causes immunoreaction among local residents and is associated with anamnestic erythema. Furthermore, screening with a second Spotted Fever Group Rickettsia indicates that significant parts of the Tyrolean population are exposed to a Rickettsia other than R. helvetica.

Molecular basis of immunity to rickettsial infection conferred through outer membrane protein B

Pathogenic rickettsiae are the causative agents of Rocky Mountain spotted fever, typhus, and other human diseases with high mortality and an important impact on society. Although survivors of rickettsial infections are considered immune to disease, the molecular basis of this immunity or the identification of protective antigens that enable vaccine development was hitherto not known. By exploring the molecular pathogenesis of Rickettsia conorii, the agent of Mediterranean spotted fever, we report here that the autotransporter protein, rickettsial outer membrane protein B (rOmpB), constitutes a protective antigen for this group of pathogens. A recombinant, purified rOmpB passenger domain fragment comprised of amino acids 36 to 1334 is sufficient to elicit humoral immune responses that protect animals against lethal disease. Protective immunity requires folded antigen and production of antibodies that recognize conformational epitopes on the rickettsial surface. Monoclonal antibodies (MAbs) 5C7.27 and 5C7.31, which specifically recognize a conformation present in the folded, intact rOmpB passenger domain, are sufficient to confer immunity in vivo. Analyses in vitro indicate this protection involves a mechanism of complement-mediated killing in mammalian blood, a means of rickettsial clearance that has not been previously described. Considering the evolutionary conservation of rOmpB and its crucial contribution to bacterial invasion of host cells, we propose that rOmpB antibody-mediated killing confers immunity to rickettsial infection.

Characterization of rickettsial adhesin Adr2 belonging to a new group of adhesins in α-proteobacteria

BACKGROUND

Rickettsia prowazekii is the etiological agent of epidemic typhus and is an obligate intracellular bacterium that grows as a parasite freely within the cytoplasm of a eukaryotic host cell. Previous studies have shown that rOmpA and rOmpB which belong to the family of rickettsial cell surface antigens are involved in vitro in the adhesion of Rickettsiae to epithelial cells. Recently, two putative rickettsial adhesins have been identified using high resolution 2D-PAGE coupled with mass spectrometry. In this study, we further characterize and describe the adhesin Adr2 from R. prowazekii.

METHODOLOGY/PRINCIPAL FINDINGS

Using an overlay assay coupled with mass spectrometry two adhesins, Adr1 (RP827) and Adr2 (RP828), were identified from the R. prowazekii proteome Recombinant R. prowazekii Adr2 was expressed through fusion with Dsbc in Escherichia coli, purified and concentrated, thus allowing production of specific monoclonal antibodies, as confirmed by western blot assays. Finally, inhibition of rickettsiae-induced cytotoxicity with monoclonal anti-Adr2 antibody has showed a greatest impact on bacterial cell entry at 8 h post-infection (ca50%) and then decreased progressively to attempt 18% of inhibition at day 7. These, correlated to the inhibition of rickettsiae-induced cytotoxicity with monoclonal anti-rOmpB antibody. Thus, Adr2 is sufficient to mediate R. prowazekii entry into the cell at early stage of mammalian cell infection.

CONCLUSIONS

Our results suggest that R. prowazekii Adr2 could be the main actor promoting the entry of rickettsiae into the host cells. The present study opens the framework for future investigations for better understanding of the Adr2 -mediated mechanisms involved in adhesion/invasion or intracellular survival of R. prowazekii.

Rickettsia phylogenomics: unwinding the intricacies of obligate intracellular life

Background

Completed genome sequences are rapidly increasing for Rickettsia, obligate intracellular α-proteobacteria responsible for various human diseases, including epidemic typhus and Rocky Mountain spotted fever. In light of phylogeny, the establishment of orthologous groups (OGs) of open reading frames (ORFs) will distinguish the core rickettsial genes and other group specific genes (class 1 OGs or C1OGs) from those distributed indiscriminately throughout the rickettsial tree (class 2 OG or C2OGs).

Methodology/Principal Findings

We present 1823 representative (no gene duplications) and 259 non-representative (at least one gene duplication) rickettsial OGs. While the highly reductive (∼1.2 MB) Rickettsia genomes range in predicted ORFs from 872 to 1512, a core of 752 OGs was identified, depicting the essential Rickettsia genes. Unsurprisingly, this core lacks many metabolic genes, reflecting the dependence on host resources for growth and survival. Additionally, we bolster our recent reclassification of Rickettsia by identifying OGs that define the AG (ancestral group), TG (typhus group), TRG (transitional group), and SFG (spotted fever group) rickettsiae. OGs for insect-associated species, tick-associated species and species that harbor plasmids were also predicted. Through superimposition of all OGs over robust phylogeny estimation, we discern between C1OGs and C2OGs, the latter depicting genes either decaying from the conserved C1OGs or acquired laterally. Finally, scrutiny of non-representative OGs revealed high levels of split genes versus gene duplications, with both phenomena confounding gene orthology assignment. Interestingly, non-representative OGs, as well as OGs comprised of several gene families typically involved in microbial pathogenicity and/or the acquisition of virulence factors, fall predominantly within C2OG distributions.

Conclusion/Significance

Collectively, we determined the relative conservation and distribution of 14354 predicted ORFs from 10 rickettsial genomes across robust phylogeny estimation. The data, available at PATRIC (PathoSystems Resource Integration Center), provide novel information for unwinding the intricacies associated with Rickettsia pathogenesis, expanding the range of potential diagnostic, vaccine and therapeutic targets.

Evaluation of conventional and real-time PCR assays for detection and differentiation of Spotted Fever Group Rickettsia in dog blood

Spotted Fever Group Rickettsia is important cause of emerging and re-emerging infectious disease in people and dogs. Importantly, dogs can serve as sentinels for disease in people. Sensitive and specific diagnostic tests that differentiate among species of infecting Rickettsia are needed. The objective of this study was to develop a sensitive and specific PCR that differentiates SFG Rickettsia infecting dog blood. Conventional and real-time PCR assays were developed using primers that targeted a small region of the ompA gene. Their sensitivity, determined by testing a cloned target sequence in the presence of host DNA, was 15-30 and 5 copies of DNA, respectively. Testing of Rickettsia cultures and analysis of Rickettsia gene sequences deposited in GenBank verified DNA could be amplified and used to differentiate species. DNA from the blood of infected dogs was also tested. Importantly, Rickettsia DNA was detected before seroconversion in some dogs. The species of infecting Rickettsia was also identified. We conclude these assays may assist in the timely diagnosis of infection with SFG Rickettsia. They may also facilitate the discovery of novel SFG Rickettsia infecting dogs, and in the investigation of dogs as sentinels for emerging rickettsioses.

Identification of two putative rickettsial adhesins by proteomic analysis

The rickettsial membrane proteins that promote their uptake by eukaryotic host cells are unknown. To identify rickettsial ligand(s) that bind host cell surface proteins, biotinylated epithelial cells were used to probe a nitrocellulose membrane containing rickettsial extracts separated by SDS-PAGE. This overlay assay revealed that two close rickettsial ligands of approximately 32-30 kDa were recognized by host cells. Both proteins were identified using high resolution 2D-PAGE coupled with mass spectrometry analysis. One protein was identified as the C-terminal extremity of rOmpB called the beta-peptide. The second interacting protein was identified as a protein of unknown function encoded by RC1281 and RP828 in Rickettsia conorii and in Rickettsia prowazekii, respectively, that shares strong similarities with other bacterial adhesins. Both proteins are highly conserved within the Rickettsia genus and might play a critical role in their pathogenicity. These data may have important implications for the development of future vaccines against rickettsial infections.

Proteome analysis of Rickettsia conorii by two-dimensional gel electrophoresis coupled with mass spectrometry

The availability of genome sequence offers the opportunity to further expand our knowledge about proteins expressed by Rickettsia conorii, strictly intracellular bacterium responsible for Mediterranean spotted fever. Using two-dimensional polyacrylamide gel electrophoresis combined with MALDI-TOF mass spectrometry, we established the first reference map of R. conorii proteome. This approach also allowed identification of GroEL as the major antigen recognized by rabbit serum and sera of infected patients. Altogether, this work opens the way to characterize the proteome of R. conorii, to compare protein profiles of different isolates or of bacteria maintained under different experimental conditions and to identify immunogenic proteins as potential vaccine targets.

Detection of R. felis and R. typhi in fleas using monoclonal antibodies

Rickettsia typhi and R. felis are flea-transmitted human pathogenic rickettsial species. To investigate the distributional dynamics of these rickettsiae we designed a micro-immunofluorescence assay (MIF) using species-specific monoclonal antibodies (MAbs) applied to flea cryosections. Our assay was performed in less than 3 h and its applicability was demonstrated by the detection of R. typhi in 50 artificially infected human body lice but in none of 50 uninfected lice. With MIF, we identified 31 positive among 32 fleas proven with PCR to be naturally infected with R. felis; and 7 positive among 32 fleas proven with PCR to be naturally infected with R. typhi. No cross-detection was observed with both MAbs. Fresh R. felis-infected fleas were significantly more MIF-positive than long conserved R. typhi-infected fleas (31/32 vs. 7/32, P < 0.01). This discrepancy may be linked to degradation of antigens by long-term freezing. For R. typhi-infected fleas, our assay was significantly more efficient when applied to fleas in early stages of infection (less than 15 days) by comparison with fleas frozen more than 20 days after infection (7/15 vs. 0/17, P = 0.01). This difference may be related to an antigenic modification caused by selection pressure in the vector and host process. The sensitivity of the described method did not exceed 47% (7/15) for R. typhi but, in contrast, was 97% for R. felis. Thus, our method appears to be useful for surveillance in R. felis infections, but requires further studies for the detection of R. typhi.

Antigenic classification of Rickettsia felis by using monoclonal and polyclonal antibodies

Rickettsia felis is a flea-transmitted rickettsia. There is a discrepancy between its reported phylogenic and phenotypic identifications. Following the first report of R. felis, it was considered by tests with serologic reagents to be closely related to another recognized flea-transmitted rickettia, R. typhi. Subsequently, it appeared to be more closely related to spotted fever group (SFG) rickettsiae by genetic analysis. In the present work, R. felis was studied by microimmunofluorescence (MIF) serologic typing and with monoclonal antibodies (MAbs). Mouse polyclonal antisera to R. felis cross-reacted only with SFG rickettsiae. A neighbor-joining analysis based on MIF indicated that R. felis is actually related to SFG rickettsiae antigenically, clustering with R. australis, R. akari, and R. montanensis. A panel of 21 MAbs was raised against a 120-kDa protein antigen or a 17-kDa polypeptide of R. felis. They cross-reacted with most members of the SFG rickettsiae but not with R. prowazekii, R. typhi, or R. canadensis of the typhus group (TG) rickettsiae. Sixty-four MAbs previously generated to seven other ricketttsial species were tested with R. felis. Three MAbs reacted with the 120-kDa antigen and were generated by R. africae, R. conorii, and R. akari, respectively. They exhibited cross-reactivities with R. felis. All our data show that R. felis harbors the antigenic profile of an SFG rickettsia.

Detection of Rickettsia prowazekii in body lice and their feces by using monoclonal antibodies

In order to identify Rickettsia prowazekii in lice, we developed a panel of 29 representative monoclonal antibodies selected from 187 positive hybridomas made by fusing splenocytes of immunized mice with SP2/0-Ag14 myeloma cells. Immunoblotting revealed that 15 monoclonal antibodies reacted with the lipopolysaccharide-like (LPS-L) antigen and 14 reacted with the epitopes of a 120-kDa protein. Only typhus group rickettsiae reacted with the monoclonal antibodies against LPS-L. R. felis, a recently identified rickettsial species, did not react with these monoclonal antibodies, confirming that it is not antigenically related to the typhus group. Monoclonal antibodies against the 120-kDa protein were highly specific for R. prowazekii. We successfully applied a selected monoclonal antibody against the 120-kDa protein to detect by immunofluorescence assay R. prowazekii in smears from 56 wild and laboratory lice, as well as in 10 samples of louse feces infected or not infected with the organism. We have developed a simple, practical, and specific diagnostic assay for clinical specimens and large-scale epidemiological surveys with a sensitivity of 91%. These monoclonal antibodies could be added to the rickettsial diagnostic panel and be used to differentiate R. prowazekii from other rickettsial species.

Genetic classification of "Rickettsia heilongjiangii" and "Rickettsia hulinii," two Chinese spotted fever group rickettsiae

To determine the phylogenetic position of two new rickettsial strains isolated from ticks in China, 16S ribosomal DNA, gltA, and ompA (apart from the tandem repeat units) genes were amplified by PCR and sequenced. The phylogenetic relationships between these strains and other rickettsiae were inferred from the comparison of sequences of the three genes by the parsimony, neighbor-joining, and maximum-likelihood methods. The results demonstrated that the 054 strain, a rickettsia pathogenic in humans, and the HL-93 strain were related and clustered together with Rickettsia japonica. Significant statistical bootstrap values (100 and 92%) supported the nodes in this cluster. Based on previous genotypic and antigenic data and the phylogenetic analysis presented here, the 054 and HL-93 strains should be considered as new species, and we formally propose that they be named "Rickettsia heilongjiangii" and "Rickettsia hulinii," respectively.

Phenotypic and genotypic homogeneity of the strains of Rickettsia japonica isolated from patients with Oriental spotted fever

Nine pathogenic strains of Rickettsia japonica isolated from patients with Oriental spotted fever were compared phenotypically and genotypically. Constitution and antigenicity of the proteins demonstrated to be the same among strains. Polymerase chain reaction (PCR) amplification of the two major outer membrane protein genes (ompA and ompB) and an intracellular spotted fever group-common antigen protein gene (rps120) produced the same sizes of products for all strains. Restriction fragment length polymorphism of the PCR products showed the same pattern among strains with each endonuclease. Thus, these strains belong to a single type, the same as the type strain YH (=ATCC VR-1363).

Taxonomic relationships among spotted fever group rickettsiae as revealed by antigenic analysis with monoclonal antibodies

The spotted fever group (SFG) is made up of more than 20 different rickettsial species and strains. Study of the taxonomic relationships among the group has been attempted by phenotypic, genotypic, and phylogenetic analyses. In this study, we determined taxonomic relationships among the SFG rickettsiae by comparative analysis of immunogenic epitopes reactive against a panel of monoclonal antibodies. A total of 98 monoclonal antibodies, which were directed against epitopes on the major immunodominant proteins or on the lipopolysaccharide-like antigens of strains of Rickettsia africae, Rickettsia conorii, Rickettsia massiliae, Rickettsia akari, Rickettsia sibirica, and Rickettsia slovaca, were used in the study. The distribution and expression of the epitopes among 29 SFG rickettsiae and Rickettsia bellii were assessed by determination of reaction titers in a microimmunofluorescence assay. The results were scored as numerical taxonomic data, and cluster analysis was used to construct a dendrogram. The architecture of this dendrogram was consistent with previous taxonomic studies, and the implications of this and other findings are discussed.